Modeling and remodeling in a developing artiodactyl calcaneus: a model for evaluating Frost's Mechanostat hypothesis and its corollaries.

The artiodactyl (mule deer) calcaneus was examined for structural and material features that represent regional differences in cortical bone modeling and remodeling activities. Cortical thickness, resorption and formation surfaces, mineral content (percent ash), and microstructure were quantified between and within skeletally immature and mature bones. These features were examined to see if they are consistent with predictions of Frost's Mechanostat paradigm of mechanically induced bone adaptation in a maturing "tension/compression" bone (Frost, 1990a,b, Anat Rec 226:403-413, 414-422). Consistent with Frost's hypothesis that surface modeling activities differ between the "compression" (cranial) and "tension" (caudal) cortices, the elliptical cross-section of the calcaneal diaphysis becomes more elongated in the direction of bending as a result of preferential (> 95%) increase in thickness of the compression cortex. Regional differences in mineral content and population densities of new remodeling events (NREs = resorption spaces plus newly forming secondary osteons) support Frost's hypothesis that intracortical remodeling activities differ between the opposing cortices: 1.) in immature and mature bones, the compression cortex had attained a level of mineralization averaging 8.9 and 6.8% greater (P < 0.001), respectively, than that of the tension cortex, and 2.) there are on average 350 to 400% greater population densities of NREs in the tension cortices of both age groups (P < 0.0003). No significant differences in cortical thickness, mineral content, porosity, or NREs were found between medial and lateral cortices of the skeletally mature bones, suggesting that no modeling or remodeling differences exist along a theoretical neutral axis. However, in mature bones these cortices differed considerably in secondary osteon cross-sectional area and population density. Consistent with Frost's hypothesis, remodeling in the compression cortex produced bone with microstructural organization that differs from the tension cortex. However, the increased remodeling activity of the tension cortex does not appear to be related to a postulated low-strain environment. Although most findings are consistent with predictions of Frost's Mechanostat paradigm, there are several notable inconsistencies. Additional studies are needed to elucidate the nature of the mechanisms that govern the modeling and remodeling activities that produce and maintain normal bone. It is proposed that the artiodactyl calcaneus will provide a useful experimental model for these studies.

Evidence of structural and material adaptation to specific strain features in cortical bone.

BACKGROUND: Functionally induced strains provide epigenetic signaling for bone modeling and remodeling activities. Strain gauge documentation of the equine third metacarpal reveals a neutral axis passing through the craniolateral cortex, resulting in a narrow band of cortex loaded predominantly in tension, with the remainder of the cortex experiencing a wide range of compression strain magnitudes that are maximal in the caudomedial cortex. This predictable strain pattern provides a model for examining the hypothesis that strain mode, magnitude, and strain energy density are potential correlates of compact bone structural and material organization. METHODS: Structural and material variables were quantified in nine equine (standard breeds) third metacarpals for comparison with the in vivo strain milieu that was evaluated in thoroughbred horses. The variables quantified included secondary osteon population density (OPD), fractional area of secondary bone (FASB), fractional area of porous spaces, collagen fiber orientation, mineral content (% ash), and cortical thickness. Each bone was sectioned transversely at 50% of length, with subsequent quantification of eight radial sectors and three intracortical regions (periosteal, middle, endosteal). Linear regression analysis compared these variables to magnitudes of corresponding regional in vivo longitudinal strain, shear strain, and strain energy density values reported in the literature. RESULTS: The craniolateral ("tension") cortex of this bone is distinguished by its 30% lower FASB and with the lateral cortex exhibits 20% darker gray level (more longitudinal collagen) compared with the average of all other locations. Conversely, the remaining ("compression") cortices as a group have a high OPD, are more extensively remodeled, and contain more oblique-to-transverse collagen. The caudal cortices (caudomedial, caudal, caudolateral) are significantly thinner (P < 0.01) and have 4% lower mineral content (P < 0.05) than all other locations. Moderately strong correlations exist between collagen fiber orientation and normal strain (r = 0.752) and shear strain (r = 0.555). When normal and shear strains were transformed to their respective absolute values, thus eliminating the effects of strain mode (tension vs. compression), these correlation coefficients decreased markedly. CONCLUSIONS: Collagen fiber orientation is related to strain mode and may function to accentuate rather than attenuate bending. These differences may represent adaptations that function synergistically with bone geometry to promote a beneficial strain distribution and loading predictability during functional loading.

