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Mobile bearing dislocation in lateral unicompartmental knee replacement
The Knee 17 (2010) 392–397
Contents lists available at ScienceDirect
The Knee
Mobile bearing dislocation in lateral unicompartmental knee replacement H. Pandit a,b, C. Jenkins b, D.J. Beard a, A.J. Price a,b, H.S. Gill a, C.A.F. Dodd b, D.W. Murray a,b,⁎ a b
Nuffield Department of Orthopaedics, Rheumatology and Musculoskeletal Sciences, University of Oxford, Headington, Oxford, UK Nuffield Orthopaedic Centre, Headington, Oxford, UK
a r t i c l e
i n f o
Article history: Received 31 March 2009 Received in revised form 19 October 2009 Accepted 19 October 2009 Keywords: Lateral unicompartmental knee replacement Mobile bearing
a b s t r a c t Despite the theoretical advantages of mobile bearings for lateral unicompartmental replacement (UKR), the failure rate in the initial published series of lateral Oxford UKR's was unacceptably high. The main cause of failure was bearing dislocation. To address this problem we first modified the surgical technique and then introduced a new design with a convex domed tibial plateau. This paper presents the results of these changes. In the original series (n = 53), implanted using a standard open approach, there were six dislocations, all of which occurred in the first year. Five of the dislocations were primary and one was secondary to trauma. In the second series (n = 65), with the modified technique, there were three dislocations, all of which were primary and occurred in the second and third year. In the third series (n = 101, 69 with a minimum 1-year follow-up), with the modified technique and the domed tibial plateau, there was one dislocation which was secondary to trauma and occurred in the second year. At 4 years the dislocation rates in the three series were 11%, 5% and 1.7% and the primary dislocation rates were 10%, 5% and 0%. Both the overall and the primary dislocation rates were significantly different (p = 0.04 and p = 0.03) in the different series. The combination of the modified surgical technique and new design with a domed tibial component appears to have reduced the early dislocation rate to an acceptable level. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In the lateral compartment of the knee during flexion and rotation there is a large amount of movement of the femoral condyle on the tibia [1,2]. Therefore, fixed bearing unicompartmental knee replacements (UKR) are likely to sustain substantial wear whereas fully congruent mobile bearing devices like the Oxford Knee (Biomet UK Limited, Swindon, UK) should have minimal wear. Despite this theoretical advantage the Oxford, initially, did not do well; In the series of Phase 1 and 2 Oxford UKR's, published by Gunther et al. in 1996 [3] the 5 year survival was only 82% hence mobile bearings were not recommended for lateral UKR. The main cause of failure was dislocation. All the dislocations in Gunther's series occurred within the first post-operative year. All but one of these dislocations were primary. The secondary dislocation was a result of significant trauma. The survival at 5 years for aseptic loosening or progression of disease to the medial compartment was 98%. This suggested that if the risk of dislocation could be decreased the results in the lateral compartment could potentially be as good as those in the medial compartment [4].
⁎ Corresponding author. Nuffield Department of Orthopaedics, Rheumatology and Musculoskeletal Sciences, Nuffield Orthopaedic Centre NHS Trust, Headington, Oxford, UK OX3 7LD, UK. Tel.: +44 1865 227457; fax: +44 1865 227671. E-mail address: [email protected] (D.W. Murray). 0968-0160/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.knee.2009.10.007
A detailed radiographic analysis of the causes of bearing dislocation in the lateral compartment was undertaken by Robinson et al. [5]. Only one of several variables relating to surgical technique was found to be associated with dislocation. The risk of dislocation increased substantially if the lateral tibial joint line was elevated. A new surgical technique was therefore introduced in which care was taken neither to remove too much bone from the distal femur, nor to over-tighten the knee, as both elevate the tibial joint line. Other modifications to the technique to help prevent dislocation included a short lateral parapatellar incision without patella dislocation and an internally rotated vertical tibial cut. In addition Phase 3 components were used which, unlike Phase 1 and 2, had multiple sizes of femoral component. The main reason why dislocation is much more common in the lateral compartment than in the medial is that, in flexion, the lateral collateral ligament is slack whereas the medial is tight. This allows the lateral compartment to be distracted on average 7 mm, whereas the medial can only be distracted on average 2 mm [6]. The Phase 1, 2 and 3 lateral bearings had only 5 mm of entrapment so are at risk of dislocation in a substantial proportion of patients. Another important difference between the medial and lateral compartment is that the lateral tibial condyle is convex, whereas the medial is concave. In high flexion the medial femoral condyle is on top of the tibia whereas the lateral femoral condyle subluxes posteriorly and inferiorly off the back of the tibial plateau [7]. Baré et al. [8] showed that if a flat lateral tibial component is used then in high flexion the lateral compartment is
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grossly over-tightened. This may compromise the clinical outcome and stretch the ligaments, which in turn will increase the risk of dislocation. In contrast, a convex tibial component did not overtighten the knee in high flexion. A new lateral UKR was therefore developed (Fig. 1). It has a spherically convex, domed tibial plateau, and a biconcave bearing that has fully congruent contact in all positions with both femur and tibia. The biconcave bearings have the advantage of increased entrapment (7 mm), which should help decrease the dislocation rate (Fig. 2). This new design was used in a series of knees with the improved surgical technique. The aim of this study was to compare the outcome of the three iterations of Oxford UKR used in the lateral compartment: in the original series (series I) a standard open approach was used; in the second series (series II) the modified surgical technique was used; in the third series (series III) the new domed tibial component with the modified surgical technique was used. As the major problem in the original series was early primary dislocation, this study has focused on early primary dislocation as the study endpoint.
