Characteristics of immediate and fatigue strength of a dual-threaded pedicle screw in cadaveric spines

Background contextNovel dual-threaded screws are configured with overlapping (doubled) threads only in the proximal shaft to improve proximal cortical fixation.PurposeTests were run to determine whether dual-threaded pedicle screws improve pullout resistance and increase fatigue endurance compared with standard pedicle screws.Study design/settingIn vitro strength and fatigue tests were performed in human cadaveric vertebrae and in polyurethane foam test blocks.Patient sampleSeventeen cadaveric lumbar vertebrae (14 pedicles) and 40 test sites in foam blocks were tested.Outcome measuresMeasures for comparison between standard and dual-threaded screws were bone mineral density (BMD), screw insertion torque, ultimate pullout force, peak load at cyclic failure, and pedicular side of first cyclic failure.MethodsFor each vertebral sample, dual-threaded screws were inserted in one pedicle and single-threaded screws were inserted in the opposite pedicle while recording insertion torque. In seven vertebrae, axial pullout tests were performed. In 10 vertebrae, orthogonal loads were cycled at increasing peak values until toggle exceeded threshold for failure. Insertion torque and pullout force were also recorded for screws placed in foam blocks representing healthy or osteoporotic bone porosity.ResultsIn bone, screw insertion torque was 183% greater with dual-threaded than with standard screws (p<.001). Standard screws pulled out at 93% of the force required to pull out dual-threaded screws (p=.42). Of 10 screws, five reached toggle failure first on the standard screw side, two screws failed first on the dual-threaded side, and three screws failed on both sides during the same round of cycling. In the high-porosity foam, screw insertion torque was 60% greater with the dual-threaded screw than with the standard screw (p=.005), but 14% less with the low-porosity foam (p=.07). Pullout force was 19% less with the dual-threaded screw than with the standard screw in the high-porosity foam (p=.115), but 6% greater with the dual-threaded screw in the low-porosity foam (p=.156).ConclusionsAlthough dual-threaded screws required higher insertion torque than standard screws in bone and low density foam, dual-threaded and standard pedicle screws exhibited equivalent axial pullout and cyclic fatigue endurance. Unlike single-threaded screws, the mechanical performance of dual-threaded screws in bone was relatively independent of BMD. In foam, the mechanical performance of both types of screws was highly dependent on porosity.     

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.     

The biomechanical effect of pedicle screw hubbing on pullout resistance in the thoracic spine

Background contextThe biomechanical fixation strength afforded by pedicle screws has been strongly correlated with bone mineral density. It has been postulated that “hubbing” the head of the pedicle screw against the dorsal laminar cortex provides a load-sharing effect, thereby limiting cephalocaudad toggling and improving the pullout resistance of the pedicle screw.PurposeTo evaluate the pullout strength (POS) of monoaxial hubbed pedicle screws versus standard fixation in the thoracic spine.Study designBiomechanical investigation.MethodsTwenty-two human cadaveric thoracic vertebrae were acquired and dual-energy X-ray absorptiometry scanned. Osteoporotic (n=16) and normal (n=6) specimens were instrumented with a 5.0×35-mm pedicle screw on one side in a standard fashion. In the contralateral pedicle, 5.0×30-mm screw was inserted with hubbing of the screw into the dorsal lamina. A difference in screw length was used to achieve equivalent depth of insertion. After 2,000 cycles of cephalocaudad toggling, screws were pulled out with the tensile force oriented to the midline of the spine and peak POS measured in newtons (N). Four additional specimens were subjected to microcomputed tomography (micro-CT) analysis to evaluate internal pedicle architecture after screw insertion.ResultsHubbed screws resulted in significantly lower POS (290.5±142.4 N) compared with standard pedicle screws (511.5±242.8 N; p=.00). This finding was evident in both normal and osteoporotic vertebrae based on independent subgroup post hoc analyses (p<.05). As a result of hubbing, half of the specimens fractured through the lamina or superior articular facet (SAF). No fractures occurred on the control side. There was no difference in mean POS for hubbed screws with and without fracture; however, further micro-CT analysis revealed the presence of internal fracture propagation for those specimens that did not have any external signs of failure.ConclusionsHubbing pedicle screws results in significantly decreased POS compared with conventional pedicle screws. Hubbing predisposes toward iatrogenic fracture of the dorsal lamina, transverse process, or SAF during insertion.     

