OA is the most common type of arthritis. Reported incidence and prevalence rates of OA in specific joints vary widely, due to differences in the case definition of OA. OA may be defined by radiographic criteria alone (radiographic OA), typical symptoms (symptomatic OA), or by both. Using radiographic criteria, the distal and proximal interphalangeal joints of the hand have been identified as the joints most commonly affected by OA, but they are the least likely to be symptomatic. In contrast, the knee and hip, which constitute the second and third most common locations of radiographic OA, respectively, are nearly always symptomatic. The first metatarsal phalangeal and carpometacarpal joints are also frequent sites of radiographic OA, while the shoulder, elbow, wrist and metacarpophalangeal joints rarely develop idiopathic OA.
Risk Factors for Osteoarthritis
- AGE: In demographic studies, age is the most consistently identified risk factor for OA, regardless of the joint being studied. Prevalence rates for both radiographic OA and, to a lesser extent, symptomatic OA rise steeply after age 50 in men and age 40 in women. OA is rarely present in individuals less than 35 years of age, and secondary causes of OA or other types of arthritis should strongly be considered in this population.
- SEX: Female gender is also a well-recognized risk factor for OA. Hand OA is particularly prevalent among women. In addition, polyarticular OA and isolated knee OA are slightly more common in women than men, while hip OA occurs more commonly in men. Interestingly, women are more likely to report pain in all affected joints, including the hip, than men.
- OBESITY: Cohort studies have demonstrated a clear association of obesity with the development of radiographic knee OA in women and a weaker association with hip OA. Whether obesity is a risk factor for the development of hand OA remains controversial. Regardless, this remains one of the most important modifiable risk factors for OA and patients should be counseled appropriately.
- JOINT STRESS: Occupation-related repetitive injury and physical trauma contribute to the development of secondary (non-idiopathic) OA, sometimes occurring in joints that are not affected by primary (idiopathic) OA, such as the metacarpophalangeal joints, wrists and ankles. Although the prevalence of knee OA is greater in adults who have engaged in occupations that require repetitive bending and strenuous activities, an association with regular, intense exercise remains controversial. While early studies in joggers failed to find a higher prevalence of OA of the knee in joggers compared to non-joggers, a recent study of the Framingham data base in elderly adults provided the first longitudinal association between high level of physical activity and incident knee OA. Low-impact and recreational exercises are unlikely to constitute a risk factor for knee OA, and are likely to benefit the cardiovascular system. Prior menisectomy is a significant risk factor in men for the development of OA in the knee.
- GENETICS: Twin studies have demonstrated an important role for genetics in the development of OA. In some cases, this is associated with a particular genetic syndrome, such as Stickler syndrome or familial chondrocalcinosis. Genome-wide studies contiue to evaluate for particular chromosomes, particularly those involved in bone or articular cartilage structure and metabolism, and associations of familial OA.