Fig 1 (see description in graphics file)
Fig 1 (see description in graphics file)
(Int J Oral Maxillofac Implants 1996;11:106-111)
The concept of creating a biocompatible relationship with titanium in bone is based on research that began in 1952. The continuing studies in the early 1960's indicated the possibility of establishing true implant anchorage in bone tissue. Radiologic and histologic analyses indicated that this anchorage, termed osseointegration by Brånemark, could maintain bridge stability for ten years. Osseointegration has been defined as "a direct structural and functional connection between ordered living bone and the surface of a load carrying implant". With the introduction of osseointegration in North America in 1982, many advances, particularly in the prosthetic applications, have been developed.
The conversion prosthesis, introduced by Balshi in 1985, is an innovation designed to create a transitional fixed implant prosthesis immediately following the placement of abutments at stage II surgery,. This technique permits stable function and the advantage of a fixed prosthesis. The conversion prosthesis is dependent on well constructed transitional dentures which provide the patient with the correct occlusal vertical dimension, esthetics, and phonetics, as well as lip and perioral muscle support.
Prosthetic management is important to the success of osseointegration and the final prosthetic reconstruction. Brånemark's initial recommendations were that all prosthetic treatment be deferred for at least two weeks after the implant placement surgery3.
Following this healing phase and after suture removal, the transitional denture is soft lined. The imperative issue in this form of treatment is the unloaded bone healing around the implants for 3 to 6 months. Experienced clinicians recognize that even indirect loading through mucosa may effect and even negate the osseointegration process.
An increasingly popular method of avoiding direct or indirect loading on early osseointegrating implants, is the Class III modification of the Brånemark method. This method maintains periodontally hopeless teeth to support a temporarily nonremovable prosthesis, suspended over the implant sites. Converting this fixed form of temporary prosthesis to a "conversion prosthesis" has been previously described.
The purpose of this article is to describe the conversion prosthesis process for the edentulous patient using a two stage implant system with recent modifications in technique.
CONVERSION PROCESS TECHNIQUE
Step 1: Abutment Connection
In the two stage implant systems, while anesthesia is administered for the surgical exposure of the implants, the denture is reinforced with clear acrylic resin and a wire on the facial and buccal surfaces of the teeth (Fig 1). The reinforcement should not interfere with the occlusion. The soft tissue is reflected to permit cover screw removal. The implants are then verified for osseointegration and appropriate abutments are securely connected.
Radiographic verification should be used to check for the complete seating of all the abutments to implants especially when the junction is not clinically visible, and to evaluate the marginal bone height and perifixtural radiopacity.
The recommended torque should be applied to the abutments using appropriate counter torque devices to ensure torque being delivered to the abutment screw and not the bone implant interface10.
Step 2: Prosthetic Coping
Modified screw retained, stainless steel, square impression copings (modified copings) are used as transitional prosthetic cylinders. The impression copings are modified to clear the occlusion. Guidepins are also modified to clear the occlusion or soft tissue. The modified copings and guide pins should be as long as possible in order to allow for the formation of tall screw access channels. In areas where interocclusal space is limited, gold prosthetic retaining screws can be used instead of the modified guidepins (Fig 2).
Step 3: Modifying the Denture
The position of the modified copings can be identified relative to the transitional denture either visually or by use of an ink stick (Dr. Thompson's Sanitary Color Transfer Applicators, Great Plains Dental Products Co., Inc., Kingmon, Kansas) transferred to the denture.
The facial wire reinforcement permits lingual reduction of the denture while minimizing the potential for fracture. The reduction should be conservative to retain adequate thickness and strength of the base and to reduce the amount of acrylic resin necessary to join the copings to the denture in order to limit shrinkage and distortion. Relief is completed when the denture occludes with the opposing arch in the same manner previously designed without contacting the implant components. Soft tissue contacts are maintained to provide a vertical stop for the denture and preserve the original vertical dimension of occlusion (Fig 3).
