Abstract
Introduction
Unicompartmental Knee Replacement (UKR) is an appealing alternative to Total Knee Replacement (TKR) when the patient has isolated compartment osteoarthritis (OA). A common observation post-operatively is radiolucency between the tibial tray wall and the bone. In addition, some patients complain of persistent pain following implantation with a UKR; this may be related to elevated bone strains in the tibia. The aim of this study was to investigate the mechanical environment of the tibia bone adjacent to the tray wall, following UKR, to determine whether this region of bone resorbs, and how altering the mechanical environment affects tibia strains.
Materials and methods
A finite element (FE) model of a cadaver tibia implanted with an Oxford UKR was used in this study, based on a validated model. A single static load, measured in-vivo during a step-up activity was used. There was a 1 mm layer of cement surrounding the keel in the cemented UKR, and the cement filled the cement pocket. In accordance with the operating procedure, no cement was used between the tray wall and bone. For the cementless UKR a layer of titanium filled the cement pocket. An intact tibia was used to compare to the cemented and cementless UKR implanted tibiae. The tibia was sectioned by the tray wall, defining the radiolucency zone (parallel to the vertical tray wall, 2 mm wide with a volume of 782.5 mm3), corresponding to the region on screened x-rays where radiolucencies are observed. Contact mechanics algorithms were used between all contacting surfaces; bonded contact was also introduced between the tray wall and adjacent bone, simulating a mechanical tie between them. Strain energy density (SED), was compared between the intact and implanted tibia for the radiolucency zone. Equivalent strains were compared on the proximal tibia between the intact and implanted tibia models. Forty patients (20 cemented, 20 cementless) who had undergone UKR were randomly selected from a database, and assessed for radiolucency.
Results
The SED in the radiolucency zone was 80% lower in the cemented and cementless tibia, compared to the intact tibia, without a mechanical tie between the tibial tray wall and adjacent bone. When a mechanical tie was introduced the SED in the radiolucency zone was 35% higher in the cemented and cementless tibia, compared to the intact tibia. The strain on the proximal tibia was reduced by 20% when a mechanical tie was used between the tray wall and adjacent bone. Radiolucency at the tray wall was observed in all forty radiographs examined.
Discussion
This work has presented a static snapshot of the load being carried through the proximal tibia following implantation with an Oxford UKR. It has been shown that by introducing a mechanical tie between the tibial tray wall and the adjacent bone, the SED in the region observed to have radiolucency is increased; this has the potential of reducing the likelihood of a radiolucency occurring in that region. Moreover, the strain observed in the proximal tibia was reduced when a mechanical tie was introduced, which may reduce the incidence of pain following implantation with a UKR. It is recommended that integration between the bone and the tray wall is important for UKR.
