Is it safe to back out pedicle screws after augmentation with polymethyl methacrylate or calcium phosphate cement? A biomechanical study.
METHODS
Biomechanical cadaveric study.


OBJECTIVE
To determine the torque required to remove pedicle screws augmented with polymethyl methacrylate (PMMA) or calcium phosphate cement (CPC); thus, proving the safety of back out of augmented screws in the osteopotoric model, which would be a more dangerous setting than the nonosteoporotic model.


BACKGROUND
To our knowledge, no earlier study has characterized the safety of backing out pedicle screw augmented with PMMA or CPC.


METHODS
Pedicle screws were inserted in 24 osteoporotic vertebrae (48 pedicles). The maximal insertion torque and pullout strength of each screw were recorded. After pullout of the pedicle screws, the vertebrae were then randomized into 2 groups of 12 (24 pedicles) each. PMMA was injected into the pedicles in the first group and CPC was injected into the second group after which the pedicle screws were inserted. The pedicle screws were inserted into the pedicle holes augmented with PMMA or CPC, respectively. Finally, all augmented screws were backed out and the maximal removal torque was recorded using a digital torque wrench.


RESULTS
Throughout the study, no incidence of pedicle or lamina fractures was observed. The average insertion torque was 0.5±0.27 and 0.45±0.29 N·m for groups 1 and 2, respectively (P=0.724). The average pullout strength was 723.1±391.7 and 671.2±383.0 N (P=0.950). After cement augmentation, the average removal torque was 0.77±0.31 and 0.81±0.26 N·m for PMMA and CPC, respectively (P=0.494).


CONCLUSIONS
The results of this study showed that pedicle screws can be easily and safely backed out after augmentation with PMMA or CPC. The result of CPC, however, may only be valid before any bony ingrowth.
Foundation for the Advancement of Spine Knowledge, Twin Cities Spine Center, Minneapolis, MN 55404, USA. woojinchomd@aol.com
Woojin
W
Cho
Chunhui
C
Wu
Xiujun
X
Zheng
Serkan
S
Erkan
Sanjeev J
SJ
Suratwala
Amir A
AA
Mehbod
Ensor E
EE
Transfeldt
D000368:Aged; D000369:Aged, 80 and over; D001696:Biomechanical Phenomena / Q000502:physiology; D001843:Bone Cements; D001863:Bone Screws / Q000009:adverse effects; D002130:Calcium Phosphates / Q000008:administration & dosage*; D020878:Device Removal / Q000009:adverse effects / Q000379:methods*; D005260:Female; D006801:Humans; D008159:Lumbar Vertebrae / Q000502:physiology / Q000601:surgery; D019904:Polymethyl Methacrylate / Q000008:administration & dosage*; D013904:Thoracic Vertebrae / Q000502:physiology / Q000601:surgery
D001843:Bone Cements; D002130:Calcium Phosphates; D019904:Polymethyl Methacrylate; C020243:calcium phosphate