Maximum Pullout Strength Research Articles

Effect of surface modification of a metal implant by micro-screw threading plus micro-arc oxidation on its biostability in osteoporotic sheep

Objective To observe whether biological performance of a metal implant can be im-proved after its surface has been modified by micro-screw threading plus micro-arc oxidation in osteoporotic sheep. Methods Osteoporosis was induced in 6 adult female sheep by bilateral ovariectomy and con-firmed by dual energy X-ray absorptiometry and test of bone density of lumbar vertebrae after hormone ad-ministration and low calcium diet for 12 months. After 6 implants were put into the right ilium of each sheep, they were divided into 3 equal groups: group A (untreated), group B (treated by micro-screw threading) and group C (treated by micro-screw threading plus micro-arc oxidation). All the animals were sacrificed at 3 months after implantation. Three of them were randomly chosen as zero point controls, in each of which 6 implants were put into the left ilium and divided into similar 3 treatment groups. Mechanical test, Micro-CT analysis and histological observation were conducted for statistical analysis of the data. Results At 3 months after implantation, the maximal pull-out strength and energy absorption in each experimental group were significantly enhanced compared with each control group (P<0.05). The maximal pull-out strength and energy absorption in group C were significantly better than in groups A and B (P<0.05). Micro-CT recon-struction showed that the bone volume fraction, trabecular thickness and bone mineral density in group C were significantly improved compared with groups A and B (P<0.05). Histological observation showed significantly improved rates of new bone formation and bone contact in group C compared with groups A and B (P<0.05). Conclusion Biological performance and stability of a metal implant in osteoporotic sheep can be significantly improved after its surface has been modified by micro-screw threading plus micro-arc oxidation. Key words: Osteoporosis; Osteogenesis; Sheep; Micro-arc oxidation; Surface treatment

Effect of Crosslinks on the Stability of the Spine and the Pedicle Screw Fixation.

To evaluate the effect of crosslinks on the stability of the spine and pedicle screws. Compression fracture of the L1 vertebra was produced in 30 fresh thoracic and lumbar vertebrae samples obtained from adult sheep, which were divided into 3 groups (n=10)with lot-drawing method. Four screws were fixed onto the superior and inferior pedicles of vertebral arch close to the fractured vertebrae, with different number of crosslinks (0 in Group A,1 in Group B, and 2 in Group C) on the rods. After fixation, the samples were subject to 10 000 times of fatigue test with 1.5 Hz load on the HY-3080 computer-control electronic universal test machine and HY-1000NM computer-control torsion test machine. The axial compressive stiffness, maximum pullout strength,and range of motion (ROM) of 6 directions, i.e., flexion, extension, left and right lateral bending, and left and right axial rotation of the 3 groups were measured and compared. There were no statistically significant differences in axial compressive stiffness as well as the ROM of flexion, extension, and left and right lateral bending (all P>0.05). The maximum pullout strength was significantly smaller in Group A and Group B than in Group C [(129.56±29.63)N vs.(294.67±23.25) N,P=0.000;(254.02±36.29)vs.(294.67±23.25)N, P=0.006]. The ROM of left axial rotation was the highest in Group A(13.35°±1.06°), followed by Group B(12.23°±1.06°)and Group C (11.04°±0.74°)(F=13.44, P=0.000; Group B vs. Group A, P=0.000; Group B vs. Group C, P=0.001; Group C vs. Group A,P=0.000). The ROM of right axial rotation was also the highest in Group A(13.56°±1.15°), lower in Group B (12.39°±1.01°) and the lowest in Group C (10.81°±0.51°) (F=21.91, P=0.000; Group B vs. Group A,P=0.002; Group B vs. Group C, P=0.001; Group C vs. Group A, P=0.000). Crosslinks may reinforce the pullout strength of the screws and improve the axial stability of the spine.

Biomechanical and finite element analyses of bone cement-Injectable cannulated pedicle screw fixation in osteoporotic bone.

