Pullout Strength Of Screws Research Articles

Drilling resistance: A method to investigate bone quality.

Bone drilling is a major part of orthopaedic surgery performed during the internal fixation of fractured bones. At present, information related to drilling force, drilling torque, rate of drill-bit penetration and drill-bit rotational speed is not available to orthopaedic surgeons, clinicians and researchers as bone drilling is performed manually. This study demonstrates that bone drilling force data if recorded in-vivo, during the repair of bone fractures, can provide information about the quality of the bone. To understand the variability and anisotropic behaviour of cortical bone tissue, specimens cut from three anatomic positions of pig and bovine were investigated at the same drilling speed and feed rate. The experimental results showed that the drilling force does not only vary from one animal bone to another, but also vary within the same bone due to its changing microstructure. Drilling force does not give a direct indication of bone quality; therefore it has been correlated with screw pull-out force to provide a realistic estimation of the bone quality. A significantly high value of correlation (r2 = 0.93 for pig bones and r2 = 0.88 for bovine bones) between maximum drilling force and normalised screw pull-out strength was found. The results show that drilling data can be used to indicate bone quality during orthopaedic surgery.

Biomechanical study of the fixation stability of broken pedicle screws and subsequent strategies.

Pedicles are often broken when screws are inserted into hard pedicles with small diameters or when the diameter of the screw itself is inadequate. However, there is a lack of biomechanical literature that addresses screw loosening as a result of broken pedicles or the resulting salvage of those screws. We performed a novel in vitro study to compare the pullout strength of screws between intact pedicles and two different types of broken pedicles; strategies to prevent screw loosening were also compared. Thirty L4 Sawbones were designed to represent intact pedicles, semi-pedicles, and nonpedicles and were prepared for screw insertion. Three sizes of polyaxial screws (diameter × length dimensions of 6.0 mm × 45 mm, 6.0 mm × 50 mm and 6.5 mm × 45 mm) were independently and randomly distributed into the intact-pedicle group (IP group, n = 30), the semi-pedicle group (SP group, n = 15), or the nonpedicle group (NP group, n = 15). The experiments were conducted across a minimum of five trials for each of the chosen screw sizes. We then analyzed the results of the imaging, pullout testing, and embedded bone volume. Any fractures or defects of the vertebrae caused by screw insertion were excluded from the study. The appropriate screw trajectory and insertional depth were confirmed using axial and sagittal X-ray imaging prior to screw pullout testing. A pullout strength of only 41% to 45% for the SP group and 29% to 39% for the NP group was retained following a broken pedicle. The use of longer or larger-diameter screws appears to be an effective salvaging procedure for the NP group (p < 0.05). The embedded bone volume percentage analysis indicated that, compared to the IP group, 68% to 76% of effective bone embedded into the screw threads in the SP group, and 58% to 65% in the NP group. There was no direct correlation between the pullout strength and the embedded bone volume; however, less effective embedded bone volume was associated with lower pullout strength. This study describes the evolution of the well-established screw pullout test being applied to the broken pedicle Sawbone testing model. The pedicle plays an important role in determining the pullout strength of a screw. However, a salvage procedure that utilizes a longer or larger-diameter screw might be a reliable clinical approach to address broken pedicles.

Pullout Strength Predictor: A Machine Learning Approach.

Study DesignA biomechanical study.PurposeTo develop a predictive model for pullout strength.Overview of LiteratureSpine fusion surgeries are performed to correct joint deformities by restricting motion between two or more unstable vertebrae. The pedicle screw provides a corrective force to the unstable spinal segment and arrests motions at the unit that are being fused. To determine the hold of a screw, surgeons depend on a subjective perioperative feeling of insertion torque. The objective of the paper was to develop a machine learning based model using density of foam, insertion angle, insertion depth, and reinsertion to predict the pullout strength of pedicle screw.MethodsTo predict the pullout strength of pedicle screw, an experimental dataset of 48 data points was used as training data to construct a model based on different machine learning algorithms. A total of five algorithms were tested in the Weka environment and the performance was evaluated based on correlation coefficient and error matrix. A sensitive study of various parameters for obtaining the best combination of parameters for predicting the pullout strength was also preformed using the L9 orthogonal array of Taguchi Design of Experiments.ResultsRandom forest performed the best with a correlation coefficient of 0.96, relative absolute error of 0.28, and root relative squared error of 0.29. The difference between the experimental and predicted value for the six test cases was not significant (p >0.05).ConclusionsThis model can be used clinically for understanding the failure of pedicle screw pullout and pre-surgical planning for spine surgeon.

