Polyaxial Pedicle Screws Research Articles

Revision of the failed pedicle screw in osteoporotic lumbar spine: Biomechanical comparison of kyphoplasty versus pedicle hole polymethylmethacrylate augmentation techniques

Over the years, spine surgeons have been obliged to revise pedicle screws due to the increasing number of the pedicle screw instrumentation in elderly osteoporotic population. There are many ways to salvage pedicle screws such as using larger and/or longer size pedicle screws, augmentation or inserting screws in a different trajectory. Lumbar vertebrae were harvested from 4 calves and the bone mineral density (BMD) value was measured. Each vertebra was then placed into a beher-glass filled with hydrochloric acid decalcifier solution for 24 h to obtain osteoporotic specimens. Stage 1 : Primary polyaxial pedicle screws were randomly inserted to left or right pedicle. The screws were pulled out in a random order at 10 mm/min with Instron Material Testing Machine. The pull-out strength (POS) results of all specimens were recorded. Stage 2 : Revision pedicle screws were randomly assigned to be replaced in the same pedicles by either pedicle hole PMMA augmentation (Group 1) or kyphoplasty (Xvoid™) augmented pedicle screws (Group 2). The pull-out tests of the specimens were repeated. The mean BMD result of the specimens before the decalcification was 1.686±227.9 and 1.432±157.1 g/cm 2 after decalcification. There was a statistically significant difference between BMD results before and after the decalcification ( p <0.001). Group 1 : The mean POS of primary screws was 3443±1086 N/m 2 . After pedicle hole augmentation, the mean POS was 2088±924 N/m 2 . The statistical difference was found between the primary and revision screws POS in group 1 ( p =0.006). Group 2 : The mean POS of primary screws was 3702±1063 N/m 2 and after kyphoplasty augmentation, the mean POS was decreased to 3664±1057 N/m 2 . There was no statistical significant difference between the primary and kyphoplasty augmented screws in group 2 ( p =0.934). There was also significant difference between the pedicle hole versus kyphoplasty augmented revision screw’s POS ( p =0.002). There was a strong correlation between POS and BMD for the primary screw ( p <0.001). However, no statistical significance was obtained between BMD score and both the revision techniques POS ( p =0.808). Even thought pedicle hole PMMA augmentation has been known to be the gold standard for the failed screw in an osteoporotic bone, kyhoplasty augmented pedicle screw seems to be the most powerful salvage technique for a failed pedicle screw.

Clinical and methodological precision of spinal navigation assisted by 3D intraoperative O-arm radiographic imaging

In recent years, the importance of intraoperative navigation in neurosurgery has been increasing. Multiple studies have proven the advantages and safety of computer-assisted spinal neurosurgery. The use of intraoperative 3D radiographic imaging to acquire image information for navigational purposes has several advantages and should increase the accuracy and safety of screw guidance with navigation. The aim of this study was to evaluate the clinical and methodological precision of navigated spine surgery in combination with the O-arm multidimensional imaging system. Thoracic, lumbar, and sacral pedicle screws that were placed with the help of the combination of the O-arm and StealthStation TREON plus navigation systems were analyzed. To evaluate clinical precision, 278 polyaxial pedicle screws in 139 vertebrae were reviewed for medial or caudal perforations on coronal projection. For the evaluation of the methodological accuracy, virtual and intraoperative images were compared, and the angulation of the pedicle screw to the midsagittal line was measured. Pedicle perforations were recorded in 3.2% of pedicle screws. None of the perforated pedicle screws damaged a nerve root. The difference in angulation between the actual and virtual pedicle screws was 2.8° ± 1.9°. The use of the StealthStation TREON plus navigation system in combination with the O-arm system showed the highest accuracy for spinal navigation compared with other studies that used traditional image acquisition and registration for navigation.

A Comparative Study of Posterior Fixation in Acute Unstable Thoracolumbar Injuries by Monoaxial and Polyaxial Pedicle Screws

The pedicle offers a strong point of attachment of the posterior elements to the vertebral body and pedicle screw instrumentation has revolutionized spine surgery. Pedicle screw fixation is considered bio-mechanically superior to other stabilization constructs or parapedicular screws and is exceptionally rigid. Additionally, pedicle screw constructs may obviate the need for neural canal dissection and potential neural element impingement by intracanal instrumentation

