Distribution of normalized relative strontium in the femur of an adult Kenyan population

Abstract

Elemental density has a determinant role in bone mineral disorders like osteoporosis and osteopetrosis. Studies in bone elemental density show that other elements exist in bones besides the predominant element, Ca. Recent studies report that relative Sr (to Ca) can be used to treat bone disorders by preventing resorption of Ca from bones into blood and by facilitating the generation of new bone units. Osteoporotic bones have been reported to be Sr deficient meaning that mapping out elemental distribution across bones could give an indication of areas of the bone that are strong or otherwise. Other studies outline the frequent breakage of the hip axis length of the femur in osteoporosis patients thereby suggesting a bone weakness of some sort. Osteoporosis prevalence rates and incidences have been reported to be the lowest in Africa by extrapolation studies but there was no bone matrix information to aid in explaining this or compare with known relative Sr values from other world populations. There is contradictory information on the nature of the relationship between bone morphometric parameters and bone mineral density. Some studies report a correlation between mass and length of bones with the bone mineral density while others report the opposite. These observations generated the need to know the distribution of relative Sr in human bones against the established threshold for normal bones hence provide vital information and clarifications for the reported inconsistencies stemming from other studies. This kind of assessment had not been carried out in Kenya at the time the main study took place yet bone mineral disorders did exist within the Kenyan population. The study was therefore the first in investigating the relative Sr distribution in selected cadaver femurs with respect to the abundant calcium and zinc. A representative sample of femurs was obtained from the Department of Human Anatomy at the University of Nairobi (UoN) having been boiled (femurs), macerated xiii and cleaned off from their tissues. These were taken to the department of Civil and Structural engineering, at UoN, where morphometric measurements were done. The length and mass of the femurs were measured using a tape measure and a weighing balance respectively. At the Institute of Nuclear Science and technology (INST) laboratory the samples were cleaned and dried using double distilled water and a hot air blower respectively. The femurs were then partitioned by labeling nine relatively equal partitions including the heads of the femurs and the condyles. A makeshift sample holding system was used to hold each femur such that the flattest part of an irradiation partition rested on the primary radiation source and detector which were part of a calibrated Energy dispersive X-ray Fluorescence (XRF) analysis system that was available at INST, UoN. This system comprised of a sample hold, primary radiation source, detector, associated electronics, a cooling system and a computer. Irradiation took 1800s per an irradiation partition ending up with a histogram of counts per second versus energies in keV. Analysis of X ray spectra by least iterative squares was used to extract net intensities of Ca, Sr and Zn. These elements were prominent in the femurs as could be seen from their net intensities on the raw data tables. The net intensities were proportional to the actual concentrations of elements in the samples which could not be fairly obtained due to the irregular surface of the bones and the absence of bona fide bone standards. As such, normalization of the net intensities using their molar masses was done so as to provide reliable data for comparative or distributive studies as was the case in the main study. The normalized net intensities were then analyzed using statistical techniques of simple averaging and correlations (including bivariate) basing on the expected result from each specific objective. In assessing the distribution of relative Sr across the femur, averages were calculated for similar partitions, for example the average net intensity for the head of the femur in the 16 femurs was determined. Differences between paired irradiation partitions, based on averaged results were then analyzed using bivariate correlations. Correlations between femur morphometric parameters and relative Sr relied on normalized net intensities that had been found by calculating the average intensity for each of the sixteen femurs, from the nine irradiation partitions per femur, then compared with values for mass and length of the femurs. It was observed that the Sr to Ca ratio was mostly homogenous across the femurs and the slight differences appeared to depend on the region and architecture of the bone. The proximal femur had the lowest Sr to Ca ratios compared to the distal femur. Porous regions had therefore more Sr to Ca ratios and vice versa. The results could explain the frequent breaking of the hip axis lengths of the femurs in osteoporotic women as there was comparatively less relative Sr at the proximal femur bearing in mind that studies had reported that relative Sr was a biomarker for bone strength. There was no significant correlation between the morphometric parameters of the femur and Sr to Ca ratios. This meant that the fracture of femurs, due to their morphometric properties, was based on other factors such as mechanical positioning and the relative stress yields or binding of zinc which as observed in this study, correlated with the mass and length of the femurs. Sr to Zn ratios also appeared to depend on the bone region, as there were different values for the femur condyles, the shaft and the distal femur. Further to this, the proximal femur showed higher Zn to Sr ratios compared to the distal femur. There were no correlations between Sr to Zn ratios and the mass or length of the femurs. Parallels were drawn with the correlation of Sr to Ca ratios and the morphometric parameter of the femur. The normalized net intensities for the prominent elements in the bone were further analyzed to include results for sum of Sr and Zn versus calcium. These (Sr+Zn and Ca) were plotted against the femur morphometric parameters where they showed no correlation with the morphometric parameters but an inverse relationship with each other. This meant that Sr or Zn substituted themselves with Ca in the femur probably depending on the anatomical site or function as there was more Sr or Zn across porous regions of the femur. Zn, as well, had been observed to be a bone growth and strength biomarker in some studies. These findings are important in mapping the regional bone strength of the femur, based on specific bone elements, and may be of use in osteology.