Pedicle Screw Design Research Articles

Mechanical Performance of Cylindrical and Dual-Core Pedicle Screws After Repeated Insertion

In Brief Study Design. Ex vivo study of the mechanical performance of cylindrical and dual-core pedicle screws after insertion, removal, and reinsertion in the same hole. Objective. To evaluate the effect of repeated use of same screw hole on the insertion torque and the retentive strength of the cylindrical and dual-core screws. Summary of Background Data. Insertion and removal of pedicle screws is sometimes necessary during surgical procedure to assess the integrity of the pilot-hole wall. However, this maneuver may compromise the implant-holding capacity. Methods. Sixty thoracolombar vertebrae (T13–L5), harvested from 10 healthy calves, were used to insert 2 different designs of pedicle screws: cylindrical (5.0-mm outer diameter) and dual-core screws (5.2-mm outer diameter). Three experimental groups were created on the basis of the number of insertions of the screws and 2 subgroups were established according to the core pedicle screw design (dual-core and cylindrical). The insertion torque was measured during initial insertion, second insertion, and third insertion. Pullout screw tests were performed using a universal testing machine to evaluate the pullout strength after initial insertion, second insertion, and third insertion. Results. Significant reductions of 38% in mean insertion torque and 30% in mean pullout strength of dual-core screw were observed between the initial insertion and the third insertion. The cylindrical screw observed significant reductions of 52.5% in mean insertion torque and 42.3% in mean pullout strength between the initial insertion and the third insertion. A reduction of mean insertion torque and pullout strength between the first insertion and the second insertion but without significance was also observed for both types of screws. Conclusion. Insertions and reinsertion of either cylindrical or dual-core pedicle screws have compromised insertion torque and pullout strength of the implants as measured by mechanical tests. An ex vivo study of the insertional torque and pullout force of cylindrical and dual core pedicle screws after insertion, removal, and reinsertion in the same hole was conducted. Insertion and reinsertion of both screw types diminished the bone-holding capacity.

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

The 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. An experimental and biomechanical study. To evaluate the geometric factors of screws affecting fixation ability after assessing pullout strength based on various pedicle screw designs and different bone densities. Nine 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. Pullout 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). Pedicle screw with an outer cylindrical and inner conical configuration witha V-shaped thread may have maximum pullout strength, regardless of bone density.

A titanium expandable pedicle screw improves initial pullout strength as compared with standard pedicle screws

Pedicle screws are now standard for spinal arthrodesis as they provide three-column spinal stabilization. Decreased vertebral body bone density because of aging reduces the stability of the bone-screw interface, potentially increasing screw pullout or pseudarthrosis. Modifications to standard pedicle screw designs to improve screw stabilization may help to compensate for the detrimental effects of decreased vertebral bone density. To evaluate differences in initial pullout strength of an expandable titanium pedicle screw as compared with a standard titanium pedicle screw. In vitro human cadaveric biomechanical investigation. Fresh thoracolumbar spines from four human cadavers were imaged using quantitative computed tomography to obtain standard lumbar osteoporosis (Dual-energy X-ray absorptiometry [DXA]) T scores. Six bodies were sectioned per spine, and standard titanium 6.5-mm diameter pedicle screws and expandable 6.5-mm diameter titanium screws (maximum expanded diameter=10 mm) were randomized to right and left sides. Screw testing, in axial pullout at 25 mm/min, was randomized to reduce the effects of testing order. Data for stiffness (N/mm), yield load (N), ultimate load (N), and energy (N mm) (area under the load-displacement curve) were analyzed using a one-way analysis of variance (p<.05). Lumbar DXA scores averaged -3.6. There were no statistical differences between screw types for stiffness. Yield load was not statistically different between groups, although the expandable screw yield load was nearly 25% greater than that of the standard screw. Ultimate load was found to be statistically greater (∼30%) for the expandable screw compared with the standard screw (p<.05). The energy required to cause bone-implant failure was also statistically greater for the expandable screw compared with the standard screw (p<.0001). Expandable titanium pedicle screws demonstrated improved screw pullout stability compared with standard titanium screws in osteopenic or osteoporotic bone. Further studies are warranted examining other loading methods to evaluate the stability provided by an expandable pedicle screw.

