What Is Osteoarthritis?

Joint pain may be a sign of osteoarthritis. The pain may be mild and annoying or so severe and debilitating it affects everyday activities. Osteoarthritis can develop in any joint and is most common in the hands, knees, and hips.

Osteoarthritis occurs when a joint breaks down, and it affects nearly half of people over age 65.¹Dieppe PA, Lohmander LS. Pathogenesis and management of pain in osteoarthritis. Lancet. 2005;365:965–73. As cited in Jefferies, MA. Osteoarthritis. In: Efthimiou P, ed. Absolute Rheumatology Review. Springer Nature Switzerland AG; 2020; chap 15. Accessed September 15, 2020. https://doi.org/10.1007/978-3-030-23022-7_15 The hallmark sign of osteoarthritis is damaged, thinned, or missing cartilage.

This article discusses how osteoarthritis develops, as well as its symptoms, causes and risk factors, diagnosis, and treatment.

• Explore joint specific articles for:
  • Shoulder (Glenohumeral) Osteoarthritis
  • Shoulder (Acromioclavicular) Osteoarthritis
  • Hip Osteoarthritis
  • Knee Osteoarthritis
  • Ankle Osteoarthritis

Why Joints Break Down

In the past, osteoarthritis has been considered the result of a previous injury and/or wear-and-tear on the joint. Research now suggests that there are many factors that play a role in the development of osteoarthritis, including low-grade inflammation throughout the body, joint biochemistry,²Jefferies, MA. Osteoarthritis. In: Efthimiou P, ed. Absolute Rheumatology Review. Springer Nature Switzerland AG; 2020; chap 15. Accessed September 15, 2020. https://doi.org/10.1007/978-3-030-23022-7_15^,3Millerand M, Berenbaum F, Jacques C. Danger signals and inflammaging in osteoarthritis. Clin Exp Rheumatol. 2019 Sep-Oct;37 Suppl 120(5):48-56. Epub 2019 Oct 15. PMID: 31621566. body composition, and genetics.

This more recent research helps account for why some people whose joints are regularly under stress—recreational joggers, for example—are not necessarily more likely to develop osteoarthritis.⁴Timmins KA, Leech RD, Batt ME, Edwards KL. Running and Knee Osteoarthritis: A Systematic Review and Meta-analysis. Am J Sports Med. 2017 May;45(6):1447-1457. DOI: 10.1177/0363546516657531^,5Alentorn-Geli E, Samuelsson K, Musahl V, Green CL, Bhandari M, Karlsson J. The Association of Recreational and Competitive Running With Hip and Knee Osteoarthritis: A Systematic Review and Meta-analysis. J Orthop Sports Phys Ther. 2017 Jun;47(6):373-390. DOI: 10.2519/jospt.2017.7137

While the underlying cause(s) of osteoarthritis are still being investigated, the signs of osteoarthritis are fairly well recognized. These signs include a loss of cartilage as well as changes to bones, the joint’s membrane and the special fluid it produces, and surrounding soft tissues.

See Understanding Joint Pain

The Damage and Loss of Joint Cartilage

The defining feature of osteoarthritis is the breakdown and loss of articular cartilage. This strong, smooth, slippery cartilage normally covers the surfaces of bones where they meet at joints—for example, the bottom of the femur (thighbone) and top of the tibia (shin bone) are covered in articular cartilage where they meet at the knee. Cartilage protects bones from impact and friction.

See What Is Cartilage?

During the development of osteoarthritis:

• Cartilage composition changes and becomes weaker during the aging process.
• Weakened cartilage may become damaged, thin, or wear away.
• The damaged cartilage may attempt to heal by growing new cells. There are typically not nearly enough new cells to replace the missing cartilage. Plus, these cells may not form as smooth a surface as the old cartilage.

Alone, damaged cartilage does not necessarily cause pain. Instead, damaged or missing cartilage contributes to exposed, uncovered bone and other joint changes that lead to pain.

Changes to the Joint’s Bones

When articular cartilage is lost or damaged, the underlying exposed bone experiences more friction and impact. Increased stress on a bone can cause it to undergo changes. For example:

• Bone spurs form on the joint’s bones. Sometimes called osteophytes, bone spurs are abnormal growths. It is believed that bones produce bone spurs to compensate for deteriorated or missing cartilage by redistributing weight loads. Bone spurs do not hurt but—depending on their size and location—they can result in painful friction and/or limit a joint’s range of motion.
• Subchondral bone sclerosis develops on bone tissue that lies just beneath cartilage. The bone tissue changes in composition and hardens. ("Chondral" means relating to cartilage; subchondral bone is located just beneath cartilage. "Sclerosis" means hardening.)
• Cysts and bone marrow lesions may also form in the bone tissue underneath damaged or missing cartilage. The bone cysts, referred to as subchondral cysts, are filled with fluid and typically do not cause pain. Bone marrow lesions are areas of abnormal swelling and may be associated with osteoarthritis pain.⁶Felson DT, Chaisson CE, Hill CL, Totterman SM, Gale ME, Skinner KM, Kazis L, Gale DR. The association of bone marrow lesions with pain in knee osteoarthritis. Ann Intern Med. 2001 Apr 3;134(7):541-9. PubMed PMID: 11281736.^,7Collins JA, Beutel BG, Strauss E, Youm T, Jazrawi L. Bone Marrow Edema: Chronic Bone Marrow Lesions of the Knee and the Association with Osteoarthritis. Bull Hosp Jt Dis (2013). 2016 Mar;74(1):24-36. Review. PubMed PMID: 26977546.

Subchondral bone sclerosis and bone marrow lesions can occur at any time but are particularly common in the later stages of arthritis.

Changes to the Joint’s Membrane and Fluid

Osteoarthritis affects joints classified as synovial joints. These joints are encapsulated by a membrane (synovial membrane) that produces fluid, called joint fluid (synovial fluid. Typically, this fluid promotes healthy cartilage and lubricates the joint.

In an arthritic joint, the membrane becomes inflamed, and both the composition and the amount of joint fluid in the knee joint can change.⁸Roemer FW, Guermazi A, Felson DT, et al. Presence of MRI-detected joint effusion and synovitis increases the risk of cartilage loss in knees without osteoarthritis at 30-month follow-up: the MOST study. Ann Rheum Dis. 2011;70(10):1804-1809. DOI: 10.1136/ard.2011.150243 For example, the membrane may produce too much synovial fluid, and this fluid may be less lubricating. Such a change may lead to joint swelling, stiffness, and friction.

See How Do Synovial Joints Work?

Changes to the Surrounding Soft Tissues

As arthritis develops, the surrounding soft tissues undergo stress, likely due to compensation. For example, the nearby tendons and ligaments may become strained and bursae may become irritated, potentially leading to more pain and loss of joint stability.

Osteoarthritis vs. Other Types of Arthritis

Commonly referred to as simply "arthritis," osteoarthritis is the most common type of arthritis. Many other types of arthritis can also cause joint pain, including but not limited to rheumatoid arthritis, ankylosing spondylitis, gout, and reactive arthritis.

These less common types of arthritis will be considered and ruled out before a diagnosis of osteoarthritis is made. While all forms of arthritis cause joint pain, the type of pain and other symptoms can vary.

Read more about Types of Arthritis