Cornie Scheffer

Benchmarking of a full-body inertial motion capture system for clinical gait analysis

In order for gait analysis to be established as part of routine clinical diagnoses, an accurate, flexible and user-friendly motion capture system is required. Commonly used optical, mechanical and acoustic systems offer acceptable accuracy and repeatability, but are often expensive and restricted to laboratory use. Inertial motion capture has seen great innovation in the last few years, but the technology is not yet considered mature enough for clinical gait analysis. In this paper we compare the kinematic reliability of inertial motion capture with optical motion capture during routine gait studies of eight able-bodied subjects. The root mean squared, RMS, and coefficient of correlation, R, was used to compare data sets. Saggital plane joint angles in the knee and hip compared very well. Corresponding transverse and frontal plane values were moderately accurate. The ankle joint angles calculated from the two systems were less accurate. This was believed to be due to the use of different rotation axis orientations used for calculation of angular rotations.

Tactile Sensing Using Force Sensing Resistors and a Super-Resolution Algorithm

This paper presents a tactile sensor consisting of an array of force sensing resistors (FSRs). The tactile sensing array can be seen as a coordinated system of touch sensors. The low spatial resolution measured with the FSRs compared to other force or pressure sensors required the use of a super-resolution algorithm. Super-resolution algorithms are often used in digital image processing to enhance the resolution of images. Multiple images taken from slightly different orientations are superimposed in such a way that a single higher-resolution image is obtained. Different touch sensors are briefly discussed and the use of FSRs is motivated. Image-registration techniques are discussed and the super-resolution algorithm developed for the application is presented. Some tests performed using the tactile sensor in a neck palpation device and the results of these tests are also presented.

Application of finite element analysis to the design of tissue leaflets for a percutaneous aortic valve

Percutaneous Aortic Valve (PAV) replacement is an attractive alternative to open heart surgery, especially for patients considered to be poor surgical candidates. Despite this, PAV replacement still has its limitations and associated risks. Bioprosthetic heart valves still have poor long-term durability due to calcification and mechanical failure. In addition, the implantation procedure often presents novel challenges, including damage to the expandable stents and bioprosthetic leaflets. In this study, a simplified version of Fungs elastic constitutive model for skin, developed by Sun and Sacks, was implemented using finite element analysis (FEA) and applied to the modelling of bovine and kangaroo pericardium. The FEA implementation was validated by simulating biaxial tests and by comparing the results with experimental data. Concepts for different PAV geometries were developed by incorporating valve design and performance parameters, along with stent constraints. The influence of effects such as different leaflet material, material orientation and abnormal valve dilation on the valve function was investigated. The stress distribution across the valve leaflet was also examined to determine the appropriate fibre direction for the leaflet. The simulated attachment forces were compared with suture tearing tests performed on the pericardium to evaluate suture density. It is concluded that kangaroo pericardium is suitable for PAV applications, and superior to bovine pericardium, due to its lower thickness and greater extensibility.

Differential Forces Within the Proximal Patellar Tendon as an Explanation for the Characteristic Lesion of Patellar Tendinopathy: An In Vivo Descriptive Experimental Study

Background Patellar tendinopathy is a common condition affecting the posterior region of the proximal patellar tendon, but the reason for this typical location remains unclear. Hypothesis The posterior region of the proximal patellar tendon is subjected to greater tendinous forces than is the corresponding anterior region. Study Design Descriptive laboratory study. Method An optic fiber technique was used to detect forces in both the anterior and the posterior regions of the proximal patellar tendon in 7 healthy persons. The optic fiber force sensor works on the principle of the amplitude modulation of transmitted light when the optic fiber is geometrically altered owing to the forces acting on it. Longitudinal strain in the tendon or ligament produces a negative transverse strain, thus causing a force that effectively squeezes the optic fiber. Measurements were recorded during the following exercises: closed kinetic chain quadriceps contraction (eccentric and concentric), open kinetic chain quadriceps contraction (eccentric and concentric), a step exercise, and a jump exercise. Results During all the exercises, the peak differential signal output in the posterior location of the proximal patellar tendon was greater than in the corresponding anterior location. The greatest differential signal output was found in the jump and squat exercises. Conclusion The posterior region of the proximal patellar tendon is subjected to greater tendinous forces than is the corresponding anterior region. This finding supports the tensile-overload theory of patellar tendinopathy. Clinical Relevance Jump activities and deep squat exercises expose the patellar tendon to very large tendinous forces.

Design challenges for camera oximetry on a mobile phone

The use of mobile consumer devices as medical diagnostic tools allows standard medical tests to be performed anywhere. Cameras embedded in consumer devices have previously been used as pulse oximetry sensors. However, technical limitations and implementation challenges have not been described. This manuscript provides a critical analysis of pulse oximeter technology and technical limitations of cameras that can potentially impact implementation of pulse oximetry in mobile phones. Theoretical and practical examples illustrate difficulties and recommendations to overcome these challenges.

The impact of backboard size and orientation on sternum-to-spine compression depth and compression stiffness in a manikin study of CPR using two mattress types

OBJECTIVES To explore how backboard orientation and size impact chest compressions during cardiopulmonary resuscitation (CPR). METHODS Experiments were conducted on a full-body CPR training manikin using a custom-built simulator. Two backboards of different sizes were tested in longitudinal (head to toe) and latitudinal (side to side) directions to assess the impact of size and orientation on chest compressions during CPR. The net sternum-to-spine displacement, combined mattress and sternal displacement as well as the axial reaction force were measured during each test. RESULTS The difference in net compression depth between the larger and smaller backboards ranged between 0.08±0.30 cm and 1.47±0.13 cm, while the difference in back support stiffness varied between 103.7±211 N/cm and 688.1±180.3 N/cm. The difference in net compression depth between the longitudinal and latitudinal backboard orientations ranged from 0.07±0.32 cm to 0.34±0.18 cm, while for the back support stiffness the difference was between 13.4±50.0 N/cm and 592.2±211.0 N/cm. CONCLUSIONS The effect of backboard size on chest compression (CC) performance during CPR was found to be significant with the larger backboard producing deeper chest compressions and higher back support stiffness than the smaller backboard. The impact of backboard orientation was found to depend on the size of the backboard and type of mattress used. Clinicians should be aware that although a smaller backboard may be easier for rescuers to manipulate, it does not provide as effective back support or produce as deep chest compressions as a larger backboard.

