Is Push-out Testing of Cage Devices Worthwhile in Evaluating Clinical Performance?

Abstract

Interbody cages are successfully used clinically to support the spine's anterior column to facilitate fusion of the motion segment. A standard, “Static Push-Out Test Method for Intervertebral Body Fusion Devices,” is under consideration by ASTM. The objective of this testing was to determine the loads required to extract interbody fusion cages from a simulated spinal motion segment (Grade 20 polyurethane foam model). The results obtained using the foam model were compared to results done using intact cadaver spinal motion segments (thoracic and lumbar). These data are contrasted with incidences of cage expulsion mentioned in the literature. Testing revealed a difference in the expulsion strength between the foam and cadaveric models. Differences between the 13 × 20 mm and the 9 × 20 mm cages were also found. The literature search revealed that posteriorly implanted cages may retropulse into the spinal canal due inadequate annular tension, undersized cages, and destabilization from laminectomy and partial facetectomy. The danger of retropulsion may be alleviated by the addition of posterior instrumentation (rods and screws). Standardized testing is a valuable tool in differentiating among designs but does not represent clinical failures.