Immediate Occlusal Loading for Fixed Prostheses

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Immediate Occlusal Loading for Fixed Prostheses
By Carl Misch, BS, DDS, MDS and Hom-Lay Wang, DDS, MSD
August 2003
Oral Health Journal
The concept of immediate implant occlusal loading root form implants for fixed restorations has received increasing interest over the last five years. Studies have discussed the factors that may influence results, including implant number, implant length, bone density, occlusal schemes and patient habits. To reduce the risks, implant numbers should be increased, occusal forces should be properly managed and implant designs should be more specific to rigid fixation and macroscopic load conditioners. It is the purpose of this article to review the scientific rationale of these parameters as they relate to bone physiology and biomechanics.
Predictable formation of a direct bone-to-implant interface is a consistent treatment goal in implant dentistry. The two stage surgical protocol established by Branemark et al to accomplish "osseointegration" consisted of several prerequisites, including: (1) countersinking the implant below the crestal bone; (2) obtaining and maintaining a soft-tissue covering over the implant for three to six months; and (3) maintaining a non-loaded implant environment for three to six months. Following this procedure, a second stage surgery is necessary to uncover these implants and place a prosthetic abutment. The primary reasons cited for the submerged, countersunk surgical approach to implant placement were: (1) to reduce and minimize the risk of bacterial infection, (2) to prevent apical migration of the oral epithelium along with the body of the implant, and (3) to reduce and minimize the risk of early implant loading during bone remodeling.1
IMMEDIATE LOADING
Immediate loading of a dental implant actually loads the implant with a provisional restoration at the same appointment, or shortly thereafter. Immediate loading was the initial protocol suggested with dental implants. These implants yielded a wide range of clinical survival.2-6 Recently studies in immediate loading have been proposed and have shown encouraging results.
There have been two approaches to immediate load in the complete edentulous patient. One protocol is to immediately load additional implants, not necessary for the final restoration. If these implants fail, the submerged implants may be uncovered after additional healing periods to restore the patients. For example, Schnitman et al. reported on immediate loading of 25 screw shaped implants in nine completely edentulous mandibles with fixed prostheses with this approach.7,8 Using this protocol, three immediate loaded implants failed before six months and one implant failed 18 months post surgery (84% survival).8 Tarnow et al. also reported on immediate loading with a fixed prosthesis, using a similar method in 10 consecutive completely edentulous cases over five years.9 Sixty-six of 69 implants were integrated in six mandibular and four maxillary completely edentulous arches (96% survival).
The other protocol for immediate loading implants in completely edentulous patients loads all the implants at the same time. Since all the implants are splinted together, the risk of overload is decreased due to a greater surface area and improved biomechanical distribution. Often more implants than the usual used in the two-stage surgery approach are inserted. Over the last few years, several authors have reported on immediate loading in the completely edentulous patient with this protocol, with 95 to 100% success rates.10-15
More recent investigations have sought to extend the understanding of crestal bone resorption surrounding endosteal dental implants with immediate loading. However, the influence of immediate loading on crestal bone loss has few animal and/or clinical reports to compare the differences of immediate loading to a more traditional bone healing time with no functional load.
In order to address the issues of immediate occlusal loading and crestal bone loss, a bone quality-based implant system (Maestro System,Ç—¢ BioHorizons, Birmingham, AL) was evaluated in a two center prospective study.15 The present article summarizes the six-year interim evaluation from an ongoing clinical evaluation, and presents a scientific rationale for this process in the completely edentulous patient.
PROSPECTIVE REPORT
A prospective two-center study of immediate implant loading was begun in August, 1996 at two different clinical centers.15 All patients were completely edentulous in the reported arch prior to implant insertion. The functional transitional prosthesis was delivered the day of surgery or at the suture removal appointment 10 to 14 days later.
