Metatarsophalangeal Joint Arthroplasty for Intractable Plantar Keratosis: A Comprehensive Surgical Guide

Introduction to Metatarsophalangeal Joint Arthroplasty

Arthroplasty of the lesser metatarsophalangeal (MTP) joints, particularly involving the second through fifth metatarsals, represents a focused surgical intervention primarily indicated for the management of isolated, intractable plantar keratosis. This condition, often localized beneath the lateral condyle of a single metatarsal head, is characterized by persistent pain and discomfort despite exhaustive conservative management. The objective of this procedure is to alleviate localized pressure on the plantar aspect of the forefoot by modifying the metatarsal head's morphology, thereby resolving the underlying mechanical etiology of the keratosis. While historically associated with a reported satisfaction rate of 85% to 90%, a thorough understanding of patient selection, biomechanical principles, meticulous surgical technique, and potential complications is paramount to achieving optimal outcomes.

Indications for MTP Joint Arthroplasty

The primary indication for MTP joint arthroplasty, in the context of the lesser toes, is an isolated, intractable plantar keratosis. This lesion typically presents as a discrete, painful callosity beneath the lateral condyle of one of the second, third, fourth, or fifth metatarsal heads.

Specific criteria for surgical consideration include:
* Localized Pain: Persistent, severe pain directly attributable to the plantar keratosis, significantly impacting ambulation and daily activities.
* Failure of Conservative Management: Exhaustive trials of non-operative treatments, including:
* Custom orthotics with metatarsal pads or offloading cut-outs.
* Appropriately fitted, wide-toe-box footwear.
* Regular debridement of the keratosis.
* Topical emollients and keratolytic agents.
* Activity modification.
* Isolated Lesion: The keratosis should be isolated to a single metatarsal head, differentiating it from diffuse metatarsalgia or multiple transfer lesions, which may warrant alternative surgical strategies such as metatarsal osteotomies.
* Radiographic Confirmation: Plain radiographs (weight-bearing anteroposterior, lateral, and oblique views) should confirm the absence of significant MTP joint arthritis, severe hallux valgus, or other deformities that might contraindicate this specific procedure or suggest a broader forefoot reconstruction is necessary. While not always evident on plain films, a prominent plantar condyle may be suspected.

Clinical Pearl

A key diagnostic differentiator is the ability to precisely localize the pain to a single metatarsal head. Palpation and direct pressure over the affected metatarsal head should reproduce the patient's symptoms. Infiltrating the area with local anesthetic can further confirm the specific pain generator.

Contraindications

While MTP joint arthroplasty can be highly effective for specific indications, several contraindications must be carefully considered:

• Diffuse Metatarsalgia or Multiple Keratoses: This procedure is not suitable for widespread forefoot pain or multiple plantar keratoses, which often indicate a more generalized biomechanical imbalance requiring metatarsal osteotomies or other reconstructive procedures.
• Systemic Neuropathy: Patients with significant peripheral neuropathy (e.g., diabetic neuropathy) are at high risk for delayed wound healing, infection, and Charcot neuroarthropathy. The altered pain perception can also mask complications.
• Peripheral Vascular Disease: Compromised vascular supply to the foot significantly increases the risk of wound complications, infection, and non-healing.
• Active Infection: Any local or systemic infection must be resolved prior to elective surgery.
• Severe MTP Joint Arthritis: While the procedure involves partial resection, severe degenerative changes throughout the joint may necessitate a different approach, such as fusion or total joint replacement (though less common in lesser MTPs).
• Significant Forefoot Deformity: Uncorrected severe hallux valgus, pes cavus, or other complex deformities may lead to transfer lesions or recurrence if not addressed concurrently.
• Unrealistic Patient Expectations: Patients must understand the goals of the surgery, potential complications, and the expected recovery period.
• Pes Cavus Deformity: As noted in the original text, a high-arched foot can predispose to increased pressure under the metatarsal heads and may lead to recurrence or transfer lesions if the underlying cavus foot biomechanics are not addressed.

Pathophysiology and Biomechanics

The development of intractable plantar keratosis beneath a lesser metatarsal head is fundamentally a biomechanical problem. The MTP joints are critical for load transfer during the stance phase of gait and for propulsion during toe-off. Normal foot mechanics distribute weight evenly across the metatarsal heads.

