Implants in Partially Edentulous Patients; The UCLA Experience

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

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Next, I would like to address the consequences of exposing implants placed in posterior quadrants and arranged in a linear configuration to nonaxial occlusal loads. When implants are arranged in a linear fashion as is seen here,

the biomechanics with respect to bone response is quite unfavorable as compared to a configuration shown here where the implants are arranged in a curvilinear fashion and there is significant anterior-posterior spread.

In the 1980's conventional dogma stated that once an implant became "osseointegrated" it did not matter whether or not occlusal loads were or were not applied axially. However, as more clinical followup data and animal research data becomes available, it is becoming increasingly obvious that in some clinical situations, namely, implant supported fixed partial dentures restoring posterior quadrants, that nonaxial loading causes sufficient load magnification at the bone-implant interface and may lead to a higher rate of bone resorption and eventually a higher rate of implant failure.

In past years theoretical studies using finite element analysis and photoelastic analysis aroused concern, because nonaxial loads, particularly those in excess of 15¡ to the long axis of the implants produced marked deformation of the section around the neck of the implant. Kinni and Caputo in a report published in 1987 tested several implant fixture designs and noted that stresses in the region around the neck of the implants were significantly increased when loads are applied at severe angles.

Cho and his associates in 1992 at Dan Kook University in South Korea, using finite element analysis studied stress distribution patterns around implants when implants were subjected to nonaxial occlusal loading at angles ranging from 5^o to 45^o to the long axis of the implant. He reported deformation of the region around the neck became particularly prominent when the load was applied at angles of 20^o or more.

These and other studies raised concern among clinicians particularly when implants were arranged in a linear configuration in posterior quadrants, but it was not known whether nonaxial loading would cause enough load magnification to indeed cause bone loss around implants. In the early 1990's clinical reports began to appear in the literature suggesting that overloading particularly nonaxial overloading could lead to compromise of the bone implant interface. At the same time Hoshaw and co-workers designed an animal experiment to test whether overloading of an implant would lead to bone loss around implants. Hoshaw et al. found that excessive occlusal loading lead to a resorptive remodeling response of the bone around the implant resulting in the loss of bone around the neck of screw shaped implants, plus a reduction in the mineral content of the bone in a zone adjacent to the body of the implant. Loads were applied in an axial direction but were quite substantial given the length of the implants tested. Hoshaw and co-authors concluded that the bone loss observed was secondary to implant overload. They correctly pointed out however, that the precise determination of what constitutes implant overload in a clinical setting was not possible at this time because of the great variability in bone quantity and quality from patient to patient, the variation of lengths of implants used in clinical situations, the variation in the biologic capacity of remodeling and repair from patient to patient, and the difficulty in determining the amount of load magnification resulting from cantilevering the restoration or from the application of nonaxial loads.

Our clinical data appears to support the assumption that nonaxial loads, particularly when implants are aligned linearly, eventually compromise the bone implant interface. Followup examinations of patients such as this one,

where the implants in the posterior maxillary quadrant were significantly buccally inclined and restored with angled or custom abutments, has revealed that a very high number of the implants restored, angled in this fashion and with implant supported fixed partial dentures demonstrated significant and progressive crestal bone loss with a higher than usual implant failure rate. Although the number of implants and number of patients is limited and the study is a retrospective one, it is our clinical impression that misangled implants, particularly maxillary implants severely tipped buccally are more susceptible to progressive bone loss leading to implant failure than those implants placed in the maxilla where occlusal forces can be directed axially.

We have observed the same phenomenon in the mandible as seen here in these patients . We have not observed a similar pattern of bone loss around implants placed in the anterior portion of the maxilla with comparable labial inclinations presumably because the magnitude of forces generated anteriorly are only about 15-20% of those generated in the posterior regions, the quantity and quality of bone anteriorly permits the placement of longer implants with better bone anchorage, and the size of the incisal table of anterior teeth is certainly less than the area of occlusal surface area of posterior teeth. So, to conclude this point, we believe that nonaxial loads cause load magnification and can lead to overloading which in turn precipitates a resorptive remodeling response of the bone around the neck of the implants.

The bone loss appears to be progressive and can lead to implant failure. Linear implant configurations in the posterior mandible and maxilla, and here shown is another case in the mandible, are prone to bone loss when loads are not applied axially . Implants should be positioned so that occlusal loads can be directed axially as is seen in this patient. The response is not universal however, and is probably dependent on the bone anchorage of the implant, the length of the implant, whether the implant supported restoration is used for incising the bolus of food or masticating the food bolus, and the magnitude of the occlusal loads applied to the implants.

Therefore we believe every attempt should he made to surgically position implants in the posterior quadrant so that occlusal forces can be directed axially, or along the long axis of the implant as is shown here . If implants are positioned in such a way the restoration is very easy and cost effective to fabricate and the implants will have a better long term prognosis.

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