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.     

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.     

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.     

The effect of pedicle screw redirection after lateral wall breach—a biomechanical study using human lumbar vertebrae

Background contextCurrently, pedicle screw segmental fixation of the spine is considered a standard of care for a number of conditions. Most surgeons employ a free-hand technique using various intraoperative modalities to improve pedicle screw accuracy. Despite continued improvements in technique, pedicle breach remains a frequent occurrence. Once a breach is detected intraoperatively, the most common corrective maneuver is to medially redirect the pedicle screw into the pedicle. To our knowledge, the biomechanical impact of medially redirecting a pedicle screw after a lateral pedicle breach has not been examined.PurposeTo compare the fixation strength of perfectly placed pedicle screws to the fixation strength of pedicle screws that were correctly placed after having been redirected (RD) following a lateral pedicle breach.Study design/settingA biomechanical study using human lumbar vertebrae.MethodsTen fresh human lumbar vertebrae were isolated from five donors. Each vertebra was instrumented with a monoaxial pedicle screw into each pedicle using two different techniques. On one side, a perfect center-center (CC) screw path was created using direct visualization and fluoroscopy. A 6.0-mm-diameter cannulated tap and a pedicle probe were used to develop the pedicle for the 7.0-mm-diameter by 45-mm-long cannulated pedicle screw, which was placed using a digital torque driver. On the contralateral side, an intentional lateral pedicle wall breach was created at the pedicle-vertebral body junction using a guide wire, a 6.0-mm-diameter cannulated tap, and a pedicle probe. This path was then redirected into a CC position, developed, and instrumented with a 7.0-mm-diameter by 45-mm-long cannulated pedicle screw: the RD screw. For each pedicle screw, we assessed four outcome measures: maximal torque, seating torque, screw loosening, and post-loosening axial pullout. Screw loosening and axial pullout were assessed using an MTS machine.ResultsThe biomechanical cost of a lateral pedicle breach and the requirement to redirect the pedicle screw are as follows: an overall drop of 28% (p<.002) in maximal insertion torque and 25% (p<.049) in seating torque, a drop of 25% (p<.040) in resistance to screw loosening, and a drop in axial pullout force of 11% (p<.047).ConclusionsCompared with a CC lumbar pedicle screw, an RD lumbar pedicle screw placed after a lateral wall breach is significantly weaker in terms of maximal insertional torque, seating torque, screw loosening force, and axial pullout strength. These significant decreases in biomechanical properties are clearly important when RD pedicle screws are placed at the cephalad or caudal end of a long construct. In this situation, augmentation of the RD screw is an option.     

Primary stability of pedicle screws depends on the screw positioning and alignment

Background contextThere is no universal consensus regarding the biomechanical aspects and relevance on the primary stability of misplaced pedicle screws.PurposeThe study is aimed to the determination of the correlation between axial pullout forces of pedicle screws with the possible screw misplacement, including mild and severe cortical violations.MethodsEighty-eight monoaxial pedicle screws were implanted into 44 porcine lumbar vertebral bodies, paying attention on trying to obtain a wide range of placement accuracy. After screw implantation, all specimens underwent a spiral computed tomography scan, and the screw placements were graded following the scales of Laine et al. and Abul Kasim et al. Axial pullout tests were then performed on a servohydraulic material testing system.ResultsDecreasing pullout forces were determined for screws implanted with increasing cortical violation. A smaller influence of cortical violations in the medial direction with respect to the lateral direction was observed. Screws implanted with a large cortical violation and misplacement in the craniocaudal direction were found to be significantly less stable than screws having comparable cortical violation but in a centered sagittal position.ConclusionsThese results provide adjunctive criteria to evaluate more accurately the fate of a spine instrumentation. Particular care should be placed in the screw evaluation regarding the craniocaudal positioning and alignment.     

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.     

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.     

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.     

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

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

Pullout strength of pedicle screws versus pedicle and laminar hooks in the thoracic spine

While the biomechanical properties of pedicle screws have proven to be superior in the lumbar spine, little is known concerning pullout strength of pedicle screws in comparison to hooks in the thoracic spine. In vitro biomechanical pullout testing was performed to evaluate the axial pullout strength of pedicle screws versus pedicle and laminar hooks in the thoracic spine with regard to surgical correction techniques in scoliosis. Nine human cadaveric thoracic spines were harvested and disarticulated. To simulate a typical posterior segmental scoliosis instrumentation, standard pedicle hooks were used between T4 and T8 and supralaminar hooks between T9 and T12 and tested against pedicle screws. The pedicle screws were loaded strictly longitudinal to their axis; the hooks were loaded perpendicular to the intended rod direction. In total, 90 pullout tests were performed. Average pullout strength of the pedicle screws was significantly higher than in the hook group (T4-T8: 531 N versus 321 N, T9-T12: 807 N versus 600 N, p < 0.05). Both screw diameter and the bone mineral density (BMD) had significant influence on the pullout strength in the screw group. For scoliosis correction, pedicle screws might be beneficial, especially for rigid thoracic curves, since they are significantly more resistant to axial pullout than both pedicle and laminar hooks.     

