From Volume 51, Issue 8, September 2024 | Pages 593-598
Ideal prosthetic and aesthetic results in implant therapy results from successful implant placement. Such success depends on a number of factors; one of which is the accurate transfer of information from the planning stage to the surgical field. Surgical templates are instrumental in allowing such transfer of information. This article aims to review various designs of implant guides and their accuracy.
It is well documented in the literature that there is clear correlation between surgical and prosthetic complications and improper implant placement.1 As such, it is imperative that methodical treatment planning on implant position is undertaken and that this is translated to the oral cavity during surgery. The use of a surgical guide facilitates this transfer with regards to implant position, angulation and depth.2
A surgical guide is the combination of a contact surface that fits onto a patient's dentition and/or edentulous space to stabilize the stent in the operative field and guiding cylinders, which assist in drill orientation.3 This article reviews different types of surgical guides used for implant placement and the level of accuracy expected.
Surgical guides have three main uses, which are highlighted in Table 1.
| To guide osteotomy drills at the optimum location, angulation and correct depth |
| To guide implant fixtures at the optimum location, angulation and correct depth |
| To guide the extent of bone removal/bone harvesting required |
Advances in medical imaging and digital software have revolutionized the treatment planning of prosthetically driven implant provision.5 In turn, this has facilitated the development of novel and advanced techniques to fabricate surgical guides that are more accurate.
Prior to fabrication of a surgical stent, a diagnostic tooth arrangement is undertaken in one of the following ways:
Ideal requisites of a surgical guide are given in Table 2.
| Provision to hold radiographic markers, so that during diagnostic imaging, there is contrast between implant trajectory sites and the guide |
| Reproducible and economical to fabricate |
| Well retained within the surgical field (i.e. does not easily move during the procedure being undertaken) |
| Allows easy access of drills etc (Figure 2) |
| A high degree of accuracy in translating pre-surgical diagnostic information to the surgical area |
Stumpel was one of the pioneers in surgical stent use for implant placement. His work led to the identification of three fabrication design concepts:9
Each of these design notions differ with regards to the extent of surgical restriction that they offer.
The simplest of the concepts is non-limiting design. It gives the surgeon only an idea of where the final prosthesis sits in relation to the proposed implant site. This type of guide does not provide information on spatial planes and, as such, does not help orientate drill angulation. This can then result in room for greater operator error regarding angulation and thus there can be great flexibility in the final implant position.10
The most common technique using this concept involves the fabrication of a clear acrylic splint over a duplicate cast of the diagnostic wax-up of the final intended prosthesis, as described by Engelman et al (Figure 3). A guide pin hole is then drilled through this to indicate the ideal implant position. However, no information regarding spatial orientation is provided and so the operator relies on adjacent and opposing teeth to determine angulation.11
Almog et al described a similar technique. However, there was also the circumferential incorporation of a lead strip on the fit surface of the clear acrylic splint, outlining the buccally, occlusal and lingual/palatal dimensions and position of the intended restoration overlying the implant site. This technique is shown in Figure 4.12
The literature widely acknowledges that these non-limiting guides can lead to unacceptable angulation and improper implant placement. As such, they are not necessarily used as surgical guides, but rather as imaging indicators during implant placement. The ability to then radiographically cross-check the final position of the implant helps the surgeon during placement to determine parallelism with adjacent structures.10
A partially limiting design directs the initial osteotomy via a guide sleeve. The rest of the implant surgery is finished freehand by the operator.9 This concept involves construction of a radiographic template. Once radiographic evaluation is complete, it can then be converted into a surgical guide. The literature has described multiple techniques for the fabrication of surgical guides that harness this concept. They vary in terms of the material used, the type of radiographic marker employed, the method of medical imaging and the way in which the imaging marker is converted into a surgical stent.13 These techniques are documented in Table 3. Although a partially limiting design offers the surgeon more information over a non-limiting design, none of the techniques described in Table 3 can completely control the angulation of drills during implant placement. This can lead to incorrect implant angulation and parallelism to adjacent structures. Furthermore, as the apico-coronal plane is not considered, there is room for error with regards to iatrogenic damage to vital structures as a result of overdrilling.