Comparison of the Pullout Strength of Pedicle Screws According to the Thread Design for Various Degrees of Bone Quality

Although dual-threaded pedicle screws have been developed, the advantages over single-threaded screws remain controversial. We aimed to investigate the biomechanical performance of two types of dual-threaded pedicle screw by comparing their pullout strength with that of a single-threaded screw in relation to bone quality. Four types of pedicle screw with different thread patterns were designed. Type I: single-threaded screw; Type II: double-threaded screw; Type III: dual-threaded screw; Type IV: a newly designed double dual-threaded screw. Five types of polyurethane foams simulating various degrees of bone quality were used. These were: Type A: cancellous bone; Type B: cancellous bone with cortical bone in the upper margin; Type C: osteoporotic cancellous bone; Type D: osteoporotic cancellous bone with cortical bone in the upper margin; and Type E: osteoporotic bone with cortical bone in the upper and lower margins. A comparison of the pullout strength of Type I, II, and III screws in Type A, B, C and D bone specimens was performed. Type C and E bone specimens were used for comparisons among Type I, II, and IV screws. Compared to the single-threaded screw, the dual-threaded pedicle screws exhibited higher pullout strength in normal-quality bone and significantly lower pullout strength in compromised osteoporotic bone. However, the double dual-threaded screw exhibited better pullout biomechanics in osteoporotic bone with bi-cortical bone.

Bone Hardness of Different Anatomical Regions of Human Radius and its Impact on the Pullout Strength of Screws.

ObjectiveTo investigate the bone hardness of different anatomical regions of the human radius and its impact on the pullout strength of screws.MethodsFresh radius bones were obtained from three donated cadavers. They were divided into three parts: proximal metaphysis, shaft, and distal metaphysis. The proximal metaphysis contains the head, neck, and radial tuberosity. The distal metaphysis includes the palmaris radius and the styloid process. The shaft of the radius was divided into nine segments of equal length. The bone hardness of three radiuses, one from each cadaver, was measured by Vickers microindentation hardness tests, and the screw pullout strength was examined in the other three radiuses using a materials testing machine. The trend between radius hardness and pullout strength was analyzed by using an analysis of variance randomized block design. Pearson correlation analysis was performed to evaluate the linear correlation between the bone hardness and the pullout strength of the human radius.ResultsThe mean hardness ranged from 33.30 HV (the head) to 43.82 HV (the diaphysis). The hardest part of the radius was the shaft, with a value of 42.54 ± 5.59 HV. The proximal metaphysis had a hardness value of 34.15 ± 6.48 HV, and the distal metaphysis hardness value was 35.24 ± 5.17 HV. The shaft was 23.5% harder than the proximal metaphysis and 20% harder than the distal metaphysis. The microhardness test demonstrated that the bone hardness value of the diaphysis was significantly higher than those of both the proximal and distal metaphysis of the radius (both P < 0.05). The mean pullout strength values ranged from 552 N (the distal metaphysis) to 2296 N (the diaphysis). The greatest pullout strength of the radius was observed for the shaft, with a pullout strength of 1727.96 ± 111.44 N. The pullout strength of the proximal metaphysis was 726.33 ± 236.39 N, and the pullout strength of the distal metaphysis was 590.67 ± 36.30 N. The pullout strength of the shaft was 138% greater than that of the proximal metaphysis and 190% greater than that of the distal metaphysis. The pullout strength was also higher in the diaphysis than at both ends of the radius (both P < 0.05). A positive correlation was found between bone hardness and pullout strength (R = 0.927, P < 0.001).ConclusionsBone hardness and screw pullout strength are higher in the diaphysis of the radius than at either end. The pullout strength is positively related to bone hardness in the human radius.

