Lumbar spondylolysis, a unilateral or bilateral stress fracture of the narrow bridge between the upper and lower pars interarticularis, is a common cause of low back pain (LBP) in adolescent athletes.  The lifetime prevalence of LBP in those aged 11-17 years has been reported to be as high as 30.4% among adolescents participating in sports.  Although a variety of disorders are likely responsible for these cases, lumbar spondylolysis must be considered in the differential diagnosis of LBP in this population.
Lumbar spondylolysis is a radiographic finding that is believed to develop, in most cases, during early childhood. Typically, it is not associated with any clinical symptomatology of significance, except in a particular subset of patients who are young and adolescent athletes participating in sports that involve repetitive spinal motion, especially lumbar flexion/extension, and to a lesser degree, rotation.
Athletes who are involved in gymnastics, diving, weight lifting, wrestling, rowing, figure skating, dancing, volleyball, soccer, tennis, and football have been found to have a higher incidence of spondylolysis.  The pars interarticularis defect is believed by most authors to represent a fatigue fracture caused by repetitive loading and unloading of this region of the vertebrae from physical activity. The natural history of the fracture appears to be relatively benign, and in most cases, there is no significant progression of the pars defect.
See Common Pediatric Sports and Recreational Injuries, a Critical Images slideshow, to help recognize some of the more common injuries and conditions associated with pediatric recreational activities.
Spondylolysis can persist in some cases to become spondylolisthesis.  Spondylolisthesis occurs when one vertebra slips forward in relation to an adjacent vertebra, usually in the lowest lumbar vertebral segment (L5). As a result, the L5 vertebral body slips forward on the S1 vertebral body. This also commonly occurs at the L4 and L5 levels. Spondylolisthesis is almost never due to trauma; however, it is usually discovered after a trauma or prolonged episode of back pain in an athlete prompts radiographic studies.
Most patients with either spondylolysis or spondylolisthesis have excellent clinical outcomes with conservative measures, and surgical intervention rarely is rarely necessary. [1, 4, 5] In selected cases, those patients unresponsive to nonoperative measures may benefit from surgical management. The approach to surgical management is dictated by the age of the patient and the degree of associated spondylolisthesis. 
This article focuses on isthmic spondylolysis as an independent entity from spondylolisthesis and its relationship to athletes, as this type of spondylolysis is a primary focus of concern in athletic adolescents.
The incidence of isthmic spondylolysis varies according to different surveys, but it has been estimated to be approximately 3-6% in the general adult population. The incidence has been found to vary amongst different ethnic groups, possibly identifying genetic factors as having a degree of influence. Roche and Lowe examined 4200 cadaveric spines and found an overall incidence of 4.2%, with an incidence of 6.4% for white males, 2.3% for white females, 2.8% for black males, and 1.1% for black females.  Lifestyle differences among cultural groups undoubtedly account for at least part of the difference in incidence among ethnic groups, and these findings must be treated with a degree of caution. 
Most studies reveal that males are consistently affected 2-3 times as often as females, and whites are affected almost 3 times as often as blacks. Most studies also show no significant change in incidence in individuals aged 20-80 years. Based on these studies, spondylolytic lesions are generally believed to occur in the early school-age years.
A prospective study demonstrated a 4.4% incidence of spondylolysis in 500 first-grade children, which increased to an incidence of 6% in adulthood, with a follow-up interval of 45 years.  The prevalence of spondylolytic lesions among adolescent athletes appears to be much higher than the prevalence among the general population. According to large-scale radiographic studies, the prevalence among adolescent athletes ranges from 8-15%; among adolescent athletes referred for evaluation of back pain, this figure has been reported to be as high as 47%. 
A large screening study in Japan obtained from children who presented with LBP and who were participating in sports found that 32% of the patients younger than 19 years had at least one or more pars interarticularis defects.  Morita et al investigated 185 adolescents younger than 19 years with spondylolysis and found 180 to be currently active in sports. 
Within competitive sports, increasing age and training more than 15 hours per week correlates with a higher incidence of spondylolytic defects.  The most common level of a spondylolytic lesion is at the L5 level, estimated at 85-95%, followed by the L4 level, estimated at 5-15%.
Further evidence supporting the role of genetics as a significant factor was found by Fredrickson et al, who discovered an increased incidence of spondylolysis in fathers, mothers, and male siblings of affected people in their study.  In an earlier study, as many as 26% of the immediate relatives of those with a demonstrable spondylolysis were found to have a similar problem. 
A strong association exists between lumbar spondylolysis and the presence of spina bifida occulta, which has been found to occur in 5-10% of the general population. [7, 13, 15] One theory is that spina bifida occulta may lead to instability of the lower lumbar segment, predisposing an individual to the development of pars interarticularis defects.  Hyperlordosis of the lumbosacral spine, such as seen in Scheuermann kyphosis, has been associated with a higher incidence of spondylolysis. 
Spondylolysis is associated with spondylolisthesis in approximately 25% of cases; however, the progression of the spondylolisthesis to any significant degree is generally uncommon in those who participate in athletics and in those who do not participate in athletics. The tendency of progression of spondylolisthesis is correlated with the pubescent growth spurt; in a study involving a 20-year follow-up of 255 patients, the mean slip progression was 4 mm.  Only 11% of adolescents and 5% of adults had slip progressions of greater than 10 mm in this radiologic review.
Spondylolysis is derived from the Greek word spondylo, which means vertebrae, and lysis, which means fracture. Spondylolysis is defined as a defect in the pars interarticularis of the vertebral arch. Often, it is described in association with spondylolisthesis, which can be found concurrently with spondylolysis. Spondylolisthesis is defined as the anterior or posterior displacement of a vertebral body on the one below it. These conditions are generally described according to the following classification of Wiltse, Newman, and Macnab  :
Type I (dysplastic): Congenital abnormalities of L5 or the upper sacrum allow anterior displacement of L5 on the sacrum, which can occur with the pars interarticularis remaining intact.
Type II (isthmic): A lesion in the pars interarticularis occurs. This type of spondylolysis is subclassified as a fatigue fracture (IIA), elongation (IIB), or acute fracture (IIC).
Type III (degenerative): This type of spondylolysis is associated with long-standing segmental instability and alterations in the articular processes with associated remodeling of the articular process.
Type IV (traumatic): Acute fractures of the vertebral arch occur in areas other than the pars.
Type V (pathologic): This type of spondylolysis is due to generalized or focal bone disease affecting the vertebral arch.
The pathogenesis of pars interarticularis injuries is likely multifactorial. This "pincer theory" hypothesizes that the inferior articular process of the superiorly adjacent vertebrae and the superior articular process of the inferiorly adjacent vertebrae act as wedges and create a shear force at the pars. For example, at the L5 pars, the inferior articular process of L4 and the superior articular process of S1 would create a shear force at the L5 pars during spinal extension.  Two studies assessing the intrafacet distance in the lumbar spine suggest that insufficient caudal increase in lumbar interfacet spacing could be another predisposing risk factor for this condition. [21, 22]
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