In vivo preclinical evaluation of the influence of osteoporosis on the anchorage of different pedicle screw designs

We investigate the anchorage of pedicle screws with different surface treatments in osteoporotic bone. Eight ewes were divided into two groups of four animals each: four sheep underwent bilateral ovariectomy (OVX Group), whereas the operation was simulated in the remaining group (SHAM Group). Eighteen months after the first operation, the Dynesys(®) System was fitted to the sheep using pedicle screws with three different surface treatments: untreated, rough blasted (uncoated) and bioactive coated (bioactive). Uncoated screws showed a significantly higher bone ingrowth value compared with the untreated screws in the OVX group (9.3%, p<0.005) and a significantly lower bone ingrowth value in the SHAM group (-11.0%, p<0.05). Furthermore, the bioactive pedicle screws had a significant lower bone ingrowth value than the untreated screws in the SHAM group (-12.1%, p<0.05). These results suggest that both tested surface treatments of pedicular screws may provide an advantage in terms of bone quality and osseointegration, when implanted in osteoporotic vertebrae.

Biomechanical Evaluation of Conical and Cylindrical Core Pedicle Screws in a Cancellous Bone Model

Despite advances in pedicle screw design, purchase and loosening in osteoporotic bone still presents a significant clinical problem. As opposed to the traditional cylindrical screw, the conical core screw was developed to maximize purchase of the pedicle screw path. Previous studies have shown the differences in pullout load between a 6mm conical and cylindrical core screw. In addition, the effect of backing out a conical or cylindrical core screw on the biomechanics has been described using porcine vertebrae with sufficient bone density and a single diameter screw.

A parametric study of cylindrical pedicle screw design implications on the pullout performance using an experimentally validated finite-element model

The present study aims to the design of a finite-element model simulating accurately the pullout behaviour of cylindrical pedicle screws and predicting their pullout force. Three commercial pedicle screws, subjected to pure pullout from synthetic bone, were studied experimentally. The results were used for the design, calibration and validation of a finite-element model. Special attention was paid to the accurate simulation of the failure inside the host material under shear. For this purpose, a bilinear cohesive zone material model was adopted, controlling the mode-II debonding of neighbouring elements in the vicinity of the screw. Comparison between experimental and numerical results proved that the implementation of this approach can significantly enhance the accuracy of the numerical simulation of a screw's mechanical behaviour under pure pullout loads. The numerical model was used for the parametric study of various factors affecting the pullout performance of a cylindrical pedicle screw. It was concluded that the major parameter influencing the pullout force is the outer radius (increasing its value by 36% increases the pullout force by 34%). The influence of the purchase length of the screw is of similar quantitative nature. The respective dependence on the thread inclination, depth and pitch was significantly weaker.

Evaluation of the intrinsic properties of pedicle screws: do diameter, manufacturing and screw design affect resistance and/or resistivity.

The pedicle screw diameter, composite and design are variables that can affect the threshold of intraoperative electromyographic monitoring. Even though we know that larger diameter objects tend to have less resistance, no study documented the effect that this variable could have on pedicle screw resistance. Using high quality equipment, resistance and resistivity of ten pedicle screws (from four manufacturers) were calculated based on known constant current and measured voltage. Voltage was measured three times for each screw to determine intraobserver measurement variability. Resistance of all screws ranged from 1.4 to 3.9 m ohm (mean = 2.69+/-0.71 m ohm). The screw with largest diameter (7.75 mm) had lower resistance than screws with other diameters. Resistivity of screws ranged from 7.12 to 12.63 micro ohm*m (mean = 9.9+/-1.82 micro ohm*m). Based on the screw design, one manufacturer's pedicle screws (A) had significantly lower resistivity compared to three other manufacturers (p<0.01). Larger diameter screws (7.75 mm in diameter) had lower resistance. Screw design (polyaxial or monoaxial) had no effect on its resistance. Screws of one manufacturer (A) showed lower resistivity compared to those manufactured by other three companies.

