Biomechanical analysis of different techniques in revision spinal instrumentation: larger diameter screws versus cement augmentation.
METHODS
Biomechanical analysis.


OBJECTIVE
To determine the relative strengths of 2 different forms of revision spinal instrumentation using a validated, constant load, cyclic testing mechanism.


BACKGROUND
Spinal fusion with instrumentation procedures are on the rise. As such, so are revision procedures. A few studies have looked at revision instrumentation techniques. Both increased pedicle screw diameter as well as cement augmentation of pedicle screw fixation have been proposed, used clinically and tested biomechanically. To our knowledge, no comparative study exists between these techniques.


METHODS
Using an instron servohydraulic loading machine, we tested pedicle screws inserted in both the anatomic (angled) and Roy-Camille (straight) insertion technique with both larger diameter (8 mm) pedicle screws, as well as standard diameter (6 mm) pedicle screws augmented with polymethylmethacrylate bone cement. Each of these techniques was subjected to constant load under cyclic conditions for 2000 cycles at 2 Hz. Computerized data collection was used at all time points. Comparisons were made between primary instrumentation data (previously published) and large diameter screws for revision. Further comparisons were made between large diameter screws and cement augmented screws.


RESULTS
The larger diameter screws compared with the cement augmented screws showed significant differences in: initial stiffness with straight insertion technique (P < 0.01), stiffness damage with straight insertion technique (P < 0.01), and creep damage with straight insertion technique (P = 0.01). There was also a significant difference between large diameter and primary instrumentation technique all calculated values (P <or= 0.05).


CONCLUSIONS
The larger diameter screws were equivocal or significantly more resilient than the cement augmented standard diameter screws at the strongest of the insertion angles for all values. Since rigidity of the instrumentation construct is one of the very few factors that is surgeon controlled, this could influence the choice of instrumentation in revision spinal arthrodesis.
Kiner
Dirk W
DW
Department of Orthopaedic Surgery, University of Tennessee College of Medicine, Chattanooga, TN 37415, USA. dirkkiner@yahoo.com
Wybo
Christopher D
CD
Sterba
William
W
Yeni
Yener N
YN
Bartol
Stephen W
SW
Vaidya
Rahul
R
D000368:Aged; D001696:Biomechanical Phenomena; D001843:Bone Cements / Q000627:therapeutic use*; D001863:Bone Screws; D005260:Female; D006801:Humans; D008159:Lumbar Vertebrae / Q000601:surgery*; D008297:Male; D008422:Materials Testing; D019904:Polymethyl Methacrylate / Q000627:therapeutic use*; D011474:Prosthesis Design; D011475:Prosthesis Failure; D012086:Reoperation; D013123:Spinal Fusion / Q000295:instrumentation / Q000379:methods*; D013314:Stress, Mechanical
D001843:Bone Cements; D019904:Polymethyl Methacrylate