Step 4: Soft Tissue Protection
Protecting the freshly sutured soft tissue adjacent to the modified copings will promote proper healing. A rubber dam is cut to cover the surgical site. It should not interfere with reseating the denture. Protective coverage should extend to the vestibule and the floor of the mouth. The transfer ink sticks are used to mark the top of either the modified impression copings or the screws. The rubber dam is pressed over the ink to identify the precise location of the holes to be punched. The rubber dam is then seated over the modified copings. The margin of the dam should be located on the modified impression coping or prosthetic cylinder, totally protecting the titanium abutment beneath (Fig 4).
Step 5: Acrylic Application
With the rubber dam in place the occlusion is verified with the denture stabilized in its correct arch position (Fig 5). Holding the denture in that position the mouth is opened to check clearance of the implant components (Fig 6). Once adequate clearance is achieved, a small amount of autopolymerizing resin is placed around the modified copings using a thin mix in a modified Monojet syringe (Sherwood Medical, St. Louis, MO) (Fig 7). The altered denture is then repositioned and acrylic added to join the denture to the copings filling the area where the denture was relieved. Before the acrylic resin sets the patient is asked to close into the previously established centric occlusion position. The clinician may help guide the denture into proper interocclusal contact and the rubber dam can be raised and manipulated to help form a smooth underside to the soft acrylic.
The screws must be completely visible prior to acrylic setting. After the acrylic resin hardens but prior to detectable heat from the exothermic reaction, the screws are removed from the modified copings allowing them to disengage the abutments.
The denture with the modified copings attached in acrylic resin is removed from the mouth and immediately placed in the chairside pressure pot for 10 minutes with water at 125o F under 30 PSI pressure.
Step 6: Reinforcement and finishing
Close inspection of the denture will show areas where the acrylic has not been completely adapted to the modified copings. Long guidepins are used to tightly secure the brass abutment analogues to the prosthetic copings. Acrylic is added to fill the voids between the pick-up acrylic and the modified copings. The brass analogues prevent the acrylic from adhering to the base of the modified copings which would interfere with the fit of the prosthesis. The long guidepins help form smooth screw access channels. The prosthesis is again placed in the pressure pot.
Only the crest of the denture base is allowed to remain in contact with the residual ridge in distal extension cantilever situations. Excessive acrylic including the flanges and palate in the maxilla is then trimmed.
The conversion prosthesis can be checked both clinically and radiographically to verify the fit of the modified copings. Any misaligned copings should be removed from the conversion prosthesis and the pickup procedure repeated for that individual site.
The occlusion is adjusted and the conversion prosthesis inspected for any unusual rough spots or angles that might prove annoying to the cheeks or tongue. It is then removed from the mouth and placed on the master cast, verifying the accuracy of the master cast.
The conversion prosthesis also rigidly records the occlusal vertical dimension and facilitates occlusal registration for mounting the master cast on the articulator. The original vertical dimension of occlusion is preserved and related on the master casts with or without the use of an interocclusal recording medium (Fig 8). After the casts are mounted the cantilevered portions of the conversion prosthesis are reduced accordingly (Fig 9). The prosthesis is then highly polished to minimize plaque retention and delivered. The screw access holes are covered with cotton, followed by either Cavit (Espe, W. Germany) or Fermit (Vivadent; Liechtenstein).
Discussion
There are significant biological and mechanical differences between the conversion prosthesis philosophy and traditional methods of prosthetic management after stage II surgery. Traditionally abutments are placed at stage II surgery with healing caps to keep the abutment screw holes free of debris. More recently healing abutments have been used to permit the mucosa to heal and mature. These are replaced by the prosthetic abutments as a third stage procedure. Both methods require relief of the denture, and in most instances, the use of a soft liner creating an overdenture, with its concomitant loading through the individual abutments to the implants and bone. Some clinicians recommended that no denture be used at least during the first three to four weeks following abutment connection to allow for undisturbed soft tissue healing. The use of a conversion prosthesis is a concept of prosthetic management following stage II surgery that helps promote patient comfort and acceptance. It provides a fixed dentition immediately following abutment connection, eliminating the need for healing abutments, healing caps and soft liners, and in effect reduces the time the patient is required to function with a removable prosthesis. Its stability provides improved functional ability and allows the patient to preview the function and esthetics of a fixed implant prosthesis.
