Radiologic Assessment of Subsidence in Stand-Alone Cervical Polyetheretherketone (PEEK) Cage

Abstract

Objective: Aim of study was to find a proper method for assessing subsidence using a radiologic measurement following anterior cervical discectomy and fusion (ACDF) with stand-alone polyetheretherketone (PEEK), Solistrade mark cage.

Methods: Forty-two patients who underwent ACDF with Solistrade mark cage were selected. With a minimum follow-up of 6 months, the retrospective investigation was conducted for 37 levels in 32 patients. Mean follow-up period was 18.9 months. Total intervertebral height (TIH) of two fused vertebral bodies was measured on digital radiographs with built-in software. Degree of subsidence (DeltaTIH) was reflected by the difference between the immediate postoperative and follow-up TIH. Change of postoperative disc space height (CT-MRDeltaTIH) was reflected by the difference between TIH of the preoperative mid-sagittal 2D CT and that of the preoperative mid-sagittal T1-weighted MRI.

Results: Compared to preoperative findings, postoperative disc height was increased in all cases and subsidence was observed only in 3 cases. For comparison of subsidence and non-subsidence group, TIH and CT-MRDeltaTIH of each group were analyzed. There was no statistically significant difference in TIH and CT-MRDeltaTIH between each group at 4 and 8 weeks, but a difference was observed at the last follow-up TIH (p=0.0497).

Conclusion: ACDF with Solistrade mark cage was associated with relatively good radiologic long-term results. Fusion was achieved in 94.5% and subsidence occurred in 8.1% by the radiologic assessment. Statistical analysis reveals that the subsidence seen later than 8 weeks after surgery and the development of subsidence does not correlate statistically with the change of the postoperative disc space height.

Keywords: Anterior cervical discectomy and fusion; Cervical PEEK cage; Fusion rate; Radiologic assessment; subsidence.

Fig. 1

Digital radiologic images on which total inter-vertebral height (TIH) of the two fused vertebral bodies. The distance between the mid-point of upper end plate of cranial vertebral body and the mid-point of lower end plate of caudal vertebral body was measured with in-built software (Marosis 5.0 PACS viewer, Marotech, Korea). TIH are measured on the postoperative lateral plain radiograph (A), postoperative mid-sagittal 2D computed tomography (B), preoperative mid-sagittal T1-weighted magnetic resonance image (C).

Fig. 2

Diagram showing measurement of total inter-vertebral height (TIH) of the two fused vertebral bodies, the black arrow head is the mid-point of upper end plate of cranial vertebral body, the white arrow head is the mid-point of lower end plate of caudal vertebral body and the arrow line indicates TIH. A showing measurement of ΔTIH (difference between the immediate postoperative TIH and the follow-up TIH in the lateral radiographs). B showing measurement of computed tomography (CT)-magnetic resonance (MR) ΔTIH (difference between TIH of the postoperative mid-sagittal 2D CT and that of the preoperative mid-sagittal T1-weighted MR). The degree of subsidence was reflected by ΔTIH and the change of postoperative disc space height was reflected by CT-MR ΔTIH.

Fig. 3

Radiographs demonstrating the cage subsidence in the immediate postoperative (A) and the last follow-up (B). Δtotal inter-vertebral height greater than 3 mm was considered relevant subsidence (ΔTIH : 34.85-31.22 = 3.63mm > 3mm).

Fig. 4

The two-dimensional computed tomography findings of fusion state. There is bony bridge formation around or inside the cage and increasing bone density inside the cage. Coronal reconstruction view (A), sagittal reconstruction view (B).

Fig. 5

The graph illustrating total inter-vertebral height (TIH) of non-subsidence group and subsidence group at each follow-up. Until the 8 week follow-up, there was no statistically significant difference in TIH between non-subsidence group and subsidence group. Statistically significant difference is observed at the last follow-up(p=0.0497), indicating that the subsidence has occurred at later than 8 weeks after surgery.