eMedicine Specialties > Radiology > Musculoskeletal

Osteoarthritis, Primary

Author: Gregory Scott Stacy, MD, Assistant Professor, Department of Radiology, University of Chicago Hospitals
Coauthor(s): Pat Auveek Basu, MD, MBA, Staff Physician, Department of Radiology, Stanford University Hospitals
Contributor Information and Disclosures

Updated: Mar 17, 2009

Introduction

Background

Osteoarthritis, the most common type of joint disease, is a heterogeneous group of conditions that result in common histopathologic and radiologic changes. It is a degenerative disorder that results from the biochemical breakdown of articular cartilage in the synovial joints. Although osteoarthritis is thought to be largely due to excessive wear and tear, secondary nonspecific inflammatory changes may also affect the joints. By definition, the etiology of primary osteoarthritis is unknown, but the pathology and pathogenesis of osteoarthritis have been extensively studied.^{1,2,3,4,5,6,7,8,9,10,11}

The definition of primary osteoarthritis is more nebulous. In the broadest sense of the term, primary osteoarthritis is an idiopathic phenomenon, occurring in previously intact joints, with no apparent initiating factor. Primary osteoarthritis is related to the aging process and typically occurs in older individuals. Some clinicians limit primary osteoarthritis to the joints of the hands (specifically the distal interphalangeal joints, proximal interphalangeal joints, and joints at the base of the thumb), whereas others include the knees, hips, spine (apophyseal articulations), and hands as potential sites of involvement. This article primarily focuses on osteoarthritis of the hand, knee, and hip joints.

The term primary or idiopathic osteoarthritis may become obsolete as underlying causes of osteoarthritis are discovered. For instance, many investigators believe that most cases of primary osteoarthritis of the hip may, in fact, be due to subtle or even unrecognizable congenital or developmental defects.

Presentation

Demographics

Age

Osteoarthritis occurrence appears to increase with patient age in a nonlinear fashion. Primary osteoarthritis is a common disorder of the elderly, and patients are often asymptomatic. Approximately 80-90% of individuals older than 65 years have evidence of primary osteoarthritis.¹² Patients with symptoms usually do not notice them until after they are aged 50 years. The prevalence of the disease increases dramatically after the age of 50 years, likely because of age-related alterations in collagen and proteoglycans that decrease the tensile strength of the joint cartilage and because of diminished nutrient supply to the cartilage.¹²

Sex

In individuals older than 55 years, the prevalence of osteoarthritis is higher among women than men.¹² Women are especially susceptible to osteoarthritis in the distal interphalangeal joints of the fingers. Women also have osteoarthritis of the knee joints more frequently than do men, and they are more prone to erosive osteoarthritis, with a female-to-male ratio of about 12:1.

Race

Primary osteoarthritis affects all races, although the prevalence and patterns of the disease appear to differ.^12,13 The disorder is more prevalent in Native Americans than in the general population. Disease of the hip is seen less frequently in Chinese patients from Hong Kong than in age-matched white populations. In persons older than 65 years, osteoarthritis is more common in whites than in blacks.

Morbidity

Osteoarthritis typically develops slowly and progresses over several years. Usually, the pain slowly worsens over time, but it may stabilize in some patients. Osteoarthritis of the knee is a leading cause of disability in elderly persons. Osteoarthritis also causes millions of Americans to miss work because of back pain.

Primary osteoarthritis occurs commonly in the hands, particularly in the distal interphalangeal joints, proximal interphalangeal joints, and first carpometacarpal joints. Clinicians who include involvement of other joints in the definition of primary osteoarthritis also see the disease in the hip joints, knee joints, first metatarsophalangeal joints, and lower lumbar and cervical regions of the spine (apophyseal articulations). Primary osteoarthritis in other joints, such as the shoulder, elbow, wrist, and ankle, is less common.

The normal articular surface of synovial joints consists of hyaline cartilage, which is composed of  chondrocytes, surrounded by an extracellular matrix that includes various macromolecules, most importantly proteoglycans and collagen. The cartilage protects the underlying subchondral bone by distributing large loads, maintaining low contact stresses, and reducing friction at the joint.

