Posterior tibial slope interpretation is method‐dependent: No universal threshold for defining abnormality in primary and recurrent ACL rupture

Abstract Purpose To determine method‐specific normal posterior tibial slope (PTS) values, define abnormal PTS using two statistical approaches, and assess how abnormal PTS classification varies in primary anterior cruciate ligament (ACL) rupture and ACL re‐rupture. Methods In this retrospective single‐centre study, all patients undergoing knee imaging during the study period were screened and categorized into three groups based on clinical and imaging findings: ACL‐intact, primary ACL rupture and ACL re‐rupture. PTS was measured on standardized lateral radiographs using five techniques. Normal values were defined as the 95% reference interval (2.5th–97.5th percentiles). Abnormal PTS was defined using both a quantile‐based approach and a standard deviation–based approach. Statistical significance was set at p < 0.05. Results A total of 609 subjects were included (mean age 30 ± 5 years; 304 males and 305 females), comprising an ACL‐intact reference group ( n = 470), a primary ACL rupture group ( n = 88) and an ACL re‐rupture group ( n = 51). Mean PTS in the reference group ranged from 5.9° to 8.2°, with increased PTS generally observed above 12–15°, depending on the technique. Using reference‐based thresholds, abnormal PTS was identified in 18.2%–22.7% of ACL patients and 35.3%–41.2% of ACL re‐rupture patients, with significantly higher proportions in the latter ( p = 0.009–0.036). Using standard deviation–based thresholds, abnormal PTS was identified in 17.0%–20.5% and 27.5%–37.3% of patients, respectively. When thresholds were derived from the ACL population, abnormal PTS classification in ACL re‐rupture decreased (2.0%–9.8%) and was no longer significantly different (all p ≥ 0.532). Conclusion PTS interpretation is highly method‐dependent. Normal ranges and abnormal classification vary according to measurement technique, statistical definition and reference population. A universal PTS cut‐off is therefore inappropriate. Level of Evidence Level III, retrospective comparative cohort study.