Summary

Optimal and maximal sizing of the tibial component is associated with better outcomes, but overhang is detrimental; rotation had less implication to outcome.

Abstract

Introduction

& aims

Tibial component positioning is understood to be a compromise between rotational alignment, maximisation of coverage and avoidance of overhang. Each of these aspects has been studied in separate studies, but to date no comprehensive evaluation of all 3 aspects on a single data set exists in the literature. This study sought to analyse the relative contribution of these three positioning aspects.

Method

A database of TKR patients operated on by six surgeons from 1-Jan-2014 who had a pre-operative and post-operative CT scan and 6-month postoperative Knee Injury & Osteoarthritis Outcome (KOOS) scores were assessed. All knee operations were performed with the Omni Apex implant range using an asymmetric tibial tray design with a mixture of alignment techniques.

3D implant and bone models from the preoperative CT scans were registered to the post-operative CT scan. The back plane of the tibia was determined and the position of the tibial component relative to the tubercle, PCL attachment point and periphery of the tibial bone determined. Coverage of the bone by the tray, rotational position and portion of the component overhanging the bone were calculated.

Results

A total of 457 patients were identified in the database. 59% (142) were female and the average age was 68.4 years (+/- 8.1).

Coverage percentage of the tibial surface by the tibial tray was 77% (± 4%), with 94% (429) knees having greater than 70% coverage and 22% (102) greater than 80%. 21% (97) of analysed knees exhibited some level of overhang, while 5% (22) had greater than 2.5% of the tibial tray area overhanging the tibial bone. Mean rotation was 2.1° (± 5.4) of rotation internal to Insall’s axis, with an average deviation (internal or external) of 4.7° (± 3.4).

No relationship with rotational position of the tibial component was found. Increased coverage was found to significantly correlate with increased KOOS score in the symptoms subdomain (coefficient=0.13, p-value = 0.008). This observation had limits, however, and when knees were subdivided into patients who had overhang and those who did not, there was a statistically significant difference in KOOS symptoms score (73.5 points with overhang, 79.1 without, p = 0.009).

Conclusions

The results suggest that ‘upsizing to the point of overhang’ is a rule consistent with the best possible patient outcomes. The lack of relationship with rotation to Insall’s axis would suggest that some level of compromise on rotation to fit bone is acceptable, though not desirable