Evidence of strain-mode-related cortical adaptation in the diaphysis of the horse radius.

The relative importance that certain strain features, including mode (e.g., tension vs. compression) and magnitude, have in affecting adaptive bone remodeling seen in normal skeletally mature bones remains controversial. The equine radius is used as a model because in vivo strain data show that the mid-to-proximal diaphysis receives a consistent history of predominantly cranial-caudal bending loads, in contrast to the distal diaphysis which receives relatively more torsional loading superimposed on cranial-caudal bending. Medial and lateral cortices serve as control regions because they correspond to a neutral axis of bending. Equine radii were sectioned transversely at 65% (proximal), 50%, and 35% (distal) of length and cortical bone from the cranial ("tension"), caudal ("compression"), medial, and lateral regions was examined to determine if one, of many, structural and material features could be distinguished as being consistently related to the distribution of the prevailing strain modes. Mineral content (percent ash) differences, though statistically significant (p < 0.01), vary less than 1% between regions of the cortex at all sections. Porosity is not significantly different between any of the regions (p = 0.13). In the 65% and 50% sections, secondary osteon population density (OPD, osteons per square millimeter) and fractional area of secondary bone (FASB) are each nearly two times as great in the caudal regions than in the other three regions (p < 0.01). The 35% section shows a pattern opposite of that in the other sections--there are more than two times as many osteons in the cranial cortex than in the caudal cortex.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of a tension/compression skeletal system: possible strain-specific differences in the hierarchical organization of bone.

BACKGROUND: Examination of a simple skeletal cantilevered beam-like bone (artiodactyl calcaneus) suggests that regional differences in strain magnitude and mode (tension vs. compression) reflect regional adaptation in the structural/material organization of bone. The artiodactyl (e.g., sheep and deer) calcaneus has a predominant loading condition typified by the unambiguous presence of prevailing compressive and tensile strains on opposite cortices. Bone habitually loaded in bending may accommodate regional disparities in loading conditions through modifications of various aspects of its organization. These include overall bone build (gross size and shape), cross-sectional shape, cortical thickness, and mineral content. METHODS & RESULTS: Cross-sections taken along the calcaneal body exhibited cranial-caudal elongation with the compression (cranial) cortex thicker than the tension cortex (P < 0.01). Mineral content (ash fraction) was significantly greater in the compression cortex (P < 0.01), averaging 6.6% greater than in the tension cortex. Strong positive correlations were found between mineral content and section location in both the tension (r2 = 0.955) and compression (r2 = 0.812) cortices. These correlations may reflect functional adaptations to the linear increases in stress that are known to occur in the distal-to-proximal direction in simple, unidirectionally loaded cantilevered beams. According to engineering principles, the roughly triangular transverse cross-sectional geometries and thicker compression cortex are features consistent with a short cantilevered structure designed to resist unidirectional bending. CONCLUSIONS: Known differences in mechanical properties of bone in tension vs. compression suggest that these regional differences in cortical thickness and mineralization may be related to differences in strain mode. These structural/material dissimilarities, however, may be related to regional variations in strain magnitude, since bending and axially directed stresses in a simple cantilevered structure produce greater strain magnitudes in the compression domain. It is possible that the superimposed habitual strain magnitudes enhance strain-mode-specific adaptive responses. We hypothesize that these structural/material differences reflect the capacity of bone to process local information and produce a regionally heterogeneous organization that is appropriate for prevailing loading conditions.

Differences in osteonal micromorphology between tensile and compressive cortices of a bending skeletal system: indications of potential strain-specific differences in bone microstructure.

