Objective As cartilage loss and bone marrow lesions (BMLs) are associated with knee joint pain and structural worsening, this study assessed whether non-invasive estimates of articular contact stress may longitudinally predict risk for worsening of knee cartilage morphology and BMLs. Design This was a longitudinal cohort study of adults aged 50-79 years with risk factors for knee osteoarthritis. Baseline and follow-up measures included WORMS classification of knee cartilage morphology and BMLs. Tibiofemoral geometry was manually segmented on baseline MRI, and 3D tibiofemoral point clouds were registered into subject-specific loaded apposition using fixed-flexion knee radiographs. Discrete element analysis (DEA) was used to estimate mean and peak contact stresses for the medial and lateral compartments. The association of baseline contact stress with worsening cartilage and BMLs in the same sub-region over 30 months was assessed using conditional logistic regression. Results Subjects (N=38, 60.5% female) had a mean±SD age and BMI of 63.5±8.4 years and 30.5±3.7 kg/m2 respectively. Elevated mean articular contact stress at baseline was associated with worsening cartilage morphology and worsening BMLs by 30-months, with OR (95%CI) of 4.0 (2.5, 6.4) and 6.6 (2.7, 16.5) respectively. Peak contact stress also was significantly associated with worsening cartilage morphology and BMLs {1.9 (1.5, 2.3) and 2.3 (1.5, 3.6)}(all p<0.0001). Conclusions Detection of higher contact stress 30 months prior to structural worsening suggests an etiological role for mechanical loading. Estimation of articular contact stress with DEA is an efficient and accurate means of predicting sub-region-specific knee joint worsening and may be useful in guiding prognosis and treatment.

Acute injury severity, altered joint kinematics, and joint incongruity are three important mechanical factors linked to post-traumatic osteoarthritis (PTOA). Finite element analysis (FEA) was previously used to assess the influence of increased contact stress due to joint incongruity on PTOA development. While promising agreement with PTOA development was seen, the inherent complexities of contact FEA limited the numbers of subjects that could be analyzed. Discrete element analysis (DEA) is a simplified methodology for contact stress computation, which idealizes contact surfaces as a bed of independent linear springs. In this study, DEA was explored as an expedited alternative to FEA contact stress exposure computation. DEA was compared to FEA using results from a previously completed validation study of two cadaveric human ankles, as well as a previous study of post-operative contact stress exposure in 11 patients with tibial plafond fracture. DEA-computed maximum contact stresses were within 19% of those experimentally measured, with 90% of the contact area having computed contact stress values within 1 MPa of those measured. In the 11 fractured ankles, maximum contact stress and contact area differences between DEA and FEA were 0.85±0.64 MPa and 22.5±11.5 mm2. As a predictive measure for PTOA development, both DEA and FEA had 100% concordance with presence of OA (KL grade ≥ 2) and >95% concordance with KL grade at 2 years. These results support DEA as a reasonable alternative to FEA for computing contact stress exposures following surgical reduction of a tibial plafond fracture.

Evaluation of abnormalities in joint contact stress that develop after inaccurate reduction of an acetabular fracture may provide a potential means for predicting the risk of developing post-traumatic osteoarthritis. Discrete element analysis (DEA) is a computational technique for calculating intra-articular contact stress distributions in a fraction of the time required to obtain the same information using the more commonly employed finite element analysis technique. The goal of this work was to validate the accuracy of DEA-computed contact stress against physical measurements of contact stress made in cadaveric hips using Tekscan sensors. Four static loading tests in a variety of poses from heel-strike to toe-off were performed in two different cadaveric hip specimens with the acetabulum intact and again with an intentionally malreduced posterior wall acetabular fracture. DEA-computed contact stress was compared on a point-by-point basis to stress measured from the physical experiments. There was good agreement between computed and measured contact stress over the entire contact area (correlation coefficients ranged from 0.88 to 0.99). DEA-computed peak contact stress was within an average of 0.5 MPa (range 0.2-0.8 MPa) of the Tekscan peak stress for intact hips, and within an average of 0.6 MPa (range 0-1.6 MPa) for fractured cases. DEA-computed contact areas were within an average of 33% of the Tekscan-measured areas (range: 1.4-60%). These results indicate that the DEA methodology is a valid method for accurately estimating contact stress in both intact and fractured hips.

viii cycle. Axial loading on the tibia is the subject body weight scaled according to the phase of gait. A spring is applied to the talus to represent the stabilization of the tibia.

Objective To determine the test-retest reliability of knee joint space width (JSW) measurements made using standing CT (SCT) imaging. Subjects and Methods This prospective two-visit study included 50 knees from 30 subjects (66% female; mean ±SD age 58.2 ± 11.3 years; BMI 29.1 ± 5.6 kg/m2; 38% KL grade 0–1). Tibiofemoral geometry was obtained from bilateral approximately 20° fixed-flexed SCT images acquired at visits two weeks apart. For each compartment, the total joint area was defined as the area with a JSW <10mm. The summary measurements of interest were the percent of the total joint area with a JSW less than 0.5mm thresholds between 2.0mm–5.0mm in each tibiofemoral compartment. Test-retest reliability of the summary JSW measurements was assessed by Intraclass Correlation Coefficients (ICC 2,1) for the percent area engaged at each threshold of JSW and root-mean-square errors (RMSE) were calculated to assess reproducibility. Results The ICC were excellent for each threshold assessed, ranging from 0.95–0.97 for the lateral and 0.90–0.97 for the medial compartment. RMSE ranged from 1.1–7.2% for the lateral and from 3.1–9.1% for the medial compartment, with better reproducibility at smaller thresholds of JSW. Conclusion The knee joint positioning protocol used demonstrated high day-to-day reliability for SCT 3D tibiofemoral JSW summary measurements repeated 2 weeks apart. Low-dose SCT provides a great deal of information about the joint while maintaining high reliability, making it a suitable alternative to plain radiographs for evaluating JSW in people with knee OA.