Implant-supported mandibular denture: planning to delivery – a case report

Diagnosis and treatment planning

A diagnosis of an atrophic mandible was made, together with an unstable and poorly retentive mandibular denture with superficial positioning of the mental foramina secondary to bone resorption. The treatment aim was ‘To provide a functional and aesthetic dentition for the mandible with a view to reduce pressure on the mental foramina and edentulous ridge of the posterior segments’. The key issue behind this case was poor bone support in the mandible and this would be improved with the following treatment options:

Option 1 was initially considered, however, after reassessment it was decided that a successful lower removable prosthesis could not be constructed that would meet the treatment aim based on the clinical situation. Option 2a was considered and, although there is good evidence8 to support the use of a mandibular two-implant-supported removable prosthesis, there was concern over the possibility of denture rotation around the two implants during function.17 This would put pressure on the posterior segments of the mandible causing soreness, an indication for the provision of a four-implant overdenture (Table 1). Option 2b would be the most appropriate for the patient. The patient already had a complete upper denture and, by providing more support across the anterior part of the mandible, it would reduce loading on the posterior mandible and the areas overlying the mental foramina. Occlusal load may then be diluted across more of the upper denture as opposed to being concentrated in the anterior maxilla (Option 3). The options were discussed with the patient and she preferred to have a removable prosthesis as she has always been used to taking her dentures out at night. The treatment plan for this patient was split into four phases: Surgical, Healing, Restorative and Review (Table 2).

Surgical planning

Planning implant placement requires careful assessment of the radiographs and clinical scenario, as well as knowledge of the risks involved. It is important to know the anatomy of the area, ensuring implant placement is planned to avoid damage to nerves and haemorrhage from blood vessels during the procedure. It is also important to ensure implants are completely surrounded by bone. A perforation through the inferior border of the mandible can make it prone to fracture. In the mandible, implant placement usually occurs between the mental foramina (interforaminal region). Placing implants posterior to this foramen can encroach upon the inferior dental canal and damage nerves and blood vessels housed within. Placing implants close to the mental foramen, but in the interforaminal region, can also lead to complications due to the presence of the anterior loop of the mental nerve. The anatomy of the inferior dental canal can take the nerves and blood vessels anterior to the mental foramen, whilst still in bone, then track back to escape from the mental foramen. Guidelines to protect against damage to a possible anterior loop state that implants should be placed at least 2 mm away from the mental foramen. However, all radiographs should be examined first for a possible anterior loop and the mental foramen probed to exclude its presence.18 If implant placement is intended to occur close to sensitive structures, such as nerves, or if, in the case of the maxilla, the maxillary sinus, a cone beam computed tomography (CBCT) scan should be undertaken which can accurately identify these structures. Implant placement can then be guided with the use of a surgical guide stent constructed from the CBCT scan. This provides a more reliable and accurate method for implant placement but does incur additional costs and radiation exposure for the patient. In the case described here, a dental panoramic radiograph and lateral cephalograph were sufficient to identify the anatomy of the mandible and, together with clinical measurements, were able to identify the dimensions of the implants required. Based on the width of the ridge (intra-oral assessment: visual and palpation) and the height of the mandible (radiographs and radiographic implant guide), four 4 mm diameter 11 mm length titanium implants (OsseoSpeed^TM, AstraTech, Sweden) were used. Positioning implants in the mandible requires knowledge of the shape of the final prosthesis and tooth position. If an implant is incorrectly placed, removal may result in a bony defect and render the bone unsuitable for implant placement without surgical bone grafting. Prior to surgery, a copy technique19 was used to produce a surgical guide from the patients' current removable mandibular complete denture. Tooth positioning and shape of this denture were both aesthetically and functionally ideal and it was planned to replicate both these qualities in the final overdenture prosthesis. The copied denture was constructed in clear acrylic and modified to allow access to the edentulous anterior mandibular ridge whilst still being able to identify tooth position. Blue cross hairs (Figure 3) identify the positions planned for implant placement based on the positioning of the teeth on the surgical guide which was used to ensure optimal positioning of the implant during placement.

Surgical procedures

A mucoperiosteal incision was made on the crest of the alveolar ridge and down into the labial sulcus (Figure 4). A flap was raised and the bony ridge was exposed (Figure 5). Both right and left mental foraminae were visualized prior to implant placement. Using the surgical guide, a pilot hole was made with a rose head bur marking implant position and then the first narrow drill was used. It is essential that both labial and lingual plates of the mandible are visualized, and can be palpated whilst preparing for implant placement, to ensure correct drill alignment and prevent a lateral perforation of the mandible. Alignment pins were then used to show the position of the implants relative to each other and their angulation in the jaw (Figure 6). If at this stage a misalignment is seen (as in the left distal implant in Figure 6), the alignment can be corrected using the second wide drill and cross checked with the alignment pins once again. Once alignment was confirmed, the implants were screwed into place (Figure 7) and either cover screws placed or, in this case healing abutments, to prevent gingival growth into the implant. Alternatively, implants can be directly loaded by screwing the abutment fixture into the implant and then the denture can either engage or rest on this abutment. The mucosa was sutured closed around the implants. The use of healing abutments allows for the placement of the final abutment without a second surgical procedure. If cover screws are used, the implants would need to be surgically exposed to place the final abutments.

