Comparison of expandable and fixed interbody cages in a human cadaver corpectomy model, part I: endplate force characteristics.
OBJECTIVE
Expandable cages are becoming more popular due in large part to their versatility, but subsidence and catastrophic failure remain a concern. One of the proposed reasons of failure is edge loading of the endplate caused by a mismatch between the sagittal alignment of the motion segment and cage. This in vitro analysis investigates the endplate forces characteristic of expandable and fixed interbody cages in a single-level human cadaver corpectomy model.


METHODS
Ten human thoracolumbar spines (T10-L2, L3-5) were biomechanically evaluated following a single-level corpectomy that was reconstructed with an expandable or fixed cage. Fixed cages were deployed with the best-fitting end cap combination, whereas expandable cages were deployed in normal, hypolordotic, and hyperlordotic alignment scenarios. The endplate forces and contact area were measured with a pressure measurement system, and the expansion torque applied by the surgeon was measured with a custom-made insertion device.


RESULTS
The contact areas of the expandable cages were, in general, higher than those of the fixed cages. The endplate forces of the expandable cages were similar to those of the fixed cages in the normal alignment scenario. Higher endplate forces were observed in the hyperlordotic scenario, whereas the endplate forces in the hypolordotic and normal alignment scenarios were similar. There was no correlation with the expansion torque and the final endplate forces.


CONCLUSIONS
Expandable cages resulted in consistently higher contact area and endplate forces when compared with the fixed cages. Because the expansion torque does not correlate with the final endplate forces, surgeons should not rely solely on tactile feedback during deployment of these cages.
Biomechanical Testing Facility, San Francisco General Hospital, San Francisco, California, USA.
Murat
M
Pekmezci
Jessica A
JA
Tang
Liu
L
Cheng
Ashin
A
Modak
R Trigg
RT
McClellan
Jenni M
JM
Buckley
Christopher P
CP
Ames
D000368:Aged; D000369:Aged, 80 and over; D001696:Biomechanical Phenomena / Q000502:physiology; D005260:Female; D006801:Humans; D016268:Internal Fixators; D008159:Lumbar Vertebrae / Q000502:physiology / Q000601:surgery*; D008297:Male; D019736:Prostheses and Implants; D013123:Spinal Fusion / Q000295:instrumentation*; D013904:Thoracic Vertebrae / Q000502:physiology / Q000601:surgery*