2. Method In the published series [3] (series I), 53 Phase 1 and 2 lateral Oxford UKRs were implanted in 51 patients between April 1983 and December 1991, through a traditional open approach as used for total knee replacement (TKR) with patella dislocation. In most cases the popliteus tendon was divided as it was felt that it might “bowstring” across the joint and cause a dislocation. Between April 1998 and October 2004, 65 flat lateral Phase 3 Oxford UKRs were implanted by the two senior authors (CAFD and DM) using a modified surgical technique (series II). The approach was a short lateral parapatellar incision without patella dislocation. The vertical tibial cut was internally rotated. A new technique was used to position the femoral component. Traditionally, the femoral component has been positioned so that the ligament tension was similar in flexion and extension. With the new technique, this was not done, instead the femoral component was positioned anatomically providing this did not over-tighten the knee in flexion. The bearing thickness was selected so as to restore normal ligament tension in full extension. This, therefore, restored pre-disease alignment in extension and normal ligament laxity in flexion. Popliteus was routinely divided.
Fig. 1. Domed Oxford UKR components.
Fig. 2. Diagram demonstrating the entrapment of the bearing, shown by an arrow, which is the amount of distraction necessary for a dislocation. The biconcave bearing has more entrapment than the standard, shown by the longer arrow, and is therefore less likely to dislocate.
Between September 2004 and May 2008, 101 domed lateral Oxford UKRs were implanted by the two senior authors (series III). Sixty-nine had a minimum follow-up of 1 year. The surgical technique used was the same as with series II except that popliteus was only divided if it caused the bearing to sublux in flexion. The indications were the same in all series. The primary indication was lateral compartment osteoarthritis with exposed bone, although two cases had lateral femoral condyle avascular necrosis. The anterior cruciate ligament (ACL) was intact and there was full thickness cartilage in the medial compartment. The valgus deformity was correctable. The state of the patello-femoral joint was not considered to be a contraindication unless there was bone loss. Patients were assessed clinically pre-operatively, post-operatively and annually thereafter. Complications and revisions were recorded. The primary outcome measure used to compare the three series was the early primary dislocation rate. Dislocations were defined as primary or secondary, secondary being those that were a result of some other event such as component loosening or major trauma resulting in ligament damage. As the number of patients followed up each year decreased with time, the dislocation rates each year during the follow-up were calculated using life table survival analysis with overall and primary dislocation respectively being the definitions of failure. 95% confidence intervals (CI) of these cumulative dislocation rates were calculated using the method described by Peto. The dislocation rates in the different series, in the first year, were compared using Fisher's Exact test. The dislocation rates over the whole length of the follow-up were compared with the log rank test. Revisions were defined as operations in which at least one of the components was changed. The revision rates at different stages during the follow-up were calculated using life table survival analysis. Series I patients had been followed with the BOA score [9]. Series II and III patients were assessed independently by physiotherapists using an assessment that included pain and range of movement as well as the Oxford Knee Score (OKS, 0 to 48 with 48 being the best) [10,11] the Knee Society Score [12], and the Tegner score [13]. As different scoring systems were used, the clinical outcome in each series were compared using pain at rest, pain on activity and range of flexion at last review in patients that had a minimum of 1-year follow-up and had not been
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revised. The t test was used to compare the differences in OKS between different series.
3. Results Demographic details of the patients in the three series are given in Table 1.
3.1. Dislocation In the first series (open approach) there were six dislocations (11%). Five were primary and one was secondary to significant trauma. All dislocations occurred in the first year giving an incidence of primary dislocation in the first year of 9%. Out of these six dislocations, two were treated by immediate conversion to TKR. The other dislocated bearings were replaced with new bearings. Two of these functioned well and the patients remained asymptomatic. In the other two, further dislocations occurred and successful conversion to TKR or arthrodesis (at the request of the patient) was carried out. In the second series (modified surgical technique) there were three dislocations (5%), which were all primary. One occurred in the second year and two in the third, so the incidence of primary dislocation in the first year was 0%. In all three cases new bearings were inserted. In two of these cases, the bearings had dislocated over the tibial wall into the intercondylar notch. Therefore, in these cases, in addition to bearing exchange, the effective height of the wall was increased by inserting screws with their heads above the wall. There have been no further dislocations. In the third series (domed tibial component) there was one dislocation, which occurred in the second year (Fig. 3). This was a secondary dislocation into the intercondylar notch and was a result of significant trauma that damaged the ACL and lateral collateral ligament (LCL). The knee was explored, screws were inserted to improve bearing stability and a new bearing was inserted (Fig. 3). No primary dislocations occurred, so the incidence of primary dislocation in the first year was 0%. The primary dislocation rate in the first year in series II and III (0%) was significantly (p = 0.002) less than in series I (9%). The cumulative primary dislocation rate at both 3 and 4 years in series I was 10% (CI 9%), in series II was 5% (CI 5%) and in series III was 0% (Fig. 4). The cumulative overall dislocation rate at 3 and 4 years in series I was 11% (CI at 3 years 9%, at 4 years 10%), in series II was 5% (CI 5%) and in series III was 1.7% (CI at 3 years 4%, at 4 years 7%). No dislocations occurred after the third year in any series therefore the cumulative dislocation rates remained constant after the third year. The log rank test indicated that the difference in both primary and overall dislocation rates between the series was statistically significant (p = 0.03 and p = 0.04 respectively).
