A frequently used method in this setting is the “ABCs” approach that systematizes eighth different features of hand and wrist arthropathies: joint alignment, bone mineralization, bone production, cartilage, calcification, distribution of affected joints, erosions and soft tissue change. After assessing these features, disease findings can be categorized into three conceptual categories: aggressive erosive diseases, osteoarthritis-like diseases and mass-like depositional diseases. These categories are useful for identifying distinguishing features between pathologies and to provide a differential diagnosis in the report. [2]

• Alignment - joint alignment is preserved by articular surfaces, tendons and ligaments of interphalangeal (IP) and metacarpophalangeal (MCP) joints. Certain arthropathies feature prominent subluxations, such as rheumatoid arthritis. [1, 2]
  • Rheumatoid arthritis is associated with ulnar and volar subluxation of metacarpophalangeal (MCP) joints (Fig. 1) and characteristic finger deformities:
    • Boutonniere (Fig. 2);
    • Swan neck (Fig. 3);
    • Hitchhiker's thumb (Fig. 4). [1, 2]
  • Psoriatic arthritis is characterized by multidirectional subluxations and in advanced stages by “telescoping fingers” due to extensive bone destruction (Fig. 5). [ 2]
  • In osteoarthritis and erosive osteoarthritis subluxations occur only in the coronal plane of IP joints (Fig. 6). [ 2]
  • CPPD and gout are not associated with joint misalignment. (Fig. 7 and Fig. 8). [ 2]

Fig 1: Patient with rheumatoid arthritis subjected to silicone arthroplasty of right hand MCP joints. Notice the ulnar subluxation of MCP joints of the left hand (arrows) as opposed to those of the right hand that were already subject to silicone arthroplasty. © Department of Radiology, Centro Hospitalar Universitário de São João, Porto/Portugal.

Fig 2: Boutonniere finger deformity (arrow) is characterized by flexion of PIP joint and hyperextension of DIP joint. © Department of Radiology, Centro Hospitalar Universitário de São João, Porto/Portugal.

Fig 3: Swan neck finger deformity of fourth and fifth fingers (arrows) occurs by hyperextension of the PIP joint and flexion of the DIP joint. © Department of Radiology, Centro Hospitalar Universitário de São João, Porto/Portugal.

Fig 4: Same patient as Fig. 1 after being subjected to silicone arthroplasty of the left hand. Alignment: Ulnar subluxation of PIP joints and Hitchhiker’s thumb (blue arrows); Bone mineralization/production: periarticular osteopenia; Cartilage/calcification: uniform narrowing of joint spaces of several PIP and MCP joints; Distribution: bilateral, symmetric and proximal joint involvement (see Fig. 5); Erosions: marginal erosions (see Fig. 13); Soft tissues changes: fusiform swelling. Typical findings of rheumatoid arthritis. © Department of Radiology, Centro Hospitalar Universitário de São João, Porto/Portugal.

Fig 5: Alignment: Multidirectional subluxations of DIP, PIP and MCP joints; Bone mineralization/production: production of fluffy bone (arrows) and bone proliferation at the bases of phalanges; Cartilage/calcification: uniform narrowing of joint spaces of several DIP, PIP and MCP joints; Distribution: asymmetric and with extensive DIP joint involvement (see Fig. 12); Erosions: marginal erosions, some are extensive and are associated with bone production create the “pencil-in-cup” deformities (arrowheads, see Fig. 14); Soft tissues changes: fusiform swelling. These features are consistent with psoriatic arthritis. © Department of Radiology, Centro Hospitalar Universitário de São João, Porto/Portugal.

Fig 6: Radiograph featuring osteoarthritis. Alignment: lateral subluxations of some DIP joints; Bone mineralization/production: osteophytes (arrows) and subchondral sclerosis; Cartilage/calcification: asymmetric narrowing of joint spaces; Distribution: DIP joints, IP joint of thumb and trapeziometacarpal joint of the left hand (see Fig. 10); Erosions: none; Soft tissues changes: asymmetric joint distortion. © Department of Radiology, Centro Hospitalar Universitário de São João, Porto/Portugal.

Fig 7: Alignment: no misalignment; Bone mineralization/production: subchondral sclerosis of radiocarpal and trapeziometacarpal joints (blue arrows), hook-like osteophyte in the second MCP joint (white arrow); Cartilage/calcification: joint space reduction of radiocarpal and trapeziometacarpal joints, calcification of triangular fibrocartilage, 3rd and 4th MCP joint spaces (see Fig. 9); Distribution: radiocarpal, trapeziometacarpal and second MCP joints; Erosions: none; Soft tissues tumefaction: none. Typical findings CPPD arthropathy. © Department of Radiology, Centro Hospitalar Universitário de São João, Porto/Portugal.

