 KNEE

The influence of surgical factors on dislocation of the meniscal bearing after Oxford medial unicompartmental knee replacement S. Y. Lee, J. H. Bae, J. G. Kim, K. M. Jang, W. Y. Shon, K. W. Kim, H. C. Lim From Guro Hospital, Korea University College of Medicine, Seoul, Korea

 S. Y. Lee, MD, Orthopaedic Surgeon, Clinical Fellow  K. M. Jang, MD, PhD, Orthopaedic Surgeon, Clinical Professor  W. Y. Shon, MD, PhD, Orthopaedic Surgeon, Professor  H. C. Lim, MD, PhD, Orthopaedic Surgeon, Professor Korea University College of Medicine, Department of Orthopaedic Surgery, 80, GuroDong, Guro-Gu, Seoul, 152-703, South Korea.  J. H. Bae, MD, PhD, Orthopaedic Surgeon, Professor  J. G. Kim, MD, PhD, Orthopaedic Surgeon, Clinical Professor  K. W. Kim, MD, Orthopaedic Surgeon, Resident Korea University College of Medicine, Department of Orthopaedic Surgery, 516, Gojan 1-Dong, Danwon-Gu, Ansan, 425-707, South Korea. Correspondence should be sent to Prof Dr Med H. C. Lim; e-mail: [email protected] ©2014 The British Editorial Society of Bone & Joint Surgery doi:10.1302/0301-620X.96B7. 33352 $2.00 Bone Joint J 2014;96-B:914–22. Received 19 October 2013; Accepted after revision 14 March 2014

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A CASE–CONTROL STUDY The aim of this study was to evaluate the risk factors for dislocation of the bearing after a mobile-bearing Oxford medial unicompartmental knee replacement (UKR) and to test the hypothesis that surgical factors, as measured from post-operative radiographs, are associated with its dislocation From a total of 480 UKRs performed between 2001 and 2012, in 391 patients with a mean age of 66.5 years (45 to 82) (316 female, 75 male), we identified 17 UKRs where bearing dislocation occurred. The post-operative radiological measurements of the 17 UKRs and 51 matched controls were analysed using conditional logistic regression analysis. The postoperative radiological measurements included post-operative change in limb alignment, the position of the femoral and tibial components, the resection depth of the proximal tibia, and the femoral component-posterior condyle classification. We concluded that a post-operative decrease in the posterior tibial slope relative to the pre-operative value was the only significant determinant of dislocation of the bearing after medial Oxford UKR (odds ratio 1.881; 95% confidence interval 1.272 to 2.779). A postoperative posterior tibial slope < 8.45° and a difference between the pre-operative and postoperative posterior tibial slope of > 2.19° may increase the risk of dislocation. Cite this article: Bone Joint J 2014; 96-B:914–22.

The mobile-bearing Oxford medial unicompartmental knee replacement (UKR) has given favourable long-term results and has consequently increased in popularity.1-4 However, there is concern that dislocation of the bearing occurs in 0% to 5.3% of cases.2,4-6 Lewold et al7 reported dislocation in 2.3%, making it the most common cause for revision. Goodfellow et al8 reported that this became less frequent as the implant design progressed from Phase I to Phase III (with a prevalence of 2.5% vs 0.2%, respectively). Although some reports have shown a decreasing frequency of dislocation, studies from East Asia have shown a high prevalence associated with the Phase III Oxford UKR.4-6 A recent systematic review revealed that dislocation of the bearing is the single most important cause for revision and that it occurs more frequently in Asian populations than in the West.9 Possible mechanisms of dislocation include imbalance of the flexion–extension gap; impingement on the bearing; delayed elongation of or damage to the medial collateral ligament; relatively frequent deep flexion positioning (which

is particularly common in the East Asian population) and malpositioning of the components, including implantation with a wide mediolateral gap between the femoral and tibial components.8 One of the likeliest causes, which has been suggested by several authors, is posterior impingement on the bearing by osteophytes in the extreme posterior region of the medial femoral condyle (MFC).4-6,8,10 However, this theory has not yet been validated. The purpose of this study was to evaluate possible causes of dislocation of the meniscal bearing by examining the post-operative radiographs. We hypothesised that post-operative limb alignment, malposition of components or posterior impingement on the bearing may be associated with dislocation of the bearing.