三维CT模拟双通道螺钉置入同一腰椎椎弓根成功率的影响因素分析

目的 通过三维CT模拟椎弓根螺钉(pedicle scrw,PS)和皮质骨通道螺钉(cortical bone tra?jectory,CBT)置入同一腰椎椎弓根,并分析置钉成功率的影响因素.方法 应用Mimics 10.0软件重建49例病人腰椎三维图像,测量双侧L1～L5椎弓根直径(横径和纵径).将横径及纵径各分为3...     

Biomechanical evaluation and preliminary clinical experience with an expansive pedicle screw design

The advantages of pedicle screw fixation depend on their ability to retain bony purchase until the fusion mass is stable. Osteoporotic bone and removal and replacement of pedicle screws in revision procedures substantially reduce screw mechanical fixation strength and can lead to clinical failure. The objective of this study was to determine if an expansive pedicle screw design could be used to improve biomechanical fixation in bone of compromised quality. Axial mechanical pullout testing was performed on paired expansive and conventional pedicle screws placed in fresh, unembalmed cadaveric vertebrae. Bone mineral density measurements (made using a dual-energy X-ray absorption meter) were used to characterize bone quality. A preliminary clinical and radiographic evaluation of 14 patients was also performed at a minimum 2-year follow-up. The mean axial pullout force in bone of all qualities was increased 30% when the expansive pedicle screw design was used. This included an appropriate 50% increase in pullout force in bone of poor quality (low bone mineral density). The preliminary clinical and radiographic results were supportive of the biomechanical design rationale and mechanical testing. The results were similar to those expected for spinal instrumentation using pedicle screws, even though compromised bone was present in two thirds of the cases in which the expansive screw was used.     

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.     

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.     

Feasibility of using tapping torque during lumbar pedicle screw insertion to predict screw fixation strength

BackgroundRigid pedicle screw fixation is mandatory for achieving successful spinal fusion; however, there is no reliable method predicting screw fixation before screw insertion. The purpose of the present study was to investigate the efficacy of measurement of tapping torque to predict pedicle screw fixation.MethodsFirst, different densities of polyurethane foam were used to measure tapping torque. The insertional torque during pedicle screw insertion and axial pullout strength were measured and compared between under-tapped and same-tapped groups. Next, for in vivo study, the tapping and insertional torque of lumbar pedicle screws using the cortical bone trajectory technique were measured intraoperatively in 45 consecutive patients. Then, correlations between tapping torque, the bone mineral density of the femoral neck and lumbar vertebrae, and insertional torque were investigated.ResultsEx vivo tapping torque significantly correlated with the insertional torque and pullout strength regardless of tapping sizes (r = 0.98, p < 0.001). The mean in vivo tapping and insertional torque were 1.48 ± 0.73 and 2.48 ± 1.25 Nm, respectively (p < 0.001). Insertional torque significantly correlated with tapping torque and two BMD parameters, and the correlation coefficient of tapping torque (r = 0.83, p < 0.001) was higher than those of femoral neck BMD (r = 0.59, p < 0.001) and lumbar BMD (r = 0.39, p < 0.001).ConclusionsTapping torque is a reliable predictor of pedicle screw fixation and allows surgeons to improve the integrity of the bone-screw interface by making modification prior to actual screw insertion.     

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.     

Tapping insertional torque allows prediction for better pedicle screw fixation and optimal screw size selection

