Biomechanical comparison of a novel multilevel hex-head pedicle screw design with a conventional head design.

The objective of the study was to determine the biomechanical effect during insertion of multilevel hex-head design pedicle screws compared to a conventional screw-head design. Eighteen lumbar vertebrae and thoracic vertebrae from human cadavers were instrumented with a novel, multilevel hexagonal head pedicle screw on one side and a conventional head pedicle screw on the contralateral side. Screws were inserted at a constant rate and insertion and removal torques were recorded. A further 14 lumbar and thoracic vertebrae were used to test alterability of screw direction and operational effort required. Electromagnetic sensors recorded the change in angular direction for both screw and screwdriver. The force applied through the insertion screwdriver required to produce the directional change was also recorded. No significant differences were found between the two screw types for insertion or removal torque in either lumbar or thoracic vertebrae. Multilevel hex-head screws had significantly greater directional alterability than conventional head screws in both lumbar and thoracic specimens. Multilevel hex-head screws also required less force applied through the screwdriver than conventional screws to alter direction of screw insertion in both lumbar and thoracic specimens. The multilevel hex-head design did not affect the insertion or removal torque in comparison to a conventional head design.     

Slippage between screwdriver and bone screw

Cortical 3.5-mm stainless steel screws with hexagonal heads and corresponding screwdrivers from two manufacturers were investigated. Measurement of dimensions and torsional testing were done to study slippage between the screw and the driver bit. There were only small differences in dimensions between the manufacturers. Ultimate torque values obtained were at the level of 2.7 N-m where reaming of the screw socket took place. Additional rotation resulted in approximately (1/2) of the maximum torque. Subsequent torque testing in the opposite direction, corresponding to removal of the screw, revealed that the torque values were equally low in that direction. Additional insertion and removal of bone screws with hexagonal sockets are hampered after only one episode of slippage. It is justified to consider new shapes of drive bits and corresponding screw head sockets, such as a fluted multiedge configuration.     

Technical difficulties of removal of locking screw after locking compression plating

Introduction  As there are few reports on the difficulties of removing the locking compression plate (LCP), we prospectively investigated the incidence and difficulties in 58 patients in whom various types of LCPs were taken. Methods  From January 2004 to December 2007, we have removed 159 5.0-self tapping locking screws and 279 3.5- self tapping locking screws. All of the operations were performed by experienced trauma surgeons. All of the screws were inserted with the use of torque limiting attachment according to the manufacturer’s recommendation. During the same period of time we have removed 198 AO-3.5 cortical and 4.0 cancellous screws from various sites. Results  All of 159 5.0-self tapping locking screws were removed without difficulties. A total of 24 out of 279 3.5- self tapping locking screws were removed with many difficulties due to the stripping of the hexagonal recess. The use of conical extraction screw which was developed especially for the removal of stripped locking screws was successful in only six screws. We have removed plates by cutting the plate with metal cutting saw. We describe useful technical trick to remove the plate when there is only one screw left stripped. Compared to the locking screws, only one of 198 3.5-cortical screws was stripped. Conclusion  Care should be taken at the time of removal of the locking compression plate, especially for the 3.5-locking screws.     