Patients and Methods After obtaining institutional review board approval, we retrospectively reviewed the medical records of all patients who underwent Oxford Phase III UKR at a single hospital between January 2001 and May 2012. Among THE BONE & JOINT JOURNAL

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Table I. Pre-operative characteristics of the patients without bearing dislocation (NDL) and with bearing dislocation (DL) (SD, standard deviation) Characteristic

NDL group (n = 51)

DL group (n = 17)

p-value

Age (yrs, mean (SD, range)) Gender (M:F) Body mass index (kg/m2, mean (SD)) Size of femoral component (n (%)) Extra small Small Medium Large Extra large Size of tibial component (n (%)) AA (smallest) A B C D (largest) Size of polyethylene bearing (n (%)) 3 (thinnest) 4 5 6 7 (thickest) Type of polyethylene bearing (n (%)) Non-anatomical Anatomical Pre-operative flexion contracture (°, mean (SD)) Pre-operative further flexion (°, mean (SD)) Pre-operative range of movement (°, mean (SD)) Pre-operative Kellgren–Lawrence grade (n (%)) I II III IV Pre-operative hip-knee-ankle angle (°, mean (SD)) Pre-operative medial proximal tibial angle (°, mean (SD)) Pre-operative posterior tibial slope (°, mean (SD))

65 (7.1, 52 to 79)) 0:51 26.1 (2.2)

65 (7.5, 53 to 79) 0:17 25.8 (2.9)

0.975* 1.000† 0.548* 0.715†

39 (76) 11 (22) 1 (2) 0 (0) 0 (0)

13 (76) 3 (18) 1 (6) 0 (0) 0 (0)

21 (41) 18 (35) 10 (20) 2 (4) 0 (0)

7 (41) 5 (29) 4 (24) 1 (6) 0 (0)

8 (16) 21 (41) 13 (25) 6 (12) 3 (6)

5 (29) 7 (41) 1 (6) 4 (24) 0 (0)

21 (41) 30 (59) 2.8 (4.2) 124.3 (8.8) 121.5 (9.6)

7 (41) 10 (59) 3.2 (4.3) 121.8 (10.4) 118.5 (13.0)

0 (0) 2 (4) 30 (59) 19 (37) 6.5 (2.6) 86.2 (1.7) 10.2 (1.3)

0 (0) 2 (12) 11 (65) 4 (23) 6.5 (3.0) 85.9 (2.2) 11.2 (3.2)

0.971†

0.276†

1.000†

0.799* 0.380* 0.308* 0.297†

0.530* 0.728* 0.174*

* Mann–Whitney U test † Fisher’s exact test

the 480 medial UKRs (391 patients), we found 17 patients who had experienced dislocation of the bearing, all of whom were included in this study. To determine the risk factors for this, we selected a control group matched by age, gender, body mass index (BMI) and type of meniscal bearing. The cases and controls were matched in a ratio of 1:3. In addition to the matching criteria, follow-up of more than 24 months after UKR was required for inclusion in the control group. This was based on a study by Lewold et al,7 who found that dislocation of the bearing after mobilebearing medial UKR usually occurred in the early postoperative period, i.e. within two years. Overall, the matching process yielded 51 controls to compare with the 17 patients with dislocations. Comparison of the pre-operative demographic data revealed no significant differences between the groups11 (Table I). The 17 patients with dislocation of the bearing had 11 anterior and 6 posterior dislocations: all the affected patients were women. The primary diagnosis was degenerative VOL. 96-B, No. 7, JULY 2014

osteoarthritis (OA) in every case. Dislocation of the bearing occurred two or more times in four patients; three patients experienced two incidents of dislocation and one patient experienced three incidents of dislocation. However, only the first episode is included in this study because there were no changes after first bearing dislocation except bearing thickness. Overall, dislocation of a bearing occurred in 3.54% of the patients during the study period. For the whole group of 480 patients, the dislocation rate was 5.19% (7/135) for the non-anatomical bearing and 2.90% (10/345) for the anatomical bearing. Surgical procedure. The surgical indications for primary medial UKR were medial OA without the involvement of other compartments, a moderate level of activity, and an intact medial collateral and anterior cruciate ligament (ACL). The contraindications were an inflammatory arthropathy, ligament instability, a range of movement ≤ 90°, and varus deformity ≥ 15°. All operations were performed by two surgeons (HCL and JHB) using a minimally

