Comparative high-resolution pQCT analysis of femoral neck indicates different bone mass distribution in osteoporosis and osteoarthritis

SUMMARY

Osteoarthritis is linked to a reduced risk of femoral fracture despite osteoporosis. Different bone distribution in the femoral neck in osteoarthritis and fracture was revealed using a peripheral quantitative computed tomography (pQCT) comparative analysis. Our findings sustain the presence of an adaptive mechanism of bone structure providing fracture protection in osteoarthritis.

INTRODUCTION

Although osteoarthritis is associated with reduced femoral fracture risk, it does not protect from bone loss. We investigated whether adaptive mechanisms are present at the arthritic joint, leading to reduced fracture risk, despite the presence of low bone mass density.

METHODS

We performed pQCT comparative analyses of human femoral neck specimens derived from 32 postmenopausal women who received hip prostheses for osteoarthritis (n = 19) or femoral fracture (n = 13) by applying an in-house automated software to extract bone structure descriptors, characterize trabecular and cortical bone distribution, and evaluate their mutual relationships.

RESULTS

The cortical bone volume and trabecular thickness were significantly (p < 0.05) higher in the osteoarthritis group than in the fracture group. Trabecular bone volume was also significantly (p < 0.05) higher in the osteoarthritis group than the fracture group at the inferior and anterior quadrants. Significance was maintained after adjusting for age, cortical bone volume, and cortical porosity thickness. Multiple linear regression analysis showed that thickness, volume, and apparent density of the trabecular region significantly (p < 0.05) correlated with the same cortical descriptors in osteoarthritis, but no significant relationship was found in the fracture group. Age differentially affected the mutual relationships in the two groups, showing a significant correlation with trabecular thickness in both groups and with apparent trabecular density only in femoral fracture group.

CONCLUSIONS

Starting from these differences in the structural descriptors, our study sustains the presence of a compensatory mechanism in osteoarthritis to preserve the mechanical competence of bone structure, despite the loss of trabecular bone, underlying lower fracture risk.

An automatic segmentation method for regional analysis of femoral neck images acquired by pQCT

We developed an automatic method for regional analysis of femoral neck images acquired by peripheral quantitative computed tomography (pQCT), based on automatic spatial re-alignment and segmentation; the segmentation method, based on a morphological approach, explicitly accounts for the presence of three different bone compartments: cortical region, trabecular region, and transition zone between cortical and trabecular compartments. The proposed method was applied on 13 femoral neck sections derived from female donors who were undergoing hip replacement surgery for primary degenerative arthritis or fracture, and a typical densitometric and structural analysis was performed both globally and regionally. The proposed segmentation method was quantitatively evaluated by comparing automatic contour and the corresponding manual contours delineated by three operators using metrics based on surface distance (average symmetric distance, ASD) and volumetric overlapping (dice similarity coefficient, DSC). The same approach was used to validate the automatic spatial orientation, considering as metric the difference between manual and automatic angle orientation. Results confirm a satisfactory agreement between automatic and manual performances (ASD < 0.41 mm, DSC > 0.91, orientation difference = 3.61°) and show that globally our algorithm performs very well. Concerning regional analysis application, from our results we can observe that significant differences are present among the four bone quadrants.

Automatic segmentation of cortical and trabecular components of bone specimens acquired by pQCT

Peripheral Quantitative Computerized Tomography (pQCT) allows the acquisition of bone specimens with a spatial resolution adequate to visualize the 3D structure of the bone cortex and the trabecular network. At present, pQCT scanners are equipped with image processing software that limits the bone analysis in two dimensions and requires strong user interaction. In this work, a method is proposed to automatically segment, in 3D, cortical and trabecular components of bone specimens acquired by pQCT, in order to facilitate and enhance the quantitative evaluation of densitometric properties of the bone.

