Joint Disease

Location

It is typically asymmetrical and affects the hands, spine and large weight-bearing joints ▸ primary and secondary OA have similar radiological appearances

Hand

The commonly affected joints: 1 ^st MCP joint, scaphotriquetral joint, IP joints ▸ distal and proximal IP joint prominences are commonly seen (Heberden's and Bouchard's nodes, respectively)

Knee

Tibial spine spiking is an early sign ▸ the medial joint compartment shows the greatest narrowing (as it is subject to greater stressors) leading to a varus deformity ▸ it usually affects the lateral facet of patellofemoral joint ▸ there is articular and meniscal cartilage chondrocalcinosis

• Chondromalacia patellae: this is a related condition in younger patients with softening of the patellar cartilage (due to repetitive trauma)

• Pellegrini–Stieda disease: ossification of the medial collateral ligament is associated with degenerative change or may occur in isolation

Hip

Loss of the superior weight-bearing joint space with marginal osteophytes, subchondral acetabular cysts (Egger's cyst) and sclerosis formation ▸ there is a tendency for superolateral femoral head subluxation (but there may be lateral restraining osteophytes) ▸ rarely medial migration can lead to protrusio acetabuli ▸ ‘buttressing’: osseous hypertrophy along the medial femoral neck

Foot/ankle

The 1st MTP joint is the most commonly affected site (and is associated with hallux valgus)

• ‘Talar beak’: the talonavicular joint is also commonly involved with formation of a dorsal bone spur

Spine

See degenerative spinal disease Section 7 Chapter 6

Erosive OA

This characteristically affects middle-aged women ▸ destructive changes outstrip productive changes and can mimic an erosive arthritis (such as psoriatic arthropathy)

XR

Central erosions and marginal osteophytes (a ‘gull wing’ pattern) affecting the PIP + DIP joints (although large joint involvement has been described)

• Differential: psoriasis and rheumatoid arthritis – however these will have no 1 ^st CMC joint involvement and will have marginal (as opposed to central) erosions with no true osteophyte formation

Plain radiographic findings in primary osteoarthritis and corresponding underlying pathophysiological causes*

Radiological finding	Pathological cause
Localized joint space narrowing	Articular cartilage fibrillation, ulceration and erosion lead to changes in collagen and protein polysaccharide structure of cartilage. This results in reduced turgor
Subchondral bony sclerosis	Increased osteoblastic activity resulting in new bone formation and increased cellularity of the subchondral bone
Osteophyte formation (most commonly marginal)	Cartilage and bone proliferation and revascularization of remaining cartilage
Bone cysts and bone collapse	Subchondral micro-fractures and passage of synovial fluid under pressure through the damaged cartilage to excavate a subchondral cyst
Gross deformity with subluxation	Ligamentous laxity resulting from mechanical force applied after the distortion of capsular structures
Loose bodies	Fragments of bone and cartilage become separated and, if not resorbed, become loose in the joint. They may reattach to the membrane, become vascularized and undergo endochondral ossification
Fibrocartilage or hyaline cartilage calcification	This is usually due to calcium pyrophosphate deposition disease (CPPD). The reparative response is usually quite florid

Location

It particularly affects the knee, elbow, ankle and shoulder ▸ small peripheral joints are rarely involved

Intraosseous subperiosteal or soft tissue pseudotumours

These can develop following the encapsulation of episodes of repeated haemorrhage ▸ it occurs particularly within the femur and around the pelvis ▸ it presents as a painless expanding mass, with soft tissue and bone destruction ▸ can be calcified ▸ mural nodules are characteristic

MRI

T1WI: a low SI thick fibrous capsule ▸ T2WI: central high SI (due to blood breakdown products)

Location

The distribution helps determine the cause (see table)

Atrophic form

This occurs early and is more acute ▸ resorption of the ends of affected bones results in sharp pointed ends ▸ there is an absence of osteoporosis, sclerosis, fragmentation or soft tissue debris ▸ it may lead to joint dislocation

Hypertrophic form

This occurs later with a slow progression ▸ it begins with a joint effusion ▸ the joint spaces are initially widened but then narrowed ▸ there is marked bony sclerosis (and no osteoporosis) ▸ fragmentation of the articular surfaces results in bony debris, which may later fuse into a large dense and well-organized corticated bony mass (± fusion with the underlying bone or dissection into the muscle planes) ▸ there is periosteal new bone formation ▸ subluxation and dislocation preceeds total joint disorganization ▸ pathological fractures can occur

• 5 ‘D's’: normal bone D ensity ▸ joint D istension ▸ bony D ebris ▸ joint D isorganization ▸ D islocation

Location

Knee > wrist ▸ also involved: feet, shoulders, elbows and hips

Early

Soft tissue swelling ▸ synovitis ▸ synovial hyperplasia and pannus formation ▸ a widened joint space (secondary to an effusion) ▸ periarticular osteopenia

• Initially there is cartilage preservation

Late

Epiphyseal overgrowth and premature closure of the physis (due to a prolonged synovitis and hyperaemia) ▸ cartilage loss ▸ there is often extensive ankylosis (affecting the CMC and mid-carpal joints) ▸ joint space narrowing

