Fusion rate: a time-to-event phenomenon
Authors: Tuli Sagun K. , Tuli Jayshree , Chen Peng , Woodard Eric J. 
Published: May 2009

Object.The term “fusion rate” is generally denoted in the literature as the percentage of patients with successful fusion over a specific range of follow up. Because the time to fusion is a time-to-event phenomenon a more accurate method of representation may be made using the Kaplan—Meier method of estimation.

Methods.The current study was performed to illustrate that fusion rate is more accurately represented by median times as calculated using survival analysis. Patients undergoing a cervical decompressive corpectomy and reconstruction formed the basis of the primary analysis. A secondary analysis was made to evaluate the difference in the fusion times for one- compared with multilevel corpectomy cases.

Data were collected at a tertiary care institution over a 5-year period with 6-month follow up after the last recruitment. Descriptive statistics of baseline patient characteristics, the extent of disease, and the surgical intervention were obtained.

Fusion was the final outcome, and it was defined as the “event.” The presence of any trabeculae bridging between the vertebral body and allograft signified the occurrence of an event. Postoperative static radiographs were evaluated by independent neuroradiologists to assess the presence of fusion.

Fusion rate was determined using the Kaplan—Meier estimate. The median time to fusion was calculated, as were the 95% confidence intervals (CIs). These were stratified for patients who underwent one- and two-level vertebrectomy. The log-rank test was used to differentiate between one-level and multilevel corpectomy. Multivariate analysis was performed using Cox regression for further evaluation, by adjusting for covariates (age, sex, smoking history).

Fifty-seven patients underwent single- or multilevel corpectomy and fusion. The male/female ratio was similar, with a median age of 53 years. Fourteen patients had a history of cigarette smoking. Thirty-six patients underwent a one-level corpectomy, 20 a two-level corpectomy, and one patient underwent a three-level corpectomy. The analysis was restricted to one- and two-level cases.

The median time to fusion for the cephalad and caudad aspect of the graft—host interface was 88 days (95% CI 82–94 days) and 85 days (95% CI 77–93 days), respectively. As generally reported in the literature, this translates to a 92% (by 2.1 years) and 93% (by 1.5 years) fusion rate, for the cephalad and caudad, respectively. The median time to fusion for the cephalad aspect of the graft for one-level vertebrectomy was 87 days (95% CI 83–91 days), whereas for two-level vertebrectomy was 90 days (95% CI 59–121 days). The median time to fusion for the caudal aspect of the graft—host interface was 85 days (95% CI 80–90 days) for one-level corpectomy and 90 days (95% CI 83–97 days) for the two-level cases.

There was no statistically significant difference in the median time to fusion for one- and two-level corpectomy at either the superior or inferior aspect of the graft (p = 0.19 and 0.84, respectively). This held true even after adjusting for covariates.

Conclusions.Fusion rate is a time-to-event phenomenon and is more accurately represented using the Kaplan—Meier method of estimation.