Medial posterior tibial slope measurements are overestimated on long radiographs and 3D CT compared to measurements on short lateral radiographs.

PURPOSE

This study assessed the measurements of the medial posterior tibial slope (MPTS) using long radiographs and three-dimensional (3D) computed tomography (CT) scans and compared them to measurements taken on short lateral knee radiographs. The study aimed to identify whether the at-risk slope measurements previously defined on the short radiographs would be similar to long radiographs and 3D CT scans.

METHODS

A retrospective radiological review of 52 cases, who underwent planning for a slope-changing high tibial osteotomy and had short and long lateral radiographs and 3D CT scans of the tibia. Two independent observers measured the MPTS on the three modalities. The MPTS was defined as the angle between a tangent to the medial tibial plateau and the referenced tibia anatomical axis. The MPTS measurements from the short and long radiographs were compared to each other and then were compared to the measurements performed on the CT scan. False positives were defined as those cases with MPTS measurements of >78° on CT scans or long radiographs while having measurements ≤78° on short radiographs. These false positive cases are the ones which would be falsely labelled as having an abnormal slope based on the previously validated short radiograph slope threshold ≥12°.

RESULTS

A total of 52 cases were analysed (67.9% males and 32.1% females). The mean age was 27 ± 5.4 years. The mean weight was 71.5 ± 7.7 kg, and the mean height was 1.8 ± 0.1 m. The mean MPTS measured on the short radiographs was 77.3 ± 2.3°; on the long radiographs, it was 75.8 ± 2.0°; and on the CT scan, it was 75.3 ± 2.1°. There was a positive correlation between the measurements taken on both the short and long radiographs ( = 0.9) ( < 0.001). Additionally, there was a positive correlation between CT tibial slope measurements and both short and long radiographs tibial slope measurements ( = 0.86,  = 0.87), respectively (both  < 0.001). False positives were 13 (25%) patients on long radiographs, and 12 (23.1%) patients on CT scans, who had their MPTS measurements ≤78° (equivalent of PTS ≥ 12°) while their measurements were >78° on the short radiographs.

CONCLUSION

Measurements of the MPTS can be overestimated by 1.5-2° on long lateral knee radiographs or 3D CT scans compared to measurements taken on short lateral radiographs. Different thresholds for the abnormal PTS measurements on long radiographs and CT scans, should be defined, considering the overestimated measurements in these modalities.

LEVEL OF EVIDENCE

Level IV case series.