Interbody fusion aims to treat painful disc disease by demobilising the spinal segment through the use of an interbody fusion device (IFD). Diminished contact area at the endplate interface raises the risk of device subsidence, particularly in osteoporosis patients. The aim of the study was to ascertain whether vertebral body (VB) cement augmentation would reduce IFD subsidence following dynamic loading. Twenty-four human two-vertebra motion segments (T6–T11) were implanted with an IFD and distributed into three groups; a control with no cement augmentation; a second with PMMA augmentation; and a third group with calcium phosphate (CP) cement augmentation. Dynamic cyclic compression was applied at 1Hz for 24 hours in a specimen specific manner. Subsidence magnitude was calculated from pre and post-test micro-CT scans. The inferior VB analysis showed significantly increased subsidence in the control group (5.0±3.7mm) over both PMMA (1.6±1.5mm, p=.034) and CP (1.0±1.1mm, p=.010) cohorts. Subsidence in the superior VB to the index level showed no significant differences (control 1.6±3.0mm, PMMA 2.1±1.5mm, CP 2.2±1.2mm, p=.811). In the control group, the majority of subsidence occurred in the lower VB with the upper VB displaying little or no subsidence, which reflects the weaker nature of the superior endplate. Subsidence was significantly reduced in the lower VB when both levels were reinforced regardless of cement type. Both PMMA and CP cement augmentation significantly affected IFD subsidence by increasing VB strength within the motion segment, indicating that this may be a useful method for widening indications for surgical interventions in osteoporotic patients.

INTRODUCTION

Over 85% of patients with multiple myeloma (MM) have bone disease, mostly affecting thoraco-lumbar vertebrae. Vertebral fractures can lead to pain and large spinal deformities requiring application of vertebroplasty (PVP). PVP could be enhanced by use of Coblation technique to remove lesions from compromised MM vertebrae prior to cement injection (C-PVP).

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: Vertebral bodies and intervertebral discs resist most of the compressive force acting on the spine. However, experiments on lumbar spines have shown that apophyseal joints can resist more than 50% of applied compression, and that the proportion varies with spinal level, disc narrowing, and posture. In the cervical spine, the situation is likely to be complicated by the presence of uncovertebral joints on the lateral margins of the disc. Load-sharing is important because it influences injury risk, and predisposition to degenerative changes. The present study aims to characterise compressive load-sharing in the cervical spine.

Methods: Sixteen cervical motion segments, consisting of two vertebrae and the intervening disc and ligaments, were dissected from nine cadaveric spines, aged 48-77 yrs (mean 63 yrs) which had been stored at -17degC. Specimens were subjected to 200N of compression while the distribution of compressive ‘stress’ was measured along the mid-sagittal diameter of the disc, using a pressure transducer side-mounted in a 0.9mm-diameter needle. ‘Stress profiles’ effectively were integrated over area to calculate the total compressive force acting on the disc. Experiments were performed with each specimen in flexion, extension and neutral posture. They were repeated after creep compressive loading (2 hrs at 150N) to simulate diurnal loading in life, and again following removal of the apophyseal joints. Eight specimens were re-tested following bi-lateral removal of the uncovertebral joints.

Results: Creep loading reduced disc height by an average 0.64mm (approximately 12%). Creep reduced overall computed disc loading by 14% and 25% in neutral and extended postures respectively (P< 0.005). Apophyseal joint removal increased disc loading in extension (only) by 14% (P< 0.05). Uncovertebral joint removal further increased disc loading in flexed, neutral and extended postures by 28%, 33% and 21% respectively (P< 0.05).

Conclusion: Creep loading of the cervical spine transfers loading to the apophyseal joints and uncus. The former effect is small, and significant only in extended postures. The latter effect is larger, and is greatest in flexed and neutral postures.

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: There are extensive differences in structure and composition between cervical and thoracolumbar discs, yet practically nothing is known about the time-dependent “creep” behaviour of cervical discs.

Methods: 41 cadaveric cervical motion segments aged 48–89 yrs were subjected to a static compressive load of 150N for 2 hrs. Specimen height was recorded by the displacement of the actuator of the testing machine. Digitized radiographs were analysed to obtain dimensions of the vertebrae and discs. A three-parameter solid viscoelastic model was fitted to experimental data using nonlinear regression. Model parameters represent compressive stiffness of the wet tissue (E₂) and of the drained solid matrix (E₁), and tissue viscosity (η₁).

Results:Model and experimental data were in good agreement (r²> 0.98) and the average absolute error was always < 2%. E₁ was 11% and 39% lower than published values for thoracic and lumbar discs, respectively, whereas E₂ was 43% and 53% higher. The ratio E₂/E₁ for cervical discs (1.63) was greater than for thoracic (1.01) and lumbar (0.66) discs. η₁ for cervical discs was 108% and 21% higher than in thoracic and lumbar discs, resulting in a creep rate (E₁/η₁) which was lower by 51% and 43% respectively. Comparisons between younger (mean age 58 yrs) and older (79 yrs) cervical discs showed that in the latter, η₁ was reduced by 32% (p=0.01), E2 reduced by 18% (p=0.06), whereas E₁/η₁ was increased by 47% (p=0.02).

Discussion: Cervical discs appear to resist water loss more than thoracolumbar discs, but this resistance falls in old age.

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.