The objectives of this study were to investigate the safety and biomechanical stability of a polymethylmethacrylate (PMMA)-augmented bone cement-injectable cannulated pedicle screw (CICPS) in cancellous bone model, and to analyze the stress distribution at the screw-cement-bone interface. The OMEGA cannulated pedicle screw (OPS) and conventional pedicle screw (CPS) were used as control groups. Safety of the CICPS was evaluated by the static bending and bending fatigue tests. Biomechanical stability was analyzed by the maximum axial pullout strength and maximum torque tests. Stress distribution at the screw-cement-bone interface was analyzed by the finite element (FE) method. The CICPS and CPS produced statistically similar values for bending stiffness, bending structural stiffness, and bending yield moment. The maximum pullout force was 53.47 ± 8.65 N in CPS group, compared to 130.82 ± 7.32 N and 175.45 ± 43.01 N in the PMMA-augmented OPS and CICPS groups, respectively (p < 0.05). The CICPS had a significantly greater torque than the OPS and CPS. The FE model did not reveal excessive stress at the screw-cement-bone interface in the CICPS group. In conclusion, PMMA-augmentation with CICPS may be a potentially useful method to increase the stability of pedicle screws in patients with osteoporosis. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 104B: 960-967, 2016.

Influence of tire cord layers and arrangement direction on mechanical properties and vibration isolation of chloroprene rubber composite materials reinforced with polyester tire cord

Abstract Polyester tire cord is often used as reinforcement in the production of chloroprene rubber (CR) composite materials. This study investigated the influences of polyester tire cord layers and arrangement directions on the mechanical properties and vibration isolation of CR composite materials. The experimental results indicate that the optimal molding condition for bonding tire cord with CR composites is 170°C for 30 min, which enables tire cord-reinforced CR composites to exhibit maximum pull-out strength. The compression set, tensile strength, and tear strength of CR composites increased with the increasing number of tire cord layers, and the stretching direction parallel to the arrangement direction of the tire cord produced optimal reinforcement effect. Moreover, the storage modulus of CR composites is better than that of pure CR at low temperature, although it reduced the tan delta at glass transition temperature. Regarding the vibration isolation properties, the transmissibility ratio decreased at resonance frequency, while the damping ratio increased when the CR is reinforced with increasing number of tire cord layers.

The effect of lateral wall perforation on screw pull-out strength: a cadaveric study.

BackgroundLateral pedicle wall perforations occur frequently during pedicle screw insertion. Although it is known that such an occurrence decreases the screw pull-out strength, the effect has not been quantified biomechanically.Materials and methodsTwenty fresh cadaveric lumbar vertebrae were harvested, and the bone mineral density (BMD) of each was evaluated with dual-energy radiography absorptiometry (DEXA). Twenty matched, 6.5-mm pedicle screws were inserted in two different manners in two groups, the control group and the experimental group. In the control group, the pedicle screw was inserted in a standard fashion taking adequate precaution to ensure there was no perforation of the wall. In the experimental group, the pedicle screw was inserted such that its trajectory perforated the lateral wall. Group assignments were done randomly, and the maximal fixation strength was recorded for each screw pull-out test with a material-testing system (MTS 858 II).ResultsThe average BMD for both groups was 0.850 g/cm2 (0.788–0.912 g/cm2). The average (and standard deviation) maximal pull-out forces were 1,015.8 ± 249.40 N for the experimental group and 1,326.0 ± 320.50 N for the control group. According to a paired t-test, the difference between the two groups was statistically significant (P < 0.001).ConclusionThe results of this study confirm that the maximal pull-out strength of pedicle screws decreases by approximately 23.4% when the lateral wall is perforated.

Should we use cortical bone screws for cortical bone trajectory?

In 2009, Santoni et al. reported cortical bone trajectory (CBT) as a method of inserting pedicle screws to obtain more solid fixation, and proposed the use of cortical trajectory screws with a more closely placed thread (cortical screws) for CBT. Since the entry trajectory in CBT differs from that in the traditional trajectory, it is unclear whether the increased strength derives from the specific trajectory or the shape of the screw thread in contact with the cortical bone. Whether the use of cortical screws is always required with CBT thus remains unclear. The authors therefore investigated the relationship between screw entry trajectory and screw thread characteristics and pullout strength in animal experiments. Lumbar vertebrae (L1-L4) from 4-month-old female pigs were randomly assigned to one of 4 groups, with cancellous screws or cortical screws inserted via the traditional trajectory or CBT. For pullout strength testing, the screw was pulled out vertically against the direction of insertion. Rod pullout testing (toggle testing) was also performed, and the peak breaking strength was measured. The maximum pullout strength was significantly greater for CBT using cortical screws than for the traditional trajectory using cancellous screws. Pullout strength tended to be higher when cortical screws were used in both CBT and the traditional trajectory, although the difference was not significant. Toggle testing showed no significant differences among the 4 groups. The specific unconventional trajectory seemed to have a major impact on the increased strength obtained with CBT.

