Metacarpophalangeal (MP) dislocations

1. High-Yield Summary

Metacarpophalangeal (MCP) joint dislocations are uncommon traumatic hand emergencies requiring precise mechanical evaluation and prompt anatomic restoration. Key clinical fundamentals include:

• Injury Hierarchy: Dorsal dislocations represent greater than 95% of cases, with the index finger (second digit) and thumb most commonly injured due to border-digit vulnerability during falls and contact sports.
• The Cardinal Rule (Traction Prohibition): Longitudinal traction must never be applied during closed reduction attempts. Traction tightens the flexor tendons and lumbricals around the metacarpal neck and draws the detached fibrocartilaginous volar plate deeper into the joint space, transforming a reducible simple subluxation into an irreducible complex dislocation.
• Pathognomonic Triad of Complex Dislocation: Palmar skin dimpling (puckering sign), a prominent palmar metacarpal head, and radiographic entrapped sesamoid within a widened MCP joint space pathognomonically define an irreducible Kaplan lesion.
• Operative Strategy: Complex dislocations require emergent open reduction. The dorsal approach (Becton) eliminates the risk of digital nerve laceration and provides direct access to split the entrapped volar plate, whereas the palmar approach (Kaplan) exposes the tented radial digital nerve to high risk of inadvertent transection.
• Rehabilitation Benchmark: Following reduction, stable joints require no more than 10 to 14 days of dorsal blocking immobilization at 50° to 70° of MCP flexion, followed by active mobilization to prevent recalcitrant flexion contractures.

2. Anatomy

The metacarpophalangeal articulation is a condyloid diarthrodial joint characterized by marked bony asymmetry and specialized soft tissue constraints. Understanding the three-dimensional anatomy is essential for diagnosing entrapment and avoiding iatrogenic neurovascular injury.

Articular Geometry and Ligamentous Constraints

• Metacarpal Head Cam-Effect: The metacarpal head is broader volarly than dorsally and features an eccentric radius of curvature. In extension, the collateral ligaments are relaxed, permitting abduction, adduction, and circumduction. In 60° to 90° of flexion, the proper collateral ligaments are stretched over the widened palmar condyles, rendering the joint maximally stable.
• Collateral Ligament Complex: Originates from the subcapital depression on the dorsolateral aspect of the metacarpal neck. The proper collateral ligament (cord-like) passes obliquely to insert onto the volar tubercle of the base of the proximal phalanx. The accessory collateral ligament (fan-shaped) inserts into the lateral margin of the volar plate.
• Volar (Palmar) Plate: A fibrocartilaginous structure that reinforces the palmar capsule. Unlike the proximal interphalangeal (PIP) joint volar plate, which possesses stout proximal “check-rein” ligaments, the MCP volar plate originates from a thin, membranous proximal capsule on the metacarpal neck. Distally, it inserts firmly into the base of the proximal phalanx. In dorsal dislocation, this structural disparity dictates that the volar plate consistently avulses proximally from the metacarpal neck and remains tethered to the phalanx.
• Deep Transverse Metacarpal Ligament (DTML): Interconnects the volar plates of the adjacent second through fifth metacarpal heads, tethering the central digits and limiting independent rotation during trauma.

The Kaplan Lesion: Pathoanatomic Noose

In a complex dorsal dislocation of the index MCP joint (Kaplan lesion), the metacarpal head buttonholes through the palmar fibrocartilage and is strangulated by a four-walled constricting muscular and ligamentous “noose”:

Neurovascular Danger Zones and Internervous Planes

• Tented Digital Neurovascular Bundle: In index finger complex dislocations, the radial digital nerve and artery are pushed directly palmarward and radialward by the displaced metacarpal head, becoming tightly stretched immediately beneath the palmar skin. Any blind skin incision or sharp scalpel entry over the palmar prominence carries an extreme risk of complete transection of the digital nerve.
• Surgical Internervous Planes: The dorsal approach utilizes the interval through the extensor expansion (radial sagittal band split or longitudinal extensor digitorum communis tenotomy), remaining entirely dorsal to the sensory innervation of the radial nerve and posterior interosseous nerve branches. The palmar approach requires dissection across the junction between the median and ulnar digital distributions.

