Histomorphometry of iliac crest bone in 346 normal black and white South African adults

BoacrmdMincral, ElseviCI

lO(l!%O) 183-199

183

BAM 00295

Histomorphometry of iliac crest bone in 346 normal black and white South African adults* C.M. Schnitzlet’~2,I.M. Pettifog, J.M. Mesquita’2, M.D.T. Bird’, E. Schnaid’ and A.E. Smyth3 ‘LkprmwuofOrdwpaedic Sqwy, Univeniry ofdw WinwKmMd ‘MRCMbwid Membolism ResearchUnit and %wtitute forB&afisricr of rk Medical Rcwwcb Council. lobmmabuq, sowlr Africa (Receiucd 21 luly 1989) (Accepted 4May 1990)

Osteoporotis fracture rates of the hip and spine are lower in North American blacks [l-6], and bone density is greater than in whites [7-121. South African studies, too. report lower hip [13] and spinal [14] fracture rates in blacks. However, metacarpal cortical density has been found to be lower in South African blacks than in whites [13,15]. The contradictory association of lower fracture rates and lower cortical bone density in blacks prompted us to investigate trahecular bone in South African blacks and whites. Moreover, this study provided an opportunity to establish histomorphometric reference values for our laboratory. South African climatic, &t?ry and possibly genetic factors, differ from those in European and North American countries where the currently available reference values were established [16-261. South Africa has a mean of 8.5 h of sunshine a day throughout the year (271 which makes vitamin D deficiency a rare event. The low calcium intake by most South African blacks [28] could adversely affect bone, although the effect of dietary calcium on bone mass remains controversial [29]. These factors, and possible racial differences, made determination of local reference values necessary.

A transiliac bone sample was obtained from each of 346 black and white subjects from the urban and periurban areas of Johannesburg. Age, sex and race distribution are given in Table 1. There was a paucity of black subjects older than 60 years, probably because many elderly individuals retire to the rural areas. Fifty-four of the bone samples were biopsies, and 292 were obtained at necropsy of previously healthy individuals who had died suddenly. We excluded bone samples of individuals who had been confined to bed before death o: who at autopsy showed evidence of organic disease which could have affected bone. In most cases the bone specimen was obtained within 2 days after death, and in the remainder within 3 days. The circumstances leading to the subjects’ death were a motor vehicle or train accident in 118 (67 blacks, 51 whites), assault in 86 (69 blacks, 17 whites), suicide in 47 (3 blacks, 44 whites), an accidental event (drowning, fall from a height, asphyxiation, electrocution, food poisoning) in 28 (12 blacks, 16 whites) and natural causes (cardiovascular event) in 13 (2 blacks, 11 whites) cases. Thirteen (3 blacks, 10 whites) of the 54 biopsies were taken at the time of upper limb surgery for painful post-traumatic conditions of the shoulder, elbow, wrist or hand, or for removal of metal implants. Thirty-four biopsies were obtained at the time of iower !Imb surgery for osteoarthritis of the foot or knee (5 blacks, 19 whites), removal of metal implants from healed fractures of the tibia, ankle or foot (6 blacks), bone grafting of non-union of a tibia (3 black), during a corrective distal femoral osteotomy for malunion of a previous fracture (1 black) and for hallux valgus (2 blacks). All traumatic events had taken place at least 2 years before the biopsy. Seven biopsies were from

I

NS NS

NS

NS NS

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NS NS

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0.90 I I4

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Tb.N

Tb.Th (urn)

Structural variables of trabecular bone in 346 normal black and white South African adults (values given are ir f SD)

Table

Table 1 (continued)

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74

13

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1.31 f0.27

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NS

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I22 f4.5

15.9 t4.4

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18.6 k5.9

20.2 3.1

(Nimm)

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6.54

697 +,,I

688 t17,

594 +x3

(urn)

187 patients with backpain not due to bone disease (previous traumatic vertebral fracture or er~oneoos radiological diagnosis of osteopenia). Socioeconomic and life-style factors of the black and white QoQuk2tions differ. Whites have a European or North American type life-style. Although some blacks live under similar conditions, most come from a poorer socioeconomic background and have larger families than whites. About 50% of urban and periorban blacks are literate (more than 6 years of schooling). Blacks tend to be physically more active than whites in that most perform manual tasks and often walk long distances. The nutritional status of urban black adults is adequate. Corn meal forms the staple diet, protein intake is adequate to low, and the calcium content of food is below the Recommended Dietary Allowance of 800 mgiday [28-301. We Pooled necropsy and biopsy samples in view of reports that histomorphometric findings in necropsy and biopsy samples did not differ significantly [31,32]. We also compared our own samples from the two sources, correcting for age, sex and race. Bone specimens