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Fig. 1 Diagrams of the radiological measurements (modified from the operating manual of the Oxford Phase III Unicompartmental Knee Replacement). Fig. 2

invasive approach, during which no ligaments were released.4 Dislocation of the bearing was treated either by replacement of the bearing with a thicker one, or by conversion to total knee replacement. This decision was based on the patient’s general health and level of activity, any loosening of the metallic components, and an intra-operative assessment of the medial flexion–extension gap. Radiological evaluation. We used pre- and post-operative standing anteroposterior (AP) and lateral radiographs of the knee or the entire lower limb for radiological evaluation. Two orthopaedic surgeons (SYL, KWK) measured each radiographs using StarPACS PiView system (INFINITT Co, Ltd, Seoul, South Korea). The image scale was about 105% of the actual size. The hip-knee-ankle (HKA) angles were measured from peri-operative standing AP radiographs of the entire lower limb. The pre-operative medial proximal tibial angle (Pre MPTA) and posterior tibial slope (Pre PTS) were also measured from standing AP and lateral radiographs of the entire lower limb. Both values were determined with respect to the anatomical axis of the tibia. The positions of the femoral and tibial components were determined using the method of radiological analysis outlined in the operating manual for the Oxford Partial Knee Surgical Technique (Biomet, Warsaw, Indiana)12,13 (Fig. 1). We obtained data about the varus/valgus angle and flexion/extension angle of the femoral component (respectively F-VarVal and F-FlexExt) as well as the varus/valgus angle and posterior inclination angle of the tibial component (respectively T-VarVal and Post PTS). The differences between Pre MPTA and T-VarVal and between Pre PTS and Post PTS were calculated as Pre-Post MPTA and Pre-Post PTS, respectively. Although the resection depth of the proximal medial tibia (RDT) can be measured intra-operatively by applying

The resection depth of the proximal medial tibia (RDT). A line was drawn at a tangent to the lateral tibial plateau on the pre-operative measurement of the MPTA relative to the anatomical axis of the tibia (white arrow). A further line, perpendicular to this tangent, was projected to the most medial point of the proximal tibia, and its length measured (black arrow).

callipers to the resected bone, the uneven upper surface of the tibia caused these measurements to vary. The RDT was therefore estimated from the standing AP radiograph of the knee. A line was drawn at a tangent to the lateral tibial plateau and at the pre-operatively measured MPTA with respect to the anatomical axis of the tibia. Another line perpendicular to this tangent was projected to the most medial point of the proximal tibia, and its length measured (Fig. 2). Posterior osteophytes and radio-opaque cement remnants that could contact the bearing or tibial plate, during deep flexion, were identified, but these lesions could not be evaluated objectively. Therefore, protruding or depressed posterior radio-opaque regions, including osteophytes and cement remnants, were classified based on the circle adjoining the outer margin of the femoral component (femoral component-posterior condyle (FCC) classification) (Fig. 3).14,15 Protrusion or depression were defined as a radio-opaque lesion above or below the circle of the femoral component. Type A (depressed posterior condyle within the circle) was the most common finding after medial UKR. In type B, the posterior condyle or cement remnant just abuts the circle (arrowheads in the Fig. 3). A type C1 was defined as a constant protrusion above the circle. Type C2 is similar but protrudes suddenly above the circle in the 6.5 mm arc length. In the East Asian population, a length of 6.5 mm suggests the possibility of contact with the posterior femoral condyle when the knee is in more than 120° of flexion. The MFC can be divided into an extension facet circle (EF), a flexion facet circle (FF) and a further extenTHE BONE & JOINT JOURNAL

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analysis. A p-value < 0.05 was taken to indicate significance. Finally, we performed receiver operating characteristic (ROC) curve analysis of the significant determinants of bearing dislocation to identify their optimal cut-off values for predicting dislocation. Statistical analysis was performed using SPSS v20.0 (SPSS Inc., Chicago, Illinois).

