Timing of development of adjacent-level ossification after anterior cervical arthrodesis with plates

Although the prevalence and severity of adjacent-level ossification development after anterior cervical plating has previously been described, there are no investigations regarding the timing of ossification development.

To determine the timing of adjacent-level ossification development and maturation and discern any differences in ossification development for patients who have a plate-to-disc distance (PDD) that is <5 mm versus =5 mm.

Retrospective study.

One-hundred twelve visible cranial or caudal adjacent discs in 62 patients, with a minimum of 24 months follow-up, were assessed. Among them, 40 had a minimum of 36 months follow-up after surgery (range, 36-91 months).

Grading system of adjacent-level ossification.

The PDD was measured on the postoperative lateral X-ray film and was used to divide the adjacent disc spaces into two groups. The first group had a PDD <5 mm, and the second group had a PDD >5 mm. The presence and severity of ossification were assessed at 3, 6, 12, and 24 months postoperatively and then annually and recorded into 4 grades: grade 0 (none) to grade 3 (complete bridging). We determined whether discs with no or mild (grade 1) ossification at a given follow-up period progressed to advanced (grade 2 or 3) by 24 months and the last follow-up (mean, 48.5 months).

Adjacent levels with even mild ossification by 3, 6, or 12 months had a high likelihood (87.5%, 62.5%, and 37.5%, respectively) of progression to advanced ossification by 24 months. The absence of ossification in the early postoperative period was no guarantee of avoiding ossification; 23.5% and 14.9% of those with no ossification at 3 and 6 months, respectively, progressed to advanced ossification by 24 months. On the other hand, only 1.8% of those with no ossification at 12 months progressed to advanced ossification. None of the 80 levels with no or grade 1 ossification at 24 months went on to advanced ossification by the last follow-up (mean, 48.5 months). The occurrence of ossification was significantly increased for levels with a PDD <5 mm (72.1%, 49/68) compared with levels with a PDD >5 mm (45.5%, 20/44). Of 28 cases with advanced ossification, 24 (78%) developed in levels with a PDD <5 mm.

We conclude that any adjacent-level ossification within the first 12 months postoperatively has a substantial likelihood of progression to advanced ossification by 24 months. However, those with no ossification at 12 or 24 months or mild ossification at 24 months are very unlikely to progress to advanced ossification.