Cervical spine pedicle screws: a biomechanical comparison of two insertion techniques

STUDY DESIGN: Biomechanical testing of the pullout strengths of pedicle screws placed by two different techniques in adult human cadaveric cervical spines. OBJECTIVES: To determine whether there is a significant difference in screw purchase of two commonly proposed methods of cervical pedicle screw insertion. SUMMARY OF BACKGROUND DATA: Wiring techniques remain the gold standard for posterior cervical fixation. However, absent or deficient posterior elements may dictate the use of alternative fixation techniques. Cervical pedicle screws have been shown to have significantly higher pullout strength than lateral mass screws. METHODS: Fifty fresh disarticulated human vertebrae (C3-C7) were evaluated with computed tomography for anatomic disease and pedicle morphometry. The right and left pedicles were randomly assigned to either a standard method or the Abumi insertion method. In the latter technique the cortex and cancellous bone of lateral mass are removed with a high-speed burr, which provides a direct view of the pedicle introitus. The pedicle is then probed and tapped and a 3.5-mm cortical screw inserted. Each screw was subjected to a uniaxial load to failure. RESULTS: There was no significant difference in the mean pullout resistance between the Abumi (696 N) and standard (636.5 N) insertion techniques (P = 0.41). There was no difference in pullout resistance between vertebral levels or within vertebral levels. Two (4%) minor pedicle wall violations were observed. CONCLUSION: In selected circumstances pedicle screw instrumentation of the cervical spine may be used to manage complex deformities and patterns of instability. Surgeons need not be concerned about reduced screw purchase when deciding between the Abumi method and its alternatives.     

Pullout resistance of thoracic extrapedicular screws used as a salvage procedure.

BACKGROUND CONTEXT: Extrapedicular screws are placed more laterally than intrapedicular screws and pass through the transverse process or rib head before entering the vertebral body. These screws are sometimes placed to salvage failed pedicle screws, but the change in pullout resistance of extrapedicular screws after salvage has not been quantified. PURPOSE: To quantify the pullout resistance of thoracic extrapedicular screws compared with intrapedicular screws and the pullout resistance of newly inserted screws compared with extrapedicular screws used as salvage for failed intrapedicular screws. STUDY DESIGN: In vitro paired comparison of screw pullout resistance in isolated thoracic vertebrae. METHODS: Tapered monoaxial pedicle screws were inserted in the left or right pedicle of 11 human cadaveric thoracic vertebrae. An extrapedicular screw was inserted on the contralateral side. Both screws were pulled out axially at 0.5 mm/s using a servohydraulic test frame while applied load was recorded. Then a fresh extrapedicular screw was inserted as a salvage screw on the intrapedicular screw side and pulled out. RESULTS: In uncompromised vertebrae, the pullout strength of extrapedicular screws was 80+/-32% of that of intrapedicular screws (p=.073, repeated-measures one-way analysis of variance/Tukey). Salvage screws restored pullout strength to 65+/-30% of that of intrapedicular screws (p=.003). CONCLUSIONS: Extrapedicular screws provided comparable but slightly lower pullout resistance to intrapedicular screws in uncompromised vertebrae. They are therefore a feasible salvage technique when a compromised pedicle precludes reinsertion of an intrapedicular screw, but the salvage screw is significantly weaker than the original screw.     

Pedicle screw augmentation with bone cement enforced Vicryl mesh

Achieving sufficient mechanical purchase of pedicle screws in osteoporotic or previously instrumented bone is technically and biologically challenging. Techniques using different kinds of pedicle screws or methods of cement augmentation have been used to address this challenge, but are associated with difficult revisions and complications. The purpose of this biomechanical trial was to investigate the use of biocompatible textile materials in combination with bone cement to augment pullout strength of pedicle screws while reducing the risk of cement extrusion. Pedicle screws (6/40 mm) were either augmented with standard bone‐cement (Palacos LV + G) in one group (BC, n = 13) or with bone‐cement enforced by Vicryl mesh in another group (BCVM, n = 13) in osteoporosis‐like saw bone blocks. Pullout testing was subsequently performed. In a second experimental phase, similar experiments were performed using human cadaveric lumbar vertebrae (n = 10). In osteoporosis‐like saw bone blocks, a mean screw pullout force of 350 N (±125) was significantly higher with the Bone cement (BC) compared to bone‐cement enforced by Vicryl mesh (BCVM) technique with 240 N (±64) (p = 0.030). In human cadaveric lumbar vertebrae the mean screw pullout force was 784 ± 366 N with BC and not statistically different to BCVM with 757 ± 303 N (p = 0.836). Importantly, cement extrusion was only observed in the BC group (40%) and never with the BCVM technique. In vitro textile reinforcement of bone cement for pedicle screw augmentation successfully reduced cement extrusion compared to conventionally delivered bone cement. The mechanical strength of textile delivered cement constructs was more reproducible than standard cementing. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:212–216, 2018.     