bone specimens were transiliac cores consisting of outer and inner conices and the intervening cancellous bone. The cores were taken from or near the standard site for bone biopsies at 2 cm below the iliac crest and 2 cm behind the anterior superior iliac spine [25]. A Mayo bone biopsy trephine with an inner diameter of 7.5 mm (Zimmcr, Warsaw, IN, USA) was used. The specimens were fixed in 70% alcohol, embedded ondecalcified in methylmethacrylate and cut on a Jung K heavy duty or a Supercut 2050 microtome. Sections of 7 pm thickness were stained with Goldner’s ttichrome stain and the central portion of cancellous bone was examined with the aid of a semi-au:oma!ic image analyser (Videoplan, Kontron, Munich, FRG) and the Osteoplan program 1331.In each specimen 99 fields, each measrring 0.15 mm*, were evaluated at a magnification of x200. All specimens were read by the same observer. The variables examined are given in Tables 1 and 2. Trabecular number (Tb.N) and separation (Tb.Sp) were calculated according to the formulae given by Parfitt and co-workers (321. Values for trabecular thickness (Tb.Th) and osteoid thickness (O.Th) were not corrected for section obliquity. The three-dimensional quantities, bone surface (BSITV), osteoid surface (OS/TV) and erosion surface (ESEV), were derived from primary two-dimensional measurements by use of the factor 4/n = 1.273, e.g., BS!TV = trabecular perimeter/tissue area X 1.273 X 1MN)mm*/cm’. The nomenclature of the measured and calculated variables is that approved by the American Society for Bone and Mineral Research [34].

The

Statisrical analysis The data were analysed for race, age and sex dependence. Analysis of co-variance (controlling for age) was used to test for racial and sex differences, and linear regression analysis to examine for age dependence. To ensure that the paucity of black subjects older than 60 years did not distort our results, we repeated all analyses for subjects aged 60 years and younger. The 5% level of significance was used throughout.

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15.27 21.10 + IIIQ t 979

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Comparison oi necropsy (Ne) and biopsy (Bi) samples (values given are mean 5 SD)

Table 3

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193 RWlltS

The results are given in Fig. 1 and Tables 1-3. Correhttion coefficients were low, reflecting a wide scatter of data. Comparison of necropsy and biopsy samples revealed only two differences: in black males aged ~50 years biopsies showed higher values for bone surface (P < 0.05) and trabecular number (P < 0.05) than necropsies (Table 3). In the remaining, non-significant necropsylbiopsy comparisons there was no tendency for biopsy samples to have either higher or lower values than necropsy samples. We therefore pooled biopsy and necropsy data. Race Bone volume (BV/TV) was greater in black males, and trabecular thickness (Tb.Tb) greater in black males and females, than in their white counterparts (Table 1). All osteoid and erosion variables showed higher values in blacks than in whites (Table 2). For many variables the racial differences were present from age 21 years onwards.

Age-related changes in structural variables were simdar in black and white males. Up to age 60 years, only bone volume (BVfTV) declined but. when older individuals were included in the analysis, age-related changes became statistically significant in other variables as well. Although the larger number of subjects could have accounted for the change in significance when ail ages were included, the graphs also showed evidence of accelerated changes of bone loss after age 60 years (Fig. 1). Among females, blacks aged ~60 years showed a decline in bone volume (BVfTV) and trabecular rbickness (Tb.Th); insufficient numbers of black females older than 60 years preclude conclusions for this age group. In white females no significant changes were noted up to age 60 years; however, when older women were included, changes of bone loss were evident in all structural variables (Fig. 1, Table 1). Static bone turnover variables did not show consistent age-related changes in any of the groups. Differing levels of significance in the 40 years vs. all ages could, for almost all variables, be exp!ained by marked changes after age 60 years (Fig. 1, Tables 1 and 2).

Structural variables did not differ between males and females, but several as&id variables showed greater values in males than in females of both races (Tables 1 and 2).

We have demonstrated racial differences in trabecular bone architecture and osteoid and erosion variables. Blacks had thicker trabeculae (Tb.Th) than whites, and black males had greater bone volume (BV/lV) than white males. The apparent

