Abstract
Introduction
In complex primary and revision total knee replacement (TKR) the operating surgeon may encounter proximal tibial bone defects. The correct management of such defects is fundamental to both the initial stability and long-term survival of the prosthesis. Block or wedge-shaped metal augments are used to address some such type II unconstrained defects1.
Aim
The aim of this finite element (FE) study was to assess the effects of block and wedge-shaped metal augments upon the shear stresses in the cement mantle at the bone-implant interface of an augmented TKR.
Materials and methods
A three-dimensional FE model was constructed from a computer tomography (CT) scan of the proximal tibia using SIMPLEWARE v3.2 image processing software. The tibial component of a TKR was implanted with either a block or wedge-shaped metal augment in-situ. The model was axially loaded with a force of 3600N, equating to four times the body weight of a 90kg patient, and the load evenly distributed between the medial and lateral tibial plateaux.
Results
Upon loading of the FE model, shear stresses in the cement-augment interface were found to act towards the centre of the prosthesis. The maximal magnitudes of these ‘reverse’ shear stresses were 3.6MPa with a block-shaped augment and 2.6MPa with a wedge-shaped augment. These values are significantly lower than the reported fatigue limit of cement of 17MPa2.
Discussion
Initially it might appear surprising that the cement shear stresses with the wedge-shaped augment are lower than those with the block-shaped augment. However, the FE modelling has shown that in both cases the implant and cement act to prevent the cortical and cancellous bone from expanding when loaded vertically thus inducing a shear stress at the cement-augment interface acting towards the centre of the prosthesis. The angled loading induced by a wedge-shaped augment creates shear stress acting away from the centre of the prosthesis thus reducing the overall cement shear stress in the wedge augmented model.
Conclusion
The FE model has demonstrated reduced cement shear stresses with a wedge-shaped rather than block-shaped augment. However, both values of maximal recorded shear stresses are below the fatigue limit of cement2. Therefore, either a wedge or block-shaped augments can be used and the choice of augment may be determined by the shape of the defect and the quality of the underlying bone.
