Approach to the Foot

A systematic assessment of foot radiographs for the manifestations of arthropathies is important, because the foot may be an early site of involvement in a systemic arthropathy such as rheumatoid arthritis, or it may be the only site of involvement in arthropathies such as gout or reactive arthritis. The foot, however, can be difficult to evaluate radiographically. Arches are present in the long and short axes of the foot that make assessment of articulations in more than one plane difficult. The wedge shape of the foot does not permit uniform exposure of the foot on a single radiograph. The hindfoot articulations are complex and often require either computed tomography (CT) or magnetic resonance (MR) imaging for accurate evaluation.

A screening study of the foot should include anteroposterior (AP), lateral, and oblique radiographs. The AP radiograph of the foot permits evaluation of the interphalangeal (IP), metatarsophalangeal (MTP), and the first and second metatarsal-tarsal (MTT) joints. The oblique radiograph is necessary to observe abnormalities of the third through fifth MTT joints, the midfoot, and any early erosive changes on the lateral aspect of the fifth metatarsal. The oblique radiograph also permits evaluation of the lateral sesamoid at the first MTP joint. The lateral radiograph provides orthogonal assessment of the forefoot articulations, the mid- and hindfoot articulations, and the calcaneus. On rare occasions, a sesamoid view may be necessary to observe the sesamoidal articulation with the first metatarsal head.

Successful assessment of the foot depends on systematically observing changes in four separate anatomic compartments: (1) the forefoot articulations (MTP, sesamoid-MTP, and IP joints), (2) the MTT joints, (3) the mid- and hindfoot articulations (tarsal joints), and (4) the ligamentous insertions about the calcaneus. As in the hand, the following radiographic changes should be assessed: soft tissue swelling, soft tissue calcification, bony mineralization, joint space narrowing, erosion, subluxation and dislocation, and bone production.

Forefoot

Arthropathies involving the IP joints and the MTP joints of the forefoot follow the same principles outlined in the chapter on the assessment of the hand. The sesamoid bones of the first MTP joint have a synovium-lined articulation with the plantar aspect of the first metatarsal head and, if involved, will demonstrate the manifestations of any of the arthropathies of the foot. This articulation should not be forgotten when assessing foot radiographs.

Soft Tissue Swelling

Symmetrical Swelling Around a Joint ( Fig. 3-1 )

Symmetrical swelling about a joint is a manifestation of synovial proliferation, effusion, and periarticular soft tissue edema associated with inflammatory arthropathies. Soft tissue swelling is easier to appreciate with digital radiographic techniques than with a film screen system.

Fusiform Swelling of an Entire Digit ( Fig. 3-2 )

The diffuse swelling of a digit resulting in a “sausage” or “cocktail hot dog” appearance is a manifestation of the spondyloarthropathies, trauma, and infection.

Lumpy, Bumpy Soft Tissue Swelling ( Fig. 3-3 )

Soft tissue masses located eccentrically about a joint associated with cortical erosions are findings most commonly associated with gout, although these changes can be seen with amyloid, xanthomas, and sarcoid.

Soft Tissue Calcification

Mass ( Fig. 3-4 )

Gouty tophi may or may not contain varying amounts of calcium. Regardless of calcium content, gouty tophi are more radiopaque than the surrounding soft tissues.

Tendinous or Ligamentous and Soft Tissue Calcification ( Fig. 3-5 )

Idiopathic hydroxyapatite deposition disease may present as calcification of the tendons of the medial flexor group (flexor hallucis longus, flexor digitorum longus, and posterior tibialis) or around the first MTP joint. Because soft tissue calcifications can be associated with renal osteodystrophy and scleroderma, these diseases must be excluded before diagnosing idiopathic disease.

Mineralization

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