The Fatigue Life of Contoured Cobalt Chrome Posterior Spinal Fusion Rods

Intraoperative contouring of posterior rods in lumbar arthrodesis constructs introduces stress concentrations that can substantially reduce fatigue life. The sensitivity of titanium (Ti) and stainless steel (SS) to intraoperative contouring has been established in the literature; however, notch sensitivity has yet to be quantified for cobalt chrome (CoCr), which is now being advocated for use in posterior arthrodesis constructs. The goal of this study is to evaluate the sensitivity of CoCr rods to intraoperative contouring for posterior lumbar screwrod arthrodesis constructs. In this paper lumbar bilateral vertebrectomy models are constructed based on ASTM F1717-01 with curved rods (26-30 degrees total curvature) and poly-axial pedicle screws. Three types of constructs are assembled: first, 5.5 mm SS rods with SS screws (6.5 x 35 mm), second, 6.0 mm Ti rods with Ti screws (7.5 x 35 mm), and third, 6.0 mm CoCr rods with Ti screws (7.5 x 35 mm). All specimens are tested at 4 Hz in dynamic axial compression-bending with a load ratio of ten and maximum load levels of 250, 400, and 700 N until run-out at 2 000 000 cycles. Results are presented that show that the fatigue life of CoCr constructs tend to be greater than Ti constructs at all levels. At the 400 N maximum loading, CoCr lasts an average of 350 000 cycles longer than the Ti constructs. The CoCr constructs are able to sustain the 250 N load until run-out at 2 000 000 cycles but they fail at high load levels (maximum 700 N). The CoCr constructs fail at the neck of the Ti screw at high loads whereas Ti screws fail at the notch induced by contouring. Since CoCr is compatible with magnetic resonance imaging and has high static strength characteristics, the results of this study suggest that it may be an appropriate substitute for Ti.

Simultaneous double-rod rotation technique in posterior instrumentation surgery for correction of adolescent idiopathic scoliosis

The authors present a new posterior correction technique consisting of simultaneous double-rod rotation using 2 contoured rods and polyaxial pedicle screws with or without Nesplon tapes. The purpose of this study is to introduce the basic principles and surgical procedures of this new posterior surgery for correction of adolescent idiopathic scoliosis. Through gradual rotation of the concave-side rod by 2 rod holders, the convex-side rod simultaneously rotates with the the concave-side rod. This procedure does not involve any force pushing down the spinal column around the apex. Since this procedure consists of upward pushing and lateral translation of the spinal column with simultaneous double-rod rotation maneuvers, it is simple and can obtain thoracic kyphosis as well as favorable scoliosis correction. This technique is applicable not only to a thoracic single curve but also to double major curves in cases of adolescent idiopathic scoliosis.

Poly-Axial Screw Removal

As the use of instrumentation in spinal surgery has become common, the need for revision surgery has increased. During revision surgery one of the most difficult steps is removal of poly-axial pedicle screws, especially if no suitable revision set is available. We describe here an easy method for poly-axial screw removal. Leaving or placing a small piece of rod, attached firmly by a nut, tightens the head of the poly-axial screw. It can no longer move freely from the distal part of the screw and the screw can be removed by turning it counterclockwise with a big clamp or needle driver, which is available in almost every surgery set.

Comparative analysis between shape memory alloy-based correction and traditional correction technique in pedicle screws constructs for treating severe scoliosis

The three-dimensional correction of severe rigid scoliosis has been improved by segmental pedicle screw instrumentation. However, there can be significant difficulty related to the use of a rigid rod, especially in the apex region of severe scoliosis. This study is a retrospective matched cohort study to evaluate the advantages of Nitinol shape memory alloy (SMA) rod-based correction by comparing the clinical and radiographic results obtained from using a temporary SMA rod and those from a standard rod in the correction of severe scoliosis. From May 2004 to September 2006, patients with matched curve type, ages at surgery, operative methods and fusion levels in our institute and instrumented with either SMA rods (n = 14) or traditional correction techniques (n = 16) were reviewed. In SMA group, the SMA rods served as a temporary intraoperative tool for deformity correction and were replaced by standard rods. The blood loss at surgery averaged 778 +/- 285 ml in the traditional group and 585 +/- 188 ml in the SMA group (P < 0.05). Operative time averaged 284 +/- 53 min in the SMA group and 324 +/- 41 min in the traditional group (P < 0.05). In the SMA group, the preoperative major curve was 92.6 degrees +/- 13.7 degrees with a flexibility of 25.5 +/- 7.3% was corrected to 29.4 degrees +/- 5.7 degrees demonstrating a 68.4% immediate postoperative correction. In the traditional group, the preoperative major curve was 88.6 degrees +/- 14.6 degrees with a flexibility of 29.3 +/- 6.6% was corrected to 37.2 degrees +/- 7.3 degrees demonstrating a 57.8% immediate postoperative correction. There was a statistic difference between the SMA group and traditional group in correction rate of the major thoracic curve. In the SMA group, one case suffered from deep infection 2 months postoperatively. In the traditional group, 6 of 16 cases suffered pedicle screw pull out or loosening during placement of the standard rod at the apex vertebrae on the concave side. In three cases, the mono-axial pedicle screws near the apex were abandoned and in five cases replaced with poly-axial pedicle screws. This study shows that the temporary use of SMA rod may reduce the operative time, blood loss, while improve the correction of the coronal plane when compared with standard techniques.

