908 RDIOLOGIC VIGNETTE OSTEOPHYTOSIS OF THE FEMORL HED ND NECK DONLD RESNICK Osteophytes are frequently considered the most characteristic abnormality of degenerative joint disease. In patients with osteoarthritis, osteophytes in the femoral head and neck are usually combined with asymmetric joint space narrowing, subchondral cysts, and bony sclerosis, and are accompanied by similar abnormalities in the acetabulum. Occasionally, osteophytosis of the femoral head and neck may indicate a disease process other than osteoarthritis, such as ankylosing spondylitis. Osteophytes, representing osseous excrescences, result from either of two types of bone formation: endochondral bone formation, in which cartilage lays down bone, or intramembranous bone formation, in which the periosteal membrane produces bone. Most typically, osteophytes arise as a revitalization or reparative response by remaining cartilage, following vascular invasion and erosion of the subchondral bone plate and calcified cartilage, with deposition of osseous tissue on the eroded surface. s such, they are most prominent in areas of the articulation that are subjected to low stress; at these sites, portions of the cartilaginous articular coat are still present, a requirement of endochondral bone formation. Therefore, the largest osteophytes are usually marginal, or peripheral, in distribution (Figure 1). s it grows, the osteophyte buries a portion of From the Department of Radiology, V Medical Center, San Diego, California and the Department of Radiology, University of California, San Diego. Supported by V Grant #7406. ddress reprint requests to Donald Resnick, MD, Chief, Radiology Service, V Medical Center, 3350 Ldolla Village Drive, San Diego, C 92161. Submitted for publication January 18, 1983; accepted in revised form February 1, 1983. the original zone of calcified cartilage, which can be recognized on radiographic and pathologic evaluation. lternatively, osteophytes may develop as a result of intramernbranous bone formation in which the synovia1 membrane, the intraarticular counterpart of the periosteal membrane, is stimulated to produce new osseous tissue. With regard to the femoral head and neck, osteophytes can be classified according to the following scheme (1,2): 1. Marginal (epiarticular) osteophyte: the outgrowths begin peripherally at the edge, or margin, of the femoral head, or centrally at the margin of the fovea capitis. They may enlarge at either site, but typically, the most prominent marginal osteophyte is apparent on the medial surface of the femoral head (Figure 1). In osteoarthritis, the growing ledge of bone in this latter location fills in the free space on the medial aspect of the articulation that was created by the narrowing of the superior, or upper, joint space. In some instances the large size of the medial femoral osteophyte, combined with the flattening of the superior surface of the femoral head, produces a deformed proximal femur in which the femoral head is no longer situated symmetrically on the femoral neck. tilt deformity is created, due to the prominence of the medial portion of the femoral head (3) (Figures 2 and 3). Some investigators have interpreted the tilt deformity as indicative of a previous epiphysiolysis (4), but generally, this is not the case (Figure 4). Rather, the deformity is a direct result of a remodeling process in the osteoarthritic femoral head, with bone resorption laterally and bone deposition medially. Correct interpretation of the nature of the tilt deformity is aided by the rthritis and Rheumatism, Vol. 26, No. 7 (July 1983)
Figure 1. Marginal (epiarticular) osteophyte in osteoarthritis. Radiograph () and photograph () of coronal section of the proximal femur reveal a large marginal osteophyte on the medial aspect of the femoral head (closed arrows). The osteophyte has buried a portion of the original zone of calcified cartilage (arrowheads). Observe cortical thickening. or buttressing. on the medial portion of the femoral neck (open arrows). subchondral cysts. and sclerosis. Figure 2. Tilt deformity in osteoarthritis. Radiograph of the hip indicates diffuse loss of joint space (closed arrows). large osteophyte on the medial aspect of the femoral head (lower open arrow) can be identified by the presence of a faint curvilinear radiodense line (arrowheads), representing the original zone of calcified cartilage. The femoral head is no longer situated normally on the femoral neck. cetabular osteophytes (upper open arrow) and buttressing (curved arrow) are seen. 909
Figure 3. Tilt deformity in osteoarthritis. radiograph of the coronal section of the femoral head (illustrated in Figure 2) reveals the original zone of calcified cartilage (open arrowheads), the medial femoral osteophyte (open arrows), and buttressing or cortical thickening of the medial portion of the femoral neck (curved arrow). Subchondral cysts (closed arrowhead) are seen. Figure 4. Possible previous epiphysiolysis with secondary osteoarthritis. radiograph () and photograph () of a macerated femur indicate that the femoral head is abnormally situated with respect to the femoral neck. lthough some of this malposition is related to a medial femoral osteophyte (arrows), a true tilting of the femoral head is also present. These findings could relate to either a proximal slipped capital femoral epiphysis or coxa vara deformity. 910
