Original Article
309
Anterior Cruciate Ligament Tunnel Placement Brian R. Wolf, MD, MS1 MOON Knee Group3,
Austin J. Ramme, MD, PhD2
1 Department of Orthopaedics and Rehabilitation, University of Iowa,
Iowa City, Iowa 2 Department of Orthopaedic Surgery, University of Iowa, Iowa City, Iowa 3 Medical School, Vanderbilt University, Nashville, Tennessee
Carla L. Britton, PhD1
Annunziato Amendola, MD1
Address for correspondence Brian R. Wolf, MD, MS, Department of Orthopaedics and Rehabilitation, University of Iowa, 2701 Prairie Meadow Drive, 160-D, Iowa City, IA 52242 (e-mail: [email protected]).
The MOON Knee Group members contributing to this article are included in Appendix 1.
Abstract
Keywords
► anterior cruciate ligament reconstruction ► femoral drilling technique ► tunnel placement variability ► CT imaging ► ACL ► surgeon experience
The purpose of this cadaveric study was to analyze variation in anterior cruciate ligament (ACL) tunnel placement between surgeons and the influence of preferred surgical technique and surgeon experience level using three-dimensional (3D) computed tomography (CT). In this study, 12 surgeons drilled ACL tunnels on six cadaveric knees each. Surgeons were divided by experience level and preferred surgical technique (twoincision [TI], medial portal [MP], and transtibial [TT]). ACL tunnel aperture locations were analyzed using 3D CT scans and compared with radiographic ACL footprint criteria. The femoral tunnel location from front to back within the notch demonstrated a range of means of 16% with the TI tunnels the furthest back. A range of means of only 5% was found for femoral tunnel low to high positions by technique. The anterior to posterior tibial tunnel measure demonstrated wider variation than the medial to lateral position. The mean tibial tunnel location drilled by TT surgeons was more posterior than surgeons using the other techniques. Overall, 82% of femoral tunnels and 78% of tibial tunnels met all radiographic measurement criteria. Slight (1–7%) differences in mean tunnel placement on the femur and tibia were found between experienced and new surgeons. The location of the femoral tunnel aperture in the front to back plane relative to the notch roof and the anterior to posterior position on the tibia were the most variable measures. Surgeon experience level did not appear to significantly affect tunnel location. This study provides background information that may be beneficial when evaluating multisurgeon and multicenter collaborative ACL studies.
Anterior cruciate ligament (ACL) reconstruction is one of the most common orthopedic surgical procedures performed today by surgeons of varying levels of experience using various surgical techniques. The location of the bone tunnels that reproduce the ACL anatomy placed during the reconstruction is a crucial part of the procedure. Three techniques that are all commonly used today for single-bundle reconstruction of the ACL differ in how the tunnel is drilled on the femoral side of the joint. This includes transtibial (TT), medial portal (MP), and two-incision (TI) techniques.1–3
The study of ACL reconstruction outcomes has demonstrated a generally high level of subjective and objective success. Reported success rates in terms of functional stability, return to activity, and relief of symptoms range from 75 to 90%.4–7 Previous studies have correlated functional outcomes with ACL tunnel placement.8,9 However, numerous questions still remain regarding the predictors of short- and long-term outcomes after ACL reconstruction with emphasis on predictors of arthrosis after ACL injury and reconstruction. The Multicenter Orthopaedic Outcomes Network (MOON) study
received September 6, 2013 accepted November 19, 2013 published online January 10, 2014
Copyright © 2013 by Thieme Medical Publishers, Inc., 333 Seventh Avenue, New York, NY 10001, USA. Tel: +1(212) 584-4662.
DOI http://dx.doi.org/ 10.1055/s-0033-1364101. ISSN 1538-8506.
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group was established to study such outcomes. The MOON collaborative research effort has enrolled ACL reconstruction patients from multiple surgeons at multiple participating centers across the United States prospectively into a large collaborative multicenter cohort study between 2002 and 2008.4,7,10–13 The MOON consortium includes surgeons with varying levels of experience after fellowship. These surgeons also use all three common single-bundle surgical techniques. On the basis of the previous literature, it has been assumed that all three techniques will produce similar ACL reconstructions.14 Despite the prevalence of ACL surgery, relatively little is known about variability of ACL tunnel placement relative to surgeon experience level and preferred technique. This information is potentially very important when analyzing past and future studies of ACL reconstruction outcomes in multicenter and multisurgeon collaborative research efforts. The purpose of this cadaveric study was to analyze variation in ACL tunnel placement between surgeons and the influence of preferred surgical technique and surgeon experience level. The goal of this study was to provide platform information that could be used when analyzing large multicenter ACL cohort studies, and provide insight into how similar or dissimilar ACL tunnels are between participating surgeons.
Methods In this study, 12 orthopedic sports medicine fellowship trained orthopedic surgeons in the MOON group participated in the study. All 12 knee surgeons participating in the study routinely perform arthroscopically assisted ACL reconstruction. The 12 surgeons were chosen based on their preferred ACL femoral tunnel drilling technique. There were four surgeons who routinely perform TT femoral drilling, four who perform MP femoral drilling, and four who perform TI technique for femoral tunnel drilling. Among the 12 surgeons, there were 6 surgeons with 6 years or less of clinical experience (new) and 6 surgeons with at least 9 years of clinical experience (experienced). The new and experienced surgeons were equally distributed in the TT, MP, and TI technique groups. A total of 72 cadaver knees were thawed to room temperature and tagged for future identification and blinded association with performing surgeon. The study was performed in a cadaveric wet laboratory designed for arthroscopic surgery. All ACL tunnel surgeries were completed during 1 day using multiple surgical stations. Surgeons were instructed to use their standard skin incisions and portals on the cadaver knee as they would during routine ACL reconstruction in the clinical setting. If a native ACL was present in the cadaver knee then the surgeon arthroscopically resected the native ACL before tunnel preparation. The 12 MOON surgeons each drilled tunnels on the tibia and femur on six cadaver knees. The goal for all surgeons was create tunnels for a primary single-bundle ACL reconstruction similar to what they would do in a routine clinical setting. The surgeons were allowed to use ACL guides and/or bony and soft tissue landmarks per surgeon discretion and preferred technique. All femoral and The Journal of Knee Surgery
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tibial tunnels were drilled using a 10-mm reamer. All tibial tunnels were drilled from outside to inside such that both the joint and the anteromedial tibial cortex were breached. Each surgeon was allowed a nonsurgeon assistant during the procedures. Surgeons were not allowed to directly watch, analyze, or influence each other during ACL tunnel drilling. The specimens were refrozen and later thawed for computed tomography (CT) scanning. During the freezing, shipping and rethawing process identification tags became detached on five specimens. Hence, CT scan data were only analyzed for 67 knees. A Siemens Sensation 64 slice CT scanner was used to collect three-dimensional (3D) voxel datasets of the knee for each specimen (matrix ¼ 1,005 512, field of view ¼ 261 133 mm, peak kilovoltage ¼ 120, current ¼ 128 mA, exposure ¼ 160 mAs) with a 0.26-mm in-plane resolution and a 0.75-mm slice thickness. The CT datasets were resampled to 1.0-mm isotropic voxels, and all left knees were mirrored along the x-axis to produce right knees for our analysis. The BRAINS2 software and 3DSlicer were used to generate 3D images.15,16 A 3D measurement system was developed for characterizing tunnels based on simulating the positioning of the drill bit originally used to create the tunnels. Virtual 10-mm drill bit cylinders were aligned within the drilled tunnels using custom software.17 The virtual drill bits were cropped at the tunnel aperture. The tunnel aperture center point was calculated and measures were taken from this location. Operating surgeons were not involved in the data collection or analysis. A 3D Cartesian coordinate system was used to analyze tunnel location as previously described.18 This coordinate system allows reproducible specimen orientation and radiographic measures based on anatomic landmarks around the knee. It is applicable to cadaveric and clinical studies.19,63 The apertures of the tunnel into the knee joint were measured spatially. On the femur, the spatial location was measured relative to the intercondylar roof, similar to the quadrant method of Bernard et al20 and similar to methods used by Forsythe et al.21 Our image analysis demonstrated that the contour of Blumensaat’s line was often quite irregular on a sagittal slice through the apex of the intercondylar notch making determination of Blumensaat’s slope difficult and unreliable. Therefore, a 35-degree intercondylar roof angle was applied to the Cartesian coordinate system measure for each knee to establish the notch roof.22 The tunnel location was measured for back to forward position within the intercondylar notch and high to low position on the lateral wall. Back to forward tunnel position was calculated as a percentage of the anterior to posterior dimension of the lateral femoral condyle (c/C) with the posterior edge of the condyle as 0% and the anterior condyle 100%. Tunnel height was analyzed to the maximal height of the intercondylar notch with the notch apex designated as 0%, and the inferior edge of the lateral femoral condyle, designated as 100% (n/N) (►Fig. 1). Tibial tunnel location was measured as a percentage of plateau width from the medial edge of the tibial plateau (m/M). In the sagittal plane, the aperture center point was measured as a percentage of the maximal sagittal depth of the tibial plateau as measured from the anterior edge and
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Tunnel Aperture Overlap Mapping Tunnel aperture overlap mapping was used to aid in largescale visualization of the data from a qualitative standpoint. A 10-mm sphere to match the drill bit used was mapped to the bone surface at the tunnel aperture center point. This was based on the central axis of the drilled tunnel. Using an iterative procedure, the number of times a surface point was encapsulated by spheres was recorded for each surface point; this point “count” data can then be represented using different colors on the surface. Regions of the bone surface that are more frequently contained within a drill tunnel aperture are represented by red, while regions less frequently contained are represented by blue. The color scale was normalized as a percentage of the total number of spheres. Statistical comparison is not feasible with overlap mapping but it does provide visual comparison of tunnel placement by technique and experience level.