Improving socket design to prevent difficult removal of locking screws

IntroductionReports of driver slippage leading to difficult locking screw removals have increased since the adoption of titanium for screw fabrication; the use of titanium is known to cause cross-threading and cold welding. Such problems occur most frequently in screws with hex sockets, and may cause serious surgical complications. This study aimed to improve screw socket design to prevent slippage and difficult screw removal.Materials and methodsThree types of small sockets (hex, Torx, and cruciate) and six types of large sockets (hex, Torx, Octatorx, Torx+ I, Torx+ II, and Torx+ III) with screw head diameters of 5.5 mm were manufactured from titanium, and corresponding screwdrivers were manufactured from stainless steel. The screw heads and drivers were mounted on a material testing machine, and torsional tests were conducted to simulate screw usage in clinical settings at two insertion depths: 1 and 2 mm. Ten specimens were tested from each design, and the maximum torque and failure patterns were recorded and compared.ResultsFor small sockets in 2 mm conditions, the hex with the largest driver core had the highest torque, followed by Torx and cruciate. In these tests, the drivers were twisted off in all specimens. However, under the 1 mm condition, the hex slipped and the torque decreased markedly. Overall, torque was higher for large sockets than for small sockets. The Octatorx, with a large core and simultaneous deformation of the driver and socket lobes, had the highest torque at almost twice that of the small hex. The hex had the lowest torque, a result of slippage in both the 1 and 2 mm conditions. Torx plus designs, with more designed degrees of freedom, were able to maintain a higher driving angle and larger core for higher torque.ConclusionsThe hex design showed slipping tendencies with a marked decrease in torque, especially under conditions with inadequate driver engagement. Large sockets allowed for substantial increases in torque. The Torx, Octatorx, and Torx plus designs displayed better performance than the hexes. Improvements to the socket design could effectively prevent slippage and solve difficult screw removal problems.     

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

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

Clinical accuracy of cervicothoracic pedicle screw placement: a comparison of the "open" lamino-foraminotomy and computer-assisted techniques

OBJECTIVE: Posterior transpedicular fixation at the cervicothoracic junction (CTJ) is increasing in popularity. However, the clinical accuracy of pedicle screw placement at the CTJ has not been specifically assessed. METHODS: Between January 2000 and July 2004, 60 consecutive patients underwent a variety of posterior spinal procedures necessitating pedicle screw placement at C7, T1, and T2. Thirty-two patients had cervicothoracic screws (3.5 to 4.5 mm) placed by an "open" technique (laminectomies or lamino-foraminotomies) and 28 patients with either a closed (before any decompression) 2-dimensional (n=19, fluoroscopy) or 3-dimensional (n=9, CT) computer-assisted technique. Screws were independently assessed for pedicle breach on postoperative CT and scored using a points-based classification system. RESULTS: The total number of screws placed was 86, 63 and 45 in the open, closed-2-dimensional and closed-3-dimensional groups, respectively. Overall, 61(70.9%), 51(81%), and 40(89%) screws were completely within the pedicle. In the open group, the majority of pedicle breaches were more than 2 mm [n=3 (<2 mm), n=20 (2-4 mm), n=2 (>4 mm)]. Screw violation occurred laterally 11/25(44%), medially 3/25(12%), inferiorly 7/25(28%), and superiorly 4/25(16%). In the closed technique, all breaches were lateral. Seventeen screws (n=11-2-dimensional, n=5-3-dimensional) breached the pedicle by a margin of less than 2 mm and 1 screw (2-dimensional) by 2 to 4 mm. Pedicle screw accuracy was significantly improved with computer-assisted techniques. However, there was no significant difference between the 2-dimensional and 3-dimensional techniques. For all patients, there were no clinically significant screw misplacements, nor any need for screw revision. CONCLUSIONS: Computer-assisted surgery allows for more accurate placement of pedicle screws at the CTJ. Although a higher proportion of major pedicular breaches occurred in the "open lamina/lamino-foraminotomy" group, no screws required revision in either group.     

Interference screw fixation using bioabsorbable screw as void filler

We evaluated interference screw fixation in a plug-tunnel construct using bioabsorbable screws as void fillers with different percentages of the screw removed. Nine-millimeter tunnels in a closed-cell foam block were filled with a 10-mm bioabsorbable screw, and 10-mm revision tunnels were placed in parallel with tunnel overlap resulting in removal of 10%, 25%, or 50% of the screw diameter. Synthetic bone plugs were fashioned to fit 10-mm tunnels. In all groups, the plugs were secured in standard interference fixation with a 9-mm metal screw between the void-filling bioabsorbable screw and plug. Failure loads for the control group (no revision tunnel) averaged 926 +/- 44 N, 10% (1024 +/- 129 N) and 25% (932 +/- 129 N) groups were not significantly different; failure load in the 50% diameter group (780 +/- 72 N) was significantly lower than all other groups (p < 0.001). Using a bioabsorbable screw as void filler provided mean load to failure not different from that of standard reconstruction when 10 and 25% of the diameter of the void-filling screw was removed. Load to failure was significantly lower when 50% of the void-filling screw diameter group was removed. This may be applicable in anterior cruciate ligament reconstruction where a previous tunnel void has to be addressed.     

