Stress 'Fracture', Bone Reaction.

Bone is a composite material of fibrous, crystalline and cellular elements and, like any other material, has its mechanical limits. All materials are elastic to some degree and will be deformed slightly by a mechanical load. Repeated stresses may cause microfractures in the material and like the aluminium skeleton of the Comet 1, the first jet airliner, the material will fail. Fortunately bone responds to stresses and is effectively plastic. Why else is the soft brain able to leave impressions in the skull vault?

Bone self-repair and self strengthening involves first osteoclastic activity to take down the inappropriate microstructure and followed by osteoblastic activity to lay down the matrix, which will ossify to become the new, mechanically appropriate bone architecture. During this process, the bone is temporarily vulnerable to the mechanical stress, if it continues. The Looser zone is a special case, where lack of vitamin "D" delays ossification of the matrix, which then accumulates, widening the microfracture untilit becomes visible. Continued stress in normal bone can also lead to widening or extension of microfractures that then cause pain and local irritation, which can stimulate a periosteal reaction. The stimulation follow the expected lines of force, as predicted by the vectors of forces within the structure. The individual bone may develop lines of increased density that follow the sites of this stimulation.

Traditionally, stress reactions in bone turn-up in adolescence, when activity and physical demands on bone normally increase. Inconveniently, this period also corresponds to the maximum incidence of malignant bone tumours. Radiologists making the diagnosis also contribute to the management, since biopsy of a stress-fracture may actually confuse the situation, where an exuberant cellular reaction to microfracture may resemble primary bone tumour.
Most examples in the database are related to sports training, where individuals fail to realise that force of character does not triumph over the physical properties of their material parts.

There is one significant set of cases that are unrelated to sport
Significant deformity from any cause will produce a stress reaction as the bone remodels to meet the changed mechanical stresses. For example, the majority of cases with deformity from Osteonecrosis or Slipped epiphysis show bands of increased bone density in the neck of the affected femur.