Cementless Oxford Unicompartmental Knee Replacements (OUKR) are known to have a lower revision rate than cemented, but have a higher incidence of early tibial fracture, and is not known how their symptoms compare. The aims of this thesis were firstly to compare the symptoms following cemented and cementless OUKR and explore factors that may influence them, and secondly to assess design changes which may reduce their risk of fracture.
A study comparing Cemented and Cementless OUKR demonstrated that most pain reported by patients is intermittent, and that 42% more patients with Cementless OUKR report having no pain compared to Cemented OUKR.
Radiolucencies at the tibial wall, and bone density around the tibial keel, are characterised for the first time in literature. Radiolucencies around the tibial component were found only where there was no porous coat, and these radiolucencies were associated with an increased likelihood of patients reporting pain. This suggests that the porous coating should be applied to all bone-implant interfaces. Bone density around the keel changes in the years following surgery, but this change is unrelated to pain.
Despite the benefits of Cementless OUKR, cemented tibial components are still used, in part due to concerns about periprosthetic tibial fractures in Cementless OUKR. These occur most commonly around very small components. The keel has the same depth for all sizes, suggesting that the relatively larger keels are an important risk factor for the smaller tibias. To address this, changes to keel design were studied using a plastic bone test in which the push-in force was considered to be related to the fracture risk and the pull-out force was considered to be related to primary fixation. From these tests, two new keel designs were proposed: a small porous keel without a window, and a small porous-walled smooth-edged keel. A more radical redesign replacing keels with two anterior angled pegs was also proposed to address fundamental limitations of a keel.
More physiologically relevant tests were developed using plastic bone blocks machined to the shape of a very small tibia that experienced a fracture. These tests used load-to-fracture to assess fracture risk and anterior lift-off with posterior loading to assess primary fixation. Important assumptions about the effect of keel size and interference are assessed with these tests. The small porous keel, compared to the standard keel, reduced fracture load, and had similar fixation. The smooth-edged keel similarly reduced the fracture load but had better fixation. The two-peg component greatly reduces fracture risk and virtually eliminates pathological implant micromotion.
The thesis concludes with proposed next steps leading to clinical use of these new designs. The small porous and porous-walled smooth-edged keels require minor modifications to manufacturing, while the two-peg device requires modifications to manufacturing, surgical technique, and instrumentation. The new small keels could be introduced relatively rapidly after short clinical studies, whereas the two-peg device would require more in-depth safety assessments. Whichever design is introduced, all surfaces interfacing with bone in the tibial component should have a porous coating
