Thumb Metacarpophalangeal Joint Arthrodesis

Surgical Indications

Absolute Indications

• Chronic ulnar collateral ligament insufficiency not amenable to reconstruction — gross instability with volar subluxation during pinch, failed prior UCL repair or graft reconstruction
• Post-traumatic MCP joint destruction with combined arthritis and instability where motion-preserving procedures (arthroplasty, osteotomy) are not viable
• Rheumatoid Z-collapse deformity with painful MCP volar subluxation as part of the zigzag collapse pattern — fusion stabilises the middle link of the collapse

Relative Indications

• Primary MCP osteoarthritis (uncommon in isolation — more often part of pan-trapezial arthritis) causing pain and functional loss refractory to non-operative treatment for greater than 6 months
• MCP instability after infection or tumour resection where joint reconstruction is not possible
• Failed MCP arthroplasty (silicone or pyrocarbon) with painful instability or loosening — revision to fusion
• Salvage after failed external fixation or failed dynamic external fixation for complex MCP peri-articular fractures

Contraindications

Absolute:

• Active infection at or adjacent to the MCP joint
• Uncorrectable soft-tissue deficit precluding wound closure over fixation hardware

Relative:

• Symptomatic CMC arthritis that should be addressed first (either staged before MCP fusion or simultaneously)
• Pre-existing IP joint arthritis — fusion at MCP may overload the IP; consider whether IP fusion alone would suffice
• Patient whose occupation requires full MCP flexion arc (some musicians, specialised manual workers) — counsel thoroughly

Evidence for Non-Operative Treatment

Splinting and Activity Modification

• A resting thumb spica splint reduces MCP loading in arthritic patients and provides symptomatic relief in early disease
• Static splinting at 10-15 degrees of MCP flexion can reduce pain during activities of daily living
• Evidence is largely from case series and expert opinion — no RCTs specifically compare splinting versus surgery for MCP arthritis
• Non-operative treatment is reasonable as a first-line measure in mild disease and when the patient is not yet ready for surgery

Corticosteroid Injection

• Intra-articular corticosteroid injection into the MCP joint provides temporary symptom relief (weeks to months) in inflammatory arthritis
• Limited role in primary OA — the MCP joint has a small capsule and injection provides inconsistent relief
• May be used as a diagnostic tool (symptom relief after injection confirms the MCP as the pain source before committing to surgery)

Evidence for Arthroplasty Versus Arthrodesis

• MCP arthroplasty (silicone, pyrocarbon, or CMC-type interposition) is an option for patients who wish to retain MCP motion
• Silicone arthroplasty has a long track record but is associated with silicone synovitis, implant fracture, and subsidence over time — outcomes are less predictable than fusion
• Pyrocarbon and other resurfacing implants have shorter-term follow-up and higher revision rates than fusion in the available literature
• Arthrodesis provides a predictable, permanent, and painless solution at the cost of MCP motion — most evidence supports fusion as the gold standard for young, active, or high-demand patients

Evidence for Arthrodesis

Functional Outcomes

• Thumb MCP arthrodesis reliably eliminates MCP pain and provides a stable platform for pinch and grip
• Loss of the small MCP flexion arc is well compensated by preserved CMC opposition and IP flexion — most patients report no subjective loss of overall thumb function
• Pinch and grip strength are maintained or improved post-operatively in the majority of series, particularly where instability was the pre-operative problem
• Patient satisfaction rates range from 80% to 95% across published series

Nonunion

• Nonunion is the most commonly reported complication, with rates between 5% and 15% depending on fixation method, bone quality and patient factors
• Compression fixation (headless screw or plate) provides more consistent union than K-wire-only fixation
• Smoking, diabetes, and rheumatoid osteopenia are the most consistently reported risk factors for nonunion
• Re-operation for nonunion involves revision fixation with bone grafting — union rates after revision are lower than primary fusion

Fixation Method Comparison

Key Decision Points

• Young, active patient with good bone stock and UCL insufficiency: headless compression screw provides the best balance of compression, low profile and no hardware removal
• Rheumatoid patient with osteoporotic bone: dorsal mini-plate provides the most rigid fixation in poor-quality bone; accept the higher rate of hardware removal
• Low-resource setting or cost-sensitive: tension band wiring with K-wires is reliable and inexpensive; plan for hardware removal at 4-6 weeks
• Concomitant CMC disease: address CMC first (trapeziectomy and LRTI), then reassess the MCP — some patients no longer need MCP fusion after CMC stabilisation
• IP joint arthritis present: if IP disease is mild, proceed with MCP fusion and monitor the IP; if IP disease is severe, consider IP fusion alone (which eliminates tip pinch) or staged procedures

