August 1, 2001
by Blake Nicolucci, BSc., DDS and Carl E. Misch, DDS, MDS
Root form implants are the most popular current implant design, and usually are the only implant designs used by most practitioners. Yet, over the last 15 years in the United States and Canada, over 2,000 implant surgeons have inserted more than 15,000 subperiosteal implants, and these services have been used in more than 17 Universities (Root Laboratory, Leewood KS, Communication 2001). Approximately 85% of subperiosteal implants are placed in the completely edentulous mandible. However, the posterior mandible may be without teeth long term, while the anterior dentition remains healthy. The posterior mandibular height is lost four times faster than the anterior after tooth loss. There are more than 10 million Americans with anterior remaining teeth and bilateral posterior edentulous sites. Many of these patients have inadequate bone height for endosteal implants in the posterior regions. In these situations, a circumferential or universal subperiosteal implant may be an alternative treatment. This is a variation of the complete mandibular subperiosteal, and circumvents existing and/or anterior teeth or root form implants.
The subperiosteal implant is a framework made of surgical Vitallium (Chrome, Cobalt, Molybdenum). Each implant is unique in that it is a custom device fabricated for each individual patient from a direct bone impression or a CT scan generated model. It is inserted directly on the bone, under the periosteum. The increased surface area of the more modern designs act and function much like a “snow shoe on snow”, by dissipating the forces over a large area. The permucosal posts are cast to the substructures thus eliminating the need for a second stage surgery after initial healing. The supra-structures to be used on the posts of a circumferential subperiosteal are generally cemented fixed partial dentures.
Although there have been many reports of complete subperiosteal implants in the literature, fewer reports exist for the circumferential subperiosteal implant. It was first published by Weber in 1950, and has been widely presented at scientific meetings on implant dentistry.
The primary indication for the mandibular circumferential subperiosteal implant is a partially edentulous Kennedy Class I patient missing all or most of the posterior teeth and with inadequate available bone for endosteal implants. Rarely are second and third molars replaced when these are the only teeth missing.
A mandibular bilateral edentulous segment with molars and premolar(s) missing and with inadequate available bone for endosteal implants has several options. A traditional removable partial denture may be fabricated with the shortcomings inherent to this treatment modality. Bone augmentation is the second option. However to gain adequate height, additional extra-oral sources may be indicated. To date, alloplastic or allografts with or without a small pore membrane are not as predictable for adequate available bone height beyond 6 mm growth in moderately resorbed posterior mandibles compared to obtaining prosthetic support with a subperiosteal implant. In addition, an extended treatment time is required for bone augmentation and implant interface development plus additional cost to the patient for grafts and two or more endosteal implants on each side. Nerve repositioning or bypassing the nerve on the lingual aspect are other alternatives. These techniques require additional specific training and yield higher risks of paresthesia and/or poor abutment position compared with the circumferential subperiosteal implant. Rarely is height augmentation or nerve repositioning indicated for endosteal implant in the posterior mandible to replace only one or two teeth, because of the risks or additional costs.
The typical implant patient for the circumferential subperiosteal mandibular implant is therefore a Kennedy Applegate Class 1, Misch/Judy Division C mandible. Inadequate available bone in the posterior mandible may result from long-term partial edentulism, severe periodontal disease or trauma before tooth loss, or endosteal implant failure. Resorption of the alveolar ridge has often progressed during the several years that elapsed between the loss of the teeth and the decision to restore a mandibular posterior area. The anterior limit of the edentulous segment is either a canine or a first premolar.
The type of restoration typically fabricated on this implant is a fixed prosthesis. The decision to splint the restoration to the remaining teeth or to let it be self-standing depends on several factors that are evaluated during the dental evaluation. In general, the final prosthesis replaces teeth to the fist molar when premolars and molars are missing, or an endosteal implant is used in the first premolar position and a subperiosteal implant replaces the second premolar and the first molar. The implant-supported prosthesis extends to the mesial one-half of the second molar when the second premolar and molars are missing.
The circumferential subperiosteal implant has three prosthetic options. When the available bone is Division C, but has several mm of height and is in a patient with immediate posterior disclusion in all excursions, an independent restoration unattached to any teeth may be fabricated. In general the dental evaluation is most critical for independent restorations. An absence of tongue thrust (anterior or posterior), little to no parafunction (clenching or bruxism), and a mutually protected or canine guidance occlusion are suggested requirements. A second prosthetic option is to splint the circumferential subperiosteal implant to one or more anterior teeth. However, the tooth-retained portion of the restoration must be rigid, with no clinical mobility. Splinting several anterior teeth may be required to achieve rigidity of the anterior segment. The third prosthetic option is to place an anterior endosteal implant, usually in front of the mental foramen. The rigid implant may be splinted with the subperiosteal for an independent implant prosthesis. This treatment option is more ideal than joining the implant to a canine tooth because lateral forces would be introduced into the implant prosthetic system during excursion.
