Guided Surgery Part 1: Conventional surgical stent
Dental implants have become a treatment options widely used for the replacement of lost teeth. The development of dental implants has had a major impact on the patients and the implant supported oral restoration has become an increasingly used treatment option for partially edentulous and completely edentulous patients, also even in patients with severe bone loss and in locations which all previously considered unsuitable for implant placement has been made possible by means of bone augmentation, regeneration and soft tissue regeneration procedures. The success of dental implants in the treatment of patients is directly related to patient evaluation and good treatment planning.
Earlier dentists were intended to place implants where the greatest amount of bone was present, with less regard to placement of final definitive restoration. In most of the times, the placement of implant is not as accurate as intended. Even a minor variation in comparison to ideal placement causes difficulties in fabrication of final prostheses. Failures arise as a result of lack of consideration of the super structure during pre-surgical planning. Accurate placement is required to achieve best functional and esthetic result. Since the oral cavity is a relatively restricted space, a high degree of accuracy in placement of implant is very important for success of the prostheses. This can be achieved by means of a surgical guide which provides adequate information regarding implant placement and at the time of surgery it fits on to the existing dentition or on to the edentulous span. This article evaluates the conventionally used radiographic templates with their fabrication, advantages, disadvantages and a review of computer aided surgical guide.
Conventional Surgical Template
The surgical template enables a predictable and a safe minimally invasive surgery. The main objective of surgical template is to direct the implant drilling system and provide an accurate placement of the implant according to the surgical treatment plan. To precisely transfer the plan to the operative site, customized conventional radiographic or computer image guided surgical templates have become a treatment of choice.
A surgical guide is the union of two components: The guiding cylinders and the contact surface. The contact surface fits either on the patient′s tissue or on the patient′s bone or teeth. Cylinders with the drill guides help in transferring the plan by guiding the drill in the exact location and orientation. The bottom and sides of implant must be covered fully by bone or bone-replacement material. There should be the care of not damaging any neighboring anatomic structures. These are in particular the mandibular nerve of mandible and the sinus membrane of maxilla and also the roots of adjacent teeth. Moreover, position of the implant has to be compatible with the intended final prosthodontic restoration.
The radiographic template is the key to the success, since it allows the transfer of the predetermined prosthetic setup to the actual implant planning. In surgical template that makes use of a conventional radiographic method, a thorough radiographic examination and proper diagnosis of the bony architecture are fundamental prerequisites. Panoramic radiography is still the standard for planning of implants. However, precise measuring of the bone architecture is impossible, because they have a magnification factor that is not always uniform. Therefore a better assessment of the bone dimensions in panoramic radiographs is by determination of the magnification factor. Conventional dental panoramic radiography and plain film radiography are usually performed with the patient wearing a radiographic template with integrated metal spheres or rods, sleeves, guide posts at the position of the wax up. Based on the magnification factor and the known dimensions of the metal, the depth and dimensions of the implants are planned. The implant placement planning is guided by quality and quantity of bone, as well as the position of the teeth for esthetics and phonetics.
Fabrication process of conventional method
Several types of surgical guides have been reported in the literature. Some are designed for placement of a single implant, while other reports present designs for implant fixed partial dentures, multiple single implants, and implant-retained overdentures. Some of the most commonly used techniques are mentioned here.
- Diagnostic casts of the dental arches are made from impressions. A diagnostic wax up of the proposed in case of an implant supported FPD is done. A putty mold of the cast with the waxed FPD is made. A clear, chemically activated acrylic resin is poured into the mold space and cured. Access holes are made according to information obtained from the cast model for initial surgical drill. Stainless steel guide sleeves of uniform length is cut and placed in access holes and cured. (fig.1a)
- Another method to prepare a radiographic guide is from vacuum formed templates. After the diagnostic wax up of the final restoration is completed, duplication is made and a cast is poured. The vacuum formed template fabricated is placed over the cast and the edentulous space is filled with radio opaque material. (fig.1b)
- In the other method, it make use of two vacuum formed templates, one over the blocked out diagnostic cast and other over the duplicate cast of the diagnostic wax up with a clear plastic sheet is made. Both the templates are returned to the unaltered diagnostic cast. The edges of the two templates are trimmed to make them coincident. The diagnostic wax template is removed and filled with clear orthodontic resin or radio opaque material. The filled template is placed over the template of the unaltered diagnostic cast. Holes are made according to information obtained from the radiograph for placement of implants, followed by placement of drill guides. (fig.1c)
[fig.1] Conventional surgical stent
The radiopaque markers help in redesigning the direction of implant placement and in comparing the angulations of radiopaque markers with the available bone and also in locating the position of the vital structures to determine the best angulations for the implant. These radio-opaque markers can be placed in the center of the occlusal surfaces of the teeth that corresponds to the screw access holes of the prosthesis.
The milling technique is an accurate technique in which it employs parallel holes in the surgical guide. This technique needs the aid of a conventional dental surveyor. All the conventional made radiographic guides can be converted to an accurate surgical guide by means of this milling technique. Limitation in this technique is, it requires special equipment not commonly available in private dental practices. In addition, the practitioner needs a certain amount of experience and knowledge to use this machine properly.
However, panoramic radiography which is still the standard and widely used, has diagnostic limitations such as expansion and distortion, setting error, positional artifacts and there is no information regarding the dimension of bone in buccolingual direction. Further these surgical templates are fabricated on dental casts, which is a rigid, nonfunctional surface without the knowledge of underlying soft tissue resiliency and bone topography. Anatomical landmarks are not precisely located, it does not show the lingual blood vessels, and approach is always two dimensional. So thereby more chances of malpositioning the implants during placement. Always there is less stability during surgery. The success of the final outcome always depends on clinician skill and alertness. It requires more chair time, leads to stress on the dentist and patient. Although conventional surgical templates will allow the placement of implant guiding, they do not provide exact 3D guidance.