In the first series, 11 (21%) knees had further operations, all of which were revisions. Six were for bearing dislocation, three for infection, one for tibial component loosening and one for tibial stress fracture. One patient also developed a permanent peroneal nerve palsy. In the second series, 16 (25%) knees required further operations, of which nine (14%) were revisions. Three were for bearing dislocation, three for progression of disease to the medial compartment, one for infection, one for recurrent haemarthrosis and one for pigmented villonodular synovitis. Three patients had arthroscopy; one was for resection of a medial meniscus tear; and two were for unexplained pain. Four additional patients had manipulation under anaesthetic to improve range of movement. In the third series, two (2%) knees required further operations, of which one (1%) was a revision. The revision was for the bearing dislocation secondary to trauma. One knee had a manipulation under anaesthetic and an arthroscopy for clearance of synovitis. One patient developed a deep vein thrombosis and non-fatal pulmonary embolism 10 days post surgery. Using revision as failure the survival at 4 years was 82% (SE 6%) in the first series, 91% (SE 4%) in the second series and 98% (SE 4%) in the third series (p = 0.057) (Fig. 5). 3.3. Clinical outcome In the first series, as there were 11 revisions, 42 knees were reviewed at an average of 5.2 years post-operatively (Table 2). Ninety-six percent had no pain or mild pain at rest and on activity. Two (5%) had severe pain, one of which had signs of tibial loosening and the other had unexplained pain that was thought not to be related to the knee replacement. The mean flexion improved from 106° preoperatively to 110°. In the second series, as there were nine revisions, 56 knees were reviewed at an average of 4.7 years post-operatively (range: 3–9 years) (Table 2). Eighty-six percent had no pain or mild pain at rest and 80% had no pain or mild pain on activity. Of the three who had arthroscopy, one had no pain after resection of a medial meniscal tear while the other two continued to have moderate pain, which is well controlled with oral medication. The mean flexion improved from 111° pre-operatively to 117°. The OKS improved from 24 to 36. In the third series, as there was one revision, 68 knees were reviewed at an average of 2.3 years post-operatively (range: 1– 4 years) (Table 2). Ninety-eight percent had no pain or mild pain at rest and 94% had no pain or mild pain on activity. No patients had severe pain. One patient who had arthroscopy for clearance of synovitis still has moderate pain, and one patient has moderate pain which may be related to a neuroma formation. The mean flexion improved from 115° pre-operatively to 125° post-operatively. The OKS improved from 22 to 41. Both the absolute and change in OKS were significantly (p = 0.006 and p = 0.003 respectively) better in the third series than in the second series. 4. Discussion
Table 1 Demographics of each series.
Number of knees Mean follow-up in years: Number of patients Female:male Age (years): mean (range)
3.2. Revisions and other complications
Original
Modified technique
Domed tibia
Domed tibia (minimum 1 year follow-up)
Series I
Series II
Series III
Series III
53 5 51 47:4 68 (40–88)
65 5 64 36:28 53 (39–83)
101 2 98 60:38 62 (41–86)
69 3 67 44:23 63 (42–85)
The study demonstrates that the previous unacceptably high dislocation rate for mobile bearing UKR in the lateral compartment can be decreased to an acceptable level by the use of a modified surgical technique and a new implant design. In the original series all the dislocations occurred in the first year and there was a 10% primary dislocation rate. In the second series, implanted with a modified technique, although there were no dislocations in the first year, dislocations occurred in the second and third year, and the primary dislocation rate was 5%. In the third series, with the domed tibial component and the modified technique, there were no primary
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Fig. 3. AP radiographs of the patient who had a secondary dislocation with a domed tibial component: (A) pre-operative, (B) post-operative, (C) following dislocation, and (D) following reduction of dislocation and insertion of screws.
dislocations so the primary dislocation rate was 0%. However, there was one dislocation secondary to significant trauma in the second year so the overall dislocation rate was 1.7%. As we implant more
domed lateral UKR there will undoubtedly be primary dislocations and more secondary dislocations. However, as the confidence intervals (CI) for the dislocations rates at 3 years are relatively
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Fig. 4. Graph showing the percentage of patients in each series that have not had a primary dislocation. (Calculated using survival analysis with primary dislocation being considered to be failure.)
Fig. 5. Graph showing the percentage of patients in each series that have not had a revision. (Calculated using survival analysis with revision being considered to be failure.)
narrow, we expect the short term dislocation rate to remain at an acceptably low level. As dislocations tend to occur early we therefore believe that mobile bearing UKR can now be considered to be a treatment option for isolated lateral OA. It would appear that both the new surgical technique and the domed implant have resulted in a decrease in dislocation rate. There were a number of changes to the surgical technique so it is not clear
which specific aspects of the technique have been responsible for reducing the dislocation rate. The recommendation, therefore, has to be that all changes in the technique should be adopted until further information becomes available. The operation is technically demanding in part because the patella and the extensor mechanism are in front of the lateral compartment and make access difficult and in part because of the difficulty in balancing the soft tissues. Errors in technique are likely to increase the dislocation rate. There are a number of aspects of the domed design that have contributed to the decreased dislocation rate. Probably the most important is that the bearing entrapment has been increased from 5 mm to 7 mm (Fig. 2). The clinical results with the domed tibial component (series III) were significantly better than with the flat components implanted with the same modified technique (series II). This is best explained by an observation we made during the early intra-operative trials of the domed component, which was subsequently confirmed by Bare et al. [8]. We found in very high flexion that the flat components overtightened the knee, whereas the domed components did not because the bearing moved distally over the back of the convex surface (Fig. 6). We believe this avoidance of over-tightening with the dome explains the decreased pain and increased flexion. The flat components in the original series (I) did not achieve as good flexion as in the modified technique series (II), presumably because of damage to the extensor mechanism caused by the open approach. As high flexion was not routinely achieved in the original series the ligaments would not have been over-tightened, perhaps explaining why there was less pain in this series than in the modified technique series. In all series the commonest direction of dislocation was medial, with the bearing ending up in the intercondylar notch (Fig. 3). When the three medial dislocations in series II and III were explored we found that it was easy for the bearing to dislocate into the notch. By placing two screws into the tibia with their heads above the wall of the tibial component it was possible to prevent further dislocations in all three cases (Fig. 3). We believe that this is a valuable technique for preventing recurrent medial dislocation. However, we cannot be certain that it will not lead to other complications. This series of cases suggests that with the new surgical technique and the domed component, mobile bearing lateral UKR can be satisfactorily undertaken with a low dislocation rate and good clinical results in the short term. Further follow-up is required to assess the long term effectiveness of this implant and the surgical technique. 5. Conflict of interest statement The authors have received benefits for personal, professional and institutional use from commercial parties related to the subject of this paper.
