Stability analysis of an enhanced load sharing posterior fixation device and its equivalent conventional device in a calf spine model

Lumbar dynamic pedicle-based stabilization versus fusion in degenerative disease: a multicenter, double-blind, prospective, randomized controlled trial

OBJECTIVE

Fusion is the standard of treatment for degenerative lumbar symptomatic instabilities. Dynamic stabilization is a potential alternative, with the aim of reducing pathological motion. Potential advantages are a reduction of surgical complexity and morbidity. The aim of this study was to assess whether dynamic stabilization is associated with a higher degree of functional improvement while reducing surgical complexity and thereby surgical duration and perioperative complications in comparison with lumbar fusion.

METHODS

This was a multicenter, double-blind, prospective, randomized, 2-arm superiority trial. Patients with symptomatic mono- or bisegmental lumbar degenerative disease with or without stenosis and instability were randomized 1:1 to instrumented fusion or pedicle-based dynamic stabilization. Patients underwent either rigid internal fixation and interbody fusion or pedicle-based dynamic stabilization. The primary endpoint was the Oswestry Disability Index (ODI) score, and secondary endpoints were pain, health-related quality of life, and patient satisfaction at 24 months.

RESULTS

Of 293 patients randomized to fusion or dynamic stabilization, 269 were available for analysis. The duration of surgery was significantly shorter for dynamic stabilization versus fusion, and the blood loss was significantly less for dynamic stabilization (380 ml vs 506 ml). Assessment of primary and secondary outcome parameters revealed no significant differences between groups. There were no differences in the incidence of adverse events.

CONCLUSIONS

Dynamic pedicle-based stabilization can achieve similar clinical outcome as fusion in the treatment of lumbar degenerative instabilities. Secondary failures are not different between groups. However, dynamic stabilization is less complex than fusion and is a feasible alternative.

A influência da incompatibilidade do macho de rosca na resistência à extração do parafuso pedicular

Objective  We aimed to study the “in vitro” pullout strength of SpineGuard/Zavation Dynamic Surgical Guidance Z-Direct Screw (DSG Screw, SpineGuard Inc, Boulder, Colorado, USA), a screw designed to be inserted using a direct insertion technique.

Methods  Dynamic Surgical Guidance Screws of 5.5 and 6.5 mm were introduced into polyurethane blocks with a density of 10 PCF (0,16g/cm ³ ). According to the experimental group, screws were inserted without pilot hole, with pilot without tapping, undertapping and line-to-line tapping. Screw pullout tests were performed using a universal test machine after screw insertion into polyurethane blocks.

Results  Screws inserted directly into the polyurethane blocks without pilot hole and tapping showed a statistically higher pullout strength. Insertion of the screw without tapping or with undertapping increases the pullout screw strength compared with line-to-line tapping.

Conclusion  Dynamic Surgical Guidance Screw showed the highest pullout strength after its insertion without pilot hole and tapping.

Usefulness of dynamic stabilisation with mobile percutaneous pedicle screw for thoracic vertebral fractures in diffuse idiopathic skeletal hyperostosis

We report a case of vertebral fracture with diffuse idiopathic skeletal hyperostosis (DISH) who underwent posterior dynamic stabilisation using mobile percutaneous pedicle screws (PPS) with 1 above-1 below and obtained good bone fusion. A 76-year-old man experienced severe low back pain after he fell backward 1 m off a stepladder during work. A 12th thoracic vertebral fracture with DISH was observed. As the fractured part was unstable due to a three-column injury, and the conservative treatment of resting was not successful, posterior dynamic stabilisation with a mobile PPS between T11-L1 was performed the 38th day after injury. Immediately after surgery, a fracture gap was observed, but 5 months later, vertebral body height was shortened by about 4 mm, and good bone fusion was observed without loosening of the screw. The mobile PPS flexibly adapts to spinal plasticity and may be useful for bone union in vertebral fractures associated with DISH.

Investigation into the biomechanics of lumbar spine micro-dynamic pedicle screw

Background

Numerous reports have shown that rigid spinal fixation contributes to a series of unwanted complications in lumbar fusion procedure. This innovative micro-dynamic pedicle screw study was designed to investigate the biomechanical performance of lumbar implants using numerical simulation technique and biomechanical experiment.

Methods

Instrumented finite element models of three configurations (dynamic fixation, rigid fixation and hybrid fixation) using a functional L3-L4 lumbar unit were developed, to compare the range of motion of the lumbar spine and stress values on the endplate and implants. An in vitro experiment was simultaneously conducted using 18 intact porcine lumbar spines and segmental motion analyses were performed as well.

Results

Simulation results indicated that the dynamic fixation and the hybrid fixation models respectively increased the range of motion of the lumbar spine by 95 and 60% in flexion and by 83 and 55% in extension, compared with the rigid fixation model. The use of micro-dynamic pedicle screw led to higher stress on endplates and lower stress on pedicle screws. The outcome of the in vitro experiment demonstrated that the micro-dynamic pedicle screw could provide better range of motion at the instrumented segments than a rigid fixation.

