An ex vivo biomechanical comparison of a novel vertebral compression fracture treatment system to kyphoplasty.

BACKGROUND: Vertebral compression fracture repair aims to relieve pain and improve function by restoring vertebral structure and biomechanics, but is still associated with risks arising from polymethylmethacrylate cement extravasation. The Kiva® Vertebral Compression Fracture Treatment System, a stacked coil implant made of polyetheretherketone and delivered over a guide-wire, is a novel device designed to provide height restoration and mechanical stabilization, while improving cement containment and minimizing disruption of cancellous bone. The objective of this study was to determine whether the Kiva system is as effective as balloon kyphoplasty at restoring mechanical properties in osteoporotic vertebral compression fractures. METHODS: Wedge fractures were created in the middle vertebra of fourteen osteoporotic three-vertebra spine segments and then repaired with either the Kiva or kyphoplasty procedure. Height, stiffness and displacement under compression of the spine segments were measured for four conditions: intact, fractured, augmented, and post-cyclic eccentric loading (50,000cycles, 200-500N, 30mm anterior lever arm). FINDINGS: No significant differences were seen between the two procedures for height restoration, stiffness at high or low loads, or displacement under compression. However, the Kiva System required an average of 66% less cement than kyphoplasty to achieve these outcomes (mean 2.6 (SD 0.4) mL v. mean 7.5 (SD 0.8) mL 0; P<0.01). Extravasations and excessive posterior cement flow were also significantly lower with Kiva (0/7 v. 4/7; P<.05). INTERPRETATION: Kiva exhibits similar biomechanical performance to balloon kyphoplasty, but may reduce the risk of extravasation through the containment mechanism of the implant design and by reducing cement volume.

The effect of dynamic posterior stabilization on facet joint contact forces: an in vitro investigation.

STUDY DESIGN: Facet contact forces in the lumbar spine were measured during flexibility tests using thin film electroresistive sensors in intact cadaveric spine specimens and in injured specimens stabilized with a dynamic posterior system. OBJECTIVE: The purpose of this study was to investigate the effect of the Dynesys system on the loading in the facet joints. SUMMARY OF BACKGROUND DATA: The Dynesys, a posterior nonfusion device, aims to preserve intersegmental kinematics and reduce facet loads. Recent biomechanical evidence showed that overall motion is less with the Dynesys than in the intact spine, but no studies have shown its effect on facet loads. METHODS: Ten human cadaveric lumbar spine specimens (L2-L5) were tested by applying a pure moment of +/-7.5 N m in 3 directions of loading with and without a follower preload of 600 N. Test conditions included an intact specimen and an injured specimen stabilized with 3 Dynesys spacer lengths. Bilateral facet contact forces were measured during flexibility tests using thin film electroresistive sensors (Tekscan 6900). RESULTS: Implanting the Dynesys significantly increased peak facet contact forces in flexion (from 3 N to 22 N per side) and lateral bending (from 14 N to 24 N per side), but had no significant effect on the magnitude of the peak forces in extension and axial rotation. Peak facet loads were significantly lower with the long spacer compared with the short spacer in flexion and lateral bending. CONCLUSION: Implantation of the Dynesys did not affect peak facet contact forces in extension or axial rotation compared with an intact specimen, but did alter these loads in flexion and lateral bending. The spacer length affected the compression of the posterior elements, with a shorter spacer typically producing greater facets loads than a longer one.

Ex vivo measurement of lumbar intervertebral disc pressure using fibre-Bragg gratings.

Methods were developed to measure intervertebral disc pressure using optical fibre-Bragg gratings (FBGs). The FBG sensor was calibrated for hydrostatic pressure in a purpose-built apparatus and the average sensitivity was determined to be -5.7 +/- 0.085 pm/MPa (mean +/- SD). The average coefficient of determination (r(2)) for the calibration data was 0.99, and the average hysteresis of the sensor was 2.13% of full scale. The FBG was used to measure intradiscal pressure response to compressive load in five lumbar functional spine units. The pressure measured by the FBG sensor varied linearly with applied compressive load with coefficients of determination ranging from 0.84 to 0.97. The FBG sensor's sensitivity to compressive load ranged from 0.702 +/- 0.043 kPa/N (mean +/- SD) in a L1-L2 specimen, to 1.07 +/- 0.069 kPa/N in a L4-L5 specimen. These measurements agree with those of previous studies in lumbar spines. Two strain gauge pressure sensors were also used to measure intradiscal pressure response to compressive load. The measured pressure sensitivity to load ranged from 0.251 kPa/N (L4-L5) to 0.850 kPa/N (L2-L3). The average difference in pressure sensitivity to load between Sensors 1 and 2 was 12.9% of the value for Sensor 1, with a range from 1.1% to 20.4%, which suggests that disc pressure was not purely hydrostatic. This may have contributed to the difference between the responses of the FBG and strain gauge sensors.