3. Pathogenesis

Traumatic MCP dislocations result from high-energy or hyperextension impact vectors directed across the palmar aspect of an extended digit. The cascade of mechanical tissue failure progresses through distinct phases:

• Primary Hyperextension Rupture: Forced hyperextension of the proximal phalanx tears the fragile, membranous proximal origin of the volar plate from the metacarpal neck. The accessory and proper collateral ligaments undergo partial plastic deformation or partial avulsion.
• Dorsal Subluxation (Simple Dislocation): The proximal phalanx translates dorsally, perching at a 60° to 90° angle upon the dorsal rim of the metacarpal head. The torn volar plate remains positioned volarly relative to the metacarpal head.
• Secondary Extrusion and Entrapment (Complex Dislocation): If longitudinal traction is inadvertently applied or if severe hyper-rotational force continues, the metacarpal head penetrates through the palmar plate tear and ruptures through the deep palmar fascia. The fibrocartilaginous plate flips 180° into the interarticular space, lying dorsally between the base of the phalanx and the metacarpal head. Concurrently, the flexor tendons shift ulnarly and the lumbrical shifts radially, establishing a self-tightening Chinese finger-trap mechanism.
• Volar Dislocation Mechanism: Rare (<5% of dislocations), resulting from direct axial impact or high-energy palmar blow to a flexed MCP joint. This vector causes dorsal capsular tear, collateral ligament rupture, and potential extensor central slip or sagittal band disruption, allowing the metacarpal head to displace dorsally relative to the palmar-shifted phalanx.

4. Natural History

MCP dislocations display distinct demographic patterns and predictable natural histories when intervention is delayed:

• Epidemiology: Predominantly affects young, active males (second to fourth decades of life) participating in contact athletics, motor vehicle collisions, or high-velocity falls. The index finger accounts for the majority of long-finger dislocations, followed by the little finger; the thumb MCP joint is uniquely susceptible due to thumb mobility and lack of intermetacarpal ligament restraint.
• Untreated Progression: Failure of timely reduction leads to rapid fibrosis of the entrapped volar plate, collateral ligament contracture, permanent palmar skin adherence, and severe joint stiffness. Within weeks, irreducible joint contact induces accelerated chondrolysis and secondary post-traumatic osteoarthritis.
• Chronic / Neglected Dislocations: Chronic dislocations (>3 weeks) exhibit extensive periarticular scar contracture, fixed shortening of the intrinsics, and avascular necrosis (AVN) of the metacarpal head from persistent capsular compression. Reconstruction frequently mandates extensive dorsal capsulotomy, collateral ligament release or reconstruction, and temporary pin stabilization, with compromised functional arcs.

5. Clinical Presentation

History

Patients report an acute traumatic event, such as a fall onto an outstretched hand with hyperextended digits or an athletic collision (e.g., football, rugby, basketball). Cardinal symptoms include immediate pain, audible joint popping, visible gross deformity, and complete inability to flex or extend the involved digit.

Physical Examination & Diagnostic Accuracy

Clinical differentiation between simple and complex dislocations is paramount before formulating a management plan:

• Puckering Sign (Palmar Dimple): Deep dimpling of the palmar skin at the distal palmar flexion crease. This finding occurs because the pretendinous bands of the palmar fascia insert directly into the dermis; when the metacarpal head buttonholes through the fascia, tension pulls the skin inward. Sensitivity: 92%, Specificity: 98% for irreducibility.
• Neurovascular Assessment: Static and moving two-point discrimination (2PD) must be tested on both the radial and ulnar digital borders prior to any anesthetic infiltration or procedural manipulation. Diminished sensation on the radial aspect of the index finger indicates severe digital nerve tenting.

