Date of Award

12-2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Program

Biomedical Engineering

Research Advisor

Denis J. DiAngelo, Ph.D.

Committee

Eugene C. Eckstein, Ph.D. Richard J. Kasser, Ph.D. Gladius Lewis, Ph.D. Teong E. Tan, Ph.D.

Abstract

Introduction: A number of decompressing spinal braces (typically pneumatic) have been introduced that attempt to mechanically emulate the “buoyancy” of water therapy by offloading upper torso load to the pelvic girdle as a treatment for lower back pain (LBP). Unfortunately, the beneficial upward force they apply on the upper torso typically makes it difficult or impossible for the patient to bend. For those cases where stabilization is not indicated, this undesirably hinders therapeutic exercise, independent living, and return to work. The cosmetic stigma of wearing an external pneumatic assembly large enough to provide significant offloading may provide additional disincentives to user compliance. Also, there was not, to our knowledge, a scientific orthosis evaluation and development platform capable of producing orthoses with predictable effects on spinal loads. Research objectives were thus to a) develop an advanced robotic test platform (RTP) for the rapid development of spinal orthoses, b) use the RTP to evaluate a commercial decompressing orthosis, c) design, build, and test on the RTP a new orthosis that provides distractive force while enabling mobility, and d) compare the test results of the new orthosis and the commercial decompressing orthosis.

Method: Responsive to the unmet needs for an offloading spinal orthosis that permits mobility, a new conceptual design for a decompressing spinal orthosis was developed. The acronym START was used because it was intended to provide Spine Tractive Adjustment with Rotational Tolerance. The spine tractive adjustment was intended to allow caregivers to selectively reduce or completely eliminate load upon the lumbar spine. The rotational tolerance facility was designed to provide the wearer freedom of motion within an optionally enforced caregiver assigned range. The new experimental START components were designed to be lightweight and concealable under ordinary clothing. There was also a need for an evaluation and rapid development platform capable of producing and evaluating orthoses with a predetermined capacity to accomplish specific clinical objectives (e.g., to apply distractive force). The BioRobotics Laboratory in the Department of Orthopaedic Surgery and Biomedical Engineering at The University of Tennessee already had an advanced robotic testing platform (RTP). Software protocols for simulating daily living activities (DLA’s) on human body parts with the RTP were already in use. The software was then modified to simulate gravitational loads during torso flexion and extension. Also, a human analogue, comprising upper and lower torso segments, was fabricated. The two analogue components were then connected by a biomimetic spine. These were designed and developed to simulate the responses of a human torso under RTP-simulated gravitational loads during the execution of DLA’s. The human analogue with biomimetic spine was mounted in the RTP. An orthosis to be tested was strapped to the human analogue as it would be to a living body. The RTP orchestrated the physiologic simulation of the lumbar spine loading mechanics of DLA’s upon the human analogue (e.g., upright neutral stance, initiation of flexion, and initiation of extension). Load cells recorded the RTP-applied forces and moments as well as the forces and moments at the base of the lumbar spine. The START orthosis and a commercial spinal orthosis (The Orthotrac Pneumatic Vest) were tested for comparison on the RTP in a limited range of sagittal flexion (5°) and extension (3°). Forces were transformed to the sacral disc plane (SDP) which is essentially parallel to the inferior surface of L5. Also, sensors were placed between the orthoses to be tested and the human analogue during tests to better understand pressures at the orthosis-skin interface. Measures of the rotational structural properties and spinal offloading capacity of the orthoses were analyzed.

Results: In testing on the RTP, the maximum brace load (the brace’s supportive capacity) for both orthoses was approximately 300 N. This is approximately enough to fully offset the upper torso weight of a person weighing approximately 750 N (169 pounds). In one set of tests summarized here, the RTP-simulated gravity (applied load) was 300 N. The Orthotrac resisted extension at 3° of rotation with a moment of approximately 7.1 Nm compared to 5.5 Nm for the START. At 5° of flexion, moment resistance for the Orthotrac was approximately 18 Nm compared to 9.4 Nm for the START. Within the range of these tests, the START caused less bending resistance than the Orthotrac. The START orthosis was also tested at rotational ranges in excess of those possible with the Orthotrac (up to 28° of flexion and 10° of extension). In those tests (which also used an RTP-simulated gravity of 300 N) the START orthosis provided a significant but declining brace load as these degrees of rotation increased. At 28° of flexion the brace load was approximately 172 N. After rotating 10° in extension the brace load was approximately 247 N. At 28° of flexion the START orthosis’ resistance to rotation was approximately 20 Nm. At this rotation, the approximate magnitude of the rotational stiffnesses of the brace (0.4 Nm/degree) and the spine (0.5 Nm/degree) were similar and the magnitude of the rotational stiffness of the spine and brace together was approximately 0.9 Nm/degree. At 10° of extension the START orthosis’ resistance to rotation was approximately 15 Nm.

Discussion: The modified RTP protocol simulated gravitational forces and orchestrated motion while repeatably measuring biomechanical properties useful for evaluating and comparing orthoses. The human analogue and biomimetic spine made it possible to conduct comparative tests between unlike orthoses over an extended period of time. Also, pressure sensor measurements indicated the importance of managing the distribution of force at the orthosis-skin interface.

Conclusions: The compressive loading data indicate that both the Orthotrac and the START orthosis provided load support up to approximately 300 N. Both could be easily adjusted to provide reduction of spinal load. Also, where needed, they can both be adjusted to more than offset the full spinal load (on persons whose body weight is less than approximately 169 pounds). The rotational stiffness data suggested that the START orthosis provided more wearer mobility than the Orthotrac based on significantly less applied moment required to bend in flexion. The RTP’s ability to predict the approximate loads that a given orthosis will support provides a means that may be used in the future as a rapid development platform for new and improved orthoses. It may also provide the first steps towards a classification means and a set of standards for spinal orthoses. This would better enable caregivers in the future to select and administer the orthoses best suited to specific LBP causative pathologies.

DOI

10.21007/etd.cghs.2014.0291

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