The fatigue strength of the lumbar neural arch in spondylolysis.

Abstract

Spondylolysis is a fracture of the bone in one or both sides of the narrow regions of the neural arch, the partes interarticulares. The fifth lumbar vertebra is the most commonly affected. Although spondylolysis is not considered to be of congenital origin (Rowe and Roche 1953; Newman and Stone 1 963), genetic factors may predispose the individual to it (Baker and McHollick 1956; Saha et al. 1970; Wiltse, Widell and Jackson 1975). There are also racial differences, the Eskimos having an unusually high incidence of the defect (Stewart 1953; Lester and Shapiro 1968; Kettelkamp and Wright 1971). The most generally favoured theory, however, is that spondylolysis is a fracture caused by mechanical stress and that the mode of failure is fatigue (Pfeil 1 97 1 ; Wiltse et al. 1975). This theory was enhanced by experiments to determine the bending strength of the lumbar neural arch (Cyron, Hutton and Troup 1976). In order to achieve a fracture, an increasing force was applied to the inferior articular facets of a lumbar vertebra and the neural arch was bent away from the vertebral body by about 0.5 centimetre. This relative movement would be difficult to accomplish in vivo, except possibly itt hyperexte nsion. When a static force acts on a material, stress is generated within the material. As the applied force increases the material will eventually fail when the stress reaches a certain value, the “ ultimate stress” . However, the material will also fail when it is subjected to cyclic repetitive stresses which never reach the ultimate stress. This mode of failure is known as fatigue failure. The fatigue life, defined as the number of cycles to failure, is dependent on the amplitude of the cyclic stresses and the method by which they are imposed. It is relatively easy to determine the fatigue life of inert materials such as metal. In bone, however, a fatigue fracture will only develop in vivo when the cellular mechanism of repair fails to keep pace with the microscopic damage caused by the repetitive force. It has been suggested that this occurs in “shin soreness” (Devas 1958). It is thought to be caused by prolonged periods of unac.customed or strenuous activities such as running. Walking with a heavy pack on the back and working in the fully flexed posture are activities that produce cyclic stresses within the partes articulares of lumbar vertebrae. If the stress generated during these activities reaches a critical value and a sufficient number of cycles is accumulated, the neural arch may fracture. This has already been considered as a likely explanation of the occurrence of spondylolysis (Stewart 1 953 ; Newman and Stone 1963). There is little published information on the fatigue properties of the lumbar neural arch. Pfeil (1971) reported on fractures resembling spondylolysis. He subjected lumbar spines from children to repetitive impact loads, but he did not measure the applied force or the number of cycles to failure. This paper describes experimental work carried out on cadaveric lumbar vertebrae to determine the fatigue life of the lumbar neural arch when subjected to forces comparable with those encountered in vivo.