Osteoarthritis

01Definition and overview

Osteoarthritis is a degenerative and repair process of synovial joints in which the articular cartilage is gradually lost, while the bone beneath it remodels: it becomes dense, forms cysts and grows osteophytes at the joint margins. It used to be seen as simple wear and tear of the cartilage; today it is understood as the failure of the whole joint as an organ, with low-grade inflammation of the synovium, changes in the subchondral bone, menisci, ligaments and capsule, and weakness of the muscles around it.

It is classified by cause and by pattern:

• Primary osteoarthritis has no single cause; it is the usual form, driven by age, sex, weight and genes.
• Secondary osteoarthritis follows a known joint insult: a fracture through the joint, a meniscal or cruciate tear, developmental dysplasia of the hip, Perthes disease or a slipped upper femoral epiphysis, avascular necrosis, inflammatory or septic arthritis, gout, calcium pyrophosphate deposition, haemochromatosis, acromegaly, haemophilia (repeated bleeds into the joint) or a neuropathic (Charcot) joint.
• By pattern: localised (knee, hip, hand, foot, spine), or generalised, in three or more joint groups, often with nodal hand disease.

02Etiology and pathophysiology

Risk factors

Bone mass runs the other way: people with high bone density have more osteoarthritis and less osteoporosis.

What happens in the joint

1. 1

   Cartilage under stress

   Healthy cartilage is mostly water held by a meshwork of type II collagen and the proteoglycan aggrecan, kept by sparse chondrocytes. Abnormal load on normal cartilage, or normal load on weakened cartilage, injures the chondrocytes.
2. 2

   Matrix breakdown

   The chondrocytes switch to a catabolic state: under IL-1, TNF and other signals, they make matrix-degrading enzymes, chiefly the collagenase MMP-13 and the aggrecanase ADAMTS-5, faster than they rebuild the matrix. The surface frays (fibrillation), splits and wears through.
3. 3

   Bone responds

   The subchondral bone thickens and becomes sclerotic, develops cysts and, on MRI, bone marrow lesions. At the margins, new cartilage forms and ossifies into osteophytes.
4. 4

   The rest of the joint

   Fragments of matrix and crystals irritate the synovium, which thickens and secretes effusions; the capsule stiffens, ligaments and menisci degenerate, and the muscles weaken from disuse and pain.

Cartilage has no nerves, so the pain comes from the bone, synovium, capsule, ligaments and periosteum, and with time from sensitisation of the spinal cord and brain. That is why the X-ray and the pain often disagree, and why mood, sleep and muscle strength predict pain better than the radiograph. The figure of "wear and tear" simplifies: the process is active and partly reparable, and some X-rays improve.

03Clinical features

Symptoms and signs

• Pain on use and weight bearing, relieved by rest; in advanced disease, pain at rest and at night.
• Stiffness after inactivity and in the morning that lasts minutes, usually under 30 (the "gelling" phenomenon).
• Loss of function: walking, stairs, rising from a chair, gripping.
• Signs: crepitus, bony enlargement, tenderness at the joint line, reduced and painful movement, deformity, muscle wasting, and sometimes a cool effusion.
• Symptoms come and go; flares of pain and swelling last days to weeks.

By joint

The metacarpophalangeal joints and the wrist are not usual sites of primary osteoarthritis: their involvement suggests rheumatoid arthritis, calcium pyrophosphate deposition or haemochromatosis (hook-like osteophytes at the second and third metacarpophalangeal joints).

When it is not just osteoarthritis

04Diagnosis and management

Making the diagnosis

In the UK osteoarthritis is diagnosed clinically, without imaging, in people who:

• are 45 or over, and
• have activity-related joint pain, and
• have no morning joint stiffness, or stiffness that lasts no longer than 30 minutes.

Imaging is not used routinely to diagnose it unless there are atypical features (recent trauma, prolonged morning stiffness, rapid worsening, a hot swollen joint, concern about infection or cancer) or features that suggest another diagnosis. Nor is it needed routinely to follow the disease.

Tests when they are needed

• X-ray, weight-bearing for the knee and with skyline views for the patella, when imaging is needed or surgery is planned:

Severity is often graded by the Kellgren-Lawrence scale from 0 (none) to 4 (large osteophytes, marked narrowing, sclerosis and deformity). In erosive hand osteoarthritis, central erosions with marginal osteophytes give a "gull-wing" outline, unlike the marginal "pencil-in-cup" erosions of psoriatic arthritis.