Finite element method-based study of pedicle screw–bone connection in pullout test and physiological spinal loads

Finite element (FE) method has been widely used to study the screw–bone connections. Screw threads are often excluded from the FE spine model to reduce computational cost. However, no study has been conducted to compare the effect of such simplification in the screw models on the predicting accuracy of the model. The effects of different screw–bone connection types on the overall spine biomechanics are also unknown. In this study, three different types of screw–bone connections were compared using FE simulations in this study: (1) screw and bone are not fully bonded (contact connection); (2) screw is rigidly bonded with the bone (bonded connection); and (3) simplified-geometry-rigid (SGR) connection. Screw pullout test and physiological spinal loading test were simulated for the screws in this study: (1) pullout test where the pedicle screws were inserted in polyurethane foam; and (2) physiological spinal loading test (flexion, extension, lateral bending, and axial rotation) where the screws were fused into previously-validated FE lumbar spine model. The FE spine model used in this study included L1–L5 spine levels and simulated major ligaments and resultant muscle forces. This study indicated that the holding capability in the screw–bone interaction is smaller and the bone and implants are subjected to larger von Mises stress (up to 44.88%) in the contact connection than those in the bonded connection. Among the four spinal loading cases tested in this study, flexion produced the highest von Mises stress in both the bone and the implants. Considerable differences were observed between simplified and non-simplified screw FE models in the von Mises stress at screw–bone contact region within spinal loading environment and the ultimate screw pullout strength in pullout test. This study concluded that both the spinal implants and the bone are subjected to higher stress immediately after the pedicle-screw-instrumented surgery and before the screw and bone are fully bonded. The screw–bone interface is less likely to fail after the screw and bone are fully bonded. SGR screw model is able to predict screw force and rod stress that are consistent with those predicted by non-simplified screw models.

Cortical bone trajectory screws used to save failed traditional trajectory screws in the osteoporotic lumbar spine and vice versa: a human cadaveric biomechanical study.

Traditional trajectory (TT) screws are widely used in lumbar fixation. However, they may require revision surgery in some instances, especially in patients with osteoporotic spines. Cortical bone trajectory (CBT) screws may potentially be used to rescue a failed TT screw and vice versa in nonosteoporotic spines. This study aimed to investigate whether a CBT screw can salvage a compromised TT screw in osteoporotic lumbar spines and vice versa. A total of 42 vertebrae from 17 cadaveric lumbar spines were obtained. Bone mineral density was measured, and a CBT screw was randomly inserted into one side of each vertebra. A TT screw was then inserted into the contralateral side. The biomechanical properties of the screws were tested to determine their insertional torque, pullout strength, and fatigue performance. After checking the screws for the failure of each specimen, the failed screw track was salvaged with a screw of the opposite trajectory. The specimen was then subjected to the same mechanical tests, and results were recorded. A repeat pullout test on TT and CBT screws was also performed. When CBT screws were used to rescue failed TT screws, the original torque increased by 50%, an average of 81% of the pullout strength of the initial TT screws was retained, and the fatigue performance was equal to that of the original screws, which were considerably stronger than the loose TT screws-that is, the TT repeat screws/TT screws were 33% of the pullout strength of the initial TT screws. When the TT screws were used to salvage the compromised CBT screws, the TT screws retained 51% of the original torque and 54% of the original pullout strength, and these screws were still stronger than the loose CBT screws-that is, the loose CBT screws retained 12% pullout strength of the initial CBT screws. Fatigue performance and the ratio of the pullout strength considerably decreased between the CBT rescue screws and the original CBT screws but slightly changed between the TT rescue screws and the original TT screws. CBT and TT screws can be applied in a revision technique to salvage each other in osteoporotic lumbar spines. Additionally, CBT and TT screws each retain adequate insertional torque, pullout strength, and fatigue performance when used for revision in osteoporotic lumbar spines.

