Arthroscopy: The Journal of Arthroscopic and Related Surgery 6(3):198-204 Published by Raven Press, Ltd. 0 1590Arthroscopy Association of North America

Arthroscopic

Strain Gauge Measurement of the Normal Anterior Cruciate Ligament

James G. Howe, M.D., Clay Wertheimer, M.D., Robert J. Johnson, M.D., Claude E. Nichols, M.D., Malcolm H. Pope, Ph.D., and Bruce Beynnon, M.S.

Summary: This article describes a new arthroscopic technique to study the anterior cruciate ligament (ACL) in vivo. A Hall effect strain transducer (HEST) is inserted arthroscopically into the anterior medial band (AMB) of the ACL. The strain is calculated from HEST displacement data. This method determines a reference length of the AMB when it becomes taut and load bearing. Data from HEST implantation in five patients with normal ACLs are reported. The HEST was implanted in the AMB with patients under local anesthesia. Strain was calculated during anterior-posterior shear testing and isometric quadriceps contractions at 30 and 90” of knee flexion. The results demonstrate that this technique is safe and reliable. Lachman testing (anterior shear testing at 30”) caused significantly higher strain in comparison to the drawer tests (anterior shear testing at 90”). A significant increase in strain occurred during isometric quadriceps contraction when the knee was flexed at 30”. No significant change in strain was measured, however, during isometric quadriceps contraction at 90”of flexion. These results confirm previous studies showing that the Lachman test is a more sensitive clinical method for evaluating the AMB. They suggest that isometric quadriceps activity at 90” of knee flexion can be prescribed for rehabilitation without risk of increased strain of the AMB. Key Words: Strain gauge-Anterior cruciate ligament-Biomechanics-Rehabilitation.

the elongation of the normal ACL directly while preserving the ligament, other joint structures, and muscle activity. The technique uses arthroscopic implantation of a displacement transducer, the Hall effect strain transducer (HEST) into the anteromedial band (AMB) of the ACL. Ligament strain is calculated from the displacement data. Strain values were obtained during anterior-posterior (A-P) shear testing of five patients with their knees flexed at 30” (Lachman test), 90” (drawer test), and during isometric quadriceps contraction at 30 and 90”. The objectives of this study were to measure ACL displacement and to calculate strain in vivo during activities that are recognized as diagnostically or therapeutically important.

Characterization of anterior cruciate ligament (ACL) biomechanics in vivo is necessary for its successful repair, reconstruction, and rehabilitation. Previous in-vitro studies of the ACL have sacrificed the ligament to quantify its contribution to joint function (l-7). These studies lacked normal muscle tone or activity, which has been shown to affect ACL behavior (8-10). Henning et al. measured ACL strain in vivo but they used injured ligaments (11). This article describes a technique for measuring From the McClure Musculoskeletal Research Center, Department of Orthopaedics and Rehabilitation, University of Vermont College of Medicine, Burlington, Vermont. Address correspondence and reprint requests to Dr. J. G. Howe at McClure Musculoskeletal Research Center, Department of Orthopaedics and Rehabilitation, University College of Medicine. Burlington. VT 05405. U.S.A. This paper won the’Richard O’Connor Research Award at the 1989 Arthroscopy Association of North America Meeting in Seattle, Washington.

MATERIALS AND METHODS The HEST developed at the McClure Musculoskeletal Research Center operates on the principle 198

