| In revision total knee arthroplasty, long intramedullary stems are frequently used to increase stability and transfer load from the damaged metaphyseal region to the diaphysis. However, no evidence based guidelines exists to determine stem size and the goal of this research is to help develop such guidelines.;Finite element models were developed from a computed tomography (CT) scan of a composite tibia bone. These models were used to investigate the effect of four stem lengths (125, 145, 175 & 200 mm) on stress distribution and stem-bone sliding under three different medial: lateral load distributions (50:50, 60:40 & 100:0). The effect of multiple medial defect types was also investigated under a single load distribution (60:40) with two stem lengths (145 & 200 mm). In-vitro testing was used to confirm a subset of the finite element results by applying a cyclic axial load with an Instron testing machine for 1000 cycles. Micromotion was measured with two linear variable displacement transducers for two stem lengths (145 & 200 mm) under three interface conditions (uncemented, cemented tray & defective) and two load distributions (60:40 & 100:0).;The finite element analysis showed that a longer stem increased the stress near the stem tip and decreased the average stress in the proximal region. The peak stem-bone sliding occurred near the stem tip and increased with a longer stem. A medial defect also had the same effect as a longer stem. However, the effect of a medial defect was smaller than changing stem length.;The in-vitro testing showed that the average micromotion decreased with a longer stem. When micromotion was compared with stem bone sliding at similar locations, a consistent trend was observed between the finite element and in-vitro results; a longer stem reduced micromotion. A more medial load distribution and a medial defect also increased micromotion independently.;The results of this thesis confirm that a longer stem can increase implant stability at the expense of increased stress transfer to the stem tip. However, further study is required in more realistic models developed from CT scans of cadaver tibias to fully develop guidelines for stem size selection. |
