The effect of arthrodesis, implant stiffness, and time on the canine lumbar spine.
METHODS
Canine posterior lumbar instrumentation and fusion.


OBJECTIVE
To study effects of implant rod size and time on the stiffness of related spine construct elements.


BACKGROUND
The ideal stiffness of posterior spinal implants to successfully treat clinical instability or deformity with minimal side effects is unknown.


METHODS
Twenty-six canines were divided into 7 groups: control, and 6 or 12-month survival after sham or lumbar L3-5 arthrodesis (facet, posterior, and posterolateral) with either 4.76 or 6.35 mm diameter rod-pedicle screw instrumentation. Axial flexion-compression stiffness of the L3-5 segment components and axial compression stiffness of the bypassed and adjacent anterior column elements were measured.


RESULTS
Posterior instrumentation initially increased flexion-compression stiffness of the L3-5 segment more than the intrinsic stiffness of the implant due to control of spinal column flexion buckling. Sham operation did likewise, apparently by posterior scar tissue tethering. The percent contribution of the implant construct to instrumented segment stiffness was significantly less at 6 months without further change from 6 to 12 months; 14% and 22% for 4.76 and 6.35 mm rod constructs, respectively. Spinal column as well as posterior column stiffness after fusion was independent of rod size at 6 months and increased at 12 months in only the 4.76 mm rod group. Bypassed L4 vertebral body stiffness decreased significantly at 6 months, was not rod size dependent and changed little between 6 and 12 months. Bypassed disk stiffness responded in a biphasic manner, apparently increasing at 6 months with significant decrease from 6 to 12 months. Adjacent disk compression stiffness progressively decreased over time independent of rod size, also decreasing after sham operation.


CONCLUSIONS
Both rod sizes were associated with 100% fusion and produced similar changes in bypassed bone and disks, and adjacent disks. There was delayed fusion stress shielding by 6.35 mm rod constructs.
Department of Orthopedic Surgery, Kansas University Medical Center, Kansas City, KS 66160-7387, USA. masher@kumc.edu
Marc A
MA
Asher
William L
WL
Carson
James W
JW
Hardacker
Richard G
RG
Lark
Sue Min
SM
Lai
D000818:Animals; D001863:Bone Screws; D004285:Dogs; D005260:Female; D008159:Lumbar Vertebrae / Q000601:surgery*; D008297:Male; D013123:Spinal Fusion / Q000295:instrumentation* / Q000379:methods*; D013997:Time Factors; D016896:Treatment Outcome