自导向空心松质骨螺钉改锥的力学测试及初步临床应用

目的 自行设计并研制一种用于取出空心松质骨螺钉的改锥,通过力学测试及临床应用对其实用性进行探讨.方法 自行设计自导向改锥由导向部分、六角部分、杆部和手柄组成.力学测试试验分5组,A组:窄心螺钉内插入导针(进入50 mm)后套入空心改锥,但改锥六角部分与螺钉不接触;B组:在A组基础卜将空心改锥六角部分插入空心螺钉;C组:将自导向改锥导向部分插入空心钉,但改锥六角部分与螺钉小接触;D组:在C组基础上将改锥六角部分插入空心钉;E组:将普通实心改锥六角部分直接插入空心钉.测试空心改锥、自导向改锥及实心改锥的稳定性.2008年6月至2009年6月共收治26例股骨颈骨折空心钉内固定术后再次入院取钉的患者,随机分为自导向改锥组和普通实心改锥组.比较两组患者的取钉手术时间、出血量及切口长度.结果 排除相对移位的影响后,A、B、C、D、E组的侧方稳定性差异有统计学意义(F=94.639,P=0.000,A组与B组、C组与D组比较差异均有统计学意义(P<0.05),E组与B、D组比较差异均有统计学意义(P<0.05),而B组与D组比较差异尤统计学意义(P>0.05).自导向改锥组中39枚螺钉均顺利取出,手术时间和切口长度较普通实心改锥组少,差异有统计学意义(P<0.05);自导向改锥组平均失血量约5 mL,普通实心改锥组平均失血量约30 mL.结论 自导向空心改锥既有普通实心改锥的强度,又能引导和限制改锥方向,从而避免螺钉溢扣.特别适用于股骨颈骨折空心松质骨螺钉固定后的螺钉取出手术.     

Syndesmosis fixation: a comparison of three and four cortices of screw fixation without hardware removal

BACKGROUND: Great variability exists in methods of stabilization for syndesmotic disruptions of the ankle. We hypothesized that syndesmotic screw fixation with 3.5-mm fully threaded cortical screws through either three or four cortices would have similar strength and rate of mechanical failure and that retention of screws after fracture healing would not result in adverse clinical symptoms. METHODS: In a prospective, surgeon-randomized study at a Level-one trauma center, 127 patients with syndesmotic disruptions were treated surgically. Seven patients were lost to followup, leaving 120 for review. Syndesmotic disruptions were stabilized with 3.5-mm fully threaded cortical screws placed through three or four cortices. Screws were removed only if symptomatic. Outcome criteria were screw failure, loss of reduction, and need for hardware removal. RESULTS: Fifty-nine patients received fixation through three cortices and 61 patients received fixation through four cortices. Mean follow-up was 150 days. In the group with stabilization through three cortices, hardware failure occurred in five patients (8%) and three had a loss of reduction. In the group with stabilization through four cortices, hardware failure occurred in four patients (7%); all were asymptomatic and did not require screw removal. There was no loss of reduction in that group. Comparing the two groups using binary logistic analysis, there was no difference in loss of reduction (p = 0.871), screw breakage (p = 0.689), or need for hardware removal (p = 0.731). CONCLUSION: The data suggest that either three or four cortices of fixation can be used when stabilizing syndesmotic injuries of the ankle. There was a trend towards higher loss of reduction in the group with tricortical fixation when weightbearing restrictions were not followed. Retention of the syndesmotic screws, even with mechanical failure, does not pose a clinical problem. Weightbearing can be allowed at 6 to 10 weeks without routine removal of screws.     