P169. Biomechanical Advantages of a Novel Dual-threaded Pedicle Screw Design vs. Traditional Single-threaded Pedicle Screws

P169. Biomechanical Advantages of a Novel Dual-threaded Pedicle Screw Design vs. Traditional Single-threaded Pedicle Screws

Comparing the Fixation of a Novel Hollow Screw Versus a Conventional Solid Screw in Human Sacra Under Cyclic Loading

The loosening rates of two monocortical pedicle screw designs (hollow and solid) were compared in a cadaveric sacrum model subjected to cyclic loading. To determine if a hollow screw would be more resistant to loosening than a solid pedicle screw when placed into the pedicles of S1 and tested under stair-cased cyclic loading. Screw loosening is a clinical problem for lumbosacral fusions. No previous literature has evaluated the use of a monocortical hollow screw within the sacrum; however, in other vertebral bodies, results of using this screw have been varied. Seven fresh-frozen cadaveric sacra were thawed and stripped of soft tissues. Solid and hollow screws were inserted contralaterally into the pedicles of S1. A materials testing machine applied alternating flexion and extension bending moments at 1 Hz, to each screw independently, via a standard connecting rod. Flexion moments were applied starting at 0.5 Nm and increased by 0.5 Nm after every 1000 cycles until the screw had visibly failed. Extension moments were maintained at 0.5 Nm. Screw rotation (flexion) relative to the sacrum was recorded using a custom optical tracking system, and analyzed using 2-way repeated measures analyses of variances and post hoc Student-Newman-Keuls tests (alpha = 0.05). Screw rotation tended to gradually increase to six degrees, after which the screw was grossly loose. Overall, the hollow screw required fewer loading cycles (P = 0.004) and less applied moment (P = 0.003) to achieve the same magnitude of screw rotation as the solid screw. To achieve 6 degrees of screw rotation, the number of loading cycles were 6301 +/- 2161 and 11151 +/- 4221 for hollow and solid screws, respectively. The corresponding applied moments were 3.5 +/- 1.0 Nm and 5.8 +/- 2.0 Nm. CONCLUSION.: The novel hollow screw was less resistant to loosening when compared with a conventional solid pedicle screw in this sacral model under cyclic loading.

Development of High Technology for Improvement of Fatigue Strength in the Intervertebral Disc Herniation's Cervical Attachment System

The fixation of the vertebral column using transpedicular screws is a well-establish technique. Multi-axial pedicle screw designs allow deviation of the screw away from the perpendicular to the longitudinal rod, which facilitates application of a screw-rod system such as Cervical Attachment System (CAS) into the curved spine. This study was performed a static (compression bending and torsion) and dynamic (compression bending fatigue) empirical analysis of CAS component such as pedicle screw, rod and set screw based on the guideline of ASTM F136- 98 using Ultra-high Molecular Weight Polyethylene (UHMWPE) blocks as test vertebral bodies.

Mechanical Performance of PRIMA OCT System under Interconnection Testing and Corpectomy Testing

The fixation of the vertebral column using transpedicular screws is a well-establish technique. Multi-axial pedicle screw designs allow deviation of the screw away from the perpendicular to the longitudinal rod, which facilitates application of a screw-rod system such as PRIMA OCT into the curved spine. This study was performed a static (compression bending and torsion) and dynamic (compression bending fatigue) empirical analysis of PRIMA OCT component such as pedicle screw, rod and set screw based on the guideline of ASTM F136-98 using Ultra-high Molecular Weight Polyethylene (UHMWPE) blocks as test vertebral bodies.

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.

A Comparison of Pullout Strength for Pedicle Screws of Different Designs

The pullout strengths of various pedicle screw designs are compared using tapped and untapped pilot holes. The objective of this study is to compare the pullout strength of various pedicle screw designs. The designs are compared using tapped and untapped pilot holes. By using several different screw designs, it is possible to gain an understanding of whether there is a correlation between tapping a pilot hole and the ultimate pullout strength. Most bone screws originally developed were intended to be installed in a pretapped pilot hole. This same technology has been carried over to the development of more modern bone screws for use in spinal fixation applications. Many pedicle screws in use today are still intended to be installed in a tapped hole. Preparing the vertebrae and tapping of a pilot hole involve additional trauma to the patient as well as increased operating time. Pedicle screws from various manufacturers are installed in tapped and untapped pilot holes and then loaded to failure. A uniform synthetic material was used to provide a consistent test of each screw design by eliminating variability seen in bone. Tapping pilot holes did not increase the pullout strength of the screws tested in this study. It was observed during testing that tapping some of the holes degraded the material. This degradation led to pullout strengths that were lower than in the untapped case, and generally larger standard deviations. The pullout strength was not increased by tapping for the screws in this study. Screws placed in untapped holes generally had higher pullout strengths and lower standard deviations. The results of this study suggest that tapping does not increase pullout strength in bone with densities near 20 lb/ft3, which correlates with low density cancellous or osteoporotic bone.