Conclusion

The micro-dynamic pedicle screw has the advantage of providing better range of motion than conventional pedicle screw in flexion-extension, without compromising stabilization, and has the potential of bringing the load transfer behavior of fusional segment closer to normal and also lowers the stress values of pedicle screws.

Dynamic stabilization for degenerative diseases in the lumbar spine: 2 years results

Following lumbar fusion, adjacent segment degeneration has been frequently reported. Dynamic systems are believed to reduce main fusion drawbacks. We conducted a retrospective study on patients with degenerative lumbar disease treated with posterior dynamic stabilization with monoaxial hinged pedicular screws and lumbar decompression. VAS and ODI were used to compare clinical outcomes. As radiological outcomes, LL and SVA were used. 51 patients were included with an average follow-up of 24 months. 13 patients were revised because of postoperative radiculopathy (n=4), subcutaneous hematoma (n=2), L5 screw malposition (n=1) and adjacent segment disease (n=6). The mean ODI score 41 preoperatively compared to 36 postoperatively. The mean VAS scores for back and leg pain were 5.3 and 4.2, respectively compared to 4.5 and 4.0 postoperatively. The mean SVA was 5.3 cm preoperatively, and 5.7 cm postoperatively. The mean LL was 47.5° preoperatively and 45.5° postoperatively. From our data, which fail to show significant improvements and reflect a high revision rate, we cannot generally recommend dynamic stabilization as an alternative to fusion. Comparative trials with longer follow-ups are required.

Biomechanical Analysis of Bilateral Facet Joint Stabilization Using Bioderived Tendon for Posterior Cervical Spine Motion Reservation in Goats

OBJECTIVES

To investigate the biomechanical properties of a novel stabilization method for posterior cervical motion preservation using bioderived freeze-dried tendon.

METHODS

Experiments were conducted both in vitro and in vivo. For the in vitro group, 15 fresh-frozen goat spines (C1-C7) were randomly divided into 3 subgroups: intact (INT-vitro, n = 5), injury model (IM-vitro, n = 5), and bilateral facet joint stabilization (BFJS-vitro, n = 5) subgroups. For the in vivo group, 15 adult goats were randomly divided into 3 experimental subgroups: INT-vivo subgroup (n = 5), IM-vivo subgroup (n = 5), and BFJS-vivo subgroup (n = 5). Goats in the in vivo group were euthanized 12 weeks after surgery. Biomechanical tests were performed to evaluate range of motion. Histologic analysis was conducted to evaluate survival and reactions associated with the bioderived tendon.

RESULTS

Compared with the INT-vitro and INT-vivo subgroups, the flexion of IM-vitro and IM-vivo subgroups increased significantly, respectively (P < 0.05). The flexion of the BFJS-vitro and BFJS-vivo subgroups was significantly smaller than in the IM-vitro and IM-vivo subgroups, respectively (P < 0.05). Significant differences between the BFJS-vitro and BFJS-vivo subgroups were observed in flexion, lateral bending, and rotation (P < 0.05). Histologic evaluation demonstrated that fibers arranged regularly and stained homogeneously. New vessels in growth indicated that the bioderived tendon was survival and processed good regeneration.

CONCLUSIONS

Bilateral facet joint stabilization can significantly limit excessive flexion motion and maintain adequate stability. Furthermore, the preservation of extension motions without limiting lateral bending and rotation ideally simulates the features of the posterior ligamentous complex. This preserves the dynamic stability of the lower cervical spine.

Biomechanical evaluation of a new pedicle screw-based posterior dynamic stabilization device (Awesome Rod System)--a finite element analysis

Background

Pedicle-screw-based posterior dynamic stabilization devices are designed to alleviate the rate of accelerated degeneration of the vertebral level adjacent to the level of spinal fusion. A new pedicle- screw-based posterior dynamic stabilization device- the Awesome Dynamic Rod System was designed with curve cuts on the rods to provide flexibility. The current study was conducted to evaluate the biomechanical properties of this new device.

Methods

Finite element models were developed for the intact spine (INT), the Awesome Dynamic Rod Implanted at L4-L5 (AWE), a traditional rigid rod system implanted at L4-L5 along with an interbody cage (FUS), and the Awesome Dynamic Rod System implanted at L4-L5 along with an interbody cage as an adjunct to fusion procedures and extension of dynamic fixation to L3-L4 (AWEFUS). The models were subjected to axial loads and pure moments and evaluated by a hybrid method on range of motion (ROM)s, disc stresses, pedicle screws stresses, and facet joint contact forces.

Results

FUS sustained the lowest L4-L5 ROM decrement in flexion and torsion. AWE demonstrated the lowest adjacent level ROM increment in all moments except for extension at L3-L4, and AWEFUS showed the greatest ROM increment at L2-L3. AWE demonstrated lowest adjacent segment disc stress in flexion, lateral bending and torsion at L3-L4. AWEFUS showed the highest disc stress increment in flexion, extension, and lateral bending, and the lowest disc stress decrement in torsion at L2-L3. AWE sustained greater adjacent facet joint contact forces than did FUS in extension and lateral bending at L3-L4, and AWEFUS demonstrated the greatest contact forces concentrating at L2-L3.

