ESTABLISHING METHODS FOR MEASURING MENISCUS DISPLACEMENTS IN-VIVO

Introduction: The meniscus plays a critical role in transmitting axially loaded forces while mitigating excessive stresses and strains on the underlying articular cartilage of the knee. Meniscus tears disrupt its mechanical abilities, leading to altered joint loading, joint instability and, if left untreated, increase one’s risk of developing osteoarthritis [1]. The current standard treatment is surgical repair. While most meniscus repair surgeries are highly successful, some studies have shown repair failure rates upwards of 20% [2,3]—which may, in part, be attributed to the lack of consensus on post-operative rehabilitation protocols [4]. Several studies have shown that early weight-bearing and mobilization leads to better patient outcomes [4,5]. However, studies have also shown that it can compromise the repair integrity and potentially lead to repair failure [4,5]. While there have been several mechanical studies in cadaveric samples, meniscus mechanics under load in-vivo has remained largely unstudied. In this study, we have addressed this gap by fabricating an in-vivo magnetic resonance imaging (MRI) compatible knee loading device to quantify meniscus mechanics under physiologic loading conditions. The objective of this study was to establish methods for measuring meniscus displacements in healthy adults. Establishing methods in healthy adults will 1) provide a baseline of typical meniscus mechanics and 2) lay the groundwork for future studies evaluating the mechanics of repaired menisci in the surgical population.

Materials and

Methods: After receiving IRB approval, the medial and lateral menisci of N=3 healthy adults (aged 32  11.1 years old, 1 female, 2 male) were evaluated using a custom MRI-compatible knee loading device with capability of applying loads up to 500N at flexion angles between 0 and 90 (Fig. 1). Each subject’s foot was placed in a modified orthopedic boot affixed to a foot plate with an embedded load cell. Load was applied by turning a threaded rod which moved the foot plate, applying a compressive force on the knee. Subject’s knees were loaded under 10% and 50% bodyweight at 0 and 30 flexion. Proton-density weighted turbo spin-echo (PD-TSE) MRIs were acquired at each load and angle condition. Meniscus displacements were calculated using image registration between loads, at each flexion angle. Additionally, as a measure of joint compression, cartilage strain was calculated by taking bone-to-bone measurements at the central contact region, as well as at a non-contact region, at each flexion angle.

Results, Conclusions, and Discussions: In all subjects the medial and lateral menisci extruded slightly (Fig. 2). On average, the displacement of the medial meniscus was 0.84 mm  0.43 mm at 0 flexion and 0.56 mm  0.30 mm at 30 flexion. The average displacement of the lateral meniscus was 0.60 mm  0.28 mm at 0 flexion and 0.39 mm  0.16 mm at 30 flexion. The average cartilage strain in the central contact region was -6.58%  0.86 and -7.03%  0.79 at 0 and 30, respectively (Fig. 3). The average cartilage strain in the non-contact region was -0.11  0.05 and -0.35%  0.32 at 0 and 30, respectively (Fig. 3).
While we only observed small displacements of both lateral and medial menisci, the cartilage strain measurements indicate that joints were experiencing relatively significant compression. At 0 flexion, cartilage strains on the order of -6% is typical of what would be expected in-vivo. The increased strain at 30 flexion suggests further compression of the joint. The small meniscus displacements, while under physiologically relevant loads, can be attributed to the healthy population tested in this study. All subjects in this study were relatively young, active and have no history of any kind of knee injury. Because of these factors we can assume that their menisci, as well as the other tissues in the knee, are strong enough to resist large displacements under 50% bodyweight. Nonetheless, while this study found small displacements in a healthy population, it has provided proof of principle for the further study of surgically repaired meniscus tears.
In summary, this study demonstrated the use of an in-vivo MRI-compatible knee loading device that is capable of applying physiologically relevant compressive loads at varying angles. Future work is aimed at quantifying meniscus mechanics in the surgical population who have received meniscus repair surgery.

Acknowledgements (Optional): REFERENCES
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