E1771. Why is Osteochondrosis in Diarthrodial Joint More Common at Convex Than Concave Surfaces: A Matter of Morphology and Biomechanics
San Antonio Military Medical Center
San Antonio Military Medical Center; Uniformed Services University Health Sciences
Osteochondrosis, osteochondritis dissecans, osteochondral fractures, and subchondral insufficiency fractures are commonly seen in clinical practice. Observationally, they occur more commonly in the convex surface than in the concave surface. The goal of this educational exhibit is to provide an explanation for this tendency.
Educational Goals / Teaching Points
The presentation will review definitions of osteochondral pathology and common locations for osteochondrosis, osteochondral fractures, and subchondral insufficiency fractures. The anatomy, morphology, and biomechanics of the diarthrodial joint are reviewed to highlight convex surface predisposition to osteochondral injury.
Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
Osteochondritis dissecans is a commonly used misnomer since osteochondritis implies an inflammatory process of bone and cartilage, which is not present. The word “dissecans,” from the word “dissect,” is only appropriate when the subchondral fragment is separated or loose. A more appropriate term is osteochondrosis, a disease of bone and cartilage growth centers in children characterized by degeneration or aseptic necrosis followed by regeneration and reossification. Osteochondrosis most commonly affects the knee, elbow, and ankle, in descending incidence. Osteochondral fracture involves the cartilage and the underlying subchondral bone. Subchondral insufficiency fracture occurs in subchondral bone, typically in the femoral head or femoral condyle, mainly in elderly, obese, or osteoporotic women with underlying osteoarthrosis or meniscal pathology. Freiberg infraction is now thought to be subchondral insufficiency fracture of the metatarsal head. A diarthrodial or movable joint consists of convex and concave articulating bones. The opposing joint surfaces are subjected to different stresses under load. The force applied to a convex joint surface is largely a compression phenomenon. The force applied to a concave joint surface is largely a tension phenomenon, driving the constituent parts away from each other. Over time, the subchondral bone of the concave surface is thicker than that of the convex surface. Underlying cancellous bone is also different: fine honeycomb structure underlies convex joint surfaces while coarser structure with dense bone lies perpendicular to concave articular surfaces. Cartilage of concave surfaces is also slightly thicker and stiffer than that of convex surfaces.
Osteochondrosis, osteochondral fractures, and subchondral insufficiency fractures in diarthrodial joints affect convex surfaces more often than concave surfaces due to the inherent morphology and biomechanics of the different surfaces. Concave surfaces have thicker subchondral bone, denser underlying cancellous bone, and thicker and stiffer cartilage. Consequently, when an acute traumatic event occurs, the convex surface is more likely than the concave surface to fail. In patients with underlying osteoarthritis or meniscal pathology, subchondral insufficiency fractures also occur more frequently at convex surfaces. Armed with this knowledge, radiologists can train their eyes to improve sensitivity in lesion detection.