ROTATING PLATFORM DISLOCATION IN LOW CONTACT STRESS TOTAL KNEE ARTHROPLASTY

Abstract

Low Contact Stress(LCS) total knee arthroplasty was developed to reduce contact stress on the bearing surface and to minimise stresses at the interface between the host bone and the implant surface leading to long term implant survival. The rotating platform was introduced as the bearing interface when both cruciates are sacrificed. It has a central cone which engages into a matching cone in the tibial tray. This allows unconstrained axial rotation of the bearing surface. However, this potentially leaves the platform susceptible to subluxation/dislocation. The reported rate of this complication varies from 0.5% to 4.65%.

In this study from a single center we report the incidence and highlight the associated causative factors found in our series. There were 1053 Low Contact Stress total knee arthroplasties performed between 1994 and 2003. We reviewed 10 knees in 9 patients who had dislocation of the polyethylene rotating platform. This amounts to 0.95% in our series. All the patients with dislocation were women. Average age was 72 years(range 62–84). Osteoarthritis was the primary diagnosis in 8 patients. One patient was suffering from rheumatoid arthritis was on long term steroid therapy and had bilateral dislocations. One patient with Osteoarthritis with Parkinson’s disease went onto have 2 recurrent dislocations. Pre-operative deformity was varus in 9 knees(range 4–10 degrees) and valgus of 15 degrees was noted in one. Time from index operation to dislocation ranged from as early as 10 days to 10 months. There was history of trauma and acute presentation only in one patient. In one patient the knee dislocated while she twisted her knee in bed and in another while climbing up the stairs. In the remaining seven patients the presentation was subacute with symptoms such as pain, decreased range of motion, swelling and a clunking sensation while walking.

Although manipulation under anaesthesia was successful in 3 patients, all of them had recurrent dislocations and two patients had revision to a deepdish platform. Failed closed reduction led to open reduction in two patients with replacement of the rotating platform to deepdish (12.5 mm) type in one. Following both procedures knee was immobilised in a cast for 6 weeks. Five patients were directly revised without attempting closed reduction to a deepdish rotating platform. At revision in all cases the platform was found to be rotated medialy and posteriorly. Soft tissue imbalance and laxity were seen in all but one. At an average followup of 48.5 months (range 11–84 months) no patient had recurrent instability.

Increasing age, questionable soft tissue integrity and varus deformity were significantly associated with rotating platform dislocation. Closed reduction may be possible but invariably leads to recurrence of dislocation and open reduction with revision of the rotating platform gives reliable results. Replacing the rotating platform with a thicker deepdish bearing provides satisfactory stability at revision surgery. Meticulous surgical technique with accurate soft tissue balancing are important in reducing the incidence of dislocations.