Estimating the Proportion of Bone Mineral Density Loss in Patients with Normal Kidney Function among South Indian Population

Background: Bone Mineral density (BMD) is considered as one of the golden tool to measure the bone quality in which two measurements namely T- Score and Z –score are used to report the BMD test results. Whenever there is deterioration of BMD it is associated with low skeletal mass. Creatinine is a chemical waste molecule that is generated from this muscle metabolism. Creatinine is a good marker for muscle mass. However, limited studies have been done which tell us the association between BMD loss and serum creatinine (SCr) levels. Methodology: 200 participants who were referred for CECT abdomen and pelvis were scanned using 128 slice Philips Brilliance CT. Using BMD software, four different vertebral bodies from L1-L4 were taken and ROI was placed at the central portion of the trabecular bone, two reference ROI’s, one in retro spinal muscle and one in fat tissue were also placed. To measure CT attenuation value, a ROI graphic tool was drawn at the trabecular bone. Average HU of BMD, T score and Z score values were taken from L1-L4. Statistical Analysis: The data was analysed using SSPS version 16.0. For assessment of normal BMD values, the results of measurements were averaged for age, creatinine level, T score and Z score and descriptive statistics was calculated. Spearman’s correlation coefficient was used to estimate the correlation between serum creatinine with T score and Z score. Results: T Score and SCr correlated negatively at -0.25. The correlation between Z Score and SCr was also negatively correlated at -0.187. Conclusion: It was evident from the study findings, that there is a non-association between SCr and T Score levels which showed that BMD of a person was not associated with normal kidney function. Thus, SCr levels cannot be used as a biomarker for osteoporosis or osteopenia.


Introduction
BMD reflects the bone rigidity which is a result of the calcium and phosphorus deposits. It is considered as one of the golden tool to measure the bone quality and density (1) . Measuring the BMD from axial and appendicular skeleton helps to assess the reason for the shrinkage of bone with age (2) . Quantitative computed tomography (QCT) is a reputed technique used to measure the BMD in the axial skeleton which can be further used for the evaluation of risk with fractures in the vertebra, degree of bone loss, follow-up of osteoporosis and other bone related metabolic ailments (3) .The measurements named, T Score and Z Score are being used to report BMD test results. Deterioration of BMD is associated with low skeletal mass (4) . SCr levels can be used as a marker to assess muscle mass in subjects with normal renal function (5) . Therefore, SCr levels could be directly proportional to BMD (6) . Subjects with poor renal function have significantly lower BMD, than those with mild-to-moderate CKD who don't have a very significant loss (7) .
The relationship between SCr levels and BMD remains unexplored and its findings might help to test the efficiency of the QCT machine and its BMD software.

Methods
The study protocol followed was reviewed and approved by the Research Committee, and ethical clearance was also obtained. All the subjects provided written informed consent to participate after a detailed explanation about the study was given by the principal investigator.

Subjects
This was a cross sectional study. A total of 200 patients who were referred for CECT of abdomen and pelvis scans, were recruited. Patients above 20 and below 60 years of age with a creatinine level in the range of 0.7 -<1.2 mg/dl were included in the study. Patients below 20 years and above 60, with fractures in spinal vertebra (L1-L5), implants in the spine, osteoporosis, osteopenia or anydiseases which causes low BMD , consumption of calcium supplementation andCKD and renal insufficiency were excluded from the study.

Image Acquisition/SCr reading
Images were acquired using Phillips Incisive 128 slice MDCT. CT images were acquired using a standard CECT protocol using the following image parameters as depicted in table 1. The serum creatinine values of each patient was collected from their blood reports available in the patient's file obtained 4 days prior to CECT. Image analysis and post-processing BMD and corresponding HU values were analysed using the BMD software and stored in the Philips intellectual space portal workstation. BMD of lumbar spine using ROI's at (L1-L4) was measured and averaged from the reconstructed plain scan images, using phantom less BMD application as shown in fi gure 1. ROI's were drawn on the vertebra avoiding any bony pathologies and two reference ROIs were used: one in retro spinal muscle (+40 HU to+120 HU) and one in fat (0 -to -100HU). The complete T score and Z score was then taken into consideration as shown in fi gure 2.

Statistical Analysis
All analyses were performed using the SPSS software package (version 16.0). The mean, standard deviation was calculated for Serum Creatinine, age, Z score and T score. Median, Q1 and Q3 of T Score and Z Score was also analysed. Spearman's correlation coeffi cient was used to fi nd the correlation between SCr with T Score and Z Score respectively.

Demographic characteristics of the participants
Out of the 200 participants aged between 20-60 years, mean age was 39 + 11.6 and creatinine levels between 0.7-1.2 mg/dl with a mean of 0.87 + 0.16. 125 patients were males (62.5%) and 75 were females (37.5%).

