Posterior tibial slope measurements show a high degree of variability

PURPOSE: Posterior tibial slope (PTS) ≥ 12° is regarded as the strongest predictor of anterior cruciate ligament (ACL) reconstruction graft failure. Several methods to measure PTS have been reported with varying degrees of accuracy. We investigate whether the sagittal plane on the axial slice of magnetic resonance imaging (MRI) and computed tomography (CT) scans, with respect to the axial series, either parallel to the ACL or perpendicular to the posterior femoral condylar axis (PFCA), affects PTS measurements.

METHODS: Twenty patients undergoing ACL revision surgery between November 2019 and September 2022 were included. Medial PTS (MPTS) and lateral PTS (LPTS) were measured and compared using the method of Hudek et al. on sagittal MRI, initial sagittal CT acquisition, sagittal CT reconstructed to correspond to the MRI and sagittal CT reconstructed perpendicular to the PFCA.

RESULTS: MRI produced greater mean PTS measurements compared to the different CT reconstructions. Mean LPTS measurements were greater than MPTS measurements across all imaging modalities, but only significant for MRI studies (p = 0.01). There was no significant difference between the PTS measurements for CT as per MRI and perpendicular to PFCA. Intraclass correlation coefficient demonstrated good agreement (0.64) for LPTS measurements on MRI but poor agreement for CT oriented to MRI (MPTS: 0.52, LPTS: 0.15) and perpendicular to PFCA (MPTS: 0.35, LPTS: 0.16).

CONCLUSION: PTS measurements vary widely between imaging modalities and raters, with no difference in measurements according to different CT reconstructions. Therefore, one must take great care with PTS measurements when considering slope-changing osteotomy during ACL revision surgery.

LEVEL OF EVIDENCE: Level IV.