Clin Should Elbow.  2022 Dec;25(4):282-287. 10.5397/cise.2022.00885.

Biomechanical investigation of arm position on deforming muscular forces in proximal humerus fractures

• Chalmers CE
• Wright DJ
• Patel NA
• Hitchens H
• McGarry M
• Lee TQ
• Scolaro JA

• KMID: 2540253
• DOI: http://doi.org/10.5397/cise.2022.00885

Abstract

Background
Muscular forces drive proximal humeral fracture deformity, yet it is unknown if arm position can help mitigate such forces. Our hypothesis was that glenohumeral abduction and humeral internal rotation decrease the pull of the supraspinatus and subscapularis muscles, minimizing varus fracture deformity.
Methods
A medial wedge osteotomy was performed in eight cadaveric shoulders to simulate a two-part fracture. The specimens were tested on a custom shoulder testing system. Humeral head varus was measured following physiologic muscle loading at neutral and 20° humeral internal rotation at both 0° and 20° glenohumeral abduction.
Results
There was a significant decrease in varus deformity caused by the subscapularis (p<0.05) at 20° abduction. Significantly increasing humeral internal rotation decreased varus deformity caused by the subscapularis (p<0.05) at both abduction angles and that caused by the supraspinatus (p<0.05) and infraspinatus (p<0.05) at 0° abduction only.
Conclusions
Postoperative shoulder abduction and internal rotation can be protective against varus failure following proximal humeral fracture fixation as these positions decrease tension on the supraspinatus and subscapularis muscles. Use of a resting sling that places the shoulder in this position should be considered.

Keyword

Proximal humerus; Biomechanical; Two-part fracture; Rotator cuff; Arm position