Masterclass: Surgical Management of Thumb Metacarpophalangeal Joint Volar Instability and Dislocations

Introduction and Epidemiology

Disruption of the restraining structures on the volar surface of the thumb metacarpophalangeal (MCP) joint leads to profound kinematic derangements, primarily characterized by excessive joint translation and abnormal hyperextension. While less frequently encountered than collateral ligament incompetence (e.g., ulnar collateral ligament ruptures or "Skier's thumb"), volar instability frequently causes significant functional deficits. The biomechanical demands placed on the thumb dictate that stable, pain-free opposition is a prerequisite for effective key pinch, tip-to-tip pinch, and power grasp.

Acute injuries to the volar restraints, including frank joint dislocations, mandate prompt and precise intervention to optimize functional outcomes and prevent chronic sequelae. Dorsal dislocations of the thumb MCP joint are overwhelmingly more common than volar dislocations. The classical mechanism of injury involves a sudden, forceful hyperextension vector of sufficient magnitude to rupture the volar plate and the associated joint capsule. This injury pattern is ubiquitous in athletic competitions—such as when a ball strikes the distal aspect of an extended thumb—or during a fall on an outstretched hand (FOOSH) that drives the proximal phalanx into forced hyperextension against a fixed metacarpal.

Acute simple dislocations are defined by the absence of soft tissue interposition and can typically be managed with closed reduction and subsequent immobilization. Conversely, complex dislocations are characterized by the interposition of soft tissues—most notably the avulsed volar plate—which renders the joint mechanically irreducible to closed manipulation. Chronic volar instability typically presents as the inevitable sequela of an unrecognized, neglected, or inadequately treated acute injury, leading to progressive attenuation of the volar structures. Clinically, these patients present with a characteristic collapse of the MCP joint into hyperextension during pinch maneuvers, which drastically reduces their pinch strength and overall hand dexterity.

Surgical Anatomy and Biomechanics

The kinematics of the thumb MCP joint exhibit significant inter-individual variability, largely dictated by the spectrum of metacarpal head geometry. Metacarpal heads presenting with a more spherical or rounded contour permit a greater arc of flexion, extension, and axial rotation. Conversely, flatter metacarpal heads constrain the joint, allowing relatively less range of motion. Because the bony architecture affords minimal intrinsic constraint, the joint relies heavily on its robust soft tissue envelope for stability.

Collateral Ligament Complex

The collateral ligament complex is composed of the proper collateral ligaments (PCL) and the accessory collateral ligaments (ACL). The proper collateral ligaments originate from the dorsal third of the lateral metacarpal condyles and course palmarly and obliquely to insert on the palmar-proximal aspect of the proximal phalanx. They are the primary stabilizers against varus and valgus stress when the joint is in flexion. The accessory collateral ligaments originate slightly more palmar and proximal to the PCLs, traversing distally and palmarly to insert on the volar plate and the sesamoids. The ACLs are taut in extension and contribute significantly to volar stability and the prevention of hyperextension.

Volar Plate and Sesamoid Anatomy

The volar plate functions as the floor of the MCP joint and is the primary static restraint to hyperextension. Distally, it is a thick, rigid fibrocartilaginous structure that firmly attaches to the volar base of the proximal phalanx. Proximally, it transitions into a thinner, more compliant membranous structure—often referred to as the "swallowtail" extensions—that attaches to the metacarpal neck. During a dorsal dislocation, the volar plate typically fails at its weaker proximal membranous attachment, though distal avulsions from the proximal phalanx can also occur.

Two sesamoid bones (radial and ulnar) are embedded within the distal, thick portion of the volar plate. This complex is heavily reinforced by the tendinous insertions of the intrinsic thenar musculature. The adductor pollicis inserts into the ulnar sesamoid, while the flexor pollicis brevis (FPB) and abductor pollicis brevis (APB) insert into the radial sesamoid. These intrinsic muscles provide critical dynamic volar support and contribute fibers to the dorsal extensor mechanism via the adductor and abductor aponeuroses, thereby conferring a degree of lateral joint stability.

Dorsal Extensor Mechanism

Dorsally, the joint is stabilized by the extensor apparatus. The extensor pollicis brevis (EPB) inserts onto the dorsal base of the thumb proximal phalanx, whereas the extensor pollicis longus (EPL) bypasses the MCP joint to insert at the base of the distal phalanx. Both tendons traverse the MCP joint and, along with the dorsal joint capsule and sagittal bands, add to the stabilizing compressive forces surrounding the articulation.

Pathoanatomy of the Complex Dislocation

In a complex (irreducible) dorsal dislocation, the proximal phalanx is displaced and rests dorsally on the metacarpal head. The metacarpal head is driven palmarly, buttonholing through a rent in the volar joint capsule and the proximal membranous portion of the volar plate. Consequently, the volar plate becomes entrapped dorsal to the metacarpal head and volar to the articular base of the proximal phalanx.

