Swan-Neck Deformity

Etiology

The swan neck deformity develops from either loss of extension mechanism at the distal phalanx or tightening or overpull of the extensor mechanism on the proximal phalanx. 

Initially, swan neck deformity can develop from loss of the extensor tendon at the terminal tendon at the distal phalanx, which over time can progress to the characteristic deformity. Injury to the terminal tendon can result from the following:

• A traumatic laceration to the extensor mechanism
• Closed avulsion from a direct blow on the distal phalanx of an extended digit
• Attrition by weakening from chronic inflammation at the DIP joint along with extensor tendon subluxation due to rheumatoid arthritis or any other inflammatory process

Hyperextension of the PIP joint can be due to the tightness of the intrinsic muscles and increased pull at the central slip. This could be secondary to rheumatoid arthritis, spasticity from traumatic brain injury, or stroke and intrinsic tightness alone.

Laxity of the volar plate is also a cause of swan neck deformity and the hyperextension seen at the PIP joint.[3]

Epidemiology

The frequency of swan neck deformity caused by traumatic insult is not more common in a specific age or sex. However, swan neck deformity related to rheumatoid arthritis is more common in women than in men.[4]

Pathophysiology

The beginning injury is to the extensor mechanism, typically a central slip insult. This results in a lax volar plate and a tight triangular ligament. The laxity of the volar plate relates to the hyperextension of the PIP joint, and the intrinsic contracture relates to the imbalance of forces on the PIP joint and DIP joint.[5]

History and Physical

The patient may have a history of inflammatory diseases such as rheumatoid arthritis. History may also include a traumatic injury to the extensor mechanism. It is important to ask about these when taking the patient's history. 

On inspection, the digit will have the visible, classic swan neck deformity of hyperextension at the PIP joint and flexion at the DIP joint. 

The range of motion should be assessed independently at the MCP, PIP, and DIP joints. It is important to examine both active and passive flexion at the PIP joint among the other joints. 

There may still be quite a bit of flexibility (active or passive) of the PIP joint in the early stages. However, if passively correctible, the PIP joint will reassume the deformity position when the full active digital extension is attempted.

To flex the PIP joint, the patient will often use the other hand to passively bring the PIP joint out of the fixed hyperextension. Once the joint is no longer hyperextended, active flexion of the joint is still possible in the earlier stages.

The PIP may be stiff and fixed in the later stages or more chronic deformities and will not be passively correctible. Degenerative arthritis at the PIP joint may lead to further joint pain and loss of motion. 

Perform the Finochietto-Bunnell test to determine whether a capsular restriction or intrinsic tightness is to blame. Normally, MCP joint extension should not restrict PIP joint motion. The Finochietto-Bunnell test involves holding the MCP in an extended position and then passively flexing the PIP, noting the available ROM. The test is repeated with the MCP flexed. If no change in motion is detected between the two trials, then capsular restriction at the PIP joint is suggested. If the motion increases when the MCP is flexed, then intrinsic muscle tightness is suggested.[6][1]

Evaluation

Along with testing for rheumatoid, x-rays are ordered to evaluate for articular disruption, severe arthritis in the later stages, or even carpal collapse.

Treatment / Management

Conservative Management 

Conservative treatment is non-operative. 

• Hand therapy for passive stretching combined with corrective splinting may benefit even the chronic conditions. This combination of treatment may increase mobility and flexibility at both the DIP and the PIP joints.
• Extension block splints may help correct the hyperextension at the PIP joint.
• Progressive extension splinting can help improve the DIP flexion deformity. 
• Although stretching and splinting may increase mobility, severe or long-standing deformities will not likely achieve true correction without surgery.[7]

Operative Management

• An intact passive motion at the PIP joint is present for patients with early stages of swan neck deformity. In these cases, a volar restraint must be developed during surgery to recreate the action of the volar plate. FDS tenodesis is a common checkrein to help prevent hyperextension. However, FDS tenodesis does not correct DIP joint mallet deformity if lateral band laxity has already formed. Conjoint lateral band tenodesis can improve both deformities if the anatomy is intact.[1]
• PIP joint hyperextension can be corrected by lengthening the central slip. PIP joint hyperextension can also be corrected from the volar side of the joint by tethering the joint in flexion using an FDS tenodesis or by performing a dermodesis (removing an ellipse of skin on the volar aspect of the joint). If intrinsic tightness is also evident, the release of the intrinsic muscle contributing to the central slip can also be performed. 
• In the later stages, if PIP arthrosis is evident, joint fusion or replacement may be indicated. These procedures are additional to the soft tissue procedures that will still be necessary. 
• If the MCP joints are also subluxing from severe degenerative arthritis, these joints may also be addressed in the form of an arthroplasty.

Differential Diagnosis

Boutonniere deformity can be mistaken for swan neck deformity. It is important to recognize that boutonniere deformity consists of hyperextension of the DIP and flexion of the PIP. Swan neck deformity is just the opposite.[1]

Staging

The severity of swan neck deformity is commonly classified by using the Nalebuff classification system. This classification system is based on the PIP joint stiffness with associated MCP joint positions.

• Type 1: PIP joint is flexible in all positions of the MCP joint. 
• Type 2: PIP joint flexion is limited in certain positions of the MCP joint.
• Type 3: PIP joint flexion is limited irrespective of the position of the MCP joint.
• Type 4: PIP joints are stiff and have a poor radiographic appearance.[8]

Complications

Complications include incomplete correction of the deformity, recurrence, stiffness at PIP joint, and loss of extension at the DIP joint due to overcorrection.[9]