Balancing Rigidity and Safety of Pedicle Screw Fixation via a Novel Expansion Mechanism in a Severely Osteoporotic Model

Many successful attempts to increase pullout strength of pedicle screws in osteoporotic bone have been accompanied with an increased risk of catastrophic damage to the patient. To avoid this, a single-armed expansive pedicle screw was designed to increase fixation strength while controlling postfailure damage away from the nerves surrounding the pedicle. The screw was then subsequently tested in two severely osteoporotic models: one representing trabecular bone (with and without the presence of polymethylmethacrylate) and the other representing a combination of trabecular and cortical bone. Maximum pullout strength, stiffness, energy to failure, energy to removal, and size of the resulting block damage were statistically compared among conditions. While expandable pedicle screws produced maximum pullout forces less than or comparable to standard screws, they required a higher amount of energy to be fully removed from both models. Furthermore, damage to the cortical layer in the composite test blocks was smaller in all measured directions for tests involving expandable pedicle screws than those involving standard pedicle screws. This indicates that while initial fixation may not differ in the presence of cortical bone, the expandable pedicle screw offers an increased level of postfailure stability and safety to patients awaiting revision surgery.

Biomechanical testing of a unique built-in expandable anterior spinal internal fixation system.

BackgroundExpandable screws have greater pullout strength than conventional screws. The purpose of this study was to compare the biomechanical stability provided by a new built-in expandable anterior spinal fixation system with that of 2 commonly used anterior fixation systems, the Z-Plate and the Kaneda, in a porcine partial vertebral corpectomy model.MethodsEighteen porcine thoracolumbar spine specimens were randomly divided into 3 groups of 6 each. A vertebral wedge osteotomy was performed by removing the anterior 2/3 of the L1 vertebral body and the T15/L1 disc. Vertebrae were fixed with the Z-Plate, Kaneda, or expandable fixation system. The 3-dimensional spinal range of motion (ROM) of specimens in the intact state (prior to osteotomy), injured state (after osteotomy), and after internal fixation were recorded. The pullout strength and maximum torque of common anterior screws, the expandable anterior fixation screw unexpanded, and the expandable anterior fixation screw expanded was tested.ResultsAfter internal fixation, the expandable device and Z-plate system exhibited higher left bending motion than the Kaneda system (5.50° and 5.37° vs. 5.04, p = 0.001 and 0.008, respectively), and the Z-plate and Kaneda groups had significantly higher left axial and right axial rotation ROM as compared to the expandable device group (left axial rotation: 5.23° and 5.02° vs. 4.53°; right axial rotation: 5.23° and 5.08° vs. 4.49°). The maximum insertion torque of the expandable device was significantly greater than of a common screw (5.10 vs. 3.75 Ns). The maximum pullout force of the expandable device expanded was significantly higher than that of the common screw and the expandable device unexpanded (3,035.48 N vs. 1,827.38 N and 2,333.49 N).ConclusionsThe built-in anterior fixation system provides better axial rotational stability as compared to the other 2 systems, and greater maximum torque and pullout strength than a common fixation screw.Electronic supplementary materialThe online version of this article (doi:10.1186/1471-2474-15-424) contains supplementary material, which is available to authorized users.

Experimental behavior of masonry wall-to-timber elements connections strengthened with injection anchors

Out-of-plane failure mechanisms observed in stone masonry buildings subjected to seismic action are often a direct result of poor connections between structural elements. During a seismic event these weak connections become incapable of assuring proper load transmission. Therefore, the need to prevent these phenomena is of critical importance in understanding the behavior of unstrengthened masonry buildings along with the necessity of developing effective strengthening solutions. This paper presents injection anchors as a viable option to improve anchorage between masonry and timber elements on historical buildings, as for example wall-to-timber framed wall or wall-to-timber diaphragm connections. The experimental campaign consisted of quasi-static monotonic and cyclic pullout tests performed on real scale specimens, representative of wall-to-timber framed wall connections found in late 19th century buildings of downtown Lisbon, Portugal. Combined cone-bond failure was obtained in all 7 tests. Boundary conditions of the specimens greatly affected the results in terms of maximum pullout force, dissipated energy, and strength degradation. Displacement ductility of the strengthened connections is high. The force–displacement curves clearly pointed out the influence on the results of the wall’s compressive stress state and the contribution of friction in the grout/masonry interface.

