75. Micro-CT Based Microarchitecture Analysis of Cryopreserved Calvarial Bone Graft in a Mouse Model of Cranioplasty

INTRODUCTION: Compared to craniotomy surgeries, wherein the removed bone is immediately replaced during the same operation, cranioplasty with cryopreserved calvarial bone graft (CCBG) is associated with up to 35% rate of failure due to impaired healing and bone flap resorption. To provide a baseline understanding of the healing process of CCBG, we utilized micro-computed tomography (Micro-CT) scan to evaluate the microarchitecture of CCBG and surrounding bone following autologous cranioplasty in mice. METHODS: Adult C56BL/6 mice were divided into experimental (delayed cranioplasty with CCBG) and control (immediate bone replacement) groups. The experimental group had their bone graft cryopreserved at -80C following craniectomy. Within each group, there were 5 cranioplasty timepoints (1,2,3,4 and 8-week). Following euthanization, skulls were scanned using Scanco mCT50 with 10 micron voxel size. Areas of interest taken were the “bone graft rim”, “defect rim”, and “central bone graft”. Mean bone volume for all areas of interest were taken to standardize all the areas analyzed. Mean gray value, which is proportional to tissue mineral density was taken for all areas. The 5th percentile Gray Level values is an indicator of new bone formation, and areas with low values have high new bone forming activity. For all areas to be compared, bone volumes were compared first. Lack of statistical significance in bone volumes (p>0.05) was desirable to minimize bias. RESULTS: In the immediate cranioplasty group, regardless of the timepoint, the bone graft rim was significantly more active than the rest of the bone graft (central BG) (P=0.033). The activity in the bone graft rim and the defect rim had similar levels of activity (P=0.075). Similarly, there was no difference in the level of remodeling activity at the defect rim and the central BG (P=0.92). When stratified by timepoints, there was no difference in the activity level across timepoints. In the delayed cranioplasty group, the BG rim similarly demonstrated significantly higher remodeling activity compared to the central bone graft (P<0.0001). The BG rim showed a higher level of remodeling activity than defect rim however, this was not significant (P=0.127). Notably, the defect rim in this group was also significantly more active than the central BG (P=0.016). When activity was stratified by timepoint, BG rim at week 1 was significantly more active than central BG (P=0.02) at the same week, and at week 8 (P=0.0037); There was no difference in activity between these areas from week 2 through 4. BG rim at all timepoints was consistently more significantly active than central at week 8. When these sites were compared in the immediate versus delayed groups, there was no difference in the level of remodeling activity at all the relevant sites compared. CONCLUSIONS: Our data demonstrates that BG rim is the most active area in term of remodeling following cranioplasty, regardless of the timing of cranioplasty. Similarly, the central bone graft may be the least active area, likely with a predominance of nonviable bone. Further studies at later timepoints beyond 8 weeks are needed to determine whether this decreased activity is associated with clinical bone resorption.