Table 2 Clinical data on all patients that have not been revised and have a minimum 1-year follow-up. Pre-op
Last review
Series
I
II
III
I
II
III
Number of knees Pain at rest
40a 11 (28%) 12 (30%) 15 (38%) 2 (5%) 21 (53%) 14 (35%) 4 (10%) 1 (3%) 106
56 15 (27%) 34 (61%) 7 (13%) 0 33 (59%) 20 (36%) 3 (5%) 0 111 (75–125) 23.6 (7.3) 43.9 (15.8) 64.6 (14.3) 2.08 (0.9)
69 26 (38%) 39 (57%) 4 (6%) 0 50 (72%) 16 (23%) 3 (4%) 0 115 (40–140) 22.1 (8.3) 43.9 (13.6) 63 (17.6) 2.15 (0.87)
42 2 (5%) 0 7 (17%) 33 (79%) 2 (5%) 0 10 (24%) 30 (71%) 110
56 3 (5%) 5 (9%) 17 (30%) 31 (55%) 4 (7%) 7 (13%) 21 (37%) 24 (43%) 117 (60–130) 35.8 (10.4) 85.2 (17.2) 85.7 (16.2) 2.8 (1.05)
69 0 1 (1%) 16 (23%) 52 (75%) 0 4 (6%) 35 (51%) 30 (43%) 125 (110–145) 40.6 (6.9) 89.9 (10.5) 87.0 (16.3) 2.8 (0.93)
Pain on Activity
Severe Moderate Mild None Severe Moderate Mild None
Flexion (in degrees) OKS (0–48) AKSS-O (0–100) AKSS-F (0–100) Tegner (0–10) a
Pre-op status of two patients unknown.
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Acknowledgements The authors wish to thank Mrs. Barbara Marks for help with preparing the manuscript. Financial support has been received from the NIHR Biomedical Research Unit into Musculoskeletal Disease, Nuffield Orthopaedic Centre and University of Oxford. References [1] Iwaki H, Pinskerova V, Freeman MA. Tibiofemoral movement 1: the shapes and relative movements of the femur and tibia in the unloaded cadaver knee. J Bone Joint Surg Br 2000;82–8:1189–95. [2] O'Connor JJ, Shercliff TL, Biden E, Goodfellow JW. The geometry of the knee in the sagittal plane. Proc Inst Mech Eng [H] 1989;203–4:223–33. [3] Gunther T, Murray DW, Miller R, Wallace DA, Carr AJ, O'Connor JJ, et al. Lateral unicompartmental arthroplasty with the Oxford meniscal knee. The Knee 1996;3: 33–9. [4] Murray DW, Goodfellow JW, O'Connor JJ. The Oxford medial unicompartmental arthroplasty: a ten-year survival study. J Bone Joint Surg Br 1998;80–6:983–9. [5] Robinson BJ, Rees JL, Price AJ, Beard DJ, Murray DW, McLardy Smith P, et al. Dislocation of the bearing of the Oxford lateral unicompartmental arthroplasty. A radiological assessment. J Bone Joint Surg Br 2002;84–5:653–7. [6] Tokuhara Y, Kadoya Y, Nakagawa S, Kobayashi A, Takaoka K. The flexion gap in normal knees. An MRI study. J Bone Joint Surg Br 2004;86–8:1133–6. [7] Nakagawa S, Kadoya Y, Todo S, Kobayashi A, Sakamoto H, Freeman MA, et al. Tibiofemoral movement 3: full flexion in the living knee studied by MRI. J Bone Joint Surg Br 2000;82–8:1199–200. [8] Bare JV, Gill HS, Beard DJ, Murray DW. A convex lateral tibial plateau for knee replacement. Knee 2006. [9] A knee function assessment chart. From the British Orthopaedic Association Research Sub-Committee. J Bone Joint Surg Br 1978;60-B-3:308–9. [10] Dawson J, Fitzpatrick R, Murray D, Carr A. Questionnaire on the perceptions of patients about total knee replacement. J Bone Joint Surg Br 1998;80–1:63–9. [11] Murray DW, Fitzpatrick R, Rogers K, Pandit H, Beard DJ, Carr AJ, et al. The use of the Oxford hip and knee scores. J Bone Joint Surg Br 2007;89–8:1010–4. [12] Insall JN, Dorr LD, Scott RD, Scott WN. Rationale of the Knee Society clinical rating system. Clin Orthop 1989;248:13–4. [13] Tegner Y, Lysholm J. Rating systems in the evaluation of knee ligament injuries. Clin Orthop Relat Res 1985;198:43–9.
Fig. 6. Photographs of a cadaver with domed components implanted and lateral soft tissues removed in (A) extension and (B) high flexion, showing that as the bearing moves posteriorly in high flexion it also moves distally so does not over-tighten the ligaments.