Cortical bone trajectory for lumbar pedicle screws

Background contextAchieving solid implant fixation to osteoporotic bone presents a clinical challenge. New techniques and devices are being designed to increase screw–bone purchase of pedicle screws in the lumbar spine via a novel cortical bone trajectory that may improve holding screw strength and minimize loosening. Preliminary clinical evidence suggests that this new trajectory provides screw interference that is equivalent to the more traditionally directed trajectory for lumbar pedicle screws. However, a biomechanical study has not been performed to substantiate the early clinical results.PurposeEvaluate the mechanical competence of lumbar pedicle screws using a more medial-to-lateral path (ie, “cortical bone trajectory”) than the traditionally used path.Study designHuman cadaveric biomechanical study.MethodsEach vertebral level (L1–L5) was dual-energy X-ray absorptiometry (DXA) scanned and had two pedicle screws inserted. On one side, the traditional medially directed trajectory was drilled and tapped. On the contralateral side, the newly proposed cortical bone trajectory was drilled and tapped. After qCT scanning, screws were inserted into their respective trajectories and pullout and toggle testing ensued. In uniaxial pullout, the pedicle screw was withdrawn vertically from the constrained bone until failure occurred. The contralateral side was tested in the same manner. In screw toggle testing, the vertebral body was rigidly constrained and a longitudinal rod was attached to each screw head. The rod was grasped using a hydraulic grip and a quasi-static, upward displacement was implemented until construct failure. The contralateral pedicle screw was tested in the same manner. Yield pullout (N) and stiffness (N/mm) as well as failure moment (N-m) were compared and bone mineral content and bone density data were correlated with the yield pullout force.ResultsNew cortical trajectory screws demonstrated a 30% increase in uniaxial yield pullout load relative to the traditional pedicle screws (p=0.080), although mixed loading demonstrated equivalency between the two trajectories. No significant difference in construct stiffness was noted between the two screw trajectories in either biomechanical test or were differences in failure moments (p=0.354). Pedicle screw fixation did not appear to depend on bone quality (DXA) yet positive correlations were demonstrated between trajectory and bone density scans (qCT) and pullout force for both pedicle screws.ConclusionsThe current study demonstrated that the new cortical trajectory and screw design have equivalent pullout and toggle characteristics compared with the traditional trajectory pedicle screw, thus confirming preliminary clinical evidence. The 30% increase in failure load of the cortical trajectory screw in uniaxial pullout and its juxtaposition to higher quality bone justify its use in patients with poor trabecular bone quality.     

Pullout strength of misplaced pedicle screws in the thoracic and lumbar vertebrae - A cadaveric study

Background:

The objective of this cadaveric study was to analyze the effects of iatrogenic pedicle perforations from screw misplacement on the mean pullout strength of lower thoracic and lumbar pedicle screws. We also investigated the effect of bone mineral density (BMD), diameter of pedicle screws, and the region of spine on the pullout strength of pedicle screws.

Materials and Methods:

Sixty fresh human cadaveric vertebrae (D10–L2) were harvested. Dual-energy X-ray absorptiometry (DEXA) scan of vertebrae was done for BMD. Titanium pedicle screws of different diameters (5.2 and 6.2 mm) were inserted in the thoracic and lumbar segments after dividing the specimens into three groups: a) standard pedicle screw (no cortical perforation); b) screw with medial cortical perforation; and c) screw with lateral cortical perforation. Finally, pullout load of pedicle screws was recorded using INSTRON Universal Testing Machine.

Results:

Compared with standard placement, medially misplaced screws had 9.4% greater mean pullout strength and laterally misplaced screws had 47.3% lesser mean pullout strength. The pullout strength of the 6.2 mm pedicle screws was 33% greater than that of the 5.2 mm pedicle screws. The pullout load of pedicle screws in lumbar vertebra was 13.9% greater than that in the thoracic vertebra (P = 0.105), but it was not statistically significant. There was no significant difference between pullout loads of vertebra with different BMD (P = 0.901).

