Can extra-articular strains be used to measure facet contact forces in the lumbar spine? An in-vitro biomechanical study

Experimental measurement of the load-bearing patterns of the facet joints in the lumbar spine remains a challenge, thereby limiting the assessment of facet joint function under various surgical conditions and the validation of computational models. The extra-articular strain (EAS) technique, a non-invasive measurement of the contact load, has been used for unilateral facet joints but does not incorporate strain coupling, i.e. ipsilateral EASs due to forces on the contralateral facet joint. The objectives of the present study were to establish a bilateral model for facet contact force measurement using the EAS technique and to determine its effectiveness in measuring these facet joint contact forces during three-dimensional flexibility tests in the lumbar spine. Specific goals were to assess the accuracy and repeatability of the technique and to assess the effect of soft-tissue artefacts.

In the accuracy and repeatability tests, ten uniaxial strain gauges were bonded to the external surface of the inferior facets of L3 of ten fresh lumbar spine specimens. Two pressure-sensitive sensors (Tekscan) were inserted into the joints after the capsules were cut. Facet contact forces were measured with the EAS and Tekscan techniques for each specimen in flexion, extension, axial rotation, and lateral bending under a ±7.5 N m pure moment. Four of the ten specimens were tested five times in axial rotation and extension for repeatability. These same specimens were disarticulated and known forces were applied across the facet joint using a manual probe (direct accuracy) and a materials-testing system (disarticulated accuracy). In soft-tissue artefact tests, a separate set of six lumbar spine specimens was used to document the virtual facet joint contact forces during a flexibility test following removal of the superior facet processes.

Linear strain coupling was observed in all specimens. The average peak facet joint contact forces during flexibility testing was greatest in axial rotation (71±25 N), followed by extension (27±35 N) and lateral bending (25±28 N), and they were most repeatable in axial rotation (coefficient of variation, 5 per cent). The EAS accuracy was about 20 per cent in the direct accuracy assessment and about 30 per cent in the disarticulated accuracy test. The latter was very similar to the Tekscan accuracy in the same test. Virtual facet loads (r.m.s.) were small in axial rotation (12 N) and lateral bending (20 N), but relatively large in flexion (34 N) and extension (35 N).

The results suggested that the bilateral EAS model could be used to determine the facet joint contact forces in axial rotation but may result in considerable error in flexion, extension, and lateral bending.

Bioactive bone cement as a principal fixture for spinal burst fracture: an in vitro biomechanical and morphologic study

STUDY DESIGN

An in vitro biomechanical and radiographic study to evaluate the properties of a newly developed bioactive bone cement for stabilization of the fractured spine, suitable for minimally invasive application.

OBJECTIVES

To determine the mechanical stability of the fractured spine after injection of the newly developed bioactive bone cement under quasi-static and cyclic loading regimens.

SUMMARY OF BACKGROUND DATA

Bone cement injection has been reported as a potentially useful, minimally invasive technique for treating vertebral body fracture or stabilizing osteoporosis. However, potential problems associated with the use of polymethylmethacrylate (PMMA) have prompted the search for alternative solutions. The use of bioactive bone cement as a potential replacement for PMMA has been reported.

METHODS

Biomechanical and radiographic analyses were used to test the mechanical stability of the fractured spine. The cement used was formed from hydroxyapatite powder containing strontium and bisphenol A diglycidylether dimethacrylate (D-GMA) resin. Twenty-six fresh porcine spine specimens (T10-L1) were divided into three groups: pilot, intact, and cemented. Spinal stiffness and failure strength were recorded in the intact group with the specimens flexed at 10 degrees. Uniform injuries were created in all specimens of the cemented group, and compressive loading was applied with 10 degrees of flexion until a fracture occurred. The bone cement was injected into the fractured spine, and stiffness was evaluated after 1 hour. Failure strength was also recorded after 3000 and 20,000 fatigue load cycles. Morphology of the specimens was observed and evaluated.

RESULTS

Results from a cell biocompatibility test indicated that the new bioactive bone cement was favorable for cell growth. Spinal stiffness significantly decreased after fracture (47.5% of intact condition). Instant stiffness of the spine recovered to 107.8% of the intact condition after bone cement injection. After 3000 and 20,000 cycles of fatigue loading, stiffness of the cemented spine was found to be 93.5% and 94.4% of intact stiffness, respectively (P < 0.05). Average failure strength of the spine was 5056 N (after 3000 cycles) and 5301 N (after 20,000 cycles) after bone cement injection and fatigue loading. Radiographs and cross-sectional observations indicated a good cement-bone bonding and fracture fill.

