Pars Interarticularis Injury

Updated: Oct 26, 2023
  • Author: Gene Tekmyster, DO; Chief Editor: Craig C Young, MD  more...
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Practice Essentials

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. [1, 2]  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. [3]  Typically, few clinical symptoms are present other than pain, with one cohort reporting up to 55% of athletes aged 10-30 years with LBP having had spondylolysis. [4]  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. [5] 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. [6]  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.

Spondylolysis can persist in some cases to become spondylolisthesis. [7]  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, 7, 8]  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. [9]

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.



Spondylolysis is considered by most to represent a fatigue fracture that results from repeated mechanical stress with microtrauma and eventual overload to the pars interarticularis rather than as a result of a single traumatic event. [10]  However, a traumatic event may result in the completion of a developing fracture. [10]  Studies have shown a remarkably low or absent rate of occurrence in newborns and very young children, as well as in those patients who have never been ambulatory.

Rosenberg et al studied 143 patients who had never walked, with an average age of 27 years, and found no cases of spondylolysis. [11]  This finding appears to support the theory that loading of the pars interarticularis during upright, weight-bearing activities plays a role in the pathogenesis of these lesions.

Another study investigating the mechanical loading of the spine tested cadaveric lumbar vertebrae that were cyclically loaded at the inferior articular processes to simulate shear force. [10]  The authors found 55 of 74 vertebrae to sustain pars fractures. They concluded that the pars interarticularis was particularly vulnerable to this type of repetitive loading. Further analysis of the vertebrae of those subjects without a fracture revealed a larger cross-sectional area of cortical bone in the pars compared with the control group. [10]  This led Wiltse et al to hypothesize that a genetic predisposition may be related to the cortical bone density of the pars. This study also suggested that the strength of the neural arch may increase up to the 4th or 5th decade of life. [10]

In an experimental model, Dietrich and Kurowski found that the greatest mechanical loads occur at L5 and S1 with flexion and extension movements. [12]  Furthermore, the greatest mechanical stress was found to occur at the region of the pars interarticularis. The investigators also noted that the loads and stresses across this region are related to the physical dimensions of the vertebrae, which may offer a partial explanation regarding the varying incidence among different races and the sexes. Repeated flexion and extension maneuvers, and to a lesser degree rotation, typically have been thought to be the movements that are responsible for generating the forces across the pars interarticularis that result in spondylolysis. [12, 13]

In a retrospective analysis of 213 young athletes, Gregory et al found left-sided lower lumbar pain was more common than the right side, and a marked increase in scintigraphic uptake was noted on the left side of the neural arch more often than the right side. [14]  Unilateral spondylolysis was identified by reverse gantry computed tomography (CT) scanning on the left pars 36 times and on the right pars 16 times. These findings support the hypothesis that asymmetric repetitive movements associated with certain sports may be responsible for the development of unilateral spondylolysis. [14]

Green et al concluded from their cadaveric study on pars interarticularis stress, which investigated mechanical loading, that activities involving alternating flexion and extension movements cause large stress reversals in the pars interarticularis, thereby creating the highest risk for developing a pars defect. [15]  The authors found compressive or axial loading to have little effect in generating these stresses likely responsible for spondylolysis. [15]  Other anatomic studies have suggested that shear stresses on the isthmic pars are the greatest with lumbar spine extension. [16]

The specific cause of LBP associated with spondylolysis and spondylolytic spondylolisthesis has not been definitively established. Theories include nerve root compression by floating laminae, intervertebral disc painlumbar facet joint pain resulting from spinal instability, or a combination of these pathologies. [17] . A fibrocartilage mass of scar tissue forms at the site of lumbar spondylolysis and eventually develops into a structure frequently indistinguishable from a normal ligament by adulthood.

Eisenstein et al identified nerve fibers in the fibrocartilage masses histologically, [18]  and Nordstrom et al detected the existence of the slow-conducting type C pain fibers and substance P in spondylolytic tissue obtained from patients who underwent resection. [19]  Mechanoreceptors were later identified in these masses as well.

Hasegawa et al concluded that these fibrocartilage masses appear to be one source of pain, as LBP was induced by injecting hypertonic saline and was blocked by injecting lidocaine into these masses in all patients in their study before resection of the lesion. [20]  The authors hypothesized that the fibrocartilage mass plays a protective role by sensing instability through the mechanoreceptor and then conveying this information through nociceptive fibers as pain, while at the same time, stabilizing this area of instability by acting as a ligamentlike structure across the defect. [20]

Given the high number of asymptomatic spondylolytic lesions, an important issue that is lacking in the literature and warrants further investigation is determining the factors that are responsible for producing pain in one patient but not another.



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. [21] 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. [22]

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. [23] 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%. [24]

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. [25] Morita et al investigated 185 adolescents younger than 19 years with spondylolysis and found 180 to be currently active in sports. [25]

A literature review by Tawfik et al found that the sports associated with the highest reported incidence of pars interarticularis injuries are diving (35.38%), cricket (31.97%), baseball/softball (26.91%), rugby (22.22%), weightlifting (19.49%), sailing (17.18%), table tennis (15.63%), and wrestling (14.74%). [26]

Within competitive sports, increasing age and training more than 15 hours per week correlates with a higher incidence of spondylolytic defects. [27] 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. [28] 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. [29]

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. [21, 28, 30] 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. [31] Hyperlordosis of the lumbosacral spine, such as seen in Scheuermann kyphosis, has been associated with a higher incidence of spondylolysis. [32]

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. [33] Only 11% of adolescents and 5% of adults had slip progressions of greater than 10 mm in this radiologic review.

A European retrospective analysis by Lemoine et al that included 717 pediatric abdominal and pelvic CT scans from 532 children who had a CT scan for a variety of non-lumber conditions reported that the prevalence of spondylolysis was 1% in children under age 3, 3.7% in children under age 6, and 4.7% total. (Note that the prevalence may have been affected by the inclusion of multiple CT studies on the same patient, since it was unclear how the researchers handled these cases, other than noting that 3 of these patients had findings of spondylosis on all their studies.) The study also found that unilateral spondylolysis was associated with a spinal malformation with normal pelvic incidence. [34]


Functional Anatomy

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 [35] :

  • 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.


Sport-Specific Biomechanics

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. [36] 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. [37, 38]



In general, early lesions have a greater chance for true bony healing. Early lesions usually yield good to excellent results. The chronic lesions have a decreased chance for true bony healing; however, even without complete bony union, the symptoms can resolve with proper therapy, rest, and sport-specific techniques. [1]


Patient Education

Overall, patient education in the prevention of low back injuries is important. Maintaining proper flexibility and spinal stabilization with a home exercise program are also strongly advised. Teaching proper technique in the specific sport can also prevent recurrence of back injury. Seasonal athletes should be encouraged to cross-train year round or undergo preconditioning before participation in the sport.

For patient education resources, see the patient education articles Back Pain and Slipped Disk.