MR Imaging of the Fingers

MP joint dislocations are uncommon, except for the thumb.⁷ Valgus or varus injuries on a flexed joint may produce a rupture of a collateral ligament,⁸ which can be confirmed by MR imaging when the diagnosis is uncertain. MR examination is also helpful in detecting complications, such as ligament entrapment⁹ or associated tears of the volar plate,¹⁰ which are clinically underestimated.

PIP joint sprains are very common and are usually considered irrelevant and are therefore underrated or misdiagnosed. As a result, these theoretically benign lesions can be transformed into a painful, stiff joint or even into an unstable one.

The extensor apparatus is thin and superficial and therefore is susceptible to lacerations or avulsions.

Pulleys are vulnerable to four main pathologies:

Annular pulley ruptures may occur during powerful flexions of the fingers with MP joint extension, PIP joint flexion, and DIP joint extension. The distal end of the A2 pulley is first partially torn, with progressive extension to complete rupture and involvement of the A3, A4, and, rarely, A1 pulleys (Fig. 11.54).¹⁰⁰ Pulley ruptures usually involve the long fingers, particularly the A2 pulley of the fourth finger of the nondominant hand, but some rare ruptures of the thumb annular pulleys have also been reported.^101,102

In acute injuries, pain and swelling may blur the depiction of flexor tendon bowstringing. Neglected tears of the annular pulleys may be complicated by scar tissue and stiffness of the PIP joint.¹⁰³ An early and accurate diagnosis can prevent these complications. The choice between a conservative or surgical approach to treatment depends on the age of the patient, the degree of the pulley tear, and the number of injured pulleys.¹⁰⁴

Ultrasonography and CT and MR imaging are all accurate for assessment of injuries of the pulley system. A dynamic study with forced flexion increases the sensitivity and is easier to perform with ultrasonography or CT.^{105,106,107,108} Ultrasound scans provide excellent visualization of the finger pulleys, except for the A5 annular pulley and the C2 and C3 cruciform pulleys.¹⁰⁹ MR also provides accurate assessment of annular pulley injuries. Although sagittal

and axial MR images directly depict the tear as a thickening and a defect of the pulley (Fig. 11.55), sagittal images, which display the indirect sign of bowstringing, are more sensitive, particularly with imaging performed in forced flexion (Fig. 11.56).

MR images also display and allow measurement of the increased distance between the flexor tendon and the phalanx.^110,111 During forced flexion, this gap increases proportionally to the number of disrupted pulleys. The gap is 2 to 5 mm in isolated complete tears and 5 to 8 mm in simultaneous complete tears. A partial tear of the A2 pulley is suspected when bowstringing does not extend proximally beyond the base of the proximal phalanx. Measurement has no significance in partial tears.¹⁰⁵

Trigger finger is a common stenosing tenosynovitis of the flexor tendons. Primary stenosing tenosynovitis is usually idiopathic and results from chondroid metaplasia of the A1 pulley. It occurs more frequently in middle-aged women. Secondary stenosing tenosynovitis of the fingers may occur in rheumatoid arthritis, diabetes mellitus, gout, and other connective tissue disorders.

The clinical diagnosis of trigger finger is usually obvious.¹¹² In recurrent cases, however, it may be difficult to assess the anatomic lesions accurately without imaging. Ultrasonography and MR imaging both depict diffuse or nodular inflammatory thickening of the A1 pulley. On sagittal images, associated focal compression of the underlying flexor tendons may be seen, as well as a more distal nodular tendonitis or a tenosynovitis (Figs. 11.57 and Fig. 11.58). Imaging is also helpful in guiding placement of steroid injections in recurrent cases¹¹³ and in depicting uncommon etiologies, particularly an old partial laceration of the FDS tendon or an intertendinous connection between the FDS and the FDP tendons in the palmar region (Fig. 11.59).^114,115

Posttraumatic arteriovenous fistulas (AVFs) are acquired high-flow lesions.¹¹⁶ Chronic longstanding traumatic AVF may simulate and be misdiagnosed as an arteriovenous malformation (AVM). They have been extensively described at the digital extremities and in the palm of the hand.¹¹⁷ Histologic features similar to hemangioendothelioma have been reported within posttraumatic AVF. Flow void artifacts on T1- and T2-weighted images indicate a high-flow lesion but are not very sensitive. Dynamic 3D gradient-echo sequences after intravenous injection of gadolinium appear to be more sensitive (Fig. 11.60).¹¹⁸

The hypothenar hammer syndrome involves the terminal portion of the ulnar artery. The superficial branch of the ulnar artery, which becomes the superficial palmar arch, is exposed to injury where it emerges from Guyon's canal and runs superficially across the hypothenar musculature for 2 cm. Clinical symptoms appear more distally, due to arterial emboli in the second through fifth digits.^116,119

Thrombosis of the common digital artery may be visualized on ultrasound, but a detailed preoperative evaluation includes digital subtraction angiography, multislice spiral CT angiography, or magnetic resonance angiography (MRA).¹²⁰ Emboli may be seen in as many as 50% of cases at MRA. The correct diagnosis is indicated by highlighting of the initial lesion of the ulnar artery at the level of the hamate (Fig. 11.61).^121,122,123

