What is osteoarthritis?
Osteoarthritis is a type of degenerative joint disease where the articular cartilage is eroded. It is the most common type of arthritis and is mainly seen in the elderly. Osteoarthritis or OA is commonly known as bone-on-bone arthritis but this term can apply to other types of joint disease where the articular cartilage is damaged, such as post-traumatic arthritis. Although osteoarthritis is seen mainly in the elderly, it can start from as early as childhood since it is long term wear and tear of the cartilage. However, it is usually in the senior years where the body’s ability to repair this daily wear and tear is impaired leading to eventual breakdown of the cartilage.
Bone on Bone Arthritis Meaning
Joints are where two bones meet in a manner that allows for movement (articulation). There are different types of joints. The major joints with greatest flexibility are synovial joints. Here the ends of the bones that comprise the joint are covered by articular cartilage. It is this cartilage which reduces friction between the two surfaces and absorbs shock thereby protecting the underlying bone. The joint is lined with synovium and surrounded by a capsule. Within the joint space is synovial fluid which lubricates the articulating surfaces and helps absorb shock.
The bone under the cartilage known as subchondral bone is smooth and provides support to the articular cartilage. However, when the cartilage is worn out, the subchondral bone is exposed in the joint. It is not a suitable surface for opposing bone movement in a joint and is quickly damaged without the articular cartilage. Essentially the movement of bone on bone breaks down the subchondral surface and permanently damages the bone.
However, the term bone on bone arthritis can be misleading. Although osteoarthritis can be literally translated into bone arthritis, the term arthritis infers that the the joint is inflamed. There is little to no inflammation in most cases of osteoarthritis. Instead it is degeneration that occurs. Secondly bone degeneration is a late consequence of osteoarthritis. In the early stages, it is primarily a condition of cartilage erosion and the bone is largely spared.
Pathophysiology of Osteoarthritis
Joint Cartilage Damage
The articular cartilage is made and maintained by a type of cell known as chondrocyte. These cells are originally known as chondroblasts and lay down the matrix outside of the cell that constitutes the bulk of the cartilage. This extracellular matrix is made up on water, proteoglycans, elastin and collagen. It makes the cartilage strong yet flexible. Those chondroblasts that are trapped within the matrix are then known as chondrocytes and are responsible for maintaining the cartilage.
Normally, the articular cartilage does not wear out with daily use. Joint structure is such that friction between the articular surfaces is minimal and the cartilage can withstand significant strain. It is constantly replaced and maintained. However, cartilage can be affected if there is :
- severe trauma to the joint
- inflammation of the joint structures even if it does not directly involve the cartilage
- disruption in cartilage repair and maintenance
It is also that a genetic component is responsible for generalized osteoarthritis. However, almost every person after the age of 75 years has some degree of osteoarthritis, whether symptomatic or not.
Tissue damage appears to be the trigger factor for osteoarthritis. The inflammatory mediators that accumulate in the joint initially causes the chondrocytes to become more active in an attempt to repair the cartilage. However, this inadvertently promotes erosion of the cartilage in the long term as the enzymes responsible for cartilage breakdown are also secreted. The chondrocytes undergo cell death (apoptosis) gradually exhausting the ability of the cartilage to regenerate.
Bone and Synovium Damage
This leaves the underlying subchondral bone exposed in the joint and without the protection of the cartilage, the bone begins to erode. This is a rather simplistic explanation but there is evidence to suggest that damage to the bone begins even before the articular cartilage is completely eroded. The same may apply to the joint lining – synovium – which secretes the synovial fluid.
Severe damage to the bone and synovium occurs once the articular cartilage is eroded. The subchondral bone hardens in an attempt to repair the damage to it. Bony outgrowths known as osteophytes also develop to stabilize the joint. The synovium thickens and secretes larger than normal quantities of synovial fluid. The tendons and ligaments around the joint become strained leading to tendonitis and contractures.Muscles around the joint go into spasm and become fatigued.
Causes of Osteoarthritis
Osteoarthritis arises with the breakdown of the joint cartilage. The exact cause is not always known but there are certain risk factors that increase the chance of osteoarthritis developing. This includes :
- Advancing age – replenishment and maintenance of healthy cartilage diminishes with age thereby increasing the chance of cartilage destruction.
- Obesity – increased body weight places greater strain on the weight bearing joints in particular.
- Injury – acute trauma or repetitive injury (occupational) to the joint serves as the trigger for the degenerative process.
- Gender – women are at a greater risk of developing osteoarthritis especially during menopause although the exact reasons are unclear.
- Sedentary lifestyle – inactivity affects cartilage nutrition and ultimately its strength.
- Genetics – specific genes may increase the likelihood of osteoarthritis in multiple joints throughout the body.
Other factors that may also increase the risk of osteoarthritis includes congenital joint deformity, hypothyrodism, diabetes mellitus, gout and Paget’s disease of the bone.
The symptoms develop gradually over years since osteoarthritis is a slow progressing joint disorder. Apart from the initial symptoms following joint injury in traumatic onset, a person may be asymptomatic for years and even decades. The main symptoms includes :
- Joint stiffness worse in the morning or after long periods of inactivity.
- Joint pain (arthralgia) worse with movement and afterwards.
- Joint tenderness which is pain when applying pressure on the joints.
- Joint noises such as a grating sound.
- Joint nodules which are bony outgrowths (osteophytes).
Most Affected Joints
The most commonly affected joints in osteoarthritis includes :
- Finger joints – proximal and distal interphalangeal joints and thumb carpometacarpal joint
- Back joints – intervertebral (IV) discs and zygapophyseal joints especially on the cervical and lumbar vertebrae
- Hip joint
- Knee joint
- Toe joints – first metatarsophalangeal joint (big toe)
Intervertebral discs and zygapophyseal joints of the cervical vertebrae are affected. Symptoms include :
- Neck stiffness
- Neck pain
- Pinched neck nerve symptoms
Finger joint osteoarthritis symptoms includes :
- Finger stiffness
- Finger pain and tenderness
- Heberden’s joints – distal interphalangeal joints
- Bouchard’s joints – proximal interphalangeal joints
Lower Back Osteoarthritis
Intervertebral discs and zygapophyseal joints of the lumbar vertebrae are affected. Symptoms include :
- Low back pain
- Leg pain
- Pinched lumbar nerve symptoms
- Hip joint pain
- Referred pain to the knee
- Loss of range of motion at the hip joint
- Grating sound at the hip joint with certain movements
- Knee joint pain
- Pain from tendonitis (muscle tendons) and contractures (ligaments)
1. Osteoarthritis. Merck Manuals
2. Osteoarthritis. Emedicine Medscape
Article reviewed by Dr. Greg. Last updated on March 7, 2012