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Objectives: Sesamoid bones, such as the patella, are found within tendons in regions that wrap around bony prominences and are often implicated in the pain and disability associated with osteoarthritis. To investigate the etiology and progression of osteoarthritis in sesamoids, we must understand the factors that regulate cartilage maintenance in sesamoids. In this study, we elucidate the role of mechanics on bone development and cartilage maintenance in sesamoids. Sesamoid bones form by the ossification of cartilages that develop in utero. Sesamoid cartilages are formed after tendon continuity is established. Prior to sesamoid cartilage formation, geometric conformity between the tendon and its articulating contact surface exists. However, nonconformity between sesamoid and articulating surface arises naturally during growth. As development progresses, the sesamoid cartilage begins ossification via one or more nuclei within its interior.3 These nuclei rapidly coalesce and spread outward while preserving a layer of cartilage at the articular surface.3
Methods: We used 2-D finite element analysis to investigate the behavior of a simple sesamoid cartilage model. The geometry was based on a sesamoid cartilage embedded within a fibrous tendon2 that wraps around a bony articulation, such as the distal femur. Patterns of cartilage ossification and maintenance were predicted using an ossification index that was previously used to predict epiphyseal ossification.1,4 We investigated if loading history and joint nonconformity contribute to the formation of multiple ossification nuclei and to the preservation of a cartilage layer at the articular surface of sesamoids.
Results: For a fully conforming joint, the ossification index did not distinguish between regions of bone formation in the sesamoid substance and a layer of preserved cartilage at the articular surface. For a nonconforming joint, the ossification index predicted regions in the sesamoid substance where ossific nuclei normally appear and a region where cartilage is maintained at the articular surface.
Conclusions: We found that a simple mechanical loading history applied to a tendon containing an embedded sesamoid cartilage in a conforming joint does not predict regions of bone formation in the sesamoid substance and cartilage maintenance at the articular surface. In nonconforming joints, however, the loading history predicts patterns of cartilage ossification and maintenance that are observed in sesamoids in vivo. The ossification index predicts a diffuse ossification stimulus within the interior of the sesamoid cartilage. These findings suggest that the ossification stimulus is not localized and may be favored in multiple regions within the sesamoid, except near the articular surface where a cartilage layer is preserved. Furthermore, our results suggest that joint nonconformity leads to a thicker cartilage layer at the articular surface of the sesamoid at least early in development. Joint nonconformity or poor joint tracking, however, may lead to cartilage degeneration and osteoarthritis later in life. Our findings could improve techniques for the prevention or rehabilitation of cartilage damage in patients with osteoarthritic joints.
References: 1) Carter and Wong (1988) JOR 6:804; 2) Giori et al. (1993) JOR 11:581; 3) Ogden (1984) Skel Rad 11:246; 4) Stevens (1997) Ph.D. Thesis, Stanford University.
Acknowledgments: Department of Veterans Affairs Rehabilitation R&D Center (Palo Alto, CA, USA), Stanford Graduate Fellowships, and National Science Foundation.