Easy retrieval of polyaxial tulip-head pedicle screws by “U” rod technique

The number of fusion surgeries increase each year which also increase the need for implant removal. In some cases, it can be extremely hard to remove a pedicle screw especially when there is a mismatch of the screw and the screwdriver. Also the screwdrivers can be contaminated during the operation, and this will cause a delay till the instruments are re-sterilized. There is a need for the removal of screws without special instruments. We describe a method for removing tulip-head polyaxial pedicle screws without special instruments. The screws are removed using an Allen key, a rod bender and a "U" shaped rod. We successfully removed 76 screws in 11 recent cases without any complications. The "U" rod technique is a simple and useful technique for the removal of tulip-head polyaxial screws.

Polyaxial screws for lumbo-iliac fixation after sacral tumor resection: experience with a new technique for an old surgical problem

Background Although numerous reports have been published about various methods for reconstruction after sacrectomies, there are still biomechanical and technical dilemmas that are unaddressed. This report describes the experience at authors' institution of five cases in which polyaxial pedicle screws construct has been successfully used for lumbo-iliac fixation after sacral tumor resection. Methods Five cases of sacral tumors, two of Ewing's sarcoma and three of giant cell tumor (GCT) underwent surgical resection and then reconstruction was done with hardware using vertical rods placed alongside the spine bilaterally, transfixing monoaxial and polyaxial pedicle screws in lower lumbar levels and polyaxial screws into the ilium bilaterally. Cross links were also used to connect the two vertical members, thus enhancing biomechanical stability of the construct. Use of autologous bone grafts was relied upon to fill the gap created by sacral resection. Results No instrumentation failure was noted and the continuity of the spine and pelvis was well established with the instrumentation and auto grafts. In follow up of these patients (1–3 years), no complications were seen. Conclusion Polyaxial pedicle screws fixation is an effective technique to transmit axial load from spine to the appendicular bone and can be used safely in patients in whom sacral integrity is compromised after surgical resection. However, the long term benefits of this technique need to be evaluated.

Analysis of Deformation and Stress Distribution of the Poly-Axial Pedicle Screw

The effect of design features of an internal spinal fixator under loading is critical to understanding of interaction between fixator and instrumented spine. In this study, we performed finite element analysis for spinal pedicle screw installed in the lumbar spine. The purpose of this study is to model and simulate the newly designed spinal pedicle screw. The deformation and stress of the screw are analyzed for the tightening process and loading process simulating the condition when it is installed in the human body as described in the ASTM F1717 procedure. We expected this study is to derive reliable results for developing a new model by analysis of design variables and fatigue behavior.

511 多軸性脊椎固定用スクリューのLocking Systemにおける生体力学的な安定性分析に関する研究(GS-1:一般セッション(1)医療)

511 多軸性脊椎固定用スクリューのLocking Systemにおける生体力学的な安定性分析に関する研究(GS-1:一般セッション(1)医療)

Posterior vertebrectomy in kyphosis, scoliosis and kyphoscoliosis due to hemivertebra

Vertebrectomy and instrumentation only via the posterior approach has been increasingly used in sagittal, frontal plane and combined deformities. The aim of this retrospective study is to evaluate the clinical and radiological results of hemivertebrectomy and instrumentation only via the posterior approach in congenital spinal deformities. Between 1998 and 2003, we performed hemivertebrectomy and interbody fusion using posterior instrumentation with titanium mesh cage via the posterior approach in 19 patients (three scoliosis, five kyphosis and 11 kyphoscoliosis). The age of the patients ranged from 2 to 22 years and they all underwent hemivertebrectomy (at thoracic level in six patients, at thoracolumbar level in eight patients and at lumbar level in five patients). A titanium mesh cage was used for anterior column support and interbody fusion in patients who had residual anterior gap preventing bone-to-bone contact. Correction and stabilization were achieved by posterior polyaxial pedicle screws. Follow-up was an average of 4.6 years (range: 2-7 years). We did not confront any loss of correction, pseudoarthrosis, and titanium mesh cage collapse or implant failure. Hemivertebrectomy and instrumentation via the posterior approach is a good one-stage surgical treatment option that can be used to avoid the surgical trauma and morbidity related to anterior surgery. It is a technically demanding surgical procedure, however, requiring extreme care and experience in spine surgery.

