Development and validation of a kinematically-driven discrete element model of the patellofemoral joint☆
Introduction
Osteoarthritis (OA) of the patellofemoral (PFJ) joint has recently gained significant attention as a major debilitating musculoskeletal disease, with studies showing incidence rates as high as 32% in individuals over the age of 50 (Hinman et al., 2014, Stefanik et al., 2013). Despite the high incidence of patellofemoral OA, the pathophysiology of this disorder remains unclear; one accepted and well-documented precursor to patellofemoral OA is patellofemoral pain (Thomas et al., 2010). Currently, one commonly accepted hypothesis for development and progression of patellofemoral pain is altered loading at the cartilage and subchondral bone levels (Powers et al., 1999, Wyndow et al., 2016). Altered PFJ loads have been found to be most directly caused by poor patella malalignment and tracking (Cahue et al., 2004, McWalter et al., 2007, Ward and Powers, 2004, Worlicek et al., 2017) that result in elevated PFJ stress (Fulkerson and Buuck, 2004, Heywood, 1961, Insall et al., 1976, Moller et al., 1989, Outerbridge, 1961).
Measuring the changes in the loading environment of the PFJ has been limited to experimental, in-vitro cadaveric tests (Ahmed et al., 1987, Haut, 1989, Huberti and Hayes, 1984, Lee et al., 1994, Lee et al., 2003). The goal of these studies was to understand the impact of altered patellofemoral alignment and tracking on patellofemoral contact mechanics. In such studies, the contact stress profile across the joint is measured to provide insight for developing improved treatments to limit pain and prevention of OA. Unfortunately, no current method exists to provide direct quantitative evidence of joint contact stress profiles in-vivo. To address this knowledge gap, recent advancements in numerical approximation techniques have allowed for development of elaborate subject-specific computational models to investigate the internal PFJ contact environment.
Current models of the PFJ incorporate the use of finite element (FEA) techniques (Besier et al., 2005, Besier et al., 2008, Cohen et al., 2001), discrete element (DEA) methods (Elias et al., 2004a, Elias et al., 2004b, Elias and Cosgarea, 2006, Elias et al., 2004a, Elias et al., 2004b, Elias and Cosgarea, 2007, Elias et al., 2010), or utilize a combination of subject-specific musculoskeletal computer models with accurate in-vivo experimental data (Adouni et al., 2012, Fernandez and Hunter, 2005, Schmitz and Piovesan, 2016). Validation of these prior models has been limited to replicating experimental kinematics. Considering the advancements in biplanar fluoroscopy and other in-vivo imaging methods for collecting accurate kinematic data (Bey et al., 2008), displacement-driven models of the PFJ offer significant potential to quantitatively evaluate treatment in patients long-term if these models are validated.
The primary objectives of this study were: (1) to develop a three-dimensional (3D), subject-specific modeling framework for estimating PFJ contact stress utilizing the DEA method driven by highly accurate knee joint kinematics and (2) to validate the model-estimated PFJ contact stress distribution in cadaveric knees. Model validation was carried out by comparing predicted PFJ contact stress distributions with those obtained from in vitro experiments.
Section snippets
Methods
A DEA framework was developed to employ specimen-specific kinematics to estimate PFJ contact stress. The DEA modeling pipeline is described in three parts: (1) creation of subject-specific model geometry; (2) experimental collection of kinematics and contact mechanics; and (3) DEA model simulations and validation (Fig. 1).
Two fresh-frozen (stored at −18 °C) cadaveric knees (specimen #1: Age = 52 years, Sex = Male, Weight = 81.7 kg, Height = 1.80 m; specimen #2: Age = 46 years, Sex = Male,
Contact stress
Overall, locations of contact stress were visually similar between the experimental and computational contact stress distributions for both specimens (Fig. 4). The computational model predicted the change in stress distribution observed experimentally across the different joint positions. Both computational and experimental data determined the greatest peak stresses for the medial facet at the externally rotated positions and greatest stresses for the lateral facet at the internally rotated
Discussion
The objective of this study was to develop a subject-specific modeling framework employing the DEA method driven by knee joint kinematics to estimate PFJ contact mechanics. Additionally, a secondary objective was to validate this modeling framework by comparing DEA-computed contact stresses with those measured experimentally at multiple knee joint positions. The predicted stress distributions were qualitatively similar to the experimental pressure distributions, both in relative magnitude and
Acknowledgements
The project described was supported by the Pittsburgh Claude D. Pepper Older Americans Independence Center (P30 AG024827) and the National Institutes of Health (K12 HD055931). The content is solely the responsibility of the authors and the study sponsors had no role in study design, data collection, analysis, manuscript preparation, or decision making for manuscript submission.
Conflict of interest statement
The authors declare that there is no conflict of interest regarding the content of this article.
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The project described was supported by the Pittsburgh Claude D. Pepper Older Americans Independence Center through (Grant number P30 AG024827) and the National Institutes of Health (Grant number K12 HD055931).