Fig. 1 Method of measuring femoral tunnel location as percentage of lateral femoral condyle (c/C) and as percentage of intercondylar notch height (n/N).
perpendicular to a reference line across the posterior tibial condyles (a/A) (►Fig. 2). The measured locations of tunnel placements were analyzed to present a range of placement between cases and surgeons. In addition, tunnel locations were compared with radiographic data on ACL footprints. The 3D footprint of the femoral ACL footprint was analyzed in a cadaveric study at the home institution.64 The applied limits of the ACL footprint using mean values plus one standard deviation were 0.20 to 0.71 for c/C and 0.33 to 0.74 for n/N. A comprehensive review of the literature provided radiographic footprint data for the tibia.23–27 For the tibia footprint limits of 0.30 to 0.55 for a/A and 0.4 to 0.51 for m/M were applied.
Fig. 2 Method of measuring tibial tunnel location as a percentage of the tibial plateau sagittal depth (a/A) and width (m/M).
Reliability testing for different users for 3D tunnel analysis was performed and the intraclass correlation coefficients ranged from 0.95 to 0.99. Standard descriptive statistics were calculated for the measurements made on each ACL tunnel on both the tibia and the femur. To address each of the specific study goals, statistical analysis was performed using SAS (Cary, NC) on each of the five measurements. Variation in tunnel placement was analyzed for ranges of tunnel locations based on applied measures. Measurements were also then grouped by the technique used to create the tunnels creating mean tunnel placement for the TT, MP, and TI groups. Finally, tunnels created by new surgeons and experienced surgeons were averaged and compared. Data normality were tested using the Shapiro–Wilk test for normality. A general linear model with a post hoc Tukey test was used to evaluate for statistically significant differences.
Results Surgeon Experience All surgeons participating in the study had completed an orthopedic sports medicine fellowship and regularly perform ACL reconstruction. In the new group, the average years being in practice was 2.5 years (range, 4 months–6 years). In the experienced group, the average years in practice were 15.8 years (range, 9–21 years). The new surgeons performed an average of 31 ACL reconstructions during the year of the study (range, 4–57). The experienced surgeons performed an average of 101 (range, 45–140) ACL reconstructions during the year of the study. The experienced surgeons placed the femoral tunnel, on average, 7% further back in the notch than new surgeons (►Table 1). This was statistically significant (p < 0.016). Only a 2% difference was found for the mean high to low position between experienced and new surgeons with the experienced surgeons placing the tunnel slightly higher. This was not statistically significant. Experienced surgeons placed the tibial tunnel aperture on average 3% more posterior on the tibia than new surgeons. This was not statistically significant. The Journal of Knee Surgery
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Statistical Analysis
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Table 1 Descriptive statistics for the femoral and tibial tunnel spatial measurements organized by technique and experience level
Technique
Femoral tunnel average (SD)
Tibial tunnel average (SD)
c/C
n/N
a/A
m/M
Medial portal
0.46 (0.07)
0.45 (12)
0.44 (0.03)
0.45 (0.03)
Transtibial
0.41 (0.09)
0.40 (0.10)
0.50 (0.05)
0.45 (0.02)
Two incision
0.30 (0.14)
0.43 (0.09)
0.48 (0.07)
0.46 (0.03)
Experienced
0.36 (0.12)
0.42 (0.11)
0.49 (0.07)
0.45 (0.03)
New
0.43 (0.11)
0.44 (0.10)
0.46 (0.07)
0.46 (0.03)
Experience level
The mean femoral tunnel measurement for back to forward placement in the notch was 0.39 with a range of 0.03 to 0.66 (►Fig. 3). One tunnel axis fell posterior to the margin of the applied measurement system. There were three outliers in terms of forward to back position with tunnel axes that fell much further back in the notch relative to the remainder of the tunnel measures. If these three tunnels are excluded, the range of tunnel back to forward position is 0.20 to 0.66; this range can be interpreted as a range of 46% of the total lateral femoral condyle radiographic dimension parallel to the notch roof angle. For superior to inferior tunnel location on the lateral wall of the notch, the mean tunnel axis measure was 0.43 with a range of 0.19 to 0.69; this range can also be interpreted as a range of 50% of the lateral wall superior to inferior radiographic dimension perpendicular to the notch roof (►Fig. 4). ►Table 1 demonstrates the mean tunnel measurements for femoral and tibial tunnel aperture locations after grouping by technique and experience level. The scatter-
plots in ►Figs. 3 and 4 demonstrate the central axis of the tunnels by technique. A range of 16% was found when comparing the mean tunnel location of the three techniques in terms of the back to forward tunnel location in the notch (c/C). The mean c/C measure was MP (46%), TT (41%), and TI (30%). Statistical testing demonstrated a significant difference between the three techniques (p < 0.0001). Post hoc Tukey testing showed the TI technique to be statistically further back in the notch than the MP technique and the TT technique with no significant difference between MP and TT. A rather small range of 5% was found when comparing the mean high to low tunnel location on the lateral wall of the notch between the three techniques. The order from lower to higher on the wall was MP (45%), TI (43%), and TT (40%). However, no significant differences were demonstrated for the three techniques for n/N. A total of 82% (55/67) of the femoral tunnel axes were located within the applied radiographic boundaries of the anatomic femoral footprint. Three femoral tunnel axes were back of, or behind, the footprint within the notch relative to the intercondylar notch roof. All three tunnels were drilled using TI technique by two experienced and one new surgeon. There were 10 tunnel axes that were superior to the applied radiographic femoral footprint. Five of these were drilled using TT, three drilled using MP, and two drilled using TI.
Fig. 3 Scatterplot of femoral tunnel depth (c/C) according to technique (no data for knees 15, 22, 61, 67, 71). The dotted line represents the average c/C (depth) position of the tunnel with 1.00 representing the maximum anterior extent of the lateral femoral condyle. The blue dashed lines represent the boundary of the femoral footprint as measured in a cadaver dataset at the authors’ institution.
Fig. 4 Scatterplot of femoral tunnel height (n/N) according to technique (no data for knees 15, 22, 61, 67, 71). The dotted line represents the average n/N (height) position of the tunnel with 0 representing the maximumsuperior height of the notch and 1.0 the inferior extent of the notch. The blue dashed lines represent the boundary of the femoral footprint as measured in a cadaver dataset at the authors’ institution.
A minimal difference of less than 1% was shown between surgeon experience groups for the tibial tunnel aperture location as plateau percentage from medial to lateral on the tibial plateau (m/M), with the new surgeons placing the tibial tunnel slightly more lateral.
Femoral Tunnel Location
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Seven of these 10 tunnels were drilled by experienced surgeons. Examples of tunnels falling outside the footprint are shown in ►Fig. 5.