Screw pull-out force is dependent on screw orientation in an anterior cervical plate construct

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

Effect of cutting flute design on cortical bone screw insertion torque and pullout strength

OBJECTIVE: To determine the effect of the number and length of cutting flutes on the insertion torque and pullout strength for self-tapping 4.5-millimeter cortical bone screws. DESIGN: Screws were self-tapped in the diaphysis of human cadaver femurs. Each of the six screw types studied had different designs with varying cutting flute lengths and numbers. Bone mineral density, insertion torque, and pullout strength were measured. SETTING: The study was conducted at an experimental biomechanics laboratory associated with a university medical center. OUTCOME MEASUREMENTS: Insertion torque and pullout strength were normalized by the local bone mineral density. RESULTS: The mean normalized insertion torque of the design with four full-length cutting flutes was less than the design with three full-length flutes and the two designs with one-third length flutes (p < 0.05). The mean normalized pullout strength of the screw with four full-length flutes was significantly greater than that of all screws with fewer than three flutes (p < 0.05). CONCLUSIONS: Priorities for a cutting flute design should ideally include ease of screw insertion, minimal soft tissue irritation, and maximal screw holding power. Screws with more than two flutes were easier to insert and did not cause cortical damage during insertion. The screw with four full-length flutes showed a trend toward being the easiest to insert and having the greatest holding strength.     

A comparison of three screw types for unicortical fixation in the lateral mass of the cervical spine.

STUDY DESIGN: In vitro comparison of three different screws for unicortical fixation in lateral masses of the cervical spine. OBJECTIVES: To compare the axial load-to-failure of cervical lateral mass screws and their revision screws in a cadaveric model. SUMMARY OF BACKGROUND DATA: Lateral mass screws are used for posterior fixation of the cervical spine. Risks to neurovascular structures have led many surgeons to advocate unicortical application of these screws, although fixation strength may vary with screw design. METHODS: Screws from three posterior cervical fixation systems were used: Axis, Starlock/Cervifix, and Summit. Tested were 3.5-mm cancellous screws, along with revision screws for each system. The C3-C6 vertebrae from three cadaveric specimens were fixed with screws inserted into the lateral masses at a depth of 10 mm with 30 degrees cephalad and 20 degrees lateral angulation. Coaxial pullout force was recorded for each primary and revision screw. RESULTS: Axial load-to-failure (mean +/- SD) of the screws was 459 +/- 60 N for Axis screws, 423 +/- 78 N for Starlock screws, and 319 +/- 97 N for Summit screws. The Axis and Starlock screws were significantly stronger than Summit screws (P = 0.017 and P = 0.067, respectively). The load-to-failure of revision screws was much lower than that of primary screws (Axis 54%, Starlock 56%, Summit 63% of the primary screw), without significant difference between screw types. CONCLUSIONS: The Axis and Starlock screws resisted significantly greater axial load-to-failure than did the Summit screws. For all three systems, the revision screws could not restore the load-to-failure of the primary screw in this model. The tested unicortical screws had a consistently higher load-to-failure than those previously tested under similar conditions, suggesting that currently available screws may be superior to those previously tested.     

Design and biomechanical study of a modified pedicle screw

Objective： In pedicle screw fixation, the heads of monoaxial screws need to be directed in the same straight line to accommodate the rod placement by backing out during operation, which decreases the insertional torque and internal fixation strength. While polyaxial screws facilitate the assembly of the connecting rod, but its ball-in-cup locking mechanism reduces the static compressive bending yield strength as compared with monoaxial screws. Our study aimed to assess the mechanical performance of a modified pedicle screw.
Methods： In this study, the tail of the screw body of the modified pedicle screw was designed to be a cylindershaped structure that well matched the inner wall of the screw head and the screw head only rotated around the cyclinder. Monoaxial screws, modified screws and polyaxial screws were respectively assembled into 3 groups ofvertebrectomy models simulated by ultra high molecular weight polyethylene （UHMWPE） blocks. This model was developed according to a standard for destructive mechanical testing published by the American Society for Testing Materials （ASTM F1717-04）. Each screw design had 6 subgroups, including 3 for static tension, load compression and torsion tests, and the rest for dynamic compression tests. In dynamic tests, the cyclic loads were 25%, 50%, and 75% of the compressive bending ultimate loads respectively. Yield load, yield ultimate load, yield stiffness, torsional stiffness, cycles to failure and modes of failure for the 3 types of screws were recorded. The results of modified screws were compared with those ofmonoaxial and polyaxial screws.
Results： In static tests, results of bending stiffness, yield load, yield torque and torsional stiffness indicated no significant differences between the modified and monoaxial screws （P〉0.05）, but both differed significantly from those ofpolyaxial screws （P〈0.05）. In dynamic compression tests, both modified and monoaxial screws showed failures that occurred at the insertion point of screw body into the UHMWPE block, while the polyaxial screw group showed screw body swung up and down the screw head because of loosening of the ball-in-cup mechanism.
Conclusions： The modified screw is well-designed and biomechanically improved. And it can provide sufficient stability for segment fixation as monoaxial screws.     

