I. Background

• Also known as degenerative joint disease.
• Most common form of arthritis.
• Classified as: Idiopathic (localized or generalized) or Secondary (traumatic, congenital, metabolic/endocrine/neuropathic and other medical causes).
• Characterized by focal and progressive loss of the hyaline cartilage of joints, underlying bony changes.
• Usually defined by symptoms, pathology or combination (1)
• Pathology = radiographic changes (joint space narrowing, osteophytes and bony sclerosis)
• Symptoms = pain, swelling, stiffness.

II. Prevalence

• Overall OA affects 13.9% of adults aged 25 and older and 33.6% (12.4 million) of those 65+; an estimated 26.9 million US adults in 2005 up from 21 million in 1990 (believed to be conservative estimate) (2)
• Radiographic OA (moderate to severe)—prevalence per 100 (knee and hip may be underestimated)
• Hand = 7.3 (9.5 female; 4.8 male) (5)
• Feet = 2.3 (2.7 female; 1.5 male) (2)
• Knee = 0.9 (1.2 female; 0.4 male) (3)
• Hip = 1.5 (1.4 female; 1.4 male) (2)
• Symptomatic OA—prevalence per 100
• Hand = 8% (8.9% female; 6.7% male) 2.9 million adults aged 60+ years (5)
• Feet = 2.0% (3.6 female; 1.6 male) aged 15–74 years (2)
• Knee = 12.1% (13.6% female; 10.0% male) 4.3 million adults aged 60+ years (3)
• Knee = 16% (18.7% female; 13.5% male) adults aged 45+ years (4)
• Data from Framingham OA Study reports similar rates:
• Knee = 6.1% all adults > age 30 (6)
• Knee = 9.5% (11.4 female; 6.8 male) ages 63-93 (6)
• Hip = 4.4% (3.6% female; 5.5% male) adults ≥55 years of age (2)

III. Incidence

• Age and sex-standardized incidence rates of symptomatic OA:
• Hand OA = 100 per 100,000 person years (7)
• Hip OA = 88 per 100,000 person years (7)
• Knee OA = 240 per 100,000 person years (7)
• Among women:
• Incident radiographic knee OA 2% per year (8)
• Incident symptomatic knee OA 1% per year (8)
• Progressive knee OA 4% per year (8)
• Incidence rates increased with age, and level off around age 80. (9)
• Women had higher rates than men, especially after age 50. (9)
• Men have 45% lower incident risk of knee OA and 36% reduced risk of hip OA than women. (10)
• Prevalent knee OA, but not hip or hand OA, is significantly more severe in women compared to men. (10)

IV. Mortality

• About 0.2 to 0.3 deaths per 100,000 population due to OA (1979–1988). (11)
• OA accounts for ~6% of all arthritis-related deaths. (11)
• ~ 500 deaths per year attributed to OA; numbers increased during the past 10 years. (11)
• OA deaths are likely highly underestimated. For example, gastrointestinal bleeding due to treatment with NSAIDs is not counted. (11)

V. Hospitalizations

• OA accounts for 55% of all arthritis-related hospitalizations; 409,000 hospitalizations for OA as principal diagnosis in 1997. (12)
• Knee and hip joint replacement procedures accounted for 35% of total arthritis-related procedures during hospitalization. (17)
• From 1990 to 2000 the age-adjusted rate of total knee replacements in Wisconsin increased 81.5% (162 to 294 per 100,000). (13)
• Rates increased most among youngest age group (45–49 years). (13)
• Costs increased from 69.4 million to 148 million dollars.
• Blacks and persons with low income have lower rates of total knee replacement but higher complications and mortality than whites. (14)

VI. Ambulatory Care

• OA accounted for 7.1 million (19.5%) of all arthritis-related ambulatory medical care visits in 1997. (15)
• 7.1 million total ambulatory care visits for OA as primary diagnosis.
• SEX: Males = 2.2 million; Females = 4.9 million. (15)
• AGE: 0–18 = 35,000; 19–44 = 355,000; 45–64 = 2.5 million; 65+ = 4.1 million. (15)

• About 39% of people with OA report inability to access needed health care rehabilitative services. (16)

VII. Costs

• $7.9 billion estimated costs of knee and hip replacements in 1997. (12)
• Average direct costs of OA ~$2,600 per year out-of-pocket expenses. (17)
• Total annual disease costs = $5700 (US dollars FY2000). (18)
• Job-related OA costs $3.4 to $13.2 billion per year. (9)

VIII. Impact on health-related quality of life (HRQOL) [AAOS Fact Sheet; NHANES III data]

• OA of the knee is 1 of 5 leading causes of disability among non-institutionalized adults. (19)
• About 80% of patients with OA have some degree of movement limitation
• and 25% cannot perform major activities of daily living (ADL’s), 11% of adults with knee OA need help with personal care and 14% require help with routine needs.
• About 40% of adults with knee OA reported their health “poor” or “fair”.
• In 1999, adults with knee OA reported more than 13 days of lost work due to health problems.
• Hip/knee OA ranked high in disability adjusted life years (DALYs) (20) and years lived with disability (YLDs). (20)

IX. Unique characteristics

• Disease in weight bearing joints has greater clinical impact.
• About 20–35% of knee OA and ~50% of hip and hand OA may be genetically determined. (21, 22)
• Established modifiable and nonmodifiable risk factors: (4, 21, 22, 23)
• Modifiable
• Excess body mass (especially knee OA).
• Joint injury (sports, work, trauma).
• Occupation (due to excessive mechanical stress: hard labor, heavy lifting, knee bending, repetitive motion).
• Men — Often due work that includes construction/mechanics, agriculture, blue collar laborers, and engineers.
• Women — Often due work that includes cleaning, construction, agriculture, and small business/retail.
• Structural malalignment, muscle weakness.
• Non-modifiable.
• Gender (women higher risk).
• Age (increases with age and levels around age 75).
• Race (some Asian populations have lower risk).
• Genetic predisposition.

NOTE: Current smoking has been shown to be protective for osteoarthritis although it is unknown if this is due to the physiological effects of smoking on collagen, bone and cartilage tissue or if it is due to some unmeasured surrogate factor.

• Other possible risk factors:
• Estrogen deficiency (ERT may reduce risk of knee/hip OA).
• Osteoporosis (inversely related to OA).
• Vitamins C, E and D – equivocal reports.
• C-reactive protein (increased risk with higher levels)