Tibial Tunnel Aperture Location
Overall, 52 of 67 (78%) tibial tunnels met all applied radiographic criteria for the tibial ACL footprint. Eleven tibial tunnels were deemed too posterior and 0 were deemed too anterior according to applied criteria. Of these 11 posterior tibial tunnels, 5 were drilled by surgeons using TT technique for the femur, 5 by TI surgeons, and 1 by MP surgeons. Seven of these tunnels were drilled by experienced and four by new surgeons. Two tunnels were too lateral according to criteria and both drilled by a new surgeon using TI technique for the femoral drilling. Finally, two tibial tunnels were found to be too medial and these were drilled by one new MP surgeon and one experienced TI surgeon.
Overall, the mean tibial tunnel axis placement was 0.47 from the anterior tibial edge (a/A) with a range of 0.32 to 0.64 (►Fig. 6). The mean tunnel axis from medial to lateral (m/M) was 0.45 with a range of 0.39 to 0.52 (►Fig. 7). Since preferred femoral tunnel drilling technique can influence tibial tunnel placement, especially for TT technique, we did also compare tibial tunnel locations by preferred femoral drilling technique groupings. A range of 6% was shown between the averages of the three methods for the tibial a/A measure (►Table 1). The order of increasing proportional position from anterior to posterior was MP (44%), TI (48%), and TT (50%). Statistical testing showed a significant difference between the three techniques (p < 0.004). Post hoc Tukey testing showed the MP technique to be significantly more anterior than the TT technique. The aperture of the tibial tunnel showed a very small range (1%) between the averages of the three femoral drilling techniques when measured as a percentage from medial to lateral.
Tunnel Aperture Overlap Mapping
Fig. 6 Scatterplot of tibia tunnel anterior-posterior position (a/A) according to technique (no data for knees 15, 22, 61, 67, 71).
Fig. 7 Scatterplot of tibia tunnel medial-lateral position (m/M) according to technique (no data for knees 15, 22, 61, 67, 71).
Aperture overlap mapping for the three techniques is shown in ►Fig. 8. The TT surgeons demonstrate the most tunnel overlap on both the tibia and femur. The overlap mapping also demonstrates the relatively more posterior position of the tibial tunnel for the TT surgeons relatively to the TI and MP tunnels. The relatively more posterior position of the TI femoral tunnels is also demonstrated by the overlap mapping. Tunnel overlap mapping demonstrates the experienced
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Fig. 5 Examples of tunnels that were too (A) superior or (B) anterior according to applied measurement criteria.
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Fig. 8 Femoral and tibial anterior cruciate ligament tunnel aperture overlap maps by technique.
Fig. 9 Femoral and tibial anterior cruciate ligament tunnel aperture overlap maps by surgeon experience.
surgeons to be less variable on the tibia and the new surgeons to be less variable on the femur (►Fig. 9).
Discussion This study provides platform data for a subset of surgeons participating in collaborative multicenter research on ACL reconstruction and associated outcomes. Experience level did not appear to strongly influence ACL tunnel locations. The data demonstrate ranges of femoral and tibial tunnel placements in a group of 12 surgeons using three common ACL techniques. The largest ranges were found in the front to back femoral tunnel position within the notch and anterior to posterior position on the tibia. A femoral tunnel that was superior to the radiographic footprint criteria on the femur or posterior to the radiographic footprint on the tibia were the most common tunnel malpositions. The Journal of Knee Surgery
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Multicenter and multisurgeon collaborative ACL research has become more common both in the United States28,29 and abroad.30–32 However, little information exists regarding the variability that may be associated with such collaboration. Further, there is little information on the influence of surgical technique or surgeon experience level. This study provides background information, albeit in a controlled cadaver study, regarding variability in tunnel placement in collaborative multisurgeon ACL research.33 Our data demonstrated that experienced surgeons placed their femoral tunnel significantly further back, 7% of the total depth of the lateral condyle on average, in the intercondylar notch compared with new surgeons. No other comparisons were significantly different. Nine of 12 tunnels that did not meet the footprint radiographic criteria were drilled by experienced surgeons. Eight of 15 tunnels that did not meet applied criteria on the tibia were drilled by experienced surgeons. Overall, these data would suggest that experience level does not significantly alter tunnel placement, at least in this study group. There is limited prior research on the impact of experience level on ACL surgery. In a plastic knee model, Burkart et al compared the accuracy of ACL guide pin placement with a robotic ACL system using TT technique34 and found significant differences between orthopedic fellows and experienced faculty. Behrend et al did a retrospective review of 50 ACL reconstructions done by 17 different surgeons of varying experience levels at a teaching institution.35 The proportion of “correctly placed” tunnels was higher in the expert group. The only prior study that evaluated volume– outcome relationships in ACL reconstruction was done by Lyman et al using 10 years of an administrative database in New York.36 This study found that patients having ACL reconstruction surgery by a low volume surgeon (less than 6 per year) were significantly more likely to be readmitted to the hospital within 90 days after surgery and to have subsequent knee surgery. These authors did not demonstrate that surgical volume was associated with the need for subsequent ACL reconstruction. Technical aspects of the ACL surgery being performed were not feasible given that study was an analysis of a large database. A wide range of femoral tunnel location from front to back within the notch was demonstrated within the entire group of cadaver knees. This appears to be influenced by preferred femoral tunnel technique, with a range of means of 16%. Our results show that the average tunnel using TI technique was further back in the notch followed by TT and then MP techniques. Again, a wide overall range was demonstrated for high to low femoral tunnel location relative to the intercondylar notch. However, the range of means between drilling techniques in femoral tunnel aperture location was only 5% in terms of superior to inferior location. Our data demonstrate that the TI and MP tunnels were slightly, but insignificantly, lower on the wall of the notch than the TT tunnels. The tibial tunnel measurements demonstrated smaller overall ranges and the average tunnel placements were more similar among the three technique groups. This is not surprising as the techniques are different due to drilling methods used for the femoral tunnel side. Yet, small
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differences were seen for the anterior to posterior tibial tunnel position. Our data demonstrate that the TT tibial tunnels were on average the most posterior with the tibia independent TI and MP groups more anterior. This may reflect a conscious effort for TT surgeons to be able to place TT femoral tunnels more posteriorly on the femur.37,38 The medial to lateral tibial tunnel location was easily the most consistent in our study. ACL tunnels were compared with radiographic measures of the ACL footprint on the femur and tibial. This was done to allow analysis beyond just overall ranges of tunnel placements and comparison of mean tunnel placements between groups. On the femur, a separate anatomic study was done to produce 3D measures of the femoral footprint using measurement methods exactly as done in this cadaveric tunnel study. Tibial tunnels were compared with literature-based values. In total, 82% of femoral tunnels and 78% of tibial tunnels met these applied footprint criteria. It is a limitation of our study that tunnels were not directly compared with each cadaveric specimen’s own ACL footprint by direct dissection or measured relative to arthroscopic landmarks that have been described.39–42 Our data demonstrated tunnels falling outside of criteria by all femoral drilling techniques. On the femur, the 12 tunnels outside of criteria were drilled by TT (5), TI (4), and MP (3). For the 15 tunnels outside of criteria on the tibia, 8 were drilled by surgeons using TI technique, 5 by TT surgeons, and 2 by MP surgeons. Unfortunately, there is no consensus on the appropriate location for ACL tunnels and a fair amount of variability still exists between researchers and clinicians as to the appropriate location, especially on the femoral side.14,21,43–45 Indeed there is a fair amount of variation in the descriptions and anatomic studies of the ACL footprint and its bundles. Kopf et al demonstrated significant variability in their systematic review of ACL footprint in anatomy studies.46 Dargel et al demonstrated substantial side-to-side variability in cadaver knees from a single donor.47,48 Some authors have advocated placing the femoral tunnel at or just posterior to the footprint of the anteromedial bundle of the ACL,14,49–52 while others have advocated for placing the femoral tunnel more central in the ACL footprint.53–57 Similarly, there is variation on recommendations for the tibial side as well with advocates for both anterior placement within the tibial footprint,53,58,59 while others have advocated a more posterior position within the footprint.60–62 It is very important to note that it was not the intent of this study to determine which surgeon or technique produced a more “correct” tunnel position. Surgeons were not given a discrete or absolutely specific target for their ACL tunnels. Rather, they were instructed to try to replicate ACL tunnels they would produce in the clinical setting. Therefore, the tunnel variabilities demonstrated reflect the interaction of personal surgeon beliefs on appropriate tunnel location, anatomic variation, and likely influences from technique used to perform tunnel drilling. How much of the variability is due to surgeon choice versus anatomy or technique cannot be deciphered from our study. This study has limitations that merit brief discussion. Participating surgeons were aware that their tunnels would
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be analyzed closely and this may have induced performance bias. Drilling ACL tunnels on cadaver models can be different than live subjects, especially if knees are arthritic or stiff which may hinder the ability to maximally flex the knee as preferred in some techniques. In addition, the surgeons resected the native ACL before drilling the tunnels; therefore, the native anatomic location of the ACL was freshly evident and may have introduced some bias as well, perhaps reducing the surgeon variability on tunnel placement. Experience level was categorically analyzed based on years in practice rather than continuously by total number of ACL reconstructions each surgeon has performed. All surgeons were sports medicine fellowship trained and hence these data may not be generalizable to all surgeon groups. ACL tunnel measures were compared with radiographic criteria for the ACL footprints. Tunnel location was not, however, directly compared with the center of the anatomic footprint or to particular bundles as this was not the a priori intent of the study. Finally, our tunnel measurement system is not directly referencing off Blumensaat’s line but rather off the anatomic axis of the femur accounting for physiologic valgus and for a 35-degree mean intercondylar roof angle. Previous works have demonstrated this mean roof angle with ranges between 25 and 45 degrees. It is possible that some of the specimens had differing roof angles in actuality and this could slightly alter given measures on the femur. However, the authors encountered impressive undulation of Blumensaat’s line on the sagittal slicing of the distal femur. This led to substantial variability in establishing a reliable landmark upon which to analyze percentages. It is feasible that other 3D analysis techniques are more adept at establishing Blumensaat’s line using 3D CT.
