Subchondral bone

The material in use as of the mid-1990s in these components is HDPE, a linear polymer which is tough, resiUent, ductile, wear resistant, and has low friction (see Olefin polymers, polyethylene). Polymers are prone to both creep and fatigue (stress) cracking. Moreover, HDPE has a modulus of elasticity that is only one-tenth that of the bone, thus it increases the level of stress transmitted to the cement, thereby increasing the potential for cement mantle failure. When the acetabular HDPE cup is backed by metal, it stiffens the HDPE cup. This results in function similar to that of natural subchondral bone. Metal backing has become standard on acetabular cups. 

Subchondral bone undergoes metabolic changes, including increased bone turnover, that appear to be precursors to tissue destruction. The normally contiguous bony surface becomes fissured. Persistent use of the joint eventually results in loss of cartilage, permitting bone-to-bone contact that ultimately promotes thickening and eburnation of exposed bone. Microfractures may appear in subchondral bone, and osteonecrosis may develop beneath the surface. 

Irregular thickening and remodeling of subchondral bone, with sclerosis and cysts... 

As the disease progresses, joint-space narrowing, subchondral bone sclerosis, and osteophytes may be detected. 

Radiology Radiography of the affected knee shows joint-space narrowing and subchondral bone sclerosis. 

Bugatti S, Caporali R, Manzo A, Vitolo B, Pitzalis C, Montecucco C. Involvement of subchondral bone marrow in rheumatoid arthritis lymphoid neogenesis and in situ relationship to subchondral bone marrow osteoclast recruitment. Arthritis Rheum 2005 52(ll) 3448-3459. 

Joint deformity may be present in the later stages as a result of subluxation, collapse of subchondral bone, formation of bone cysts, or bony overgrowths. 

Osteoarthritis proves to be a more complex disease than autoimmune disease, with multiple variable manifestations like knee, hip, hand, DIP, elbow, shoulder, and spinal joints OA, which have different risk factors. The etiology of OA is multifactorial with inflammatory, metabolic and mechanical causes. A number of personal and environmental risk factors, such as obesity, occupation, and trauma, may initiate various pathological pathways. OA comprises degeneration of articular cartilage together with changes in subchondral bone of the joint margins and mild intraarticular inflammation. 

Figure 11.9 Schematic of articular cartilage structure showing the superficial, middle and deep zones, the tidemark boundary between the noncalcified and calcified cartilage layer, and the subchondral bone that underlies the articular cartilage.

Iwata Y, Mort JS, Tateishi H et al (1997) Macrophage cathepsin L, a factor in the erosion of subchondral bone in rheumatoid arthritis. Arthritis Rheum 40 499-509... 

Figure 3.23. Diagram illustrating the zonal structure of articular cartilage. The superficial zone contains aligned collagen fibrils the intermediate zone contains unoriented collagen fibrils the deep zone contains collagen fibrils perperdicular to the subchondral bone.

A step towards organ culture represents the culture of full thickness cartilage with subchondral bone, cultured for 24 h on moist lens tissue (Chayen et al. 1994). 

Joint deformity may be present in the later stages as a result of subluxation, collapse of subchondral bone, formation of bone cysts, or bony overgrowths. Physical examination of the affected joints reveals tenderness, crepitus, and possible joint enlargement. Heberden s and Bouchard s nodes are bony enlargements (osteophytes) of the DIP and PIP joints, respectively. 

In the deep zone, type II collagen is ubiquitous and is oriented perpendicular to the cartilage surface. Although articular cartilage is unmineralized in its surface layers it contains a layer of calcified tissue adjacent to the subchondral bone. 

Fibrillation or splitting of the noncalcifled cartilage, likely related to the biochemical changes described earlier loss of cartilage exposes the underlying subchondral bone and may lead to microfractures... 

Radiographic changes often absent Progression of OA Joint space narrowing Subchondral bone sclerosis Marginal osteophytes Late OA... 

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