Accuracy of cut-off acetabular reamers for minimally invasive THA.

Cut-off reamers have been introduced for minimally invasive hip replacement to make reamer insertion through the small incision easier. However, the accuracy of cut-off reamers in comparison to traditional hemispherical reamers has not been documented. We reamed four human cadaveric hips using a cut-off reamer and three hips using a standard reamer. We started with smallest size reamer to remove subchondral bone, and the size was progressively increased until breaching the acetabular floor. We performed computed tomography scans for each reamer size to digitally determine the true dimensions and sphericity of the reamed acetabula. The cut-off reamers breached the acetabulum at a smaller size than with a standard reamer in two specimens, and at the same size as the standard reamer in one specimen. The accuracy of each reamer size was determined by quantifying the percentage of the reamed acetabular surface that was within 0.5 mm of the hemispherical reamer size. The average accuracy of the cut-off reamers was 70% compared with 81% for the standard reamers. The cut-off acetabular reamers showed a trend toward decreased accuracy that may be attributable to a tendency of the reamer to wobble in use.

Biomechanical characterization of the three-dimensional kinematic behaviour of the Dynesys dynamic stabilization system: an in vitro study.

The Dynesys, a flexible posterior stabilization system that provides an alternative to fusion, is designed to preserve intersegmental kinematics and alleviate loading at the facet joints. Recent biomechanical evidence suggests that the overall range of motion (ROM) with the Dynesys is less than the intact spine. The purpose of this investigation was to conduct a comprehensive characterization of the three-dimensional kinematic behaviour of the Dynesys and determine if the length of the Dynesys polymer spacer contributes to differences in the kinematic behaviour at the implanted level. Ten cadaveric lumbar spine segments (L2-L5) were tested by applying a pure moment of +/-7.5 Nm in flexion-extension, lateral bending, and axial rotation, with and without a follower preload of 600 N. Test conditions included: (a) intact; (b) injury; (c) injury stabilized with Dynesys at L3-L4 (standard spacer); (d) long spacer (+2 mm); and (e) short spacer (-2 mm). Intervertebral rotations were measured using an optoelectronic camera system. The intersegmental range of motion (ROM), neutral zone (NZ), and three-dimensional helical axis of motion (HAM) were calculated. Statistical significance of changes in ROM, NZ, and HAM was determined using repeated measures analysis of variance (ANOVA) and Student-Newman-Keuls post-hoc analysis with P<0.05. Implantation of the standard length Dynesys significantly reduced ROM compared to the intact and injured specimens, with the least significant changes seen in axial rotation. Injury typically increased the NZ, but implantation of the Dynesys restored the NZ to a magnitude less that that of the intact spine. The Dynesys produced a significant posterior shift in the HAM in flexion-extension and axial rotation. The spacer length had a significant effect on ROM with the long spacer resulting in the largest ROM in all loading directions without a follower preload. The largest differences were in axial rotation. A 4 mm increase in spacer length led to an average intersegmental motion increase of 30% in axial rotation, 23% in extension, 14% in flexion, and 11% in lateral bending. There were no significant changes in NZ with different spacer lengths. Typically, the short spacer caused a greater shift and a greater change in orientation of the HAM than the long spacer. The long spacer resulted in a ROM and a motion pattern, as represented by the HAM, that was closer to that seen in an intact specimen. The results of this study suggest that the length of the Dynesys spacer altered the segmental position and therefore affected kinematic behaviour.

Accuracy and repeatability of a new method for measuring facet loads in the lumbar spine.

We assessed the repeatability and accuracy of a relatively new, resistance-based sensor (Tekscan 6900) for measuring lumbar spine facet loads, pressures, and contact areas in cadaver specimens. Repeatability of measurements in the natural facet joint was determined for five trials of four specimens loaded in pure moment (+/- 7.5 N m) flexibility tests in axial rotation and flexion-extension. Accuracy of load measurements in four joints was assessed by applying known compressive loads of 25, 50, and 100 N to the natural facet joint in a materials testing machine and comparing the known applied load to the measured load. Measurements of load were obtained using two different calibration approaches: linear and two-point calibrations. Repeatability for force, pressure, and area (average of standard deviation as a percentage of the mean for all trials over all specimens) was 4-6% for axial rotation and 7-10% for extension. Peak resultant force in axial rotation was 30% smaller when calculated using the linear calibration method. The Tekscan sensor overestimated the applied force by 18 +/- 9% (mean+/-standard deviation), 35 +/- 7% and 50 +/- 9% for compressive loads of 100, 50, and 25 N, respectively. The two-point method overestimated the loads by 35 +/- 16%, 45 +/- 7%, and 56 +/- 10% for the same three loads. Our results show that the Tekscan sensor is repeatable. However, the sensor measurement range is not optimal for the small loads transmitted by the facets and measurement accuracy is highly dependent on calibration protocol.