In vitro testing of a new transpedicular stabilization technique

Surface hardening of Ti-Al-V superalloy spinal implant by using the boronization method

BACKGROUND

We compared the raw Ti-Al-V super alloy transpedicular implant screws with boronized and surface-hardened transpedicular implant screws.

OBJECTIVE

To improve patients' postoperative prognosis with the production of harder and less fragile screws.

METHODS

Surface hardening was achieved by applying green-body encapsulation of the specimen with elemental boron paste which is sintered at elevated temperatures to ensure the boron-metal diffusion. Boron transported into the Ti-Al-V super alloy matrix gradually while suppressing aluminum and a homogeneously boronized surface with a thickness of ∼15 microns was obtained. The uniform external shell was enriched with TiB2, which is one of the hardest ceramics. The Ti-Al-V core material, where boron penetration diminishes, shows cohesive transition and ensures intact core-surface structure.

RESULTS

Scanning electron microscope images confirmed a complete homogeneous, uniform and non-laminating surface formation. Energy-dispersive X-ray monitored the elemental structural mapping and proved the replacement of the aluminum sites on the surface with boron ending up the TiB2. The procedure was 8.6 fold improved the hardness and the mechanical resistance of the tools.

CONCLUSIONS

Surface-hardened, boronized pedicular screws can positively affect the prognosis. In vivo studies are needed to prove the safety of use.

Novel force-displacement control passive finite element models of the spine to simulate intact and pathological conditions; comparisons with traditional passive and detailed musculoskeletal models

Passive finite element (FE) models of the spine are commonly used to simulate intact and various pre- and postoperative pathological conditions. Being devoid of muscles, these traditional models are driven by simplistic loading scenarios, e.g., a constant moment and compressive follower load (FL) that do not properly mimic the complex in vivo loading condition under muscle exertions. We aim to develop novel passive FE models that are driven by more realistic yet simple loading scenarios, i.e., in vivo vertebral rotations and pathological-condition dependent FLs (estimated based on detailed musculoskeletal finite element (MS-FE) models). In these novel force-displacement control FE models, unlike the traditional passive FE models, FLs vary not only at different spine segments (T12-S1) but between intact, pre- and postoperative conditions. Intact, preoperative degenerated, and postoperative fused conditions at the L4-L5 segment for five static in vivo activities in upright and flexed postures were simulated by the traditional passive FE, novel force-displacement control FE, and gold-standard detailed MS-FE spine models. Our findings indicate that, when compared to the MS-FE models, the force-displacement control passive FE models could accurately predict the magnitude of disc compression force, intradiscal pressure, annulus maximal von Mises stress, and vector sum of all ligament forces at adjacent segments (L3-L4 and L5-S1) but failed to predict disc shear and facet joint forces. In this regard, the force-displacement control passive FE models were much more accurate than the traditional passive FE models. Clinical recommendations made based on traditional passive FE models should, therefore, be interpreted with caution.

A Biomechanical Model for Testing Cage Subsidence in Spine Specimens with Osteopenia or Osteoporosis Under Permanent Maximum Load

BACKGROUND

Intervertebral fusions in cases of reduced bone density are a tough challenge. From a biomechanical point of view, most current studies have focused on the range of motion or have shown test setups for single-component tests. Definitive setups for biomechanical testing of the primary stability of a 360° fusion using a screw-rod system and cage on osteoporotic spine are missing. The aim of this study was to develop a test stand to provide information about the bone-implant interface under reproducible conditions.

METHODS

After pretesting with artificial bone, functional spine units were tested with 360° fusion in the transforaminal lumbar interbody fusion technique. The movement sequences were conducted in flexion/extension, right and left lateral bending, and right and left axial rotation on a human model with osteopenia or osteoporosis under permanent maximum load with 7.5 N-m.

RESULTS

During the testing of human cadavers, 4 vertebrae were fully tested and were inconspicuous even after radiological and macroscopic examination. One vertebra showed a subsidence of 2 mm, and 1 vertebra had a cage collapsed into the vertebra.

CONCLUSIONS

This setup is suitable for biomechanical testing of cyclical continuous loads on the spine with reduced bone quality or osteoporosis. The embedding method is stable and ensures a purely single-level setup with different trajectories, especially when using the cortical bone trajectory. Optical monitoring provides a very accurate indication of cage movement, which correlates with the macroscopic and radiological results.

