Introduction: Vertebral body osteophytes are common in elderly spines, but their mechanical function is unclear. Do they act primarily to reduce compressive stress on the vertebral body, or to stabilise the spine in bending? How do they influence estimates of vertebral strength based on bone mineral density (BMD)?

Methods: Spines were obtained from cadavers aged 51–92 yrs (mean 77 yrs) with radiographic evidence of vertebral osteophytes (mostly antero-lateral). Twenty motion segments, from T5-T6 to L3–L4, were dissected and loaded a) in compression to 1.5 kN, and b) in bending to 10–25 Nm. Vertebral movements were tracked at 50 Hz using an optical MacReflex system. Bending tests were performed in random order, in flexion, extension, and lateral bending. Resistance to bending and compression was measured before and after surgical excision of all osteophytes. The bone mineral content (BMC) and density (BMD) of each vertebra was measured in the antero-posterior direction, using DXA. Density measurements were repeated after excision of all osteophytes. ANOVA was used to detect changes after osteophyte excision, and regression was used to examine the influence of osteophyte size and BMC.

Results: Removal of osteophytes reduced-vertebral BMD by 9% (SD 13%). Compressive stiffness was affected rather more, being reduced by an average 17% (p< 0.05). Bending stiffness was reduced in flexion and extension by 50% and 39% respectively (p< 0.01), and in left and right lateral bending by 41% and 49% respectively (p< 0.01). Osteophyte removal increased the neutral zone and range of motion in each mode of bending. Most mechanical changes were proportional to osteophyte mass, and to changes in BMC (p< 0.01).

Conclusions: Vertebral body osteophytes primarily stabilise the spine in bending, and do not play a major role in resisting compression. Animal models show that osteophytes grow in response to experimentally-induced instability, so their formation can be seen as mechanically-adaptive (restoring stability) rather than degenerative. The influence of typical osteophytes on compressive stiffness is greater than their influence on vertebral BMD (17% vs 9%) so predictions of vertebral compressive strength based on BMD measurements are likely to be under-estimates if osteophytes are present.

Introduction: Vertebral body osteophytes are common in elderly spines, but their mechanical function is unclear. Do they act primarily to reduce compressive stress on the vertebral body, or to stabilise the spine in bending?

Methods: Spines were obtained from cadavers aged 51–92yrs (mean 77yrs) with radiographic evidence of vertebral osteophytes (mostly antero-lateral). Twenty motion segments, from T5-T6 to L3-L4, were dissected and loaded a) in compression to 1.5kN, and b) in bending to 10–25Nm. Vertebral movements were tracked at 50Hz using an optical MacReflex system. Bending tests were performed in random order, in flexion, extension, and lateral bending. Resistance to bending and compression was measured before and after surgical excision of all osteophytes. Bone mineral content (BMC) of osteophytes was measured using DXA. ANOVA was used to detect changes after osteophyte excision, and regression was used to examine the influence of osteophyte size.

Results: Compressive stiffness was reduced by an average 17% following osteophyte removal (p< 0.05). In flexion and extension, bending stiffness was reduced by 60% and 79% respectively (p< 0.01). In left and right lateral bending, stiffness was reduced by 42% and 49% respectively. Osteophyte removal increased the neutral zone and range of motion in each mode of bending, and changes were proportional to osteophyte mass and BMC (p< 0.01).

Conclusion: Vertebral body osteophytes primarily stabilise the spine in bending, and do little to resist compression, despite their considerable BMC. Predictions of vertebral compressive strength based on BMC measurements are likely to be over-estimates if large osteophytes are present.

Introduction: Kyphoplasty is a modification of the basic vertebroplasty technique used to treat osteoporotic vertebral fracture. This study evaluated whether kyphoplasty conferred any short-term mechanical advantage when compared with vertebroplasty.

Methods: Pairs of thoracolumbar “motion segments” were harvested from nine spines (42–84 yrs). Specimens were compressed to failure in moderate flexion to induce vertebral fracture. One of each pair underwent vertebroplasty, the other kyphoplasty. Specimens were then creep loaded at 1.0kN for 2 hours to allow consolidation. At each stage of the experiment, motion segment stiffness in bending and compression was determined, and the distribution of compressive “stress” was measured in flexed and extended postures by pulling a pressure- sensitive needle through the mid-sagittal diameter of the disc whilst under 1.5kN load. Stress profiles indicated the intradiscal pressure (IDP), stress peaks in the posterior annulus (SPP), and neural arch compressive load-bearing (FN).

Results: Vertebral fracture reduced bending and compressive stiffness by 37% and 55% respectively (p< 0.0001), and IDP by 55%–83%, depending upon posture (p< 0.001). SPP increased from 0.188 to 1.864 MPa in flexion, and from 1.139 to 3.079 MPa in extension (p< 0.05). FN increased from 13% to 37% of the applied load in flexion, and from 29% to 54% in extension (p< 0.001). Vertebroplasty and kyphoplasty partially reversed these changes, and their immediate mechanical effects were mostly sustained after creep-loading. No differences were found between vertebroplasty and kyphoplasty.

Conclusion: Kyphoplasty and vertebroplasty are equally effective in reversing fracture-induced changes in motion segment mechanics. In the short-term, there is no mechanical advantage associated with kyphoplasty.