Fig. 3 Posterior radio-opaque lesions, including osteophytes and cement remnants, were classified based on the circle adjoining the outer margin of the femoral component (femoral component-posterior condyle classification). Depression within the circle was defined as type A. In type B, the posterior lesions just abut the circle (arrowheads). In type C1, those lesions protrude above the circle (arrow). A sudden protrusion in arc length within 6.5 mm was defined as a type C2 (empty arrow).

sion of the FF that articulates in an extremely flexed position (the extreme posterior portion of the MFC that contacts only the posterior horn of the meniscus (PHF)). Usually, flexion of 0° to 20° occurs along the EF, and flexion of 20° to 120° along the FF. The PHF articulates only in > 120° of flexion. The arc angle of the PHF was previously reported to be 24°.14 As inter-ethnic differences can occur, we calculated the possible contact length in deep flexion (> 120°) based on the report by Yue et al15 (Fig. 4). Statistical analysis. The reliability of the measurements was assessed using intraclass correlation coefficients (ICCs) and κ statistics (Table II). After matching cases with a control group, pre-operative variables were compared using the Mann–Whitney U or Fisher’s exact tests. Post-operative values of the radiological measurements were also compared between groups using the same statistical methods. The post-operative variables were analysed using univariate conditional logistic regression analysis. Only covariates with resulting p-values < 0.20 were included in the backward stepwise multivariate conditional logistic regression VOL. 96-B, No. 7, JULY 2014

Results The mean Pre-Post PTS and Post PTS differed significantly between the groups (Table III). Univariate analysis of the radiological parameters yielded p-values < 0.20 for PrePost HKA, Pre-Post PTS, T-VarVal, RDT and Post PTS (Table IV). Multivariate analysis indicated that only PrePost PTS was a significant determinant of bearing dislocation (odds ratio (OR) 1.881; 95% confidence interval (CI) 1.272 to 2.779) (Table V). The significant determinants of anterior dislocation of the bearing from univariate analysis were Pre-Post MPTA, Pre-Post PTS, T-VarVal and Post PTS (Table IV). Multivariate analysis revealed that Pre-Post PTS was the only significant determinant (OR 2.568; 95% CI 1.180 to 5.588) (Table V). However, after multivariate analysis, none of the measurements was a significant determinant of posterior bearing dislocation (Table V). A ROC plot was used to determine the cut-off values of Post PTS and Pre-Post PTS, which differed significantly between the dislocation and non-dislocation groups. A Post PTS < 8.45° or Pre-Post PTS > 2.19° may predict bearing dislocation (Fig. 5). Discussion The single most significant determinant of dislocation of the bearing in this study was a decreased post-operative posterior tibial slope (PTS) compared with the pre-operative PTS. Surprisingly, only a few previous studies have addressed the effects of the PTS on the results of medial UKR. A recent cadaveric study has shown that the posterior proximal tibial strain increased and the anterior proximal tibial strain decreased as the PTS increased.16 By contrast, a finite element analysis of medial UKR showed that the proximal tibial strains were similar regardless of the posterior inclination of the tibial components.17 However, none of these studies considered the original PTS and analysed the effect of the change. Chatellard et al18 reported that prosthesis survival after medial UKR was significantly shorter when the PTS changed by >2° from the physiological value. Goodfellow et al8 stated that it is reasonable to customise the PTS relative to the individual’s pre-operative PTS. The flexion and extension gaps are measured with the tibial template in place, but those authors concluded that a constant inclination of 7° posterior is sufficient for medial mobile-bearing UKRs.8 There are two possible explanations for the influences of PTS on bearing dislocation. The first is a change in the gap or the pressure between the femoral and tibial components; the second is a change in the rotational kinematics of the knee.

