Screw Pullout Force Research Articles

Sol–gel derived titania coating with immobilized bisphosphonate enhances screw fixation in rat tibia

A variety of surface modifications have been tested for the enhancement of screw fixation in bone, and locally delivered anti-osteoporosis drugs such as bisphosphonates (BP) are then of interest. In this in vivo study, the impact of surface immobilized BP was compared with systemic BP delivery and screws with no BP. After due in vitro characterization, differently treated stainless steel (SS) screws were divided into four groups with 10 rats each. Three of the groups received screws coated with sol-gel derived TiO(2) and calcium phosphate (SS+TiO(2)+CaP). One of these had no further treatment, one had alendronate (BP) adsorbed to calcium phosphate mineral, and one received systemic BP treatment. The fourth group received uncoated SS screws and no BP (control). The screw pullout force was measured after 4 weeks of implantation in rat tibiae. The immobilized amount and release rate of alendronate could be controlled by different immersion times. The SS+TiO(2)+CaP coating did not increase the pullout force compared to SS alone. Surface delivered alendronate enhanced the pullout force by 93% [p = 0.000; 95% Confidence Interval (CI): 67-118%] compared to SS, and by 39% (p = 0.044; 95% CI: 7-71%) compared to systemic alendronate delivery. Both surface immobilized and systemically delivered alendronate improved implant fixation. Also, locally delivered, that is, surface immobilized alendronate showed a better fixation than systemically delivered. Using sol-gel derived TiO(2) as a platform, it is possible to administer controllable amounts of a variety of BPs.

Biomechanical Evaluation of Parasagittal Occipital Plating

Biomechanical evaluation of occipitocervical instrumentation techniques. Compare methods of occipital instrumentation by quantifying load sharing of occipital screws and measuring motion across instrumented occipitocervical spines. Newer occipitocervical plate/screw systems that attach to longitudinal rods have been developed to improve fixation. These devices place screws in the center of occipital bone or off-midline. Midline plates offer screw purchase in thicker bone. Off-midline systems may increase the effective moment arm for torsional and lateral bending control. Measurement of screw loads within occipital plates is useful for determining optimal plate configuration. Ten cadaveric specimens (occiput-C4) were tested in flexion/extension (FE), lateral bending (LAT), and axial rotation (ROT) over +/-3 Nm pure moment. After intact testing, 4 occipitocervical fixation constructs were tested using washer load cells to assess loading across screws used to fix the plates to the occiput. Parasagittal occipital plates were positioned either convex or concave side facing medially. Each plate was first fixed using 3 screws (rostral, middle, caudal), then with the caudal screw eliminated (simulated failure). Range of motion (ROM) and peak screw loads are reported. ROM decreased from intact to any of the 4 fusion plate configurations in FE, LAT, and ROT (P << 0.05), but not between plate configurations. Screw load significantly decreased from medially convex to medially concave configurations in LAT, but no significant changes were observed in FE or ROT. With caudal screws removed, middle screws peak loads significantly increased in FE and LAT (P < 0.05), but not ROT. Occipital screw placement off-midline improves screw loads under lateral bending forces on occipitocervical constructs, though loads for FE and ROT are unchanged. As screws pullout, the loads may be redistributed, resulting in increased screw pullout forces above. Despite the improvement in screw loads for laterally based plates during lateral bending, overall ROM across the occipitocervical junction is unchanged.