Pedicle screw fixation strength: pullout versus insertional torque

Background context Researchers studying early pedicle screw designs have suggested that pullout strength and insertional torque are correlated. For surgeons using pedicle screws, insertional torque is widely believed to be a good predictor of pullout strength and initial stability of the screw and construct. How appropriate is this assumption when applied to new screw and thread designs? Purpose This study investigated the correlation between insertional torque and pullout strength of three different pedicle screw designs, with different insertional torque characteristics. We hypothesized that a significant increase in insertional torque would indicate a commensurate increase in pullout strength. Study design Biomechanical analysis of instrumented vertebral specimens. Methods Calf lumbar vertebra were prepared and instrumented with one of three pedicle screws. Pilot hole preparation was standardized and coaxial orientation was confirmed by direct inspection. Screws did not penetrate the pedicle cortex or abut or penetrate the anterior vertebral cortex. Any specimen with pedicle wall breach was discarded. The pedicles were instrumented with one of three screws: 1) 7.5×40 mm conical, asymmetric progressive thread (Xia; Stryker Spine, Allendale, NJ), 2) 7.5×40 mm conical with traditional V-shaped thread (Osteonics, Stryker Spine, Allendale, NJ)) or 3) 6.5×40 mm cylindrical with V thread (Osteonics, cylindrical). Paired testing allowed individual screws to be directly compared with a contralateral “control.” Insertional torque and peak torque values were recorded for each rotation up to full insertion. Pullout testing was conducted at a rate of 1 mm/minute. Load-displacement data were recorded at 20 Hz. Stiffness was considered the slope of the most linear part of the curve before the yield point. Results Peak loads for 7.5 conical Xia screws measured 1,783±589.1 N compared with 1,943.0±625.8 N for 7.5 conical Osteonics screws and 1,641.0±356.7 N for 6.5 cylindrical Osteonics screws. The peak insertional torque values were 6.7±1.9 Nm (158% greater than control), 4.5±1.1 Nm (73% greater than control) and 2.6±0.7 Nm, respectively. Insertional torques for Xia screws were significantly greater than conical (p=.001) and cylindrical Osteonics screws (p<.0001), and insertional torques for Osteonics conical screws were significantly greater than those of cylindrical screws (p<.0001). Although pullout loads for the conical Osteonics screws were consistently higher than either the Xia or Osteonics cylindrical screws, the differences were not significant (p>.05). There was no significant correlation between pullout strength and insertional torque (p>.05). Conclusions This unexpected result is best explained by the progressively narrowing flutes of the Xia screw, which compact the trabeculae into a smaller volume as the screw nears full insertion. The trapezoidal threads also increase contact with the cortical surface area and compress trabeculae toward the cortex, thus creating greater friction and higher torque values. This increase in torque did not translate into a commensurate increase in pullout strength, where trabeculae fail in shear.

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.

Multiaxial pedicle screw designs: static and dynamic mechanical testing.

