Abstract
Background
Under- or oversizing of either component of a total knee implant can lead to early component loosening, instability, soft tissue irritation or overstuffing of joint gaps. All of these complications may cause postoperative persistent pain or stiffness. While survival of primary TKA's is excellent, recent studies show that patient satisfaction is worse. Up to 20% of the patients are not satisfied with the outcome as and residual pain is still a frequent occurrence.
The goal of this study was therefore to evaluate if the sizing of the femoral component, as measured on a 3D-reconstructed projection, is related to patient reported outcome measures.
From our prospectively collected TKA outcome database, all patients with a preoperative CT and a postoperative X-ray of their operated knee were included in this study. Of these 43 patients, 26 (60,5%) were women and 17 (39,5%) were men. The mean age (+/−SD) was 74,6 +/− 9 years.
Methods
CT scans were acquired. All patients underwent TKA surgery in a single institution by one surgical team using the same bi- cruciate substituting total knee (Journey II BCS, Smith&Nephew, Memphis, USA). Using a recently released X-ray module in Mimics (Materialise NV, Leuven, Belgium), this module allows to align the post-operative bi-planar x-rays with the 3D- reconstructed pre-operative distal femur and to determine the 3D position of the bone and implant models using the CAD- file of the implant. This new technique was validated at our department and was found to have a sub-degree, sub-millimeter accuracy. Eleven zones of interest were defined. On the medial and the lateral condyle, the extension, mid-flexion and deep flexion facet were determined. Corresponding trochlear zones were defined and two zones were defined to evaluate the mediolateral width. In order to compare different sizes, elastic deforming mesh matching algorithms were implemented to transfer the selected surfaces from one implant to another. The orthogonal distances from the implant to the nearest bone were calculated. Positive values represent a protruding (oversized) femoral component, negative values an undersized femoral component. The figure shows the marked zones on the femoral implant. The KOOS subscores and KSS Satisfaction subscore were evaluated.
Results
Two-step cluster analysis based on the clinically relevant zones on both medial (zone 12, 14 and 17) and lateral (zone 2, 5 and 9) femoral condyle of the implant, led to the formation of two clusters. Cluster 1 contained 23 patients with, in general, an undersized femoral component (negative values) whilst cluster 2 contained 20 patients with in general an oversized femoral component (positive values). (see graph) No significant differences were found between both clusters regarding demographics.
Regarding PROM data, a significant difference was found for KOOS Symptoms (p=0.037) and a KOOS Pain (p=0.05). Other PROMs are not significantly different between both clusters.
Conclusion
Our data shows that undersizing the femoral component results in less postoperative pain and symptoms. The clinical consequence of this study is that in case of in between femoral component sizes, the smallest size should be chosen to diminish the occurrence of postoperative pain and symptoms.
