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.

Biomechanical Study of 3 Osteoconductive Materials Applied in Pedicle Augmentation and Revision for Osteoporotic Vertebrae: Allograft Bone Particles, Calcium Phosphate Cement, Demineralized Bone Matrix.

This study assessed biomechanical properties of pedicle screws enhanced or revised with 3 materials. We aimed to compare the efficacy of these materials in pedicle augmentation and revision. One hundred twenty human cadaveric vertebrae were utilized for in vitro testing. Vertebrae bone density was evaluated. Allograft bone particles (ABP), calcium phosphate cement (CPC), and demineralized bone matrix (DBM) were used to augment or revise pedicle screw. Post the implantation of pedicle screws, parameters such as insertional torque, pullout strength, cycles to failure and failure load were measured using specialized instruments. ABP, CPC, and DBM significantly enhanced biomechanical properties of the screws. CPC augmentation showed superior properties compared to ABP or DBM. ABP-augmented screws had higher cycles to failure and failure loads than DBM-augmented screws, with no difference in pullout strength. CPC-revised screws exhibited similar strength to the original screws, while ABP-revised screws showed comparable cycles to failure and failure loads but lower pullout strength. DBM-revised screws did not match the original screws' strength. ABP, CPC, and DBM effectively improve pedicle screw stability for pedicle augmentation. CPC demonstrated the highest efficacy, followed by ABP, while DBM was less effective. For pedicle revision, CPC is recommended as the primary choice, with ABP as an alternative. However, using DBM for pedicle revision is not recommended.

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.

Pullout strength and failure mode of industrially manufactured and self-made all-suture anchors: a biomechanical analysis

This study presents a new technique for assembling an all-suture anchor from existing medical products. The biomechanical characteristics of this self-made anchor (SMA) are compared with those of an industrially manufactured all-suture anchor. The SMAs were made from established medical products (FiberWire #2and 2-mm FiberTape; Arthrex, Naples, FL, USA). Pretesting was performed in biphasic polyurethane foam blocks. In the next step, 10 SMAs and 10 industrially made anchors (IMAs; 1.8-mm double-loaded Y-KnotFlex all-suture anchor; ConMed Linvatec, Largo, FL, USA) were applied with an insertion tool and tested in fresh porcine femora using a servohydraulic testing system, with a preload of 10 N and a displacement rate of 12.5 mm/s. Pullout strength and failure mode were recorded. The mean load at failure in the foam blocks was 459 ± 124 N in the SMA group and 538 ± 83 N in the IMA group. In porcine bone, failure occurred at 461 ± 102 N in the SMA group and 431 ± 135 N in the IMA group. The differences in pullout strength between the 2 types of anchor were not statistically significant, either in the foam blocks (P = .17) or in porcine bone (P = .62). A handmade all-suture anchor using 2 high-strength sutures woven into a 2-mm strip of high-strength tape did not show statistically different failure loads in polyurethane foam or in porcine metaphyseal bone in comparison with a commercially produced double-loaded all-suture anchor. The principal mode of failure in porcine bone in both groups was anchor pullout.

Screw insertion torque as parameter to judge the fixation. Assessment of torque and pull-out strength in different bone densities and screw-pitches

BackgroundPull-out strength is a critical parameter to judge screw fixation in orthopaedic implants. However, the insertion torque is the main feeling in the hand of a surgeon relating to the strength of synthesis. The correlation between pull-out strength and torque is not completely understood. This creates uncertainty about the key-question: Should the torque be considered a valid parameter to judge the quality of fixation? MethodsUsing the ASTM F543 as reference, three screws differing only in pitch (1.5, 2.1, 2.8 mm pitches) were tested in three foam-block densities (10, 15, and 20 pcf). The correlation was investigated by assessing the role of density and screw geometry. FindingsTorque was related to pull-out strength in all configurations (R = 0.979, P = 0.000). No difference in pull-out strength was found when screws were tightened to a range of 71.6%, SD = 7.6, of torque to fail (P > 0.05). Torque and pull-out strength were stratified according to density that influenced the two parameters up to 524% (P < 0.000). Pitch determined pull-out strength up to 33% (P < 0.000) while the 2.1 mm screw pitch showed the highest pull-out strength and torque in all configurations. InterpretationInsertion torque was demonstrated to be a valid parameter to judge the quality of bone under fixation and therefore, the strength of the synthesis. Surgeons should not tighten the screws to values approaching torque to fail to obtain the highest pull-out strength. Density was the main factor influencing pull-out strength and torque. Pitch is another parameter deciding screw holding capacity.

