In this study, we investigated the progression rates to SLS grade 2 from SL and SLS grade 1 in a population of 1521 recruits according to their profession (CU, MU, DU, AU) during 36 months follow up. The main findings were that the incidence rate of progression of SL or SLS grade 1 to SLS grade 2 was 1.02% and that the RR for developing severe SLS was higher in category 4 compared to category 3 in AU (RR = 4.7, p < 0.05). The overall incidence of SL/SLS in the population from which the cohort was drawn can not be published, though the SL/SLS categories’ overall incidence may be seen in Bar Dayan et al. publication [10].
In skeletally immature individuals, the tendency for lumbosacral slip progression is well known. Slip progression is most likely to occur in adolescents younger than 15 years of age, usually during the adolescent growth spurt [11]. In contrast to the well-documented slip progression in children and adolescents [3, 7, 8], especially in athletes [7], the adult progression of isthmic SLS has rarely been described or discussed in the literature [11, 12]. Some authors even dispute its existence and clinical importance [2, 8]. Harris and Weinstein [12] reported the outcome of 11 patients in whom Grade-III and IV SLS were treated non-operatively over an average eighteen-year follow-up and found that 5/11 had one or more neurological findings, but none were incontinent. In his work, Floman [11] described 18 patients, ages 32 to 55 years, with documented adult isthmic slip progression who reported incapacitating low back pain, accompanied in most by significant sciatica. Documented slip progression ranged from 9 to 30% (average, 14.6%), and occurred from 2 to 20 years. Seitsalo et al. [13] followed-up 272 patients with SLS aged 14.3 for 14.9 years and found that 23% of adult patients exhibited some slip progression. Virta and Osterman [14] reported a 5.6% slip increase over a 17-year follow-up in 40 adult patients with SLS who had not undergone operations. Our study reflects a mean annual progression of 0.34% compared to 2% [6], 1.5% [12] and 0.32% in other studies. The variance between incidence rates emerges from different severity classifications, different age groups, and different follow-up times. Fredrickson et al. [2] estimated that approximately 15% of individuals with a pars interarticularis lesion had progression to SLS. The slip was seen predominately during the growth spurt, with minimal change after sixteen years. Others have accepted this concept [8]. Our cohort of 1521 patients is larger than all the previous studies [11,12,13,14] summed together (341 patients) and proves that the risk of progression of SL or low-grade SLS to a more significant slip is not ignorable and is 1.02% in 3 years among 18-year-old males.
We had hypothesized that there would be different progression rates in different severity scales of SL/SLS patients, specifically that the progression rate would be higher in Cat4. The low progression rate enabled statistical significance only in the large sample group (AU, that comprised two-thirds of our study population). In this group, the incidence of progression rate to Cat5 (SLS Grade 2 or higher) from Cat3 (Painful SL or asymptomatic grade 1 SLS) and category 4 (Grade 1 SLS with pain) was 0.5 and 2.2%, i.e. the RR was 4.7 (p < 0.05). This significance did not appear in the overall calculations, possibly due to the exclusion of Cat4 in CU. We did not find similar results in other studies; however, the incidence in Seitsalo et al. [13] for SLS patients’ progression was 2% yearly but without any comparison within subgroups.
We also hypothesized to find higher CU progression rates than AU and MU since they have more physical and strenuous activities and training during their services. It is known, that sports professionals have a significantly greater prevalence of SL and SLS, for instance, gymnastics (30%), American football (20%), weight lifting (23–30%), and wrestling (30–35%) [7, 11, 15]. The literature also shows an approximately fourfold increase of SL and SLS in adolescent dancers than the general population [16]. This increased incidence is mainly attributed to the repetitive shear forces of hyperextension positions and the poor mechanics and alignment often used by young dancers to obtain their positions. Ishimoto et al. [17] found, in a nested case-control study (722 vs. 605, mean age 70), that occupational driving and working in the agricultural/fishing industry were associated with radiographic spondylolisthesis (Odds ratio: 2.4 and 3.5, respectively). We indeed found a difference in the incidence rate between CU (1.8%) to AU (1%) and MU (0.8%), but those differences were not statistically significant. We estimate that the difference in activities and the short period of follow-up is the reason for the negative results.
At first glimpse, it not clear why the only subgroup that progressed with a significant RR was the AU. Two factors may contribute to this: almost 70% of the cohort was in AU; the second largest group was CU, from which Cat4 was banned.
The present study has several strengths. The first being that it is based on a large cohort (1521 subjects). Second, SL and SLS’s definition is exact and decided by a military medical committee based to a great extent on imaging (particularly crucial for Cat5 - SLS grade 2, since it is dependent on imaging). Third, SL/SLS data were collected from computerized data, guaranteeing a high coverage of progression. Fourth, all soldiers were conscripts in obligatory service of uniform age, rank, living conditions, and diet provided for each occupation.
There are several limitations to this study. We could not publish the actual number of subjects from which our cohort was drawn due to military censor restrictions. The study includes only male recruits, and therefore the conclusions are limited to males and possibly soldiers in general. The tendency to allocate higher grades of SL/SLS to less strainful occupations could have caused bias. Since the incidence of progression from SL to SLS is generally small, the size of our cohort, despite being much more prominent than other studies, is still not big enough to provide overall statistically significant results, beyond the stated subgroups. Moreover, despite collecting the baseline data prospectively, our study is retrospective. Data was not collected for research purposes and prevented differentiation between unilateral and bilateral SL and might have skipped asymptomatic SL/SLS progression.