Background contextThere is currently no reliable technique for intraoperative assessment of pedicle screw fixation strength and optimal screw size. Several studies have evaluated pedicle screw insertional torque (IT) and its direct correlation with pullout strength. However, there is limited clinical application with pedicle screw IT as it must be measured during screw placement and rarely causes the spine surgeon to change screw size. To date, no study has evaluated tapping IT, which precedes screw insertion, and its ability to predict pedicle screw pullout strength.PurposeThe objective of this study was to investigate tapping IT and its ability to predict pedicle screw pullout strength and optimal screw size.Study designIn vitro human cadaveric biomechanical analysis.MethodsTwenty fresh-frozen human cadaveric thoracic vertebral levels were prepared and dual-energy radiographic absorptiometry scanned for bone mineral density (BMD). All specimens were osteoporotic with a mean BMD of 0.60±0.07 g/cm². Five specimens (n=10) were used to perform a pilot study, as there were no previously established values for optimal tapping IT. Each pedicle during the pilot study was measured using a digital caliper as well as computed tomography measurements, and the optimal screw size was determined to be equal to or the first size smaller than the pedicle diameter. The optimal tap size was then selected as the tap diameter 1 mm smaller than the optimal screw size. During optimal tap size insertion, all peak tapping IT values were found to be between 2 in-lbs and 3 in-lbs. Therefore, the threshold tapping IT value for optimal pedicle screw and tap size was determined to be 2.5 in-lbs, and a comparison tapping IT value of 1.5 in-lbs was selected. Next, 15 test specimens (n=30) were measured with digital calipers, probed, tapped, and instrumented using a paired comparison between the two threshold tapping IT values (Group 1: 1.5 in-lbs; Group 2: 2.5 in-lbs), randomly assigned to the left or right pedicle on each specimen. Each pedicle was incrementally tapped to increasing size (3.75, 4.00, 4.50, and 5.50 mm) until the threshold value was reached based on the assigned group. Pedicle screw size was determined by adding 1 mm to the tap size that crossed the threshold torque value. Torque measurements were recorded with each revolution during tap and pedicle screw insertion. Each specimen was then individually potted and pedicle screws pulled out “in-line” with the screw axis at a rate of 0.25 mm/sec. Peak pullout strength (POS) was measured in Newtons (N).ResultsThe peak tapping IT was significantly increased (50%) in Group 2 (3.23±0.65 in-lbs) compared with Group 1 (2.15±0.56 in-lbs) (p=.0005). The peak screw IT was also significantly increased (19%) in Group 2 (8.99±2.27 in-lbs) compared with Group 1 (7.52±2.96 in-lbs) (p=.02). The pedicle screw pullout strength was also significantly increased (23%) in Group 2 (877.9±235.2 N) compared with Group 1 (712.3±223.1 N) (p=.017). The mean pedicle screw diameter was significantly increased in Group 2 (5.70±1.05 mm) compared with Group 1 (5.00±0.80 mm) (p=.0002). There was also an increased rate of optimal pedicle screw size selection in Group 2 with 9 of 15 (60%) pedicle screws compared with Group 1 with 4 of 15 (26.7%) pedicle screws within 1 mm of the measured pedicle width. There was a moderate correlation for tapping IT with both screw IT (r=0.54; p=.002) and pedicle screw POS (r=0.55; p=.002).ConclusionsOur findings suggest that tapping IT directly correlates with pedicle screw IT, pedicle screw pullout strength, and optimal pedicle screw size. Therefore, tapping IT may be used during thoracic pedicle screw instrumentation as an adjunct to preoperative imaging and clinical experience to maximize fixation strength and optimize pedicle “fit and fill” with the largest screw possible. However, further prospective, in vivo studies are necessary to evaluate the intraoperative use of tapping IT to predict screw loosening/complications.     

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.     

颈椎经关节椎弓根螺钉固定与标准椎弓根螺钉固定的生物力学比较

目的 比较颈椎经关节椎弓根螺钉固定和标准椎弓根螺钉固定的拔出强度.方法 取10具新鲜尸体颈椎标本(C_3～T_1),游离成三个颈椎运动节段(C_(3,4),C_(5,6),C_7T_1).在椎体两侧随机进行经关节椎弓根螺钉固定或标准椎弓根螺钉固定,置入直径3.5 mm皮质骨螺钉.经关节椎弓根螺钉固定以上位椎骨侧块外下象限中点为进钉点,在直视椎弓根下,螺钉在冠状面内倾约45°、矢状面尾倾约50°.由上位椎骨下关节突经关节突关节、下位椎骨的椎弓根,进入下位椎骨的椎体内.标准椎弓根螺钉固定以侧块外上象限中点为进钉点,在直视椎弓根下,螺钉方向参考CT测量结果 ,尽量与椎弓根倾斜角度保持一致,在横断面上内倾约45°、矢状面上螺钉指向椎体的上1/3.在生物力学试验机上行拔出强度试验,比较两种螺钉固定的最大轴向拔出力.结果 颈椎经关节椎弓根螺钉固定平均最大轴向拨出力为(694±42)N,标准椎弓根螺钉固定为(670±36)N,两者比较差异有统计学意义(P<0.05).结论 颈椎后路经关节椎弓根螺钉固定的拔出强度大干标准椎弓根螺钉固定,从生物力学强度方面考虑经关节椎弓根螺钉固定可以作为标准椎弓根螺钉固定的一种补充方法.