Ganglion cysts are cystic lesions filled with a thick gelatinous fluid. Their cause is unknown, although trauma has been postulated as an inciting factor. They occur more commonly in women (female to male ratio of 2.6:1), and most patients are in their third to fifth decade. The middle finger is most commonly affected, and 69% of the ganglions are located at the weak point between the A1 and A2 pulleys (Fig. 11.63).⁹⁹ More distal locations at the PIP and DIP joints are less common and may be related to osteoarthritis in patients older than 65 years of age.¹²⁷, ^128,129 Rarely an ossified ganglion is depicted on plain films.¹³⁰ Treatment is percutaneous punctures or surgery, but recurrences are common (89%).

Imaging may be useful in preoperative planning.¹³¹ On ultrasound scans the lesion appears as one or several anechoic masses with an associated tenosynovitis.¹³² The depiction of a peduncle may be more difficult, as is visualization of underlying joint disease. MR images show a well-delineated homogeneous mass with low to intermediate signal on T1-weighted images, depending on protein concentration. On T2-weighted images the cyst displays very high signal and common intralesional septa are better depicted. After intravenous injection of gadolinium, there is a thin and regular enhancement of the peripheral wall and septa (Figs. 11.64 and Fig. 11.65). Hemorrhage may change the appearance of the cyst, which will display high signal intensity on T1-weighted images and low signal intensity on T2-weighted images. Fluid–fluid levels are exceptional. Sometimes T2-weighted images display a thin and irregular pedicle communicating with an underlying PIP or DIP joint.

Rarely, extensions of ganglions have been found in theextensor tendon at the level of the metacarpals.^133,134,135 Thecyst may be large and hard enough to block sliding of the extensor tendon beneath the extensor retinaculum.¹³⁶ The EDC and EIP tendons are more commonly involved. Axial T2-weighted

images show intratendinous extension of the cyst with peripherally displaced thin tendinous walls (Fig. 11.66). Tenosynovitis is commonly associated at the level of the wrist. A painful cyst may be due to extrinsic nerve compression or, rarely, intraneural infiltration of a proper digital nerve (Fig. 11.67).¹³⁷ Intraosseous ganglions in the phalanx have also been reported.¹³⁸

Granulomatous inflammation of the synovium may be encountered in a variety of diseases, such as infection with Mycobacterium tuberculosis or atypical mycobacteria, certain fungal infections, or Brucella infections. Inflammatory disorders, such as sarcoidosis, crystal-associated arthritis, gout, or foreign body reactions, may also present as granulomas.

Tenosynovial chondromatosis is due to cartilaginous metaplasia of the synovium of a tendon sheath. Cartilaginous bodies are first produced in the synovium and later are released into the joint or the tendon sheath. At this point they may be ossified,¹⁵⁷ and patients present with a swollen and stiff finger.

Plain films may be negative early in the process, or they may show calcified nodules.¹⁵⁸ MR imaging depicts a lobulated mass with signal intensity similar to cartilage. Intralesional signal voids are due to calcifications. The synovium is thickened and enhances strongly after gadolinium administration (Fig. 11.70).

Penetrating trauma of the finger is common, and foreign bodies are usually promptly removed by the patient or a physician. Sometimes, however, the initial trauma is overlooked or the foreign body cannot be removed, resulting in a foreign body granuloma. The patient has a painful mass on the finger. Radiopaque foreign bodies can be detected on plain films, and most others can be seen on ultrasound.¹⁵⁹ MR imaging can also be used to display foreign bodies and granulomatous reaction. On T2-weighted images, longstanding foreign bodies (e.g., glass, wood splinter) are seen as low-signal areas surrounded by a high-signal granuloma. Post-contrast studies show thick peripheral enhancement (Fig. 11.71).¹⁶⁰

Dupuytren's contracture, a superficial palmar fibromatosis, is common, occurring in 1% to 2% of the general population. The palmar aponeurosis of the hand is the main target, but more distal extension to the PIP joint may also be seen. Initially a small subcutaneous nodule is palpated in the palm at the level of the distal palmar crease. Longitudinal cords appear later, with collagen fibers oriented parallel and superficial to the flexor tendons. The overlying skin thickens and retracts, resulting in a flexion contracture. The annular (ring) finger is commonly involved.¹²⁵

T1- and T2-weighted MR images depict low-signal cord-like structures arising from the palmar aponeurosis. The distal expansion appears as subcutaneous thin strands or small nodules. These nodules are depicted in nearly 60% of patients and present different signal characteristics and variable gadolinium enhancement depending on the degree of cellularity (Fig. 11.72).¹⁶¹ It is not uncommon to find several intralesional low-signal dots. Nodules with lower signal intensity on T2-weighted images correlate with lesions with less cellularity. These lesions tend to recur less frequently (18%) than the more cellular lesions (70%).

