Introduction

Recent evidence has found that an increased posterior tibial slope (PTS) angle is associated with increased risk for ACL injury [1,2,3,4,5]. Biomechanically, as the PTS degree increases, the shear force, anterior translation, and rotational force all increase on the tibia, resulting in increased force transmission to the ACL during movement [6, 7]. Furthermore, females have osseous morphology predisposing them to increased PTS, and a relationship between an increased PTS and risk for non-contact ACL injuries has been reported in females [8,9,10,11]. While investigation into the relationship between non-contact ACL injuries and PTS angle has been examined, the relationship between contact ACL injuries and PTS has not been as thoroughly investigated.

A study by DePhillipo et al. analyzed the difference in the lateral PTS angle between contact ACL tears compared to matched controls with non-contact ACL tears [12]. The group found that mean lateral PTS was not significantly different between the contact and non-contact ACL cohorts. However, given that non-contact ACL injuries typically occur when an athlete is trying to pivot, slow down, or land from a jump; whereas, contact ACL injuries are from a direct blow to the knee, there is reason to believe the intrinsic anatomic risk factor of an increased PTS would be greater in the non-contact cohort [13, 14]. Additional investigations are required to improve the Orthopedic discipline’s understanding of the relationship between PTS and contact ACL injuries.

Therefore, the purpose of this study was to determine whether patients with non-contact ACL injury have a higher PTS angle than those with contact ACL injury. The hypothesis was that patients who sustain non-contact ACL injuries would have an increased PTS angle compared to those with contact ACL injuries.

Methods

Study design

Institutional Review Board (protocol i19-01430) approval was obtained before commencing study activities. A retrospective review of patients who underwent a primary ACL reconstruction from August 2010 to June 2023 at a single academic medical institution was conducted. All patients who underwent a unilateral primary ACL reconstruction were initially included. Patients were then excluded if they were below 18 years of age, did not have a lateral knee X-ray for radiographic evaluation or had prior knee surgery. Patients were excluded from the analysis if their lateral knee X-ray was of inadequate quality for analysis including if the knee was severely rotated with non-overlapping femoral condyles, if there was less than 10cm of tibia visualized on the X-ray or if the image quality of the X-ray was poor (low image contrast, resolution and excessive noise which impair the ability to accurately assess the X-ray) [15]. The electronic medical record (EMR) was queried for demographic data such as age, sex, and body mass index (BMI). Additionally, the EMR were reviewed for mechanism of ACL injury (contact vs. non-contact). Subjects were divided into two cohorts, those who had experienced a contact ACL injury and those who had a non-contact ACL injury. Additionally, a cohort of 152 patients with intact ACLs was identified. Patients in the contact cohort were propensity score matched to patients in the non-contact cohort and intact ACL cohort by age, sex and BMI.

Radiographic analysis

The measurement of the posterior tibial slope (PTS) on lateral radiographs followed the methodology outlined by Hashemi et al which gives a composite angle of the medial and lateral plateau slopes [15]. Two circular markers were positioned below the tibial tuberosity: the first circle was drawn 5 cm distal to the tibial tuberosity, and the second as distal as possible. Each of these circles was extended to the outer cortex level. The longitudinal axis of the tibia was determined by the line intersecting the centers of these two circles. Posterior tibial slope was defined as the angle formed between a tangent line to the tibial plateau and a perpendicular line to the longitudinal axis, as depicted in Fig.1. A second reader was employed to measure posterior tibial slope in the contact ACL injury cohort to ensure inter-rater reliability of posterior tibial slope measurement throughout the study.

Fig.1
figure 1

Radiographic Measurement of Posterior Tibial Slope

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The assessment of the tibial slope on a lateral radiograph involved drawing two circles beneath the tibial tubercle at the outer cortex level to approximate the anatomical tibial axis. The longitudinal axis of the tibia was determined by the line connecting the centers of these two circles. In the image above the posterior tibial slope was measured to be 16.9°.

Statistical analysis

Statistical analysis was completed using SPSS, Version 24 (IBM Corp., Armonk, NY). Normally distributed continuous variables between cohorts were compared using the two-sample Student t test. The Mann–Whitney U test was used for nonnormally distributed variables. Chi-squared analysis was done to compare categorical and binomial variable. Logistic regression analysis was used for binary categorical dependent variables; while, linear regression was used for continuous dependent variables. Findings were considered significant at p ≤ 0.05. Inter-rater reliability was measured through calculation of intraclass correlation coefficient (ICC) using a two-way mixed model for absolute agreement. Evaluation of ICC acceptability was defined as in Koo et al. [16]

Propensity score matching of the non-contact and contact ACL injury cohorts was conducted using age, sex and BMI as predictor variables. The protocol for propensity score matching was carried out through conducting logistic regression analysis on the group indictor of contact versus non-contact ACL injury for the predictor variables age, sex and BMI with the resulting propensity variable used to match patients in each group with the nearest neighbor algorithm.

