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...