Tenosynovial giant cell tumors, also called giant cell tumor of the tendon sheath or localized nodular tenosynovitis, originate from the tendon sheaths and share histopathologic features with pigmented villonodular synovitis.^162,163 Tenosynovial giant cell tumors of the fingers are not uncommon; in fact, they are the second most common tumors of the soft tissues in the hand.^164,165 These tumors are primarily found in adults 30 to 50 years of age.^166,167 Most often the tumor is seen as a solitary nodule close to a tendon sheath on the palmar aspect of the first three fingers. Clinically the patient reports an asymptomatic, slowly enlarging mass attached to deep structures (tendon sheath, capsule). The lesion may be painful when neurovascular elements are compressed. Surgery is mandatory, and local recurrences may occur in as many as 30% of cases.¹⁶²

Imaging evaluation of tenosynovial giant cell tumors includes plain films, ultrasound, and MR imaging. Plain films may depict a pressure erosion and less commonly a periosteal reaction (Fig. 11.73). There are no calcifications, as would be

seen with synovial sarcomas. Bone and joint invasion is possible.¹⁶⁸ Ultrasonography shows a nonspecific solid mass with a variable color Doppler signal. MR images are specific, depicting a well-defined mass with hemosiderin deposits. Typical signal void artifacts are seen on all sequences, particularly on gradient-echo images, and a more heterogeneous and predominantly low signal is found on T2-weighted images.^162,166,169 The lesion typically enhances after intravenous injection of gadolinium (Fig. 11.74). The tendon sheath of the flexor digitorum tendons is usually partially or totally enveloped; the extensor tendons are less commonly involved. Some lesions may be more aggressive, and diffuse lesions are seen in multiple locations with invasion of both the flexor and extensor tendons (Fig. 11.75).¹⁷⁰

Fibromas of the tendon sheath demonstrate similar signal characteristics but are much less common in the fingers.

Vascular manifestations of a variety of diseases may involve the fingers. A fluctuant soft tissue mass is a common presentation of a vascular malformation. Plain radiographs are useful in the search for a periosteal reaction or phlebolith,¹⁷¹ and noninvasive vascular assessment can be performed with color Doppler imaging and CT or MRA. Occasionally, digital subtraction angiography (DSA) may be necessary for preoperative planning and ultimately for therapeutic purposes. DSA also allows imaging of the entire upper extremity vascular tree from its origin at the aortic arch to the digits. Doppler imaging and MRA may be useful in following the patient's response to therapy.

There remains widespread confusion regarding properterminology of vascular masses of the fingers. The term

“hemangioma” is often used as a generic term, although it represents a well-defined and rather unusual entity. Hemangiomas result from endothelial hyperplasia and consist of lobules of microcapillaries. They are absent or discrete at birth and grow rapidly in successive phases of proliferation and regression. Vascular malformations do not exhibit endothelial abnormalities; they are present at birth and their growth follows that of the child; and they never regress. In 1996, the International Society for the Study of Vascular Anomalies (ISSVA) classification was proposed by Enjolras and Mulliken, based on the initial classification of Mulliken and Glowacki proposed in 1982 (Table 11.3).¹⁷² In this classification low-flow malformations include venous, lymphatic, and capillary malformations. High-flow malformations are represented by AVMs and AVFs.¹⁷³ The distinguishing features of infantile hemangioma and vascular malformations are presented in Table 11.4.

Superficial cutaneous vascular lesions of childhood are easily recognized clinically; usually no imaging is necessary. Deeper vascular lesions may be discovered in adolescents and young adults, sometimes after trauma. They occur much less frequently and have no sex predilection. Although these lesions are present at birth and grow with the child, they may not be discovered until much later. They may be quiescent and cause no symptoms, but once they become symptomatic they rarely regress.

Lipomas are benign soft tissue masses of mature adipocytes usually seen in elderly patients. Most occur proximally in the thenar and hypothenar eminences; very rarely they occur more distally.¹⁹⁰ Fingers are an uncommon location (less than 5% of cases). The clinical presentation is of a slowly growing asymptomatic mass. Pain and paresthesia, when present, are due to deep nerve compression.

Lipomas are depicted on plain films and CT as low-density soft tissue masses (Fig. 11.82). Parosteal lipomas are associated with cortical irregularities of the phalanx and possible intralesional calcifications.¹⁹¹ The ultrasound appearance of lipomas varies widely (from hypoechoic to hyperechoic), depending on the number of internal interfaces between fatty and connective tissue.¹⁹²

The diagnosis is obvious on MR images, which display a mass with high signal intensity on T2-weighted images and a lack of signal on fat-suppressed images, identical to subcutaneous fat. Heterogeneities are due to intralesional strands of fibroconnective septa (Fig. 11.83). On post-gadolinium fat-suppressed images, lipomas enhance, and nodular enhancement must raise the suspicion of a rare liposarcoma.

