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Surgical Principles for the Mandibular Two-Implant Overdenture

While exact implant position is critical for fixed prostheses, it is equally important for removable prostheses where improper placement may negatively affect tooth position, attachment mechanism, and flange extension. Prior to implant placement surgery, the clinician must determine the intended final position of the artificial tooth position and the form of the overdenture. Often, there is a tendency to consider implant placement first and artificial tooth position and overdenture form later.

 

Implant Selection 

Successful treatment with the two-implant overdenture has been documented with multiple implant designs (eg, hex, Morse taper, internal connection) and many implant systems. Clinicians may select implants for retention of the two-implant overdenture according to personal experience and preference with confidence that treatment success will not be determined by the selection made. This is due primarily to the anatomy and density of the bone in the anterior mandible

 

SURGICAL CONSIDERATIONS FOR THE TWO-IMPLANT OVERDENTURE 

1. Final Prosthetic Tooth Form 

To communicate prosthetic requirements to the surgeon, the final denture should be duplicated in clear acrylic resin as the surgical guide. As an alternative, the final denture can also be prepared (ie, with access windows/holes) to serve as the guide and later repaired (Figure 1).

 

2. Available Bone 

The two-implant overdenture will help stabilize bone in the mandible, which resorbs in an anterior, inferior, and lateral manner and thus affects implant placement.

 

3. Final Restorative Design 

Implant placement is restoration-specific, meaning the final position is as unique as each final restorative design. Fixed prostheses are different from removable designs, and the bar overdenture is vastly different from the unsplinted two-implant overdenture.

 

4. Number of Implants: Two 

One way to significantly diminish the cost of implant treatment is simply to reduce the number of implants needed and to keep them unsplinted (eliminating the bar and its laboratory fees). Utilization of two implants has been demonstrated in numerous, long-term studies worldwide to be clinically successful, economically advantageous to the patient, and structurally sufficient to retain an overdenture. In the mandibular anterior region, a smaller number of implants will not adversely affect success rates, meaning fewer implants can be equally effective.1,2 Additionally, randomized controlled clinical trials have demonstrated that patients are equally satisfied with two implants retaining an overdenture as compared to multiple implants.3 Therefore, there is consensus that two implants splinted or unsplinted should be considered the minimal objective for mandibular overdenture treatment. 

 

5. Implant Position: Ideally Canine or Lateral Location 

Implants in the anterior mandible should be placed in the canine or lateral positions. Implants positioned in this slightly more anterior position reduce the tendency for the denture to rotate around the fulcrum provided by the denture. The denture base may lift when the patient incises anteriorly if implants are placed too far distally.

 

NOTE: The unsplinted overdenture is not constrained by specific inter-implant space requirements, meaning no such measurements are necessary. Although not a standard procedure, when a bar (ie, splinted) two-implant overdenture is fabricated, an inter-implant distance of no more than 15 mm to 20 mm is needed to accommodate at least one clip and for metallurgic considerations.

 

6. Surgical Protocol: Ideally One-Stage Procedure 

Comparable clinical success rates have been reported with one-stage versus two-stage implant treatment, including the absence of significant differences in marginal bone resorption and the attainment of similar tissue health.4-6 This and other evidence-based literature is sufficient to support modification of the original two-stage surgical protocol to a one-stage nonsubmerged approach. In addition, one-stage treatment allows use of early loading protocols. The selection of the loading protocol has a significant influence on the course of surgery as well as restorative treatment and must be determined during treatment planning.

 

7. Attach Keratinized Tissue 

The final healing abutments should be surrounded by a circumferential zone of attached healing tissue.

 

Indications for One-Stage Surgery 

Although the one-stage approach is the desired treatment, it is surgically determined and may be altered accordingly. When the following conditions are NOT present, a two-stage surgical protocol is to be used:

  • Simple and uncomplicated implant placement: When no auxiliary procedures (eg, hard and soft tissue grafting) are required.
  • Adequate primary stability must be attained: Resistance of at least 30 Ncm or implant stability quotient of >60 can be achieved at the time of placement.

