Significant Difference In Pullout Strength Research Articles

Biomechanical Comparison of Impaction Techniques and Cross-Sectional Femoral Stem Shapes for Cementless Total Hip Arthroplasty.

Traditional mallet broaching and stem seating in cementless total hip arthroplasty (THA) can result in femoral stem misalignment, potentially reducing implant longevity. This study aimed to compare the pullout strength of cementless THA femoral stems with different cross-sectional designs achieved through the powered impactor method versus the traditional mallet method. The authors utilized 24 polyurethane foam femurs and two femoral bone preservation stems with different proximal cross-sectional shapes (double taper: ACTIS®, size 5; flat taper: TRI-LOCK®, size 5). A single orthopedic surgeon broached each femur from size 0 to size 5 using either the powered impactor or mallet impaction methods. Broaching time and component implantation times were recorded. A load-to-failure pullout strength test was conducted, and the ultimate pullout load was recorded. The broaching time for the TRI-LOCK® stem showed a statistically significant difference between the two impaction methods (powered: 37±7 seconds, mallet: 75±29 seconds, F[3, 20] = 4.56, p = 0.002), but no statistically significant difference was detected for the ACTIS® stem between the two impaction methods (powered: 47±22 seconds, mallet: 59±9 seconds, F[3, 20] = 4.56, p = 0.304). There was a statistically significant difference in pullout strength between the two impaction groups, and this strength was influenced by the implant cross-sectional shape (ACTIS®: 774±75N versus 679±22N, F(3,20) = 16.38, p = 0.018; TRI-LOCK®: 616±57N versus 859±85N, F(3, 20) = 16.38, p <0.001). The technique used for femoral bone preparation (powered impactor versus mallet) and the cross-sectional design of the cementless femoral stem are crucial factors that affect initial stem stability and operation time.

Effect of liquid environment on adhesion strength of bioactive glass-based adhesives to bone

Glass Polyalkenoate Cements (GPCs) are composed of ionomeric glass having setting reaction with aqueous poly(acrylic) acid (PAA). The mechanical properties of GPCs are normally affected by various factors such as the concentration of the acid solution, the molar mass of the polyacid, and the powder:liquid ratio. A previous study using fracture testing of double cantilever beam specimens made with a GPC (GPC A) adhered to bovine bone showed that the mode I energy release rate decreased significantly when the specimens were prepared in a flooded liquid environment, simulating a worst-case surgical sternal closure application. The present study used pull-out and butt strength tests to determine the extent to which the adhesion to bovine bone could be improved in wet environments by using a more viscous adhesive (GPC B). The pull-out samples were prepared in both dry and flooded liquid environments and incubated in both humid and flooded liquid environments at 37 °C for two time periods: one and seven days. Overall, the pull-out strength of GPC B was found to be between 56% and 808% higher than GPC A, indicating that the more viscous formulation could mitigate the effects of the wet environment. The GPC B samples prepared and incubated in a flooded liquid environment (L-LI) had a significantly lower pull-out strength than all three other conditions: (1) when prepared in a dry environment and incubated in either a liquid (D-LI) or (2) humid environment (D-H), and (3) when prepared in a liquid environment and incubated in a humid one (L-H), which were all of similar strength. No significant difference in pull-out strength between the samples tested after 1 and 7 days of incubation was found indicating that robust adhesion strength was obtained rapidly. To further assess the adhesive strength for augmenting sternal closure, GPC B and bovine bone were used to create butt joints which were tested at strain rates typical of breathing and coughing. The tests revealed that GPC B's viscoelastic properties resulted in a higher butt joint strength for coughing loads compared to breathing. Overall, the results of the present study show that when the adhesives are expected to be subjected to a flooded liquid environment while they set, significant gains in adhesive strength can be obtained by using more viscous GPC adhesives.

