Knee joint kinematics with dynamic augmentation of primary anterior cruciate ligament repair
The current study biomechanically analyzed AP translational knee kinematics with dynamic augmentation of primary ACL repair in an environment simulating 50’000 gait cycles i.e. the early post-operative phase. AP translation was maintained very close to the immediate post-operative level for all flexion angles except for 30°, where an increase of 1.4 mm was measured. This is the first study assessing the course of AP translational knee laxity with dynamic augmentation of primary ACL repair subjected to dynamic loading.
There are no existing data on in-vivo ACL and ACL-graft forces during rehabilitation. We therefore limited our loading protocol to cyclic flexion-extension of 0°-70°-0° at 1 Hz occurring during walking, and 134 N AP translational force at the time-points and flexion angles of examination (Arnold et al. 2005; Kadaba et al. 1990). The cyclic motion protocol was designed to simulate the early post-operative phase. Preliminary data from a prospective randomized trial at the university hospital in Münster (Germany) showed that DIS patients wearing step counters performed on average 52’000 steps (i.e. 26’000 gait cycles) during the first 3 post-operative weeks (personal notice Dr. Schliemann). Given that physical activity of patients will increase over the period of rehabilitation, the 50’000 cycles, simulated in this study, represent an average post-operative rehabilitation period of 4 to 5 weeks.
The present study design represented a worst-case scenario with regard to load transmission through the implant system over time, considering a relatively high degree of flexion (70°) and exclusion of biological ACL healing. A further reason for the latter assumption was that no data on the mechanical strength of a healing ACL is available in the literature.
Our results best compare to those of Arnold et al., who tested bone-patellar-tendon-bone autografts preconditioned on a tension board for 20 min (Arnold et al. 2005). Knees were tested for 1500 flexion-extension cycles with 0°-70°-0° flexion and at cycles 0, 500, and 1500, knee laxity was measured under 90 N of anterior tibial force at 20° of flexion. Arnold et al. found that anterior laxity increased 1.3 mm after 500 cycles and 1.6 mm after 1500 cycles. Our testing of DIS showed an increase of 0.0 mm in 15° flexion and 1.4 mm in 30° flexion after 50’000 cycles.
Another study performed by Boguszewski et al. used a robotic system to apply 250 cycles of alternating anteroposterior and posteroanterior force of 134 N on ten human knees and then measured the increase in AP tibial translation (Boguszewski et al. 2015). The ACL was reconstructed either with bone-patellar-tendon-bone, bone-achilles-tendon, hamstring-tendon or tibialis tendon and different pretensioning protocols varying in load and duration were conducted. Average increases in AP translation ranged from 1.9 to 3.1 mm depending on the preconditioning and graft type and 75 % of the total increase occurred within the first 125 cycles. All values were thus well above the maximum increase of 1.4 mm we found in our investigation.
It is so not surprising that we recorded the highest values for AP translation in 15° and 30° as this is probably due to the generally reduced additional ligamentous constraint arising from reduced capsular and collateral ligament tension in slight flexion. However, a direct comparison of the dynamic augmentation technique to any type of ACL-reconstruction is associated with limitations because the former serves as an internal brace to protect the healing ACL whereas the latter substitutes the native ACL. AP translational knee laxity with ACL repair techniques finally depends on a stable scar tissue formation at the healing site whereas with ACL reconstruction remodeling of the graft plays a pivotal role (Heitmann et al. 2014; Janssen Scheffler 2014). Ideally, an augmentation would fully stressshield the ACL repair during the first days after surgery to provide a calm environment for fibrin clot formation and then continuously decrease in stiffness and therefore transfer back the load to the ACL in order to generate a stable scar tissue with longitudinal orientation of collagen and elastin fibres. However, load distribution between the two parallel systems “ACL repair” and “dynamic augmentation” as well as the amount of mechanical stimuli needed for optimal ACL healing are unknown and could be subject to future investigations. Nevertheless, our measurements show that dynamic augmentation is capable of supporting an ACL repair during the initial and very likely most important phase of biological healing.
The limitations of this study ARE similar to those inherent to all cadaveric studies. A limited number of specimens were used, thus restricting generalization to actual patients. In addition, degradation of the knee specimens was a concern, muscle tension was not present and therefore knee motion was uniquely passive and femoral and tibial tunnel positions during implantation were not assessed. However, drying-out of the specimens was prevented by leaving the joint surrounding soft tissue intact, including the skin. Paired knees of four donors were used resulting in a reduced statistical power and overall sample size as left and right knees of the same donor might show similar behavior. The translational knee laxity was measured with a clinically widely used instrument (Rolimeter) and, although the anteriorly directed force executed by the examiner was standardized to 134 N in order to reduce inter-tester variability, variability can still occur (Herbort et al. 2013; Loh et al. 2003; Petersen et al. 2007; Schliemann et al. 2015).
The strength of our study is that it replicated the clinical situation much better than a simple ex-situ uniaxial quasi-static loading test. The donor age of the specimens represented the young age of the typical patient population experiencing ACL ruptures. Bone quality, joint integrity and kinematics therefore highly represented a realistic scenario. Freezing and defrosting of the specimens does not have a relevant effect on the quality and on the mechanical properties of bone and ligament tissue (Linde Sørensen 1993; Woo et al. 1986). The intra-articular milieu was best replicated with artificial synovial fluid and Lachman/KT-1000 testing was simulated applying a standardized force of 134 N to reduce inter-tester variability. Instrumentation was performed by an experienced knee surgeon (PH).