Characterisation of size and direction of osteophyte in knee osteoarthritis: a radiographic study. (Extended Report).

Objectives: To examine the size and direction of osteophyte in knee osteoarthritis (OA) and to determine associations between osteophyte size and other radiographic features.

Methods: Knee radiographs (standing extended anteroposterior and 30 degrees flexion skyline views) were examined from 204 patients referred to hospital with symptomatic knee OA (155 women, 49 men; mean age 70, range 34-91 years). A single observer assessed films for osteophyte size and direction at eight sites; narrowing in each compartment; varus/valgus angulation; patellofemoral subluxation; attrition; and chondrocalcinosis using a standard atlas, direct measurement, or visual assessment. For analysis, one OA knee was selected at random from each subject.

Results: Osteophyte direction at the eight sites was divisible into five categories. At all sites, except for the lateral tibial plateau and the medial patella, osteophyte direction varied according to (a) the size of osteophyte and (b) the degree of local narrowing. At the medial femur, medial tibia, and lateral femur osteophyte direction changed from being predominantly horizontal to predominantly vertical with increasing size. The size of osteophyte correlated positively with the severity of local narrowing, except for the medial patellofemoral compartment where osteophyte size correlated positively with the severity of narrowing in the medial tibiofemoral compartment. Logistic regression analysis showed that osteophyte size was associated not only with local narrowing but also with local malalignment and bone attrition, and that chondrocalcinosis was positively associated with osteophyte size at multiple sites.

Conclusion: In patients referred to hospital with knee OA different patterns of osteophyte direction are discernible. Osteophyte size is associated with local compartmental narrowing but also local alignment and attrition. Chondrocalcinosis is associated with osteophytosis throughout the joint. These data suggest that both local biomechanical and constitutional factors influence the size and direction of osteophyte formation in knee OA.

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Osteoarthritis (OA) is the most prevalent form of arthritis. (1) It shows a strong association with aging and selective targeting of certain joints such as the knee. (1) A variety of genetic, constitutional and environmental risk factors for OA are recognised, which vary according to joint site. (2) The defining radiographic features of OA are (a) focal cartilage loss, resulting in "joint space narrowing" and (b) accompanying endochondral ossification at the joint margins that produces "marginal osteophyte". (1 3)

Although osteophyte is viewed as a remodelling and reparative feature of OA, the factors that determine osteophyte formation and growth are unknown. Growth factors influence both chondrocyte synthesis and osteophyte formation in experimental joint damage, (4 5) and evidence from animal (6) and human studies (7-9) shows that cartilage damage initiates "secondary" osteophyte growth. However, osteophyte may also develop as an isolated feature associated with age (10) and precede rather than follow cartilage loss in animal studies. (11) Joint instability has been emphasised as a biomechanical trigger to osteophyte formation, with osteophyte and bone remodelling being viewed as an attempt to stabilise and broaden the compromised joint to better withstand loading forces. (12 13) Chondrocalcinosis due to calcium pyrophosphate crystals has also been suggested to be associated with a tendency to osteophyte formation and a "hypertrophic" form of OA. (14) Possibly, therefore, multiple factors may influence osteophyte formation and contribute to the marked heterogeneity of OA.

Assessment of individual radiographic features is the main outcome measure for evaluating structural changes in OA. (15 16) There is little information, however, on the morphology of knee osteophyte seen on radiographs. The present study aimed at (a) describing the size and direction of osteophyte in the OA knees of a group of patients in hospital and (b) assessing possible associations between osteophyte size and other factors visible on standard knee radiographs that may influence osteophyte growth.

PATIENTS AND METHODS

Approval for the study was obtained from the local research ethics committee.

Patients and radiographs

Routine radiographs of patients seen for symptomatic knee OA (new and follow up) over a nine month period in a hospital rheumatology clinic were examined for the study. No patient had coexisting inflammatory arthropathy as determined by clinical inquiry, examination, and limited laboratory and radiographic investigation. Radiographic knee OA was defined as the presence of joint space narrowing and osteophyte in any knee compartment. No radiograph showing patellectomy or joint replacement was included. All radiographs were obtained under standardised conditions and included (a) weightbearing, full extension anteroposterior views (55 kV, 8 mA/s, full scale deflection 100 cm; Kodak film) and (b) skyline 30 degrees flexion views …