• Bone mineralization - general osteopenia is assessed by evaluating the summed cortical thickness-to-2nd/3rd-MC shaft width ratio, which is considered abnormal when it falls below 50%. It only occurs in late RA (excluding systemic processes). Periarticular osteopenia is nonspecific. [1]
• Bone production is seen in two primary types: reparative and proliferative bone.
  • OA/EOA are slow degenerative processes, allowing time for reparative bone to occur in the form of subchondral sclerosis, osteophytes, and articular surface widening (Fig. 6). Reparative bone also occurs in diseases that produce secondary OA such as CPPD arthropathy, hemochromatosis, or acromegaly. [ 2]
  • The hallmark of peripheral spondyloarthropathies, most commonly PsA in the hands, is the presence of bone proliferation: fluffy periostitis adjacent to erosions creating a “fuzzy” appearance (Fig. 4). [2] It is also important to note that bone proliferation might occur in some areas without the presence of erosions. [3]
  • Gout is an indolent process, allowing for periosteal new bone formation at the site of erosion that results in overhanging edges (Fig.8). [ 2]

Fig 8: Alignment: no misalignment; Bone mineralization/production: overhanging edges; Cartilage/calcification: preserved joint spaces; Distribution: gout has a random distribution in the hands; Erosions: well corticated extra articular erosions around the PIP and MCP joints of the second finger with overhanging edges (blue arrows, see Fig 7.); Soft tissue tumefaction and increased density around the second PIP joint represents a tophus (white arrow). This radiograph is pathognomonic for gout. © Department of Radiology, Centro Hospitalar Universitário de São João, Porto/Portugal.

• Cartilage - assessment of joint space allows us to deduce cartilage condition. All of the arthropathies produce uniform narrowing of joint spaces except for OA/EOA and gout.
  • Nonuniform narrowing of joint spaces occurs in OA/EOA due to noneven cartilage wear
  • Joint space is usually preserved even in advanced gout. [1,2]

• Calcification occurs in three sites:
  • Soft tissues - tophi in gout occur due to uric acid deposits that are radiolucent, often calcium coprecipitates making a tophus denser (Fig. 8);
  • Tendons - CPPD or hydroxyapatite deposition disease (HADD);
  • Hyaline and fibrocartilage chondrocalcinosis are a characteristic radiographic feature of CPPD (Fig. 9). [2]

Fig 9: Same radiograph as on Fig. 7. Detail of triangular fibrocartilage, 3rd and 4th MCP joint spaces calcifications (arrows). © Department of Radiology, Centro Hospitalar Universitário de São João, Porto/Portugal.

• Distribution patterns are typical to each disease process:
  • Primary OA and EOA most often affect the trapeziometacarpal and DIP joints, followed by the PIP and thumb IP joints (Fig. 10). [2]

Fig 10: Same radiograph as on Fig. 6. Typical OA distribution is highlighted: trapeziometacarpal and DIP joints, followed by PIP and thumb IP joints involvement. © Department of Radiology, Centro Hospitalar Universitário de São João, Porto/Portugal.

• • Rheumatoid arthritis is characterized by bilateral, symmetric and proximal joint involvement and usually spares the DIP joints (Fig. 11). [2,3]

Fig 11: Same radiograph as in Fig. 4. Bilateral, symmetric and proximal (PIP, MCP and radiocarpal) joint involvement is highlighted. The DIP joints are usually spared in rheumatoid arthritis. © Department of Radiology, Centro Hospitalar Universitário de São João, Porto/Portugal.

• • Psoriatic arthritis usually involves the hands and has several patterns of distribution:  
    • Asymmetric DIP and PIP joint predominant (Fig. 12);
    • “Ray” distribution in which all joints of two to three fingers are affected;
    • Symmetric polyarticular pattern that is similar to RA but usually affects one or more DIP joints. [2]

Fig 12: Same radiograph as in Fig. 5. Asymmetric and predominantly distal joint involvement of PsA is highlighted. © Department of Radiology, Centro Hospitalar Universitário de São João, Porto/Portugal.

• • Gout has random distribution in the hands.

• Erosions are classified based on their location in the joint and if they are aggressive or non-aggressive.
  • Aggressive erosions have non corticated margins and occur in RA, PsA and septic arthritis. Indolent processes such as gout and EOA allow for bone repair to occur at the margins of erosion and thus are corticated.
  • The location of the erosion is also an essential feature to evaluate.
    • Erosions in inflammatory arthritis such as RA (Fig. 13) and PsA (Fig. 14) start in the bare area of a joint - the area that occurs in the margin of a joint between the cartilage and the joint capsule insertion - and then progress centripetally. [2,3]

Fig 13: Same radiograph as in Fig.4. Detail of marginal erosions (blue arrows) in RA. © Department of Radiology, Centro Hospitalar Universitário de São João, Porto/Portugal.