Anterior root form implants may be inserted into a completely edentulous mandible. However, if the patient has a square arch form, posterior cantilevers of the prosthetics may be contraindicated. If the patient desires or needs posterior implant support, but is a Division C bone volume, a circumferential subperiosteal may be indicated with the anterior root form implants. The prosthetic option may be fixed or removable, depending upon the other force factors found with the patient.
The dental examination identifies conditions that may contraindicate the procedure or be detrimental to the long-termed predictable success. Linkow identified the noxious effect of lateral forces such as those generated by a lateral tongue thrust on unilateral subperiosteal implants. This may constitute a contraindication because the volume and geometry of the remaining ridge do not permit stabilization of the implant bucco-lingually by engaging the mylohyoid and medial ramus regions. Severe parafunction, especially bruxism, may generate lateral forces on the implant and compromise its long-termed survival. In presence of strong masticatory dynamics, the restoration may need greater support and additional abutments. Extending the anterior aspect of the implant substructure past the canine helps resist the dislodging lateral or protrusive forces. However, extension of the prosthesis to the canine introduces additional lateral forces and should be avoided whenever possible.
The existing occlusion, functional scheme (canine disclusion vs. group function), and number of missing teeth all affect the treatment plan. For example, the location of the maxillary second molar is closely evaluated. If extruded, this tooth may interfere with the ascending ramus posts of the subperiosteal implant and lead to complications. In small vertical spaces, the maxillary second molar may need to be extracted or at least the occlusal surface recontoured to be above the regular plane of occlusion to accommodate the necessary space of the ramus bar.
A CASE REPORT
The surgical technique for the procedure mirrors that of the complete subperiosteal implant. As a consequence, the portions of the technique that are different will be addressed. The soft tissue reflection is similar for the two stage surgery requiring a bone impression and the surgery required for the implant insertion. Two surgeries, one for a bone impression and another for implant placement, are the norm. However, CT scan models have improved over the years, and is often used instead of a bone impression surgery. In these cases, a one stage implant insertion surgery is most typical. The case that is being presented is a bone impression and implant insertion technique.
The following is a case report of a 50-year-old woman (Fig. 1) who decided against bilateral monocortical block grafts to the posterior mandible for endosseous implants, and chose the circumferential subperiosteal implant for the restoration of the posterior segments of her mandible. Her choice was made on the basis of lower cost, and faster treatment time (and she didn’t want bone grafting). She presented with a B-w ridge, and would have been ideal for a ramus or Symphysis graft, but she made the decision to have a subperiosteal implant placed when all the choices were given, and possible complications explained (Figs. 2 & 3).
A midcrestal incision was made from the retro-molar pad on each aspect of the mandible to the most distal natural tooth in the quadrant. This incision was continued as a sulcular incision around the lingual of the remaining dentition for the reflection of a lingual full thickness flap. The sulcular incision of the facial continues to the mesial of the canines. A bilateral vertical release incision was made at this point. At the superior aspect of the retromolar pad, the incision is carried obliquely on the ascending ramus superiorly and laterally for a distance of about 10 mm. Caution was directed on the lingual aspect of this incision to avoid the lingual nerve, which is usually 2 to 3 mm under the periosteum and medial to the retromolar pad.
A full thickness reflection of the soft tissue was performed bilaterally on the lingual from the mylohyoid area in the first molar region, anteriorly to the digastric fossae, and to the mid-line at the superior genial tubercles. Two cross-arch tie back sutures were placed from the cuspid of one quadrant to the first molar on the opposing side. This held the tongue back and allowed ample room for the impression material. Reflection of the facial tissues permitted identification of the mental foramen, and neurovascular bundles. After this was accomplished, the bundles were marked by placing a tie-back sutures at the corresponding position on the facial flap (Figs. 4 & 5). This suture may be left long for easy identification during the impression stage. Posteriorly, the masseter region was reflected from the lateral aspects of the ramus (Fig. 6). Whether for the bone impression (2 stage implant) or implant placement (1 or 2 stage), specific landmarks were identified before making the impression or implant insertion and include: the neurovascular bundles and the mental foraminae, the superior genial tubercles, the digastric fossae, the mylohyoid ridges, the submandibular fossae, the internal oblique ridges, the external oblique ridges, and the lateral borders of the mandible.
