The concept of immediate occlusal loading of root form implants for fixed restorations has received increasing interest over the last 5 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, occlusal force should be properly managed, and implant designs should be more specific to rigid fixation and macroscopic load conditioners. This article reviews the scientific rationale of these parameters as they relate to bone physiology and biomechanics.
TWO-STAGE SURGICAL PROTOCOL
Predictable formation of a direct bone-to-implant interface is a consistent treatment goal in implant dentistry. The 2-stage surgical protocol established by Branemark et al1 to accomplish “osseointegration” consists of several prerequisites, including the following: (1) countersinking the implant below the crestal bone; (2) obtaining and maintaining a soft-tissue covering over the implant for 3 to 6 months; and (3) maintaining a nonloaded implant environment for 3 to 6 months. Following this procedure, 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 are the following: (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.
TWO APPROACHES TO 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 shown encouraging results.
There have been 2 approaches to immediate loading in the completely edentulous patient. One protocol is to immediately load additional implants that are 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 al7,8 reported on immediate loading of 25 screw-shaped implants in 9 completely edentulous mandibles with fixed prostheses. Using this protocol, 3 immediate-loaded implants failed before 6 months and one implant failed 18 months postsurgery (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 5 years.9 Sixty-six of 69 implants were integrated in 3 mandibular and 3 maxillary completely edentulous arches (96% survival).
The other protocol for immediate-loading implants in completely edentulous patients is to load all the implants at the same time. Since all the implants are splinted together, the risk of overload is decreased because of a greater surface area and improved biomechanical distribution. Often, more implants than the usual number used in the 2-stage surgery approach are inserted. Over the last few years, several authors have reported on immediate loading in the completely edentulous patient using this protocol, with 95% to 100% success rates.10-15
EVALUATION OF IMMEDIATE LOADING AND CRESTAL BONE LOSS
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) was evaluated in a 2-center prospective study.15 The following section summarizes the 6-year interim evaluation from an ongoing clinical evaluation and presents a scientific rationale for this process in the completely edentulous patient.
Study Design
A prospective 2-center study of immediate implant loading was begun in August 1996.15 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.
Procedures and Results
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| Figure 1. An immediate postoperative panograph of 10 maxillary BioHorizons implants and an immediate occlusal loaded transitional prosthesis. | Figure 2. An intraoral view of the immediate-loaded maxillary implants after 6 months. |
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| Figure 3. The implants are widely distributed from the molar to the central incisor and splinted together in the prosthesis. | Figure 4. The final PFM implant prosthesis for this patient is cement-retained. |
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| Figure 5. An intraoral view of the maxillary final restoration supported by 10 BioHorizons implants opposing natural teeth and a removable partial denture. | Figure 6. The final postoperative radiograph with the final prosthesis in place. |
Thirty-one arches were restored in 30 patients during a 3-year period and have been evaluated over the last 6 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 implant surgery and 15 arches loaded 10 to 14 days after implant surgery. After 4 to 7 months, 30 of the final restorations were fabricated. (One restoration was not finally restored for almost 2 years for financial reasons.) The average follow-up period was 3.6 years (Figures 1 through 6).
The number of implants in the mandible ranged from 5 to 10 implants per arch, with a mode of 7 implants. There were 108 implants in the maxilla, with a range of 6 to 12 implants and a mode of 8 or 9 implants. All implants in the maxilla were at least 12 mm long, and all but 4 implants in the mandible (9 mm) were also 12 mm or more in length.
| Table 1. Summary of BioHorizons implant type and prosthesis survival | ||||||||||||||||||||||||
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Of the 244 implants, no failures were found. The prosthesis survival has been 100% within the time frame reported16-17 (Table 1).
Discussion
Using the conventional healing approach, the interface bone is ready for loading at 3 to 6 months. Most of the surgery-related regional acceleratory phenomenon at this point is abated, and the remodeling rate due to trauma is reduced.18 Remodeling (also called bone turnover) not only repairs damaged bone but also allows the implant interface to adapt to its biomechanical situation.19 The interface remodeling rate 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 the trigger for this process is strain rather than the trauma of implant placement.