Conclusion In summary, our data demonstrate ranges of ACL tunnel locations. The location of the femoral tunnel aperture in the front to back plane relative to the notch roof and the anterior to posterior position on the tibia were the most variable measures. Surgeon experience level did not appear to significantly affect tunnel location. This study provides background information that may be beneficial when evaluating multisurgeon and multicenter collaborative ACL studies.
Acknowledgments This study was supported by the National Institute of Health Mentored Clinical Research Scholar Program at Iowa (5K12RR017700–04) and the Vanderbilt Sports Medicine Research Fund. DonJoy Orthopaedics provided the specimens and wet laboratory facility for the study.
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ligament tunnel placement. Am J Sports Med 2012;40(12): 2737–2746 Bernard M, Hertel P, Hornung H, Cierpinski T. Femoral insertion of the ACL. Radiographic quadrant method. Am J Knee Surg 1997; 10(1):14–21, discussion 21–22 Forsythe B, Kopf S, Wong AK, et al. The location of femoral and tibial tunnels in anatomic double-bundle anterior cruciate ligament reconstruction analyzed by three-dimensional computed tomography models. J Bone Joint Surg Am 2010;92(6): 1418–1426 Howell SM, Barad SJ. Knee extension and its relationship to the slope of the intercondylar roof. Implications for positioning the tibial tunnel in anterior cruciate ligament reconstructions. Am J Sports Med 1995;23(3):288–294 Colombet P, Robinson J, Christel P, et al. Morphology of anterior cruciate ligament attachments for anatomic reconstruction: a cadaveric dissection and radiographic study. Arthroscopy 2006; 22(9):984–992 Doi M, Takahashi M, Abe M, Suzuki D, Nagano A. Lateral radiographic study of the tibial sagital insertions of the anteromedial and posterolateral bundles of human anterior cruciate ligament. Knee Surg Sports Traumatol Arthrosc 2009;17(4):347–351 Edwards A, Bull AM, Amis AA. The attachments of the anteromedial and posterolateral fibre bundles of the anterior cruciate ligament: Part 1: tibial attachment. Knee Surg Sports Traumatol Arthrosc 2007;15(12):1414–1421 Lintner DM, Dewitt SE, Moseley JB. Radiographic evaluation of native anterior cruciate ligament attachments and graft placement for reconstruction. A cadaveric study. Am J Sports Med 1996;24(1):72–78 Lorenz S, Elser F, Mitterer M, Obst T, Imhoff AB. Radiologic evaluation of the insertion sites of the 2 functional bundles of the anterior cruciate ligament using 3-dimensional computed tomography. Am J Sports Med 2009;37(12):2368–2376 Morgan JA, Dahm D, Levy B, Stuart MJ; MARS Study Group. Femoral tunnel malposition in ACL revision reconstruction. J Knee Surg 2012;25(5):361–368 Spindler KP, Parker RD, Andrish JT, et al; MOON Group. Prognosis and predictors of ACL reconstructions using the MOON cohort: a model for comparative effectiveness studies. J Orthop Res 2013; 31(1):2–9 Ahldén M, Samuelsson K, Sernert N, Forssblad M, Karlsson J, Kartus J. The Swedish National Anterior Cruciate Ligament Register: a report on baseline variables and outcomes of surgery for almost 18,000 patients. Am J Sports Med 2012;40(10):2230–2235 Granan LP, Bahr R, Steindal K, Furnes O, Engebretsen L. Development of a national cruciate ligament surgery registry: the Norwegian National Knee Ligament Registry. Am J Sports Med 2008; 36(2):308–315 Røtterud JH, Sivertsen EA, Forssblad M, Engebretsen L, Arøen A. Effect of meniscal and focal cartilage lesions on patient-reported outcome after anterior cruciate ligament reconstruction: a nationwide cohort study from Norway and Sweden of 8476 patients with 2-year follow-up. Am J Sports Med 2013;41(3):535–543 Bedi A, Raphael B, Maderazo A, Pavlov H, Williams RJ III. Transtibial versus anteromedial portal drilling for anterior cruciate ligament reconstruction: a cadaveric study of femoral tunnel length and obliquity. Arthroscopy 2010;26(3):342–350 Burkart A, Debski RE, McMahon PJ, et al. Precision of ACL tunnel placement using traditional and robotic techniques. Comput Aided Surg 2001;6(5):270–278 Behrend H, Stutz G, Kessler MA, Rukavina A, Giesinger K, Kuster MS. Tunnel placement in anterior cruciate ligament (ACL) reconstruction: quality control in a teaching hospital. Knee Surg Sports Traumatol Arthrosc 2006;14(11):1159–1165 Lyman S, Koulouvaris P, Sherman S, Do H, Mandl LA, Marx RG. Epidemiology of anterior cruciate ligament reconstruction: trends, readmissions, and subsequent knee surgery. J Bone Joint Surg Am 2009;91(10):2321–2328
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traditional transtibial single-bundle anterior cruciate ligament reconstruction evaluated by three-dimensional computed tomography. J Bone Joint Surg Am 2010;92(6):1427–1431 Lopez-Vidriero E, Hugh Johnson D. Evolving concepts in tunnel placement. Sports Med Arthrosc 2009;17(4):210–216 Purnell ML, Larson AI, Clancy W. Anterior cruciate ligament insertions on the tibia and femur and their relationships to critical bony landmarks using high-resolution volume-rendering computed tomography. Am J Sports Med 2008;36(11):2083–2090 Siebold R, Ellert T, Metz S, Metz J. Femoral insertions of the anteromedial and posterolateral bundles of the anterior cruciate ligament: morphometry and arthroscopic orientation models for double-bundle bone tunnel placement—a cadaver study. Arthroscopy 2008;24(5):585–592 Siebold R, Ellert T, Metz S, Metz J. Tibial insertions of the anteromedial and posterolateral bundles of the anterior cruciate ligament: morphometry, arthroscopic landmarks, and orientation model for bone tunnel placement. Arthroscopy 2008;24(2):154–161 Zantop T, Wellmann M, Fu FH, Petersen W. Tunnel positioning of anteromedial and posterolateral bundles in anatomic anterior cruciate ligament reconstruction: anatomic and radiographic findings. Am J Sports Med 2008;36(1):65–72 Amis AA, Jakob RP. Anterior cruciate ligament graft positioning, tensioning and twisting. Knee Surg Sports Traumatol Arthrosc 1998;6(Suppl 1):S2–S12 Giron F, Cuomo P, Edwards A, Bull AM, Amis AA, Aglietti P. Doublebundle “anatomic” anterior cruciate ligament reconstruction: a cadaveric study of tunnel positioning with a transtibial technique. Arthroscopy 2007;23(1):7–13 Kaseta MK, DeFrate LE, Charnock BL, Sullivan RT, Garrett WE Jr. Reconstruction technique affects femoral tunnel placement in ACL reconstruction. Clin Orthop Relat Res 2008;466(6):1467–1474 Kopf S, Musahl V, Tashman S, Szczodry M, Shen W, Fu FH. A systematic review of the femoral origin and tibial insertion morphology of the ACL. Knee Surg Sports Traumatol Arthrosc 2009;17(3):213–219 Dargel J, Pohl P, Tzikaras P, Koebke J. Morphometric side-to-side differences in human cruciate ligament insertions. Surg Radiol Anat 2006;28(4):398–402 Dargel J, Schmidt-Wiethoff R, Fischer S, Mader K, Koebke J, Schneider T. Femoral bone tunnel placement using the transtibial tunnel or the anteromedial portal in ACL reconstruction: a radiographic evaluation. Knee Surg Sports Traumatol Arthrosc 2009; 17(3):220–227 Girgis FG, Marshall JL, Monajem A. The cruciate ligaments of the knee joint. Anatomical, functional and experimental analysis. Clin Orthop Relat Res 1975;(106):216–231 Hefzy MS, Grood ES, Noyes FR. Factors affecting the region of most isometric femoral attachments. Part II: The anterior cruciate ligament. Am J Sports Med 1989;17(2):208–216 Hoogland T, Hillen B. Intra-articular reconstruction of the anterior cruciate ligament. An experimental study of length changes in different ligament reconstructions. Clin Orthop Relat Res 1984; (185):197–202 Shelbourne KD, Whitaker HJ, McCarroll JR, Rettig AC, Hirschman LD. Anterior cruciate ligament injury: evaluation of intraarticular reconstruction of acute tears without repair. Two to seven year followup of 155 athletes. Am J Sports Med 1990;18(5):484–488, discussion 488–489
