Pathology: disease of the articular cartilage. Continuing mechanical stimulation follows by initial increase in water and cartilage thickness. Gradual loss of proteoglycans and ground substance. Fissuring/splitting. Chondrocytes are damaged and release enzymes into the joint. Cystic progression and further cartilage loss. Subchondral bone is denuded and exposed to mechanical stresses. It becomes hypervascular forming osteophytes. Subchondral cysts and bone thickening/sclerosis develop.
Imaging plays a crucial role in Dx/grading and management
Grade 4: severe JSN, large osteophytes, marked subchondral sclerosis and definite bony deformity
Typical report language will state:
Minor, mild, moderate or severe aka advanced arthrosis
Technique
Radiography: AP weight-bearing knees: note severe JSN of the medial compartment more severely with lateral knee compartment. Osteophytes and marked genu varum deformity and bone deformation
Typically medial femoro-tibial compartment is affected early and more severely
Patellofemoral compartment is also affected and best visualized on the lateral and Sunrise views
Impressions: severe tri-compartmental knee arthrosis
Recommendations: referral to the orthopedic surgeon
Moderate JSN
B/L AP weight bearing view (above top image): Moderate JSN primarily of the medial femorotibial compartment. Osteophytosis, subchondral sclerosis and mild bone deformation (genu varum)
May present as asymptomatic chondrocalcinosis, CPPD arthropathy resembling DJD with predominance of lare subchondral cysts. Often found as isolated PFJ DJD
Pseudogout with acute attack of knee pain resembling gouty arthritis
Radiography is the 1st step and often reveals the Dx
Arthrocenthesis with polarized microscopy may be helpful to DDx between CPPD and Gouty arthritis
Rheumatoid Arthritis
RA: autoimmune systemic inflammatory disease that targets soft tissues of joints synovium, tendons/ligaments, bursae and extra-articular sites (e.g. eyes, lungs, cardiovascular system)
RA is the m/c inflammatory arthritis, 3% of women and 1% of men. Age: 30-50 F>M 3:1, but may develop at any age. True RA is uncommon in children and should not be confused with Juvenile Idiopathic Arthritis
RA most often affects small joints of the hands and feet as symmetrical arthritis (2nd 3rd MCP, 3rd PIPs, wrists & MTPs, sparing DIPs of fingers and toes)
Radiographically: RA presents with joint effusion leading to hyperemia and marginal erosions and periarticular osteoporosis. In the knee, lateral compartment is affected more frequently leading to valgus deformity. Uniform aka concentric/symmetrical JSN affects all compartments and remains a key Dx clue
Absence of subchondral sclerosis and osteophytes. Popliteal cyst (Baker’s cyst) may represent synovial pannus and inflammatory synovitis extending into popliteal region that may rapture and extend into posterior leg compartment
N.B. Following initial RA joint destruction, it is not unusual to note superimposed 2nd OA
Radiography is the 1st step but early joint involvement may be undetectable by x-rays and can be helped by US and/or MRI.
Final Dx is based on Hx, clinical exam, labs and radiology
Clinical pearls: patients with RA may present with a single knee being affected
Most patients are likely to have bilateral symmetrical hands/feet RA.
Cervical spine, particularly C1-2 is affected in 75-90% of cases throughout the course of the disease
N.B. Sudden exacerbation of joint pain in RA should not underestimate septic arthritis because patients with pre-existing RA are at higher risk of infectious arthritis. Joint aspiration may help with Dx.
Radiographic DDx
RA (above left) vs. OA (above right)
RA: concentric (uniform) joint space loss, lack of osteophytes and juxta-articular osteopenia.
Clinical Pearls: patients with RA may present radiographically with subchondral sclerosis d/t superimposed DJD. The latter feature should not be interpreted as OA but instead considered as secondary OA
AP Knee Radiograph
Note marked uniform JSN, juxta-articular osteopenia and subchondral cystic changes
Clinical Pearls: subcortical cysts in RA will characteristically lack of sclerotic rim noted in OA-associated subcortical cysts.
MRI Sensitivity
MRI is very sensitive and may aid during early Dx of RA.
T2 fat-sat or STIR and T1 + C gad contrast fat-suppressed sequences may be included
MRI Dx of RA: synovial inflammation/effusion, synovial hyperplasia and pannus formation, decreased cartilage thickness, subchondral cysts and bone erosions
MRI is very sensitive to reveal juxt-articular bone marrow edema, a precursor to erosions
Intra-articular fibrinoid fragments known as “Rice bodies” are characteristic MR sign of RA
Note: T2 fat-sat sagittal MRI revealing large inflammatory joint effusion and pannus synovial proliferation (above arrow heads). No evidence of radiographic or MRI bone erosions present. Dx: RA
STIR MR Slices
Note: STIR MR slices in the axial (above bottom image) and coronal planes (above top image) demonstrate extensive synovitis/effusion (above arrowheads) and multiple erosions in the medial and lateral tibial palteau (above arrows)
Additionally, scattered patchy areas of bone marrow edema are noted (above asterisks) such marrow edema changes are indicative and predictive of future osseous erosions.
Additional features: note thinning and destruction of joint cartilage