An a priori power analysis was carried out using the data in DePhillipo et al. which determined an effect size of 53 for their study analyzing the difference in PTS between contact and non-contact ACL injury patients [12]. With a desired statistical significance level of 0.05, a desired statistical power of 0.80 we determined that a minimum sample size necessary to detect a clinically significant difference would be 57 patients.

Results

A total of 1700 patients who underwent primary ACL reconstruction between January 2011 and June 2023 at a single academic institution were initially identified. From this initial cohort, 102 patients with contact injury were identified and 1598 patients with non-contact injuries were identified. Of these 102 patients with a contact ACL injury, only 67 had knee X-rays that were suitable for measurement. These 67 patients were propensity score matched to 67 noncontact patients by age, sex and BMI. Additionally, the 67 patients with contact ACL injury were propensity score matched to 67 patients with intact ACL from a cohort of 150 patients who had intact ACL for which PTS was measured. The intraclass correlation coefficient (ICC) between reader 1 and reader 2 was determined to be moderate with an absolute agreement of 0.504.

Patient demographics

There were no significant differences between contact and non-contact cohorts with respect to age (28.7± 6.3 vs. 27.1±6.5, p = 0.147), sex (Female: 36.0% vs. 34.3%, p = 0.858), or BMI (26.7± 5.6 vs 26.1±3.4, p = 0.475). (Table 1).

Table 1 Cohort Demographics
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However, when comparing the demographics of the non-contact and contact ACL cohort to the group of patients intact ACLs with an ANOVA analysis there was a significant difference in age (27.1±6.5 vs. 28.7± 6.3 vs. 42.0±16.7, p < 0.001) and a chi square analysis determined that there was a significant difference in sex between cohorts (Female: 36.0% vs. 34.3% vs. 46.3%, p = 0.040).

PTS angle analysis

There was no significant difference between PTS angle between contact versus non-contact ACL injury patients (11.6±3.0 vs.11.6±2.8, p = 0.894). Furthermore, a logistic regression analysis for an association between contact versus non-contact injury status and PTS angle which controlled for age, sex and BMI as covariates was not significant (β = − 0.010, p = 0.866). Additionally, a regression analysis demonstrated no significant association between age (β = 0.046, p = 0.260), sex (β = − 0.353, p = 0.521) or BMI (β = 0.025, p = 0.663) with PTS angle.

However, an ANOVA analysis comparing the contact, non-contact and intact ACL cohorts demonstrated a significant difference with respect to PTS (11.6±3.0 vs.11.6±2.8 vs. 10.0±3.9, p = 0.008). Additionally, in a post hoc independent samples t test there was a significant difference in PTS between the contact ACL injury and the intact cohort (11.6±3.0 vs. 10.0±3.9, p = 0.010) and the non-contact ACL injury and the intact cohort (11.6±2.8 vs. 10.0±3.9, p = 0.010).

Furthermore, in a logistic regression model controlling for age, sex and BMI determined PTS was found to be a significant predictor of ACL tear (OR = 0.873, 95% CI: 0.785-0.972, p = 0.013).

Male versus female sub-analysis

There was no significant difference in PTS angle in men with non-contact versus contact ACL injury (11.5±2.8 vs. 11.9±3.0, p = 0.618). There was also no significant difference in PTS angle in women with non-contact versus contact ACL injury (11.5±3.4 vs. 11.0±3.0, p = 0.558). Among patients who had a contact ACL injury, there was no significant difference between men and women with respect to PTS (11.8±3.0 vs. 11.0±3.0, p = 0.271). For patients who had a non-contact ACL injury there was no significant difference between men and women with respect to PTS (11.5±2.8 vs. 11.6±3.4, p = 0.981).

There was no significant difference between PTS angle in the non-contact and intact ACL cohorts among male patients (11.6±2.8 vs. 10.1±4.2, p = 0.069) or female patients (11.6±3.4 vs. 9.9±3.6, p = 0.090). Males who sustained contact ACL injury had a higher PTS than males who had intact ACLs (11.9±3.0 vs. 10.1±4.2, p = 0.033). However, there was no significant difference in PTS for females who sustained contact ACL injury and those with intact ACLs (11.8±3.0 vs. 10.0±4.2, p = 0.237).

