Pullout Strength Of Screws Research Articles

Finding the right fit: studying the biomechanics of under-tapping with varying thread depths and pitches

Background ContextCompromise of pedicle screw purchase is a concern in maintaining rigid spinal fixation, especially with osteoporosis. Little consistency exists among various tapping techniques. Pedicle screws are often prepared with taps of a smaller diameter, which can further exacerbate inconsistency. PurposeThe objective of this study was to determine whether a mismatch between tap thread depth (D) and thread pitch (P) and screw D and P affects fixation when under-tapping in osteoporotic bone. Study DesignThis study is a polyurethane foam block biomechanical analysis. Materials and MethodsA foam block osteoporotic bone model was used to compare pullout strength of pedicle screws with a 5.3 nominal diameter tap of varying D's and P's. Blocks were sorted into seven groups: (1) probe only; (2) 0.5-mm D, 1.5-mm P tap; (3) 0.5-mm D, 2.0-mm P tap; (4) 0.75-mm D, 2.0-mm P tap; (5) 0.75-mm D, 2.5-mm P tap; (6) 0.75-mm D, 3.0-mm P tap; and (7) 1.0-mm D, 2.5-mm P tap. A pedicle screw, 6.5 mm in diameter and 40 mm in length, was inserted to a depth of 40 mm. Axial pullout testing was performed at a rate of 5 mm/min on 10 blocks from each group. ResultsNo significant difference was noted between groups under axial pullout testing. The mode of failure in the probe-only group was block fracture, occurring in 50% of cases. Among the other six groups, only one screw failed because of block fracture. The other 59 failed because of screw pullout. ConclusionsIn an osteoporotic bone model, changing the D or P of the tap has no statistically significant effect on axial pullout. Osteoporotic bone might render tap features marginal. Our findings indicate that changing the characteristics of the tap D and P does not help with pullout strength in an osteoporotic model. The high rate of fracture in the probe-only group might imply the potential benefit of tapping to prevent catastrophic failure of bone.

Rotary ultrasonic bone drilling: Improved pullout strength and reduced damage

Bone drilling is one of the most common operations used to repair fractured parts of bones. During a bone drilling process, microcracks are generated on the inner surface of the drilled holes that can detrimentally affect osteosynthesis and healing. This study focuses on the investigation of microcracks and pullout strength of cortical-bone screws in drilled holes. It compares conventional surgical bone drilling (CSBD) with rotary ultrasonic bone drilling (RUBD), a novel approach employing ultrasonic vibration with a diamond-coated hollow tool. Both techniques were used to drill holes in porcine bones in an in-vitro study. Scanning electron microscopy was used to observe microcracks and surface morphology. The results obtained showed a significant decrease in the number and dimensions of microcracks generated on the inner surface of drilled holes with the RUBD process in comparison to CSBD. It was also observed that a higher rotational speed and a lower feed rate resulted in lower damage, i.e. fewer microcracks. Biomechanical axial pullout strength of a cortical bone screw inserted into a hole drilled with RUBD was found to be much higher (55–385%) than that for CSBD.

Pullout strength of monocortical and bicortical screws in metaphyseal and diaphyseal regions of the canine humerus.

Monocortical screws are commonly employed in locking plate fixation, but specific recommendations for their placement are lacking and use of short monocortical screws in metaphyseal bone may be contraindicated. Objectives of this study were to evaluate axial pullout strength of two different lengths of monocortical screws placed in various regions of the canine humerus compared to bicortical screws, and to derive cortical thickness and bone density values for those regions using quantitative computed tomography analysis (QCT). The QCT analysis was performed on 36 cadaveric canine humeri for six regions of interest (ROI). A bicortical, short monocortical, or 50% transcortical 3.5 mm screw was implanted in each ROI and axial pullout testing was performed. Bicortical screws were stronger than monocortical screws in all ROI except the lateral epicondylar crest. Short monocortical metaphyseal screws were weaker than those placed in other regions. The 50% transcortical screws were stronger than the short monocortical screws in the condyle. A linear relationship between screw length and pullout strength was observed. Cortical thickness and bone density measurements were obtained from multiple regions of the canine humerus using QCT. Use of short monocortical screws may contribute to failure of locking plate fixation of humeral fractures, especially when placed in the condyle. When bicortical screw placement is not possible, maximizing monocortical screw length may optimize fixation stability for distal humeral fractures.

