Central Slip Injuries

The central slip is the middle band of the extensor mechanism, and its injury at the PIP joint is the zone III extensor injury. It is frequently missed in the acute setting because the intact lateral bands often preserve PIP extension at first. Left alone, the finger develops a boutonniere deformity insidiously over 3-6 weeks; early recognition and splinting prevent it.

Mechanism. The central slip is torn by force, a blade or a dislocation:

• Forced flexion - ball catching, the jammed finger
• Direct laceration - a dorsal PIP wound
• Crush - industrial accidents
• Volar PIP dislocation - reduction can leave the central slip torn even though the joint is now congruent

What it costs the patient. Loss of PIP extension power weakens grip. The deformity is conspicuous and the usual reason for referral, but an established boutonniere is often more disfiguring than disabling, because PIP flexion and the ability to make a fist are preserved. Chronic cases develop secondary PIP arthritis, and hyperextension of the adjacent DIP joint causes pain.

The extensor mechanism at the PIP. Over the proximal phalanx the extensor mechanism divides into three slips: the central slip and two lateral bands. The central slip inserts on the dorsal base of the middle phalanx and is the primary PIP extensor. The lateral bands assist PIP extension and continue distally to join as the terminal tendon, which extends the DIP; the triangular ligament connects them dorsally and prevents them separating.

How the boutonniere forms. When the central slip ruptures, the lateral bands lose their dorsal tether, the triangular ligament attenuates and the bands migrate volar. Once they lie volar to the PIP axis they change from PIP extensors into PIP flexors and exert increased extension force on the DIP, producing PIP flexion with DIP hyperextension. The migration runs to a timetable:

• Weeks 1-2 - the lateral bands begin to migrate volar
• Weeks 3-6 - they cross volar to the PIP axis
• Later - a fixed contracture develops over weeks, and once fixed the deformity is difficult to correct without surgery

Why extension survives at first. In the acute injury the lateral bands are still dorsal to the PIP axis, so the patient can extend the PIP through them, weakly. Extension is lost over the following weeks as they migrate, so a finger that extends on the day of injury has not had a central slip rupture excluded.

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Injuries are classified by timing from injury, which predicts the outcome of conservative treatment, and by mechanism.

History. Establish the mechanism, forced flexion or laceration, and whether the presentation is acute or delayed. The patient reports difficulty extending the finger, grip weakness, and pain over the dorsum of the PIP that is worse with flexion.

Examination. Inspect for PIP swelling, an open wound and the resting posture. Active PIP extension is weak or absent, although it can be near normal acutely through the lateral bands. Passive extension is full in the acute injury and limited by contracture in the chronic one, and DIP function checks the integrity of the terminal tendon.

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The Elson test. The patient's PIP is flexed to 90° over the edge of a table, and the examiner resists attempted PIP extension while palpating the DIP. What the DIP does gives the answer:

• Intact central slip - strong PIP extension force, and the DIP stays floppy because the lateral bands are slack in PIP flexion
• Ruptured central slip - weak PIP extension, and the DIP goes rigid in extension because all the extensor force is transmitted through the lateral bands to the DIP, bypassing the PIP

The rigid, fixed-extended DIP is the positive finding; a floppy DIP is the normal result. The test is regarded as pathognomonic, but two cautions come from the Evidence Base: the original description reported no accuracy data, and in a painful acute finger the test should be performed under digital block.

Pseudo-boutonniere is the trap. It follows a hyperextension injury with volar plate damage and produces a fixed PIP flexion contracture, but the DIP is normal and the Elson test is negative. Mistaking it for a true boutonniere leads to extension splinting that fails.

The diagnosis of a central slip injury is clinical. Radiographs are primarily to rule out a bony avulsion and to assess PIP congruity, and treatment does not wait for an MRI: if the Elson test is positive, splint immediately.

Radiographs. PA, lateral and oblique views of the affected finger are first line. Most central slip injuries are purely soft tissue and the films are normal; what they are looking for is:

• An avulsion fracture of the dorsal base of the middle phalanx
• PIP dislocation
• PIP arthritis, in chronic cases

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CT is added when a fracture is seen. It measures the size and displacement of the fragment against the fixation threshold below and plans the fixation, screws or a tension band.

MRI has its place in the chronic boutonniere, where it assesses tendon quality and joint cartilage: a scarred central slip, volar lateral bands and arthritic change at the PIP. It helps decide between repair, reconstruction and arthroplasty.

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The bony equivalent of a central slip rupture is managed differently from the pure soft-tissue tear, and deserves its own description.

What it is. Instead of tearing in mid-substance, the central slip avulses a fragment of dorsal cortex from the base of the middle phalanx. The fragment marks the detached central slip insertion, and the injury carries the same boutonniere risk as the closed rupture.

How it is seen. The true lateral radiograph is the key view: look for a small dorsal flake off the middle-phalanx base, analogous to the dorsal distal-phalanx flake of a bony mallet. A subtle fragment is easily missed on the AP, so a dedicated lateral is mandatory in any PIP dorsal injury.

When to fix. A small, minimally displaced, congruent fragment is treated exactly like the soft-tissue injury, with PIP extension splinting and the DIP free, because reducing the extended PIP reduces the fragment. Operative fixation is reserved for:

• A large fragment, commonly quoted as more than about a quarter (over 25%) of the articular surface
• Significant displacement
• PIP joint incongruity or subluxation

The techniques include a small screw, a pull-out suture, or a dorsal tension band or anchor.

The pitfall. Do not mistake it for a stable chip. An unrecognised displaced dorsal-base avulsion produces the same fixed boutonniere as a missed soft-tissue rupture, plus articular incongruity and secondary PIP arthritis.

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The relative-motion extension orthosis is an increasingly used alternative to rigid static splinting.

The principle. A small yoke across the dorsum holds the injured finger's MCP joint in slightly more extension than the adjacent MCPs, roughly 15-20° more. Altering the relative MCP posture offloads the healing central slip, through the quadriga/juncturae linkage, while still permitting active PIP and DIP motion, so the finger is not rigidly immobilised.

Why it is attractive. Early controlled active motion during healing can reduce PIP stiffness and adhesion and lets the patient keep using the hand. The splint is generally better tolerated and easier to comply with than 6-8 weeks of continuous rigid PIP extension, and compliance is the main determinant of outcome.

How it is applied. In zone III it is typically used as a relative-motion extension orthosis, often with a short additional PIP extension component early, and progressed by a hand therapist. It suits selected acute closed injuries and rehabilitation after repair rather than fixed chronic deformity.

The honest caveat. Systematic-review evidence shows no clear long-term superiority of relative-motion or dynamic regimens over static extension splinting for zone III. The choice is pragmatic and therapist-led, an option rather than a mandate, and rigid static splinting remains a perfectly acceptable default.

Prevention is the best treatment. The most important complication to prevent is the boutonniere deformity itself. Once established it is difficult to correct and often results in permanent functional impairment, even though, as above, it is often more disfiguring than disabling. Prevention rests on a high index of suspicion in any PIP injury, an Elson test, and splinting started immediately and continued for 6-8 weeks.

Prognostic factors. The best prognosis belongs to an acute injury in a compliant patient, splinted full time for 6-8 weeks with early DIP mobilisation. The poorest follows diagnosis delayed beyond 6 weeks, a fixed PIP contracture, PIP arthritis on imaging, or non-compliance. The threshold to remember is 3 weeks from injury: outcomes decline significantly after it.