Assessment of the Healing of Vascularized Fibula Bone Graft in the Reconstruction of the Mandible Using Computed Tomorgraphy

Purpose: Fibula free flap represents the workhorse for simultaneous bone and soft tissue reconstruction of the Mandible. The aim of this study is to quantify bone formation, if any, in the graft-mandible and graft-graft gaps using computed tomography (CT) scans by developing a reliable threshold-based post-imaging processing tool, compare the healing of fibula to the mandible to the healing of the fibula to itself using this tool, and to investigate potential factors affecting bone formation specifically the linear distance between the bony edges during surgery. 

Patients and Methods: This is a multicenter study centered at Boston medical center. DICOM images were analyzed using Osirix software (V.3.7.1, 32 bits) after blinding identifying data. The inclusion criteria for this study: 1) patients received a vascularized Fibula free flap for Mandible reconstruction; 2) patients who have at least 2 postoperative CT scans with at least one month interval; 3) the first CT is within the first 3 months after the surgery; 4) no signs of clinical failure of the graft or hardware failure. The reliability of this technique was tested using two independent blinded examiners. Each blinded examiner tested each scan three times. Pearson’s correlation coefficient was used to assess inter-rater reliability while the mean, Standard deviation error, and standard deviation of the mean assessed the intra-rater reliability. Paired T-test was used to compare the amount of volume change over time in participants who had both graft-graft gaps and graft-Mandible gaps. Multiple linear regressions were used to investigate the relation between the initial linear distance between the bony edges of the gap, age, and time interval against the percentage of change in gap volume. All statistics were conducted using Microsoft excel software and SPSS.

 Results: Twenty bony gaps from nine subjects were included in this study. This includes five graft-graft gaps and fifteen graft-Mandible gaps. The first post-operative CT scan was done within first three months after surgery (range= 2-77 days, mean= 22.2 days). Each subject had two CT scans with time interval ranging between 33 days to 390 days (mean= 191.1 days). The subjects’ age ranged between 30 and 72 years (mean= 56.1 years). 12 bony gaps were used for assessing inter-rater and intra-rater reliability. The Pearson’s correlation coefficient for inter-rater reliability was 0.94. Inter-reliability standard deviation error average was 0.03 and the standard error of the mean average was 0.003. Two-tailed paired T-test comparing the interval change in volume of graft-graft gaps to graft-Mandible gaps was 0.304. We found a significant negative correlation between absolute volume change and distance in mm (Pearson =-0.476, p-value=0.017). 22.7% of the variability in volume change can be explained by the initial linear distance between the bony edges of the gaps in millimeter.

Conclusion: Small bony gaps between the fibula bone graft and the mandible after mandibular reconstruction can be reliably assessed. The healing of the fibula to itself was not found to be significantly different from the healing of fibula to the mandible in the same subject. The initial linear distance between the bone edges of the gap is inversely related to subsequent bone formation. It is recommended to adapt the bony segments as close as possible to increase bone formation. 

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