Flexible non-fusion scoliosis correction systems reduce intervertebral rotation less than rigid implants and allow growth of the spine: a finite element analysis of different features of orthobiom™

The orthobiom non-fusion scoliosis correction system consists of two longitudinal rods, polyaxial pedicle screws, mobile and fixed connectors and a cross-connector. The mobile connectors can move along and around the rod, thus allowing length adaptation during growth. The aim of this study was to determine the effects of different features of this novel implant on intervertebral rotations, to calculate the movement of the mobile connectors along the rods for different loading cases and to compare the results with those of a rigid implant construct. A finite element analysis was performed using six versions (M1-M6) of a three-dimensional, nonlinear model of a spine ranging from T3 to L2. The models were loaded with pure moments of 7.5 N m in the three main anatomical planes. First, the validated intact model (M1) was studied. Then, the orthobiom implant system was inserted, bridging the segments between T4 and L1 (M2). The effect of pedicle screws only in every second vertebrae was investigated (M3). For comparison, three connection variations of screws and rods were investigated: (1) an implant with rigid screws and mobile connectors (M4), (2) an implant with non-locking polyaxial screws and fixed connectors (M5) and (3) a completely rigid implant construct (M6). For flexion, extension and lateral bending, intervertebral rotation was reduced at all implant levels due to the implants. A rigid implant construct (M6) and an implant with non-locking polyaxial screws and fixed connectors (M5) led to the strongest reduction of intervertebral rotation. The orthobiom non-fusion implant system (M2, M3) allowed much more intervertebral rotation than a rigid implant (M6). Differences in intervertebral rotations were small when polyaxial screws were placed at every second level only (M3) instead of at every level (M2). For axial rotation, intervertebral rotation was strongly reduced by a rigid implant construct (M6) and by an implant with rigid screws and mobile connectors (M4). For rotation, an implant with non-locking polyaxial screws (M2, M3, M5) led to nearly the same intervertebral rotations as in an intact spine without an implant (M1). The predicted maximum translation of the mobile connectors along the rod was 4.2 mm for extension, 3.1 mm for lateral bending, 1.6 mm for flexion and 0.8 mm for axial rotation. The movement of the connectors was highest for those closest to the ends of the rods. With rigid screws, the maximum translation was significantly reduced. This study, conducted under a load-controlled loading protocol, showed that intervertebral rotation was reduced much less by the non-fusion orthobiom system than by a rigid implant. The mobile connectors moved considerably along the rod when the spine was bent. It can be expected that the connectors also move along the rod as the adolescent grows, possibly leaving the discs intact until the patient is fully grown.

Biomechanical effects of polyaxial pedicle screw fixation on the lumbosacral segments with an anterior interbody cage support

BackgroundLumbosacral fusion is a relatively common procedure that is used in the management of an unstable spine. The anterior interbody cage has been involved to enhance the stability of a pedicle screw construct used at the lumbosacral junction. Biomechanical differences between polyaxial and monoaxial pedicle screws linked with various rod contours were investigated to analyze the respective effects on overall construct stiffness, cage strain, rod strain, and contact ratios at the vertebra-cage junction.MethodsA synthetic model composed of two ultrahigh molecular weight polyethylene blocks was used with four titanium pedicle screws (two in each block) and two rods fixation to build the spinal construct along with an anterior interbody cage support. For each pair of the construct fixed with polyaxial or monoaxial screws, the linked rods were set at four configurations to simulate 0°, 7°, 14°, and 21° lordosis on the sagittal plane, and a compressive load of 300 N was applied. Strain gauges were attached to the posterior surface of the cage and to the central area of the left connecting rod. Also, the contact area between the block and the cage was measured using prescale Fuji super low pressure film for compression, flexion, lateral bending and torsion tests.ResultsOur main findings in the experiments with an anterior interbody cage support are as follows: 1) large segmental lordosis can decrease the stiffness of monoaxial pedicle screws constructs; 2) polyaxial screws rather than monoaxial screws combined with the cage fixation provide higher compression and flexion stiffness in 21° segmental lordosis; 3) polyaxial screws enhance the contact surface of the cage in 21° segmental lordosis.ConclusionPolyaxial screws system used in conjunction with anterior cage support yields higher contact ratio, compression and flexion stiffness of spinal constructs than monoaxial screws system does in the same model when the spinal segment is set at large lordotic angles. Polyaxial pedicle screw fixation performs nearly equal percentages of vertebra-cage contact among all constructs with different sagittal alignments, therefore enhances the stabilization effect of interbody cages in the lumbosacral area.

