Biomechanical Function and Size of the Anteromedial and Posterolateral Bundles of the ACL Change Differently with Skeletal Growth in the Pig Model

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

Pediatric anterior cruciate ligament (ACL) injuries are on the rise, and females experience higher ACL injury risk than males during adolescence. Studies in skeletally immature patients indicate differences in ACL size and joint laxity between males and females after the onset of adolescence. However, functional data regarding the ACL and its anteromedial and posterolateral bundles in the pediatric population remain rare. Therefore, this study uses a porcine model to investigate the sex-specific morphology and biomechanics of the ACL and its bundles throughout skeletal growth. Hind limbs from male and female Yorkshire pigs aged early youth to late adolescence were imaged using magnetic resonance imaging to measure the size and orientation of the ACL and its bundles, then biomechanically tested under anterior-posterior drawer using a robotic testing system. Joint laxity decreased (p < 0.001) while joint stiffness increased (p < 0.001) throughout skeletal growth in both sexes. The ACL was the primary stabilizer against anterior tibial loading, while the functional role of the anteromedial bundle increased with age (p < 0.001), with an earlier increase in males. ACL and posterolateral bundle cross-sectional area and ACL and anteromedial bundle length were larger in males than females during adolescence (p < 0.01 for all), while ACL and bundle sagittal angle remained similar between sexes. Additionally, in situ ACL stiffness versus cross-sectional area regressions were significant across skeletal growth (r²  = 0.75, p < 0.001 in males and r²  = 0.64, p < 0.001 in females), but not within age groups. This study has implications for age and sex-specific surgical intervention strategies and suggests the need for human studies.

Age- and sex-specific differences in ACL and ACL bundle size during adolescent growth

Anterior cruciate ligament (ACL) injuries are increasingly common in adolescents, and injuries in this age-group are associated with many unique challenges. Recent large animal studies suggest that the size and function of the major bundles of the ACL change differently throughout skeletal growth. To better aid clinical treatment of pediatric partial ACL tears and better predict outcomes from age-specific treatments, there is a need to measure changes in ACL bundle size in humans during growth. As such, the objective of this study was to compare changes in the length and cross-sectional area (CSA) of the ACL and its primary bundles in adolescent human subjects. Magnetic resonance imaging (MRI) scans were analyzed to determine the visibility and integrity of the ACL and its anteromedial and posterolateral bundles. MRI scans were considered from a retrospective database of subjects ranging from 10 to 18 years of age. The ACL and its anteromedial and posterolateral bundles were segmented and reconstructed into 3D models, and length and CSA were calculated. Total ACL length and CSA were greater in males compared with females, with a statistically significant interaction between age and sex for CSA. Sex had a significant effect on the CSA of both bundles. These sex-dependent differences emerge with moderate to large effect sizes (range: d = 0.50 to d = 1.23) beginning around 13 years of age. Along with ACL bundle structure-function relationships previously established in preclinical animal models, these findings may point toward biomechanical changes in the adolescent human ACL.

Anterior cruciate ligament remnant-preserving and re-tensioning reconstruction: a biomechanical comparison study of three different re-tensioning methods in a porcine model

BACKGROUND

With the developments in the arthroscopic technique, anterior cruciate ligament (ACL) remnant-preserving reconstruction is gradually gaining attention with respect to improving proprioception and enhancing early revascularization of the graft. To evaluate the mechanical pull-out strength of three different methods for remnant-preserving and re-tensioning reconstruction during ACL reconstruction.

METHODS

Twenty-seven fresh knees from mature pigs were used in this study. Each knee was dissected to isolate the femoral attachment of ACL and cut the attachment. An MTS tensile testing machine with dual-screw fixation clamp with 30° flexion angle was used. The 27 specimens were tested after applying re-tensioning sutures with No. 0 polydioxanone (PDS), using the single stitch (n = 9), loop stitch (n = 9), and triple stitch (n = 9) methods. We measured the mode of failure, defined as (1) ligament failure (longitudinal splitting of the remnant ACL) or (2) suture failure (tearing of the PDS stitch); load-to-failure strength; and stiffness for the three methods. Kruskal-Wallis test and Mann-Whitney U-test were used to compare the variance of load-to-failure strength and stiffness among the three groups.

RESULTS

Ligament failure occurred in all cases in the single stitch group and in all but one case in the triple stitch group. Suture failure occurred in all cases in the loop stitch group and in one case in the triple stitch group. The load-to-failure strength was significantly higher with loop stich (91.52 ± 8.19 N) and triple stitch (111.1 ± 18.15 N) than with single stitch (43.79 ± 11.54 N) (p = 0.002). With respect to stiffness, triple stitch (2.50 ± 0.37 N/mm) yielded significantly higher stiffness than the other methods (p = 0.001).

CONCLUSIONS

The results suggested that loop stitch or triple stitch would be a better option for increasing the mechanical strength when applying remnant-preserving and re-tensioning reconstruction during ACL reconstruction.

Joint laxity varies in response to partial and complete anterior cruciate ligament injuries throughout skeletal growth

Anterior cruciate ligament (ACL) injuries are increasingly common in the skeletally immature population. As such there is a need to increase our understanding of the biomechanical function of the joint following partial and complete ACL injury during skeletal growth. In this work, we aimed to assess changes in knee kinematics and loading of the remaining soft tissues following both partial and complete ACL injury in a porcine model. To do so, we applied anterior-posterior tibial loads and varus-valgus moments to stifle joints of female pigs ranging from early juvenile to late adolescent ages and assessed both kinematics and in-situ loads carried in the bundles of the ACL and other soft tissues including the collateral ligaments and the menisci. Partial ACL injury led to increased anterior tibial translation only in late adolescence and small increases in varus-valgus rotation at all ages. Complete ACL injury led to substantial increases in translation and rotation at all ages. At all ages, the medial collateral ligament and the medial meniscus combined to resist the majority of applied anterior tibial load following complete ACL transection. Across all ages and flexion angles, the contribution of the MCL ranged from 45 to 90% of the anterior load and the contribution of the medial meniscus ranged from 14 to 35% of the anterior load. These findings add to our current understanding of age-specific functional properties of both healthy and injured knees during skeletal growth.

In Situ Joint Stiffness Increases During Skeletal Growth but Decreases Following Partial and Complete Anterior Cruciate Ligament Injury

Partial and complete anterior cruciate ligament (ACL) injuries occur in both pediatric and adult populations and can result in loss of joint stability and function. The sigmoidal shape of knee joint function (load-translation curve) under applied loads includes a low-load region (described by slack length) followed by a high-load region (described by stiffness). However, the impact of age and injury on these parameters is not fully understood. The current objective was to measure the effects of age and injury on the shape of joint function in a porcine model. In response to an applied anterior-posterior tibial load, in situ slack did not change (p > 0.05), despite sevenfold increases in joint size with increasing age. Joint stiffness increased from an average of 10 N/mm in early youth to 47 N/mm in late adolescence (p < 0.05). In situ ACL stiffness increased similarly, and changes in in situ joint stiffness and ACL stiffness were highly correlated across ages. With complete ACL injury, in situ slack length increased by twofold to fourfold and in situ stiffness decreased threefold to fourfold across ages (p < 0.05). Partial ACL injury resulted in less dramatic, but statistically significant, increases in joint slack and significant decreases in in situ joint stiffness in the adolescent age groups (p < 0.05). This work furthers our understanding of the interaction between joint biomechanics and ACL function throughout growth and the impact of ACL injury in the skeletally immature joint.