Thumb Metacarpophalangeal Joint

Articular Surface Geometry

• The thumb MCP is a condyloid joint with a biconcave proximal phalanx base articulating with a biconvex metacarpal head
• The metacarpal head is wider volarly than dorsally — this shape contributes to the natural slight flexion of the resting thumb
• The collateral ligaments are eccentrically attached: taut in flexion, lax in extension — this is the basis for examining the UCL with the MCP in flexion
• The joint capsule is thin dorsally (allowing surgical access) and reinforced volarly by the volar plate

Range of Motion

• Total MCP flexion-extension arc: 5-10 degrees of extension to 50-60 degrees of flexion
• Useful functional arc: most thumb function (pinch, grip, opposition) occurs with the MCP between 5 degrees and 20 degrees of flexion — hence a fusion at 10-15 degrees sacrifices relatively little
• Lateral deviation: minimal — the tight collateral ligaments restrict abduction-adduction
• The CMC joint provides the majority of thumb opposition and circumduction; the IP joint provides tip flexion for fine pinch

Collateral Ligament Complex

Proper Ulnar Collateral Ligament (UCL)

• Origin: ulnar side of the metacarpal head, from the dorsal tubercle to the volar edge
• Insertion: base of the proximal phalanx on the ulnar side
• Function: primary restraint to valgus (radial deviation) stress during pinch — prevents volar subluxation of the proximal phalanx
• Testing: apply valgus stress with the MCP in 30 degrees of flexion — this tightens the UCL and isolates it from the accessory ligament
• Clinical relevance: the Stener lesion (UCL torn and displaced superficial to the adductor aponeurosis) prevents healing and requires surgical repair; chronic insufficiency after failed repair leads to MCP fusion

Accessory Ulnar Collateral Ligament

• Origin: more volar on the metacarpal neck
• Insertion: volar plate and ulnar sesamoid
• Function: stabilises the volar plate; taut in extension
• Not the primary valgus restraint — it is the proper UCL that is tested in the flexed position

Radial Collateral Ligament

• Less commonly injured than the UCL
• Provides restraint to varus (ulnar deviation) stress
• Rarely requires reconstruction or fusion as an isolated pathology

Extensor Tendon Anatomy (Dorsal Approach)

Extensor Pollicis Longus (EPL)

• Passes over Lister's tubercle at the wrist, then crosses dorsally to insert on the distal phalanx base
• At the MCP level the EPL is the most ulnar of the dorsal extensors
• Forms the ulnar boundary of the dorsal approach interval

Extensor Pollicis Brevis (EPB)

• Inserts on the proximal phalanx base, dorsal and slightly radial
• Forms the radial boundary of the dorsal approach interval
• May be absent or fused with the APL in up to 30% of individuals

Intervals for Dorsal Approach

The standard dorsal approach to the thumb MCP proceeds between the EPB (radial) and EPL (ulnar). This interval is identified proximal to the MCP joint and developed distally. The joint capsule is exposed directly beneath the interval.

Abductor Pollicis Longus (APL)

• Lies radial to the EPB, inserting on the first metacarpal base
• Forms the radial border of the first dorsal compartment
• Not routinely encountered in the MCP approach unless the incision is extended proximally

Neurovascular Anatomy — Dorsal Approach Danger

Radial Digital Nerve

• Runs volar to the extensor tendons, crossing into the dorsal plane in the proximal portion of the wound
• Lies superficially in the subcutaneous fat, vulnerable during skin-flap elevation
• Must be identified and protected before deepening between EPB and EPL
• Injury produces a painful neuroma and loss of sensibility over the radial pulp

Ulnar Digital Nerve

• Lies on the volar-ulnar side of the thumb, not routinely encountered in the dorsal approach unless dissection extends volarly
• Less at risk than the radial digital nerve in the dorsal approach, but must be protected during any volar release or plate placement

Dorsal Digital Nerves

• Small sensory branches over the MCP dorsum from the radial nerve (superficial branch)
• Transected during the skin incision — they provide limited sensation to the MCP dorsum and are not critical, but warn the patient about a small area of numbness over the MCP

Vascular Supply

• Dorsal metacarpal arteries supply the dorsal skin and MCP capsule
• The volar blood supply is dominant — avoid excessive dorsal periosteal stripping to preserve cancellous vascularity for fusion healing

Positioning and Preparation

Patient position: Supine, arm abducted 90 degrees on hand table, forearm in full supination. A non-sterile forearm tourniquet at 200-250 mmHg is applied but may not need to be inflated if the WALANT technique is used.

