Calf Vertebrae Research Articles

A new alternative to expandable pedicle screws: Expandable poly-ether-ether-ketone shell

Screw pullout is a very common problem in the fixation of sacrum with pedicle screws. The principal cause of this problem is that the cyclic micro motions in the fixation of sacrum are higher than the other regions of the vertebrae that limit the osteo-integration between bone and screw. In addition to that, the bone quality is very poor at sacrum region. This study investigated a possible solution to the pullout problem without the expandable screws' handicaps. Newly designed poly-ether-ether-ketone expandable shell and classical pedicle screws were biomechanically compared. Torsion test, pullout tests, fatigue tests, flexion/extension moment test, axial gripping capacity tests and torsional gripping capacity tests were conducted in accordance with ASTM F543, F1798 and F1717. Standard polyurethane foam and calf vertebrae were used as embedding medium for pullout tests. Classical pedicle screw pullout load on polyurethane foam was 564.8 N compared to the failure load for calf vertebrae's 1264 N. Under the same test conditions, expandable poly-ether-ether-ketone shell system's pullout loads from polyurethane foam and calf vertebrae were 1196.3 and 1890 N, respectively. The pullout values for expandable poly-ether-ether-ketone shell were 33% and 53% higher than classical pedicle screw on polyurethane foam and calf vertebrae, respectively. The expandable poly-ether-ether-ketone shell exhibited endurance on its 90% of yield load. Contrary to poly-ether-ether-ketone shell, classical pedicle screw exhibited endurance on 70% of its yield load. Expandable poly-ether-ether-ketone shell exhibited much higher pullout performance than classical pedicle screw. Fatigue performance of expandable poly-ether-ether-ketone shell is also higher than classical pedicle screw due to damping the micro motion capacity of the poly-ether-ether-ketone. Expandable poly-ether-ether-ketone shell is a safe alternative to all other expandable pedicle screw systems on mechanical perspective.

Bicortical Versus Unicortical Pedicle Screws in Direct Vertebral Rotation

In vitro experimental study. This study aimed to evaluate the biomechanical properties of bicortically placed and laterally oriented screws, which may represent an alternative approach for challenging sites during direct vertebral rotation (DVR). DVR corrects the transverse plane deformity and the thoracic hump in idiopathic scoliosis. However, instrumentation of the convex side of the scoliosis apex may pose a challenge, not allowing the placement of suitable sized screws in adequate direction. Forty-eight calf vertebrae were used and each vertebral body was instrumented with 1 pedicle screw as follows: unicortical group (n=16), a short screw was unicortically placed and directed laterally; bicortical group (n=16), a short screw was bicortically placed again in lateral direction; control group (n=16), a screw with ideal length and direction was placed. Vertebral bodies were rigidly anchored in a custom device. Each screw was rotated using a constant length lever arm while collecting "force to failure" data. Significantly better results were obtained with bicortical screwing when compared with unicortical screwing (335.4±45.6 vs. 239.5±58.50 N, P<0.001). However, mean "force to failure" was significantly higher in the controls than in the bicortical group (415.8±49.2 vs. 335.4±45.6 N, P<0.001). Bicortical screw placement may provide a biomechanically superior construct than unicortical screw placement for resisting DVR maneuver during scoliosis correction. This technique may represent an effective and safe approach, particularly for the convex side of the scoliosis apex, with increased resistance to derotational forces and decreased risk of bone failure. Further clinical studies are warranted for firmer conclusions.

Design and biomechanical testing of pedicle screw for osteoporotic incidents

In this study, geometrical features of pedicle screws have been modified and their performances are compared. Performance analysis has been made in terms of pull-out strength and torsional strength. The parameters investigated are core diameter, holes drilled normal to screw axis, angle between sequential holes and distance between holes. Three different core diameter have been studied, which are 4 mm (normal core diameter), 5 mm (medium core diameter) and 5.5 mm (high core diameter). Distance between sequential holes has been arranged such that there is either one hole per pitch or one hole per two pitches. Angle between sequential holes is either 90 degrees or 120 degrees. According to the test results, the screw, with medium core diameter (5 mm) containing one hole per two pitches with 90 degrees angle between sequential holes, has exhibited the optimum performance considering torsional strength and pull-out strength requirements. Its torsional strength is slightly higher than and, when Grade 40 polyurethane foam was used as bone simulating material, its pull-out strength is as good as, an undrilled normal core diameter screw, which is already being used in surgical operations. The fatigue performance of this best performed screw has also been found satisfactory according to the related standard. Its pull-out strength is also tested on a calf vertebra and a promising result has been obtained.

