INDUCIBLE MOVEMENTS OF FIXED MODULAR TIBIAL POLYETHYLENE BEARINGS IN TOTAL KNEE ARTHROPLASTY – AN IN VIVO STUDY USING RSA

Abstract

Introduction and Aims: Movements between the poly-ethylene insert and the metal tibial tray in modular fixed bearing total knee arthroplasty (TKA) due to poor locking mechanism is said to occur. The resulting ‘backside wear’ is proposed to be one cause to osteolysis and subsequent loosening. The purpose of this study was to determine in vivo the magnitude of movements between the poly and the tibial tray in modular metal-backed fixed bearing TKA using RSA.

Method: Four patients (six knees), mean age 70 years, operated with cemented NexGen modular fixed bearing TKA were studied 12 to 18 months after surgery. The tibial baseplates and the polyethylene liners were prepared for RSA with tantalum markers. The patients stood with the foot of the investigated leg fully weight bearing on a rotating platform. The platform (and thereby the knee) was subjected to an internal or external directed torque of 10 Nm. At the first RSA examination the patient resisted an internal directed torque, and at the second examination an external directed torque. Rotation of the polyethylene liner in relation to the base plate between these two examinations was recorded. Also the insert motion index (IMI) was determined.

Results: In three knees there was no inducible rotation of the polyethylene (rotation less than 0.03 degrees, IMI less than 93um). In the other three knees, however, rotation varied between 0.16 and 0.64 degrees, and the IMI varied between 210um and 420um.

Conclusion: This study shows that the fixed polyethylene liner in some knees can display rotatory motions in relation to the base plate. The magnitude of the motions is comparable to that found for implants extracted at revision or autopsy. Whether this is due to inferior locking mechanism, or gradual deformation of the part of the polyethylene in contact with the locking mechanism cannot be determined in this study.