• Bone erosions are uncommon

Peripheral joints

Marked radial head enlargement may be seen ▸ a widened intercondylar notch of the knee ▸ overtubulated diaphyses ▸ enlarged and osteoporotic epiphyses with compression fractures ▸ florid periosteal new bone formation within the phalanges, metacarpals and metatarsals (cf. RA) ▸ enlarged, irregular and squared carpal bones (due to erosion and repair)

• Misalignment and subluxation are uncommon

Spine/sacroiliac joints

The cervical spine is the most commonly affected region (it is rare within the thoracolumbar spine or sacroiliac joints) ▸ atlantoaxial subluxation is common ▸ compression fractures ▸ scoliosis (with advanced disease)

• There can be underdevelopment and block fusion of the vertebral bodies and intervertebral discs (due to disc and apophyseal joint ankylosis which can also mimic Klippel–Feil syndrome)

Spine

T7 to T12 are commonly affected (it is typically right sided as the pulsating aorta inhibits ossification on the left) ▸ ossification of the anterior longitudinal ligament is seen at the affected level (up to 2 cm)

• New bone formation within the cervical spine can cause dysphagia

• Ossification of the posterior longitudinal ligament can cause spinal stenosis

Extraspinal features

• Enthesopathy: heel and elbow spurs and a whiskered pelvic appearance

• Ossification: pelvic tendons and ligaments ▸ superior of the sacroiliac joints ▸ symphysis pubis ▸ calcaneus ▸ tarsal bones ▸ patella ▸ olecranon ▸ humerus ▸ hands

Location

A symmetrical polyarthritis of the small joints of the hands and feet > larger joints > axial skeleton (usually affecting the cervical spine)

• Early: juxta-articular osteopenia ▸ symmetrical soft tissue swelling ▸ joint space widening ▸ tenosynovitis

• Later: diffuse osteoporosis ▸ marginal erosions involving the bare area (the bone between the edge of the articular cartilage and the joint capsule attachment) ▸ joint space narrowing ▸ reduced soft tissue swelling ▸ subchondral cysts ▸ joint subluxation and dislocation ▸ joint ankylosis (this is uncommon but typically affects the carpus)

• End stage: pancompartmental loss of the joint spaces ▸ resorption of the carpal bones and ‘arthritis mutilans’

Osteopenia

Early: periarticular ▸ late: it becomes generalized due to steroid use or limitation of movement

Joint space changes

Early widening (due to synovial hypertrophy, effusion or pannus interposition between the articular surfaces) is followed by narrowing (due to cartilage destruction by pannus) ▸ joint alignment abnormalities are caused by tendonitis, tendon rupture or synovitis weakening the capsule

Erosions

These are classic periarticular marginal erosions involving the ‘bare areas’ of bone ▸ they less commonly involve the larger joints but are often more destructive in nature due to the greater stresses involved

Periostitis

This is less commonly seen than with a seronegative arthropathy ▸ if present it is commoner within the feet

Soft tissue changes

Fusiform swelling (due to capsular distension and oedema) over the IP and MCP joints ▸ swelling over the ulnar styloid (due to local involvement of the extensor carpi ulnaris tendon sheath) ▸ distension of the knee joint capsule (due to lateral displacement of the normally barely visible fat planes adjacent to the distal femur) ▸ rheumatoid nodules

Hand

Proximal distribution (carpal/MCP/PIP joints) with sparing of the DIP joints (cf. OA and psoriatic arthritis) ▸ commonly affected sites: the 2 ^nd and 3 ^rd MCP joints (initially affecting the radial side – more distal erosions are less commonly seen) ▸ the ulnar and radial styloid processes ▸ the distal radioulnar joint and carpus

• Ulnar deviation (due to MCP subluxation)

• Volar subluxation and dislocation of the phalanges at the MCP joint

• Rotary subluxation of the scaphoid and drift of the entire carpus in an ulnar (± volar) direction

• Boutonnière deformity: proximal IP joint flexion and distal IP joint extension

• Swan neck deformity: proximal IP joint extension and distal IP joint flexion

• ‘Telescope’ fingers: phalangeal dislocation with subsequent shortening

Foot

Changes lag behind those in the hands ▸ disease initially involves the MTP joints (particularly the 4 ^th and 5 ^th ) with erosions on the bare areas of the metatarsal heads ▸ lateral phalangeal subluxation with PIP joint dorsiflexion deformities ▸ bony ankylosis with chronic disease

Hips

There is predominantly medial joint involvement (cf. superior joint involvement in OA) ▸ medial migration and acetabular resorption results in protrusio acetabuli

Shoulder

Lateral clavicular resorption ▸ upward subluxation of the humeral head

Spine

Disease usually affects the cervical spine (the thoracic and lumbar spine are rarely involved) ▸ cervical spine facet joint erosions may result in subluxation and nerve entrapment ▸ basilar invagination is a late feature

• Atlantoaxial subluxation: this is due to cruciate ligament damage (separation in flexion of > 2.5 mm in adults or 5 mm in children is abnormal)

• Odontoid erosion: this results from involvement of the synovial joint between the odontoid peg and cruciate ligament or involvement of the small bursa adjacent to the odontoid

• Odontoid fracture: this is due due to erosions or osteopenia