At Dynamic Disc Designs, we believe research to be the foundation of our spine models so practitioners in musculoskeletal health feel confident in the use of an accurate model while they educate patients about their findings.  Historically, models have been inaccurate and most critically, static, making it very difficult for the doctor to be convincing to the patient in the accuracy of diagnosis.

Research is at the roots of any practice. It fuels practice guidelines and directs both the patient and practitioner down the best path of care. Our models help support that voyage. We have worked hard to bring the best to practitioners of musculoskeletal science by scouring databases of spine science, to arrive at the most accurate model for teaching possible.

With over 1000 papers read in full text, Dr. Jerome Fryer leads the way by making sure our models are keeping up to the standards of best evidence. Weekly literature searches on keywords that surround musculoskeletal health are at the core roots of Dynamic Disc Designs.

Stuart McGill, ddd spinal models

In an online interview with Bill Morgan, President of Parker University, world-renowned spine researcher and scientist, Stuart McGill, uses dynamic disc models from Dynamic Disc Designs to explain lumbar disc herniations, extrusions, and the mechanisms for lumbar disc injuries and treatments.

When treating spinal injuries, McGill stresses the importance of recognizing that the cause of most disc extrusions and herniations is a combination of factors, occurring over time. The cumulative array of factors may present as an acute condition causing pain, but in most cases, the disruption has not been created by a single loading event.

McGill uses the analogy of cloth to explain how repetitive loading and movement fray the collagen fibers that cover the socket joints, eventually working a hole into the fibers by repetitive stress strains occurring in a back and forth motion.

“The disc is layer upon layer of collagen fibers held together with [a tightly woven lamination matrix]. If you keep moving the disc under load, the hydraulic pressure of the pressurized nucleus slowly starts to work its way through the delamination that forms because of the movement,” he says.

He explains that when the collagen is intact and supple, a person has full range-of-motion without danger of creating tears, but when the spine is stiff and has become adapted to bearing heavy loads, it is in danger of injury.

“The problem comes when you combine the two worlds and confuse the adaptation process,” he says.

“In a modern lifestyle, you might have a person who sits at a computer for eight or more hours in a flexion stressed position which—on its own—may not be that bad. But then they go to the gym for an hour every night and start lifting loads. They’re taking their spine through the range of motion, so cumulatively, the collagen is asked to move, but it’s also pressurized. The nucleus behind gets pressurized and slowly works its way through the delaminated collagen.”

Stuart McGill, Models

Stuart McGill and the many ddd models he uses.

McGill, Dynamic Disc Designs

Professor Stuart McGill and Dynamic Disc Designs endorsement.

Recreating Compression Loading, Disc Bulge, and Proper Thrust Line with our Dynamic Model

Using the disc model, McGill demonstrates how the gel inside the disc remains pressurized under compression, but in cases where the collagen has become delaminated, bending the spine under a load creates a disc bulge.

“This is exactly what we see on dynamic MRI,” he says, manipulating the disc model to demonstrate. “In the laboratory we would inject the nucleus with various radio-opaque markers. We would watch the migration as the bulge would come through. Touch a nerve root and now you would match where the disc bulges with the precise anatomic pathway. If you sit for 20 minutes slouched and your right toe goes on fire, we know it’s the right ring and that’s exactly where the disk bulge is.”

McGill stacks the disc model into a thrust line and squeezes the spine segment to show how proper alignment adapts the movement experience.

“The whole disc is experiencing movement, but there’s no pressure, and nothing comes out to touch the nerve root,” he says.

Empowering the Patient with Simple Posture and Stress Exercise

McGill says his insight is based upon years of experiments studying the exact mechanisms of spinal injury and pain. He recommends using improved posture and stress—lying on the stomach for five minutes with two fists under their chin—to help,” mitigate the dynamics of that very dynamic disc bulge.”

He says the immediate relief provided by this simple exercise can empower a patient with discogenic pain and help alleviate the potential psychological trauma of feeling hopeless at not understanding the source of, or how to mitigate, pain.

intradiscal, endplate

A study 1 on the efficacy of intradiscal biologic therapy, where new cells or genes are implanted into the degenerated disc matrix to reduce inflammation and increase matrix cell production, found that degenerated discs may not have the necessary nutrient transport capabilities to ensure proper disc nutrition during this form of therapy. The authors of the study emphasize the importance of research into the determining factors influencing disc cell nutrient transport in informing targeted treatments and strategies to improve disc nutrition in degenerated discs.

What’s at Stake?

