Computer-assisted orthognathic surgery: evaluation of mandible registration accuracy and report of the first clinical cases of navigated sagittal split ramus osteotomy.

Intraoperative navigation is a helpful tool in complex anatomical regions or procedures. The mobility of the mandible in relation to the skull base limits the use of navigation tools on the lower jaw if the reference device is installed on the forehead. A new workflow that allows navigation-assisted sagittal split osteotomy in orthognathic surgery using a separate non-invasive mandibular registration technique has been developed. An evaluation of accuracy in different anatomical regions and with different registration techniques was performed on skull models and skulls with movable mandibles. The mean inaccuracy was 1.51mm, with no significant difference between anatomical sites. Using a splint-based reference device allows the movable mandible to be registered independently from the midface. Registration using metal points in the splint provides higher accuracy than using interdental anatomical landmarks. The workflow could be transferred successfully to patient treatment. Navigation-assisted osteotomy by Obwegeser-Dal Pont technique was performed without any complication in six patients. The mean deviation from the planned osteotomy line was 1.52mm. The navigated sagittal split ramus osteotomy seems to be a suitable technique to increase patient safety.

Mechanical resistance of the periorbita and the orbital floor complex--are isolated orbital floor fractures only a soft tissue problem?

The primary aims of orbital floor reconstruction are to prevent enophthalmos and herniation of the orbital contents in order to achieve correct globe position. Theoretically, the mechanical load of the orbital floor is approximately 0.0005N/mm(2) (30g orbital content onto 600mm(2) of orbital floor area). Therefore, low mechanical stress from orbital floor reconstruction materials is expected. The periorbita and orbital floor complex (bony orbital floor with periorbita) of 12 human cadavers were investigated for their mechanical resistance to distortion and compared to different absorbable pliable reconstruction materials after modification with pores (Bio-Gide, Creos, and PDS). The human periorbita resistance (approximately 1.4N/mm(2)) was comparable to that of the absorbable membranes (Creos, Bio-Gide), and the resistance of PDS (approximately 2.3N/mm(2)) was comparable to that of the orbital floor complex. The periorbita has a higher stability than the bony orbital floor. Therefore, in isolated orbital floor fractures with a traumatized bony orbital floor and periorbita, reconstruction of the soft tissue as a periorbita equivalent with a resorbable membrane appears to be adequate to prevent enophthalmos and herniation of the orbital contents.

Mechanical properties of collagen membranes modified with pores--are they still sufficient for orbital floor reconstruction?

Adequate mechanical strength is essential for materials used to reconstruct the orbital floor, and collagen membranes have recently been suggested for the repair of isolated fractures of the orbital floor. However, their mechanical properties after modification with pores for increased drainage of blood into the sinus have not been sufficiently investigated. We have tested the mechanical resistance of polydioxanone foils (PDS) to distortion and compared it with that of 3 resorbable collagen membranes (Smartbrane(®), Bio-Gide(®), and Creos(®)) in mint condition and when artificially aged (3 weeks, 6 weeks, and 8 weeks) after modification with pores (diameter 2mm) in a standard configuration (n=12 in each group). PDS and Creos(®) had comparable initial values for mechanical resistance of about 2.3N/mm(2), and Bio-Gide(®) and Smartbrane(®) had about 20% and 80% lower initial mechanical resistance, respectively. All materials tested had lower values after artificial ageing. After eight weeks of ageing, PDS lost about 99% of its initial mechanical resistance, Creos(®) about 66%, Bio-Gide(®) about 30%, and Smartbrane(®) about 95%. After 3 weeks the mechanical resistance in all groups was significantly less than the initial values (p=0.05), but there was no difference between samples aged artificially for 6 compared with 8 weeks. The mechanical resistance of the tested materials was not influenced by the presence of pores in a standard configuration and was in the appropriate range for moderate fractures of the orbital floor. We recommend further clinical investigations of collagen membranes modified with pores.