Bone mass in osteoarthritis.

Since 1970 a number of studies have noted that osteoarthritis of the hip is uncommon in patients with femoral neck fracture.l The negative association between osteoporosis and osteoarthritis has been the subject of a detailed review by Dequeker,s who painted an attractive picture of two very different groups, the osteoporotic, slender with osteopenia and little degenerative joint change, and the osteoarthritic, with above normal bone mass, more fat, greater muscle strength, and fewer fractures, though occasionally both conditions may be present in the same subject.6 Although osteoarthritic patients have more bone than osteoporotic patients, controversy still exists as to whether osteoarthritic patients have greater bone mass than normal subjects. Part of the controversy is due to selection of study and control groups. Two different study designs have been used, the 'population based' study and the 'patient based' study. The first method selects a large number of 'normal' subjects and separates them into 'osteoarthritic' or 'non-osteoarthritic', usually on the basis of radiological change in the hands, hips, or spine, and compares the two groups. The second method compares patients with osteoarthritis requiring either medical or surgical treatment with 'normal' people from other sources. The 'normal' group, however, may include people with asymptomatic radiological osteoarthritis.7 8 Thus study and control groups are different in the two types of investigation. In addition, bone mass may be measured at a number of sites containing differing amounts of trabecular and cortical bone, and over the past 20 years many different techniques have been used for measurement. These are not the only problems. Bone mass is affected by several factors, including age, weight, sex, menopausal age, and some clinical conditions and drugs. These factors have not always been taken into consideration, particularly in earlier studies. The effect of body weight is complex. Obesity appears to protect against osteoporosis either through the peripheral conversion of androstenedione to oestrone in adipose tissue9 or through increased loading, and is also associated with osteoarthritis.1o 0 1 The diagnosis of osteoarthritis is not always easy and there are many clinical variants. 12 The most recent definition of osteoarthritis included the words 'a heterogeneous group of conditions leading to the clinical syndrome of joint symptoms and signs'; and classification criteria ard still being perfected. 13 In general, two osteoarthritic subgroups have been studied: primary generalised osteoarthritis and osteoarthritis of the hip. Osteoarthritis of the hip In two of the earliest studies cortical bone mass was measured at the second metacarpal in osteoarthritic men and women and controls by radiographic morphometry.4 " Both studies showed increased bone mass in the osteoarthritic subjects, though one study'4 included tuberculous and psychiatric patients in the normal controls. In both studies large numbers of patients with severe osteoarthritis were studied (100 patients with osteoarthritis undergoing hip surgery,4 75 patients with primary osteoarthritis Kellgren's grade 3-4 at the hip).'4 Neither study took weight or menopausal status into account. In contrast, another large study of 105 men and women with 'advanced' osteoarthritis of the hip3 showed that these patients had bone mass at the metacarpal similar to that of normal subjects. Although 94% of the osteoarthritic patients were described as endomorphic mesomorphs, direct comparison of body weight was not made with controls. Menopausal age was not discussed, though all subjects were aged 55 or more. The trabecular pattern in the proximal femur has been classified by the Singh grade,'5 which is related to bone density.15 16 Radiographs of 314 men and women over 50 attending for radiology for non-skeletal problems were assessed for osteoarthritis ofthe hip by Kellgren's grading and bone density by Singh grade. ' There was a significant negative association between low bone density and osteoarthritis. However, the authors were unable to obtain body weights. Single photon absorptiometry is a useful technique for measuring bone mass in the appendicular skeleton, most commonly the distal radius comprising mainly trabecular bone, and the mid-radius, a predominantly cortical site, though the precise site on the radius used may vary. Bone mineral content (BMC, g/cm) may be measured or the results adjusted for skeletal size by dividing by bone diameter to give the bone mineral density (BMD, g/cm2); the technique is described in detail by Wahner. 8 Three studies of bone mass by this technique in the forearm of men and women with primary osteoarthritis of the hip gave conflicting results. One group of 53 patients 9 was scheduled for surgery and the other two groups of 31 and 29 patients respectively20 21 had osteoarthritis graded as Kellgren's 3 and 4. Women with osteoarthritis were shown to have a higher BMC and BMD at both cortical and trabecular sites in the radius in one study,20 but only the BMC at the predominantly trabecular site of the distal radius was increased in Rheumatology and Rehabilitation Research Unit, University of Leeds L D Hordon V Wright MRC Bone Mineralisation Unit, University of Leeds M A Smith