Abstract
Three cases of posterior dislocation of the Kinemax Posterior- Stabilized total knee replacement are reported, and predisposing factors, including operative technique and prosthesis design, are discussed.
All three patients underwent posterior-stabilised knee replacement surgery at the Avon Orthopaedic Centre for osteoarthritis, between 1984 and 2000. In all cases the patient represented to the Emergency Department of a local hospital with posterior dislocation, at between 9 months and 6 years postoperatively. The mechanism for dislocation was hyperflexion of the knee.
The dislocations could not be reduced under sedation because of obstruction by the protruding tibial insert, and required general anaesthesia to disengage the components. In all cases posterior dislocation became recurrent problem, and further surgery was required to address the instability. Two of the three patients underwent exchange of their stabilised tibial inserts for thicker versions of the same design, in order to reduce the excessive laxity present in flexion. The third patient underwent exploratory surgery and it was found that his patellar button had separated from the underlying bone. The patella was therefore resurfaced, restoring the integrity of his extensor mechanism. No further dislocations have occurred in any of the three patients.
The causes of posterior dislocation of posterior-stabilized total knee replacements are multifactorial. They include malrotation of the tibial component, although this was not found to be the case in the three patients reported here. The design of the prosthesis may also contribute, and the upsloping and relatively shallow tibial spine of the Kinemax prosthesis (Howmedica) appears to be less forgiving than others. This is particularly the case if soft tissue lateral release or excessive resection of the posterior condyles has produced an increased flexiongap and therefore excessive flexion laxity. Our cases demonstrate the pitfalls that can produce this uncommon but serious complication, some of which can be predicted preoperatively, particularly in the patient with a valgus knee or deficiency of the extensor mechanism.
The abstracts were prepared by Mr R. B. Smith. Correspondence should be addressed to him at the British Orthopaedic Association, Royal College of Surgeons, 35-43 Lincoln’s Inn Fields, London WC2A 3PN.
