Physical characteristics of polyaxial-headed pedicle screws and biomechanical comparison of load with their failure.
METHODS
Pedicle screw strength or load to failure was biomechanically evaluated, and the geometric characteristics of pedicle screw instrumentation systems were compared.


OBJECTIVE
To compare the features of pedicle screw systems, and to demonstrate the failure point of the polyaxial pedicle screw head.


BACKGROUND
Many pedicle screw instrumentation systems are currently available to the spine surgeon. Each system has its unique characteristics. It is important for the surgeon to understand the differences in these pedicle screw systems. Pedicle screw load to failure has not been subjected to a comparison study.


METHODS
The physical characteristics of each pedicle screw instrumentation system were determined. Features of rods, instruments, and pedicle screws were cataloged. Biomechanical testing of the pedicle screw construct was performed to determine the site and force of the load to failure. Nine pedicle screw systems were evaluated. Testing was performed with a pneumatic testing system under load control. Three polyaxial screws were used for each test at a load rate of 100 N/second. The load failure value was the force at which the pedicle screw or polyaxial head-screw interface initially deflected.


RESULTS
Biomechanical testing demonstrated in all instances that the polyaxial head coupling to the screw was the first failure point. Although there have been subtle design differences in the instruments over time, the features of the pedicle screw instrument sets have become remarkably similar.


CONCLUSIONS
Biomechanical pedicle screw load-to-failure data demonstrated that the polyaxial head coupling to the screw is the first to fail and may be a protective feature of the pedicle screw, preventing pedicle screw breakage. Knowing the physical characteristics of the available pedicle screw instrumentation systems may allow the choice of pedicle screw best suited for a given clinical situation.
Fogel
Guy R
GR
Department of Orthopedic Surgery, Baylor College of Medicine, Houston, Texas 77030, USA.
Reitman
Charles A
CA
Liu
Weiqiang
W
Esses
Stephen I
SI
D001696:Biomechanical Phenomena; D001863:Bone Screws / Q000592:standards*; D004867:Equipment Design; D004868:Equipment Failure; D008422:Materials Testing / Q000379:methods*; D019637:Orthopedic Procedures / Q000295:instrumentation