Randomized investigation of multiaxial pedicle screw mechanical properties. Measure static yield and ultimate strengths, yield stiffness, and fatigue resistance according to an established model. Compare these measured properties with expected loads in vivo. Multiaxial pedicle screws provide surgical versatility, but the complexity of their design may reduce their strength and fatigue resistance. There is no published data on the mechanical properties of such screws. Screws were assembled according to a vertebrectomy model for destructive mechanical testing. Groups of five assemblies were tested in static tension and compression and subject to three cyclical loads. Modes of failure, yield, and ultimate strength, yield stiffness, and cycles to failure were determined for six designs of screw. Static compression yield loads ranged from 217.1 to 388.0 N and yield stiffness from 23.7 to 38.0 N/mm. Cycles to failure ranged from 42 x 10(3) to 4,719 x 10(3) at 75% of static ultimate load. There were significant differences between designs in all modes of testing. Failure occurred at the multiaxial link in static and cyclical compression. Bending yield strengths just exceeded loads expected in vivo. Multiaxial designs had lower static bending yield strength than fixed screw designs. Five out of six multiaxial screw designs achieved one million cycles at 200 N in compression bending. "Ball-in-cup" multiaxial locking mechanisms were vulnerable to fatigue failure. Smooth surfaces and thicker material appeared to be protective against fatigue failure.

Mechanical performance of the new posterior spinal implant: effect of materials, connecting plate, and pedicle screw design.

A newly designed spinal implant was tested to evaluate multicycle stiffness and fatigue resistance. To investigate the effect of different materials, connecting plate, and pedicle screw design on the mechanical performance of the spinal implant. The addition of cross-linkages did not significantly increase implant compression/flexion stiffness, but accelerated fatigue failure at the rod junctions. Both Ti-6Al-4V spinal implants and the 316L stainless-steel counterparts have been used extensively for clinical cases; however, design factors establishing the proposed superiority of the Ti-6Al-4V implant for fatigue resistance have not, as yet, been extensively studied. Twenty implants with connecting plates (two materials by two screw designs by five implants) and five implants without connecting plates were assembled to UHMWPE blocks and cyclically loaded from 60 N to 600 N at a frequency of 5 Hz. Failure sites for the tested prototypes were at the cephalic screw hubs or rod-plate junctions. All Ti-6Al-4V implants demonstrated reduced stiffness compared to the structurally identical 316L analogs. The use of connecting plates raised the stiffness of the 316L prototypes without cross-links. However, elimination of the connecting plate avoided stress concentration at the rod/plate junctions and increased fatigue life. The Ti-6Al-4V new system with the minimal notch effect at the screw hubs achieved greater fatigue resistance than its 316L counterpart. By contrast, enlargement of the inner-hub diameter resulted in greater gains for fatigue resistance than for stiffness, especially for Ti-6Al-4V variants. Although Ti-6Al-4V was superior to 316L for endurance-limit properties, structural design of the Ti-6Al-4V implant dramatically affects fatigue resistance. This may explain the differences between existing studies and the current report, comparing fatigue life for implants made from these two materials. Our results reveal that Ti-6Al-4V must be carefully treated because of sensitivity to notch, with special consideration given to screw-hub design.

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.

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.

Insertional torque and pull-out strengths of conical and cylindrical pedicle screws in cadaveric bone.

Insertion torque and pull-out strengths of conical and cylindrical pedicle screws were compared in human cadaveric vertebral bodies. To compare the performance of the conical design with the cylindrical design, and to determine whether insertional torque correlates with pull-out strength. A tapered pedicle screw design may lessen the likelihood of implant failure. Its effect on thread purchase is not known. Previous studies of cylindrical designs on the relation between insertion torque and pull-out strength have been conducted in bovine and synthetic bone. Seventy-eight pedicles were assigned randomly to one of the following pedicle screw: Texas Scottish Rite Hospital (Sofamor-Danek, Memphis, TN), Steffee VSP (Acromed, Cleveland, OH), Diapason (Dimso, Paris, France), AO Schanz (Synthes, Paoli, PA), or Synthes USS (Synthes, Paoli, PA). Pedicle screws were inserted with a torque screwdriver. Each screw was extracted axially from the pedicle at a rate of 1.0 mm/sec until failure using an MTS machine (Bionix 858, Minneapolis, MN). Force data were recorded. The conical design had the highest insertion torque. There were no significant differences in pull-out between any of the screw types. Correlation between insertional torque and pull-out strength was statistically significant only with the Texas Scottish Rite Hospital and Steffee VSP in L4 and AO Schanz in L5. A conical screw profile increases insertion torque, although insertional torque is not a reliable predictor of pull-out strength in cadaveric bone. Screw profile (with similar dimensions) has little effect on straight axial pull-out strengths in cadaveric bone.