Surgical Principles for the Mandibular Two-Implant Overdenture
Joseph R. Carpentieri, DDS • Dennis P. Tarnow, DDS
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.
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
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
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
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.
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:
and uncomplicated implant placement: When no auxiliary procedures (eg, hard and
soft tissue grafting) are required.
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.
Influence of the Restorative Protocol on Surgical Approach
implant placement--No denture--Conventional
implant placement--Denture--Early loading
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:
flap with less resultant discomfort and swelling because muscle attachments are
flap with less resultant bone loss; and
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.
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
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).
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).
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
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.
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).
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.
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).
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
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.
- Mericske-Stern RD, Taylor
TD, Belser U. Management of the edentulous patient. Clin Oral Impants Res
- Visser A, Raghoebar GM, Meijer RH, et al.
Mandibular overdentures supported by two or four implants. A 5-year prospective
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- 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
- 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
- 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.
- Cochran DL. A comparison of endosseous dental
implant surfaces. J Periodontol 1999;70(12):1523-1539.
- Mericske-Stern R, Zarb GA. Overdentures: An
alternative implant methodology for edentulous patients. Int J Prosthodont
- 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.
- 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.
- 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.