Methods
Participants
Participants in the study represent a subset of the Joondalup Spinal Health Study (JSHS) cohort. Approval to conduct the JSHS was granted by Curtin University and Edith Cowan University Human Research Ethics Committees and all participants provided written, informed consent.
The JSHS is a community-based cohort study which aims to explore familial aspects in spinal pain presentations by studying families where no member reported a history of LBP in the previous 12 months (control families) and families where at least one parent and one child reported chronic (≥ 3 months duration either continuously, or intermittently such that pain was experienced at least once per week), disabling LBP within the past month (pain families). 'Disabling LBP' was defined as pain impacting on at least 3 of the following areas: lifting, walking, sitting, sleeping, social interaction, travel, need to take medication, need to see a health professional, consistent with an earlier study. A total of 231 participants were recruited for the JSHS for which full datasets were available for 227 (98.3%) participants. A detailed description of the JSHS and the recruitment method has been reported previously.
The sample for the current study was derived from adult participants (N = 151) within the JSHS cohort. The sub-group for this study was generated after several exclusion criteria were applied based on historical risk factors for osteoporosis. These risk factors were assessed via questionnaire, completed by each participant independently at home, in order to minimise the potential confounding effects on DXA measures. This resulted in a final sample size of 29 adults with disabling CLBP and 42 adults with no history of LBP in the last 12 months (N = 71). These criteria for exclusion and the number of participants excluded (n) were:
Aged greater than 60 years to minimise the potential effect of age-related bone loss and spinal degenerative conditions (n = 3 excluded).
Currently smoking on all or most days or a history of smoking on all or most days in the last 10 years (n = 20 excluded).
A period of immobilisation of ≥ 6 weeks within the last 12 months to minimise the deleterious effects of immobilisation on bone mass (n = 4 excluded).
Health conditions known to affect aBMD other than primary osteoporosis and osteopenia (rheumatoid arthritis, osteomalacia, Padget's disease, Cushing's syndrome, ankylosing spondylitis) to exclude the influence of co-morbidities on aBMD (n = 3 excluded: rheumatoid arthritis [n = 2] and osteomalacia [n = 1]).
Medications known to affect aBMD (oestrogen, progesterone, bisphosphonates and other osteoporosis therapies) which have been taken for ≥ 6 months to exclude pharmacologically-mediated effects on bone (n = 1 excluded).
Menarche delayed beyond 16 years of age, cessation of normal menstrual periods prior to the age of 45 other than cases of hysterectomy and women more than 5 years post menopause to exclude the potential deleterious effect of reduced circulating oestrogen on aBMD (n = 4 excluded).
Adults recruited in the JSHS who failed to meet inclusion criteria for CLBP or control groups (n = 24 excluded).
Body mass index (BMI) < 18 or > 30 to exclude the influence of low body weight and obesity on aBMD measures and accuracy of densitometer performance (n = 21 excluded).
Questionnaire Data
Demographic and clinical characteristics related to LBP were collected via questionnaires, completed independently by each participant. All participants responded to the modified Nordic Musculoskeletal Pain Questionnaire to ascertain the presence and duration of LBP. Psychological wellbeing was measured using the 21-item Depression Anxiety and Stress Scale (DASS-21). The psychological correlates of CLBP including depression, anxiety and stress have been linked to reduced bone density and reduced bone turnover among pre-, peri-and postmenopausal women. The DASS-21 has three sub-scales (depression, stress and anxiety) each consisting of seven items measured on a 4-point scale (0-did not apply to me at all, to 3-applied to me very much or most of the time). The internal consistency (α = 0.92–0.95) and reliability (r = 0.65–0.78) of the DASS-21 have been established previously. The volume of weekly, non-work-related vigorous physical activity of at least 10 min duration was assessed using a standard question from the International Physical Activity Questionnaire. All participants indicated whether they were currently taking analgesic or non-steroidal anti-inflammatory medications.