Contents lists available at ScienceDirect
The Knee
Mobile bearing dislocation in lateral unicompartmental knee replacement H. Pandit a,b, C. Jenkins b, D.J. Beard a, A.J. Price a,b, H.S. Gill a, C.A.F. Dodd b, D.W. Murray a,b,⁎ a b
Nuffield Department of Orthopaedics, Rheumatology and Musculoskeletal Sciences, University of Oxford, Headington, Oxford, UK Nuffield Orthopaedic Centre, Headington, Oxford, UK
a r t i c l e
i n f o
Article history: Received 31 March 2009 Received in revised form 19 October 2009 Accepted 19 October 2009 Keywords: Lateral unicompartmental knee replacement Mobile bearing
a b s t r a c t Despite the theoretical advantages of mobile bearings for lateral unicompartmental replacement (UKR), the failure rate in the initial published series of lateral Oxford UKR's was unacceptably high. The main cause of failure was bearing dislocation. To address this problem we first modified the surgical technique and then introduced a new design with a convex domed tibial plateau. This paper presents the results of these changes. In the original series (n = 53), implanted using a standard open approach, there were six dislocations, all of which occurred in the first year. Five of the dislocations were primary and one was secondary to trauma. In the second series (n = 65), with the modified technique, there were three dislocations, all of which were primary and occurred in the second and third year. In the third series (n = 101, 69 with a minimum 1-year follow-up), with the modified technique and the domed tibial plateau, there was one dislocation which was secondary to trauma and occurred in the second year. At 4 years the dislocation rates in the three series were 11%, 5% and 1.7% and the primary dislocation rates were 10%, 5% and 0%. Both the overall and the primary dislocation rates were significantly different (p = 0.04 and p = 0.03) in the different series. The combination of the modified surgical technique and new design with a domed tibial component appears to have reduced the early dislocation rate to an acceptable level. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In the lateral compartment of the knee during flexion and rotation there is a large amount of movement of the femoral condyle on the tibia [1,2]. Therefore, fixed bearing unicompartmental knee replacements (UKR) are likely to sustain substantial wear whereas fully congruent mobile bearing devices like the Oxford Knee (Biomet UK Limited, Swindon, UK) should have minimal wear. Despite this theoretical advantage the Oxford, initially, did not do well; In the series of Phase 1 and 2 Oxford UKR's, published by Gunther et al. in 1996 [3] the 5 year survival was only 82% hence mobile bearings were not recommended for lateral UKR. The main cause of failure was dislocation. All the dislocations in Gunther's series occurred within the first post-operative year. All but one of these dislocations were primary. The secondary dislocation was a result of significant trauma. The survival at 5 years for aseptic loosening or progression of disease to the medial compartment was 98%. This suggested that if the risk of dislocation could be decreased the results in the lateral compartment could potentially be as good as those in the medial compartment [4].
⁎ Corresponding author. Nuffield Department of Orthopaedics, Rheumatology and Musculoskeletal Sciences, Nuffield Orthopaedic Centre NHS Trust, Headington, Oxford, UK OX3 7LD, UK. Tel.: +44 1865 227457; fax: +44 1865 227671. E-mail address: [email protected] (D.W. Murray). 0968-0160/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.knee.2009.10.007
A detailed radiographic analysis of the causes of bearing dislocation in the lateral compartment was undertaken by Robinson et al. [5]. Only one of several variables relating to surgical technique was found to be associated with dislocation. The risk of dislocation increased substantially if the lateral tibial joint line was elevated. A new surgical technique was therefore introduced in which care was taken neither to remove too much bone from the distal femur, nor to over-tighten the knee, as both elevate the tibial joint line. Other modifications to the technique to help prevent dislocation included a short lateral parapatellar incision without patella dislocation and an internally rotated vertical tibial cut. In addition Phase 3 components were used which, unlike Phase 1 and 2, had multiple sizes of femoral component. The main reason why dislocation is much more common in the lateral compartment than in the medial is that, in flexion, the lateral collateral ligament is slack whereas the medial is tight. This allows the lateral compartment to be distracted on average 7 mm, whereas the medial can only be distracted on average 2 mm [6]. The Phase 1, 2 and 3 lateral bearings had only 5 mm of entrapment so are at risk of dislocation in a substantial proportion of patients. Another important difference between the medial and lateral compartment is that the lateral tibial condyle is convex, whereas the medial is concave. In high flexion the medial femoral condyle is on top of the tibia whereas the lateral femoral condyle subluxes posteriorly and inferiorly off the back of the tibial plateau [7]. Baré et al. [8] showed that if a flat lateral tibial component is used then in high flexion the lateral compartment is
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grossly over-tightened. This may compromise the clinical outcome and stretch the ligaments, which in turn will increase the risk of dislocation. In contrast, a convex tibial component did not overtighten the knee in high flexion. A new lateral UKR was therefore developed (Fig. 1). It has a spherically convex, domed tibial plateau, and a biconcave bearing that has fully congruent contact in all positions with both femur and tibia. The biconcave bearings have the advantage of increased entrapment (7 mm), which should help decrease the dislocation rate (Fig. 2). This new design was used in a series of knees with the improved surgical technique. The aim of this study was to compare the outcome of the three iterations of Oxford UKR used in the lateral compartment: in the original series (series I) a standard open approach was used; in the second series (series II) the modified surgical technique was used; in the third series (series III) the new domed tibial component with the modified surgical technique was used. As the major problem in the original series was early primary dislocation, this study has focused on early primary dislocation as the study endpoint.
