Mechanical and morphological studies on cadaveric spines. To explain how spinal level and age influence disc degeneration arising from endplate fracture. 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. 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. 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 (R = 22%, P < 0.001), and increased with age (R = 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 (R = 7%, P < 0.001). Endplate thickness increased by approximately 50% between T11 and L5 (R = 21%, P < 0.001). 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. N/A.

Mechanical study on cadaver motion segments. To determine whether high gradients of compressive stress within the intervertebral disc are associated with progressive disc degeneration. 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. 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 disc's 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. 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. 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. N/A.

Mechanical testing of cadaveric spines. To determine whether vertebral body osteophytes act primarily to reduce compressive stress on the intervertebral discs, or to stabilize the spine in bending. The mechanical significance of vertebral osteophytes is unclear. 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. 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. 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.

Study Design. Biomechanical study of vertebroplasty in cadaver motion segments. Objectives. To determine how the volume of injected cement influences: (a) stress distributions on fractured and adjacent vertebral bodies, (b) load-sharing between the vertebral bodies and neural arch, and (c) cement leakage. Summary of Background Data. Vertebroplasty is increasingly used to treat vertebral fractures, but there are problems concerning adjacent level fracture and cement leakage, both of which may depend on the volume of injected cement. Methods. Nineteen thoracolumbar motion segments from 13 cadavers (42–91 years) were loaded to induce fracture. Fractured vertebrae received 2 sequential injections (VP1 and VP2) of 3.5 cm³ of polymethylmethacrylate cement. Before and after each intervention, motion segment stiffness was measured in compression and in bending, and “stress profilometry” was used to quantify the distribution of compressive stress in the intervertebral disc (which presses equally on fractured and adjacent vertebrae). Stress profiles were obtained by pulling a pressure transducer through the disc while the motion segment was compressed in flexed and extended postures. Stress profiles yielded the intradiscal pressure (IDP), the magnitude of stress peaks in the anterior and posterior (SP_P) anulus, and the percentage of the applied compressive force resisted by the neural arch (F_N). Cement leakage and vertebral body volume were quantified using water-immersion, and the percentage cement fill was estimated. Results. Bending and compressive stiffness fell by 37% and 50% respectively following fracture, and were restored only after VP2. Depending on posture, IDP fell by 59–85% after fracture whereas SP_P increased by 107– 362%. VP1 restored IDP and SP_P to prefracture values, and VP2 produced no further changes. Fracture increased F_N from 11% to 39% in flexion, and from 33% to 59% in extension. F_N was restored towards prefracture values only after VP2. Cement leakage increased after VP2 and was negatively correlated to vertebral body volume. Following VP2, increases in IDP and compressive stiffness were proportional to percentage fill. Conclusion. About 3.5 cm³ of PMMA largely restored normal stress distributions to fractured and adjacent vertebral bodies, but 7 cm³ were required to restore motion segment stiffness and load-sharing between the vertebral bodies and neural arch. Cement leakage, IDP and compressive stiffness all increased with percentage fill.