Two-piece Versus Single-piece Patient-specific Titanium Subperiosteal Implants

After obtaining a thorough medical and dental history and performing necessary laboratory investigations, the following steps will be undertaken:

Preoperative prosthetic preparation:

• For eligible patients who are already wearing upper and lower complete dentures: The dentures will be checked for adaptation, fit, stability and occlusion.
• For eligible patients who are not wearing upper and lower complete dentures:

Preliminary alginate impressions will be made using a perforated stock tray for both the maxillary and mandibular arches. Impressions are then poured into dental stone to obtain the diagnostic models. Occlusion blocks will be constructed over the diagnostic models, followed by bite registration, try-in and delivery of upper and lower complete dentures.

Afterwards, approved complete dentures will be duplicated using laboratory condensation silicone to construct a radiographic stent. The radiographic stent is prepared by using a ratio of 3:1 acrylic resin to barium sulphate mixture, and then the patient is sent for a Cone Beam Computed Tomography (CBCT) scan.

CBCT scanning and virtual design of the patient-specific implant (PSI):

The CBCT scan of the patients - in the form of DICOM files - will be imported into surgical planning software (Mimics 19, Materialise, Leuven, Belgium). A segmentation process will be accomplished by using the two dimensional CT cuts to define the image thresholds based on Hounsfield's units, excluding soft tissue and only highlighting hard tissue (bone). The maxilla/mandible region will be further isolated by cropping and/or region-growing functions to remove artifacts and select only the area of interest, followed by the 3D calculation of the selected regions. The 3D models will be exported in standard triangulation language (STL) format.

The STL files will then be imported into computer-aided design (CAD) software (3-matic) in order to create the design of the PSI. The subperiosteal part of the PSI will be drawn on the surface of the region planned to receive the PSI and then extruded to a thickness of 2.5mm. The PSIs will be meshed out with 2.5 mm holes to be compatible with 2.0 mm fixation screws. For the single-piece PSI, the abutments will be placed, guided by the opposing teeth and the radio-opaque dentures. As for the two-piece PSI, the same design will be repeated without the addition of the abutment posts.

Fabrication of the patient-specific implant (PSI):

The STL file of the final design will be sent for 3D printing via additive manufacturing using selective laser sintering (SLS) machine from medical titanium grade (Ti-6Al-4V) Extra Low Interstitials (ELI).

PSI surface treatment:

The PSI will be subjected to acid etching following a protocol proposed by Juodzbalys et al to attain micro roughness on the fitting surface of the implant. The PSIs will be etched with sulfuric acid (H2SO4) for 72 hours followed by Hydrochloric acid (HCl) for 30 hours. After removal from the acids PSIs will be washed in distilled water.

Cleaning and sterilization:

Implants will be cleaned in an ultrasonic bath of absolute ethyl alcohol solution at ambient temperature for 30 minutes to ensure the removal of any residues on the fitting surface of the implant. After being thoroughly cleaned, the PSIs will be wrapped and sterilized using a class B autoclave.

Surgical phase:

For both groups, under general anesthesia; a mucosal incision 3-5 mm crestal to the vestibule will be made in the anterior region similar to the genioplasty incision, followed by dissection over the orbicularis oris and mentalis muscle and incising down to bone using a scalpel blade #15c mounted on a Bard-Parker® surgical blade handle size 3, to expose the proposed implant site. The incision will be completed posteriorly after localization of the mental nerves. After bone exposure the implant will be installed in place and fixed with 2.0mm grade 5 titanium screws.

For two-piece PSIs, the framework will be designed with 3-4 mm endosseous projections at the anticipated area where the final abutments will be fastened, guided by the position of the teeth in the proposed prosthetic set-up. The endosseous projections will be designed in order to accommodate the height of the internal connection that will receive the final abutment in a second-stage surgery. Unlike single piece PSIs, bone-supported surgical guides will be designed to aid in the placement and seating of the two-piece PSIs corresponding to the preplanned osteotomy sites. The flap will be sutured in two layers; the muscle layer will be closed with 3/0 poliglecaprone resorbable suture material and the mucosa with 4/0 Polypropylene non-resorbable suture material.

Postoperative care and instructions:

The intra oral wounds will be evaluated for any signs of bleeding, hematoma, infection, wound dehiscence or PSI exposure after 48 hours, then weekly till the end of the first month. The following medications will be administered:

Long acting corticosteroids: Methylprednisolone Acetate 40 mg/ml intramuscular injection single dose immediate postoperative.

Anti-inflammatory analgesic: Ketorolac tromethamine 30 mg/amp/2mL intramuscular injection every 12 hours for the first 24 hours followed by diclofenac potassium 50 mg tablets (every 8 hours for 4 days) Oral antibiotic: Ampicillin/sulbactam 375 mg tablets (every 8 hours for 10 days) Strict oral hygiene measures in the form of regular use of chlorohexidine Gluconate 125 mg/100 mL antiseptic mouth wash (every 8 hours for two weeks).

Prosthetic phase:

The steps of fabrication of the final prosthesis will be initiated 3 months after implant placement for both groups. For the single-piece PSI group, a closed impression will be made. For the two-piece PSI group, final abutments will be fastened to the corresponding connection in the framework in a second stage surgery, followed by a final closed impression a week later. The final restoration will be constructed from a high impact polymer composite (HIPC) material via computer numerically controlled (CNC) milling. Occlusal adjustments will be performed followed by temporary cementation for two weeks until a final approval is obtained from the patient. Later, the prosthesis will be cemented using glass ionomer self-cured luting cement.