Subsidence and nonunion after anterior cervical interbody fusion using a stand-alone polyetheretherketone (PEEK) cage

Abstract

Background: The purposes of the present study are to evaluate the subsidence and nonunion that occurred after anterior cervical discectomy and fusion using a stand-alone intervertebral cage and to analyze the risk factors for the complications.

Methods: Thirty-eight patients (47 segments) who underwent anterior cervical fusion using a stand-alone polyetheretherketone (PEEK) cage and an autologous cancellous iliac bone graft from June 2003 to August 2008 were enrolled in this study. The anterior and posterior segmental heights and the distance from the anterior edge of the upper vertebra to the anterior margin of the cage were measured on the plain radiographs. Subsidence was defined as ≥ a 2 mm (minor) or 3 mm (major) decrease of the segmental height at the final follow-up compared to that measured at the immediate postoperative period. Nonunion was evaluated according to the instability being ≥ 2 mm in the interspinous distance on the flexion-extension lateral radiographs.

Results: The anterior and posterior segmental heights decreased from the immediate postoperative period to the final follow-up at 1.33 ± 1.46 mm and 0.81 ± 1.27 mm, respectively. Subsidence ≥ 2 mm and 3 mm were observed in 12 segments (25.5%) and 7 segments (14.9%), respectively. Among the expected risk factors for subsidence, a smaller anteroposterior (AP) diameter (14 mm vs. 12 mm) of cages (p = 0.034; odds ratio [OR], 0.017) and larger intraoperative distraction (p = 0.041; OR, 3.988) had a significantly higher risk of subsidence. Intervertebral nonunion was observed in 7 segments (7/47, 14.9%). Compared with the union group, the nonunion group had a significantly higher ratio of two-level fusion to one-level fusions (p = 0.001).

Conclusions: Anterior cervical fusion using a stand-alone cage with a large AP diameter while preventing anterior intraoperative over-distraction will be helpful to prevent the subsidence of cages. Two-level cervical fusion might require more careful attention for avoiding nonunion.

Keywords: Anterior cervical fusion; Nonunion; PEEK cage; Subsidence.

Fig. 1

(A) The anterior segmental height (ASH) and posterior segmental height (PSH) were measured as the distance between the upper end plate of the upper vertebra and the lower end plate of the lower vertebra at the anterior and posterior margins, respectively. (B) The cage distance was measured as the distance between the anterior edge of the upper vertebra and the anterior margin of the cage. (C) Magnification differences were corrected by the ratio between the anteroposterior diameters at the middle of the upper vertebral body on each plain radiograph (A-a) and MRI (B-b). Every length measured on the plain radiographs was multiplied by the ratio (B-b/A-a). (D) Segmental lordosis was measured using Cobb's method.

Fig. 2

A case of anterior subsidence without nonunion. The anterior subsidence was 2.8 mm, and segmental stability on the flexion and extension radiographs was observed. Subsidence is not necessarily accompanied with nonunion. F/U: follow-up.

Fig. 3

A case of nonsubsidence with nonunion. Instability with a difference of 5 mm between the interspinous distances on the flexion and extension radiographs was observed without subsidence. Nonunion can occur without subsidence. F/U: follow-up.