Wide-Diameter Implants: Surgical, Loading, and Prosthetic Considerations

The initial treatment plan for implant dentistry should include the ideal implant size, based primarily upon biomechanic and aesthetic considerations. When a tooth is replaced in traditional prosthetics, the abutment teeth are already provided by nature. For example, the missing posterior teeth have posterior abutments, and the missing maxillary anterior teeth have anterior abutments. When teeth are replaced with dental implants, the implant team should preselect the ideal abutment size, based upon the ideal size for an aesthetic restoration within biomechanical guidelines.
Traditionally, the size of an implant was primarily determined by the existing bone volume in height, width, and length.¹ The surgeon would select longer implants for the anterior regions and shorter ones in the posterior regions that were limited by the mandibular canal and maxillary sinus. The width of the implant would primarily be a single-diameter implant (3.75 mm) used in all situations.
Misch² has proposed over the years a biomechanical approach to dental implant treatment plans to decrease the most common complications – those related to stress. The prosthesis is first planned, including whether the restoration is fixed or removable, how many teeth are replaced, and the aesthetic considerations for the restoration. The patient force factors are then evaluated in terms of the magnitude and type of force applied to the restoration. The bone density is assessed in the regions of potential implant abutments. The key implant positions and the implant number are then considered, related to the force factors and bone density for the individual patient.
For example, when the patient has parafunction and/or the bone is less dense and/or when a cantilever is present, the greater force on the terminal abutment transfers stresses to the implant-bone interface. The next consideration in this ideal treatment plan sequence is the implant size.³
Over the past 5 decades of endosteal implant history, implants have gradually increased in width. The pin implants of Scialom in the 1960s and 70s were less than 2 mm wide. The Branemark implant in the 1980s first presented a primary implant body diameter of 3.75 mm.¹ Over the years, many manufacturers have provided a wide range of implant diameters.
Dental implants function to transfer loads to surrounding biologic tissues.⁴ Biomechanical load management depends on two factors: the character of the applied force and the functional surface area over which the load is dissipated.⁵ The size of the implant directly affects the functional surface area, which distributes a load that is transferred to the prosthesis. The aesthetic considerations for the implant are also determined relative to implant size when the restoration is within the aesthetic zone.
In this article the surgical and prosthetic advantages of a wide-diameter implant are addressed. In addition, this article will build upon and apply basic biomechanic principles to demonstrate how the width of a dental implant affects the distribution of occlusal load.

SURGICAL ADVANTAGES OF WIDE-DIAMETER IMPLANTS

The surgical advantages of a wide-diameter implant primarily relate to its use as a rescue implant, when the regular body size does not adequately fixate to the surrounding bone.¹ Under these conditions, the regular diameter implant may be removed and replaced with a wide-body implant. In addition, when an implant fails due to lack of osteointegration or fracture, the implant may be removed and the wide-body implant immediately inserted.⁶  This eliminates the need for bone grafting, the time required for bone augmentation healing, and the additional surgery to replace the implant. The same concept may be used for the immediate placement of an implant after the extraction of a tooth.⁷ Since the diameter of most teeth is greater than 4 mm, a larger diameter implant leaves a smaller defect space between the alveolus and the wide implant body (Table 1).

Loading Advantage

The presence of fibrous tissue has long been known to decrease the long-term survival of a root-form implant.¹ Excessive loads on an osteointegrated implant may result in mobility of the supporting device, even after a favorable bone-implant interface has been obtained.⁸ In addition, although several conditions may cause crestal bone loss, one of these causative factors may be prosthetic overload.⁹ Excessive loads on the bone increase strain in the bone.¹⁰ These bone microstrains may affect the bone remodeling rate and cause pathologic overload, which results in the loss of bone. The amount of bone strain is directly related to the amount of stress applied to the implant-bone interface. The greater the stresses throughout the implant-bone interface, the greater the risk factor for crestal bone loss and subsequent implant failure.¹¹ Therefore, the stress/strain relationship has been shown to be an important parameter for crestal bone maintenance and implant survival.
The surface area over which the occlusal forces are applied is relevant and inversely proportional to the stress observed within the implant system (stress=force/surface area). It can be clearly seen from this basic engineering equation that in order to reduce stress, the force must decrease or the surface area must increase. Hence, an increase in implant size is beneficial to decrease the stress applied to the system. The size of an implant may be modified in either length and/or diameter.       

Figure 1. Three-dimensional finite element analysis studies demonstrate the stresses to an implant interface are greatest at the crest. Wider diameter implants have as much as a 3.5-fold decrease in crestal stresses compared to narrower diameter implants.	Figure 2. The forces in the posterior regions are greater than in the anterior areas of the mouth. As a result, wider diameter molar implants offer advantages.
Figure 3. Wider diameter implants are used in the molar regions, especially when a shorter implant is selected. This periapical x-ray illustrates two 5-mm-diameter implants that are 12 mm and 9 mm long, replacing the first and second molars. (Implants used were manufactured by BioHorizons Dental Implants.)	Figure 4. For each 0.25-mm increase in diameter, the surface area is increased by 5% (with a cylinder implant design). A 4.0-mm-diameter implant has 25% greater surface area than a 3.0-mm implant.