A variety of factors, most notably age,^14,15 lead to the development of primary osteoarthritis; however, primary and secondary osteoarthritis are not separable on a pathologic basis. Most investigators believe that degenerative alterations primarily begin in the articular cartilage, as a result of either excessive loading of a healthy joint or relatively normal loading of a previously disturbed joint. External forces accelerate the catabolic effects of the chondrocytes and disrupt the cartilaginous matrix.^16,17,18,19

Enzymatic destruction increases cartilage degradation, which is accompanied by decreased proteoglycans and collagen synthesis. Changes in the proteoglycans render the cartilage less resistant to compressive forces in the joint and more susceptible to the effects of stress. The decreased strength of the cartilage is compounded by adverse alterations of the collagen. Elevated levels of collagen degradation place excessive stresses on the remaining fibers, eventually leading to mechanical failure. The diminished elastic return and reduced contact area of the cartilage, coupled with the cyclic nature of joint loading, causes the situation to worsen over time.^20,21,22,23

Microscopically, flaking and fibrillations develop along the normally smooth articular cartilage surface. The loss of cartilage results in the loss of the joint space. Progressive erosion of the damaged cartilage occurs until the underlying bone is exposed. Bone denuded of its protective cartilage continues to articulate with the opposing surface. Eventually, the increasing stresses exceed the biomechanical yield strength of the bone. The subchondral bone responds with vascular invasion and increased cellularity, becoming thickened and dense (eburnation) at areas of pressure.²⁴

Furthermore, the traumatized subchondral bone may undergo cystic degeneration, due to either osseous necrosis secondary to chronic impaction or the intrusion of synovial fluid. At nonpressure areas along the articular margin, vascularization of subchondral marrow, osseous metaplasia of synovial connective tissue, and ossifying cartilaginous protrusions lead to irregular outgrowth of new bone (osteophytes). Fragmentation of these osteophytes or of the articular cartilage itself results in intra-articular loose bodies (joint mice).

Deep, achy, joint pain exacerbated by extensive use is the primary symptom. Also, reduced range of motion and crepitus are frequently present. Joint malalignment may be visible. Heberden nodes, which represent palpable osteophytes in the distal interphalangeal joints, are characteristic in women but not men. Inflammatory changes are typically absent or at least not pronounced.

Subsets of primary osteoarthritis

Certain diseases are often categorized as subsets of primary osteoarthritis. These disorders include primary generalized osteoarthritis (PGOA), erosive inflammatory osteoarthritis, and chondromalacia patellae.

Kellgren and Moore described PGOA in 1952.^1,25 The disease is characterized by familial and often premature development of Heberden and Bouchard nodes, as well as the precocious degeneration of the articular cartilage of multiple other joints, including the first carpometacarpal joints, knee joints, hip joints, and spine articulations. The radiographic appearance of PGOA is indistinguishable from that of nonfamilial primary osteoarthritis, although the disease typically progresses relatively rapidly and appears severe on images.

Erosive (ie, inflammatory) osteoarthritis is a form of primary osteoarthritis marked by a greater degree of inflammation, with erosive abnormalities and, in some cases, osseous ankylosis. The disease most commonly occurs in postmenopausal women, and it may be hereditary. Laboratory findings are generally uninformative. Erosive osteoarthritis is typically bilateral and symmetrical, and it occurs in the interphalangeal, particularly distal interphalangeal, joints of the hands (see Image 24). Rarely, patients may have erosive osteoarthritis at the base of the first metacarpal or even in the feet.²⁶

Radiologists may aid in the treatment of osteoarthritis by administering image-guided intra-articular injections of steroids. After the introduction of the needle into the joint and prior to steroid administration, aspiration of as much synovial fluid as possible should be attempted. This procedure often provides symptomatic relief for the patient and allows laboratory evaluation of the fluid, if necessary. Infected joint fluid and bacteremia are contraindications to steroid injection.

If these treatments are ineffective, surgical intervention, which ranges from arthroscopic procedures to total joint arthroplasty,³⁰ may be necessary in some patients.³¹

In a study by Kirkley et al, it was found that arthroscopic surgery for osteoarthritis of the knee provided no additional benefit to optimized physical and medical therapy.² In an accompanying editorial, Marx stated, however, that osteoarthritis is not a contraindication to arthroscopic surgery and that arthroscopic surgery remains appropriate in patients with arthritis in specific situations in which osteoarthritis is not believed to be the primary cause of pain.^32,33

Preferred Examination

Osteoarthritis is typically diagnosed on the basis of clinical and radiographic findings.^34,35,36,37 Radiographic findings may be normal in the early stages of the disease, because cartilage is not directly visualized. Eventually, cartilage loss manifests as joint-space narrowing.

For excellent patient education resources, visit eMedicine's Arthritis Center. Also, see eMedicine's patient education article Osteoarthritis.

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