August 1, 2003
by Carl Misch, BS, DDS, MDS and Hom-Lay Wang, DDS, MSD
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 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, 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.
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.
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).
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 n
o 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
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.
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 increased number of implants also increases the retention of the restoration and reduces the number of pontics. The increased retention minimizes the occurrence of partially retained restorations during healing, which can overload the implants still supporting the restoration. The decrease in pontics may decrease the risk of fracture of the transitional restoration, which also may be a source of overload to the remaining implants supporting the prostheses. In this report, more implants were used in the maxilla compared to the mandible (Figs. 7-11). This approach helps compensate for the less dense bone often found in the upper arch.
The functional surface area of each implant support system is primarily related to the size and the design of the implant. Wider root form implants provide a greater area of bone contact than narrow implants (of similar design). The crest of the ridge is where the occlusal stresses are greatest. As a result after interface integration, width is more important than length of implant. However, the immediate loaded implant does not have a histological attachment of bone to the interface. As a result, length is a more important parameter during the initial loading condition.
The major increase in tooth size occurs in the molar regions for natural teeth, where root surface area doubles compared to the rest of the teeth. Hence in this clinical report, implant diameter was often increased in the molar region.
The surface area of implant support may also be increased by the length of the implant. The length of the implant in most systems increases in increments of 2 to 4mm. Each 3mm increase in length can improve surface area support by more than 20%.26 However, the benefit of increased length is not found at the crestal bone interface, but rather in initial stability of the bone-implant interface. Since the immediate loaded implant requires rigid fixation the day of placement, this is a more important factor than when a non-loaded condition exists. Hence, implant length is more important for immediate loading protocols.
Implant Body Design
The implant body design is more specific for immediate loading, since the bone has not had time to grow into recesses or undercuts in the design prior to the application of occlusal load. For example, a press fit implant designed as (i.e. a cylinder) does not have bone integration the day of implant placement. A cylinder implant requires a healed bone interface to transmit stress along the sides of the implant. A press-fit implant with undercuts (i.e. plateaus or mini balls) does not have bone in the undercut region to gain support during the initial immediate bone loading. For example, an implant body with a series of horizontal plates with a press fit surgical placement does not have bone present between the plates the day of surgical placement. Macro-spheres do not have bone present around the balls on the surface of the implant the day of implant placement. Hence, press fit implants have a major disadvantage for immediate load applications.
The goal for immediate load is to create a stable bone implant interface from the first day, so the bone can grow and/or attach to the interface over the next few months. Hence, rigid fixation is more conducive for immediate load applications. A tapered implant has less surface area and less initial fixation than a non-tapered implant. Since the tapered implant has a tapered osteotomy, the implant does not engage the bone until it is almost completely seated into the osteotomy. This makes surgical placement easier, but also means less fixation, coupled with less surface area to resist the initial load. A tapered implant also has less depth to the thread design, which also decreases surface area of load and provides less rigid fixation for immediate load.
A parallel walled, threaded implant insertion permits bone to be present into the depth of the threads the day of surgery. The deeper the thread, the more surface area to resist the initial loads, and the greater the initial fixation. The number of threads also are relative. The fewer the threads, the less the fixation and the less surface area.27 Nonetheless, future studies in this area are needed to determine how the implant thread design may influence the outcome of immediate occusal loading (Table 3).
The greater the occusal force applied to the prosthesis, the greater the stress at the implant bone interface, and the greater the strain to the bone. Therefore, force conditions, which increase occusal load, make immediate loading more at risk. Parafunctional forces of bruxism and clenching are significant force factors, because magnitude of the force is increased, the duration of the force is increased, the direction of the force is more horizontal than axial to the implants and the type of force is more shear (in which the bone is 70 percent weaker compared to compressive loads). Balshi and Wolfinger have reported that 75% of all failure in immediate occusal loading occurred in patients with bruxism.25 In their report, 130 implants were placed in 10 patients, with 40 implants immediate loaded. An 80% survival for immediately loaded implants was reported. Parafunctional loads also increase the risk of abutment screw loosening, unretained prostheses or fracture of the transitional restoration used for immediate loading.
