50 - Stability of Stand-Alone Cervical Interbody Fusion Cages Over Functional Cervical ROM During Activities of Daily Living

Introduction:
Standalone interbody cervical fusion cages are often used alternatives to anterior plating in anterior cervical discectomy and fusion (ACDF) procedures. Stand-alone cages use integrated screws or blades that allow fixation to adjacent vertebral bodies of the spinal segment. Integrated cages combine the functionality of an interbody spacer with some of the mechanical fixation benefits of an anterior cervical plate, while trying to minimize complications such as dysphagia. Previous studies have documented the stability of stand-alone cervical cages by measuring segmental range of motion (ROM) using maximal motion endpoints before and after cage placement. However, cervical functional ROM required to execute commonly performed activities of daily living (ADL) occurs in a much smaller region than the maximum ROM measured between motion extremes. This study is the first to quantify standalone cage stability over the entire arc of motion between the motion extremes.

Objectives:
To analyze stability of standalone cervical interbody fusion cages over functional ROM in flexion-extension (FE) with and without compressive preload, lateral bending (LB), and axial rotation (AR).

Methods:
6 human cervical spine specimens (age: 36.8±13.2 years) were tested intact and after cage implantation. Two different standalone cervical interbody cages were used, one with integrated screw fixation, the other with blade fixation. Specimens were implanted with both cage types with an equal number of each type at the cranial and caudal level. There were two specimen groups: Group 1 had C4-C5 and C6-C7 standalone cages and Group 2 had C3-C4 and C5-C6 standalone cages. Segmental motion was calculated at 0.05Nm applied moment increments to ±1.5Nm in FE, left- and right-LB, and left- and right-AR. Percent reduction in motion was calculated in all three test modes as [(cage - intact)/intact]x100.

Results:
Maximum stability of the cages occurred within the high flexibility region where large angular motion occurred with minimal applied moments. In flexion without compressive preload, this corresponded to the region between 0Nm and 0.5Nm (1/3 of the maximum flexion moment of 1.5Nm) or approximately 2/3 of the flexion ROM (Figures 1A and 1B). In this region, the cages yielded a flexion-motion restriction of up to 56% versus 34% at maximum ROM (1.5Nm flexion moment). Similar behavior was noted for lateral bending and axial rotation. Integrated cages, in the absence of compressive preload, were ineffective in blocking segmental extension.

Conclusions:
The arthrodesis rate and clinical outcomes of standalone cages are comparable to plated ACDF. Our results demonstrate integrated cages have the highest stability within the high flexibility region, which is where most ADL take place. With increased effort/moment needed to produce full ROM, integrated cage effectiveness in limiting segmental motion substantially decreases. Previous studies show that, patients only use a small percent of their maximum ROM during ADL (median of less than 20%). Our results offer a possible explanation for the positive outcomes of standalone interbody fusion, despite allowing more motion than anterior plating for ACDF. The results may also offer guidelines for ADL during the first weeks after surgery or time to start of physical therapy.