Digital Mucous Cyst

Etiology

The etiology for digital mucous cysts is unknown at this time. Digital mucous cysts are typically associated with underlying osteoarthritis of the DIP joint of the finger.[1]

Epidemiology

Digital mucous cysts are a type of ganglion commonly found on the hand. Sixty percent of tumors found in the wrist and hand are ganglions. Females are more likely to be affected, with a rate three times as many as males. Mucous cysts typically affect middle age to elderly patients.[2]

Pathophysiology

It has been proposed that mucous cysts could be an outpouching of the synovial lining of a joint. Studies have been performed where dye is injected into the joint and has shown communication between the joint and the cyst. However, when the dye is injected into the cyst, there does not appear to be a communication of the cyst to the joint. This could indicate that there is a one-way valve, where the fluid is originating from the joint but it is unable to go back into the joint. Others have proposed that there could be a rent in the tendon sheath or the capsule surrounding the joint, causing irritation and a local reaction and thus the formation of the cyst. Another theory is that the connective tissue surrounding the joint is undergoing mucoid degeneration, with byproducts of collagen breakdown collecting in the cyst.[2] In a study by Eaton, all mucous cysts excised with marginal osteophyte resection were observed to communicate with the DIP joint.[3]

Histopathology

Looking grossly at the pathology of a digital mucous cyst will show a multilobulated cyst. Under a microscope, the cyst is made up of several layers of collagen fibers that are randomly oriented, which make up the outer wall of the cyst. This structure is mostly acellular and composed of mesenchymal cells and fibroblasts.[4] The inside of a mucous cyst is composed of viscous mucin, which can be clear to yellow in color.[5] This is composed of hyaluronic acid, globulin, glucosamine, and albumin.[2]

History and Physical

As with any soft tissue diagnosis, it is important to obtain a good history from the patient. It is important to ask the patient about any pertinent underlying medical problems that could be contributing to the disease as well as pertinent family history. In regards to the history, it is important to ascertain how fast the mass has been growing, whether or not it is causing them pain/night pain, if they have noticed color changes, or if there was any history of recent trauma to the area.[6] Digital mucous cysts of the hand typically present as a mass overlying the dorsal aspect of the DIP joint that is slow-growing in nature and located in the subcutaneous tissue. Upon palpation, the mass is not very mobile, can be transilluminated with a light, and feels firm. Mucous cysts are typically round and dome-shaped and can vary in size.[5] The mass typically arises off midline from the DIP joint due to the mass being pushed over by the extensor tendon but is still attached to the DIP joint by a stalk.[4] These lesions sometimes present with deformities of the nail if the cyst is applying pressure on the germinal matrix.[2][7] This may lead to longitudinal grooves.[4] The skin overlying the cyst should be assessed to determine the thickness and the possible need for skin graft if surgical intervention is undertaken. Skin can also commonly be ulcerated, and cysts may present open and draining, and possibly infected.

Evaluation

After a physical exam is performed, radiographs of the hand should be obtained. Typically on x-ray, there will be evidence of underlying osteoarthritis of the DIP joint, including sclerosis of the subchondral bone, osteophytes, and narrowing of the joint space.[7]

Treatment / Management

Initial treatment can be with conservative management, starting with observation, especially with small cysts (several millimeters in size). Aspiration and excision are also options. It is recommended to avoid using corticosteroid injections as these can cause the skin to become thinner in this region of the body.[6] If the skin is becoming thin over the cyst or if a painful deformity of the nail is present, more invasive treatment may be pursued at that time. Due to its common association with underlying osteoarthritis of the DIP joint, patients should be aware that after cyst excision alone, they may still experience pain secondary to the remaining underlying osteoarthritis. Patients who wish to avoid surgery may proceed with an aspiration of the mucous cyst, however, they should be aware that there is a high recurrence rate around 50%. Surgical management is chosen to decrease the risk of recurrence or because of the failure of conservative treatment. The stalk should be excised, dorsal capsule removed, and osteophytes removed. Care should be taken to protect the skin during surgical dissection to avoid any postoperative complications. A digital block without additional anesthesia can be used with a tourniquet placed at the base of the finger.[2]  

Surgical Excision:  Meticulous technique is required to avoid damage to the germinal matrix, which is often in close 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 dorsum of the DIP joint, the transverse limb of the H is located at the dorsal DIP crease or centered along the cyst. The longitudinal limbs are placed along the midaxillary line on both the radial and ulnar side of the digit. After identifying the cyst, careful dissection around the stroma should occur down to the stalk. The stalk can be tied off or ligated, and any osteophytes removed with a rongeur.[1] T, inverted U, or transverse incision configuration are also options.

Removal of osteophytes at the time of surgery can also decrease the rate of recurrence (<10%). If there is a concern for the thickness of the skin after removal, the surgeon should consider performing a full-thickness graft (thenar crease of palm), a local advancement flap, or a rotational flap.[6][8] Bipedicled flap transfers can be performed as well but can cause donor site morbidity.  Digital artery perforator flaps can also be used, especially for larger defects (>2cm), however, this is more technically difficult requiring localizing and dissecting out the vascular supply to the flap.[6][6](B3)

DIP joint arthrodesis is the only way to guarantee no recurrence postoperatively.[8] If a patient decides to undergo DIP joint arthrodesis, there are many options to consider in regards to technique. Currently, there are many options. Kirschner wires, intraosseous wires, headless compression screws, as well as headed screws, are all options. Risks involved with these techniques include pin tract infections (k-wires), hardware prominence and dorsal skin necrosis (intraosseous wires), and size mismatch (headless compression screws), as well as the risk of implant breakage. With headless compression screws and size mismatch, this may also put pressure on the nail bed leading to nail deformities, such as a split nail. The nonunion rate has been reported to be the highest in interosseous wiring, at as high as 12% of cases. The other techniques have reported union rates ranging from 92 to 100%.[9](B3)

Differential Diagnosis

• Gout
• Giant cell tumor
• Heberden's node[2]

Prognosis

After surgical excision, there is a recurrence rate of 2%, but only if 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%.  The recurrence rate has been as high as 68-100% after corticosteroid injection into the cyst. If aspiration is attempted, there is a 50% chance of recurrence. Arthrodesis can also be performed if the patient is experiencing pain from underlying degenerate joint disease of the DIP joint of the involved finger.[2] If a patient decides to undergo surgery without addressing the underlying DIP joint arthritis, they should be warned they may still experience postoperative pain.[8]

Complications

• Recurrence
• Postoperative pain from underlying DIP joint arthritis
• Soft tissue defect
• Soft tissue infection
• Draining sinus tract
• Joint stiffness
• Nailbed injury/deformity
• Damage to the extensor tendon
• Osteomyelitis[8]