Conclusion

The results demonstrate that the Awesome Dynamic Rod System preserved more bridged segment motion than did the traditional rigid rod fixation system except in extension. However, the Awesome Dynamic Rod System bore a greater facet joint contact force in extension. The Awesome Dynamic Rod System did protect the adjacent level of fusion segments, but led to much greater ROM, disc stresses, and facet joint contact forces increasing at the adjacent level of instrumented segments.

How does free rod-sliding affect the posterior instrumentation for a dynamic stabilization using a bovine calf model?

STUDY DESIGN

A biomechanical cadaveric study in lumbar calf spine.

OBJECTIVE

Evaluation of the effects of selected degrees of freedom (df) on the dynamic stabilization of the spine in terms of segmental range of motion (RoM), center of rotation (CoR), and implant loadings.

SUMMARY OF BACKGROUND DATA

For dorsal stabilization, rigid implant systems are becoming increasingly complemented by numerous dynamic systems based on pedicle screws and varying df. However, it is still unclear which df is most suitable to accomplish a physiologically related dynamic stabilization, and which loadings are induced to the implants. Human and calf specimens are reported to show certain similarities in their biomechanics. Young healthy calf specimens are not degenerated and show less interindividual differences than elderly human specimens. However, the existing differences between species limit the conclusions in a preclinical setting.

METHODS

Six calf specimens from level L3-L4 were analyzed in flexion and extension with a 6-df robotic spine simulator. A clinical functional radiological examination tool was used and parameters such as RoM, CoR, and implant loadings were determined for 6 configurations: (1) intact, (2) defect, (3) rigid fixation, (4) free craniocaudal (CC) rod-sliding, (5) free polyaxiality, and (6) combined free rod-sliding and free polyaxiality. The location of the CoR was determined relative to vertebral body dimensions. A CoR repositioning was defined as sufficient when its median differed less than 5% of the vertebral body dimensions.

RESULTS

Free rod-sliding in the CC direction restored the CoR from the defect back to the intact condition. The RoM could be significantly reduced to approximately 1/2 of the intact condition. Compared with the rigid condition, the implant bending moments increased from 0.3/-0.8 Nm (flexion/extension) to 1.3/-1.2 Nm for the free CC rod-sliding condition.

CONCLUSION

Free CC rod-sliding restores the intact conditions of the tested kinematic parameters most suitably and at the same time reduces the RoM. Stabilization toward the intact condition could decrease the risk of stress shielding and the progress of segment degeneration.

LEVEL OF EVIDENCE

N/A.

Dynamic stabilization for challenging lumbar degenerative diseases of the spine: a review of the literature

Fusion and rigid instrumentation have been currently the mainstay for the surgical treatment of degenerative diseases of the spine over the last 4 decades. In all over the world the common experience was formed about fusion surgery. Satisfactory results of lumbar spinal fusion appeared completely incompatible and unfavorable within years. Rigid spinal implants along with fusion cause increased stresses of the adjacent segments and have some important disadvantages such as donor site morbidity including pain, wound problems, infections because of longer operating time, pseudarthrosis, and fatigue failure of implants. Alternative spinal implants were developed with time on unsatisfactory outcomes of rigid internal fixation along with fusion. Motion preservation devices which include both anterior and posterior dynamic stabilization are designed and used especially in the last two decades. This paper evaluates the dynamic stabilization of the lumbar spine and talks about chronologically some novel dynamic stabilization devices and thier efficacies.

Biomechanical evaluation of a posterior non-fusion instrumentation of the lumbar spine

PURPOSE

Numerous posterior non-fusion systems have been developed within the past decade to resolve the disadvantages of rigid instrumentations and preserve spinal motion. The aim of this study was to investigate the effect of a new dynamic stabilization device, to measure the screw anchorage after flexibility testing and compare it with data reported in the literature.

METHODS

Six human lumbar spine motion segments (L2-5) were loaded in a spine tester with pure moments of 7.5 Nm in lateral bending, flexion/extension and axial rotation. Specimens were tested intact, after instrumentation of the intact segment, after destabilization by a nucleotomy and after instrumentation of the destabilised segment with the new non-fusion device (Elaspine). After flexibility testing all screws were subjected to a pull-out test.

RESULTS

Instrumentation of the intact segment significantly reduced the RoM (p < 0.002) in flexion, extension and lateral bending to 49.7, 44.6 and 53% of the intact state, respectively. In axial rotation, the instrumentation resulted in a non-significant RoM reduction to 95% of the intact state. Compared to the intact segment, instrumentation of the destabilized segment significantly (p < 0.05) reduced the RoM to 69.8, 62.3 and 79.1% in flexion, extension and lateral bending, respectively. In axial rotation, the instrumented segment showed a significantly higher RoM than the intact segment (137.6% of the intact state (p < 0.01)). The pull-out test showed a maximum pull-out force of 855.1 N (±334) with a displacement of 6.1 mm (±2.8) at maximum pull-out force.

