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IMPLANTOLOGY: Endosteal Implants in the Edentulous Posterior Maxilla: Rationale and Clinical Report

August 1, 2000
by Carl E. Misch, DDS, MDS; Yvan Poitras, DMD; Francine Dietsh-Misc


ABSTRACT: The maxillary posterior region of the mouth sustains greater bite forces compared to the anterior, yet often present the poorest bone density. A biomechanical approach, often presented to decrease risk factors in regions of high stress or poor bone density, is to increase implant surface area. Most manufacturers provide implants in variable lengths. Sinus grafts permit longer implants, however, finite element analyses support the hypothesis that implant length is a secondary parameter for stress distribution. A more beneficial approach to enhance implant surface area in the posterior regions has been primarily to increase the implant diameter. However, when conventional designs and diameter are used, this only increases surface area by 30%, yet bite forces increase by more than 300% in the posterior regions. A change in implant diameter and thread design (i.e. BioHorizons implant systems) may increase surface area by more than 300%. This clinical report demonstrates an implant surgical success rate of 99.4% in the posterior maxilla using the Bone Quality-based implant system, BioHorizons. In addition, there were no early loading failures and no prosthetic failures. Crestal bone loss during early loading averaged .71 mm or less, dependent upon a one stage or two stage surgical approach. The increases in surface area of this design, coupled with the compressive load thread design may indeed be responsible for the decrease in early loading implant failure and also contribute to a decrease in crestal bone stresses, which may reduce crestal bone loss.

The maxillary posterior edentulous region presents many unique and challenging conditions in implant dentistry, such as poor bone quality and limited volume due to the presence of the maxillary sinus. In the past, these conditions resulted in the lowest implant survival region in the mouth. However, today there are proven treatment modalities which render treatment in this region as predictable as in any other intraoral region. Most noteworthy are sinus grafts to increase available bone height, increased number of implants to support the prosthesis, improved implant designs and surface conditions, modified surgical and treatment approaches related to bone density, and progressive bone loading during the prosthodontic phase of reconstruction. This article addresses the treatment options specific to the maxillary posterior edentulous region based on available bone, and presents a clinical report of 131 patients and restorations supported by a Bone Quality based Implant System (BioHorizons Dental Implants).

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LITERATURE REVIEW

Over the years, several strategies have been advocated to restore the posterior maxilla and address its deficiency of bone volume. These approaches can be categorized as follows:

Avoid the sinus and place implants anteriorly, posteriorly, or medially1-3

Place implants and perforate the sinus floor.4-5

Use subperiosteal implants.6-7

Perform horizontal osteotomy, interpositional bone grafting, and endosteal implants.8-9

Elevate sinuses during implant placement.10-14

Perform lateral wall approach sinus grafts, and simultaneous or delayed implant placement.10-23

Early loading implant failure in the posterior maxilla has repeatedly been reported to be the highest of all areas of the jaws. For example, Schnitman observed a 22% failure when Brnemark implants were used for prosthesis support in the posterior maxilla, compared to a 7% failure in the anterior maxilla.24 Jaffin and Berman, in a 5 year retrospective analysis, reported a 65% failure in the maxilla compared to a 10% failure in the anterior mandible, when soft bone was present.25 The implant failure at Stage II surgery was 8.3%, so most of these implants failed once they were loaded. Drago also reported a poorer success rate (71.4%) in the posterior maxilla, compared to any other anatomic location in the mouth.26

A prospective study of BioHorizons, a bone quality-based implant system with four different implant designs, was begun in 1996. A report in 1998 demonstrated the combined surgical survival of 975 implants was 99.4%, with a survival of 100% in the softest bone type (D4).27 There were no early loading implant failures in the first 103 consecutive patients restored with 360 implants and 105 prostheses. All patients with implants were in satisfactory to optimum health according to the Misch Implant Quality Scale.28 The mean early loading bone loss was 0.29 mm.27