The Effects of Rod Contouring on Spinal Construct Fatigue Strength

In vitro fatigue loading using a corpectomy model outfitted with posterior pedicle screw instrumentation. The purpose of this study was to detect differences in fatigue resistance of titanium and stainless steel spinal constructs that use rods contoured using a French Bender, and to compare differences in fatigue resistance of contoured and straight titanium rods. Instrumentation failure is generally thought to be caused by fatigue or cyclic loading. Intraoperative contouring of the posterior rods is almost always required to match the native kyphotic (thoracic) or lordotic (cervical or lumbar) spinal curvature. How bending these rods affects their overall fatigue resistance is not well described. In addition, changes in fatigue resistance may be a function of material type. Spinal constructs were evaluated using the ASTM F1717-01 model. Two different titanium-based rods (Ti6AL4V and CpTi) and two different steel-based rods (Orthinox and 316L stainless steel) were evaluated in this study (n = 6 for each group). Rods were contoured at two points using a French Bender and were rigidly coupled to polyaxial pedicle screws within UHMWPE vertebral bodies. Constructs were cycled at a load ratio of 10 between a minimum and maximum loading regime of -250 N/-25 N and -700 N/-70 N at a frequency of 4 Hz. Estimated maximum nominal stresses at various points of interest in the spinal constructs were calculated using beam theory. Effects of the rod material, load, and stress on the number of cycles to failure were analyzed using Cox proportional hazards regression. All of the spinal constructs with contoured CpTi rods and contoured Ti6Al4V rods failed at one of the bends in the rods. Almost all of the spinal constructs with straight CpTi rods and straight Ti6Al4V rods failed where the blocker screw fastens the rod to the coupler of the polyaxial screw head. Contoured titanium constructs demonstrated significantly lower fatigue life than contoured 316L constructs. Contouring tended to lower the fatigue life of both the Ti6Al4V and CpTi constructs. Intraoperative rod contouring using a French Bender significantly reduces the fatigue life of titanium spinal constructs.

Biomechanical Comparison of Anatomic Trajectory Pedicle Screw versus Injectable Calcium Sulfate Graft-Augmented Pedicle Screw for Salvage in Cadaveric Thoracic Bone

Many salvage options for failed thoracic pedicle screws exist including the use of a different trajectory or the augmentation of the screw with polymethylmethacrylate cement. Although polymethylmethacrylate immediately increases the construct stiffness and the pull-out strength, it may cause bone necrosis, toxin relaxation, and/or neural injury. On the other hand, calcium sulfate bone grafts have a high potential for biologic incorporation and no thermal damage effect. In the current study, polyaxial pedicle screws were first inserted with a straightforward approach on both sides in 17 fresh human cadaveric thoracic vertebrae. The maximal insertion torque for each screw was measured and then the pull-out strengths were recorded. Afterward, these pedicle screws were randomly assigned to be replaced either by graft augmentation or by anatomic trajectory technique for salvage. The graft-augmented screws were placed using the previous holes. The maximum insertional torque for each anatomic trajectory screw was measured. Finally, the pull-out strengths of the revision screws were recorded. The mean maximum insertional torque decreased with the anatomic trajectory salvage technique when compared with the straightforward approach, 0.23 versus 0.38 Nm, respectively (P=0.003). The anatomic trajectory revision resulted in decreased pull-out strength when compared with the pull-out strength of the straightforward technique, 297 versus 469 N, respectively (P=0.003). The calcium sulfate graft augmentation increased the pull-out strength when compared with the pull-out strength of the straightforward technique, 680 versus 477 N, respectively (P=0.017). The mean pull-out strength ratio of revised screw to original was 0.71 for anatomic trajectory and 1.8 for graft-augmented screws, a statistically significant difference (P=0.002).