437 Percutaneous Juxtapedicular Cement Salvage of Failed Pedicle Screws: A Cadaveric Biomechanical Study

INTRODUCTION: Posterior spinal fixation with pedicle screw-based constructs is the gold standard for stabilizing the thoracolumbar spine; however, these constructs sometimes fail due to fatigue loading, necessitating revision surgery. A common hardware revision strategy involves open surgery for screw upsizing, with or without cement augmentation, and extension of fusion. Others have described good clinical outcomes following percutaneous juxtapedicular cement salvage of failing pedicle screws. The relative biomechanical properties of these various approaches has not been evaluated. METHODS: Thirty-two fresh, un-embalmed vertebrae from four cadaveric thoracolumbar spines were used for the study. The study was conducted in two phases. The first phase involved primary pedicle screw insertions and pullout strength testing to model critical pedicle screw failure. The second phase involved revision of failed pedicle screws followed by pullout testing. The four revision cohorts were: 1) Screw Upsizing, 2) Vertebroplasty (cement augmentation with replacement of primary screw), 3) Fenestrated Screws with cement augmentation, and 4) Juxtapedicular Cement Salvage. RESULTS: There was no statistically significant difference in primary fixation strength between the cohorts. The Screw Upsizing cohort exhibited inferior pullout strength compared to the other groups which involved cement augmentation. There was no significant difference in pullout strength between Vertebroplasty, Fenestrated Screws, or Juxtapedicular Cement Salvage cohorts. CONCLUSIONS: Cement augmentation confers superior pullout strength compared to screw upsizing only. The key finding of this study is that the technique of percutaneous juxtapedicular cement salvage is biomechanically non-inferior to other revision strategies that require an open procedure to perform.

Comparison of Initial Stability of Oblong, Large Circular, and Multiple-Plug "Snowman" Osteochondral Autografts for Elongated Focal Cartilage Lesions: A Biomechanical Study in a Porcine Model.

Background:Distal femoral osteochondral allograft transplantation (OAT) is an effective treatment of osteochondral lesions in the knee measuring >2 cm2 in select patients. Prior studies have demonstrated that the morphology of the plug can affect graft-host interference fit. To our knowledge, there are no data comparing the initial biomechanical stability of standard cylindrical plugs with multiple-plug and oblong-plug morphologies.Hypothesis:Large cylindrical single-plug (LCSP) and oblong single-plug (OSP) grafts will have greater pull-out strength, and therefore greater initial stability, than multiple-plug (MP) grafts in a cadaveric porcine femur model.Study Design:Controlled laboratory study.Methods:A total of 55 porcine distal femurs were divided into 3 groups—LCSP (n = 18), OSP (n = 19), and MP (n = 18)—according to the plug morphology used. The method of graft harvesting and implantation was based on technique guides for the respective implant systems. The sizes (length × width × depth) of the osteochondral defects created in each of the groups were approximately 20.2 × 20.2 × 9.4–mm for the LCSP group, 14.4 × 30.5 × 7.9–mm for the OSP group, and 14.8 × 14.8 × 9.9–mm for the MP group. Tensile testing was performed on each graft to determine pull-out strength.Results:The pull-out strength was significantly lower in the OSP group (65.7 N) versus the LCSP (133 N; P = .0005) and the MP (117.6 N; P = .001) groups. There was no statistically significant difference in pull-out strength between the LCSP and MP groups (P = .42). There were no statistically significant differences in displacement at maximum load among any 2 of the 3 groups.Conclusion:These findings suggest that while initial stability may play a role in the clinical outcomes of osteochondral allograft (OCA) implantation, the biological milieu in vivo for each graft setting perhaps has a greater impact on the success of an OAT procedure. Further study is needed on the relationship between OCA biomechanics and clinical outcomes of OAT.