STRAIN

GAUGE MEASUREMENT

that voltage is generated when a current-carrying conductor is placed in a magnetic field. The device is 5-7 mm long and 2 mm wide. A stainless steel encased magnet slides within a nonstick (Teflon)coated stainless steel tube. The steel tube is coupled to an F. W. Bell Hall generator, which produces a voltage proportional to the linear displacement of the magnetic core (12). The linear displacement accuracy is .2% strain. The HEST is highly compliant because it does not require contact between the sliding tubes. A force less than .5 g expands or contracts the HEST. The ligament can, therefore, be measured with minimal influence on the normal displacement pattern of the tissue. The HEST does not distort or “prestrain” the ligament. It is compliant enough to measure both shortening and lengthening. During implantation, the long axis of the HEST is aligned with the fiber bundles of the ligament. This alignment assures that the natural elongation pattern of the ligament is measured by the HEST. Previous in-vitro experiments demonstrated the precision, accuracy, and reliability of the HEST (9,12,13). The HEST is coated with parylene, allowing it to be immersible in both synovial and arthroscopy fluid without electrical failure. Stainless steel barbs hold the HEST in the ligament and allow for easy implantation and removal (Fig. 1).

OF NORMAL

ACL

199

IMPLANTATION The HEST is implanted via standard arthroscopic knee portals. A tined insertion tool mates with the HEST and holds it at mid working range. The HEST is secured onto the tool with removable mersilene sutures (Fig. 1). The HEST is implanted by inserting a 9-mm stainless steel sleeve through the anterolateral portal with a blunt trochar (Fig. 2B). The trochar is withdrawn and the HEST insertion tool assembly is passed through the sleeve (Fig. 2C). Using arthroscopic visualization from the anteromedial portal, the HEST is aligned with the fibers of the anteromedial band (AMB). The HEST barbs are pushed into the ligament with the insertion tool and the HEST is then released from the tool by retracting the tines. The insertion tool is then withdrawn through the sleeve, the sleeve is removed, and the portals are closed around the HEST electrical wires and sutures (Fig. 2D). The portals are then covered with a sterile dressing, and the HEST electrical wires and sutures are taped to the lateral aspect of the patient’s leg. The arthroscopic tools and techniques were developed in cadaver experiments before their application in vivo. The HEST is removed from the ligament and delivered through the portal by pulling on the mersilene sutures attached to its barbs.

TESTING PROTOCOL Preliminary in-vivo experiments determined much of the protocol described in this study. These preliminary studies demonstrated that (a) active muscle tone alters ACL strain and, therefore, ACL strain is most accurately recorded with the patient under local anesthesia, and (b) knee flexion, tibialfemoral rotation, and load must be controlled and monitored during testing. Five healthy male volunteers between 18 and 40 years of age (mean age, 30 years) who required diagnostic or therapeutic arthroscopy were tested in this study. All patients had normal ACLs by clinical and arthroscopic examination. The experimental protocol was reviewed and accepted by The Human Research Committee. Arthroscopy and all testing were accomplished with patients under local anesthesia. Immediately after diagnostic or therapeutic arthroscopy a HEST was implanted into the AMB. The patient was positioned sitting on the end of a modified operating room table. The femur was oriArthroscopy,

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J. G. HOWE ET AL.

Anterolateral

Portal

Blunt Trochar

D

C FIG. 2. Hall effect strain transducer (HEST) implantation.

ented in the horizontal plane with the tibia placed over the end of the table. The thigh was secured to the table with a strap to prevent further hip flexion or thigh elevation. The leg was attached by a strap to an adjustable T-bar that maintained knee flexion angle and served as a platform for mounting a force plate against the patient’s tibia (Fig. 3). The force plate was used to measure loads generated during isometric quadriceps contraction (Fig. 4). An Acufex (Not-wood, MA, USA) knee signature sys-

A-P LOAD CELL

FIG. 3. Anterior-posterior

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Vol. 6, No. 3, 1990

HALL EFFECT TRANSDUCER

(A-P) load applied at 30”.

ACL, anterior cruciate ligament.

tern electrogoniometer was placed on the patient’s leg to provide a continuous recording of flexionextension, internal-external rotation, and A-P translation of the knee. A-P loads were applied with a custom-designed, hand-held load cell positioned perpendicular to the tibial shaft at the level of the tibiaI tubercle (Fig. 3). The output from the HEST, electrogoniometer, load cell, and isometric quadriceps force plate were transferred to a Techmar Data Acquisition board (Solon, Cleveland, OH, USA) for analog-to-digital conversion at 10 samples per secHALL

EFFECT

TRANSDUCER

FIG. 4. Isometric quadriceps contraction

at 30”.