Oblique Terminal Screw Placement: Does It Improve Fixation in Plate Osteosynthesis?

The technique of placing an oblique screw in the terminal hole of a plate to increase screw pullout strength is widely taught in the operating room. The origin of this technique is unclear; however, it may have been used simply as a means to identify radiographs and misinterpreted to have some biomechanical benefit. The objective of this study was to measure the structural effect of oblique terminal screw placement (OTSP) during plate osteosynthesis. Foam blocks and limited contact dynamic compression plates and screws were used along with a custom fixture device. The terminal screw was placed in either an oblique (30-degree angle outward) or perpendicular fashion. A load was applied perpendicular to the plate in cantilever bending until failure. The oblique screw construct was significantly weaker than the perpendicular screw construct (399N vs. 465N, P < 0.001), independent of the block of material used. Post hoc analysis showed that the screw angle (P < 0.001) was a significant determinant of the load required to cause screw pullout. OTSP led to a decrease in pullout strength compared with a perpendicular screw in a deformable foam medium similar in density to osteoporotic bone. In patients with poor bone quality, OTSP may create a suboptimal fracture fixation construct.

P22 - Divergent Configuration Improves Insertion Torque and Pullout Strength of Anterior Cervical Screws

P22 - Divergent Configuration Improves Insertion Torque and Pullout Strength of Anterior Cervical Screws

Pull out strength calculator for pedicle screws using a surrogate ensemble approach

Background and objectivePedicle screw instrumentation is widely used in the treatment of spinal disorders and deformities. Currently, the surgeon decides the holding power of instrumentation based on the perioperative feeling which is subjective in nature. The objective of the paper is to develop a surrogate model which will predict the pullout strength of pedicle screw based on density, insertion angle, insertion depth and reinsertion. MethodsA Taguchi's orthogonal array was used to design an experiment to find the factors effecting pullout strength of pedicle screw. The pullout studies were carried using polyaxial pedicle screw on rigid polyurethane foam block according to American society for testing of materials (ASTM F543). Analysis of variance (ANOVA) and Tukey's honestly significant difference multiple comparison tests were done to find factor effect. Based on the experimental results, surrogate models based on Krigging, polynomial response surface and radial basis function were developed for predicting the pullout strength for different combination of factors. An ensemble of these surrogates based on weighted average surrogate model was also evaluated for prediction. ResultsDensity, insertion depth, insertion angle and reinsertion have a significant effect (p < 0.05) on pullout strength of pedicle screw. Weighted average surrogate performed the best in predicting the pull out strength amongst the surrogate models considered in this study and acted as insurance against bad prediction. ConclusionsA predictive model for pullout strength of pedicle screw was developed using experimental values and surrogate models. This can be used in pre-surgical planning and decision support system for spine surgeon.

Pull-Out Strength of Screws in Screw Chases: Part 2

This paper (Part 2 of 2) presents a study of the structural behavior of tension-loaded screws installed in screw chases in extruded aluminum profiles. This connection is commonly used to resist outward wind load on glass in curtainwalls and skylights. See Part 1 for details of the pull-out testing. Two types of chase, each with 1/4 in. (6.35 mm) diameter stainless steel screws were used: a flat chase with AB tapping screws (1/4-14; 14 threads per 25.4 mm) and F tapping screws (1/4-20; 20 threads per 25.4 mm), and a longitudinally ribbed chase with UNC-thread machine screws (1/4-20). The extrusions included six cross-section shapes (three with flat chases and three with ribbed chases) and three alloy-tempers.

