Li. Mosekilde

Adipocyte tissue volume in bone marrow is increased with aging and in patients with osteoporosis

Aging of the human skeleton is characterized by decreased boneformation and bone mass and these changes are more pronounced inpatients with osteoporosis. As osteoblasts and adipocytes share a commonprecursor cell in the bone marrow, we hypothesized that decreased boneformation observed during aging and in patients with osteoporosis is theresult of enhanced adipognesis versus osteoblastogenesis from precursorcells in the bone marrow. Thus, we examined iliac crest bone biopsiesobtained from 53 healthy normal individuals (age 30–100) and 26patients with osteoporosis (age 52–92). Adipose tissue volumefraction (AV), hematopoietic tissue volume fraction (HV) and trabecularbone volume fraction (BV) were quantitated as a percentage of totaltissue volume fraction (TV) (calculated as BV + AV + HV) usingthe point-counting method. We found an age-related increase in AV/TV(r = 0.53, p < 0.001, n =53) and an age-related decline in BV/TV (r =−0.46, p < 0.001, n = 53) as well asin the HV/TV (r = −0.318, p <0.05, n = 53). There was an age-related inversecorrelation between BV/TV and AV/TV (r =−0.58, p < 0.001). No significant correlation betweenthe AV/TV and the body mass index (r = 0.06, n.s.,n = 52) was detectable. Compared with age-matchedcontrols, patients with osteoporosis exhibited an increased AV/TV(P < 0.05) and decreased BV/TV (P < 0.05)but no statistically significant difference in HV/TV. Our datasupport the hypothesis that with aging and in osteoporosis an enhancedadipogenesis is observed in the bone marrow and that these changes areinversely correlated to decreased trabecular bone volume. The cellularand molecular mechanisms mediating these changes remain to bedetermined.

Biomechanical competence of vertebral trabecular bone in relation to ash density and age in normal individuals

Cylindrical trabecular bone samples from the central part of the first lumbar vertebral body were obtained from 42 normal people (27 females and 15 males) aged 15-87 years and analyzed by a compression test in either vertical or horizontal direction. Maximum stress, maximum stiffness, energy absorption capacity, and strain at maximum stress (compressibility) were calculated from the load-deformation curves. The ash density of the samples was measured after incineration. From age 20 to 80 years, the decline (P less than 0.01) in ash density was 48-50% compared to a 75-80% decrease (P less than 0.001) in vertical stress, stiffness, and energy absorption capacity. Qualitatively similar age-related changes (P less than 0.01) were observed by horizontal compression, but the absolute values were smaller. In both directions, highly significant positive correlations (P less than 0.01) were observed between the biomechanical properties and ash density stress, stiffness and energy absorption capacity still showed significant decreases with age (P less than 0.01). The vertical maximum strain values, which increased (P less than 0.05) with age, were inversely related to the other biomechanical variables (P less than 0.05) and to the ash density (P less than 0.05). It is concluded from the study that the biomechanical competence of vertebral trabecular bone depends not only on bone mass (ash density) but also on the continuity of the trabecular lattice, which changes with increasing age.

Age-related changes in vertebral trabecular bone architecture—assessed by a new method

Cylindrical trabecular bone specimens (d = 7 mm) were drilled in a vertical direction from the central part of the third vertebral body (L3) from 23 normal individuals aged 15-87 years (10 males and 13 females). The bone samples were embedded in methylmetacrylate and sawn in 400 microns thick sections with an arbitrary rotation but a fixed vertical axis. The sections were investigated in polarized light at a magnification of x 8. By using this technique, vertical and horizontal trabeculae were clearly separated due to different colors. Photographs were taken. These were magnified, and trabecular thickness and intertrabecular distance were measured, using a Zeiss-integration plate II. A significant age-related decrease was found in the mean horizontal trabecular thickness (r = -0.71, p less than 0.001), while the mean thickness of the vertical trabeculae was unchanged with age (r = 0.06, n.s.). Furthermore, a significant increase was found both for the mean distance between the horizontal trabeculae (r = 0.79, p less than 0.001) and between the vertical trabeculae (r = 0.75, p less than 0.001). The present study gave a clear and striking visual presentation of both the thinning and disappearance of the horizontal supporting struts in the vertebral trabecular lattice and the total removal of some of the vertical trabeculae--leading to the dramatic loss of bone strength previously demonstrated.

Biologically meaningful determinants of the in vitro strength of lumbar vertebrae

Applying unbiased stereological methods and a new stereological parameter, star volume of cancellous bone, the bone structure of the first vertebral body was examined and compared with the compressive strength of the second lumbar vertebra. The material came from eight males, aged 33-69 years (mean 49 years) and seven women, aged 22-87 years (mean 52 years) without malignant or metabolic bone disease. From these individuals, first and second lumbar vertebral body were obtained at autopsy. The heights and weights of the individuals were recorded. The following structural parameters were estimated on undecalcified, seven-microns, Goldner-Trichrome stained vertical sections: fractional volume of trabecular bone (BV/TV%), mean trabecular thickness (Tb.Th.l1 microns), trabecular star volume (V*tr mm3), marrow space star volume (V* m.space mm3), and mean thickness of the lateral cortical ring (microns). The compressive strength of whole vertebral body, mean cross sectional area (cm2), and ash density (g/cm3) were estimated and the data were compared to bone histomorphometric estimates. A significant decrease with age for all parameters was found except for marrow space star volume, which increased. With compressive strength as the dependent variable and all other parameters as independent variables, it was shown by standard multiple regression analysis that the in vitro tested compressive strength could be predicted from mean cortical thickness, mean cross sectional area, and marrow space star volume or ash density with a multiple, squared coefficient of regression (r2) of 0.95 when the height and sex of the individual were known.(ABSTRACT TRUNCATED AT 250 WORDS)

Age-related changes in the biochemical properties of human cancellous bone collagen: relationship to bone strength.