INFLUENCE OF DIAMETER AND GEOMETRY IN THE TAPPING OF THE PILOT HOLE IN PEDICLE SCREWS

ABSTRACT Objective: To evaluate the insertion torque and the pulling force of each screw with different diameters and tap. Methods: Polyurethane blocks with a pilot hole of 2.7 mm were used in the study. An experimental group with 5 blocks was formed, the insertion torque was evaluated with a torque meter, and the pullout strength of each Globus screw of 5.5 mm and 6.5 mm was assessed. Results: The comparison of the insertion torque on the 5.5 mm screws with pilot hole without tapping and with a smaller diameter than that of the screw (4.5 mm) and a different thread, and with the tapping with the same diameter as that of the screw (5.5 mm) and equal or different thread presented a statistical difference with a higher value of the insertion torque in the group in which the tapping was not performed. As for the pulling force of the 5.5 mm screw, the non-tapping of the pilot hole resulted in statistical difference with the same diameter of the screw (5.5 mm) and with a different thread of the screw. The pullout force on the 6.5 mm screw was higher in the group where the pilot hole was not tapped according to the non-parametric Kruskal-Wallis test, with significance level of p &lt;0.05 in the comparison of the groups. Conclusions: Pilot hole tapping reduced insertion torque and pullout resistance of the pedicle screw influencing the fixation with tapping with the same screw diameter and different thread design.

Improved bone ingrowth of tricalcium phosphate filled Poly(methyl methacrylate) (PMMA) bone cements in vivo

The cement augmentation technique using poly(methyl methacrylate) (PMMA) is considered to be the most effective method for fixation of osteoporotic fracture because of its outstanding mechanical properties. However, the low bio-degradation of PMMA, which hinders bone ingrowth, limits it potential for a broad range of applications. Previous studies have shown blending β-tricalcium phosphate (β-TCP) with PMMA was able to improve the osteoconduction while the material still suffered poor mechanical properties due to the poor compatibility between two components. In this study, we explored the use of hydroxyethyl methacrylate (HEMA) as a compatibilizer for improving the affinity of TCP/PMMA blends system. The resultant TCP/HEMA/PMMA(THP) composite cements exhibited similar setting properties to PMMA cement but much better mechanical properties than composite systems without HEMA as a compatibilizer. No cytotoxicity or hemolysis was observed in the THP bone cement system by biochemical assays. Bone ingrowth was significantly improved at 12 weeks. The reinforced pull-out strength of screws was observed at 4, 8, 12 weeks through in vivo studies performed on rabbit femurs. The THP system, as an injectable bone cement material, shows great potential for use in the cement augmentation technique.

Pedicle Screw Designs in Spinal Surgery

An experimental laboratory-based biomechanical study. The objective of this study was to evaluate, in a synthetic bone model, the difference in primary and revision pull-out strength using pedicle screws of different thread designs. Over the past few decades, there has been a growing interest in optimizing the screw pull-out strength using various screw designs (single-thread, mixed-thread, dual-thread). Although primary pull-out strength has been studied previously, little is known about revision pull-out strength of different pedicle screws. The pull-out strengths of three different pedicle screw designs (single-thread, mixed-thread, dual-thread) were tested in standardized polyurethane foam in three sequences. Sequence 1: A 6.5 mm screw was inserted into the foam block and the primary pull-out strength measured. Sequence 2: A 6.5 mm screw was inserted, removed, and then reinserted into the same foam block. The revision pull-out strength was then measured. Sequence 3: A 6.5 mm screw was inserted, removed and a 7.5-mm screw of the same thread design was reinserted. The revision pull-out strength was then measured. The primary pull-out strength was similar across screw designs, although dual-thread screws showed higher primary pull-out strength (2628.8 N) compared to single-thread screws (2184.4 N, P < 0.05). For revision pull-out strength, the mixed-thread screws had significant reduction in revision pull-out strength of 18.6% (1890.2 N, P = 0.0173). Revision with a larger diameter screw improved the pull-out strength back to baseline. Single and dual-thread screws showed no significant reduction in revision pull-out strength. The dual-threaded screws provided the strongest primary pull-out strength for spinal fixation. The mixed-thread screws, however, had the poorest revision pull-out strength, decreasing by 18.6% compared to other screw designs. In cases in which mixed-threaded screws have to be revised (at the index or revision surgery), surgeons should consider the use of larger diameter screws to improve the pull-out strength. 5.