Key biomechanical factors contributing to keratosis formation include:
* Prominent Plantar Condyle: An anatomical variant where the plantar aspect of the metatarsal head is excessively prominent, leading to focal pressure concentration.
* Relative Metatarsal Length: An abnormally long metatarsal, particularly the second, can bear disproportionately high loads.
* Metatarsal Plantarflexion Deformity: A metatarsal that is excessively plantarflexed relative to its neighbors will experience increased ground reaction forces.
* MTP Joint Extension Deformity: Chronic MTP joint extension (e.g., in hammertoe or claw toe deformities) can lead to a "buckling" effect, driving the metatarsal head further plantarward and increasing pressure.
* Loss of Fat Pad Cushioning: Atrophy or displacement of the plantar fat pad can reduce the natural cushioning beneath the metatarsal heads, exacerbating pressure effects.
* Intrinsic Muscle Weakness: Weakness of the intrinsic foot muscles can lead to MTP joint instability and altered load distribution.

The constant, repetitive shear and compressive forces on the skin overlying the prominent metatarsal head stimulate hyperkeratosis as a protective response. When these forces are excessive and unrelieved, the keratosis becomes painful and intractable. The goal of arthroplasty is to reduce this focal pressure by excising the offending plantar prominence of the metatarsal head, thereby restoring a more physiological load distribution.

Preoperative Assessment

A comprehensive preoperative assessment is crucial for patient selection and surgical planning.

Clinical Examination

• Visual Inspection: Assess for the presence, size, and location of the plantar keratosis. Note any associated toe deformities (hammertoe, claw toe), MTP joint extension, or hallux valgus.
• Palpation: Precisely localize the area of maximal tenderness. Assess the mobility of the MTP joints and the flexibility of any associated toe deformities.
• Gait Analysis: Observe the patient's gait for any compensatory mechanisms or abnormal pressure points.
• Vascular and Neurological Status: Perform a thorough neurovascular examination, including palpation of pedal pulses, capillary refill time, and assessment of protective sensation (e.g., Semmes-Weinstein monofilament testing).

Imaging Studies

• Weight-Bearing Radiographs:
  • Anteroposterior (AP) View: Evaluate metatarsal length patterns (e.g., metatarsal parabola), MTP joint alignment, and any signs of arthritis.
  • Lateral View: Assess metatarsal declination angle and MTP joint sagittal plane alignment. A prominent plantar condyle may be subtly visible.
  • Oblique View: Can provide additional information on metatarsal head morphology.
• Magnetic Resonance Imaging (MRI) or Ultrasound: Rarely indicated for isolated keratosis, but may be considered if there is suspicion of associated soft tissue pathology, stress fracture, or Freiberg's infraction.

Surgical Goals

The primary surgical goals of MTP joint arthroplasty for intractable plantar keratosis are:
1. Alleviation of Focal Plantar Pressure: By excising the prominent plantar aspect of the metatarsal head.
2. Resolution of Plantar Keratosis: Leading to pain relief and improved function.
3. Preservation of MTP Joint Function: Minimizing loss of motion and maintaining joint stability.
4. Prevention of Transfer Lesions: By carefully balancing the metatarsal parabola and avoiding excessive shortening or dorsiflexion.
5. Correction of Associated Deformities: Addressing any MTP joint extension contracture or toe hammering.

Patient Positioning and Anesthesia

• Anesthesia: General anesthesia or regional anesthesia (e.g., ankle block or popliteal block) combined with sedation is typically employed.
• Patient Positioning: The patient is positioned supine on the operating table. A tourniquet is applied to the thigh or ankle to ensure a bloodless field. The foot is prepped and draped in a sterile fashion, allowing full access to the forefoot.
• Tourniquet Time: Standard tourniquet protocols should be followed, typically not exceeding 90-120 minutes.

Surgical Approaches and Techniques

Two primary approaches are described for MTP joint arthroplasty: the dorsal approach (with modifications) and the plantar approach. Each has distinct advantages and disadvantages.

Dorsal Approach (Mann and DuVries Technique)

This traditional approach provides excellent visualization of the dorsal aspect of the MTP joint and is well-suited for addressing dorsal contractures.

