Novel mechanical impact simulator designed to generate clinically relevant anterior cruciate ligament rupture

INTRODUCTION: There is a need for laboratory-based landing simulators that can consistently induce anterior cruciate ligament (ACL) rupture and reproduce the distribution of injuries observed in the clinical setting.1 The objective of this investigation is to develop and analyze a novel impact test device that consistently induces ACL failure in a clinically relevant manner. It is hypothesized that the that improvements to external loads and muscle force application simulated about the knee joint would lead to ACL ruptures more consistent with the clinically observed patterns of ACL injury. METHODS: The novel mechanical impact simulator is a gravity-driven mechanical testing apparatus designed to generate impulse forces at the knee on lower extremity joints that represent the in vivo loading induced during jump landing. It was designed around two sleds on a minimal-friction, slide-rail track used to deliver an impact load to the foot of an inverted lower extremity specimen oriented in a position representative of initial ground contact during jump landing.1; 2 The inferior sled served as the ground, resting on the base of a specimen's foot; the superior sled was suspended 31 cm above by electromagnets. An electrical trigger released the suspended sled, allowing it to fall and impact the ground in alignment with the specimen's tibial shaft. Additionally, variable external tibial loads (anterior shear, internal rotation, knee abduction) and constant muscle forces (quadriceps, hamstrings) were applied about the knee via pneumatic actuators to simulate various degrees of relative injury risk loading (Figure 1). Pneumatic magnitudes were based on in vivo kinetics collected from a cohort of 68 athletes divided into no (1st percentile), low (33rd percentile), middle (67th percentile), and high (100th percentile) risk categories. Three cadaveric specimens (33.0 ± 13.1 years; 90.1 ± 28.1 kg) were subjected to a randomized series of sub-failure impacts of the presented external tibial loading profiles. Once sub-failure impacts were complete, failure testing consisted of increased external tibial loads in 20% increments from the 100th percentile until structural damage was incurred. During impact, knee joint loads were recorded by a 6 degree of freedom load cell aligned with the femoral shaft, while ACL and medial collateral ligament (MCL) strains were recorded by implanted strain gauges. An orthopedic exam was performed by a board certified orthopedic surgeon before and after impacts to confirm the functional integrity of each structure within the knee. Investigation methods were approved by Mayo Clinic IRB. RESULTS SECTION: All three specimens tested survived the randomized sub-failure protocol and sustained ACL rupture during failure protocol (Table 1; Figure 2). No additional structural damage was visually assessable; however, specimens did exhibit MCL medial laxity during clinical examination. The mean peak ACL strain prior to failure was 18.8 ± 6.2% with a range of 14.4-25.9%. ACL failure was confirmed with a Grade 3 Anterior Drawer and Lachman orthopedic tests in all specimens. At the time of ACL failure, the average peak MCL strain was 7.9 ± 5.9% with a range of 1.9%-13.8%. Leading up to failure, increases in ACL strain magnitude corresponded with increases in externally applied pneumatic loads (Figure 2). DISCUSSION: The hypothesis that the novel impact simulator would produce a more physiologically representative loading scenario that insights clinical modes of ACL rupture was supported. Anecdotal evidence from orthopedic surgeons indicate that the most frequently observed location of ACL rupture is at or near the femoral insertion. Intra-substance tears are the second most common location of clinical ACL ruptures, while patients less frequently present with tibial-sided tears. Therefore, injury locations in the present study resemble clinically observed patterns of disruption, which is a superior physiologic outcome compared to any previously investigated impact-based knee injury simulator.1 This result may be attributed to the use of dynamic pneumatic actuators that better represent in vivo physiological dampening as compared to the static suspended-weight employed by previous impact simulators.1-3 Presently reported failure strains are near identical to previous investigation (18.7 ± 10.0%).1 Presently reported MCL strains were well below the failure threshold of 17.1 ± 1.5%.4 The present ACL:MCL strain ratio of 2.4 is in agreement with previous literature that demonstrated ACL:MCL strain ratios of 1.7.2 This data exhibits how the ACL might be susceptible to rupture without concomitant MCL injury in 70% of cases.