Pullout Strength of Pedicle Screws Following Redirection After Lateral or Medial Wall Breach.

A cadaveric biomechanical study designed to test the pullout strength of pedicle screws. To evaluate the pullout strength of redirected pedicle screws with a larger diameter following lateral wall breach, redirected pedicle screws of the same diameter following medial wall breach, and redirected pedicle screws with a larger diameter following medial wall breach. Screw malposition is one of the main pitfalls of inserting pedicle screws. Intraoperatively a malpositioned screw is redirected and inserted along the correct axis. Forty-seven vertebrae (T9-L5) were harvested from eight fresh cadaveric spines. The 18 pedicle screws that breached the lateral wall were then removed and redirected using a pedicle screw of 1 mm larger in diameter. The 16 pedicle screws that had breached the medial wall were then removed and redirected using a pedicle screw of the same diameter. The other 13 pedicle screws that had breached the medial wall were then removed and redirected using a pedicle screw of 1 mm larger in diameter. The pullout strength was measured. Following lateral wall breach, mean pullout strength for the larger redirected screws was 46.9% greater than that of the correctly aligned screws. Following medial wall breach, mean pullout strength for the redirected screws of the same diameter was 20.6% less than that of the correctly aligned screws. Mean pullout strength for the larger pedicle screws following medial wall breach was 27.3% more than that of the correctly aligned screws. Redirected pedicle screws of larger diameter after a lateral or medial pedicle breach show recovery of pullout strength. However, the pullout strength of redirected pedicle screws of the same diameter after a medial pedicle breach is significantly less than that of correctly aligned screws. 1.

Biomechanical Fixation Properties of the Cortical Bone Trajectory in the Osteoporotic Lumbar Spine

Selecting optimal strategies for improving fixation in osteoporotic lumbar spine is an important issue in clinical research. Cortical bone trajectory (CBT) screws have been proven to enhance screw pullout strength, but biomechanical efficacy of these screws remains understudied. The aim of this study was to evaluate biomechanical efficacy of CBT screws in osteoporotic lumbar spine. Thirty-one vertebrae from 14 cadaveric lumbar spines were obtained. All specimens were measured by computed tomography; the diameter of pedicles, excluding those of vertebral bodies with very small pedicle developments, was calculated. After measuring bone mineral density, the CBT screw was randomly inserted into 1 side, and the traditional trajectory (TT) screw was inserted into the contralateral side. Maximum insertional torque was recorded after screw insertion. Of vertebrae, 21 were subjected to pullout testing at a rate of 5 mm/minute, and 10 were subjected to cyclic fatigue testing. Each construct was loaded until exceeding 5 mm. Average bone mineral density was 0.567 ± 0.101 g/cm2. CBT screws had higher maximum insertional torque (degrees of freedom = 30, t= 5.78, P < 0.001, 0.333 N-m vs. 0.188 N-m) and higher axial pullout strength (degrees of freedom = 20, t= 7.41, P < 0.001, 394 N vs. 241 N) than TT screws. Increased bone mineral density was not significantly associated with higher pullout load. Compared with TT screws, CBT screws showed better resistance to fatigue testing and required more cycles to exceed 5 mm (degrees of freedom = 9, t= 5.62, P < 0.001, 6161 cycles vs. 3639 cycles). Failure load for displacing screws was also significantly greater for CBT screws than for TT screws (degrees of freedom = 9, t= 5.75, P < 0.001, 443 N vs. 317 N). CBT screws had better biomechanical fixation in osteoporotic lumbar spine compared with standard pedicle screws.

The Effect of Pitch Variation and Diameter Variation on Screw Pullout.

