Insertion loads of the X STOP interspinous process distraction system designed to treat neurogenic intermittent claudication

Abstract

An interspinous process implant has been developed to treat patients suffering from neurogenic intermittent claudication secondary to lumbar spinal stenosis. As most patients who suffer from spinal stenosis are over the age of 50 and may have weaker bones, it is imperative to know how bone mineral density (BMD) correlates with lateral spinous process strength. The study was undertaken to characterize the lateral failure loads of the spinous process, correlate the failure loads to BMD, and compare the failure loads to the loads required to insert an interspinous process implant. Spinous process lateral failure loads were assessed, correlated to BMD, and compared to the loads required to insert an interspinous process implant. Mean spinous process failure loads were significantly greater than the lateral insertion load of the interspinous process implant. There was a significant relationship between the BMD and spinous process failure load. The technique used to insert the interspinous implant poses little risk to spinous process failure. There is ample margin of safety between the insertion loads and spinous process failure loads. The significant relationship between BMD and spinous process failure load suggests that patients with lower BMD must be approached with more caution during the implant insertion procedure.

Fig. 1

A schematic of the X STOP interspinous implant

Fig. 2

Schematic of the procedure used to measure the insertion load of the X STOP. a The X STOP is placed above the interspinous space. b The crosshead of the axial loading frame is advanced until the implant is placed between the spinous processes

Fig. 3

A schematic of the three loading positions used to test the lateral failure strength of the spinous processes. The processes randomized into the Cranial group were contacted by the implant at 75% of spinous process height from the caudal surface. The Middle and Caudal groups were contacted by the implant at 50% and 25% of the spinous process height from the caudal surface respectively

Fig. 4

The mean insertion load of the X STOP and lateral failure loads of the spinous processes

Fig. 5

The relationship between lateral spinous process failure load and bone mineral density (BMD). The load increases with the square of BMD. The solid horizontal line represents the mean insertion load (65.6 N), the adjacent dashed lines represent ± one standard deviation (46.2 N)