26th January 2024

Differential diagnoses 

Background

OA is a clinical diagnosis; however in the presence of atypical features, further investigations are required to distinguish between other rheumatological conditions which can present similarly. It is vital to exclude the other conditions discussed below, as they have different pathological processes and subsequent management pathways. However, it is worth being aware that there can often be overlap between the features of different conditions and that they can co-exist in the same patient.

Rheumatoid arthritis (RA) 

A number of features can be used to distinguish between RA and OA, especially the number and distribution of affected joints. RA tends to cause a symmetrical small-joint polyarthritis. The joints most commonly affected are the metacarpophalangeal (MCP) joints in the hands, with sparing of the distal interphalangeal joints (DIPS). Patients with RA generally experience morning stiffness for longer than 30 minutes, which can be relieved with activity and can often present with systemic symptoms such as fever and fatigue. In terms of investigations, CRP and ESR are raised in RA, rheumatoid factor and anti-CCP are positive, and characteristic features can be seen on imaging, such as loss of joint space, erosions, soft tissue swelling and soft bones (osteopenia). Nevertheless, it can be difficult to distinguish between OA and inflammatory arthropathies, and the two conditions can often co-exist. 

Psoriatic arthritis

Psoriatic arthritis often presents with asymmetric joint involvement, mostly affecting the DIPs. However, in certain types of psoriatic arthritis where only DIPS are affected, it can be difficult to differentiate from OA. A diagnosis of psoriatic arthritis can be suspected if there are psoriatic skin changes; however, these can often either be absent or manifest many years after arthritic symptoms. 

Bursitis

Greater trochanteric bursitis in the hip and pes anserine bursitis in the knee can also present with localised pain in the hip and knee respectively. However, in these conditions, there is often local tenderness on examination, which is usually absent in OA. Local anaesthetic and corticosteroid injections can be used to treat bursitis.

Gout and pseudogout

Gout and pseudogout usually present with an acute monoarthritis; however, they can both mimic an acute exacerbation of OA. A joint affected by gout or pseudogout is usually warm, erythematous and tender to palpation. Joint fluid analysis can be used to distinguish between the conditions. The most commonly affected joint in gout is the first metatarsophalangeal joint. Gout will be suspected in the presence of sodium monourate crystals present in the synovium. The serum uric acid level would also be raised (> 450 µmol/l). Pseudogout usually occurs in the wrist and knee but can also involve MCPs. Pyrophosphate crystals would be found on joint fluid analysis, and x-rays would demonstrate features such as chondrocalcinosis.

Avascular necrosis (AVN)

AVN presents sub-acutely, usually in the hip and knee joints. Patients often have risk factors such as corticosteroid use. There may be localised bony tenderness in the affected joint. AVN can be distinguished from OA with MRI imaging, which is the most sensitive test for AVN. 

Internal derangements (e.g., meniscal tears)

Meniscal tears tend to present acutely and can cause hugely debilitating symptoms. Patients usually have a history of preceding trauma, although this can be minor. Joint locking can occur in both conditions; however, MRI can be useful in distinguishing between them. 

Malignancy 

Malignancy, including bony metastases, can present with persistent and severe bone pain unrelated to activity, joint pain, rapid progression of symptoms, unexplained swelling or potentially neurological features. Systemic features such as weight loss and night sweats may also be reported. Patients who present with these red-flag symptoms are required to be investigated further with suitable imaging modalities. 

Septic arthritis 

Sudden onset of a red, hot, swollen joint associated with systemic features such as fever can trigger suspicion of septic arthritis, especially in the presence of risk factors such as prior joint damage, joint surgery, presence of joint prosthesis or immunodeficiency. Joint fluid analysis and aspiration with microscopy and gram staining can be used to identify organisms in septic arthritis. 

Other differentials to consider

Other alternative diagnoses according to affected joints:

Hands

• Ganglion
• De Quervain’s tenosynovitis 
• Trigger thumb 

Hips

• Greater trochanteric pain syndrome 
• Meralgia paraesthetica 
• Sciatica 
• Spinal stenosis 

Knees

• Patellofemoral pain 
• Anterior cruciate ligament tear 

References 

• Wenham CY, Grainger AJ, Conaghan PG. The role of imaging modalities in the diagnosis, differential diagnosis and clinical assessment of peripheral joint osteoarthritis. Osteoarthritis and Cartilage. 2014 Oct 1;22(10):1692-702.
• Conaghan PG, Dickson J, Grant RL. Care and management of osteoarthritis in adults: summary of NICE guidance. Bmj. 2008 Feb 28;336(7642):502-3.
• Graham DJ, Campen D, Hui R, Spence M, Cheetham C, Levy G, Shoor S, Ray WA. Risk of acute myocardial infarction and sudden cardiac death in patients treated with cyclo-oxygenase 2 selective and non-selective non-steroidal anti-inflammatory drugs: nested case-control study. The Lancet. 2005 Feb 5;365(9458):475-81.
• National Institute for Health and Care Excellence (Great Britain).Osteoarthritis: care and management. National Institute for Health and Care Excellence (NICE); 2014
• Doherty M, Abhishek A, Hunter D. Clinical manifestations and diagnosis of osteoarthritis. UpToDate [Internet]. 2017:1-30.