Introduction: There are many screw and thread designs commercially available to surgeons for bone fixation. There is a paucity of literature on comparative mechanical properties of various screw and thread designs including variable pitch screws, and tapered screws. This purpose of this study was to test whether varying a screws thread pitch and/or tapering a screws core diameter alters the mechanical performance of screws. Methods: A mechanical pullout test was performed on 4 different screw designs, including a variable pitch screw, a constant pitch screw, and variations of these in a straight and tapered screw design. Three-dimensional printing technology was used to manufacture the metal screws in order to control for as many variables as possible. Results: The pullout strength of the constant pitch screws (304.9 ± 25.3 N, P < .001) was significantly greater than the variable pitch screws (259.7 ± 23.4 N). The pullout strength was also significantly greater for screws with a tapered diameter (305.4 ± 24.1 N) than a constant diameter (259.1 ± 23.5N, P < .001). Tapered diameter variable pitch screws had the largest stiffness overall, which was statistically significant against all other groups (P ⩽ .001). Conclusion: The pullout strength is significantly greater for screws with a tapered diameter than a constant diameter and greater for screws with a constant pitch than for a variable pitch. Results of stiffness testing is mixed depending on the screw taper. The clinical significance of this study is that it provides data on the effects that thread design and tapering have on the pullout strength of screws. Levels of Evidence: Level V: Mechanical study.

Effects of the Insertion Type and Depth on the Pedicle Screw Pullout Strength: A Finite Element Study.

Purpose The pedicle screw is a surgical device that has become widely used in spinal fixation and stabilization. Postsurgical complications such as screw loosening due to fatigue loading and screw breakage still need investigations. Clinical parameters such as the screw insertion type and depth, the bone density, and the patient degree of mobility greatly affect the mechanisms of the implant's failure/success. Methods The current finite element study focused on the prediction of the pedicle screw pullout strength under various conditions such as insertion type, insertion depth, bone quality, and loading mode. Results As depicted in this study, the preservation of the pedicle cortex as in the N1 insertion technique greatly enhances the pullout resistance. In addition, the higher the screw-anchoring depth, permitting to gear a maximum number of threads, the better the protection against premature breakouts of pedicle screws. Conclusions In agreement with experimental data, the type of insertion in which the first screw thread is placed immediately after the preserved pedicle cortex showed the best pullout resistance for both normal and osteoporotic bone.

The Effect of Pitch Variation and Diameter Variation on Screw Pullout in 3D Printed Screws

Category: Basic Sciences/Biologics Introduction/Purpose: There are many screw and thread designs available to the foot and ankle surgeon for bone fixation. Comparing screws between companies is difficult because properties including core diameter, thread diameter, and pitch confound direct comparisons. There is a paucity of literature on comparative mechanical properties of various screw designs including variable pitch screws and tapered screws. The purpose of this study was to determine the effect of varying thread pitch and/or tapering a screws core diameter. Our hypothesis was that variable pitch screws and tapered screws would have decreased strength compared to screws with constant diameter and constant pitch. Methods: 3D metal printing technology was used to manufacture screws that varied in only the test variables. Four different screws designs were created including variable and constant pitch screws in both straight and tapered diameter versions. These screws were pulled out of synthetic bone at a constant rate while force was measured. Pull out strength and stiffness was calculated for each screw. A generalized linear model and ANOVA were completed to assess the effect on pull out strength and stiffness of the pitch and taper variables. Results: The pull out strength of the constant pitch screws (304.9N+-25.3 N, p&lt;0.001) was significantly greater than the variable pitch screws (259.7N+-23.4 N). The pull out strength was also significantly greater for screws with a tapered diameter (305.4N+- 24.1 N) than a constant diameter (259.1N+-23.5 N, p&lt;0.001). Tapered diameter variable pitch screws had the largest stiffness overall (mean stiffness 556.6 N/mm), which was greater than all other screw designs (p=0.001). The difference in stiffness between all screws with either constant pitch or constant diameter was not statistically significant (p=0.060). Conclusion: The pull out strength is significantly greater for screws with a constant pitch than for a variable pitch and greater for a tapered diameter than a constant diameter. The clinical significance of this study is that it provides data on the effects that thread design and tapering have on the pullout strength of screws providing some evidence that variable pitch screws may have lower pull out strength than constant pitch screws. Utilizing 3D printed screws allowed for the control of all other variables, which may otherwise confound the comparison of different commercially available screws.