Failure analysis and the effect of material properties on the screw pull-out behaviour of polymer composite sleeper materials

The low pull-out resistance and the limited understanding of how the sleeper materials affect the screw pull-out behaviour is one of the key reasons for the lack of confidence in the wide adoption of available composite railway sleepers. In this study, the pull-out strength and the failure analysis through hole microstructure of different sleeper technologies including timber, synthetic composite, recycled plastic and Particulate Filled Resin sleepers were investigated. Also, the effect of material properties on the screw pull-out strength and failure behaviour was determined. The results showed a statistically significant difference in pull-out strength among different sleeper technologies and the paired samples test revealed a high correlation between the pull-out strength and the shear strength of the materials. A simplified prediction model was developed to predict the pull-out resistance for railway sleepers based on shear strength of the composite sleepers. This proposed equation shows a better agreement with the experimental results than an existing theoretical equation regardless of different pull-out behaviour.

Biomechanical Evaluation of Achilles Tendon Midsubstance Repair: The Effects of Anchor Angle and Position.

The percutaneous knotless repair technique for Achilles tendon ruptures utilizes a Percutaneous Achilles Repair System (PARS) device for suturing the proximal tendon and 2 suture anchors for fixing the sutures into the calcaneus. Determining the best position of the suture anchors may optimize the strength of this repair. Twelve pairs of human ankle cadaveric specimens were randomly assigned to receive suture anchors placed at 45°, 90°, or 135° from the sagittal plane. The anchors were tensioned according to a protocol representing progressive, postoperative rehabilitation. Load, number of loading cycles, displacement, and mode of failure were recorded. With the anchors placed at 45°, 90°, and 135°, the ultimate failure loads were mean 265 ± 64 N, 264 ± 75 N, and 279 ± 40 N, and the total number of loading cycles were mean 459 ± 166, 466 ± 158, and 469 ± 110, respectively. The effect of anchor angle on failure load, number of loading cycles, and displacement was not statistically significant. Visually, the anchors at 45° and 90° demonstrated sutures cutting through the bone. We found no statistically significant difference in pullout strength between the 3 different anchor angles. Sutures cutting through the bone may be a concern with acute anchor angles. This suggests that a 135° anchor angle may result in a lower risk of tendon elongation with the percutaneous knotless repair technique. Cadaveric laboratory study.

Bioactive pedicle screws prepared by chemical and heat treatments improved biocompatibility and bone-bonding ability in canine lumbar spines.

BackgroundTitanium (Ti)-6Al-4V alloy, which is widely used in spinal instrumentation with a pedicle screw (PS) system. However, significant clinical problems, including loosening and back-out of PSs, persist. During the last decade, a novel technology that produces bioactive Ti from chemical and heat treatments has been reported that induces the spontaneous formation of a hydroxyapatite (HA) layer on the surface of Ti materials. The purpose of this study was to study the effect of bioactivation of Ti-6Al-4V PSs on the ability of HA formation in vitro and its biocompatibility and bone-bonding ability in vivo.MethodsTi-6V-4Al alloy PSs were prepared and bioactivated by NaOH-CaCl2-heat-water treatments. The HA-forming ability of bioactive PSs in simulated body fluid (SBF) was evaluated by field emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray analysis (EDX). Six 11-month-old female beagle dogs were used for the in vivo study. Bioactive and control (without bioactivation) PSs were left and right randomly placed from L1 to L6. One and three months after surgery, lumbar spines were removed for biomechanical and histological analyses.ResultsIn vitro: The surface analysis of bioactive PSs by FE-SEM and EDX showed substantial HA deposits over the entire surface. In vivo: The mean extraction torque was significantly higher for bioactive PSs compared to controls PSs (P<0.01); there was no significant difference in pull-out strength between control and bioactive PSs. Histologically, the contact area between bone tissue and screw surface showed no significant trend to be greater in bioactive PSs compared to control PSs (P = 0.06).ConclusionsBioactive PSs prepared by chemical and heat treatments formed layers of HA on the surface of screws in vitro that improved biocompatibility and bonding ability with bone in vivo. Bioactive PSs may reduce screw loosening to overcome the obstacles confronted in spinal instrumentation surgery.