Anaesthesia: General anaesthesia, regional (brachial plexus block), or WALANT. WALANT allows intraoperative assessment of pinch position before committing to fixation. Inject 3-5 mL of 1% lidocaine with 1:100,000 epinephrine along the planned incision 15-20 minutes before incision.

Tourniquet: If used, inflate to 200-250 mmHg on the forearm. Exsanguinate the limb with an Esmarch bandage. Limit total tourniquet time to less than 90 minutes (shorter if WALANT is not used).

Fluoroscopy: Position the mini C-arm so AP and lateral views of the thumb MCP can be obtained without moving the hand. Confirm the image intensifier is draped and the surgeon can see the screen easily.

Consent: Specifically counsel regarding nonunion (5-15%), malposition requiring revision, hardware prominence and possible removal (10-20% with plate), infection (less than 1%), IP joint overload, CMC overload, and the certainty of permanent loss of MCP motion.

Equipment: Loupe magnification (2.5x minimum). Headless compression screw set (or plate and screw set, or K-wire and cerclage wire set — chosen pre-operatively). Small osteotomes, curettes, and burr. Intraoperative fluoroscopy. Image intensifier verified.

Dorsal Approach — Step-by-Step

Step 1: Marking and Incision

Mark the thumb MCP joint by palpating the metacarpal head and MCP flexion crease. Mark a longitudinal or gently curving dorsal incision centred on the MCP joint, approximately 3-4 cm in length. The incision should extend from the mid-shaft of the proximal phalanx to the distal metacarpal shaft.

Step 2: Subcutaneous Dissection and Nerve Identification

Incise skin sharply with a number 15 blade. Elevate thin skin flaps with care in the subcutaneous plane. Identify the radial digital nerve running in the proximal wound, crossing from volar to dorsal. Retract it gently with a vessel loop.

Step 3: Identify the Extensor Interval

Identify the EPB (radial) and EPL (ulnar) tendons proximal to the MCP joint. The EPB inserts on the proximal phalanx base; the EPL passes over the MCP to reach the distal phalanx. The intertendinous interval between EPB and EPL is developed sharply. Retract EPB radially and EPL ulnarly.

Step 4: Capsular Exposure and Release

The MCP joint capsule is visible between the retracted tendons. Make a longitudinal capsular incision in the midline from proximal to distal, extending the capsular flaps with small releasing incisions at the corners for exposure. Preserve as much capsule as possible for later closure over the fusion site.

Step 5: Joint Preparation — Surface Resection

Flex and extend the MCP to visualise the full articular surface. Using a small oscillating saw, flat osteotome or burr, resect the metacarpal head articular surface to flat, bleeding cancellous bone. Resect the proximal phalanx base articular surface similarly.

The goal is two flat, parallel surfaces with maximal cancellous contact. A cup-and-cone configuration (convex metacarpal, concave phalanx) or a fish-mouth (step-cut) can increase contact area but requires more precision. For most cases, flat surfaces with compression fixation provide reliable union.

Step 6: Positioning the Fusion

Place a single 1.0 mm K-wire across the joint to hold the provisional reduction at 10-15 degrees of MCP flexion with slight pronation. Check the position clinically:

• The thumb pulp should approximate the index finger pad in a comfortable pinch posture
• The thumb should not be hyperextended or excessively flexed
• Confirm the alignment on AP and lateral fluoroscopy

Adjust the K-wire position as needed until clinical and radiographic alignment are satisfactory.

Step 7: Definitive Fixation

Option A — Headless Compression Screw (preferred in good bone stock): Remove the provisional K-wire. Insert a guide wire from the dorsal cortex of the proximal phalanx base, directed across the fusion site and into the metacarpal shaft. Confirm trajectory on fluoroscopy. Measure screw length. Overdrill the near cortex, then insert a headless compression screw (Acutrak mini or Herbert-type) of appropriate length. The screw should provide compression across the fusion surfaces as it is fully seated.

Option B — Dorsal Mini-Plate (preferred in osteoporotic bone or rheumatoid disease): Select a 2.0 or 2.4 mm titanium mini-plate (T-plate or straight plate with 4-6 holes). Contour the plate to sit on the dorsal metacarpal neck and proximal phalanx base. Fix proximally with 2-3 screws in the metacarpal shaft and distally with 2-3 screws in the proximal phalanx. Ensure the plate does not impinge on the EPL tendon. Apply compression across the fusion site through the plate or with a separate lag screw.

Option C — Tension Band Wiring: Insert two parallel 1.0-1.2 mm K-wires across the fusion site from the proximal phalanx into the metacarpal. Pass an 0.8-1.0 mm cerclage wire in a figure-of-eight configuration over the K-wire ends on the dorsal surface and through a transverse drill hole distal to the fusion site. Tighten the figure-of-eight to generate compression. Bend the K-wire ends and bury them beneath the dorsal skin or paratenon.

