224. INFLUENCE OF THE DESIGN OF A TOTAL KNEE ARTHROPLASTY WITH A PURE ROTATIONAL MOBILE PLATEAU ON INTRA- AND POSTOPERATIVE MOBILITY: PROSPECTIVE STUDY OF THREE MODELS: ULTRACONGRUENT, POSTEROSTABILISED, AND LARGE FLEXION POSTEROSTABILISED

Abstract

Purpose of the study: Certain patients with a total knee arthroplasty (TKA) require large range of flexion postoperatively to enable squatting and sitting cross-legged. Several factors have an effect on this flexion, including prosthesis design. The purpose of this study was to examine the influence of prosthesis design on intra- and postoperative flexion of three modes of TKA with a pure rotational mobile plateau: two NexGen posterostabilised (PS) prostheses (LPS-Standard and LPS-Flex) and one ultracongruent prosthesis (SAL). It was hypothesised that PS prostheses would have a better flexion than the ultracongruent prosthesis and that the flexion would be greater with the LPS-Flex than the LPS-standard.

Material and methods: This was a prospective randomised study of consecutive patients from January 2006 to January 2007 to compare maximal flexion for each model. All patients requiring a first-intention TKA were included in this study. Seventy-tow TKA were studied: LPS-standard (n=24), LPS-Flex (n=22), SAL (n=26). Flexion was measured pre and postoperatively goniometrically. Intraoperative measures were made with the navigation system (Navitrack-Orthosoft). Minimum follow-up was one year.

Results: There was a significant difference in flexion, intraoperatively and at last follow-up, in favour of the PS models over the ultracongruent SAL. Conversely, there was no significant difference between the LPS-standard and the LPS-Flex. The analysis of factors predictive of flexion were the SAL model with a negative influence (loss of 8° intraoperatively [p< 10-4] and 15° at one year [p< 10-4] compared with the LPS models). Preoperative flexion appeared to be a positive predictive factor (p=0.00023).

Discussion: The design of the TKA has an influence on knee flexion: from implantation, flexion of the PS models was superior to the ultracongruent model, a difference which persisted late after the operation. For the PS models, the LPS-Flex model presented as a large flexion model did not, in our study, demonstrate its superiority over the LPS-standard model, irrespective of the time of the comparison. It should be noted that for implantation the LPS-Flex model required a posterior cut 2mm more than for the LPS-standard. Good preoperative flexion is an essential factor for obtaining good postoperative flexion, irrespective of the model implanted.