RESULTS
Totally, 31 arches have been restored in 30 patients during a three-year period and have been evaluated over the last six years. Nineteen mandibular and 12 maxillary arches were restored (one patient with both arches). A total of 244 implants were used to support 31 restorations, for an average of 7.8 implants per prosthesis. There were 16 arches loaded the day of surgery and 15 arches 10 to 14 days after implant surgery. After four to seven months, 30 of the final restorations were fabricated (one restoration was not finally restored for almost two years for financial reasons). The average follow-up period was 3.6 years (Figs. 1-6).
The number of implants in the mandible ranged from five to 10 implants per arch, with a mode of seven implants. There were 108 implants in the maxilla, with a range of six to 12 implants, and a mode of eight or nine implants. All implants in the maxilla were at least 12mm long, and all but four implants in the mandible (9mm) were also 12 mm or more in length.
No implant failures16,17 (out of 244 implants) were found. The prosthesis survival has been 100% within the time frame reported. One maxillary final prosthesis was delayed for almost two years (22 months) for financial considerations, prior to finishing the restoration (Table 1).
DISCUSSION
Using the conventional healing approach the interface bone is ready for loading at three to six months. Most of the surgical related Regional Acceleratory Phenomenon (RAP) at this point is abated, and the remodeling rate due to trauma is reduced.18 Remodeling is also called bone turnover, and not only repairs damaged bone, but also allows the implant interface to adapt to its biomechanical situation.19 The interface Remodeling Rate (RR) is the period of time for bone at the implant interface to be replaced with new bone. Once the bone is loaded by the implant prosthesis, the interface begins to remodel again, but this time the trigger for this process is strain, rather than the trauma of implant placement.
The classic 2-stage surgical approach to implant dentistry permitted the surgical repair of the implant to be separated from the early loading response by three to months months. Hence, the majority of the woven bone, which formed to repair the initial surgical trauma was replaced with lamellar bone. Lamellar bone is stronger and able to respond to the mechanical environment of occlusal loading.20 Therefore, a rationale for immediate loading is not only to reduce the risk of fibrous tissue formation (which results in clinical failure), but also to minimize woven bone formation and promote dense lamellar bone maturation to sustain occlusal load.
The immediate implant loading concept challenges the conventional healing time of three to six months of no loading, prior to the restoration of the implant. Often the risks of this procedure are perceived to be during the first week from the implant insertion surgery. In reality, the developing bone interface is stronger on the day of implant placement, compared to the time period a few weeks later. Therefore, the greatest risk of immediate loading may not be during the first few days when the bone is stronger than three months later, but at a time frame around three to five weeks after implant insertion. A clinical report by Buchs et al found immediate loaded implant failure primarily between three to five weeks after implant insertion, and occurred as mobility without infection.21 On the other hand, the immediate loaded implant has no opportunity for bone to grow into the implant design, or attach itself to the implant. Therefore, implant design is more specific and implant surface condition less important during the first few weeks of immediate load. More important factors such as implant number and position, or patient force factors (as parafunction) should be considered for immediate load situations.
IMMEDIATE OCCLUSAL LOADING
Bone Microstrain
One goal for an immediate loaded implant/prosthesis system is to decrease the risk of occlusal overload and its resultant increase in the remodeling rate of bone. The lower the stress applied to the bone (force divided by the functional surface area which receives the load), the lower the microstrain in the bone. Therefore, one method to decrease microstrain and the RR in bone is to provide conditions, which increase functional surface area to the implant bone interface.22 The surface area of load may be increased in a number of ways, i.e. implant number, implant size and implant design. Another method to decrease microstrain to bone is to reduce the force applied to the implant. Methods that affect the amount of force include patient conditions and implant position.
SURFACE AREA
Implant Number
The functional surface area of occlusal load at an implant interface may be increased by implant number23 (Table 2). Hence, rather than four to six implants to support a full arch fixed restoration,24,25 it is more prudent to use additional implants when immediate loading is planned. Immediate loading reports in the literature with the lowest percentage survival correspond to fewer implants loaded.7,8,25 On the other hand, when more implants were inserted per arch, implant survival may be above 97 percent.10,15 The incre