Introduction: Injuries to the ACL are multi-factorial and recent research has focussed on anatomical variations and the correlation to ACL rupture and graft failure. Recently, there has been a focus on posterior tibial slope and the variations between the lateral and medial side. The literature has been heterogenous and contradictory with variable results reporting inconsistent differences between lateral and medial slopes. Additionally, biomechanical cadaveric studies have demonstrated the effects of increased posterior slope and rotational instability. The aim of this study was to compare the lateral and medial posterior tibial slopes in ACL deficient knee and its correlation to rotational instability as determined by a pivot shift test. Methods: Patients were recruited through two channels. 149 patients were recruited from the Queensland Children’s Hospital ACL Injury Registry and 150 patients were recruited from a concurrently run study from the Brisbane Orthopaedics and Sports Medicine Research Group. The lateral and medial slopes were analysed from MRI scans using a method validated by Hudek1 and documented as LPTS and MPTS respectively. Tibial slope asymmetry (LPTS – MPTS), slope ratio (LPTS/MPTS) and cumulative slope (LTPS + MPTS) were calculated. A pivot shift test was performed on an anaesthetised patient by a senior consultant as per the concurrent running study and grade documented. Intra and Inter-rater observer reliability was measured using intraclass coefficient with two way random agreement. Results: A total of 259 patients were included with 40 patients excluded due to sub-optimal MRI scans for accurate slope assessment. The intra-rater and inter-rater reliability were excellent for radiographical assessment of slope. The average age was 21.28 ± 10.77 with a similar number of participants in either sex (male 53.6%). Slope analysis LPTS = 6.22 ± 3.36, MPTS 4.32 ± 2.50, Slope Ratio 3.37 ± 8.82, Slope Asymmetry 1.89 ± 3.30 and Cumulative Slope 10.54 ± 4.91. Logistic regressions between pivot shift grade and lateral slope, medial slope, slope ratio, slope asymmetry and cumulative slope were not statistically significant indicating that the former variable were not predictive of pivot shift grade. Conclusion: We were unable to find any positive correlation between lateral and medial tibial slope, slope ratio or slope asymmetry and rotational instability. This is contrary to the published literature. We recognise that the pivot shift exam is subjective, and that rotational instability is multi-factorial with several patient factors contributing. Our study to date is one of the largest to directly look at the difference between lateral and medial tibial slope and its relationship with rotational instability in a clinical setting. Our results suggest that there may be other anatomical variations which contribute to high grade rotational instability.
Shales et al. (Thu,) studied this question.