Abstract
Introduction
Pre-clinical assessment of total knee replacements (TKR) can provide useful information about the constraint provided by an implant, and therefore help the surgeon decide the most appropriate configurations. For example, increasing the posterior tibial slope is believed to delay impingement in deep flexion and thus increase the maximal flexion angle of the knee, however it is unclear what effect this has on anterior-posterior (AP) constraint.
The current ASTM standard (F1223) for determining constraint gives little guidance on important factors such as medial- lateral (M:L) loading distribution, flexion angle or coupled secondary motions. Therefore, the aim of the study was to assess the sensitivity of the ASTM standard to these variations, and investigate how increasing the posterior tibial slope affects TKR constraint.
Methods
Using a six degree of freedom testing rig, a cruciate-retaining TKR (Legion; Smith & Nephew) was tested for AP translational constraint. In both anterior and posterior directions, the tibial component was displaced until a ‘dislocation limit’ was reached (fig. 1), the point at which the force-displacement graph started to plateau (fig. 2). Compressive joint loads from 710 to 2000 N, and a range of medial-lateral (M:L) load distributions, from 70:30% to 30:70% M:L, were applied at different flexion angles with secondary motions unconstrained. The posterior slope of the tibial component was varied at 0°, 3°, 6° and 9°.
Results
AP translation was significantly larger at 60° and 90° flexion (22 ± 1 mm and 24 ± 1 mm respectively) than at 0° (14 ± 1 mm), whilst increasing the compressive joint load increased the force required to translate the tibia to limits of AP constraint at all flexion angles tested. When the M:L load distribution was shifted medially, a coupled internal rotation was observed with anterior translation and external rotation with posterior translation; this was reversed with a lateral shift in load distribution.
It was also found that increasing the posterior slope of the tibial tray moved the neutral position of the tibia relative to the femur more anteriorly at all flexion angles tested. The constraint under anterior drawer was then reduced with increasing slope, which meant that the tray dislocated at lower drawer force and translations.
Conclusions
When intraoperative tibial bone cuts are made, surgeons should be aware that by increasing posterior slope angles the TKR may offer less anterior constraint under body-weight loads, therefore relying more heavily on surrounding soft-tissue and muscle action to prevent dislocation.
The ASTM test protocol could be refined to stipulate the variation of the M:L loading distribution. It has been shown to vary between patients and activities, and the AP constraint and associated secondary motions in this study were very sensitive to this distribution. The secondary motions observed should be measured and recorded to provide more information about the device's stability characteristics. The tests could also be extended to include a higher axial load such as 2000 N, approximately three times body weight, in order to investigate coupled rotations and M:L distribution effects whilst under normal walking gait loads.