A comprehensive approach for the validation of lumbar spine finite element models investigating post-fusion adjacent segment effects

Spinal fusion surgery is usually followed by accelerated degenerative changes in the unfused segments above and below the treated segment(s), i.e., adjacent segment disease (ASD). While a number of risk factors for ASD have been suggested, its exact pathogenesis remains to be identified. Finite element (FE) models are indispensable tools to investigate mechanical effects of fusion surgeries on post-fusion changes in the adjacent segment kinematics and kinetics. Existing modeling studies validate only their intact FE model against in vitro data and subsequently simulate post-fusion in vivo conditions. The present study provides a novel approach for the comprehensive validation of a lumbar (T12-S1) FE model in post-fusion conditions. Sixteen simulated fusion surgeries, performed on cadaveric specimens using various testing and loading conditions, were modeled by this FE model. Predictions for adjacent segment range of motion (RoM) and intradiscal pressure (IDP) were compared with those obtained from the corresponding in vitro tests. Overall, 70% of the predicted adjacent segment RoMs were within the range of in vitro data for both intact and post-fusion conditions. Correlation (r) values between model and in vitro findings for the adjacent segment RoMs were positive and greater than 0.84. Most of the predicted IDPs were, however, out of the narrow range of in vitro IDPs at the adjacent segments but with great positive correlations (r ≥ 0.89). FE modeling studies investigating the effect of fusion surgery on in vivo adjacent segment biomechanics are encouraged to use post-surgery in vitro data to validate their FE model.

Biomaterials in Spinal Implants: A Review

The aim to find the perfect biomaterial for spinal implant has been the focus of spinal research since the 1800s. Spinal surgery and the devices used therein have undergone a constant evolution in order to meet the needs of surgeons who have continued to further understand the biomechanical principles of spinal stability and have improved as new technologies and materials are available for production use. The perfect biomaterial would be one that is biologically inert/compatible, has a Young’s modulus similar to that of the bone where it is implanted, high tensile strength, stiffness, fatigue strength, and low artifacts on imaging. Today, the materials that have been most commonly used include stainless steel, titanium, cobalt chrome, nitinol (a nickel titanium alloy), tantalum, and polyetheretherketone in rods, screws, cages, and plates. Current advancements such as 3-dimensional printing, the ProDisc-L and ProDisc-C, the ApiFix, and the Mobi-C which all aim to improve range of motion, reduce pain, and improve patient satisfaction. Spine surgeons should remain vigilant regarding the current literature and technological advancements in spinal materials and procedures. The progression of spinal implant materials for cages, rods, screws, and plates with advantages and disadvantages for each material will be discussed.

Biomechanical evaluation of an integrated fixation cage during fatigue loading: a human cadaver study

OBJECTIVE

Lumbar cages with integrated fixation screws offer a low-profile alternative to a standard cage with anterior supplemental fixation. However, the mechanical stability of integrated fixation cages (IFCs) compared with a cage with anterior plate fixation under fatigue loading has not been investigated. The purpose of this study was to compare the biomechanical stability of a screw-based IFC with a standard cage coupled with that of an anterior plate under fatigue loading.

METHODS

Eighteen functional spinal units were implanted with either a 4-screw IFC or an anterior plate and cage (AP+C) without integrated fixation. Flexibility testing was conducted in flexion-extension (FE), lateral bending (LB), and axial rotation (AR) on intact spines, immediately after device implantation, and post-fatigue up to 20,000 cycles of FE loading. Stability parameters such as range of motion (ROM) and lax zone (LZ) for each loading mode were compared between the 2 constructs at multiple stages of testing. In addition, construct loosening was quantified by subtracting post-instrumentation ROM from post-fatigue ROM.

