Technical note

“Shoe-heel design” for custom-made miniplates. A new design for an easier screwing on the proximal segments in guided bilateral sagittal split osteotomies

The placement of the osteosynthesis screws in the proximal end of the plate during Bilateral Sagittal Split Osteotomies (BSSOs) may be difficult to carry out transorally owing to the obliquity of the path of insertion of the instruments against the bone surface.

Despite muscular relaxation obtained by general anesthesia, soft tissues often prevent trans-oral screwing to the bone surface. The screws do not engage in the holes reserved in the miniplates and they slide away from the bone during trans-oral procedure.

Because computer aided desing - computer aided manufacturing technology enables the design and manufacture of custom-made plates,[¹²] the use of a “shoe-heel design” for the plates improves transoral access to the screw holes on the plate at the proximal segment by incorporating an oblique angle facilitating screw placement from a transoral approach [Figures 1 and 2]. This improves the accessibility and guidance during screw placement.

Figure 1: “Shoe Heel Design” facilitates oblique screwing. View from below

Figure 2: Lateral view

Due to increased thickness, the screw holes present an obliquity and increased length. These plates present a deeper counter sink which enables the screw to be entirely incorporated within the plate design.

The geometrical data of our latest eighteen guided BSSO plates use this new configuration. They were compared to the data of eighteen previous guided BSSOs using the former design. The geometrical comparisons were realised from STL files.

Each custom-made plate has four screw holes for the osteosynthesis of the proximal segment [Figures 2 and 3]. Consequently, the data of 144 posterior screw cylinders of 36 “new design” plates were compared to the data of 144 posterior screw holes of 36 “former design” plates.

Figure 3: Per-operatory view. The positioning of the custom made BSSO plate is passive. Note the increased thickness opposite the posterior-inferior part of the proximal screw hole

The design of the plates and the geometrical measures were realised with 3-matic Medical Version 15.0 (Materialise).

Angle measures were done with the “Angle Measure” tool with a “Line to plane” measure. Length measures were done with the “Measure Radius” tool and the surface contact area measures with the “Properties > Surface area “ tool.

Comparisons show:

• Regarding angulation between screw cylinder axes and bone surface mean plan: an absolute difference of the means of 8.14° and a relative difference of the means of 69.9%
• Regarding screw cylinder length through the plates: an absolute difference of the means of 0.20 mm and a relative difference of the means of 44.9%.
• Regarding contact area between screw heads and countersinks: an absolute difference of the means of 1.93 mm² and a relative difference of the means of 34.8% [Table 1].

Table 1: Comparisons between former design and Shoe Heel design for BSSO Custom-made plates

The angulation of screw cylinders at the proximal fragment facilitates transoral posterior osteosynthesis by ensuring a better access and improves control of the screws by having a longer length. The thicker plates avoid trauma of the periosteum by burying the screw head, despite the increased angulation of the screws.

The “Shoe-Heel Design” facilitates optimal transoral access and avoids a transbuccal approach while also offering greater control of the screw due to placement through a longer oblique guiding cylinder.

This “Shoe-Heel design concept” is applied to BSSO cutting and drilling guides for a better access and a better control of transoral drilling.[³] As both its positioning and axis are identical to the drilling and despite the obliquity of the screwing, there is no migration of the plate when screws are tightened.

Patient agreement

All patients have given their written agreement to the use of their personal anonymous computer-assisted-design data used for this technical note.

Ethics statement

Not Needed. All private practice patients received extensive information about splintless guided orthognathic surgery and CAD-CAM plates and guides. All private practice patients paid for their custom-made devices.

Financial support and sponsorship

Data processing and data analysis performed with the support of Materialise Malakoff - France.

Conflicts of interest

There are no conflicts of interest.

Acknowledgements

The author wishes to thank Dr Colin Maciver (Consultant Mafraq Hospital - Abu Dhabi · Maxillofacial - Head and Neck Surgery UAE) for his writing assistance. The author thanks Materialise Malakoff – France, for data processing and data analysis assistance.