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

Neo-natal castration leads to subtle differences in porcine anterior cruciate ligament morphology and function in adolescence

Female adolescent athletes are at a higher risk of tearing their anterior cruciate ligament (ACL) than male counterparts. While most work related to hormones has focused on the effects of estrogen to understand the increased risk of ACL injury, there are other understudied factors, including testosterone. The purpose of this study was to determine how surgical castration in the male porcine model influences ACL size and function across skeletal growth. Thirty-six male Yorkshire crossbreed pigs were raised to 3 (juvenile), 4.5 (early adolescent), and 6 months (adolescent) of age. Animals were either castrated (barrows) within 1-2 weeks after birth or were left intact (boars). Post-euthanasia, joint and ACL size were assessed via MRI, and biomechanics were assessed via a robotic testing system. Joint size increased throughout age, yet barrows had smaller joints than boars (p<0.001 for all measures). ACL cross-sectional area (CSA), length, volume, and stiffness increased with age (p<0.0001), as did ACL anteromedial (AM) bundle percent contribution to resisting loads (p=0.012). Boar ACL, AM bundle, and PL bundle volumes were 19% (p=0.002), 25% (p=0.003), and 15% (p=0.04) larger than barrows across ages. However, CSA, stiffness, and bundle contribution were similar between boars and barrows (p>0.05). The barrows had smaller temporal increases in AM bundle percent function than boars, but these data were highly variable. Thus, early and sustained loss in testosterone leads to subtle differences in ACL morphology, but may not influence measures associated with increased injury risk, such as CSA or bundle forces in response to applied loads.

Age- and Sex-Specific Joint Biomechanics in Response to Partial and Complete Anterior Cruciate Ligament Injury in the Porcine Model

CONTEXT

Pediatric anterior cruciate ligament (ACL) injury rates are increasing and are highest in female adolescents. Complete ACL tears are typically surgically reconstructed, but few guidelines and very limited data exist regarding the need for surgical reconstruction or rehabilitation for partial ACL tears in skeletally immature patients.

OBJECTIVE

To evaluate the effects of partial (anteromedial bundle) and complete ACL transection on joint laxity and tissue forces under anterior and rotational loads in male and female stifle joints throughout skeletal growth in the porcine model.

DESIGN

Descriptive laboratory study.

SETTING

Laboratory.

PATIENTS OR OTHER PARTICIPANTS

We studied 60 male and female Yorkshire crossbreed pigs aged 1.5, 3, 4.5, 6, and 18 months (n = 6 pigs per age per sex).

MAIN OUTCOME MEASURE(S)

Joint laxity was measured in intact, anteromedial bundle-transected, and ACL-transected joints under applied anterior-posterior drawer and varus-valgus torque using a robotic testing system. Loading of the soft tissues in the stifle joint was measured under each condition.

RESULTS

Anterior-posterior joint laxity increased by 13% to 50% (P < .05) after anteromedial bundle transection and 75% to 178% (P < .05) after ACL transection. Destabilization after anteromedial bundle transection increased with age (P < .05) and was greater in late female than late male adolescents (P < .05). In anteromedial bundle-transected joints, the posterolateral bundle resisted the anterior load. In ACL-transected joints, the medial collateral ligament (MCL) contribution was largest, followed by the medial meniscus. The MCL contribution was larger and the medial meniscus contribution was smaller in male versus female specimens.

CONCLUSIONS

Partial ACL transection resulted in moderate increases in joint laxity, with the remaining bundle performing the primary ACL function. Destabilization due to partial ACL transection (anteromedial bundle) was largest in late adolescent joints, indicating that operative treatment should be considered in active, late-adolescent patients with this injury. Increased forces on the MCL and medial meniscus after ACL transection suggested that rehabilitation protocols may need to focus on protecting these tissues.

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.

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.