Pull-Out Strength of Screws in Screw Chases: Part 1

This paper presents test results and a proposed pull-out strength equation for tension-loaded screws installed in chases in extruded aluminum profiles. The screws are perpendicular to the extrusion's length. This connection is commonly used to resist outward wind load on glass in curtain walls and skylights. Two types of chase, each with 1/4 in. (6.35 mm) diameter 300 series stainless steel screws were used: a flat chase with AB tapping screws (1/4-14; 14 threads per 25.4 mm) and F tapping screws (1/4-20; 20 threads per 25.4 mm), and a longitudinally ribbed chase with UNC-thread machine screws (1/4-20). The extrusions included six cross-section shapes (three with flat chases and three with ribbed chases) and three alloy-tempers. Pull-out tests of 150 single screws were conducted. The Aluminum Design Manual [1] does not include pull-out strength equations for screws in chases; this research was intended to develop such equations. Pull-out strength increased with an increase in engaged length of screw threads with the chase. The shape and dimensions of the chase also affected the pull-out strength.

The Pullout Strength of Pedicle Screws Following Redirection After Lateral Wall Breach or End-plate Breach.

Using fresh cadavers, the biomechanical testing were used to examine the pullout strength of each pedicle screw. The aim of this study was to evaluate pullout strength of (1) a redirected pedicle screw following lateral wall breach; (2) a redirected pedicle screw following end-plate breach; and (3) a pedicle without redirection after end-plate breach without redirection. Screw malposition, such as lateral wall breach or end-plate breach, is one of the main pitfalls of inserting pedicle screws. From 17 fresh spines, 54 vertebrae were harvested. In each vertebra on one pedicle, the screw was inserted correctly down the axis of the pedicle, while on the other pedicle, the screw was inserted to breach the lateral wall or the end-plate. The 18 pedicle screws that breached the lateral wall were then removed and redirected along the correct axis of the pedicle. The 18 pedicle screws that breached the end-plate were removed and redirected along the correct axis of the pedicle. The 18 other pedicle screws that had breached the end-plate were not removed. The pullout force of pedicle screws was measured. First, the mean pullout strength for the redirected screws following lateral wall breach was 24.0% less as compared with the correctly aligned screws. Second, the mean pullout strength for the redirected screws following end-plate breach was 23.3% less as compared with the correctly aligned screws. Third, the mean pullout strength for the pedicle screws end-plate breach was 7.6% less as compared with the correctly aligned screws. The pullout strength of redirected pedicle screws after either a lateral pedicle breach or end-plate breach is significantly less than the pullout strength of correctly aligned screw. A pedicle screw that is not redirected after end-plate breach is weaker than a pedicle screw correctly aligned; however, the difference is not significant. N/A.

Larger screw diameter may not guarantee greater pullout strength for headless screws - a biomechanical study.

Headless compression screws (HCSs) are commonly utilized devices for small bone fracture fixation. The Mini-Acutrak 2 and headless reduction (HLR) screws are the newer version types, in which both have fully threaded and variable pitch design. Specifically, the HLR is characterized by two thread runouts to facilitate implantation. With the thread runouts, the holding strength of the screw may be compromised. To the best of our knowledge, no study has examined the pullout force of the global sizes of a HCS. We sought to determine the pullout strength of the HLR and compare the strength of this screw with that of the Mini-Acutrak 2. Synthetic bone blocks with simulated transverse fractures were used to conduct the tests. Four commonly used sizes of the HLR were examined, and one Mini-Acutrak 2 was employed for comparison. Five screws of each size were tested. The pullout force of all screws that were tested in this study ranged from 45.23 to 233.22 N. The results revealed that the pullout force increased as the screw diameter increased. Interestingly, we found that one small screw outperformed the Mini-Acutrak 2, which has a larger diameter. This study provided extensive knowledge regarding the pullout strength of fully threaded HCSs of different sizes. An unexpected finding is that a small screw has higher holding power than a large one because of its increased number of threads. Therefore, we suggest that the thread number should be a critical consideration for the design of size distribution of HCSs.