Insertional torque and pullout strength of pedicle screws with or without repositioning: a porcine study.

PURPOSE. To evaluate the insertion torque and pullout strength of pedicle screws with or without repositioning. METHODS. 20 fresh porcine lumbar vertebrae of similar size were used. The entry point was at the site just lateral and distal to the superior facet joint of the vertebra, and to a depth of 35 mm. A 6.2-mm-diameter, 35-mm-long pedicle screw was inserted parallel to the superior end plate on one side as control. On the other side, an identical screw was first inserted 10º caudal to the superior end plate, and then repositioned parallel to the superior end plate. The insertional torque and pullout strength were measured. RESULTS. Three of the specimens were excluded owing to pedicle fractures during the pullout test. Repositioned pedicle screws were significantly weaker than controls in terms of the maximum insertional torque (3.20 ± 0.28 vs. 2.04 ± 0.28 Nm, 36% difference, p<0.01) and pullout strength (1664 ± 378 vs.1391 ± 295 N, p<0.01). CONCLUSION. Repositioning pedicle screws should be avoided, especially when the pedicle wall is breached. If repositioning is deemed necessary, augmentation with polymethyl methacrylate or a screw with a larger diameter should be considered.

Biomechanical evaluation of expansive pedicle screws and injectable absorbable bone cement augmentation in the osteoporotic vertebral environment

Objective To evaluate the fixation strengths of expansive pedicle screws in human cadaveric vertebrae with different bone mineral densities and with the augmentation by newly-developed injectable absorbable bone cement(IABC).Methods Fresh human cadaveric spines used for this study were grouped into 4 levels according to the measurements of bone mineral density:normal,osteopenia,osteoporosis and severe osteoporosis.The vertebrae were bilaterally instrumented with 4 pedicle screw protocols:conventional pedicle screw without augmentation,expansive pedicle screw without augmentation,conventional pedicle screw with augmentation and expansive pedicle screw with augmentation.The statistical method of balanced incomplete randomized blocks was used to assign the pedicles for the 4 protocols,resulting in 9 screws per group per level.Screw pullout tests were conducted.On the load-stroke curve,the maximum pullout strength and stiffness were determined.The differences between groups at the same bone mineral density level but with different screw protocols,or between groups at different bone mineral density levels but with the same screw protocol,were compared.Results Given the same bone mineral density level,the maximum pullout strength and stiffness of expansive screws were significantly higher than those of conventional screws (P ＜ 0.05).When the same type of screw was used,the maximum pullout strength in the augmented group was significantly higher than in the non-augmented group (P ＜ 0.05),but the cement augmentation had insignificant impact on the stiffness of either conventional or expansive pedicle screws (P ＞ 0.05).The pullout strength and stiffness of expansive screws,augmented conventional screws and augmented expansive screws at the osteoporotic level were comparable to those of conventional screws at the osteopenic lcvel (P ＞ 0.05).However,the pullout strength of any of the 4 protocols in the severe osteoporosis group was significantly lower than that of conventional screws in the osteopenia group (P ＜ 0.05).Conclusions Expansive pedicle screws appear feasible and safe in either osteopenic or osteoporotic spines.Augmentation by the newly-developed IABC can further improve the initial fixation strength of either conventional or expansive pedicle screws. Key words: Osteoporosis; Bone screw; Biomechanics; Cement; Pedicle screw augmentation

Biomechanical and histological evaluation of roughened surface titanium screws fabricated by electron beam melting.