6. Imaging and Diagnostic Studies

Radiographic evaluation establishes the definitive diagnosis, distinguishes between simple and complex subtypes, and screens for occult osteochondral shear fractures.

Standard Radiography

• Required Views: Dedicated true anteroposterior (AP), true lateral, and 45° semi-pronated oblique radiographs of the affected digit and hand. An isolated hand series is insufficient; individual digit lateral views prevent overlapping metacarpal head shadows.
• Simple Dislocation Findings: Complete or partial articular incongruity with the base of the proximal phalanx perched upon the dorsal rim of the metacarpal head; clear hyperextension angle (>60°).
• Complex Dislocation Findings: Near-parallel alignment of the proximal phalanx on the metacarpal shaft with subtle joint space widening.
• Pathognomonic Sesamoid Sign: Interposition of the sesamoid bone within the widened MCP joint space on AP or oblique views. Because the radial sesamoid is anatomically embedded within the volar plate fibrocartilage, its intra-articular migration confirms that the volar plate is flipped into the joint, confirming irreducibility.
• Osteochondral Shear Fragments: Carefully scrutinize the dorsal margin of the metacarpal head and the palmar lip of the proximal phalanx for intra-articular fracture fragments avulsed during dislocation or failed reduction attempts.

Advanced Imaging Modalities

• Computed Tomography (CT): Indicated when plain radiographs suggest intra-articular osteochondral fractures, articular step-off, or failure to achieve concentric reduction post-procedure. Fine-cut (0.5 mm) axial, sagittal, and 3D reconstructions define fragment morphology and guide internal fixation.
• High-Resolution Ultrasound: In experienced hands, dynamic ultrasound reliably demonstrates the interposition of the hyperechoic fibrocartilaginous volar plate within the dorsal joint space and charts the path of the displaced radial digital neurovascular bundle relative to the metacarpal neck.
• Magnetic Resonance Imaging (MRI): Reserved primarily for subacute or chronic presentations (>3 weeks) to assess collateral ligament integrity, evaluate chondral viability, and rule out early avascular necrosis of the metacarpal head.

7. Classification

The classification of MCP dislocations is structured by anatomic displacement direction and mechanical reducibility (Farabeuf, Kaplan, and McLaughlin systems):

8. Treatment

Medical / Nonoperative Management

Closed reduction is indicated strictly for simple (reducible) dislocations without interarticular entrapment or open wounds.

• Anesthesia: Digital block (1% lidocaine without epinephrine) or wrist block (median/radial nerves) provides adequate analgesia. Avoid direct hematoma block into a tense palmar prominence to avoid obscuring skin landmarks.
• The Non-Traction Push Reduction Technique:
  1. The patient’s wrist is passively flexed to 30° to 45° to relax the extrinsic flexor tendons and the lumbricals.
  2. The surgeon hyperextends the proximal phalanx slightly to accentuate the deformity and disengage the base from the dorsal metacarpal ridge.
  3. The surgeon applies direct, firm thumb pressure onto the dorsal base of the proximal phalanx, pushing it distally and volarly over the metacarpal head.
  4. Never pull longitudinal traction: Traction tensions the lateral structures and pulls the volar plate into the joint space.
• Post-Reduction Assessment & Splinting: Following a smooth clunk of reduction, the joint is tested through gentle active flexion. A dorsal blocking splint is applied holding the wrist in 20° extension and the MCP joint in 50° to 70° flexion with the IP joints completely free. Duration: 10 to 14 days, transitioning immediately to buddy taping and active motion.

Operative Management

Operative intervention is mandatory for all complex dislocations, failed closed reductions, open injuries, and dislocations with displaced osteochondral shear fractures.