194 lack of a difference in bone volume between black and white females was unexpected in view of the lower osteoporotic fracture rate in black women, and cannot be explained. Greater trabecular thickness without associated greater trabecular bone volume in black females could reflect the fact that trabecular architecture is a more important determinant of bone strength than is bone volume [35,36]. Given equal trabecular contiguity, a bone sample with thick trabeculae will be stiffer than one with thin trabeculae [37]. The effect of trabecular thickness is such that a rod of double the sectional area withstands quadruple (exponential) the buckling load [38]. Greater trabecular thickness thus confers disproportionately greater strength on bone. A simple calcu!ation based on the assumption that trabeculae are rod shaped [39] will demonstrate this. Two women in this series, one black aged 52 and one white aged 55 years, had similar bone volume but trabecular thickness in the black woman was greater (150.8ym) than in the white woman (110.3&. Trabecular thickness in the black woman was 1.37-times (150.8/110.3 = 1.37) and sectional area 1.87-times greater than in the white woman ((150.8i2)z x n = 17860pmr, (110.3/2)2 x n = 9555rm’; 1786CV9555= 1.87). The bucklingloadwouldbe 1.87* = 3.5times greater in the black woman. Thus, the one third greater trabecular thickness in the black woman would confer a 3.5-times greater buckling load than in the white woman, despite similar bone volume. On the basis of this principle, greater trabecular thickness in blacks may cnntribute to the lo-wer vertebral fracture rate. An extrapolation from iliac crest to vertebral body bone is perntissible in view of similar trabecular thickness at the two sites 140,411, although ideally vertebra1 bone itself should be studied. The other commonly examined structural variables, trabecular number (TbN) and trabecular spacing (TbSp), showed signs of bone loss only when the over 60 years age-groups were included in the analysis, which suggests that these changes are characteristic of the elderly; unfortunately, no data are available yet for elderly black women. Normal trabecular bone is a three-dimensional network of curved plates which are connected to each other. Perforation of these plates, conversion of the plates to rods and, finally, complete removal of the rods (36,413 constitutes irreversible bone loss. Such loss of trabeculae makes an even greater contribution to the decline in bone strength than does a reduction in trabecular thickness [35,36], and a better correlation exists between trabecular connectivity (or number) and bone stiffness than between bone volume and stiffness [37]. Most importantly, bone strength declines exponentially with loss of connecting trabeculae 136). This biomechanical principle explains the occurrence of fractures in vertebral bodies where loss of horizontal connecting trabeculae has been well documented [39,42]. The trabecular architectural difference between blacks and whites would explain adequately the lower spinal fracture rates in blacks. Yet, there is suggestive evidence that bone quality, too, may be better in blacks than in whites. Although the greater extent of osteoid and erosion surfaces in blacks could be an expression of prolonged remodelling periods, it could equally reflect a larger number of remodelling units, i.e., greater hone turnover. The wider osteoid seams (O.Th) in blacks (a feature of high bone turnover [43] rather than of prolonged remodelling) favour the

195 high bone turnover explanation. Tetracycline bone labelling studies are however needed to establish bone turnover status in blacks and whites. If bone turnover were indeed greater in blacks, then bone would be renewed more frequently, the same bone tissue would accumulate fewer loading cycles and would therefore be less prone to fatigue failure [44,45]. While such an assumed relationship bctwcan bone turnover and fatigue failure is theoretically plausible, it remains to be tested in viva. In whites, high bone turnover has been shown to lead to accelerated bone loss [46], especially in the aged. This is due to an age-related decline in mean wall thickness of osteons [47], i.e.. with every remodelling cycle, less bottc is put back than was removed. Comparison of erosion depth and mean wall thickness of ostcons in blacks and whites would help clarify whether blacks have a mote positive bone balance at Ihe remodelling unit than whites. If this were so, blacks would be protected against accelerated bone loss in the presence of high bone turnover. The causes for possible greater bone turnover in blacks are unknowr. Skeletal loading associated with physically demanding work, low calcium intake [28] and genetic itiucnccs may ail play a mle. The use of patients and cadavers as a source of normal reference values presents problems because neither are truly representative of the general population. A limp due to lower limb pain could conceivably lead to the changes of disuse in iliac crest bone, and alcohol abuse by victims of sudden death in traffic accidents [48,49] may adversely affect bone. The lower values for bone surface (Bslrv) and trabecular number (Tb.N) in necropsy samples of young black malts may bc a reflection of such an effect. In spite of this, and a higher necropsylbiopsy ratio in blacks than whites, blacks had higher values in two structural and all static variables. These findings, and the lack of a bias in favour of biopsy values, encouraged us to continue pooling nccropsy and biopy bone samples. We are nevertheless aware that normal reference data should be obtained only from compbtely healthy individuals [SO]. Comparative hiitomorphometric studies in blacks and whites are scant. Weinstein and Bell’s [Sl] findings differ from outs. They compared North American blacks with whites and found not only no difference in trabecular bone volume, trabecular thickness and trabecular spacing, but also lower bone turnover rates in b!acks than in whites. The reasons for the contradictory findings in structural variables between their study and ours are unclear. Differences in genetic background and in life-style between South African and North American blacks could play a role. We compared our age- and sex-dependent data with published reports, which are presumably based on predominantly white populations. Our finding of an age-related decline in trabecular bone volume (BVEV) is in keeping with that of all other investigators [16,1&L20,23,25,26,52-551. Accelerated bone loss in women over SO years of age was observed by some workers [21,31,41]; in our white women, accelerated bone loss was seen only after the age of 60 years. Bt-ne volume in all groups began to decline fmm the age of 21-U) years onwards; the age of onset of t&ecular bone loss reported in the literature varies between 21 and 30 years [21,40,52] and 45 and 60 years [16,18-20,23,2S,S3,SS]. The absence of a sex difference in tra-