The material of choice for the direct bone impression technique was addition silicone (polyvinylsiloxane), which is very accurate and as important, non-toxic.
To make the bone impression, small amounts of impression material putty was packed under the periosteum starting in the ramus region, then lingual to the teeth, followed by material on the residual crest and the facial on the body and around the mental foraminae. A shortened stock tray (that has been adjusted to fit over the existing dentition without interference) (Fig. 7) was then placed over the impression packed ridge. The bone bite for a circumferential subperiosteal uses the existing dentition to determine the vertical dimension and arch position (Fig. 11). Extra material was placed over the bone impression tray prior to removal, and the patient was instructed to bite firmly to re-establish occlusal contact (Fig. 8). This seats the impression material completely and records the vertical dimension and occlusion. After the impression material set, the neurovascular bundles were checked for freedom from under the impression material (Fig. 9). If impression material flows under the neurovascular bundle, it must be very carefully cut away prior to removing the impression to prevent damage to the structure. Because this material is not radio-opaque, care was taken to remove all material on the bone from under the soft tissue prior to soft tissue closure. The tissues were re-approximated with 3’0′ Vicryl sutures.
The procedures for the laboratory techniques are quite specific if the implant is to fit properly. A two-stage pour of the impression with the stone is similar to the complete subperiosteal procedures (Figs. 12 & 13). When a CT scan model is obtained from a computer model, it may be duplicated in stone, prior to implant fabrication.
After the impressions are poured and set, (Fig. 14), the model is used to design the custom implant device (Figs. 15 & 16). The primary peripheral struts are similar to the complete subperiosteal design, as is the secondary struts and location of the ramus bars, which follows the criteria of Linkow, James, Tatum, and/or Misch. The design impression and model are then sent to the laboratory for implant fabrication (Fig. 17).
After fabrication and passivation, the implant internal aspect (bone surface) is coated with hydroxylapatite (HA) (Fig. 18). This promotes intimate bone contact with the implant, and helps insure a direct bone implant interface. Before insertion, the implant must be sterilized. This is usually accomplished with dry heat sterilization at 325F for 2 hours.
The second surgery, usually 8 to 10 weeks after the bone impression, allows the tissues to heal (Fig. 19) before a secondary implant placement. This allows enough time for the tissues to reestablish a blood supply and decreases the risk of incision line opening. This in turn will make the final suturing much more predictable, and primary closure will be more easily attained (Figs. 20-22).
If the subperiosteal casting is stable, but has voids between the bone and the struts, demineralized freeze-dried bone and an alloplast may be used under the primary and secondary struts. (Fig. 23). This will fill the void, but will rarely form bone unless a blood supply is established from the bone. Suturing may now be completed using 3’0′ Vicryl sutures. Suturing is started in the midline. This will allow proper orientation of the flaps and positioning of the interdental papillae. Ideally, the sutures should be removed in 2 weeks. After healing, the tissues should look healthy, and have good contact around the permucosal posts (Figs. 24 & 25).
A temporary acrylic fixed prosthesis was inserted with no occlusal contacts after soft tissue healing. Lack of micro movement between the implant and bone is important, since the goal is to establish a direct osseous connection between the two.
Final restorations were fabricated after three months. The prosthesis on a circumferential sub is almost always a fixed prosthesis and separate from natural teeth (Figs. 26 & 27). The occlusal surface should be gold or porcelain with reduced occlusal table width, and no lateral contacts during excursions.
During the last 15 years, the authors have inserted 43 circumferential subperiosteal implants. Two of these devises have been removed after 10 and 13 years, for an overall survival rate of 95%. Two other implants have had portions of the posterior struts removed, as a consequence of repeated soft tissue inflammation and infection. The remaining 39 implants and two altered implants are in function with a James, Misch Implant Quality Scale of 1 or 2.
The implant dentist will be faced with a myriad of classifications, combinations, quantities and qualities of bone. The endosteal implant is one of the most frequently used implants around the world, and its very high success rate has been widely documented. The subperiosteal implant is one of the oldest forms of dental implants still in use today. There have been many ten and twenty year studies that indicate these implants have success rates similar to root forms placed in similar ridges. In addition, the circumferential subperiosteal implant does not require nerve repositioning, and/or advance bone-grafting procedures. As a consequence, it may often be the treatment of choice for posterior support of a fixed prosthesis.
Dr. Blake Nicolucci is Oral Health’s editorial board member for Implantology. He maintains a private practice, London, ON, Canada.
Dr. Carl Misch is Associate Professor, Univ. of Michigan, Department of Periodontics. He maintains a private practice in Birmingham, MI, USA.
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