Anatomical defects in the vertebral pars interarticularis are commonly referred to as spondylolysis, and slippage of the body of the vertebra immediately below is referred to as spondylolisthesis. Lumbar spondylolysis is a frequently encountered problem and usually arises as a result of increased stress on the pars interarticularis. This stress may be related to monotonous extension, flexion, or rotation of the lumbar spine during bipedal movement in humans [12] or activities that aggravate such stresses [34]. Spondylolisthesis has two important causes. The first is associated with the degeneration of the intervertebral disc or posterior facet joints and is referred to as the degenerative type. The second occurs in relation with spondylolysis and is known as the isthmic type. Isthmic spondylolisthesis most commonly occurs at the L5/S1 level [5], while degenerative spondylolisthesis most commonly occurs at the L4/L5 level [67].

In a symptomatic population, the prevalence of spondylolysis and spondylolisthesis has been adequately studied. The reported frequency of spondylolisthesis associated with low back pain is 67.4% [8], and most previous studies have been conducting using plain radiographs [9]. However, computed tomography (CT) is considered to represent a particularly accurate tool for evaluating pars defects as it can often reveal a pars fracture early without the development of spondylolisthesis [1011]. In a study in an adult community-based population, CT examinations showed that the prevalence of lumbar spondylolysis was 11.5% [12].

This study aimed to evaluate the prevalence of spondylolysis and spondylolisthesis in a general adult population unrelated to lower back pain. Most studies on this topic have been reported from developed countries. As the present study was conducted in a population in a developing country for the first time, our data provide a fresh viewpoint and represent a valuable addition to the existing literature.

A total of 4,348 patients were included. The mean±standard deviation age was 42.74±16.01 years. Our study featured more males (n=2,320, 53.4%) than females (n=2,028, 46.6%). The clinical characteristics of patients with spondylolysis and those with spondylolisthesis are shown in Table 1.

Spondylolysis was identified in 266 patients (6.1%), while spondylolisthesis was observed in 142 patients (3.3%). Of the 266 patients with spondylolysis, a break in the pars interarticularis at L3 was observed in 5 (1.9%), L4 in 44 (16.5%), and L5 in 217 (81.6%). There were 247 patients (92.9%) with fractures of the bilateral pars interarticularis, 8 patients (3%) with fractures of the pars interarticularis on the left side and 11 (4.1%) with fractures on the right side.

Of the 142 patients with spondylolisthesis, grade I spondylolisthesis was most commonly observed and was identified in 124 patients (87.3%); grade II spondylolisthesis was observed in 18 patients (12.7%). None of our patients had grade III or grade IV spondylolisthesis. Most patients (106, 74.6%) had spondylolisthesis of L5 over S1, followed by L4 over L5 35 (24.6%), and only one patient (0.7%) had spondylolisthesis of L3 over L4 (Figs. 1, 2).

The mean±standard age of patients with spondylolysis was significantly higher than that of patients without spondylolysis (47.19±15.45 vs. 42.5±15.96 years, p<0.001). Similarly, the mean±standard deviation age of patients was higher among patients with spondylolisthesis than those without spondylolisthesis (53.01±15.31 vs. 42.44±15.88 years, p<0.001). However, gender was only significantly associated with spondylolisthesis (p=0.029) (Table 2).

Stratification of age showed that there was a significant association of spondylolysis and gender among patients aged 31–40 years (p=0.03) and patients aged >60 years (p=0.018), while a significant association of spondylolisthesis with gender was found only among patients aged >60 years (p=0.025) (Table 3). A comparison of the anatomical site and spondylolisthesis also showed significant association (p=0.002).

In contrast with earlier studies, the findings in the present study revealed a slightly lower prevalence of spondylolysis [121314]. However, two previous studies conducted in Asia showed a slightly lower prevalence than existing studies, thus concurring with our current findings [1516]. We believe that the lower prevalence in our study is due to the difference in study population and the possible role of genetic factors

In the present study, a greater number of patients with spondylolysis had a bilateral fracture of the pars interarticularis with a predominant involvement of the L5 vertebra. Unilateral spondylolysis was identified in less than 10% of patients, which was lower than that reported in previous studies [1516]; this difference may be attributed to a compensatory response to mechanical stress.

In terms of gender distribution, our study revealed a surprisingly high incidence of spondylolysis among females. This is in contrast to previously published studies, which have shown that spondylolysis is more common in males [14151617]. Previous studies have revealed different male-to-female ratios for spondylolysis, including 1.5:1 [14] and 2:1 [1516], and according to one study, this ratio ranged from 2:1 to 4:1 [2]. All these studies reported a higher chance of developing a break in the pars interarticularis in males. However, in our present study, the male-to-female ratio was approximately 1:1. This may be due to the fact that majority of the females in underdeveloped countries usually begin working from a very young age. These defects are more common at the age of 7 or 8 years [18] and usually occur following chronic repetitive stress at the pars interarticularis. Thus, it can be hypothesized that starting work from an early age and engaging in laborious work leads to continuous stress at the pars interarticularis that may subsequently lead to damage.

Multilevel spondylolysis has been described by some authors [1920], and a high prevalence of multilevel spondylolysis was observed in a radiographic-based study [19]. Nine patients showed spondylolysis at multiple levels in the present study with prevalence almost equal to that in a CT-based study [16] but lower than that in another study [19]. This could have been due to the greater accuracy of CT in depicting pars fractures as well as the different population of patients being evaluated.

In our study, the prevalence of spondylolisthesis was very similar to that reported by an earlier study [15]. However, the prevalence of spondylolisthesis in patients with a fracture of the pars interarticularis was slightly lower than that reported in a previous study [16].

Defects in the pars interarticularis do not always progress to spondylolisthesis. This was observed in our study, where approximately more than 45% of patients with spondylolysis did not develop spondylolisthesis. Other authors have also reported a similar finding: up to 55% of patients diagnosed with spondylolysis did not progress to spondylolisthesis [212223].

The prevalence of isthmic spondylolisthesis was also higher among females in the current study. This was also surprisingly high compared to a previous study [17]. Spondylolisthesis, or slippage of the vertebral body, is more common in a person who has not yet reached maturity [24]. Slippage usually occurs between the bony and cartilaginous end plates and can also include the growth plate, which represents the weak link to straining forces [2526]. Thus, we can speculate that fragile growth plates in females are more prone to slippage and thus led to the higher prevalence of spondylolisthesis observed in the current study.

Our study demonstrated that with advancing age, there is a gradual increase in the prevalence of spondylolysis. These findings are not in agreement with those previously reported in a large CT-based study, which stated that the prevalence of spondylolysis is not significantly different between younger and older patients [14]. However, a study based on large plain radiographs reported that the prevalence of lumbar spondylolysis increased throughout life [9].

Among those patients identified with spondylolysis, approximately half had degenerative changes at multiple spinal levels. These changes included anterior and posterior osteophytes, vacuum phenomena, and facet joint arthrosis. Other significant findings associated with vertebral bodies were found in patients with spondylolysis.

A major limitation of our study was that it was a single institution retrospective study. Despite this limitation, we believe that this is represents the largest CT-based study to determine the prevalence of spondylolysis and spondylolisthesis from a developing country such as Pakistan. However, prospective studies in developing countries are recommended to further validate the findings of this study.

The prevalence of pars interarticularis fracture observed higher with gradual increase in the prevalence with advancing age. In particular, preponderance was significantly higher among older females.