Step-by-Step Technique:

1. Incision: A "hockey-stick" incision is made. It commences in the web space adjacent to the involved toe, extends distally over the dorsal aspect of the metatarsal head, and then curves proximally along the distal third of the metatarsal shaft (Fig. 83-35A in original text context). This curvilinear extension allows for better exposure and minimizes tension on the incision.
2. Dissection:
   • Carefully dissect through the subcutaneous tissue, protecting the dorsal neurovascular structures.
   • Identify and retract the extensor digitorum longus (EDL) tendon to one side (typically medially or laterally, depending on the specific metatarsal and surgeon preference).
   • Identify the deep transverse metatarsal ligament (DTML) on both sides of the involved metatarsal head. This ligament connects the metatarsal heads and helps stabilize the forefoot.
3. Ligament Release:
   • Incise the DTML on both the medial and lateral sides of the involved metatarsal head. This release is crucial for mobilizing the metatarsal head and facilitating exposure of the plantar aspect.
   • Perform a dorsal capsulotomy of the MTP joint through a longitudinal incision to gain access to the joint.
   • Sever the collateral ligaments (medial and lateral) of the MTP joint. This further mobilizes the proximal phalanx relative to the metatarsal head and allows for significant plantarflexion of the toe.
4. Exposure of Metatarsal Head:
   • Firmly plantarflex the involved toe.
   • Simultaneously, apply dorsal pressure to the distal metatarsal shaft (Fig. 83-35B in original text context). This maneuver "presents" the plantar aspect of the metatarsal head into the surgical field.
5. Resection of Metatarsal Head:
   • Using a thin, small osteotome or a power saw with a 4-mm blade, remove 2 to 3 mm of the distal portion of the metatarsal head. This initial cut is typically made perpendicular to the long axis of the metatarsal.
   • With continued traction on the toe (in plantarflexion) and dorsal pressure on the metatarsal, the plantar lip of the condyle becomes clearly visible.
   • Carefully excise the prominent plantar lip of the metatarsal head. The goal is to remove approximately 50% of the metatarsal head's plantar aspect, creating a smooth, rounded contour that eliminates the focal pressure point. Avoid excessive resection, which can lead to instability or transfer lesions.
6. Smoothing and Reduction:
   • Use a rongeur or rasp to meticulously smooth any sharp edges or irregularities on the resected metatarsal head.
   • Reduce the joint, ensuring smooth articulation and appropriate alignment.

Surgical Warning

Excessive resection of the metatarsal head can lead to MTP joint instability, shortening of the toe, and potential transfer metatarsalgia to adjacent metatarsal heads due to altered load distribution. Aim for a precise, limited resection.

Modified Dorsal Approach (Hutaifortho's Modification)

Our experience has led to several modifications to the traditional dorsal approach, particularly useful when an extension posture of the MTP joint is present.

Key Modifications:

• Addressing Extension Contracture: An extension posture of the MTP joint is frequently observed and must be relieved. This is achieved through a comprehensive dorsal capsulotomy. If necessary, the collateral ligaments are also incised to ensure full release.
• Targeted Plantar Condyle Excision: It is often not necessary to remove the entire articular surface of the metatarsal head. Instead, the focus is on excising only the projecting plantar portion of the condyle.
  • To achieve this, a curved Hohmann retractor is used to lift the metatarsal dorsally.
  • The toe is acutely flexed more than 90 degrees.
  • This maneuver provides excellent exposure of the plantar projection of the metatarsal head.
  • Only the projecting plantar portion of the condyle is osteotomized, flush with the metatarsal shaft, using a small osteotome or fine saw blade.
• Fragment Control: A small key elevator placed behind the plantar condyles of the metatarsal heads can be invaluable in preventing migration of the osteotomized fragment, ensuring precise removal.
• Temporary Fixation: The toe is held in the proper position on the metatarsal head post-resection, either by a carefully applied dressing or, if deemed necessary for stability, by a small Kirschner wire (K-wire) inserted across the MTP joint. The K-wire is typically removed at 3-4 weeks.

Plantar Approach

The plantar approach offers direct access to the plantar aspect of the metatarsal head, allowing for precise excision of the plantar condyle without disrupting the dorsal capsule, collateral ligaments, or the deep transverse intermetatarsal ligament. This approach is particularly advantageous when the primary pathology is solely a plantar prominence without significant dorsal contracture.