BackgroundVarious fabrication methods are used to improve the stability and osseointegration of screws within the host bone. The aim of this study was to investigate whether roughened surface titanium screws fabricated by electron beam melting can provide better stability and osseointegration as compared with smooth titanium screws in sheep cervical vertebrae.MethodsRoughened surface titanium screws, fabricated by electron beam melting, and conventional smooth surface titanium screws were implanted into sheep for 6 or 12 weeks (groups A and B, respectively). Bone ingrowth and implant stability were assessed with three-dimensional imaging and reconstruction, as well as histological and biomechanical tests.ResultsNo screws in either group showed signs of loosening. Fibrous tissue formation could be seen around the screws at 6 weeks, which was replaced with bone at 12 weeks. Bone volume/total volume, bone surface area/bone volume, and the trabecular number were significantly higher for a define region of interest surrounding the roughened screws than that surrounding the smooth screws at 12 weeks. Indeed, for roughened screws, trabecular number was significantly higher at 12 weeks than at 6 weeks. On mechanical testing, the maximum pullout strength was significantly higher at 12 weeks than at 6 weeks, as expected; however, no significant differences were found between smooth and roughened screws at either time point. The maximum torque to extract the roughened screws was higher than that required for the smooth screws.ConclusionsElectron beam melting is a simple and effective method for producing a roughened surface on titanium screws. After 12 weeks, roughened titanium screws demonstrated a high degree of osseointegration and increased torsional resistance to extraction over smooth titanium screws.

Extracellular matrix-coating pedicle screws conduct and induce osteogenesis

The purpose of the present study was to study the effects of the extracellular matrix-coating pedicle screws on the conduction and induction of bone formation in young sheep. Pedicle screws [coated with collagen/chondroitin sulfate (coll/CS), hydroxyapatite (HA), and coll/CS/HA or uncoated] were randomly implanted into the L2-L5 pedicles of sheep. In the first stage, a static experiment was performed. In the second stage, a loading test was performed by implanting connecting rods. After 3months, the lumbar vertebrae with the screws were removed and examined by micro-CT, histological, and biomechanical analyses. Under non-loading conditions, there is bone formation around the surfaces of coated screws. Bone formation on the surface of the coll/CS/HA coating of pedicle screws was the highest. In terms of the trabecular bone morphology parameters of the region of interest around the surface of the pedicle screws, those associated with coll/CS/HA coatings were highest under non-loading conditions, the pullout strength of the coll-/CS-/HA-coated screws was the highest and that of the uncoated screws was minimal. Under loading conditions, the maximum pullout strength of each group of pedicle screws was less than that of the pedicle screws in the non-loading state. Under non-loading conditions, the organic and inorganic components of the titanium pedicle screw coatings can conduct or induce bone formation around the surface of the screws. The ability of the coll/CS/HA coating to induce bone formation was the strongest. Under loading conditions, a large amount of connective tissue formed around the surface of the screws in each group.

A Biomechanical Research of Growth Control of Spine by Shape Memory Alloy Staples

Shape memory alloy (SMA) staples in nickel titanium with shape memory effect are effective for spinal growth control. This study was designed to evaluate the biomechanical properties of the staples and observe the stability of the fixed segments spine after the staples were implanted. According to the vertical distance of the vertebrae, SMA staples of 5, 6.5, and 8 mm were designed. The recovery stress of 24 SMA staples in three groups was measured. The pullout strength of SMA staples and stainless steel staples in each functional spinal unit was measured. Each of the six fresh specimens was divided into three conditions: normal, single staple, and double staples. Under each condition, the angle and torque of spinal movements in six directions were tested. Results show that the differences in recovery stress and maximum pullout strength between groups were statistically significant. In left and right bending, flextion, and extention, the stability of spine was decreased in conditions of single staple and double staples. Biomechanical function of SMA staples was superior to stainless steel staple. SMA staples have the function of hemiepiphyseal compression and kyphosis and scoliosis model of thoracic vertebrae in goat could be successfully created by the fusionless technique.