Biomechanical Comparative Study of Postoperative Achilles Pull-out Strength After Open Versus

Category: Hindfoot Introduction/Purpose: The risk of Achilles avulsion is the limiting factor in return to full weight bearing and early activity following calcaneoplasty for Haglund’s syndrome. The most commonly performed surgical procedure is an open calcaneoplasty through a posterior approach. Another option is an endoscopic decompression, which theoretically causes less disruption of the Achilles insertion, and allows a more rapid return to full weight bearing and activity. Despite this potential benefit no study has assessed the difference in Achilles tendon pull-out strength after open versus endoscopic techniques. The purpose of this study was to investigate those changes in a cadaveric model and provide objective data upon which to base postoperative recovery. Methods: Six matched pairs of cadaveric specimens(mid-tibia to toes) were randomly divided into two surgical groups.Group#1 was treated with an endoscopic decompression under fluoroscopic guidance using a 4 mm burr.Group#2 was treated with a traditional calcaneoplasty through a posterior approach using a microsaggital saw.The Achilles was repaired back to the calcaneus with two nonsabsorbable suture anchors in Group #2.Fouroscopic guidance was used to exactly match the amount of bone removed from each matched pair.Pre and post procedure true lateral x-rays were obtained in both groups to quantify bone loss.These radiographic images were imported into Matlab computational software(MathWorks, Inc., Natick, MA) for digital analysis. The distal aspect of each calcaneus was potted and held at a 20 degree angle.The Achilles tendon was secured in a freeze-clamp, which was attached to a mechanical testing system. Specimens were then loaded to failure. Outcome measures include: Height of bony resection, angle of bone resection and load to failure. Results: To date our results show significantly higher pullout forces in specimens that had an endoscopic calcaneoplasty (average failure load: 1368 ±445 N) compared to those that received an open surgical procedure (450±184 N), p&lt;0.05. On average, the endoscopic technique had a 204% greater pull out strength compared to the open cohort. The calcaneal resection angle (37 degrees vs 39 degrees, p=.6) and the height of bony resection (49% vs 51%, p=.39) were not statistically significant between the open and endoscopic techniques. Conclusion: This study describes the first biomechanical comparison of Achilles pull-out-strength between open and endoscopic calcaneoplasty.The endoscopic-technique had 204%greater pull-out-strength compared to the open-technique.Even though equal amounts of bone were resected in each technique, the open-technique requires more disruption of the Achilles insertion to accommodate the saw.This marked difference in pull-out-strength demonstrates the vulnerability of the Achilles insertion following open treatment.Our results support protected weight-bearing and cautious return to activity following open surgery. Also an endoscopic-technique results in significantly increased post-surgical mechanical strength compared to an open-technique. Therefore, earlier weight-bearing and return to activity may be warranted after endoscopic calcaneoplasty.

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 standard vs. cement-augmented rotator cuff repair anchors in cadaveric bone

BackgroundWe evaluate a novel method of rotator cuff repair that uses arthroscopic equipment to inject bone cement into placed suture anchors. A cadaver model was used to assess the pullout strength of this technique versus anchors without augmentation. MethodsSix fresh-frozen matched pairs of upper extremities were screened to exclude those with prior operative procedures, fractures, or neoplasms. One side from each pair was randomized to undergo standard anchor fixation with the contralateral side to undergo anchor fixation augmented with bone cement. After anchor fixation, specimens were mounted on a servohydraulic testing system and suture anchors were pulled at 90° to the insertion to simulate the anatomic pull of the rotator cuff. Sutures were pulled at 1 mm/s until failure. FindingsThe mean pullout strength was 540 N (95% confidence interval, 389 to 690 N) for augmented anchors and 202 N (95% confidence interval, 100 to 305 N) for standard anchors. The difference in pullout strength was statistically significant (P < 0.05). InterpretationThis study shows superior pullout strength of a novel augmented rotator cuff anchor technique. The described technique, which is achieved by extruding polymethylmethacrylate cement through a cannulated in situ suture anchor with fenestrations, significantly increased the ultimate failure load in cadaveric human humeri. This novel augmented fixation technique was simple and can be implemented with existing instrumentation. In osteoporotic bone, it may substantially reduce the rate of anchor failure.

Performance of laser sintered Ti–6Al–4V implants with bone-inspired porosity and micro/nanoscale surface roughness in the rabbit femur

Long term success of bone-interfacing implants remains a challenge in compromised patients and in areas of low bone quality. While surface roughness at the micro/nanoscale can promote osteogenesis, macro-scale porosity is important for promoting mechanical stability of the implant over time. Currently, machining techniques permit pores to be placed throughout the implant, but the pores are generally uniform in dimension. The advent of laser sintering provides a way to design and manufacture implants with specific porosity and variable dimensions at high resolution. This approach enables production of metal implants that mimic complex geometries found in biology. In this study, we used a rabbit femur model to compare osseointegration of laser sintered solid and porous implants. Ti–6Al–4V implants were laser sintered in a clinically relevant size and shape. One set of implants had a novel porosity based on human trabecular bone; both sets had grit-blasted/acid-etched surfaces. After characterization, implants were inserted transaxially into rabbit femora; mechanical testing, micro-computed tomography (microCT) and histomorphometry were conducted 10 weeks post-operatively. There were no differences in pull-out strength or bone-to-implant contact. However, both microCT and histomorphometry showed significantly higher new bone volume for porous compared to solid implants. Bone growth was observed into porous implant pores, especially near apical portions of the implant interfacing with cortical bone. These results show that laser sintered Ti–6Al–4V implants with micro/nanoscale surface roughness and trabecular bone-inspired porosity promote bone growth and may be used as a superior alternative to solid implants for bone-interfacing implants.

