Evaluation of a laser Doppler flowmetry implantable fiber system for determination of threshold thickness for flow detection in bone

Laser Doppler flowmetry (LDF) with the use of a standard metal shafted probe, has been successfully used to study bone blood flow in a wide variety of settings. The use of the standard probe is limited by the requirement that the probe be replaced onto the bone surface or driven into the bone with a trocar. In response to this, a system of implantable, detachable fibers was developed. This system allows repeated measurement over time without repeated surgical manipulation of the area of interest. This study was performed to evaluate the implantable fiber system by determining threshold thickness for flow detection in bone. A flow chamber with perpendicular and end-on flow was designed to collect results under controlled conditions for flow detection. Threshold thickness for bovine cortical and cancellous bone samples was determined after sequentially grinding the specimens and placing them in the flow chamber. A 2% solution of latex circulated in the chamber and each specimen was exposed to both flow directions and both types of probe. The laser Doppler probe was able to detect flow by resting on top of the sample, with the latex on the other side of the sample. The results showed a significantly greater threshold thickness for the standard probes than for the implantable probes, and a significantly greater threshold thickness for trabecular bone than for cortical bone. Despite the reduced threshold thickness with the implantable fiber, this new system was able to consistently detect depth of perfusion of 80-90% of the values for the standard probe. The minimum value for threshold thickness in cortical bone with the implantable fiber was 2.43 mm. This was well within a clinically relevant range for flow detection. Threshold thickness and the LDF output were not affected by the flow direction. The study provides baseline information on the utility of this technique and demonstrates the potential usefulness of the implantable fiber system.