Relationship of Anterior Cruciate Ligament Volume and T2* Relaxation Time to Anterior Knee Laxity

Abstract

Background: High anterior knee laxity (AKL) has been prospectively identified as a risk factor for anterior cruciate ligament (ACL) injuries. Given that ACL morphometry and structural composition have the potential to influence ligamentous strength, understanding how these factors are associated with greater AKL is warranted. Hypothesis: Smaller ACL volumes combined with longer T2* relaxation times would collectively predict greater AKL. Study Design: Cross-sectional study; Level of evidence, 3. Methods: College-aged active male (n = 20) and female (n = 30) participants underwent magnetic resonance imaging (MRI) and AKL testing. T2-weighted MRI scans were used to assess ACL volumes, and T2* relaxation times were used to assess ACL structural composition. AKL was measured via a commercial knee arthrometer. Forward stepwise linear regression with sex and weight (first step; suppressor variables) as well as ACL volume and T2* relaxation time (second step; independent variables) was used to predict AKL (dependent variable). Results: After initially adjusting for sex and weight ( R 2 = 0.19; P = .006), smaller ACL volumes combined with longer T2* relaxation times collectively predicted greater AKL ( R 2 = 0.52; P < .001; R 2 Δ = 0.32; P Δ < .001). A smaller ACL volume was the primary predictor of greater AKL ( R 2 Δ = 0.28; P < .001), with a longer T2* relaxation time trending toward a significant contribution to greater AKL ( R 2 Δ = 0.04; P = .062). After adjusting for ACL volume and T2* relaxation time, sex (partial r = 0.05; P = .735) and weight (partial r = 0.05; P = .725) were no longer significant predictors. Conclusion: AKL was largely predicted by ACL volume and to a lesser extent by T2* relaxation time (and not a person’s sex and weight). These findings enhance our understanding of how AKL may be associated with a structurally weaker ACL. The current study presents initial evidence that AKL is a cost-effective and clinically accessible measure that shows us something about the structural composition of the ACL. As AKL has been consistently shown to be a risk factor for ACL injuries, work should be done to continue to investigate what AKL may tell a clinician about the structure and composition of the ACL.

Sex‐specific biomechanics and morphology of the anterior cruciate ligament during skeletal growth in a porcine model

“…In fact, a recent study in humans found that sex and weight did not impact anterior knee laxity after accounting for ACL volume and T2* relaxation time. 40 Therefore, evaluating the effect of underlying factors, such as bundle-specific properties and in situ slack and stiffness, on ACL injury risk may help us better understand age and sex-dependent injury rates. Although not measured in this study, there is also evidence that bony anatomical features, 13,14 neuromuscular biomechanics, [41][42][43] sex hormones, [44][45][46] and oral contraceptive use [46][47][48] impact ACL injury risk and likely age and sex-dependent injury rates.…”

“…However, this difference in joint laxity is small in magnitude and likely depends on several underlying factors including ACL and bundle size, orientation, and mechanical properties. In fact, a recent study in humans found that sex and weight did not impact anterior knee laxity after accounting for ACL volume and T2* relaxation time 40 . Therefore, evaluating the effect of underlying factors, such as bundle‐specific properties and in situ slack and stiffness, on ACL injury risk may help us better understand age and sex‐dependent injury rates.…”

Quadriceps muscle volume positively contributes to ACL volume

“…Smaller ACLs have also been observed in the contralateral knee of those who have torn their ACL compared to healthy matched controls 10–12 . Smaller ACLs have been associated with less linear stiffness, 13,14 lower load at failure, 13,14 and greater anterior knee laxity 15 . In turn, greater anterior knee laxity is associated with a greater risk of ACL injury, particularly in females 16,17 .…”

“…[10][11][12] Smaller ACLs have been associated with less linear stiffness, 13,14 lower load at failure, 13,14 and greater anterior knee laxity. 15 In turn, greater anterior knee laxity is associated with a greater risk of ACL injury, particularly in females. 16,17 Understanding the factors that contribute to interindividual and intersex variability in ACL size, and subsequently lead to a smaller and weaker ACL in females, may lead to novel prevention strategies aimed at improving the resiliency of the ACL and reducing injury risk.…”

Methods for improved kinematic measurements of the lower extremities

“…Additionally, such kinematic metrics may be useful for screening uninjured athletes for ACL injury risk 43 . It has been found that smaller ACLs are associated with less linear stiffness 10 , lower load at failure 10 , and greater anterior knee laxity 65 . As those with greater knee laxity have been associated with a greater ACL injury risk 58 , objective and noninvasive metrics could be used in preventative training protocols.…”