Diagnosis of knee osteoarthritis is made on the basis of clinical findings, confirmed by imaging studies and arthroscopy. The process starts with breakdown of the cartilage matrix followed by fibrillation and erosion of the cartilage surface. This progresses through the full thickness of the cartilage until bone is exposed. Knee OA can include pathology on the femoral, tibial and patellar surfaces in either of the three joint compartments, lateral, medial and patella-femoral. Each may have different presentations and prognoses.
Knee OA typically presents as increasing pain on extensive activity, relieved by rest. As symptoms progress pain at rest may also be experienced. Pain may be accompanied by crepitus and swelling of the joint. Early morning stiffness rarely lasting more than thirty minutes may be present, and stiffness during rest may be noted. As the condition progresses, instability, joint deformity and restricted range of movement may all develop.
Pathology may be restricted to only one of the three joint compartments, with pain typically restricted to that compartment. Wear in only the medial compartment may lead to varus deformity while wear in the lateral compartment may lead to valgus deformity. Patellofemoral OA typically causes pain rising from a seat, on using stairs or walking up and down hill.
Diagnosis is generally confirmed by imaging, usually X-ray examination which shows progressive narrowing of the joint space due to loss of the articular cartilage. Osteophytes may frequently be observed in addition to bone cysts and bone sclerosis. MRI may show more detail of any cartilage destruction, along with other abnormalities to menisci and subchondral bone. Arthroscopy will confirm the presence of cartilage pathology and is usually required for accurate grading of knee OA.
Knee osteoarthritis can be graded by symptom, or by radiological classification. As noted above, many patients experience knee pain without any radiographic changes. On the other hand, many people with early radiographic changes experience no symptoms, and some patients with marked radiographic changes are able to function well with reasonable mobility including participation in sports. Other factors may be involved when there is an unexplained presence of severe symptoms with only mild radiographic changes, including secondary gain. Generally however there is consensus that patients who reach a certain level of objective radiological change are likely to have significant symptoms and significant impairment.
Kellgren-Lawrence grading system. Kellgren and Lawrence developed a radiographic classification in the 1950s which is generally used today. Based on four features, joint space narrowing, osteophyte formation, subchondral sclerosis and subchondral cysts, the following grades are described:
Grade 0. No radiographic features of OA.
Grade 1. Doubtful joint space narrowing and possible osteophyte lipping.
Grade 2. Definite osteophytes and possible joint space narrowing.
Grade 3. Multiple osteophytes, definite joint space narrowing, sclerosis and possible bony deformity.
Grade 4. Large osteophytes, marked joint space narrowing, severe sclerosis and definite bony deformity.
Outerbridge classification. Arthroscopic classifications such as that by Outerbridge assess articular cartilage damage, and are often used specifically in arthroscopy reports. Four grades are described:
I. Softening and swelling
II. Fragmentation and fissuring of less than 0.5 inches
III. Fragmentation and fissuring of greater than 0.5 inches
IV. Erosion down to subchondral bone.
Classifications based on symptoms include the Cincinnati Knee Rating System, Activities of Daily Living Scale and Sports Activity Scale of the Knee Outcome Survey, the Lysholm Scoring Scale and others.