Disc degeneration (DD) is a chronic condition that causes spinal pain in aging adults worldwide. The process of DD involves biomechanical modeling of the entire disc matrix and frequently leads to surgical intervention to remove the offending disc and restore functionality to the spine. For many patients, surgical procedures are unsuccessful, however. A noninvasive treatment that has demonstrated recent promise involves regenerating the DD by injecting it with genes, growth factors, small molecules, or implanted cells. These procedures are intended to reduce inflammation and catabolism and assist in the creation of a new disc matrix. But a cell-rich disc requires increased nutrients, and the cartilage endplate (CEP) of the DD may not have the capacity to deliver these nutrients to the matrix. In this study, researchers examined the effects of CEP transport properties in DD on nutrient diffusion and cell function and survival.

The Study

In order to isolate the variable of how nutrient supply affects the nucleus pulposus (NP) cell function, the researchers involved in this study mimicked the in vivo, diffusion-poor disc environment by creating diffusion chambers with similar parameters to isolate the NP nutrient supply mechanics. The cells of the NP receive nutrients that are diffused through the CEP matrix. Cells at the center of the lumbar discs can be up to 10mm from a capillary, while other cells can be just beside a CEP.

Researchers provided glucose and oxygen to cultured NP cells within the chambers. These nutrients were delivered through diffusion from human CEP’s from the open sides of the chamber. Metabolites were expelled into the culture medium by CEP diffusion. The functioning and survival of the cells require a balance between CEP transport properties and cell density, allowing for the request and supply of nutrients. The researchers reproduced the disc matrix environment and physiologic transport conditions in their CEP tissue cultures and diffusion chambers to monitor the effects of NP cell viability and gene expression across the different conditions of nutrient transport.

Specifically, intact human CEP’s from human cadaveric lumbar spines were used for the study. Full-thickness samples of the CEP’s and surrounding calcified cartilage were frozen and sectioned. The researchers calculated the diffusivity of each full-thickness CEP sample through fluorescence and photo-bleaching and using the Axelrod method. They measured each CEP’s biochemical composition spatially via imaging. They created special maps of the collagen, aggrecan, and mineral-to-matrix ratio of the CEP samples with the highest and lowest diffusivities. They measured CEP thickness with photomicrographs and then determined the average measurement across the five chambers.

Bovine NP cells were used in the study (similar to human NP cells). Post-incubation cell viability was determined using a cytotoxicity assay involving gel-stains and low-magnification imagery. Each L4-L5 donor CEP was analyzed for cell density and the anabolic and catabolic gene expressions were examined after chamber incubation. A regression model of fluorescence intensity was used to determine the NP cell gene expression and distance from the CEP. Spatial fluctuations of the CEP composition were described based upon regression models.

Results

The diffusive transport of nutrients varied widely between the CEP samples, affecting the function, health, and survival potential of the NP cells. In fact, there was a four-fold variation in small solute diffusivity in our human CEP sample array. Those allowing less diffusive transport reduced the supply of nutrients to the NP and shortened the viable distance within the diffusion chambers up to 51 percent with typical cell density. Those permitting poor diffusion seemed to downregulate anabolic and catabolic NP cell gene expression. This may mean that a reduced number of disc cells are capable of being sustained through low nutrient CEP diffusion, and the cell’s ability to retain its matrix homeostatic condition is hindered.

When we increased cell density, there was a reduction in cell viability caused by the CEP transport properties, though increasing cell density should raise nutritional demands and shorten the viable distance.  The CEP’s in our study that exhibited low diffusive transport were unresponsive to doubling the cell density, perhaps because they did not provide enough nutrient diffusion to nurture the cell.

We imaged the CEP’s to identify any differences between those with low or high intradiscal diffusivity. Our data found that those with low-diffusivity (and shortened viable distance) contained more collagen and aggrecan, mineral, and lower cross-link maturity. This could explain the blockage of solute penetration and diffusion. At any rate, there appears to be a strong correlation between NP cell survival or function and the availability and mobility of the nutrient supply in the CEP. Compositional defects with the CEP matrix can inhibit nutrient diffusion and undermine biologic therapies that depend upon an increased supply of nutrients to the cell matrix to succeed.

Summary

Our findings suggest that the composition of CEP can contribute to or detract from the function and viability of NP cells. Deficits within the CEP matrix can cause poor nutrient diffusion and block solute passages. This can cause an abundance of collagen and aggrecan, as well as mineral, and lower cross-link maturity. When cell density is increased, CEP’s developed transport deficits, decreasing the cell’s viability. It appears NP function and survival are dependent on the proper CEP composition, as an imbalance in this makeup can reduce the supply of nutrients to the cells, reducing the success rates of biologic therapies.