Fibrolipohamartomas, also called lipomatous hamartomas or neural fibrolipomas, are very rare tumors of children and young adults in which fatty and fibrous tissues infiltrate the epineurium and perineurium and surround the median nerve and its terminal branches.¹⁹³ The mass involves mainly the forearm and the wrist, but more distal extension may occur. The radial and ulnar nerves are occasionally involved. Pain and paresthesia are due to carpal tunnel syndrome.¹⁹⁴ Nearly

one third of cases are associated with macrodactyly (macrodystrophia lipomatosa) with bony and soft tissue overgrowth.¹⁹⁵

Plain films show macrodactyly of the second and third digits, which are subject to early osteoarthritis. CT and MR demonstrate a characteristic cable-like appearance of the tumor, with a hypertrophic appearance of the median nerve, common digital nerves, and proper digital nerves. Low-signal-intensity thickened nerve bundles and fibrous strands are embedded within a predominantly fatty mass.^196,197 Rare cases of predominant involvement of the proper digital nerves have been reported (Fig. 11.84).¹⁹⁸

Enchondromas are the most common bone tumors of the hand. The absolute incidence is not known since most lesions are asymptomatic and go undiagnosed. The median age at presentation is 35 years, and the main site is the phalanges. Most tumors are solitary, but multiple enchondromas in a predominantly unilateral distribution are seen in Ollier's disease and are associated with multiple hemangiomas of the skin in Maffuci's syndrome (Fig. 11.87).²¹¹ When in multiple locations, tumors that show recent growth or cause pain should be further studied for malignant transformation to a chondrosarcoma. A painful solitary enchondroma is most likely due to a fracture (Fig. 11.88).

MR studies of enchondromas are not usually needed, since the radiographic appearance of a lobulated radiolucent defect with bone expansion and flecks of calcification is specific. In enchondromas of small bones, calcifications are frequently lacking. A pathologic fracture is usually diagnosed on plain films. In atypical cases, MR may be used to display the cartilaginous content, which is displayed with low signal intensity on T1-weighted images and high signal intensity, identical to that of water, on T2-weighted images. There is post-contrast enhancement of the periphery and the septa (Fig. 11.89), which delineate typical cartilaginous lobules (Fig. 11.90).

Juxtacortical or periosteal chondromas are also common in the phalanges. The tumor is located adjacent to and partially embedded in the shaft of the phalanx or the metacarpal. The proximal third of the phalanx is most commonly involved. Plain films show a soft tissue mass adjacent to the diaphysis of the phalanx or the metacarpal, with a variable indentation and a thin bony shell.²¹²

These patterns may be very subtle on radiographs, and CT more accurately depicts the calcifications and cortical abnormalities. MR scans confirm the presence of intralesional cartilaginous lobules and frequently display bone marrow invasion (Fig. 11.91).^213,214,215

Osteochondromas are much less prevalent than enchondromas in the hand and most so-called osteochondromas or exostoses of the finger are actually reactive processes. These posttraumatic subperiosteal calcifications involve mainly the hands and feet. Several entities have been identified, with a common mechanism of subperiosteal hematoma,²¹⁶ although previous trauma is noted in less than half of the cases. The pattern of ossification depends on the involved bone, on the extensibility of the anatomic compartment where the hematoma develops, and on the periosteal reaction.

Osteoid osteomas are small benign tumors with a central nidus of highly vascularized osteoblastic tissue, surrounded by a reactive sclerotic bone. Four percent of all bone tumors are osteoid osteomas, and they occur in young patients, usually in the first three decades. About 8% of osteoid osteomas involve the phalanges. They usually cause severe pain, swelling of the finger, and, when they occur in distal locations, widening and thickening of the nail plate. The pain is worst at night and is somewhat alleviated by salicylates. Probing with a pin locates a painful pressure point.

Osteoid osteomas in the phalanges often present with atypical clinical and radiologic characteristics. Radiographs may depict a more or less centrally calcified nidus smaller than 1 to 1.5 cm. Peripheral osteosclerosis and solid periostitis are commonly associated findings and may be particularly aggressive in lesions with a less centrally located nidus and in those that are more distally located (Fig. 11.97).²⁴⁷ On plain films this reaction may conceal the nidus. Additional

misleading features in osteoid osteoma include the following (see Fig. 11.97):^248,249

These unusual presentations may cause a delay in diagnosis and treatment. Bone scintigraphy is very helpful in these cases by highlighting a high uptake in the phalanx or metacarpal bone (see Fig. 11.97). CT scanning with thin contiguous slices is the best imaging technique for detecting the nidus. Accuracy may be improved by prior bone scintigraphy (Fig. 11.98). On MR examination the osteoid tissue is hyperintense on T2-weighted images and enhances after contrast administration. When the nidus is highly calcified and in cases of extensive bone edema and inflammation of the surrounding

soft tissues, the nidus may be difficult to highlight with MR imaging (Fig. 11.99). Percutaneous radiofrequency ablation may be performed in the tubular bones of the hand.²⁵⁰

Giant cell tumors are benign and aggressive bone tumors with a high postoperative recurrence rate. Most occur in patients 15 to 45 years of age. The bones of the hand are rarely involved, but when they are the metacarpals are the most common location and the lesions may be associated with other bone tumors in other locations (multicentric giant cell tumors) or lung metastases.^251,252,253 The duration of symptoms is shorter than that seen in giant cell tumor of bone occurring in sites other than the hand.²⁵⁴