RESULTS

IFC and AP+C configurations exhibited similar stability (ROM and LZ) at every stage of testing in FE (p ≥ 0.33) and LB (p ≥ 0.23) motions. In AR, however, IFCs had decreased ROM compared with AP+C constructs at pre-fatigue (p = 0.07) and at all post-fatigue time points (p ≤ 0.05). LZ followed a trend similar to that of ROM in AR. ROM increased toward intact motion during fatigue cycling for AP+C and IFC implants. IFC specimens remained significantly (p < 0.01) more rigid than specimens in the intact condition during fatigue for each loading mode, whereas AP+C construct motion did not differ significantly (p ≥ 0.37) in FE and LB and was significantly greater (p < 0.01) in AR motion compared with intact specimens after fatigue. Weak to moderate correlations (R2 ≤ 56%) were observed between T-scores and construct loosening, with lower T-scores leading to decreased stability after fatigue testing.

CONCLUSIONS

These data indicate that a 4-screw IFC design provides fixation similar to that provided by an AP+C construct in FE and LB during fatigue testing and better stability in AR motion.

Transpedicular plate fixator as effective system of spine stabilisation: biomechanical characteristics

INTRODUCTION

Zespol fixator, which was created in Poland by Ramatowski and Granowski, has an angular stable connection of screws and plate. These properties of this plate fixator, that is effective and not an expensive system of osteosynthesis of shaft of long bone widely used in Poland, impelled us to adapt it as a transpedicular plate fixator of spine.

AIM

The aim of our in vitro study was to measure loads acting on spine stabilized by transpedicular plate fixator and to determine if its stability is comparable with uninjured spine. We also hypothesized that the spine stability with examined fixator had similar properties as spine fixators constructed with screws and rods.

MATERIALS AND METHODS

We tested its biomechanical properties and compared it with a CD device by using specimens of four human spines. Each spine with damage induced in laboratory conditions was stabilised by one of those stabilisers in one (L4-L5) or two (Th12-L2) motion segments and subsequently were subject to load. The spines without and with one of transpedicular stabilization were subject to an unsymmetrical shift of +3/-4 mm for extension-compression and symmetrical shift for bending, in the frontal plane (+0.14/-0.14 rad) and the sagittal plane (+0.11/-0.11 rad), respectively.

RESULTS

Loads during extension-compression and bending in the sagittal plane were similar to the uninjured spine for short stabilization by using both stabilizers and amounted to 92.3 and 98.26%, respectively, of the load range sums of healthy spines. For long stabilization these loads amounted to 93.2 and 84.4%, respectively. Only following short and long stabilization for both devices in case of bending in the frontal plane the increase in loads up to 144.2 and 163.3% of the range sums of uninjured spines was achieved.

CONCLUSION

It corroborates the fact that the application of the modified Zespol device for spine stabilisation provides the possibility of restoring its load transfer capacity similar to that in the healthy spine and comparable with the CD device.

A prospective randomized study of unilateral versus bilateral instrumented posterolateral lumbar fusion in degenerative spondylolisthesis

STUDY DESIGN

Prospective randomized study on 82 patients with degenerative lumbar spondylolisthesis, having undergone posterolateral fusion with bilateral or unilateral instrumentation.

OBJECTIVE

To determine the effectiveness of unilateral pedicle instrumentation in clinical outcome and rate of union in comparison with the classic bilateral system.

SUMMARY OF BACKGROUND DATA

Instrumentation has proved to have advantages and disadvantages related to its rigidity. The use of less rigid systems applied to posterior lumbar fusions proved promising according to the results achieved in both experimental and clinical field.

METHODS

Eighty-two patients were randomized into 2 groups: Group 1 (n = 42) had had bilateral instrumentation, and Group 2 (n = 40) had only had unilateral instrumentation. One case from Group 1, L3-S1 dropped out; only fusions of 1 or 2 levels remained in the study. Length of time spent on operating, blood loss, blood transfusion, hospital stay, complications, clinical results measured by SF-36v2, and radiologic assessment of union and of loss of height of adjacent discs were analyzed and compared by means of chi2 test, t test, and Fisher exact test.

RESULTS

Statistically, there was no significant difference between the 2 groups in relation to demographics, blood loss, need of transfusion, hospital stay, complications, clinical results, rate of union, and effect on adjacent discs. The operating time needed for Group 2 was significantly shorter in than the time needed for Group 1 (P < 0.001). In Group 1, 3 of 186 screws violated the pedicle cortex requiring reoperation because root irritation versus no complication on a total of 90 screws in Group 2.