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Fig. 4 The possible contact length during deep flexion (> 120°). The arc angle of the posterior extension of the flexion facet circle (PHF) was previously reported to be 24°.14 As inter-ethnic differences occur, we developed a model based on the report by Yue et al15 and calculated the possible contact length in deep flexion (as shown in the drawing). (Modified from Iwaki et al14)

Table II. Intraclass correlation coefficient (ICC) and κ values for the new measurements. (Interpretation of the ICC and κ values: < 0.20, poor reliability; 0.21 to 0.40, fair reliability; 0.41 to 0.60, moderate reliability; 0.61 to 0.80, good reliability; 0.81 to 1.00, excellent reliability).

Inter-tester ICC (95% confidence interval (CI)) κ value Tester 1 ICC (95% CI) κ value Tester 2 ICC (95% CI) κ value

RDT*

FCC classification†

0.895 (0.816 to 0.938) -

0.878

0.922 (0.846 to 0.957) -

0.970

0.903 (0.835 to 0.942) -

0.848

* RDT, resection depth of the proximal tibia † FCC classification, femoral component-condyle classification

Initially, we supposed that dynamic changes in the flexion–extension gap or pressure in the femoral componentbearing articulation might cause dislocation of the bearing, because the static flexion–extension gap can only be approximated at operation. Small et al16 reported that because the bearing used in medial Oxford UKR translates up to 7 mm during movement of the knee, an extreme slope can cause a significant disparity in loading on the bearing placed anteriorly or posteriorly.16 By contrast, a PTS lower than the original value may also cause a disparity in AP

loading during movement of the knee. Although the flexion–extension gaps are checked as being equal using the feeler gauge at operation, the loading condition of the anterior or posterior portion of the knee may vary according to its angle of flexion. This variation will be higher during transition from the stance phase to the swing phase of gait, and vice versa. Some studies have reported that changing the PTS can change the loading on the proximal tibia.16,17,19,20 A recent study has reported that a decrease in the PTS increases the translation and wear of the bearing.20 THE BONE & JOINT JOURNAL

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Table III. Post-operative radiological values of the groups Patient groups*

DL group

Post HKA (°, mean (SD), +ve: varus) Pre-Post HKA (°, mean (SD), +ve: varus change) Pre-Post MPTA (°, mean (SD), +ve: varus change) Pre-Post PTS (°, mean (SD), +ve: decreased post-operative tibial slope than pre-operative tibial slope) F-VarVal (°, mean (SD), +ve: varus) T-VarVal (°, mean (SD), +ve: varus) RDT (mm, mean (SD)) F-FlexExt (°, mean (SD), +ve: flexion) Post PTS (°, mean (SD), + ve: posterior inclination) FCC classification Type A (n, %) Type B (n, %) Type C1 (n, %) Type C2 (n, %)

NDL (n = 51)

ADL (n = 11)

p-value†

PDL (n = 6)

p-value‡

All (n = 17)

p-value§

0.83 (3.15) –5.62 (2.61) –2.31 (2.16) 0.17 (1.30)

0.31 (2.25) –6.54 (2.36) –1.41 (2.43) 3.43 (3.94)

0.521¶ 0.308¶ 0.196¶ 0.004¶

–1.18 (4.42) –7.13 (2.31) –1.85 (2.93) 3.46 (2.71)

0.156¶ 0.224¶ 0.673¶ 0.012¶

–0.21 (3.13) –6.75 (2.29) –1.57 (2.53) 3.44 (3.47)

0.206¶ 0.161¶ 0.355¶ < 0.0001¶

–0.41 (4.02) 1.51 (2.13) 4.99 (1.35) 2.98 (4.15) 9.98 (1.35)

0.68 (6.86) 2.74 (1.55) 5.31 (1.27) 4.50 (6.62) 7.82 (3.05)

0.283¶ 0.065¶ 0.295¶ 0.612¶ 0.009¶ 0.849**

1.40 (5.85) 2.14 (2.51) 6.52 (2.63) 3.44 (9.64) 7.54 (3.64)

0.581¶ 0.721¶ 0.119¶ 0.378¶ 0.119¶ 0.449**

0.93 (6.35) 2.53 (1.89) 5.74 (1.88) 4.13 (7.53) 7.72 (3.16)

0.231¶ 0.067¶ 0.066¶ 0.987¶ 0.002¶ 0.408**

35 (68) 9 (18) 6 (12) 1 (2)

