Pre-tension effects from tightening the ligature on spinous process fracture risk in interspinous process device implantation

Ligatures are used with many types of interspinous process devices to prevent instability in flexion movement by wrapping around upper and lower spinous processes. In this study, the effects of the pre-tension by tightening the ligature on the spinous process fracture were investigated using a finite element method. Five different interspinous process devices, Coflex-F-A (R), Wallis((TM)), Diam((TM)), Viking, and Spear(A (R)), were chosen, and implanted after creating a defect consisting of bilateral facetectomy with a resection of the ligamentum flavum. Flexion and extension motions were simulated with various pre-tension values of the ligatures in the range of 0 to 400 N at 100 N increments. While stability at the surgical level in the flexion increased with increasing pre-tension of the ligatures, the spinous process fracture risk also increased with increasing ligature pre-tension. Furthermore, the fatigue life of the spinous process also decreased. Thus, the amount of pre-tension of the ligatures should be carefully selected depending on the materials and design of implants.