Repeatability of an off-the-shelf, full body inertial motion capture system during clinical gait analysis

Background and aims: To establish gait analysis as part of routine clinical diagnoses, physicians demand accurate and flexible motion capture (Mocap). Currently, popular optical, mechanical and acoustic Mocap systems offer acceptable repeatability, but fall short of the spatial and economic benefits accompanying inertial motion capture (IMC). However, IMC is considered adolescent due to limited testing in gait analysis. This paper aims to address this hindrance through within-day and between test day repeatability studies. Methods: To determine the repeatability of IMC, routine gait studies were done on 30 able-bodied males. Repeatability was quantified using the coefficient of multiple determination (CMD) and the coefficient of multiple correlation (CMC). Results: IMC-recorded kinematics were highly repeatable for within-day (CMD: 0.786–0.984 or CMC: 0.881–0.992) and between-day trials (CMD: 0.771–0.991 or CMC: 0.872–0.995). The results compare well to those from similar repeatability studies in the literature based on optical Mocap systems.

Reducing subsidence risk by using rapid manufactured patient-specific intervertebral disc implants

BACKGROUND CONTEXT Intervertebral disc implant size, shape, and position during total disc replacement have been shown to affect the risk of implant subsidence or vertebral fracture. Rapid manufacturing has been successfully applied to produce patient-specific implants for craniomaxillofacial, dental, hip, and knee requirements, but very little has been published on its application for spinal implants. PURPOSE This research was undertaken to investigate the improved load distribution and stiffness that can be achieved when using implants with matching bone interface geometry as opposed to implants with flat end plate geometries. STUDY DESIGN The study design comprises a biomechanical investigation and comparison of compressive loads applied to cadaveric vertebrae when using two different end plate designs. METHODS Four spines from male cadavers (ages 45-65 years, average 52 years), which had a total of n=88 vertebrae (C3-L5), were considered during this study. Bone mineral density scans on each spine revealed only one to be eligible for this study. Twenty remaining vertebrae (C3-L3) were potted and subjected to nondestructive compression tests followed by destructive compression tests. Custom-made nonfunctional implants were designed for this experiment. Ten implants were designed with matching end plate-to-bone interface geometry, whereas the other 10 were designed with flat end plates. Testing did not incorporate the use of a keel in either design type. I-Scan pressure sensors (Tekscan, Inc., MA, USA) were used during the nondestructive tests to assess the load distribution and percentage surface contact. RESULTS Average percent contact area measured during nondestructive tests was 45.27% and 10.49% for conformal and flat implants, respectively-a difference that is statistically significant (p<.001). A higher percent contact area was especially observed for cervical vertebrae because of their pronounced end plate concavity. During destructive compression tests, conformal implants achieved higher failure loads than flat implants. Conformal implants also performed significantly better when stiffness values were compared (p<.0001). CONCLUSIONS One of the main expected benefits from customizing the end plate geometry of disc implants is the reduced risk and potential for subsidence into the vertebral bone end plate. Subsidence depends in part on the stiffness of the implant-bone construct, and with a 137% increase in stiffness, the results of this study show that there are indeed significant potential benefits that can be achieved through the use of customization during the design and manufacture of intervertebral disc implants.

Development of a diagnostic glove for unobtrusive measurement of chest compression force and depth during neonatal CPR

Optimizing chest compression (CC) performance during neonatal cardiopulmonary resuscitation (CPR) is critical to improving survival outcomes since current clinical protocols often achieve only a fraction of the native cardiovascular perfusion. This study presents the development of a diagnostic tool to unobtrusively measure the CC depth and force during neonatal CPR using sensors mounted on a glove platform. The performance of the glove was evaluated by infant manikin tests using the two-thumb (TT) and two-finger (TF) methods of CC during simulated, unventilated neonatal CPR. The TT method yielded maximum CC depths and forces of as much as 25.7 ± 3.2 mm and 35.9 ± 2.2 N while the TF method produced CC depths and forces of as much as 21.6 ± 2.2 mm and 23.7 ± 2.9 N. These results are consistent with clinical findings which suggest that TT compression is more effective than TF compression since it produces greater CC depths and forces.

Contact stresses in a patient-specific unicompartmental knee replacement

BACKGROUND Unicompartmental knee replacement has gained popularity in recent times, showing improved success rates. The main reasons for the failure of unicompartmental knee replacement are the wear of the polyethylene bearing, aseptic loosening and wear in the opposite compartment. The contact stresses involved are significant contributing factors to these causes of failure. METHODS In this study, a patient-specific unicompartmental knee replacement is proposed using a methodology based on neural network modeling of a database of healthy knee geometries. This custom implant was then compared to two conventional implant designs in terms of contact stress in a validated finite element model. FINDINGS The custom implant experienced lower contact stresses at the tibio-femoral joint compared to a fixed-bearing design and also displayed more uniform stress distribution at the bone-implant interface than any of the other implant designs. INTERPRETATION Custom unicompartmental knee replacements therefore have the potential of providing good contact stress distribution, preserve bone stock and could be more anatomically accurate.