The characteristic radiographic appearance is of a metaepiphyseal osteolytic metacarpal lesion with a geographic pattern of bone destruction. The lesion may demonstrate internal ridges and smooth margins and possible cortical destruction (Fig. 11.100). Signal characteristics are nonspecific on MR images. The lesion may be heterogeneous, with hemorrhage, fibrosis, and highly vascularized foci (Fig. 11.101). A secondary aneurysmal bone cyst may be suspected in case of numerous fluid–fluid levels. Giant cell tumors of the hand are more likely to recur than are those in other locations (36% to 79% of cases, depending on the type of surgery performed).²⁵⁴

Aneurysmal bone cysts (ABCs) are tumor-like lesions composed of multiple cavities filled with blood. ABCs may present the same radiographic patterns as giant cell tumors of bone and may coexist in the same lesion. ABCs, however, are more expansile, are delineated by a thin bony shell, and are more likely to occur in a younger patient population. The metacarpals are the most frequently affected bones in the hand, but ABCs may be located on any other bone, including the sesamoids.²⁵⁵ MR findings of multiple cavities filled by blood and with fluid–fluid levels are highly evocative but not pathognomonic. Prompt therapeutic intervention is indicated because of the potential for aggressive local behavior.²⁵⁶

These lesions are discussed below in the section on Ungual and Periungual Pseudotumors and Tumors.

The nail plate itself, which is composed of keratin, a highly organized scleroprotein that causes dramatic shortening of relaxation times, does not produce a signal on MR images.²⁶² It is, however, outlined by other structures and is thus indirectly depicted in its full length on sagittal images. The ventral aspect of the nail plate is well defined by the high signal of the epidermal layer of the matrix and the nail bed, but its dorsal aspect is not visible since no signal is produced at the interface with air. Application of petroleum jelly to the nail plate before positioning the hand in the coil provides a high level of contrast level with the dorsal aspect of the nail plate, regardless of the sequencing protocol used (Fig. 11.104). Because of the chemical shift artifact, nail thickness may be underestimated by approximately 0.2 mm, which is significant since the normal nail plate thickness is only 0.5 to 1 mm.^263,264,265 On 3D gradient-echo images there may be a magnetic susceptibility artifact, seen as a low-signal area around the free edge of the nail plate. The nail plate is very thin close to the nail root and is surrounded by the thin high-signal-intensity matrix cul-de-sac (see Fig. 11.104; Fig. 11.105). The nail plate progressively thickens as it approaches its free edge, a finding in agreement with ultrasonography reports.^266,267 It is not possible to distinguish the different histologic layers of the nail plate on MR images. Whatever the sequence used, the plate always remains

devoid of signal throughout its thickness. With ultrasonography, two layers of different hydration in the nail plate can be distinguished.²⁶⁸ Unlike MR images, however, ultrasound scans do not display the epidermis layer of the nail bed.

The DIP joint has a close relationship with the nail cul-de-sac and is clearly displayed on sagittal images (see Fig. 11.104). The terminal extensor tendon inserts on the dorsal aspect of the base of the distal phalanx. The proximal matrix and distal matrix, surrounding the nail root, are also well defined on this view but are isointense with the adjacent epithelial layer of the nail bed and the ventral aspect of the proximal nail fold (or eponychium). The transition between the matrix and the nail bed is identified by the change in thickness of the epithelial layer. The epithelial layer thickens at the level of the free edge of the nail plate (the hyponychium). The lack of fat in the nail bed, except for a thin band under the nail root, eliminates chemical shift artifact.

An oval area of high signal in the deep dermis beneath the nail matrix is always depicted on T2-weighted sagittal images (Fig. 11.106). This structure is related to the lunula.²⁶⁹ The length of this area is highly correlated to the length of the lunula. When the lunula is not visible, this dermal area is still present but does not cross the borders of the eponychium. There is a strong, homogeneous, and well-defined enhancement after injection of gadolinium. Histologic studies show a more uniform dermis in this area, with a looser connective tissue than that of the more distal nail bed. Microvascularization in this area presents a more regular vascular network than in the nail bed (Fig. 11.107).²⁷⁰ This area may be confused with a vascular tumor or may blur the periphery of a tumor. In doubtful cases complementary MR angiography may be helpful.

The proximal or posterior nail fold, the extensor and flexor digitorum tendons, the medial and lateral collateral ligaments, and the volar plate of the DIP joint are well displayed on axial images. The transverse curvature of the nail root is regular and is surrounded dorsally by the proximal matrix and ventrally by the distal matrix (Fig. 11.108). The posterolateral borders of the nail plate are contiguous with the matricophalangeal ligaments (Fig. 11.109). The lateral nail folds emerge on more distal axial images and cover the lateral borders of the nail plate (see Fig. 11.108). The deep rima ungualum is a passage area between the nail bed and the pulp. The Flint lateral interosseous ligament forms its superficial boundary. Vascular arcades course through the rima ungualum, providing blood supply to the fingertip (Fig. 11.110). The epithelial crests of the nail bed (Fig. 11.111), which may be depicted with high-resolution axial MR images (more easily visible with chronic inflammatory hypertrophy), appear as a thin, high-signal-intensity structure. The underlying dermis is visualized as a thin superficial layer of low signal intensity and a thick deep layer that demonstrates heterogeneous signal

intensity. This heterogeneity is emphasized by the injection of gadolinium, which results in enhancement of the numerous glomus bodies of the nail bed (see Fig. 11.111; Fig. 11.112). Glomus bodies, which function as arteriovenous shunts, are highly concentrated in the fingertips, particularly beneath the nail plate.¹²⁸ Each glomus body is a tiny encapsulated oval organ 300 μm long. The nail beds of fingers and toes contain 93 to 501 glomus bodies per square centimeter.