Step 8: Confirm Fixation and Bone Grafting (if needed)

Check fixation stability by gentle manual stressing. Take final AP and lateral fluoroscopic images to confirm position, alignment and hardware position. If a bone defect exists after resection, pack morsellised autograft (from the resected bone surfaces) into the gap before final fixation.

Step 9: Wound Closure

Irrigate the wound thoroughly. Close the joint capsule with 3-0 or 4-0 absorbable suture. Repair the extensor interval between EPB and EPL if needed. Close subcutaneous tissue with 4-0 absorbable sutures. Close skin with 4-0 non-absorbable interrupted sutures.

Apply a well-padded thumb spica splint or short-arm thumb spica cast with the MCP in the fusion position, the IP free, and the wrist in neutral. Ensure the plaster does not extend beyond the IP joint so that IP motion can begin immediately.

Post-operative Protocol

Phase 1: Immobilisation (Weeks 0-4)

• Thumb spica: Well-padded short-arm thumb spica cast or splint, with the MCP in the fusion position and the IP joint free
• IP joint: Encourage active IP flexion and extension from day 1 within the thumb spica — the IP joint must not stiffen while the MCP is healing
• CMC joint: Gentle CMC opposition exercises within the cast — the patient can perform opposition stretches with the CMC moving inside the well-padded cast
• Elevation: Hand above heart for first 48 hours to reduce swelling
• Analgesia: Paracetamol and NSAIDs; opioids for the first 24-48 hours only
• Suture removal: 10-14 days for non-absorbable sutures

Phase 2: Early Mobilisation (Weeks 4-6)

• X-ray assessment: At 4 weeks, take AP and lateral radiographs of the thumb. If early bridging trabeculae are visible and fixation is stable, transition to a removable thumb spica splint.
• Protected motion: Allow the patient to remove the splint for gentle exercise sessions (active IP flexion/extension, CMC opposition, gentle pinch with light resistance) and wear the splint for all other activities.
• Pinch: Allow light lateral pinch and pulp pinch against mild resistance. Avoid forceful grip or sustained pinch.
• Scar management: Begin scar massage and desensitisation at the incision site.

Phase 3: Progressive Loading (Weeks 6-8)

• X-ray assessment: Repeat radiographs at 6-8 weeks. Look for progressive bridging trabeculae across the fusion site.
• Splint weaning: If union is progressing, discontinue the thumb spica. Transition to a soft hand-based thumb support during heavier activities for a further 2-4 weeks.
• Strengthening: Begin progressive pinch and grip strengthening with therapy putty, hand exerciser, and functional tasks.
• Hardware removal discussion: If a plate was used, discuss the timing of planned removal (usually after confirmed union at 4-6 months).

Phase 4: Full Activity (Weeks 8-12)

• X-ray confirmation of union: Bridging trabeculae across the fusion site on AP and lateral views; no hardware loosening or lucency.
• Return to work: Office and light manual work by 8-12 weeks. Heavy manual work (construction, agriculture) by 3-4 months, depending on the job and union status.
• Driving: When the patient can comfortably grip the steering wheel and perform an emergency stop — usually 6-8 weeks.
• Sport: Non-contact sport at 8-12 weeks; contact sport and racket sports at 3-4 months.

Rehabilitation: Key Principles

• IP motion must start day 1: The IP joint compensates for the lost MCP arc and must remain supple. A stiff IP after MCP fusion is a poor functional result.
• CMC motion within the cast: Encourage gentle opposition exercises even during the immobilisation phase to prevent CMC stiffness.
• Pinch retraining: The biomechanics of pinch change after MCP fusion — the thumb now operates as a single unit from CMC to IP with the MCP fixed. A hand therapist can guide progressive pinch strengthening and functional retraining.
• Hardware awareness: If a plate is in place, counsel the patient to expect a dorsal bump. Symptoms of EPL irritation (triggering, pain) warrant early review.

Expected Outcomes

• Union: 85-95% across all fixation methods (highest with plate, lowest with K-wires alone)
• Time to union: 8-14 weeks in most patients (longer in smokers, diabetics, rheumatoid bone)
• Pain relief: 85-95% of patients report significant or complete pain relief at the MCP
• Patient satisfaction: 80-95% satisfaction across published series
• Grip and pinch strength: Maintained or improved in most patients, particularly those with pre-operative instability
• Return to work: 8-12 weeks for most occupations; heavy manual work by 3-4 months