Dental implants have been widely used to retain and support cross-arch fixed partial dentures. Implant position is often as important as implant number. For example, it is recommended to eliminate cantilevers on two implants supporting three teeth, rather than positioning the implants next to each other with a cantilever. The cross arch splint forming an arch is a very effective design to reduce stress to the entire implant support system.24 Hence, when multiple implants are positioned around an arch and splinted together in the transitional prostheses, it is advantageous for immediate load (Table 4). Cantilevers increase the risk of overload on the implants and also increase the complication of partially uncemented restorations, which may also cause overload to the remaining implants.
The majority of clinical reports reveal similar survival rates between immediate loaded and 2-stage unloaded healing approaches in the
completely edentulous patient. In our prospective study, 31 arches received immediate loaded restorations in 30 patients, supported by 244 implants. All implants were followed from a minimum of two years post prosthesis delivery to as long as six years. The implant and final prosthesis survival were 100% during this time frame. Nonetheless, these findings do not imply a submerged surgical approach is no longer necessary or prudent in many cases. Future studies may find indications based upon surgical, host, implant and occusal related conditions more beneficial for one versus the other. For example, bone density was not addressed in these papers. The strength of bone and the modulus of elasticity are both directly related to bone density. The softest bone type maybe 10 times weaker than for the most dense types. The microstrain mismatch of titanium and the softest bone is much greater than the most dense bone. As a consequence, higher implant failure and greater crestal bone loss seem likely, but as yet is not reported in the literature.
A biomechanical treatment approach to increase surface area and decrease forces applied to the immediate restorations is logical to increase implant survival. Conditions that decrease strain to a developing interface include increasing implant number, implant size, implant thread number, and depth. Patient factors, as parafunction, may increase forces to the implant interface, while implant position may be used to decrease forces, especially when a splinted arch form is created. Tables three and four list guidelines for immediate occusal loading. As a general principle, the clinician should be able to increase surface area while minimizing occusal force to ensure long-term success.
Dr. Misch is Adjunct Professor, Department of Periodontics/Prevention/ Geriatrics, School of Dentistry, University of Michigan, Ann Arbor. MI. He maintains a private practice in Birmingham, MI.
Dr. Wang is Professor and Director, Graduate Periodontics program, Department of Periodontics/Prevention/Geriatrics, School of Dentistry, University of Michigan, Ann Arbor. MI.
Oral Health welcomes this original article.
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TABLE 1 – Summary of BioHorizon Implant Type and Prosthesis Survival
TABLE 2 – Factors which Influence Immediate Occlusal Loading (STRESS = FORCE/AREA)
TABLE 3 – Guidelines for Immediate Loading
Treatment Plan Guidelines for this prospective report for completely edentulous patients, used a biomechanical approach to reduce stress and reduce microstrain at the developing interface. These guidelines may be used for future reports on clinical application and include:
SURFACE AREA FACTORS:
1. Implant Number
1. Eight splinted implants or more for the maxillary arch and 6 splinted implants or more for the mandible-more implants if very soft bone (D4) is present, or force factors ar
e greater (eg. crown height, parafunction).
2. Implant Size
1. Larger diameter implants in the posterior regions of the mouth. If larger diameter is not possible, bone grafting or greater implant number is suggested (eg. 2 implants for each molar)
3. Implant Design
a. Parallel walled threaded implants (tapered implants contraindicated)
b.High surface area implants (more threads, deeper threads)
c. Compressive vs. shear loads (square shape threads)
TABLE 4 – Force Factors:
1. Patient Conditions
a. Parafunction, crown height, muscular dynamics require more implant surface area.
2. Implant Position:
a. In the maxilla, anterior implants should be in the central position and posterior implants in the first to second molar position for the largest anterior – posterior dimension. In the mandible the largest anterior posterior dimension possible should be used. No posterior cantilevers should exist on the transitional prosthesis.
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