Next, we'll be hearing about the quantitative and qualitative analysis of bone flap reabsorption in patients with cranioplasty following decompressive craniectomy with Tommi Koronen. Okay, I'm an MD-PhD student from the University of Oulu, Finland, and the topic of my speech today is quantitative and qualitative analysis of bone flap reabsorption after otologous cranioplasty after decompressive craniectomy, which is the tertiary option of treating increased intracerebral pressure, or intracranial hypertension, but due to its nature, it later requires reconstruction in cranioplasty after the acute phase has subsided. Despite primarily good outcomes, it is notably often the case that in the long follow-up, the outcome is partial otograft reabsorption, at least partial, and the aims of the present study were that, as we have seen the advent of major cranioplastic surgery during the last 20 years or so, it has become a necessity to look at the long-term outcome of those bone flaps, and especially assess the worldwide concern of whether all of those bone flaps reabsorb in the long run, and well, to investigate this, we invited all of our available patients for a follow-up CT scan, and as you all know, cranioplasty can be seen as a routine operation to the experienced neurosurgeon, but despite this, it is associated with a notably high complication risk of almost 40%, and the early complications include the conventional surgical complications, such as infections, hematomas, and seromas, but the late phase complications include infections and aseptic bone flap reabsorption, which is especially interesting in otograft cranioplasty. It looks like this. The CT scan on the left is the immediate post-operative CT scan, and the two to the right are the situation exactly one year after the primary CT scan, and in the 3D reconstruction here, it is seen that there is no bone union of the bone flap with the surrounding calvarium. There is also notable thinning of the bone flap and numerous holes to the bone flap. So our study questions were, how common is bone flap reabsorption, and what are the risk factors for it, and does the extent of bone flap reabsorption depend on the length of the follow-up, and whether all flaps eventually reabsorb. So to investigate this, we identified all of our otologous cranioplasty patients in our hospital, and all 45 patients we identified, and 4 had to be excluded due to missing data, and the remaining patients were follow-up images with the CT scan. So we employed a novel method of following bone flap reabsorption, which enables us to not only follow the presence of bone flap reabsorption, but also the extent of bone flap reabsorption, with the summation of area method. And in it, the bone flap volume is calculated by measuring axial bone flap areas on different levels of the bone flap, separated by a vertical interval. We used a vertical interval of one centimeter, and this was done for the follow-up CT scan and for the immediate post-operative CT scan. The measurements must be made on exactly the same level of the bone flap in order to get reasonable results. And then the bone flap volumes in follow-up and in the initial CT scan are compared to find the remaining bone volume, which we are interested in. We also measured bone quality in terms of Hounsfield units, which can't be used as absolute values, so we first have to calculate the HU value weighted by the bone flap area of the bone flap here, then compare it with the adjacent calvarium to find the relative HU value of the bone flap. This is demonstrated, and a more detailed explanation of the method is described in a manuscript that will be soon published in the Journal of Neurosurgery. This is done for the follow-up and immediate post-operative CT scans, and then the relative HU values of the bone flap are compared from the follow-up CT scan and the initial CT scan to find the bone quality change in the follow-up. So about the results, we found that over 90% of our patients had some level of bone flap resorption in a median follow-up time of 3.79 years, which should be enough for any resorption to occur. The remaining bone volume was less than 80% in 13 of our patients, so almost a third of our patients, but still only four required re-cranioplasty to do the bone flap resorption. And of note, the time between craniectomy and cranioplasty had no effect on the bone flap volume. Perhaps most importantly, the progression or the degrees of the bone flap volume is non-linear in relation to time, which means that not all of the bone flaps are going to resorb even in the long follow-up. And patients who were younger than 30 years of age at the time of cranioplasty had, as a mean, 16% less bone remaining than the rest of the cohort. In terms of the bone quality results that we measured, the relative bone radii density ranged from about 70% to 140%, but these results are very hard to interpret because there are many types of bone flap resorption, as shown here. There's bone flap resorption mainly affecting the diplo area of the bone here, which would increase the relative HU value of the bone or the bone quality that we measured. And then there's hole formation, thinning of the bone flap here. This would also increase the bone quality or the HU value that we measured. And there's cavity formation in the bone flap here and here. And even within the same bone flap, there is cavity formation and thinning of the bone flap, which make the HU results very hard to interpret. And interestingly, there are some signs of coinciding dural osteogenesis along with resorptive changes in the bone flap. And this is the optimal outcome that we're aiming for. This shows the fusage of the cranioplasty with the surrounding calvarium, and there is even some dural osteogenesis noted here. While these are negative results, we can get back to them if there are some questions. A future direction of ours is to validate a new grading system for bone flap resorption in order to better interpret those HU values and perhaps use it for treatment planning of these patients. And as seen here, the ovary resorption score correlates quite well with the bone volume that was measured in the present study. We would like to conclude that we found that most of our patients, over 90%, had some signs of bone flap resorption, which led us to postulate that a moderate extent of bone flap resorption could be due to revitalization reaction of the deep frozen cranioplasty that is planted within the calvarium. But most importantly, it looks like all of the bone flaps are not going to resorb in the long follow-up as the decrease of the bone volume is non-linear. And the third point is that in the absence of additional symptoms, it seems that routine CT scans for monitoring bone flap resorption seem unnecessary. Thank you. So I've got a quick question for you before you run off. Did you look at the type of fixation, because it looked like you used exclusively one type? The fixation of the bone flap? Yes. Do you look at plates and screws at all? We didn't analyze that, but they were screw-fixated with grating. Perfect. I have a question. Have you done any studies on people who had their bone flaps buried in the ground? No. No, we don't perform that in our hospital. We deep freeze all of the bone flaps. That would be an interesting study, anyway. I think there's more. Did you look at the yield resorption, 90K of it? Sorry, can you repeat? I couldn't hear. Did you look at the yield resorption, 90K of the bone flaps? No, these were after decompressive craniotomy. I'm trying to compare it. What is the difference between the resorption of the bone? You just put the bone back immediately, and then you put it back later on? Yes, we did not look at craniotomy patients, only delayed craniotomy. It would be very interesting, though. Go ahead. Do you notice any relation between the size of the flap and the resorption? No. We measured that, but there was no correlation or association of the bone flap size. I must say that we used the 2D scout images to find the size of the bone flap. If you mean the area of the bone flap. That method underestimates the size of the bone flap. But using that method, no correlation was found. We have time for one last question. You mentioned that it didn't carry good tissue. Did you find any difference between whether it's towards the bone flap, for example, in the subcutaneous phase of the abdomen, versus eating it in a refrigerator? Yes, that was addressed earlier. In our hospital, we only deep freeze the implants or the outcrafts. I think your paper is going to generate a lot of interest when it's published, so we very much look forward to that.

bone flap reabsorption

cranioplasty

decompressive craniectomy

long-term outcome

risk factors