CONCLUSIONS

The effect of the investigated motion preservation device on the RoM of treated segments is in the range of other devices reported in the literature. Compared to the most implanted and investigated device, the Dynesys, the Elaspine has a less pronounced motion restricting effect in lateral bending and flexion/extension, while being less effective in limiting axial rotation. The pull-out force of the pedicle screws demonstrated anchorage comparable to other screw designs reported in the literature.

Surgical results of dynamic nonfusion stabilization with the Segmental Spinal Correction System for degenerative lumbar spinal diseases with instability: Minimum 2-year follow-up

BACKGROUND

When spinal fusion is applied to degenerative lumbar spinal disease with instability, adjacent segment disorder will be an issue in the future. However, decompression alone could cause recurrence of spinal canal stenosis because of increased instability on operated segments and lead to revision surgery. Covering the disadvantages of both procedures, we applied nonfusion stabilization with the Segmental Spinal Correction System (Ulrich Medical, Ulm, Germany) and decompression.

METHODS

The surgical results of 52 patients (35 men and 17 women) with a minimum 2-year follow-up were analyzed: 10 patients with lumbar spinal canal stenosis, 15 with lumbar canal stenosis with disc herniation, 20 with degenerative spondylolisthesis, 6 with disc herniation, and 1 with lumbar discopathy.

RESULTS

The Japanese Orthopaedic Association score was improved, from 14.4 ± 5.3 to 25.5 ± 2.8. The improvement rate was 76%. Range of motion of the operated segments was significantly decreased, from 9.6° ± 4.2° to 2.0° ± 1.8°. Only 1 patient had adjacent segment disease that required revision surgery. There was only 1 screw breakage, but the patient was asymptomatic.

CONCLUSIONS

Over a minimum 2-year follow-up, the results of nonfusion stabilization with the Segmental Spinal Correction System for unstable degenerative lumbar disease were good. It is necessary to follow up the cases with a focus on adjacent segment disorders in the future.

Nonfusion stabilization of the degenerative lumbar spine

Object

The goal of this study was to assess whether a stable but nonrigid nonfusion implant can stabilize the spine in degenerative diseases and also prevent instability following decompression. Instrumented spondylodesis is a recognized surgical treatment in degenerative disease of the lumbar spine. However, pain can develop at the bone graft donor site and the operative trauma can be very stressful in elderly patients, and it is suspected that there may be increased degenerative changes in the adjacent segments. In 2002, a nonrigid but rotationally stable pedicle screw and rod system was introduced, which could be used without additional fusion (referred to hereafter as the Cosmic system).

Methods

A total of 139 patients with degenerative disease of the lumbar spine underwent spinal stabilization with the Cosmic system without additional spondylodesis. Seventy patients had an additional decompression. The minimum follow-up was 2 years. The perioperative course, the clinical results, and the erect anteroposterior and lateral radiographs were recorded and compared with the preoperative data. The data were obtained from 6 different spine centers in Europe and documented on an Internet platform.

Results

The Oswestry Disability Index score improved from 48.9% to 22.5%, and the visual analog scale score decreased from 7.3 to 2.5. Lumbar lordosis did not change, nor did the adjacent disc height. Eleven patients underwent revision, 4 of them for implant failure. Of the 139 patients, 110 assessed the result as excellent, very good, or good; 24 as fair; and 5 as poor. A total of 122 patients would undergo surgery again. There were no significant differences between patients with or without an additional decompression.

Conclusions

The Cosmic system is a stable but nonrigid posterior nonfusion system. Implant complications are low and the clinical outcome is good. Longer follow-up is necessary to confirm the 2-year results.

[Dynamic posterior stabilization with the cosmic system]

OBJECTIVE

Stabilization of unstable motion segments with a stable but non-rigid implant system without an additional spondylodesis.

INDICATIONS

Neurogenic claudication with instability; discogenic pain; in combination with a fusion (hybrid technique); elongation of a preexisting fusion; second recurrence of a herniated disk.

CONTRAINDICATIONS

Increased instability; correction and reduction; instrumentation of more than three levels.

SURGICAL TECHNIQUE

Muscle-sparing approach to the posterior lumbar spine under anteroposterior and lateral image control. Use of special instruments with a slotted sleeve connected to the screw head for rod implantation. Alternatively: conventional midline approach with detachment of muscles from the posterior spine.

POSTOPERATIVE MANAGEMENT

Mobilization on the day after surgery. Limited physical activities and no work load for 6 weeks.

RESULTS

In 139 patients (77 females, 62 males, average age 55 years) with a follow-up of 2 years, Oswestry score improved from 49.0% preoperatively to 22.5% and VAS (visual analog scale) from 7.3 preoperatively to 2.5 after 2 years. No change of the lordosis. Eleven revisions (7.9%). Two broken screws (0.3%) and 17 screws (2.5%) with a radiolucent halo.