STRAIN

GAUGE MEASUREMENT

OF NORMAL

Isometric quadriceps contraction 90 and 30” The patient’s distal tibia was secured and the knee was positioned at the appropriate flexion angle. The isometric quadriceps force plate, attached to the steel T-bar, was positioned against the distal tibia above the malleoli (Fig. 4). The moment arm from knee joint line to the center of the force plate was measured and recorded for torque calculation. The patient was then asked to perform four maximal isometric contractions. STATISTICAL ANALYSIS Strain values at selected applied anterior shear loads were compared during Lachman and drawer tests. Strain values at selected torques were compared for isometric quadriceps contractions at 30 and 90” of knee flexion. Comparisons of strain were made within each patient; each patient served as their own control. For statistical comparisons, negative strain values that indicated a palpably slack ligament was assigned a value of zero. A Student’s t test was used to determine statistical significance @ < 0.05).

201

RESULTS

ond. As data were sampled they were simultaneously stored on a personal computer. Drawer and La&man tests (90”) The knee was maintained at 90” of flexion for drawer testing. The hand-held A-P load cell was used to apply measured loads to the tibia. The load was applied in a continuous fashion, anteriorly and then posteriorly in four cycles to limits of 200 N. The leg was repositioned at 30” of flexion for the Lachman test and the procedure was repeated. Flexion and tibial rotation were maintained within a 25” range during applied loading.

ACL

Clinical data No intraoperative or postoperative complications occurred in the five patients studied. Clinical data are presented in Table 1. Calculation of Lo and strain Strain is defined as the change in length of the ligament divided by its reference length, Lo. Percent strain is defined as this ratio multiplied by 100: (L-Lo)/Lo x 100. Previous in-vitro studies arbitrarily defined the reference length as the HEST displacement measured at full passive extension of the knee (9,13). In this study we selected a reference length that corresponds to the condition of the ligament when it initially becomes load bearing. The ACL can lengthen when it is slack as well as when it is taut or load bearing. To distinguish between displacements measured by the HEST when the ligament was slack from displacements measured when the ligament was taut, we measured AMB displacements with the HEST during A-P shear loading of the knee at 30” and simultaneously palpated tension in the ligament. Tension was palpated with a barbed instrument arthroscopically positioned into the AMB next to the HEST. A distinct change in ligament tension was felt and arthroscopically visualized by the examiner during an applied anterior load. A switch was closed to mark the transition point. This Lachman test is graphically depicted in Fig. 5. The curve characterizes the hysteresis behavior of the AMB during continuous A-P loading. The inflection point of the curve corresponded to the slack-taut transition point visualized, palpated, and recorded by the examiner. In Fig. 5, the AMB became palpably taut or load bearing when the HEST measured a displacement of 5.33 mm. This transition point was produced by

TABLE 1. Clinical data Procedure

Patient no.

Age (yr)

1

24

Lateral men&al R knee

Pre-op Dx

2

28

Loose body L knee

3

39

Ganglion R knee

4

34

5

28

Medial meniscus tear R knee Medial meniscus tear L knee

tear

Diagnostic arthroscopy

Partial medial meniscectomy; resection suprapatellar plica Arthroscopy Removal of mass quadriceps tendon Partial medial meniscectomy Partial medial meniscectomy

Outcome No complications Returned to ski instructing by 2 mo follow-up No complications Back to work 1 week postop No complications No complaints 3 mo postop No complications Without complaints at 1 week postop No complications Without complaints at 1 week postop

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J. G. HOWE ET AL,.