CONCLUSION

Unilateral instrumentation used for the treatment of degenerative lumbar spondylolisthesis is as effective as bilateral instrumentation when performed in addition to 1- or 2-level posterolateral fusion. The cost of this method is lower, saves time, and reduces possible risk inserting screws in only one side.

A stronger bicortical sacral pedicle screw fixation through the s1 endplate: an in vitro cyclic loading and pull-out force evaluation

STUDY DESIGN

The insertion torque and pull-out force after cyclic loading of the bicortical sacral pedicle screw through the S1 endplate were tested using human cadaveric specimens.

OBJECTIVES

The purpose of this study was to (1) evaluate the effect of cyclic loading on the pull-out force of two different techniques of bicortical sacral pedicle screw fixation and (2) to correlate the pull-out force after cyclic loading with the screw insertion torque.

SUMMARY OF BACKGROUND DATA

Biomechanical studies using conventional sacral pedicle screw fixation techniques have demonstrated reduction in stiffness and strength after cyclic loading. Technical difficulties with anterior sacral cortex penetration and frequent screw loosening have been reported in clinical studies. In the authors' center, a new method of sacral pedicle screw fixation bicortically through the S1 endplate has been used successfully in the clinical setting. However, the mechanical stability of this new technique after undergoing cyclic loading has not been documented in the literature.

METHODS

Seven-millimeter compact Cotrel-Dubousset sacral screws were randomly assigned by side (left vs. right) and inserted bicortically either anteromedially through the anterior sacral cortex or superiorly through the S1 endplate of 17 fresh frozen human sacrum. The tk;4maximum insertion torque for each screw was measured. Cyclic loading from 40 N to 400 N was applied to each screw at a frequency of 2 Hz for 20,000 cycles. Pull-out tests were conducted after completion of the cyclic tests.

RESULTS

The mean maximum insertion torque and mean pull-out force following cyclic loading were significantly higher for bicortical fixation through the S1 endplate (mean 3.17 N.m and 1457 N) than bicortical fixation through the anterior sacral cortex (mean 1.98 N.m and 1122 N). Both S1 endplate and anterior cortical fixation techniques demonstrated significant correlations between insertion torque and pull-out force following cyclic loading.

CONCLUSIONS

In sacral pedicle screw fixation, screw trajectory through the S1 endplate was significantly stronger than screws penetrating the anterior sacral cortex. Insertion torque was a good intraoperative indicator of screw pull-out force after cyclic loading.

Multiaxial pedicle screw designs: static and dynamic mechanical testing

STUDY DESIGN

Randomized investigation of multiaxial pedicle screw mechanical properties.

OBJECTIVES

Measure static yield and ultimate strengths, yield stiffness, and fatigue resistance according to an established model. Compare these measured properties with expected loads in vivo.

SUMMARY OF BACKGROUND DATA

Multiaxial pedicle screws provide surgical versatility, but the complexity of their design may reduce their strength and fatigue resistance. There is no published data on the mechanical properties of such screws.

MATERIALS AND METHOD

Screws were assembled according to a vertebrectomy model for destructive mechanical testing. Groups of five assemblies were tested in static tension and compression and subject to three cyclical loads. Modes of failure, yield, and ultimate strength, yield stiffness, and cycles to failure were determined for six designs of screw.

RESULTS

Static compression yield loads ranged from 217.1 to 388.0 N and yield stiffness from 23.7 to 38.0 N/mm. Cycles to failure ranged from 42 x 10(3) to 4,719 x 10(3) at 75% of static ultimate load. There were significant differences between designs in all modes of testing. Failure occurred at the multiaxial link in static and cyclical compression.

CONCLUSIONS

Bending yield strengths just exceeded loads expected in vivo. Multiaxial designs had lower static bending yield strength than fixed screw designs. Five out of six multiaxial screw designs achieved one million cycles at 200 N in compression bending. "Ball-in-cup" multiaxial locking mechanisms were vulnerable to fatigue failure. Smooth surfaces and thicker material appeared to be protective against fatigue failure.

