Cartilage calcification (chondrocalcinosis) can occur in both fibrocartilage and hyaline cartilage. Fibrocartilaginous calcifications are most common in the menisci of the knee, triangular fibrocartilage of the wrist, symphysis pubis, annulus fibrosus of the intervertebral disc, and acetabular and glenoid labra but also can be seen also within the discs of the sternoclavicular and acromioclavicular joints.
Fibrocartilaginous deposits appear more thick, shaggy, and irregular when compared to the deposits in hyaline articular cartilage.
Hyaline cartilage calcifications may be detected by plain radiography in many joints but most commonly in the knee, wrist, hip, elbow, and shoulders. These deposits are typically thin, punctate and linear or curvilinear and are parallel to but separated from the subchondral bone plate.
Synovial calcification is a relatively common feature of calcium pyrophosphate dihydrate (CPPD) crystal deposition disease. Such synovial deposits are most frequently detected by plain radiography in the knee, wrist, and metacarpophalangeal and metatarsophalangeal joints.
CPPD crystal arthropathy affecting the joint can have features similar to those seen in osteoarthritis, such as joint space narrowing, sclerosis, and cyst formation; however, distribution of involved joints (and/or specific joint compartments) can help differentiate this particular arthropathy from primary, degenerative joint disease. Examples are isolated involvement of the patellofemoral compartment of the knee, isolated radiocarpal or trapezioscaphoid joint involvement in the wrist, and isolated talocalcaneonavicular joint involvement.
Prominent subchondral cyst formation, variable osteophyte formation, and, in some cases, destructive bone changes that are severe and progressive (resembling those seen on neuropathic arthropathy) also help in distinguishing primary CPPD deposition disease from osteoarthritis.
The characteristic homogeneous cloudlike appearance of basic calcium phosphate (BCP) crystal deposition and a predilection for characteristic sites help distinguish it from other disorders that can be also associated with periarticular calcifications.
BCP crystal deposition can be confused sometimes with gout due to radiographic and clinical similarities, and the fact that some tophi are faintly calcified, but BCP deposition should not be confused with the more linear, punctate and diffuse CPPD crystal calcifications.
BCP and CPPD crystal deposition can coexist in the same joint.
In this chapter, we review the imaging appearance of calcium pyrophosphate dihydrate (CPPD) crystal deposition disease and of basic calcium phosphate (BCP) crystal deposition related arthropathy (which is here termed hydroxyapatite deposition disease [HADD]). This chapter focuses on plain radiography but also discusses advanced imaging. High-resolution ultrasound is separately reviewed (see Chapter 26 ) in this book. Importantly, Chapter 24 describes the use of dual-energy computed tomography (DECT) to specifically distinguish tophi from pathologic calcifications, and DECT will not be further discussed here.
Calcium Pyrophosphate Dihydrate Crystal Deposition Disease
A variety of terms have been used to describe different manifestations of CPPD crystal deposition disease. The European League Against Rheumatism (EULAR) has proposed a specific nomenclature. However, in this review, nomenclature will be employed that features use of the terms “chondrocalcinosis” and “pyrophosphate arthropathy.” Chondrocalcinosis is a more general term that should be reserved for radiologically or pathologically evident cartilage calcification. Such cartilage deposition can be due to CPPD, dicalcium phosphate dihydrate, or calcium hydroxyapatite crystals, or a combination of the three.
“CPPD crystal deposition disease” is a more specific term for a specific disorder characterized by the exclusive presence of CPPD crystals in or around joints.
The term “pseudogout” is not a radiologic diagnosis; this term should be reserved for the gout-like clinical syndrome produced by CPPD crystal deposition and characterized by intermittent acute attacks of arthritis.
The term “pyrophosphate arthropathy” (discussed later) here refers to a particular pattern of structural joint damage that can develop in patients with CPPD crystal deposition disease.
General Imaging Features
Articular and Periarticular Crystal Deposition
Crystalline deposits of CPPD can be located in the cartilage, synovium, capsule, tendons, bursae, ligaments, and soft tissues.
Cartilage calcification (chondrocalcinosis) can occur in both fibrocartilage and hyaline cartilage. Fibrocartilaginous calcifications are most common in the menisci of the knee, triangular fibrocartilage of the wrist, symphysis pubis, annulus fibrosus of the intervertebral disc, and acetabular and glenoid labra. These deposits can be seen also within the discs of the sternoclavicular and acromioclavicular joints. Fibrocartilaginous deposits appear more thick, shaggy, and irregular compared to the deposits in hyaline cartilage. Hyaline cartilage calcifications may occur in many joints but are most common in the knee, wrist, hip, elbow, and shoulders. These deposits are typically thin, punctate and linear or curvilinear and are parallel to but separated from the subchondral bone plate.