Clinical outcomes of degenerative lumbar spinal stenosis treated with lumbar decompression and the Cosmic "semi-rigid" posterior system

Background

Although some investigators believe that the rate of postoperative instability is low after lumbar spinal stenosis surgery, the majority believe that postoperative instability usually develops. Decompression alone and decompression with fusion have been widely used for years in the surgical treatment of lumbar spinal stenosis. Nevertheless, in recent years several biomechanical studies have shown that posterior dynamic transpedicular stabilization provides stabilization that is like the rigid stabilization systems of the spine. Recently, posterior transpedicular dynamic stabilization has been more commonly used as an alternative treatment option (rather than rigid stabilization with fusion) for the treatment of degenerative spines with chronic instability and for the prevention of possible instability after decompression in lumbar spinal stenosis surgery.

Methods

A total of 30 patients with degenerative lumbar spinal stenosis (19 women and 11 men) were included in the study group. The mean age was 67.3 years (range, 40–85 years). Along with lumbar decompression, a posterior dynamic transpedicular stabilization (dynamic transpedicular screw–rigid rod system) without fusion was performed in all patients. Clinical and radiologic results for patients were evaluated during follow-up visits at 3, 12, and 24 months postoperatively.

Results

The mean follow-up period was 42.93 months (range, 24–66 months). A clinical evaluation of patients showed that, compared with preoperative assessments, statistically significant improvements were observed in the Oswestry and visual analog scale scores in the last follow-up control. Compared with preoperative values, there were no statistically significant differences in radiologic evaluations, such as segmental lordosis angle (α) scores (P = .125) and intervertebral distance scores (P = .249). There were statistically significant differences between follow-up lumbar lordosis scores (P = .048). There were minor complications, including a subcutaneous wound infection in 2 cases, a dural tear in 2 cases, cerebrospinal fluid fistulas in 1 case, a urinary tract infection in 1 case, and urinary retention in 1 case. We observed L5 screw loosening in 1 of the 3-level decompression cases. No screw breakage was observed and no revision surgery was performed in any of these cases.

Conclusions

Posterior dynamic stabilization without fusion applied to lumbar decompression leads to better clinical and radiologic results in degenerative lumbar spinal stenosis. To avoid postoperative instability, especially in elderly patients who undergo degenerative lumbar spinal stenosis surgery with chronic instability, the application of decompression with posterior dynamic transpedicular stabilization is likely an important alternative surgical option to fusion, because it does not have fusion-related side effects, is easier to perform than fusion, requires a shorter operation time, and has low morbidity and complication rates.

Pedicle screw-based dynamic stabilization of the thoracolumbar spine with the Cosmic-system: a prospective observation

OBJECT

The objective of the study was to generate prospective data to assess the clinical results after dynamic stabilization with the Cosmic system (Ulrich Medical).

PATIENTS AND METHODS

Between April 2006 and December 2007, 103 consecutive patients were treated with Cosmic for painful degenerative segmental instability +/- spinal stenosis. The preoperative workup included radiological (MRI and myelography/CT) and clinical parameters (general/neurological examination, visual analogue scale (VAS), Oswestry disability index (ODI), SF-36, Karnofsky (KPS)). At pre-defined intervals (at discharge, 6 weeks, 3 months, 6 months, 12 months, and yearly) the patients were reevaluated (X-ray/flexion/extension, neurological status, VAS, ODI, SF-36, KPS, and patient satisfaction). Data were collected in a prospective observational design.

RESULTS

Data collection was completed in 100 of 103 operated patients (mean follow-up, 15 +/- 0.6 months). Dynamic stabilization was performed as first-tier surgery in 43 cases and as second-tier therapy in 60 cases. Additional decompression was performed in 83 cases. Dynamic stabilization led to significant reduction of back pain-related disability (ODI pre-op, 51 +/- 1%; post-op, 21 +/- 1%) and improvement of pain (VAS pre-op, 65 +/- 1; post-op, 21 +/- 2), mental/physical health (norm-based SF-36: mental pre-op, 44; post-op, 48; physical pre-op, 41; post-op, 46), and mobility (KPS pre-op, 70 +/- 1; post-op, 82 +/- 31). Early reoperation was necessary in 12 patients (n = 3 symptomatic misplaced screws, n = 8 CSF pseudocele, rebleeding, or impaired wound healing, n = 1 misjudged instability/stenosis in adjacent segment). Reoperations within the follow-up period were necessary in another 10 patients due to secondary screw loosening (n = 2), persistent stenosis/disk protrusion in an instrumented segment (n = 3), symptomatic degeneration of an adjacent segment (n = 6), or osteoporotic fracture of an adjacent vertebra (n = 1), respectively. Patient satisfaction rate was 91%.

CONCLUSIONS

Dynamic stabilization with Cosmic achieved significant improvement of pain, related disability, mental/physical health, and mobility, respectively, and a high rate of satisfied patients. A reoperation rate of 10% during follow-up seems relatively high at first glance. Comparable data, however, are scarce, and a prospective randomized trial (spondylodesis vs. dynamic stabilization) is warranted based on these results.