Abstract. The metabolism of bone collagen has received little attention in relation to age-related loss of bone mass and strength. The aim of the present study was to analyze bone collagen content and metabolism in human bone with respect to age. The material consisted of iliac crest bone biopsies from 94 individuals: 46 women (ages 18–96, mean age 60.8 years) and 48 men (ages 23–92, mean age 59.5 years). Excluded from the study were all individuals with known osteoporotic lumbar vertebral fractures and renal, hepatic, or malignant diseases. Prior to collagen analysis the biopsies were scanned in a pQCT scanner for density assessment and then tested biomechanically. The results showed a decline in apparent bone density with age (P < 0.0001), a decline in maximum stress, Youngs modulus, and energy absorption with age (P < 0.001). Concomittantly, there was an age-related decline in the intrinsic collagen content with age (P < 0.001). However, there were no biochemical modifications of the bone collagen during aging. There were no significant differences between women and men in the slopes of the regressions-curves. When multiple regression analyses were performed, only apparent bone density came out as a significant contributor in the correlation to biomechanical properties. Nevertheless, the decrease in bone collagen content with age might indicate an increase in the mineralization degree (probably due to decreased bone turnover) and thereby a change in material properties of bone. In conclusion, the present study has shown that loss of bone mass plays the major role in loss of bone strength. However, there is also a change in bone composition during normal aging, leading to a decrease in collagen content and an increase in the degree of mineralization. At this skeletal site, in a normal population there was no change in the biochemical properties of bone collagen.

Reconstruction of the resorptive site in iliac trabecular bone: A kinetic model for bone resorption in 20 normal individuals

The variations with time in function and morphology at the resorptive site in normal iliac crest trabecular bone were reconstructed from tetracycline double-labeled iliac crest bone biopsies from 20 normal individuals. Resorption depths below osteoclasts, mononuclear cells, and preosteoblast-like cells were measured by counting the number of lamellae of known thickness eroded. Mean resorption depth below osteoclasts was 19.0 (+/- 4.9) micron. Lacunae containing mononuclear cells were deeper (P less than less than 0.0001), with a mean resorption depth of 49.1 (+/- 10.2) micron. The deepest lacunae were lacunae containing preosteoblast-like cells, with a mean resorption depth of 62.6 (+/- 12.5) micron. This depth was significantly deeper (P less than 0.001) than the resorption depth found below mononuclear cells. Median total resorption period was 48 days (31-68; 95% confidence interval). Median osteoclastic function period was 8 days (6-12), median mononuclear cell function period was 34 days (24-48), and median period where preosteoblast-like cells were present in lacunae before matrix synthesis started was 9 days (6-13). Distribution curves describing the occurrence of the three cell types in relation to resorption depth showed that osteoclasts occupied the more superior parts of the resorption lacunae, with mononuclear cells and preosteoblast-like cells situated in the deeper parts. The distribution curves support the hypothesis that osteoclasts precede mononuclear cells, which again precede preosteoblast-like cells. Based on this hypothesis, curves showing the variation in resorption depth with time were constructed in duplicate. Resorption rates were calculated for three periods.(ABSTRACT TRUNCATED AT 250 WORDS)

Lumbar vertebral body compressive strength evaluated by dual-energy X-ray absorptiometry, quantitative computed tomography, and ashing

Bone densitometry with DXA (dual energy X-ray absorptiometry) and QCT (quantitative computed tomography) techniques are used for in vivo assessment of bone strength and thereby prediction of fracture risk. However, only few in vitro studies have investigated and compared these techniques ability to determine vertebral compressive strength. The aim of the present study was to (1) assess the predictive value of DXA, QCT, and pQCT (peripheral QCT) for vertebral bone compressive strength assessed by mechanical testing; (2) describe both linear and power relationship between density and strength; and (3) evaluate whether gender-related differences in the above relations were present. The material comprised human lumbar vertebrae L3 from 51 women and 50 men (age range: 18 to 96 years). The study showed that both DXA and CT techniques (QCT and pQCT) have a high predictive value for vertebral strength. The DXA BMD had a high correlation with maximum compressive load (r2 = 0.86). The QCT and pQCT had high correlations with maximum compressive stress (r2 = 0.75 and r2 = 0.86, respectively). The correlation between ash density of the biomechanically tested specimen and maximum compressive stress was r2 = 0.88. There were no differences between linear and power fit in the degree of determination between density and strength. There was no gender-related difference in the relationship between volumetric density and maximum compressive stress. In conclusion, it was demonstrated that DXA, QCT, and pQCT are ex situ equally capable of predicting vertebral compressive strength with a degree of determination (r2) between 75% and 86%. No differences were found between linear and power analysis of the relationship between density and strength, and no difference was found in the density strength relationship between women and men.

