The Effect of Articular Cartilage Focal Defect Size and Location in Whole Knee Biomechanics Models

Articular cartilage focal defects are common soft tissue injuries potentially linked to osteoarthritis (OA) development. Although several defect characteristics likely contribute to osteoarthritis, their relationship to local tissue deformation remains unclear. Using finite element models with various femoral cartilage geometries, we explore how defects change cartilage deformation and joint kinematics assuming loading representative of the maximum joint compression during the stance phase of gait. We show how defects, in combination with location-dependent cartilage mechanics, alter deformation in affected and opposing cartilages, as well as joint kinematics. Small and average sized defects increased maximum compressive strains by approximately 50% and 100%, respectively, compared to healthy cartilage. Shifts in the spatial locations of maximum compressive strains of defect containing models were also observed, resulting in loading of cartilage regions with reduced initial stiffnesses supporting the new, elevated loading environments. Simulated osteoarthritis (modeled as a global reduction in mean cartilage stiffness) did not significantly alter joint kinematics, but exacerbated tissue deformation. Femoral defects were also found to affect healthy tibial cartilage deformations. Lateral femoral defects increased tibial cartilage maximum compressive strains by 25%, while small and average sized medial defects exhibited decreases of 6% and 15%, respectively, compared to healthy cartilage. Femoral defects also affected the spatial distributions of deformation across the articular surfaces. These deviations are especially meaningful in the context of cartilage with location-dependent mechanics, leading to increases in peak contact stresses supported by the cartilage of between 11% and 34% over healthy cartilage.

Local Versus Global Mechanical Effects of Intramural Swelling in Carotid Arteries

Glycosaminoglycans (GAGs) are increasingly thought to play important roles in arterial mechanics and mechanobiology. We recently suggested that these highly negatively charged molecules, well known for their important contributions to cartilage mechanics, can pressurize intralamellar units in elastic arteries via a localized swelling process and thereby impact both smooth muscle mechanosensing and structural integrity. In this paper, we report osmotic loading experiments on murine common carotid arteries that revealed different degrees and extents of transmural swelling. Overall geometry changed significantly with exposure to hypo-osmotic solutions, as expected, yet mean pressure-outer diameter behaviors remained largely the same. Histological analyses revealed further that the swelling was not always distributed uniformly despite being confined primarily to the media. This unexpected finding guided a theoretical study of effects of different distributions of swelling on the wall stress. Results suggested that intramural swelling can introduce highly localized changes in the wall mechanics that could induce differential mechanobiological responses across the wall. There is, therefore, a need to focus on local, not global, mechanics when examining issues such as swelling-induced mechanosensing.

Effects of Cartilage Constitutive Model on Specimen-Specific Validation and Predictions of Cartilage Mechanics in the Human Hip

The initiation and progression of hip osteoarthritis (OA) may be predicted by mechanical factors [1]. Contact stress (CS), maximum shear stress (MSS) at the osteochondral interface and first principal strain (FPS) at the articular surface have been identified as parameters that alter the physical integrity and metabolism of cartilage [1]. Although these parameters are difficult to measure in-vivo, they can be predicted using finite element (FE) models. However, the reliability of model predictions and the effects of model assumptions are largely unknown. Direct validation of FE models against experimental measurements for a series of specimens shows the reliability of predictions across specimen geometry [1], although to date this has only been performed for a single hip [2].

Finite Element Predictions of Cartilage Contact Mechanics in Hips With Retroverted Acetabula

Acetabular retroversion, defined by the crossover sign (COS) on plain film radiographs, (Figure 1) is thought to cause early onset osteoarthritis (OA). When the COS is present, the anterior acetabular rim is lateral to the posterior acetabular rim proximally. As the rim progresses distally, the posterior rim crosses lateral to the anterior rim, creating the COS. Clinical studies have demonstrated that hips with OA have a higher incidence of acetabular retroversion than normal hips [1–3]. There are two possible mechanical links between acetabular retroversion and OA. First, decreased area in the posterior acetabulum may cause abnormally high cartilage contact stresses in the posterior acetabulum during activities of daily living. Alternatively, pincer femoroacetabular impingement may cause posterior damage via posterior subluxation [4–6]. Although clinical studies suggest that retroverted hips have altered cartilage mechanics, cartilage mechanics cannot be measured in-vivo. However, finite element (FE) modeling can be used to predict mechanics, and thereby identify possible mechanical mechanisms of OA development in patient populations. Therefore, the objective of this study was to compare cartilage contact mechanics between normal hips and hips with acetabular retroversion using a validated approach to subject-specific FE modeling [7].

Cartilage Contact Mechanics Following Treatment of Acetabular Retroversion With Peri-Acetabular Osteotomy: A Case Study

Acetabular retroversion is a form of dysplasia where the acetabulum is tilted excessively in the anterior plane with a loss of posterior coverage. Excessive anterior coverage may cause impingement, and reduced acetabular coverage may increase cartilage contact pressures in the posteroinferior region, thereby causing hip osteoarthritis (OA). Treatment of retroversion is controversial: patients receive debridement of the anterior acetabular rim or posteriorly directed reorientation of the acetabulum via peri-acetabular osteotomy (PAO). Improved methods to quantify pre- and post-operative cartilage mechanics could be used to standardize treatment.