The relation of microscopic mineralization to intrinsic bone strength

Abstract

It is obvious that the "strength" of a bone depends on a number of variables involving both gross morphology and microscopic structure. To a large extent, the size, shape and mass of a bone control the total load which it can support. Short, thick bones, for example, can withstand greater impact and static loading than long, slender bones. The thickness of the cortical walls of long bones also has a significant effect on bone strength since such a bone under stress is essentially a cylindrical tube.One must consider, however, that there are two interpretations of bone strength: ( a ) the strength of the whole bone, and ( b ) the strength of instrinsic regions within the bone. Surprisingly there is no obvious relationship between the two. Recent studies involving the strength of osteoporotic femurs, for example, have indicated that while gross bone strength may be reduced by as much as 40% in osteoporosis, the intrinsic strength of finite regions within the cortex may be 30% higher than in normal bones (Vose, Stover and Mack, '61). In other words, the material which comprises the bone cortex is inherently strong in osteoporosis, but there is simply not enough of it. The present report is concerned with the intrinsic strength of bone -this is the strength of the basic materials which compose the bone instead of the architectural strength of the whole bone.More and more the need for microscopic analyses in bone strength studies has become apparent. Evans ('58) has reported preliminary work involving the relationship of various aspects of microscopic bone structure on ultimate tensile strength, and his work indicates that cortical bone having a relatively small number of large osteones has a higher ultimate tensile strength than bone with a large number of small osteones. In the present report this relationship has been investigated in a number of human tibias and, in addition, the ultimate yield loading of machined bone specimens has been studied with respect to: ( 1 ) The per cent of bone consisting of Haversian and erosion canals, (2) the per cent of bone consisting of Haversian bone, ( 3 ) The per cent of bone consisting of interstitial bone, ( 4 ) the degree of osteoporosis as diagnosed by cortical wall thickness, ( 5 ) the physical density of the specimen, ( 6 ) the microradiographic density of interstitial bone, (7) the size of Haversian canals, (8) the frequency of Haversian canals, ( 9 ) ash content of the specimen. MATERIALS AND METHODSBone samples were removed from the anterior cortex of 14 human tibias at a site approximately 10 cm below the proximal end of the shaft. The bones were those of Caucasian males and females all beyond 60 years of age and were preserved by freezing upon autopsy. Three rectangular sections ( 4 X 4 X 30 mm) were prepared from each specimen using an Aloe rotary bone grinder. The 42 machined samples were then dried at lOO"C, weighed, and the breaking load of each was determined by use of a hydraulically-actuated lever arm. The breaking Ioads of the identically-machined pieces fell...

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“…Ourrey (1959) stated that bone without Haversian systems had a greater tensile strength than bone with Haversian systems and calculated a strong negative correlation (r=-0.6629) between tensile strength and percentage of "Haversian system" type bone present. Vose (1962) found no correlation between ultimate yield loading and the size and frequency of Haversian canals. The question of the effect of porosity was studied by preparing sections from 125 specimens from four subjects that had been tested to failure under identical conditions.…”

“…(Wertheim, 1847;Rauber, 1876;Olivo et al, 1937;Dempster and Liddicoat, 1952;Evans, 1952Evans, , 1961Knese et al, 1955;Smith, J. W. and Walmsley, 1957;Amprino, 1958;Currey, 1959;Hollinghaus, 1959;Bassett and Becker, 1962;Taysum et al, 1962;Vose, 1962;Ascenzi and Bonucci, 1964;Mack, 1964;McElhaney, 1964;Hirsch and Evans, 1965;Sedlin and Hirsch, 1965;Smith, R. W. and Keiper, 1965). Utilization of intact bones can minimize the time between obtaining the sample and actual testing and obviates some of the questions that arise from the use of small samples, i.e.…”

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“…T h e results obtained are critically dependent on the rate of loading (McElhaney 1966, Burstein & Frankel 1968, Panjabi et al 1973, the shape and structure of the test specimen (Dempster & Coleman 1961), the stage of mineralization of the bone (Vose 1962, Currey 1969, the relation between the fibre orientation and the forces acting (Ascenzi & Bonucci 1967, Vincentelli & Evans 1971 and the effects of the storage and post-mortem changes (Sedlin & Hirsch 1966, Stromberg & Dalen 1976a.…”

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“…The mechanical strength of bone depends on its content of collagen and mineral [48][49][50][51]. Young [51] suggested that collagen gives bone its tensile strength and mineral its compressive strength.…”