Osteoarthritis of the knee

Definition

Osteoarthritis of the knee, known medically as gonarthrosis, is a chronic degenerative condition of the knee joint in which the joint cartilage is progressively broken down. The hyaline cartilage, which serves as a shock-absorbing gliding surface on the joint surfaces of the thigh bone (femur), shin bone (tibia) and kneecap (patella), loses elasticity and thickness.

As it progresses, the exposed ends of the bones rub against each other, which can lead to pain, inflammation, changes in the capsule, new bone formation (osteophytes) and a change in the whole architecture of the joint. This is an irreversible process, but its progress can be slowed with targeted measures, or its symptoms treated effectively.

Causes

Osteoarthritis of the knee (gonarthrosis) develops through a complex interplay of factors. A distinction is drawn in principle between primary (idiopathic) and secondary osteoarthritis. Primary osteoarthritis of the knee is an age-related, "natural" process of wear of the joint cartilage with no identifiable direct trigger. It frequently rests on a genetic weakness of the connective tissue or an age-related decline in cartilage regeneration. This form usually appears from middle age onwards and develops gradually over years.

Secondary osteoarthritis of the knee arises as a consequence of clearly identifiable causes. The most common triggers are:

• Malalignment of the leg axis, such as knock knees or bow legs, which leads to uneven pressure in the knee joint
• The consequences of injury, for example after a cruciate ligament tear, meniscal damage or a fracture involving the joint, which affect mechanical stability and the way the joint moves
• Chronic instability, for instance through repeated twisting injuries or ligament injuries that did not heal fully
• Excess weight (obesity), which markedly increases the load on the knee joint and wears the cartilage down faster
• Sustained occupational or sporting load, for instance through frequent kneeling, heavy lifting or intensive running
• Inflammatory joint conditions, rheumatoid arthritis for example, which can damage the cartilage even without mechanical load
• Malalignment of the kneecap or other congenital anatomical features

Several of these factors often act at the same time. The sooner the underlying loads are recognised and treated, the better the progress of the osteoarthritis can be slowed or even stopped.

Grades of knee osteoarthritis (Kellgren-Lawrence classification)

The radiological Kellgren and Lawrence classification is frequently used to grade the severity of osteoarthritis. It is based on typical features on X-ray.

Radiological grade according to Kellgren & Lawrence

• Grade 0: No sign of osteoarthritis – a normal X-ray with no changes in the bone.
• Grade 1: Very slight, questionable narrowing of the joint space, minimal osteophyte formation possible – frequently without symptoms.
• Grade 2: Early form – clear osteophyte formation, possible narrowing of the joint space – first clinical symptoms such as pain under load.
• Grade 3: Advanced osteoarthritis with clear narrowing of the joint space, marked osteophytes and beginning deformity of the bone – pain possible at rest as well.
• Grade 4: End stage – complete loss of cartilage, marked sclerosis, large osteophytes, deformity of the joint – severe pain, movement severely restricted.