1. The incidence of non-union is disproportionately high in intracapsular fractures of the neck of the femur, except in those which are either impacted or treated by accurate reduction and internal fixation. Factors entering into this high incidence are the frequent occurence of death of the head fragment from disruption of the blood vessels of the neck, displacement of fragment ends, poor immobilization, and failure of peripheral callus formation due to absence of a cambium layer on the neck.
2. If time head fragment survives, it undergoes atrophy of disuse of the same degree that of time distal fragmnent, and the two fragments are of equal density on the roentgenogram. Treatment to produce union is usually successful and extensive degenerative changes in the joint do not occur.
3. If the blood supply to the proximal fragment is severed, so that the head dies, disuse atrophy cannot occur, and the head eventually appears on the roentgenogram to be more dense than time surrounding atrophic, living bone. As slow invasion of the head by vascular fibrous tissue occurs, with absorption of the necrotic bone and replacement by canceilous new bone, the roentgenogram shows areas of reduced density.
4. The necrotic articular cartilage remains relatively unaltered until a blood supply reaches it, when it is replaced either by an imperfect type of fibrocartilage or in part by bone.
5. Transformation of a dead head takes place first at the margins of the fracture and at the fovea, and the advancing replacement zone is the weakest part of the bone. Pathological fracture between the dead and living portions may result in collapse and delayed union or non-union.
6. If union has occurred, either primarily or as the result of surgical intervention in the presence of a dead head, and if weight-bearing is begun before replacement is complete, the head will probably collapse, and degenerative changes will be severe on both sides of the joint. If time head is adequately protected, it may be transformed without any collapse; the degenerative changes in the joint are minimal; and a good functional result is obtained.