Anterior vertebral body screw pullout testing. A comparison of Zeilke, Kaneda, Universal Spine System, and Universal Spine System with pullout-resistant nut

STUDY DESIGN

A biomechanical study of pullout of anteriorly implanted screws in cadaveric vertebral bodies.

OBJECTIVES

To investigate and compare the pullout strength of the Zielke, Kaneda, Universal Spine System (USS) pedicle screw, and USS pedicle screw with a new pullout-resistant nut.

SUMMARY OF BACKGROUND DATA

A common problem with anterior purchase regardless of the implant system is screw pullout at the proximal and distal ends of multilevel constructs. There is limited information on a solution to this problem.

METHODS

The L1 to L4 vertebral bodies from four cadavers had one each of Zielke and Kaneda pedicle screws (Acromed Corp., Cleveland, OH), USS pedicle screw (Synthes Spine, Paoli, PA), and USS pedicle screw with pullout-resistant nut implanted transversely across the center of the vertebral body with bicortical purchase in a similar fashion as would be used clinically. The screws were extracted using a servohydraulic material testing system. The maximum axial forces were recorded.

RESULTS

The Zielke and Kaneda screws had no significant difference in mean pullout strength (P = 0.542). The USS screw alone was less strong (P = 0.009). The USS screw and pullout-resistant nut increased the pullout strength by twofold (P = 0.00006). In the screw pullout tests, the mode of failure was at the screw thread's interface. The USS screw and pullout-resistant nut failed by imploding the body around the nut. With the USS screw and pullout-resistant nut, the pullout strength was determined by the compressive strength of the bone.

CONCLUSIONS

The addition of a pullout-resistant nut to an anterior vertebral body screw improves the pullout strength by twofold and changes the mode of failure to rely ultimately on the inherent vertebral body strength rather than the screw's characteristics. The addition of a pullout-resistant nut may be applicable to multilevel implant constructs to prevent screw pullout at the top and bottom.

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

STUDY DESIGN

Insertion torque and pull-out strengths of conical and cylindrical pedicle screws were compared in human cadaveric vertebral bodies.

OBJECTIVES

To compare the performance of the conical design with the cylindrical design, and to determine whether insertional torque correlates with pull-out strength.

SUMMARY OF BACKGROUND DATA

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.

METHODS

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.

RESULTS

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.

CONCLUSIONS

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.

The strength of iliosacral lag screws and transiliac bars in the fixation of vertically unstable pelvic injuries with sacral fractures

This study compares the relative strengths of iliosacral lag screws and transiliac bars in the fixation of vertically unstable pelvic injuries with sacral fractures. A vertical sacral fracture was artificially induced by vertical loading in eight pelvises from cadavers, which were then fixed with two 6.5 mm iliosacral lag screws or two 6.4 mm transiliac bars. The pelvises were then loaded again to failure. The mean strength of iliosacral lag screw fixation was 819 newtons and for transiliac bars it was 1066 newtons, but the study was too small for the difference to be judged as statistically significant. Various advantages and disadvantages of each method of fixation are discussed.

Biomechanical analysis of hemipelvic deformation after corticospongious bone graft harvest from the posterior iliac crest

STUDY DESIGN

The stiffness of the hemipelvis during simulated physiologic loading and the bone deformation in the remaining posterior ilium after the harvest of increasing sizes of corticospongious bone graft were compared with values in intact bone.

OBJECTIVES

To quantify the biomechanical effects of the removal of bone grafts from the posterior ilium and to relate the size of graft removed to the stiffness of the hemipelvis and deformation of the remaining bone.

SUMMARY OF BACKGROUND DATA

Bone fractures and pelvic instability have been reported to complicate graft harvest from the posterior iliac crest. There is no quantitative data relating graft size to the mechanical properties of the remaining ilium.

METHODS

Seven cadaveric hemipelves were loaded with a materials testing machine through the superior sacrum while supported at the acetabulum and stabilized with a cable fixed to the ilium. Force and displacement histories and deformation in the greater sciatic notch were recorded for the intact bone and after removal of corticospongious bone graft in 1.5-cm increments from the posterior iliac crest.

RESULTS

If the length of the removed corticospongious bone graft exceeded 3.0 cm, the stiffness of the posterior pelvic ring decreased, and deformation in the remaining bone increased substantially.

CONCLUSIONS

Removal or bone graft in excess of 3 cm from the posterior ilium increases the risk of iatrogenic fatigue fracture.