Figure 1117

Sagging multilocated mucoid cyst (arrowed) of the proximal nail fold—sagittal T2-weighted image.

forms (22%) it was not possible to detect a connection with the distal interphalangeal joint. These cysts may develop independently from the underlying joint and result from increased production of hyaluronic acid due to the metaplasia of fibroblasts. This process may be compared to the cutaneous myxomas with a focal storage of mucoid material in the dermis.

Figure 11.18

Mucoid cyst of the proximal nail fold—axial T2-weighted image. A dorsal osteophyte (black arrowheads) lifts up the terminal band of the extensor tendon (white arrowheads). Note the underlying peduncle of the mucoid cyst (arrow).

Figure 11.18

Mucoid cyst of the proximal nail fold—axial T2-weighted image. A dorsal osteophyte (black arrowheads) lifts up the terminal band of the extensor tendon (white arrowheads). Note the underlying peduncle of the mucoid cyst (arrow).

A peduncle connecting the cyst and the distal interphalangeal joint is visible by MRI in most cases. In all these cases the peduncle is lateral, beneath the insertion of the extensor digitorum tendon on the base of the distal phalanx (Figure 11.18). If surgical treatment is chosen, the peduncle must be detected and tied up or removed to avoid frequent recurrences. The peroperative injection of methyl blue mixed with hydrogen peroxide into the palmar aspect of the distal interphalangeal joint to colour this peduncle has been proposed to aid in its identification, but this is not always easy to do.

Mucoid cysts extend into the nail bed in 30% of cases, a location that has been neglected by research. Symptoms may lead to misdiagnosis of glomus tumour when the cyst is painful. High-resolution MRI is able to detect 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. The cyst is in the dermis beneath the nail matrix, close to the distal interphalangeal j oint (Figure 11.19). Matrix compression may induce a fissure of the nail plate with a claw deformity. Most often, the cyst is bilobar, with a component in the proximal nail fold (more rarely in the pulp) associated with the nail bed component. The submatrical extension may be

Figure 11.19

Subungual mucoid cyst (arrows), (a) Axial 7^-weighted image, (b) Sagittal r2-weighted image. Note bone erosion (arrowhead).

Figure 11.19

Subungual mucoid cyst (arrows), (a) Axial 7^-weighted image, (b) Sagittal r2-weighted image. Note bone erosion (arrowhead).

clinically occult and responsible for recurrence. Detection of a peduncle is crucial, because its resection may be enough to collapse the cyst and avoid direct access to the matrix.

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