Differentials
Bursitis
SIGNS / SYMPTOMS
Greater trochanteric bursitis in the hip and pes anserine bursitis in the knee present with pain over the lateral aspect of the hip and over the medial aspect of the knee, respectively. There is local tenderness in these areas that is usually absent in simple OA.
INVESTIGATIONS
Local anesthetic and corticosteroid injection might be therapeutic and diagnostic if it relieves symptoms to a significant degree.
Gout
SIGNS / SYMPTOMS
The onset of arthritis in gout is usually more acute (over a period of a few hours), but could mimic an exacerbation of acute OA.
In acute attacks of gout, the affected joint is usually erythematous, hot, and acutely tender.
Gout commonly involves the foot, especially the first metatarsophalangeal (MTP) joint.
INVESTIGATIONS
Arthrocentesis and joint fluid analysis, which shows leukocytes >2000 cells/mm³, and the presence of sodium monourate crystals.
Pseudogout
SIGNS / SYMPTOMS
The onset of arthritis in pseudogout (calcium pyrophosphate deposition [CPPD]) is usually more acute (over a period of a few hours), but could mimic an exacerbation of acute OA. Associated with other conditions (e.g., hemochromatosis) and results in secondary pseudo-OA, which often involves the metacarpophalangeal joints.
In acute attacks of pseudogout, the affected joint is usually erythematous, hot, and acutely tender.
Pseudogout often involves the wrist and knee, although it may affect almost any joint.
INVESTIGATIONS
Arthrocentesis and joint fluid analysis, which shows leukocytes >2000 cells/mm³, and the presence of pyrophosphate crystals.
Radiographs: chondrocalcinosis; in cases of hemochromatosis, hooklike osteophytes in the second and third metacarpal heads.
Rheumatoid arthritis (RA)
SIGNS / SYMPTOMS
Number and distribution of the involved joints helps to differentiate RA from OA.
RA usually causes a symmetric small joint polyarthritis in the hands, particularly affecting the metacarpophalangeal joints and sparing the distal interphalangeal joints. Typically, RA is associated with more prolonged morning stiffness than OA. Patients with acute RA may also feel generally ill, with fatigue and low mood.
Differentiation is sometimes challenging for hand involvement, and OA and RA can coexist.
INVESTIGATIONS
In RA, erythrocyte sedimentation rate and CRP are abnormal and rheumatoid factor and anticyclic citrullinated antibodies are positive. Typical RA erosive changes are seen on x-ray, MRI, or ultrasound.
Psoriatic arthritis
SIGNS / SYMPTOMS
Psoriatic arthritis can occur in the absence of skin psoriasis and often affects the distal interphalangeal (DIP) joints.
In psoriatic arthritis, the joint involvement is usually asymmetric, but inflammatory OA can be difficult to distinguish from certain cases of psoriatic arthritis with only DIP involvement.
INVESTIGATIONS
In psoriatic arthritis, x-ray might show typical erosive changes. Ultrasound and MRI are usually more sensitive in showing enthesitis, tenosynovitis, and erosions.
Avascular necrosis (AVN)
SIGNS / SYMPTOMS
This is common in the hip and knee joints.
The onset is subacute and there is usually a risk factor such as corticosteroid use. Early on, the joint exam is unremarkable, except for possible localized bony tenderness in the knee.
INVESTIGATIONS
MRI is the most sensitive test for AVN. In the early stages, localized subchondral edema is characteristic. In 50% of all cases, accompanying joint effusion may be found. Due to necrosis of the cells of bone marrow and bone fibrovascular tissue, reactions with hyperemia can be delineated.[86]
Internal derangements (e.g., meniscal tears)
SIGNS / SYMPTOMS
The onset of meniscal tears is usually acute and debilitating, with preceding trauma, although the trauma can be minor.
Patients may describe true locking (normal flexion, but an inability to extend the affected knee).
INVESTIGATIONS
MRI is sensitive in detecting both acute meniscal and cruciate ligament tears, although degenerative meniscal tears are common in OA.
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