CLINICAL REPORT

Materials and Methods

The purpose of this article is to present the results of posterior maxillae restored by the authors with BioHorizons Dental Implants from 1996 until 1999. In this retrospective report, 131 patients received implants in this region of the mouth. There were 84 females and 47 males with age ranging from 17 to 78 years (Table 1). The prosthetic reconstruction included 30 single tooth implants, 63 fixed prostheses, and 38 overdentures which were completely implant supported. All 63 fixed prostheses were completely implant supported, with 20 that were less than four splinted units and 43 with greater than 4 splinted units (Table II).

A total of 456 BioHorizons Maestro implants were inserted to support the restorations. Almost all implants (429) were placed in adequate bone volume (Division A, Misch, Judy Classification).29 (Fig. 1) 27 implants were inserted into C-h bone (an HA coated shorter implant design, 9 to 10 mm in height). There were 15 D2 implants, 110 D3 implants, 302 D4 implants and 27 C-h implants. The diameter of these implants were one 3.5 mm, 301 4 mm, and 154 5 mm diameter respectively (Table III).

Pre-operative radiographic evaluation consisted of panoramic radiographs, supplemented with lateral cephalograms, periapical radiographs and computerized tomograms when indicated. The standardized surgical protocol for the bone quality-based implant system has been previously published.27 The platform of the implants were inserted level with the height of the facial contour of bone for D1, D2 and D3 implants, and countersunk with D4 implants 0.5 to 1 mm below the crest of bone. The implants were allowed to heal from 4 to 8 months, depending on the bone density (longer for less dense bone) and the patient’s compliance with appointments scheduled.

Out of 456 implants, 57 were placed in a one stage procedure and 396 were placed in a submerged manner. Immediately loaded implants were not included in this study. The clinical surgical failure of the implant was assessed by the clinician on the basis of any one of the following: Lack of rigid fixation, presence of persistent and irreversible pain or infection, periimplant radiolucency, loss of bone support over more than half the length of the implant, uncontrolled exudate, improper placement angulation, and/or implants unable to be used in the final restoration (“sleepers”).28

Intraoral radiographs (vertical bite wings and periapical) were usually taken at the time of presurgical assessment and first-stage surgery, then were routinely taken at second-stage surgery, prosthesis insertion, 6 months and 1 year post prosthesis insertion, and yearly thereafter. The known thread pitch of each implant design was used to calibrate the measurements for each implant, thereby adjusting for the effect of any misalignment of the film plane relative to the implant axis on the apparent crestal bone position.

The platform of the implant crest module was used as a reference point for measuring crestal bone changes. The differences between mean bone level measurements at stage I and stage II surgeries, at the prosthesis delivery to the first year of loading were calculated and analyzed statistically. Any bone levels above the reference point of the platform were recorded as 0 bone loss, rather than a positive number, which would decrease the overall bone loss data.

RESULTS

From July, 1996 to May, 1999, 456 implants were placed in 131 patients (15 D2, 110 D3 and 304 D4 implants and 27 C-h implants). A total of 3 implants were lost from stage I implant placement surgery to stage II un
covery and permucosal. abutment connection (1 D3 and 2 D4) (Table III). The overall surgical and bone healing survival rate of the 456 consecutive implants was 99.4% in this study. The three implants lost were in 2 female patients. One of these patients did not stop smoking after surgery, and incision line opening led to one 4 mm D3 and one 5 mm D4 becoming contaminated and caused their eventual loss before the scheduled stage II uncovery. The other female lost one 4 mm diameter D4 implant, most likely from parafunction on a removable transitional restoration (Table III).