The Relationship Between Insertional Torque and Pullout Strength of Surgical Screws

Objectives: To investigate the role of maximal insertional torque (T max) of surgical screws and pullout strength (POS) in human cadaveric tibiae. Methods: Self-tapping surgical screws (4.5 mm) were inserted into 9 segments of cadaveric tibial cortical bone, measuring T max and POS of screws inserted at 50%, 70%, 90%, and 100% T max. Computed tomography was also performed, allowing calculation of bone mineral density (BMD) in K2HPO4 concentration (mg/mL) and cortical thickness (mm). Results: POS measurements were highest and similar in screws inserted at 70% and 90% of the established T max. Screws inserted at 90% T max offered a significantly different POS than those inserted at 100% T max. No significant differences were found between samples in BMD or cortical thickness. No significant correlations were found between cortical thickness or BMD and T max. Conclusions: Surgeons should tighten screws to a torque less than T max to avoid loss of purchase of the screw and reduced POS.

In vitro biomechanical study of pedicle screw pull-out strength based on different screw path preparation techniques

Background:Poor screw-to-bone fixation is a clinical problem that can lead to screw loosening. Under-tapping (UT) the pedicle screw has been evaluated biomechanically in the past. The objective of the study was to determine if pedicle preparation with a sequential tapping technique will alter the screw-to-bone fixation strength using a stress relaxation testing loading protocol.Materials and Methods:Three thoracolumbar calf spines were instrumented with pedicle screws that were either probed, UT, standard-tapped (ST), or sequential tapped to prepare the pedicle screw track and a stress relaxation protocol was used to determine pull-out strength. The maximum torque required for pedicle screw insertion and pull-out strength was reported. A one-way ANOVA and Tukeys post-hoc test were used to determine statistical significance.Results:The pedicle screw insertion torques for the probed, UT, ST and sequentially tapped (SQT) techniques were 5.09 (±1.08) Nm, 5.39 (±1.61) Nm, 2.93 (±0.43) Nm, and 3.54 (±0.67) Nm, respectively. There is a significant difference between probed compared to ST (P ≤ 0.05), as well as UT compared to both ST and SQT (P ≤ 0.05). The pull-out strength for pedicle screws for the probed, UT, ST and SQT techniques was 2443 (±782) N, 2353(±918) N, 2474 (±521) N, and 2146 (±582) N, respectively, with no significant difference (P ≥ 0.05) between techniques.Conclusions:The ST technique resulted in the highest pull-out strength while the SQT technique resulted in the lowest. However, there was no significant difference in the pull-out strength for the various preparation techniques and there was no correlation between insertion torque and pull-out strength. This suggests that other factors such as bone density may have a greater influence on pull-out strength.

Biomechanical evaluation of monosegmental pedicle instrumentation in a calf spine model and the role of fractured vertebrae in screw stability.

BackgroundMonsegmental pedicle instrumentation (MSPI) has been used to treat thoracolumbar fractures. However, there are few reports about the biomechanical characteristics of MSPI compared with traditional short-segment pedicle instrumentation (SSPI) in management of unstable thoracolumbar fractures, and the influence of vertebral fracture on screw stability is still unclear.MethodsThis study was to compare the immediate stability between MSPI and SSPI in management of unstable L1 fracture, and to evaluate the role of fractured vertebrae in screw stability. Two studies were performed: in the first study, sixteen fresh calf spines (T11-L3) were divided into two groups, in which unstable fractures at L1 were produced and then instrumented with MSPI or SSPI respectively. The range of motion (ROM) and lax zone (LZ) of specimens were evaluated with pure moment of 6 Nm loaded. The second study measured and compared the pullout strength of screws inserted in to 16 intact and fractured vertebrae of calf spines (L1-3) respectively. The correlation of pullout strength with load sharing classification (LSC) of fractured vertebrae was analyzed.ResultsNo significant difference in the ROM and LZ of the destabilized segments after fixation between MSPI and SSPI, except in axial rotation of ROM (P < 0.05). After fatigue cyclic loading, the MSPI showed a significant increase of ROM during lateral bending and axial rotation (P < 0.05); however, there were no significant differences in the LZ during all loading models between groups (P > 0.05). The mean pullout strength of pedicle screws in fractured vertebrae decreased by 13.7 %, compared with that of intact vertebrae (P > 0.05), and had a low correlation with LSC of the fractured vertebrae (r = 0.293, P > 0.05).ConclusionsMSPI can provide effective immediate stability for management of unstable thoracolumbar fractures; however, it has less fatigue resistance during lateral bending and axial rotation compared with SSPI. LSC score of fractured vertebrae is not a major influence on the pullout strength of screws.