Post surgical care

A temporary prosthesis can be placed on the day of surgery or after initial healing has occurred. A temporary prosthesis was made by copying19 the patient's current lower denture and modifying to allow space for the healing abutments. A tissue conditioner, Viscogel (Dentsply, DeTrey, GmbH, Germany), was used to line the temporary lower denture around the surgical site to aid comfort and improve stability. At 8 weeks following surgery, initial healing had occurred and it was discovered that the left distal healing abutment had been covered over by mucosa (Figure 8). A second procedure was undertaken to expose the healing abutment and suture down the mucosa. A longer healing abutment was placed to ensure that the mucosa did not grow over the abutment.

Restorative planning

Basic restorative planning occurs before treatment commences. Prior knowledge of the system to be used to retain and support the prosthesis is essential before implants are placed.

Assessment should include consideration of the placement of artificial teeth following traditional complete denture planning. This should include reference to the anatomy of the denture-bearing area and the neutral zone, together with the level of retention and support required. The patient should be involved with the treatment planning options (informed consent). To decide upon which attachment system (bar or stud) to use, an assessment of interocclusal space and denture shape was required. Interocclusal space was assessed with the use of study models articulated to the correct occlusal vertical dimension for the patient. The analysis revealed limited space between the occlusal plane and mandibular denture-bearing area. An option would be to increase the height of the occlusal plane, increasing the area below, however, this would affect the overall aesthetics of the final denture set. The bucco-lingual width of the denture and angle of taper towards the occlusal plane were measured. The narrow shape of the denture, together with a limited vertical dimension, excluded the use of a bar as an attachment system without increasing the bulk of the denture and changing the aesthetics. A stud attachment, such as the Locator^® system (Zest Anchors, USA) is a low profile stud used in situations where space for the attachment system is limited. The patient preferred a system that would make the least changes to her current denture shape and aesthetics. The attachment system used was the Locator^® system, the height of which is determined by the thickness of the mucosa overlying the implants. Measurements are made from the implant to the surface mucosa and specific height abutments are screwed into the implant with the cuff standing at least 1 mm higher than the level of the mucosa (Figure 9).

Impression taking

Impressions involving implants may either be at implant level or abutment level. Implant level impressions allow for the production of superstructures (bar) that fit directly on to the implant and are held in by screws without the need for an intervening abutment. This allows for greater flexibility when designing and producing the prosthesis. Abutment level impressions are taken where retentive attachments such as studs or magnets are to be utilized. The attachment is incorporated directly into the abutment. An impression was taken at abutment level since the Locator^® abutments and sizes had already been confirmed and were present in the mouth. Pick-up impression copings were placed before the impression was taken and used to transfer the position of the abutment on to an impression. Once the impression was removed and decontamination had taken place, abutment replicas were placed into the copings. It is important to check the accuracy of your final impression ensuring there are no defects around where the abutments/implants are recorded in the impression material, but also to ensure that the impression has been functionally moulded and has recorded a good level of surface detail of the edentulous ridge. As well as the superstructure, a denture will also have to be closely adapted to the denture-bearing area. In the laboratory, the impression was cast and the abutment replicas retained within the dental stone, providing a representation of the mandibular denture-bearing area with abutment positioning (Figure 10).

Jaw registration and try-in

A wax registration rim with a rigid acrylic baseplate was constructed on the lower master cast. Embedded into the fit surface of the baseplate were four metal housings with laboratory (black) Locator^® inserts corresponding with the positions of the abutments in the mouth (Figure 11). The lower wax registration rim was assessed for stability and retention. Failure to achieve correct seating of the registration rim is a possible indication of inaccuracies at the impression-taking or casting stage and the lower impression would require re-taking. The patient's upper denture was copied using silicone material and stock trays.19

The wax and acrylic template produced by copying the denture was registered against the lower rim in the retruded contact position. The trial dentures were assessed for retention, stability and aesthetics, ensuring a balanced occlusal scheme had been created offering minimal lateral displacement forces during function.

Denture delivery

Once the lower denture was processed in heat-cured acrylic, the laboratory Locator^® inserts were replaced with coloured inserts. Each coloured insert offers a differing level of retention from high (blue) to low (green). The lowest retention inserts, red and green, do not have a central plug in the insert which allows them to be used when implants are angled more than 20 degrees from each other. The level of retention chosen is based on how easily the denture is displaced in the mouth during function. The denture should not be made too retentive otherwise the patient may not be able to remove the denture on his/her own. The correct retention level was chosen and the patient was shown how to seat and remove the prosthesis unaided. The patient was advised that initially she may feel some discomfort from either the soft tissue or from bone surrounding the implant due to functional load transferred from the denture to the implant. The patient was advised not to bite heavily into food and to start off eating slowly until she got used to the new experience of having an implant-retained and supported denture (Figure 12).

Review and maintenance

There were few problems with the mandibular implant-supported and retained denture. Numbness and pain that was a feature of her old dentures was now absent and chewing food was more efficient. She was also pleased with her appearance. Initially, regular reviews are required to ensure that the patient takes care of the implants and dentures correctly. A lapse in oral hygiene after the initial joy of being able to keep dentures in the mouth may occur, which can result in peri-implantitis and loss of the implant.20 The nylon Locator^® inserts can degrade with use, decreasing the retention level of the denture. Replacement of these inserts is dependant on masticatory load21 (Figure 13).