cruciate ligament reconstruction using one-third of the patellar ligament, augmented by extra-articular tendon transfers. J Bone Joint Surg Am 1982;64(3):352–359 Garofalo R, Mouhsine E, Chambat P, Siegrist O. Anatomic anterior cruciate ligament reconstruction: the two-incision technique. Knee Surg Sports Traumatol Arthrosc 2006;14(6):510–516 Loh JC, Fukuda Y, Tsuda E, Steadman RJ, Fu FH, Woo SL. Knee stability and graft function following anterior cruciate ligament reconstruction: Comparison between 11 o’clock and 10 o’clock femoral tunnel placement. 2002 Richard O’Connor Award paper. Arthroscopy 2003;19(3):297–304 Muneta T, Yamamoto H, Sakai H, Ishibashi T, Furuya K. Relationship between changes in length and force in in vitro reconstructed anterior cruciate ligament. Am J Sports Med 1993; 21(2):299–304 Odensten M, Gillquist J. Functional anatomy of the anterior cruciate ligament and a rationale for reconstruction. J Bone Joint Surg Am 1985;67(2):257–262 Goble EM, Downey DJ, Wilcox TR. Positioning of the tibial tunnel for anterior cruciate ligament reconstruction. Arthroscopy 1995; 11(6):688–695 Good L, Odensten M, Gillquist J. Precision in reconstruction of the anterior cruciate ligament. A new positioning device compared with hand drilling. Acta Orthop Scand 1987;58(6):658–661 Howell SM. Principles for placing the tibial tunnel and avoiding roof impingement during reconstruction of a torn anterior cruciate ligament. Knee Surg Sports Traumatol Arthrosc 1998;6 (Suppl 1):S49–S55 Howell SM, Taylor MA. Failure of reconstruction of the anterior cruciate ligament due to impingement by the intercondylar roof. J Bone Joint Surg Am 1993;75(7):1044–1055 Musahl V, Burkart A, Debski RE, Van Scyoc A, Fu FH, Woo SL. Anterior cruciate ligament tunnel placement: Comparison of insertion site anatomy with the guidelines of a computer-assisted surgical system. Arthroscopy 2003;19(2):154–160 Wolf BR, Ramme AJ, Wright RW, et al. Variability in ACL tunnel placement: observational clinical study of surgeon ACL tunnel variability. Am J Sports Med 2013;41(6):1265–1273 Westermann R, Sybrowsky C, Ramme A, Amendola A, Wolf BR. Three-dimensional characterization of the anterior cruciate ligament's femoral footprint. J Knee Surg 2014;27(1):53–58
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Appendix 1 The MOON Knee Group members contributing to this article include the following: Warren R. Dunn, MD, MPH, Kurt P. Spindler, MD, James L. Carey, MD, MPH, Charles L. Cox, MD, MPH, Christopher C. Kaeding, MD, David C. Flanigan, MD, Rick W. Wright, MD, Matthew J. Matava, MD, Robert H. Brophy, MD, Matthew V. Smith, MD, Eric C. McCarty, MD, Armando F. Vidal, MD, Michelle Wolcott, MD, Robert G. Marx, MD, MSc, Richard D. Parker, MD, Jack F. Andrish, MD, and Morgan H. Jones, MD, MPH.
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Anterior Cruciate Ligament Tunnel Placement Brian R. Wolf, MD, MS1 MOON Knee Group3,
Austin J. Ramme, MD, PhD2
1 Department of Orthopaedics and Rehabilitation, University of Iowa,
Iowa City, Iowa 2 Department of Orthopaedic Surgery, University of Iowa, Iowa City, Iowa 3 Medical School, Vanderbilt University, Nashville, Tennessee
Carla L. Britton, PhD1
Annunziato Amendola, MD1
Address for correspondence Brian R. Wolf, MD, MS, Department of Orthopaedics and Rehabilitation, University of Iowa, 2701 Prairie Meadow Drive, 160-D, Iowa City, IA 52242 (e-mail: [email protected]).
The MOON Knee Group members contributing to this article are included in Appendix 1.
Abstract
Keywords
► anterior cruciate ligament reconstruction ► femoral drilling technique ► tunnel placement variability ► CT imaging ► ACL ► surgeon experience
The purpose of this cadaveric study was to analyze variation in anterior cruciate ligament (ACL) tunnel placement between surgeons and the influence of preferred surgical technique and surgeon experience level using three-dimensional (3D) computed tomography (CT). In this study, 12 surgeons drilled ACL tunnels on six cadaveric knees each. Surgeons were divided by experience level and preferred surgical technique (twoincision [TI], medial portal [MP], and transtibial [TT]). ACL tunnel aperture locations were analyzed using 3D CT scans and compared with radiographic ACL footprint criteria. The femoral tunnel location from front to back within the notch demonstrated a range of means of 16% with the TI tunnels the furthest back. A range of means of only 5% was found for femoral tunnel low to high positions by technique. The anterior to posterior tibial tunnel measure demonstrated wider variation than the medial to lateral position. The mean tibial tunnel location drilled by TT surgeons was more posterior than surgeons using the other techniques. Overall, 82% of femoral tunnels and 78% of tibial tunnels met all radiographic measurement criteria. Slight (1–7%) differences in mean tunnel placement on the femur and tibia were found between experienced and new surgeons. The location of the femoral tunnel aperture in the front to back plane relative to the notch roof and the anterior to posterior position on the tibia were the most variable measures. Surgeon experience level did not appear to significantly affect tunnel location. This study provides background information that may be beneficial when evaluating multisurgeon and multicenter collaborative ACL studies.
Anterior cruciate ligament (ACL) reconstruction is one of the most common orthopedic surgical procedures performed today by surgeons of varying levels of experience using various surgical techniques. The location of the bone tunnels that reproduce the ACL anatomy placed during the reconstruction is a crucial part of the procedure. Three techniques that are all commonly used today for single-bundle reconstruction of the ACL differ in how the tunnel is drilled on the femoral side of the joint. This includes transtibial (TT), medial portal (MP), and two-incision (TI) techniques.1–3
The study of ACL reconstruction outcomes has demonstrated a generally high level of subjective and objective success. Reported success rates in terms of functional stability, return to activity, and relief of symptoms range from 75 to 90%.4–7 Previous studies have correlated functional outcomes with ACL tunnel placement.8,9 However, numerous questions still remain regarding the predictors of short- and long-term outcomes after ACL reconstruction with emphasis on predictors of arthrosis after ACL injury and reconstruction. The Multicenter Orthopaedic Outcomes Network (MOON) study
received September 6, 2013 accepted November 19, 2013 published online January 10, 2014
Copyright © 2013 by Thieme Medical Publishers, Inc., 333 Seventh Avenue, New York, NY 10001, USA. Tel: +1(212) 584-4662.