The effect of insertion technique and surgeon experience on the pullout strength of orthopaedic screws

Background: We tested the null hypothesis that the manner of orthopaedic screw insertion (hand-driven or electrically-driven) and the experience of the screw inserter (orthopaedic trainees or experienced orthopaedic surgeons) have no effect on the pullout strength of orthopaedic screws. Methods: Four fellowship-trained orthopaedic surgeons and four orthopaedic chief residents each inserted 10 cancellous bone screws into a polyurethane foam cancellous bone substitute. Five screws were inserted with a standard orthopaedic screw driver, and five screws were inserted with a battery-powered electric driver. Samples were loaded to failure, and pullout strength was measured using a electromechanical tensile tester. Analysis of variance was used to compare experimental groups. Results: No significant difference was found between the pullout strengths of screws inserted by hand and the pullout strengths of screws inserted with an electric driver. Screws inserted by experienced orthopaedic surgeons, however, demonstrated significantly higher pullout strengths than screws inserted by orthopaedic trainees. Conclusions: Hand-driven and electrically-driven screws demonstrate similar pullout strengths in a model for orthopaedic screw insertion into bone. Screws inserted by experienced surgeons, however, did exhibit increased pullout strength. This suggests a technical nuance that allows experienced surgeons to better insert screws. If identified, this nuance would be of interest to orthopaedists in general.

Comparative Analysis of Effect of Density, Insertion Angle and Reinsertion on Pull-Out Strength of Single and Two Pedicle Screw Constructs Using Synthetic Bone Model.

Study DesignBiomechanical study.PurposeTo determine the effect of density, insertion angle and reinsertion on pull-out strength of pedicle screw in single and two screw-rod configurations.Overview of LiteraturePedicle screw pull-out studies have involved single screw construct, whereas two screws and rod constructs are always used in spine fusions. Extrapolation of results using the single screw construct may lead to using expensive implants or increasing the fusion levels specifically in osteoporotic bones.MethodsSingle screw and two screw pull-out strength tests were carried out according to American Society for Testing and Materials F 543-07 on foam models to test the effect of density, insertion angle and reinsertion using poly axial pedicle screws.ResultsBone density was the most significant factor deciding the pull-out strength in both single and two screw constructs. The difference in pull-out strength between single screw and two screw configurations in extremely osteoporotic bone model (80 kg/m3) was 78%, whereas in the normal bone model it was 48%. Axial pull-out value was highest for the single screw configuration; in the two screw configuration the highest pull-out strength was at 10°–15°. There was an 18% reduction in pull-out strength due to reinsertion in single screw configuration. The reinsertion effect was insignificant in the two screw configuration.ConclusionsA significant difference in response of various factors on holding power of pedicle screw between single and two-screw configurations is evident. The percentage increase in pull-out strength between single and two screw constructs is higher for osteoporotic bone when compared to normal bone. Reinsertion has no significant effect on pull-out strength in the two screw rod configuration.

Prediction of Stress Shielding Around Orthopedic Screws: Time-Dependent Bone Remodeling Analysis Using Finite Element Approach

Loosening of bone screws caused by stress shielding, and subsequent unbalanced bone remodeling processes, results in bone loss around the screws, which can ultimately lead to bone fixation failure. Predicting the effects of the structural and material properties of screws on bone remodeling is crucially important in screw design and fixation reliability. Many studies have attempted to assess the significance of orthopedic screw parameters on screw pullout strength, but few have considered bone remodeling processes around the screw threads. The main objective of this study was to investigate the effects of various engineering designs of bone screws on stress shielding and screw loosening in a bone-screw system using a two-dimensional finite element model (FEM), including cortical and cancellous bone, and an orthopedic screw. In order to do so, an adaptive bone strain energy density algorithm was combined with FEM to study a 4-year follow-up of bone remodeling around the screw implant. A dimensionless set of stress transfer parameters (STP), and a newly defined parameter, called the strain energy density transfer parameter (SEDTP), were developed to quantify the screw-bone load sharing patterns. Lower STP and SEDTP values indicate a weaker transfer of mechanical stimuli to the neighboring bone. The results of this study show that increasing the major diameter of the screw, increasing the number of threads, and decreasing the minor screw diameter, pitch, and elastic modulus decrease the probability of screw loosening. The approach used here is not limited to the analysis of orthopedic screws, but may be used for other implant devices.

