Coordinate Measuring Machine (CMM) Analysis of the tibial insert
The angle between the centre of the medial and lateral wear scars was 13° relative to the coronal plane (13° external rotation).
Volumetric wear (mm3)
- Lateral - 30.21
- Medial - 139.09
- Total - 169.3
Planar deviation (μm)
- Lateral - 154.0
- Medial - 157.8
- Total - 157.8
The most significant findings were at the tibial tray:
- Microscopic analysis of the superior surface of the tibial tray showed widespread pitting (see additional images on the next page).
- There was minimal retained bone/cement on the underside of the tray. There were pronounced abrasive changes to the underside of the tray in the form of arcuate scratches – clear evidence of rotational movement. The patient was exposed to significant amounts of cobalt chrome wear debris which likely contributed to the osteolysis observed at revision.
In addition, there was batwing deformation (the margins of the insert were deformed inferiorly (blue areas on the CMM reconstruction)) of the PE backside surface with significant material loss around the peripheral edges of the PE. Such a situation will likely increase the frictional torque acting on the insert and the tray, leading to micromotion.
The maximal site of wear of the PE insert was located anteromedially. This is an uncommon finding; maximum wear is normally located centrally or posteromedially. The volumetric wear rate for the tibial insert was estimated at 8 mm³/year, which is within the expected range for contemporary polyethylene inserts. On the non-bearing surface of the femoral component there was less than 50% retained cement cover. The retained femoral bone on the lateral side was blackened - ? metallosis.
Macroscopic analysis of the tibial post showed discolouration at the base mainly on the posterior side. There was deformation of the tibial post most pronounced at the posterolateral aspect of the base. This damage corresponded with external rotation of the femur in extension.