The Role of Posterior Tibial Slope in Revision Anterior Cruciate Ligament Surgery: A Systematic Review and Meta-Analysis Using a Ski Slope Classification System

Tom Murphy¹, Georgia Brady¹, Maher Ghandour MD ², Matthieu Ollivier MD, PhD², Michael Dan MBBS, PhD ³
1 The University of Notre Dame, Australia2 Institute for Locomotion, Aix‐Marseille University, APHM, CNRS, ISM, Sainte‐Marguerite Hospital, Marseille, France3 Surgical and Orthopaedic Research Laboratory, Prince of Wales Clinical School, UNSW, Australia

Introduction:

Anterior cruciate ligament (ACL) injury is one of the most common knee injuries, with incidence in Australia rising annually (1). Despite significant progress in ACL reconstruction surgery, there remains a considerable incidence of revision, reported as anywhere from 3% to 15% (2, 3). Many studies have examined the reasons for failure and have concluded that it is multifactorial. However, increasing evidence has found that morphological features of the knee, in particular the posterior tibial slope (PTS), may play a significant role in ACLR failure. Biomechanical data indicates that a steeper PTS can increase axial loading through the knee, placing greater stress on the ACL (4).

The aim of this review is to provide a risk stratification tool for use by clinicians to aid in the identification of high risk knees, where greater consideration may be given to surgical augmentation of the posterior tibial slope.

Aim:

This study aims to examine the relationship between increased posterior tibial slope (PTS) and the risk of anterior cruciate ligament (ACL) revision surgery.  The primary outcome of the review is the incidence of ACL graft failure associated with varying degrees of PTS, stratified based on standard deviations above a mean slope (control group).

Additional outcomes include: the identification of a threshold PTS value where the risk of graft failure is significantly increased. A further additional outcome is the creation of a stratification system that can be utilised regardless of imaging modality or measurement technique used to calculate PTS. This stratification system can be used to assist clinicians in surgical planning.

Hypothesis:

Increased posterior tibial slope (PTS) is associated with an increased risk of anterior cruciate ligament (ACL) graft failure. As PTS increases above the population mean (control), the rate of ACL graft failure will rise accordingly. Knees that are categorised as being in the highest risk category i.e. black, double black, will have a significant risk of graft failure.

Knees that fall into these higher risk categories may be surgical candidates for slope augmentation.

Methods:

This systematic review was completed using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRIMSA) guidelines and was pre-registered with PROSPERO (ref no:CRD42024606313).

Medline, SCOPUS, Embase and Cochrane were searched from inception through to August 2024. The specific search terms were ‘anterior cruciate ligament’ OR ‘ACL’ AND ‘slope’ OR ‘tibial slope’. The screening process involved title/abstract screening followed by full-text review conducted by two independent reviewers (TM and GB).

A total of 2149 studies were identified, following exclusion of duplicates 1177 studies were assessed for eligibility by 2 independent reviewers. Specific inclusion and exclusion criteria were applied. Patient demographics and study details were extracted, including country of origin (determined by the location of the primary author), type of study, number of knees, sex, age, time point of re-rupture and type of graft used in the reconstruction. Data specific to the PTS included: imaging modality, measurement technique, location of PTS measurement and the PTS measurement. 

The data for the imaging modality and technique used to measure the posterior tibial slope was extracted and codified to group studies accordingly. Radiographs were divided into three categories based on the specific measurement technique: (X1) the proximal anatomic axis, which is defined by a line through the midpoint of the proximal tibia, (X2) posterior tibial cortex, defined by a line between the posterior tibial cortex of the proximal tibia, and (X3) the angle between the medial tibial plateau and the lateral mechanical axis of the leg (entire tibia used). MRIs were divided into four categories: (M1) the method proposed by Hashemi et al., (M2) the method proposed by Matsuda et al., (M3) the method proposed by Hudek et al. and (M4) the method proposed by Alici et al.

A risk of bias assessment was conducted using the Methodological Index for Non-Randomized Studies (MINORS) tool.

PTS measurements will be extracted and pooled based on imaging modality (MRI or radiograph) and measurement technique (codified during data extraction). Extracted and pooled outcomes of interest and related standard error will be performed using the DerSimonian-Laird method. Heterogeneity between studies will be quantified using the I^2 statistic. Participants will be stratified based on the ski slope-like classification system of PTS (e.g. green (low risk), blue, black, double black (high risk)) to assess whether specific PTS thresholds are associated with higher graft failure risk.

References:

1. Zbrojkiewicz, D., Vertullo, C., & Grayson, J. E. (2018). Increasing rates of anterior cruciate ligament reconstruction in young Australians, 2000-2015. The Medical journal of Australia, 208(8), 354–358. https://doi.org/10.5694/mja17.00974
2. Liukkonen, R. J., Ponkilainen, V. T., & Reito, A. (2022). Revision Rates After Primary ACL Reconstruction Performed Between 1969 and 2018: A Systematic Review and Metaregression Analysis. Orthopaedic journal of sports medicine, 10(8), 23259671221110191. https://doi.org/10.1177/23259671221110191
3. Kamien, P. M., Hydrick, J. M., Replogle, W. H., Go, L. T., & Barrett, G. R. (2013). Age, Graft Size, and Tegner Activity Level as Predictors of Failure in Anterior Cruciate Ligament Reconstruction With Hamstring Autograft. The American Journal of Sports Medicine, 41(8), 1808–1812. https://doi.org/10.1177/0363546513493896
4. Bernhardson, A. S., Aman, Z. S., Dornan, G. J., Kemler, B. R., Storaci, H. W., Brady, A. W., Nakama, G. Y., & LaPrade, R. F. (2019). Tibial Slope and Its Effect on Force in Anterior Cruciate Ligament Grafts: Anterior Cruciate Ligament Force Increases Linearly as Posterior Tibial Slope Increases. The American Journal of Sports Medicine, 47(2), 296–302. https://doi.org/10.1177/0363546518820302