Pullout strength of all suture anchors in the repair of rotator cuff tears: a biomechanical study

We evaluated the biomechanical strength of two all suture anchors (ASA) of reduced diameter (1.4 mm) and compared them with the standard screw anchor (SA) with larger diameter (5.5 mm) used in rotator cuff tears. We conducted 30 uniaxial vertical pullout tests using Material Testing System Instron 5566A until failure of the anchorage defined as rupture of the threads or anchor or detachment of the anchor. Anchor fixation was on tuberosities of fresh bovine humerus bone. ASAs were spaced four millimetres apart and were compared with a control SA implanted on the same greater tubercle at two centimetres. The tests were all performed at room temperature in a dry environment. Tensile loads (10 mm/min) were applied parallel to the axis of insertion. A preloading of 10 N was used to overcome loading artifacts of the test sample at the beginning of the test. Student's t test showed no statistically significant difference between anchors in terms of load to failure (ASA: force 265.06 ± 87.25 N versus SA : 325.35 ± 113.46 N; p = 0.09) and mean elongation at rupture (ASA : 23 ± 7 mm versus SA : 21 ± 6 mm; p = 0.46). In vitro, this experimental study showed no statistically significant difference in pullout strength and displacement between ASA and SA at a chosen level of significance (p < 0.05).

Pullout Strength Of Anterior Lumbar Interbody Fusion Plates: Fixed Versus Variable Angle Screw Designs

Abstract Purpose: To compare the pullout loads of variable versus fixed angle screws in anterior lumbar interbody fusion (ALIF) plate/screw constructs in both foam and cadaveric models. Methods: An ALIF plate was secured to a 0.160 g/cm3 polyurethane foam block (N = 10) with fixed or equivalently positioned variable angle screws and pulled out at 1 mm/min to failure. Embalmed human vertebral pairs (N = 10) were similarly utilized to compare fixed versus variable angle plate/screw constructs for pullout strength. Results: There was no statistically significant difference between fixed and variable angle plates for either foam (p = 0.6) or vertebral specimens (p = 0.3) based on maximum load or stiffness. The maximum load for the vertebral specimens was on average 39% of that for the foam (p = 0.001) and the stiffness was about 55% of that for the foam (p = 0.001). Conclusions: In this study, no statistically significant difference in pullout strength or stiffness was found between fixed and variable angle ALIF plate/screw constructs. However, the embalmed cadaveric bone model was shown to be statistically weaker in pullout strength and stiffness when compared to the foam model. This study suggests that the foam model may not always be representative of the cadaveric model in the magnitude of load, but may still provide good comparative results between different designs.

Pedicle Screw Design and Cement Augmentation in Osteoporotic Vertebrae

Experimental, human cadaveric study. To assess the fixation effects of injecting cement augmentation before screw insertion or after insertion of fenestrated screws; the effect of modulating cement viscosity; and the effects of these techniques on screw removal. It seems clear that cement augmentation can enhance pedicle screw fixation in osteoporotic bone. What remains to be demonstrated is the aspects of optimal technique such that fixation is enhanced with the greatest safety profile. Part I: Human osteoporotic vertebrae were instrumented with solid (nonaugmented) screws, solid screws with polymethylmethacrylate (PMMA), partially cannulated fenestrated (Pfen) screws, or fully cannulated fenestrated (Ffen) screws through which PMMA was injected. Screw fixation was tested in pullout. Part II: Ffen screws were augmented with standard low-viscosity PMMA versus high-viscosity PMMA. Part III: Sample cohorts were extracted from vertebrae to assess required torque and characterize difficulty of extraction. Part I: Pfen screws demonstrated the greatest fixation with mean failure force of 690 ± 182 N. All methods of cement augmentation demonstrated significant increases in screw fixation. Part II: Ffen screws did not demonstrate a significant difference in pullout strength when high-viscosity PMMA was used as compared with low-viscosity PMMA. Part III: Mean extraction torque values for solid augmented screws, Ffen screws, and Pfen screws were 1.167, 1.764, and 1.794 Nm, respectively, but these differences did not reach significance. None of the osteoporotic vertebrae sustained catastrophic failure during augmented screw extraction. Polymethylmethacrylate cement augmentation clearly enhances pedicle screw fixation in osteoporotic vertebrae when tested in pure pullout. The technique used for cement injection and choice of specialty screws can have a significant impact on the magnitude of this effect. Fenestrated screws have the capacity to confine cement placement in the vertebral body and may provide enhanced safety from cement extrusion into the spinal canal. It is feasible to inject high-viscosity PMMA through this fenestration geometry, and higher-viscosity cement may enhance the fixation effect.