Dynamic lumbar pedicle screw-rod stabilization: in vitro biomechanical comparison with standard rigid pedicle screw-rod stabilization

OBJECT

It is unclear how the biomechanics of dynamic posterior lumbar stabilization systems and traditional rigid pedicle screw-rod systems differ. This study examined the biomechanical response of a hinged-dynamic pedicle screw compared with a standard rigid screw used in a 1-level pedicle screw-rod construct.

METHODS

Unembalmed human cadaveric L3-S1 segments were tested intact, after L4-5 discectomy, after rigid pedicle screw-rod fixation, and after dynamic pedicle screw-rod fixation. Specimens were loaded using pure moments to induce flexion, extension, lateral bending, and axial rotation while recording motion optoelectronically. Specimens were then loaded in physiological flexion-extension while applying 400 N of compression. Moment and force across instrumentation were recorded from pairs of strain gauges mounted on the interconnecting rods.

RESULTS

The hinged-dynamic screws allowed an average of 160% greater range of motion during flexion, extension, lateral bending, and axial rotation than standard rigid screws (p < 0.03) but 30% less motion than normal. When using standard screws, bending moments and axial loads on the rods were greater than the bending moments and axial loads on the rods when using dynamic screws during most loading modes (p < 0.05). The axis of rotation shifted significantly posteriorly more than 10 mm from its normal position with both devices.

CONCLUSIONS

In a 1-level pedicle screw-rod construct, hinged-dynamic screws allowed a quantity of motion that was substantially closer to normal motion than that allowed by rigid pedicle screws. Both systems altered kinematics similarly. Less load was borne by the hinged screw construct, indicating that the hinged-dynamic screws allow less stress shielding than standard rigid screws.

Anatomy of large animal spines and its comparison to the human spine: a systematic review

Animal models have been commonly used for in vivo and in vitro spinal research. However, the extent to which animal models resemble the human spine has not been well known. We conducted a systematic review to compare the morphometric features of vertebrae between human and animal species, so as to give some suggestions on how to choose an appropriate animal model in spine research. A literature search of all English language peer-reviewed publications was conducted using PubMed, OVID, Springer and Elsevier (Science Direct) for the years 1980-2008. Two reviewers extracted data on the anatomy of large animal spines from the identified articles. Each anatomical study of animals had to include at least three vertebral levels. The anatomical data from all animal studies were compared with the existing data of the human spine in the literature. Of the papers retrieved, seven were included in the review. The animals in the studies involved baboon, sheep, porcine, calf and deer. Distinct anatomical differences of vertebrae were found between the human and each large animal spine. In cervical region, spines of the baboon and human are more similar as compared to other animals. In thoracic and lumbar regions, the mean pedicle height of all animals was greater than the human pedicles. There was similar mean pedicle width between animal and the human specimens, except in thoracic segments of sheep. The human spinal canal was wider and deeper in the anteroposterior plane than any of the animals. The mean human vertebral body width and depth were greater than that of the animals except in upper thoracic segments of the deer. However, the mean vertebral body height was lower than that of all animals. This paper provides a comprehensive review to compare vertebrae geometries of experimental animal models to the human vertebrae, and will help for choosing animal model in vivo and in vitro spine research. When the animal selected for spine research, the structural similarities and differences found in the animal studies must be kept in mind.

Non-fusion instrumentation of the lumbar spine with a hinged pedicle screw rod system: an in vitro experiment

In advanced stages of degenerative disease of the lumbar spine instrumented spondylodesis is still the golden standard treatment. However, in recent years dynamic stabilisation devices are being implanted to treat the segmental instability due to iatrogenic decompression or segmental degeneration. The purpose of the present study was to investigate the stabilising effect of a classical pedicle screw/rod combination, with a moveable hinge joint connection between the screw and rod allowing one degree of freedom (cosmicMIA). Six human lumbar spines (L2-5) were loaded in a spine tester with pure moments of +/-7.5 Nm in lateral bending, flexion/extension and axial rotation. The range of motion (ROM) and the neutral zone were determined for the following states: (1) intact, (2) monosegmental dynamic instrumentation (L4-5), (3) bisegmental dynamic instrumentation (L3-5), (4) bisegmental decompression (L3-5), (5) bisegmental dynamic instrumentation (L3-5) and (6) bisegmental rigid instrumentation (L3-5). Compared to the intact, with monosegmental instrumentation (2) the ROM of the treated segment was reduced to 47, 40 and 77% in lateral bending, flexion/extension and axial rotation, respectively. Bisegmental dynamic instrumentation (3) further reduced the ROM in L4-5 compared to monosegmental instrumentation to 25% (lateral bending), 28% (flexion/extension) and 57% (axial rotation). Bisegmental surgical decompression (4) caused an increase in ROM in both segments (L3-4 and L4-5) to approximately 125% and approximately 135% and 187-234% in lateral bending, flexion/extension and axial rotation, respectively. Compared to the intact state, bisegmental dynamic instrumentation after surgical decompression reduced the ROM of the two-bridged segments to 29-35% in lateral bending and 33-38% in flexion/extension. In axial rotation, the ROM was in the range of the intact specimen (87-117%). A rigid instrumentation (6) further reduced the ROM of the two-bridged segments to 20-30, 23-27 and 50-68% in lateral bending, flexion/extension and axial rotation, respectively. The results of the present study showed that compared to the intact specimen the investigated hinged dynamic stabilisation device reduced the ROM after bisegmental decompression in lateral bending and flexion/extension. Following bisegmental decompression and the thereby caused large rotational instability the device is capable of restoring the motion in axial rotation back to values in the range of the intact motion segments.

