Is a full-length weight-bearing lateral radiograph necessary to evaluate the posterior tibial slope and anterior tibial translation? A prospective observational study

Objectives: Increased posterior tibial slope (PTS) leads to more anterior translation of the tibia (ATT) and thus increased risk of anterior cruciate ligament (ACL) injury. It was hypothesized that there is a correlation between PTS and ATT in radiographs, and the obtained PTS value will be influenced by the length of the tibia chosen during measurements. Materials and Methods: Full and half-length single-leg standing lateral tibial radiographs were taken for the ACL-injured knee of a patient, and also their normal knee. ATT and PTS were calculated from these lateral radiographs and analyzed. Results: A total of 126 knees were included in this study. There was a positive correlation between PTS and ATT on full-length lateral tibial radiographs, which was statistically significant. Mean PTS measurements were 12.09 ± 2.6 and 10.6 ± 3.6° in full-length and half-length radiographs, respectively, for ACL-injured knees, and the average difference was found to be 1.5 ± 0.7, which had a strong and significant linear relationship (r = 0.86, p <0.05) among 49.2% ( n = 31) of the study subjects. Conclusion: There was a positive, statistically significant correlation between PTS and ATT on full-length lateral tibial radiographs. The obtained PTS measurements were strongly associated with the length of the lateral tibial radiograph. It is necessary to accurately measure PTS values when we plan to perform a slope reduction osteotomy. Hence, it is advisable to use full-length lateral tibial radiographs for PTS measurement. Increased PTS is correlated to increased anterior tibial translation and thereby increases the risk of ACL injury.