| Reference | Material for template construction | Radiographic marker | Medical imaging | Conversion process | Notes |
|---|---|---|---|---|---|
| Engelman et al11 | Self-polymerizing acrylic resin | Metal ball-bearings | Orthopantomogram | Trim lingual aspect to leave only the facial surface of the proposed restoration overlying the implant site | An economical and simple method that accounts for good visibility and space for irrigation to prevent overheating |
| Adrian et al14 | Self-polymerizing acrylic resin | Lead strip over upper and lower incisors, over one side of the mandibular occlusal place and on the fit surface of a mandibular trial denture | Lateral cephalometry | Radiopaque images used to determine implant angulation. Use of a lateral ceph tracing to transfer data to resin plane joining both maxilla and mandible | Helps to guide implant position and angulation. Also assists operator access by retracting the tongue and acting as a bite block |
| Marino et al15 | Heat-cured acrylic resin | A combination of coloured chalk and dual-cured composite resin | Computed tomography | Trim buccal of the proposed restoration overlying the implant site | Used in partially dentate patients |
| Takeshita et al16 | In completely edentulous patients, a denture is constructed with self-curing acrylic for the denture base, and the teeth are constructed from a combination of resin polymer and barium sulphate monomer | Stainless-steel sleeves | Orthopantomogram and computed tomography | Removal of the sleeve sprues | The radiopaque barium sulphate highlights the predetermined superstructure and the stainless-steel sleeves indicate implant position and trajectory A good method for completely edentulous cases |
| Ku and Shen17 | Vacuum-formed stent filled with self-curing acrylic resin | Gutta-percha | Computed tomography | Remove the radiographic marker with carbide bur | Suitable for single implant placement or short-span implant-retained prostheses |
| Becker and Kaiser8 | Vacuum-formed thermoplastic stent and orthodontic resin | 5/32- and 3/16-inch brass tubes | - | The 3/16-inch tube attaches to the stent and the 5/32-inch tube guides the initial drilling | A more precise partially limiting design than previously described |
| Almog et al12 | Vacuum-formed thermoplastic stent over duplicate cast of diagnostic wax-up | Lead strip placed on the buccal, occlusal and lingual aspect of proposed restoration overlying implant site | Computed tomography | Remove lead strip | Has been shown to be less-accurate in bucco-lingual aspect of implant placement |
| Gutta-percha | Remove gutta-percha | Allows flexibility in surgical latitude during initially osteotomy | |||
| Tsuchida et al19 | Self-curing acrylic resin | Silicone impression material | Computed tomography | Remove silicone material. Trim the buccal/lingual aspects of the surgical stent | Silicone markers are beneficial as they do not cause artefacts during CT scanning |
| Arfai and Kiat-Amnuay20 | Self-curing acrylic resin | 3/32-inch brass rods | Peri-apical radiography | Remove brass rods | Using a dental surveyor further improves accuracy |
This concept does what it says on the tin. It limits the trajectory of all the instruments used for the osteotomy in both bucco-lingual and mesio-distal plane. In addition, the addition of a depth stop ensures of correct length of preparation apico-coronally. As the design of the surgical guide becomes more restrictive, there is less flexibility intra-operatively.
The traditional method of fabrication of a surgical guide with a completely limiting design involved the use of a cast-based guided surgical guide.13 Initially the technique of bone sounding is employed to determine the thickness of the overlying soft tissue (Figure 5). These values are then subtracted from the alveolar ridge width to give an indication of the volume of bone at the sites measured.9 Perez et al demonstrated that this technique is reproducible, but slightly underestimates the volume of bone, thereby providing surgeons with a reliable method that also has a small degree of safety.21 Secondly, peri-apical radiographs of the root structure are manipulated using software, and are then transposed onto the cast (Figure 6). Thereafter, the cast is sectioned at the predetermined implant site (Figure 7); bone sounding values are also applied, to aid the trajectory of the drill bit during cast osteotomy. A laboratory analogue is inserted (Figure 8), along with a guide sleeve modified with wires to create a mesh around the teeth (Figure 9). Finally, polysiloxane silicone is used to create a superstructure (Figure 10).
