Associations between frontal plane joint stiffness and proprioceptive acuity in knee osteoarthritis

Osteoarthritis (OA) of the knee is a debilitating musculoskeletal condition which involves the progressive loss of articular cartilage, development of osteophytes, joint malalignment, and muscle atrophy. The pathological alterations in the joint are associated with a number of biomechanical and neurological impairments, such as muscle weakness, joint laxity, proprioceptive deficits, and pain, which contribute to disease-associated functional limitations, and ultimately, disability (1). In a vicious cycle, these physical impairments may lead to further progression of the disease. For example, cartilage loss and subsequent joint space narrowing result in ligament insertion sites moving closer together, often causing increased joint laxity, particularly in the frontal plane of the knee (2). This may lead to abnormal movement of the tibia with respect to the femur, causing detrimental stresses on the articular cartilage (2). The decrease in passive joint stability potentially places a greater burden on the musculoskeletal system to maintain joint stability and prevent harmful loading. In this context, enhancing joint proprioception may aid in developing a motor control strategy to compensate for changes in joint mechanics. However, as proprioception relies on sensory feedback from specialized mechanoreceptors located in muscle, tendon, ligament, and skin (3), it has been suggested that proprioceptive deficits in knee OA may be associated with the disease-related changes in joint laxity (4). The exact nature of this association is not well understood. To our knowledge, only two studies have reported correlations between joint laxity and proprioception in a knee OA population, though in each case, this association was not the primary outcome of the study. Both Pai et al. (5) and van der Esch et al. (6) reported no significant correlation between varus-valgus joint laxity and proprioception in knee flexion-extension. A potential limitation of these analyses, however, was that each metric was assessed in a different plane of movement and presumably targeted different joint structures (knee flexion-extension primarily engages the muscles, while frontal plane rotation targets the collateral ligaments and capsule). As such, a lack of correlation is perhaps not surprising and the contribution of joint laxity to proprioceptive acuity in knee OA remains largely unknown. While knee joint proprioceptive acuity has typically been measured in the sagittal plane (knee flexion/extension), we have recently reported on proprioceptive capabilities in the frontal plane of the knee (7–8). The experimental apparatus allows for characterization of both proprioceptive acuity and passive joint stiffness in the same plane of movement. Unlike the aforementioned studies (5–6), this allows for a more direct assessment of the association between proprioception and the intrinsic mechanics of the joint. Accordingly, the purpose of this study was threefold: first, to assess frontal plane proprioceptive acuity in patients with moderate knee OA and healthy age-matched control participants; secondly, to quantify frontal plane passive joint stiffness in each group; and, thirdly, to delineate the associations between frontal plane proprioceptive acuity and joint stiffness. Similar to previous reports, we hypothesize that both proprioceptive acuity (4–6, 9–11) and passive stiffness (2, 12) will be reduced in knee OA participants compared with the control group. Further, we hypothesize that decreased proprioceptive acuity will be associated with reduced joint stiffness in the knee OA population. While it has been proposed that these two disease-related impairments may be associated (4), this will be the first study to provide quantitative assessments of these metrics in the same plane of movement. In this context, our goal is to examine the relative contribution of passive joint stiffness to the proprioceptive abilities in the OA population. Exploring these potential interactions will help elucidate the neuro-mechanical contributions to knee OA and may provide a basis for the continued improvement of therapies and treatments in OA.