Conclusion:

The mean pullout strength was less with lateral misplaced pedicle screws while medial misplaced pedicle screw had more pullout strength. The pullout load of 6.2 mm screws was greater than that of 5.2 mm pedicle screws. No significant correlation was found between bone mineral densities and the pullout strength of vertebra. Similarly, the pullout load of screw placed in thoracic and lumbar vertebrae was not significantly different.     

Biomechanical study of pedicle screw fixation in severely osteoporotic bone.

BACKGROUND CONTEXT: Obtaining adequate purchase with standard pedicle screw techniques remains a challenge in poor quality bone. The development of alternate insertion techniques and screw designs was prompted by recognition of potential fixation complications. An expandable pedicle screw design has been shown to significantly improve fixation compared to a conventional screw in poor quality bone. PURPOSE: The purpose of this study was to determine if polymethylmethacrylate (PMMA) bone cement augmentation of an expandable pedicle screw can further improve fixation strength compared to the expandable screw alone in severely osteoporotic bone. A technique for cement insertion into the pedicle by means of the cannulated central portion of the expandable screw is also described. STUDY DESIGN: The axial pullout strength, stiffness and energy absorbed of cemented and noncemented expandable pedicle screws was determined in cadaveric vertebrae. METHODS: Twenty-one fresh unembalmed vertebrae from the thoracolumbar spine were used. Radiographs and bone mineral density measurements (BMD) were used to characterize bone quality. Paired cemented and noncemented pedicle screw axial pullout strength was determined through mechanical testing. Mechanical pullout strength, stiffness and energy to failure was correlated with BMD. RESULTS: Overall, there was a 250% increase in mean pullout strength with the cemented expandable screw compared with a noncemented expandable screw including a greater than twofold increase in pullout strength in the most severely osteoporotic bone. The mean stiffness and energy absorbed to failure was also significantly increased. A cemented conventional screw achieved a pullout strength similar to the noncemented expandable screw. CONCLUSIONS: PMMA cement augmentation of the expandable pedicle screw may be a viable clinical option for achieving fixation in severely osteoporotic bone.     

The biomechanical consequences of rod reduction on pedicle screws: should it be avoided?

Background contextRod contouring is frequently required to allow for appropriate alignment of pedicle screw-rod constructs. When residual mismatch is still present, a rod persuasion device is often used to achieve further rod reduction. Despite its popularity and widespread use, the biomechanical consequences of this technique have not been evaluated.PurposeTo evaluate the biomechanical fixation strength of pedicle screws after attempted reduction of a rod-pedicle screw mismatch using a rod persuasion device.MethodsFifteen 3-level, human cadaveric thoracic specimens were prepared and scanned for bone mineral density. Osteoporotic (n=6) and normal (n=9) specimens were instrumented with 5.0-mm–diameter pedicle screws; for each pair of comparison level tested, the bilateral screws were equal in length, and the screw length was determined by the thoracic level and size of the vertebra (35 to 45 mm). Titanium 5.5-mm rods were contoured and secured to the pedicle screws at the proximal and distal levels. For the middle segment, the rod on the right side was intentionally contoured to create a 5-mm residual gap between the inner bushing of the pedicle screw and the rod. A rod persuasion device was then used to engage the setscrew. The left side served as a control with perfect screw/rod alignment. After 30 minutes, constructs were disassembled and vertebrae individually potted. The implants were pulled in-line with the screw axis with peak pullout strength (POS) measured in Newton (N). For the proximal and distal segments, pedicle screws on the right side were taken out and reinserted through the same trajectory to simulate screw depth adjustment as an alternative to rod reduction.ResultsPedicle screws reduced to the rod generated a 48% lower mean POS (495±379 N) relative to the controls (954±237 N) (p<.05) and significantly decreased work energy to failure (p<.05). Nearly half (n=7) of the pedicle screws had failed during the reduction attempt with visible pullout of the screw. After reduction, decreased POS was observed in both normal (p<.05) and osteoporotic (p<.05) bone. Back out and reinsertion of the screw resulted in no significant difference in mean POS, stiffness, and work energy to failure (p>.05).ConclusionsIn circumstances where a rod is not fully seated within the pedicle screw, the use of a rod persuasion device decreases the overall POS and work energy to failure of the screw or results in outright failure. Further rod contouring or correction of pedicle screw depth of insertion may be warranted to allow for appropriate alignment of the longitudinal rods.     