Coronal images are less commonly acquired for the nail unit because they are subject to partial volume artifact. Numerous elements of the nail unit are tangential to this plane. However, coronal images may be a useful addition for the assessment of the DIP joint and the distal phalanx, as well as lateral lesions invading the rima ungualum (Fig. 11.113).

Ganglia of the fingertip, also called mucoid cysts or pseudocysts, usually originate from the posterior nail fold. They are common in elderly patients and present therapeutic difficulties. The recurrence rate is high despite numerous proposed treatments, and accurate preoperative imaging may be helpful. Plain films usually show severe osteoarthritis of the DIP joint with huge dorsal osteophytes. The eponychium appears thickened on the lateral view (Fig. 11.114). Erosion of the dorsal cortex of the distal phalanx may also be depicted (Fig. 11.115).

Most of these cysts are solitary and located on the proximal nail fold. Their appearance on MR images is specific (Figs. 11.116 and 11.117) and includes:

Intracystic septa, seen in 39% of cases, are best visualized on T2-weighted images.²⁷¹ Administration of gadolinium produces early faint peripheral enhancement that with time spreads toward the center of the cyst. This diffusion of contrast is similar to the intra-articular diffusion of IV gadolinium through the synovium at the level of the knee.²⁷² However, a true synovial membrane is not found in digital cysts, apart from a possible peduncle.²⁷³

In most cases a peduncle connecting the cyst and the DIP joint is identified on MR examination. When present, the peduncle is lateral, beneath the insertion of the extensor digitorum tendon on the base of the distal phalanx (Fig. 11.118). Preoperative identification of the peduncle is important in making decisions about surgical technique, since in the traditional surgical procedure the peduncle must be tied off or removed to avoid frequent recurrences.²⁷⁴ With the skin flap technique, a newly proposed procedure, the communication between the joint and the cyst may seal itself during the healing process that occurs after a flap is raised and the cyst resected from the undersurface of the flap extending to the DIP joint. In this procedure no skin is excised, and removal of the osteophytes is not necessary.²⁷⁵ Identification of the peduncle by preoperative injection of methylene blue mixed with hydrogen peroxide into the palmar aspect of the DIP joint has been proposed, but it is technically difficult and time-consuming.²⁷⁶

Satellite cysts, when present, are also well displayed by MRI (Fig. 11.119).¹³⁰

In 22% of cases, mucoid cysts have a sagging multiloculated pattern.²⁷⁷ This type of cyst may be difficult to detect clinically unless the typical signs of swelling and discharge of a thick fluid from the proximal nail fold are found. On MR examination, no connection with the DIP joint is seen (Fig. 11.120). These cysts may develop independently from the underlying joint and are caused by increased production of hyaluronic acid due to fibroblast metaplasia. This process is not unlike that which occurs in cutaneous myxomas with a focal storage of mucoid material in the dermis.

Although mucoid cysts develop into the nail bed in 30% of cases,²⁷⁷ this location has not been well studied and is rarely mentioned in the literature. If the patient presents with a painful cyst, misdiagnosis of glomus tumor is possible, and MR imaging is helpful in detecting and correctly diagnosing this type of cyst. When the cyst is large, erosion of the cortex of the underlying phalanx may occur in the confined space of the nail bed (Fig. 11.121). These cysts are located in the dermis beneath the nail matrix, close to the DIP joint. Matrix compression may induce a fissure of the nail plate with a claw deformity. Most often, the cyst is bilobed with a proximal component in the posterior nail fold (Fig. 11.122). Less commonly, the cyst extends into the pulp (Fig. 11.123). The submatrical extension may be clinically occult and responsible for recurrence.

Glomus tumors arise from glomus bodies, which are particularly numerous in the dermis of the nail bed. High-resolution MR imaging can depict normal glomus bodies on T2-weighted images and after injection of gadolinium (see Figs. 11.111 and 11.112). The classic triad of severe intermittent pain, localized tenderness, and cold sensitivity is evocative but occurs infrequently. Plain films are not sensitive, and a bony erosion of the distal phalanx, similar to that seen with epidermal inclusion cysts, is seen in less than 20% of cases (Fig. 11.127). MR imaging is helpful in the diagnosis, depicting the tumors in 68% of cases.¹ The mean diagnostic delay, reported as ranging from 4 to 7 years, needs to be shortened.^{291,292,293,294}

Glomus tumors, which may be considered hamartomas, result from hyperplasia of one or several elements of the glomus bodies.²⁹¹ In 1924, Masson described several histologic variants.²⁹⁵ Because histologic composition has no prognostic significance, it is not routinely mentioned in pathologic reports. However, the MR signal characteristics of a glomus tumor depend on its histologic composition, and knowledge of these variants is important in understanding the variations in tumor signal.¹ Three types of variants are generally seen:

It is not unusual for glomus tumors to contain a combination of these three histologic elements (Fig. 11.131).