Acute and Long-term Stability of Atlantoaxial Fixation Methods

In vitro human cadaveric biomechanical study. The aims of this project were to evaluate the acute segmental fixation and long-term screw stability afforded by 3 C2 fixation techniques: intralaminar, pars, and pedicle. C2 intralaminar screws offer the advantages of avoiding the vertebral artery; however, direct biomechanical comparison of this technique to the other methods of instrumenting C2 has not been performed. Fourteen cadaveric specimens were dual energy radiograph absorptiometry scanned and segregated into 2 groups (n = 7/group) matching the C2 bone mineral density. All specimens were instrumented with C1 lateral mass and C2 intralaminar screws while measuring the insertional torque (IT). In group 1 C2 pars screws were inserted while in group 2 pedicle screws were placed. Nondestructive testing was performed in axial rotation, flexion/extension (FE), and lateral bending. The odontoid was then resected and loading repeated. Subsequently, specimens were disarticulated about C2 and individually loaded for 2000 cycles in the cephalocaudad plane. The screws were then failed by a tensile load directed in the parasagittal plane. Full range of motion over C1-C2 and peak screw pull-out force was quantified. RESULTS.: Transpedicular technique generated significantly higher IT than the pars screws and marginally greater IT than intralaminar screws. With the intact atlantoaxial ligamentous complex, intralaminar fixation was superior to pars and similar to pedicle instrumentation at limiting axial torsion. After odontoid destabilization, however, this technique was less effective at reducing the lateral bending range of motion. Destructive loading revealed the highest pull-out forces with the pedicle screws, followed by intralaminar and pars screws. Our results suggest that C2 intralaminar fixation provides a viable alternative to pedicle screws and is superior to pars instrumentation in cases with preserved atlantoaxial ligamentous attachments. In the presence of a traumatic dens fracture, however, intralaminar fixation may not be the optimal choice.

Assessment of different screw augmentation techniques and screw designs in osteoporotic spines

This is an experimental study on human cadaver spines. The objective of this study is to compare the pullout forces between three screw augmentation methods and two different screw designs. Surgical interventions of patients with osteoporosis increase following the epidemiological development. Biomechanically the pedicle provides the strongest screw fixation in healthy bone, whereas in osteoporosis all areas of the vertebra are affected by the disease. This explains the high screw failure rates in those patients. Therefore PMMA augmentation of screws is often mandatory. This study involved investigation of the pullout forces of augmented transpedicular screws in five human lumbar spines (L1-L4). Each spine was treated with four different methods: non-augmented unperforated (solid) screw, perforated screw with vertebroplasty augmentation, solid screw with vertebroplasty augmentation and solid screw with balloon kyphoplasty augmentation. Screws were augmented with Polymethylmethacrylate (PMMA). The pullout forces were measured for each treatment with an Instron testing device. The bone mineral density was measured for each vertebra with Micro-CT. The statistical analysis was performed with a two-sided independent student t test. Forty screws (10 per group and level) were inserted. The vertebroplasty-augmented screws showed a significant higher pullout force (mean 918.5 N, P = 0.001) than control (mean 51 N), the balloon kyphoplasty group did not improve the pullout force significantly (mean 781 N, P > 0.05). However, leakage occurred in some cases treated with perforated screws. All spines showed osteoporosis on Micro-CT. Vertebroplasty-augmented screws, augmentation of perforated screws and balloon kyphoplasty augmented screws show higher pullout resistance than non-augmented screws. Significant higher pullout forces were only reached in the vertebroplasty augmented vertebra. The perforated screw design led to epidural leakage due to the position of the perforation in the screw. The position of the most proximal perforation is critical, depending on screw design and proper insertion depth. Nevertheless, using a properly designed perforated screw will facilitate augmentation and instrumentation in osteoporotic spines.

Augmentation of screw fixation with injectable calcium sulfate bone cement in ovariectomized rats

The objective of this study was to determine the effect of augmenting screw fixation with an injectable calcium sulfate cement (CSC) in the osteoporotic bone of ovariectomized rats. The influence of the calcium sulfate (CS) on bone remodeling and screw anchorage in osteoporotic cancellous bone was systematically investigated using histomorphometric and biomechanical analyses. The femoral condyles of 55 Sprague-Dawley ovariectomized rats were implanted with screw augmented with CS, while the contralateral limb received a nonaugmented screw. At time intervals of 2, 4, 8, 12, and 16 weeks, 11 rats were euthanized. Six pair-matched samples were used for histological analysis, while five pair-matched samples were preserved for biomechanical testing. Histomorphometric data showed that CS augmented screws activated cancellous bone formation, evidenced by a statistically higher (p < 0.05) percentage of osteoid surface at 2, 4, and 8 weeks and a higher rate of bone mineral apposition at 12 weeks compared with nonaugmented screws. The amount of the bone-screw contact at 2, 8, and 12 weeks and of bone ingrowth on the threads at 4 and 8 weeks was greater in the CS group than in the nonaugmented group (p < 0.05), although these parameters increased concomitantly with time for both groups. The CS was resorbed completely at 8 weeks without stimulating fibrous encapsulation on the screw surface. Also, the cement significantly increased the screw pull-out force and the energy to failure at 2, 4, 8, and 12 weeks after implantation, when compared with the control group (p < 0.05). These results imply that augmentation of screw fixation with CS may have the potential to decrease the risk of implant failure in osteoporotic bone.