Introduction: The cervical spine can be severely loaded in bending during sporting injuries and ‘whiplash’. Compressive loading could also be high if some advanced warning of impact stimulated vigorous (‘protective’) contraction of the neck muscles. Combined bending and compression can cause some lumbar discs to herniate in-vitro (1) but the outcome depends on spinal level, and may not be applicable to cervical discs. We test the hypotheses: a) that cervical discs can prolapse in-vitro, and b) that prolapse leads to irregular stress distributions inside the disc.

Material and methods: Human cervical ‘motion segments’ (two vertebrae and intervening soft tissues) were obtained from cadavers aged 51–88yrs. Specimens were secured in cups of dental stone and subjected to static compressive loading (150N) for 20s. During this time, the distribution of vertically-acting compressive ‘stress’ was recorded along the postero-anterior diameter of the disc by pulling a 0.9mm-diameter pressure transducer through it (2). Injury was induced by compressing each specimen at 1mm/s while positioned in 20 deg of flexion, 15 deg of extension, or 8 deg of lateral bending. The distribution of compressive stress within the disc was then re-measured. Specimens were sectioned at 2mm intervals in order to ascertain soft tissue disruption.

Results: In all six specimens tested to date, one or both of the apophyseal joint capsules were ruptured by the complex loading. Intervertebral disc prolapse also occurred in all six specimens, with the herniated nucleus appearing on the anterior, posterior and postero-lateral disc surface in extension, flexion and lateral bending respectively. All modes of failure affected intradiscal stresses: on average, nucleus pressure decreased by 75% (STD 7%), while stress concentrations in the annulus increased by 130% (STD 21%).

Discussion: These preliminary results confirm that severe complex loading can cause cervical discs to prolapse. No particular state of disc degeneration is required, provided the loading is sufficiently severe. Indeed, the altered stress distributions suggest that cell-mediated changes would probably follow prolapse.

Introduction: ‘Stress profilometry’ involves pulling a pressure transducer through a loaded intervertebral discs in order to characterise the intensity of loading within it. The technique has been used to explore how stress distributions vary with age, spinal level, degeneration, creep loading, and injury. However, can the output of the strain-gauged transducer (which is calibrated in a fluid) really quantify stress perpendicular to its membrane when inserted into the fibrous matrix of degenerated discs?

Methods: Thirteen full-depth cylinders, 7mm in diameter, were cut from inner, middle and outer regions of the anterior and lateral annulus of two human upper-lumbar discs aged 74 and 82 yrs. Specimens were confined within a metal cylinder of internal diameter 7 mm. Two vertical slots on opposite sides of the metal cylinder allowed a pressure transducer, side-mounted near the tip of a 0.9 mm-diameter needle, to be pulled through the annulus sample. Constant compressive loading was applied for 20s to the top of the annulus sample, using a plane-ended 6.9 mm-diameter indenter, while the transducer was pulled through the sample. Transducer output was sampled at 25Hz. ‘Stress profiles’ were repeated with the transducer orientated vertically and horizontally, and with 6-21 values of compressive load, corresponding to stresses up to 3MPa. Average values of measured ‘stress’ were compared to applied stress (compressive force/indenter area).

Results: Measured (average) vertical compressive stress was linearly related to applied stress, with Rsq values averaging 0.97. The gradient of the line averaged 0.98 (range 0.77 – 1.28) indicating that measured stress values approximated to applied stress, and were not merely proportional to it. For horizontal measurements, the Rsq and gradient averaged 0.97 and 0.92 respectively. Abnormal results in 3/13 specimens appeared to be affected by transducer damage and were disregarded.

Conclusion: Stress profilometry can quantify compressive stress within the annulus of degenerated intervertebral discs. This fibrous tissue appears to be sufficiently deformable to allow efficient coupling of stress between the matrix and transducer membrane.

Introduction: Little is known about how the cervical spine resists the high complex loading to which it is often subjected in life. In this study, such loading was applied to cadaveric cervical motion segments in order to a) measure their strength in forward and backwards bending, b) indicate which structures resist bending most strongly, and c) indicate how compressive injury influences the bending properties.

Methods: Ten human cervical spines aged 65–88yrs were obtained post-mortem, dissected into 14 motion segments, and stored at −20°C. Subsequently, motion segments were defrosted and secured in dental plaster for testing on a hydraulic materials testing machine. An optical motion capture system recorded specimen movement simultaneously. Specimens were loaded in 2.5sec in combined bending and compression to reach their elastic limit in flexion, and then extension. Experiments were repeated following creep loading, removal of spinous processes, removal of apophyseal joints, and vertebral body compressive damage.

Results: On average, full flexion was reached at an angle of 7.2° and a bending moment of 6.8Nm. Full extension occurred at 9.2° and 9.0Nm. Creep loading reduced specimen height by 0.37mm, increased flexion by 1.5° (P< 0.01) but had little effect on extension. After creep, resistance to flexion came from the spinous processes and related ligaments (46%), apophyseal joints (30%), and disc (24%). Resistance to extension came from spinous processes (23%), apophyseal joints (45%), and disc (32%). The compressive strength of discvertebral body specimens was 1.87kN (STD 0.63kN). Compressive damage reduced specimen height by 0.83mm (STD 0.29mm). This reduced the disc’s resistance to flexion by 44% and extension by 18%.

Conclusion: Cervical motion segments have approximately 20% of the bending strength, and 45% of the compressive strength, of lumbar specimens of similar age. The relative weakness of the cervical spine in bending may influence the patterns of injury seen in “whiplash”.