• Blood tests are normal: ESR and CRP normal or barely raised, rheumatoid factor and anti-CCP negative. They are done only to exclude other diseases.
• Synovial fluid, if an effusion is aspirated: clear and viscous with few cells (under about 2000 white cells per microlitre), sometimes with calcium pyrophosphate crystals.
• MRI shows meniscal tears and bone marrow lesions, but meniscal tears are common in painless knees and rarely change management of osteoarthritis.

Osteoarthritis or inflammatory arthritis

Core treatment for everyone

European and UK guidance agree that the core is the same for everyone with osteoarthritis, agreed in a shared plan:

• Information: what osteoarthritis is, that it is diagnosed clinically, that it is not inevitably progressive, that exercise does not wear out the joint, and how to manage flares.
• Therapeutic exercise, tailored to the person: local muscle strengthening (the quadriceps for the knee, the hip abductors), aerobic fitness, flexibility and balance, such as tai chi, alone, in groups, in water or with digital support, often supervised at first. Pain may rise at the start; regular exercise kept up over the long term reduces pain and improves function.
• Weight loss for those overweight or obese: any loss helps, and losing 10 per cent of body weight is likely to be better than 5.
• Walking aids, such as a stick in the opposite hand, and suitable footwear and adaptations at home and work. Insoles, braces and splints only for instability or abnormal loading when exercise alone is not enough; a thumb splint helps thumb-base disease.
• Not recommended in the UK: acupuncture and dry needling, TENS and other electrotherapy; manual therapy only alongside exercise for the hip or knee.

Drugs

Drugs are used alongside exercise, at the lowest effective dose for the shortest time, and reviewed.

Guidance elsewhere differs in places. A European society for osteoporosis and osteoarthritis (ESCEO) still proposes prescription crystalline glucosamine sulfate or chondroitin sulfate as a background treatment for knee osteoarthritis, and European hand osteoarthritis recommendations allow chondroitin sulfate for symptom relief; American guidance and OARSI accept duloxetine, particularly with widespread pain, and American guidance accepts tramadol in some patients. No drug has been shown to slow or reverse the disease: conventional and biological DMARDs (including hydroxychloroquine, methotrexate and TNF blockers) do not work in hand osteoarthritis and are not used.

Surgery

• Joint replacement is considered for hip, knee or shoulder osteoarthritis when symptoms (pain, stiffness, reduced function, progressing deformity) substantially affect quality of life and non-surgical care has failed or is unsuitable. The decision is clinical, not by a numerical score, and in the UK people are not excluded because of age, sex, smoking, other illnesses or weight, although these change the risks, which are discussed.
• Total hip and total knee replacement relieve pain and restore function in most patients. Unicompartmental knee replacement suits disease limited to one compartment; realignment osteotomy suits younger patients with malalignment.
• Trapeziectomy for the thumb base; fusion of the first metatarsophalangeal joint or of painful finger joints.
• Arthroscopic washout and debridement are not offered for osteoarthritis, because trials, including one against sham surgery, showed no lasting benefit; in the UK arthroscopy is kept for knee osteoarthritis with a clear history of true mechanical locking.
• The serious risks of replacement are venous thromboembolism, infection, dislocation (hip), loosening and fracture around the implant, and persistent pain, reported by about one in five patients after knee replacement and fewer after hip replacement.

05Epidemiology

About 595 million people had osteoarthritis in 2020, 7.6 per cent of the world's population, more than double the number in 1990, and the number is projected to rise by about three quarters for the knee and hip by 2050, as people live longer and heavier. The knee is the commonest site, then the hand and hip. Prevalence rises steeply after 40 to 50 and is higher in women, especially for the knee and hand. Symptomatic knee osteoarthritis affects more than 1 in 10 people over 60, and radiographic changes are present in most people over 65, many without symptoms. In people aged 70 and over it is the seventh leading cause of years lived with disability.

06Prognosis

Osteoarthritis is chronic but not inevitably progressive: many patients stay stable for years, some improve, and symptoms fluctuate. Nodal hand disease usually ends with good function once the inflammatory phase settles, while knee and hip disease cause the most disability. Progression is faster with obesity, malalignment, bone marrow lesions on MRI, generalised disease and weak quadriceps.

Joint replacement changes the outlook for end-stage disease: by registry data, about 82 per cent of total knee replacements and about 58 per cent of total hip replacements last 25 years. Younger patients have more years in which to need a revision. People with knee or hip osteoarthritis die earlier than others of their age, mostly from cardiovascular disease, and walking disability is one of the strongest risk factors; inactivity, the comorbidities and drug harms are the likely reasons, which is one more reason to keep them moving and to treat their cardiovascular risk. Follow-up is usually started by the patient when management is not working, and planned when treatments need monitoring or symptoms are severe.