Biomechanical comparison of pull-out force of unicortical versus bicortical screws in proximal phalanges of the hand: A human cadaveric study

Backround Bicortical fixation of proximal phalangeal fractures may damage underlying flexor tendons secondary to drilling and screw protrusion in the dorsal–ventral direction. The aim of this study was to measure and compare the pull-out force of unicortical screws compared to bicortical screws in human cadaveric proximal phalanges to identify optimal configuration for internal fixation. Methods Forty proximal phalanges were harvested. Bicortical and unicortical self tapping 1.7 mm screws were inserted into paired phalanges at the distal and proximal metaphysis and at the mid-diaphysis placed in the dorso-ventral direction. Pull-out force of unicortical and bicortical screws were measured and compared. Findings Bicortical screw pull-out force is significantly higher than that of unicortical screws in the dorsal–ventral direction. Pull-out strength of unicortical screws at the mid-diaphysis was significantly higher than the pull-out strength of bicortical screws at the proximal metaphysis (181.8 N versus 31.5 N, P < 0.0001). Interpretation Diaphyseal fixation is stronger than metaphyseal for both unicortical and bicortical configurations. Unicortical mid-diaphyseal is stronger than bicortical proximal metaphyseal screw pull-out strength. This study provides biomechanical data that may be helpful for individualizing fracture fixation techniques at the proximal phalanx.

Screw orientation and plate type (variable- vs. fixed-angle) effect strength of fixation for in vitro biomechanical testing of the Synthes CSLP

Background context Two common justifications for orienting cervical screws in an angled direction are to increase pullout strength and to allow use of longer screws. This concept is widely taught and has guided implant design. Fixed- versus variable-angle systems may offer strength advantages. Despite these teachings, there is a paucity of supporting biomechanical evidence. The purpose of our study is to test the influence of screw orientation and plate design on the maximum screw pullout force. Purpose This study evaluates the effect of screw orientation and plate type (fixed- vs. variable-angle) on screw pullout strength. Study design Anterior cervical plates of both a fixed- and variable-angle CSLP, were tested for peak pullout strength in a direct plate pullout model using polyurethane foam bone, which models osteoporotic bone. Methods Self-tapping, locking screws (4.0×14 mm and 4.0×16 mm) were used. Screws were oriented in the fixed-angle plate in the standard fashion. In the variable plate, screws were instrumented in three different orientations. Biomechanical testing was performed on an Instron DynaMight 8841 servohydraulic testing machine, measuring maximum pullout force under a displacement-controlled pullout rate of 1 mm/min. Five samples were tested per group. Results When all screws were placed 90° to the plate, there was a significantly increased peak pullout strength (412.8±22.2 N) compared with when all screws were placed 12° “up and in” (376.2±24.3 N, p less than or equal to .03). When the 90° construct was reproduced using 14-mm screws and compared with 16-mm screws oriented 12° “all up and in,” there was still significantly higher pullout strength with the all 90° construct (434.2±28.7 N vs. 376.2±24.3 N, p less than or equal to .009). The fixed-angle plate had a significantly decreased peak pullout strength (288.2±15.7 N) compared with the variable-angle plate (416.6±12.6 N) (p less than .00001) when the screws were placed in the same orientation. Overall, the variable-angle plate, regardless of the orientation of screws, had a significantly greater pullout strength than the fixed-angle plate (p less than .001). Conclusions In this system, a variable-angle plate has greater pullout strength than a fixed-angle plate, regardless of the orientation of screws. With the variable-angle plate, a construct of all screws 12° “up and in” is the weakest configuration. We found that with the 90° construct, both 16- and 14-mm screws performed significantly better than 16-mm convergent screws. These findings are remarkable because they contradict the current doctrine. This may be a function of plate-dependent factors and should not be applied universally to all plate systems. Further study of screw orientation in additional plating systems is warranted.