Experimental study of three dimensional navigation assisted spinal surgery by multi-segment registration technology based on structured light scanning

To study the registration method based on structured light scanning for navigation assisted spinal surgery and assess its accuracy so as to construct a registration system for the navigation assisted spinal surgery using structured light scanning. Both the computed tomographic (CT) dataset and the structured light scanning images of thoracic vertebra were obtained. The pre-registration and multi-segment iterative closest point (ICP) algorithm were used for the registration of CT images and structured light images. Four segmentations were selected from the surface of thoracic vertebra and placed into different combinations. The accuracy for each combination was studied. Noise and perturbation were exerted to structured light and registration accuracy was studied. And calf vertebra was used for further verification. A combination of pre-registration and multi-segment iterative closest point (ICP) algorithm was competent for the registration of CT scanning data and the structured light scanning data. The registration error was less than 1 mm when two and more segments were selected for registration combination. The registration error was less than 1 mm when noise was exerted. With a high accuracy and a perturbation resistance, a combination of pre-registration and multi-segment registration algorithm based on structured light scanning is competent for the registration of CT scanning data and structured light scanning data.

The effect of immersion times on the solubility and bonding stress of hydroxyapatite coated Ti6al4v pedicle screws.

Main aim of the present study was to test and show the dissolution of coatings onto Ti6Al4V pedicle screws by both in vitro and in vivo analysis. Hydroxyapatite (HA) was coated on Ti6Al4V by electrophoretic deposition (EPD). The dissolution of the coatings was tested in vitro. The coating was characterized before and after implantation via pull-out tests, X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The coating materials were immersed in a simulated body fluid (SBF) from 1 day to 6 weeks, in vitro, and the elemental analysis were undertaken in a calf spine, in vivo. Fourteen coated (7) and uncoated (7) Ti6Al4V pedicle screws were implanted in a calf spine for 12 weeks and the elemental concentration was analyzed using the wavelength dispersive X-ray fluorescence (WDXRF) technique. The analysis of elemental concentrations by WDXRF are presented and 97% of the dissolved elements were determined as Ca, P, Ti, O and 3% of K, Na, Fe, Ni, Zn, Mg, S in total. The solubility of the coating materials increased with increasing immersion time and, interestingly, it increased substantially for reasonably longer times, for example, 6 weeks. Through the in vivo experiments, on the HA coated and uncoated pedicle screws implanted on a calf vertebra, the elemental concentrations by WDXRF analysis showed the Ca(%) around the pedicle screws increased by 30% after re-implantation process. The results may lead us to reconsider such ceramic coatings to test in vitro and/or in vivo for longer periods before clinical applications.

Efeito do macheamento do orifício piloto na resistência ao arrancamento e no torque de inserção dos parafusos pediculares com alma cônica

OBJETIVO: Avaliar a influência do macheamento do orifício piloto na resistência ao arrancamento e no torque de inserção dos parafusos pediculares com alma cônica. MÉTODOS: Ensaios mecânicos de arrancamento em máquina universal de ensaios foram realizados com parafusos pediculares com alma cônica inseridos nos pedículos da quinta vértebra lombar de vitelo. As medidas do torque de inserção foram realizadas com torquímetro com capacidade de 10Nm, sendo considerado o maior valor de torque. O preparo do orifício piloto foi realizado por meio de sonda com 3,8 mm de diâmetro externo e o macheamento do orifício com macho de rosca com as mesmas dimensões e características de rosca do parafuso. RESULTADOS: Foi observada redução do torque de inserção e da resistência ao arrancamento dos implantes no grupo com o macheamento prévio do orifício piloto. CONCLUSÕES: O macheamento do orifício piloto reduziu o torque de inserção e a força de arrancamento dos parafusos pediculares com alma cônica inseridos no pedículo da quinta vértebra lombar de vitelos.