 

Facet Tropism - Disc Bulge

A study examining the relationship between facet joint angulation, joint tropism, and Degenerative Spondylolisthesis (DS) found a clinically significant link between DS and facet tropism, as well as observing facet tropism in non-DS disc levels of the study subjects. This supports the theory that tropism may pre-exist and contribute to the development of DS, rather than being a by-product of the condition.

 

What’s at Stake?

DS is a common condition affecting middle-aged and the elderly population—especially women. Frequently occurring at the L4-L5 spinal level, the condition has been associated with a number of potential causes, including facet joint orientation. Patients with DS may have more sagittal-oriented facet joints, which allows anterior gliding of their superior vertebra. When a patient’s left and right facet joints are asymmetrical by a minimum of 8 degrees, the condition is considered to be tropism. The authors of this study compared patients with DS with a control group of patients who had no DS to determine how facet joint angulation and/or the presence of facet tropism might play a role in the development of DS.

 

The Study

A retrospective radiographic study of 45 patients with single-level DS, presenting with lower back pain (LBP), leg pain with or without neurological effects, and neurogenic claudication compared the images of the subjects in Group A with a control group (B) of 45 non-DS patients surgically treated for disc prolapse or stenosis, matched in sex and age. Patients with previous spinal surgery or trauma, tumors, vertebrae or congenital anomalies, degenerative lumbar scoliosis, and isthmic spondylolisthesis, as well as those with flawed imaging, were excluded from the group.

MRI axial images of various disc levels were processed and analyzed with PACS software in order to calculate the facet joint angles. A difference of 8 degrees of angulation was termed facet tropism. An independent and case-blinded observer assessed the images of both groups, and an analysis was conducted as to the orientation of the facet joints at three levels in both groups.

Results

Group A was comprised of 15 male subjects and 30 female subjects between 38 and 79 years of age, with a mean age of 62.2. Of the 45 Group A patients, 8.8 percent (4/45) presented with DS, two of which (50%) had facet tropism at index level. All four of these subjects also presented with facet tropism at an adjacent distal level. A total of 37 patients (82.2 percent) showed DS in the L4-5 level, and of those patients, 14 (37.8 percent) also had facet tropism at index level. Eleven patients (29.7 percent) presented with tropism at adjacent proximal level, and 29.7 percent (11) showed the condition at adjacent distal level. Four subjects had DS at L5-S1 level, and all of thse patients had facet propism at index level. A single patient also had tropism at adjacent L4-5 level, as well.

Twenty of the 45 Group A patients (44.4 percent) demonstrated facet tropism at the level of DS. IN addition, 12 of the patients (26.6 percent) had it at a proximal  level to DS level, and 15 (33.3 percent) at level distal to the DS level. Nineteen of the subjects (42.2 percent) had it at a single level, 9 showed tropism at two levels, and 4 (8.8 percent) had it at all three of the levels examined. In all, 71.1 percent of the patients in Group A had facet tropism at one or more levels.

The numbers in Group B were considerably lower, with 2 patients showing facet tropism at L3-4, 5 at L4-5, and 2 at L5-S1. Five of the subjects had single-level tropism, and 2 had it at two levels. None of the Group B patients had tropism at all three levels. In all, only 15.5 percent of the Group B subjects had facet tropism.

Conclusion

The study confirms the association between facet joint tropism and DS. More notably, the observation of higher numbers of facet joint tropism at adjacent non-DS levels in the DS group suggests that facet tropism could contribute to the development of DS, rather than being a secondary symptom of the condition. Patients presenting with single level DS should be followed up closely to monitor adjacent spinal segments that could become symptomatic in the future.

 

 

 

 

 

ddd models, dynamic disc models

A systematic clinical literature review 1 found evidence that high intensity zones (HIZ) on MRI scans may indicate a potential risk factor in lower back pain (LBP). The review authors suggest further studies are needed to understand the relevance of lumbar biomarkers in imaging to properly diagnose and classify LBP as it relates to HIZ.

What’s at Stake?

Various lumbar phenotypes have been identified and studied in the past to determine their effects on patients suffering from LBP. MRI is a common LBP diagnostic tool used by practitioners treating patients with LBP, but its effectiveness in identifying the sources of LBP has been questioned by researchers over the years. For three decades, the debate over whether and how imaged biomarkers may relate to LBP has remained inconclusive. This extensive literature review was conducted to seek clarity on how HIZ in MRI may indicate a reliable diagnostic tool for clinicians treating patients with LBP.