Langer and Sullivan state that immediate coronal stability of joined implants reduces the potential rotation effect on the implants when compared to those not joined. The conversion prosthesis provides a more uniform load distribution to the implants through splinting. There may also be a benefit in maintaining the patient for a period of time with acrylic occlusal surfaces because this provides some load dampening to the implants.
Although the combined surgical and prosthetic procedure creates a lengthy treatment session, it reduces the overall treatment visits. Care is taken when fabricating the conversion prosthesis to avoid complications. A rubber dam is placed, completely covering the titanium abutment, so that the acrylic does not "lock in" to the undercuts created between the abutments. Screw access holes are cleared prior to setting of acrylic, or a high speed drill is necessary to expose the screw heads for removal. Both the rubber dam and rapid removal of the conversion prosthesis prior to the release of heat from the setting of acrylic is essential to avoid trauma to the soft tissue and bone to implant interface.
When fabricating the conversion prosthesis, the clinician is able to visualize potential problem areas such as screw access holes emerging in unfavorable sites and change an abutment to improve the position of a screw access hole prior to taking a final impression. An impression of the conversion prosthesis provides the laboratory with a model for fabrication of the final prosthesis. Patient preferences are noted and correlations are made in the final prosthesis.
The conversion prosthesis provides a most stable unit upon which the interocclusal records are obtained and accurately transferred to the master cast and articulated (Fig 8). When transferred it is then used as a verification jig, similar to the ones described by Rasmussen and Henry, to verify the accuracy of the master cast prior to framework fabrication.
Advantages in long term maintenance of the patient have been realized by using the conversion prosthesis. In situations where complications have occurred with the final fixed detachable prosthesis which have required an extended period of laboratory repair time, the conversion prosthesis has been used as a provisional fixed replacement. In addition the conversion prosthesis acts as a record to preserve the original vertical dimension of occlusion. After a period of extended function, some patients have exhibited a reduced vertical dimension of occlusion due to wear of the prosthetic teeth. In these situations the conversions prosthesis has been placed on the master cast and mounted on the articulator against the opposing arch with a new interocclusal record. The final prosthesis is then placed on the master cast and the appropriate laboratory procedures are carried out to restore or replace the worn prosthetic material while the patient functions in the meantime with the conversion prosthesis.
Recognizing its advantages, this prosthesis has been used routinely by the authors for the past decade. Since its introduction some of the steps have been modified to the technique currently described above.
The original protocol involved the use of gold cylinders incorporated into the conversion prosthesis. This was costly since there was no intention to retrieve the cylinders after the delivery of the final prosthesis. The gold cylinders were replaced with modified stainless steel square impression copings. Using the impression copings rather than gold cylinders have resulted in a cost saving of approximately 80%. When compared to the traditional method of using healing abutments and healing caps, the savings is approximately 67%. And when compared to using healing caps only, the cost is very similar.
The early recommendation of using a periodontal pak beneath the conversion prosthesis to control soft tissue has since been abandoned. The conversion prosthesis ultimately acts as a shield to the sutured soft tissue and allows for relatively undisturbed healing.
Confusion in the literature
In an article in 1992, Harry Rosen inadvertently referred to a technique which he reported as the "conversion prosthesis". The prosthesis he described is a "modified immediate provisional removable prosthesis constructed with salvaged fixed prosthetic restorations and executed chairside". The CONVERSION PROSTHESIS which was first introduced by Balshi in 19856,7 is totally opposite to that described by Rosen. Rosen converts a failing fixed prosthesis to a provisional removable prosthesis; while Balshi's conversion prosthesis converts a removable denture to a fixed implant supported prosthesis.
SUMMARY
An updated technique for converting a transitional removable denture to a fixed provisional implant supported prosthesis, termed the "CONVERSION PROSTHESIS", has been described. The advantages of this technique are numerous for the patient, the prosthodontist, and the laboratory technician. This technique provides superior comfort and satisfaction and improves the quality of care provided.
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