The Immediate Loading of Dental Implants

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Evolving Patient Care
Individuals perceive the complete dentition as a state of good health, and edentulous patients perceive themselves in a better light when a functional dentition is established with endosseous dental implants.1 This contrasts with the aversion reported for the use of removable partial dentures or the frustration reported for the use of complete dentures for mandibular edentulism.2,3 Implant success rates for single-tooth replacement rival or exceed the clinical performance of fixed partial dentures.4,5 Implant-supported dental prostheses offer multiple advantages for patients.

The conventional process of implant-based dental rehabilitation was founded on prospective clinical cohort studies that demonstrated the long-term success of root-form titanium dental implants.6,7 High success rates of dental implant therapy have been repeatedly reported. The reports were reviewed by Fiorellini and colleagues, performed according to the established staged protocols with a three- to six-month healing period that avoided direct masticatory loading from the prosthesis to the implant.8 However, the complexities and long duration of implant therapy may discourage some patients and clinicians from electing an implant-based strategy for dental rehabilitation.9

Nearly a decade ago, some coalescence of opinion regarding surgical approaches to implant therapy was attained in light of emerging clinical evidence that one-stage and

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More recently, several clinical investigations reported similarly high survival rates for endosseous dental implants placed in the mandibular parasymphysis and loaded either immediately after implant placement or within weeks after implant placement (Table 1).15-49 First offered as expendable or transitional fixtures, these reports of immediate loading of dental implants provided new insight into the biological capacity of the mandibular parasymphysis to support the process of osseointegration under diverse clinical conditions. Also, these successful initiatives suggested that immediate loading of endosseous dental implants was, in fact, a feasible clinical enterprise.

As the experience of immediate loading of endosseous dental implants has expanded throughout several centers worldwide, activities have grown to include maxillary complete arch prostheses, single-tooth implants, and even posterior fixed partial dentures and single-unit molar crowns. It remains to be demonstrated if all of these procedures will achieve high success over long periods of time.

Defining Immediate Loading
Immediate loading is variably defined, depending on the restorative protocol used at various investigating centers. The interval between placement of the implant and the restoration has varied between 0 and 20 days. However, from a patient's perspective, immediate loading should refer to the placement and restoration of an endosseous dental implant during the same clinical visit.

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Primary stability is the clinical means of controlling micromotion between the implant and the new, forming interfacial tissue.57 This helps to establish the proper mechanical environment for osteogenesis. Immediate provisionalization and immediate loading scenarios superimpose micromotion on interfacial tissue. How much micromotion is permissible or precisely how masticatory function relates to interfacial micromotion has not been fully addressed. When precursor osteoblastic cells are exposed to limited physical deformation that models micromotion in a laboratory setting, differentiation is enhanced in cell culture experiments.58 Despite limitations of interpretation, some range of microstrain is considered advantageous for osteoblastic differentiation,59 bone ingrowth60 and osseointegration.61 Current in vivo studies suggest that micromotion greater than 150 μm (direction and frequency remain ill-defined) limits osseointegration.57

Clinical guidelines for gaining and enhancing implant primary stability include careful evaluation of the recipient bone site, careful osteotomy preparation, undersized osteotomy, self-tapping implant insertion, osteotome preparation of the site, and use of improved implant designs. It must be acknowledged that little data exists regarding the relationship of osteotomy dimension, implant placement and resulting bone formation or resorption. Current clinical approaches to immediate loading advocate attaining high levels of primary stability and an array of methods for assessing implant stability are available (Table 3).25,28,29,39,62-70

Initial studies of immediate loading suggested that insertion torque values of 40 Ncm to 45 Ncm were required; more recently, values of 30 Ncm to 32 Ncm have been reported. Additional analytical values of correlation of insertion torque or stability values with dental implant outcomes are needed.