The Review

A total of 756 studies were scanned for data relating to search terms that were indicative of their usefulness to the researchers involved in this review. Six studies—five comparison studies, and one cross-sectional population-based study—were ultimately chosen for their relevance, and their data was reviewed in the context of an association between HIZ and LBP. The literature chosen was published between 2000 and 2015 and involved studies of symptomatic subjects and asymptomatic controls between the ages of 21 to 50 years of age.

Results

Three of the comparative studies demonstrated a clinically-significant association between HIZ and LBP. In one study, over 32 percent of the patients with LBP exhibited HIZ in at least one disc. Of these patients, 5.3 percent showed multi-segmental HIZs, with 3.9 percent showing HIZs in the adjacent discs. Furthermore, 57.5 percent of the HIZs subjects had symptoms of LBP, while only .02 percent of the patients without HIZs were symptomatic. There was a correlation between higher LBP incidence and HIZs in the lower lumbar spine or with multiple HIZs, but these statistics were considered clinically-insignificant. In another study, 61 percent of patients with HIZs experienced LBP, compared to only 32 percent of those without HIZs. The median rate of HIZs was lower in subjects without LBP than in those who were symptomatic.

While the data studied in this review indicates a higher prevalence of LBP in patients with identifiable HIZs in imaging studies, other studies have found little-to-no evidence of this correlation, indicating the need for further studies and reviews on the nature of HIZs and LBP in symptomatic and asymptomatic patients.

Conclusion

This systematic literature review suggests an association between HIZs and LBP. However, the authors express the need for further study of the LBP pathology and HIZs morphology/topography as they relate to various spinal phenotypes to determine how variant biomarkers on MRI studies may help determine the existence and source of LBP in patients.

inflammation, re-absorption

A review  1 of the clinical literature regarding lumbar disc herniation (LDH), particularly as it relates to the phenomenon known as “spontaneous LDH regression,” in which the herniation reduces or resolves without surgical treatment, concludes that the inflammatory response that contributes to nerve pain and damage may also be responsible for the spontaneous re-absorption of the herniation. Therefore, except in extreme cases where a neurological deficit or intolerable pain is experienced by the LBP patient, treatment for LDH should be conservative and custom-tailored to address the specific biochemical mechanisms at play in the patient.

What’s at Stake?

The pain and economic disability caused by LDH affects roughly 9 percent of the world’s population and is strongly associated with the aging process. Recent studies have indicated that the malady is more often caused by a failure in the endplate junction, rather than an annulus fibrosis (AF) failure, with its associated nerve ingrowth. This explains why up to 40 percent of patients diagnosed with LHD after imaging tests are asymptomatic.

Typically, LDH and degenerative disc treatments may be surgical, or conservative (non-surgical), with the decision about which approach is appropriate determined cooperatively by the clinician and patient. Because disc herniations often regress spontaneously, without surgical intervention, the authors of this review emphasize the need for clinicians to better understand the biomechanisms at work in LDH in order to make better-informed decisions about which treatment approach might be best for their patients.

Subtypes of LDH that More Frequently Regress

Magnetic Resonance Imaging (MRI) and CT Scan evidence of LDH regression indicate that particular subsets of herniations are more likely to spontaneously regress than others. Specifically, large-sized and sequestered herniations at the L4-L5 spinal segment level are more apt to partially or completely regress than other types of herniations. It is thought the regression is facilitated by the herniation’s exposure to the epidural vascular supply when the posterior longitudinal ligament ruptures. In fact, MRI studies have shown that the spontaneous regression of herniated disc materials is associated less with the size of the rupture and more with the vessels extending beyond the ligaments and supplying blood and nutrients to the inflamed herniation.

Of 36 analyzed herniations imaged in one study, 25 of them resolved spontaneously—17 percent subligamentous, 48 percent transligamentous, and a whopping 82 percent of sequestered herniations, respectively. This suggests the size of the hernia is less of a factor than the PLL rupture. In another study, all sequestered discs self-resolved within 9 months, while extruded discs took a full 12 months to resolve. Disc protrusions did not resolve, even after a full year.