Participants who reported currently experiencing CLBP (pain duration of ≥ 3 months) responded to additional pain-specific questions to characterise the nature of their pain. LBP intensity in the past week was quantified with a numeric pain rating scale (0–10 where 0 = no pain and 10 = extreme pain). The personal impact of LBP was measured by asking the participants to indicate the number of LBP episodes in the last year (1–3, 4–10, > 10 episodes), number of work days missed in the last year (0, 1–2, 3–7, 15–30, 181–365 days), and any interference with normal daily activities and recreational activities (yes/no). LBP-related disability was measured with the Oswestry Disability Index (ODI). The ODI contains 10 questions, each with six ordinal responses scored as 0–5. The total score is expressed as a percentage with a higher score representing higher disability. Internal consistency (Cronbach's alpha (α)) of the ODI in adults is reported to range from α = 0.71–0.87.
Physical Data
Anthropometric and other physical characteristics were measured in a standardised fashion and in a random order in a University laboratory by trained research officers. Height (cm) was measured for each participant without shoes using a standing stadiometer while mass (kg) was measured using electronic scales. The height and mass data were used to calculate BMI (kg/m). Hip and waist girths (cm) were measured using body girth measuring tapes. Measurement of back muscle endurance (BME) was performed using the Biering-Sorenson test. The test requires participants to hold their trunk horizontal in a prone position unsupported for a maximum of 360 s, while the pelvis and lower limbs remain supported on a plinth. Moderate-high reliability (r = 0.66–0.98), in both LBP and non-LBP populations, has been reported. A Hologic Discovery A densitometer (Hologic, Inc, Bedford, MA, USA) was used to measure bone mineral content (BMC) (g) and aBMD (g/cm) in lumbar spine. Apparent volumetric BMD (ap.vBMD) (g/cm) was also measured in the lumbar spine using the vertebral width-adjustment feature of the Hologic analysis software version 12.4:3. The lumbar spine was scanned with a matched PA-lateral scan sequence in high-definition mode using the rotating C-arm feature of the densitometer. All participants lay supine on the scanner bed with hips and knees flexed to 90°, supported by a Hologic positioning device. The upper limbs remained elevated with palms resting behind the back of the head. Analysis of the scans was performed using the automated method with Hologic analysis software version 12.4:3. The L3 vertebra was chosen as the level of interest since previous work suggests that precision of densitometric parameters is maximised at this level. Further, the potentially over-riding influence of the ribs and ilia at L2 and L4, respectively, are overcome by measuring properties at L3 in isolation. DXA variables of interest included L3 BMC (PA and lateral projection), L3 aBMD (PA and lateral projection), total spine BMC and aBMD (PA projection), and L3 width-adjusted ap.vBMD (lateral projection). The T-scores for the PA-projection total spine parameters were also acquired. Quality control of the scanner was monitored by regularly scanning a Hologic phantom. All scanning and analysis was performed by one of three trained research officers. The reliability of performing PA and lateral scanning of the lumbar spine has been established previously. The reproducibility of DXA parameters for densitometer used in the laboratory has been established previously with data from a pilot study for PA-projection aBMD in the lumbar spine indicating excellent short-term reproducibility (%CV: 1.1%).
Data Analysis
Questionnaire-derived and physical characteristics were compared between the CLBP groups and genders using t-tests for continuous data and chi square tests for categorical data. Univariate linear regression models were used to examine the association between vertebral DXA variables and predictor variables including the DASS-21 scores, volume of physical activity and back muscle endurance for males and females separately. Subsequently, multiple linear regression models were used to examine the association between study group (CLBP and no LBP) and vertebral DXA-derived variables using the 'enter' method. Separate models were run for each DXA variable of interest and for each gender. The DXA-variables were defined as the dependent variable for each model while 'group' was defined as a dichotomous predictor variable. Each model also included height, mass and age as constant predictor variables so that any association between a given DXA variable and group was adjusted for these factors which are known to influence BMC and aBMD values. Predictor variables identified as significantly-associated with DXA variables in the univariate analyses were also included in the multivariate models. The regression coefficient (B) in each multivariate model represents the mean difference in each DXA variable between groups, adjusted for age, height and mass and other predictor variables. The proportion of variance in each DXA variable accounted for by all the predictor variables in each model was expressed with an R value. Data were analysed SPSS Statistics version 17.0 (SPSS, Inc).