Prosthetically CAD-CAM–Guided Bone Augmentation of Atrophic Jaws Using Customized Titanium Mesh: Preliminary Results of an Open Prospective Study

Since the 1990s, several studies have described the use of titanium mesh for regeneration of the atrophic maxilla and mandible bones.^1–15  Clinical studies of titanium mesh^1–16  have produced important data regarding exposures or removal: the percentage of exposures ranged between 0%¹⁴  and 52.7%,⁷  and the number of early removals was between 0%^{1,3,4,6,9–11,13,15,16}  and 30.4%,⁷  depending on the protocols used by different surgeons (Table 1). A more recent study¹⁷  documented the complete workflow of prosthetically guided bone regeneration (PGBR), describing the step-by-step digital design and rapid prototyping (RP) procedures required to obtain custom-made mesh for minimal intervention surgery. Moreover, Sumida et al¹⁸  compared computer-aided design–computer-aided machining (CAD-CAM) custom-made mesh with conventional mesh for the bone augmentation of atrophic maxillary arches. The operation time and the number of retaining screws were significantly lower than those in the conventional group (both P < .01); the incidence of mucosal rupture with mesh exposure and the number of infections were not significantly different between groups (P = .27 for both variables).

This study presents preliminary data on 9 patients undergoing bone augmentation of partially or totally atrophic maxillary arches using customized titanium mesh prior to implant surgery. The aims of this pilot study were to evaluate the outcomes of customized titanium mesh loaded with autologous bone chips and anorganic bovine bone (Bio-Oss, Geistlich Pharma, Wolhusen, Switzerland) in a 1:1 ratio and used for prosthetically guided bone augmentation, to calculate the vertical bone volume gain of the mandible and maxilla, and to evaluate complications, such as mesh exposure.

The study was approved by the S. Orsola Hospital Ethics Committee in May 2013 (approval No. 121/2013/O/Disp). Nine healthy patients, 6 women and 3 men, with a mean age of 50 years (range: 25 to 68 years), with no systemic contraindications for treatment (diabetes mellitus or heavy smoking, >10 cigarettes a day) were included in this preliminary data analysis.

Each implant site was evaluated in either the mandible and/or the maxilla by positioning an implant that was at least 8-mm long (WinSix, Biosafin Srl, Ancona, Italy; Keystone Dental, Burlington, Mass) and observing the required bone regeneration, via data obtained with cone-beam computerized tomography (CT; NewTom, Cefla, Verona, Italy). Using NobelClinician software (Nobel Biocare, Kloten, Switzerland), each dental element was positioned as a function of the occlusion in relation to the buccopalatal/lingual position of the opposite masticatory arch, according to the concept of PGBR (Figure 1). Because NobelClinician does not include volume calculation for grafting procedures, the 2-dimensional project screenshots (upper, frontal, and lateral views) were imported into the Mimics Innovation Suite software (version 17.0, Materialise, Leuven, Belgium) for conversion into 3-dimensional (3D) models. The skull of the maxilla and the mandible was reconstructed by setting the same threshold values used in NobelClinician. Then, the plan was exported into general-purpose CAD software (Freeform Modelling Plus, version 13.0, 3D Systems, Rock Hill, SC) to design the bone augmentation, guided by the positions of the implants and corresponding containment mesh.

The 1.5-mm safety zone around each implant was virtually modeled to provide the minimal bone width/height of available bone to enhance the clinical safety of the implant procedure (Figure 2a and b). Over this augmented bone volume, we projected an overcontouring of 1.5-mm thickness to compensate for the biological formation of the pseudo-periosteum (connective tissue)¹⁹  that typically develops between the titanium mesh and the bone (Figure 2c). The mesh was calibrated at a 0.3-mm thickness, and holes in the mesh were calibrated at 1-mm diameter for printing with the direct metal laser sintering (DMLS) technique, by using a rapid-prototyping machine.

