Impact of Mechanical Stability on the Progress of Bone Ongrowth on the Frame Surfaces of a Titanium-Coated PEEK Cage and a 3D Porous Titanium Alloy Cage: in Vivo Analysis Using CT Color Mapping

To determine the impact of mechanical stability on the progress of bone ongrowth on the frame surfaces of a titanium-coated polyether ether ketone (TCP) cage and a three-dimensional porous titanium alloy (PTA) cage following posterior lumbar interbody fusion (PLIF) until 1 year postoperatively. A total of 59 patients who underwent one- or two-level PLIF for degenerative lumbar disorders since March 2015 were enrolled. Bone ongrowth of all cage frame surfaces (four surfaces per cage: TCP, 288 surfaces and PTA, 284 surfaces) was graded by 6-month and 1-year postoperative computed tomography color mapping (grade 0, 0‒25% of bone ongrowth; grade 1, 26‒50%; grade 2, 51‒75%; and grade 3, 76‒100%). Bone ongrowth (≥ grade 1) was observed on 58.0% and 69.0% of the surfaces of TCP and PTA cages 6 months postoperatively and on 63.5% and 75.0% of those 1 year postoperatively, respectively. In the TCP cages, bone ongrowth grade increased from 6 months to 1 year postoperatively only in the union segments (median, 1 [interquartile range, IQR, 0–2] to 1 [IQR, 0–3], p = 0.006). By contrast, in the PTA cages, it increased at 6 months postoperatively in the union (1 [IQR, 1–2] to 2 [IQR, 1–3], p = 0.003) and non-union (0.5 [IQR, 0–2] to 1 [IQR, 0–2.75], p = 0.002) segments. Early postoperative mechanical stability has a positive impact on the progress of bone ongrowth on both the TCP and PTA cage frame surfaces after PLIF.