Elizabeth A. Friis

Negative Poisson's ratio polymeric and metallic foams

Foam materials based on metal and several polymers were transformed so that their cellular architecture became re-entrant, i.e. with inwardly protruding cell ribs. Foams with re-entrant structures exhibited negative Poissons ratios as well as greater resilience than conventional foams. Foams with negative Poissons ratios were prepared using different techniques and materials and their mechanical behaviour and structure evaluated.

Prevalence of carpal tunnel syndrome and other work-related musculoskeletal problems in cardiac sonographers

Cardiac sonographers at a regional medical center have experienced carpal tunnel syndrome symptoms and other work-related musculoskeletal injuries. The nationwide incidence of these problems was not known. A questionnaire pertaining to possible causes of work-related injuries was developed and distributed to 225 cardiac sonographers. A 47% response rate was achieved with 72% female respondents. Eighty-six percent reported one or more physical symptoms. Only 3% of respondents had been diagnosed with carpal tunnel syndrome. Posture correlated significantly with other work-related musculoskeletal injuries. High-pressure hand grip correlated significantly with carpal tunnel syndrome symptoms. No other strong relations with physical symptoms were found. The contribution of specific factors to musculoskeletal problems experienced by cardiac sonographers was difficult to determine.

Negative pressure intrusion cementing technique for total knee arthroplasty.

Negative pressure intrusion (NPI) is an alternative cementing technique for the tibial baseplate of total knee arthroplasty that uses a suction cannula in the proximal tibia to remove excess fluids and fat before cementing. This technique was compared with standard third-generation positive pressure intrusion (PPI) techniques in an in vitro implantation and analysis of 6 pairs of cadaveric tibiae. Six matched pairs of fresh frozen tibiae were prepared by cutting the tibial surfaces, standard cleaning and surface drying, then performing PPI and NPI on 1 of each pair. No objective differences were noted on radiographs or direct cement depth measurement analysis. Scanning electron micrograph evaluation revealed that the PPI specimens had consistently more voids in the cement-bone composite, and the NPI specimens had consistently narrower empty spaces between bone and cement, resulting in tighter fill in NPI specimens. NPI was shown to enhance characteristics known to improve tensile and shear strength in cement-bone composites.

Effect of Bone Block Removal and Patellar Prosthesis on Stresses in the Human Patella

Thermoelastic stress analysis was used to examine stresses on the anterior surface of patellae after patellar bone block excision for autogenous graft anterior cru ciate ligament reconstruction. Complications of anterior cruciate ligament injury often lead to degenerative changes in the knee that can require total knee joint replacement. It was hypothesized that stresses in a bone block-compromised patella may be increased even further by insertion of a patellar prosthesis. All pa tellae were first tested intact and then were retested after a sequence of surgical modifications including pa tellar prosthesis implantation, tapered bone block ex cision, square bone block excision, and both shapes of excised bone blocks with a patellar prosthesis in place. Stresses in patellae with bone blocks excised were sig nificantly greater than stresses in intact patellae. The anterior surface stress pattern in the loaded patella was significantly altered by excision of a bone block. There were no significant differences between maximum stress in patellae with tapered and square bone blocks excised. A finite element analysis showed that excision of a larger trapezoid-shaped bone block greatly in creased maximum stress levels. Insertion of a patellar prosthesis did not significantly alter stress patterns or maximum stress levels in the patella.

Mechanical analysis of the human cadaveric thoracic spine with intact rib cage

The goal of this study was to characterize the overall in-plane and basic coupled motion of a cadaveric human thoracic spine with intact true ribs. Researchers are becoming increasingly interested in the thoracic spine due to both the high prevalence of injury and pain in the region and also innovative surgical techniques that utilize the rib cage. Computational models can be useful tools to predict loading patterns and understand effects of surgical procedures or medical devices, but they are often limited by insufficient cadaveric input data. In this study, pure moments to ±5 Nm were applied in flexion-extension, lateral bending, and axial rotation to seven human cadaveric thoracic spine specimens (T1-T12) with intact true ribs to determine symmetry of in-plane motion, differences in neutral and elastic zone motion and stiffness, and significance of out-of-plane rotations and translations. Results showed that lateral bending and axial rotation exhibited symmetric motion, neutral and elastic zone motion and stiffness values were significantly different for all modes of bending (p<0.05), and out-of-plane rotations and translations were greater than zero for most rotations and translations. Overall in-plane rotations were 7.7±3.4° in flexion, 9.6±3.7° in extension, 23.3±8.4° in lateral bending, and 26.3±12.2° in axial rotation. Results of this study could provide inputs or validation comparisons for computational models. Future studies should characterize coupled motion patterns and local and regional level biomechanics of cadaveric human thoracic spines with intact true ribs.

Mechanical Contribution of the Rib Cage in the Human Cadaveric Thoracic Spine.

