ETIOLOGY
Although much debate has arisen regarding the true nature of isthmic spondylolysis, there is little doubt that physical forces are major factors in its production. Wiltse52 theorized that the defect has two causes operating dependently. The lumbar lordosis necessary for bi-pedal locomotion imparts stress on a neural arch weakened by an inherited defect in its cartilage model resulting in failure. This is supported by the absence of neural arch defects in other primates and mammals. In a study of nonambulatory adult cerebral palsy patients, not a single pars defect was identified.35 Conversely, the importance of physical forces in the pathogenesis of isthmic spondylolysis is suggested by its high incidence in adolescent athletes participating in sports such as gymnastics,7, 21, 26 diving,47 football,56 weight lifting, 17 and wrestling.17, 36 The likelihood that regions of high stress intensity in the vertebra will fracture over time in vivo is supported by numerous biomechanical studies.8, 11,16,23,44
There is definite evidence that many of these lesions have some basis in heredity . Spondylolisthesis has been described in identical twins,50 and a high familial incidence has been reported in the literature,41, 47, 52, 57 with up to 50% of first-degree relatives of index cases similarly afflicted.57 A prevalence of up to 50% among Alaskan natives has been found.43
On the other hand, Cirillo and Jackson7 found in gymnasts a fourfold increase in the prevalence of spondylolysis compared with the general nonathletic population. Also, athletically acquired spondylolysis usually produces symptoms and presents later than the "silent" pars fracture often picked up incidentally on screening radiographs in childhood. Although heredity probably plays some part, spondylolysis in the athlete in some respects is a unique entity .
CLINICAL PRESENTATION
The majority of patients with spondylolysis and spondylolisthesis are asymptomatic. In LaFond's series of 415 patients, only 9% sought medical attention as children or adolescents.24 In a long-term follow-up study by Sarasta,37 only 13% reported periods of disabling pain. Again, athletes present a different picture. They will first note pain associated with certain activities during their training regimen. They later will complain of a chronic midline ache at the lumbosacral junction worsened by extension maneuvers. The .pain may radiate into the buttocks and thighs. Although more leg pain may be noted with higher grade slips, many patients can be completely symptom free.
With minimal slippage, the physical examination is often normal. A palpable or even visible discontinuity at the lumbosacral junction may be found with higher grade slips. With hypertrophy of the fibrocartilaginous mass at the defect fifth lumbar or first sacral nerve root, irritation may ensue.