Where should botulinum toxin be injected in the posterior tibialis muscle?

Botulinum Toxin Injection Site for Posterior Tibialis Muscle

The posterior tibialis muscle should be injected at approximately 22% of the distance along a posterior midline axis from the fibular head to the intermalleolar line, within 1 cm of the midline, using ultrasound guidance for optimal accuracy 1.

Anatomical Localization

The motor point of the tibialis posterior is located at a consistent anatomical position:

• Surface landmark axis: Draw a line from the head of the fibula proximally to the intermalleolar axis distally
• Injection depth: The nerve enters the muscle at 22.1% down this axis 1
• Medial-lateral position: Within 1 cm of the posterior midline 1

In chronic stroke patients with spastic equinovarus foot, ultrasound evaluation reveals:

• Mean muscle depth: 26.5 mm from the medial surface
• Mean muscle thickness: 10.1 mm 2

Injection Approaches

There are three validated approaches for targeting the tibialis posterior, each with distinct advantages 3:

1. Medial Approach (Most Common)

• Access from the medial surface of the middle lower leg
• Muscle depth averages 26.5 mm in spastic patients 2
• Requires penetration through overlying muscles
• Best for patients with adequate medial access

2. Posterior Approach

• Access from the posterior leg
• Allows visualization of multiple deep flexors simultaneously
• A novel technique enables single-puncture injection of tibialis posterior, flexor digitorum longus, and flexor hallucis longus through one ultrasound window 4
• Improves efficiency and patient comfort

3. Anterior Approach

• Less commonly used
• May be considered based on individual anatomy 3

Critical Technical Considerations

Ultrasound guidance is essential for this deep muscle 2, 3. Both ultrasound and electrical stimulation guidance are effective, though ultrasound provides superior visualization of muscle depth and surrounding structures 5.

Key Safety Points:

• The tibialis posterior is a deep-seated muscle requiring precise needle placement
• Muscle thickness inversely correlates with depth (p < 0.001) 2
• Echo intensity (muscle quality) directly correlates with depth (p = 0.004) 2
• Confirm needle placement before injection to avoid spread to adjacent structures

Dosing Framework

While specific dosing for tibialis posterior varies by clinical context:

• Lower limb spasticity: OnaBoNT-A and aboBoNT-A are safe and effective formulations that should be offered 6
• Tibialis posterior is frequently targeted in post-stroke spasticity patterns 7
• Dosing should account for total limb treatment and patient-specific factors

Common Pitfalls to Avoid

1. Inadequate depth: The muscle averages 26.5 mm deep—superficial injection will miss the target 2
2. Blind injection: Without ultrasound, accuracy is significantly compromised for this deep muscle 2, 3
3. Ignoring muscle quality: Higher echo intensity (fibrotic muscle) may require technique adjustment 2
4. Multiple punctures: Consider the posterior approach for simultaneous treatment of multiple deep flexors 4

Practical Algorithm

1. Position patient: Supine or prone depending on approach selected
2. Identify landmarks: Fibular head to intermalleolar axis for medial approach
3. Apply ultrasound: Transverse view at middle leg level
4. Locate muscle: 22% down the axis, 26.5 mm deep from medial surface
5. In-plane needle insertion: Visualize needle trajectory throughout
6. Confirm placement: Verify needle tip within muscle belly
7. Inject slowly: Monitor spread within target muscle

The posterior approach with transverse probe placement offers the advantage of treating tibialis posterior along with flexor digitorum longus and flexor hallucis longus through a single skin puncture, which may improve procedural efficiency 4.