Navicular Bone Fusion: Congenital, Acquired, Biomechanics, and Management

What is a fusion of the navicular bone?

A fusion of the navicular bone refers to the abnormal or surgical joining of the navicular bone with one or more adjacent bones in the foot, leading to a loss of motion at the affected joint(s) and altered foot biomechanics.

Understanding the Navicular Bone

The navicular bone is a boat-shaped tarsal bone located on the medial (inner) side of the foot, situated between the talus (above) and the three cuneiform bones (in front). It plays a critical role in the midfoot's structure and function.

• Anatomy and Location: The navicular bone articulates with the head of the talus posteriorly, the cuboid laterally (in some individuals), and the three cuneiform bones anteriorly. Its medial prominence, the navicular tuberosity, serves as the primary insertion point for the tibialis posterior tendon, a major stabilizer of the medial longitudinal arch.
• Role in Foot Biomechanics: The navicular bone is a keystone of the medial longitudinal arch, contributing significantly to its stability and shock absorption capabilities. It is integral to the proper functioning of the talonavicular and cuneonavicular joints, which are essential for coordinated foot movements during gait, allowing for adaptation to uneven surfaces and efficient propulsion.

What is Bone Fusion (Arthrodesis)?

Bone fusion, medically known as arthrodesis, is a procedure or condition where two bones that normally articulate at a joint become permanently joined together, eliminating motion at that joint.

• Definition: Arthrodesis can occur naturally (congenital or pathological) or be surgically induced. The goal is to create a solid bony bridge between two or more bones.
• Purpose of Fusion: In a pathological context, fusion might occur as a compensatory mechanism or due to disease. Surgically, it's typically performed to alleviate pain, correct deformity, or provide stability in a severely damaged or unstable joint that hasn't responded to other treatments.

Navicular Bone Fusion: Congenital vs. Acquired

Fusion involving the navicular bone can occur congenitally (present from birth) or be acquired later in life, often through surgical intervention.

Congenital Tarsal Coalition (Specifically Navicular Coalition)

• Definition: Tarsal coalition is an abnormal connection (fusion) between two or more bones in the hindfoot or midfoot. When it involves the navicular, it's often a talonavicular coalition (fusion between the talus and navicular) or a calcaneonavicular coalition (fusion between the calcaneus and navicular). These coalitions result from a failure of segmentation during fetal development.
• Prevalence and Types: Tarsal coalitions are relatively uncommon, often bilateral, and may be asymptomatic for years. They can be:
  • Fibrous: A bridge of fibrous tissue.
  • Cartilaginous: A bridge of cartilage.
  • Osseous: A complete bony bridge (true fusion).
• Symptoms: Symptoms typically arise in late childhood or adolescence as the cartilaginous or fibrous connections ossify (turn to bone), leading to increased rigidity. Common symptoms include:
  • Chronic or recurrent foot pain, especially with activity.
  • Stiffness or limited range of motion in the subtalar and/or midfoot joints.
  • Recurrent ankle sprains.
  • Peroneal spastic flatfoot deformity (muscle spasm leading to a rigid flatfoot).
  • Difficulty with gait or participating in sports.

Acquired Fusion (Surgical Arthrodesis)

Surgical fusion involving the navicular bone is a procedure performed to address severe foot conditions.

• Reasons for Surgical Fusion: This is typically considered when conservative treatments have failed for conditions such as:
  • Severe arthritis: Degenerative changes in the talonavicular or cuneonavicular joints causing intractable pain and deformity.
  • Post-traumatic deformity: Irreparable damage to the joint surfaces or severe malalignment following an injury.
  • Instability: Chronic instability that cannot be managed by other means.
  • Failed non-operative treatment: When persistent pain, deformity, or instability significantly impact quality of life.
• Procedure Overview: Surgical navicular fusion involves removing the damaged cartilage from the joint surfaces, shaping the bones to maximize contact, and then securing them with screws, plates, or staples. Bone graft (autograft or allograft) is often used to promote healing and solidify the fusion.

Biomechanical Implications of Navicular Fusion

Whether congenital or acquired, a fusion involving the navicular bone profoundly alters the biomechanics of the foot.

• Impact on Foot Mobility: The primary consequence is a significant reduction or complete elimination of motion at the fused joint(s).
  • Talonavicular Fusion: Dramatically limits pronation and supination movements of the midfoot, affecting the foot's ability to adapt to uneven terrain and absorb shock.
  • Calcaneonavicular Fusion: Primarily restricts subtalar joint motion, which is crucial for shock absorption and gait mechanics.
• Altered Load Distribution: The fused segment becomes rigid, preventing the normal distribution of forces across the midfoot. This can lead to:
  • Increased stress on adjacent, unfused joints (e.g., ankle, other midfoot joints, forefoot).
  • Development of secondary arthritis in compensating joints over time.
• Potential for Adjacent Joint Stress: The body will try to compensate for the lost motion by increasing movement in other areas, potentially leading to overuse injuries or premature degeneration in the ankle, subtalar joint (if not fused), or forefoot. Gait deviations may also become apparent.