After completing the CAD design, the STL file was 3D-printed using an EOSINT M270 (Electro Optical Systems, Munich, Germany), a DMLS machine. The metal powder used for RP was titanium Ti64, a prealloyed Ti₆AIV₄ alloy in fine powder form with excellent mechanical properties and corrosion resistance, low specific weight, and high biocompatibility. The thickness of the mesh was reduced up to 0.1 mm by laminating the metal after printing without increasing its fragility.

The success criteria were defined as follows:

1. Ability to insert implants as previously planned on a virtual surgery simulator (according to a planned minimal intervention) and its influence on the final outcome

2. Evaluation of the vertical bone gain

3. Presence and timing of mesh exposure

The observation period ranged from 6 to 18 months after implant placement. Patients were scheduled to undergo the bone augmentation procedure under general or local anesthesia, depending on whether the autologous bone graft was harvested from the iliac crest or the mandibular ramus. A midcrestal incision was made, maximizing the keratinized mucosa on each side of the incision over the atrophic edentulous ridge. Perforations into the marrow space were produced using a round bur to improve vascularization and incorporation of the graft (Figure 3a). The mesh was loaded with autologous bone chips (harvested from the mandibular ramus or the iliac crest) and anorganic bovine bone (Bio-Oss, Geistlich Pharma) in a 1:1 ratio and was fixed with 1, or at most 2, osteosynthesis screws (Synthes, West Chester, Penn; Figure 3b). Releasing incisions of the periosteum along the buccal flap allowed extension of the flap coronally over the mesh to enable passive primary closure. The flap was sutured with Vicryl 4-0 (Johnson & Johnson Intl, Ethicon GmbH, Norderstedt, Germany). Postoperatively, patients received antibiotic (amoxicillin and clavulanic acid) and analgesic (ibuprofen) and were instructed to rinse twice daily for 2 weeks (digluconate chlorhexidine, 0.12%).

After a 6- to 8-month healing period, a new CT scan data set was collected to verify the bone augmentation prior to implant surgery. In all patients, the regenerated bone was sufficient to position the planned implants (n = 26). At the time of implant surgery, the titanium mesh was removed, and the implants were inserted using a 2-step technique (submerged implants). After 4-month (maxilla) and 3-month (mandible) healing periods, the implants were uncovered, and prosthetic procedures for construction of the prosthesis were performed (Figure 4). No implant failure was observed after a 2-year follow-up.

The CT data of postoperative results after implant surgery were used to calculate the bone gain obtained with regenerative therapy. The linear vertical bone augmentation was calculated based on superimposition of the preoperative and postoperative CT. A virtual calculation of bone augmentation was performed in relation to each planned implant site (Figures 5 and 6). Table 2 shows all results of the pre/postoperative bone augmentation, the incidence and timing of mesh exposure, and its removal time during follow-up.

The vertical bone augmentation was digitally measured in relation to the long axis of each programmed implant site. The single values of the augmentation are presented in Table 2. Results data were not considered as mean values to compare with other studies because of their individual specific planning for minimal bone augmentation. However, the data showed 1.72 mm to 4.1 mm of bone reconstruction in the mandible and 2.14 to 6.88 mm in the maxilla.

Table 2 provides data on the exposure and removal time of the mesh: 3 of 9 patients showed premature exposure of the mesh (2 to 4 weeks), and 3 of 9 patients showed a delayed exposure (10–24 weeks). All meshes except one, which was removed at 3 months with evident discharge of purulent exudate, were removed at 6–8 months after surgery. The mesh exposures were examined every 2 months for clinical signs of infection, and the patients were instructed to use a soft toothbrush to apply chlorhexidine digluconate gel 1% (Corsodyl gel, GlaxoSmithKline, Baranzate, Italy) twice a day to the exposed mesh.

Several studies have supported the excellent mechanical properties of commercial (noncustomized) titanium mesh: sufficient rigidity, plasticity for manual bending, and elasticity for the prevention of flap compression.^4,19–22  Ciocca et al²³  described a further problem with commercial mesh. After superimposition of the mesh postoperative CT data on the planned volume of bone to restore, they registered a compression of the mesh in the palatal area that was interpreted as possibly being due to wearing the prosthesis (full denture) during follow-up for regenerative surgery or as a probable further deformation due to screw fixation of the mesh in the palate.