BACKGROUND: It has been hypothesized that bone has the capacity to accommodate regional differences in tension and compression strain mode and/or magnitude by altering its osteonal microstructure. We examined a simple cantilevered bone to determine whether regional differences in particular strain-related features are reflected in the microstructural organization of compact bone. METHODS & RESULTS: The artiodactyl (e.g., sheep and deer) calcaneus has a predominant loading condition which is typified by prevailing compressive and tensile strains on opposite cortices, and variations in strain magnitudes across each of these cortices. Microscopic examination showed osteon density and cortical porosity differences between tension (caudal) and compression (cranial) cortices, averaging 11.4% more osteons in the compression cortex (P < 0.01) and 80.2% greater porosity in the tension cortex (P < 0.01). There is 43.5% more interstitial bone in the compression cortex (P < 0.01). Osteons in the compression cortex also have smaller areas in contrast to the larger area per osteon in the tension cortex. Although no definite transcortical gradient in osteonal density or cortical porosity is found, fractional area of interstitial bone is larger and osteon population density is lowest in the endocortical regions of both tension and compression cortices. The endocortical regions also have greater porosity than their corresponding middle and pericortical regions (P < 0.01). CONCLUSIONS: These osteonal microstructure and cortical porosity differences may be adaptations related to regional differences in strain mode and/or strain magnitude. This may be related to the disparity in mechanical properties of compact bone in tension vs. compression. These differences may reflect a capacity of bone to process local and regional strain-related information.

On the application of linear relative risk regression models.

Motivations for considering linear relative risk models are described. These include covariate data reduction and the testing of additivity of covariate effects on the relative risk. A simulation study was conducted in order to study properties of asymptotic distributional approximations and iterative convergence properties. Parameter transformations and likelihood ratio approximations are considered for confidence interval calculation.

HLA and disease: relative-risk regression methods and multiple testing considerations.

This paper discusses certain statistical issues arising in an analysis to relate HLA-B antigens to the incidence of graft-versus-host disease in a clinical study. The Cox regression method is used to develop a global test of the hypothesis of no association and to produce estimated relative-risk factors corresponding to the presence of each particular allele. Multiple-testing considerations are central to any attempt to identify pairs of alleles having significantly different relative-risk factors. A number of simultaneous-testing approaches are considered including cumulative P-value plotting and a direct simulation of the null distribution of the maximum pairwise ratio.

Serum cholesterol, other risk factors, and cardiovascular disease in a Japanese cohort.

The relationship of serum cholesterol and other risk factors to cardiovascular disease was investigated in a 16-year cohort of 16,711 residents of Hiroshima and Nagasaki. Examined in detail were the relationship of serum cholesterol, and the joint relationships of serum cholesterol, systolic blood pressure, diastolic blood pressure, and other risk factors to coronary heart disease (CHD), cerebral infarction (CI), and cerebral hemorrhage (CH). Baseline and biennially collected risk factor data were analyzed. The latter type of measurement permitted separate investigation of both the short-term and long-term effects of cholesterol measurements. In both types of analyses, both serum cholesterol and blood pressure showed strong associations with CHD incidence. In particular, there were strong associations with short-term and delayed CHD incidence. Furthermore, the association of cholesterol with short-term CHD incidence could not be explained by its association with delayed CHD incidence, or vice versa. Multivariate analyses that also included several other risk factors (smoking habits, clinical diagnosis of diabetes mellitus, left ventricular hypertrophy or strain on electrocardiogram, relative body weight, hematocrit, and proteinuria) for which data were available showed such risk factors to be of lesser, but generally non-negligible, importance in this population. In the case of CH and CI, serum cholesterol was found to be weakly or not at all related to incidence of either disease while blood pressure remained a strong correlate. For CI some suggestion of a statistical interaction between blood pressure and serum cholesterol was found. Discussed are implications for theories of disease pathogenesis for CHD, CI and CH.

Relationship of cigarette smoking and radiation exposure to cancer mortality in Hiroshima and Nagasaki.