Hexarelin, a growth hormone - releasing peptide, counteracts bone loss in gonadectomized male rats

The age-related decline in growth hormone (GH) secretion has been implicated in the pathogenesis of involutional bone loss. Whether restoration of GH secretion might be helpful in maintaining and/or improving bone mass during aging is still unsettled. The aim of the present study was to examine the effects of 30-day treatment with hexarelin (HEXA, 50 microg/kg subcutaneously b.i.d.), a highly effective GH-releasing compound, on bone metabolism and bone mineral density (BMD) in intact and osteopenic gonadectomized (GDX) mature male rats. Serum total alkaline phosphatase (ALP, bone formation marker) and bone resorption markers (lysylpyridinoline, LP and hydroxylysylpyridinoline, HP) were measured before and 7, 14 and 30 days after treatment. BMD was measured by dual-energy X-ray absorptiometry at lumbar vertebrae, femoral metaphysis and diaphysis before and at the end of the experiment. In intact rats, HEXA significantly (P<0.05) decreased LP (-36.3%) and HP (-22.8%) excretion at day 7, whereas it did not change serum ALP activity and BMDs. In GDX rats, HEXA completely prevented the significant (P<0. 01) increase in urinary excretion of both LP (+143.8%) and HP (+119. 4%), the early decrease in ALP activity (-26.5%) and the significant (P<0.05) decrease in BMDs in the femoral metaphysis (-7.9%) and lumbar vertebrae (-6.8%) caused by androgen deficiency. The bone-protective effects of HEXA could be attributed, at least in part, to its GH-releasing activity since chronic-treated rats maintained the GH response to an acute challenge with HEXA. The evidence that HEXA, unlike GH, inhibits bone resorption indicates that other mechanisms contribute to the bone sparing effect of HEXA.

Characterization of maize elongation factor 1A and its relationship to protein quality in the endosperm

The protein synthesis elongation factor 1A (eEF1A) is a multifunctional protein in eukaryotic cells. In maize (Zea mays L.) endosperm eEF1A co-localizes with actin around protein bodies, and its accumulation is highly correlated with the protein-bound lysine (Lys) content. We purified eEF1A from maize kernels by ammonium sulfate precipitation, ion-exchange, and chromatofocusing. The identify of the purified protein was confirmed by microsequencing of an endoproteinase glutamic acid-C fragment and by its ability to bundle actin. Using purified eEF1A as a standard, we found that this protein contributes 0.4% of the total protein in W64A+ endosperm and approximately 1% of the protein in W64Ao2. Because eEF1A contains 10% Lys, it accounts for 2.2% of the total Lys in W64A+ and 2.3% of the Lys in W64Ao2. However, its concentration predicts 90% of the Lys found in endosperm proteins of both genotypes, indicating that eEF1A is a key component of the group of proteins that determines the nutritional quality of the grain. This notion is further supported by the fact that in floury2, another high-Lys mutant, the content of eEF1A increases with the dosage of the floury2 gene. These data provide the biochemical basis for further investigation of the relationship between eEF1A content and the nutritional quality of cereals.

Comparison between urinary pyridinium cross-links and hydroxylysine glycosides in monitoring the effects of ovariectomy and 17 beta-estradiol replacement in aged rats

This study was undertaken to assess the sensitivity of hydroxylysylpyridinoline (HP), lysylpyridinoline (LP), galactosylhydroxylysine (GHyl) and glucosylgalactosylhydroxylysine (GGHyl) to monitor bone response to estrogen deficiency and replacement by comparing their excretory patterns in ovariectomized aged (11-14 months old) rats. The ovariectomized (OVX) rats were randomized into two groups: (1) OVX plus vehicle; (2) OVX plus 17 beta-estradiol (17-beta E, 10 micrograms/kg, s.c., 4 days/week). Treatment with 17-beta E started immediately after OVX and continued for 60 days. The collagen catabolites were measured in urine for 1 month before OVX and thereafter for 60 days. In temporal coincidence with urine collection, bone area and bone mineral density (BMD) of lumbar vertebrae, femoral diaphysis and distal metaphysis were measured by dual-energy X-ray absorptiometry. In the untreated rats, BMD of the femoral metaphysis and lumbar vertebrae decreased significantly and the urinary excretion of LP, HP, GHyl and GGHyl increased with different patterns. In the treated rats, 17-beta E replacement prevented the increment in LP excretion, partially prevented the increase in HP excretion, but had no effect on the excretion of GHyl and GGHyl. In conclusion pyridinolines and glycosides have different sensitivities to the bone response to OVX. Glycoside excretion after OVX also reflects metabolic processes not strictly related to bone loss and, in contrast with LP, is not sensitive to estrogen replacement.

Time-relationship between bone growth and increment of bone mineral content in growing rats

The aim of the study was to describe the time-relationship between the growth rate of the rat tibia and the increase rate of its mineral content. Appositional and endochondral bone growth rates were derived from sequential X-rays measurements of tibial widening and lengthening, respectively; the increase rate of bone mineral content was derived from sequential photon absorptiometry measurements of the proximal tibio-fibular site. The time-relationship of appositional growth rate and endochondral growth rate versus bone mineral content increase rate was mathematically described. The results allow a better understanding of the time-course of two distinct features of bone growth: increase in size and increase in mineral content.

