Qingan Zhu

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.

Biomechanical evaluation of the Total Facet Arthroplasty System: 3-dimensional kinematics.

Study Design. An in vitro biomechanical study to quantify 3-dimensional kinematics of the lumbar spine following facet arthroplasty. Objectives. To compare the multidirectional flexibility properties and helical axis of motion of the Total Facet Arthroplasty System™ (TFAS™) (Archus Orthopedics, Redmond, WA) to the intact condition and to posterior pedicle screw fixation. Summary of Background Data. Facet arthroplasty in the lumbar spine is a new concept in the field of spinal surgery. The kinematic behavior of any complete facet arthroplasty device in the lumbar spine has not been reported previously. Methods. Flexibility tests were conducted on 13 cadaveric specimens in an intact and injury model, and after stabilization with the TFAS and posterior pedicle screw fixation at the L4–L5 level. A pure moment of ±10 Nm with a compressive follower preload of 600 N was applied to the specimen in flexion-extension, axial rotation, and lateral bending. Range of motion (ROM), neutral zone, and helical axis of motion were calculated for the L4–L5 segment. Results. ROM with the TFAS was 81% of intact in flexion (P = 0.035), 68% in extension (P = 0.079), 88% in lateral bending (P = 0.042), and 128% in axial rotation (P = 0.013). The only significant change in neutral zone with TFAS compared to the intact was an increase in axial rotation (P = 0.011). The only significant difference in helical axis of motion location or orientation between the TFAS and intact condition was an anterior shift of the helical axis of motion in axial rotation (P = 0.013). Conclusions. The TFAS allowed considerable motion in all directions tested, with ROM being less than the intact in flexion and lateral bending, and greater than the intact in axial rotation. The helical axis of motion with the TFAS was not different from intact in flexion-extension and lateral bending, but it was shifted anteriorly in axial rotation. The kinematics of the TFAS were more similar to the intact spine than were the kinematics of the posterior fixation when applied to a destabilized lumbar spine.

Anterior cervical plate fixation: a biomechanical study to evaluate the effects of plate design, endplate preparation, and bone mineral density.

Study Design. A biomechanical study using multidirectional flexibility testing in a human cadaveric cervical spine model of a flexion-distraction injury. Objectives. To compare the immediate postoperative stabilizing effect of dynamic and rigid anterior cervical plates and to assess the confounding effects of bone mineral density (BMD) and endplate preparation technique. Summary of Background Data. Dynamic plate designs presumably increase load sharing between the plate and graft, but their effect on spinal stabilization has not been assessed in a traumatic flexion-distraction model. Methods. Twenty-four fresh frozen human cervical functional spinal units were dual-energy x-ray absorptiometry scanned for bone mineral density and allocated into 4 groups by the type of plate, dynamic (ABC, Aesculap, Germany) versus rigid (Cervical Spine Locking Plate, Synthes USA, Paoli, PA), and the technique of endplate preparation, intact versus removed. Each functional spinal unit had all posterior ligaments transected and both inferior facets excised, after which anterior discectomy, grafting, and plating was performed. Nondestructive testing applied a 1.5 Nm pure moment, whereas ranges of motion and neutral zones were measured in flexion/extension, lateral bending, and rotation. Ratios of the range of motion and neutral zone of the plated to the intact site were analyzed. The load sharing between the plate and the functional spinal unit was measured via strain gauges mounted on the plate. Results. There were no significant differences in the range of motion or neutral zone ratios between the 2 plate designs, except for the range of motion ratio in extension, where the dynamic plate exhibited better stabilization than the rigid plate (P = 0.02). There was a consistent interaction whereby endplate removal resulted in better stabilization for the dynamic plate, but less stabilization for the rigid plate. Significantly less motion was observed with increasing bone mineral density in all loading directions. In flexion and extension, the dynamic plate measured one-third less strain than the rigid plate. Conclusions. The dynamic plate appeared to provide better stabilization in extension, and the technique of endplate preparation has some effect on immediate stabilization, dependent on the type of plate employed. Bone mineral density of the specimen was a strong determinant of the degree of stabilization achieved, regardless of the type of plate used.

Failure Characteristics of the Thoracic Spine with a Posteroanterior Load : Investigating the Safety of Spinal Mobilization