2. Method In the published series [3] (series I), 53 Phase 1 and 2 lateral Oxford UKRs were implanted in 51 patients between April 1983 and December 1991, through a traditional open approach as used for total knee replacement (TKR) with patella dislocation. In most cases the popliteus tendon was divided as it was felt that it might “bowstring” across the joint and cause a dislocation. Between April 1998 and October 2004, 65 flat lateral Phase 3 Oxford UKRs were implanted by the two senior authors (CAFD and DM) using a modified surgical technique (series II). The approach was a short lateral parapatellar incision without patella dislocation. The vertical tibial cut was internally rotated. A new technique was used to position the femoral component. Traditionally, the femoral component has been positioned so that the ligament tension was similar in flexion and extension. With the new technique, this was not done, instead the femoral component was positioned anatomically providing this did not over-tighten the knee in flexion. The bearing thickness was selected so as to restore normal ligament tension in full extension. This, therefore, restored pre-disease alignment in extension and normal ligament laxity in flexion. Popliteus was routinely divided.
Fig. 1. Domed Oxford UKR components.
Fig. 2. Diagram demonstrating the entrapment of the bearing, shown by an arrow, which is the amount of distraction necessary for a dislocation. The biconcave bearing has more entrapment than the standard, shown by the longer arrow, and is therefore less likely to dislocate.
Between September 2004 and May 2008, 101 domed lateral Oxford UKRs were implanted by the two senior authors (series III). Sixty-nine had a minimum follow-up of 1 year. The surgical technique used was the same as with series II except that popliteus was only divided if it caused the bearing to sublux in flexion. The indications were the same in all series. The primary indication was lateral compartment osteoarthritis with exposed bone, although two cases had lateral femoral condyle avascular necrosis. The anterior cruciate ligament (ACL) was intact and there was full thickness cartilage in the medial compartment. The valgus deformity was correctable. The state of the patello-femoral joint was not considered to be a contraindication unless there was bone loss. Patients were assessed clinically pre-operatively, post-operatively and annually thereafter. Complications and revisions were recorded. The primary outcome measure used to compare the three series was the early primary dislocation rate. Dislocations were defined as primary or secondary, secondary being those that were a result of some other event such as component loosening or major trauma resulting in ligament damage. As the number of patients followed up each year decreased with time, the dislocation rates each year during the follow-up were calculated using life table survival analysis with overall and primary dislocation respectively being the definitions of failure. 95% confidence intervals (CI) of these cumulative dislocation rates were calculated using the method described by Peto. The dislocation rates in the different series, in the first year, were compared using Fisher's Exact test. The dislocation rates over the whole length of the follow-up were compared with the log rank test. Revisions were defined as operations in which at least one of the components was changed. The revision rates at different stages during the follow-up were calculated using life table survival analysis. Series I patients had been followed with the BOA score [9]. Series II and III patients were assessed independently by physiotherapists using an assessment that included pain and range of movement as well as the Oxford Knee Score (OKS, 0 to 48 with 48 being the best) [10,11] the Knee Society Score [12], and the Tegner score [13]. As different scoring systems were used, the clinical outcome in each series were compared using pain at rest, pain on activity and range of flexion at last review in patients that had a minimum of 1-year follow-up and had not been
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revised. The t test was used to compare the differences in OKS between different series.
3. Results Demographic details of the patients in the three series are given in Table 1.
3.1. Dislocation In the first series (open approach) there were six dislocations (11%). Five were primary and one was secondary to significant trauma. All dislocations occurred in the first year giving an incidence of primary dislocation in the first year of 9%. Out of these six dislocations, two were treated by immediate conversion to TKR. The other dislocated bearings were replaced with new bearings. Two of these functioned well and the patients remained asymptomatic. In the other two, further dislocations occurred and successful conversion to TKR or arthrodesis (at the request of the patient) was carried out. In the second series (modified surgical technique) there were three dislocations (5%), which were all primary. One occurred in the second year and two in the third, so the incidence of primary dislocation in the first year was 0%. In all three cases new bearings were inserted. In two of these cases, the bearings had dislocated over the tibial wall into the intercondylar notch. Therefore, in these cases, in addition to bearing exchange, the effective height of the wall was increased by inserting screws with their heads above the wall. There have been no further dislocations. In the third series (domed tibial component) there was one dislocation, which occurred in the second year (Fig. 3). This was a secondary dislocation into the intercondylar notch and was a result of significant trauma that damaged the ACL and lateral collateral ligament (LCL). The knee was explored, screws were inserted to improve bearing stability and a new bearing was inserted (Fig. 3). No primary dislocations occurred, so the incidence of primary dislocation in the first year was 0%. The primary dislocation rate in the first year in series II and III (0%) was significantly (p = 0.002) less than in series I (9%). The cumulative primary dislocation rate at both 3 and 4 years in series I was 10% (CI 9%), in series II was 5% (CI 5%) and in series III was 0% (Fig. 4). The cumulative overall dislocation rate at 3 and 4 years in series I was 11% (CI at 3 years 9%, at 4 years 10%), in series II was 5% (CI 5%) and in series III was 1.7% (CI at 3 years 4%, at 4 years 7%). No dislocations occurred after the third year in any series therefore the cumulative dislocation rates remained constant after the third year. The log rank test indicated that the difference in both primary and overall dislocation rates between the series was statistically significant (p = 0.03 and p = 0.04 respectively).
