Medialised loading position may increase risk of pain after unicompartmental knee arthroplasty

Introduction
Persistent pain is an important cause of patient dissatisfaction after unicompartmental knee arthroplasty (UKA) and
some studies have correlated pain with localised tibial strain. The position of femoral loading through the medial
side is dependent upon surgical factors (femoral and tibial component position) as well as patient factors (gait, varus
deformity). The purpose of this study was to examine whether load position has an effect on tibial strain.
Patients & Methods
Using a previously validated nite element analysis (FEA) model [1], material properties of the bone were assigned
using the Hounseld units of the CT scan. Components were implanted according to the operative technique and the
loads were calculated using measurements from an instrumented knee prosthesis. The tibia was cut 100 mm below
the tibial plateau, this shortened model was validated. A mesh size of 2.4 mm was used after performing a mesh
convergence study. Analyses were performed with the medial load positioned between 1 mm lateral to 7 mm medial
from the centre, and 14 mm posterior to 14 mm anterior from the centre.
Results
Implanting the components increased the strain by 20%; this increase was signicant. Movement of the loading
position beyond 10 mm anteriorly and 8 mm posteriorly caused a signicant increase in the mean von Mises strain in
the proximal exterior cortex. Fluorscopy studies have shown anterior and posterior movement rarely exceed 7 mm so
the movement is outside of this range and thought not to be of clinical impact. However, movement in excess of 3 mm
medially also caused a signicant increase in strain and is within the range of movement which could occur in vivo
and potentially cause pain.
Discussion/Conclusion
Previous studies have found that patient with a tibial tray overhang of greater than 3 mm are more likely to have a
poor outcome, the present study indicates the poor outcome may be due to medialisation of the load. These results
suggest that a cause of early pain after UKR may be strain. Surgical factors which may move the load position
medially, such as the use of a small tray in a large patient or medial placement of the tibial tray, should be avoided to
minimise strain and possible consequent pain.
References
[1] Gray HA, Taddei F, Zavatsky AB, et al. 2008. Experimental Validation of a Finite Element Model of a Human
Cadaveric Tibia. J Biomech Eng 130: 031016