We will develop a noninvasive technique to measure strain on orthopedic screws through tissue.
We aim to measure residual screw strain as an early quantitative indication of loosening, and to measure load-sharing between the screw and fracture callus to assess state of fracture healing and determine when safe load- bearing can begin. The objective of this exploratory research project is to develop and test first tension- indicating orthopedic screw prototypes. Optical tension-indicating bolts have been described in the patent literature since the 1970s for measuring tension on mechanical structures such as railroad ties, however these cannot be used for non-invasive measurement through tissue. The novelty of our approach is that we use upconversion and radioluminescence materials to provide an essentially background-free near infrared signal that can propagate deeply through tissue for orthopedic applications. In addition, the calibration is insensitive to spectral distortion from passing through the tissue because it is based upon spectral features in a narrow spectral bandwidth. We will fabricate the screws modifying commericial orthopedic screws with a luminescent strain-indicator for either elongation or bending. We will test the prototype load-displacement curves as a function of material, load, and screw design, and compare strain indicator resolution, reproducibility, and hysteresis. Initial prototype design will measure screw elongation and bending using a reflective color displacement indicator. We will then modify the indictor for luminescence measurements with a portable fiber- coupled spectrometer system. We will characterize the sensitivity of the luminescent strain measurements through varying tissue thicknesses 1-20 mm. Finally, we will validate the use of tension-indicating screws in a simulated interochanteric femur fixation model with intact and fractured sawbone and cadaveric specimens. The research is relevant to public health because, when fully developed, the strain sensors can indicate fracture complications and allow for interventions to avoid hardware failure.
This research project develops a novel type of orthopedic screw that changes its luminescence color according to screw tension. The proposed research is relevant to public health because it provides a means to non- invasively assess biomechanical fracture healing in vivo and detect pathologies such as delayed/non-union and early loosening. 1