8 (73) 1 (9) 2 (18) 0 (0)

4 (66) 0 (0) 1 (17) 1 (17)

12 (70) 1 (6) 3 (18) 1 (6)

* NDL, patients without bearing dislocation; DL, patients with any bearing dislocation; ADL, patients with anterior bearing dislocation; PDL, patients with posterior bearing dislocation; SD, standard deviation; Post HKA, post-operative hip–knee–ankle angle; Pre-Post HKA, difference between preoperative and post-operative HKA; Pre-Post medial proximal tibial angle (MPTA), difference between pre-operative and post-operative MPTA; Pre-Post Posterior Slope (PTS), difference between pre-operative and post-operative PTS; F-VarVal, varus/valgus position of the femoral component; T-VarVal, varus/valgus position of the tibial component; RDT, the resection depth of the proximal medial tibia; F-FlexExt, flexion/extension position of the femoral component; Post PTS, post-operative PTS; FCC classification, femoral component-posterior condyle classification † ADL vs matched NDL (n = 33) ‡ PDL vs matched NDL (n = 18) § DL vs matched NDL (n = 51) ¶ Mann–Whitney U test ** Fisher’s exact test. Table IV. Univariate conditional logistic regression analysis of risk factors for bearing dislocation. Patient groups*

Overall bearing dislocation Odds ratio (OR) (95% confidence intervals (CI))

Anterior bearing dislocation

Posterior bearing dislocation

p-value OR (95% CI)

p-value

OR (95% CI)

p-value

0.884 (0.658 to 1.187) 0.746 (0.461 to 1.207) 1.009 (0.690 to 1.475) 1.885 (0.993 to 3.579) 1.049 (0.853 to 1.290) 1.171 (0.705 to 1.943) 1.738 (0.803 to 3.764) 1.018 (0.887 to 1.168) 0.612 (0.346 to 1.082)

Reference 0.835 (0.089 to 7.852)

0.391 0.299 0.189 0.017 0.359 0.110 0.553 0.397 0.021 0.860 0.875

0.413 0.232 0.964 0.053 0.651 0.542 0.161 0.801 0.091 0.941 1.000

Post HKA Pre-Post HKA Pre-Post MPTA Pre-Post PTS F-VarVal T-VarVal RDT F-FlexExt Post PTS FCC classification Type A Type B

0.893 (0.740 to 1.078) 0.847 (0.678 to 1.059) 1.132 (0.907 to 1.413) 1.934 (1.271 to 2.942) 1.064 (0.943 to 1.201) 1.290 (0.963 to 1.728) 1.339 (0.935 to 1.918) 1.040 (0.941 to 1.148) 0.568 (0.385 to 0.838) Reference 0.372 (0.045 to 3.052)

0.239 0.149 0.273 0.002 0.312 0.087 0.111 0.444 0.004 0.631 0.357

0.900 (0.707 to 1.146) 0.878 (0.687 to 1.122) 1.221 (0.906 to 1.644) 1.969 (1.131 to 3.427) 1.072 (0.924 to 1.243) 1.357 (0.933 to 1.972) 1.153 (0.721 to 1.842) 1.065 (0.921 to 1.230) 0.539 (0.320 to 0.909)

Type C1

1.415 (0.258 to 7.755)

0.690

1.672 (0.209 to 13.369) 0.628

Type C2

2.668 (0.165 to 43.066)

0.489

-

-

Reference 1.000 (0.053 to 18.915) 2.449 (0.151 to 39.723) 0.884 (0.658 to 1.187)

0.529 0.413

* Post HKA, post-operative hip-knee-ankle angle; Pre-Post HKA, difference between pre-operative and post-operative HKA; Pre-Post medial tibial proximal angle (MPTA), difference between pre-operative and post-operative MPTA; Pre-Post posterior tibial slope (PTS), difference between pre-operative and post-operative PTS; F-VarVal, varus/valgus position of the femoral component; T-VarVal, varus/valgus position of the tibial component; RDT, the resection depth of the proximal medial tibia; F-FlexExt, flexion/extension position of the femoral component; Post PTS, post-operative posterior tibial slope; FCC classification, femoral component-posterior condyle classification

