Axial twist of the lumbar spine: Mechanical responses to twisted postures and potential factors for workplace injury

Abstract

While a link between magnitudes of spinal axial twist motions and the various modes of associated injury, pain reporting, and lost time claims has been tentatively established, there is need for greater investigation and understanding of the mechanical impact of axial twist motions. Researchers have compiled data sets demonstrating the relationship between twisting motions and moments and low back injury outcomes, but do not create a link to gross occupational exposures. Further, few studies can create a direct relationship between workstation design, trunk postures, and spine joint specific pain and failure mechanisms. When this limited mechanistic understanding is paired with injury prevalence statistics, they highlight a clear need to investigate the role of tissue-level axial twist exposures on occupational injury risk and workstation design guidelines to mitigate that risk. The global objective of this research was focused on developing a relationship between working axial twist postures and intervertebral joint injury risk. The four specific questions asked were (1) What is the relationship between externally measured thoracopelvic axial twist and the actual segmental axial twist motion of the intervertebral joints? (2) Can we use ultrasound as a modality to consistently and accurately measure vertebral axial twist motion? (3) What amount of lumbar axial twist presents an elevated injury risk for working populations? (4) What movement strategies do people use to perform reaching tasks at different hand locations, and how do task parameters impact these strategies?

Study 1: Ultrasound has the potential for use to evaluate boney movement during axial twist of the lumbar spine in both in vivo and in vitro evaluations. Such segmental rotations could then be measured under controlled external thoracic axial twist conditions and in response to mechanical loading. The purpose of this study was to measure vertebral segmental rotations in a porcine model of the human lumbar spine using an ultrasound imaging protocol, and to validate use of this imaging technique with an optical motion capture system. Twelve porcine functional spinal units were fixed to a mechanical testing system, and compression (15% of compressive tolerance), flexion-extension, and axial twist (0, 2, 4, or 6 degrees) were applied. Axial twist motion was tracked using an optical motion capture system and posterior surface ultrasound. Correlation between the two measurement systems was greater than 0.903 and absolute system error was 0.014 across all flexion-extension postures. These findings indicate that ultrasound can be used to track axial twist motion in an in vitro spine motion segment and has the potential for use in vivo to evaluate absolute intervertebral axial twist motion.

Study 2: The relationship between externally measured and internal spine axial twist motion is not well understood. Ultrasound is a validated technique (Study 1) for measurement of vertebral axial twist motion and has the potential for measuring segmental vertebral axial twist in vivo. The purpose of this study was to evaluate lumbar segmental axial twist in relation to external thoracopelvic twist using an ultrasound imaging technique. Sixteen participants kneeled in a custom-built axial twist jig which isolated motion to the lumbar spine. Participants twisted from neutral to 75% of maximum twist range of motion in an upright flexion-extension posture. Thoracopelvic motion was recorded with a motion capture system and L1 to S1 vertebral axial twist was recorded using ultrasound. Maximum thoracopelvic axial twist motion was 41.1 degrees. The majority of axial twist motion occurred at the L2-L3 (46.8% of lumbar axial twist motion) and L5-S1 (33.5%) intervertebral joints. Linear regression fits linking axial twist at each vertebral level to thoracopelvic axial twist ranged from 0.43 to 0.79. These findings demonstrate a mathematical relationship between internal and external axial twist motion, and suggest that classic use of L4-L5 to represent lumbar spine motion may not be appropriate for axial twist modeling approaches.

Study 3: Axial twisting exposures have been repeatedly identified as a risk factor for occupational low back pain and injury, but there is a need for an improved understanding of the role of axial twist magnitude and associated moment as modifiers of the cumulative load tolerance of intervertebral joints. The purpose of this study was to mathematically characterize the relationship between axial twist motion magnitudes and the cumulative load tolerance of porcine cervical functional spinal units. Twenty-four porcine functional spinal units were fixed in a mechanical testing system under compressive load (15% of compressive tolerance) and in a neutral flexion-extension posture. Specimens were axially twisted to 5, 7.5, 10, 12.5, 15 or 17.5 degrees at 1 Hz until failure or 21 600 total cycles. Cumulative applied axial twist was recorded, and exponential functions were fit to the twist magnitude-cumulative twist moment recordings. Weighting-factor functions for cumulative axial twist moment injury risk were developed based on absolute axial twist magnitude and twist normalized to maximum range of motion. The non-linear weighting-factors have potential use in assessment of cumulative axial twist injury risk in occupational tasks.

Study 4: The magnitude of axial twist in the lumbar spine in relation to reaching tasks is currently unknown. Therefore, the purpose of this study was to investigate lumbar spine axial twist during simulated occupational tasks across a range of forward and lateral reach distances, task heights, and exertion directions. Twenty-four participants performed single-handed, right-handed exertions against a load cell in three directions (upward, downward, forward push), at two heights (shoulder, elbow), and at 11 different hand target locations corresponding to current ergonomic reach guidelines. Thoracopelvic and right upper limb postures were recorded using an optical motion capture system, and trunk muscle activation was recorded using surface electromyography. Participants performed a contralateral twist at both the thoracopelvic spine and pelvis about the feet for directly forward hand targets, and twisted up to 19.9 degrees and 12.1 degrees at the lumbar spine and pelvis, respectively, at the most lateral hand target locations. Lumbar flexion and shoulder elevation each increased with reach distance to a maximum of 5.6 degrees and 64.9 degrees, respectively, at the furthest, directly forward hand target location. Hip and abdominal muscle activation exceeded 10% MVC for the most lateral hand target locations, and exhibited the highest activation for upward and forward push exertions. These findings suggest that future ergonomics guidelines should assess reaching and exertion tasks to hand target locations beyond 60-degrees from the midline of the body and consider them as non-optimal zones. The collection of studies in this thesis was structured to improve current ergonomics reach guidelines and provide a physiological and biomechanical basis for reach distance recommendations incorporating the low back. The findings from these studies have important implications for researchers, ergonomists, and clinicians assessing injury risk related to twisted occupational postures.