Bone flap survival and resorption after autologous cranioplasty

Thesis event information

Date and time of the thesis defence

Place of the thesis defence

University of Oulu, Leena Palotie Auditorium 101A

Topic of the dissertation

Bone flap survival and resorption after autologous cranioplasty

Doctoral candidate

Bachelor of Medicine Tommi Korhonen

Faculty and unit

University of Oulu Graduate School, Faculty of Medicine, Research Unit of Clinical Neuroscience

Subject of study



Professor Peter Hutchinson, University of Cambridge


Docent Sami Tetri, Oulu University Hospital and University of Oulu

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Bone flap survival and resorption after autologous cranioplasty

The present thesis examined the factors affecting the results of autologous cryopreserved cranial reparation surgery, cranioplasty. The bone flap used for the reconstruction surgery had been removed from the patient in an earlier operation that had most commonly been conducted in order to reduce increased intracranial pressure due to traumatic brain injury or stroke. Cranioplasty using cryopreserved autologous bone is associated with bone flap resorption. A resorbing bone flap begins to soften and cavities form within the flap. Eventually, the brain may lose its protective bony cover: a bone defect practically identical to that preceding cranioplasty may recur. The mechanisms behind bone flap resorption are unclear, and no means to reduce its incidence is known.

The prevalence of complications associated with autologous cryopreserved cranioplasty was evaluated in a multicenter setting in cooperation with Oulu, Turku and Kuopio University Hospitals. 40 % of the patients developed any complication following surgery. Half of the complications required removal of the bone flap and re-cranioplasty with a synthetic implant. Bone flap resorption and surgical site infections were the cause of 90 % of all the complications requiring bone flap removal. Young age at surgery was associated with bone flap resorption and smoking predisposed the patient to surgical site infections.

The study utilized conventional follow-up imaging data in a novel manner in following the surgical result of the patients of the Oulu University Hospital. Changes in autograft volume and radiodensity were calculated as a function of follow-up time. 90 % of the patients had a decreased bone flap volume in the follow-up suggesting varying degrees of resorption. On the level of the whole cohort, the progression of bone flap resorption was non-linear, and most of the bone flaps do not appear to resorb enough to require removal even in the long-term follow-up. Instead, a moderate extent of bone flap resorption is likely a revitalization reaction.

Despite being a common phenomenon, the definition of bone flap resorption has been unclear in the literature, limiting inter-study comparison. In the present thesis, a new computed tomography-based scoring system (the Oulu Resorption Score) was developed in order to standardize the interpretation of imaging findings and to guide treatment planning. The scores range from 0 to 9 with increasing values indicating increasingly severe bone flap resorption. Compared with evaluations of independent neurosurgeons on the requirement of re-cranioplasty, a score of ≥5 was found to depict clinically relevant bone flap resorption. The maximum score indicated bone flap failure.

The new data on the factors affecting the results of cranial reparation surgery could be applied in selecting the optimal reparation material in a patient-by-patient manner. It was found that the presence of mild bone flap resorption changes did not predict the development of severe resorption. Thus, part of the routine imaging studies could be replaced with purely clinical controls. The Oulu Resorption Score enhances inter-study comparison and assists the clinician in the choosing of the treatment scheme.
Last updated: 1.3.2023