Tobin N. Gerhart

Mechanical properties of metaphyseal bone in the proximal femur.

We used a three-point bending test to investigate the structural behavior of 123 rectangular flat plate specimens harvested from the metaphyseal shell of the cervical and intertrochanteric regions of five fresh/frozen human proximal femora. For comparison purposes, 36 specimens of similar geometry were also fabricated from bone of the femoral diaphysis. All specimens were oriented in either the local longitudinal or transverse directions. The mean longitudinal elastic modulus was 9650 +/- 2410 (SD) MPa and demonstrated a 24% decrease from that measured for the diaphysis (12500 +/- 2140 MPa) using the same testing technique. However, the transverse elastic moduli did not differ significantly between the proximal (5470 +/- 1720 MPa) and diaphyseal (5990 +/- 1520 MPa) specimens. The globally averaged values for the ultimate tensile strengths of the metaphyseal shell were 101 +/- 26 MPa in the longitudinal and 50 +/- 12 MPa in the transverse directions. These compared with diaphyseal values of 128 +/- 16 MPa and 47 +/- 12 MPa, respectively. While these differences were largely due to the reduced density of the proximal specimens, a slight decrease in transverse anisotropy for the proximal specimens was also noted by comparing the ratio of longitudinal to transverse moduli (1.76) and tensile strength (2.02) to the diaphyseal values (2.09 and 2.71, respectively). Use of these data should lead to improved performance of analytical models for the proximal femur, and thus help focus increased attention on the structural contribution of trabecular bone to the strength and rigidity of the proximal femur.

Quantitative Histology and Mechanical Testing of a Biodegradable Bone Cement

We have developed a particulate composite bone cement consisting of a particulate phase of tricalcium phosphate (TCP) particles bound together by a polymeric matric phase (PPF). This matrix hardens through a free radical polymerization reaction in vivo within several minutes after mixing. The initial mechanical strength of our particulate composite bone cement results from the matrix, but over time this degrades and the strength is augmented by bone ingrowth and incorporation of the tricalcium phosphate particles. Possible orthopaedic applications include fixation of fractures, augmenting fixation of implants in osteoporetic bone, and temporary stabilization of bone ingrowth prostheses.