Are the spines of calf, pig and sheep suitable models for pre-clinical implant tests?

Pre-clinical in vitro tests are needed to evaluate the biomechanical performance of new spinal implants. For such experiments large animal models are frequently used. Whether these models allow any conclusions concerning the implant's performance in humans is difficult to answer. The aim of the present study was to investigate whether calf, pig or sheep spine specimens may be used to replace human specimens in in vitro flexibility and cyclic loading tests with two different implant types. First, a dynamic and a rigid fixator were tested using six human, six calf, six pig and six sheep thoracolumbar spine specimens. Standard flexibility tests were carried out in a spine tester in flexion/extension, lateral bending and axial rotation in the intact state, after nucleotomy and after implantation. Then, the Coflex interspinous implant was tested for flexibility and intradiscal pressure using another six human and six calf lumbar spine segments. Loading was carried out as described above in the intact condition, after creation of a defect and after implantation. The fixators were most easily implantable into the calf. Qualitatively, they had similar effects on ROM in all species, however, the degree of stability achieved differed. Especially in axial rotation, the ROM of sheep, pig and calf was partially less than half the human ROM. Similarly, implantation of the Coflex interspinous implant caused the ROM to either increase in both species or to decrease in both of them, however, quantitatively, differences were observed. This was also the case for the intradiscal pressure. In conclusion, animal species, especially the calf, may be used to get a first idea of how a new pedicle screw system or an interspinous implant behaves in in vitro flexibility tests. However, the effects on ROM and intradiscal pressure have to be expected to differ in magnitude between animal and human. Therefore, the last step in pre-clinical implant testing should always be an experiment with human specimens.

Mechanical performance of cylindrical and dual core pedicle screws in calf and human vertebrae

INTRODUCTION

Failure of pedicle screws by loosening and back out remains a significant clinical problem. Pedicle screw fixation is determined by bone mineral density, pedicle morphology and screw design. The objective of this study was to compare the holding strength of newly developed dual core pedicle screws having a cylindrical design in terms of outer diameter and two cylindrical inner core regions connected by a conical transition with conventional cylindrical pedicle screws.

MATERIALS AND METHODS

Fifty bovine lumbar vertebrae and 40 human lumbar vertebrae were used. Five different screws were tested in nine experimental "settings" and ten specimens each. The screws were tested for cranial displacement and pullout strength before and after 5,000 cycles of cranio-caudal loading. The tests included a setting with fully inserted and 4 mm backed out screws. For statistical analysis the incomplete balanced block design was used.

RESULTS

Cyclic loading led to a decrease of pullout force between 24 and 31% and a 9% increase of displacement. The cylindrical screw designs were affected more than the dual core designs. The pullout force of cylindrical screws was smaller than of dual core screws. Even in a backed out condition dual core screws showed a significantly smaller displacement than cylindrical screws.

CONCLUSION

Pedicle screws with the dual core design provide good anchorage in the vertebra.

Biomechanical evaluation of a dynamic pedicle screw fixation device

BACKGROUND

Recent innovations in dynamic devices have promised a reduction in stress shielding, protection of adjacent segment degeneration, and decreased implant failure. However, there have been few studies comparing the biomechanical properties of a rigid device in comparison to a dynamic posterior fixation device. The purpose of this study was to compare the immediate stability of a new dynamic pedicle screw fixation device with an equivalent rigid device.

METHODS

Six thoracolumbar cadaver spines (T10-L4) were fixed in a biomechanical testing frame. Pure moments of 10Nm were loaded in six directions: flexion, extension, right and left lateral bending, and right and left axial rotation. For each spine, four different stages were tested: intact, destabilization of the middle segment, fixation with the dynamic device, and fixation with the rigid device. Ranges of motion were measured using stereophotogrammetry. The specimens with each device were then subjected to flexion-compression loading for five cycles on a MTS 858 Universal Testing Machine. The average stiffness of the last three cycles was recorded.

FINDINGS

Both dynamic and rigid devices were found to provide stability for the injured segment in flexion-extension and lateral bending. In axial rotation, the devices could restore the stability to levels similar to those in an intact spine. Results also indicated a slight increase in range of motion in flexion-extension and significant reduction in stiffness of flexion-compression with the dynamic device (P < 0.01), in comparison to the rigid device.

INTERPRETATION

The dynamic device offers a system that may alter favorably the movement and load transmission of a spinal motion segment without sacrificing construct stability.