Purpose of the study: To determine if cement type, bone mineral density (BMD), disc degeneration and fracture severity influence the restoration of spinal load-sharing following vertebroplasty.

Methods: Fifteen pairs of thoracolumbar motion-segments (51–91 yrs) were loaded to induce fracture. Vertebroplasty was performed so that one of each pair was injected with Cortoss, the other with Spineplex. Specimens were then creep loaded at 1.0kN for 2 hours. At each stage of the experiment, stress” profiles were obtained by pulling a pressure-sensitive needle through the disc whilst under 1.5kN load. From these profiles, the intradiscal pressure (IDP), posterior stress peaks (SP_P), and neural arch compressive load (F_N) were determined. BMD was measured using dual photon X-ray absorptiometry. Severity of fracture was quantified from height loss.

Results: Fracture reduced IDP (p< 0.001) but increased SP_P and F_N (p< 0.001). Following vertebroplasty, these effects were significantly reversed, and in most cases persisted after creep-loading. However, no differences were observed between PMMA- and Cortoss-injected specimens. After fracture, decreases in IDP, and increases in SP_P and F_N, were greater in specimens with lower BMD or greater height loss (p< 0.05). After vertebroplasty, specimens with lower BMD showed greater increases in IDP, and those with more degenerated discs showed greater reductions in SP_P (p< 0.05).

Conclusions: Changes in spinal load-sharing following fracture were partially restored by vertebroplasty, and this effect was independent of cement type. The effects of fracture and vertebroplasty were influenced by BMD, disc degeneration, and fracture severity. People with more severe fractures, low BMD and degenerated discs may gain most mechanical benefit from vertebroplasty.

Introduction: We have shown that vertebroplasty increases stiffness and partly restores normal load-sharing in the human spine following vertebral fracture. The present study investigated how this restorative action is influenced by type of cement injected, bone mineral density (BMD), and fracture severity.

Methods: Fifteen pairs of thoracolumbar motion-segments (51–91 yrs) were loaded on a hydraulic materials testing machine to induce vertebral fracture. One from each pair underwent vertebroplasty with polymethyl-methacrylate (PMMA) cement, the other with a biologically- active resin (Cortoss). Specimens were then creep loaded at 1.0kN for 2 hours. At each stage of the experiment, bending and compressive stiffness were measured, and ‘stress’ profiles were obtained by pulling a pressure-sensitive needle through the disc whilst under 1.5kN load. Profiles indicated the intradiscal pressure (IDP) and neural arch compressive load (FN). BMD was measured using dual photon X-ray absorptiometry. Severity of fracture was quantified from height loss. Changes were compared using repeated measures ANOVA.

Results: Fracture reduced bending and compressive stiffness by 31% and 41% respectively (p< 0.0001), and IDP by 43%–62%, depending upon posture (p< 0.001). In contrast, FN increased from 14% to 37% of the applied load in flexion, and from 39% to 61% in extension (p< 0.001). Following vertebroplasty, these effects were significantly reversed, and in most cases persisted after creep-loading. No differences were observed between PMMA- and Cortoss-injected specimens. The decrease in IDP and increase in FN after fracture were correlated with BMD in flexion and with height loss in extension (p< 0.01). After vertebroplasty, restoration of IDP and FN in flexion were correlated with their loss after fracture (p< 0.01). The former was also related to BMD (p< 0.05).

Conclusions: Changes in spinal load-sharing following fracture were partially restored by vertebroplasty, and this effect was independent of cement type. The effects of fracture and vertebroplasty on spinal load-sharing were influenced by severity of fracture, and by BMD.

These findings suggest that people with more severe fractures and low BMD may gain most mechanical benefit from vertebroplasty.

Introduction: To evaluate whether a biologically-active cement “Cortoss” confers any short-term mechanical advantages when compared with a polymethylmethacrylate bone cement “Spineplex” which is currently in widespread use.

Methods: Two thoracolumbar motion segments were harvested from each of six spines (51 – 82 yrs). Specimens were compressed to failure in moderate flexion to induce vertebral fracture. Pairs of specimens were randomly assigned to undergo vertebroplasty with either Cortoss or Spineplex. Compressive stiffness and compressive stress on the disc were measured before and after fracture, and after vertebroplasty. Compressive stress was measured by pulling a pressure- sensitive needle through the mid-sagittal diameter of the disc whilst under 1.5kN load. Intradiscal pressure (IDP), peak stress in the annulus and neural arch compressive load were obtained from the resulting stress profiles.

Results: No differences in IDP, annulus stress, neural arch load bearing and compressive stiffness were observed between the groups before fracture, after fracture or after vertebroplasty (p> 0.05). After fracture, IDP decreased from 1.02 to 0.68 MPa in flexion and from 0.75 to 0.34 MPa in extension (p< 0.05), neural arch load bearing increased from 13% to 37% of the applied load in flexion (p< 0.05), and compressive stiffness decreased from 2441 to 1478 N/mm (p< 0.05). After vertebroplasty, these changes were largely reversed: IDP increased to 0.45 MPa in extension (p< 0.05), neural arch load bearing fell to 20% in flexion (p=0.1), and compressive stiffness increased to 1799 N/mm (p< 0.05).

Conclusion: Vertebroplasty using either Cortoss or Spineplex was equally effective in reversing fracture-induced changes in motion segment mechanics.