gms | German Medical Science

Objectives: Restoration of the native posterior femoral offset has been described as a significant prognostic factor, mainly for completion of a good range of motion in flexion. The measurement of the posterior offset prior to the TKR on standard X-ray or CT-scan may be inaccurate because the thickness of the remaining cartilage cannot be assessed. The measurement of the posterior offset prior to the TKR on MRI is not routinely performed. The post-operative evaluation of offset restoration may also be inaccurate, as the rotational positioning of the femur may impact the 2D image of the implant and the measure performed. We hypothesized that the measurement of the restoration of the posterior femoral offset by a navigation system during the implantation is more precise than the conventional post-operative measurement on post-operative standard X-rays.

Methods: 100 cases of navigated TKR have been analyzed. The native offset was measured on pre-operative standard sagittal X-rays with a conventional technique (distance between the distal anterior femoral cortex and the most posterior point of the femoral condyles). The thickness of the bone ± cartilage resection performed was measured intra-operatively by the navigation system, independently on the medial and lateral condyles. The thickness of the femoral implant at the level of the posterior femoral resection was recorded. The final offset was measured on post-operative standard sagittal X-rays with the same conventional technique as above. The actual offset change (thickness of resection - thickness of the implant) was compared to the measured offset change (pre-operative - post-operative X-ray offset). Both measures were compared by a paired Wilcoxon test and calculation of the Spearman correlation coefficient at a 5% level of significance.

Results: The mean actual offset change was +0.5 ± 2.0 mm on the medial condyle and +3.3 ± 2.9 mm on the lateral condyle ; the average was +2.7 ± 2.4 mm. The mean measured offset change was +1.1 ± 2.1 mm. There was a significant difference between the paired measurements of the actual offset and of the measured offset. There was a poor correlation between the two paired measurements (r² = 0.33).

Discussion: We observed a significant difference between the actual offset change and the measured offset change. This may be explained by several biases: 1) rotational malposition of the femur which may be different between pre- and post-operative views; 2) uncertainty between the definition of the contours of both medial and lateral condyles; 3) uncertainty about the thickness of the remaining cartilage; 4) uncertainty about the amount of bone loss to be corrected. According to the present study, the pre-operative X-ray measurement of the femoral offset cannot be used reliably to plan the expected bone resection. Furthermore, the post-operative standard X-ray cannot be used as a quality control to assess the reconstruction of the femoral offset.