Pullout strength of pedicle screws with cement augmentation in severe osteoporosis: A comparative study between cannulated screws with cement injection and solid screws with cement pre-filling

BackgroundPedicle screws with PMMA cement augmentation have been shown to significantly improve the fixation strength in a severely osteoporotic spine. However, the efficacy of screw fixation for different cement augmentation techniques, namely solid screws with retrograde cement pre-filling versus cannulated screws with cement injection through perforation, remains unknown. This study aimed to determine the difference in pullout strength between conical and cylindrical screws based on the aforementioned cement augmentation techniques. The potential loss of fixation upon partial screw removal after screw insertion was also examined.MethodThe Taguchi method with an L8 array was employed to determine the significance of design factors. Conical and cylindrical pedicle screws with solid or cannulated designs were installed using two different screw augmentation techniques: solid screws with retrograde cement pre-filling and cannulated screws with cement injection through perforation. Uniform synthetic bones (test block) simulating severe osteoporosis were used to provide a platform for each screw design and cement augmentation technique. Pedicle screws at full insertion and after a 360-degree back-out from full insertion were then tested for axial pullout failure using a mechanical testing machine.ResultsThe results revealed the following 1) Regardless of the screw outer geometry (conical or cylindrical), solid screws with retrograde cement pre-filling exhibited significantly higher pullout strength than did cannulated screws with cement injection through perforation (p = 0.0129 for conical screws; p = 0.005 for cylindrical screws). 2) For a given cement augmentation technique (screws without cement augmentation, cannulated screws with cement injection or solid screws with cement pre-filling), no significant difference in pullout strength was found between conical and cylindrical screws (p >0.05). 3) Cement infiltration into the open cell of the test block led to the formation of a cement/bone composite structure. Observations of the failed specimens indicated that failure occurred at the composite/bone interface, whereas the composite remained well bonded to the screws. This result implies that the screw/composite interfacial strength was much higher than the composite/bone interfacial strength. 4) The back-out of the screw by 360 degrees from full insertion did not decrease the pullout strength in any of the studied cases. 5) Generally, larger standard deviations were found for the screw back-out cases, implying that the results of full insertion cases are more repeatable than those of the back-out cases.ConclusionsSolid screws with retrograde cement pre-filling offer improved initial fixation strength when compared to that of cannulated screws with cement injection through perforation for both the conically and cylindrically shaped screw. Our results also suggest that the fixation screws can be backed out by 360 degrees for intra-operative adjustment without the loss of fixation strength.

Evaluation of pullout strength and failure mechanism of posterior instrumentation in normal and osteopenic thoracic vertebrae