The bone loss observed at stage II uncovery and/or after bone healing was, in part, related to the surgical approach. Most single tooth implants used a one stage procedure, with the permucosal soft tissue healing abutment placed at the initial surgery (Figs. 2-6). From the 57 one stage implants group, four implants had no bone loss, 18 exhibited 0.5 mm bone loss, and 35 had 1 mm bone loss (Table IV). The remaining 396 two stage implants exhibited no bone loss in 383 implants, 5 implants exhibited 0.5 mm, 4 implants had lost 1 mm, 1 implant had lost 2 mm, 2 implants lost 2.5 mm and 1 implant with 3.0 mm bone loss. (Table V) This stage I to stage II bone loss was most likely from bone remodeling from surgery, but the 3 implants with more than 2 mm bone loss were located under a removable soft tissue borne prosthesis, and bone loss may also be related to parafunction on this device.

The 131 prostheses were fabricated on a total of 453 implants. The prosthetic protocol used a progressive bone loading approach for all fixed restorations (Figs. 7-11). The prostheses and implants for these 131 patients have been observed for 12 to 46 months, with a mean average of 25 months. During this time frame, no implants have been lost and no prostheses have been lost or remade (Tables VI & VII).

The amount of the early loading bone loss on the 453 successful implants at stage II or bone healing, was also related to the one stage or two stage surgical approach. The implants with a two stage surgical approach lost an average of 0.71mm during the first year of bone loading from stage 11 uncovery (Fig. 12). The implants with a one stage surgical approach lost an average of 0.55 mm of bone during the first year of loading (Table VIII).

DISCUSSION

A key to long-term success of posterior maxillary implants is the design of an adequate treatment plan, which includes the presence of anterior teeth or implant(s). Therefore the treatment plan should provide for the maintenance or restoration of healthy anterior teeth or Division A bone in the premaxilla for implant placement. A minimum of a healthy natural canine tooth or two implant abutments in the canine to central incisor region for each quadrant are required before posterior implants are considered.30

In the past, implants were inserted in the posterior maxilla, without modifying the maxillary sinus topography. Small implants were often placed below the antrum. The decreased surface area compounding the poor bone quality resulted in poor implant stability. Attempts to place larger endosteal implants posterior to the antrum and into tuberosity and pterygoid plates also resulted in compromised situations. In addition, although feasible from a surgical standpoint, rarely are third or fourth molar abutments indicated for proper prosthodontic support. This approach also often requires three or more pontics between the anterior and posterior implants. The typical span results with excessive flexibility of the prosthesis, unretained restorations, excess stresses, and implant failure.

In 1974, Tatum developed a modified Caldwell-Luc procedure for elevation of the sinus membrane and subantral augmentation.10 In 1975 Tatum developed a lateral approach surgical technique that allowed the elevation of the sinus membrane and implant placement in the same surgical appointment.11 In 1981 Tatum developed a submerged titanium implant for use in the posterior maxilla.12 The advantages of submerged healing, the use of titanium as a biomaterial, improved biomechanics, and improved surgical technique made subantal bone grafting more predictable. The sinus graft procedure has been the most predictable method to grow bone height compared to any other intraoral bone grafting technique (Figs. 12-18). However, the bone density is still reduced, and the bone strength is the weakest of any region in the mouth.32 Therefore, to improve implant survival in these bone grafts this rate requires a further enhancement of treatment plans other than improving bone quantity.

IMPLANT NUMBER

The natural dentition in the posterior maxilla is provided with the largest diameter teeth, the greatest number of roots, and the greatest amount of root surface area. These are all biomechanical advantages to sustain greater forces. Implant treatment plans should attempt to simulate the conditions found with natural teeth. Because stresses are the major cause of implant complications or failures, biomechanical concepts to minimize their noxious effects are implemented. Implant number is an excellent method to decrease overall stresses.31 At least one implant for each missing tooth is often indicated in the posterior maxilla (Figs. 19-23). When forces are higher than usual, one implant for each buccal root (i.e. 2 for each molar) may even be indicated.32 Implants are splinted together to biomechanically reduce stresses to the bone.