Screw Pull-out Force is Dependent on Screw Orientation in an Anterior Cervical Plate Construct

Two common justifications for orienting cervical screws in an angled direction is to increase pull-out strength and to allow use of longer screws. This concept is widely taught and has guided implant design. Fixed versus variable angle systems may offer strength advantages. The purpose of our study is to test the influence of screw orientation and plate design on the maximum screw pull-out load. Variable and fixed angle 4.0 x 15 mm and 4.0 x 13 mm self-tapping screws were used to affix a Medtronic Atlantis cervical plate to polyurethane foam bone samples (density 0.160/cm). This synthetic product is a model of osteoporotic cancellous bone. The fixed angle screws can only be placed at 12 degrees convergent to the midline and 12 degrees in the cephalad/caudal ("12 degrees up and in") direction. Three groups were tested: (1) all fixed angle screws, (2) variable angle, all screws 12 degrees up and in, (3) variable angle, all screws 90 degrees to the plate. Plate constructs were pulled off with an Instron DynaMight 8841 servohydrolic machine measuring for maximum screw pull-out force. There was no difference between group 1, fixed angle (288.4 +/- 37.7 N) (mean +/- SD) and 2, variable angle group (297.7 +/- 41.31 N P< or =0.73). There was a significant increase in maximum pull-out force to failure for the construct with all screws at 90 degrees (415.2+/-17.4 N) compared with all screws 12 degrees "up and in" (297.4 +/- 41.3 N, P< or =0.0016). Group 3 done with 13 mm screws, showed a trend toward better pull-out strength, compared to group 2 w/15 mm screws (345.2 +/- 20.5 vs. 297.4 +/- 41.3, P< or =0.06). In this plate pull-out model, screw orientation influences maximum force to failure. When all 4 screws are 90 degrees to the plate the construct has the greatest ability to resist pullout. Fixed angle designs show no advantage over variable angle. These findings are contrary to current teaching.

Cortical Screw Pullout Strength and Effective Shear Stress in Synthetic Third Generation Composite Femurs

The use of artificial bone analogs in biomechanical testing of orthopaedic fracture fixation devices has increased, particularly due to the recent development of commercially available femurs such as the third generation composite femur that closely reproduce the bulk mechanical behavior of human cadaveric and/or fresh whole bone. The purpose of this investigation was to measure bone screw pullout forces in composite femurs and determine whether results are comparable to cadaver data from previous literature. The pullout strengths of 3.5 and 4.5 mm standard bicortical screws inserted into synthetic third generation composite femurs were measured and compared to existing adult human cadaveric and animal data from the literature. For 3.5 mm screws, the measured extraction shear stress in synthetic femurs (23.70-33.99 MPa) was in the range of adult human femurs and tibias (24.4-38.8 MPa). For 4.5 mm screws, the measured values in synthetic femurs (26.04-34.76 MPa) were also similar to adult human specimens (15.9-38.9 MPa). Synthetic femur results for extraction stress showed no statistically significant site-to-site effect for 3.5 and 4.5 mm screws, with one exception. Overall, the 4.5 mm screws showed statistically higher stress required for extraction than 3.5 mm screws. The third generation composite femurs provide a satisfactory biomechanical analog to human long-bones at the screw-bone interface. However, it is not known whether these femurs perform similarly to human bone during physiological screw "toggling."

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

Failure of pedicle screws by loosening and back out remains a significant clinical problem. Pedicle screw fixation is determined by bone mineral density, pedicle morphology and screw design. The objective of this study was to compare the holding strength of newly developed dual core pedicle screws having a cylindrical design in terms of outer diameter and two cylindrical inner core regions connected by a conical transition with conventional cylindrical pedicle screws. Fifty bovine lumbar vertebrae and 40 human lumbar vertebrae were used. Five different screws were tested in nine experimental "settings" and ten specimens each. The screws were tested for cranial displacement and pullout strength before and after 5,000 cycles of cranio-caudal loading. The tests included a setting with fully inserted and 4 mm backed out screws. For statistical analysis the incomplete balanced block design was used. Cyclic loading led to a decrease of pullout force between 24 and 31% and a 9% increase of displacement. The cylindrical screw designs were affected more than the dual core designs. The pullout force of cylindrical screws was smaller than of dual core screws. Even in a backed out condition dual core screws showed a significantly smaller displacement than cylindrical screws. Pedicle screws with the dual core design provide good anchorage in the vertebra.