In the first series, 11 (21%) knees had further operations, all of which were revisions. Six were for bearing dislocation, three for infection, one for tibial component loosening and one for tibial stress fracture. One patient also developed a permanent peroneal nerve palsy. In the second series, 16 (25%) knees required further operations, of which nine (14%) were revisions. Three were for bearing dislocation, three for progression of disease to the medial compartment, one for infection, one for recurrent haemarthrosis and one for pigmented villonodular synovitis. Three patients had arthroscopy; one was for resection of a medial meniscus tear; and two were for unexplained pain. Four additional patients had manipulation under anaesthetic to improve range of movement. In the third series, two (2%) knees required further operations, of which one (1%) was a revision. The revision was for the bearing dislocation secondary to trauma. One knee had a manipulation under anaesthetic and an arthroscopy for clearance of synovitis. One patient developed a deep vein thrombosis and non-fatal pulmonary embolism 10 days post surgery. Using revision as failure the survival at 4 years was 82% (SE 6%) in the first series, 91% (SE 4%) in the second series and 98% (SE 4%) in the third series (p = 0.057) (Fig. 5). 3.3. Clinical outcome In the first series, as there were 11 revisions, 42 knees were reviewed at an average of 5.2 years post-operatively (Table 2). Ninety-six percent had no pain or mild pain at rest and on activity. Two (5%) had severe pain, one of which had signs of tibial loosening and the other had unexplained pain that was thought not to be related to the knee replacement. The mean flexion improved from 106° preoperatively to 110°. In the second series, as there were nine revisions, 56 knees were reviewed at an average of 4.7 years post-operatively (range: 3–9 years) (Table 2). Eighty-six percent had no pain or mild pain at rest and 80% had no pain or mild pain on activity. Of the three who had arthroscopy, one had no pain after resection of a medial meniscal tear while the other two continued to have moderate pain, which is well controlled with oral medication. The mean flexion improved from 111° pre-operatively to 117°. The OKS improved from 24 to 36. In the third series, as there was one revision, 68 knees were reviewed at an average of 2.3 years post-operatively (range: 1– 4 years) (Table 2). Ninety-eight percent had no pain or mild pain at rest and 94% had no pain or mild pain on activity. No patients had severe pain. One patient who had arthroscopy for clearance of synovitis still has moderate pain, and one patient has moderate pain which may be related to a neuroma formation. The mean flexion improved from 115° pre-operatively to 125° post-operatively. The OKS improved from 22 to 41. Both the absolute and change in OKS were significantly (p = 0.006 and p = 0.003 respectively) better in the third series than in the second series. 4. Discussion
Table 1 Demographics of each series.
Number of knees Mean follow-up in years: Number of patients Female:male Age (years): mean (range)
3.2. Revisions and other complications
Original
Modified technique
Domed tibia
Domed tibia (minimum 1 year follow-up)
Series I
Series II
Series III
Series III
53 5 51 47:4 68 (40–88)
65 5 64 36:28 53 (39–83)
101 2 98 60:38 62 (41–86)
69 3 67 44:23 63 (42–85)
The study demonstrates that the previous unacceptably high dislocation rate for mobile bearing UKR in the lateral compartment can be decreased to an acceptable level by the use of a modified surgical technique and a new implant design. In the original series all the dislocations occurred in the first year and there was a 10% primary dislocation rate. In the second series, implanted with a modified technique, although there were no dislocations in the first year, dislocations occurred in the second and third year, and the primary dislocation rate was 5%. In the third series, with the domed tibial component and the modified technique, there were no primary
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Fig. 3. AP radiographs of the patient who had a secondary dislocation with a domed tibial component: (A) pre-operative, (B) post-operative, (C) following dislocation, and (D) following reduction of dislocation and insertion of screws.
dislocations so the primary dislocation rate was 0%. However, there was one dislocation secondary to significant trauma in the second year so the overall dislocation rate was 1.7%. As we implant more
domed lateral UKR there will undoubtedly be primary dislocations and more secondary dislocations. However, as the confidence intervals (CI) for the dislocations rates at 3 years are relatively
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Fig. 4. Graph showing the percentage of patients in each series that have not had a primary dislocation. (Calculated using survival analysis with primary dislocation being considered to be failure.)
Fig. 5. Graph showing the percentage of patients in each series that have not had a revision. (Calculated using survival analysis with revision being considered to be failure.)
narrow, we expect the short term dislocation rate to remain at an acceptably low level. As dislocations tend to occur early we therefore believe that mobile bearing UKR can now be considered to be a treatment option for isolated lateral OA. It would appear that both the new surgical technique and the domed implant have resulted in a decrease in dislocation rate. There were a number of changes to the surgical technique so it is not clear
which specific aspects of the technique have been responsible for reducing the dislocation rate. The recommendation, therefore, has to be that all changes in the technique should be adopted until further information becomes available. The operation is technically demanding in part because the patella and the extensor mechanism are in front of the lateral compartment and make access difficult and in part because of the difficulty in balancing the soft tissues. Errors in technique are likely to increase the dislocation rate. There are a number of aspects of the domed design that have contributed to the decreased dislocation rate. Probably the most important is that the bearing entrapment has been increased from 5 mm to 7 mm (Fig. 2). The clinical results with the domed tibial component (series III) were significantly better than with the flat components implanted with the same modified technique (series II). This is best explained by an observation we made during the early intra-operative trials of the domed component, which was subsequently confirmed by Bare et al. [8]. We found in very high flexion that the flat components overtightened the knee, whereas the domed components did not because the bearing moved distally over the back of the convex surface (Fig. 6). We believe this avoidance of over-tightening with the dome explains the decreased pain and increased flexion. The flat components in the original series (I) did not achieve as good flexion as in the modified technique series (II), presumably because of damage to the extensor mechanism caused by the open approach. As high flexion was not routinely achieved in the original series the ligaments would not have been over-tightened, perhaps explaining why there was less pain in this series than in the modified technique series. In all series the commonest direction of dislocation was medial, with the bearing ending up in the intercondylar notch (Fig. 3). When the three medial dislocations in series II and III were explored we found that it was easy for the bearing to dislocate into the notch. By placing two screws into the tibia with their heads above the wall of the tibial component it was possible to prevent further dislocations in all three cases (Fig. 3). We believe that this is a valuable technique for preventing recurrent medial dislocation. However, we cannot be certain that it will not lead to other complications. This series of cases suggests that with the new surgical technique and the domed component, mobile bearing lateral UKR can be satisfactorily undertaken with a low dislocation rate and good clinical results in the short term. Further follow-up is required to assess the long term effectiveness of this implant and the surgical technique. 5. Conflict of interest statement The authors have received benefits for personal, professional and institutional use from commercial parties related to the subject of this paper.