SLACK - TAUT (Nt

I I

-*O” 5.2

5.1

5.4 5.3 HEST DISP. (mm)

5.5

5.6

5. Lachman anterior-posterior shear load versus Hall effect strain transducer (HEST) displacement (30” of knee flexion). FIG.

a 20 N anterior shear load. The reference length for calculation of strain, Lo, was assigned the value of 5.33 mm in this patient. Any displacements measured ~5.33 mm would result in a negative strain DRAWER AM6 A-P SHEAR TESTING CT0 DEGREESOF KYEE FLEW)&1

IN-VW0

ILOADING

200 -I

-4

-3

-2

-1 0 STRAIN

I

m

I

2

3

4

(%I

LACHMAN IN-VIVO AMB A-P SHEAR TESTING (3zGREES OF KNEE FLEXION) --7---m-m

-. 1

200 ANTERIOR E

IOO-

A R

k ; N

0

-100

I-

POSTkRIOR

-200

Drawer and La&man tests Figure 6 shows the typical load versus strain behavior of the ACL during drawer and Lachman tests. Table 2 lists percent strain values for drawer and Lachman tests for the five subjects when an anterior shear force of 150 N was applied to their knees. For sample size of five patients, the mean strain values for Lachman and drawer tests were found to be significantly different with a corresponding p < .Ol. Strain developed in the AMB during Lachman testing was greater than during the drawer test. Isometric quadriceps contraction at 90” and 30” Figure 7 shows a typical torque versus strain curve for isometric quadriceps contraction at 90 and 30” of knee flexion. Isometric quadriceps contraction causes no increase in strain when the knee is held at 90”, but strain values increased during isometric quadriceps contraction at 30” of flexion. Table 3 lists the load-bearing strain for all patients measured at rest (0 N-m) and during isometric quadriceps contractions yielding 27 N-m of torque with the knee held at 90 and 30”of flexion. Table 4 shows that for all patients no significant change in strain occurred during isometric quadriceps contraction at 90” of knee flexion. Strain increased significantly, however, during isometric quadriceps contraction at 30” of knee flexion (p < .05). Table 4 also shows that the strain measured during contraction yielding 27 N-m at 30” is significantly greater than the strain measured at the same torque when the knee is held at 90” of flexion (p s .Ol).

-

I -4

-3

-2

-1

0 % STRAIN

, 1

2

3

4

FIG. 6. Drawer (upper panel) and La&man (lower panel) invivo anterior cruciate ligament anterior medial band anteriorposterior shear testing at 90 and 30” knee flexion, respectively.

Arthroscopy,

value and correspond to the slack or unloaded condition of the AMB whereas displacements measured >5.33 mm would result in positive strain values and would correspond to the taut or loadbearing condition of the AMB. Although the slacktaut transition point was palpated in only one patient, we found that the inflection point of the HEST displacement versus applied anterior shear load curve consistently occurred at a 20 N applied load for all patients. Therefore, our reference for calculation of strain was defined as the HEST displacement that resulted from an anterior shear load of 20 N in the four other patients examined.

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DISCUSSION We were able to determine a load-bearing reference length for calculation of strain by determining

STRAIN

GAUGE MEASUREMENT

OF NORMAL

ACL

203

TABLE 2. Lachman and drawer strain at I50 N load Subject no.

Lachman % Strain

Drawer % Strain

1 2 3 4 5

4.1 4.4 3.1 4.6 2.8 Mean = 4.0 2 0.795

2.0 3.6 2.7 1.8 1.6 Mean = 2.3 2 0.813

the point when the AMB becomes taut or slack. Unlike our previous in-vitro work, this reference length was not arbitrarily chosen (9,13). We justified the choice of this reference length based on the following three observations: (a) The load versus displacement curve (Fig. 3) is a hysteresis loop. The values of the curve to the right of the inflection point are repeatable indicating a taut AMB while values to the left of the inflection point are not repeatable, characteristic of a slack structure. (b) The anterior shear load corresponding to the inflection point of this curve consistently measured 20 N of force in all patients. This corresponds well to that force required to offset the gravity vector of the leg as it is held over the edge of the operating table. Thus, it seems reasonable that the posterior shear force from the weight of the leg must be overcome by an anteriorly directed force on the knee before the ligament begins to get taut and becomes load bearing. (c) The taut-slack transition point palpated directly by placing an instrument into the fibers of the ligament corresponded to the inflection point on the load versus displacement curve (Fig. 5). This study has demonstrated that A-P shear testing at 30” of knee flexion (the Lachman test) produces more load-bearing strain than at 90” (drawer

