Resistance of the sheep skull after a monocortical cranial graft harvest

Cranial bone grafts are commonly used for preimplant or facial reconstructive surgery. However, removing bone may weaken the parietal bone and lead to a loss of strength. This loss has never been quantified. Bone harvest site reconstruction is being carried out more frequently than in the past, but its effect on the strength of the donor site is unknown. The aim of our study is to quantify the loss of strength due to a monocortical cranial bone graft harvest in sheep.

Thirty-four fresh sheep cadaver heads were used for the study. We performed a monocortical bone graft harvest on the posterior part of the right frontal bone. We used a surgical navigation system with optoelectronic tracking to measure bone thickness. To evaluate the resistance of the skull to an impact we developed a pendulum Charpy impact testing machine. The impact force hit a defined target frontal area.

The total thickness on both sides ranged from 3 mm to 10 mm with a mean of 6 mm (SD = 1.4 mm). The loss of strength between the intact left side and the harvested right side varied with a mean of 49% (SD = 17%) and was significant (p = 6.10(-10)).

This study has demonstrated that there is a loss of strength in the skull on the side where a bone graft has been harvested. Reconstruction of the harvested site using biomaterials reduces the poor aesthetic outcome due to depression at the site, but we do not know its effects on strength. This kind of study cannot be performed in humans for ethical reasons. Data obtained from this study will allow us to carry out a study in sheep to evaluate strength of the frontal area of a skull with a harvest site reconstructed with hydroxyapatite cement.