An in vitro optimized injectable calcium phosphate cement for augmenting screw fixation in osteopenic goats

This study reports the proportioning and standardized mixing procedures for preparing a hydroxylapatite cement (tetracalcium phosphate and dicalcium phosphate) of desired viscosity and mechanical strength reproducibly for application in trauma surgery. The behavior and the biomechanical properties of the resulting bone cement in screw augmentation were then evaluated in our osteopenic goat model. The use of a shaker standardized the mixing procedure. The optimal volume of Na2HPO4 used to prepare the injectable cement was 0.45 mL/g, with averaged in vitro compressive strength of 48.29 +/- 5.62 MPa. Histology showed increasing tightly-coupled bone apposition on the cement surface without fibrous encapsulation as observed in the screw-only controls with time in the osteopenic goat model. The cement increased the initial screw pull-out force (54.7%, p = 0.005) significantly and the energy required to failure (54.7%, p < 0.05) significantly, and remained higher than the screw-only controls after 3 months (9.8% and 20.2%, respectively) and 6 months (20.2% and 44.7%, respectively). These results imply potential in the prevention of interfacial micromotions and subsequent fibrous tissue formation at the implant-bone interface resulting in a decreased risk of implant failure. The optimized cement in this study may serve as a good candidate for augmenting fixation of osteoporotic bone.

Biocompatibility and functionality of the degradable polymer alkylene bis(dilactoyl)‐methacrylate for screw augmentation in vivo

Recently, a new degradable polymer has been developed on the basis of alkylene bis(dilactoyl)-methacrylate as an alternative material for screw augmentation. The polymer has been investigated in vitro and in a short-term experiment in rabbits exhibiting promising results. The aim of the present study was to investigate its long-term biocompatibility and mechanical functionality in a large animal model. The polymer was used for screw augmentation in the cancellous bone of the femoral condyle and tibia epiphysis of 12 sheep and was compared to polymethylmethacrylate (PMMA) augmented and nonaugmented screws. After an implantation period of 6 months, bone, regional lymph nodes, and several organs were histologically evaluated. The mechanical efficacy was investigated by a biomechanical pullout test. A lot of mononuclear macrophages and multinuclear foreign body giant cells with incorporated polymer particles indicate strong inflammatory reactions. Large osteolysis zones with osteoclasts were found in the surrounding polymer. The polymer was fragmented but not substantially degraded. Polymer particles were also found in the regional lymph nodes. Lung, liver, kidney, and spleen did not show any pathological signs. The pullout force of screws augmented with the new polymer was significantly reduced in comparison to PMMA augmented and nonaugmented screws, respectively. It was concluded that the material has poor biocompatibility and cannot be recommended for clinical application as screw augmentation material.

Hydroxyapatite-coating of pedicle screws improves resistance against pull-out force in the osteoporotic canine lumbar spine model: a pilot study

Background context In patients with spinal osteoporosis, the early achievement and maintenance of a biological bond between the pedicle screw and bone is important to avoid screw loosening complications. There are few reports of in vivo investigations involving biomechanical and histological evaluations in the osteoporotic spine. Purpose To evaluate the effect of hydroxyapatite (HA)-coating on the pedicle screw in the osteoporotic lumbar spine and to investigate the relationship between resistance against the screw pull-out force and bone mineral density (BMD) of the vertebral body. Study design/setting Mechanical and pathological investigations in the lumbar spine. Methods Two 24-month-old female beagle dogs were fed a calcium-free dog chow for 6 months after ovariectomy (OVX). BMD (in g/cm 2) was measured by dual energy X-ray absorptiometry at pre-OVX and 6 months after OVX. Pedicle screws were placed from L1 to L6 at 6 months after OVX. Twenty-four pure titanium cortical screws (Synthes, #401-114) were used as pedicle screws (Ti-PS). Of these, 12 screws had HA-coating (HA-PS). The HA-PS screws were inserted into the right pedicles and the Ti-PS were inserted into the left pedicles. Ten days after this procedure, the lumbar spines were removed en bloc for screw pull-out testing and histological evaluation. Results The mean BMD value of the lumbar vertebrae 6 months after the OVX was 0.549±0.087 g/cm 2, which was significantly less than the pre-OVX mean BMD of 0.603±0.092 g/cm 2 (p<0.001). The mean resistance against the pull-out force for the HA-PS was significantly greater at 165.6±26.5N than in the Ti-PS (103.1±30.2N, p<.001). The histological sections in the HA-PS clearly revealed new bone bonding with the apatite coating but only fibrous tissue bonding in the Ti-PS. Conclusions The results of this study showed that the resistance to the pull-out force of HA-PS is 1.6 times that of Ti-PS. Furthermore, HA-PS has superior biological bonding to the surrounding bone, as early as 10 days after surgery in this osteoporotic spine model. Thus, in patients with osteoporosis, coating of the pedicle screw with HA may provide better stability and bonding between the pedicle screw and bone in the early postoperative period.