Diagnosis and Assessment

Diagnosis of navicular fusion, whether congenital or acquired, involves a thorough clinical evaluation and advanced imaging.

• Clinical Examination: Assessment typically includes:
  • Palpation for tenderness and swelling.
  • Range of motion testing of the ankle, subtalar, and midfoot joints.
  • Gait analysis to identify compensatory patterns.
  • Observation for foot deformities (e.g., rigid flatfoot).
• Imaging:
  • X-rays: Initial imaging to identify bony coalitions or degenerative changes. Specific oblique views are often required to visualize coalitions.
  • CT Scans (Computed Tomography): The gold standard for confirming and characterizing tarsal coalitions, providing detailed 3D views of the bony anatomy and the type of fusion (fibrous, cartilaginous, osseous). Also excellent for assessing surgical fusion healing.
  • MRI (Magnetic Resonance Imaging): Useful for evaluating soft tissue involvement, such as fibrous or cartilaginous coalitions, inflammation, or associated tendon pathologies.

Management and Rehabilitation Considerations

Management approaches differ based on whether the fusion is congenital and symptomatic, or post-surgical.

Conservative Management (for Symptomatic Congenital Fusions)

For symptomatic congenital fusions, especially in younger individuals or those with mild symptoms, non-surgical options are often tried first:

• Rest and Activity Modification: Limiting activities that exacerbate pain.
• Immobilization: Short-term casting or bracing to reduce inflammation and pain.
• Orthotics: Custom foot orthoses to provide arch support, improve alignment, and redistribute pressure.
• NSAIDs: Non-steroidal anti-inflammatory drugs to manage pain and inflammation.
• Physical Therapy: Focus on pain management, improving gait, and strengthening surrounding musculature, though joint mobility cannot be restored.
• Injections: Corticosteroid injections may provide temporary pain relief.

Post-Surgical Rehabilitation (if Acquired)

Rehabilitation following surgical navicular fusion is a critical, multi-phase process designed to ensure proper healing and optimize functional outcomes.

• Immobilization Phase (Weeks 0-6/12):
  • Non-weight bearing in a cast or boot to protect the fusion site.
  • Focus on managing swelling, pain, and maintaining range of motion in non-involved joints (e.g., knee, hip).
  • Isometric exercises for surrounding muscles (e.g., quadriceps, glutes).
• Mobilization and Strengthening Phase (Weeks 6/12-16+):
  • Gradual transition to weight-bearing as determined by radiographic healing.
  • Gait training with assistive devices (crutches, walker) and progression to independent ambulation.
  • Progressive strengthening exercises for the foot, ankle, and lower limb to improve stability and control.
  • Balance and proprioception exercises.
• Gait Training: Re-education of walking patterns to accommodate the altered foot mechanics, focusing on smooth transitions and appropriate load distribution.
• Return to Activity (Months 4-12+):
  • Gradual reintroduction to daily activities, then low-impact exercises, and eventually sport-specific training.
  • Emphasis on listening to the body and avoiding activities that cause pain or excessive stress on adjacent joints.

Long-Term Biomechanical Considerations

Individuals with navicular fusions, whether congenital or acquired, must understand the long-term biomechanical implications. Regular follow-ups with a podiatrist or orthopedic surgeon are advisable to monitor for:

• Adjacent Joint Arthritis: Increased stress on unfused joints can lead to premature degenerative changes.
• Gait Deviations: Persistent altered walking patterns.
• Compensatory Pain: Pain in other areas of the kinetic chain (e.g., knee, hip, lower back) due to compensatory movements.
• Need for Supportive Footwear and Orthotics: Custom orthotics are often beneficial long-term to manage forces and provide support.

Conclusion

A fusion of the navicular bone, whether a congenital tarsal coalition or a surgically induced arthrodesis, represents a significant alteration to the foot's complex biomechanics. While congenital fusions may remain asymptomatic, symptomatic cases and surgical fusions necessitate comprehensive management and rehabilitation. Understanding the specific type of fusion, its impact on foot function, and the importance of targeted rehabilitation is crucial for optimizing outcomes, minimizing secondary complications, and enabling individuals to maintain the highest possible level of function and quality of life.