Estimating the proportion of bone loss
105 patients had no bone loss (52.5%) and 95 had bone loss (47.5%). Out of the 95 patients with bone loss 74 (77.8%) patients had osteopenia and 21 (22.2%) patients had osteoporosis. All these values were recorded prospectively. The evaluation of normal vs bone loss was done according to world health organization (WHO) set guidelines, which states that a T score between -1 and +1 is reflected as a standard, a T score of -1 to -2.5 is considered osteopenic and if the T score is -2.5 or lower is considered to be osteoporotic. The frequency and percentage of bone loss where "positive" indicates the presence of bone loss and "negative" indicates no bone loss as depicted in table 2. SCr in relation to T Score and Z score A mean T Score of -0.84+1.46 was found after analysing the BMD data. T Score and SCr showed negatively correlation at -0.25. SCr in relation to Z Score was also analysed with a mean of -0.08+1.3. The correlation between Z Score and SCr was negatively correlated at -0.187 as depicted in table 3.

Discussion
Osteoporosis being the one of the most common bone ailment, it has been affecting our societies from a very long time and is mainly described as the disruption of the equilibrium between bone formation and resorption (8) . Subsequently this loss of bony constituent creates changes in bone microarchitecture which markedly favours the incidence of fractures, all of which makes it closely related with high morbidity and mortality rates (9) .
Reduction in skeletal muscle mass can be linked to decline BMD (10) . Since SCr can serve as an indicator for muscle mass, Huh et al assessed the association between SCr and BMD in an elderly population whose kidney function was normal and came to a conclusion that SCr imitated muscle mass, and low levels of SCr was related to low levels of BMD independently (11) . Greater muscle mass is said to be related to better results and long life in people with CKD states (12) . Proper lifestyle choices, facilitated by choosing a balanced diet and regular exercise can help improve an individual's maximum bone potential at an early age (13) . If there is a vitamin D receptor genotype variant such patients are susceptible to low bone density (14) .
Robert H et al., evaluated the linkage between BMD and fractures in partakers with or without CKD by taking health, age and body composition into consideration. They came to a conclusion that BMD values provided information pertaining to fracture risk in the elderly with or without moderate CKD (15) . Therefore, by looking at the values we can predict the risk of future fractures in such individuals irrespective of their renal health. Simerjot K Jassal et al., conducted a study to determine the relationship between kidney function and BMD and related it to bone loss, and osteoporotic fracture. They found that measured renal function had declined with age. They concluded that there is an associations between kidney function and BMD which is the strongest when CKD is high, which shows that if a patient has declining renal function the BMD values may be low (16) .

Myong Jun-Pyo Myong et al.,conducted a study in
Korea to find the relationship between BMD and CKD among general population in Korea and concluded that there was a link between eGFR and BMD in men and women. This study shows that if GFR decreases, there can be a decline in BMD. Therefore, there's higher risk of osteoporosis or osteopenia in people with reduced renal function (17) . Using modern therapy for osteoporosis, it is important to use newer pathogenetic approaches which aim towards the elimination of any imbalances between the relationship of osteoclast -osteoblast while using anabolic support of all bone cells. "Osteomed Forte" is a drug that complies these requirements (18) .
In this cross sectional study, 95 participants out of 200 were detected with bone loss (47.5%) and it was observed that most of the participants with bone loss were from the age range from (41-60) about 62.1%. Therefore, as age increases bone density decreases even with good renal health. We also evaluated the average of normal creatinine levels (0.87+0.16) with BMD and independently associated it with the average values of T Score (-0.84+1.46) and Z Score (-0.08+1.32), and the principle finding was that T Score (-0.25) and Z Score (-0.187) were not associated with normal creatinine levels. A negative correlation between BMD and normal kidney function done by evaluating serum creatinine levels cannot give an explanation about the bone health of an individual.
QCT permits measurement of volumetric bone density without any superimposition of cortical bone and other surrounding soft tissue (19) . The site most commonly used to measure BMD using QCT is the lumbar spine. The trabecular bone is the main site for osteoporotic bone loss, therefore the density loss in that region is higher than that other sites like in cortical bone (20) . Phantom-less CT scans can be used to estimate lumbar BMD with accuracy similar to that of dual energy x-ray absorptiometry (DEXA) scans which is considered a gold standard in detecting BMD. QCT is largely applied to both prospective and retrospective studies which can assess patient bone density and therefore can be helpful for research and clinical practice (21) . The QCT machine used for this study was Philips Incisive 128 slice CT. The Philips BMD software can therefore be used as an effective tool to diagnose osteoporosis/osteopenia. In this study we found that out of the 95 patients with bone loss 74 (77.8%) patients had osteopenia and 21 (22.2%) patients had osteoporosis. Moreover, we tested the efficiency of our MDCT system in evaluating BMD without any extra radiation, time and cost to the patient.

Conclusion
The present study has provided information about the relationship between Serum creatinine and T Score levels. It was evident from this study that there is a non-association between SCr and T Score levels which showed that BMD of a person was not associated with normal kidney function thus SCr levels cannot be used as a biomarker for osteoporosis or osteopenia. The results showed a loss in bone health in patients with good kidney function, this loss may be associated with other pathophysiological conditions other than normal SCr which weren't taken into account in this study.

'Declarations'
Ethics approval and consent to participate-The study protocol followed was reviewed and approved by the Research Committee of Manipal College of Health Profession and Manipal Academy of Higher Education, and ethical clearance was also obtained by Kasturba Medical College and Hospital, MAHE, Manipal. A detailed explanation about the study was given by the principal investigator after which they provided consent for publication. All the patients included in this research gave written informed consent to publish the data contained within this study.
· Availability of data and material-The data has been uploaded as supplementary files in the upload section.