Pullout strength for cannulated pedicle screws with bone cement augmentation in severely osteoporotic bone: influences of radial hole and pilot hole tapping.
BACKGROUND
Pedicle screw fixation in a severely osteoporotic spine remains a challenge for orthopedic surgeons. The previous literature does not adequately address the effects of radial holes for cannulated screws with cement injection and pilot hole tapping on the bone/screw interfacial strength.


METHODS
Specially designed cannulated pedicle screws, with or without radial holes, were installed in tapped and untapped pilot holes and then injected with cement. A uniform synthetic bone (test block) was used to provide a platform for each screw design. Specimens with inserted screws were then tested for axial pullout failure.


RESULTS
(1) Cannulated screws with cement augmentation significantly increased the pullout strength in comparison to solid screws. Additionally, the amount of cement exuded from the cannulated screws increased with an increasing number of radial holes, leading to an increase in the average ultimate pullout strength for cannulated screws with a large number of radial holes. (2) Radiological examination indicated that the cement was exuded from the most proximal holes at the very beginning of its flow path, whereas no cement exudation was found at the remaining distal holes. (3) Cement exudation from the holes of cannulated screws into the open cell of the test block led to a composite (cement/bone) structure at the area of cement exudation. Observations of the failed specimens indicated that failure occurred at the composite/bone interface, while the composite was well bonded to the screws. This implies that the screw/composite interfacial strength was much higher than the composite/bone interfacial strength. (4) Tapping pilot holes decreased the pullout strength of the screws. Generally, larger standard deviations were found for the tapped cases, implying that untapped cases results are more repeatable than tapped cases results.


CONCLUSIONS
Cannulated pedicle screws with radial holes combined with PMMA cement augmentation but without tapping may be a viable clinical option for achieving fixation in severely osteoporotic bone.
Chen
Lih-Huei
LH
Department of Orthopaedic Surgery, Chang Gung Memorial Hospital, Chang Gung University, Taoyuan, Taiwan. LHCHEN2132@adm.cgmh.org.tw
Tai
Ching-Lung
CL
Lai
Po-Liang
PL
Lee
De-Mei
DM
Tsai
Tsung-Tin
TT
Fu
Tsai-Sheng
TS
Niu
Chi-Chien
CC
Chen
Wen-Jer
WJ
D000268:Adhesiveness; D001843:Bone Cements / Q000737:chemistry*; D001863:Bone Screws; D001842:Bone and Bones / Q000601:surgery*; D017276:Friction; D006801:Humans; D010024:Osteoporosis / Q000601:surgery*; D019919:Prosthesis Implantation / Q000379:methods*
D001843:Bone Cements