Pedicle and transverse process screws of the upper thoracic spine. Biomechanical comparison of loads to failure

STUDY DESIGN: An In vitro biomechanical load-to-failure test. OBJECTIVES: To determine the comparative axial pullout strengths of pedicle screw versus transverse process screws in the upper thoracic spine (T1-T4), and to compare their failure loads with bone density as seen on computed tomography. SUMMARY OF THE BACKGROUND DATA: The morphology of the upper thoracic spine presents technical challenges for rigid segmental fixation. Though data are available for failure characteristics of cervical-lateral mass screws, analogous data are wanting in regard to screw fixation of the upper thoracic spine. METHODS: Ten fresh-frozen human spines (T1-T4) were quantitatively scanned using computed tomography to determine trabecular bone density at each level. The vertebrae were drilled and tapped for the insertion of a 3.5-mill meter-diameter cortical bone screw in either the pedicle or the transverse process position. A uniaxial load to failure was applied. RESULTS: The mean ultimate load to failure for the pedicle screws (658 N) was statistically greater than that of the transverse process screws (361 N; P < 0.001). The T1 pedicle screw sustained the highest load to failure (775 N). No significant difference was found between load to failure for the pedicle and transverse process screws at T1. A trend toward decreasing load to failure was seen for both screw positions with descending thoracic level. Neither pedicle dimensions nor screw working length correlated with load to failure. CONCLUSIONS: Upper thoracic pedicle screws have superior axial loading characteristics compared with bicortical transverse process screws, except at T1. Load behavior of either of these screws was not predictable based on anatomic parameters.     

Evaluation of calcium sulfate paste for augmentation of lumbar pedicle screw pullout strength.

BACKGROUND CONTEXT: Many authors have evaluated the components responsible for ultimate pullout strength of pedicle screws. In these studies, one important variable has been the screw fixation. Because pedicle screw fixation has increased in popularity over recent years, so has the need for augmentation in difficult situations. Polymethylmethacrylate (PMMA) has been established as the gold standard in terms of strength of fixation but has the potential for severe complications when applied in spine surgery. Calcium sulfate is an alternative to PMMA, because it lacks the exothermic reaction, is potentially osteoconductive and is resorbed by the body in 30 to 60 days. PURPOSE: To determine the strength of a new calcium sulfate cement in terms of pedicle screw augmentation. The purpose was to evaluate calcium sulfate versus PMMA in terms of pullout strength. PMMA was considered the gold standard in terms of strength for this experiment. STUDY DESIGN: Lumbar vertebrae implanted with pedicle screws were subjected to axial pullout tests. The force required to cause implant failure was measured and compared for three methods of fixation. OUTCOME MEASURES: Force to failure (Newtons) for each pedicle test was recorded and analyzed with results from similarly augmented pedicles. METHODS: Lumbar vertebrae were harvested from four cadavers and implanted with pedicle screws. These screws were either placed in native bone or augmented with either calcium sulfate paste or PMMA. In those pedicles that had augmentation, the material was permitted to set for a minimum of 24 hours. Axial pullout tests were then performed using an MTS (Materials Testing System Corporation, Minneapolis, MN) testing machine. The screws were pulled out over a distance of up to 6 mm at 0.25 mm/sec. This rate and distance ensured failure in each case. The load to failure was recorded for each pedicle. RESULTS: Calcium sulfate augmentation improved pedicle screw pullout strength significantly when compared with native bone (p=.0003). This represented an average increase of 167% over the native bone. Likewise, PMMA improved the pullout strength significantly over native bone (p<.0001) for an average increase of 199%. There were no significant differences between the calcium sulfate and PMMA augmentation (p=.0791), although the PMMA averaged 119% of the strength of calcium sulfate. CONCLUSIONS: Although PMMA is considered the gold standard in terms of fixation strength, its use around the spine has been limited because of concern over complications. The calcium sulfate bone paste tested in this study demonstrated strength similar to PMMA without the major risks to the spinal cord. In addition, the calcium sulfate paste allows for possible osteoconduction to aid in spinal fusion. Further study is needed to determine the applicability of this calcium sulfate paste in the clinical setting of spine surgery.     