Testing Pullout Strength of Pedicle Screw Using Synthetic Bone Models: Is a Bilayer Foam Model a Better Representation of Vertebra?

Study DesignA biomechanical study.PurposeA new biomechanical model of the vertebra has been developed that accounts for the inhomogeneity of bone and the contribution of the pedicle toward the holding strength of a pedicle screw.Overview of LiteraturePullout strength studies are typically carried out on rigid polyurethane foams that represent the homogeneous vertebral framework of the spine. However, the contribution of the pedicle region, which contributes to the inhomogeneity in this framework, has not been considered in previous investigations. Therefore, we propose a new biomechanical model that can account for the vertebral inhomogeneity, especially the contribution of the pedicles toward the pullout strength of the pedicle screw.MethodsA bilayer foam model was developed by joining two foams representing the pedicle and the vertebra. The results of the pullout strength tests performed on the foam models were compared with those from the tests performed on the cadaver lumbar vertebra.ResultsSignificant differences (p <0.05) were observed between the pullout strength of the pedicle screw in extremely osteoporotic (0.18±0.11 kN), osteoporotic (0.37±0.14 kN), and normal (0.97±0.4 kN) cadaver vertebra. In the monolayer model, significant differences (p <0.05) were observed in pullout strength between extremely osteoporotic (0.3±0.02 kN), osteoporotic (0.65±0.12 kN), and normal (0.99±0.04 kN) bone model. However, the bilayer foam model exhibited no significant differences (p >0.05) in the pullout strength of pedicle screws between osteoporotic (0.85±0.08 kN) and extremely osteoporotic bone models (0.94±0.08 kN), but there was a significant difference (p <0.05) between osteoporotic (0.94±0.08 kN) and normal bone models (1.19±0.05 kN). There were no significant differences (p >0.05) in pullout strength between cadaver and bilayer foam model in normal bones.Conclusions The new synthetic bone model that reflects the contribution of the pedicles to the pullout strength of the pedicle screws could provide a more efficacious means of testing pedicle-screw pullout strength. The bilayer model can match the pullout strength value of normal lumbar vertebra bone whereas the monolayer foam model was able to match that of the extremely osteoporotic lumbar vertebra.

P.141 Is occipito-condylar screw a better alternative fixation point for occipitocervical stabilization? 1. Review of the literature

Background: Occipitocervical fusion (OCF) using screws and rods offers immediate stability and an high fusion rates. However, multiple cranial fixation points are required in order to compensate for the poor bony purchase. Methods: The aim of this study was to compare the occipital condyle screw with the standard OCF techniques as well as to compare available techniques of the occipital condyle screw insertion. Materials and Methods: A comprehensive “Medline” and “Web of science” database search was performed. Cadaveric, radiographic and case studies were included. Results: The occipital condyle screw in comparison to the occipital plate enables an increased screw length, greater screw pullout strength, lower profile of the hardware and extended grafting surface. Both constructs have similar biomechanical properties (range of motion restriction, stiffness). Proximity of the vertebral artery and hypoglossal canal presents the greatest technical challenge of occipital condyle screw. Four surgical techniques with different entry points, cranial-caudal and medial angulations were described. None of these techniques is superior to the other. Conclusions: Occipital condyle screw is a viable alternative to standard OCF techniques. Challenges exist due to the proximity of the vital anatomical structures. Choice between four available techniques depends on unique patient’s anatomy

Development and optimization of biodegradable calcium phosphate-based nanocomposite for the application of spinal fixation