Conclusion

Clinicians should pay particular attention to the subset type of LDH in their patients when deciding whether to treat their condition surgically or conservatively. Further study into the biochemical mechanisms involved in LDH and its potential for self-resolution would be beneficial in long-term LDH patient outcomes and should be a focus of research for clinicians treating patients with LDH.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

degenerative, MRI, low back pain

A retrospective magnetic resonance imaging (MRI) analysis 1 of lumbar degenerative changes in 283 patients with chronic low back pain (CLBP) found more severe disc degeneration (DD), lower disc height, and more extreme disc displacement at the L4–L5 and L5-S1 of patients with work-related CLBP. The results of the study help to elucidate MRI-visible changes and clinical attributes of work-related CLBP.

What’s at Stake

Lower back pain (LBP) affects up to 84 percent of the world population at some point in life and can contribute to acute or chronic disability in up to 12 percent of those affected. As of 2016, LBP was the leading cause of years lived with a disability, and the U.S. economic burden of LBP is estimated to be somewhere between 84.1 billion to 624.8 billion dollars. Understanding the various stages and degenerative characteristics of LBP can help with appropriate and timely treatment, which may help to reduce cases of CLBP. MRI allows physicians to recognize pathologies, so they can appropriately plan treatment for their LBP patients.

Study Design

The study involved the retrospective review of medical records of adults who had sought treatment for CLBP that had lasted for a period of greater than three months. Inclusion requirements included MRI scans of the entire lumbar spine and clinical lumbar evaluations. Excluded were patients under 18, or those whose LBP was intermittent or had not occurred every day for at least three months. Those experiencing pain that outranked their LBP elsewhere in their body were also excluded from the study, as were patients who could not have an MRI, who had a lumbar infection, spinal trauma, tumor, deformities, or spontaneous septic spondylodiscitis or epidural abscess, previous back surgery, osteoarthritis of the hip, and significant psychological disturbances.

Subject demographics were collected and analyzed, including their occupations, how many hours per week they worked, heavy lifting or lengthy desk sitting involved in their jobs, and the age, sex, body mass index, education level, smoking history, and duration of their LBP. Their LBP scores were recorded using a Visual Analog Scale (VAS) from 0 to 10 (no pain to worst pain). The Oswestry Disability Index (ODI) was used to rank each subject’s functional capacity, where those with a lower percentage were rated healthier.

Imaging Analysis Results

Pre-treatment MRIs of three positions—neutral, flexion, and extension—were performed on each subject by two experienced radiologists and then independently evaluated by an orthopedic surgeon. The subjects were grouped according to their MRI results. The four groups included: normal disc (ND), degenerative disc (DD), bulging disc (BD), and herniated disc (HD). Statistical analysis was performed using special software, and clinically-significant value was assigned. Of the 283 patients with CLBP taking part in the study, 110 were women, and 173 were men, and they ranged in age from 18 to 80, with a mean age of 41.8. The post-MRI groups included 37 subjects in the ND group, 85 in the DD group, 123 in the BD group, and 38 in the HD group. The mean age of the patients in the ND group was significantly lower (31.9) than that of the DD group patients (42.8), HD group (39.3), and the BD group (44.9). The ratio of male to female across all groups was 6:4, but the ratio in the HD group was 84.2 % male to 15.8 % female. The duration of CLBP across all groups was roughly 25 months, but when analyzed group-to-group, it progressively ranged from 15 to 25 months, with the ND group at the lowest range, followed by the DD, BD, and HD groups. The duration of pain was significantly increased from the ND group to the BD group. There were few differences in age, smoking history, or education levels across the groups.

The subjects were further categorized into 10 groups based on their occupations. The three groups that were most prominently represented in the ND, DD, BD, and HD groups were manual workers, desk workers, and technicians. They were similarly represented within their groups. Working hours were also similar across these groups, between 59.7 and 63.2 hours per week. The percentage of subjects who were required to manually handle weighty objects at work was significantly lower than those with no manual handling. The number of working hours spent sitting at a desk was much higher in the DD group, as compared to the other three groups.

When comparing clinical CLBP, the VAS pain scores in the DD, BD, and HD groups were much higher than those of the ND group members. The ODI scores of these three groups were also higher than those of the ND group, and those in the HD group were significantly higher than subjects in the DD and BD groups, indicating less functionality.

The MRI looked for the degree of DD in the neutral, flexion, and extension positions, as well as the vertebral height (anterior and posterior), slipping distance of spondylolisthesis in all three positions, height of the L1-S1 discs, disc bulge or herniation distance, AP diameter of the spinal canal, and translational motion. The data was analyzed and classified indicating the severity of disfunction or damage. The worst degeneration was at the L-4/L-5 and L-5/S-1 level, followed in severity by L-3/L-4.

