Abstract
Introduction Movement between the polyethylene insert and the metal tibial tray in modular fixed bearing total knee arthroplasty (TKA) due to a poor locking mechanism is said to occur. The resulting “backside wear” is proposed as one cause of osteolysis and subsequent loosening. In vitro analysis has revealed movements between the poly and the tibial tray both in non-implanted designs as well as in implants extracted during revisions and at autopsy. Scratch marks on the surfaces at the tray-poly interface have been found indicating rotatory movements between the components. The purpose of this study was to determine in vivo the existence and magnitude of movements between the poly and the tibial tray in modular metal-backed TKA.
Methods Ten patients (median age 71) operated with the NextGen modular fixed bearing TKA due to gonarthrosis were analyzed. This TKA has lipped edges around the entire periphery of the implant to capture and secure the poly insert. The metal tray of the tibial component was equipped with five tantalum markers, and the polyethylene insert with six markers. Radiostereometric (RSA) investigation was performed within one week post-op, and at 12 months. Change in position of the poly insert in relation to the metal tray between the post-op and the 12 months investigations was analyzed as rotations about, and translations along the cardinal axis of the knee. The Insert motion index according to Engh et al (2001) was calculated.
Results Between the surgery and 12 months post-operatively there was a median external rotation of the poly in relation to the metal tray of 0.4 (range: 0.09 to 0.73). Median insert motion index was 0.36 mm (range 0.2 mm to 0.6 mm). Subsidence and lift-off of the polyethylene were very small and below the detection limit of RSA (< 0.08 mm).
Conclusions This study shows for the first time that movements do occur in vivo between the polyethylene insert and the metal tray in modular fixed bearing TKA, even in designs with a full peripheral capture mechanism. The movements occur in the plane of the metal tray (i.e. external rotation, medial-lateral and anteroposterior translation), but no movements are detected in directions out of this plane (proximal-distal translation). The magnitudes of these translations are equivalent to those found in in vitro studies of explanted components. This study thus questions the efficacy of the locking mechanism in modular fixed bearing TKA.
In relation to the conduct of this study, one or more of the authors is in receipt of a research grant from a non-commercial source.
The abstracts were prepared by Mr Jerzy Sikorski. Correspondence should be addressed to him at the Australian Orthopaedic Association, Ground Floor, William Bland Centre, 229 Macquarie Street, Sydney NSW 2000, Australia.
