Preclinical testing of a wedge-rod system for fusionless correction of scoliosis.
METHODS
Biomechanical study.


OBJECTIVE
This paper describes the biomechanical comparison of calf spines instrumented with a wedge alone versus a wedge-rod construct for the fusionless correction of scoliosis.


BACKGROUND
Current techniques for the correction of scoliosis require either anterior or posterior spinal fusion for correction. We propose a technique allowing correction via vertebral body osteotomies along with fixation using a wedge-rod construct without the requirement of intervertebral segment fusion.


METHODS
Calf spines were used to test the biomechanical difference between the intact spine, transverse osteotomized spine with wedge-rod reconstruction, and transverse osteotomized spine with wedge alone reconstruction. Unconstrained segments (L1-L5) were first tested under five nondestructive static loading conditions to evaluate the intact stability of the operative motion segments in axial compression (-600 N), axial rotation (+/-5.0 Nm, 150 axial preload), flexion and extension (+/-5.0 Nm), and lateral bending (+/-5.0 Nm). Following the intact analysis, vertebral wedge osteotomies in the transverse plane were performed at the L2, L3, and L4 levels. The defects were reconstructed using the Sofamor Danek Wedge Spacer, and stainless steel TSRH one-quarter inch single rod with modified CD HORIZON 6.5-mm diameter vertebral body screws at each level (L2-L4). Standard CD HORIZON 6.5-mm bone screws and staples were used at the superior and inferior ends of the five-level construct. The wedge was on the left side and the rod and screw heads on the right side. After testing the reconstructed specimen, the TSRH rod was removed and the construct retested to evaluate the stability of the wedge alone reconstruction.


RESULTS
Construct stiffness was calculated as the peak applied load (N or Nm) divided by the corresponding segmental displacement (mm or degrees) normalized to the intact specimen. Reconstruction static data are expressed as a percentage change from the intact condition. Statistical analysis included descriptives, a one-way analysis of variance, and the Student-Newman-Keuls test for multiple comparisons among the reconstruction groups. Axial compression: under axial compressive loads, the stiffness of the wedge-rod construct was approximately equal to that of the intact group. The stiffness of the wedge alone construct was 56% less than that of the intact group except for extension and left lateral bending. For the other modes of loading (right rotation, left rotation, flexion, extension, right lateral bending, and left lateral bending), the wedge-rod construct was stiffer than that of the intact group. The stiffness of the wedge alone construct was consistently less than that of the intact group.


CONCLUSIONS
Based on the results of this biomechanical comparison, the calf spines instrumented with the wedge-rod system for fusionless correction were significantly stiffer as compared to the intact calf spine. The wedges alone were not as stiff as the intact spine. This suggests that the theory of performing transverse osteotomies of vertebral bodies with fixation with wedge-rod construct for 8 to 12 weeks, followed by removal of the rod, could provide adequate fixation and correction of a scoliotic deformity without requiring fusion of motion segments.
Betz
Randal R
RR
Shriners Hospitals for Children, Philadelphia, Pennsylvania 19140, USA. rbetz@shrinenet.org
Cunningham
Bryan
B
Selgrath
Christopher
C
Drewry
Troy
T
Sherman
Michael C
MC
D000818:Animals; D001696:Biomechanical Phenomena; D002417:Cattle; D008159:Lumbar Vertebrae / Q000601:surgery; D023421:Models, Animal; D010027:Osteotomy / Q000295:instrumentation / Q000379:methods*; D012399:Rotation; D012600:Scoliosis / Q000473:pathology / Q000601:surgery*; D013131:Spine / Q000601:surgery*