Repeated compression and decompression of the bearing, accompanied by significantly increased translation, may result in sudden dislocation of the meniscal bearing from its articulation. Second, we interpreted our results on the assumption that they were related to change in the rotational mechanics VOL. 96-B, No. 7, JULY 2014

of the knee. Nelitz et al21 reported that a greater PTS reduces the capacity for tibial rotation. In other words, reducing the posterior tibial slope increases the capacity for tibial rotation under various conditions of load.21 Under axial loading with an internal rotation moment, a decrease in the PTS led to a statistically significant increase in

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Table V. Multivariate conditional logistic regression analysis of risk factors for bearing dislocation.

Overall bearing dislocation Anterior bearing dislocation Posterior bearing dislocation

Variables*

Adjusted odds ratio (OR) (95% confidence intervals (CI))

p-value

Pre-Post PTS T-VarVal Pre-Post PTS Pre-Post MPTA Pre-Post PTS

1.881 (1.272 to 2.779) 1.356 (0.888 to 2.069) 2.568 (1.180 to 5.588) 1.649 (0.971 to 2.799) 1.885 (0.993 to 3.579)

0.002 0.158 0.017 0.064 0.053

* Pre-Post medial tibial proximal angle (MPTA), difference between pre-operative and post-operative MPTA; Pre-Post posterior tibial slope (PTS), difference between pre-operative and post-operative PTS; T-VarVal, varus/valgus position of the tibial component

1.0

Sensitivity

0.8 Pre-post PTS Post PTS Reference line

0.6

0.4 Cut-off value (Post PTS): 8.45° Sensitivity 0.647, Specificity 0.863, AUC 0.752, 95% CI 0.584 to 0.920

0.2

Cut-off value (Pre-Post PTS): 2.19° Sensitivity 0.706, Specificity 0.941, AUC 0.802, 95% CI 0.643 to 0.960

0.0 0.0

0.2

0.4 0.6 0.8 Specificity

1.0

Fig. 5 Receiver operating characteristics (ROC) curves of the postoperative posterior tibial slope (Post PTS, positive value indicates posterior slope (Pre-Post PTS, positive value indicates a decreased Post PTS over the Pre PTS). The optimal cut-off values of the Post PTS and Pre-Post PTS for balancing sensitivity and specificity were 8.45° and 2.19°, respectively (AUC, area under curve; 95% CI, 95% confidence interval).

internal rotation of the tibia. Although the amount of migration and the exact position of the bearing differ according to the state of loading, the meniscal bearing moves posterolaterally when the knee flexes.8,22 Therefore, allowing greater internal rotation of the tibia relative to the femur would increase the contact between the femoral component and the medial rim of the bearing (Fig. 6). Because the height of the medial rim is smaller than that of the posterior rim, increasing its contact with the femoral component reduces the required jump height for dislocation of the bearing. The mean pre-operative PTS in our study was 10.4° (SD 2), which is similar to that found in previous studies using the same line of reference.23,24 However, some studies of Western populations using a method similar to ours yielded lower PTS values.25,26 Haddad et al25 showed that the PTS is significantly higher in Asian than in Western populations. We suggest that further research is needed to determine whether uniform implantation of the tibial component at a 7° posterior angle regardless of ethnicity is, in

fact, ideal. As we suppose that the original concept of medial UKR was based on resurfacing joint replacement, we prefer to re-establish the pre-operative PTS. Unlike varus and valgus alignment of the tibial component, which can be approximated to the original angle under direct vision at operation, the original posterior inclination of the tibial component of the knee is difficult to re-establish owing to an inability to see the posterior portion of the proximal tibia before cutting it. Nonetheless, we are wary of creating an excessive post-operative PTS relative to the pre-operative value, because several studies have shown that a greater tibial slope is related to loosening of the tibial component, increased anterior translation of the knee and increased strain on the ACL.27-29 Further study is needed to establish the optimal PTS for medial UKR. Previous reports have suggested that posterior impingement on the bearing is one of the most common causes of dislocation.8,10 Our study classified posterior femoral condylar osteophytes and cement remnants using a novel radiological classification based on the arc of the femoral THE BONE & JOINT JOURNAL

