Karen J. Reynolds

Effect of screw torque level on cortical bone pullout strength

Objectives: The objectives of this study were 2-fold: (1) to perform detailed analysis of cortical screw tightening stiffness during automated insertion, and (2) to determine the effect of 3 torque levels on the holding strength of the bone surrounding the screw threads as assessed by screw pullout. Methods: Ten pairs of ovine tibiae were used with 3 test sites spaced 20 mm apart centered along the shaft. One side of each pair was used for measuring ultimate failure torque (Tmax). These Tmax and bone-density values were used to predict Tmax at contralateral tibia sites. Screws were inserted and tightened to 50%, 70%, and 90% of predicted Tmax at the contralateral sites to encompass the average clinical level of torque (86% Tmax). Pullout tests were performed and maximum force values were normalized by cortical thickness. Results: Torque to failure tests indicated tightening to 86% Tmax occurs after yield and leads to an average 51% loss in stiffness. Normalized pullout strength for screws tightened to 50% Tmax, 70% Tmax, and 90% Tmax were 2525 ± 244, 2707 ± 280, and 2344 ± 346 N, respectively, with a significant difference between 70% Tmax and 90% Tmax groups (P < 0.05). Conclusions: Within the limitations of our study involving the testing of 1 type of screw purchase in ovine tibiae, results demonstrate that clinical levels of lag screw tightening (86% Tmax) are past the yield point of bone. Tightening to these high torque levels can cause damage leading to compromised holding strength. Further research is still required to establish the appropriate level of torque required for achieving optimal fracture fixation and healing.

A multi-stage neural network classifier for ECG events

In this paper, a multi-stage network including two multilayer perceptron (MLP) and one self organizing map (SOM) networks is presented. The input of the network is a combination of independent features and the compressed ECG data. The proposed network as a form of data fusion, performs better than using the raw data or individual features. We classified six common ECG waveforms using ten ECG records of the MIT/BIH arrhythmia database. An average recognition rate of 0.883 was achieved within a short training and testing time.

Reducing power line interference in digitised electromyogram recordings by spectrum interpolation.

Interference from power lines (50 or 60 Hz) is the largest source of extraneous noise in many bio-electric signals and is within the bandwidth of many such signals. In this study, two different methods were compared for their efficacy in removing 50 Hz noise added to surface electromyogram (EMG) signals free of power line interference. The first was a simple second-order recursive digital notch filter. The second was an approach called spectrum interpolation, in which it is assumed that the magnitude of the original 50 Hz component of the EMG signal can be approximated by interpolation of the amplitude spectrum of the signal. When the spectrum was based on records containing an integer number of cycles of 50 Hz interference, and the frequency resolution was finer than 1 Hz, spectrum interpolation performed similarly to, or significantly better than, the notch filter (p<0.01). It was also possible to make spectrum interpolation more robust than the notch filter. The Pearson squared correlation coefficient r2 between clean signals and signals processed using the notch filter was reduced from 0.98 to 0.65 when the interference frequency was increased by 0.5 Hz, but r2 for spectrum interpolation at 0.2 Hz resolution was only reduced from 0.99 to 0.85 if spectral values between approximately 49.5 and 50.5 Hz were modified by interpolation.

ECG noise cancellation using digital filters

A digital filter structure is proposed to maximally remove noise from the ECG signals. This structure is based on cascading a zero-phase bandpass, an adaptive filter, and multi-band-pass filter. It provides an efficient method for removing noise from the ECG signals. This filter structure has low implementation complexity and introduces little noise into a typical ECG. It can be applied to real-time applications particularly automatic cardiac arrhythmia classifiers.

Micro-CT examination of human bone: from biopsies towards the entire organ

Micro-CT systems are available that facilitate ex vivo examinations of human specimens as big as entire vertebrae, with spatial resolutions in the 10-micrometer range. This opens a new way for looking at entire bones in 3D. Accurate description of the internal microarchitecture of the entire organ can be obtained, at spatial resolutions previously achievable only on excised biopsies. These high resolution scans produce large datasets and come with costs and benefits, which have to be considered in the successful planning of an experiment. The aim of this paper is to present examples of human vertebrae scanned at high resolution (17 µm/pixel), allowing the visualization and quantification of the microarchitecture, and to discuss some aspects of using high resolution scans of such large specimens. The datasets were down-sampled to 34 µm and 68 µm pixel size, and their morphometric parameters compared to those obtained at 17 µm pixel size, in relation to data size and calculation time.