CONCLUSIONS

A new bioactive bone cement without cytotoxic effect has been developed. Results show that minimally invasive techniques to apply this cement to porcine spines results in augmentation of mild burst fractures such that the original stiffness and strength of the vertebra are recovered. This new cement therefore shows potential as an augmentation to traditional instrumentation in the surgical management of vertebral fractures. The potential for further clinical applications is currently under investigation.

The effect of cervical traction combined with rotatory manipulation on cervical nucleus pulposus pressures

OBJECTIVE

To study experimentally the effect of cervical traction combined with rotatory manipulation on the pressure of the cervical nucleus pulposus.

DESIGN

Randomized experimental study.

SETTING

Institute of Clinical Anatomy and Biomechanics, the First Military Medical University, GuangZhou, China.

SUBJECTS

Twelve fresh cervical spines, from C7 to occipital bone, were obtained from cadavers of patients who had died from acute brain death.

INTERVENTION

State A: under different traction forces, the cervical spine was rotated. State B: the cervical spine was rotated first and then tractioned. State C: the cervical spine was tractioned and rotated simultaneously.

MAIN OUTCOME MEASURES

The cervical spine was tractioned and rotated by the Material Test System (MTS), and pressures of the cervical nucleus pulposus of C3-4, C4-5 and C5-6 were measured using pressure sensors.

RESULTS

(a) When the traction force increased, the pressure fell continuously in the 200-N tractioned spines; the pressure increased slightly when the sample was rotated. (b) The pressure fell to a certain extent when the state of cervical spine was restored. (c) In state A, the pressure fell obviously and increased slightly under a 200-N traction force and then the sample was rotated; in state B, the pressure first increased to a certain extent and then fell slightly and in state C, the pressure underwent no change in the main.

CONCLUSIONS

The results of this research suggest that rotatory manipulation of cervical spine under traction was the safest of the three procedures and the traction force used in clinical treatment may be a little smaller.

Effects of total coumarins of Cnidium monnieri on bone density and biomechanics of glucocorticoids-induced osteoporosis in rats

AIM

To evaluate the effects of total coumarins from dried fruits of Cnidium monnieri (TCCM) on glucocorticoids (GC)-induced osteoporosis (OP) in rats.

METHODS

Single photon absorptiometric and biomechanical character measurements of femurs were used.

RESULTS

The bone density (BD) indices in proximal, middle, and distal segments in GC group were decreased by 12% (P < 0.05), 14% (P < 0.05), and 12% (P < 0.05), respectively vs control group. The BD on proximal, middle, and distal segments in GC-TCCM group were increased by 26% (P < 0.01), 34% (P < 0.01), and 31% (P < 0.01), respectively vs GC group. The biomechanical competence in femoral middle segments in GC group tended to decrease vs control group. In GC-TCCM group, the torsional strength, energy, maximal torsional angle, and rigidity were increased by 15% (P < 0.05), 32% (P < 0.05), 14% (P > 0.05), and 13% (P > 0.05), respectively vs the GC group.

CONCLUSION

TCCM not only prevented glucocorticoids-induced osteoporosis but also increased the torsional strength of femurs in rats.

Biomechanical characteristics of human trabecular bone

OBJECTIVE: To evaluate the effect of strain rate (varepsilon), apparent density (rho(a)) and tissue density (rho(t)) on Young's modulus (E), strength (sigma(u)) and ultimate strain (varepsilon(u)) on intervertebral bone from a Chinese population. METHODS: Testing was performed by uniaxial compression at five strain rates on 36 human trabecular bone specimens from three male T(12) thick similar L(4) vertebrae. RESULTS: Apparent density ranged between 0.46 and 0.71 g/cm(3). Tissue density ranged between 1.02 and 1.54 g/cm(3). Non-linear regression analyses using strength, Young's modulus or ultimate strain as dependent variables (Y) and strain rate and apparent density or tissue density as independent variables were performed using the following equation: Y = arho(b)varepsilon(c). The exponent of apparent density and strain rate to Young's modulus were 1.88 and 0.07, separately (P = 0.0007). The variation of strength was explained only by apparent density with an exponent of 1.29 (P = 0.0107). The variation of Young's modulus was explained equally by the quadratic and cubic relationship to apparent density or tissue density (P < 0.01). Ultimate strain varied independently of apparent density or tissue density and strain rate.