Digital Mucous Cyst

Etiology

The etiology of digital mucous cysts remains unknown; however, they are commonly associated with underlying osteoarthritis of the DIP joints of the fingers or toes.[1]

Epidemiology

Digital mucous cysts are ganglion cysts most commonly found on the fingers, although they may also occur on the toes. Ganglion cysts account for approximately 60% of soft-tissue tumors of the wrist and hand. Women are affected about 3 times more often than men, and digital mucous cysts typically occur in middle-aged to older adult patients.[3]

Pathophysiology

Digital mucous cysts may arise as an outpouching of the synovial joint lining. Dye studies have shown that digital mucous cysts communicate with the joint when injected from the joint side, but not when injected from the cyst. These findings support a one-way valve mechanism, with fluid flowing from the joint into the cyst but not in reverse.

A rent in the tendon sheath or joint capsule may cause local irritation and inflammation, leading to cyst formation. Another theory is that the connective tissue surrounding the joint undergoes mucoid degeneration, with collagen breakdown products accumulating in the cyst.[3] In a study by Eaton et al, all mucous cysts excised with marginal osteophyte resection were found to communicate with the DIP joint.[4]

Histopathology

Gross evaluation of a digital mucous cyst shows a multilobulated cyst. Under the microscope, the cyst is composed of several layers of randomly oriented collagen fibers that form the outer wall. This structure is mostly acellular and contains mesenchymal cells and fibroblasts.[5] The interior of the cyst is filled with viscous mucin, which can be clear to yellow in color and is composed of hyaluronic acid, globulin, glucosamine, and albumin.[3][6]

History and Physical

As with any soft-tissue diagnosis, it is important to obtain a thorough patient history, including relevant medical and family history, as well as details about the lesion, such as its rate of growth, presence of pain or night pain, color changes, and any history of recent trauma to the area.[7] Digital mucous cysts of the fingers or toes typically manifest as slow-growing masses located in the subcutaneous tissue overlying the dorsal aspect of the DIP joint.

There are 3 main locations where digital mucous cysts may arise. The first is the most common and obvious, occurring on the dorsal aspect of the DIP joint. Deep cysts in this area can cause deformities in nail growth, and superficial cysts can rupture. The second location is in the space between the ventral and dorsal aspects of the proximal nail fold.[8] Cysts in this location cause deep longitudinal grooves of the nail plate. The third location is under the nail matrix, which can cause tenting of the nail plate and a violaceous-to-deep-blue discoloration of the proximal matrix. When located under the nail matrix, digital mucous cysts can disrupt nail plate formation, leading to atrophy.[9] 

Upon palpation, the digital mucous cysts are firm and immobile and can be transilluminated with a light source. They are typically round and dome-shaped and can vary in size.[6] The mass usually arises off midline from the DIP joint, displaced by the extensor tendon, but remains attached to the joint by a stalk.[5] 

Depending on the tumor's location, nail deformities may occur if the cyst exerts pressure on the germinal matrix.[3][10] This also may lead to longitudinal grooves.[5] The skin overlying the cyst should be assessed to determine the thickness and the possible need for a skin graft if surgical intervention is undertaken. Ulceration is also common, and cysts may present as open, draining lesions with possible infection.

Evaluation

Following physical examination, radiographs of the hand or foot can be obtained, but this is not necessary in most cases, especially if surgical intervention is not being pursued. Typically, radiographic evaluation demonstrates underlying osteoarthritis of the DIP joint, including sclerosis of the subchondral bone, osteophytes, and narrowing of the joint space.[10]

Treatment / Management

Initial management may be conservative, beginning with observation, particularly for small cysts measuring only a few millimeters. Aspiration and surgical excision are also treatment options for digital mucous cysts. Avoidance of corticosteroid injections is recommended due to the risk of cutaneous atrophy in the hands and feet.[7][11] More invasive treatment may be considered if the overlying skin thins or a painful nail deformity develops. 

Surgical management may be chosen to reduce the risk of recurrence or after failure of conservative treatment. In-office surgical management performed by a trained clinician is associated with high cure rates and low complication rates.[12] The stalk should be excised, including the dorsal capsule and associated osteophytes. Care should be taken to protect the overlying skin during surgical dissection to avoid postoperative complications. A digital block with a tourniquet placed at the base of the finger is typically sufficient for anesthesia.[3]  

Meticulous technique is required during surgery to avoid damage to the germinal matrix, which is often in proximity to or underlying the cyst. The germinal matrix of the nail can be up to 5 mm proximal to the fold of the eponychium. An H-shaped incision is performed overlying the dorsal aspect of the DIP joint; the transverse limb of the H is located at the dorsal DIP joint crease or centered along the cyst. The longitudinal limbs are placed along the midaxillary line on both the radial and ulnar sides of the digit. After identifying the cyst, careful dissection around the stroma should occur down to the stalk, which can be tied off or ligated. Any osteophytes should be removed with a rongeur.[1] Other options include T-, inverted U-, or transverse incision configurations.

Removal of osteophytes during surgery can also reduce the recurrence rate (<10%). If there is concern about skin thickness after removal, the surgeon should consider performing a full-thickness graft (from the thenar crease of the palm), a local advancement flap, or a rotational flap.[7][13] Bipedicled flap transfers can also be performed, but can cause donor site morbidity. Digital artery perforator flaps can also be used, especially for larger defects (>2 cm); however, this is more technically demanding, requiring localization and dissection of the flap's vascular supply.[7][14] Results from a study found that postoperative splinting of the digit did not reduce recurrence rates.[15](B2)

Because DIP joint osteoarthritis is commonly associated with digital mucous cysts, patients should be aware that excision alone may not eliminate the pain from the residual osteoarthritis. For patients who prefer to avoid surgery, aspiration of the mucous cyst may be performed; however, recurrence rates are high, approaching 50%. Arthrodesis of the DIP joint is the only procedure that reliably prevents postoperative recurrence of a digital mucous cyst.[13] (B3)

If a patient elects to undergo arthrodesis, several technical options are available, including Kirschner wires, intraosseous wires, headless compression screws, and headed screws. Each technique carries specific risks: K-wires may cause pin tract infections; intraosseous wires may result in hardware prominence or dorsal skin necrosis; and headless compression screws may create a size mismatch, potentially exerting pressure on the nail bed and causing nail deformities, such as a split nail. Implant breakage is also a potential complication. Nonunion rates are highest with interosseous wiring, reported in up to 12% of cases, whereas union rates with the other techniques range from 92% to 100%.[16](B3)

Differential Diagnosis

The differential diagnosis of digital mucous cysts includes gout, giant cell tumor, and Heberden node, which can present with similar soft-tissue or joint findings and should be carefully considered and ruled out.[3] 

Prognosis

After surgical excision of digital mucous cysts, recurrence occurs in approximately 2%, provided the excision includes the stalk of the cyst. If the cyst is removed without the stalk, the recurrence rate can be as high as 25% to 50%. Recurrence rates of up to 68% to 100% have been reported after corticosteroid injection into digital mucous cysts.

If aspiration is attempted, the recurrence rate is 50%. Arthrodesis can also be performed if the patient is experiencing pain due to an underlying degenerative disease of the DIP joint of the involved digit.[3] If a patient elects to undergo surgery without addressing underlying DIP joint osteoarthritis, they should be warned that they may still experience pain postoperatively.[13]

Complications

Complications following treatment of digital mucous cysts may include recurrence, postoperative pain related to underlying DIP joint arthritis, soft tissue defects, soft tissue infection, formation of a draining sinus tract, joint stiffness, nailbed injury or deformity, damage to the extensor tendon, and osteomyelitis.[13]