There is limited data on the pullout strength of spinal fixation devices in the thoracic spine among individuals with different bone quality. An in vitro biomechanical study on the thoracic spine was performed to compare the pullout strength and the mechanism of failure of 4 posterior fixation thoracic constructs in relation to bone mineral density (BMD). A total of 80 vertebrae from 11 fresh-frozen thoracic spines (T2-12) were used. Based on the results from peripheral quantitative CT, specimens were divided into 2 groups (normal and osteopenic) according to their BMD. They were then randomly assigned to 1 of 4 different instrumentation systems (sublaminar wires, pedicle screws, lamina claw hooks, or pedicle screws with wires). The construct was completed with 2 titanium rods and 2 transverse connectors, creating a stable frame. The pullout force to failure perpendicular to the rods as well as the pattern of fixation failure was recorded. Mean pullout force in the osteopenic Group A (36 vertebrae) was 473.2 ± 179.2 N and in the normal BMD Group B (44 vertebrae) was 1414.5 ± 554.8 N. In Group A, no significant difference in pullout strength was encountered among the different implants (p = 0.96). In Group B, the hook system failed because of dislocation with significantly less force than the other 3 constructs (931.9 ± 345.1 N vs an average of 1538.6 ± 532.7 N; p = 0.02). In the osteopenic group, larger screws demonstrated greater resistance to pullout (p = 0.011). The most common failure mechanism in both groups was through pedicle base fracture. Bone quality is an important factor that influences stability of posterior thoracic implants. Fixation strength in the osteopenic group was one-fourth of the value measured in vertebrae with good bone quality, irrespective of the instrumentation used. However, in normal bone quality vertebrae, the lamina hook claw system dislocated with significantly less force when compared with other spinal implants. Further studies are needed to investigate the impact of different transpedicular screw designs on the pullout strength in normal and osteopenic thoracic spines.

Mechanical Evaluation of a 4-mm Cancellous “Rescue” Screw in Osteoporotic Cortical Bone: A Cadaveric Study

Obtaining sufficient fracture fixation in osteoporotic bone is challenging. The purposes of the current study were (1) to compare the pullout strength of a 4-mm cancellous screw (cancellous screw group) with that of a 3.5-mm cortical screw (cortical screw group), and (2) to measure the pullout strength of a 4-mm cancellous screw placed as a rescue screw (rescue screw group) in a stripped 3.5-mm cortical screw (stripped screw group) hole while controlling for bone density and cortical thickness. We inserted 4 screws, one from each experimental group, into 11 osteoporotic cadaveric radii, while recording the insertion torque. Radii were mounted on a servohydraulic testing machine, and each screw was pulled out at a rate of 5 mm/min. Pullout strength was recorded. The effects of cortical thickness (near, far, and total), bone density, insertion torque, and the experimental screw group (cortical, cancellous, stripped, and rescue screw groups) on pullout strength were analyzed using multiple linear regression with random effects. Statistical significance was set at P < 0.05. There was no significant difference in pullout strength between the cortical and cancellous screw groups. The rescue screw group had significantly less pullout strength than did the cortical and cancellous screws, and only partly increased pullout strength compared with stripped screws. Bone density significantly affected pullout strength, but insertion torque and cortical thickness were not significant covariates. There seems to be no advantage in using a cancellous screw over a cortical screw in bicortical fixation in osteoporotic bone. Although the rescue screw provided greater pullout strength than the stripped screw, it is unknown if the purchase it provides is clinically sufficient.

Strength of Single- Versus Double-Anchor Repair of Type II SLAP Lesions: A Cadaveric Study

This study was designed to examine whether there was a difference in pullout strength along with the mode of failure between 1 suture anchor and 2 suture anchors. Ten matched pairs of cadaveric shoulders were used in the study. A type II SLAP lesion, according to the classification of Snyder et al., was created. In 10 shoulders 1 anchor with 2 sutures was used to repair the lesion. In the other group of 10 matched shoulders, fixation was done with 2 anchors, with each anchor having only 1 suture. An Instron servohydraulic test machine (Instron, Canton, MA) was used to pull the long head of the biceps tendon until failure occurred. The single-anchor group failed at a mean load of 278.5 +/- 101.5 N. The double-anchor group failed at 242.5 +/- 96.5 N. A paired 2-sample Student t test showed that there was no significant difference in pullout strength between the 2 groups (P = .090). The most common mode of failure was soft-tissue failure. There was 1 anchor pullout in the single-anchor group and 2 anchor pullouts in the double-anchor group. The results of this study imply that using 1 anchor with 2 sutures is biomechanically equivalent to 2 anchors with 1 suture each for repairing type II SLAP lesions. Using 1 suture anchor is sufficient to repair a type II SLAP lesion.

