Development and validation of a new method for the radiologic measurement of the tibial slope

The posterior tibial slope has a huge influence on the kinematics of the knee. In several orthopedic interventions such as high tibial osteotomy and unicondylar or bicondylar knee replacement changing, the tibial slope can result in altered knee mechanics. Therefore, an exact preoperative measurement of the posterior tibial slope is mandatory. Several methods are used on conventional radiographs and CT scans, but until now there is no standard validated method. The aim of this study was to compare several methods and imaging techniques to measure the posterior tibial slope and to establish a standard and reliable measurement method by radiography.

Fourteen knees (seven cadavers) were scanned by a 64-slice CT, a 3T-MRI, and true lateral radiographs were performed. The anatomical references (TPAA = tibial proximal anatomical axis; ATC = anterior tibial cortex; PTC = posterior tibial cortex) and the new computed reference (MPA = mean of PTA and ATC) were compared by short as well as long radiographs, CT scan and MRI. The influence of a malrotation in radiographs of the knees was also analyzed.

CT scan and MRI are suitable for the measurement of the medial and lateral posterior tibial slopes, the results of the radiographs varied depending on the method used. The new method (MPA) showed the best correlation to the CT scan (r = 0.997), even on short radiographs (10 cm distal the joint line).

The measurement of the posterior tibial slope on a short lateral radiograph using the MPA is a reliable method and should be established as a standard.

Diagnostic study, Level II.