Volumetric Evaluation of Gingival Augmentation Around Dental Implants

Introduction Adequate soft tissue thickness at the vestibular aspect of dental implants are crucial for satisfactory implant engraftment and aesthetic outcome. To pursue this objective, numerous surgical procedures have been adopted to augment the peri-implant gingiva, including autologous soft tissue grafting, xenogeneic collagen matrix, acellular dermal matrix, and roll/pedicle flap techniques: however, the available data do not suffice to clearly indicate which technique yields the best results.

The pouch roll techniques, originally proposed by Abrams in 1980 and subsequently improved, are particularly effective because they maintain vascularity of the grafted flap, which greatly facilitates its survival. Most of these procedures use a scalpel to shape the flap and remove the keratinized epithelial layer, but this conventional pouch roll technique is hampered by the fact that flap thickness must be no less than 3 mm and it is very difficult to perform hand scalpel gingival de-epithelization of the without damaging the underlying connective tissue, thereby causing bleeding and predisposing to scarring in the long term. More recently, laser-aided epithelial photoablation has been proposed as a possible solution to these issues. In fact, diode laser operating at 810 nm wavelength (λ) has been successfully used for selective de-epithelialization for several periodontal therapy purposes, e.g. palatal grafts for gingival recession defects, gingival hyperpigmentation, intra/extra pocket decontamination in chronic periodontitis and placement to second-stage surgery for buried implant exposure. Laser photoablation offers several advantages, namely excellent hemostasis that results in a dry surgical field and better view of the operative field, tissue surface sterilization and shallow penetration limited to the gingival epithelium with minimal injury and inflammation of the underlying lamina propria. On this ground, the investigators successfully combined diode laser photoablation (λ 810 nm) to conventional roll flap technique for mild vestibular ridge deficiencies: in a preliminary, limited series of patients (n=3) this improved method resulted in fast healing, satisfactory clinical and aesthetic results, rapid prosthetic restoration and no need for local or systemic analgesics. The present study aims on one hand at expanding the case series to consolidate the preliminary findings, on the other hand at evaluating another key clinical outcome parameter of this laser-aided roll flap procedure, namely the actual, objective gingival soft tissue augmentation over time. To this purpose, the volume of the treated regions, acquired by a digital intra-oral scanning, was measured and analyzed by appropriate 3D software and compared before roll flap surgery and implant placement and upon 12-month follow-up. Post-operative discomfort and aesthetic satisfaction of the patients were also assessed.

Material and methods

Volumetric measurements and analysis - Volume changes of the vestibular gingiva before and after the laser-aided roll-flap augmentation were evaluated and compared on digital intra-oral scanning impressions taken with an intra-oral scanner (Dentsply Syrona, Rome, Italy) and 3D reconstruction software (Exocad, Darmstadt, Germany). To reduce possible deviations and ensure higher accuracy of 3D analysis, quadrant instead of full-arch scans were recorded. These were converted to standard tessellation language (STL) format. Pre and post-operative STL files were collected, aligned and superimposed by selecting 10 reproducible anatomical points as landmarks prior to be analyzed with an appropriate software (Rhyno3D, Washington DC, USA). This software allowed: i) to demarcate an exact 3D region of interest (ROI), ii) to superimpose the same ROI taken at 2 time points, i.e. before surgery (day 0) and at 12-month follow-up, while to measure the volume of the 2 corresponding ROI and to objectively assess their differences at the different time points. Each selected ROI corresponded to a single or multiple implant site. According to previous studies, ROI in the single implant sites was demarcated on the vestibular gingival aspect using the following criteria:

1. gingiva-to-implant margin (or coronal gingival ridge, if before implant placement) as horizontal coronal border;
2. muco-gingival junction as horizontal apical border;
3. two vertical lines, 2 mm aside the surgical flap incision and corresponding to the mid-point of the interdental papillas, as mesial and distal borders.

Due to individual variations in oral anatomy, ROI were obviously different among the patients, but substantially identical in the same patient at successive time points. The software was set to a spatial resolution of ±50 nm between the STL files: in this way, negligible or no variations <50 nm in the merged STL file appeared green in a visual pseudo-color range. To assess reproducibility of the method, a trained blinded examiner (M.P.), performed 2 measurements at 1-week interval: intra-class correlation coefficient was 0.87 (95% CI 0.82-0.91).