Study Design. In vitro biomechanical study of human cadaveric thoracic spine segments and one intact cadaver and applied load measurements in human volunteers. Objectives. To quantify failure load and pattern of midthoracic vertebrae under a posteroanterior load and to compare failure load in vitro with applied load in vivo. Summary of Background Data. Osteoporosis and back pain are common alone and in combination among older adults. Spinal mobilization techniques have been shown to relieve back pain and improve function in various clinical settings. However, whether controlled spinal mobilization can cause vertebral fracture in individuals with osteoporosis is not known. Methods. Twelve T5–T8 cadaveric specimens (mean age, 77 years) were scanned using bone densitometry, radiographed, and measured for bone size. The authors measured failure load, failure site, and intervertebral motion (using a precision optoelectronic camera system) when a posteroanterior load was applied at the spinous process of T6 using a servohydraulic material testing machine. Post-test radiography and CT scan were used to verify failure site. These tests were repeated in an intact cadaver using a Tekscan I-Scan sensor to measure applied loads. The authors also quantified in vivo applied loads during posteroanterior mobilization during seven trials by two experienced physiotherapists. Results. Mean (SD) in vitro failure load of 479 N (162 N) was significantly higher than the mean (SD) in vivo applied load of 145 N (38 N) (P = 0.0004). Macroscopic observation revealed a fracture at the T6 spinous process in 11 specimens and one at the T7 spinous process. These fractures were detected by plain radiography in three of 12 cases and by CT scan in six of 12 cases. Conclusions. The results suggest a reasonable margin between failure load in vitro and applied mobilization load in vivo.

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

Abstract 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.

Anterior Occiput to Axis Screw Fixation : Part II : A Biomechanical Comparison With Posterior Fixation Techniques

Study Design. This biomechanical study used flexibility testing on fresh-frozen human cadaveric specimens (occiput to C3) and compared the range of motion and neutral zone for three occipitocervical fixation techniques. Objectives. To contrast the stabilization provided by a new technique of anterior occipitocervical screw fixation with two other commonly used posterior occipitocervical fixation techniques. Summary of Background Data. There are no published reports describing this novel technique of anterior occipitocervical screw fixation. Methods. Six human occipitocervical spine specimens were mounted in a custom-designed, spine-testing machine that applied a pure moment in flexion–extension, lateral bending, and axial rotation. The specimens were tested intact, after an odontoid osteotomy with capsular injury, and after each of three fixation methods: posterior wiring, posterior plate fixation with C1–C2 transarticular screws, and finally with anterior occipitocervical screws. Intervertebral motion was measured with an optoelectronic measurement system, and the range of motion and neutral zone were the kinematic variables measured and used for analysis. Results. In flexion and extension testing, the posterior plate with transarticular screws provided greater stabilization than posterior wiring or anterior occipitocervical screws. In lateral bending and rotation, the anterior screws were similarly effective to the posterior plate, both of which were more effective than posterior wiring. Conclusion. The anterior screw fixation technique was as effective as a posterior plate with transarticular screws in stabilizing between the occiput and C2 in axial rotation and lateral bending. In extension and flexion, the anterior screw technique was not as effective as a posterior plate with transarticular screws in providing stability.

The effect of cement augmentation and extension of posterior instrumentation on stabilization and adjacent level effects in the elderly spine.

Study Design. An in vitro cadaveric study comparing different implant fixation techniques using a repeated measures design. Objective. To compare the effects of cement augmentation of pedicle screws and extension of posterior fixation on (i) 3-dimensional stabilization, and (ii) adjacent level effects in the aging spine. Summary of Background Data. Device loosening and adjacent level effects are concerns in implant fixation in the elderly spine. Extension of posterior fixation and cement augmentation of pedicle screws have not been previously compared with respect to stabilization and adjacent level effects. Methods. Twelve T9 to L3 cadaveric specimens were tested in flexion-extension (FE), lateral bending (LB), and axial rotation (AR) with applied pure moments of ±5 Nm. A T11 corpectomy was reconstructed with a vertebral body replacement device and T10 to T12 posterior instrumentation. Further stabilization was provided by posterior rod extension to L1 (flexible or rigid rods) and/or cement augmentation of T12 and L1 screws. The effects of cement augmentation and posterior rod extension on intersegmental motion were compared using the hybrid flexibility-stiffness protocol. Two-way repeated measures ANOVA and SNK post hoc tests (99% significance level) were used. Results. Range of motion at the corpectomy T10 to T12 levels significantly decreased after cement augmentation (AR 43%, LB 71%, FE 68%), and posterior rod extension (rigid rods: AR 26%, LB 64%, FE 57%) (flexible rods: AR 16%, LB 53%, FE 39%). Posterior rod extension significantly reduced range of motion at the rod extension level. Motion at the distal noninstrumented L1 to L2 level was increased significantly by posterior rod extension and cement augmentation. There were however, smaller magnitudes of increase in motion across L1 to L2 level with flexible rod and cement augmentation (AR 12%, LB 45%, FE 31%). Conclusion. Cement augmentation of pedicle screws resulted in the most stable vertebral reconstruction, whereas flexible rod extension minimized changes in range of motion at both adjacent rod extension and distal noninstrumented levels.

