Phillip Pollintine

Discogenic Origins of Spinal Instability

Study Design. Cadaveric motion segment experiment. Objective. To show how two physical aspects of disc degeneration (dehydration and endplate disruption) contribute to spinal instability. Summary of Background Data. The origins of spinal instability and its associations with back pain are uncertain. Methods. Twenty-one cadaveric thoracolumbar motion segments aged 48 to 90 years were secured in cups of dental plaster and loaded simultaneously in bending and compression to simulate full flexion, extension, and lateral bending movements. Vertebral movements, recorded using a two-dimensional “MacReflex” motion analysis system, were analyzed to calculate neutral zone (NZ), range of motion (ROM), bending stiffness (BS), horizontal translational movements, and the location of the center of rotation (COR). Intradiscal “stresses” were measured by pulling a miniature pressure transducer through the disc along its midsagittal diameter. All experiments were repeated after each of two treatments, which simulated physical aspects of disc degeneration: creep loading to dehydrate the disc and compressive overload to disrupt the endplate. Results were analyzed using ANOVA and linear regression. Results. Motion segment height was reduced by 1.0 (SD 0.3) mm during creep and by a further 1.7 (0.6) mm after endplate disruption. In flexion and lateral bending, the combined treatments increased NZ and ROM by 89% to 298%, and increased the “instability index” (NZ/ROM) by 43% to 61%. Translational movements increased by 58% to 86%, whereas BS decreased by 42% to 48%. In extension, ROM and NZ were little affected, although the COR moved closer to the apophyseal joints. Measures of instability increased most in lateral bending, and following endplate disruption. Stress concentrations in the posterior anulus fibrosus increased markedly after endplate disruption. Conclusions. Two physical aspects of disc degeneration (dehydration and endplate disruption) cause markedsegmental instability. Back pain associated with instability may be attributable to stress concentrations in degenerated discs.

Intervertebral disc degeneration can predispose to anterior vertebral fractures in the thoracolumbar spine

Mechanical experiments on cadaveric thoracolumbar spine specimens showed that intervertebral disc degeneration was associated with reduced loading of the anterior vertebral body in upright postures. Reduced load bearing corresponded to locally reduced BMD and inferior trabecular architecture as measured by histomorphometry. Flexed postures concentrated loading on the weakened anterior vertebral body, leading to compressive failure at reduced load.

Can vertebroplasty restore normal load-bearing to fractured vertebrae?

Study Design. Cadaver motion segments were used to evaluate the effects of vertebroplasty on spinal loading following vertebral fracture. Objectives. To determine if vertebroplasty reverses fracture-induced changes in the distribution of compressive stress in cadaver motion segments. Summary of Background Data. Vertebroplasty involves reinforcement of vertebrae by injection of cement and is now being used increasingly to treat osteoporotic vertebral fractures. However, its effects on spinal load-bearing are largely unknown. We hypothesize that vertebroplasty, following vertebral fracture, helps to equalize stress acting on the intervertebral disc and adjacent vertebral bodies. Methods. Nineteen cadaver thoracolumbar motion segments (age 64–90 years) were induced to fracture by compressive overload. Specimens were then subjected to vertebroplasty, and subsequently creep loaded for 1 hour at 1.5 kN. The compressive stress acting on the intervertebral disc was measured before and after fracture, after vertebroplasty, and after creep, by pulling a pressure transducer mounted in a 1.3-mm needle across the disc’s midsagittal diameter. This information was then used to calculate neural arch load-bearing. At each time point, measurements were also made of compressive stiffness. Results. Vertebral fracture reduced motion segment compressive stiffness, decompressed the adjacent nucleus, increased stress concentrations in the posterior anulus, and increased neural arch load-bearing, all by a significant amount. Vertebroplasty partially, but significantly, reversed all of these fracture-induced changes. Conclusions. Vertebroplasty reduces stress concentrations in the anulus and neural arch resulting in a more even distribution of compressive stress on the intervertebral disc and adjacent vertebral bodies.

Intervertebral disc decompression following endplate damage: implications for disc degeneration depend on spinal level and age.