% Strain (Lachman-drawer) difference 2.1 0.8 :.: 1:2 Mean = 1.7 ? O.% t = 3.96 p < .Ol

test). This result is in contrast to our earlier in-vitro work (13). Our earlier studies, however, did not make comparison of strain values at common magnitudes of shear load, and used an arbitrary reference length for calculation of strain. Applied loads were, therefore, not consistent when strain values were recorded at 30 and 90”. In addition, these studies lacked active, living musculature, which we now know affects the behavior of the ligament (g-10). Our in-vivo results do concur with previously published clinical impressions and studies using instrumented knee laxity testing. DeHaven (14), Johnson (15), Jonsson et al. (16), and Torg et al. (17) have argued that the Lachman test is the clinical examination of choice to evaluate the ACL. Torzilli et al. (18) roentgenographically demonstrated in vivo that the drawer test is insufficient to be of clinical use. Markolf et al. (4) showed that clinical stress tests documenting A-P laxity are best performed at 20”. Daniel et al. have chosen the Lachman as the basis for ACL evaluation with the KT1000 arthrometer (19). Henning et al. showed that the Lachman test produced greater elongation of the anteromedial fibers of the ACL than did the anterior drawer test (11). Our experiments using the HEST quantitatively describe why the Lachman test is better than the TABLE 3. Isometric quadriceps contraction data

% Strain at 90”

0

1

3

4

FIG. 7. In-vivo anteromedial band strain pattern isometric quadriceps activity at 30 and 90” of knee flexion.

Patient no.

Rest (0 N-m) R90

: 3 4 5

1.6 0” 0 0 0.01

Contraction (27 N-m) C90 1.6 0 0 0 0.20

% Strain at 30” Rest (0 N-m) R 30 2.5 ::; 0 3.3

Contraction (27 N-m) C 30 3.4 2.5 2.9 4.5 6.0

DNegative values for percent strain are defined as zero (see text).

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J. G. HOWE

TABLE 4. Isometric quadriceps and differences

contraction: changes in load-bearing strain at two different jlexion angles

Change in strain during isometric contraction Patient # 1 2 3 4 5

at 90” @9o-C901 0” 0 0 0 0.19 NS

at 30 (R&&J 0.9 1.0 1.9 4.5 2.7 f = 3.34 p c 0.05

Differences in Strain at 30 and 90” during contraction (27 N-m) (C,&,) 1.8 2.5 2.9 4.5 5.8 r = 5.18 p c 0.01

NS = not significant a Negative values for percent strain are defined as zero.

drawer for evaluating the integrity of the AMB. The AMB is minimally strained when the knee rests at 20-30” of flexion (13). It becomes maximally strained at this flexion angle when an anterior shear load is applied to the knee. The Lachman test therefore allows the examiner to appreciate the excursion of the tibia on the femur where the primary constraint is undergoing its maximum change in length. Torzilli showed that the drawer test was ineffective for demonstrating ACL disruptions because, as our results show, ACL strain at this flexion angle is significantly diminished. Our findings are also pertinent to knee rehabihtation. It appears that the AMB first becomes elongated during isometric quadriceps contraction somewhere between 30 and 90” of knee flexion. Isometric quadriceps strengthening should, therefore, be safe in the anterior cruciate injured knee if the knee is maintained at 90” of flexion. At 30” of knee flexion isometric quadriceps contraction produces an increase in strain and, thus, this exercise should be used cautiously. Grood et al. (20) have concluded from a cadaver model that a potentially deleterious effect on a repaired or reconstructed ACL can occur during leg-extension exercises in the range of t&30“. The amount of flexion where isometric quadriceps contraction starts to strain the AMB and may be unsafe remains to be delineated in vivo.

