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
Normal
Generally bone mineralization is maintained in the spondyloarthropathies, thus distinguishing these erosive arthropathies from rheumatoid arthritis.
Juxta-Articular Osteoporosis ( Fig. 3-6 )
Juxta-articular osteoporosis is a nonspecific finding that can be seen in nonarthropathic conditions. In an arthropathy it is most commonly associated with inflammatory disease. Juxta-articular osteoporosis may be difficult to appreciate when the joints are diffusely involved. Resorption of subcortical bone in the medial aspect of the metatarsal head may be an indication of osteoporosis. Juxta-articular osteoporosis in acute reactive arthritis is the only feature that can radiographically distinguish this disease from psoriatic arthropathy.
Diffuse osteoporosis
Generalized osteoporosis can be documented by assessing the cortical width in relation to the shaft width of the metatarsal bone. Generalized osteoporosis is usually seen in patients with rheumatoid arthritis.
Joint Space Change
Widening of Joint Space ( Fig. 3-7 )
Widening of a joint is often observed in acromegaly. Sometimes a joint involved by psoriatic arthropathy may appear widened secondary to fibrotic material replacing the joint.
Normal ( Fig. 3-8 )
The joint space is typically maintained in gout in the face of periarticular well-corticated erosions with overhanging edges produced by tophaceous deposits.
Uniform Narrowing
Uniform joint space narrowing reflects uniform loss of the cartilage and is associated with both inflammatory arthropathies and deposition diseases.
Nonuniform Narrowing
Asymmetrical loss of cartilage is associated with mechanical osteoarthritis. Osteoarthritis is most commonly seen at the first MTP joint.
Ankylosis ( Fig. 3-9 )
Distal IP and PIP joint ankylosis is associated with the spondyloarthropathies. Ankylosis of the MTP joints is rare.
Erosion
Aggressive Erosions
Aggressive erosions do not have corticated margins and are a manifestation of the inflammatory arthropathies. They tend to occur at the “bare area” of a bone, which is the area between where the synovial lining joins bone and the edge of the articular cartilage. Erosions are best seen on the AP view of the foot, which assesses the IP joints and the medial aspects of the second through fifth metatarsal heads ( Fig. 3-10 ). The lateral aspect of the fifth metatarsal head is the earliest site of erosive disease in rheumatoid arthritis of the foot and is best seen on the oblique view ( Fig. 3-11 ). The “pencil-in-cup” deformity is seen most frequently at the first IP joint and is most commonly associated with psoriatic arthritis, although it is also a manifestation of reactive arthritis ( Fig. 3-12 ). Erosion of the distal tuft (acro-osteolysis) may be seen with reactive arthritis and psoriatic arthropathy ( Fig. 3-13 ). Patients with acro-osteolysis in psoriasis may have nail changes that may be seen radiographically.
Nonaggressive Erosions
An erosive process that is indolent will allow a reparative response to occur. This reparative response will be manifested by a sclerotic margin to the erosion. Such an erosion is classically associated with gout. The erosions of gout are most commonly seen at the dorsomedial aspect of first MTP or IP joint and may simulate subchondral cysts in the AP view. Careful evaluation of the oblique view or the lateral view will demonstrate the dorsally located well-corticated erosion with overhanging cortex associated with a soft tissue mass (tophus) ( Fig. 3-14 ). Patients with inflammatory arthropathies whose disease is in remission may also exhibit erosions with sclerotic margins, but these erosions tend to be located along the plantar aspect of the foot ( Fig. 3-15 ). Normally there is loss of joint space accompanying healed inflammatory erosions, whereas there is maintenance of the joint space with the erosions of gout.
Bone Production
There are two different kinds of new bone production. One is in the form of added bone as a periostitis or enthesitis. The second is in the form of a reparative response.
New Bone of Enthesopathies
Periosteal new bone and bone formed at tendinous insertions are associated with the spondyloarthropathies. The periostitis of the spondyloarthropathies tends to be seen along the shafts of the phalanges and the metatarsal bones ( Fig. 3-16 ). Periosteal new bone can also be seen at the base of the fifth metatarsal and along the medial aspect of the tarsal navicular ( Fig. 3-17 ). New bone may be identified at any ligamentous attachment. New bone can also be seen within or just behind an erosion, giving the erosion a “paint brush” appearance ( Fig. 3-18 ).
Reparative Response
Overhanging Edge of Cortex. The slow erosive process of a gouty tophus will permit a reparative response and elevation of the cortex, which will manifest as an overhanging edge ( Fig. 3-19 ).
Related posts:
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
