The proximal humerus is fixed to the body by the scapulohumeral and thoracohumeral muscles. The latter, consisting of the latissimus dorsi muscle, pulls the humeral shaft medially and rotates it internally. The scapulohumeral rotatory muscles result in a medial and caudal directed force, whereas the deltoid and biceps muscle antagonize the proximal humerus against caudally vectored muscle force and gravity.3
Fracture morphology depends on the mechanism of injury, number of and muscular forces on fracture fragments, and connection of fragments to periosteum.