The tumor margins are usually well defined by a peripheral pseudocapsule. This capsule, a reactive response of the surrounding connective tissue, demonstrates very low signal intensity on all sequences, although it is more readily visible on T2-weighted images or 3D gradient-echo images (Fig. 11.132). In some cases, the tumor margins are ill-defined, and injection of gadolinium, particularly with MR angiography, may depict small foci of tumor extending into the nearby nail bed (Fig. 11.133). In these cases some adhesions with the nail bed are often noted during surgery.

The importance of local invasion of the capsule, which is found on histologic examinations in less than 2% of cases,³⁰¹ is debatable. However, the risk of recurrence is definitely high if some tumor tissue is left in situ during surgery of these ill-defined lesions. The reported recurrence rate ranges from 12% to 24%.^294,302,303 MR imaging appears to be particularly helpful in evaluating cases of recurrent pain after surgery (Fig. 11.134).^298,304

Rarely, patients present with multiple recurrences of tumors with ill-defined borders. In this situation, glomangiosarcoma, a rare malignant variant of the glomus tumor, should be considered.³⁰⁵ Glomangiosarcomas have been reported in several anatomic locations, but there is only one report of its localization to the hand, despite the fact that its benign counterpart is most frequently found there. There is also only one report of a glomangiosarcoma that has metastasized.

Multiple glomus tumors may arise in the hand or in the same fingertip (Fig. 11.135), and some cases have been reported in association with neurofibromatosis type 1.³⁰⁶ MR angiography depicts these multiple sites and is essential in planning an appropriate surgical approach.³⁰⁷

In most cases, the tumor is located in the subungual area, in the supporting tissue of the nail bed or the matrix. The lesion is usually deep, close to the periosteum of the underlying phalanx. In this location tumors smaller than 3 mm are difficult to depict with ultrasonography.^308,309 On MR images cortical bone erosion is often seen on axial images although it was occult on radiographs (see Fig. 11.128). Axial images are needed to distinguish tumors on the median line from those of the lateral part of the nail bed, which sometimes extend into the pulp via the rima ungualum (see Fig. 11.133). The surgical approach is planned according to the size and location of the tumor. Lateral-type tumors may be excised by a lateral approach; median tumors may require a transungual approach. Sagittal images are used to determine the relationship between the tumor and the nail matrix.

Occasionally the tumor is located in the pulp or the posterior nail fold (Figs. 11.136 and 11.137). In this case, the fatty tissue of the hypodermis surrounding the tumor completely changes the signal characteristics and contrast between healthy

tissue and tumor. On T1-weighted images the low-signal-intensity tumor is seen, surrounded by high-signal-intensity fat. After gadolinium administration, tumor enhancement is only visible on fat-suppressed images (see Fig. 11.136).

Glomus tumors are easily distinguished from other vascular lesions, such as venous hemangiomas and arteriovenous malformations, by their characteristic blood flow artifacts and vascular pedicles. Subungual extraskeletal chondromas may mimic a glomus tumor, but the peripheral enhancement pattern is different.³¹⁰

Benign epithelial tumors of the nail apparatus (e.g., warts, eccrine poroma, subungual papilloma, subungual linear keratotic melanonychia, keratin cysts) are numerous, and in most cases MR imaging is not needed because the clinical findings are obvious and it would not be helpful because specific abnormalities cannot be depicted. Two lesions, however (distal digital keratoacanthomas and onychomatricomas), may be aggressive or present specific MR features and are discussed below.

Fibrous tumors can develop in the perionychium and present with a wide range of clinical patterns despite relative histologic uniformity. In fact, Koenen's tumor, acquired fibrokeratoma, and dermatofibroma may represent different stages in a clinical continuum.

Fibrokeratomas may be multiple and located beneath the nail plate, but most emerge from the proximal nail fold and grow in a splint of the nail plate.³²⁰ Biopsy is the rule, since Bowen's disease may simulate a fibrokeratoma.^321,322 Fibrokeratomas are thought to consist of newly formed collagen.³²³ The lesion may be septated.

MR images highlight the deep implantation close to the nail root, and the component emerging from the proximal nail fold is clearly depicted.²⁷⁷ The signal characteristics of the tumor vary depending on its histologic type. Dense tumors with numerous collagen bundles demonstrate very low signal on all sequences, and those with a mucoid stroma demonstrate high signal on T2-weighted images (Fig. 11.140). Overlying

acanthotic epidermis displays signal intensity identical to that of normal epidermis. On MR images displaying involvement of the ventral aspect of the proximal nail fold, epithelial invagination (see Fig. 11.140) may be visualized. This invagination acts as an incidental matrix and produces a pseudonail of collagen.