Screw design alters the effects of stress relaxation on pullout

Stress relaxation during pullout of a pedicle screw decreases the peak load and stiffness of the bone-screw interface. However, it is unknown whether this can be generalized to all types of screw designs. This study aimed to show whether screw design altered the effects of stress relaxation on the mechanical performance of the pedicle screw during pullout. Twelve calf vertebrae were obtained: six vertebrae were instrumented with 7.5x40 mm conical pedicle screws and the other six with 5.0x40 mm cylindrical pedicle screws. The screws with two different designs were pulled out using either a standard pullout or a stress relaxation pullout protocol. Both bone-screw interfaces had lower stiffness in the stress relaxation pullout model than in the standard pullout model, but it was significant in only the cylindrical design group (P<0.05). However, the stress relaxation and standard pullout models did not yield any difference in peak loads in either screw type. Although stress relaxation at the bone-screw interface can alter the mechanical performance of the screw, this may be eliminated by modifying the screw design. A better understanding of viscoelastic properties of the bone-screw interface may help improve implant design and thus, clinical outcomes.

A demineralized calf vertebra model as an alternative to classic osteoporotic vertebra models for pedicle screw pullout studies

Screws, clamps and other spinal instrumentation materials are tested using healthy animal and healthy human vertebrae, but the application of similar tests to an osteoporotic vertebra is generally neglected because of high costs and limited availability of high quality and consistent osteoporotic vertebrae. The objective of this study is to develop an in-vitro method to decrease the mineral content of an animal vertebra utilizing decalcifying chemical agents that alters the bone mineral density and some biomechanical properties to such an extent that they biomechanically mimic the osteoporotic spine. This study was performed on 24 fresh calf lumbar vertebrae. Twelve out of these 24 vertebrae were demineralized and the others served as control. A hole was opened in the pedicles of each vertebrae and the bone mineral density was measured. Each vertebra was then placed into a beher-glass filled with hydrochloric acid decalcifier solution. The decalcifier solution was introduced through the holes in the pedicles with an infusion pump. The vertebrae were then subjected to DEXA to measure post process BMD. Pedicle screws were introduced into both pedicles of each vertebrae and pullout testing was performed at a rate of 5 mm/min. The difference of BMD measurements between pre- and post-demineralizing process were also statistically significant (p < 0.001). The difference of pullout loads between pre- and post-demineralizing process were also statistically significant (p < 0.001). The acid demineralizing process may be useful for producing a vertebra that has some biomechanical properties that are consistent with osteopenia or osteoporosis in humans.

Effects of angle and laminectomy on triangulated pedicle screws

We aimed to demonstrate the effect of angle and laminectomy on paired pedicle screws to determine whether a 90° screw angle is optimal as has been previously suggested. According to the angle between right and left screws, 28 calf vertebrae were divided into three groups and instrumented as follows: Group I: 60° screw angle; Group II: 90° angle; Group III: 60° angle with laminectomy. The screws were connected using rods and cross-fixators and tested to peak pullout force. Triangulated pedicle screws provided 76.5% more pullout strength than single screws. Most of the specimens failed through loss of convergence angle (toggling of screws on the rods) and subsequent uni- or bilateral screw pullout. Mean ± SD peak loads were: Group I: 2071 ± 622 N; Group II: 1753 ± 497 N; Group III: 2186 ± 587 N. The differences were not significant ( p > 0.05). 90° triangulation was not associated with a superior pullout performance versus conventional 60° triangulation, suggesting that achieving additional triangulation angle is not necessary to obtain increased pullout strength. Laminectomy did not alter the effect of triangulation on fixation strength.

Evaluation of Cyanoacrylate Augmentation of Transpedicular Screw Pullout Strength

Pedicle screw fixation of osteoporotic bone in the elderly is a challenge. Various augmentation methods have been studied by many authors. Although polymethylmethacrylate (PMMA) augmentation is believed to be a standard method, its usage is fraught with complications. Butyl-2-cyanoacrylate is an alternative to PMMA as it is bioresorbable, biocompatible, inexpensive, and noninfective. The objective of the current study was to determine the pullout strength of the pedicle screws when butyl-2-cyanoacrylate is used for augmentation. Fresh calf lumbar vertebrae were obtained from male calves weighing 100-120 kg and implanted with pedicle screws. The screws were placed in native, unaugmented bone (group 1), butyl-2-cyanoacrylate-augmented bone (group 2), and PMMA-augmented bone (group 3). Axial pullout tests were done by an Instron 4411 universal testing machine. Statistical analysis was performed using the SPSS 9.0 for Windows program. Paired samples t test was used, and P < 0.05 was considered significant. The mean bone mineral density of the vertebrae was 1.6 +/- 0.1 g/cm2. The mean pullout strengths were 1.55 +/- 0.23 kN for group 1, 1.62 +/- 0.42 kN for group 2, and 2.55 +/- 0.22 kN for group 3. There was no statistically significant difference between groups 1 and 2. PMMA augmentation increased the pullout strength significantly when compared with butyl-2-cyanoacrylate augmentation and native bone (P = 0.002 and P = 0.001, respectively). The results of this study show that butyl-2-cyanoacrylate has no contribution to the augmentation of pedicle screw fixation in a calf model when compared with native bone or PMMA augmentation. Further studies are required to evaluate the effectiveness of butyl-2-cyanoacrylate in osteoporotic specimens and under cyclic loading in calf vertebra and animal and cadaver models before dispensing with its utility as an augmentation method in the clinical setting.