Primary stability of anterior lumbar stabilization: interdependence of implant type and endplate retention or removal

This is a comparative in vitro biomechanical study of the primary stability of an anterior lumbar interbody stabilization. The objective was to compare the stability of a interbody stabilizing titanium cage with and without the retention of the bordering vertebral endplates, as well as to compare the titanium cage with a tricalcium phosphate block when the endplates are removed. An adequate blood supply is critical for interbody fusion, which suggests surgical treatment of the bordering endplates. On the other hand, primary stability is improved by the retention of the endplates. Furthermore, bone substitute materials are finding more frequent use due to complications associated with autologous bone grafts. Ten bovine lumbar spine motion segments (average age 6 months) were investigated. Pure bending loadings as well as eccentric axial compression loadings were applied. A titanium cage and tricalcium phosphate block, were tested in conjunction with an anterior augmentation (MACS). Range of motion, neutral zone (NZ) and bending stiffness were measured under pure bending to 10 Nm, and bending stiffness under axial loads of up to 1,500 N. Range of motion of both implants in flexion-extension was significantly smaller than physiologic (cage without endplates 4.3 degrees , cage with 2.8 degrees , block without 3.4 degrees , and physiologic 6.6 degrees , all p<0.001). The cage with endplates and the block without endplates were both significantly stiffer than physiologic in all directions except left lateral bending. The block without endplates and the cage with endplates were both stiffer than the cage without endplates. The results suggest that the use of the bone substitute block provides better stability than the cage when the endplates are removed.

Comparative biomechanical testing of anterior and posterior stabilization procedures

STUDY DESIGN

This is a comparative in vitro biomechanical study in a calf lumbar spine model.

OBJECTIVES

The objective was to compare the primary stability of an anterior instrumentation, an intercorporal cage in combination with an anterior instrumentation, and a posterior instrumentation for monosegmental spondylodesis.

SUMMARY OF BACKGROUND DATA

Spondylodesis can be achieved through a posterior lumbar fusion, posterior lumbar intercorporal fusion, or an anterior lumbar intercorporal fusion. The posterior lumbar fusion is the gold standard, although the anterior approach offers some potential advantages to the transpedicular posterior techniques.

METHODS

Stability testing was performed on 30 calf lumbar spine motion segments in a physiologic state (n = 30), with either an isolated anterior (MACS) or posterior instrumentation (SOCON), and with an anterior instrumentation augmented with an intercorporal cage (MACS-Cage, n = 10, respectively). Range of motion, neutral zone, and bending stiffness were measured under pure bending to 10 Nm, and bending stiffness under axial loads of up to 1500 N.

RESULTS

The isolated posterior instrumentation was found to be more stable than the isolated or augmented anterior instrumentation in flexion/extension, although no significant differences were observed in lateral bending or axial rotation. The results of this biomechanical study suggest that an augmented anterior instrumentation provides similar stability for bony fusion as does the golden standard posterior instrumentation, with the exception of flexion/extension.

CONCLUSION

An augmented anterior instrumentation may provide similar stability for bony fusion as does the posterior instrumentation.

Effect of constrained posterior screw and rod systems for primary stability: biomechanical in vitro comparison of various instrumentations in a single-level corpectomy model

Cervical corpectomy is a frequently used technique for a wide variety of spinal disorders. The most commonly used approach is anterior, either with or without plating. The results for single-level corpectomy are better than in multilevel procedures. Nevertheless, hardware- or graft-related complications are observed. In the past, constrained implant systems were developed and showed encouraging stability, especially for posterior screw and rod systems in the lumbar spine. In the cervical spine, few reports about the primary stability of constrained systems exist. Therefore, in the present study we evaluated the primary stability of posterior screw and rod systems, constrained and non-constrained, in comparison with anterior plating and circumferential instrumentations in a non-destructive set-up, by loading six human cadaver cervical spines with pure moments in a spine tester. Range of motion and neutral zone were measured for lateral bending, flexion/extension and axial rotation. The testing sequence consisted of: (1) stable testing; (2) testing after destabilization and cage insertion; (3a) additional non-constrained screw and rod system with lateral mass screws, (3b) with pedicle screws instead of lateral mass screws; (4a) constrained screw and rod system with lateral mass screws, (4b) with pedicle screws instead of lateral mass screws; (5) 360 degrees set-up; (6) anterior plate. The stability of the anterior plate was comparable to that of the non-constrained system, except for lateral bending. The primary stability of the non-constrained system could be enhanced by the use of pedicle screws, in contrast to the constrained system, for which a higher primary stability was still found in axial rotation and flexion/extension. For the constrained system, the achievable higher stability could obviate the need to use pedicle screws in low instabilities. Another benefit could be fewer hardware-related complications, higher fusion rate, larger range of instabilities to be treated by one implant system, less restrictive postoperative treatment and possibly better clinical outcome. From a biomechanical standpoint, in regard to primary stability the constrained systems, therefore, seem to be beneficial. Whether this leads to differences in clinical outcome has to be evaluated in clinical trials.