Step-by-Step Technique:

1. Incision: A longitudinal incision, approximately 4 to 5 cm in length, is made on the plantar aspect of the foot. It is crucial to place this incision a few millimeters medial to the weight-bearing portion of the metatarsal head to avoid direct pressure on the scar postoperatively.
2. Dissection:
   • Carefully dissect through the skin and subcutaneous tissue.
   • Identify and retract the neurovascular bundle (plantar digital nerve and artery) medially to protect it from injury.
   • The sheath of the flexor tendons (flexor digitorum longus and brevis) is identified and incised longitudinally.
   • Retract the flexor tendons laterally, exposing the plantar plate.
3. Plantar Plate Incision: The plantar plate, a crucial stabilizer of the MTP joint, is incised longitudinally. This provides excellent direct exposure of the projecting plantar condyle of the metatarsal head.
4. Resection of Plantar Condyle:
   • Under direct visualization, the prominent plantar portion of the metatarsal condyle is osteotomized flush with the metatarsal shaft using a small osteotome or fine saw blade.
   • The advantage here is that the dorsal capsule, collateral ligaments, DTML, and the articular surface of the metatarsal head are left undisturbed, potentially preserving more MTP joint motion and stability.
5. Closure:
   • The plantar plate is meticulously closed with 3-0 absorbable sutures. A curved needle, bent more acutely, can facilitate passage in the tight plantar space.
   • The flexor tendons are allowed to resume their normal course; the flexor pulley is typically not repaired.
   • The subcutaneous tissue is closed.
   • The skin is closed with nonabsorbable 2-0 or 3-0 monofilament nylon sutures.

Clinical Pearl

We have found the plantar incision to be highly useful in many instances of forefoot surgery. It affords excellent exposure that would often be unobtainable through a dorsal incision and, in our experience, results in no greater incidence of incisional complications than the dorsal approach, provided meticulous technique and careful scar placement are observed.

Postoperative Care and Rehabilitation

Postoperative care is critical for optimizing outcomes and minimizing complications.

Immediate Postoperative Period

• Dressing: A bulky, compressive dressing is applied to maintain toe alignment, reduce swelling, and provide comfort. If a K-wire was used, it is incorporated into the dressing.
• Elevation: The foot should be elevated above heart level for the first 48-72 hours to minimize swelling.
• Pain Management: Appropriate analgesia, including oral pain medications, is prescribed.
• Weight Bearing:
  • Dorsal Approach: A wooden-soled shoe or stiff-soled postoperative shoe is typically worn for 4 weeks. Weight-bearing to tolerance is usually allowed immediately in this protective footwear.
  • Plantar Approach: Weight-bearing to tolerance in a postoperative stiff-soled shoe is allowed immediately.
• Suture Removal: Skin sutures are typically removed between 14 and 21 days postoperatively.

Rehabilitation

• Early Motion: Active range-of-motion (ROM) exercises for the MTP joint are initiated as soon as comfort allows, typically within the first week post-suture removal, or earlier if no K-wire is present. The goal is to prevent stiffness and maintain joint mobility.
• K-wire Removal: If a K-wire was used, it is typically removed at 3-4 weeks postoperatively, followed by more aggressive ROM exercises.
• Progressive Weight Bearing: As pain subsides and healing progresses, patients gradually transition from the stiff-soled shoe to comfortable, wide-toe-box athletic shoes.
• Activity Modification: High-impact activities and prolonged standing should be gradually reintroduced over several weeks to months.
• Orthotics: Custom orthotics may be prescribed to help redistribute plantar pressure and prevent recurrence, especially if underlying biomechanical issues persist.

Expected Outcomes

• Joint Motion: Patients should be counseled that some loss of MTP joint motion (approximately 25%) is common after this procedure, but this rarely results in clinical significance or functional impairment.
• Pain Relief: The primary goal is significant reduction or resolution of pain associated with the plantar keratosis.
• Keratosis Resolution: The plantar keratosis should resolve over time as the pressure point is eliminated.

Complications

While generally successful, MTP joint arthroplasty is not without potential complications:

• Dorsal Contracture of the MTP Joint with Hammering of the Affected Toe: This is a common complication, especially with the dorsal approach if inadequate soft tissue release (capsulotomy, collateral ligament release) is performed or if postoperative splinting/dressing is insufficient to maintain plantarflexion.
• Fracture of the Metatarsal Head: Can occur during osteotomy if excessive force is applied or if the bone is osteoporotic.
• Osteonecrosis of the Metatarsal Head: A rare but serious complication, typically due to disruption of the blood supply to the metatarsal head during extensive dissection or aggressive resection.
• Medial or Lateral Drift of the Toe: Can result from imbalanced soft tissue release, inadequate fixation, or excessive resection leading to instability.
• Limitation of Motion of the MTP Joint: Some degree of stiffness is expected, but severe limitation can be functionally debilitating.
• Transfer Lesions: The most common complication, occurring in fewer than 5% of patients. This happens when the pressure is shifted to an adjacent metatarsal head due to altered biomechanics (e.g., excessive shortening or dorsiflexion of the treated metatarsal).
• Infection: As with any surgical procedure, superficial or deep infection can occur.
• Nerve Injury: Damage to the dorsal or plantar digital nerves can lead to numbness, paresthesia, or painful neuroma formation.
• Delayed Wound Healing/Dehiscence: More common in patients with compromised vascularity or diabetes.
• Recurrence of Keratosis: If the underlying biomechanical issue is not adequately addressed or if the resection is insufficient.

Pitfall

Inadequate release of dorsal soft tissues and collateral ligaments during a dorsal approach is a common cause of persistent MTP joint extension contracture and subsequent hammering of the toe. Ensure a thorough release to allow for full plantarflexion.

Alternative Procedures for Diffuse Plantar Keratosis and Metatarsalgia

While MTP joint arthroplasty is specific for isolated lesions, diffuse plantar keratoses and metatarsalgia often necessitate different surgical strategies, primarily metatarsal osteotomies. Our experience with metatarsal osteotomy for isolated plantar keratoses has been unsatisfactory due to complications, and we emphasize exhausting all conservative measures first. However, for diffuse issues, specific osteotomies are indicated.

Metatarsal Osteotomy for Excessive Metatarsal Length and Diffuse Plantar Keratosis and Metatarsalgia

Many procedures have been developed for diffuse plantar keratoses associated with metatarsalgia and a long metatarsal. In patients, especially women who must wear fashionable shoes, intractable pain under a long second metatarsal often requires surgical intervention.

Weil Osteotomy

We recommend the Weil osteotomy (see Technique 83-9 in original text context) for these patients due to its reported lower complication rates and superior results compared to other shortening osteotomies.
* Advantages:
* Precise Shortening: Allows for controlled shortening and elevation of the metatarsal head.
* Reduced Transfer Lesions: Less chance of transfer symptoms to adjacent metatarsal heads compared to shaft shortening osteotomies.
* Metaphyseal Bone: The osteotomy incorporates metaphyseal bone, which has a rich blood supply, leading to a lower chance of delayed union or nonunion.
* Technique: A dorsal approach is used. A long, oblique osteotomy is performed from dorsal-proximal to plantar-distal, allowing the distal fragment to be translated proximally and fixed with a screw. This effectively shortens and dorsiflexes the metatarsal head.

Clinical Pearl

While dorsiflexion osteotomies through the metaphyseal bone at the base of the metatarsal can be performed, controlling the amount of dorsiflexion and the relative position of the metatarsal head can be challenging, and this procedure is generally less effective in shortening the metatarsal than the Weil osteotomy.

Shortening Oblique Metatarsal Osteotomy

This procedure is also described for intractable plantar keratosis beneath metatarsal heads two, three, four, and five (Fig. 83-37 in original text context).
* Goal: To reduce the pressure beneath the metatarsal head that produces the keratosis.
* Outcomes: Callus resolution and pain relief have been reported in 50% to 90% of patients.
* Contraindications: This procedure is contraindicated in patients with pes cavus or with diabetes mellitus, similar to MTP arthroplasty, due to the increased risk of complications.
* Original Technique: The original technique, described in 1954, involved a step-cut osteotomy. Modern variations aim for more stable fixation and controlled shortening.

Conclusion

Arthroplasty of the lesser metatarsophalangeal joint remains a valuable surgical option for patients suffering from isolated, intractable plantar keratosis beneath a prominent metatarsal head. Success hinges on meticulous patient selection, a thorough understanding of the underlying biomechanics, and precise surgical execution. Both dorsal and plantar approaches offer distinct advantages, and the choice should be tailored to the specific pathology and surgeon's preference. While complications such as dorsal contracture and transfer lesions can occur, careful attention to surgical detail and comprehensive postoperative rehabilitation can lead to high patient satisfaction and significant improvement in quality of life. For diffuse metatarsalgia or excessive metatarsal length, metatarsal osteotomies, particularly the Weil osteotomy, offer more appropriate solutions.