Cortical bone screw fixation in ionically modified apatite cements

Hydroxyapatite cements are used in reconstruction of the face; usually in well-defined cavities where the cement can be stabilized without the need for internal fixation. A hydroxyapatite cement that could enable screw fixation and some loading therefore has considerable potential in maxillofacial reconstruction. It has been demonstrated recently that water demand of calcium phosphate cements can be reduced by ionically modifying the liquid component. This study investigated the capacity of an ionically modified precompacted apatite cement to retain self-tapping cortical bone screws. Screw pullout forces were determined in the direction of the screw long axis and perpendicular to it, using cortical bone and polymethylmethacrylate cement as a control. In bending pullout tests, measured forces to remove screws from ionically modified precompacted cement were insignificantly different from cortical bone. However, pullout forces of bone screws from hydroxyapatite cement decreased with aging time in vitro.

A Stronger Bicortical Sacral Pedicle Screw Fixation Through The S1 Endplate

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. 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. 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. 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. 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. 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.

Insertion torque and pullout force of rescue screws for anterior cervical plate fixation in a fatigued initial pilot hole.

The purpose of this study was to investigate whether thicker-core-diameter screws increase fixation strength in the cervical spine. Bone mineral density (BMD) was determined for each vertebral body (VB) obtained in six human C4-7 segments. Based on their BMD, the specimens were assigned to one of two groups in which torque and pullout force were tested. Two initial pilot holes were drilled into the VBs and tests were first performed using a standard screw. The test was repeated using a thicker rescue screw inserted into the same initial pilot hole. The mean value of peak torque and pullout force resulting from the single left/right measurements was used for statistical analysis. A t-test was performed to determine the effect of screw design on peak torque and pullout force. Moment correlation coefficients were calculated to determine the effect of BMD on peak torque and pullout force. Mean insertional peak torque for the standard screw was 82.1 N/cm and that for the rescue screw was 47.6 Ncm (p < 0.001). There was a strong correlation between insertional peak torque and BMD for both standard screws (r = 0.71, p = 0.02) and rescue screws (r = 0.59, p = 0.07). The mean pullout force for standard screws was 464.7 N, whereas it was 164.5 N for rescue screws (p < 0.001). There was a strong correlation between pullout force and BMD for both standard (r = 0.75, p = 0.0081) and rescue screws (r = 0.7, p = 0.025). Uncemented rescue screws that have been inserted into a fatigued hole in the cervical VB do not strengthen the screw-bone interface compared with the strength initially conferred by a standard screw.

Tensile failure of C2 pedicles and of subsequent direct repair in a porcine model.

This repeated-measures biomechanical study evaluated the tensile force required to cause bipedicular (hangman's) fractures in isolated porcine C2 specimens, and the subsequent force to failure after direct fracture repair with bipedicular lag screws. To assess the pullout strength of direct lag screw fixation of hangman's fracture, relative to the strength of the intact specimens. Clinical studies have reported successful treatment of hangman's fractures by direct screw repair followed by a collar after surgery. However, to the authors' best knowledge, there has not yet been a biomechanical analysis of the force required for screw pullout after direct repair. Of 60 mounted porcine C2 specimens subjected to tensile anteroposterior force, hangman's fracture occurred in 15 cases, 12 of which were fixed with bipedicular 4.0-mm lag screws. These fixed specimens were retested to failure in the same manner. Most specimens had laminar fractures after the application of tensile force, with 15 of 60 (25%) showing bipedicular fractures. The force to bipedicular failure was 3259.1 +/- 148.5 N (mean +/- standard error of the mean). After screw fixation, the force to failure of the same specimens was 882.0 +/- 108.5 N (mean +/- standard error of the mean), or 27.3% of the intact bone. The pullout strength was substantial (882 N), although the relative strength of fixation was only 27.3% of the fracture strength shown by the intact specimens. The findings are discussed in relation to previous studies. To the authors' best knowledge, this is the first study to examine screw pullout forces after direct repair of hangman's fracture.

