Screw omission and the stability of posterior pedicle screw constructs for short-segment stabilization.
To determine the net contribution of a spinal construct to stability, and whether extending the construct to another level in situations in which a defective pedicle cannot have a screw inserted, we performed biomechanical tests in which we evaluated three-, four-, and five-level synthetic spinal constructs in which the location and number of pedicle screws were varied above and below a vertebrectomy defect. We subjected all constructs to axial, compression, lateral bending, flexion, extension, and torsional forces with the use of an Instron biaxial machine. Left-right symmetrical constructs were more stable than asymmetrical ones. Three-level constructs were statistically stiffer than the longer ones in compression, left bending, and flexion. Torsional stability, however, was greater in the longer constructs. Five-level constructs with both end screws in place had greater torsional stiffness than when they were missing a screw. In vertebrectomy defects, if four screws cannot be placed across it, then the engagement of two screws is indicated. The stability provided by a single screw at a spinal level is minimal. Additional screws augment the purchase of the construct in the bone; however, they do not afford further protection to the defect.
Margulies
J Y
JY
Department of Orthopaedic Surgery, Montefiore Medical Center, Bronx, NY 10467, USA.
Casar
R S
RS
Neuwirth
M G
MG
Margulies
S D
SD
Kummer
F J
FJ
D001863:Bone Screws; D004867:Equipment Design; D005069:Evaluation Studies as Topic; D008953:Models, Anatomic; D009038:Motion; D011312:Pressure; D013314:Stress, Mechanical