IMPLANT LENGTH

An implant treatment plan axiom has long been the use of the longest implant possible in the available bone.33 The surface area of a root form implant increases relative to its length. Most every implant manufacturer provides implants in variable lengths from 7 to 16 mm, often in 2 or 3 mm increments. For every 3 mm increase in length, the surface area of a cylinder shaped implant increases an average of 20 to 30 percent.29 The use of the longest possible implant is most commonly recommended to maximize implant surface area; it also allows engagement of the opposing cortical plate, a dense region which provides implant immobilization during trabecular bone interface remodeling. The anterior mandibular region serves as an ideal example of this philosophy. However, in the posterior maxilla, there is no dense opposing cortical plate to engage the implant. When sinus grafts are performed, longer implants may be used, but the functional surface area may not be improved. Hence, functional surface area rather than total surface area should be addressed. Most stresses to the implant bone interface are located in the crestal one-third of the implant.34 This is a critical area for stress distribution. Longer implants, although of greater total surface area, may transfer very little stress to the apical region and do not minimize stress in the most critical crestal regions. Hence, the greatest functional surface area is required in the crestal 1/2 of the implant body.

IMPLANT DESIGN

Implant dentistry has evolved into a greater understanding of biomechanics and the importance of stress reduction to minimize the risks of crestal bone loss and early implant failure. However, conventional implants limit themselves to implant length and diameter which are much less effective surface area enhancers than thread designs. Modified implant designs with increased functional surface area (instead of total surface area) allow shorter implants with greater surface areas to be used in all regions of the mouth. This is most important in the posterior maxillary regions, where forces are greater and bone strengths are reduced, and available bone height is often less than in the anterior regions. Interfacial loads to an implant should be compressive in nature, because of bone’s ability to best resist compressive loads, with a decrease in strength of 30% when subjected to tensile loading, and a decrease in strength of 65% in shear loading35 (Fig. 2). Functional (or active) surface area does not include the portion of the implant which is passive or transfers shear loads to bone.36 For example, if large size balls are attached t
o the surface of an implant, only the bottom 1/3 of the sphere can load the bone under an axial occlusal load. The top 1/2 to 2/3 of the sphere does not actively load the bone, but instead transfers passive or shear loads. Likewise, the functional surface area of a thread is that portion of the thread which participates in compressive or tensile load transmission under axial occlusal loads.37

Thread Geometry

Biomechanical engineering can help improve the transosteal region, in order to decrease the length of implants and minimized crestal stress around endosteal implants.36 Functional surface area per unit length of the implant may be modified by varying thread geometry parameters: i.e., thread pitch, and thread depth.37 Thread pitch is defined as the distance between adjacent threads, or the number of threads per unit length in the same axial plane and on the same side of the axis.38 The smaller (or finer) the pitch, the more threads on the implant body for a given unit length, and thus the greater surface area per unit length of the implant. Restated, a decrease in the distance between threads will increase the number of threads per unit length. For example, the thread pitch of the Steri-Oss implant body is 0.625 mm, and the thread pitch of the Nobel BioCare implant body is 0.60 mm, the latter exhibiting a greater number of threads per unit length.37 Therefore if force magnitude is increased or bone density decreased, the thread pitch may be decreased to increase the functional surface area. Traditionally, manufacturers have provided implant systems with a constant pitch and surface area per unit length, regardless of the character of forces or the bone density of the recipient site. The first bone quality based implant system designed (BioHorizons Maestro Dental Implants), increases the number of threads on the implant body as the recipient bone site decreases in density (Fig. 1).

The thread depth refers to the distance between the major and minor diameter of the thread.37 The greater the thread depth, the greater the surface area. The Steri-Oss thread depth is 0.28 mm, the Nobel BioCare is 0.375 mm and the BioHorizon thread depth in the crestal region is 0.419 mm, and therefore, each have a different surface area.