Cancer mortality among 40,498 Hiroshima and Nagasaki residents was examined in relation to cigarette smoking habits and estimated atomic bomb radiation exposure level. Relative risk (RR) models that are either multiplicative or additive in the two exposures were emphasized. Most analyses were directed toward all nonhematologic (ANH) cancer, stomach cancer, lung cancer, or digestive tract cancer other than stomach cancer, for which there were, respectively, 1,725, 658, 281, and 338 deaths in the follow-up period for this study. Persons heavily exposed to both cigarette smoke and radiation were found to have significantly lower cancer mortality than multiplicative RR models would suggest for ANH cancer, stomach cancer, and digestive tract cancer other than stomach cancer. Surprisingly, the RR function appeared not only to be submultiplicative for some of these cancer site categories but also may be subadditive. The lung cancer RR function could not be distinguished from either a multiplicative or an additive form. The number of deaths was sufficient to permit some more detailed study of ANH cancer mortality: RR functions appeared to be consistent between males and females, though a paucity of heavy smoking females limits the precision of this comparison. The submultiplicative nature of the RR function mentioned above was particularly pronounced among persons who were relatively young (less than or equal to 30 yr of age) at the time of radiation exposure. The RR function for these younger subjects depends strongly on both radiation and cigarette smoke exposure levels. Even light smoking (approximately 5 cigarettes/day) for an extended period of time was associated with a large estimated RR. Implications of these findings are discussed in relation to human carcinogenesis models. As a byproduct, cancer mortality of several sites is significantly related to radiation exposure in this population, after accommodation for the possible confounding effects of cigarette smoking.

Leukocyte counts and coronary heart disease in a Japanese cohort.

Coronary heart disease incidence during 1958-1974, in a cohort of Hiroshima and Nagasaki residents, was found to relate significantly (p = 0.01) to total leukocyte count, taken an average of two years earlier. Relative risks, as a function of leukocyte count, did not appear to depend on sex or cigarette smoking status, but may be larger for subjects less than 65 years of age than for older persons. This study examines, for the first time, differential leukocyte counts and percentages in relation to coronary heart disease incidence. Both neutrophil count and eosinophil count significantly relate to coronary heart disease incidence, while there is also a suggestion for a relationship with monocyte count. A possible immunopathologic basis for such associations is mentioned. It will be important for other studies to confirm these relationships, however, as the present data do not allow one to clearly show the relative risk, at a specified total leukocyte count, to depend on the differential leukocyte fractions. Available data also leave uncertainty concerning the "independence" of leukocyte counts in the specification of coronary heart disease risk, relative to such known risk factors as cigarette smoking habits and serum cholesterol levels.

Serial blood pressure measurements and cardiovascular disease in a Japanese cohort.

A cohort of 16,711 residents of Hiroshima and Nagasaki have participated in a program of biennial clinical examination and history taking that began in 1958. During 1958-1974, a total of 621 confirmed cases of cerebrovascular disease and 218 confirmed cases of coronary heart disease were incident. This study makes a detailed examination of the relationship between a series of biennial examination blood pressure (BP) measurements and cardiovascular disease risk. Two aspects are emphasized: the quantitative relationship between risk and both systolic blood pressure (SBP) and diastolic blood pressure (DBP), and the predictive value of BP levels some years in the past, given more recent BP determinations. Cerebral hemorrhage incidence is shown to depend markedly on recent DBP level, while earlier DBP levels make an additional important contribution to risk prediction. Corresponding SBP levels have little additional predictive value. With cerebral infarction, SBP is the more important predictor, though elevated DBP conveys some additional risk at high SBP levels. Recent BP levels are more strongly predictive for cerebral infarction than are BP levels some years earlier. Elevated SBP is also an important predictor of coronary heart disease risk in this population, while elevated DBP does not appear to be predictive at specified SBP. SBP levels several years in the past are more closely associated with coronary heart disease risk than are recent SBP readings. The dependence of BP relative risk functions on sex and age is examined and some departures from the results just listed are noted at younger ages. Implications for disease mechanism and hypertensive therapy are discussed.

Leukocyte counts and cerebrovascular disease.

Elevated total leukocyte count in a biennial examination period is shown to be a significant (p = 0.001) predictor of cerebral infarction (CI) incidence in the subsequent 2 yr examination period, in a large Japanese cohort study. This association is not explainable on the basis of corresponding age, sex or blood pressure levels. The extent to which the association might be attributable to cigarette smoking habits could not be thoroughly examined with available data. Relative risks associated with a specific elevated leukocyte count may be larger among persons less than 65 yr of age than among older persons. When counts of specific leukocyte cell lines are considered a significant (p = 0.0006) role for neutrophil count emerges, while an additional predictive role for other leukocyte cell types could not be detected. In contrast, there is a suggestion that cerebral hemorrhage (CH) risk may be lower following an elevated leukocyte count. In particular, a negative association between lymphocyte count and CH incidence in the subsequent biennial examination cycle, is nearly significant (p = 0.07), in spite of a rather small number of CH cases in the sample.