Bone Flap Changes after Cranioplasty Using Frozen Autologous Bone Flaps: A Three-Dimensional Volumetric Reconstruction Study

Abstract

Purpose: Bone flap resorption (BFR) after cranioplasty with an autologous bone flap (ABF) is well known. However, the prevalences and degrees of BFR remain unclear. This study aimed to evaluate changes in ABFs following cranioplasty and to investigate factors related with BFR.

Materials and methods: We retrospectively reviewed 97 patients who underwent cranioplasty with frozen ABF between January 2007 and December 2016. Brain CT images of these patients were reconstructed to form three-dimensional (3D) images, and 3D images of ABF were separated using medical image editing software. ABF volumes on images were measured using 3D image editing software and were compared between images in the immediate postoperative period and at postoperative 12 months. Risk factors related with BFR were also analyzed.

Results: The volumes of bone flaps calculated from CT images immediately after cranioplasty ranged from 55.3 cm³ to 175 cm³. Remnant bone flap volumes at postoperative 12 months ranged from 14.2% to 102.5% of the original volume. Seventy-five patients (77.3%) had a BFR rate exceeding 10% at 12 months after cranioplasty, and 26 patients (26.8%) presented severe BFR over 40%. Ten patients (10.3%) underwent repeated cranioplasty due to severe BFR. The use of a 5-mm burr for central tack-up sutures was significantly associated with BFR (p<0.001).

Conclusion: Most ABFs after cranioplasty are absorbed. Thus, when using frozen ABF, patients should be adequately informed. To prevent BFR, making holes must be kept to a minimum during ABF grafting.

Keywords: Autografts; bone resorption; cranioplasty; decompressive craniectomy; skull.

Fig. 1. Flowchart of cases included in or excluded from the study. CP, cranioplasty.

Fig. 2. Volume measurement with 3D modeling software. Using Materialise Mimics, we selected autologous bone flaps from CT scans of the skull and reconstructed them into 3D images. The reconstructed images were converted into STL files for 3D images. The STL file of the bone flap was imported by Netfabb to calculate the volume. 3D, three dimensional; STL, stereolithography.

Fig. 3. Distribution of volumes of remnant bone flap.

Fig. 4. Changes in volume of ABF of 13 patients with serial computed tomography scans at 3, 6, and 12 months in the bone flap resorption group. On average, the volume of ABF at 12 months was preserved by an average of 52.1%. ABF, autologous bone flap.

Fig. 5. Changes in the skull and ABF after cranioplasty in the BFR group. In general, BFR began in the parietal bone and the temporal bone (left). If wide holes existed on ABF, BFR was also initiated at the holes (right). ABF, autologous bone flap; BFR, bone flap resorption.

Fig. 6. Changes in the volume of ABF after 12 months in the bone flap resorption group. Unlike before 12 months, the decline decreased after 12 months. After an additional 12 months, the volume of ABF decreased only by an average of 5.8%. ABF, autologous bone flap.

Fig. 7. Three-dimensional images of the cranium following cranioplasty. The upper image was obtained from immediate postoperative CT scans, and the lower image was obtained from CT scans at 12 months after surgery. The volume of the remaining autologous bone flap was 67% of its original volume, and repeated cranioplasty was performed due to appearance issues that caused depression in the forehead area.