Evaluation of indication-based use of transpedicular instrumentations with different rigidity for lumbar spinal fusion: a prospective pilot study with 3 years of follow-up

In a prospective cohort study in 94 patients with 3 years' follow-up the efficacy of rigid and semi-rigid transpedicular instrumentation for lumbar spine fusion was evaluated via three established scores. Patient groups were similar in respect of anthropometric data. The indication for using the semi-rigid technique was a fairly stable intraoperative situation; for the more common unstable situations, the rigid technique was chosen. Selecting implant rigidity on these criteria led to results with an improvement rate well within the upper success range reported in the literature. Among people in employment, a lengthy preoperative sick leave was an important predictor for unsatisfactory outcome.

Characteristics of pullout failure in conical and cylindrical pedicle screws after full insertion and back-out

BACKGROUND CONTEXT

Biomechanical studies show that bone-mineral density, pedicle morphology, and screw thread area affect pedicle screw pullout failure. The current literature is based on studies of cylindrical pedicle screw designs. Conical screws have been introduced that may provide better "fit and fill" of the dorsal pedicle as well as improved resistance to screw bending failure. However, there is concern about loss of fixation if conical screws must be backed out after insertion.

PURPOSE

To determine that conical screws have comparable initial stiffness and fixation strength compared with standard, cylindrical screws, and to assess whether conical screw fixation deteriorates when screws are backed out from full insertion.

STUDY DESIGN/SETTING

This biomechanical analysis compared pullout strength of cylindrical and conical pedicle screw designs, using porcine lumbar vertebrae in a paired testing format.

METHODS

Porcine lumbar vertebrae were instrumented with conical and cylindrical pedicle screws with the same thread pitch, area and contour, and an equivalent diameter at the pedicle isthmus, 1.2 cm distal to the hub. Axial pullout was performed at 1.0 mm/minute displacement. Pullout loads, work and stiffness were recorded at 0.02-second intervals. Conical versus cylindrical screws were tested using three paired control configurations: fully inserted, backed out 180 degrees and backed out 360 degrees. Fully inserted values were compared with each set of back-out values to determine relative loss of fixation strength. Screw pullout data were analyzed using a Student's t test.

RESULTS

Pullout loads in these porcine specimens were comparable to data from healthy human vertebrae. Conical screws provided a 17% increase in the pullout strength compared with cylindrical screws (P<.10) and a 50% increase in initial stiffness (P<.05) at full insertion. There was no loss in pullout strength, stiffness or work to failure when conical or cylindrical screws were backed out 180 or 360 degrees from full insertion.

CONCLUSIONS

Conical screws offer improved initial fixation strength compared with cylindrical screws of the same size and thread design. Our results suggest that appropriately designed conical screws can be backed out 180 to 360 degrees for intraoperative adjustment without loss of pullout strength, stiffness or work to failure. Intraoperative adjustments of these specific conical screws less than 360 degrees should not affect initial fixation strength. These results may not hold true for screws with a smaller thread area or larger minor diameter.

Loosening of sacral screw fixation under in vitro fatigue loading

Sacral screw fixation is frequently used for fusion of the lower lumbar spine, but sacral screws appear to offer less secure fixation than lumbar pedicle screws, and failure due to loosening under fatigue loading is common. The aim of this study was to examine in vitro the stability of medial and lateral bicortical and unicortical sacral screw fixation under a physiologically relevant fatigue-loading pattern. Bone mineral density, screw insertion torque, and screw-fixation stiffness were measured prior to cyclic loading between 40 and 400 N compression at 2 Hz for 20,000 cycles. The screw-fixation stiffness was measured every 500 cycles, and the axial pullout strength of the screws was recorded following loading. All of the lateral insertions loosened under the applied loading, but some of the medial insertions remained stable. Medial insertions proved stiffer and stronger than lateral insertions, and bicortical fixations were stronger than unicortical fixations. Bone mineral density and insertion torque were correlated with screw stiffness and pullout strength, although better correlation was found for insertion torque than bone mineral density. Bone mineral density is a good preoperative indicator of sacral screw-fixation strength, and insertion torque is a good intraoperative indicator. An insertion torque greater than 1.5 Nm is suggested as an indicative value for a stable medial unicortical insertion, whereas an insertion torque greater than 2 Nm suggests a stable medial bicortical insertion. It appears that, apart from the choice of technique (screw orientation and depth), minimizing the load on the screws during the initial part of the fusion process is also critical to maintain stability of the fused section and to obtain a solid fusion mass.

