Biomechanics of Nested Transforaminal Lumbar Interbody Cages.
BACKGROUND
Arthrodesis is optimized when the structural graft occupies most of the surface area within a disc space. The transforaminal corridor inherently limits interbody size.


OBJECTIVE
To evaluate the biomechanical implications of nested interbody spacers (ie, a second curved cage placed behind a first) to increase disc space coverage in transforaminal approaches.


METHODS
Seven lumbar human cadaveric specimens (L3-S1) underwent nondestructive flexibility and axial compression testing intact and after transforaminal instrumentation at L4-L5. Specimens were tested in 5 conditions: (1) intact, (2) interbody, (3) interbody plus bilateral pedicle screws and rods (PSR), (4) 2 nested interbodies, and (5) 2 nested interbodies plus PSR.


RESULTS
Mean range of motion (ROM) with 1 interbody vs 2 nested interbodies, respectively, was: flexion, 101% vs 85%; extension, 97% vs 92%; lateral bending, 127% vs 132%; and axial rotation, 145% vs 154%. One interbody and 2 nested interbodies did not differ significantly by loading mode (P > .10). With PSR, ROM decreased significantly compared with intact, but not between interbody and interbody plus PSR or 2 interbodies plus PSR (P > .80). Mean vertical height during compressive loading (ie, axial compressive stiffness) was significantly different with 2 nested interbodies vs 1 interbody alone (P < .001) (compressive stiffness, 89% of intact vs 67% of intact, respectively).


CONCLUSIONS
Inserting a second interbody using a transforaminal approach is anatomically feasible and nearly doubles the disc space covered without affecting ROM. Compressive stiffness significantly increased with 2 nested interbodies, and foraminal height increased. Evaluation of the clinical safety and efficacy of nested interbodies is underway.
*Division of Neurological Surgery, Barrow Neurological Institute, St. Joseph's Hospital and Medical Center, Phoenix, Arizona; ‡Division of Neurological Surgery, Spine Group Arizona, HonorHealth Research Institute, Scottsdale, Arizona.
Hector
H
Soriano-Baron
Anna G U S
AG
Newcomb
Devika
D
Malhotra
Kaylee
K
de Tranaltes
Eduardo
E
Martinez-Del-Campo
Phillip M
PM
Reyes
Neil R
NR
Crawford
Nicholas
N
Theodore
Luis M
LM
Tumialán
D000368:Aged; D001696:Biomechanical Phenomena / Q000502:physiology; D001863:Bone Screws / Q000592:standards; D005260:Female; D006801:Humans; D016268:Internal Fixators / Q000592:standards; D008159:Lumbar Vertebrae / Q000000981:diagnostic imaging / Q000601:surgery*; D008297:Male; D008875:Middle Aged; D018583:Pliability / Q000502:physiology*; D011187:Posture / Q000502:physiology; D011859:Radiography; D016059:Range of Motion, Articular / Q000502:physiology; D012399:Rotation; D013123:Spinal Fusion / Q000295:instrumentation / Q000379:methods