ABSTRACTClinical practices in spinal surgery have shown that pedicle screw fixation augmented by polymethyl methacrylate (PMMA) bone cement has improved outcomes for osteoporotic patients, Here, a biodegradable calcium phosphate-based nanocomposite (CPN) exhibiting better anti-pullout ability but similar fluidity and dispersing ability compared to clinically-used polymethyl methacrylate (PMMA) was developed. The present study showed that the axial pullout strength of cement-augmented pedicle screws was increased as the liquid/solid ratio (L/S) of CPN decreased. Mechanical evaluation using ASTM-standard Sawbones showed that the axial pullout force of CPN could reach ∼720 N at the L/S ratio of 0.4 mL/g, while PMMA cannot be injected through the screw. In vitro evaluation in decalcified sheep vertebral models showed that the axial pullout forces of CPN and PMMA were similar, both reaching ∼800 N. The excellent anti-pullout property of CPN clearly suggests its potential for replacing PMMA in the application of cannulated pedicle-screw fixation, worth further study and development for clinical uses.

Fixation Strength of Pedicle and Cortical Lumbar Vertebral Screws after Laminectomy: A Cadaver Study.

Cortical screws were proposed as an alternative to the traditional pedicle screws. Diverse experimental results support the biomechanical superiority of cortical screws compared to pedicle screws. Laminectomy is often part of multilevel lumbar surgeries. Laminectomy might weaken the medial bony edge at the entry of the divergently oriented screw and, thereby, the screw purchase. This study investigated the biomechanical strength of lumbar cortical screw after laminectomy. To compare the fixation strength of cortical screws and traditional pedicle screws after lumbar laminectomy. A total of 120 pedicles from 60 lumbar vertebrae of 12 cadavers (8 men, 4 women) were assessed. The mean age of the cadavers was 73.4 ± 6.2 years (range: 62-82 years). Using a posterior midline approach, we inserted the traditional pedicle screws into one and the cortical screws into the other side of each vertebra. Laminectomy was performed after screw insertion. Vertical pullout strength and toggle strength testing were performed to compare the fixation strength between the two sides. After laminectomy, the pullout strength of the cortical screws was 718.92 ± 340.76 N, and that of the pedicle screws was 625.78 ± 287.10 N (p = 0.183). The toggle strength of the cortical screws was 544.83 ± 329.97 N; that of the pedicle screws was 613.17 ± 311.70 N (p = 0.145). No significant difference was found in biomechanical strength between the two types of screws. Despite laminectomy, lumbar cortical screws offers comparable pullout and toggle biomechanical strength as traditional pedicle screws.

Comparison of the Pullout Strength of Different Pedicle Screw Designs and Augmentation Techniques in an Osteoporotic Bone Model.

Study DesignMechanical study.PurposeTo compare the pullout strength of different screw designs and augmentation techniques in an osteoporotic bone model.Overview of LiteratureAdequate bone screw pullout strength is a common problem among osteoporotic patients. Various screw designs and augmentation techniques have been developed to improve the biomechanical characteristics of the bone–screw interface.MethodsPolyurethane blocks were used to mimic human osteoporotic cancellous bone, and six different screw designs were tested. Five standard and expandable screws without augmentation, eight expandable screws with polymethylmethacrylate (PMMA) or calcium phosphate augmentation, and distal cannulated screws with PMMA and calcium phosphate augmentation were tested. Mechanical tests were performed on 10 unused new screws of each group. Screws with or without augmentation were inserted in a block that was held in a fixture frame, and a longitudinal extraction force was applied to the screw head at a loading rate of 5 mm/min. Maximum load was recorded in a load displacement curve.ResultsThe peak pullout force of all tested screws with or without augmentation was significantly greater than that of the standard pedicle screw. The greatest pullout force was observed with 40-mm expandable pedicle screws with four fins and PMMA augmentation. Augmented distal cannulated screws did not have a greater peak pullout force than nonaugmented expandable screws. PMMA augmentation provided a greater peak pullout force than calcium phosphate augmentation.ConclusionsExpandable pedicle screws had greater peak pullout forces than standard pedicle screws and had the advantage of augmentation with either PMMA or calcium phosphate cement. Although calcium phosphate cement is biodegradable, osteoconductive, and nonexothermic, PMMA provided a significantly greater peak pullout force. PMMA-augmented expandable 40-mm four-fin pedicle screws had the greatest peak pullout force.