becular bone volume in our study is in agreement with findings by mosi other workers [21,25,52,53,55]. However, Eger and colleagues [40] observed that females up to the age of 40 years have greater trabecular bone volume thaa males. Our values for extent of bone surface (BSrrV) tended to parallel those for bone volume, i.e., bone surface tended to decline with age. This is in agreement with findings in most other studies [20,23,26,32,52,54]. Ow finding of an age-related decline in trabecular thickness (Tb.Th) in all groups (except black males) is in agreement with some reports [56-581, but not with others which recorded either thinning in males only [17], or no age-related decline [19,23,26,32,50,54,59$0]. The reason for this discrepancy in published data may have been explained by Parfitt and co-workers [32] who attributed the small decline in trabecular thickness with age to the fact that their measurements included some thicker peripheral plates which may have arisen by subendosteal tunnelling of the cortex. Since we excluded subcortical trabaculae from our measurements, this argument does not apply to our data. Neither did we find a definite bimodality of trabecular thickness with age as observed by Parfitt and co-workers [32], possibly for the same reason. We found no difference in trahecular thickness between males and females, which accords with other reports [17&l]. Trabecular number (Tb.N) declined with age. especially in the elderly. A reduction in trabecular number with age was also r.oted by other investigators [17,19,32, 36,39,50&O]. Like others [32], we saw no difference in trabecular number between males and females. Trabecular separation (Th.Sp) increased with age, particularly in the elderly, and did not differ between the sexes; these findings, too, accord with reports in the literature [17,32,50,54,61]. The rise in osteoid surface (OS/BS) with age in our black subjects and in white females, accorded with observations by some investigators [56]. A rise after an initial decline as was seen in our white males is in keeping with findings of other studies [20,23,24,52]. While the lack of a sex-difference in osteoid surface (OS/BS) in our white subjects agrees with observations by some investigators [17,31,55], our finding of greater osteoid surface (OS/BS) in black males than black females accords with that of others [24]. Reports of either no change [23,26] or a decline with age [20.52] in osteoid surface (OS/TV) agree with oar data for males, but not with those for females whose values rose. We could find no sex-specificdata for this variablein

the literature. A rise in osteoid volume (OViEXV)with age, as was seen in most of our subjects, was also observed by some investigators (either with [53] or without ao initial decline [SS]) but not by others [23-261. In blacks OWTV, and in whites OViJ3V and OVRV, were greater in males than in females. Only one of the studies which analysed males and females separately also recordedhighervalues for osteoid volume (OVEN) in males [24];others found no sex difference for osteoid volume (OVIEtV or OVfIV) 125,551. Reports on the effects of age on osteoid volume (OVflV) vary between an upward trend [55], no change [23], and a downward trend [25]; om

findingsin white males agreed with those of the latter authors, those in black femaleswith the first, and in blackmales and white women withthe second authors. Our findingof an age-relateddecrease in osteoid thickness(O.Th) (whites only)

is in agreement with observations by some workers [20.24], but not with those by others who recorded no change [25,26,50,55]. The decline with age in only whites suggests less vigorous bone formation in this group. We demonstrated greater OSteoid thickness in white men than white women: other workers recorded no such difference [24,25,55]. An age-related increase in erosion surface (ESBS) as was seen in blacks was also found by some workers [l&25,52], but not by others [17,20,22,26]. The absence of a sex difference in erosion surfacz in our study is in keeping with reports in the literature [22,25,52,55]. The absence of age-related changes in erosion surface (E&TV) in whites is also in agreement with published data [23,26,52]. Values for the variable ‘mineralized erosion surface’ (ES/Md.S) are included to provide reference values for the interpretation of the extent of erosion surfaces in osteomalacia and the hyperosteoidcxes. This study has shown that South African blacks not only have sturdier trabecuiae and, in males, greater bone quantity, but possibly also better bone quality. These differences may contribute to the lower osteoporotic fracture rates in blacks.

Acknowkdgemenls This work was carried out with financial assistance from the South African Medical Research Council and the C J Petrow Orthopaedic Chair Trust Fund. We thank Cc+ ral Gordon for her secretarial help.

I Moldawer M. Zimmerman

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thickness

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