The mechanical properties of cancellous bone in the proximal tibia of ovariectomized rats

The "mature rat model" is an effective and often-used surrogate for studyin g mechanisms and characteristics of estrogen-deficient osteopenia, The purp ose of this study was to extend our understanding of this animal model to i nclude the mechanical properties of cancellous bone in the proximal tibia, Female Sprague-Dawley rats were divided into two groups (n = 13 each) at 14 weeks of age: an ovariectomized group (OVX) and a sham-operated control gr oup (sham), The study terminated after a duration of 5 weeks. Specimens 2 m m long were cut from the proximal tibial metaphysis just below the growth p late and tested using two methods: (1) "whole-slice" compression, in which the entire specimen is loaded between two larger flat platens and (2) "redu ced-platen" compression (RPC), which uses platens sized and aligned to load only the cancellous bone in the center of the sample. Three-point bending tests also were conducted on the femur. The short duration of estrogen defi ciency yielded only minimal differences (<10%) in femoral cortical bone but dramatic reductions (similar to 60%) in cancellous bone properties as dete rmined by the RPC method. Ultimate stress was 7.23 MPa +/- 1.97 MPa for OVX versus 18.1 MPa +/- 5.21 MPa for sham and elastic modulus was 252 MPa +/- 104 MPa for OVX versus 603 MPa +/- 180 MPa for sham, These changes In mecha nical properties are similar in many respects to the dramatic effects repor ted in histomorphometric studies. For the whole-slice method, differences i n mechanical properties between the two groups were not as large because th e test directly loads both cancellous and cortical bone, and the latter is not affected as severely by estrogen deficiency. In this case, ultimate str ess and elastic modulus were only 30% (or less) lower for the OVX group.