Other studies reported the results of customized titanium mesh in terms of accuracy when using RP techniques.^24,25  The CAD-CAM technology used in this study allowed for programming of the ideal position of implants for atrophic maxillary/mandibular total or partial arches, taking into account the occlusal intermaxillary relationship by using CAD software; moreover, this procedure permitted projecting the least bone augmentation required for implant surgery. When designing the mesh, the virtual project must consider, as an overcontouring with respect to the expected bone augmentation, the width (1.2–1.5 mm) of the connective tissue that develops between the mesh and the regenerated bone (pseudo-periosteum). Moreover, the time savings afforded by intraoperative mesh cutting, the bending and adaptation, and the postoperative site morbidity due to the limited bone harvest (in turn due to the planned augmentation) represent advantages of this protocol.

In terms of bone augmentation, 2 main variables were analyzed: the increase in the vertical bone level and mesh exposure. For the first variable, the method described by Ciocca et al²⁶  was used to calculate the amount of bone (Figure 6). The premature (within 4–6 weeks) or delayed (after 4–6 weeks) exposure of the mesh did not limit bone augmentation (mean value: 3.83 mm for the mandible and 3.95 mm for the maxilla) for purposes of the implant surgery, as all programmed implants were positioned according to the treatment plan, even after premature mesh exposure. However, in this case series study, the incidence of mesh exposure was important, and consequently, there was a possibility of infection that could jeopardize the desired augmentation. In this regard, 6 patients reported mesh exposure (66%), and 1 needed to remove the mesh early (3 months) to prevent infection risks. Although all instances of wound dehiscence were controlled with local antibacterial rinse and gel during the 6- to 8-month follow-up before mesh removal, the continuous monitoring of these situations is a weakness of this procedure. This rate of mesh exposure might be due to the stiffness of the titanium mesh, which could cause mechanical irritation to the mucosal flap²³  that may be worsened if a provisional removable prosthesis is used in the edentulous augmented crest during the healing period. Mesh exposure may be also due to the learning curve associated with CAD-CAM mesh projecting: when virtual design of the mesh is performed, the mesiodistal end of the titanium mesh should be separated at least 2 mm from the adjacent tooth, to avoid creating a pathway for the penetration of bacteria. Moreover, the external shape of the mesh should be projected as smoothly as possible to avoid flap tension that can lead to mucosal rupture. However, no flap resuturing was required, and even if these complications occurred, they did not influence implant treatment. No implant failure or bone loss was noted during the follow-up or after the prosthetic rehabilitation.

A disadvantage of this technique may be the cost of the CAD-CAM process needed to prototype the titanium mesh: in comparison with a standard commercially available mesh, the cost was slightly higher (US $425), but the surgical advantage of operation time reduction seems to compensate for this problem.

No sample sizing or power evaluation was performed in this study because of the limited number of implant sites, which does not permit inferential considerations. Further studies are required to develop new resorbable and sufficiently rigid and smoothed mesh that may be directly printed using modern bioprinting technologies.

This study suggests a cautious approach to this procedure, because the CAD-CAM technology used for prosthetically guided bone augmentation was less successful than expected, especially in terms of postoperative morbidity due to mesh exposure. However, this protocol allowed positioning of all programmed implants according to the treatment plan and simplified the surgery by reducing the number of retaining screws to 1, or at most, 2.

Abbreviations

3D

3-dimensional

CAD-CAM

computer aided design–computer aided machining

CT

computerized tomography

DMLS

direct metal laser sintering

PGBR

prosthetically guided bone regeneration

RP

rapid prototyping

The authors thank Dr Andrea Sandi for his valuable work with the CAD and for prototyping the titanium mesh.

No financial funding was received for this research. No conflicts of interest exist for all authors.