Pedicle screw cement augmentation. A mechanical pullout study on different cement augmentation techniques

Pedicle screws with polymethyl methacrylate (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 remains unknown. This study aimed to determine the difference in pullout strength between different cement augmentation techniques. Uniform synthetic bones simulating severe osteoporosis were used to provide a platform for each augmentation technique. In all cases a polyaxial screw and acrylic cement (PMMA) at medium viscosity were used. Five groups were analyzed: I) only screw without PMMA (control group); II) retrograde cement pre-filling of the tapped area; III) cannulated and fenestrate screw with cement injection through perforation; IV) injection using a standard trocar of PMMA (vertebroplasty) and retrograde pre-filling of the tapped area; V) injection through a fenestrated trocar and retrograde pre-filling of the tapped area. Standard X-rays were taken in order to visualize cement distribution in each group. Pedicle screws at full insertion were then tested for axial pullout failure using a mechanical testing machine. A total of 30 screws were tested. The results of pullout analysis revealed better results of all groups with respect to the control group. In particular the statistical analysis showed a difference of Group V (p = 0.001) with respect to all other groups. These results confirm that the cement augmentation grants better results in pullout axial forces. Moreover they suggest better load resistance to axial forces when the distribution of the PMMA is along all the screw combining fenestration and pre-filling augmentation technique.

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

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

The effect of hubbing on the pull-out strength of lateral mass screws in the cervical spine: a biomechanical experiment.

This was a cadaveric biomechanical experiment. To compare the pull-out strength of polyaxial screws that are either hubbed or not hubbed when inserted into the lateral mass. It has been shown in a study on pedicle screws in the thoracic spine that "hubbing" the head of the screw against the dorsal laminar cortex results in significantly lower pull-out strength of the screws. Fifteen segments of the human cervical spine (from C3 to C7) were prepared. Polyaxial screws 3.5 mm in diameter were used. On one side screws 12 mm in length were inserted until the screw head touched the lateral mass; they were then turned 2.5 more times until they were fully hubbed (hubbed screws). On the other side screws 14 mm in length were inserted until the screw head just touched the lateral mass (nonhubbed screws). The 2 mm difference in length was to ensure that the screws were buried to the same length. All screws inserted into the lateral masses underwent tensile pull-out by applying a tensile force down the long axis of the screw. The difference in pull-out strength between the 2 groups was evaluated using a nonparametric paired test (the Wilcoxon signed rank test), which compared side to side on each vertebra. One specimen was excluded because of cement breakage during the biomechanical test. A total of 14 vertebrae were tested. Four vertebrae in the hubbed group showed small fractures or cracks around the screw hole after screw insertion. In a side to side comparison, the hubbed screws had significantly lower pull-out strengths as compared with the nonhubbed screws (P=0.033). Hubbing of lateral mass screws lowers the potential pull-out strength of the screws as compared with the pull-out strength of nonhubbed screws. Thus, hubbing of lateral mass screws, on the basis of the parameters applied in this study, is not recommended.

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).

Considerations for the Use of C7 Crossing Laminar Screws in Subaxial and Cervicothoracic Instrumentation

Radiographical and biomechanical analyses. To determine the applicability of C7 laminar screw fixation using radiographical and biomechanical analysis. The unique anatomy of C7 creates a challenge during instrumentation at the caudal aspect of the cervical spine and cervicothoracic junction. The C7 lateral mass is often smaller, resulting in increased difficulty for pedicle screw placement. The use of crossing laminar screw fixation is common in the upper cervical and thoracic spine; its use at the C7 level, however, has only recently appeared in the literature. Radiographical: Computed tomographic scans from 72 patients were used to measure laminar thickness, spinolaminar angle, and length (i.e., from the spinolaminar junction to the contralateral lamina-lateral mass junction) for each C7 vertebrae. Biomechanical: The C2 and C7 vertebrae from 13 cadaveric cervical spines were obtained, scanned using pQCT (Stratec Electronics, Pforzheim, Germany) for bone mineral density, and then instrumented in the following manner: (1) bilateral crossing intralaminar screws in C2, (2) bilateral crossing intralaminar screws in C7, and (3) bilateral pedicle screws in each C7 specimen after completion of laminar screw biomechanical testing. Each specimen was cyclically loaded for 5000 cycles after which axial screw pullout tests were performed. Radiographical: Mean laminar thickness and length were 5.67 ± 1.00 mm and 25.49 ± 2.73 mm, respectively. Biomechanical: The mean load to failure was 610.3 ± 251 N for C7 laminar screws, 666.33 ± 373N for C7 pedicle screws, and 355 ± 250 N for C2 laminar screws. A student t test indicated no statistical difference in pullout strength between C7 laminar and C7 pedicle screws (P = 0.6). The radiographical anatomy at C7 suggests that intralaminar screws can be placed in the majority of patients. The in vitro biomechanical analysis performed indicates that C7 laminar screws are as strong as C7 pedicle screws and significantly stronger than laminar screws at C2. N/A.

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.