MTP Joint Fusion: Optimal Position, Biomechanics, and Patient Factors

What is the position for MTP joint fusion?

For first metatarsophalangeal (MTP) joint fusion, the optimal position is typically 10-20 degrees of dorsiflexion and 0-10 degrees of valgus, with neutral rotation, designed to facilitate a functional gait cycle and comfortable footwear.

Understanding MTP Joint Fusion

Metatarsophalangeal (MTP) joint fusion, also known as arthrodesis, is a surgical procedure that permanently joins the bones of the toe and the metatarsal, eliminating movement at that specific joint. This procedure is most commonly performed on the first MTP joint (the joint at the base of the big toe), but can also be applied to the lesser MTP joints. The primary indications for MTP joint fusion include:

• Severe Arthritis: Conditions like osteoarthritis, rheumatoid arthritis, or post-traumatic arthritis that cause chronic pain, stiffness, and deformity.
• Hallux Rigidus: A progressive degenerative arthritis of the big toe MTP joint, leading to loss of motion and pain.
• Severe Deformity: Such as a severe bunion (hallux valgus) or other toe deformities that are not correctable by other means.
• Neuropathic Arthropathy (Charcot Foot): To stabilize the foot in cases of severe joint destruction.

The overarching goal of MTP joint fusion is to alleviate pain, correct deformity, and provide a stable, functional platform for weight-bearing and propulsion during walking.

The Optimal Position for First MTP Joint Fusion

The precise positioning of the fused MTP joint is critical for successful long-term outcomes, directly impacting gait, balance, and the ability to wear shoes comfortably. For the first MTP joint, the generally accepted optimal position is:

• Dorsiflexion: Typically 10 to 20 degrees relative to the ground or the long axis of the first metatarsal. This angle allows for toe-off during the propulsion phase of gait.
• Valgus: Approximately 0 to 10 degrees of valgus (abduction away from the midline of the body). This slight outward angulation helps prevent rubbing against the second toe and ensures proper weight distribution.
• Rotation: Neutral rotation, meaning the toenail faces directly upwards, preventing pronation or supination of the toe.

While these ranges provide a guideline, the exact position may be individualized based on patient-specific factors. For lesser MTP joints, the goal is often a more neutral, slightly plantarflexed position to ensure ground clearance and prevent hammertoe deformities.

Biomechanical Rationale for the Fusion Position

The specific dorsiflexion and valgus angles are not arbitrary; they are carefully chosen to optimize foot function during the gait cycle:

• Facilitating Push-Off: The 10-20 degrees of dorsiflexion is crucial for the "toe-off" or propulsion phase of gait. During normal walking, the big toe dorsiflexes significantly to allow the body to roll over the foot. While fusion eliminates this motion, the fixed dorsiflexed position mimics the functional demand, allowing the foot to clear the ground during the swing phase and effectively transfer weight during terminal stance.
• Weight Distribution: This position ensures that weight is distributed evenly across the forefoot during the stance phase, preventing excessive pressure points on other parts of the foot, which could lead to pain or calluses.
• Shoe Compatibility: The chosen angles allow the foot to fit comfortably into most standard footwear without undue pressure on the top of the shoe (due to excessive dorsiflexion) or the side (due to excessive valgus/varus).
• Minimizing Compensation: A well-positioned fusion reduces the need for compensatory movements in other joints, such as the ankle or midfoot, which could otherwise lead to secondary pain or arthritis in those areas.
• Balance and Stability: A stable, appropriately positioned big toe provides a critical tripod point for balance, essential for standing and dynamic activities.

Factors Influencing Position Selection

While general guidelines exist, the surgeon will consider several individual factors when determining the precise fusion angle:

• Patient's Activity Level: Highly active individuals may benefit from a slightly more dorsiflexed position to aid in propulsion, while less active individuals might prefer a more neutral position for comfort.
• Contralateral Foot: If the other foot is healthy, the surgeon may attempt to match its functional position to maintain symmetry in gait.
• Shoe Preferences: Consideration of the patient's typical footwear choices can influence the optimal angle.
• Presence of Other Deformities: Existing deformities in the midfoot or hindfoot might necessitate slight adjustments to the MTP fusion angle to accommodate overall foot mechanics.
• Bone Quality: The quality of the bone can influence the stability of the fixation and, indirectly, the achievable final position.

Post-Fusion Implications for Movement and Function

A fused MTP joint, by definition, eliminates motion at that joint. This has several implications:

• Altered Gait Mechanics: The most significant change is the loss of dynamic dorsiflexion at the MTP joint. While the fixed position aids push-off, the foot cannot adapt as flexibly. Patients often develop a slightly stiffer toe-off, relying more on ankle plantarflexion and midfoot flexibility.
• Footwear Modifications: Many individuals find rocker-bottom sole shoes beneficial. These shoes have a rounded sole that helps roll the foot forward during gait, compensating for the lack of MTP joint motion and reducing stress on the fused joint.
• Impact on Specific Activities: Activities requiring significant forefoot flexibility, such as squatting deeply, running, jumping, or wearing high heels, may be limited or require adaptation.

Rehabilitation and Adaptation

Post-fusion rehabilitation focuses on restoring overall foot and ankle function and adapting to the new biomechanics:

• Initial Immobilization: The foot is typically immobilized in a cast or boot to allow the bones to fuse.
• Gradual Weight Bearing: A progressive return to weight-bearing activities is guided by the surgeon.
• Gait Retraining: Patients work with physical therapists to re-learn efficient walking patterns that accommodate the fused joint.
• Proximal Strengthening: Strengthening of the ankle, knee, and hip muscles can help compensate for the altered foot mechanics.
• Footwear Guidance: Essential advice on appropriate footwear, including the potential benefit of rocker-bottom soles, is provided to maximize comfort and function.

Conclusion

The position for MTP joint fusion, particularly for the first MTP, is a carefully determined angle of dorsiflexion and valgus, chosen to optimize the biomechanics of gait, facilitate comfortable shoe wear, and provide long-term pain relief and stability. While the procedure eliminates motion at the joint, the precise fixed position allows the foot to function effectively during daily activities. Understanding this critical aspect of the surgery highlights the intricate relationship between anatomy, biomechanics, and functional outcomes in foot health.