Characteristics of pullout failure in conical and cylindrical pedicle screws after full insertion and back-out

BACKGROUND CONTEXT: Biomechanical studies show that bone-mineral density, pedicle morphology, and screw thread area affect pedicle screw pullout failure. The current literature is based on studies of cylindrical pedicle screw designs. Conical screws have been introduced that may provide better "fit and fill" of the dorsal pedicle as well as improved resistance to screw bending failure. However, there is concern about loss of fixation if conical screws must be backed out after insertion. PURPOSE: To determine that conical screws have comparable initial stiffness and fixation strength compared with standard, cylindrical screws, and to assess whether conical screw fixation deteriorates when screws are backed out from full insertion. STUDY DESIGN/SETTING: This biomechanical analysis compared pullout strength of cylindrical and conical pedicle screw designs, using porcine lumbar vertebrae in a paired testing format. METHODS: Porcine lumbar vertebrae were instrumented with conical and cylindrical pedicle screws with the same thread pitch, area and contour, and an equivalent diameter at the pedicle isthmus, 1.2 cm distal to the hub. Axial pullout was performed at 1.0 mm/minute displacement. Pullout loads, work and stiffness were recorded at 0.02-second intervals. Conical versus cylindrical screws were tested using three paired control configurations: fully inserted, backed out 180 degrees and backed out 360 degrees. Fully inserted values were compared with each set of back-out values to determine relative loss of fixation strength. Screw pullout data were analyzed using a Student's t test. RESULTS: Pullout loads in these porcine specimens were comparable to data from healthy human vertebrae. Conical screws provided a 17% increase in the pullout strength compared with cylindrical screws (P<.10) and a 50% increase in initial stiffness (P<.05) at full insertion. There was no loss in pullout strength, stiffness or work to failure when conical or cylindrical screws were backed out 180 or 360 degrees from full insertion. CONCLUSIONS: Conical screws offer improved initial fixation strength compared with cylindrical screws of the same size and thread design. Our results suggest that appropriately designed conical screws can be backed out 180 to 360 degrees for intraoperative adjustment without loss of pullout strength, stiffness or work to failure. Intraoperative adjustments of these specific conical screws less than 360 degrees should not affect initial fixation strength. These results may not hold true for screws with a smaller thread area or larger minor diameter.     

Pullout strength of pedicle screws using cadaveric vertebrae with or without artificial demineralization

OBJECTIVESTo evaluate the differences in the pullout strength and displacement of pedicle screws in cadaveric thoracolumbar vertebrae with or without artificial demineralization.METHODSFive human lumbar and five thoracic vertebrae from one cadaver were divided into two hemivertebrae. The left-side specimens were included in the simulated osteopenic model group and the right-side bones in a control group. In the model group, we immersed each specimen in HCl (1 N) solution for 40 minutes. We measured bone mineral density (BMD) using dual-energy X-ray absorptiometry and quantitative computerized tomography. We inserted polyaxial pedicle screws into the 20 pedicles of the cadaveric lumbar and thoracic spine after measuring the BMD of the 2 hemivertebrae of each specimen. We measured the pullout strength and displacement of the screws before failure in each specimen using an Instron system.RESULTSThe average pullout strength of the simulated osteopenic model group was 76% that of the control group. In the control and model groups, the pullout strength was 1678.87±358.96 N and 1283.83±341.97 N, respectively, and the displacement was 2.07±0.34 mm and 2.65±0.50 mm, respectively (p<.05). We detected positive correlations between pullout strength and BMD in the control group and observed a negative correlation between displacement and BMD in the model group.CONCLUSIONSBy providing an anatomically symmetric counterpart, the human cadaveric model with or without demineralization can be used as a test bed for pullout tests of the spine. In the simulated osteopenic model group, pullout strength was significantly decreased compared with the untreated control group.CLINICAL SIGNIFICANCEDecreased bone mineral density may significantly reduce the pullout strength of a pedicle screw, even though the range is osteopenic rather than osoteoporotic.     

下颈椎椎弓根螺钉内固定技术在临床中的应用

目的 :探讨下颈椎椎弓根螺钉内固定技术的临床应用。方法 :对2011年9月至2013年7月行下颈椎椎弓根螺钉内固定的32例患者进行回顾性分析,男20例,女12例;年龄21~78岁,平均56.4岁。其中10例为创伤性颈髓损伤,9例为颈椎管内肿瘤,7例颈椎后纵韧带骨化症,6例多节段颈椎病。所有患者术前行X线、CT、MRI及椎动脉MRA等影像学检查,术后及随访时行X线片及CT平扫明确螺钉的位置情况。根据Lee等4级分类法评价置钉的准确性,创伤性患者行ASIA分级评价脊髓功能变化,非创伤性患者采用JOA评分评价神经功能改善情况。结果:32例患者成功置入144枚下颈椎椎弓根螺钉,术后CT显示,0级132枚,1级5枚,2级5枚,3级2枚。有12枚螺钉穿破椎弓根,其中8枚螺钉穿破椎弓根外侧皮质,2枚螺钉穿破椎弓根下侧皮质,穿破椎弓根内侧、上侧皮质螺钉各1枚。术后随访12~33个月,平均(21.0±1.5)个月,6例完全性颈髓损伤患者术后神经功能虽无恢复,但截瘫平面下降1~3个脊髓节段。4例不完全性颈髓损伤患者术后按ASIA损伤分级提高1~2级。22例非创伤性患者术后6个月JOA评分平均(15.9±0.6)分,较术前(11.5±0.8)分明显提高(P<0.01)。所有患者未发现钉棒系统松动、断裂情况。结论:下颈椎椎弓根螺钉固定能提供优秀的三维稳定性。合理选择适应证,术前充分准备以及根据椎弓根形态个体化置钉可以最大限度的降低手术风险及手术并发症,值得临床应用推广。     