The effects of cyclic loading on pull-out strength of sacral screw fixation: an in vitro biomechanical study

STUDY DESIGN

The pull-out strength of sacral screw fixation after cyclic loading was tested using young human cadaveric specimens.

OBJECTIVES

To evaluate the effects of fatigue loading on the pull-out strength of medial and lateral unicortical and bicortical sacral screws and to correlate the pull-out strength with sacral bone density and the screw insertion torque.

SUMMARY OF BACKGROUND DATA

The immediate biomechanical effects of depth of penetration, screw orientation, and bone density on sacral screw fixation have been studied in aged cadaveric specimens. The effect of cyclic loading on the pull-out strength of sacral screw fixation is unknown, however, and data from young specimens is rare.

METHODS

Eleven fresh specimens of human sacrum were used in this study. Bone mineral density at the vertebral body and the ala were determined by peripheral quantitative computed tomography. Seven-millimeter compact Cotrel-Dubousset sacral screws were inserted into the sacrum anteromedially and anterolaterally, both unicortically and bicortically, and the insertion torque for each screw was measured. Cyclic loading from 40 to 400 N was applied to each screw at a frequency of 2 Hz up to 20,000 cycles. Pull-out tests were conducted after completion of the fatigue tests.

RESULTS

The average bone density was 0.38 +/- 0.08 g/mL at the S1 body and 0.24 +/- 0.05 g/mL at the S1 ala. The insertion torque and average pull-out force after cyclic loading were significantly higher for bicortical fixation than for unicortical fixation for a particular screw alignment. The pull-out strength and insertion torque of medially oriented fixation was always higher than that for lateral fixation, however, regardless of whether the insertion was unicortical or bicortical. The pull-out force of unicortical and bicortical medial screw fixations after cyclic loading showed significant linear correlations with both the insertion torque and the bone mineral density of the S1 body.

CONCLUSIONS

In a young population, screw orientation (anterolateral or anteromedial) was more important in determining pull-out strength than screw depth (unicortical or bicortical) after fatigue loading, anteromedially directed screws being significantly stronger than laterallyplaced screws. Bone mineral density of the S1 body andinsertion torque were good preoperative and intraoperative indicators of screw pull-out strength.

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

STUDY DESIGN

An in vitro test of calf spine lumbar segments to compare biomechanical stabilization of a rigid versus a dynamic posterior fixation device.

OBJECTIVES

To compare flexibility of a dynamic pedicle screw fixation device with an equivalent rigid device.

SUMMARY OF BACKGROUND DATA

Dynamic pedicle screw device studies are not as prevalent in the literature as studies of rigid devices. These devices contain the potential to enhance load sharing and optimize fusion potential while maintaining stability similar to that of rigid systems.

METHODS

Load-displacement tests were performed on intact and stabilized calf spines for the dynamic and rigid devices. Stability across a destabilized L3-L4 segment was restored by insertion of either a 6 mm x 40 mm dynamic or rigid pedicle screw fixation device across the L2-L4 segment. The screws then were removed, 7 mm x 45 mm pedicle screws of the opposite type were inserted, and the construct then was re-tested. Axial pull-out tests were performed to assess the likely effects of pedicle screw replacement on the load-displacement data.

RESULTS

Results indicated a 65% reduction in motion in flexion-extension and a 90% reduction in lateral bending across the destabilized level for both devices, compared with intact spine values. Reduction in axial rotation motion was much smaller than in other modes. Axial pull-out tests showed no weakening of the bone-screw interface.

CONCLUSIONS

Both devices provided significant stability of similar magnitudes in flexion, extension, and lateral bending. In axial rotation, the devices only could restore stability to levels similar to those in an intact spine. The dynamic device offers a design that may enhance load sharing without sacrificing construct stability.

Biomechanical testing sequelae relevant to spinal fusion and instrumentation

The increasing prevalence of spinal disorders and associated treatments has produced a dramatic increase in the number of available devices. The biomechanical evaluation leading to the design, development, and implementation of spinal instrumentation has resulted in a number of in vitro and in vivo testing methods. This article reviews some of the methods and associated results obtained by various evaluation techniques of spinal fusion hardware. Current work and future considerations also are presented.