Triggered electromyographic threshold for accuracy of thoracic pedicle screw placement in a porcine model.

STUDY DESIGN: A porcine model of thoracic pedicle screw insertion was used to determine the effect of screw position on triggered electromyographic response. OBJECTIVE: To develop a model of intraoperative detection of misplaced thoracic pedicle screws. SUMMARY OF BACKGROUND DATA: Triggered electromyographic stimulation has been a valuable aid in determining appropriate placement of lumbar pedicle screws. The use of pedicle screws is increasing in the thoracic spine. Misplaced thoracic pedicle screws may have significant implications if the spinal cord is injured. This study was an attempt to determine whether the established lumbar model can be used for thoracic pedicle screws. METHODS: Five 120- to 150-lb domestic pigs had 85 pedicle screws placed bilaterally in the thoracic spine at each level from T6 to T15. Screws were inserted entirely in the pedicle (Group A). After removal of the medial pedicle wall, the screws were reinserted in the pedicle with no neural contact (Group B). The screws were then placed with purposeful contact with the neural elements (Group C). The screws were stimulated, eliciting an electromyographic response in the intercostal muscles for each instrumented level. The type of response noted was classified as either primary (response from appropriate nerve root), secondary (response at different root) or no response (response at different root, no response at appropriate root). RESULTS: Two hundred fifty responses were recorded. A primary response was noted in 72% of recordings. There was a relatively consistent decrease in the triggered electromyographic response from Group A (mean 4.15 +/- 1.80 mA) to Group C (mean 3.02 +/- 2.53 mA) screws (P = 0.0003). There was little difference in the response obtained from Group A to Group B (mean 4.37 +/- 2.48 mA) screws (P > 0.05). When a primary response was recorded, the mean threshold electromyographic response recorded was significantly lower than recordings with secondary and no response recordings (P < 0.05). CONCLUSION: Even though there was a consistent decrease between the A and C screws that was more definitively separated when a primary response was elicited, it was not possible to determine a cutoff trigger electromyographic level that would consistently differentiate intraosseous from epidural pedicle screw placement. Furthermore, this method could not differentiate screws clearly in the pedicle from screws with medial pedicle wall breakthrough. A more direct method of spinal cord monitoring must be established to provide the surgeon with early warning of the potential of neural injury in the placement of thoracic pedicle screws.     

Biomechanical properties of off-axis screw in Pauwels III femoral neck fracture fixation: Bicortical screw construct is superior to unicortical screw construct

ObjectivesThe purpose of this study is to determine the biomechanical properties of the bicortical off-axis screw fixation for stabilizing of Pauwels III femoral neck fractures compared with other fixation methods.MethodsEighteen synthetic femurs (Sawbones Pacific Research Laboratories, Vashon, WA) were divided into three groups. The osteotomy was made vertically to mimic the Pauwels type III femoral neck fracture. Group A (n = 6) was fixed with traditional inverted triangle cannulated screws. Group B (n = 6) was fixed with a unicortical off-axis screw and two parallel cannulated screws. Group C (n = 6) was fixed with a bicortical off-axis screw and two parallel cannulated screws. Each group was tested with a nondestructive axial compression test at a 7° of valgus followed with 1000 cycles of cyclic loading test from 100 N to 1000 N. Finally, a destructive axial compression test was applied until catastrophic failure.ResultsThe average axial stiffness from group A to group C was 856.5, 934, and 1340 N/mm, respectively. The average ultimate failure load from group A to group C was 2612.7, 2508.8, and 3706 N, respectively. Group C exhibited significantly greater axial stiffness and a higher ultimate failure load than the other two groups (P < 0.05). Regarding the interfragmental displacement, the values from group A to group C were 0.41, 0.83, 0.36, respectively, and group B exhibited significantly larger fracture gap formation after the cyclic loading test.ConclusionsThe results of this biomechanical study show statistically significant increases in axial stiffness and ultimate failure load for the off-axis screw placed in bicortical fashion. Once the off-axis screw was positioned unicortically, the largest fracture diastasis was observed as compared to the other two methods.     