Table 2 Clinical data on all patients that have not been revised and have a minimum 1-year follow-up. Pre-op
Last review
Series
I
II
III
I
II
III
Number of knees Pain at rest
40a 11 (28%) 12 (30%) 15 (38%) 2 (5%) 21 (53%) 14 (35%) 4 (10%) 1 (3%) 106
56 15 (27%) 34 (61%) 7 (13%) 0 33 (59%) 20 (36%) 3 (5%) 0 111 (75–125) 23.6 (7.3) 43.9 (15.8) 64.6 (14.3) 2.08 (0.9)
69 26 (38%) 39 (57%) 4 (6%) 0 50 (72%) 16 (23%) 3 (4%) 0 115 (40–140) 22.1 (8.3) 43.9 (13.6) 63 (17.6) 2.15 (0.87)
42 2 (5%) 0 7 (17%) 33 (79%) 2 (5%) 0 10 (24%) 30 (71%) 110
56 3 (5%) 5 (9%) 17 (30%) 31 (55%) 4 (7%) 7 (13%) 21 (37%) 24 (43%) 117 (60–130) 35.8 (10.4) 85.2 (17.2) 85.7 (16.2) 2.8 (1.05)
69 0 1 (1%) 16 (23%) 52 (75%) 0 4 (6%) 35 (51%) 30 (43%) 125 (110–145) 40.6 (6.9) 89.9 (10.5) 87.0 (16.3) 2.8 (0.93)
Pain on Activity
Severe Moderate Mild None Severe Moderate Mild None
Flexion (in degrees) OKS (0–48) AKSS-O (0–100) AKSS-F (0–100) Tegner (0–10) a
Pre-op status of two patients unknown.
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Acknowledgements The authors wish to thank Mrs. Barbara Marks for help with preparing the manuscript. Financial support has been received from the NIHR Biomedical Research Unit into Musculoskeletal Disease, Nuffield Orthopaedic Centre and University of Oxford. References [1] Iwaki H, Pinskerova V, Freeman MA. Tibiofemoral movement 1: the shapes and relative movements of the femur and tibia in the unloaded cadaver knee. J Bone Joint Surg Br 2000;82–8:1189–95. [2] O'Connor JJ, Shercliff TL, Biden E, Goodfellow JW. The geometry of the knee in the sagittal plane. Proc Inst Mech Eng [H] 1989;203–4:223–33. [3] Gunther T, Murray DW, Miller R, Wallace DA, Carr AJ, O'Connor JJ, et al. Lateral unicompartmental arthroplasty with the Oxford meniscal knee. The Knee 1996;3: 33–9. [4] Murray DW, Goodfellow JW, O'Connor JJ. The Oxford medial unicompartmental arthroplasty: a ten-year survival study. J Bone Joint Surg Br 1998;80–6:983–9. [5] Robinson BJ, Rees JL, Price AJ, Beard DJ, Murray DW, McLardy Smith P, et al. Dislocation of the bearing of the Oxford lateral unicompartmental arthroplasty. A radiological assessment. J Bone Joint Surg Br 2002;84–5:653–7. [6] Tokuhara Y, Kadoya Y, Nakagawa S, Kobayashi A, Takaoka K. The flexion gap in normal knees. An MRI study. J Bone Joint Surg Br 2004;86–8:1133–6. [7] Nakagawa S, Kadoya Y, Todo S, Kobayashi A, Sakamoto H, Freeman MA, et al. Tibiofemoral movement 3: full flexion in the living knee studied by MRI. J Bone Joint Surg Br 2000;82–8:1199–200. [8] Bare JV, Gill HS, Beard DJ, Murray DW. A convex lateral tibial plateau for knee replacement. Knee 2006. [9] A knee function assessment chart. From the British Orthopaedic Association Research Sub-Committee. J Bone Joint Surg Br 1978;60-B-3:308–9. [10] Dawson J, Fitzpatrick R, Murray D, Carr A. Questionnaire on the perceptions of patients about total knee replacement. J Bone Joint Surg Br 1998;80–1:63–9. [11] Murray DW, Fitzpatrick R, Rogers K, Pandit H, Beard DJ, Carr AJ, et al. The use of the Oxford hip and knee scores. J Bone Joint Surg Br 2007;89–8:1010–4. [12] Insall JN, Dorr LD, Scott RD, Scott WN. Rationale of the Knee Society clinical rating system. Clin Orthop 1989;248:13–4. [13] Tegner Y, Lysholm J. Rating systems in the evaluation of knee ligament injuries. Clin Orthop Relat Res 1985;198:43–9.
Fig. 6. Photographs of a cadaver with domed components implanted and lateral soft tissues removed in (A) extension and (B) high flexion, showing that as the bearing moves posteriorly in high flexion it also moves distally so does not over-tighten the ligaments.