Powered hammering a diffuse reflectance spectroscopy equipped K-wire

Abstract

Background Inter- and intra patient morphological differences and absence of visual feedback make the placement of pedicle screws technically very demanding. As the spine is surrounded by neurovascular tissue, complications can have severe consequences, including paralysis. Guidance techniques are often very expensive and can overexpose surgeons to radiation, while there is a high variability in reported accuracy rates. Diffuse reflectance spectroscopy (DRS) is a technique that allows for real-time tissue discrimination using an optical fiber which can e.g. be placed in a K-wire. It can detect difference in fat fraction between cortical and cancellous bone.
Purpose This master thesis aims at investigating the capability of the DRS equipped K-wire to anticipate cortical breach by measuring a decrease in fat fraction, while being oscillated with a power hammering tool. Mechanical experiments investigate the amount of control over penetration depth during powered hammering insertion of a K-wire.
Methods This thesis contains three experiments. Two focus on mechanical properties and one on optical properties. In the mechanical experiments, a 1.6 X 60 mm trocar tip K-wire without integrated optical fibers is used. In the first experiment, the K-wire and its oscillator are attached to a carriage on a horizontal guide rail. A mechanical bone phantom is placed at the end of the rail and is penetrated at varying oscillation frequencies (0 – 50 Hz). To move the carriage towards the bone phantom, masses are attached in discrete steps of 250 grams. Penetration depth is measured as insertion mass increases at constant oscillation frequency. In the second mechanical experiment, the same K-wire and K-wire oscillator are attached to a vertical linear stage. The K-wire penetrates the mechanical bone phantom at an oscillation frequency of 0 Hz or 35 Hz. Feed rates are varied (0.5 – 2.5 mm/s) and force on the bone phantom is continuously measured. For the optical experiment, a 1.6 X 300 mm fiber-optic K-wire is used with fiber distance equalling 1.22 mm. The K-wire with the oscillator are attached to a linear stage, and lowered at constant feed rate (0.5 mm/s) into an optical bone phantom. NIR spectra are acquired with a sampling frequency of 1.6 Hz. The effect of varying oscillation frequency (0 – 50 Hz) on measured fat fractions is quantified.
Results The first mechanical experiment showed that for every oscillation frequency, penetration started at the first weight increment (0.1 kg). Without oscillations, phantom penetration initiated at a mass of 1.85 Kg. Regression analysis proved the penetration depth depends on the square root of applied axial force. In the mass range of 0 – 1.6 kg, 45 Hz led to the largest penetration depth (25 mm), with a relatively narrow 95% confidence interval (17 – 33 mm). The second mechanical experiment showed that measured mean force is lower when oscillating the K-wire (1.4 – 5.8 N vs. 9.5 – 12.3 N, p<0.0001). Effect of feed rate on measured mean force is found not significant (p=0.15 for oscillating and p=0.61 for not oscillating). In the optical test, it was found that look ahead distance (LAD) increases with increasing oscillation frequency and amplitude. 50 Hz was the only frequency where the LAD was larger than its oscillation amplitude (1.22 mm and 0.5 mm, respectively).
Conclusion Powered hammering of a K-wire requires a lower axial force for penetration and therefore allows for more control over penetration depth. From the tested range, this effect is most significant for oscillations with high frequencies, adequate forces and low amplitudes. It is possible to measure a decrease in fat fraction with a fiber-optic k-wire during penetration. High oscillation frequencies with low amplitudes allow for earliest anticipation. An adequate sized sampling frequency is required to allow for breach anticipation.