Study Design. An in vitro biomechanical human cadaveric study of T1–T12 thoracic specimens was performed with 4 conditions (with and without rib cage, instrumented and uninstrumented) in flexion-extension, lateral bending, and axial rotation. Objective. The objective was to understand the influence of the rib cage on motion and stiffness parameters of the human cadaveric thoracic spine. Hypotheses tested for overall motion in all modes of bending for both uninstrumented and instrumented specimens were (i) in-plane range of motion and neutral and elastic zones will be greater without the rib cage, (ii) neutral and elastic zone stiffness values will be different for specimens without the rib cage, and (iii) out-of-plane rotations will be different for specimens without the rib cage. Summary of Background Data. The rib cage is presumed to provide significant stability to the thoracic spine, but no studies have been conducted to determine the influence of the rib cage in both uninstrumented and instrumented conditions in the full thoracic human cadaveric specimens. Methods. Seven human cadaveric spine specimens (T1–T12) with 4 conditions (with and without rib cage, instrumented and uninstrumented) were subjected to 5 N·m pure moments in flexion-extension, lateral bending, and axial rotation. Range of motion, neutral and elastic zones, neutral and elastic zone stiffness values, and out-of-plane rotations were calculated for the overall specimen. Results. In-plane range of motion was significantly higher without a rib cage for most modes of bending. Out-of-plane motions were also influenced by the rib cage. Neutral zone stiffness was significantly higher with a rib cage present. Conclusion. Testing without a rib cage yields different motion and stiffness measures, directly impacting the translation of research results to clinical interpretation. Researchers should consider these differences when evaluating the mechanical impact of surgical procedures or instrumentation in cadaveric or computational models. Level of Evidence: 5

Effects of follower load and rib cage on intervertebral disc pressure and sagittal plane curvature in static tests of cadaveric thoracic spines

The clinical relevance of mechanical testing studies of cadaveric human thoracic spines could be enhanced by using follower preload techniques, by including the intact rib cage, and by measuring thoracic intervertebral disc pressures, but studies to date have not incorporated all of these components simultaneously. Thus, this study aimed to implement a follower preload in the thoracic spine with intact rib cage, and examine the effects of follower load, rib cage stiffening and rib cage removal on intervertebral disc pressures and sagittal plane curvatures in unconstrained static conditions. Intervertebral disc pressures increased linearly with follower load magnitude. The effect of the rib cage on disc pressures in static conditions remains unclear because testing order likely confounded the results. Disc pressures compared well with previous reports in vitro, and comparison with in vivo values suggests the use of a follower load of about 400N to approximate loading in upright standing. Follower load had no effect on sagittal plane spine curvature overall, suggesting successful application of the technique, although increased flexion in the upper spine and reduced flexion in the lower spine suggest that the follower load path was not optimized. Rib cage stiffening and removal both increased overall spine flexion slightly, although with differing effects at specific spinal locations. Overall, the approaches demonstrated here will support the use of follower preloads, intact rib cage, and disc pressure measurements to enhance the clinical relevance of future studies of the thoracic spine.

Effect of follower load on motion and stiffness of the human thoracic spine with intact rib cage

Researchers have reported on the importance of the rib cage in maintaining mechanical stability in the thoracic spine and on the validity of a compressive follower preload. However, dynamic mechanical testing using both the rib cage and follower load has never been studied. An in vitro biomechanical study of human cadaveric thoracic specimens with rib cage intact in lateral bending, flexion/extension, and axial rotation under varying compressive follower preloads was performed. The objective was to characterize the motion and stiffness of the thoracic spine with intact rib cage and follower preload. The hypotheses tested for all modes of bending were (i) range of motion, elastic zone, and neutral zone will be reduced with a follower load, and (ii) neutral and elastic zone stiffness will be increased with a follower load. Eight human cadaveric thoracic spine specimen (T1-T12) with intact rib cage were subjected to 5Nm pure moments in lateral bending, flexion/extension, and axial rotation under follower loads of 0-400N. Range of motion, elastic and neutral zones, and elastic and neutral zone stiffness values were calculated for functional spinal units and segments within the entire thoracic section. Combined segmental range of motion decreased by an average of 34% with follower load for every mode. Application of a follower load with intact rib cage impacts the motion and stiffness of the human cadaveric thoracic spine. Researchers should consider including both aspects to better represent the physiologic implications of human motion and improve clinically relevant biomechanical thoracic spine testing.

Validation of a Novel Spine Test Machine

A novel spine test machine was developed for physiological loading of spinal segments. It can be used in conjunction with external motion-capture systems (EMCS) to measure angular displacement, but can also measure in-plane rotations directly, though the inherent error is unknown. This study quantified error inherent in the displacement measurement of the machine. Synthetic specimens representative of cadaveric spinal specimens were tested. Machine displacement was compared to EMCS displacement. The maximum machine displacement error was <2 deg for lumbar and thoracic specimens. The authors suggest that researchers use EMCS in conjunction with the test machine when high accuracy measurements are required.

Theoretical model of a piezoelectric composite spinal fusion interbody implant

Failure rates of spinal fusion are high in smokers and diabetics. The authors are investigating the development of a piezoelectric composite biomaterial and interbody device design that could generate clinically relevant levels of electrical stimulation to help improve the rate of fusion for these patients. A lumped parameter model of the piezoelectric composite implant was developed based on a model that has been utilized to successfully predict power generation for piezoceramics. Seven variables (fiber material, matrix material, fiber volume fraction, fiber aspect ratio, implant cross-sectional area, implant thickness, and electrical load resistance) were parametrically analyzed to determine their effects on power generation within reasonable implant constraints. Influences of implant geometry and fiber aspect ratio were independent of material parameters. For a cyclic force of constant magnitude, implant thickness was directly and cross-sectional area inversely proportional to power generation potential. Fiber aspect ratios above 30 yielded maximum power generation potential while volume fractions above 15% showed superior performance. This investigation demonstrates the feasibility of using composite piezoelectric biomaterials in medical implants to generate therapeutic levels of direct current electrical stimulation. The piezoelectric spinal fusion interbody implant shows promise for helping increase success rates of spinal fusion.