Smart tools for orthopaedic surgery with sensing capability: a novel approach

Low back pain is a very common and unsolved health problem and a major cause of disability, affecting work performances and general well-being. It is estimated that it affects roughly 60–80% of the adult population in the US. The strategies for treating this pathology range from conservative management to surgery. Novel biological technologies, such as regenerative medicine and bio-artificial organs, are already being tested in human pilot clinical trials. These approaches are an emerging and promising therapeutic tool that might stop, delay or reverse intervertebral disc degeneration. All the above surgical procedures are commonly delivered by injections into the nucleus pulposus through the annulus fibrosus route. However, even small needle annulus fibrosus punctures (25 gauge) may affect intervertebral disc biomechanics, cellularity and biosynthesis. An alternative approach to the annulus fibrosus route, namely the transpedicular route, has been developed and tested: it allows to preserve the annulus fibrosus intact, while the nucleus pulposus is reached via the endplate route. For this purpose, it is necessary to create a deep hole into the vertebra along the transpedicular trajectory. Generally, rotative drills are used, which can cause intraoperative complications: damaging of surrounding soft tissues and overheating of adjacent tissues up to necrosis are not uncommon. Moreover, the procedure is manually performed: the surgeon dexterity and experience, in fact, play a crucial role in maintaining the route toward the end plate of the vertebra during drilling. The accurate and safe creation of a single transpedicular bone hole to access to intervertebral disc space represents therefore an engineering challenge in the new transpedicular procedure. With this aim, a novel system has been developed for guiding surgical tools used for intervertebral access creation. The proposed positioning system (PS) allows to enhance the positional accuracy of drilling task. The PS allows the manual advancement of the drilling tool constrained along a fixed trajectory, in order to preserve the natural haptic perception of the surgeon, who remains in charge of modulating the drilling force and the feeding rate according to the position of the drill bit in the bone. Furthermore, a trajectory planning algorithm has been implanted for guiding the insertion towards the intervertebral space based only on the acquisition of a few fluoroscopic images. The insertion orientation, and therefore the regulation of the system joints configuration, can be planned by a software based on two perpendicular C-arm fluoroscopic images and starting from the identification of the insertion point and of the insertion direction drawn on the images by the surgeon. The system has been dimensioned so to be compatible with the size of C-arm workspace and to minimally interfere with the surgical procedure and with the work of the medical staff. Furthermore, in order to enhance the safety of the procedure, ultrasonic technology has been adopted for developing a new ultrasonic drill for deep hole creation. Ultrasonic bone cutting has advantages in controlling tissue damages because it selectively cuts only mineralized tissues, avoiding damages to soft tissues for frequencies in the range of 25 - 35kHz. In this work a development process for bone ultrasonic drill has been proposed. The whole ultrasonic drill has been designed in COMSOL Multiphysics environment, exploiting the multi-domain tools for simulation. After the manufacturing, the ultrasonic drill has been characterized in terms of vibration properties. The experimental results are in agreement with the simulations. The safety issues in spinal transpedicular procedures cannot be solely exhausted by taking into account the aforementioned solutions. Generally, the accuracy problem in bone drilling consists in finding the right drill termination moment to guarantee that the rear hole will not be widened. Different hardware and software solutions have been developed in order to estimate the drill position during drilling for detecting the bone layers breakthrough. Nevertheless, the main disadvantage of these techniques is the use of the pushing force generating drill bit advancement as effective signal to discriminate among different bone tissues when the feed rate is constant. Therefore, the adoption of these methods is ineffective in the proposed surgical positioning system, where the surgeon can manually control the advancement of the drill bit, thus preserving the haptic feedback that she/he receives from the interaction between the tool and the bone tissues. To address the problem of tissue characterization while leaving the surgeon in complete control of the transpedicular procedure, the average impedance parameter has been introduced. A custom drill, embedded with force and position sensors, which allows the evaluation this new parameter, has been developed. In this work the average impedance estimation results on porcine models are discussed. In addition, the average impedance results obtained on human vertebrae drilling tests are presented and compared with bone mineral density evaluated from CT scans. Finally, in this work it is shown the feasibility to use the ultrasonic drill in the process for the bone layers breakthrough detection, thanks to its capability of sensing of the thrust force applied during bone drilling. This represents a huge advantage, since: i) all the advantages of the ultrasonic cutting, already described, are assured; ii) a single device allows cutting and sensing features simultaneously.