Study Design. Mechanical and morphological studies on cadaveric spines. Objective. To explain how spinal level and age influence disc degeneration arising from endplate fracture. Summary of Background Data. Disc degeneration can be initiated by damage to a vertebral body endplate, but it is unclear why endplate lesions, and patterns of disc degeneration, vary so much with spinal level and age. Methods. One hundred seventy-four cadaveric motion segments, from T7–T8 to L5–S1 and aged 19 to 96 years, were subjected to controlled compressive overload to damage a vertebral body. Stress profilometry was performed before and after damage to quantify changes in intradiscal pressure, and compressive stresses in the annulus. Eighty-six of the undamaged vertebral bodies were then sectioned in the midsagittal plane, and the thickness of the central bony endplate was measured from microradiographs. Regression analysis was used to compare the relative influences of spinal level, age, disc degeneration, and sex on results obtained. Results. Compressive overload caused endplate fracture at an average force of 3.4 kN, and reduced motion segment height by an average 1.88 mm. Pressure loss in the adjacent nucleus pulposus decreased from 93% at T8–T9 to 38% at L4–L5 (R2 = 22%, P < 0.001), and increased with age (R2 = 19%, P < 0.001), especially in male specimens. Stress concentrations in the posterior annulus increased after endplate fracture, with the effect being greatest at upper spinal levels (R2 = 7%, P < 0.001). Endplate thickness increased by approximately 50% between T11 and L5 (R2 = 21%, P < 0.001). Conclusion. Endplate fracture creates abnormal stress distributions in the adjacent intervertebral disc, increasing the risk of internal disruption and degeneration. Effects are greatly reduced in the lower lumbar spine, and in young specimens, primarily because of differences in nucleus volume, and materials properties, respectively. Disc degeneration between L4 and S1 may often be unrelated to endplate fracture. Level of Evidence: N/A

Annulus fissures are mechanically and chemically conducive to the ingrowth of nerves and blood vessels.

Study Design. Mechanical and biochemical analyses of cadaveric and surgically removed discs. Objective. To test the hypothesis that fissures in the annulus of degenerated human discs are mechanically and chemically conducive to the ingrowth of nerves and blood vessels. Summary of Background Data. Discogenic back pain is closely associated with fissures in the annulus fibrosus, and with the ingrowth of nerves and blood vessels. Methods. Three complementary studies were performed. First, 15 cadaveric discs that contained a major annulus fissure were subjected to 1 kN compression, while a miniature pressure transducer was pulled through the disc to obtain distributions of matrix compressive stress perpendicular to the fissure axis. Second, Safranin O staining was used to evaluate focal loss of proteoglycans from within annulus fissures in 25 surgically removed disc samples. Third, in 21 cadaveric discs, proteoglycans (sulfated glycosaminoglycans [sGAGs]) and water concentration were measured biochemically in disrupted regions of annulus containing 1 or more fissures, and in adjacent intact regions. Results. Reductions in compressive stress within annulus fissures averaged 36% to 46%, and could have been greater at the fissure axis. Stress reductions were greater in degenerated discs, and were inversely related to nucleus pressure (R2 = 47%; P = 0.005). Safranin O stain intensity indicated that proteoglycan concentration was typically reduced by 40% at a distance of 600 &mgr;m from the fissure axis, and the width of the proteoglycan-depleted zone increased with age (P < 0.006; R2 = 0.29) and with general proteoglycan loss (P < 0.001; R2 = 0.32). Disrupted regions of annulus contained 36% to 54% less proteoglycans than adjacent intact regions from the same discs, although water content was reduced only slightly. Conclusion. Annulus fissures provide a low-pressure microenvironment that allows focal proteoglycan loss, leaving a matrix that is conducive to nerve and blood vessel ingrowth.