DOI http://dx.doi.org/ 10.1055/s-0033-1364101. ISSN 1538-8506.
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group was established to study such outcomes. The MOON collaborative research effort has enrolled ACL reconstruction patients from multiple surgeons at multiple participating centers across the United States prospectively into a large collaborative multicenter cohort study between 2002 and 2008.4,7,10–13 The MOON consortium includes surgeons with varying levels of experience after fellowship. These surgeons also use all three common single-bundle surgical techniques. On the basis of the previous literature, it has been assumed that all three techniques will produce similar ACL reconstructions.14 Despite the prevalence of ACL surgery, relatively little is known about variability of ACL tunnel placement relative to surgeon experience level and preferred technique. This information is potentially very important when analyzing past and future studies of ACL reconstruction outcomes in multicenter and multisurgeon collaborative research efforts. The purpose of this cadaveric study was to analyze variation in ACL tunnel placement between surgeons and the influence of preferred surgical technique and surgeon experience level. The goal of this study was to provide platform information that could be used when analyzing large multicenter ACL cohort studies, and provide insight into how similar or dissimilar ACL tunnels are between participating surgeons.
Methods In this study, 12 orthopedic sports medicine fellowship trained orthopedic surgeons in the MOON group participated in the study. All 12 knee surgeons participating in the study routinely perform arthroscopically assisted ACL reconstruction. The 12 surgeons were chosen based on their preferred ACL femoral tunnel drilling technique. There were four surgeons who routinely perform TT femoral drilling, four who perform MP femoral drilling, and four who perform TI technique for femoral tunnel drilling. Among the 12 surgeons, there were 6 surgeons with 6 years or less of clinical experience (new) and 6 surgeons with at least 9 years of clinical experience (experienced). The new and experienced surgeons were equally distributed in the TT, MP, and TI technique groups. A total of 72 cadaver knees were thawed to room temperature and tagged for future identification and blinded association with performing surgeon. The study was performed in a cadaveric wet laboratory designed for arthroscopic surgery. All ACL tunnel surgeries were completed during 1 day using multiple surgical stations. Surgeons were instructed to use their standard skin incisions and portals on the cadaver knee as they would during routine ACL reconstruction in the clinical setting. If a native ACL was present in the cadaver knee then the surgeon arthroscopically resected the native ACL before tunnel preparation. The 12 MOON surgeons each drilled tunnels on the tibia and femur on six cadaver knees. The goal for all surgeons was create tunnels for a primary single-bundle ACL reconstruction similar to what they would do in a routine clinical setting. The surgeons were allowed to use ACL guides and/or bony and soft tissue landmarks per surgeon discretion and preferred technique. All femoral and The Journal of Knee Surgery
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tibial tunnels were drilled using a 10-mm reamer. All tibial tunnels were drilled from outside to inside such that both the joint and the anteromedial tibial cortex were breached. Each surgeon was allowed a nonsurgeon assistant during the procedures. Surgeons were not allowed to directly watch, analyze, or influence each other during ACL tunnel drilling. The specimens were refrozen and later thawed for computed tomography (CT) scanning. During the freezing, shipping and rethawing process identification tags became detached on five specimens. Hence, CT scan data were only analyzed for 67 knees. A Siemens Sensation 64 slice CT scanner was used to collect three-dimensional (3D) voxel datasets of the knee for each specimen (matrix ¼ 1,005 512, field of view ¼ 261 133 mm, peak kilovoltage ¼ 120, current ¼ 128 mA, exposure ¼ 160 mAs) with a 0.26-mm in-plane resolution and a 0.75-mm slice thickness. The CT datasets were resampled to 1.0-mm isotropic voxels, and all left knees were mirrored along the x-axis to produce right knees for our analysis. The BRAINS2 software and 3DSlicer were used to generate 3D images.15,16 A 3D measurement system was developed for characterizing tunnels based on simulating the positioning of the drill bit originally used to create the tunnels. Virtual 10-mm drill bit cylinders were aligned within the drilled tunnels using custom software.17 The virtual drill bits were cropped at the tunnel aperture. The tunnel aperture center point was calculated and measures were taken from this location. Operating surgeons were not involved in the data collection or analysis. A 3D Cartesian coordinate system was used to analyze tunnel location as previously described.18 This coordinate system allows reproducible specimen orientation and radiographic measures based on anatomic landmarks around the knee. It is applicable to cadaveric and clinical studies.19,63 The apertures of the tunnel into the knee joint were measured spatially. On the femur, the spatial location was measured relative to the intercondylar roof, similar to the quadrant method of Bernard et al20 and similar to methods used by Forsythe et al.21 Our image analysis demonstrated that the contour of Blumensaat’s line was often quite irregular on a sagittal slice through the apex of the intercondylar notch making determination of Blumensaat’s slope difficult and unreliable. Therefore, a 35-degree intercondylar roof angle was applied to the Cartesian coordinate system measure for each knee to establish the notch roof.22 The tunnel location was measured for back to forward position within the intercondylar notch and high to low position on the lateral wall. Back to forward tunnel position was calculated as a percentage of the anterior to posterior dimension of the lateral femoral condyle (c/C) with the posterior edge of the condyle as 0% and the anterior condyle 100%. Tunnel height was analyzed to the maximal height of the intercondylar notch with the notch apex designated as 0%, and the inferior edge of the lateral femoral condyle, designated as 100% (n/N) (►Fig. 1). Tibial tunnel location was measured as a percentage of plateau width from the medial edge of the tibial plateau (m/M). In the sagittal plane, the aperture center point was measured as a percentage of the maximal sagittal depth of the tibial plateau as measured from the anterior edge and
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Tunnel Aperture Overlap Mapping Tunnel aperture overlap mapping was used to aid in largescale visualization of the data from a qualitative standpoint. A 10-mm sphere to match the drill bit used was mapped to the bone surface at the tunnel aperture center point. This was based on the central axis of the drilled tunnel. Using an iterative procedure, the number of times a surface point was encapsulated by spheres was recorded for each surface point; this point “count” data can then be represented using different colors on the surface. Regions of the bone surface that are more frequently contained within a drill tunnel aperture are represented by red, while regions less frequently contained are represented by blue. The color scale was normalized as a percentage of the total number of spheres. Statistical comparison is not feasible with overlap mapping but it does provide visual comparison of tunnel placement by technique and experience level.
Fig. 1 Method of measuring femoral tunnel location as percentage of lateral femoral condyle (c/C) and as percentage of intercondylar notch height (n/N).
perpendicular to a reference line across the posterior tibial condyles (a/A) (►Fig. 2). The measured locations of tunnel placements were analyzed to present a range of placement between cases and surgeons. In addition, tunnel locations were compared with radiographic data on ACL footprints. The 3D footprint of the femoral ACL footprint was analyzed in a cadaveric study at the home institution.64 The applied limits of the ACL footprint using mean values plus one standard deviation were 0.20 to 0.71 for c/C and 0.33 to 0.74 for n/N. A comprehensive review of the literature provided radiographic footprint data for the tibia.23–27 For the tibia footprint limits of 0.30 to 0.55 for a/A and 0.4 to 0.51 for m/M were applied.
Fig. 2 Method of measuring tibial tunnel location as a percentage of the tibial plateau sagittal depth (a/A) and width (m/M).
Reliability testing for different users for 3D tunnel analysis was performed and the intraclass correlation coefficients ranged from 0.95 to 0.99. Standard descriptive statistics were calculated for the measurements made on each ACL tunnel on both the tibia and the femur. To address each of the specific study goals, statistical analysis was performed using SAS (Cary, NC) on each of the five measurements. Variation in tunnel placement was analyzed for ranges of tunnel locations based on applied measures. Measurements were also then grouped by the technique used to create the tunnels creating mean tunnel placement for the TT, MP, and TI groups. Finally, tunnels created by new surgeons and experienced surgeons were averaged and compared. Data normality were tested using the Shapiro–Wilk test for normality. A general linear model with a post hoc Tukey test was used to evaluate for statistically significant differences.
