In vivo assessment of second generation implantable laser Doppler flowmetry fibres for bone blood flow determination

Abstract

Background: Standard laser Doppler flowmetry (LDF) requires application of a probe directly onto tissue for blood flow measurement. Repeated surgical exposures are necessary for assessments over time, making nonanaesthetized measurements unfeasible. Implantable fibres offer a method of assessing tissue perfusion without surgical reexposure and additional time expenditure. Prior in vivo studies showed that first generation implantable LDF fibres experienced early fatigue failure due to their percutaneous exit site, predisposing them to trauma. A second generation of implantable fibres has been developed with silastic coatings to improve durability.

Aims: This investigation determined the reliability of second generation LDF fibres' blood flow measurements compared to standard LDF probe readings, using a segmental tibial fracture model in 11 large adult dogs.

Methods: The fractures were stabilized by compression plates. LDF was used to determine cortical bone perfusion: (1) prefracture (2) postfracture (3) immediately postplating (4) at 5 weeks and (5) at 10 weeks, at four sites along the tibia. Two probe designs were used: (1) standard metal-sheathed probe (2) modified probe with a coupling unit for attachment to 1-mm, flexible silastic-coated fibres. The proximal end of the fibre was implanted in cortical bone, and the distal end subcutaneously for protection. At 5 weeks postplating, minimal surgical dissection was needed to locate the distal fibre tips for measurements. Following euthanasia after 10 weeks, lower extremity lymph nodes and soft tissues surrounding the fibres were removed for histology.

Results: At 5 weeks, 42 of 44 fibres (95%) implanted were intact. At 10 weeks, 75% were intact. The fibres did not incite a foreign body response at 10 weeks. At 5 weeks postplating, implantable fibre LDF values increased over acute postplating levels (p = 0.009), indicating that the fibres could detect cortical bone revascularization. Significant correlations between the standard probe and implantable fibre bone blood flow values occurred in 22% of measurements. This approaches the 33% correlation between standard probe and cerium microsphere entrapment for blood flow measurement.

Conclusion: The second generation fibres were more durable over time than first generation fibres. While implantable fibres can assess bone blood flow without repeated tissue exposure, further modifications are needed for use in clinical settings.