A RETRIEVAL ANALYSIS OF MOBILE BEARING KNEES

Abstract

Clinical implantation represents the ultimate experiment of any component and often demonstrates areas of strengths and weaknesses not predicted from in vitro testing. Mobile bearing knees incorporate an additional articulating interface between the flat distal PE insert and a highly polished metal tibial tray. This can allow the proximal interface to retain high conformity whilst leading to reduced stresses at the bone – prosthesis interface by permitting complex distal interface compensatory motion to occur (rotation and/or translation). Retrieval reports on many of the new generation of mobile bearing implants remains scarce. This study presented a retrieval analysis of 9 mobile bearing inserts that had be in situ for less than 24 months.

Nine cemented mobile bearing implants (6 AP Glide, 1 LCS, 1 MBK and 1TRAK) were received into our Implant Retrieval Program. The femoral component, tibial tray and PE insert were macroscopically examined under a stereo-zoom microscope for evidence of damage. The PE inserts were graded for wear based on optical and SEM assessments. The proximal and distal surfaces of the PE inserts were subsequently assessed for surface roughness following ISO 97 (Ra and Rp) using a Surfanalyzer 5400 (Federal Products, Providence, RI). Virgin, unused PE inserts were analysed and served as a comparison to the retrieved implants.

Time in situ time for these implants ranged from 6 months to 24 months (mean 18.6). The implants were revised for instability and pain (AP glide) or dislocation (TRAK). Damage to the femoral components, in general, was minimal with some evidence of a transfer film of PE. The proximal surface of the tibial trays presented evidence of PE transfer as well as some scratches but in general were intact. The proximal PE and distal PE articulating surfaces demonstrated significant areas of damage due to third body wear which was identified on EDAX to be PMMA. Areas of burnishing were also present at the proximal and distal interface. The damage, in part, correlated with the complex kinematics of each design.