Single osteons varying in length from 0.10-7.0 mm were dissected from human cortical bone for the purpose of studying their structure and dynamic mechanical properties.

Osteons of three basically different appearances when observed with polarized light were dissected from cross-sections 100-300μ thick, decalcified in EDTA and mechanically disturbed to create spaces between lamellae, large enough to render possible direct observation of collagen fibers by means of scanning electron microscopy (SEM). Separation of the osteon lamellae in whole bone and subsequent deorganification with hydrazine allows observation of the mineral phase. Besides osteonic bone, circumferential, endosteal and cancellous bone were similarly studied. SEM and x-ray micro-diffraction studies indicate that the structure of human osteons is not as simple as previously described by Smith (1960)ⁱ and Ascenzi and Bonucci (1968)². Only the structure of one of the three osteon types was found to be in agreement with these authors. Also, circumferential bone was found to consist of longitudinal fibers, endoste al bone of longitudinal and nearly longitudinal fibers and cancellous bone of transverse fibers lying in the lamellar planes.

Single osteons were tested in dynamic torsional, cyclic tensile-compressive and tensile modes with laboratory built apparati capable of delivering microforces and of detecting microstrains. The shear storage and loss moduli were found to be strain dependent and the sheer storage modulus was also found to be frequency dependent as expected for a viscoelastic material. Single osteons tested dry at a frequency of about 10 hertz, were found to have an average value of 7.5×10ⁱ⁰ dynes/cm² and 3.1×10⁹ dynes/cm² for the storage and loss moduli respectively, an average value of ≲ 5.5% for the loss tangent, typical strain values of ≲1% and ~5% for the onset of nonlinearity and plasticity respectively and an approximate 10% decrease in storage modulus with decreasing frequency over the range of 750-1 hertz. The average value obtained for the shear storage modulus is comparable to Bonfield and Lee's (1967)³ value of 5.7×10ⁱ⁰ dynes/cm² obtained for cylindrical samples of human cortical bone 1/4 mm in diameter-i.e. comparable to osteon diameters-and one inch in length, tested in torsion at much lower strain rates. For a sample containing about 5 osteons, considerably more dispersion was found over approximately the same frequency range. Increasing the number of osteons in the sample also results in a decrease of the shear storage and loss moduli and in a decrease of the strain values for the onsets of nonlinearity and plasticity. Effects due to full hydration and decalci fication were also investigated. The values and strain dependence features of the moduli for a rodlike piece of human cancellous bone were found to be comparable to those obtained for cortical bone.

The elastic modulus of tibial single osteons tested both dry and wet in the cyclic tensile-compressive mode at frequencies near 1000 hertz was investigated and an average value of ~1.5×10¹¹ dynos/cm² for the modulus and average strain values of 2% and 4% for the onsets of nonlinearity and plasticity respectively were found. The average value obtained for the storage modulus is comparable to the value of ~1.3×10¹¹ dynes/cm² obtained by Smith and Keiper (1965)⁴ for tibial samples at frequencies ranging from 500-3500 hertz and by Black (1972)⁵ for tibial and femoral samples at a frequency of ≳ 300 hertz.

Dry single osteons stressed in the tensile mode at low load rates until failure were found to have ultimate tensile strengths varying from 0.2-1.9×10⁹ dynes/cm². As the lowest values may be due to sample defects, the higher values are believed to be more reliable. These higher values are comparable to the average value of ~2×10⁹ dynes/cm² obtained by Ascenzi and Bonucci (1965)⁶ for osteon portions.