Head-shaft angle changes during internal and external shoulder rotations: 2-D angulation in 3-D space.

Area of Science:

• Orthopedic surgery
• Biomechanics
• Radiology

Background:

• Accurate restoration of the native head-shaft angle (HSA) is crucial for successful proximal humerus fracture treatment.
• Previous investigations have not adequately considered the influence of humeral rotation on HSA measurements.

Purpose of the Study:

• To analyze the relationship between humeral rotation and HSA at 1° increments.
• To clarify the serial changing pattern of HSA with varying degrees of humeral rotation.

Main Methods:

• CT scans of eight cadaveric humeri were analyzed using computer-aided design.
• Head-shaft angle (HSA) was measured in neutral, internally rotated, and externally rotated positions at 1° increments.
• Measurements were documented as ratios relative to the neutral position.

Main Results:

• The average HSA in the neutral position was 133±1.93°.
• HSA was underestimated by 8±1.9% with maximum external rotation and overestimated by 20±5.1% with maximum internal rotation.
• HSA changed by 1% for every 5.8° of external rotation and 1.5° of internal rotation.

Conclusions:

• Humeral rotation significantly impacts HSA measurements, leading to underestimation with external rotation and overestimation with internal rotation.
• Understanding these rotational effects is vital for surgeons aiming to restore native HSA in proximal humerus fracture treatment.