Mechanical Axis Method to Determine First Intermetatarsal Angle and Tibial Sesamoid Position

Area of Science:

• Orthopedic Surgery
• Biomechanics
• Radiographic Analysis

Background:

• The mechanical axis is crucial for analyzing bony deformities and surgical planning.
• Accurate measurement of foot alignment is essential for successful orthopedic interventions.

Purpose of the Study:

• To determine the normal mechanical axes of the first and second metatarsals.
• To establish the first/second mechanical intermetatarsal angle (mIMA) and mechanical tibial sesamoid position (mTSP).
• To compare mechanical measurements with traditional anatomic measurements.

Main Methods:

• Retrospective analysis of radiographs from 50 normal feet (40 patients).
• Calculation of mechanical axes, mIMA, mTSP, and anatomic intermetatarsal angle (aIMA), anatomic tibial sesamoid position (aTSP).
• Assessment of angular differences (anatomic-mechanical angle [AMA]) and reliability using Pearson's correlation coefficients.

Main Results:

• Mean first/second aIMA was 8.6 ± 3.0 degrees; mean first/second mIMA was 8.6 ± 2.6 degrees.
• First metatarsal AMA was 1.1 ± 1.0 degrees; second metatarsal AMA was 2.0 ± 1.6 degrees.
• Mean mTSP was 2.8 ± 1.1, comparable to aTSP (2.9 ± 1.0).

Conclusions:

• The mechanical axis method for obtaining mIMA and mTSP is reproducible and robust against anatomic variations.
• The mechanical axis offers a stable reference point for surgical planning and measurements in foot surgery.
• Recommended for preoperative and postoperative assessments, especially for forefoot conditions like hallux valgus.