This displacement creates a constricting "noose" around the metacarpal neck, analogous to a Kaplan's lesion in the index finger. Ulnarly, the flexor pollicis longus (FPL) tendon acts as a tight restrictive band. Radially, the intrinsic thenar muscles (FPB and APB) block reduction. Dorsally, the displaced volar plate prevents the proximal phalanx from translating palmarly. Distally, the natatory ligament tightens across the metacarpal neck. Applying longitudinal traction to the thumb in this state only tightens this anatomical noose further, rendering closed reduction impossible and mandating open surgical intervention.

Indications and Contraindications

The decision algorithm for operative management is dictated by the acuity of the injury, the reducibility of the joint, the presence of soft tissue interposition, and the degree of chronic functional deficit.

Pre Operative Planning and Patient Positioning

Thorough preoperative evaluation relies heavily on high-quality orthogonal radiographs of the thumb. True anteroposterior (AP), lateral, and oblique views are mandatory to assess joint congruity, fracture presence, and soft tissue interposition markers.

Radiographic Evaluation

In the setting of acute dorsal dislocations, the lateral radiograph will clearly demonstrate the proximal phalanx resting dorsal to the metacarpal head. The position of the sesamoids serves as a critical radiographic marker. Because the sesamoids are intimately embedded within the distal volar plate, their anatomical location acts as a proxy for the position of the volar plate itself. If the sesamoids are visualized within the joint space or lying dorsal to the metacarpal head, this definitively confirms a complex dislocation with volar plate interposition. Furthermore, widening of the joint space on the AP view may indicate interposition of the capsule or collateral ligaments.

For chronic volar instability, stress radiographs are highly beneficial to quantify the degree of hyperextension and compare it symmetrically to the contralateral, uninjured thumb. While Magnetic Resonance Imaging (MRI) is rarely indicated for acute dislocations, it is highly valuable in chronic cases to evaluate the integrity of the volar plate, the collateral ligaments, and the presence of chondral damage prior to committing to a soft tissue reconstruction versus an arthrodesis.

Patient Positioning and Equipment Setup

The patient is positioned supine on the operating table with the affected upper extremity extended on a radiolucent hand board.

1. Anesthesia: Regional anesthesia (supraclavicular or axillary brachial plexus block) is generally preferred to provide excellent intraoperative conditions and postoperative analgesia. Local anesthesia with intravenous sedation or general anesthesia can be utilized based on patient factors and surgeon preference.
2. Tourniquet: A well-padded pneumatic upper arm tourniquet is applied to ensure a bloodless surgical field, typically inflated to 250 mmHg after exsanguination with an Esmarch bandage.
3. Equipment: A mini C-arm fluoroscopy unit should be draped and brought into the field from the head or foot of the table to confirm reduction and hardware placement. A standard hand surgery tray is required, including fine tenotomy scissors, Freer elevators, small Hohmann retractors, and skin hooks. For chronic reconstructions, micro-suture anchors (typically 1.3 mm to 1.5 mm) and Kirschner wires (0.035 or 0.045 inch) must be available.

Detailed Surgical Approach and Technique

The surgical management strategy diverges significantly between acute complex dislocations and chronic volar instability. Both scenarios, however, demand meticulous handling of the soft tissue envelope to restore physiological kinematics without inducing severe iatrogenic stiffness.

Open Reduction of Acute Complex Dislocations

For an irreducible dorsal dislocation, the surgeon must select between a volar and a dorsal approach. Historically, the volar approach was championed because it provided direct access to the torn volar structures. However, the dorsal approach has gained widespread favor in contemporary practice due to a significantly lower risk of iatrogenic neurovascular injury and excellent access to the entrapped volar plate.

The Dorsal Approach

1. Incision: A longitudinal or slightly curved dorsal incision is made over the MCP joint, centered directly over the extensor mechanism.
2. Extensor Mechanism Splitting: The extensor mechanism is identified. A longitudinal incision is made between the extensor pollicis longus (EPL) and the extensor pollicis brevis (EPB). Alternatively, the EPB can be retracted radially and the EPL ulnarly to expose the dorsal capsule.
3. Capsulotomy: The dorsal joint capsule is incised longitudinally to expose the articular surface.
4. Identification of the Volar Plate: Looking directly down into the joint from dorsal to volar, the surgeon will identify the entrapped volar plate draped over the metacarpal head. The articular surface of the proximal phalanx will be observed resting on the dorsal aspect of the volar plate.
5. Reduction Maneuver: A Freer elevator or a blunt probe is carefully passed dorsal to the volar plate and volar to the proximal phalanx. The volar plate is gently pushed palmarly over the metacarpal head. Simultaneously, the proximal phalanx is flexed. Once the volar plate clears the metacarpal head, the joint will reduce with a palpable and audible clunk.
6. Stability Assessment: The joint is taken through a full range of motion. Typically, once the soft tissue block is removed and the joint is reduced, it is highly stable. The dorsal capsule and extensor mechanism are meticulously repaired with non-absorbable or slowly absorbable sutures.