 

Table 1. Influence of the Restorative Protocol on Surgical Approach

  1. Two-stage implant placement--No denture--Conventional loading protocol
  2. One-stage implant placement--Denture--Early loading protocol

 

SURGICAL PROTOCOL FOR IMPLANT PLACEMENT 

1. Try in the surgical guide to assess available restorative space and determine flap design (Figure 2 and 3).

The restorative space in the facial plane should be evaluated from the buccal bone to the inner aspect of the lingual denture base, not from bone to the incisal edge position. Space evaluation is most easily performed prior to reflection of the surgical flap. Contingent upon the type of overdenture abutment planned, the minimum restorative space (ie, 7mm) should then be verified (Figure 3). This space is necessary to accommodate the height of the abutments, the retentive elements, and an adequate thickness of acrylic without overcontouring the lingual or buccal flange.

 

2. Design the incision 

Incision design will depend on the overall prosthetic needs. Options include:

 

A. A traditional midcrestal incision ending slightly distal to the canine position, followed by a full-thickness flap and buccal and lingual reflection to gain access that will allow final evaluation of the shape, size, and trajectory of the remaining bone (Figures 4-5-6). This is the technique of choice for optimal access and is indicated when osseous recontouring is needed.

 

B. Modification of the traditional approach with a midcrestal incision starting slightly distal to the canines but not crossing the midline (ie, two mini-flaps). This approach is indicated when osseous recontouring is not needed or in the case the edentulous ridge with a wide circumference (Figure 6a). Advantages include:

  • Smaller flap with less resultant discomfort and swelling because muscle attachments are uninvolved;
  • Smaller flap with less resultant bone loss; and
  • Smaller area to reline (ie, sectional reline versus a full reline).

 

C. The punch and flapless technique through intact tissue is indicated for a broad, flat ridge when osseous recontouring is not needed, and an adequate zone of attached tissue is present (Figure 7). Of the three options, this is the most conservative approach in that it results in the least bone loss and smallest area to reline.

 

NOTE: that a distal incision in the zone of keratinized tissue allows attached gingivae on the buccal and lingual sides of the implant after healing. This is recommended for better long-term results, ease of hygiene, and comfort for the patient.

 

3. Re-seat the surgical template 

Using previously established records, the surgical guide is placed in position, taking care that the reflected flap does not impede proper seating of the guide (Figure 8). With the guide in position.

A. FACIAL PLANE: Re-evaluate the inferior-superior dimension and modify space as needed (Figure 9). If additional restorative space is needed and it has been determined that it cannot be obtained prosthetically by increasing the vertical dimension of occlusion, recontour the residual ridge sufficiently to accommodate the overdenture components (Figures 10-11-12-13). It should be remembered that this strategy undermines the function of implant treatment to preserve bone and prevent additional resorption. The technique should be as conservative as possible while maintaining sufficient volume of bone for implant placement.

NOTE: It is important that the osseous crest is flat to minimize the height of the overdenture abutment.

CAUTION: If the osseous crest is inclined, a higher abutment will be required, resulting in an undesirable reduction in overall restorative space.

B. OCCLUSAL PLANE: Evaluate the buccolingual dimension. Implant position may be slightly more lingual as compared to a fixed prosthesis depending on how much ridge resorption has occurred. In the case of the minimally resorbed mandible, a slight lingual position is more ideal (Figure 14). When more extensive resorption is present, the implants could be placed “under” the denture teeth since there is a sufficient bulk of acrylic. To achieve a “layering concept” in the anterior-posterior dimension, implants should be positioned slightly more to the lingual and apical but with the top of the implant angled toward the buccal to minimize bulk lingually. This will provide the sufficient space for overdenture abutment retentive elements, adequate thickness of acrylic, and a full-denture tooth that is modified minimally.

C. SAGITTAL PLANE: Evaluate space in the sagittal plane. Implant position is prosthetically driven. Since the path of draw of the prosthesis is determined by the trajectory of the remaining bone, implants must be placed in this plane (Figure 15). If the trajectory of the bone is facial, the first implant must be as parallel as possible to the facial. The second implant should be parallel to the first. For an extreme trajectory or large facial undercuts, minor osteoplasty is indicated.

 

4. Create the osteotomy 

Osteotomy technique will be based on the implant manufacturer’s recommendations (Figures 16-17-18-19-20).

5. Place the implants as indicated by the surgical template 

Final placement of the implants follows the principles of ideal implant parallelism and maximum initial stabilization, and path of draw (Figures 19-20-21-22).

NOTE: Generally, studies indicate that failure to achieve ideal implant parallelism will result in higher maintenance needs for the unsplinted overdenture patient. Therefore, implant parallelism is of considerable importance from a prosthetic and aftercare perspective.

