Osteoarthritis

KEY FACTS

Terminology

• Osteoarthrosis, osteoarthropathy, degenerative joint disease

Imaging

• Joint space narrowing, marginal osteophytosis, capsular swelling, synovitis, & effusion are US signs of OA
• Most commonly affects knee, hip, fingers, 1st carpometacarpal, triscaphe, wrist, & acromioclavicular & tarsal joints
• US is increasingly used to determine inflammatory component of OA & differentiate OA from crystal deposition disease or inflammatory arthropathy
• Inflammatory element of OA is characterized by

Capsular swelling

Joint effusion

Synovial proliferation

Synovial & pericapsular hyperemia

Edema of periarticular soft tissues

Clinical Issues

• Only modest correlate between US severity of OA & clinical symptoms

Diagnostic Checklist

• US is best for assessing degree of synovitis

Size of effusion, degree of synovial proliferation, & level of hyperemia to gauge inflammatory component of OA

Presence of echogenic comet-tail artifacts as indicator of crystal arthropathy

Joint aspiration to check for crystals or infection
• In most situations, excellent at guiding needle placement for joint aspiration, therapeutic injection, or synovial biopsy
• Degree of inflammatory component helps differentiate inflammatory-type OA from inflammatory arthropathy

Less pronounced in OA & commensurate with severity of underlying OA

Inflammatory arthropathy associated with more pronounced synovitis ± synovitis

TERMINOLOGY

Abbreviations

• Osteoarthritis (OA)

Definitions

• Degenerative disease with thinning of articular cartilage & associated bone change, soft tissue change, positional change & inflammation

Bone change: Marginal osteophytosis, subchondral reactive changes (oedema, cyst formation, sclerosis, attenuation)

Soft tissue change: Usually mild synovial proliferation, capsular swelling

Positional change: Joint subluxation ± misalignment

Inflammation: Variable degree of inflammation present
• Secondary: Accelerated OA due to existing joint disorder or deformity

IMAGING

General Features

• Best diagnostic clue

Articular cartilage thinning accompanied by marginal osteophytosis, subchondral sclerosis, cyst formation ± inflammatory component
• Location

Most commonly affects knee, hip, fingers, 1st carpometacarpal, scaphotrapezium-trapezoid articulation, wrist, acromioclavicular & tarsal joints

May affect only localized area or whole of joint

Radiographic Findings

• Radiography

Primary screening modality for OA

Joint space narrowing, subchondral sclerosis, subchondral cysts, & osteophyte formation

– Mild subluxation, especially in small joints

Focal cartilage loss may be present with no joint space narrowing radiographically

MR Findings

• T1WI

Cartilage thinning, subchondral cysts, marrow fat signal extending into osteophytes
• T2WI

Subchondral edema

Joint effusion ± intraarticular particulate debris

Reactive-type synovitis

Periarticular edema

Ultrasonographic Findings

• Increasingly used to determine inflammatory component of OA & differentiate OA from crystal or inflammatory arthropathy

Radiography can determine presence of OA but US can reveal inflammatory component

One can also detect OA when US performed for other suspected pathology
• Joint space narrowing, marginal osteophytosis, capsular swelling, synovitis, & effusion

Focal cartilage thinning or defects cannot be fully evaluated with US

Joint space narrowing of some joints (e.g., patellofemoral) may not be readily appreciable
• Joint effusion (may be anechoic or slightly echogenic)

Medial & lateral patellar recesses of knee usually distend prior to suprapatellar recess
• Mild synovial proliferation is common finding in OA
• Inflammatory element is characterized by

Capsular swelling

Joint effusion

Synovial proliferation (synovial thickening & fronds)

– Important sign of inflammatory component present in OA

Pericapsular & synovial hyperemia on color Doppler

– Not very sensitive sign in OA

– Color Doppler imaging is often negative, even with inflammatory component present in OA

Indicative of low-grade inflammation seen usually in OA

– Capsular-synovial hyperemia is difficult to depict in deeper joints such as hip, shoulder, & even knee

– Contrast-enhanced US is more sensitive

Edema of periarticular tissues (better appreciated with MR than US)
• Clinical picture, radiographic findings, & degree of inflammatory component helps differentiate inflammatory-type OA from inflammatory arthropathy

Inflammatory arthropathy has more pronounced synovial proliferation & inflammation

– ± erosions & other clinical/radiographic features of inflammatory arthropathy
• Particulate debris may be visible within joint fluid

US can detect radiographically occult intraarticular debris

– Though limited in assessing whether this particulate debris is free or attached

Central aspects of joint cannot be fully visualized with US (better seen with MR)
• Joint subluxation can be exaggerated on US as dependent on transducer angulation & alignment
• Medial meniscal extrusion is common feature in moderate to severe medial femorotibial OA
• Associated Baker cysts, parameniscal cysts, or ganglia identified by US with high level of accuracy

Inflammatory component, crystal aggregates, & synovial debris can extend into Baker cysts & ganglia
• US-guided joint aspiration is helpful to exclude infective, inflammatory arthropathy or crystal deposition disease

< 1,000 WBC/μL synovial fluid is indicative of OA (normally ~ 200 WBC/μL)

> 5,000 WBC/μL synovial fluid is indicative of acute inflammatory or infective arthropathy

Specimen for crystal identification should be sent fresh as alcohol will dissolve crystals