Reconstruction of the formative site in iliac trabecular bone in 20 normal individuals employing a kinetic model for matrix and mineral apposition

A stereologic procedure for the reconstruction of matrix and mineralized bone growth curves at formative sites in trabecular bone is presented. Iliac crest bone biopsies obtained from twenty normal individuals after tetracycline double-labeling were investigated histomorphometrically. Corresponding values for osteoid width and apparent distance between bone markers and width of mineralized bone walls were classified using a sector plotting system. Observed structure widths were converted to three-dimensional structure thicknesses by a stereologic unfolding procedure. Osteoid thickness, calcification rate, and fractional labeling of osteoid varied characteristically with increasing wall thickness and permitted the construction of curves describing the time-dependent variations in matrix and wall thicknesses. The mean thickness of completed walls was 61.9 +/- 1.5 micron (SE), and the mean bone formation period (Sigma f) was 145 days (124-168; 95% confidence interval.). The initial appositional rates for bone matrix (2.1 microns3/microns2/day) (1.4-2.9) and bone mineral (1.1 micron3/micron2/day) (0.4-1.9) declined gradually toward zero at the end of Sigma f. The initial mineralization lag time was 15 days (12-24) and increased to a maximum of 27 days during the first 45% of Sigma f. Thereafter, it decreased gradually toward zero. The height of the osteoblast nuclei gradually declined from 6.7 +/- 0.5 micron at the start of bone formation to 1.2 +/- 0.1 micron at the end. The study demonstrates that it is possible to reconstruct growth curves for trabecular bone walls based on three-dimensional values for structure thicknesses using different sections for light and fluorescence microscopy and avoiding classification according to osteoblastic nuclear morphology.(ABSTRACT TRUNCATED AT 250 WORDS)

Trabecular bone remodeling and bone balance in hyperthyroidism

In vivo tetracycline double-labeled iliac crest bone biopsies from 15 hyperthyroid patients were used for the reconstruction of curves describing the variation of resorption depth and formation thickness with time. The curves emerging were compared to curves reconstructed from 13 age- and sex-matched normal individuals (mean age 44 years). The median function period for resorptive cells in hyperthyroid patients (16 days) was about one-third the resorptive period in normals (51 days). No significant difference between the osteoclast-, mononuclear-, or preosteoblast-like cell resorption depths could be demonstrated between the two groups. Consequently, the median resorption rate in hyperthyroid patients (3.8 microns/day) was more than 3 times higher than the value in the control group (1.1 micron/day). Median Sigma, was shorter in the hyperthyroid group (109 days) than in the control group (151 days, P less than 0.05), as was the median initial mineralization lag time (5 and 16 days, respectively, P less than 0.01). No significant difference between the measured mean completed wall thickness (mcwT) values in the hyperthyroid groups and the control group could be demonstrated (58.1 and 60.5 micron respectively). Median initial mineralization rate in the hyperthyroid group (1.2 micron3/micron2 per day) was not significantly higher than the value calculated in the control group (0.9 micron3/micron2 per day), but median initial matrix appositional rate in hyperthyroid (4.8 microns3/micron2 per day) was 3 times higher than the value calculated for normals (1.6 micron3/micron2 per day) (P less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Trabecular bone remodeling and balance in primary hyperparathyroidism

The total remodeling sequences in 19 primary hyperparathyroid patients and 16 approximately age-matched and sex-matched controls were reconstructed from histomorphometric analyses of bone specimens obtained after intravital tetracycline double labeling. In the primary hyperparathyroid group the total amount of work performed by resorptive cells was reduced, as indicated by the significantly lower three-dimensional mononuclear and preosteoblast-like cell resorption depths (35.8 microns vs 44.5 microns in normals, P less than 0.01 and 45.3 microns vs 56.6 microns in normals, P less than 0.01, respectively). The active resorption period (i.e., the function period for osteoclasts and mononuclear cells) was reduced to 19 days compared to 29 days in normals (P less than 0.05), but no difference with respect to bone resorption rates could be demonstrated between the two groups. The median bone formation period (Sigmaf) in primary hyperparathyroid patients was not different from the value obtained in normals (172 days vs 134 days, respectively), and the matrix appositional rate (Ama), as well as the mineralization lag time (tm), were also unchanged. The initial mineralization rate (Ami(i)) was not significantly different from the value obtained in normals, but averaged over the total bone formation period, a reduced mineralization rate could be demonstrated (0.32 micron3/micron2 per day vs 0.46 micron3/micron2 per day in normals, P less than 0.01). The measured final three-dimensional thickness of bone formed during Sigmaf (mcwTm) was reduced in the primary hyperparathyroid group (51.1 microns vs 55.9 microns in normals, P less than 0.02).(ABSTRACT TRUNCATED AT 250 WORDS)