Abstract
Loosening and migration of tibial prostheses have been identified as causes of early total knee replacement (TKR) failure often due to defected bone in the proximal tibia compromising fixation and alignment. Clinical studies using metal augments have shown these to be an alternative to other means of defect treatment. Finite element (FE) analysis can be used to identify regions prone to loosening and migration and to advise surgical teams on appropriate course of action when dealing with defects. 2D initial stability studies were carried out with two defect angles considered. Analysis indicated that the use of a metal block results in the lowest stress values near the defect margin and cement-bone interface shear stresses in both defect sizes. Stem extensions were more beneficial in treatment of larger defects. A bone remodelling algorithm is described and applied to the 2D models. Results showed more bone loss for the metal augment models compared with the cement augment models. It is believed that the extra bone loss is unlikely to result in augment loosening. 3D FE models of non-defect TKR showed good stability with stresses produced within the maximum thresholds for compressive, tensile and shear strengths. The stem extension was shown to reduce displacements and proximal stresses in the cancellous bone while not contributing to excessive stress-shielding. 3D FE models of TKR defect models treated with block augments showed that where defects are less than 15 mm, a standard stem is adequate. The use of cement to fill proximal defects is not considered suitable as stresses carried by the cement block exceed those of the fatigue limit. Metal augments are considered superior to that of cement augments as they reduce bone stresses. Both conical augments were shown to produce high stresses at the bone/augment interfaces and stem tip. There is evidence that press-fit stems are capable of reducing these stress concentrations. Thus it is recommended that press-fit stems be used in treating severe defects with conical augments. The large conical augment was used to treat an uncontained defect with symmetrical and asymmetrical loading. The use of block augments should be considered by the surgeon to treat large uncontained defects. Conical augments should be considered when the cancellous bone contains large central cavitary defects. The use of stem extensions is recommended in this case as they significantly reduce the unstable high cancellous bone stresses produced by the peripheral defect.