Motion: Sagittal plane motion runs parallel to this plane, generally in a forward, backward, up and/or down direction – Imagine being trapped in a narrow hallway. To our knowledge, only one study has reported frontal plane joint stiffness in addition to joint laxity in the knee OA population ( 25 ). Consistent with our results, it was found that mid-range stiffness, which correlates to neutral stiffness in the present study, was decreased in the OA population ( 25 ). Yet, unlike the current study results, Creaby et al ( 25 ) found no significant differences in end-range stiffness estimates between knee OA and control populations.
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.
While we believe that we were directly targeting the medial and lateral ligamentous structures of the joint, the overall joint reaction moment measured is also a function of bony congruence (i.e. osteophytes) and other connective tissues ( 25 ). The contribution of individual tissues to joint stiffness/laxity cannot be assessed from these macroscopic measurements of the joint and there is currently not a viable method to isolate the contribution of ligamentous tissues in vivo.
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.
A decreased contribution of the ligamentous-capsular tissues, as indicated by reduced joint stiffness, may place a greater burden on the musculoskeletal system to maintain stability and prevent damaging loading on the articular cartilage ( 2 ). Evidence suggests that people with knee OA use co-contraction of the quadriceps and hamstrings to compensate for varus/valgus laxity.
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.
Creaby et al ( 25 ) performed testing with the knee at 20° flexion, which engages the joint soft tissues differently than at a neutral flexion/extension angle used in the current study ( 27 ). Further, Creaby et al ( 25 ) utilized a torque-based joint rotation, estimating end-range stiffness over the last 25% of movement, whereas the current study estimated joint stiffness at set joint rotations.