Factors affecting the interface of cervical spine facet screws placed in the technique by Roy-Camille et al.

The objective of the study was to investigate the influence of bone cement, length of burr hole and bone density on pullout force and insertional screw torque of cervical spine facet screws. Both facets of 24 human cervical vertebrae were scanned for bone mineral density (BMD) and assigned to two groups for measuring of insertional screw torque and pullout strength. Maximal insertional screw torque was measured and removal of the screws was performed in displacement control (0.25 mm/s) without bone cement (PMMA), with 0.1 ml of PMMA and with the burr hole completely filled with PMMA. Screw torque was 59.1 N cm (+/-25.7 N cm), pullout force was 382.8 N (+/-140.5 N) without PMMA. Injection of 0.1 ml PMMA did not change significantly both screw torque (p=0.73) and pullout (p=0.129). Filling of the burr holes with PMMA increased significantly both screw torque (p<0.0001) and pullout force (p=0.028) when compared with injection of 0.1 ml of PMMA. A positive, moderate correlation was seen between BMD and screw torque before (r=0.501; p=0.097) and after filling with PMMA (r=0.514; p=0.088), BMD and pullout force before (r=0.441; p=0.152) and after complete filling with PMMA (r=0.673; p=0.047). The PMMA does increase both screw torque (p<0.0001) and pullout force (p=0.028) of facet screws significantly if the burr hole is filled with PMMA completely when compared with injection of 0.1 ml PMMA. Bone mineral density of the cervical facets moderately correlates with peak insertional torque and pullout force. This is true for a facet without PMMA and for a facet filled with PMMA. The length of the burr hole seems to be less important.

Influence of a screw with a hinge on the pull-out force at the cranial end vertebra in ventral derotation spondylodesis (VDS).

Pull-out of the cranial end-vertebra screw following a correction of a scoliosis with the VDS implant is a common complication, especially in the thoracic region. It requires an extension of the fusion length or at least reduces the outcome of the correction. We therefore looked for a new approach to solve the problem of end-vertebra screw pull-out. It was postulated that the axial pull-out force at the end-vertebra screw is reduced by a screw with a hinge between the head and threaded part. The aim of our study was to calculate theoretically and then determine experimentally the screw pull-out force of regular and modified VDS screws with a hinge during lateral shifting of the screw heads in the frontal plane. The use of a modified end-vertebra screw with a hinge on the cranial end vertebra of the correction region changes the shape of the threaded rod between the last two screws. Theoretically estimated, this leads to a reduction of the pull-out force to one-fourth of the value for the original VDS screw. Experimentally, the mean pull-out force for the modified end-vertebra screw was 110+/-8.6 N in comparison with the original screw, where the pull-out force was 214+/-7.2 N. A reduction of screw pull-out can be expected when a screw with a hinge is used for the cranial end vertebra.

Wood‐flour‐reinforced polyethylene: Viscoelastic behavior and threaded fasteners

AbstractThe tensile stress relaxation and flexural creep properties of wood‐fiber‐reinforced low‐density polyethylene (WFRP) were measured at various temperatures and stress levels. Power law relations were used to correlate the data, and time‐temperature superposition was applied to tensile stress relaxation results. Stress relaxation in WFRP was similar to that of LDPE and greater than that in spruce. The clamping force and torque characteristics of self‐threading screws and internally threaded inserts were measured in WFRP, and the viscoelastic model was extended to predict the relaxation in screw clamping force as a function of time. Both screw pullout force and the amount of clamping force relaxation were greater in WFRP than in spruce.