Boutonniere Deformity 

Boutonnière deformity (BD) can manifest itself acutely following trauma, but most BDs are found weeks following the injury or as the result of progressive arthritis. The proximal interphalangeal (PIP) joint of the finger is flexed, and the distal interphalangeal (DIP) joint is hyperextended, as seen in the image below. Treatment options depend partly on etiology of the deformity and are discussed in the sections to follow.^{[1, 2, 3]}

A boutonniere deformity in the finger is due to deformity or disruption of the central slip, which is a key component of the extensor mechanism at the PIP joint.^[4] The flexion-extension mechanisms at the PIP joint are among the most complex in the hand. Weakening or disruption of the central slip with compromise of the triangular ligament subjects the lateral bands to migration volar to the axis of rotation of the PIP joint, as seen in the images below. The delicate balance between the extensor mechanism over the dorsal PIP joint and the flexors volarly is upset. As the deformity progresses, the now dominant flexor superficialis creates constant flexion at the PIP joint. Initially, the DIP joint exhibits an extensor lag.^{[5, 6]}

As the lumbrical and interosseous muscles (intrinsics) lose their insertion into the middle phalanx due to the incompetent central slip, their force of action is diverted entirely through the lateral bands. Over time, these lateral bands migrate palmarly and contract. This is accompanied by secondary shortening of the oblique retinacular ligaments. Together, these changes cause hyperextension at the DIP.

BD of the thumb consists of a spectrum of instability patterns caused by imbalance between the extensor and flexor mechanisms. This complex problem is beyond the scope of this discussion.

Frequency

The frequency of occurrence depends on the etiology. A review of 43 of 71 patients treated in an emergency department with the diagnosis of jammed or sprained finger over a 14-month period revealed that 2 of the 43 patients went on to develop a BD. Up to 50% of patients with rheumatoid arthritis (RA) are estimated to develop a boutonniere deformity in at least 1 digit.^[7]

The 3 main etiologies described are mechanical trauma, RA and other inflammatory arthritides, and burns and infections.

The pathophysiology varies depending on the etiology.

Several mechanisms of trauma can lead to a boutonniere deformity. A laceration over the joint may involve the central slip. Axial loading or forced flexion with the PIP in extension can cause closed disruption of the central slip. Volar dislocation of the PIP can cause avulsion of the central slip as well. Finally, any combination of the above may be responsible for a BD.^{[8, 9, 10, 11]}

Traumatic boutonnière deformity may develop after unsuccessful primary treatment of a lesion of the extensor tendon at the level of the proximal interphalangeal joint.^[4] Haerle et al note that knowledge of the mechanisms leading to this deformity is fundamental for making the correct diagnosis and choosing and executing reconstructive procedures. Because none of these procedures has been shown to be successful in all situations, Haerle et al recommend a staged reconstructive approach in their report but caution that even then, the deformity often results in incomplete reconstruction.

The mechanism in RA is quite different from that associated with trauma. The PIP is slowly forced into flexion by chronic synovitis of the joint, elongating the central slip and ultimately leading to rupture. Subsequent volar displacement of the lateral bands below the axis of the PIP rotation creates increased tension on the DIP extensor mechanism, leading to hyperextension and limited flexion of the DIP.^{[12, 13, 14, 15]}

Full-thickness burns may disrupt the central slip, but most commonly, BD occurs from secondary infection. Rarely, increased pressure over the PIP from the rigid eschar may cause ischemic necrosis of the central slip.^[16]

Although the boutonniere deformity is a well-known deformity in the fingers, it can, in rare cases, occur in a lesser toe. In one reported case, after reduction of a traumatic planter dislocation of the PIP joint of the second toe, boutonniere deformity resulted from a rupture of the central slip of the extensor tendon and the shift of the lateral bands to the planter side. Yoshino et al reported that surgical repair of the extensor mechanism brought good results.^[17]

The history varies depending on the primary etiology of the deformity. Even though most patients who present with a tender, swollen, and stiff PIP joint have sustained a recent closed injury (usually an axial load), they all should be questioned as follows:

• Have you experienced any recent trauma to the involved digit?
• Do you have any history of rheumatoid or other inflammatory arthritis?
• Have you experienced any recent severe burns or infections near the involved joint?

Trauma

The digit is held in a semiflexed position, and active motion, especially PIP joint extension and DIP joint flexion, is decreased. Except in late presentations, full extension can be achieved passively. Patients with jammed fingers do not develop classic boutonniere deformity until 2-3 weeks following the initial injury. Typically, the initial central slip disruption is either misdiagnosed or undertreated on first presentation. Common splinting techniques with the PIP joint partially flexed serve to accentuate the deformity.^{[8, 9, 10, 11]}

In lacerations of the central slip, acute presentations of BD can occur. The digit is held slightly flexed at the PIP joint, with full extension passively. With attempted active extension of the digit, the PIP joint flexes, and the DIP joint hyperextends due to mechanisms described above.

Volar PIP dislocations commonly disrupt the central slip insertion. However, when the patient presents, the joint already may be reduced. With volar dislocations and any jammed finger, the examiner must maintain a high index of suspicion for a central slip injury to avoid development of a BD.

Dorsal avulsion fractures or any fractures involving the base of the middle phalanx are at high risk for developing a BD. Open or closed fractures appear to have the same incidence of subsequent BD formation.

Rheumatoid arthritis

BD in patients with RA can be classified into 1 of the following 3 stages, which serve as a guide to the appropriate management:

• Stage I (mild) is the earliest stage and is the result of PIP joint synovitis with mild extensor lag that still can be corrected passively. The metacarpophalangeal (MP) joint usually is normal, and the DIP may or may not be hyperextended.
• Stage II (moderate) is characterized by 30-40° of flexion contracture at the PIP joint and hyperextension of the MP joint as a compensatory mechanism. The finger has increased functional loss. Early passive extension still is possible. With time, soft-tissue contractures develop, and passive extension becomes restricted.
• Stage III (severe) begins when the PIP joint can no longer be extended passively. Radiographs demonstrate destruction of the joint surfaces.

Pseudoboutonnière deformity is a condition marked by PIP joint flexion contracture and restricted flexion of the DIP joint. The characteristic hyperextension of the DIP in BD is not present. It often is the result of a hyperextension injury causing inflammation and contracture of the checkrein ligaments, the oblique retinacular ligaments, and, possibly, the first cruciate pulley. Pseudoboutonnière deformity must be distinguished from BD because pathophysiology and treatment are different.

Most patients receive conservative treatment by splinting initially (see Medical therapy).

The indications for surgical intervention initially include open injuries, unreducible volar dislocation of the PIP joint, and displaced avulsion fractures of the dorsal base of the middle phalanx.

Once a boutonniere deformity is established, surgery may be required if splinting techniques fail.

In patients with RA, only stage I disease can be treated with splinting techniques. Once stage II develops, surgical intervention is necessary (see Surgical therapy).

The relevant anatomy involved is discussed above. Please see the diagram in the image below for orientation and clarification.

Remember that boutonniere deformity is a functional deformity. Attempts at surgical reconstruction risk flexion of the joint and can render the joint less functional. For that reason, the patient and physician must carefully weigh and measure the risks and benefits prior to embarking on surgical planning. A relative contraindication to surgical reconstruction of the central slip is failure to achieve acceptable range of motion in the PIP passively.