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ET AL.

REFERENCES 1. Butler DL, Noyes FR, Grood ES. Ligamentous restraints to anterior-posterior drawer in the human knee. A biomechanical study. J Bone Joint Surg 1980;62A:259-70. 2. Fukabavashi T. Torzilli P. Sherman M. Warren R. An invitro b&mechanical evaluation of anterior-posterior motion of the knee. J Bone Joint Surg 1984:64A;258-64. 3. Hsieh HH, Walker PS. Stability mechanisms of the loaded and unloaded knee joint. J Bone Joint Surg 1976;58A:87-93. 4. Markolf KL. Mensch JS. Amstutz HC. Stiffness and laxitv of the knee-the contributions of the supporting structures. A quantitative in-vivo study. J Bone Joint Surg 1976;58A: 583-94. 5. Markolf K, Kochan A, Amstutz M. Measurement of knee stiffness and laxity in patients with documented absence of the anterior cruciate ligament. J Bone Joint Surg 1984;66A: 242-53. 6. Piziali RL, Rastegar JC, Nagel D. Measurement of the nonlinear, coupled stiffness characteristics of the human knee. J Biomech 1977;10:45-51. 7. Piziali RL, Seering WP, Nagel D, Schurman D. The function of the primary ligaments of the knee in anterior-posterior and medial-lateral motions. J Biomech 1980;13:777-84. 8. Kain C, McCarthy J, Arms S, et al. An in-vivo analysis of the effect of transcutaneous electrical stimulation of the quadriceps and hamstrings on anterior cruciate ligament deformation. Am J Sports Med 1988;16:147-51. 9. Renstrom P, Arms SW, Stanwyck TS, et al. Strain within the anterior cruciate ligament during hamstring and quadriceps activity. Am J Soorts Med 1986:14:83-7. 10. Solomon M, Baratta R, Zhon BM, et al. The synergistic action of the anterior cruciate ligament and thigh muscles in maintaining joint stability. Am J Sports Med 1987;15:207-13. 11. Henning CE, Lynch MD, Glick KR. An in-vivo strain gauge study of elongation of the anterior cruciate ligament. Am J Sports Med 1985;13:224. 12. Fischer RA, Arms S, Johnson R, Pope M. The functional relationship of the posterior oblique ligament to the medial collateral ligament of the human knee. Am J Sports Med 1985;13:3!&7. 13. Arms SW, Pope MH, Johnson RJ, et al. The biomechanics of anterior cruciate ligament rehabilitation and reconstruction. Am J Sports Med 1984;12:118. 14. DeHaven KE. Diagnosis of acute knee injuries with memarturosis. Am J Sports Med 1980;8:9-14. 15. Johnson RJ. The anterior cruciate. A dilemma in sports medicine. Int J Sports Med 1982;3:71-9. 16. Jonsson T, Althoff B, Peterson L, Renstrom P. Ruptures of the ACL: a clinical diagnosis of comparative study of Lachman and drawer. Am J Sports Med 1982;1O:lt%2. 17. Torg J, Conrad W, Kalen V. Clinical diagnosis of ACL instability. Am J Sports Med 1976;4:84-92. 18. Torzilli P, Greenberg R, Hood R, Pavlov H, Insall J. Measurement of anterior-posterior motion of the knee in injured patients using a biomechanical stress technique. J Bone Joint Surg 1984;66A:1438-42. 19. Daniel D, Malcolm L, Losse Cl, Stone M, Sachs R, Burks R. Instrumented measurement of anterior laxity of the knee. J Bone Joint Surg 1984;66A:725-33. 20. Grood ES, Suntay WJ, Noyes FR, Butler DL. Biomechanics of the knee-extension exercise. J Bone Joint Surg [Am] 1984;66:725-33.