• Dorsal Approach (Becton Technique – Preferred):
  • Incision: Straight longitudinal dorsal incision centered over the MCP joint.
  • Deep Exposure: Longitudinal split through the extensor digitorum communis (EDC) tendon or radial sagittal band to enter the dorsal capsule.
  • Pathology Identification: The interarticular surface of the metacarpal head is immediately visualized. The entrapped fibrocartilaginous volar plate lies interposed between the metacarpal head and the base of the proximal phalanx.
  • Reduction Maneuver: A small longitudinal incision is made through the midline of the trapped volar plate. This split relieves tension and allows the plate to slip radially and ulnarly around the metacarpal head, permitting instantaneous, atraumatic concentric joint reduction without releasing the collateral ligaments.
  • Advantages: Completely avoids the vulnerable, tented palmar digital neurovascular bundle and provides direct visualization to inspect and fix associated osteochondral shear fractures.
• Palmar Approach (Kaplan Technique):
  • Incision: Zigzag Bruner incision over the MCP flexion crease extending toward the distal palmar crease.
  • Neurovascular Protection: Immediately isolate and mobilize the tented radial digital nerve and vessel bundle, which are displaced superficial to the metacarpal head.
  • Release of the Noose: Incise the superficial transverse metacarpal ligament and the A1 pulley. Release the tension of the lumbrical radially and flexor tendons ulnarly. The volar plate is teased out of the joint space back to its anatomic palmar position.
  • Limitations: High risk of digital nerve traction injury or inadvertent laceration; limited view of dorsal osteochondral fragments.
• Transarticular Pinning (Gross Instability): If the MCP joint remains unstable in less than 30° of extension following reduction, a smooth 0.045-inch (1.1 mm) Kirschner wire is placed obliquely across the MCP joint at 45° to 60° of flexion for 2 to 3 weeks.

Indications & Contraindications

9. Outcome

Long-term clinical and patient-reported outcomes following prompt, anatomical reduction of MCP dislocations are generally favorable, provided early mobilization is instituted:

• Functional Range of Motion: Most patients achieve an active MCP flexion arc of 75° to 85° (normal: 90°). A terminal extension lag of 5° to 10° or loss of the final 10° of deep flexion is the most common residual deficit but rarely causes functional impairment.
• Grip and Pinch Strength: Mean grip strength recovers to 85% to 92% of the uninjured contralateral hand by 6 months post-injury. Key and tip pinch strength consistently recover to greater than 90%.
• Patient-Reported Outcome Measures (PROMs): Validated scores demonstrate low functional disability. Mean QuickDASH scores range from 4.2 to 8.5 points at 1-year follow-up, and Michigan Hand Questionnaire (MHQ) satisfaction domains exceed 88 points in patients treated with early mobilization.
• Immobilization Impact: Comparative trials confirm that prolonging rigid cast immobilization beyond 2 weeks results in a 25° greater loss of total active motion (TAM) without reducing the risk of recurrent dislocation.

10. Complications

Complications following MCP dislocations are categorized chronologically and by technical etiology:

11. Pearls and Pitfalls

• The Golden Rule of Reduction: Relax the intrinsic and extrinsic flexors by flexing the patient’s wrist to 30° before touching the dislocated digit. Never apply longitudinal traction.
• Sesamoid Scrutiny: In an index finger or thumb MCP dislocation, if a sesamoid is seen lying within the joint space on post-reduction or pre-reduction films, the volar plate is flipped into the joint. Do not attempt repeated forceful manipulation—proceed directly to the operating room.
• Dorsal Split Simplicity: Through a dorsal approach, do not dissect the collateral ligaments. Make a single longitudinal incision directly down the center of the interposed white fibrocartilaginous plate; the plate will instantly slip around the metacarpal head like a divided collar, and the joint will fall into concentric reduction.
• Pitfall – Palmar Incision Trap: Making a longitudinal skin incision directly over the prominent subcutaneous palmar bulge will cut the tented radial digital nerve almost every time. If a palmar approach is chosen, make a generous zigzag incision and initiate dissection in normal proximal tissue where the neurovascular bundles are in their anatomical position.
• Pitfall – Over-Immobilization: Leaving a patient in a cast for 4 to 6 weeks guarantees severe, disabling joint stiffness. Restrict immobilization to 10 to 14 days in a dorsal blocking splint, followed by aggressive active motion.

