How are adjacent spinal levels affected by vertebral fracture and by vertebroplasty? A biomechanical study on cadaveric spines

Background Context Spinal injuries and surgery may have important effects on neighboring spinal levels, but previous investigations of adjacent-level biomechanics have produced conflicting results. We use “stress profilometry” and noncontact strain measurements to investigate thoroughly this long-standing problem. Purpose This study aimed to determine how vertebral fracture and vertebroplasty affect compressive load-sharing and vertebral deformations at adjacent spinal levels. Study Design We conducted mechanical experiments on cadaver spines. Methods Twenty-eight cadaveric spine specimens, comprising three thoracolumbar vertebrae and the intervening discs and ligaments, were dissected from fourteen cadavers aged 67–92 years. A needle-mounted pressure transducer was used to measure the distribution of compressive stress across the anteroposterior diameter of both intervertebral discs. “Stress profiles” were analyzed to quantify intradiscal pressure (IDP) and concentrations of compressive stress in the anterior and posterior annulus. Summation of stresses over discrete areas yielded the compressive force acting on the anterior and posterior halves of each vertebral body, and the compressive force resisted by the neural arch. Creep deformations of vertebral bodies under load were measured using an optical MacReflex system. All measurements were repeated following compressive injury to one of the three vertebrae, and again after the injury had been treated by vertebroplasty. The study was funded by a grant from Action Medical Research, UK ($143,230). Authors of this study have no conflicts of interest to disclose. Results Injury usually involved endplate fracture, often combined with deformation of the anterior cortex, so that the affected vertebral body developed slight anterior wedging. Injury reduced IDP at the affected level, to an average 47% of pre-fracture values (p<.001), and transferred compressive load-bearing from nucleus to annulus, and also from disc to neural arch. Similar but reduced effects were seen at adjacent (non-fractured) levels, where mean IDP was reduced to 73% of baseline values (p<.001). Vertebroplasty partially reversed these changes, increasing mean IDP to 76% and 81% of baseline values at fractured and adjacent levels, respectively. Injury also increased creep deformation of the vertebral body under load, especially in the anterior region where a 14-fold increase was observed at the fractured level and a threefold increase was observed at the adjacent level. Vertebroplasty also reversed these changes, reducing deformation of the anterior vertebral body (compared with post-fracture values) by 62% at the fractured level, and by 52% at the adjacent level. Conclusions Vertebral fracture adversely affects compressive load-sharing and increases vertebral deformations at both fractured and adjacent levels. All effects can be partially reversed by vertebroplasty

Vertebroplasty reduces progressive ׳creep' deformity of fractured vertebrae

Elderly vertebrae frequently develop an “anterior wedge” deformity as a result of fracture and creep mechanisms. Injecting cement into a damaged vertebral body (vertebroplasty) is known to help restore its shape and stiffness. We now hypothesise that vertebroplasty is also effective in reducing subsequent creep deformations. Twenty-eight spine specimens, comprising three complete vertebrae and the intervening discs, were obtained from cadavers aged 67–92 years. Each specimen was subjected to increasingly-severe compressive loading until one of its vertebrae was fractured, and the damaged vertebral body was then treated by vertebroplasty. Before and after fracture, and again after vertebroplasty, each specimen was subjected to a static compressive force of 1 kN for 1 h while elastic and creep deformations were measured in the anterior, middle and posterior regions of each adjacent vertebral body cortex, using a 2D MacReflex optical tracking system. After fracture, creep in the anterior and central regions of the vertebral body cortex increased from an average 4513 and 885 microstrains, respectively, to 54,107 and 34,378 microstrains (both increases: P<0.001). Elastic strains increased by a comparable amount. Vertebroplasty reduced creep in the anterior and central cortex by 61% (P=0.006) and 66% (P=0.017) respectively. Elastic strains were reduced by less than half this amount. Results suggest that the beneficial effects of vertebroplasty on the vertebral body continue long after the post-operative radiographs. Injected cement not only helps to restore vertebral shape and elastic properties, but also reduces subsequent creep deformation of the damaged vertebra

How are adjacent spinal levels affected by vertebral fracture, and by vertebroplasty? A biomechanical study on cadaveric spines

Background Context: Spinal injuries and surgery may have important effects at neighbouring spinal levels, but previous investigations of adjacent-level biomechanics have produced conflicting results. We use ‘stress profilometry’ and non-contact strain measurements to investigate thoroughly this long-standing problem. Purpose: To determine how vertebral fracture and vertebroplasty affect compressive load-sharing and vertebral deformations at adjacent spinal levels. Study Design: Mechanical experiments on cadaver spines. Methods: 28 cadaveric spine specimens, comprising three thoracolumbar vertebrae and the intervening discs and ligaments, were dissected from 14 spines aged 67-92 yrs. A needle-mounted pressure transducer was used to measure the distribution of compressive stress across the antero-posterior diameter of both intervertebral discs. ‘Stress profiles’ were analysed to quantify intradiscal pressure (IDP), and concentrations of compressive stress in the anterior and posterior annulus. Summation of stresses over discrete areas yielded the compressive force acting on the anterior and posterior halves of each vertebral body, and the compressive force resisted by the neural arch. Creep deformations of fractured and adjacent vertebral bodies under load were measured using an optical MacReflex system. All measurements were repeated following compressive injury to one of the three vertebrae, and again after the injury had been treated by vertebroplasty. The study was funded by a grant from Action Medical Research, UK ($143,230). Authors of this study have no conflicts of interest to disclose. Results: Injury usually involved endplate fracture, often combined with deformation of the anterior cortex, so that the affected vertebral body developed slight anterior wedging. Injury reduced IDP at the affected level, to an average 47% of pre-fracture values (P<0.001), and transferred compressive load-bearing from nucleus to annulus, and also from disc to neural arch. Similar but reduced effects were seen at adjacent (non-fractured) levels, where mean IDP was reduced to 73% of baseline values (P<0.001). Vertebroplasty partially reversed these changes, increasing mean IDP to 76% and 81% of baseline values at fractured and adjacent levels, respectively. Injury also increased creep deformation of the vertebral body under load, especially in the anterior region where a 14-fold increase was observed at the fractured level and a three-fold increase at the adjacent level. Vertebroplasty reversed these changes also, reducing deformation of the anterior vertebral body (compared to post-fracture values) by 62% at the fractured level, and by 52% at the adjacent level. Conclusions: Vertebral fracture adversely affects compressive load-sharing and increases vertebral deformations at both fractured and adjacent levels. All effects can be partially reversed by vertebroplasty

Vertebroplasty: Only small cement volumes are required to normalize stress distributions on the vertebral bodies

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 (SPP) anulus, and the percentage of the applied compressive force resisted by the neural arch (FN). 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 SPP increased by 107– 362%. VP1 restored IDP and SPP to prefracture values, and VP2 produced no further changes. Fracture increased FN from 11% to 39% in flexion, and from 33% to 59% in extension. FN 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