Results Surgeon Experience All surgeons participating in the study had completed an orthopedic sports medicine fellowship and regularly perform ACL reconstruction. In the new group, the average years being in practice was 2.5 years (range, 4 months–6 years). In the experienced group, the average years in practice were 15.8 years (range, 9–21 years). The new surgeons performed an average of 31 ACL reconstructions during the year of the study (range, 4–57). The experienced surgeons performed an average of 101 (range, 45–140) ACL reconstructions during the year of the study. The experienced surgeons placed the femoral tunnel, on average, 7% further back in the notch than new surgeons (►Table 1). This was statistically significant (p < 0.016). Only a 2% difference was found for the mean high to low position between experienced and new surgeons with the experienced surgeons placing the tunnel slightly higher. This was not statistically significant. Experienced surgeons placed the tibial tunnel aperture on average 3% more posterior on the tibia than new surgeons. This was not statistically significant. The Journal of Knee Surgery
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Statistical Analysis
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Table 1 Descriptive statistics for the femoral and tibial tunnel spatial measurements organized by technique and experience level
Technique
Femoral tunnel average (SD)
Tibial tunnel average (SD)
c/C
n/N
a/A
m/M
Medial portal
0.46 (0.07)
0.45 (12)
0.44 (0.03)
0.45 (0.03)
Transtibial
0.41 (0.09)
0.40 (0.10)
0.50 (0.05)
0.45 (0.02)
Two incision
0.30 (0.14)
0.43 (0.09)
0.48 (0.07)
0.46 (0.03)
Experienced
0.36 (0.12)
0.42 (0.11)
0.49 (0.07)
0.45 (0.03)
New
0.43 (0.11)
0.44 (0.10)
0.46 (0.07)
0.46 (0.03)
Experience level
The mean femoral tunnel measurement for back to forward placement in the notch was 0.39 with a range of 0.03 to 0.66 (►Fig. 3). One tunnel axis fell posterior to the margin of the applied measurement system. There were three outliers in terms of forward to back position with tunnel axes that fell much further back in the notch relative to the remainder of the tunnel measures. If these three tunnels are excluded, the range of tunnel back to forward position is 0.20 to 0.66; this range can be interpreted as a range of 46% of the total lateral femoral condyle radiographic dimension parallel to the notch roof angle. For superior to inferior tunnel location on the lateral wall of the notch, the mean tunnel axis measure was 0.43 with a range of 0.19 to 0.69; this range can also be interpreted as a range of 50% of the lateral wall superior to inferior radiographic dimension perpendicular to the notch roof (►Fig. 4). ►Table 1 demonstrates the mean tunnel measurements for femoral and tibial tunnel aperture locations after grouping by technique and experience level. The scatter-
plots in ►Figs. 3 and 4 demonstrate the central axis of the tunnels by technique. A range of 16% was found when comparing the mean tunnel location of the three techniques in terms of the back to forward tunnel location in the notch (c/C). The mean c/C measure was MP (46%), TT (41%), and TI (30%). Statistical testing demonstrated a significant difference between the three techniques (p < 0.0001). Post hoc Tukey testing showed the TI technique to be statistically further back in the notch than the MP technique and the TT technique with no significant difference between MP and TT. A rather small range of 5% was found when comparing the mean high to low tunnel location on the lateral wall of the notch between the three techniques. The order from lower to higher on the wall was MP (45%), TI (43%), and TT (40%). However, no significant differences were demonstrated for the three techniques for n/N. A total of 82% (55/67) of the femoral tunnel axes were located within the applied radiographic boundaries of the anatomic femoral footprint. Three femoral tunnel axes were back of, or behind, the footprint within the notch relative to the intercondylar notch roof. All three tunnels were drilled using TI technique by two experienced and one new surgeon. There were 10 tunnel axes that were superior to the applied radiographic femoral footprint. Five of these were drilled using TT, three drilled using MP, and two drilled using TI.
Fig. 3 Scatterplot of femoral tunnel depth (c/C) according to technique (no data for knees 15, 22, 61, 67, 71). The dotted line represents the average c/C (depth) position of the tunnel with 1.00 representing the maximum anterior extent of the lateral femoral condyle. The blue dashed lines represent the boundary of the femoral footprint as measured in a cadaver dataset at the authors’ institution.
Fig. 4 Scatterplot of femoral tunnel height (n/N) according to technique (no data for knees 15, 22, 61, 67, 71). The dotted line represents the average n/N (height) position of the tunnel with 0 representing the maximumsuperior height of the notch and 1.0 the inferior extent of the notch. The blue dashed lines represent the boundary of the femoral footprint as measured in a cadaver dataset at the authors’ institution.
A minimal difference of less than 1% was shown between surgeon experience groups for the tibial tunnel aperture location as plateau percentage from medial to lateral on the tibial plateau (m/M), with the new surgeons placing the tibial tunnel slightly more lateral.
Femoral Tunnel Location
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Seven of these 10 tunnels were drilled by experienced surgeons. Examples of tunnels falling outside the footprint are shown in ►Fig. 5.
Tibial Tunnel Aperture Location
Overall, 52 of 67 (78%) tibial tunnels met all applied radiographic criteria for the tibial ACL footprint. Eleven tibial tunnels were deemed too posterior and 0 were deemed too anterior according to applied criteria. Of these 11 posterior tibial tunnels, 5 were drilled by surgeons using TT technique for the femur, 5 by TI surgeons, and 1 by MP surgeons. Seven of these tunnels were drilled by experienced and four by new surgeons. Two tunnels were too lateral according to criteria and both drilled by a new surgeon using TI technique for the femoral drilling. Finally, two tibial tunnels were found to be too medial and these were drilled by one new MP surgeon and one experienced TI surgeon.
Overall, the mean tibial tunnel axis placement was 0.47 from the anterior tibial edge (a/A) with a range of 0.32 to 0.64 (►Fig. 6). The mean tunnel axis from medial to lateral (m/M) was 0.45 with a range of 0.39 to 0.52 (►Fig. 7). Since preferred femoral tunnel drilling technique can influence tibial tunnel placement, especially for TT technique, we did also compare tibial tunnel locations by preferred femoral drilling technique groupings. A range of 6% was shown between the averages of the three methods for the tibial a/A measure (►Table 1). The order of increasing proportional position from anterior to posterior was MP (44%), TI (48%), and TT (50%). Statistical testing showed a significant difference between the three techniques (p < 0.004). Post hoc Tukey testing showed the MP technique to be significantly more anterior than the TT technique. The aperture of the tibial tunnel showed a very small range (1%) between the averages of the three femoral drilling techniques when measured as a percentage from medial to lateral.
Tunnel Aperture Overlap Mapping
Fig. 6 Scatterplot of tibia tunnel anterior-posterior position (a/A) according to technique (no data for knees 15, 22, 61, 67, 71).
Fig. 7 Scatterplot of tibia tunnel medial-lateral position (m/M) according to technique (no data for knees 15, 22, 61, 67, 71).
Aperture overlap mapping for the three techniques is shown in ►Fig. 8. The TT surgeons demonstrate the most tunnel overlap on both the tibia and femur. The overlap mapping also demonstrates the relatively more posterior position of the tibial tunnel for the TT surgeons relatively to the TI and MP tunnels. The relatively more posterior position of the TI femoral tunnels is also demonstrated by the overlap mapping. Tunnel overlap mapping demonstrates the experienced
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Fig. 5 Examples of tunnels that were too (A) superior or (B) anterior according to applied measurement criteria.
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Fig. 8 Femoral and tibial anterior cruciate ligament tunnel aperture overlap maps by technique.
Fig. 9 Femoral and tibial anterior cruciate ligament tunnel aperture overlap maps by surgeon experience.
surgeons to be less variable on the tibia and the new surgeons to be less variable on the femur (►Fig. 9).