Hydroxyapatite Dip Coated and Uncoated Titanium Poly-Axial Pedicle Screws: An In Vivo Bovine Model

A 1-year-old calf was implanted with titanium pedicle screws either uncoated (n = 7) or coated with hydroxyapatite (n = 7) by the dipping method on the pedicles of vertebrae (t10-l3). To evaluate biomechanical and histomorphologic responses to titanium pedicle screws uncoated and coated with hydroxyapatite. Failure of fixation caused by loosening of pedicle screws is a problem in spinal surgery. Enhancement of the fixation ability of screws by coating with biocompatible materials may improve prognosis of surgery. The calf was euthanized 4 months after implantation for determination of insertion and extraction torques for screws, and histologic and scanning electron microscopic examinations of areas screw embedded. Insertion torques did not differ by the kind of screws (99 +/- 5.7 Ncm). However, extraction torques for screws coated with hydroxyapatite were higher than for screws uncoated (249 vs. 133 Ncm, P < 0.01). As compared with uncoated screws, implanted areas for hydroxyapatite-coated screws were denser, had more cracks, and provided better bonding. Fibrous tissue and new bone formation were observed around the areas of uncoated and hydroxyapatite-coated screws embedded, respectively. Hydroxyapatite coating of titanium pedicle screws by the dipping method improved fixation and vertebral bone-implant interface, suggesting a decreased risk of a screw-loosening problem.

Unstable upper and middle thoracic fractures. Preliminary experience with a posterior transpedicular correction-fixation technique

A number of conservative and operative approaches have been described for the treatment of unstable traumatic upper and middle thoracic fractures. The advantage of surgical correction and fixation/fusion lies in its potential to restore sagittal and coronal alignment, thereby indirectly decompressing the spinal cord. A consecutive series of 8 patients with unstable traumatic upper and middle thoracic fractures is reviewed. In all patients, polyaxial pedicle screws were inserted bilaterally into the two levels above and below the fracture. Rods that were less contoured ("undercontoured") than the regional hyperkyphosis at the injured level, were anchored to the caudal four screws. The cranial four screws, with the vertebrae to which they were inserted, were then progressively pulled posteriorly onto the undercontoured rods with rod reducers, thus correcting the hyperkyphosis and anterolisthesis. The mean follow-up was 15 months. The mean regional kyphosis was 23 degrees preoperatively, 17 degrees postoperatively and 18 degrees at follow-up. The mean anterolisthesis was 8 mm preoperatively, 1 mm postoperatively and 1 mm at follow-up. No hardware failure occurred. Five patients with complete spinal cord injury at presentation made no neurological recovery, two patients with incomplete spinal cord injury initially (ASIA B), recovered substantially (to ASIA D), and the patients who were neurologically intact at presentation remained so.

Effects of polyaxial pedicle screws on lumbar construct rigidity.

Pedicle screw constructs have been shown to increase fusion rates in the lumbar spine. Manufacturers have created pedicle screws with one or two degrees of freedom built into the screw head to allow for easier incorporation of the interlocking rod, but the effects of these screws on construct stiffness has not been tested. The purpose of this study is to compare and contrast the stiffness of lumbar pedicle screw constructs with and without the use of polyaxial pedicle screws. Nontapered, self-taping pedicle screws (6.0-mm diameter x 30-mm length, titanium) were used in the fixation of porcine spines from L3-L5. Group 1 (n = 5) contained six standard pedicle screws from one manufacturer. Group 2 (n = 5) contained six standard pedicle screws from a second manufacturer. Group 3 (n = 5) contained four standard pedicle screws placed at L3 and L5, as well as two polyaxial screws placed at L4. Group 4 (n = 5) contained six polyaxial pedicle screws. A rotational variable differential transformer was used to record angular displacement between vertebrae in the construct as it is loaded in flexion, extension, right bend, left bend, clockwise torque, and counterclockwise torque. Stiffness curves were linear throughout the range of applied force. The average r2 value for the generated stiffness graphs was 0.94 (SD = 0.06). No construct failure occurred during any of the testing. There were no significant differences (p < 0.05, two-way analysis of variance) in moment versus angle noted in any of the four groups tested. For torque tests, the all-polyaxial screw constructs showed significantly increased stiffness compared with the other groups. The current study has shown that the incorporation of polyaxial screws in pedicle screw constructs did not significantly decrease the construct stiffness. There is a suggestion that the use of all polyaxial screws may increase the resistance to torque by allowing better purchase of intervertebral rods.