可灌注骨水泥椎弓根螺钉的生物力学研究

目的 探讨可灌注骨水泥椎弓根螺钉(novel perfusional pedicle screw,NPPS)在骨质疏松椎体内的生物力学稳定性.方法 选取平均年龄73岁的完整湿润脊柱标本(T11～L5)6具,共42个椎体,平均骨密度为(0.696±0.14)g/cm2.所有椎体均任取一侧椎弓根置入可灌注螺钉后,使用配套的骨水泥推杆和灌注筒向椎体内灌注骨水泥2ml,对侧椎弓根置入常规螺钉,作为对照组.随机选取3个椎体,剖开椎体观察骨水泥分布情况.余39个椎体随机分成3组,每组13个,分别行最大轴向拔出力、最大旋出力矩、周期抗屈试验.另取10个可灌注螺钉和10个对照组螺钉行三点弯曲实验.结果 所有椎体均没有观察到骨水泥渗漏.可灌注螺钉的最大轴向拔出力为(760±178)N,对照组为(355±87)N;可灌注螺钉的最大旋出力矩为(1.347±0.377)N·m,对照组为(0.488±0.205)N·m.4枚(4/13,占30.8%)可灌注螺钉发生松动,其平均载荷为(150±46)N;未松动的螺钉中,松动位移半均(0.661±0.289)mm,对照组中所有的螺钉在最大负荷介于50～200 N时开始松动(位移>2.000 mm),平均载荷(104±35)N.可灌注螺钉的平均极限弯曲载荷为(3082±144)N,对照组螺钉为(3357±263)N.结论 可灌注椎弓根螺钉,结合骨水泥推杆和灌注筒能有效控制骨水泥渗漏,明显增强椎弓根螺钉在骨质疏松椎体内的稳定性.     

Biomechanical comparison of pedicle screws versus spinous process screws in C2 vertebra: A cadaveric study

Background:

Biomechanical studies have shown C2 pedicle screw to be the most robust in insertional torque and pullout strength. However, C2 pedicle screw placement is still technically challenging. Smaller C2 pedicles or medial localization of the vertebral artery may preclude safe C2 pedicle screw placement in some patients. The purpose of this study was to compare the pullout strength of spinous process screws with pedicle screws in the C2.

Materials and Methods:

Eight fresh human cadaveric cervical spine specimens (C2) were harvested and subsequently frozen to −20°C. After being thawed to room temperature, each specimen was debrided of remaining soft tissue and labeled. A customs jig as used to clamp each specimen for screw insertion firmly. Screws were inserted into the vertebral body pairs on each side using one of two methods. The pedicle screws were inserted in usual manner as in previous biomechanical studies. The starting point for spinous process screw insertion was located at the junction of the lamina and the spinous process and the direction of the screw was about 0° caudally in the sagittal plane and about 0° medially in the axial plane. Each vertebrae was held in a customs jig, which was attached to material testing machine (Material Testing System Inc., Changchun, China). A coupling device that fit around the head of the screw was used to pull out each screw at a loading rate of 2 mm/min. The uniaxial load to failure was recorded in Newton''st dependent test (for paired samples) was used to test for significance.

Results:

The mean load to failure was 387 N for the special protection scheme and 465 N for the protection scheme without significant difference (t = −0.862, P = 0.403). In all but three instances (38%), the spinous process pullout values exceeded the values for the pedicle screws. The working distances for the spinous process screws was little shorter than pedicle screws in each C2 specimen.

Conclusion:

Spinous process screws provide comparable pullout strength to pedicle screws of the C2. Spinous process screws may provide an alternative to pedicle screws fixation, especially with unusual anatomy or stripped screws.     