Strength of the Cervical Spine in Compression and Bending

Study Design. Cadaveric motion segment experiment. Objectives. To compare the strength in bending and compression of the human cervical spine and to investigate which structures resist bending the most. Summary of Background Data. The strength of the cervical spine when subjected to physiologically reasonable complex loading is unknown, as is the role of individual structures in resisting bending. Methods. A total of 22 human cervical motion segments, 64 to 89 years of age, were subjected to complex loading in bending and compression. Resistance to flexion and to extension was measured in consecutive tests. Sagittal-plane movements were recorded at 50 Hz using an optical two-dimensional “MacReflex” system. Experiments were repeated 1) after surgical removal of the spinous process, 2) after removal of both apophyseal joints, and 3) after the disc-vertebral body unit had been compressed to failure. Results were analyzed using t tests, analysis of variance, and linear regression. Results were compared with published data for the lumbar spine. Results. The elastic limit in flexion was reached at 8.5° (SD, 1.7°) with a bending moment of 6.7 Nm (SD, 1.7 Nm). In extension, values were 9.5° (SD, 1.6°) and 8.4 Nm (3.5 Nm), respectively. Spinous processes (and associated ligaments) provided 48% (SD, 17%) of the resistance to flexion. Apophyseal joints provided 47% (SD, 16%) of the resistance to extension. In compression, the disc-vertebral body units reached the elastic limit at 1.23 kN (SD, 0.46 Nm) and their ultimate compressive strength was 2.40 kN (SD, 0.96 kN). Strength was greater in male specimens, depended on spinal level and tended to decrease with age. Conclusions. The cervical spine has approximately 20% of the bending strength of the lumbar spine but 45% of its compressive strength. This suggests that the neck is relatively vulnerable in bending.

When are intervertebral discs stronger than their adjacent vertebrae

Study Design. Mechanical testing of cadaveric tissues. Objective. To compare the strength of discs and vertebrae from the same spines in order to determine which are more vulnerable to injury, and to determine how their relative vulnerability depends on age and gender. Summary of Background Data. Vertebrae can strengthen and weaken according to mechanical demands, but the avascular intervertebral discs may be unable to “keep up.” Little is known about the relative strength of discs and vertebrae. Methods. Forty-seven thoracolumbar motion segments were obtained from 30 cadavers 48 to 91 years of age. Each was compressed until a vertebra fractured, and vertebral yield compressive stress (force per unit area) was calculated. Adjacent undamaged intervertebral discs were removed, and circumferential slices, 2.2 mm thick, were cut from the inner, middle, and outer regions of the anterolateral anulus. Slices were stretched to failure to determine their ultimate tensile stress. Results. Yield compressive stress of male and female vertebrae decreased by 69% and 75%, respectively, in the age range of 48 to 91 years (P < 0.001). In contrast, the ultimate tensile stress of the adjacent anulus did not fall significantly with age, except in the outer region of male discs, where it fell by 66% (P < 0.01). Disc strength was proportional to vertebral strength, but only for the outer anulus, and in male spines (r2 = 24%, P = 0.019, n = 22). Conclusion. The outer anulus can adapt to mechanical demands because it is the most metabolically active region of the disc. Disc and bone properties are better matched in male spines because male vertebrae are less affected by variable hormonal changes. The low adaptive potential of intervertebral discs makes them relatively weak in the strengthening spines of young men but relatively strong in the weakening spines of elderly women.

ISSLS Prize winner: Mechanical influences in progressive intervertebral disc degeneration.

Study Design. Mechanical study on cadaver motion segments. Objective. To determine whether high gradients of compressive stress within the intervertebral disc are associated with progressive disc degeneration. Summary of Background Data. Mechanical loading can initiate disc degeneration but may be unimportant in disease progression, because degenerative changes cause the disc to be increasingly “stress-shielded” by the neural arch. However, the most typical feature of advanced disc degeneration (delamination and collapse of the annulus) may not depend on absolute values of compressive stress but on gradients of compressive stress that act to shear annulus lamellae. Methods. A total of 191 motion segments (T7–T8 to L5–S1) were dissected from 42 cadavers aged 19 to 92 years. Each was subjected to approximately 1 kN compression, while intradiscal stresses were measured by pulling a pressure transducer along the discs midsagittal diameter. “Stress gradients” in the annulus were quantified as the average rate of increase in compressive stress (MPa/mm) between the nucleus and the region of maximum stress in the anterior or posterior annulus. Measurements were repeated before and after creep loading and in simulated flexed and erect postures. Disc degeneration was assessed macroscopically on a scale of 1 to 4. Results. As grade of disc degeneration increased from 2 to 4, nucleus pressure decreased by an average 68%, and maximum compressive stress in the annulus decreased by 48% to 64%, depending on location and posture. In contrast, stress gradients in the annulus increased by an average 75% in the anterior annulus (in flexed posture) and by 108% in the posterior annulus (in erect posture). Spearman rank correlation showed that these increases were statistically significant. Conclusion. Despite stress-shielding by the neural arch, gradients of compressive stress increase with increasing grade of disc degeneration. Stress gradients act to shear adjacent lamellae and can explain progressive annulus delamination and collapse. Level of Evidence: N/A

