Mechanical characterization and optoelectronic measurement of parietal bone thickness before and after monocortical bone graft harvest: design and validation of a test protocol.

Parietal bone grafts are commonly used in craniomaxillofacial surgery. However, bone removal may weaken the parietal bone and lead to deleterious complications. The aim of our study was to design a test protocol for characterization of the impact resistance of parietal bone before and after monocortical bone graft harvest and to validate an optoelectronic measurement of parietal bone thickness. Twelve fresh human cadaver heads were used for the validation study. To evaluate impact resistance, we developed a pendulum Charpy impact testing machine. The impact force was gradually increased until failure (fracture) of the defined parietal bone area. According to the protocol, we measured the maximum absorbable energy or impact force to failure. With our test setup, measurement of the impact resistance of parietal bone was accurate to within 0.025 J. We defined a range of values and particularly a threshold value. The initial maximal impact must not to exceed 4 J. For more accuracy, we compared 5 nondestructive measurement methods using a surgical navigation system with optoelectronic tracking. We achieved an algorithm based on 2 methods that ensured a measurement resolution of 0.1 mm. Validation of this protocol will allow us to evaluate the loss of strength resulting from bone removal and the correlation between strength and thickness of the parietal bone.