The disc bulge distances of L-3/L-4 and L-4/L-5 were higher in the BD and HD subject groups. Also, the distance of L-4/L-5 was much higher in the HD group than in the BD group in the neutral position. The distances of L-4/L-5 were much higher in the BD and HD groups than in the ND and DD groups during flexion position, and that of L-3/L-4 was much higher in the HD group than in the ND and DD groups. The distances of L-4/L-5 and L-5/S-1 were much higher in the BD and HD groups during extension MRIs.

Conclusion

This study used MRI to analyze and compare four types of lumbar disc degeneration in patients with CLBP and found that the ND group represented a significantly younger demographic than that of the other three group members. This suggests that age is a likely contributor to DD in CLBP. The subjects in the BD group had a much longer mean pain duration than those in the ND group, suggesting a less successful clinical future outcome for those patients. There appeared to be little-to-no association between BMI and smoking history and CLBP in any of the subjects involved in this study.

There was a positive correlation between hours worked sitting at a desk—with those in the BD and HD groups working on average more than 60 hours per week and those in the ND and DD groups working fewer hours. Interestingly, the data collected indicated that most CLBP patients did not perform heavy manual labor at work and were highly educated—suggesting a strong connection between office work and CLBP. The MRI scans showed that lower lumbar disc segments (L-4/L-5 and L-5/S-1) were the most significantly degenerated in the CLBP patients, with lower disc height and displacement.

 

 

 

properties of the annulus, shear force

An in vivo study 1 of the effects of shear force loading applied to the L5-L6 spinal segment of lab rats revealed histological evidence of IVD degeneration in the unit and surrounding discs of sacrificed study rats that had been exposed to shear force via a custom-designed loading device, while no such evidence was evident in the post-mortem rat control group. The results of the study showed that shear force, applied at .33 MPa (a lower level of compressive stress than previously shown to cause IVD degeneration in rat tail discs), creates degeneration of rat IVDs. This information may be instrumental in providing preventative and treatment-oriented care for people who may be at risk of developing IVD degeneration.

The Study of Shear Force

Researchers tested the hypothesis that sustained shear force on a spinal segment would create IVD degeneration in rat lumbar spines. They used 15 young male rats divided into three groups—one sham control group, and two experimental loading groups that would be exposed to loading for one, and two weeks. The shear loading device used was created especially for the experiment and was made of stainless steel. It was applied to the L5 and L6 vertebral rat bones and delivered a static shear load of up to 4 N.

When the shear loading experiment was completed (1 week, and 2 weeks), the rats were sacrificed. The lumbar segments were removed, viewed microscopically, and tested histologically. A degenerative score from 0 to 3 was assigned each sample, with “0” representing no changes, “1” showing minimal changes, “2” representative of samples with moderate changes, and “3” assigned to those samples showing severe changes, including some with NP disappearance. The slides were blinded and randomized to prevent observer bias.

Results

All the rats involved in the study survived the surgery and post-op period, with no signs of distress. Each of the rats that underwent shear loading had IVD degeneration in most of their lumbar discs, across all levels. The sham control rats, however, demonstrated no degeneration after the experiment.

There were differing levels of degeneration in the IVDs of the shear stress-exposed rats. After the shear loading, the posterior annulus of the exposed rats curved into the dorsal area of the NP, creating a reduction in demarcation in these samples and a disappearance of notochordal cells. The anterior NP remnants were disaggregated, collapsing into smaller sections composed of multiple cells, which, along with the NP, later disappeared. There was also a blending of NP, AF, and CE, and it was difficult to see where one began and another ended. The lamellar wall of the inner and middle annulus dissolved, creating disorganization in the AF.

Discussion

Isolating the effects of different loading modes on IVD degeneration and response is helpful in developing a more complete understanding of IVD biomechanics. Understanding the consequences of shear force applied during compressive spinal loads through in vitro studies can elucidate how shear applied during bending and torsion loading can cause damage to the IVD at the microstructural level and contribute to AF degeneration and failure.

The results of this in vivo study on the disc segments of rats undergoing shear force stress on the L5-L6 IVD segment demonstrated evidence of degenerative changes in all the rats exposed to shear force, while no degeneration occurred in the rat sham control group. The disc damage noted in the experiment groups occurred not only at the L5-L6 levels, but was also evident at adjacent levels (L3-L4, L4-L5, L6-S1). This is further confirmation that the effects of shear force can create damage, proteoglycan depletion, NP content loss and/or collapse, and severe degeneration to disc segments within one week of exposure.