Saha, Acta Orthop Scand 1973 - Mechanics of Elevation of Glenohumeral Joint. Its Application in Rehabilitation of Flail Shoulder in Upper Brachial Plexus Injuries and Poliomyelitis and in Replacement of the Upper Humerus by Prosthesis - Article Summary

Mechanics of Elevation of Glenohumeral Joint. Its Application in Rehabilitation of Flail Shoulder in Upper Brachial Plexus Injuries and Poliomyelitis and in Replacement of the Upper Humerus by Prosthesis

Saha·Acta Orthop Scand·1973·71 citations·Biomechanical study

Saha's 1973 paper defines the biomechanical requirements for stable glenohumeral elevation using cadaveric surface measurements, simultaneous 8-channel EMG, and mathematical modeling. It establishes the concept of the 'zero-position' and the minimum muscular requirements for elevation. These principles are then applied directly to muscle transfer planning for flail shoulder and to design criteria for proximal humeral prostheses.

Key Findings

Three components are required for stable shoulder elevation: a prime mover (raises the arm), a vertical steerer, and a horizontal steerer. The short rotators roll and stabilize the humeral head in the glenoid at each instant, while the deltoid primarily lifts the arm. This three-part framework is the foundation for all flail shoulder reconstruction planning.
Normal humeral retrotorsion is 20-30 degrees. Increased retrotorsion produces anterior instability. Decreased retrotorsion is rare and causes posterior instability only when combined with gross glenoid retrotilt. These thresholds directly guide the decision to add derotation osteotomy when performing muscle transfers.
Glenoid retrotilt is the normal variant in 73.5% of individuals, averaging 7.4 degrees. The remainder have antetilt (2-10 degrees) or neutral tilt. This posterior glenoid tilt is not pathologic — it is part of the normal anatomy that times steerer activation during elevation.
The glenohumeral index quantifies bony stability as the ratio of glenoid to humeral head diameter:
- Vertical index: mean 75.3 (SD ±3.9)
- Transverse index: mean 57.6 (SD ±5.6)
- A smaller index means less bony containment and greater reliance on muscular stabilization.
The glenohumeral joint is not a perfect ball-and-socket. Three joint types exist based on relative radii of curvature. In Type C joints (humeral radius greater than glenoid radius), contact forms a circular band and all stress concentrates on the anterior glenoid rim during elevation. The mechanical basis for anterior labral vulnerability.
For flail shoulder reconstruction, the transfer template follows the three-component rule:
- Prime mover: trapezius (transferred as far down the shaft as possible)
- Vertical steerer: levator scapulae or sternocleidomastoid
- Horizontal steerer: upper two serratus anterior digitations or pectoralis minor
- Add derotation osteotomy if retrotorsion exceeds the normal range.
Proximal humeral prostheses must replicate normal mechanics to function. The implant should incorporate 5-10 degrees of retrotorsion and include three tunnels at subscapularis, supraspinatus, and infraspinatus insertion sites to allow steerer reattachment. Enabling overhead elevation without instability.

Board PearlStable shoulder elevation requires three muscles: a prime mover, a vertical steerer, and a horizontal steerer — this framework drives all flail shoulder reconstruction.

Supplemental

Mechanics of Elevation of Glenohumeral Joint. Its Application in Rehabilitation of Flail Shoulder in Upper Brachial Plexus Injuries and Poliomyelitis and in Replacement of the Upper Humerus by Prosthesis.