Toward Computationally Optimized Orthopedics: Creating and Improving Finite Element Models to Inform Clinical Orthopedics Practice
Creators: name: Rosario, Ryan
Publisher: name: My University
Year: 2022
Subjects: subject: Tissue mechanics
subject: Finite elements
subject: Orthopedics
subject: Anterior cruciate ligament
subject: Articular cartilage
subject: Mechanical Engineering
subject: FOS: Mechanical engineering
subject: Engineering

description: This work advances the application of finite element (FE) models to study interventions for knee orthopedic injuries. This is accomplished in two ways: (1) current FE methods are leveraged to study osteochondral allograft transplantation (OAT), and (2) limitations of current FE methods for modeling the anterior cruciate ligament (ACL) are addressed. Articular cartilage and the ACL are the focus of this work because they are the most frequently injured knee tissues. OAT is a surgical procedure used to treat articular cartilage injuries where a graft of cadaveric articular cartilage and subchondral bone is implanted into a prepared articular cartilage injury site. While able to alleviate symptoms associated with injury, OAT suffers from high failure rates. These high failure rates may be caused by thickness mismatches between graft and native articular cartilage. This work examines how transplanted patellar geometry affects the size of the stress concentrations caused by cartilage thickness mismatches. Three key geometric parameters that affect the size of the stress concentrations in articular cartilage are identified from 3D models of transplanted patellae: recipient cartilage thickness, the donor-to-recipient cartilage thickness ratio, and cartilage surface angulation. The physiologic ranges of each of these variables are quantified using 3D models, and the effects of these variables were studied in 2D models. Ultimately, a scorecard is proposed to estimate the size of the region in a transplanted patella with stresses that are high enough to kill chondrocytes and precipitate graft failure. Recommendations from FE studies for orthopedics clinical practice currently need to be verified in follow-up in vitro, animal, or clinical studies because the inherent limitations of FE methods. The remaining studies in this work address some of these limitations, working toward a future where the findings from FE studies may be more directly applied to orthopedics clinical practice. The implications of the following ACL mechanical properties on knee FE models are examined to address some of these current limitations: the mechanical behavior of the noncollagenous matrix, spatially heterogeneous anisotropy, and spatially heterogeneous in situ strain. The mechanical behavior of the noncollagenous matrix has not been quantified for most knee ligaments. To enable solution of the FE model, researchers input an assumed value for the stiffness of the noncollagenous matrix. Based on data from other ligaments, these assumed values are likely orders of magnitude stiffer than the real behavior. This work demonstrates that these assumptions reduce knee joint motion and alter cartilage deformation patterns, introducing systematic errors in knee FE models. In addition to inaccurate models of the noncollagenous matrix, FE models of the ACL use a single stiff fiber direction rather than the spatially heterogeneous fiber direction found in real tissues. Incorporating this spatially heterogeneous fiber direction in an FE model reduces the apparent stiffness of the tissue and recapitulates experimentally observed deformation patterns. Lastly, to fill in current data gaps, the in situ strain of the ACL is quantified in physiologic knee positions. The in situ strain of the ACL is spatially heterogeneous and flexion-angle dependent. Surprisingly, the ACL exhibits fiber-direction tensile strains in positions where the tissue is slack. Incorporating accurate models of the noncollagenous matrix, spatially heterogeneous fiber direction, and in situ strain of the ACL into knee FE models would enable more accurate predictions of knee joint motions and tissue deformations.