The Volar Approach

1. Incision: A volar zigzag (Brunner) incision is made over the MCP joint to prevent flexor contractures.
2. Neurovascular Protection: This is the most critical step of the volar approach. The radial digital nerve is frequently tented tightly over the prominent, palmarly displaced metacarpal head and lies directly in the subcutaneous tissue just beneath the dermis. It must be meticulously identified, mobilized, and protected with a vessel loop.
3. Release of the A1 Pulley: The A1 pulley is released to mobilize the FPL tendon, which is often displaced ulnarly and contributing to the constricting noose.
4. Releasing the Noose: The entrapped metacarpal head is visualized. The tension on the intrinsic muscles and the natatory ligament is assessed. To facilitate reduction, a small longitudinal incision can be made in the volar plate, or the flexor sheath can be further released.
5. Reduction: The proximal phalanx is hyperextended, and direct pressure is applied to the volar plate to push it distally and dorsally back to its anatomic position, allowing the metacarpal head to seat properly within the proximal phalanx.
6. Repair: The volar plate is repaired to its anatomic footprint on the proximal phalanx if avulsed, utilizing micro-bone anchors or transosseous sutures.

Reconstruction for Chronic Volar Instability

Chronic volar instability requires a tightening, imbrication, or advancement of the volar structures to eliminate the hyperextension deformity. Volar capsulodesis or volar plate advancement remains the procedure of choice.

Volar Plate Advancement Technique

1. Exposure: A volar Brunner incision is utilized. The neurovascular bundles are identified and retracted safely.
2. Mobilization: The A1 pulley is divided. The FPL tendon is retracted laterally to expose the underlying volar plate and sesamoid complex.
3. Preparation of the Insertion Site: The distal attachment of the volar plate on the base of the proximal phalanx is identified. If the volar plate is attenuated or stretched, it is sharply detached from the proximal phalanx. A small rongeur or curette is used to decorticate the volar base of the proximal phalanx, creating a bleeding bony trough to optimize soft tissue-to-bone healing.
4. Anchor Placement: One or two micro-suture anchors (1.3 mm to 1.5 mm) are inserted into the prepared bony trough at the base of the proximal phalanx.
5. Advancement and Fixation: The sutures from the anchors are passed through the distal edge of the volar plate in a horizontal mattress fashion. The MCP joint is held in 15 to 20 degrees of flexion to eliminate the hyperextension laxity. The sutures are then tied securely, advancing the volar plate distally into the bony trough.
6. Joint Pinning: To protect the repair during the initial healing phase and prevent early elongation of the reconstruction, a 0.045-inch Kirschner wire is driven obliquely across the MCP joint, locking it in 15 to 20 degrees of flexion. The pin is cut beneath the skin to prevent pin tract infections.

Sesamoidesis Alternative

In chronic cases where the volar plate is severely deficient or irreparably attenuated, but the sesamoids remain robust, a sesamoidesis can be performed. This technique involves anchoring the sesamoids directly to the base of the proximal phalanx or to the metacarpal neck, depending on the exact site of instability. This effectively utilizes the sesamoid-intrinsic muscle complex as a robust, dynamic block to hyperextension.

Complications and Management

Surgical intervention for thumb MCP joint instability carries specific risks. Meticulous surgical technique and strict adherence to postoperative rehabilitation protocols are essential to mitigate these complications.

Stiffness is by far the most common complication. The thumb MCP joint is notoriously unforgiving of prolonged immobilization. While a slight loss of terminal extension is often the goal of the surgery (to prevent hyperextension), excessive flexion contractures or loss of flexion can severely impair grasp. Therefore, achieving a robust surgical repair that allows for early, protected motion is the holy grail of this procedure.

Digital nerve injury must be actively prevented. The radial digital nerve is uniquely vulnerable during the volar approach because the displaced metacarpal head pushes the nerve directly into the subcutaneous space, altering its normal anatomic depth. The initial skin incision must be made with extreme care, using blunt dissection until the nerve is positively identified and protected with a vessel loop.

Post Operative Rehabilitation Protocols

The postoperative rehabilitation protocol must delicately balance the need for soft tissue healing with the prevention of debilitating joint stiffness. The protocol varies slightly depending on whether the procedure was an acute reduction or a chronic reconstruction.

For acute reductions (especially those managed via the dorsal approach without extensive volar plate repair), the thumb is immobilized in a thumb spica splint with the MCP joint in 20 degrees of flexion for 3 to 4 weeks. Following this period, active range of motion is initiated. Extension block splinting may be utilized during sporting activities to prevent recurrent hyperextension injuries for an additional 4 to 6 weeks.

For chronic reconstructions involving volar plate advancement or sesamoidesis, the transarticular K-wire is typically maintained for 4 weeks. Upon removal of the K-wire, patients are transitioned to a hand therapist. Active flexion is encouraged immediately, while extension is protected using a dorsal extension-block splint set at 10 to 15 degrees of flexion. Passive extension is strictly avoided for at least 8 weeks postoperatively. Full unrestricted activity and return to contact sports are generally permitted at 10 to 12 weeks, provided that dynamic stability and pinch strength have been adequately restored.

Clinical & Radiographic Imaging