Product Notes and Case Studies

Physical Characteristics of Polyaxial-Headed Pedicle
Screws and Biomechanical Comparison of Load With
Their Failure

Guy R. Fogel, MD · Charles A. Reitman, MD
Weiqiang Liu, PhD · Stephen I. Esses, MD
SPINE Volume 28, Number 5, pp 470–473
© 2003, Lippincott Williams & Wilkins, Inc.

Polyaxial heads have made the pedicle screw more versatile, particularly improving ease of connecting rod application. Clinically, the senior author has observed that the incidence of broken pedicle screws has diminished over the past several years coincidentally with the usage of polyaxial screws. A hypothesis was developed to explain the decreased breakage of pedicle screws. It may be that there is a subtle loosening or failure of the polyaxial head that removes some of the stress from the pedicle screw. By decreasing the stiffness in the coupling of the polyaxial head to the pedicle screw, the bending stresses on the pedicle screw would be lessened. A MEDLINE search showed no biomechanical evidence in the English spine literature regarding the testing of polyaxial screws. In addition, polyaxial-headed pedicle screw load-to-failure testing has not been subjected to a comparison study.

Methods

Nine pedicle screw systems were evaluated: the Silhouette (Sulzer Spine-Tech, Minneapolis, MN), Blackstone (Blackstone Medical, Springfield, MA), Click-X (Synthes, Paoli, PA), Xia (Stryker-Howmedica, Warsaw, IN), M8 (Sofamor-Danek, Memphis, TN), Miami-MOSS, Monarch, and Magnum (Depuy-AcroMed, Cleveland, OH), and the SD-90 (Surgical Dynamics, Memphis, TN). Not all of the commercially available pedicle screws were available for testing. The screw lengths were standardized at 45 mm, and the diameters varied from 6 to 7.5 mm.

Each tested screw was mounted perpendicularly on the appropriate rod provided by the vendor at the manufacturer’s recommended torque settings. The distal half of the screw body was potted in the shape of a ceramic cylinder to enhance contact with the MTS machine. Testing was performed with a materials testing system (Model 1321; Instron, Canton, MA) (Figure 1). The MTS force was applied at a point 30 mm from the rod perpendicular to the long axis of the screw. The MTS compressed the screw at a load rate of 100 N/second until failure occurred. The load failure value was the force at which the pedicle screw initially deflected or uncoupled from the polyaxial head.

Results


Figure 1. Setup of the Instron MTS. The pedicle screw is encased in a ceramic cylinder, and the rod construct is held in a large chuck in a vise. The screw then is compressed at a load rate of 100 N/second until failure occurs.
The geometry of the coupling between the screw and head shows a conforming hemispherical interface that allows for polyaxial motion of the head on the screw. The fixation of the polyaxial head to the rod is with an internal screw, external nut, or both, pushing the rod into the slot of the head. The Silhouette has an external nut securing the rod into the head of the screw, whereas the Miami MOSS and the Magnum have an inner screw head and an external nut both securing the rod into the screw head. The other five systems have an internal screw device securing the rod to the pedicle screw head. To gain final fixation, all the screws and nuts require torque to specified levels except the Surgical Dynamics SD-90. The SD-90 has a helical wedge that requires only a 90° turn of the inner screw to gain the final fixation. The Monarch also has a dovetail top with a center screw that locks the head to the rod. The Silhouette had the lowest mean failure load of 213 N, whereas the Magnum was the highest at 486 N. The full results from load-to-failure testing of the pedicle screws are shown in Table 1. Figure 2 shows that the statistical differences ranged from a P of 0.13 to a P of 0.0009. The statistical power was limited in some cases by the small sample size.

Discussion

Pedicle screw systems have undergone continual modifications over the past several years. As recently as 1 to 2 years ago, there were substantial differences in the design features of these sets. However, the systems have evolved to match the strengths of each other such that there currently are very few differences between instrument sets (Table 2). There have been design changes in the screws themselves. The crosslink systems have been updated, and there currently is a wide selection of screw diameters, lengths, and sizes of cross-links. Two of the systems do offer more than one rod diameter.

The results from the load-to-failure testing of the polyaxial pedicle screws demonstrates that the weakest point of the construct is the head-to-screw coupling (Table 1 and Figure 2). This failure of the polyaxial head may be a protective factor for the pedicle screw shaft, preventing early breakage.

The polyaxial head has three tasks: 1) to secure the rod to the head, 2) to prevent the head from deforming in diameter, and 3) to secure the polyaxial head to the pedicle screw. The outside nut, pin-nut, helical or dovetail wedges, and thicker walled polyaxial head are designed to prevent the head from deforming. The inner screw locks the rod in the head and the head to the screw. The helical wedge actually can do all three tasks with one locking device. All pedicle screw heads have some method to stabilize the diameter of the head and a method to hold the rod in the head. The single outside nut–locking mechanism was statistically weaker than any other design.

