Augmentation procedure

A fully edentulous patient with extreme atrophy of the edentulous upper jaw underwent a maxillary augmentation procedure using iliac crest bone grafts. During this procedure special attention was paid to the placement of the osteosynthesis screws (2.0-mm Champy System; KLS Martin, Tüttlingen, Germany); these were placed perpendicular to the original alveolar process.

Implant planning scan

Two weeks prior to implant installation a cone beam computed tomography (CBCT) scan was acquired using an i-CAT 3D Imaging System (Imaging Sciences International Inc., Hatfield, PA, USA), using a setting of 120 kV peak, pulses of 3–8 mA, 8 cm scan height, and an exposure time of 20 s. This was then reconstructed with 0.3-mm isotropic voxel size. This scan was registered according to the double scan procedure to identify the relationship between the denture and osseous structures.

Implant planning

From the CBCT data, a three-dimensional (3D) model was reconstructed of both the denture and the patient’s osseous structures. Both were then registered using the markers from the double scan procedure. Subsequently, six NobelReplace Straight Groovy implants (Nobel Biocare, Zürich, Switzerland) were planned using Maxilim software (Medicim NV, Mechelen, Belgium). After importing the 3D datasets of the implants, these were virtually placed in an optimal position with respect to both the available bone volume on the one hand and prosthetic demands on the other.

Surgical template creation

During planning, the patient’s denture was used as a basis for the surgical template. By including the exported 3D computer models of the planned implants, a full surgical template was created with the aid of Autodesk 3ds Max Design software (version 2012; Autodesk Inc., San Rafael, CA, USA).

To provide extra stability to the surgical template during surgery and to limit its rotations and translations, the osteosynthesis screws were used to support the surgical template. First, the osteosynthesis screws were segmented and reconstructed from the scan using a grey value threshold for metallic objects (2500 in this case). As these segmented screws showed a very rough outer surface, 3D computer-aided design (CAD) models of the osteosynthesis screws were created with equal diameter and length. These CAD models were registered to the segmented screws to obtain the exact location of the screw head and screw central axis. A surface-based registration was used for this.

To complete this new type of surgical template an additional support structure was created based on the locations of the screws. With an offset of 3 mm buccally over the central axis from the head of the screw CAD models, a half tube structure was created to provide an exact base of support for the later partly unscrewed osteosynthesis screws ( Fig. 1 ). From this virtual model of the surgical template a high temperature resistant biocompatible resin model was manufactured.

Virtual model of the surgical template with drill guide sleeves (blue arrow) at the implant positions and half tube structures (red arrow) at the osteosynthesis screw locations. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Implant placement

During the implant placement procedure, the patient received general nasotracheal anaesthesia. To avoid a discrepancy in the fit due to swelling of the palate and alveolar process, no local anaesthesia was applied. The first step was to place the surgical template into the oral cavity, only supported by the palate and the alveolar process. Guided by the half tubes of the surgical template, it was easy to locate the osteosynthesis screws; a small incision was made, after which the screws were unscrewed by about 2 mm. At this time point, the surgical template was fully supported by the osteosynthesis screws, which were resting in the half tubes, thus providing a stable base for guided implant placement ( Fig. 2 ).

Surgical template resting on the osteosynthesis screws during surgery.

Implant placement was performed according to the NobelGuide procedure. First, one implant was installed in the left maxilla, followed by another implant in the right maxilla, after which the remaining implants were installed alternately. When interference between a screw and a planned implant was noticed during implant placement, two other non-interfering implants were first placed, thereby sufficiently stabilizing the template. Thereafter, the interfering screws could be removed safely without jeopardizing template stability. After implant installation the surgical template was removed together with the remaining osteosynthesis screws.

Validation with a validation tool and IPOP method

A postoperative CBCT scan was acquired using the same settings as for the preoperative scan and registered to the preoperative scan using voxel-based registration. Subsequently, the installed implants were segmented and reconstructed from the scan using a grey value threshold for metallic objects (2500 in this case). Voxel models of the implants of equal diameter and length were imported into the Maxilim software. These voxel models were then registered using voxel-based registration to obtain the exact location of the implant tip and shoulder. Next, the registered implants were exported and the tip and shoulder coordinates extracted. From the resulting 3D coordinates, deviations of the shoulder, tip, angle, and depth were calculated. Implant positions were validated by computing the 3D deviation of the tip and the shoulder point between the planned and final implant position. The depth difference was computed by (orthogonal) projection of the longitudinal axis of the installed implant on that of the planned implant. The 3D inclination was calculated as the difference in angle between the longitudinal axis of the planned versus the installed implant. Following this, the clinically relevant implant deviations in the mesiodistal (MD) and buccolingual (BL) direction were calculated using the Implant Orthogonal Projection (IPOP) validation method. The variables ‘implant tip’, ‘implant shoulder’, ‘angulation’, and ‘depth’ were calculated in the buccolingual and also in the mesiodistal direction. Finally, rotations and translations of the surgical template were mapped.