The aging spine: The effect of cyclic loading, simulated degeneration and prolonged sitting on joint stiffness across age

Background: Low back pain is estimated to have a lifetime prevalence as high as 84%, and both the severity and frequency of low back pain reporting have a dependency on age. The nucleus pulposus and annulus fibrosis of the intervertebral disc undergo significant structural and compositional changes with increases in age. As the Canadian working population ages, an understanding of mechanical properties of spine tissue across age is needed to understand pain generating pathways and functional changes. The aim of this thesis was to determine if spine stiffness changes with age and to determine how the mechanical properties of the osteo-ligamentous spine and the annulus contribute to these changes in different loading scenarios. The thesis implemented both in-vitro (Studies I and II) and in-vivo (Studies III and IV) approaches to meet the objectives of the global thesis question. \n \nStudy I: The effect of age and a cyclic loading protocol on the stiffness in porcine functional spine units (FSUs) was explored in study I. A total of 40 FSU specimens, with 21 young (aged 6-8 months) and 19 mature (aged 1.5-8 years) were cyclically loaded at 1 Hz to a range of motion of 8.5 degrees in flexion and extension around the midpoint of each specimen’s neutral zone for 3000 cycles with 1400 N of compression. Neutral zone stiffness was reduced in all specimens following the cyclic loading protocol, indicating no significant differences in temporal responses to repetitive loading across age. However, mature specimens were found to have greater neutral zone stiffness at both the C34 and C56 levels compared to younger specimens. This baseline differences between older and younger spines may alter load distributions in the disc and predispose mature discs to different types of injuries compared to younger specimens. \n \nStudy II: The aim of study II was to isolate stiffness changes in isolated samples of the annulus in response to simulated aging. Low pH in the disc caused by lactic acid has been linked with cell death in the nucleus, discogenic pain and is a hypothesized initiator of disc degeneration. A total of 79 multilayer samples of porcine annulus fibrosis tissue obtained from young (aged 6-8months) spines were immersed in one of four pH and concentration controlled solutions of lactic acid in phosphate buffered saline (PBS) for a duration of 6 hours. The solutions included; (i) pH 7.2 PBS, (ii) pH 3.5 Lactic acid in PBS (15 mmol/L), (iii) pH 6 Lactic acid in PBS (15 mmol/L) or (iv) pH 7 Lactic acid in PBS (15 mmol/L). Following immersion, Specimens were biaxially loaded in tension in both the circumferential and axial directions to 20% strain at a rate of 2%/cycle for 100 cycles. The results of the study showed that circumferential peak stress was significantly higher in C56 specimens immersed in pH 3.5 solution compared to other solution groups. Circumferential stiffness was higher in the C56 specimens in a low pH 3.5 environment compared to the other solution groups. Exposure to a low pH environment altered the mechanical properties of the annulus fibrosis, including higher peak stress and increased stiffness. These changes demonstrate that the annulus is a contributor to increased spine stiffness changes with age. Furthermore, discs with accumulated lactic acid also have an altered mechanical environment that could put older discs at greater risk of annulus damage, such as delamination or fissures in the tissue. \n \nStudy III: The purpose of study III was to determine the effect of age on lumped passive trunk stiffness, postures and discomfort responses during prolonged seated exposures. Participants in Studies III and IV were collected in the same session and shared a common cohort of 34 participants across younger and older age groups, with average (standard deviation) ages of 23.8 (5.0) years and 63.7 (3.9) years, respectively. Passive torso stiffness was measured in flexion before and after sitting continuously (90 minutes) while completing a controlled task on a desktop computer. Discomfort was reported to be higher among older adults in the neck, right shoulder and middle back regions during the prolonged sitting protocol compared to younger adults. There were no significant differences in passive torso stiffness between older and younger adults in flexion postures representing 10%, 20% and 30% of maximum. However, during the sitting protocol, younger adults adopted 19 degrees more flexion compared to older adults. Differences in seated postures across age may be explained by changes to passive tissues in older adults that affect the end range of functional motion, which may have implications for acute pain development during sitting. \n \nStudy IV: The aim of study IV was to determine the effect of participant age, prolonged sitting and lift type on peak thoracic, lumbar, hip and knee postures and ratings of perceived effort. A secondary purpose was to quantify the effect of age on baseline lumbar range of motion about the mediolateral axis. All lifting tasks were floor to knuckle lifts and included, (i) 7 kg symmetrical, (ii) 4.5 kg symmetrical and (iii) 4.5 kg asymmetrical (box located 45 degrees to participant right). Lifting tasks were completed before and after the prolonged sitting protocol. The results of the study demonstrated lower peak lumbar flexion angles following 90 minutes of continuous sitting compared to prior to sitting. While there was no age-related difference noted in response to the prolonged sitting protocol, reduced peak flexion during the lifting tasks following sitting could represent swelling of the intervertebral disc in response to static sitting. Older adults adopted 12 degrees less lumbar flexion during the performance of all lifting tasks compared to younger adults. Older adults had reduced maximum range of motion about the mediolateral axis in the flexion direction compared to younger adults. However, when peak lumbar angles during lifting were expressed as a percentage of maximum flexion, angles were similar between groups with an average 71% and 65% among young and mature participants respectively. This could indicate that functional range of motion in the spine is reduced in older adults, with high flexion tasks entering a zone of higher stiffness. \n \nGeneral Conclusions: Together, the findings from this thesis indicate that osteo-ligamentous functional spine units and the annulus increase in stiffness with age providing a mechanistic understanding of age-related mechanical changes to disc tissue. These changes may partially contribute to the reduction in maximum range of low back motion observed in older adults. Lumped passive stiffness was not significantly different at low flexion postures, but, maximum range of spine motion and peak flexion angles during high flexion tasks were reduced with increasing age. Higher stress in the posterior annulus of aged specimens could predispose older adults to greater risk of annulus disruption and could be a potential source of discogenic low back pain.

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