The effect of pedicle morphometry on pedicle screw loading. A synthetic model.
METHODS
Static nondestructive bending analysis of pedicle screws inserted into vertebral analogues was conducted. Pedicle screw bending load was studied as a function of pedicle morphometry.


OBJECTIVE
To determine how sagittal bending moment in pedicle screws is affected by changes in pedicle height, length, and width.


BACKGROUND
An unexpectedly high rate of clinical failure has been observed in pedicle screws used in short-segment instrumentation for axially unstable fractures. The majority of screws fail in sagittal bending within the pedicle. To date, little is known of the exogenous factors that affect in situ loads incurred by pedicle screws.


METHODS
Synthetic vertebral analogues were fabricated, varying pedicle height, length, or width independently. Pedicle screws internally instrumented with strain gages were used as load transducers to determine screw bending moments within the pedicle and body of the analogue. Analogues were loaded in compression to simulate loading of an unstable burst fracture.


RESULTS
Screw bending moments within the pedicle increased incrementally with increasing pedicle length, rising 30% as length increased from 8.0 mm to 12.0 mm. Screw moment increased 20% when pedicle height dropped below 15.0 mm, consistent with a threshold effect. Changes in pedicle width did not affect screw loads within the pedicle.


CONCLUSIONS
In situ pedicle screw loads increased significantly as pedicle length increased and as pedicle height decreased. Pedicle screws instrumented internally with strain gages are an effective research instrument allowing measurement of in situ loading along the axis of the screw.
McKinley
T O
TO
Department of Orthopaedic Surgery, University of California, Davis, USA.
McLain
R F
RF
Yerby
S A
SA
Sarigul-Klijn
N
N
Smith
T S
TS
D001696:Biomechanical Phenomena; D001863:Bone Screws; D008422:Materials Testing; D008953:Models, Anatomic; D013123:Spinal Fusion / Q000295:instrumentation*; D014520:Urethane
D014520:Urethane