Design and biomechanical study of bone cement injectable canulated pedicle screw

Objective To investigate the design and mechanical properties of bone cement injectable canulated pedicle screw (CICPS) so as to provide a safe and effective internal fixation for osteoporotic spinal disorder. Methods ( 1 ) Bone cement injection test was performed in vitro,and within osteoporotic cancellous bone models and osteoporotic vertebrae respectively.The distribution of bone cement and screw-bone interface were observed by X-ray films and CT.(2) Ten CICPSs already injected with bone cement and ten conventional pedicle screws were respectively examined by shear strength test.(3) CICPS in the cancellous bone models was augmented with 2-3 ml of bone cement.Then,the maximum axial pull-out strength of the CICPS was measured and were compared with that of conventional screws. Results Bone cement overflowed from each side hole of the CICPS and distributed only around the front of screws in an even and extensive way.Also,no bone cement leakage was founded.The mean shear load of CICPS and conventional screws was ( 10 600.8 ± 360.1 ) N and ( 15 458.1 ± 31 1.4) N respectively ( P ＜0.05 ).The mean maximum axial force at pull-out was ( 209.3 ± 13.3 ) N for the CICPS and ( 27.0 ± 5.0) N for the conventional screws ( P ＜ 0.05 ). Conclusions By dispersing bone cement uniformly,CICPS reduces the risk of cement leakage and significantly improves the pullout strength of screws in osteoporotic vertebrae.CICPS with good efficacy and safety provides theoretic basis for its clinical application. Key words: Osteoporosis; Biomechanics; Poymethyl mathacrylate; Pedicle screw

Effects of screw and host factors on insertion torque and pullout strength

To experimentally study the effects of altering implant length, outer diameter, cortical bone thickness, and cortical bone density on the primary stability of orthodontic miniscrew implants (MSIs). Maximum insertion torque (IT) and pullout strength (POS) of 216 MSIs were measured in synthetic bone with different cortical densities (0.64 g/cc or 0.55 g/cc) and cortical thicknesses (1 mm or 2 mm). Three MSIs were evaluated: 6-mm long/1.75-mm outer diameter, 3-mm long/1.75-mm outer diameter, and 3-mm long/2.0-mm outer diameter. To test POS, a vertical force was applied at the rate of 5 mm/min until failure occurred. The 6-mm MSIs displayed significantly (P < .001) higher IT and POS than the 3-mm MSIs did. The 3-mm MSIs with 2.0-mm outer diameters showed significantly higher (P < .001) IT and POS than the 3-mm MSIs with 1.75-mm outer diameters. The IT and POS were significantly (P < .001) greater for the MSIs placed in thicker and denser cortical bone. Both outer diameter and length affect the stability of MSIs. Increases in cortical bone thickness and cortical bone density increase the primary stability of the MSIs.

Effects of Pilot Hole Size and Bone Density on Miniscrew Implants' Stability

How does pilot hole size and bone density affect the primary stability of miniscrew implants (MSIs)? Using 120 MSIs divided equally into six groups, this 2 × 3 factorial design evaluated the effects of synthetic bone density (0.64 g/cc vs 0.8 g/cc cortices) and pilot hole size (no pilot hole, 1.0 mm pilot hole, and 1.4 mm pilot hole) on maximum insertion torque and pullout strength. The maximum placement torque was measured as the last thread of the MSIs was inserted. The pullout strength test applied a vertical force at 10 mm/min until failure. The insertion torque and pullout strength values were significantly (p ≤ .05) greater for the MSIs placed in high-density than in low-density cortical bone. The insertion torque and pullout strength decreased as pilot hole size increased, with significant (p ≤ .05) differences between all three subgroups. Insertion torque and pullout strength were significantly intercorrelated for all subgroupings, with stronger correlations in denser bone having smaller or no pilot holes. Depending on bone density, pilot holes of limited size can be used to optimize primary stability by decreasing insertion torque while maintaining the pullout strength of bone.

Assessment of pedicle screw pullout strength based on various screw designs and bone densities—an ex vivo biomechanical study

The pedicle screw fixation system has been used for various spinal disorders. Many studies have been conducted on the fixation ability of the pedicle screw, but variable results have been reported based on bone qualities, pedicle screw properties, insertion techniques, and experimental methods. An experimental and biomechanical study. To evaluate the geometric factors of screws affecting fixation ability after assessing pullout strength based on various pedicle screw designs and different bone densities. Nine types of pedicle screws were prepared according to the outer diameter shape (cylindrical or conical), the inner diameter shape (cylindrical or conical), and thread shape (V shape, buttress shape, and square shape). The pedicle screws were inserted into standardized polyurethane foams of Grades 5, 15, and 20. The pullout strength of each pedicle screw was determined using an MTS 858 machine (Material Testing System Corp., Minneapolis, MN, USA), and the results were analyzed statistically. Pullout strength based on the shape of thread was the highest in the V shape and lowest in the square shape for all foam grades (p<.05). The outer cylindrical and inner conical shape of pedicle screw showed the highest pullout strength in Grades 5 and 15 foam (p<.05). An outer cylindrical and inner conical shape with a V-shaped thread showed the highest pullout strength in all foam grades (p<.05). Pedicle screw with an outer cylindrical and inner conical configuration witha V-shaped thread may have maximum pullout strength, regardless of bone density.