Synovial calcification is a relatively common feature of CPPD crystal deposition disease. Such synovial deposits are most frequent in the knee, wrist, and metacarpophalangeal and metatarsophalangeal joints. When present, synovial calcifications are usually combined with chondrocalcinosis but in some instances can be the dominant radiographic feature. These synovial deposits may be cloudlike, particularly at the margins of the joint. In addition, detached pieces of calcified synovium can be seen. The overall appearance may resemble idiopathic synovial chondromatosis.
Capsular calcification is more frequently seen in the elbow, knee, metacarpophalangeal, and glenohumeral joints. These deposits usually appear as fine or irregular linear calcifications along the capsule spanning the joint.
Tendon, bursa, and ligament calcification is another feature that can be seen in patients with CPPD crystal deposition disease. Commonly involved tendons include the Achilles, triceps, quadriceps, gastrocnemius, popliteus, gluteal, and sometimes the tendons of the rotator cuff. Such calcifications along tendons appear thin and linear and may be quite extensive. Common involved ligaments include the intercarpal ligaments of the wrist as well as the cruciate ligaments of the knee.
Soft tissue calcification can occasionally be seen in patients with CPPD crystal deposition disease particularly about the elbow, pelvis, and wrist. These calcifications are usually poorly defined and sometimes can be quite prominent and tumor-like, particularly around the digits and the temporomandibular joint, as well as in the retro-odontoid region. Other reported sites include the elbow, hip, wrist, knee, glenohumeral, and acromioclavicular joints. Because of the overall imaging appearance of these lesions, which can be fairly impressive and sometimes aggressive-looking with erosion of the adjacent osseous structures, different designations have been coined on reported cases including terms such as “tophaceous pseudogout,” “pseudotumor,” “massive CPPD,” “tumoral CPPD,” and “destructive CPPD arthropathy,” among others.
The term “pyrophosphate arthropathy” is not a generally adopted nomenclature but it can be used for a specific type of arthropathy related to CPPD crystal deposition disease, with characteristic structural changes depending on the joint involved. The wrist, knee, and metacarpophalangeal joints are the most commonly affected but it may occur in any joint. Although usually bilateral, symmetric changes may not be always present. Features similar to those seen in osteoarthritis, such as joint space narrowing, sclerosis, and cyst formation occur; however, there are several characteristics that differentiate this particular arthropathy from the classic degenerative joint disease. For instance, although pyrophosphate arthropathy can be found in weight-bearing joints (i.e., knee and hip), it can also occur in articulations that are less commonly involved in degenerative joint disease such as the wrist, elbow, and glenohumeral joints. Furthermore, its distribution is unusual in some joints. For instance, isolated patellofemoral compartment of the knee, isolated radiocarpal or trapezioscaphoid joint involvement in the wrist, and isolated talocalcaneonavicular joint involvement in the foot are highly suggestive of pyrophosphate arthropathy. Additional features such as prominent subchondral cyst formation, variable osteophyte formation, and, in some cases, destructive bone changes that are severe and progressive (resembling those seen on neuropathic arthropathy) also help in its distinction.
The knee is the most commonly involved joint in CPPD crystal deposition disease. Chondrocalcinosis affecting both the fibrocartilage of the meniscus and the hyaline articular cartilage can be seen as well as synovial and capsular calcifications ( Fig. 25-1 ). Tendinous and ligamentous deposits of CPPD, particularly along the quadriceps, gastrocnemius, and popliteus tendons, as well as along the cruciate and collateral ligaments, can be also seen. Rarely, large calcified pseudotumoral soft tissue masses can be found about the knee.
Pyrophosphate arthropathy of the knee most commonly involves the medial femorotibial compartment followed by the patellofemoral compartment and, less frequently, the lateral femorotibial compartment. However, isolated or severe patellofemoral compartment changes should suggest this particular diagnosis. Along with these changes, the presence of a distal femoral excavation or erosion along the anterior cortex of the distal femur is highly suggestive of pyrophosphate arthropathy ( Fig. 25-2 ). Isolated involvement of the lateral femorotibial compartment should also suggest this diagnosis. Stress fractures and collapse of the articular surface in either the medial or lateral compartment resembling the features of spontaneous osteonecrosis of the knee can be seen.
The wrist is another commonly involved joint in CPPD crystal deposition disease. Chondrocalcinosis can be seen affecting the triangular fibrocartilage as well as the hyaline cartilage of the radiocarpal, midcarpal, and common carpometacarpal joints ( Fig. 25-3 ). Synovial and capsular calcifications can occur as well as ligamentous deposits, particularly along the scapholunate and lunotriquetral intrinsic ligaments, which may rupture with resultant widening of the interosseous distances and carpal malalignment. Depending on the location of the deposits, areas of significant synovitis, tenosynovitis, and even tendon ruptures can also occur.
