Biomechanical Characterization of a New, Noninvasive Model of Anterior Cruciate Ligament Rupture in the Rat

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1 Biomechanical Characterization of a New, Noninvasive Model of Anterior Cruciate Ligament Rupture in the Rat Tristan Maerz, MS Eng 1, Michael Kurdziel, MS Eng 1, Abigail Davidson, BS Eng 1, Kevin Baker, PhD 1, Kyle Anderson, MD 1, Howard Matthew, PhD 2. 1 Beaumont Health System, Royal Oak, MI, USA, 2 Wayne State University, Detroit, MI, USA. Disclosures: T. Maerz: None. M. Kurdziel: None. A. Davidson: None. K. Baker: None. K. Anderson: None. H. Matthew: None. Introduction: The rate of posttraumatic osteoarthritis (PTOA) following anterior cruciate ligament (ACL) rupture has been reported as high as 90%. Animal models are important for characterizing physiological events following these injuries. The current standard for ACL injury in the rat is surgical transection, which fails to reproduce high-magnitude trauma and the resultant biologic events following ACL injury. The purpose of this study was to develop and characterize a new model of noninvasive rat ACL rupture to facilitate future PTOA studies. Methods: Lewis rats were euthanized and placed prone on a custom fixture attached to a materials testing system. To induce ACL rupture, both limbs underwent loading by the application of a compressive axial load to the tibia with the knee rested on a stage flexed to 100 (Fig 1A). After preloading and preconditioning, an axial displacement was applied to the tibia at four testing conditions: low and high speed (1 mm/s, 8 mm/s) and low and high displacement (2mm, 3mm). The four testing groups were: high-speed, high-displacement (HSHD); high-speed, low-displacement (HSLD); low-speed, low-displacement (LSLD), and low-speed, high-displacement (LSHD). 3D motion data was quantified using a four-camera motion capture system to track displacement of the tibia relative to the femur via markers placed in the greater trochanter, distal femoral metaphysis, tibial tubercle and distal tibia. Total and net motion of the proximal tibia relative to the distal femur was calculated individually in all three 2D planes (transverse, sagittal, and coronal) as well as total 3D motion. ACL rupture was identified by an audible pop, a drop in load and resultant anterior laxity (Fig 1B,C). Following mechanical loading, whole limbs were disarticulated at the hip joint to quantify the degree of joint laxity in a custom laxity testing fixture. Anterior-Posterior (AP) laxity was measured by applying 2 N axially to the tibia at 90 of knee flexion to induce anterior subluxation. Joint laxity was measured by displacement (mm) and compliance (mm/n) at 2 N, and results were compared between groups. Varus-valgus laxity was measured by applying a 1 N varus or valgus load at the distal tibia at 15 of knee flexion. Displacement (mm) at 1 N was compared between groups. Uninjured limbs were used as laxity controls. One-way ANOVA or Kruskall-Wallis tests were used to compare normal and non-normal variables between groups, respectively. Variables were also grouped by high-speed/low-speed, high-displacement/lowdisplacement, and rupture/non-ruptured. Results: ACL rupture occurred in 100% of rats in HSHD, 33.33% in HSLD, 55.56% in LSHD, and 0% in LSLD. Rupture incidence was significantly associated with both endpoint displacement (P < 0.001) and displacement speed (P = 0.019). Motion-capture data indicates that there are profound differences in joint motion between groups even if rupture occurs (Table 1). HSHD induced the highest amount of transverse, coronal, sagittal, and overall 3D motion. Motion-capture data also shows that total motion

2 after peak load in the coronal plane (medial-lateral and superior-inferior motion) was significantly higher in specimens that ruptured (2.58 mm ± 1.65) compared to specimens that did not rupture (1.25 mm ± 0.97, P = 0.031). There was significantly higher net displacement in the transverse plane (medial-lateral and anterior-posterior motion) in ruptured specimens (2.15 mm ± 0.87) compared to non-ruptured specimens (1.45 mm ± 0.69, P = 0.034). High-speed groups had a significantly more motion after peak load in the coronal plane (2.59 mm ± 1.64) compared to low-speed groups (1.35 mm ± 1.1, P = 0.033). Comparing high- and low-displacement groups, there was a significant difference in total 3D motion (High-displacement: 7.17 mm ± 1.29; low-displacement: 5.19 mm ± 2.38, P = 0.008), 3D motion after peak load (high-displacement: 2.84 mm ± 1.57; low-displacement: 1.70 mm ± 1.19, P = 0.036), motion after peak load in the coronal plane (high-displacement: 2.69 mm ± 1.66; low-displacement: 1.22 mm ± 0.88, P = 0.014), total motion in the coronal plane (high-displacement: 6.58 mm ± 1.37; lowdisplacement: 4.41 mm ± 2.27, P = ), motion after peak load in the transverse plane (highdisplacement: 2.74 mm ± 1.49; low-displacement: 1.43 mm ± 1.04, P = 0.016), total motion in the transverse plane (high-displacement: 4.70 mm ± 1.07; low-displacement: 3.20 mm ± 1.76, P = 0.013), as well as total motion in the sagittal plane (high-displacement: 5.46 mm ± 0.92; low-displacement: 3.72 mm ± 1.76, P = 0.002). Only HSHD exhibited a significant increase in AP and varus laxity compared to uninjured limbs, with corresponding decreases in respective compliance (Table 2). Rupture did not induce valgus laxity in any group. There was a significant difference in AP laxity between high- and lowspeed groups (High-Speed: mm ± 0.456; Low-speed: mm ± 0.61; P = 0.026) and between high- and low-displacement groups (High-displacement: mm ± 0.53; Low-displacement: mm ± 0.42; P = 0.003). There was a significant difference in varus laxity between high- and low-displacement groups (High-displacement: 3.29 mm ± 0.93; Low-displacement: 2.55 mm ± 1.03; P = 0.031) but not highand low-speed groups (High-speed: 3.25 mm ± 0.99; Low-speed: 2.59 mm ± 0.99; P= 0.064). Valgus laxity did not differ as a function of displacement or speed. Discussion: The application of an axial tibial load via the ankle in 100 of knee flexion is able to induce a noninvasive ACL rupture in the rat with concomitant varus laxity. The incidence of ACL rupture is highly dependent upon both speed and endpoint displacement, with endpoint displacement appearing statistically more determinative of rupture. ACL rupture is more repeatable in the high-speed, highdisplacement protocol, and both AP and varus laxity are induced due to ACL rupture. Varus laxity suggests lateral collateral ligament (LCL) injury, a concomitant injury commonly observed in humans in combination with ACL rupture. Motion capture data indicates that joint motion after peak load is most prevalent in the coronal and transverse planes, and this motion is significantly higher in ruptured specimens. This additional motion after rupture may represent translation, rotation, or both, and since LCL laxity is only increased in ruptured specimens, LCL laxity is likely being induced by this post-rupture motion. A clinically-relevant noninvasive ACL rupture model is useful for future studies assessing biologic events after ACL rupture, such as the onset and progression of PTOA. Future studies will utilize this model to assess degenerative changes in the rat joint. Significance: A clinically-relevant noninvasive ACL rupture protocol has been developed and characterized. Since ACL transection does not reproduce high-magnitude joint trauma and introduces confounding variables due to surgical cutting, this noninvasive ACL rupture protocol is able to more closely mimic human biology after ACL rupture.

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5 ORS 2015 Annual Meeting Poster No: 1193