Reduced cement volume does not affect screw stability in augmented pedicle screws

Cement augmentation of pedicle screws is able to improve screw anchorage in osteoporotic vertebrae but is associated with a high complication rate. The goal of this study was to evaluate the impact of different cement volumes on pedicle screw fatigue strength. Twenty-five human vertebral bodies (T12–L4) were collected from donors between 73 and 97 years of age. Bone density (BMD) was determined by quantitative computed tomography. Vertebral bodies were instrumented by conventional pedicle screws, and unilateral cement augmentation was performed. Thirteen vertebrae were augmented with a volume of 1 ml and twelve with a volume of 3 ml bone cement. A fatigue test was performed using a cranial–caudal sinusoidal, cyclic load (0.5 Hz) with increasing compression force (100 N + 0.1 N/cycles). The load to failure was 183.8 N for the non-augmented screws and was increased significantly to 268.1 N (p < 0.001) by cement augmentation. Augmentation with 1 ml bone cement increased the fatigue load by 41% while augmentation with 3 ml increased the failure load by 51% compared to the non-augmented screws, but there was no significant difference in fatigue loads between the specimens with screws augmented with 1 ml and screws augmented with 3 ml of bone cement (p = 0.504). Cement augmentation significantly increases pedicle screw stability. The benefit of augmentation on screw anchorage was not significantly affected by reducing the applied volume of cement from 3 ml to 1 ml. Considering the high risk of cement leakage during augmentation, we recommend the usage of a reduced volume of 1 ml bone cement for each pedicle screw. These slides can be retrieved under Electronic Supplementary Material .     

后路单节段椎弓根螺钉内固定选择性治疗胸腰椎骨折

目的：探讨后路单节段椎弓根螺钉内固定选择性治疗胸腰椎骨折的临床疗效。方法：自2005年6月至2008年6月,行后路单节段椎弓根螺钉内固定治疗胸腰椎骨折28例,男17例,女11例;年龄19-60岁,平均36岁。骨折根据AO分型：A1型19例,A2型9例。观察骨折椎的椎弓根螺钉位置及手术前后骨折椎的复位情况。结果：术后CT示骨折椎56枚椎弓根螺钉均在正常骨性结构内,均未通过骨折线。术后无脊髓损伤、感染等并发症发生。骨折椎体前缘压缩率从术前平均（42．0±5．6）％恢复至（12．4±1.4）％（P〈0．05）;骨折椎后凸Cobb角从术前平均（25．8±5．1）°废复至（1．9±1．3）°（P〈0．05）。28例患者术后均获随访,平均18.3个月（3-36个月）,骨折全部获得骨性愈合,骨折椎体高度无明显丢失,无钉棒弯曲、松动或断裂。结论：只要手术适应证选择正确,后路单节段椎弓根螺钉内固定可用于治疗胸腰椎骨折。     

Cervical pedicle screws vs. lateral mass screws: uniplanar fatigue analysis and residual pullout strengths

BACKGROUND CONTEXT: Although successful clinical use of cervical pedicle screws has been reported, anatomical studies have shown the possibility for serious iatrogenic injury. However, there are only a limited number of reports on the biomechanical properties of these screws which evaluate the potential benefits of their application. PURPOSE: To investigate if the pull-out strengths after cyclic uniplanar loading of cervical pedicle screws are superior to lateral mass screws. STUDY DESIGN: An in vitro biomechanical study. METHODS: Twenty fresh-frozen disarticulated human vertebrae (C3-C7) were randomized to receive both a 3.5 mm cervical pedicle screw and lateral mass screw. The screws were cyclically loaded 200 times in the sagittal plane. The amount of displacement was recorded every 50 cycles. After cyclical loading, the screws were pulled and tensile load to failure was recorded. Bone density was measured in each specimen and maximum screw insertion torque was recorded for each screw. RESULTS: During loading the two screw types showed similar stability initially, however the lateral mass screws rapidly loosened compared to the pedicle screws. The rate of loosening in the lateral mass screws was widely variable, while the performance of the pedicle screws was very consistent. The pullout strengths were significantly higher for the cervical pedicle screws (1214 N vs. 332 N) and 40% failed by fracture of the pedicle rather than screw pullout. Pedicle screw pullout strengths correlated with both screw insertion torque and specimen bone density. CONCLUSIONS: Cervical pedicle screws demonstrated a significantly lower rate of loosening at the bone-screw interface, as well as higher strength after fatigue testing. These biomechanical strengths may justify their use in certain limited clinical applications.