Pyrophosphate arthropathy in the wrist has characteristic features including the predilection for the radiocarpal compartment with evidence of joint space narrowing, subchondral sclerosis, and cyst formation. The scaphoid can move proximally close to the radius while the lunate may move distally close to the capitate with a resultant “stepladder’ appearance, which is characteristic of this arthropathy ( Fig. 25-4 ). The overall appearance is similar to the so-called scapholunate advanced collapse (SLAC) initially described in the setting of a wrist injury. The midcarpal compartment is the second most commonly involved after the radiocarpal compartment. Moreover, isolated changes of the trapezioscaphoid joint are suggestive of the disease. Cases of carpal tunnel syndrome have been reported in the setting of CPPD crystal deposition disease.
Features of CPPD crystal deposition disease at the level of the metacarpophalangeal joints include calcifications in the cartilage, capsule, and synovium as well as changes of arthropathy including joint space narrowing, subchondral sclerosis, and cyst formation, in addition to collapse of the metacarpal heads. Such structural joint changes show a characteristic predilection for the second and third metacarpophalangeal joints ( Fig. 25-5 ). Milder changes can be seen occasionally in the interphalangeal joints. The bone collapse is more prominent in this arthropathy than in degenerative joint disease, which, in addition to the predilection of the metacarpophalangeal involvement over the interphalangeal joint, helps in its differentiation from osteoarthritis. The structural changes seen in the second and third metacarpophalangeal joints can also be seen in hemochromatosis; however, in this latter condition, similar changes in the fourth and fifth metacarpophalangeal joints are common. The “drooping” or hooklike osteophytes along the radial aspects of the metacarpal heads that can be seen in patients with CPPD crystal deposition disease are also characteristic of hemochromatosis.
A different metacarpophalangeal arthropathy related to manual labor, the “Missouri metacarpal syndrome,” can have structural changes similar to pyrophosphate arthropathy; however, in patients with this syndrome, there is no evidence of CPPD crystal deposition disease or hemochromatosis.
Chondrocalcinosis, capsular, and synovial calcifications, as well as deposits along the flexor, extensor, distal biceps, and triceps tendons, can occur ( Fig. 25-6 ). Features of pyrophosphate arthropathy in the elbow include joint space narrowing, subchondral sclerosis, and cyst formation, as well as osseous resorption in the proximal radius and ulna. Surrounding bursae about the elbow, including the olecranon and bicipitoradial (cubital) bursa, can contain CPPD crystals. Tumoral CPPD or “tophaceous pseudogout” has been reported in the elbow. Depending on the specific location, cases of cubital tunnel syndrome and posterior interosseous nerve entrapment have been described.
Chondrocalcinosis affecting both the fibrocartilage of the acetabular labrum and the hyaline articular cartilage of the femoroacetabular joint can be seen as well as capsular and synovial calcifications. The fibrocartilaginous deposits can present as small radiodensities along the periphery of the superolateral aspect of the acetabulum ( Fig. 25-7 ). The deposits along the articular hyaline cartilage can present as a radiodense curvilinear line that parallels the femoral head but is separated from the subchondral bone plate. Surrounding tendons, including the rectus femoris, hamstrings, and adductor insertion, may also calcify. CPPD crystals can deposit in the surrounding bursae of the hip including the trochanteric bursa. The initial structural changes in CPPD-related arthropathy of the hip can resemble osteoarthritis, with subchondral sclerosis, lateral osteophyte formation, and joint space narrowing in the superolateral aspect of the joint. However, concentric joint narrowing with axial migration can also occur, mimicking the appearance of an inflammatory arthropathy such as rheumatoid arthritis. A pattern of rapid and extensive destruction of the hip has been described, and although its cause is not entirely clear, some of the cases may be related to CPPD crystal deposition disease ( Fig. 25-8 ). Cases of tumoral CPPD have also been reported about the hip.
Chondrocalcinosis affecting both the fibrocartilage of the glenoid labrum and the hyaline articular cartilage of the glenohumeral joint can be seen as well as capsular, tendinous, and bursal calcifications. The deposits along the articular hyaline cartilage can present as a radiodense curvilinear line that parallels the humeral head but is separated from the subchondral bone plate ( Fig. 25-9 ). Rotator cuff tears are common in patients with CPPD crystal deposition disease. The structural changes can resemble degenerative joint disease with joint space narrowing, bony eburnation, cysts, and osteophyte formation. Similar to the hip, a pattern of advanced destructive changes may be occasionally apparent ( Fig. 25-10 ). Whether CPPD has a role in the Milwaukee shoulder syndrome (discussed later in this chapter) is controversial and not entirely clear. While CPPD crystals have been found in patients with Milwaukee shoulder, other crystals have also been detected. Some authors believe that the structural joint damage is more related to BCP crystals. Milwaukee shoulder syndrome probably represents a severe example of mixed BCP crystal deposition.
In addition to the glenohumeral joint, the acromioclavicular joint can also be affected in patients with CPPD crystal deposition disease about the shoulder. Calcification is most common in the articular disc; however, calcified deposits above the joint can be seen as well as adjacent calcified or cystic masses intimate with the acromioclavicular joint ( Fig. 25-11 ). The joint itself can exhibit arthropathic structural changes and destruction.