Biomechanical evaluation of a new fixation device for the thoracic spine

The technology used in surgery for spinal deformity has progressed rapidly in recent years. Commonly used fixation techniques may include monofilament wires, sublaminar wires and cables, and pedicle screws. Unfortunately, neurological complications can occur with all of these, compromising the patients' health and quality of life. Recently, an alternative fixation technique using a metal clamp and polyester belt was developed to replace hooks and sublaminar wiring in scoliosis surgery. The goal of this study was to compare the pull-out strength of this new construct with sublaminar wiring, laminar hooks and pedicle screws. Forty thoracic vertebrae from five fresh frozen human thoracic spines (T5-12) were divided into five groups (8 per group), such that BMD values, pedicle diameter, and vertebral levels were equally distributed. They were then potted in polymethylmethacrylate and anchored with metal screws and polyethylene bands. One of five fixation methods was applied to the right side of the vertebra in each group: Pedicle screw, sublaminar belt with clamp, figure-8 belt with clamp, sublaminar wire, or laminar hook. Pull-out strength was then assessed using a custom jig in a servohydraulic tester. The mean failure load of the pedicle screw group was significantly larger than that of the figure-8 clamp (P = 0.001), sublaminar belt (0.0172), and sublaminar wire groups (P = 0.04) with no significant difference in pull-out strength between the latter three constructs. The most common mode of failure was the fracture of the pedicle. BMD was significantly correlated with failure load only in the figure-8 clamp and pedicle screw constructs. Only the pedicle screw had a statistically significant higher failure load than the sublaminar clamp. The sublaminar method of applying the belt and clamp device was superior to the figure-8 method. The sublaminar belt and clamp construct compared favorably to the traditional methods of sublaminar wires and laminar hooks, and should be considered as an alternative fixation device in the thoracic spine.

Pullout strength of suture anchors in comparison with transosseous sutures for rotator cuff repair

Suture anchors are increasingly gaining importance in rotator cuff surgery. This means they will be gradually replacing transosseous sutures. The purpose of this study was to compare the stability of transosseous sutures with different suture anchors with regard to their pullout strength depending on bone density. By means of bone densitometry (CT scans), two groups of human humeral head specimens were determined: a healthy and a osteopenic bone group. Following anchor systems were being tested: SPIRALOK 5.0 mm (resorbable, DePuy Mitek), Super Revo 5 mm (titanium, Linvatec), UltraSorb (resorbable, Linvatec) and the double U-sutures with Orthocord USP 2 (partly resorbable, DePuy Mitek) and Ethibond Excel 2 (non-resorbable, Ethicon). The suture anchors/double U-sutures were inserted in the greater tuberosity 12 times. An electromechanical testing machine was used for cyclic loading with power increasing in stages. We recorded the ultimate failure loads, the system displacements and the modes of failure. The suture anchors tended to bring about higher ultimate failure loads than the transosseous double U-sutures. This difference was significant in the comparison of the Ethibond suture and the SPIRALOK 5.0 mm-both in healthy and osteopenic bone. Both the suture materials and the SPIRALOK 5.0 mm showed a significant difference in pullout strength on either healthy or osteopenic bone; the titanium anchor SuperRevo 5 mm and the tilting anchor UltraSorb did not show any significant difference in healthy or osteopenic bone. There was no significant difference concerning system displacement (healthy and osteopenic bone) between the five anchor systems tested. The pullout strength of transosseous sutures is neither on healthy nor on osteopenic bone higher than that of suture anchors. Therefore, even osteopenic bone does not constitute a valid reason for the surgeon to perform open surgery by means of transosseous sutures. The choice of sutures in osteopenic bone is of little consequence anyway since it is mostly the bone itself which is the limiting factor.

