Implants in Partially Edentulous Patients; The UCLA Experience

John Beumer, D.D.S., M.S.

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Let us next talk about the anatomic limitations that may prevent placement of implants of adequate length in the posterior quadrant.

As we know, in the posterior quadrants the maxillary sinus limits the lengths used in the maxilla and the inferior alveolar nerve limits the lengths of the implants used in the mandible. We also know that in the posterior quadrant of the maxilla the bone-implant interface or the bone anchorage for osseointegrated implants is poor because the bone quality is poor.

Primary anchorage for a screw type implant is provided by the implant engaging the cortical bone of the floor of the sinus and the cortical bone on the alveolar ridge. There is generally very little bone implant interface along the course of the implant and if the tip is not properly anchored in the cortical bone in the floor of the sinus, as is seen here on the right, the implant may fail upon occlusal loading .

The use of a cylindrical type of implant with a titanium plasma spray surface shown here may improve the bone anchorage in poor quality bone but still may not provide all patients with sufficient anchorage to support fixed partial dentures of the type we are discussing. We know now that the length of the implant is the most important factor in regards to stress distribution. If the implants are too short, occlusal loading will lead to bone resorption and loss of the implants. Preliminary followup data gathered by our group at UCLA shows the success rates of implants placed in the posterior maxilla.

You will note that the failure rates for 7mm implants placed in the maxilla are almost 30% at 2 years followup. At 5 years followup these figures approach 40%. The 10mm implants are also beginning to show increased failure rates. Note here that at 2 years the failure rates are 8.7%. At 5 years these figures approach 20%. We believe these failures are caused by occlusal loading and therefore recommend that in the posterior maxilla, when implants are arranged in a linear fashion, longer implants be used.

When you look at the finite element analysis data and some of the animal data you can see why the loss rate for 7mm and 10mm implants is high. This study was reported by Cho in 1992 .Note the difference in stress distribution patterns between the short 7mm implant on the left and this longer implant on the right. The poor load carrying capacity of the short implants leads to a resorptive remodeling response of the surrounding bone leading to bone loss around the neck of the implants and eventual implant failure. This resorptive remodeling response has been nicely demonstrated in an animal model by Hoshaw (1994). Based on the work of Hoshaw, Cho and our own clinical data we believe that the minimum length that should be used in linear quadrant cases restoring the posterior maxilla is 13mm.

Since a great many patients possess insufficient bone in the posterior maxillary quadrant to receive an implant of this length the bone augmentation of the alveolar ridge or maxillary sinus has been suggested. Free bone grafts taken from the chin, iliac crest or other sites, often mixed with bone substitutes, are now being used to supplement the existing bone at these sites. The question is what is the quality of bone created by these grafts and is this grafted bone capable of maturing into dense lamellar bone and withstanding the forces of masticatory function.

To answer this question we must briefly review the biology of bone grafting. When a free bone graft is placed into a site the osteogenic cells brought with the graft initially form an immature bone known commonly as woven bone as shown here .

Note that this bone has an irregular calcification pattern and is not as dense as normal lamellar bone and the bone implant interface or bony anchorage created for the implant does not appear to be as good as that which can be created in mature dense lamellar bone.

In our experience when using bone grafts in clinical situations to reconstruct localized alveolar ridge defects, predictable implant results can only be obtained when augmenting a labial-lingual or horizontal deficiencies, in the anterior maxilla.

Augmenting local vertical defects with bone grafts in posterior quadrants as was attempted in this patient is not predictable. In these cases the bone graft material fails to organize and consistently form dense lamellar bone. When the implants are loaded the grafted immature woven bone around the neck of the implant is susceptible to resorption, resulting in bone loss in some patients and to implant failure as was the case of the patient on the right.

Grafting alveolar ridge sites that demonstrate horizontal defects is most predictable in the anterior region, presumably because the neck of the implant and the tip of the implant are secured in dense lamellar cortical bone. When a vertical defect is augmented on the alveolar ridge side, the neck of the implant will be surrounded by the immature woven bone that I showed earlier. For some reason this bone does not consistently mature into dense lamellar bone perhaps because of lack of proper physiologic stresses, and upon occlusal loading is much more subject to resorption.

Augmenting posterior maxillary sites by elevating the sinus membrane and placing bone grafts in this region is more successful particularly if there is a minimum of 5mm or more of residual bone available over the sinus with which to anchor the neck of the implant. The success rates however, appear to decrease when there is less than 5mm of bone available over the sinus presumably because of the fact that a majority of the site is composed of grafted bone. This procedure however does permit the restoration of many patients such as the one seen here, but long term clinical followup studies are still not available.

In the mandible the limiting factor with respect to implant placement is the presence of the inferior alveolar nerve. Based on our clinical data we feel the minimum length for implants used to restore posterior mandibular quadrants is 10mm. Implants less than 10mm should not be used because the bone anchorage or bone implant interface is not sufficient to support posterior occlusal forces. In addition, the tip of the implant terminates in the middle of the marrow space rather than the cortical bone of the inferior border of the mandible. These phenomenon, in combination with the linear configuration of the implants, suggests that 10mm is the minimum length that should be used for implant supported fixed partial dentures used to restore posterior mandibular quadrants as is shown here.

When there is insufficient bone for placement of a 10mm implant as you see in this patient,

some surgeons have recommended that the inferior alveolar nerve be dissected out of its canal and lateralized as is shown here.

Implant can then be used that extend all the way to the inferior border of the mandible providing bicortical stabilization. The implant success rates obtained with this technique are very high but unfortunately the risk of injury to the inferior alveolar nerve is significant. In one report from a German group, the incidence of disethesias, anesthesias, and hyperesthesias was close to 25%. I therefore strongly recommend against using this technique. If the patient has reasonable dentition anteriorly a distal extension removable partial denture is still a good option.

It should be remembered that the mastication efficiency of patients fitted with properly extended and properly designed distal extension removable partial dentures is equivalent to the chewing efficiency of patients fitted with implant supported fixed partial dentures restoring distal extension areas.

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