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Fig. 6 Diagrams of the meniscal bearing and its position in the knee. Permitting greater internal rotation of the tibia relative to the femur would increase the contact between the femoral component and the medial rim of the bearing. Because the height of the medial rim is lower than that of the posterior rim, increasing its contact on the femoral component reduces the required jump height for bearing dislocation. (Modified from Goodfellow et al).8

component. However, our results showed that residual posterior osteophytes or cement were not a significant factor in dislocation of the bearing. We identified only four cases of protruding lesions on the posterior end of the femoral component in UKRs with dislocation of the bearing (Table III). Of course, the FCC classification does not encompass lesions that are undetectable on plain radiographs, such as tiny cement remnants or hypertrophied dense soft tissue in the posterior capsule. This study has several limitations. Firstly, the measurements were only made from plain radiographs. This could have resulted in errors caused by the rotational position of the knee at the time the radiograph was taken. Secondly, we could not assess the horizontal spinning of the bearing as a risk factor for dislocation. Goodfellow et al8 have stated that setting the femoral component and the bearing too far from the lateral wall of the tibial component can cause dislocation of the bearing by allowing it to spin. They concluded that an ideal position of the bearing in the extended knee is 2 mm to 3 mm away from the lateral wall of the tibial component. Because a small amount of rotation of the knee or the bearing exaggerates the coronal gap between the bearing and the wall of the tibial component, which is extremely small and difficult to measure, we were unable to develop a consistent method for measuring the position of the meniscal bearing on AP radiographs. We noticed a significant reduction in the frequency of dislocation after the introduction of the anatomical bearing. As the anatomical bearing was designed to restrict the horizontal rotation of the bearing, this could have been a significant confounding factor in the multivariate analysis. VOL. 96-B, No. 7, JULY 2014

Therefore, we decided to include the type of meniscal bearing used as a matching criterion. Thirdly, we did not measure the intra-operative flexion–extension gap more accurately than with the feeler gauges. Finally, we could not find a way to quantify the extent to which our patients performed high flexion during daily life, which has been suggested to be a risk factor for dislocation. Although we checked each patient’s range of movement in the outpatient clinic, these data did not represent the frequency or amplitude of high-flexion behaviour. Consequently, a more reliable method of investigating the association between high flexion and dislocation of the bearing is needed. Despite these limitations, this is, to the best of our knowledge, the first study to evaluate the risk factors for dislocation of the bearing after medial Oxford UKR using post-operative radiological measurements, and includes two novel methods of measurement. In conclusion, a post-operative decrease in posterior tibial slope relative to the pre-operative value is the main risk factor for dislocation of the bearing after medial Oxford UKR. A post-operative posterior tibial slope < 8.45° and a difference between the pre-operative and the post-operative posterior tibial slopes of > 2.19° may increase the risk for dislocation of the bearing. Individual and ethnic differences in the posterior tibial slope seem to warrant reconsideration of the aim to achieve a consistent posterior tibial slope of 7° in all patients. However, further investigation of the significance of the coronal gap between the tibial component and the meniscal bearing, which can allow horizontal rotation of the bearing, is still needed.

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S. Y. LEE, J. H. BAE, J. G. KIM, K. M. JANG, W. Y. SHON, K. W. KIM, H. C. LIM

No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article.

15. Yue B, Varadarajan KM, Ai S, et al. Gender differences in the knees of Chinese population. Knee Surg Sports Traumatol Arthrosc 2011;19:80–88.

This article was primary edited by G. Scott and first proof edited by A. Ross.

16. Small SR, Berend ME, Rogge RD, et al. Tibial loading after UKA: evaluation of tibial slope, resection depth, medial shift and component rotation. J Arthroplasty 2013;28(9Suppl):179–183.

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The influence of surgical factors on dislocation of the meniscal bearing after Oxford medial unicompartmental knee replacement: a case-control study.

The aim of this study was to evaluate the risk factors for dislocation of the bearing after a mobile-bearing Oxford medial unicompartmental knee repla...
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