This technique, although widely used, has inherent flaws that reduce its accuracy. The guide is constructed on a dental cast that is rigid and gives no indication of the resiliency of the soft tissue or the underlying bone morphology. Furthermore, anatomical landmarks, such as nerve supply or location of the floor of the sinus, are not located exactly. Consequently, even though an attempt at three-dimensional planning is attempted, the method is still two-dimensional. The use of panoramic imaging is also a source for inaccuracy owing to the risk of distortion and positional artefacts. Notwithstanding, the lack of bucco-lingual bone width information also limits its diagnostic value. All the above introduce small errors along the workflow, which can greatly influence implant misposition.
With the evolution of technology, computer-aided systems have become commonplace to overcome the limitations described with the conventional cast technique.
This technology uses data from computed tomography (CT scans) or cone-beam CT (CBCT). These images are deciphered and extrapolated using planning software. CAD involves placement of virtual images on screen to determine anatomical relationships, as well as being able to perform accurate measurements.7 This pre-surgical plan can be transferred to the operative field using CAM whereby stereolithography is used to fabricate three-dimensional surgical drill guides. Stereolithography involves a laser beam traversing above a photoreactive fluid acrylic, causing polymerization in layers so as to fabricate a surgical guide as designed with the software. Stainless-steel sleeves are then positioned within the guide at the pre-determined implant positions.
Nokar et al has studied the accuracy of implant surgery using this technology in vitro. The results showed that there was little difference between the pre-planned implant trajectory in both bucco-lingual and mesio-distal planes and length of implant and the final implant position in situ. As such, it was concluded that CAD/CAM offered improved accuracy in implant placement over the conventional cast technique.22 When considering the evidence base, one must also consider the quality of the evidence. Being an in vitro study, the results must be used with caution owing to the inherent difficulties in replicating the unique surgical situation intra-orally with that achieved within a laboratory setting.
Nonetheless, the many advantages of this novel technique are well described in the literature, as evidenced in Table 4. However, the technique does have some drawbacks, documented in Table 5.
| Allows precise placement of implants in all three planes |
| Documented accuracy of 0.1 mm between the pre-planned implant position within the software and that achieved after surgery10 |
| 3D planning allows anatomic structures, such as the inferior alveolar canal to be accurately evaluated, thereby reducing the risk of iatrogenic damage |
| Precise pre-surgical evaluation of bony topography |
| Reduced surgery time using a method which is reproducible |
| Reduced patient morbidity as the flapless technique results in less post-operative pain and swelling |
| Transparent guides help with visualization intra-operatively |
| As the process is becoming more mainstream, the costs are coming down, but can still be more expensive than a conventionally fabricated guide |
| It has been postulated that there is less tactile control when using these guides |
| The sleeves for drill pieces are narrow to prevent any change in drill angulation. As such, there can be an issue with adequate irrigation during drilling, resulting in overheating of bone |
| Errors in construction such as errors with the stereolithography can result in errors in implant positioning |
| A flapless technique prevents the surgeon from directly visualizing the proposed implant site. As such, any abnormality in density/morphology not detected in the planning stage may result in errors during surgery |
| The use of a CT scan exposes patients to a higher radiation dose |
When researching factors that affect accurate implant placement, three main factors are shown to be significant, namely, the surgical guide design, the experience level of the surgeon, and the size of the edentulous zone. Electronic and hand searching of the evidence base revealed that the most common classification for surgical guide designs ranged from the simple non-limiting to partially limiting and finally the completely limiting guide design. As the guide becomes more restrictive, there is less intra-operative decision-making and reduced flexibility in surgical execution. Despite the fact that the completely limiting concept is acknowledged as better, many surgeons still employ a partially limiting design owing to its long history of use and cost-effectiveness. Moreover, the evidence shows that clinicians routinely use cross-sectional imaging to aid pre-surgical planning with a view to facilitate guidance during surgical placement. With the advent of CAD/CAM technology, precision has improved and the degree of uncertainty prior to placement has reduced, allowing clinicians to undertake comprehensive rehabilitation with more confidence. Nonetheless, linear and angular discrepancies can still exist even with CAD/CAM.
Despite all the advances in technology, one thing remains the same – the need for comprehensive pre-surgery treatment planning with consideration of anatomical constraints and prosthetic requirements. Without this, it is difficult to avoid intra-operative complications. The use of surgical guides helps with predictable placement and, in turn, a better prosthetic outcome, providing optimum function, aesthetics and hygiene maintenance.