Cortex of the pedicle of the vertebral arch. Part I: Deformation characteristics during screw insertion

OBJECT: Elastic deformation has been proposed as a mechanism by which vertebral pedicles can maintain pullout strength when conical screws are backed out from full insertion. The response to the insertion technique may influence both the extent of deformation and the risk of acute fracture during screw placement. The aim of this study was to determine the deformation characteristics of the lumbar pedicle cortex during screw placement. METHODS: Lumbar pedicles with linear strain gauges attached at the lateral and medial cortices were instrumented using 7.5-mm pedicle screws with or without preconditioning by insertion and removal of 6.5-mm screws. The strains and elastic recoveries of the medial and lateral cortices were determined. RESULTS: Mean medial wall strains tended to be lower than mean lateral wall strains when the 6.5-mm and 7.5-mm screw data were pooled (p = 0.07). After the screws had been removed, 71 to 79% of the deformation at the lateral cortex and 70 to 96% of the deformation at the medial cortex recovered. When inserted first, the 7.5-mm screw caused more plastic deformation at the cortex than it did when inserted after the 6.5-mm screw. Occasional idiosyncratic strain patterns were observed. No gross fracture was observed during screw placement. CONCLUSIONS: Screw insertion generated plastic deformation at the pedicle cortex even though the screw did not directly contact the cortex. The lateral and medial cortices responded differently to screw insertion. The technique of screw insertion affected the deformation behavior of the lumbar pedicles. With myriad options for screw selection and placement available, further study is needed before optimal placement parameters can be verified.     

Dual pitch screw design provides equivalent fixation to upsized screw diameter in revision pedicle screw instrumentation: a cadaveric biomechanical study

Background ContextThere are situations that require the replacement of pedicle screws. They are often exchanged when loose or broken or to accommodate a different sized rod or pedicle screw system. Traditionally, pedicle screws are replaced by up-sizing the core diameter until an interference fit is obtained. However, this method carries a risk of pedicle screw breach.PurposeTo determine if dual pitch screws, with cancellous pitch in the vertebral body and cortical pitch throughout the pedicle, allows for in-line screw revision without upsizing screw diameter.Study DesignCadaveric biomechanical StudyPatient SampleNot applicableOutcome MeasuresNot applicableMethodsPedicle screws were tested in the lumbar vertebrae from eleven cadavers. Standard pitch 5.5 mm screws were inserted and loaded using a "break-in" protocol. Screws were removed and replaced with one of four screw types: 5.5 mm Standard Pitch, 5.5 mm Dual Pitch, 6.0 mm Standard Pitch, or 6.0 mm Dual Pitch. Failure testing was done using a stepwise increasing cyclic loading protocol for 100 cycles at each increasing load level. The loading consisted of a combined axial and bending load simulating the load seen by the most inferior screw.ResultsFailure was consistent, with the tip of the screw displacing inferiorly into the vertebral body while simultaneously pulling out. Failure strength was lowest in the 5.5mm Standard (135.8±29.4N) followed by 6.0mm Standard (141.8±38.6N), 5.5mm Dual (158.1±53.8N), and 6.0mm Dual (173.6±52.1N, p=.023). There was no difference in the failure strength between the 5.5mm Dual and 6.0mm Standard. Lumbar level (p=.701) and donor spine (p=.062) were not associated with failure strength.ConclusionsAfter pedicle screw removal, screws with a larger core diameter or with a dual pitch have similar failure strengths. Dual pitch screws may allow for in-line revision of screws without upsizing screw diameter, minimizing the risk of pedicle breach or fracture.Clinical SignificanceDual pitch screws, with cancellous pitch in the vertebral body and cortical pitch through the pedicle, allows for in-line revision of pedicle screws without upsizing screw diameter; reducing the risk of pedicle breach or fracture when exchanging screws.     