Biomechanical tests of pedicle screw constructs have demonstrated the fundamental importance of the bone implant interface, bone density, and screw pullout strength.4,5,7,9,11,12,15,16 The essential need for fit and fill of the screw in the isthmus of the pedicle has been proved.7,15 The direct relation between pullout strength and insertional torque has been well demonstrated,5,6 and the fundamental improvement in pullout strength obtained by cross-linking has been documented.14 The stabilizing influence of using converging screws has been shown.1 The major diameter of a pedicle screw has been shown to control pullout strength.2,3 Bicortical purchase increased pullout strength fundamentally both in individual vertebrae and in the sacrum.

Figure 2. Comparison of load-tofailure data for nine pedicle screw systems.

However, bicortical purchase, except at the sacrum, has not been widely adopted by surgeons because of the risk for vascular injury.8,10,13 Nevertheless, bicortical sacral purchase has been proved extremely safe and has gained widespread acceptance.8,10,13

Biomechanical testing of the load characteristics of the polyaxial pedicle screw was not reported in the literature reviewed for this study. This testing has further detailed the biomechanical characteristics of the polyaxial pedicle screw. The results of this testing demonstrate a range of load tolerances for the various systems. Although the testing does show that some polyaxial pedicle screws are stronger, it is important to note that it is not clear whether a stronger coupling of pedicle screw to head is better. There may be instances in which more or less rigid fixation is preferable. Further study is needed to investigate the physical properties and their clinical application.

Conclusion

Biomechanical pedicle screw load-to-failure data demonstrated that the polyaxial head coupling to the screw was the first failure point andmaybe a protective feature of the pedicle screw and rod, preventing pedicle screw or rod breakage. Knowing the physical characteristics of the available pedicle screw instrumentation systemsmayallow the choice of pedicle screw best suited for a given clinical situation.

Key Points

  • The polyaxial head coupling of the pedicle screw is the first feature to fail.
  • This may protect the pedicle screw from breaking.
  • There is a wide range of pedicle screw construct failure loads.

References

  1. Barber JW, Boden SD, Ganey T, et al. Biomechanical study of lumbar pedicle screws: Does convergence affect axial pullout strength? J Spinal Disord 1998; 11:215–20.
  2. Daftari TK, Horton WC, Hutton WC. Correlations between screw hole preparation, torque of insertion, and pullout strength for spinal screws. J Spinal Disord 1994;7:139–45.
  3. Hirano T, Hasegawa K, Takahashi HE, et al. Structural characteristics of the pedicle and its role in screw stability. Spine 1997;22:2504–9, discussion 10.
  4. Kostuik JP, Munting E, Valdevit A. Biomechanical analysis of screw load sharing in pedicle fixation of the lumbar spine. J Spinal Disord 1994;7:394– 401.
  5. Liljenqvist U, Hackenberg L, Link T, et al. Pullout strength of pedicle screws versus pedicle and laminar hooks in the thoracic spine. Acta Orthop Belg 2001;67:157–63.
  6. Lill CA, Schlegel U, Wahl D, et al. Comparison of the in vitro holding strengths of conical and cylindrical pedicle screws in a fully inserted setting and backed out 180 degrees. J Spinal Disord 2000;13:259–66.
  7. Lim TH, Eck JC, An HS, et al. Biomechanics of transfixation in pedicle screw instrumentation. Spine 1996;21:2224–9.
  8. Lu WW, Zhu Q, Holmes AD, et al. Loosening of sacral screw fixation under in vitro fatigue loading. J Orthop Res 2000;18:808–14.
  9. McCormack T, Karaikovic E, Gaines RW. The load sharing classification of spine fractures. Spine 1994;19:1741–4.
  10. Mirkovic S, Abitbol JJ, Steinman J, et al. Anatomic consideration for sacral screw placement. Spine 1991;16:S289–94.
  11. Oktenoglu BT, Ferrara LA, Andalkar N, et al. Effects of hole preparation on screw pullout resistance and insertional torque: A biomechanical study. J Neurosurg 2001;94:91–6.
  12. Ono A, Brown, MD, Latta LL, et al. Triangulated pedicle screw construct technique and pullout strength of conical and cylindrical screws. J Spinal Disord 2001;14:323–9.
  13. Robertson PA, Plank LD. Pedicle screw placement at the sacrum: Anatomical characterization and limitations at S1. J Spinal Disord 1999;12:227–33.
  14. Suzuki T, Abe E, Okuyama K, et al. Improving the pullout strength of pedicle screws by screw coupling. J Spinal Disord 2001;14:399–403.
  15. Tencer AF, Hampton D, Eddy S. Biomechanical properties of threaded inserts for lumbar interbody spinal fusion. Spine 1995;20:2408–14.
  16. Vaccaro AR, Garfin SR. Pedicle screw fixation in the lumbar spine. J Am Acad Orthop Surg 1995;3:263–74.