Computer Aided Surgical Guide

Digital revolution is the shift from mechanical and analogue electronic technology to digital electronics with the adoption of computers, and it is rapidly transforming the world. Today, computers and digital devices are inevitable part of our daily life, and dentistry is no exception. Dentistry has recently been dominated by various digital technologies such as cone beam computed tomography (CBCT), computer aided design and computer aided manufacturing (CAD/CAM), stereolithography, intraoral scanners, and 3-dimensional (3-D) implant planning software for virtual implant treatment planning and surgical guide fabrication. To overcome the limitations associated with conventional radiographic surgical stent, computer aided surgical guide have been evolved. A computer generated surgical guide provides a link between our treatment plan and the actual surgery by transferring the simulated plan accurately to surgical site. This surgical guide is made using stereolithography process and is custom manufactured for each patient.

Stereolithography, a rapid prototyping technology, a newer outcome in dentistry allows the fabrication of surgical guides from 3D computer generated models for precise placement of the implants. The surgical templates fabricated by this technology are preprogrammed with individual depth, angulations, mesiodistal and labiolingual positioning of the implant.

Fabrication of computer aided surgical guide requires CBCT images. In CBCT, multi planar reformatting allows one to reformat a volumetric dataset in sagittal, axial, and coronal planes and also helps in building multiple cross-sectional and panoramic views. Shaded surface display and volume rendering methods generate 3D reconstructions of the entire dental arch and their relevant structures, including nerves, which makes dental CBCT the most precise and comprehensive radiologic technique for dental implant planning. Software′s specially planned has been adapted to allow practitioners to virtually view the implant site and plan location, angle, depth, and diameter of virtual implants, which are superimposed on the 3D data.

Computer guided implant placement by using the information from CBCT imaging, 3-D implant planning software and stereolithographic surgical guide has recently become a popular treatment modality. Moreover, this technique allowed flapless implant placement, which offers a few advantages such as shorter surgical time, faster recuperation, less post-operative pain and swelling, and better esthetic outcomes.

Requirements for CAD/CAM surgical guide

1. CBCT scan (fig.2)
2. PVS impression or Intraoral scan (fig.3)

[fig.2] CBCT images

[fig.3] Scan data

Procedure

1. Merging CBCT data to Scan data from impression or intraoral scanner (fig.4a)
2. Implant Planning with virtual crowns (fig.4b~d)
3. Design surgical guide (fig.4e)
   
   (a) (b)
   
   (c) (d)
   
   (e)

   [fig.4] Procedure of implant planning

4. Send pictures and surgical report for confirmation to dental office (fig.5,6)
5. After Approval, fabricate surgical guide using 3D printer and metal sleeves
6. Sterilization
7. Delivery Surgical report, Drill protocol and Surgical guide

[fig.5] Implant information

[fig.6] Images for approval

Advantages

1. More precise placement of implants even if septum exists.(fig.7a, 7b)
2. High geometrical accuracy of 0.1 mm.
3. Shorter treatment times, surgery times.
4. Less invasive, flapless surgery and therefore less chance of swelling.
5. Less post-operative strain on dentist and patient.
6. Dentist can view the virtual 3D model from different angles using the software to customize the treatment plan.

(a) (b)
[fig.7] Precise placement of implant using CAD software

Considerations for implant planning

1. Ideal placement of implant
   The position of implant within the bone is important to minimize stress to the fixture. An axial force over the long axis of fixture generates compressive stress. However, cantilever forces and lateral forces induce compressive stress, shear stress and flexural stress. High flexural stress and shear stress might provoke severe bone loss or failure of implant. The position of implant should be thoroughly considered when planning to prevent those detrimental stresses to the implants.
   • Anterior
     • Cement retained crown; Slightly lingual to the incisal edge
     • Screw retained crown; Cingulum
   • Posterior: Central fossa
2. Distances from implant to other structures should be followed the criteria in [table.1] to avoid complication or bone loss induces implant failure. (fig.7b, fig.8)

   
   (a) (b)
   [fig.8] Distances from implant to other structures
   [Table.1] Distances from implant to other structures
   [fig.9] Examples should be avoided
   

3. In the apicocoronal dimension, implant should be positioned 3.0mm from free gingival margin for ideal emergence profile especially at anterior region. (fig. 10a)
4. Stress in the posterior mandible with poor bone quality is influenced by both implant diameter and length. Moreover, implant diameter played more significant roles in reducing cortical bone stress and enhancing implant stability, while implant length was more effective in reducing cancellous bone stress under both axial force and lateral force. In the posterior region, minimum 4.5 mm diameter of implant should be used to avoid failure of implant due to bone loss or strong occlusal force. Implant diameter exceeding 4.0mm and length exceeding 12mm is a relatively optimal selection for a screwed dental implant in the posterior mandible with poor bone quality.

   (a) (b)
   [fig.10] (a) Ideal apicocoronal dimension, (b) Anchor pin

5. Retentive anchor pin (fig.10b)
   As you can see above, the anchor pins produce retentions of surgical guide in edentulous arch. But it might cause damage to the gum and bone, fracture of bone, discomfort and pain and increasing possibility of infection. Also it makes recovery time longer.
6. The density of jaw bones
   Bone density could be used as a reference to decide implant diameter and length. (fig.11a)
   1. Type of bone (by Misch, 1988)
      1. D1: Dense cortical bone
      2. D2: Thick porous cortical bone on the crest and coarse trabecular bone underneath
      3. D3: Thinner porous cortical crest and fine trabecular bone within
      4. D4: almost no crestal cortical bone and fine trabecular bone composes almost all of the total volume of bone
         
         (a) (b)
         [fig.11] Bone density
   2. (2) Average density of bone (Housefield units, HU)
      
      

Possibilities

Conventional surgical stent and CAD/CAM surgical guide were reviewed and the considerations for implant placement were mentioned. The advent of CAD/CAM technologies in the dentistry has brought new possibilities. The development of digital surgical stent in implantology is a revolution of treatment planning. It brings incredible benefits for the patients and the practitioners such as efficiency, precision and minimally invasive surgery.

The exponential evolution of computer aided design and manufacturing makes a very bright future for guided surgery. More and more options are becoming available, with increasing precision, faster turnaround times, and cheaper fabrication costs. The software options available allow for more complete integration of surgical and prosthetic planning than before.