Virtual reality for medical training: the state-of-the-art

Virtual reality (VR) medical simulations deliver a tailored learning experience that can be standardized, and can cater to different learning styles in ways that cannot be matched by traditional teaching. These simulations also facilitate self-directed learning, allow trainees to develop skills at their own pace and allow unlimited repetition of specific scenarios that enable them to remedy skills deficiencies in a safe environment. A number of simulators have been validated and have shown clear benefits to medical training. However, while graphical realism is high, realistic haptic feedback and interactive tissues are limited for many simulators. This paper reviews the current status and benefits of haptic VR simulation-based medical training for bone and dental surgery, intubation procedures, eye surgery, and minimally invasive and endoscopic surgery.

Development and validation of a generic 3D model of the distal femur

The development and validation of a virtual generic 3D model of the distal femur using computer graphical methods is presented. The synthesis of the generic model requires the following steps: acquisition of bony 3D morphology using standard computed tomography (CT) imaging; alignment of 3D models reconstructed from CT images with a common coordinate system; computer graphical sectioning of the models; extraction of bone contours from the image sections; combining and averaging of extracted contours; and 3D reconstruction of the averaged contours. The generic models reconstructed from the averaged contours of six cadaver femora were validated by comparing their surface geometry on a point to point basis with that of the CT reconstructed reference models. The mean errors ranged from 0.99 to 2.5 mm and were in agreement with the qualitative assessment of the models.

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.

The accuracy of three-dimensional reconstructions of the ovine knee: dissectional validation.

A single, complete, ovine knee was imaged using conventional MRI then dissected allowing comprehensive dimensional measurement of the intra-articular structures. A three-dimensional (3-D) computer model of the knee was generated from the MR sections. Thirty-two individual structural measures were recorded from the image output for which there was an available surgical measure for direct comparison. The results of Pearsons correlation testing show a rounded score of 1.00, suggesting an exceptional linear correlation between direct anatomical measurement and the 3-D image output. Further analysis of the data revealed an average error of measurement of 0.2mm across the 32 measures. The findings of this preliminary study suggest that 3-D reconstruction from MR data may be an appropriate, and accurate, means for making dimensional measurements of the bony and soft tissue structures of the ovine knee. It is unlikely that the measurement error would be of any great clinical significance. There is evidence in the literature to suggest that an ovine knee may be considered an acceptable model for substitution for the human knee in diagnostic assessment studies. Therefore, such findings may be considered clinically relevant in the field of human knee assessment.

Toward photorealism in endoscopic sinus surgery simulation

Background Endoscopic sinus surgery (ESS) is the surgical standard treatment for chronic rhinitis/rhinosinusitis and nasal polyposis. There is a reported complication rate of 5–10% associated with this type of surgery. Simulation has been advocated as a means to improve surgical training and minimize the rates of complication and medical error. This study aimed to show how a virtual reality ESS simulator was developed, with particular emphasis on achieving satisfactory photorealism and surgical verisimilitude. Methods Sinus computed tomography scans were processed to create a triangle-based three-dimensional mesh model; this was incorporated into a spring-damper model of thousands of interconnected nodes, which is allowed to deform in response to user interactions. Dual haptic handpiece devices were programmed to simulate an endoscope and various surgical instruments. Textures and lighting effects were added to the mesh model to provide an accurate representation of the surgical field. Effects such as vasoconstriction in response to “virtual” decongestant were added. Results The final simulated endoscopic view of the sinuses accurately simulates the moist and glossy appearance of the sinuses. The interactive tissue simulation system enables the user to interactively cut and remove tissue while receiving accurate haptic feedback. A working prototype of the simulator has been developed that leverages recent advances in computer hardware to deliver a realistic user experience, both visually and haptically. Conclusion This new computer-based training tool for practicing ESS provides a risk-free environment for surgical trainees to practice and develop core skills. The novel use of customized precision force feedback (haptic) devices enables trainees to use movements during training that closely mimic those used during the actual procedure, which we anticipate will improve learning, retention, and recall.