Impact of screw location and endplate preparation on pullout strength for anterior plates and integrated fixation cages

Background ContextNumerous integrated fixation cages (IFCs) have recently been introduced to the market with “zero-profile” designs that incorporate screw fixation through the vertebral endplate. It is unclear whether differences in bone quality and quantity in this insertion location may affect fixation compared with screws used in traditional anterior plate (AP) fixation. Moreover, endplate preparation for IFC implantation may affect fixation strength. PurposeThis study aimed to compare pullout strength of screws used in IFCs with screws used for AP implantations. Study DesignA biomechanical cadaveric study. MethodsT12 and L1 vertebrae from 13 human cadaver spines were prepared for pullout testing. End plates in T12 vertebrae were scraped according to surgical practice for fusion procedures. Conversely, endplates in L1 vertebrae were kept intact (unscraped). Integrated fixation cage screws were implanted at a 45° angle into the endplate and AP screws were implanted horizontally into the same vertebral body. Pullout testing was performed on all screws, and peak pullout force (PPF) and work were compared between groups to determine fixation strength. In addition, micro-CT imaging was used to assess bone quantity and quality parameters such as trabecular bone volume fraction, endplate and anterior cortex thickness at screw insertion location, endplate mineralization, and anterior cortex mineralization. ResultsPeak pullout force for IFC screws (176±68 N) with scraped endplates was similar (p=.26) to AP screws (192±84 N). However, PPF for IFC screws (231±75 N) with unscraped endplates was significantly greater (p<.01) than AP screws (176±50 N). Peak pullout force for IFC screws with scraped endplates was significantly lower (p=.03) than IFC screws with unscraped endplates. Scraped endplates group (0.17±0.05 mm) were thinner (p=.05) than unscraped endplates (0.21±0.06 mm) by approximately 40 µ on average. ConclusionsThese data indicate that IFC and AP screws exhibited similar fixation behavior when the endplate is prepared according to common surgical practices. However, endplate scraping reduces endplate thickness by 20% on average, resulting in a decrease in fixation strength when compared with the unscraped endplates and provides bounds for IFC screw fixation strength.

Fenestrated Screws Augmented With PMMA Increase the Pullout Strength of Sacral Pedicle Screws.

Biomechanics. The objective of this study was to compare the fixation in osteoporotic specimens of a bicortical sacral pedicle screw to the following constructs: a unicortical solid screw augmented with polymethylmethacrylate (PMMA) (TRACT), a unicortical fenestrated screw augmented with PMMA (FEN), and a bicortical screw/sacral alar screw combination (PED/ALA). Rigid posterior sacral fixation of osteoporotic spines is problematic due to poor bone quality. Options to improve fixation include bicortical or tricortical screw trajectory, addition of S2 alar screws, or PMMA augmentation. Eleven osteoporotic cadaveric sacra were potted in 2-part filler compound. As a baseline fixation method, a 6.5-mm bicortical pedicle screw was placed into 1 pedicle of each sacra. The contralateral side was randomly assigned to receive a unicortical fenestrated 6.5-mm pedicle screw augmented with PMMA, a unicortical solid 6.5-mm pedicle screw augmented with PMMA, or a bicortical pedicle screw/alar screw construct. Pedicle screws were failed in cantilever pullout using a materials testing machine. Data were normalized into a test index by taking the ratio of the test screw pullout strength to the intraspecimen bicortical control screw pullout strength. The fixation strengths as expressed by the test index of the TRACT, FEN, and PED/ALA groups were 101%, 157%, and 167%, respectively. Although there was no statistical difference detected between the FEN and PED/ALA groups, the TRACT group had a trend toward lower pullout strength than the FEN (P=0.06) or PED/ALA (0.06) groups. Although underpowered, this study did not detect biomechanical inferiority for any of the 3 test configurations when compared with standard bicortical S1 screws. The data indicate that the fixation strength of a fenestrated, PMMA-augmented construct or a combined S1 pedicle/S2 alar screw construct may be better than either standard bicortical or unicortical PMMA screw-tract augmented screws.