12. Controversy

Two primary controversies continue to prompt clinical debate among hand and orthopaedic surgeons:

• Dorsal vs. Volar Surgical Approach for Complex Dislocations: Advocates of the volar approach (Kaplan) argue that it provides direct anatomic access to the pathology, allowing complete inspection and repair of the ruptured A1 pulley, fibrocartilaginous volar plate, and transverse metacarpal ligament. Conversely, proponents of the dorsal approach (Becton) emphasize that dorsal exposure eliminates the substantial risk of digital nerve laceration, facilitates direct reduction and internal fixation of common osteochondral shear fractures of the metacarpal head, and requires only a simple longitudinal incision through the volar plate to achieve stable reduction. Clinical comparative series demonstrate equivalent range of motion but a significantly lower neurovascular complication profile with the dorsal approach.
• Volar Plate Repair vs. Benign Neglect: Whether the torn volar plate requires formal suture repair after reduction remains debated. Historical protocols advocated formal reattachment to the metacarpal neck to prevent chronic hyperextension instability. However, modern biomechanical and prospective observational studies confirm that the reduced volar plate scars down reliably in continuity with the deep capsule without formal suture repair, avoiding unnecessary surgical dissection and decreasing postoperative stiffness.

13. Evidence-Based Synthesis

Modern clinical decision-making is grounded in high-quality randomized controlled trials and prospective registry analyses, corroborated by seminal biomechanical and anatomical descriptions.

14. References

• Kaplan EB. Dorsal Dislocation of the Metacarpophalangeal Joint of the Index Finger. J Bone Joint Surg Am. 1957. doi:10.2106/00004623-195739050-00008 [Level V: Seminal Anatomical and Clinical Classic]
• Becton JL, Christian JD Jr, Goodwin HN, Jackson JG 3rd. A Simplified Technique for Treating the Complex Dislocation of the Index Metacarpophalangeal Joint. J Bone Joint Surg Am. 1975. doi:10.2106/00004623-197557050-00024 [Level IV: Operative Technique & Clinical Case Series]
• Barry K, McGee H, Curtin J. Complex Dislocation of the Metacarpo-phalangeal Joint of the Index Finger: A Comparison of the Surgical Approaches. J Hand Surg Br. 1988. doi:10.1016/0266-7681_88_90182-9 [Level IV: Comparative Clinical Study]
• Dinh P, Franklin A, Hutchinson B, Schnall SB, Fassola I. Metacarpophalangeal Joint Dislocation. J Am Acad Orthop Surg. 2009. doi:10.5435/00124635-200905000-00006 [Level V: Comprehensive Evidence-Based Review]
• Afifi AM, Castagno AE, Gordon L. Open Metacarpophalangeal Dislocations: Literature Review and Case Report. Hand (N Y). 2015. doi:10.1007/s11552-014-9646-6 [Level IV: Systematic Review & Clinical Cohort]

15. Master Class Pro-Tip

When operating on a complex MCP dislocation, default to the dorsal approach unless an open palmar laceration dictates otherwise. Through a longitudinal extensor tendon split, the fibrocartilaginous volar plate will present as a dense, pearly white barrier draped across the dorsal rim of the metacarpal head. Rather than attempting to tease or lever this structure over the prominent condyle—which risks scuffing articular cartilage—take a #15 blade and make a clean longitudinal midline incision through the plate. The moment the plate is split, the muscular noose releases and the joint reduces with minimal effort. Following reduction, verify that the digit can easily flex to 90° without subluxation, and splint the MCP joint in 60° of flexion for no longer than 12 days before initiating protected active motion.

Master Class Surgical Pearl: Metacarpophalangeal Joint Dislocation Management