Abstract
This paper describes a new type of miniplate system that is designed and custom made during virtual surgery planning based on an individual patient’s osteotomy. These miniplates are prefabricated with commercially pure porous titanium using direct metal laser sintering. The principles that guide the conception and production of this new miniplate are presented. The surgical procedure from the stage of virtual surgery planning until the final Le Fort I osteotomy and bone fixation are described using a case example.
When performing Le Fort I osteotomies, precise simulation preoperatively and precise execution of the osteotomy with exact positioning and fixation of the osteotomized fragments are essential. Traditionally, the positioning of the freed maxillary segment of the Le Fort I osteotomy is based on preoperative model surgery, using articulated dental models made from plaster casts of the patient’s occlusion. Traditional model surgery allows the fabrication of oral splints, which in the best of circumstances, ensure that the antero-posterior (A-P) and transverse planes are accounted for during positioning and fixation of the maxillary segment. This, however, does not allow for reproducible and dependable vertical positioning of the maxillary segment in cases of maxillary impaction or downward displacement.
With the advent of computer simulation, Le Fort I osteotomies have become precise and predictable thanks to the availability of preoperative virtual surgery planning and stereolithographic modeling, which allows for use of surgical guides to place the freed Le Fort I segment in three-dimensional (3D) space. The authors present the next step in the use of computer-modeling; the use of prefabricated custom-made titanium miniplates, made for each individual patient during virtual surgery planning.
Materials and methods
Informed consent was obtained from the patient for the operation and this study; the Helsinki Protocol was followed.
The Le Fort I ostetomy is simulated during virtual surgery planning. An osteotomy guide is produced using stereolithographic modeling. Rather than using a stereolithographic positioning guide, the 3D positioning of the freed Le Fort I maxillary plateau segment intraoperatively is controlled by the use of a prefabricated custom-made titanium miniplate system, which serves as both positioning guide and osteosynthesis fixation.
These miniplates are fabricated using commercially pure porous titanium (CPPTi) under direct metal laser sintering (DSLM). The size and form of the custom-made miniplate system correspond to the patient’s skeletal anatomy as well as the desired 3D positioning and fixation of the maxillary segments elucidated during planning. The use of this system allows the surgeon to position and securely fixate the osteotomy fragments rapidly without the intraoperative bending of traditional miniplates.
The use of DSLM of CPPTi is common in the production of 3D titanium cranioplasty plates based on computed tomography (CT), or in oral maxillofacial surgery in the preoperative production of oral splints. In contrast, this article presents the novel method of virtual surgery planning in the production of custom-made miniplates, which serve both as a positioning guide and stable fixation intraoperatively. It must be noted that the successful use of the custom-made miniplates is based on precise virtual surgery planning.
Two computer software programs were used for this: the SIMPLANT-OMS ® (Materialise Dental, Leuven, Belgium) program was used to simulate the Le Fort I osteotomies and the 3-matic ® (Materialise, Leuven, Belgium) program allowed for the design and fabrication of the stereolithographic osteotomy guide as well as the custom-made titanium miniplate system.
After computer simulation of the planned Le Fort I osteotomy, the DICOM data for the simulation were sent to the biomedical engineer who drew the prototype based on the recommendations of the surgeon. There are several factors related to the design of the osteotomy guide. It should be of a small size and smooth outline to allow for ease of positioning without impinging on soft tissues or trapping structures, such as the infraorbital nerve. The guide should sit precisely on bone and have screw holes placed for securing during cutting. These holes should be placed away from where the final osteosynthesis screws will be placed ( Fig. 1 ).
The guide is made to fit the width of the cutting instrument (the working tip of the ultrasonic insert of the piezosurgery hand-piece in A-P movements and with two cutting planes for maxillary impaction) allowing the precise removal of the correct amount of bone as determined in presurgical planning. Once the design of the osteotomy guide is validated by the surgeon, it is produced using sterilizable polyamide by stereolithography (OBL, Chatillon, France).
The custom-made titanium miniplate system is designed with the 3-matic ® software program by the biomedical engineer according to the recommendations of the surgeon. Consideration must be given to the size and form of the system, which must precisely fit the skeletal anatomy of the patient and allow the placement of the miniplate system through the muco-periosteal incision. The miniplate system must lie on the maxilla in a completely passive fashion. Indentations are made to accommodate the prominence of the nasal spine and the foot of the nasal septum.
An important design modification is the placement of abutments that rest against the two sides of the osteotomy as this ensures the precise fit of the metalwork against the two sides of the osteotomy. The custom-made miniplates are made as a single unity, initially joined together by adjoining titanium wires, which allow for their use as a positioning guide. This allows for maximal congruent contact between the maxillary segments and the miniplates and thus enables the precise positioning of the skeletal segments freed by the osteotomy. The miniplates are joined together in a 4 by 4 configuration (all four of the miniplates at the bilateral nasofrontal and zygomatico-maxillary buttresses are joined) and allows precise positioning of all the plates at once onto the maxilla for osteosynthesis.
The positioning and depth of the miniscrews for osteosynthesis are also simulated. Using the ‘transparent mode’ in the software program allows one to choose the best placement for the miniscrews in order not to injure any adjacent vital structures (e.g. dental roots, nerves, intubation tube). The simulation of the position and insertion of the miniscrews in the ‘transparent mode’ and the ‘cutting mode’ also allows one to place them in the areas of thickest cortical bone.
The custom-made titanium miniplate system is made using CPPTi with the DSLM method for rapid retooling (OBL, Chatillon, France). This method of fabrication is done layer by layer and allows the production of medical implants based on a 3D design. The successive steps of this ‘implant’ are as follows. The implant is virtually ‘cut’ with the help of the piloting software of the machine (3-matic ® ), with each layer being sent into production one after another. The machine spreads each new layer of CPPTi into the fusion chamber. A laser beam sweeps across the bed of grade II titanium powder in order to fuse the powder. Before each successive layer, new CPPTi is brought into the fusion chamber above the previous chamber ( Fig. 2 ). The process is repeated until the implant is made. The implant then undergoes treatment to anodize its exterior surface. It is cleaned, degreased and conditioned to be sterilizable in the autoclave machine.