Axial pullout strength comparison of different screw designs: fenestrated screw, dual outer diameter screw and standard pedicle screw.

BackgroundThe pullout strength of pedicle screws is influenced by many factors, including diameter of the screws, implant design, and augmentation with bone cement such as PMMA. In the present study, the pullout strength of an innovative fenestrated screw augmented with PMMA was investigated and was compared to unaugmented fenestrated, standard and dual outer diameter screw.MethodsTwenty four thoracolumbar vertebrae (T10-L5, age 60 to 70 years) from three cadavers were implanted with the four different pedicle screws. Twelve screws of each type were instrumented into either left or right pedicle with standard screw paired with unaugmented and dual outer diameter screw paired with augmented fenestrated screw in any given vertebra. Axial pullout testing was conducted at a rate of 5 mm/min. Force to failure (Newtons) for each pedicle screw was recorded.ResultsThe augmented fenestrated screws had the highest pullout strength, which represented an average increase of 149%, 141%, and 78% in comparison to unaugmented, standard, and dual outer diameter screws, respectively. Pullout strength of unaugmented screws was comparable to that of standard screws, however it was significantly lower than dual outer diameter screws.ConclusionsFenestrated screws augmented with PMMA improve the fixation strength and result in significantly higher pullout strength compared to dual outer diameter, standard and unaugmented fenestrated screws. Screws with dual outer diameter provided enhanced bone-screw purchase and may be considered as an alternative technique to increase the bone-screw interface in cases where augmentation using bone cement is not feasible. Unaugmented screws can be left in the pedicle even without cement and provide similar pullout strength to standard screws.

Comparative failure analysis of allograft constructs stabilized with compression plates utilizing unicortical versus bicortical screws with or without PMMA

Orthopaedic internal fixation is subject to high stress concentration around screw holes leading to screw loosening. We compared the screw pullout strength (n=8), and bending and torsional stiffness (n=6) for cadaveric (young adult donors age 45years or less) diaphyseal bone constructs with a gap and stabilized with Zimmer 4.5 narrow stainless steel dynamic compression plates with 4.5mm cortical stainless steel screws placed in a unicortical or bicortical manner with and without intramedullary PMMA. The pullout strength for unicortical fixation with PMMA (4.3±.6kN) was not significantly different with bicortical fixation with PMMA (5.3±1.5kN), n=8, P>.5; and for the group with bicortical fixation without PMMA (4.3±1.1kN), n=8, P>.5. The difference in bending stiffness for the group with two unicortical screws fixation with PMMA (324.1±59.7kN/m) was also not statistically significant when compared with the groups with four bicortical screws fixation without PMMA (278.7±89.6kN/m) and two bicortical screws fixation with PMMA (347.9±66.7kN/m), n=6, P>.1. The difference in torsional stiffness for the group with two unicortical screws fixation with PMMA (173±44Nm/deg) was not statistically significant when compared with the groups with four bicortical screws fixation without PMMA (190±22Nm/deg), two bicortical screws fixation with PMMA (179±36Nm/deg), and two bicortical screws fixation with PMMA (193±31Nm/deg), n=6, P>.5. The use of PMMA in allograft fixation appears to allow the use of fewer screws, and perhaps unicortical fixation without significant compromise of strength. Clinically, two unicortical screws with PMMA may replace the use of two bicortical screws without PMMA, or two bicortical screws with PMMA may replace four bicortical screws. The clinical relevance would be the ability to use fewer screws and reduce violation of the far cortex of the allograft.