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The Role of Bone Resorption
As previously suggested, maintaining implant stability is a key aspect of immediate loading success and depends on bone formation and the adaptive bone remodeling that occurs at dental implants after placement. The complex nature of the load experienced by dental implant interfacial tissues is beyond the scope of this report74; however, accepted generalizations (often cited as Wolff's law) include concepts of moderate and controlled loading environments that support or enhance osteogenesis, higher loads that induce bone resorption, and reduced loading environments hat lead to tissue atrophy. However, intervening resorption of crestal bone is a consequence of the transcortical implant placement.51 It is unlikely that a loading environment associated with tissue atrophy exists at an unloaded healing dental implant; continuous bone loss is not revealed at titanium root-form implants. More importantly, when primary stability is achieved, it is likely that a loading environment associated with osteogenesis is present. Preclinical histology from primate and canine models revealed that immediate loading of dental implants led to greater bone-to-implant contact, with incrementally more bone formed at the loaded, relative to unloaded, endosseous dental implants.75-77 A possible conclusion is that the loading environment created by immediate loading at a primary stable implant is favorable.

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Bone resorption is the result of cell and molecular regulation of osteoclasts.79 At least four key molecular aspects of osteoclast activation are now well defined: 1) a specific transmembrane receptor and its ligand (RANK and RANKL) are essential for osteoclast differentiation; 2) cell adhesion via a specific transmembrane receptor (ß3 integrin) are required for osteoclastic activity; 3) a key intracellular mediator of inflammatory signals (NF-kB) promotes osteoclastogenesis (lipopolysaccharides from oral bacteria are powerful inducers of this particular osteoclastic signal); and 4) mechanical strain of bone induces osteoclastogenesis. A combination of mechanical status and inflammatory environment at the implant surface determines the extent of local bone resorption, and thus affects implant stability during the osseointegration process.

Clinical guidelines for immediate loading success should also focus on reducing cortical or crestal bone resorption. Suggestions, in part derived from experience in immediate placement,80 include avoiding elevation of mucoperiosteal flaps when feasible, careful and precise osteotomy preparation, and avoiding instrumentation of the buccal plate of the socket. Tooth resorption leads to buccal alveolar bone resorption that must be anticipated.81 Engaging a thin buccal plate with the implant places the implant at risk of loosening should subsequent resorption occur. The control of the peri-implant inflammation also necessitates implant placement at the appropriate axial depth and the use of components that preclude abutment loosening or experience retrograde bacterial colonization at the implant/abutment interface (unitary design, one-stage or modular, solid, conus design implants and implant-abutment interfaces that lack micromotion). Extended prescription of antimicrobial rinses can be valuable in limiting bacteria-associated inflammation at the healing implant site.

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The elimination and control of functional contacts is advocated for unsplinted implants. Eliminating tooth contacts at the maximum intercuspation position is possible. Excursive contacts are more difficult to control; however, development of contacts can be avoided, delayed or strategically arranged. It is essential to check the provisional contacts during the first week follow-up visit and at the three-to four-week visit. This is particularly important after orthodontic tooth movement, where minor changes in tooth position can evoke unintended contacts in centric or eccentric positions.

The nature of the provisional restoration and abutment-provisional crown finishing line are important factors in promoting peri-implant mucosal health and limiting inflammation. Provisional crown margins should not approximate the implant-bone interface; therefore, UCLA-type abutments are not preferred because they place an interface with potential for micromotion and bacterial population at the crestal bone. Recommended are titanium or ceramic abutments placed opposing as much of the peri-implant mucosa as possible. Dense acrylic denture teeth provide an ideal starting point for creating a provisional crown for single-tooth replacement. The fit of the provisional restoration should be refined on the abutment or abutment/fixture analogs using an extraoral step for finishing and refinement to keep restorative particulate materials from the healing tissue.

Cementation of the interim prosthesis is an important step in the immediate loading scenario. Permanent cements (e.g., glass ionomer and polycarbonate) offer an additional level of security against debonding and uncontrolled or unintended loading because of a loose prosthesis.

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Whether the interim prosthesis is fixed by a screw or cement, it should be retained for the six- to 12-week healing period. Excluding the short-term removal of immediately loaded implant prostheses for implant evaluation led to improved success rates.43 Clinical manipulations, such as forming impressions, provisional restoration delivery or debonding, could perturb the osseointegration process.