According to the literature, two standard diameter implants at least 10 mm in length are generally sufficient to provide long-term retention and support for an overdenture prosthesis.7,8 Although the successful use of shorter implants has been reported,9 at this time, there is a lack of available data supporting the use of shorter implants with newer surface topographies or other nanochemical enhancments as a routine procedure. Further research is needed in this area.

NOTE: It cannot be overemphasized how critical it is to avoid lingual perforation during implant placement—hemorrhage of the floor of the mouth is a potentially serious complication (Figures 24 and 25).10 

Supracrestal placement should be considered the ideal surgical endpoint. Countersinking may be needed, however, for clearance for the prosthetic components at times. Generally, this can be avoided with proper planning (Figure 23).

 

6. Place healing abutments 

Since a one-stage protocol is the treatment of choice, the final healing abutment is placed at the time of surgery (Figure 26). Placement of the superior aspect of the healing abutment approximately 1 mm to 2 mm above the final flap position will allow for healing and maturation of the soft tissue. If a two-stage protocol is indicated, a surgical cover screw is placed until the second-stage surgery.

NOTE: Placement of the healing abutment at an excessive height (eg, 4 mm to 5 mm) above the final flap will lead to excessive adjustment of the denture base, resulting in reduced acrylic thickness and an increased risk of denture base fracture. In addition, excessive height in the healing abutment may increase the incidence of micromovement of the implant and makes relieving the denture base considerably more difficult.

 

7. Suture to achieve final closure 

Interrupted sutures are sufficient for closure, but the final decision is at the discretion of the surgeon (Figures 27 and 28).

 

Final Overdenture Abutment Versus Healing Abutment (Figures 29 and 30) 

ADVANTAGES of placement of final abutment

  • Use of less components reduces treatment cost to the patient;
  • Restorative dentist not required to select an abutment; and
  • Restorative dentist’s need for implant instrumentation eliminated.

 

DISADVANTAGES of placement of final abutment

  • Approximating height of the final component becomes more difficult because tissue has not healed. Height may have to be changed later depending on tissue healing; and
  • Difficulty of approximating height may increase the risk of micromovement for the inexperienced practitioner.

Placement of a final abutment is generally easier in the advanced resorbed patient because more space is available and the exact height of the overdenture abutment is less important.

An understanding and proper execution of these surgical principles will significantly simplify the restorative aspect of two-implant overdenture treatment.

 

References: 

 

  1. Mericske-Stern RD, Taylor TD, Belser U. Management of the edentulous patient. Clin Oral Impants Res 2000;11(1):108-250.
  2. Visser A, Raghoebar GM, Meijer RH, et al. Mandibular overdentures supported by two or four implants. A 5-year prospective study. Clin Oral Implants Res 2005;16(1):19-25.
  3. Wismeijer D, van Waas MA, Mulder J. Clinical and radiologic results of patients treated with three implant modalities for overdentures on implants of the ITI Dental Implant System. Clin Oral Implants Res 1999;10(4):297-306.
  4. Szmukler-Moncler S, Piattelli A, Favero GA, Dubruille JH. Considerations preliminary to the application of early and immediate loading protocols in dental implantology. Clin Oral Implants Res 2000;11(1):12-25.
  5. Tawse-Smith A, Payne AG, Kumara R, Thomson WM. A one-stage operative procedure using 2 different implant systems: A prospective study on implant overdentures in the edentulous mandible. Clin Implant Dent Relat Res 2001;3:185-193.
  6. Cochran DL. A comparison of endosseous dental implant surfaces. J Periodontol 1999;70(12):1523-1539.
  7. Mericske-Stern R, Zarb GA. Overdentures: An alternative implant methodology for edentulous patients. Int J Prosthodont 1993;6(2):203-208.
  8. Mericske-Stern R, Piotti M, Sirtes B. 3-D in vivo force measurements on mandibular implants supporting overdentures. A comparative study. Clin Oral Implants Res 1996;7(4):387-396.
  9. Deporter D, Watson P, Pharoah M, et al. Five to six year results of a prospective clinical trial using the Endopore dental implant and mandibular overdenture. Clin Oral Impl Res 1999:10:95-102.
  10. Bruggenkate CT, Krekeler G, Kraaijenhagen H, et al. Hemorrhage of the floor of the mouth resulting from lingual perforation during implant placement: A clinical report. Int J Oral Maxillofac Impl 1993;8:329-334.

 

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