Swan neck deformity is characterized by proximal interphalangeal (PIP) joint hyperextension and flexion of the distal interphalangeal (DIP) joint. There is also reciprocal flexion noted of the metacarpophalangeal (MCP) joint. This is a result of an imbalance of the extensor mechanism of the digit. The imbalance could be due to a significant laceration or stretching of the extrinsic tendon forces on the distal phalanx or from tightness and pulling of the extensor mechanism, both intrinsic and extrinsic, at the PIP joint.
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:
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.
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.
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.
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.
In the early stages, there may still be quite a bit of flexibility (active or passive) of the PIP joint. 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.
In the later stages or more chronic deformities, the PIP may be stiff and fixed 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.
Along with testing for rheumatoid, x-rays are ordered to evaluate for articular disruption, severe arthritis in the later stages, or even carpal collapse.
Conservative treatment is non-operative.
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.
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.
Complications include incomplete correction of the deformity, recurrence, stiffness at PIP joint, and loss of extension at the DIP joint due to overcorrection.
The hand should be immobilized in a plaster splint with the MCP joints in full extension and PIP joints flexed.
Active flexion exercises should begin within the first 4 to 5 days if flexor tenosynovectomy has been performed.
Dorsal extension blocking should be placed until 4 to 6 weeks postoperatively.
The management of swan neck deformity is an interprofessional. While the primary care provider and nurse practitioner may follow the patient is conservative management is undertaken, patients who undergo surgery need to be followed by a hand or orthopedic surgeon. The outlook for patients with swan neck deformity is guarded. Complete recovery is not possible and some degree of deformity or limitations in range of motion are common. The key is to enroll the patient in a physical therapy program.(Level V)
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