Fig 14: Same radiograph as in Fig.5. Detail of extensive erosions (blue arrows), the narrowed metacarpal heads inserting in the proximal phalanges create a “pencil-in-cup” apperance. © Department of Radiology, Centro Hospitalar Universitário de São João, Porto/Portugal.

• • • Erosions in EOA occur in the central area of a joint and spare the marginal bone forming a gull-wing appearance (Fig. 15). [2]

Fig 15: Detail of central erosion in EOA with typical gull-wing appearance. © Department of Radiology, Centro Hospitalar Universitário de São João, Porto/Portugal.

• • • Extra-articular erosions occur adjacent to mass-like depositional processes in the soft tissues, most commonly in gout. In gout the tophus slowly erodes into the bone producing a corticated erosion that might be associated with overhanging edges due to periosteal new bone formation. (Fig. 16). [1,2]

Fig 16: Detail of erosion seen in gout (same radiograph as in Fig. 8). Notice the erosion is well corticated and is associated with bone production forming overhanging edges (arrows). © Department of Radiology, Centro Hospitalar Universitário de São João, Porto/Portugal.

• Soft tissue changes occur in the form of symmetric fusiform joint swelling in inflammatory arthritides. “Sausage digit” appearance occurs in dactylitis and is a highly specific feature of spondyloarthropathies and is a criterion in the CASPAR diagnostic criteria for PsA. [1, 4] In OA and EOA joint distortion occurs due to osteophyte formation forming Heberden (DIP joints) and Bouchard (PIP joints) nodules. Lumpy soft tissue swelling occurs due to the infiltration of foreign substances into the normal tissues, most commonly by urate crystals in gout and more rarely by other depositional processes like amyloidosis or sarcoidosis. [1]

Based on these features diseases can be categorized into three conceptual categories: aggressive erosive diseases, osteoarthritis-like diseases and mass-like depositional diseases. [2]

• Aggressive erosive diseases are characterized by uniform joint space loss, symmetric swelling and non-corticated marginal erosions. This category includes septic arthritis, RA and PsA. Any time there is an aggressive erosive arthropathy confined to a single joint, septic arthritis must be excluded. It is also important to consider that treated RA or PsA might have corticated erosions due to bone healing. Some distinguishing features help differentiate RA and PsA:
  • RA is purely erosive, with the exception of possible carpal bones sclerosis and ankylosis, while PsA can produce IP joint ankylosis;
  • PsA can have symmetric polyarticular distribution like RA, however it usually affects a DIP joint which is exceptionally rare in RA.

• Osteoarthritis-like diseases all feature non uniform joint space loss and osteophyte formation leading to asymmetric joint disportion. OA, EOA, CPPD and hemochromatosis comprise this category. The typical distribution of OA/EOA is the main feature that helps distinguish them from the others:
  • CPPD arthropathy features changes typical for OA in the wrong distribution: it usually involves 2nd and 3rd MCP and/or radioscaphoid joints while sparing the DIP joints. [1, 2] Chondrocalcinosis is associated with CPPD, but may occur as an isolated process. [2] Laboratory (arthrocentesis) or radiographic demonstration of chondrocalcinosis is required for the diagnosis of CPPD.[2]
  • Hemochromatosis has a very similar presentation to CPPD arthropathy, distinguishing features are: OA-like hook osteophytes at the 4th and 5th MCP joints and typically no chondrocalcinosis is present [2].

• Mass-like depositional diseases' most important feature is the presence of a soft tissue lump/bump. Another distinguishing feature is that joint spaces are usually preserved. By far gout is the most common of these diseases, but in the presence of a single mass neoplasm should be considered. Rarely other diseases create this appearance such as amyloidosis, sarcoidosis, and rheumatoid nodules. There is a delay of 5 to 10 years between the first symptoms of gout and the development of radiographic tophi and periarticular erosions, and then they only occur in some patients. Symmetric soft tissue swelling may be the sole radiographic finding. [2] The radiographic features of chronic tophaceous gout are pathognomonic. [1]

The features already mentioned above can then further help in providing the most probable diagnosis (summarized in table. 1).

Table 1: Summary of hand and wrist arthritis features by the ABCs approach. PsA, psoriatic arthritis; RA, rheumatoid arthritis; OA, osteoarthritis; EOA, erosive ostheoarthritis; CPPD, calcium pyrophosphate deposition disease, DIP, distal interphalangeal joint; PIP, proximal interphalangeal joint.