Aaron Mohtar

Predicting cancellous bone failure during screw insertion

Internal fixation of fractures often requires the tightening of bone screws to stabilise fragments. Inadequate application of torque can leave the fracture unstable, while over-tightening results in the stripping of the thread and loss of fixation. The optimal amount of screw torque is specific to each application and in practice is difficult to attain due to the wide variability in bone properties including bone density. The aim of the research presented in this paper is to investigate the relationships between motor torque and screw compression during powered screw insertion, and to evaluate whether the torque during insertion can be used to predict the ultimate failure torque of the bone. A custom test rig was designed and built for bone screw experiments. By inserting cancellous bone screws into synthetic, ovine and human bone specimens, it was established that variations related to bone density could be automatically detected through the effects of the bone on the rotational characteristics of the screw. The torque measured during screw insertion was found to be directly related to bone density and can be used, on its own, as a good predictor of ultimate failure torque of the bone.

A remote laboratory for microelectronics fabrication

During the last decade there has been a move towards using remote laboratories in engineering education. These labs allow students to control either electrical or mechanical systems remotely via the internet. At the University of South Australia (UniSA), we have been developing a remote lab that will allow users to visually inspect and test microelectronic circuits under a microscope. In this paper, we describe the architecture of the laboratory and discuss the obstacles that are faced in designing and building a remote laboratory that deals with the accurate manipulation of micro-probes for testing microelectronic circuits. Challenges include mechanical construction of probes, motor control for accurate positioning of probes and the development of a realistic graphical user interface (GUI) which will give students an environment that closely resembles that of a real lab. This laboratory is the extension of the existing, fully functional UniSA remote lab, NetLab, which has already been successfully used in teaching students on-campus as well as off-shore.

Airflow resistance and CO2 rebreathing properties of anti-asphyxia pillows designed for epilepsy.

PURPOSE Seizure related unconscious face-down positioning could contribute to sudden unexpected death in epilepsy via asphyxia. Low airflow resistance lattice foam pillows have been advocated for this group. However, data to support this approach remain lacking, and low airflow resistance per se may not negate asphyxia risk from expired gas rebreathing. This study was designed to compare the airflow resistance and CO2 rebreathing properties of lattice vs conventional pillows. METHODS Airflow resistance and inspired CO2 levels during replicate 10 min periods of simulated adult ventilation and CO2 rebreathing were compared between cotton, latex and two lattice pillows designed for use in epilepsy (one commercially available, one prototype). Kaplan-Meier and Cox regression analyses were used to examine the hazard of exceeding 10% inspired CO2 within 10-min of rebreathing. RESULTS Inspiratory resistance was significantly lower in the commercially available and prototype lattice compared to cotton and latex pillows (mean±SD; 3.2±0.8, 2.6±0.4, 26.1±3.5, 4.6±0.4 cm H2O l(-1)s respectively at 0.2l s(-1)). During simulated rebreathing, inspired CO2 exceeded 10% within 2 min with cotton and latex pillows, compared to an upper asymptote around 8-9% at 10 min with lattice pillows. The hazard of exceeding 10% inspired CO2 was therefore markedly reduced with lattice compared to cotton and latex pillows (hazard ratio vs cotton pillow; commercial 0.04 [0.01-0.18], prototype 0.08 [0.02-0.26], latex 0.79 [0.33-1.87]). CONCLUSION Conventional pillows can rapidly accumulate potentially life-threatening CO2 levels during simulated rebreathing. Lattice pillows appear to reduce asphyxia risk but accumulated CO2 may still reach levels threatening to health and survival.

Novel Handheld Magnetometer Probe Based on Magnetic Tunnelling Junction Sensors for Intraoperative Sentinel Lymph Node Identification

Using magnetic tunnelling junction sensors, a novel magnetometer probe for the identification of the sentinel lymph node using magnetic tracers was developed. Probe performance was characterised in vitro and validated in a preclinical swine model. Compared to conventional gamma probes, the magnetometer probe showed excellent spatial resolution of 4.0 mm, and the potential to detect as few as 5 μg of magnetic tracer. Due to the high sensitivity of the magnetometer, all first-tier nodes were identified in the preclinical experiments, and there were no instances of false positive or false negative detection. Furthermore, these preliminary data encourage the application of the magnetometer probe for use in more complex lymphatic environments, such as in gastrointestinal cancers, where the sentinel node is often in close proximity to other non-sentinel nodes, and high spatial resolution detection is required.

