The Neer classification system is used to describe humeral fractures and includes 4 segments -- I, II, III, and IV. The 4 segments are as follows:
According to Neer, a fracture is displaced when there is more than 1 cm of displacement and 45° of angulation of any one fragment with respect to the others.
Muscle pulls cause displacement. The supraspinatus and infraspinatus pull the greater tuberosity superiorly and the subscapularis pulls the lesser tuberosity medially, while the pectoralis major adducts the shaft medially.
Two-part fractures involve any of the 4 parts and include 1 fragment that is displaced. Three-part fractures include a displaced fracture of the surgical neck in addition to either a displaced greater tuberosity or lesser tuberosity fracture. Four-part fractures include displaced fractures of the surgical neck and both tuberosities.
The importance of the classification of this grossly displaced four part fracture is that there is complete vascular isolation of the humeral head meaning that it will infarct (osteonecrosis).