三维平板透视在脊柱椎弓根螺钉置入中的应用

目的比较C臂机透视与三维平板透视辅助椎弓根螺钉置钉的准确性。方法将206例接受脊柱椎弓根螺钉内固定术治疗的患者根据术中透视方法不同分为对照组(C臂机透视,95例)和观察组(三维平板透视,111例)。术后行X线和CT复查,并按Richter标准比较两组椎弓根螺钉在位情况。结果共置入椎弓根螺钉916枚,观察组492枚,对照组424枚,两组比较差异无统计学意义(P>0.05)。螺钉位置优良率观察组高于对照组(P<0.05)。术后13例失访,193例患者获得随访,时间6~24个月。末次随访,104例患者取出内固定物,其余89例患者尚未取出内固定物。对照组有2例螺钉位置差的患者出现典型神经根刺激症状(下肢放射痛、麻木、无力等),下肢运动功能恢复不佳。两组均无钉、棒断裂及脱落现象。结论三维平板透视在脊柱椎弓根螺钉内固定术中的置钉准确率高于C臂机透视。     

A biomechanical evaluation of a cannulated compressive screw for use in fractures of the scaphoid.

The compressive force generated by a 3.5 mm ASIF cannulated cancellous screw with a 5 mm head was compared with that generated by a standard 3.5 mm ASIF screw (6 mm head), a 2.7 mm ASIF screw (5 mm head), and a Herbert screw. The screws were evaluated in the laboratory with the use of a custom-designed load washer (transducer) to the maximum compressive force generated by each screw until failure, either by thread stripping or by head migration into the specimen. Testing was done on paired cadaver scaphoids. To minimize the variability that occurs with human bone, and because of the cost and difficulty of obtaining human tissue specimens, a study was also done on polyurethane foam simulated bones. The 3.5 cannulated screw generated greater compressive forces than the Herbert screw but less compression than the 2.7 mm and 3.5 mm ASIF cortical screws. The 3.5 mm cannulated screw offers more rigid internal fixation for scaphoid fractures than the Herbert screw and gives the added advantage of placement over a guide wire.     

The best location for proximal locking screw for femur interlocking nailing: A biomechanical study

Background:

Proximal locking screw deformation and screw fracture is a frequently seen problem for femur interlocking nailing that affects fracture healing. We realized that there is lack of literature for the right level for the proximal locking screw. We investigated the difference of locking screw bending resistance between the application of screws on different proximal femoral levels.

Materials and Methods:

We used a total of 80 proximal locking screws for eight groups, 10 screws for each group. Three-point bending tests were performed on four types of screws in two different trochanteric levels (the lesser trochanter and 20 mm proximal). We determined the yield points at three-point bending tests that a permanent deformation started in the locking screws using an axial compression testing machine.

Results:

The mean yield point value of 5 mm threaded locking screws applied 20 mm proximal of lesser trochanter was 1022 ± 49 (range 986–1057) (mean ± standard deviation, 95% confidence interval). On the other hand, the mean yield point value of the same type of locking screws applied on the lesser trochanteric level was 2089 ± 249 (range 1911–2268). Which means 103% increase of screw resistance between two levels (P = 0.000). In all screw groups, on the lesser trochanter line we determined 98–174% higher than the yield point values of the same type of locking screws in comparison with 20 mm proximal to the lesser trochanter (P = 0.000).

Conclusion:

According to our findings, there is twice as much difference in locking screw bending resistance between these two application levels. To avoid proximal locking screw deformation, locking screws should be placed in the level of the lesser trochanter in nailing of 1/3 middle and distal femur fractures.