Resistance Nut Welding: Improving the Weldability and Joint Properties of Ultra High Strength Steels

This study covers resistance projection welding of three different M6 and M8 weld nut types to high- and ultra high strength thin sheet steels ranging up to 1 500 MPa tensile strength. Static tensile testing, torsion testing and fracture evaluation as well as metallographical examinations were used to evaluate the suitability of the selected nut types for applications in high and ultra high strength steels. Different resistance welding equipment were compared and both alternating current and direct current were evaluated. A new etchant was used for examining the fusion zone in the nut joints, which indicated that no significant melting of the interfaces had occurred. This was evident for all the weld nut types welded to the galvanized thin sheet steels. The electrical characteristic of the power source plays an important role for the weld result. A rapid current build-up during the first period (AC) of weld time increased the initial heating of the weld nut projections that improved the joint strength. Significant differences in pull-out strength were recorded in joints welded with different AC power sources. Current curve modification for MFDC welding machines is a promising way of increasing the joint strength. A welding cycle with an initial high peak current pulse followed by a current plateau improved the pull-out strength by more than 100%.

Pullout Strength of Self-Tapping Screws Inserted to Different Depths

The purpose of this study was to determine whether the depth of insertion through the far cortex of self-tapping screws significantly affects pullout strength. Fifty, Synthes, 3.5-mm, self-tapping screws were inserted into synthetic bone blocks and divided into 5 groups. Group 1 had screws with their tips inserted 1 mm short of the far cortex. Group 2 had screws inserted flush with the far cortex. Groups 3, 4, and 5 had screws inserted 1 mm, 2 mm, and 3 mm past the far cortex respectively. Pullout strength was then tested. Institutional research laboratory. Pullout strength (peak force) was measured for each group and analyzed using a single factor analysis of variance-balanced incomplete block design. Peak force values presented as mean +/- SD for the 5 groups were as follows: group 1 (1380 +/- 69 N), group 2 (1566 +/- 137 N), group 3 (1956 +/- 137 N), group 4 (2013 +/- 184 N), group 5 (2044 +/- 174 N). With a P < or = 0.05, it was found that groups I and II had statistically different pullout strengths than all other groups. However, there was no significant difference in pullout strength between groups 3, 4, and 5. Synthes self-tapping screws exhibit their highest pullout strength when inserted 1 mm past the far cortex, and there is no significant increase in pullout strength with deeper insertion depths.

Strength of fixation of anterior vertebral body screws.

The technique of hole preparation and the placement or orientation of anterior transvertebral screws in relation to the endplates were the subject of two experiments. Experiment 1--to determine if anterior transvertebral body screws have higher pull-out strengths after unicortical predrilling compared with a bicortical predrilling; Experiment 2--to determine the effect of the screw's orientation in the vertebral body on its resistance to loosening. There are a variety of surgical techniques for using anterior transvertebral screws in the vertebral body without any clear guidelines as to which techniques optimize the strength of fixation. In experiment 1, 31 cadaveric vertebral bodies were tested for pull-out strength of transvertebral body screws placed after unicortical predrilling compared with bicortical predrilling. In experiment 2, 48 cadaveric vertebral bodies were tested with transvertebral screws inserted using four different screw orientations. There is no statistically significant difference in pull-out strength of anterior transvertebral body screws inserted after unicortical compared with bicortical predrilling (Experiment 1). The resistance to loosening is greatest when transvertebral screws are oriented in the "superior oblique" position (Experiment 2). Experiment 1--preparation of the far cortex by predrilling is not necessary. Experiment 2--transvertebral screws should be obliquely oriented so that the forces applied to the screws are resisted by both of the endplates.

The effect of repetitive screw hole use on the retentive strength of pretapped and self-tapped screws

Multiple systems for internal screw fixation are presently in use in oral and maxillofacial surgery. These systems differ in screw diameter, material, and design. This study evaluated the uniaxial pullout strength of five screw systems. Two-millimeter, 2.7-mm, and 3.5-mm screws were initially evaluated for pullout strength. Pretapped and self-tapped screws were then inserted into the same hole multiple times before pullout testing. Results of pullout testing revealed no significant difference in pullout strength of pretapped and self-tapped screws of equal diameter ( P > .05). Increased screw diameter produced increased pullout strength. No significant difference in pullout strength was noted in pretapped or self-tapped screws inserted into the same hole one, two, or three times before pullout testing ( P > .05).