ASGSB 1999 Annual Meeting Abstracts

IN-VIVO METHOD FOR CHRONIC ASSESSMENT OF BONE STRAIN. B.A. Rabkin¹, J.A. Szivek², J Schonfeld³ and B.P. Halloran¹. ¹Medicine and Physiol, U. Calif. and VAMC, San Francisco, CA; ²Dept. Surg. U. Arizona, Tucson, AZ and ³NASA- Ames Research Center, CA.

To better understand the mechanism by which mechanical strain influences bone, it is necessary to quantify in-vivo strains of bones over an extended time period and under varying conditions. In this study a model for measuring the in-vivo strains on the surfaces of a rat tibia for over 4 weeks was tested.

A water proofed hydroxyappetite (HA) coated 120 ohm strain gauge (EA-06-015UW-120 Measurements Groups Inc.) was implanted on the right tibia of a three month old 380g male Sprauge Dowley rat. The rat was tested at 7, 8, 9 and 12 weeks post implant. Zero (base) strain was obtained with the rat sitting and the rat was run at 8, 12, and 16m/min on a treadmill. After 12 weeks post implant both tibias were explanted and a gauge was glued with cyanoacrylate to the left tibia in the same position as that found on the right tibia. Both tibia were bent in a hydraulic press and the difference in strain patterns observed and used to quantify the percent attachment of the HA coated gauge.

Peak to trough strains averaged 240, 300 and 341 µstrain at week 7, 270, 395, and 380 µstrain at week 8 and 264, 394, and 380 µstrain at week 9, to 255, 327, and 303 µstrain at week 12 for speeds of 8, 12 and 16-m/min respectively. Post implant testing showed the HA coated gauge to be 102% attached.

After an initial 6 weeks bonding period, the HA coated gauge remained bonded and active for 12 weeks post implant. The mean peak to trough strain rates from week 8 to 9 did not vary more then 5 µstrains for the three paces. Lower strain rates at week 7, may indicate the gauge was not fully attached at this time. Lower strain rates at week 12 maybe the result of bone morphology changes, such as degree of mineralization, that occur with increased age. The consistent strain readings over the 5-week period combined with the 102% attachment found after explant validates the use of HA coated strain gauges for long term assessment of bone surface strains in the rat.

 (Supported by NASA-Ames DDF.)

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