Poster Q5 - The Acute Compressive Strain Response of Osteoarthritic Cartilage: An In Vivo Study

Introduction: Articular cartilage is a specialized connective tissue that functions as a biological bearing, supporting load and enabling low friction joint articulation. Previous studies have shown that acute in vivo loading of cartilage leads to compressive strain, while unloading leads to strain recovery (passive recovery). Recently, in a young asymptomatic cohort, we showed that walking, a loaded dynamic activity, was also capable of strain recovery. Surprisingly, the recovery rate during activity was more than 3 times faster than passive recovery. We refer to this novel recovery mode as active recovery. In this study we aimed to determine if active recovery is conserved in symptomatic osteoarthritis (OA). Given that OA is associated with a damaged collagen network and a reduction in proteoglycan content, we hypothesize that OA cartilage will exhibit larger compressive strains and slower recovery.

Materials and

Methods: In this study we recruited eight symptomatic OA participants, who met the following criteria defined by the National Institute for Health and Care Excellence (UK): age 45+, activity related knee pain, and no morning knee stiffness. All participants completed a Knee Injury and Osteoarthritis Outcome Score (KOOS) questionnaire for pain and symptoms, where 0 is extreme pain and dysfunction, and 100 is no pain or impact on daily life. The KOOS pain score ranged from 50 to 89 with the average being 69. Participants performed a fixed series of tasks shown in Fig 1. The order of the tasks was as follows: stand for 30 minutes (induce compressive strain), walk for 10 minutes (active recovery), stand for 30 minutes (induce compressive strain), and lie supine for 50 minutes (passive recovery). Following each task, the affected knee of each participant was imaged with a 3T magnetic resonance scanner using a sagittal plane proton density turbo spin echo sequence. The resulting voxel size was 0.34 x 0.34 x 1.5 mm. The scan duration was 3:43 min.
The compressive strain was calculated as the change in the tibio-femoral bone to bone distance relative to the first scan (Ref) in both the medial and lateral compartments. To establish the reliability and robustness of the method, we performed an intra-rater repeatability assessment. The results are shown in Fig 2 using a Bland-Altman plot. The bias ± standard deviation was found to be -0.12 ± 0.81% strain.

Results, Conclusions, and Discussions:

Results: In the OA group, the mean strain in the medial and lateral compartments following standing for 30 minutes = -11.8% and -7.9% strain, walking for 10 minutes = -12.1% and -5.7% strain, and lying supine for 50 minutes = -7.7 and -7.1% strain. When compared to our prior work in an asymptomatic cohort, OA led to significantly (p < 0.0003, Kruskal-Wallis) increased compressive strain across all conditions. In addition, several important trends can be observed. First, the OA cohort exhibited increased strain between the first and second period of standing (-9.8% vs -13.0% strain, mean of both compartments ), which is not consistent with the healthy cohort (-5.0% vs -5.1% strain), see Fig 3. Second, walking for OA partici-pants did not always lead to strain recovery, which is particularly evident in the medial compartment where on average, walking increased the compressive strain from -11.8% strain to -12.1% strain. Third, the passive recovery rates between the medial and lateral compartment appear to have mostly a linear recovery response, which is in agreement with the asymptomatic cohort, but the rate of recovery differs from the asymptoatic cohort (0.08%/min, mean of both compartments) and between OA compart-ments (0.11%/min medial and 0.0325%/min lateral). Fourth, there is much greater variance the compressive strain response between OA participants than asymptomatic participants.

Conclusion: This study shows that although walking is a form of active recovery for asymptomatic participants, this may not be true for those with OA. Depending on the severity of the OA, walking may actually add strain to the cartilage. In our future work will correlate each participant’s self reported function to their task specific compressive strain response.

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