Biomechanical evaluation of the expansive cannulated screw for fixation of femoral neck fractures

Femoral neck fracture is one of the common clinical traumas, especially amongst elder patients. This study aims to test, compare and evaluate the bone-screw interface strengths, the fatigue strengths, and the stabilities of our newly designed expansive cannulated screw (ECS) and the common cannulated compression screw (CCS) in the fixation of femoral neck fracture, which is a summary of recent research. Twenty-four pairs (48) of fresh femur specimens were randomly divided into four groups with six pairs (12) in each. To simulate one-legged standing, the maximum compressive strength and the single-screw axial pull-out force were compared between the fixed femoral necks treated with two ECSs and two CCSs, two ECSs and three CCSs or three ECSs and three CCSs, respectively. The screws were also subjected to 1,000,000 cycles of a loaded fatigue test and the results were recorded. When the same number of screws was used, the ECS showed significantly greater maximum compressive strength than the CCS (P<0.05), but no significant difference in fixation effectiveness was detected between the two ECSs and the three CCSs groups. The maximum axial pull-out strength of the ECS was also significantly greater that of the CCS (P<0.01); however, there was no sign of fatigue in both the ECS and CCS after 1,000,000 cycles of loaded fatigue test. The ECS shows better fixation performance than the currently and commonly used CCS; under certain circumstances, fixation with two ECSs can achieve the same effect as that with traditional three CCSs.

Which is the Preferred Revision Technique for Loosened Iliac Screw?

An in vitro biomechanical cadaver study. To evaluate the pull-out strength after 5000 cyclic loading among 4 revision techniques for the loosened iliac screw using corticocancellous bone, longer screw, traditional cement augmentation, and boring cement augmentation. Iliac screw loosening is still a clinical problem for lumbo-iliac fusion. Although many revision techniques using corticocancellous bone, larger screw, and polymethylmethacrylate (PMMA) augmentation were applied in repairing pedicle screw loosening, their biomechanical effects on the loosened iliac screw remain undetermined. Eight fresh human cadaver pelvises with the bone mineral density values ranging from 0.83 to 0.97 g/cm were adopted in this study. After testing the primary screw of 7.5 mm diameter and 70 mm length, 4 revision techniques were sequentially established and tested on the same pelvis as follows: corticocancellous bone, longer screw with 100 mm length, traditional PMMA augmentation, and boring PMMA augmentation. The difference of the boring technique from traditional PMMA augmentation is that PMMA was injected into the screw tract through 3 boring holes of outer cortical shell without removing the screw. On an MTS machine, after 5000 cyclic compressive loading of -200∼-500 N to the screw head, axial maximum pull-out strengths of the 5 screws were measured and analyzed. The pull-out strengths of the primary screw and 4 revised screws with corticocancellous bone, longer screw and traditional and boring PMMA augmentation were 1167 N, 361 N, 854 N, 1954 N, and 1820 N, respectively. Although longer screw method obtained significantly higher pull-out strength than corticocancellous bone (P<0.05), the revised screws using these 2 techniques exhibited notably lower pull-out strength than the primary screw and 2 PMMA-augmented screws (P<0.05). Either traditional or boring PMMA screw showed obviously higher pull-out strength than the primary screw (P<0.05); however, no significant difference of pull-out strength was detected between the 2 PMMA screws (P>0.05). Wadding corticocancellous bone and increasing screw length failed to provide sufficient anchoring strength for a loosened iliac screw; however, both traditional and boring PMMA-augmented techniques could effectively increase the fixation strength. On the basis of the viewpoint of minimal invasion, the boring PMMA augmentation may serve as a suitable salvage technique for iliac screw loosening.