Remarkably, such a repair process occurred with time, although never demonstrably to normal hyaline cartilage. Marshall Urist (the Editor-in-Chief of Clinical Orthopaedics and Related Research from 1966–1993) demonstrated this process 50 years ago in histologic sections from a series of 15 patients [9] using a unique form of arthroplasty [8]. He noted two distinct forms of response: (1) progressive disintegration of the joint (typically in patients with posttraumatic and other forms of osteonecrosis; (2) progressive repair (typically in patients with old acetabular fractures or developmental conditions). The latter apparently required good collateral circulation, and progressed to fibrocartilage (Fig. 1) as had been described by Smith-Petersen [7]. That cartilage, he noted, was not normal hyaline cartilage, and rather had smaller cells (often paired presumably from recent divisions) and lacked the typical organizational features such as “chondrones” (a concept more recently popularized by Poole and colleagues [6] suggesting a coherent structure of the cell and surrounding matrix) and Benninghoff’s arcades of collagen fibers extending from the subchondral bone to the cartilage surface [1]. Contrary to common opinion at the time, this repair process occurring after a period of progressive deterioration suggests when submitted to a new mechanical environment cartilage does have a capacity to partially restore itself. Similar radiographic and histologic restoration naturally occurs spontaneously in rare cases of advanced hip osteoarthritis [4]. Urist’s work and that of others raises a question as to what conditions lead to durable restoration.

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