Discussion

The principal finding of the present study was that there was no significant difference in the degree of PTS angle between patients with contact and non-contact ACL tears who were matched by age, sex, and BMI. The hypothesis is therefore rejected, namely, that patients with non-contact ACL injuries would have increased PTS relative to contact ACL injury patients. Though it was conjectured that the inherent anatomic risk factor of an increased PTS would play a more important role in the mechanism of non-contact ACL injuries, given these injuries are often associated with pivoting and jumping rather than the high energy collisions associated with contact ACL injuries, the results of this study do not support this conclusion.

The primary analysis in the current study found no difference in PTS in patients with contact versus non-contact ACL injuries. The mean PTS of the contact cohort was found to be 11.5±2.8; while, the mean angle for the non-contact cohort was found to be 11.9±3.0. These values fall well within the 0–18° range previously reported for adults of comparable age [17, 18]. Only one prior study has made a similar investigation, DePhillipo et al., which compared lateral PTS in patients with contact and noncontact ACL injuries measured on MRI [12]. They conducted a retrospective analysis of 56 contact ACL injury patients matched by age, sex and BMI to 56 non-contact ACL injury patients. They demonstrated no significant difference in lateral PTS angle between the contact and non-contact cohort (9.1 ± 2.9 vs. 9.9 ± 3.0). These results are in agreement with the findings of the present study, which also found no significant difference in PTS between contact and non-contact ACL injury patients. Additionally, Hudek et al. found that the mean lateral PTS reading was 3.4° smaller on MRI scans compared with lateral knee radiographs which supports the finding in the present study of higher PTS angle measurements than those found in DePhillipo et al. [12, 19] Furthermore, the small discrepancy in measurement of PTS between DePhillipo et al. and the present study is likely attributable to differing methods of measuring PTS. While DePhilipo measured lateral PTS, in the present study PTS was measured using the two circle method by Haseshemi et al. which measures a composite angle of the medial and lateral plateau slopes [15]. As the medial plateau PTS has been shown to be higher than the lateral plateau PTS this could be another explanation for the slightly higher PTS values observed in the present study [20]. Therefore, based on the available evidence, it appears that increased PTS does not preferentially lead to non-contact ACL injuries when compared to contact ACL injuries.

PTS was also found to not be significantly different between male and female cohorts. Existing evidence is mixed with regards to the interplay between PTS, ACL injury mechanism, and sex. Hohmann et al. conducted an analysis of 316 non-contact ACL injury matched to 316 patients without ACL injury [9]. They found that there was a significantly higher PTS among those with non-contact ACL injuries than uninjured controls. These findings are in agreement with the present study which found a significant difference in PTS between the non-contact ACL injury cohort and the intact control ACL cohort. Furthermore, Hohmann et al. found that PTS was significantly higher in the non-contact ACL cohort than the intact ACL cohort when analyzing male patients and female patients separately. These findings are in disagreement with those in the present study which did not find significant differences between the non-contact and intact ACL cohorts when male and female patients were analyzed separately. On the other hand, the present study only found that male patients with contact injury had higher PTS than intact controls. However, it is highly likely that the comparison between female patients was underpowered to detect a significant difference and suffered from type II error given the relatively small cohort sizes. Additionally, DePhillipo et al. found no significant differences in the degree of lateral PTS between males and females in the noncontact and contact patient groups [12]. This finding is in agreement with the present study which also found no difference in PTS between males and females in either the contact ACL injury or non-contact ACL injury cohorts. Therefore, similar to the general population, PTS does not appear to preferentially increase non-contact ACL injury risk compared to contact ACL injury risk based on sex. Increased PTS still may however increase the risk of obtaining a non-contact ACL injury, as suggested by the work by Hohmann et al.

Limitations

This study had several limitations. This was a retrospective review and as such had biases inherent to the methodology of the study including selection biases. Additionally, we relied on patient history documented in the chart to obtain the majority of the data in this study, including the classification of contact versus non-contact injury status, which possibly introduces recall bias. We cannot rule out the possibility that some patients may have recounted their injury mechanism inaccurately. Moreover, while this study did match cohorts based on age, sex, and BMI and control for these variables in regression analyses there are other covariates, including ethnicity, which have been shown to influence PTS but were not accounted for in the present study [20]. Lastly, while the present study was adequately powered to detect a statistically significant difference in PTS between the contact and non-contact cohorts there were many sub-analyses conducted in the present study which many have been underpowered and suffered from type II error.

Conclusion

There was no significant difference in PTS angle between patients with contact versus non-contact ACL injuries. The contact and non-contact ACL injury cohorts had significantly increased PTS when compared to patients with intact ACLs.