Evidences suggests that degeneration starts at a very early stage of life where mild changes are seen in the first decade of life and more significant changes from second decade onwards [3,11,21]. It is reported that LBP, lumbar disc herniation and degeneration are common in the fourth to fifth decade of life [5,12,22]. One study has indicated that mean age for LDH as 37 years , while other studies have reported mean ages as 45 ± 13 years and 42 ± 10 years  and 41 ± 10 years . Mean ages in all study groups of the present study were in fifth decade of life which was similar to the previous reported findings mentioned above. However, contrast to our findings, one study has recorded 61 - 70 years as the peak age for LDHD in both genders .
We observed a significant difference in BMI between cases and control indicating majority of the patients in LDHD group (67.7%) and LDH group (62.1%) were in overweight or obese categories according to BMI. However, control group had 51% subjects with normal BMI. Although there were many heterogenous data available regarding the association of BMI and LDH, majority of the data emphasized that increased BMI or obesity is a risk factor for LDH. Present study also confirms the above fact as BMI less than 25 kgm-2 as a protective factor with odds ratio of 0.31 (95% CI=0.13-0.72) in LDHD group and 0.39 (95 % CI=0.20-0.77) in the LDH group. Studies conducted on histological assessment of intervertebral disc tissue further confirmed that high degree of degeneration is also associated with elevated BMI . As overweight and obesity encounters an increased pressure and weight on the intervertebral tissue thus, initiate herniation and degeneration of the intervertebral discs.
Study findings regarding gender and LDH in the present study are in accordance with reported similar studies. A study conducted with 205 surgical patients reported that men to women ratio in patients who are undergoing lumbar surgery was 1.5:1 in surgical setting . This was in accordance to the study carried out by Kelsey and co-workers (1984). However, in non-surgical setting it was reported that men to women ratio was1:1. In addition, another study showed that prevalence ratio for male:female was 1:0.61 with a significance of p=0.0001 . Similar observations were noted in a study which recruited 48 patients with LBP. Above study affirmed similar male prominent gender distribution with 67% males and 33% females . Further, a reported study has also indicated that LDH is found in 4.8% men over 35 years and 2.5% women over 35 years suggesting that men are more prone to LDH . The present study finding in Sri Lankan subjects with LDH also adds to the study findings that males are more prone to develop LDH compared to females.
Majority of cases (both LDHD and LDH groups) and controls had secondary or higher educational level. Experts suggest that subjects who are employed with higher education level having more sedentary life style and lack of exercise on back muscles weaken the power of the muscles. This could trigger the herniation of the intervertebral disc, when sudden load to the vertebral column increases. However, according to the present study control group also had a good educational level, hence this phenomenon cannot be applied to the present scenario.
In the present study there was a significant difference in smoking among cases and controls (p=0.012) with high frequency of smoking reported in cases (16.3%) compared to controls (4.8%). Our findings are in accordance with previous studies which affirm the association between smoking and LDH. Studies have reported that smoking in past years is associated with increased risk of LDH . Further studies have highlighted that nicotine in cigarettes may cause narrowing of blood vessels hence impair the blood flow to the disc tissue causing disc degeneration [22,24]. A twin study reported by Battie et al (1995) remarked that there was 18% greater mean disc degeneration scores in lumbar spine of smokers when compared to non-smokers. Interestingly, a study has stated that smoking cannot be regarded as a risk factor for disc degeneration although there was considerable percentage (41 %) of smokers in the study . Therefore, this present study finding on smoking further adds evidence to previously reported studies on the positive association between LDH/LDHD and cigarette smoking. Further, studies have identified that intervertebral disc being the largest avascular tissue in the human body, narrowing of blood vessels by nicotine can interrupt the diffusion process via cartilage end plate, thus leading the disc to degenerate.
To further strengthen the study, the present study also attempted to distinguish the relationship between the sleeping postures and type of mattress used in LDH subjects. These factors are considered as critical conventional factors contributing to LBP associated with LDH. However, present study did not find any significant association with types of sleeping systems used and LDH/LDHD. However, there were limited literature on these parameters. A study conducted in 313 adults with LBP has proven that medium form mattresses had better outcome for pain while in bed (OR=2.35; 95% CI=1.13-4.93) compared to the pain on rising on the same mattresses type (OR=1.92; 95% CI=0.97-3.86) than in patients using firm mattresses. Finally, authors have concluded that medium firm mattresses could improve the pain and disability in patients with chronic lower back pain . Further, it was also believed that mattresses with soft surfaces increase LBP due to incorrect support to the vertebral column and decrease the quality of sleep .
It is believed that loading of the intervertebral disc as an important factor which determines the LDHD and LDH. Therefore, different impact on the disc by different sleeping postures could not be disregarded in the etiology of LDHD and LDH. However, studies done on direct measurement of spinal loading is limited and studies on sleeping postures are scarce. Interestingly, present study has observed that sleeping in supine posture as a significant risk factor with odds ratio of 2.09 (95% CI=1.09-4.06) in patients with LDH. However, this phenomenon could not be observed in patients with LDHD. It was stated that proper sleeping system could align the spine on to its neutral posture as do in upright position, whereas non-neutral postures can apply unbalanced loading on intervertebral discs and facet joints. Further, intervertebral discs tend to restore and grow through hydration during sleeping. As the gravity changes during sleeping, intervertebral disc tissues are unloaded and can rehydrate to restore its elasticity .
Therefore findings related to sleeping postures and type of mattress used adds valuable insight to the studies on risk factors associated with LDH and LDHD.