The thread depth for conventional implants is similar for all implant diameters. As a result, an increase in surface area with greater diameter implants is primarily the result of an increased circumference, and approximates 30%.37 However, thread depth can be increased in larger diameter implants. Hence, the surface area in the larger diameter implant body may result from a combined increase in circumference and thread depth, and be increased up to 300% as a consequence (i.e. BioHorizons Dental Implants)36 (Table IX). Thread geometry may modify surface area to such an extent, that smaller length implants may have a greater surface area than implants of wider and/or longer dimensions, but of a different design. These factors are most critical in the posterior maxillary regions of the mouth, since the opposing landmarks often limit the length of the implant (Fig. 13).

Implant Surface Condition

Strategic choices to increase bone contact are suggested in the posterior maxilla to offset the poor strength and decreased bone density. Hydroxyapatite (HA) coating on the implant has been shown to increase the rate of osseous adaptation to implants39 give greater initial rigid fixation,40 increase the surface of bone contact,41 increase the amount of lamellar bone,39 and give relative greater strength of the coronal bone around the HA-coated implants when compared with titanium implants.40 The space or gap between the bone and implant at initial placement is greatest in the soft bone of the posterior maxilla compared to the other regions. Gap healing may be enhanced by HA coatings.40 Therefore, HA coatings are strongly suggested in the D4 softest bone category, where the aforementioned benefits outweight the potential problems associated with HA coating technology42 (Fig. 24).

The clinical trial reported in this paper demonstrates that implant number and design may be successfully used to restore the posterior maxilla. Implant surgical success was 99.4% even though more than two-thirds of the implants were in D4 bone. The 453 successful implants which restored 131 patients had no early loading failures and no prostheses were lost. The average bone loss during the first year of loading was 0.71 mm or less. Hence, all restored implants have remained in moderate to optimum health during the first year of prostheic loading.

CONCLUSION

In the past, force reduction and surface area were difficult to balance in the posterior regions of the mouth. Studies clearly demonstrate forces are often 300% greater in the posterior compared to the anterior regions of the mouth. Bone densities and strengths are 50% to 200% weaker in the posterior region. Yet, the implants with greater surface area (through length) were inserted in the anterior regions. It is noteworthy that natural teeth do not have longer roots in the posterior regions of the mouth, where stresses are greater. Instead, increased surface area is achieved by an increase in diameter and a change in root design.

This article reports on 456 BioHorizons Dental Implants, which are 13 mm or less in length, may be used successfully in the posterior maxilla, with 99.4% surgical survival. In addition, there were no early loading failures and no prosthetic failure with follow-up as long as 46 months. The functional surface area of the bone quality-based implant system is designed to increase as bone quality decreases or when prosthetic loads increase. Hence, design modification rather than implant length changes was shown to be very clinically predictable in this report.

Methods to increase the functional surface area of implants is warranted in the posterior regions, since the implants have greater force and weaker bone types. Conventional implant designs increase surface area by 20% to 30% when their diameter is increased. This may not be adequate to compensate for a force increase of more than 300% in the posterior regions, especially when the bone density is also less. When thread pitch and depth are increased along with the diameter, the functional surface area of a dental implant may increase more than 300%. Hence, a more scientific approach to treatment planning may be accomplished when the biomechanical aspects of implant design are considered.

ACKNOWLEDGMENTS

The authors recognize the financial assistance of BioHorizons Implant Systems, Inc. in providing some of the support and supplies for this study. Special thanks to Denise Zuzow for processing the implant survival and related bone loss data. Thanks also to Jill Bertelson for typing and coordinating this manuscript.

r. Carl E. Misch is a consultant for BioHorizons Implant Systems, Inc and is a member of Oral Health’s Board of Directors.

Oral Health welcomes this original article.

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Table I:

Patients (n = 131)
Age/Years Female Male
17-78 84 47

Table II:

Prosthetics (n = 131)
Single Tooth FPD < 4 FPD > 4 O.


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