LDH is a commonly regarded as a high-risk degenerative disease of the elderly. The current sedentary and physically inactive lifestyle of many young adults, leads to long-term excessive pressure on the waist, which causes chronic lumbar muscle strain. Over time, structural variation in the lumbosacral vertebrae and pathological changes in the lumbar disc, can result in the occurrence of LDH. Previous epidemiological studies have found that sex, age, BMI, smoking, occupation type and level of physical activity are the main risk factors for lumbar disc degeneration and herniation . Nevertheless, a study on twins indicated that genetic sources exist in over 70% of patients with LDH . The study found that compared to acquired environmental factors, congenital or developmental factors may be more critical in the occurrence and development of LDH.
The developmental difference of lumbosacral bone structure has an important effect on its stability. The posterior longitudinal ligament divides the spine into anterior and posterior parts. Previous research has reported that the anterior structure of the lumbar spine mainly bears the compression load and absorbs the shock generated by the spine, while the posterior structure mainly controls the complex activities of different types of lumbar vertebrae, such as flexion and extension, lateral bending, and rotation . However, King and Yang et al. [18, 19] reported that the lumbar posterior structure also has statics function that cannot be ignored, but that the compression load it bears varies greatly with different postures.
Lumbar lamina is the skeletal structure in the posterior part of lumbar vertebrae. Its integrity plays a vital role in maintaining the stability of lumbosacral vertebrae. Biomechanical experimental studies have analyzed the function of the lumbar lamina and shown that laminectomy reduced torsion moment to failure and torsion stiffness by approximately 18% in lumbar vertebrae . In addition, the strength was reduced by 44.2% and the stiffness was reduced by 19.9% when shear loading [21, 22]. Raj D. Rao et al. reported that laminectomy caused more instability of motion segment of the spine compared to laminotomy and increased the stress to the anterior intervertebral disc annulus . Cunningham et al.  also reported in their study that the pressure in the nucleus pulposus will increase significantly after complete instability of rear structure (including partial laminectomy). Pressure changes in the intervertebral disc can result in altered metabolism and apoptosis of cells, leading to long-term disc degeneration [25–27]. Consequently, lumbar posterior instability causes weight-bearing transfer from the posterior component to the anterior component, which may increase the risk of LDH. The most prominent segment of LDH is L4/L5 and/or L5/S1. The L5 lamina is the important skeletal structure at the back of the lower lumbar vertebra. Unlike laminectomy, our objective was to investigate a congenital or developmental defect or weakness in the L5 lamina, thus increasing the possibility of lower LDH, which was supported by our results.
Lamina horizontalization is considered to be one of the causes of the lumbar sagittal instability.  Lamina horizontalization can be manifested as a decrease in the height of the lamina under the anteroposterior lumbar spine X-ray. This is consistent with our conclusion and can be used as a special form of lamina height reduction. Spina bifida occulta (SBO) is a common deformity in the lumbosacral region, where the lamina of one or more vertebrae is not completely closed. The vast majority of individuals with SBO remain asymptomatic for life and the incidence of SBO varies from 0.6–25% [29, 30]. A study by Avrahami et al.  reported a higher prevalence of LDH among SBO patients, which was shown to increase with age. SBO patients have defects in the posterior lumbosacral vertebrae that leads decreased spinal stability, and increases the risk of posterior disc herniation, which supported our conclusions. However, the difference in the current study is that the L5 laminae in the LDH group had a developmental defect rather than reaching the degree of failed fusion.
Appropriate control is essential for clinical trials. Compared to our previous study , the control group enrolled young cases without LDH and low back pain, which can be more precisely reflecting the influence of the height of L5 lamina on the onset of lower LDH. Regarding the risk factors associated with LDH, numerous studies have investigated the effect of sex, BMI, physical labor intensity, smoking history, and family history [15, 32–36]. In the present study, we excluded the confounding factors, such as sex, BMI, smoking, etc. In addition to skeletal structures, spinal ligaments are important in stabilizing intervertebral discs . When selecting participants for enrollment, we used MRI to exclude any adverse conditions of ligament damage and calcification, to ensure the integrity and strength of the lumbosacral ligament.
Plain radiographs are rarely used in the diagnosis of LDH as MRI or CT are the preferred methods . Our study provides orthopedic surgeons with a simpler and more affordable imaging technique that can be used to predict the risk of lower LDH in asymptomatic young adults. We applied ROC curve analysis to predict the onset risk of lower LDH, where the cutoff value of the “h/H” ratio was 0.315. Based on this, patients with “h/H” ratio less than 0.315, were more likely to suffer from lower LDH. Our results show that, as the proportion of "h/H" decreased, the protrusion segment tended to be L4/5. We determined that was related to the physiological lordosis of the lumbosacral vertebra and the characteristics of upright walking. Lumbar posterior instability causes weight-bearing transfer from the posterior component to the anterior component, so the L4/5 intervertebral disc, which is more affected by the vertical load, can accelerate degeneration more easily. Additionally, the iliac lumbar ligament mainly arises from the L5 transverse process, which can stabilize the lumbosacral union and thus reduce the impact of lumbar posterior instability on this segment .
Several limitations should be considered in interpreting the results of this study. The study had a relatively small number of participants, who were selected from a single hospital. Additionally, anatomical differences in the lumbosacral vertebra may vary among different ethnicities, the population of this study is from northern China. The anteroposterior lumbar spine X-ray used for imaging, was restricted to the coronal plane of the L5 lamina, limiting analysis of its three-dimensional shape. Further studies on the three-dimensional reconstruction and mechanical analysis of L5 lamina should be conducted, to accurately reflect the effect of L5 lamina height on the degeneration of lumbosacral vertebrae.