In each patient, the 2 ROI volumes (cubic mm) measured at day 0 and 12 months were compared to evaluate the changes of the vestibular gingival soft tissue over time. Exocad software also allowed to determine the linear dimensional changes (LD), i.e. the linear distance (mm) between the 2 ROI surfaces measured at 1, 3 and 5 mm along a vertical midline traced between the gingival free margin and the muco-gingival junction, as previously reported.

Clinical peri-implant parameters and calibration - At 12 months, a trained and blinded examiner (L.L.) evaluated PD and BoP (within 30 s after probing) at 6 sites per implant (3 buccal, 3 palatal) and PI (yes/no, 4 sites per implant), using a conventional manual periodontal probe (PCP15, Hu-Friedy, Milan, Italy) with about a 0.3 N force. Operator calibration was considered satisfactory when the percent fitting between the first and second measurements taken at a 24-h gap was ≥ 90%.

Primary outcome variables

1. Objectively measurable volumetric changes (cubic mm) of the ROI regions between day 0 and 12 months.
2. Objectively measurable linear dimensional changes (LD, mm) of the ROI regions between day 0 and 12 months;

Secondary outcome variables

1. Changes in periodontal probing depth PD (mm)
2. Changes in periodontal bleeding on probing BoP (yes/no),
3. Changes in plaque index PI (yes/no)
4. Subjective perception of discomfort and pain during surgery and the post-operative period, semi-quantitatively assessed by Visual Analogue Scale (VAS) questionnaires administered 5 h after surgery and each morning for the next 7 days (score: 0,no pain to 10,extremely painful);
5. analgesics taken (number);
6. willingness to undergo the same treatment again (yes/no).
7. Color and texture differences between peri-implant and natural gingiva at 12 months, subjectively evaluated by the patients at a mirror from speaking distance (visible/not visible).

Operators - Laser-aided roll-flap technique was performed by a dental practitioner (M.G) with long-lasting expertise in periodontal surgery, implant dentistry and use of dental laser technology.

Surgical treatments - The delivered laser-aided roll flap technique was performed as previously described. It consisted in the following steps:

1. De-epithelialization of a vestibular gingival area, 2 mm larger than the cover screw diameter, using a λ 810-nm diode GaAlAs laser (4x4 Dental Laser System, General Project Ltd., Montespertoli, Florence, Italy) equipped with a ∅ 600-μm optical fiber and set as follows: pulse mode, frequency 8,000 Hz, pulse energy 69 μJ, pulse duration 18 μs, corresponding to 0.6 W
2. mean power (peak 3.8 W) and 40 J/square cm irradiation energy. Photoablation was performed in fiber tip contact mode at a constant speed of about 2.5 mm/s under airflow cooling and thermographic feedback to prevent tissue over-heating and damage. The procedure was made under a high-resolution stereomicroscope (Extaro 300, Zeiss, Milan, Italy).
3. Full-thickness U-shaped incision of the photoablated mucosa was made with the same λ 810-nm diode laser set at pulse energy 22 μJ, peak power 3.14 W, frequency 20 kHz, pulse width 7 μs, in contact mode and equipped with a ∅ 300-μm optical fiber. The incision margins were extended to the vestibular gingiva for 2 to 3 mm to facilitate flap displacement.
4. Elevation and rolling of the full-thickness flap into a vestibular pouch, no surgical sutures.
5. Relieve or remove any soft tissue or bone interferences by adjusting the abutment or the tissues.
6. Connection of titanium abutments or healing abutment to the implants. Overall time of the surgical procedure was measured, starting from the end of local anesthesia to connection of titanium abutment.

Post-surgical instructions and prosthetic treatments - No local or systemic antibiotics, mouthwashes or toothpastes containing triclosan, chlorhexidine, or other disinfectants were prescribed. The patients were instructed to avoid brushing of the treated zone for about 6 hours from surgery and then to perform normal oral hygiene.

Prosthetic treatment including the application of a temporary crown and the final rehabilitation was performed according to a standard treatment plan.

Statistical analysis - This was performed with Prism5 software (GraphPad, Boston, MA, USA). The ROI of each treated implant/tooth site was taken as unit of analysis. Differences in ROI volumes (mean ±SEM) measured before the treatment (day 0) and at 12-month follow-up were analysed statistically by Student's t test for paired values. A p value ≤0.05 was considered significant.