Knuckle pads are asymptomatic, persistent, keratotic nodular plaques that involve the dorsal aspect of the interphalangeal joints.¹²⁵ Histology reveals hyperkeratosis associated with thickened collagen bundles similar to the palmar nodules in Dupuytren's disease.³²⁴ Knuckle pads may produce contraction by invasion of the extensor apparatus.³²⁵ Ultrasonography and MR images accurately show the relationship between the extensor apparatus and the plaque (Fig. 11.141). MR signal characteristics are not specific, and knuckle pads, like other fibrous lesions rich in collagen, demonstrate a rather low and heterogeneous signal on T1- and T2-weighted images.²⁷⁷

Fibromas are painless slow-growing nodules that can develop in any epidermal structure of the nail unit.³²⁶ Multiple and bilateral periungual fibromas sometimes represent an oligosymptomatic form of tuberous sclerosis.^327,328 Radiographs may depict bone erosion and thickening of the soft tissue. There are no calcifications. Ultrasonography documents the solid and poorly vascularized pattern of the lesion but is nonspecific. Findings on MR are similar to those of knuckle pads. Superficial acral fibromyxoma is a recently reported entity characterized by a rich mucoid stroma visualized with high signal intensity on T2-weighted images.^326,329

Abnormalities are similar in both early and chronic OA and include:

Cartilage degradation is progressive and diffuse in these non-weight-bearing joints, but diffuse thinning of the normally 1-mm-thick cartilage may be difficult to appreciate on MR images.³ The commonly associated subchondral changes are often more obvious.

MR examinations are now used extensively in cross-sectional and observational studies as well as controlled clinical trials to assess disease activity and joint damage in rheumatoid arthritis (RA). Further studies are needed to determine which regions should be scanned for optimal assessment of joint damage and disease activity. The number of sequences may be reduced if only limited MR outcome data are required. Although low-field devices may be used in clinical trials, resolution and tissue discrimination must not be sacrificed.

The finger joints are affected early and are often the first joints involved in rheumatoid arthritis.³⁴² They are also good markers for assessment of overall disease activity. Clinical studies have often focused on the MP joints because they are easier to image than the more complex wrist and the smaller interphalangeal joints. The Outcome Measures in Rheumatology Clinical Trials (OMERACT) and the European League Against Rheumatism (EULAR) have studied the role of MR in RA and developed a scoring system based in part on findings in the MP joint. The OMERACT 2002 RA MRI scoring system (RAMRIS) is based on the following:^343,344,345

It is difficult to image all the joints of both hands with high spatial resolution, and standardized protocols are necessary.³⁴⁶ The core set of basic MR sequences should include the following:^{343,344,345,347,348}

Fat suppression with IV injection of gadolinium is necessary to differentiate joint effusion from synovial pannus. This technique is also useful in monitoring the patient's response to therapy.³⁴⁹ Dynamic contrast-enhanced MR imaging is not routinely performed but is the best technique to assess hypervascularized synovium. Evaluation of cartilage destruction of the fingers is limited by the insufficient spatial resolution.

Both ultrasonography with color Doppler and MR imaging are useful in detecting early RA before changes become apparent on radiographs. MR diagnosis is based on the triad of synovitis, bone erosion, and bone edema.

MR measurement of synovitis and bone erosion is a valid and reliable tool for assessing disease progression and treatment response in RA of wrist and finger joints. The importance of the depiction of bone edema remains more problematic. MR may become as routinely used by clinicians as plain films. Active inflammation is detected by enhancement of synovium and subchondral tissue after injection of gadolinium. The degree of enhancement depends on the vascularization of the synovium. Several subsets of synovial changes have been identified:

Hypervascularized synovium shows an early maximum enhancement after 1.5 minutes and after gadolinium spreads toward the joint fluid.²⁷² Therefore, early post-enhanced images are more appropriate in the assessment of the true thickness of active synovium.^365,367 The amount of synovium in the finger joints and the volume of the enhanced pannus may be efficient parameters for follow-up.^368,369,370 Volumetric quantification has a 10% error rate. The synovial volume of fingers is larger in clinically active RA. The transition of synovitis to pannus represents the beginning of irreparable joint damage in the course of RA.³⁶⁴ There may be discrepancies between clinical, laboratory, and imaging assessment of disease activity.³⁴⁶

The detection of early erosive RA may have prognostic value for future bone destruction activity.^{364,371,372,373} The bone edema score in early RA is an excellent prognostic factor as it predicts both radiographic damage and functional outcome.^356,372,373 This bone edema is correlated with disease activity (synovitis and clinical scores) and appears as a pre-erosive lesion on outcome studies.^{351,356,372,373} Longitudinal studies show that new erosions at the MP joints are preceded by MR synovitis.³⁵¹ The median time interval between detection of an erosion using MR and its appearance on plain films is about 2 years.³⁵⁶ Scoring MR erosions is an outcome measure with a high sensitivity to change. It appears more sensitive than plain films and allows short-term studies (3 months, for example).^361,372,373