Introduction and Aims: Mobile bearings have been introduced in total knee arthroplasty (TKA) as a means to improve kinematics and reduce wear. Another theoretical advantage may be a reduction of the torsional and shear stresses at the baseplate – bone interface, and thereby improving the fixation of the tibial component. The aim of this study was to analyse and compare the fixation of tibial components with fixed or mobile bearings in a prospective randomised study using RSA.

Method: Fifty-two consecutive patients (31 women, 21 men, mean age 72) with knee osteoarthrosis underwent primary cemented total knee arthroplasty. Included were patients with arthrosis stage III–V and age over 62. Patients were randomised at operation by opening of sealed envelopes to either Mobile Bearing (MB) or Fixed Bearing (FB). The FB knees received a NexGen TKA with titanium tibial baseplate, and the MB knees a NexGen mobile bearing TKA with CrCo tibial baseplate. All components were cemented using vacuum-mixed Palacos-Gentamicin bone cement. The stem of the component was not cemented. Patellar components were not used. Each tibial baseplate was equipped with five tantalum markers on the undersurface by the manufacturer. The peri- and post-operative management was in all cases identical. Radiostereometric analysis (RSA) was performed three, 12, and 24 months post-op. Clinical results were assessed with Knee Society Knee and Function Scores.

Results: There were no complications. One patient (MB) died two months after operation in myocardial infarction. The Knee Society Knee and Function scores and range of knee motion improved after surgery in both groups with no differences between the groups, reaching 89 at both 12 and 24 months. The rotations of the tibial baseplates did not differ significantly between the two groups. In both groups, anterior-posterior tilting was somewhat larger than varus-valgus tilting. Maximum subsidence was 0.3 ± 0.1 mm (MB) and 0.2 ± 0.1 mm (FB), and maximum migration was 0.6 ± 0.2 mm (MB) and 0.5 ± 0.1 mm (FB) (P = 0.3 – 0.4).

Conclusion: This study could not detect any positive effects on the fixation of the cemented tibial baseplate when a mobile polyethylene insert was used. It may be that in cemented fixation the theoretical advantages of lower shearing and torsional forces at the interface are not important, at least during the initial 24 months post-op.

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.

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.