Contact of hydroxyapatite spacers with split spinous processes in double-door laminoplasty for cervical myelopathy

We developed a new type of spacer made of hydroxyapatite (the STSS spacer) for double-door laminoplasty, and evaluated the contact of 93 STSS spacers with the split spinous processes in 20 patients with double-door laminoplasty. Contact was assessed by measuring the extent of touch of the spacer to the spinous processes, classified into four categories based on computed tomography (CT) images: excellent, complete touch on both sides of the spacer to the spinous process; good, complete touch on one side and more than half touch on the other side; fair, more than half touch on both sides; poor, half or less touch on at least one side. Excellent contact was achieved in 65 spacers (69.9%); good, in 13 (14.0%); fair, in 11 (11. 8%); and poor, in 4 (4.3%). The percentages of excellent or good categories were 75.0% at the C3 level, 73.7% at the C4 level, 78.9% at the C5 level, 90.0% at the C6 level, and 100% at the C7 level. The contact rate of the STSS spacer with the spinous process was better than that achieved with other spacers, probably because the characteristic shape of the STSS spacer was compatible with the widened space between the bilateral spinous processes; i.e., it is trapezoidal on both the axial and the frontal sections. However, the appropriate size of the spacer must be selected in accordance with the size of the spinous process to obtain higher percentages of excellent or good contact.