Mobile bearing dislocation in lateral unicompartmental knee replacement
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].
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 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 over-tighten 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.
Section snippets
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
Results
Demographic details of the patients in the three series are given in Table 1.
Discussion
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
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.
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.
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