The classic 2-stage surgical approach to implant dentistry permits the surgical repair of the implant to be separated from the early loading response by 3 to 6 months. Hence, the majority of the woven bone that forms to repair the initial surgical trauma is 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 3 to 6 months of no loading prior to the restoration of the implant. Often, the risks of this procedure are perceived to be at their highest during the first week after the implant insertion surgery. In reality, the developing bone interface is stronger on the day of implant placement, compared to 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 3 months later but at a time frame of around 3 to 5 weeks after implant insertion. A clinical report by Buchs et al21 found immediate-loaded implant failure primarily between 3 to 5 weeks after implant insertion, and it occurred as mobility without infection. 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 such 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 that receives the load), the lower the microstrain in the bone. Therefore, one way to decrease microstrain and the remodeling rate in bone is to provide conditions that increase functional surface area to the implant-bone interface.22 The surface area of load may be increased in a number of ways; eg, implant number, implant size, and implant design. Another way to decrease microstrain in bone is to reduce the force applied to the implant. Factors that affect the amount of force include patient conditions and implant position.
| Table 2. Factors that influence immediate occlusal loading |
(STRESS = FORCE/AREA)
Force Factors Area Factors •Parafunction •Implant Number •Implant Positions •Implant Size •Implant Design |
The functional surface area of occlusal load at an implant interface may be increased by implant number23 (Table 2). Hence, rather than 4 to 6 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 are inserted per arch, implant survival may be above 97%.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 the study previously discussed,15 more implants were used in the maxilla than in the mandible (Figures 7 through 11). This approach helps compensate for the less-dense bone often found in the maxillary arch.
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| Figure 7. An intraoral view of 11 maxillary implants that were immediately loaded with a transitional restoration for 6 months. | Figure 8. The laboratory-fabricated PFM fixed prosthesis, replacing both tooth and lost gingiva. |
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| Figure 9. An intraoral view of the final full-arch restoration in the maxilla. | Figure 10. An intraoral view of the final maxillary PFM fixed prosthesis opposing a traditional fixed prosthesis and natural teeth. |
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| Figure 11. A panoramic radiograph demonstrating the final restoration in place. |
Implant Size
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, the width of implant is more important than the length. 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 with the rest of the teeth. Hence, in the clinical report reviewed above,15 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 4 mm. Each 3-mm 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 nonloaded 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 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 (ie, 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. Macrospheres 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 nontapered 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.
| 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 2. Implant Size 3. Implant Design |
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 is 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 occlusal loading (Table 3).
The greater the occlusal 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 that increase occlusal load make immediate loading more at risk. Parafunctional forces of bruxism and clenching are significant force factors because (1) the magnitude of the force is increased, (2) the duration of the force is increased, (3) the direction of the force is more horizontal than axial to the implants, and (4) the type of force is more shear (in which the bone is 70% weaker compared to compressive loads). Balshi and Wolfinger25 reported that 75% of all failure in immediate occlusal loading occurred in patients with bruxism. In their report, 130 implants were placed in 10 patients, with 40 implants immediate-loaded. An 80% survival for immediate-loaded implants was reported. Parafunctional loads also increase the risk of abutment screw loosening, unrestrained prostheses, or fracture of the transitional restoration used for immediate loading.
Implant Position
| Table 4. Force factors |
| 1. Patient Conditions •Parafunction, crown height, muscular dynamics require more implant surface area. 2. Implant Position |
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 2 implants supporting 3 teeth, rather than positioning the implants next to each other with a cantilever.23 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.
SUMMARY
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 a minimum of 2 years after prosthesis delivery to as long as 6 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 occlusal-related conditions more beneficial for one versus the other. The strength of bone and the modulus of elasticity are both directly related to bone density. The softest bone type may be 10 times weaker than the most dense types. The microstrain mismatch of titanium and the softest bone is much greater than with the densest bone. As a consequence, higher implant failure and greater crestal bone loss seem likely but as yet has not been 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, and implant thread number and depth. Patient factors such 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 3 and 4 list guidelines for immediate occlusal loading. As a general principle, the clinician should be able to increase surface area while minimizing occlusal force to ensure long-term success.
References
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Dr. Misch, past director of the oral implantology residency p