Elongation factor 1 alpha concentration is highly correlated with the lysine content of maize endosperm

Lysine is the most limiting essential amino acid in cereals, and for many years plant breeders have attempted to increase its concentration to improve the nutritional quality of these grains. The opaque2 mutation in maize doubles the lysine content in the endosperm, but the mechanism by which this occurs is unknown. We show that elongation factor 1 alpha (EF-1 alpha) is overexpressed in opaque2 endosperm compared with its normal counterpart and that there is a highly significant correlation between EF-1 alpha concentration and the total lysine content of the endosperm. This relationship is also true for two other cereals, sorghum and barley. It appears that genetic selection for genotypes with a high concentration of EF-1 alpha can significantly improve the nutritional quality of maize and other cereals.

[Dual-energy X-ray absorptiometry of the vertebrae: determining which vertebrae should be scanned]

This study was aimed at verifying whether bone mineral density (BMD) and its loss with aging are different in the lumbar vertebrae and whether the region of interest--which is usually limited to the L2-L4 segment--may be extended to L1-L4. BMD was measured by means of dual-energy X-ray absorptiometry (Hologic QDR 1000) in 74 women, aged 37 to 78 years, not affected with any disease or subjected to any treatment known to interfere with bone metabolism. The relationship between age and BMD was expressed by the following equations for L1, L2, L3: BMD-L1 = 1181.68-7.85 x age, BDM-L2 = 1251.57-7.70 x age, BMD-L3 = 1231.66-6.57 x age, as shown by linear regression analysis. The behavior of the bone density of L4 with age appeared to be different and could not be described by linear regression curves and was therefore not comparable with that of the other vertebrae. BMD was different in the four lumbar vertebrae as shown by: a) the diversity of the intercept of the regression lines describing age-bone density relationships (F = 7.7, p < 0.001); b) the bone density of L1 being lower than the mean bone density of the L2-L4 region (p < 0.005); c) the bone density of L4 being higher than the mean bone density of the L1-L3 region (p < 0.001). In order to evaluate the effect of senile degenerative changes of the vertebrae on the relevant bone density, two groups of women were considered, according to age--i.e., pre- or iuxtamenopausal and late postmenopausal. It appeared that the BMD of L1 was always more correlated to the bone density of L2 and L3 than the BMD of L4. Our results suggest that L1 is homogeneous to the L2-L3 region, relative to both bone density and its loss with aging. Therefore, L1 should be included in the region of interest during the examination. Even though the bone mineral content of L4 and its loss with aging seem to be different, our results do not allow the exclusion of L4 from the scanned area.

A new mathematical model to study bone turnover in growing rats

A new mathematical model for the study of bone turnover in growing rats was developed. The model predicts a linear relationship between bone mineral content (BMC) and biochemical markers (BMK) of bone turnover assuming that rats are growing, bone turnover is profoundly affected by skeletal maturation, and resorption and formation are physiologically balanced. The model validation was performed by measuring galactosyl-hydroxylysine (GHYL) and hydroxyproline (HYP) in urines. This mathematical evidence supports our proposed use of the specific bone resorption marker GHYL to predict bone mineral content. Further studies on bone turnover will be possible by the application of the same approach.

[A method for the in vivo measurement of the mineral content in an experimental animal]

Bone mineral content measurement is not as widely used in experimental conditions as in clinical practice because of the lack of adequate experimental methods. An in vivo measurement method of bone mineral content in the rat is here presented. Measurements were made at the proximal tibio-peroneal segment, which has a mainly trabecular structure, by means of single photon absorptiometry. The presence of the fibula, having cortical structure, has been shown not to affect the read values, which can be therefore attributed to the tibial metaphysis, having a mainly trabecular structure. Expressing mineral content values as a function of the measured bone diameter has proven useless. Limb repositioning on the instrument holding device is critical for the reproducibility of the measurement, but the latter is not affected by the repositioning method. Method reproducibility (depending on either the intrinsic instrumental error or the repositioning error) is 5-6%. This relatively low reproducibility of the suggested method does not prevent its use in the study of physiological and pathological variations of bone mineral content in rats, and of the means to influence it.