Pediatric and adult three-dimensional cervical spine kinematics: effect of age and sex through overall motion

Study Design. Cross-sectional study. Objective. To determine the effect of age and sex on the three-dimensional kinematics of the cervical spine. Summary of Background Data. Spine kinematics information has important implications for biomechanical model development, anthropomorphic test device development, injury prevention, surgical treatment, and safety equipment design. There is a paucity of data of this type available for children, and it is unknown whether cervical spine kinematics of the pediatric population is different than that of adults. The helical axis of motion (HAM) of the spine provides unique information about the quantity and quality (coupling etc.) of the measured motion. Methods. Ninety subjects were recruited and divided into 6 groups based on sex and age (young children aged 4–10 years, older children aged 11–17 years, adults aged 25+ years). Subjects actively moved their head in axial rotation, lateral bending, and flexion/extension. An optoelectronic motion analysis system recorded the position of infrared markers placed on the first thoracic vertebrae (T1) and on tight-fitting headgear worn by the subjects. HAM parameters were calculated for the head motion with respect to T1. Results. HAM location in axial rotation and flexion/extension was more anterior in young females compared to adult females. Young females had a more anterior HAM location in flexion/extension compared to young males, indicating an effect of sex. For females, the HAM locations of adults were superior to those of children in flexion/extension and lateral bending whereas in males the HAM locations of adults were inferior to those of children. Age-related differences in HAM orientation were also observed in axial rotation and lateral bending. Conclusion. Cervical spine kinematics vary with age and sex. The variation in spine mechanics based on age and sex found in the present study may indicate general trends that would grow stronger in even younger children (age <4 years).

Differential Histopathological and Behavioral Outcomes Eight Weeks after Rat Spinal Cord Injury by Contusion, Dislocation, and Distraction Mechanisms

Abstract The objective of this study was to compare the long-term histological and behavioral outcomes after spinal cord injury (SCI) induced by one of three distinct biomechanical mechanisms: dislocation, contusion, and distraction. Thirty male Sprague-Dawley rats were randomized to incur a traumatic cervical SCI by one of these three clinically relevant mechanisms. The injured cervical spines were surgically stabilized, and motor function was assessed for the following 8 weeks. The spinal cords were then harvested for histologic analysis. Quantification of white matter sparing using Luxol fast blue staining revealed that dislocation injury caused the greatest overall loss of white matter, both laterally and along the rostrocaudal axis of the injured cord. Distraction caused enlarged extracellular spaces and structural alteration in the white matter but spared the most myelinated axons overall. Contusion caused the most severe loss of myelinated axons in the dorsal white matter. Immunohistochemistry for the neuronal marker NeuN combined with Fluoro Nissl revealed that the dislocation mechanism resulted in the greatest neuronal cell losses in both the ventral and dorsal horns. After the distraction injury mechanism, animals displayed no recovery of grip strength over time, in contrast to the animals subjected to contusion or dislocation injuries. After the dislocation injury mechanism, animals displayed no improvement in the grooming test, in contrast to the animals subjected to contusion or distraction injuries. These data indicate that different SCI mechanisms result in distinct patterns of histopathology and behavioral recovery. Understanding this heterogeneity may be important for the future development of therapeutic interventions that target specific neuropathology after SCI.

Enhancing pedicle screw fixation in the aging spine with a novel bioactive bone cement: an in vitro biomechanical study.

Study Design. A paired biomechanical study of pedicle screws augmented with bone cement in a human cadaveric and osteoporotic lumbar spine model. Objectives. To evaluate immediate strength and stiffness of pedicle screw fixation augmented with a novel bioactive bone cement in an osteoporotic spine model and compare it with polymethylmethacrylate (PMMA) cement. Summary of Background Data. A novel bioactive bone cement, containing nanoscale particles of strontium and hydroxyapatite (Sr-HA), can promote new bone formation and osteointegration and provides a promising reinforcement to the osteoporotic spine. Its immediate mechanical performance in augmenting pedicle screw fixation has not been evaluated. Methods. Two pedicle screws augmented with Sr-HA and PMMA cement were applied to each of 10 isolated cadaveric L3 vertebrae. Each screw was subjected to a toggling test and screw kinematics were calculated. The pedicle screw was subjected to a pullout test until failure. Finally, the screw coverage with cement was measured on computed tomographic images. Results. Screw translations in the toggling test were consistently larger in the Sr-HA group than in the PMMA group (1.4 ± 1.2 mm vs. 1.0 ± 1.1 mm at 1000 cycles). The rotation center was located closer to the screw tip in the Sr-HA group (19% of screw length) than in the PMMA group (37%). The only kinematic difference between Sr-HA and PMMA cements was the screw rotation at 1000 cycles (1.5° ± 0.9° vs. 1.3° ± 0.6°; P = 0.0026). All motion parameters increased significantly with more loading cycles. The pullout force was higher in the PMMA group than the Sr-HA group (1.40 ± 0.63 kN vs. 0.93 ± 0.70 kN), and this difference was marginally significant (P = 0.051). Sr-HA cement covered more of the screw length than PMMA cement (79 ± 19% vs. 43 ± 19%) (P = 0.036). Conclusion. This paired-design study identified some subtle but mostly nonsignificant differences in immediate biomechanical fixation of pedicle screws augmented with the Sr-HA cement compared with the PMMA cement.