Time-dependent compressive deformation of the ageing spine: relevance to spinal stenosis

Study Design. Mechanical testing of cadaveric spines. Objective. To test the hypothesis that, in the ageing spine, vertebrae deform more than discs, and contribute to time-dependent creep. Summary of Background Data. Intervertebral discs and vertebrae deform under load, narrowing the intervertebral foramen and increasing the risk of nerve root entrapment. Little is known about compressive deformations when elderly spines are subjected to sustained physiologic loading. Methods. A total of 117 thoracolumbar motion segments, aged 19 to 96 yrs (mean, 69), were subjected to 1kN compressive loading for 0.5, 1, or 2 hours. Deformations during the first 7 seconds were designated “elastic” and subsequent deformations as “creep”. A 3-parameter model was fitted to experimental data in order to characterize their viscous modulus E1, elastic modulus E2 (initial stiffness), and viscosity &eegr; (resistance to fluid flow). Intradiscal pressure (IDP) was measured using a miniature needle-mounted transducer. In 17 specimens loaded for 0.5 hours, an optical MacReflex system measured compressive deformations separately in the disc and each vertebral body. Results. On average, the disc contributed 28% of the spines elastic deformation, 51% of the creep deformation, and 38% of total deformation. Elastic, creep, and total deformations of 84 motion segments in 2-hour tests averaged 0.87, 1.37, and 2.24 mm respectively. Measured deformations were predicted accurately by the model (average r2 = 0.97), but E1, E2, and &eegr; depended on the duration of loading. E1 and &eegr; decreased with advancing age and disc degeneration, in proportion to falling IDP (P < 0.001). Total compressive deformation increased with age, but rarely exceeded 3 mm. Conclusion. When the ageing spine is compressed, vertebral bodies show greater elastic deformations than intervertebral discs, and creep by a similar amount. Responses to axial compression depend largely on IDP, but deformations appear to be limited by impaction of adjacent neural arches. Total compressive deformations are sufficient to cause foraminal stenosis in some individuals.

Mechanical Function of Vertebral Body Osteophytes, as Revealed by Experiments on Cadaveric Spines

Study Design. Mechanical testing of cadaveric spines. Objective. To determine whether vertebral body osteophytes act primarily to reduce compressive stress on the intervertebral discs, or to stabilize the spine in bending. Summary of Background Data. The mechanical significance of vertebral osteophytes is unclear. Methods. Thoracolumbar spines were obtained from cadavers, aged 51 to 92 years, with vertebral body osteophytes, mostly anterolateral. Twenty motion segments, from T5–T6 to L3–L4, were loaded in compression to 1.5 kN, and then in flexion, extension, and lateral bending to 10 to 25 Nm (depending on specimen size) with a compressive preload. Vertebral movements were tracked using an optical 2-dimensional MacReflex system. Tests were performed in random order, and were repeated after excision of all osteophytes. Osteophyte function was inferred from (a) changes in the force or moment resisted and (b) changes in tangent stiffness, measured at maximum displacement or rotation angle. Volumetric bone mineral density (BMD) was measured using dual photon x-ray absorptiometry and water immersion. Results were analyzed using repeated measures analysis of variance. Results. Resistance to compression was reduced by an average of 17% after osteophyte removal (P < 0.05), and resistance to bending moment in flexion, extension, and left and right lateral bending was reduced by 49%, 36%, 36%, and 35%, respectively (all P < 0.01). Changes in tangent stiffness were similar. Osteophyte removal increased the neutral zone in bending (P < 0.05) and, on average, reduced motion segment BMD by 7% to 9%. Results were insensitive to applied loads and moments, but several changes were proportional to osteophyte size. Conclusion. Vertebral body osteophytes resist bending movements more than compression. Because they reverse the instability in bending that can stimulate their formation, these osteophytes seem to be adaptive rather than degenerative. Results suggest that osteophytes could cause clinical BMD measurements to underestimate vertebral compressive strength.