Figure 5. Central (subarticular) osteophyte in osteoarthritis. photograph of a macerated coronal section of the femoral head in a patient with osteoarthritis demonstrates widespread osteophytosis (closed arrows) which is developing by reduplication and shifting of the cartilage-bone junction. calcified zone of cartilage can be identified (arrowhead). marginal osteophyte (open arrow) is also seen. Figure 6. Femoral neck osteophyte in osteoarthritis. radiograph () and photograph () of a coronal section of the proximal femur demonstrate a prominent osseous excrescence protruding from the femoral neck (arrows). Marginal osteophytes on both the medial and lateral portions of the femoral head (open arrows), cyst formation, and sclerosis are seen. 91 1
912 RESNICK Figure 7. Rheumatoid arthritis with secondary osteoarthritis. radiograph () and photograph () of a macerated femur in a cadaver with rheumatoid arthritis demonstrate diffuse osseous erosions of the femoral head and osteophytes (arrows) at the femoral head-neck junction. identification of the original zone of calcified cartilage on radiographic examination. 2. Central (subarticular) osteophyte: the appearance of osteophytes in the central, or interior, portions of the femoral head is a less well recognized manifestation of osteoarthritis. Their pathogenesis is similar to that of marginal osteophytes, the bony outgrowths resulting from hypervascularity and stimulation of cartilaginous remnants in the central portion of the joint. The resulting excrescences are smaller than marginal osteophytes. They are typically flat, or button-like, in configuration (Figure 5). On occasion, a central osteophyte simulates an intraarticular osseous body on radiographic examination, as the site of femoral attachment of the outgrowth cannot be identified. The visualization of a well demarcated radiodense line, representing the original zone of calcified cartilage, at the base of the osteophyte is important in the correct interpretation of the radiographic image. Central osteophytes frequently lead to a bumpy or irregular articular surface. 3. Femoral neck osteophyte: in addition to marginal osteophytes occurring at the junction of the femoral head and femoral neck, osteophytes and plaques of new bone may develop further down the femoral neck, especially medially (Figures 1 and 6). This phenomenon, commonly termed buttressing, may be related to synovial and capsular inflammation and fibrosis, as well as intraarticular osteocartilaginous debris and adhesions (5). The periosteal membrane and its counterpart, the synovial membrane, lay down osseous tissue through the process of intramembranous bone formation, perhaps due to displacement of these structures from the femoral neck due to capsular traction. uttressing is not specific for osteoarthritis; it is also apparent in osteonecrosis, rheumatoid arthritis, ankylosing spondylitis, and congenital subluxation of the hip, as well as in the presence of an osteoid osteoma. lthough osteoarthritis is the most common disease leading to osteophyte formation in the femoral head and neck, other disorders can produce such outgrowths. In rheumatoid arthritis, synovial inflammation leads to diffuse loss of articular cartilage which, in most cases, prevents significant osteophyte
RDIOLOGIC VIGNETTE 913 Figure 8. Osteonecrosis of the femoral head. nteroposterior () and frog lateral () radiographic views indicate avascular necrosis characterized by osseous collapse, osteolvsis, and osteosclerosis. Note buttressing (arrowhead) of the femoral neck and limited marginal osteophyte formation (arrows). formation. However, in some patients with rheumatoid arthritis, secondary degenerative abnormalities are identified (Figure 7). In these individuals, the resulting osteophytes are generally small in size, although they can be marginal or central in location. Similarly, in osteonecrosis of the femoral head, secondary degenerative abnormalities may be evident, leading to osteophytes and bone formation in the femoral head and neck (Figure 8). These abnormalities, when severe, create problems in differential diagnosis; it may be difficult to decide if the primary disease process is osteonecrosis with secondary degenerative alterations or osteoarthritis with secondary collapse of the femoral head. bnormalities of the hip are not infrequent in ankylosing spondylitis. characteristic alteration of the femoral head, consisting of a focal area of new bone formation on its lateral aspect, can represent an important clue to the early diagnosis of hip involvement in this disease (6). This osseous bump is related to a small marginal osteophyte developing at the femoral head-femoral neck junction. With progression of the disease, a ring or collar of osteophytes may be seen at this junction. The resulting appearance of the outgrowths is similar to that in osteoarthritis, although the degree of bone formation is less, and a large medial femoral osteophyte is not apparent. Furthermore, in ankylosing spondylitis, intraarticular bony ankylosis may be evident. This ankylosis may be associated with an apparently intact joint space since the outline of the femoral head, representing the original zone of calcified cartilage that is buried by the new bone, can be identified. REFERENCES 1. Resnick D: Patterns of migration of the femoral head in osteoarthritis of the hip. JR 124(1):62-74, 1975 2. Jeffrey K: Osteophytes and the osteoarthritic femoral head. J one Joint Surg 57(3):314-324, 1975 3. Resnick D: The tilt deformity of the femoral head in osteoarthritis of the hip: a poor indicator of previous epiphysiolysis. Clin Radiol 27:355-363, 1976 4. Murray R: The aetioiogy of primary osteoarthritis of the hip. r J Radiol 38:810-824, 1965 5. Lloyd-Roberts GC: The role of capsular changes in osteoarthritis of the hip joint. J one Joint Surg 35:627-642, 1953 6. Dwosh IL, Resnick D, ecker M: Hip involvement in ankylosing spondylitis. rthritis Rheum 19:683-692, 1976