A Detachable Electronic Device for Use With a Long White Cane to Assist With Mobility

Vision-impaired individuals often use a long white cane to assist them with gathering information about their surroundings. However, these aids are generally not used to detect obstacles above knee height. The purpose of this study is to determine whether a low-cost, custom-built electronic device clipped onto a traditional cane can provide adequate vibratory warning to the user of obstacles above knee height. Sixteen normally sighted blindfolded individuals participated in two mobility courses which they navigated using a normal white cane and a white cane with the electronic device attached. Of the 16 participants, 10 hit fewer obstacles, and 12 covered less ground with the cane when the electronic device was attached. Ten participants found navigating with the electronic device easier than just the white cane alone. However, the time taken on the mobility courses, the number of collisions with obstacles, and the area covered by participants using the electronic device were not significantly different (p > 0.05). A larger sample size is required to determine if the trends found have real significance. It is anticipated that additional information provided by this electronic device about the surroundings would allow users to move more confidently within their environment.

Time-elapsed screw insertion with microCT imaging

Time-elapsed analysis of bone is an innovative technique that uses sequential image data to analyze bone mechanics under a given loading regime. This paper presents the development of a novel device capable of performing step-wise screw insertion into excised bone specimens, within the microCT environment, whilst simultaneously recording insertion torque, compression under the screw head and rotation angle. The system is computer controlled and screw insertion is performed in incremental steps of insertion torque. A series of screw insertion tests to failure were performed (n=21) to establish a relationship between the torque at head contact and stripping torque (R(2)=0.89). The test-device was then used to perform step-wise screw insertion, stopping at intervals of 20%, 40%, 60% and 80% between screw head contact and screw stripping. Image data-sets were acquired at each of these time-points as well as at head contact and post-failure. Examination of the image data revealed the trabecular deformation as a result of increased insertion torque was restricted to within 1mm of the outer diameter of the screw thread. Minimal deformation occurred prior to the step between the 80% time-point and post-failure. The device presented has allowed, for the first time, visualization of the micro-mechanical response in the peri-implant bone with increased tightening torque. Further testing on more samples is expected to increase our understanding of the effects of increased tightening torque at the micro-structural level, and the failure mechanisms of trabeculae.

Turn-of-the-Nut Method Is Not Appropriate for Use in Cancellous Bone.

Objective: The level to which bone screws are tightened is determined subjectively by the operating surgeon. It is likely that the tactile feedback that surgeons rely on is based on localized tissue yielding, which may predispose the screw–bone interface to failure. A limited number of studies have investigated the ratio between clinical tightening torque and stripping torque. The purpose of this study was to measure, for the first time, the ratio between yield torque (Tyield) and stripping torque (Tmax) during screw insertion into the cancellous bone and to compare these torques with clinical levels of tightening reported in the literature. Additionally, a rotational limit was investigated as a potential end point for screw insertion in cancellous bone. Methods: A 6.5-mm outer diameter commercial cancellous bone screw was inserted into human femoral head specimens (n = 89). Screws were inserted to failure, while recording insertion torque, compression under the screw head, and rotation angle. Results: The median, interquartile ranges, and coefficient of variation were calculated for each of the following parameters: Tyield, Tmax, Tyield/Tmax, slope, Tplateau, and rotation angle. The median ratio of Tyield/Tmax and rotation angle was 85.45% and 96.5 degrees, respectively. The coefficient of variation was greatest for the rotation angle compared with the ratio of Tyield/Tmax (0.37 vs. 0.12). Conclusions: The detection of yield may be a more precise method than the rotation angle in cancellous bone; however, bone–screw constructs that exhibit a Tyield close to Tmax may be more susceptible to stripping during insertion. Future work can identify factors that influence the ratio of Tyield/Tmax may help to reduce the incidence of screw stripping.