[Posterior luxation of the tibia on total knee prosthesis: apropos of 6 cases].

PURPOSE OF THE STUDY: Posterior dislocation of Total knee arthroplasty is an infrequent but serious complication. Six cases of this complication were treated from 1979 to december 1993, all occurring on primary arthroplasties.
MATERIAL AND METHODS: Knee arthroplasty was performed once for rheumatoid arthritis, five times for osteoarthritis. All cases occurred with a semi constrained prosthesis sacrifying posterior cruciate ligament: 2 Total condylar without posterior stabilisation, and 4 posterior stabilised prosthesis. In one case the dislocation occurred on a very severe rheumatoid arthritis: the patient, confined in a wheel chair, was not reoperated. In two patients, the dislocation was due to rotatory malposition of the tibial component. In the last three cases, we did not found any cause to the dislocation, except ligament laxity: 2 of these patients had, pre operatively, a valgus deformity. In 6 cases, we found only 2 problems on extensor system (one patellar dislocation and one patellectomy).
RESULTS: 5 patients required a surgical treatment In 2 cases, we used a more constrained prosthesis, with poor results, but the knee was stable. Once, by changing the position of the tibial component, and using a thicker plate. In 2 patients we only put a thicker tibial polyethylene component. These 3 patients had a good stability: 2 have an excellent result with H.S.S. rating system, the third one has a poor result explained by patellar pseudoarthrosis occurring after traumatic patellar fracture. DISCUSSION: With our patients and cases published in North American works, we have studied the different mechanisms of such a posterior dislocation: rotatory disorder on tibial component, ligament laxity in flexion, extensor system deficiency, valgus deformity with important postero lateral release.
CONCLUSION: The causes of posterior dislocation on Total knee arthroplasties must be known: we have to try to prevent such a complication. If it occurs, a precise analysis will permit a logical curative treatment, which must avoid constrained prosthesis.