Biomechanical evaluation of an expansive pedicle screw in calf vertebrae

The main objective of the present study is to evaluate biomechanically a newly designed expansive pedicle screw (EPS) using fresh pedicles from calf lumber vertebrae in comparison with conventional pedicle screws, (CDH) CD Horizon, Universal Spine System pedicle screw (USS) and Tenor (Sofamor Denek). Pull-out and turning-back tests were performed on these pedicle screws to compare their holding strength. Additionally, revision tests were undertaken to evaluate the mechanical properties of EPS as a "rescue" revision screw. A fatigue simulation test using a perpendicular load up to 1,500,000 cycles was also carried out. The results showed that the turning back torque (Tmax) and pull-out force (Fmax) of EPS screws were significantly greater than those of USS, Tenor and CDH screws (6.5x40 mm). In revision tests, the Fmax of both types of EPS screws (6.5x40 mm; 7.0x40 mm) were significantly greater than that of CDH, USS, and Tenor screws (P<0.05). Furthermore, no screws were broken or bent at the end of fatigue tests. The findings from the current study suggest that expansive pedicle screws can significantly improve the bone purchase and the pull-out strength compared to USS, Tenor and CDH screws of similar dimensions before and after a failure simulation.

Bone cement or bone substitute augmentation of pedicle screws improves pullout strength in posterior spinal fixation.

Pedicle screws are widely used to fix posterior spinal implants. However, in some situations, such as at the ends of long constructs in scoliosis correction, the screws may pull out of the pedicles. This limits the use of pedicle screw fixation where bone quality is poor. The aim of this study was to investigate the effect of using either a low-viscosity bone cement (Palacos LV) or a bone augmentation material (Cortoss) on the pullout strength of typical pedicle screws (5 mm USS Schanz screws). Ten lumbar calf vertebrae were implanted with pedicle screws. One screw was inserted as normal, and the contralateral screw was augmented with Palacos LV or Cortoss. A plate was then cemented to the posterior surface of each pedicle and the screws were pulled out using a tensile testing machine. The pullout strength of the non-augmented screws was 1203+/-260 N, while the pullout strength of the augmented screws was 1970+/-220 N (Palacos LV) and 2021+/-342 N (Cortoss). Both Palacos LV and Cortoss significantly increased the pullout strength (p=0.0213 and p=0.0029, respectively). There was no significant difference between the Palacos LV and Cortoss groups (p=0.79).

Correlations between screw hole preparation, torque of insertion, and pullout strength for spinal screws.

The bone-screw interface is critical in the use of spinal instrumentation. The purpose of these experiments described here was twofold. First, to determine whether a correlation existed between torque generated during screw insertion and the pullout strength. Second, to determine how differing surgical methods of screw hole preparation influenced torque of insertion and screw pullout strength. A series of experiments were carried out in which screws were inserted into synthetic bone (experiment 1) and into calf vertebrae (experiment 2). The method of screw hole preparation (i.e., diameter of entrance hole and pilot hole) was varied while the resulting torque of insertion and the pullout strength of the screw was measured in each case. A torque screwdriver was used to measure the torque of insertion of the screws. Screw pullout strength was measured using a materials testing machine. Two important results emerged from these experiments. First, a higher torque of insertion correlated with a higher screw pullout force. This correlation may be useful intraoperatively in evaluating fixation. Second, torque of insertion and pullout force were more influenced by cortex over-drill diameter than pilot hole diameter. These experiments show the importance of the dorsal cortex in pedicle screw fixation.