Discussion This study provides platform data for a subset of surgeons participating in collaborative multicenter research on ACL reconstruction and associated outcomes. Experience level did not appear to strongly influence ACL tunnel locations. The data demonstrate ranges of femoral and tibial tunnel placements in a group of 12 surgeons using three common ACL techniques. The largest ranges were found in the front to back femoral tunnel position within the notch and anterior to posterior position on the tibia. A femoral tunnel that was superior to the radiographic footprint criteria on the femur or posterior to the radiographic footprint on the tibia were the most common tunnel malpositions. The Journal of Knee Surgery
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Multicenter and multisurgeon collaborative ACL research has become more common both in the United States28,29 and abroad.30–32 However, little information exists regarding the variability that may be associated with such collaboration. Further, there is little information on the influence of surgical technique or surgeon experience level. This study provides background information, albeit in a controlled cadaver study, regarding variability in tunnel placement in collaborative multisurgeon ACL research.33 Our data demonstrated that experienced surgeons placed their femoral tunnel significantly further back, 7% of the total depth of the lateral condyle on average, in the intercondylar notch compared with new surgeons. No other comparisons were significantly different. Nine of 12 tunnels that did not meet the footprint radiographic criteria were drilled by experienced surgeons. Eight of 15 tunnels that did not meet applied criteria on the tibia were drilled by experienced surgeons. Overall, these data would suggest that experience level does not significantly alter tunnel placement, at least in this study group. There is limited prior research on the impact of experience level on ACL surgery. In a plastic knee model, Burkart et al compared the accuracy of ACL guide pin placement with a robotic ACL system using TT technique34 and found significant differences between orthopedic fellows and experienced faculty. Behrend et al did a retrospective review of 50 ACL reconstructions done by 17 different surgeons of varying experience levels at a teaching institution.35 The proportion of “correctly placed” tunnels was higher in the expert group. The only prior study that evaluated volume– outcome relationships in ACL reconstruction was done by Lyman et al using 10 years of an administrative database in New York.36 This study found that patients having ACL reconstruction surgery by a low volume surgeon (less than 6 per year) were significantly more likely to be readmitted to the hospital within 90 days after surgery and to have subsequent knee surgery. These authors did not demonstrate that surgical volume was associated with the need for subsequent ACL reconstruction. Technical aspects of the ACL surgery being performed were not feasible given that study was an analysis of a large database. A wide range of femoral tunnel location from front to back within the notch was demonstrated within the entire group of cadaver knees. This appears to be influenced by preferred femoral tunnel technique, with a range of means of 16%. Our results show that the average tunnel using TI technique was further back in the notch followed by TT and then MP techniques. Again, a wide overall range was demonstrated for high to low femoral tunnel location relative to the intercondylar notch. However, the range of means between drilling techniques in femoral tunnel aperture location was only 5% in terms of superior to inferior location. Our data demonstrate that the TI and MP tunnels were slightly, but insignificantly, lower on the wall of the notch than the TT tunnels. The tibial tunnel measurements demonstrated smaller overall ranges and the average tunnel placements were more similar among the three technique groups. This is not surprising as the techniques are different due to drilling methods used for the femoral tunnel side. Yet, small
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differences were seen for the anterior to posterior tibial tunnel position. Our data demonstrate that the TT tibial tunnels were on average the most posterior with the tibia independent TI and MP groups more anterior. This may reflect a conscious effort for TT surgeons to be able to place TT femoral tunnels more posteriorly on the femur.37,38 The medial to lateral tibial tunnel location was easily the most consistent in our study. ACL tunnels were compared with radiographic measures of the ACL footprint on the femur and tibial. This was done to allow analysis beyond just overall ranges of tunnel placements and comparison of mean tunnel placements between groups. On the femur, a separate anatomic study was done to produce 3D measures of the femoral footprint using measurement methods exactly as done in this cadaveric tunnel study. Tibial tunnels were compared with literature-based values. In total, 82% of femoral tunnels and 78% of tibial tunnels met these applied footprint criteria. It is a limitation of our study that tunnels were not directly compared with each cadaveric specimen’s own ACL footprint by direct dissection or measured relative to arthroscopic landmarks that have been described.39–42 Our data demonstrated tunnels falling outside of criteria by all femoral drilling techniques. On the femur, the 12 tunnels outside of criteria were drilled by TT (5), TI (4), and MP (3). For the 15 tunnels outside of criteria on the tibia, 8 were drilled by surgeons using TI technique, 5 by TT surgeons, and 2 by MP surgeons. Unfortunately, there is no consensus on the appropriate location for ACL tunnels and a fair amount of variability still exists between researchers and clinicians as to the appropriate location, especially on the femoral side.14,21,43–45 Indeed there is a fair amount of variation in the descriptions and anatomic studies of the ACL footprint and its bundles. Kopf et al demonstrated significant variability in their systematic review of ACL footprint in anatomy studies.46 Dargel et al demonstrated substantial side-to-side variability in cadaver knees from a single donor.47,48 Some authors have advocated placing the femoral tunnel at or just posterior to the footprint of the anteromedial bundle of the ACL,14,49–52 while others have advocated for placing the femoral tunnel more central in the ACL footprint.53–57 Similarly, there is variation on recommendations for the tibial side as well with advocates for both anterior placement within the tibial footprint,53,58,59 while others have advocated a more posterior position within the footprint.60–62 It is very important to note that it was not the intent of this study to determine which surgeon or technique produced a more “correct” tunnel position. Surgeons were not given a discrete or absolutely specific target for their ACL tunnels. Rather, they were instructed to try to replicate ACL tunnels they would produce in the clinical setting. Therefore, the tunnel variabilities demonstrated reflect the interaction of personal surgeon beliefs on appropriate tunnel location, anatomic variation, and likely influences from technique used to perform tunnel drilling. How much of the variability is due to surgeon choice versus anatomy or technique cannot be deciphered from our study. This study has limitations that merit brief discussion. Participating surgeons were aware that their tunnels would
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be analyzed closely and this may have induced performance bias. Drilling ACL tunnels on cadaver models can be different than live subjects, especially if knees are arthritic or stiff which may hinder the ability to maximally flex the knee as preferred in some techniques. In addition, the surgeons resected the native ACL before drilling the tunnels; therefore, the native anatomic location of the ACL was freshly evident and may have introduced some bias as well, perhaps reducing the surgeon variability on tunnel placement. Experience level was categorically analyzed based on years in practice rather than continuously by total number of ACL reconstructions each surgeon has performed. All surgeons were sports medicine fellowship trained and hence these data may not be generalizable to all surgeon groups. ACL tunnel measures were compared with radiographic criteria for the ACL footprints. Tunnel location was not, however, directly compared with the center of the anatomic footprint or to particular bundles as this was not the a priori intent of the study. Finally, our tunnel measurement system is not directly referencing off Blumensaat’s line but rather off the anatomic axis of the femur accounting for physiologic valgus and for a 35-degree mean intercondylar roof angle. Previous works have demonstrated this mean roof angle with ranges between 25 and 45 degrees. It is possible that some of the specimens had differing roof angles in actuality and this could slightly alter given measures on the femur. However, the authors encountered impressive undulation of Blumensaat’s line on the sagittal slicing of the distal femur. This led to substantial variability in establishing a reliable landmark upon which to analyze percentages. It is feasible that other 3D analysis techniques are more adept at establishing Blumensaat’s line using 3D CT.
Conclusion In summary, our data demonstrate ranges of ACL tunnel locations. The location of the femoral tunnel aperture in the front to back plane relative to the notch roof and the anterior to posterior position on the tibia were the most variable measures. Surgeon experience level did not appear to significantly affect tunnel location. This study provides background information that may be beneficial when evaluating multisurgeon and multicenter collaborative ACL studies.
Acknowledgments This study was supported by the National Institute of Health Mentored Clinical Research Scholar Program at Iowa (5K12RR017700–04) and the Vanderbilt Sports Medicine Research Fund. DonJoy Orthopaedics provided the specimens and wet laboratory facility for the study.
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Appendix 1 The MOON Knee Group members contributing to this article include the following: Warren R. Dunn, MD, MPH, Kurt P. Spindler, MD, James L. Carey, MD, MPH, Charles L. Cox, MD, MPH, Christopher C. Kaeding, MD, David C. Flanigan, MD, Rick W. Wright, MD, Matthew J. Matava, MD, Robert H. Brophy, MD, Matthew V. Smith, MD, Eric C. McCarty, MD, Armando F. Vidal, MD, Michelle Wolcott, MD, Robert G. Marx, MD, MSc, Richard D. Parker, MD, Jack F. Andrish, MD, and Morgan H. Jones, MD, MPH.
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