Implant Restoration in the edentulous mandible
Patient who wears conventional mandibular denture often will complaint about the difficulty in functioning, unstable and slip out of place during speech, poor chewing ability, and most importantly continuous bone resorption which requires relining of denture and/or compromises the retention of the prosthesis. The most serious problem is when the bone resorption approaching the proximity of inferior alveolar nerve, of which the patient will feel the pressure and pain from the force exerted directly over the alveolar ridge. As the ridge continue to wear down, retention becomes severely compromise. Implant support allows for force distribution into the mandible and away from the tissue.
One of the better option is the implant supported overdenture. It is a regular denture that can be removed and inserted by the patient. However, instead of the denture resting on the gum with all the support came from the soft tissue, the implant support overdenture usually is supported by a bar connecting four or more implants with the use of clip or retentive mechanism. The cast metal bar helps to stabilize the implant and provides a retentive mechanism (usually a clip) to hold the denture in place. Another design is the use of attachment(ERA) or better yet, the locator. This design usually is chosen in the case where interarch distance is inadequate for the cast metal bar design. The locator provides the denture retention through the male-female attachment. Ring of various tightness allows the patient to choose light, moderate, or tight retention. The implant gives the denture substantial increase in bite force, much more retention during function, relieve of full palatal coverage which help in speech, and finally, tremendous reduction of ridge resorption due to direct compressive pressure on the alveolar ridge.
Another option is the implant-supported fixed complete denture. This denture is supported by five or more implants and is fixed (not removable). The restoration is retrievable or removable only by the dentist for cleaning at each recall appointment. The disadvantage of this option is the inaccessibility for proper oral hygiene. Difficulty cleaning in the area of implants leading to more implant failure, halitosis (bad breath), and tissue inflammation. Another disadvantage is that more implants required and thus the treatment is more costly.
In my opinion, the best treatment for the complete edentulous patient is the implant supported overdenture with the cast gold bar or the use of locator. I preferred dental gold over metal due to the hygienic and inert property of gold. It is most cost effective and patient can remove the denture for its cleaning and also he or she can maintain better oral hygiene at the implant sites. This will aid greatly to the longevity of the implant services. All the treatment protocol explained below will pertain to the implant supported overdenture as the treatment of choice.
What is involved with the treatment?
First we must determine the number of implants, implant positioning, and final prosthesis design are determined. We must anticipate the end result before we begin the treatment. Treatment planing phase can never be completed without a mounted diagnostic cast, from which we constructed a surgical guide for our implant placement. A well mounted diagnostic cast also provides the interarch maxillo-mandibular relationship to determine our occlusion and also the interarch width from which to choose our retention mechanism (cast metal bar or locator/resilient attachment). The smile line should be registered as the wax trial denture setup performed. Radiographic included a panoramic x-ray. Tomogram or bone sounding should be performed in the implant site where adequate bone width is questionable. In the mandible, the good number of implants is four. I prefer one extra implant in the central incisor area as to provide better support during biting in the anterior area. Four implant allowed for maximal implant support, increased retention, increased stability due to shorter cantelever distance. The cantelever distance is the distance from the most posterior functioning tooth to the closest adjacent implant, also referred to as the length of the occlusal surface from the most distal abutment to the distalmost contact. The maximum cantelever distance (cantilever limitation) is calculated by measuring the distance from the most anterior implant to the most distal implant, then multiply by 1.5 or 2. Finally, implant positioning is governed by the patient maxillo-mandibular relation(normal, overbite, or underbite). Implant should be positioned such that to maximize the bone support during function.
Next is the implant surgical stage. I always graft the ridge width simultaneously using bone graft and membrane barrier if the ridge is inadequate. Remember the more bone supported under the prosthesis, the better for longevity of the prosthesis function. This is especially true for the mandible because there is no palatal coverage or support. All implants should be placed as parallel as much as possible. This will provide stability and better force distribution to the implant site. Moreover, the use of locator only allowed 15 degree of play in the path of draw. The path of draw of the prosthesis should be parallel to the angulation of the implant in the jaw.
After 6-8 months in the mandible from the date of surgical implant placement(to ensure osseointegration of the implant), we uncover the implant and attach the transmucosal component which emerges through the gum tissue. We then proceed to begin fabrication of the implant bar and the overdenture prosthesis after 28 days of soft tissue healing. The occlusal gingival height of the bar should be 6-8mm or the bar will flex especially at the cantelever portion. Flexing of the bar will put lateral stress to the implant abutment, cause inflammation to the surrounding tissue, and lead to implant failure. The fabrication of denture involves preliminary impression, final impression, wax try-in, bar try-in, and insertion of denture (at least 5 appointments, 1-2 weeks apart). In the mandible, careful consideration of the most distal occlusion should be made based on the number of implant and the cantelever distance. In the maxillary, complete palatal coverage is recommended unless patient have five or more implants placed. Another trick to add strength to the reduced palatal coverage prosthesis is to insert a metal framework into the acrylic(similar to that of the partial). My experience have shown that a definitive implant overdenture should have a palatal finish line or be reinforced with a metal framework. For more information on fabrication of maxillary implant supported overdenture please refer to Implant Support Overdenture, Maxillary Bar under services menu. Adjustment(s) are made for several appointments after insertion of denture. Once all the adjustment have been made, patient will report better function, speech, stronger bite force, and better retention.
How is the bone density affect the treatment planning?
Especially in the maxilla where less densed bone have been encountered, more implants needed to be placed if the prosthesis is to be fully implant support. The average number of implants used for the maxilla is five and the mandible is four. This is especially true in the situation where the bar is used. Bone density have been classified by Carl Misch as followed (from most dense to least dense)
- Dense compact
- Porous compact
- Coarse trabecular
- Fine trabecular
The dense compact bone is very dense and found in the resorb anterior mandible. A threaded implant will provide immediate fixation and initial stabilization. This bone type represents the greatest percentage of bone at the implant interface (80%). On the opposite end of the spectrum, the fine trabecular bone has very light density and no cortical crestal bone. It can be found in the posterior maxilla of the long term edentulous patient. It is very porous and thus bone height and width is crucial in term of implant initial stability during placement.