Illustrating Immediate Loading for Clinically Validated Scenarios
Whether the interim prosthesis is fixed by a screw or cement, it should be retained for the six- to 12-week healing period. Excluding the short-term removal of immediately loaded implant prostheses for implant evaluation led to improved success rates.43 Clinical manipulations, such as forming impressions, provisional restoration delivery or debonding, could perturb the osseointegration process. mandibles of adequate bone quality and shape. The Cochrane collaboration found three relevant randomized clinical trials and two trials including 68 patients of high scientific merit.15 Statistical evaluation of this data indicated there were no differences on measures of prosthesis failure, implant failure and marginal bone loss on intraoral radiographs when immediately loaded implants were compared with conventionally loaded implants in the parasymphyseal mandible. Several additional cohort trials have been published that suggest high implant and prosthesis short-term survival. This conclusion supports the immediate provisionalization of mandibular overdentures (Figure 1a, Figure 1b, Figure 1c, Figure 1d, Figure 1e, Figure 1f, Figure 1g) and immediate loading of implant-supported fixed dentures (Figure 2a, Figure 2b, Figure 2c, Figure 2d, Figure 2e, Figure 2f, Figure 2g, Figure 2h, Figure 2i, Figure 2j) for comprehensive rehabilitation of mandibular edentulism.

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Illustrating Immediate Loading for the Yet-to-be-Fully-Validated Clinical Scenarios
Additional short-term data suggests that immediate placement and provisionalization of single-tooth implant may be achieved with success. Thirteen machined implants (Bio-Dent) were placed immediately after anterior maxillary tooth extraction and provisionalized without occlusal contacts. Failures were not detected.44 A similar result with a similar protocol using Steri-Oss implants (Nobel Biocare; Yorba Linda, Calif.) was reported after a 12-month evaluation of 35 patients.17 Thirty-five SLA surface implants were placed in maxillary single-tooth extraction sockets and provisional crowns without occlusal contacts were placed at surgery. The six- to 12-month evaluation of these implants indicated no implant failure.19

Evaluations of single-tooth replacement by immediate dental implant placement and loading suggest that the expected success will be defined, and uniform clinical procedures will be established (Figure 3a, Figure 3b, Figure 3c, Figure 3d, Figure 3e, Figure 3f, Figure 3g, Figure 3h, Figure 3i).

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Several cohort investigations of immediate loading protocols have included maxillary rehabilitation.24,27,36-39,42 These limited reports have included the placement of eight to 12 implants restored using provisional splinted prostheses. Development of protocols for reproducible management of the maxilla using immediate loading protocols is ongoing. One possible approach is the use of six to eight implants loaded using a cement-retained interim fixed denture composed of acrylic resin (Figure 4a, Figure 4b, Figure 4c, Figure 4d, Figure 4e, Figure 4f, Figure 4g, Figure 4h, Figure 4i). An alternative approach involves the computer-aided fabrication of a surgical guide and final prosthesis for delivery immediately after surgery.36 The immediate loading of splinted implants for maxillary rehabilitation has shown great promise.40 However, one preliminary report indicated that for the 95 percent survival recorded at the implant level, nearly one-third of the patients had experienced an implant failure during the provisionalization period.85

There is less clinical information for unilateral fixed partial dentures. Compelling data have been reported by Glauser and colleagues that includes implant-supported prostheses in low-density posterior regions.26 Histological evidence for successful osseointegration has been provided.22 The potential value of immediate loading of unilateral fixed partial dentures can be described for the replacement of failed fixed partial dentures. As illustrated (Figures 4a-4i), rapid restoration of function can be achieved by replacement of failed abutment teeth using dental implants and immediate loading with a provisional fixed partial denture. While this has been attempted in select cases with comprehensive informed consent, there remains only limited published data and experience to support this procedure in clinical practice.

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Once primary stability is established, loading environments and potential inflammatory changes must be controlled to permit maintained implant stability in support of osseointegration. Modification of implant surgery and provisional prosthesis techniques can promote tissue integration. Improved implant components are one aspect of clinical success for immediate loading. With detailed planning and execution, the parasymphyseal mandible and anterior single-tooth implants placed into an intact alveolus appear successful in the short term. The generalized and widespread application of immediate loading requires additional evaluation and development.

Originally published in: Cooper L. The immediate loading of dental implants. Compendium. 2007;28(4):216-25. Copyright © 2007 to AEGIS Communications. All rights reserved.