In the current analysis of the study, we specially focused on the association between physical workload and LDH. Accordingly, results of this case-control study on occupational risk factors associated with LDH are well correlated with the reported studies on similar theme. Heavy physical work such as lifting and carrying heavy objects are proposed risk factors for LDH associated LBP . Another study on identical twins also found similar findings . It is also reported that heavy lifetime occupational and physical loading have an association with disc degeneration in upper lumbar levels (p=0.055 - 0.01) whereas sedentary work was associated with less significant degeneration (p=0.006) . Contrast to our findings, observations by a different study conducted in monozygotic twins stated that there was no significant difference observed in the level of leisure time physical activities when the monozygotic twins were compared to entire twin cohort in Finland . Similarly, a review has shown that workers with many sedentary activities had higher prevalence rates for LBP symptoms and sick leaves due to LBP [OR=1.46; (95% CI=1.18–1.29) for sedentary leisure activities)]. They have also indicated that physical activities in leisure time (either sports or daily physical activities) do not associate with prevalence rates for low back morbidity . Review study concluded contradictory findings stating that sedentary lifestyle and leisure time is not associated with LBP . Our study further confirmed that severity of daily physical activities causing strain to back have a considerable effect on LDHD.
Occupation was recorded as a risk factor by Manek and MacGregor (2005). The authors stated that occupations with night shifts, lifting, bending, twisting, pulling and pushing favours LDHD . According to the present study authors found heavy lifting, bending and twisting as severe or moderate risk occupations that had a strong significant association with LDH [OR=5.96 (95% CI=1.22–29.18)]. Another study also emphasized that main causes for LBP associated with LDH in work place are heavy lifting, repeated loads from manual handling, work postures incurring postural stress and whole body vibrations . Contradictory to our findings a twin population study stated that there is no significant association with occupational loading and LDHD . Therefore, our findings with perceived work strains on LDH cannot be disregarded.
There are several reported literatures that suggest the relationship between sports and LDHD. Hence, present study also hypothesized sports as a contributory factor for LDH. However, authors could not find significant association with LDH and sports. According to published literature, evidences have stated that there was high incidence of radiographic abnormalities of spondylolysis in college level football players (80.5%) . In addition, above study also stated that spondylolysis as a significant risk factor for LBP in football players. Observations from another study was in agreement with previous studies stating that football players were at increased risk of developing LBP and disc degeneration . Another study conducted in Japan among rugby players (n=327) also supported the above relationship of LBP and strenuous sporting activities. That study also emphasized radiographic abnormalities seen in spondylolysis as a significant radiological risk factor for LBP in high school rugby players . A similar study conducted in elite athletes also revealed that disc degeneration is significantly higher in elite athletes (75%) when compared to non-athletes (31%) . However, a similar case control study carried out in former elite athletes showed that odds ratios for back pain was significantly lower among athletes than among control subjects suggesting contradictory findings of the above report. Authors have stated that LBP is less common in athletes when compared to control subjects [OR=0.62; (95 % CI=0.37 – 0.98) for endurance sports: OR=0.60; (95% CI=0.44–0.82) for sprinting and games: OR= 0.67; (95% CI=0.47-0.96) for contact sports such as wrestling and boxing]. The study further commented that maximal weight lifting is associated with disc degeneration of the entire lumbar spine, whereas soccer associated degeneration confined to lower lumbar spine region only. Authors further emphasized that there was no accelerated disc degeneration in runners and shooters . Number of factors could have interfered with the results of present study with LDH and sports. Majority of participants of the present study were unable to mention the duration of involvement in sports, reason for stop playing and unable to recall the specific sporting activities they were engaged during school time. Therefore, these factors could have greatly reduced the specificity of sports definitions and might also lead to numerous misclassifications of the type of sports (strenuous sports or mild strenuous or etc.). Though there is no significant association between sports and LDH, present study could highlight some valuable information regarding sports and LDH. According to the history of involvement in sports among the recruited subjects in our study emphasizes that improper training or lack of back muscle strengthening exercise may attribute for sports associated LBP and LDHD in Sri Lankan context. Further, according to expertise experience it is hypothesized that people who have engaged in sports have developed a good muscle tone during the period of active involvement in sports, but when they quit or stop regular sporting activities the developed muscle tone will decrease and as a result when they participate in strenuous work or sports, the load that comes to the body will directly pass through the vertebral column without involvement of back muscles. Hence, the intervertebral disc tends to herniate which is enhanced by the excessive load that triggers degeneration.
Traditionally it was believed that traumatic occupations and heavy physical/mechanical loading were the major contributing factors that leads LBP and LDHD [7, 9]. However, according to the present study more than half of the study subjects (56.7%) did not have any of the above predisposing factors associated with LDH. Therefore, it is suggested that there could be other factors associated with regular or occupational behavior that is related to LBP in this cohort of patients.
Recurrence of LDH in the present study was 13.5% and was in agreement to previous findings of recurrence of lumbar disc disease (5–15%) [37, 38]. However, above published studies further commented that there was no significant association of age, sex and level of herniation and the recurrence of LDH.
The limitations of the present study include a convenience sample with case-control study design. Secondly, assessment of BMI had a limitation as some of the cases were reluctant to measurement of height and weight due to severity of pain while some controls were also refused remove foot ware to measure the weight. Further, social behavior also had a limitation as it was based on direct questioning of the participants only. Also, self-reported data on sleeping posture and data on daily physical activities were regarded as limitations of the study. There are several notable strengths in our study such as assessing of sleeping postures, types of sleeping systems and developing of a regression model associated with LDH considered as some strengths. Though the sample size was adequate to detect the hypothesized effects of socio-demographic, behavioural and occupational factors associated with LDH among Sri Lankan subjects, large studies would add more comprehensive findings in the etiology of LDH.