Since occlusal stresses to the implant interface are concentrated at the crest of the ridge, width appears more important than height once a minimum height has been obtained for initial fixation and resistance to torque and bending loads. A comparative evaluation of strains in the alveolar crest of implants with different diameters was performed by Petrie and Williams.¹² This 3-dimensional finite element analysis found as much as a 3.5-fold reduction in stress when wider diameter implants (up to 6 mm) were compared to narrow diameters (3.5 mm), as seen in Figure 1. A study by Aparicio and Orozco¹³ used Periotest values (PTVs) to clinically confirm less stress transferred to the implant-bone complex with wide-diameter implants. The observed PTVs from 5-mm- diameter implants in the maxilla and mandible were 1.1 and 0.6 units lower, respectively, than for 3.75- mm-diameter implants in the same patients, which indicated wider implants have less stress transferred to the interface.
Since the loading advantages of a wide-diameter implant relate to a greater surface area, especially in the crestal region of the implant, the greater surface area is of benefit when the patient force factors are greater. For example, parafunction, increased crown height, increased masticatory dynamics, and the molar regions of the mouth are all conditions that increase force and would benefit from a wider diameter implant (Figure 2). The greater surface area is also an advantage when a cantilever is necessary to restore the dentition, either in a mesio-distal or facial-lingual direction. For example, the most distal implant with a posterior cantilever acts as a fulcrum and receives the greatest force.⁵ A wider diameter implant in this location reduces the risk of overload. An angled load to the implant body also increases the magnitude of the force at the crestal marginal bone. A larger diameter implant reduces the magnitude of force to the entire implant system and reduces the risk of angled loads to the bone.

Methods to increase the functional surface area are especially warranted in the posterior regions where greater forces are generated (Figure 3). The logical method to increase functional surface area in this region is by increasing the diameter of the implant, since the opposing anatomical landmarks limit the implant length. Wider root form designs exhibit a greater area of bone contact than narrow implants of similar design, in part due to an increase in circumferential bone contact. Each increase of 1 mm in implant diameter may increase the functional surface area by 30% to 200% (Figure 4), dependent upon the implant design, eg, cylinder versus thread¹⁴ (Table 2).

PROSTHETIC ADVANTAGES

Figure 5. A 4-mm-diameter implant replacing a molar has a steep emergence angle at the gingival region. This periapical x-ray illustrates the poor emergence profile. Also note bone loss at the crestal bone region.	Figure 6. A periapical x-ray demonstrating a 5.0-mm-diameter implant replacing a first molar with an improved emergence profile. The periapical x-ray also illustrates ideal crestal bone levels.

The prosthetic advantages of the wide-diameter implant include an improved emergence profile for the crown (Figures 5 and 6). The greater the implant diameter, the closer the emergence profile to that of a natural tooth, especially in the posterior region of the mouth. This contour may improve the aesthetics of the restoration. The wider crown contour may also decrease the interproximal space and the incidence of food impaction during function. The wide-diameter implant may also improve sulcular daily oral hygiene. A proper crown emergence permits access to the sulcus to obtain periodontal probing depths or cleaning.¹⁵
In 1997, Jarvis¹⁶ emphasized the biomechanical advantage of wide-diameter implants, particularly in reducing the magnitude of stress delivered to the various parts of the implant. An increase in implant diameter also increases the strength of the implant body, which decreases the risk of fracture. The bending fracture resistance of an implant is related to the diameter at the fourth power.¹⁴ In other words, a 4-mm-diameter implant is 16 times stronger than a 2-mm- diameter implant, and 16 times weaker than an 8-mm-diameter implant. Hence, when forces are greater than usual, a larger diameter implant will decrease the risk of fracture.

In 1999, Boggan et al¹⁷ showed that the force on an abutment screw is reduced with a larger diameter implant. The larger diameter implants, which have a larger prosthetic platform, transfer less force and stress to the abutment screw and therefore are likely to reduce screw loosening. In a clinical article by Cho, et al¹⁸ in 2004, wide-diameter implants had 5.8% screw loosening compared to 14.5% for implants of a standard diameter (Table 3).

CLINICAL REPORTS

Figure 7. A maxillary full-arch implant prosthesis supported by 10 endosteal implants.	Figure 8. A panoramic x-ray of Figure 7 with full-arch maxillary and mandibular fixed prostheses.
Figure 9. The high smile line of the patient in Figures 7 and 8.	Figure 10. A periapical x-ray demonstrating 4-mm-diameter implants in the premolar region and a 5-mm- diameter implant in the molar position.

Clinical reports indicate improved implant survival with wide-diameter implants.¹⁹ Griffin and Cheung²⁰ reported on short, wide implants in posterior areas with reduced bone height for 168 HA-coated implants 6 mm in diameter and 8 mm long in 167 patients. The overall cumulative survival rate for up to 68 months (mean 34.9 months) after loading was 100%. Anner, et al²¹ in 2005 reported a 100% survival rate in 45 implants with a mean loading period of 2 years with a 6-mm-wide, tapered, HA-coated implant. Graves, et al¹⁵ in 1994 reported 96% survival over a 2-year period with 268 wide implants in 196 patients. All failures occurred before stage II surgery due to nonintegration of the implant. In 2006 Misch, et al²² compared 4.0-mm and 5.0-mm implants 7 and 9 mm long in the posterior maxilla and mandible (Figures 7 to 9). The 5-year retrospective report found 100% implant success for the 5.0-mm implant, while the 4.0-mm implant had a 98% survival. Hence, these reports seem to find the larger diameter implants have a similar or improved implant survival compared to the standard 3.75-mm-diameter implant body.

DISADVANTAGES OF WIDE-DIAMETER IMPLANTS

The disadvantages of a wide- bodied implant are discussed in reports that indicate a higher failure rate. For example, Eckert, et al23 in 2001 found implant loss of 19% in the mandible and 29% in the maxilla in a study of 85 wide-platform MK II implants  (Nobel Biocare) in 63 patients. In 2003, Attard and Zarb24 compared the success rate of the standard-diameter 3.75 mm at 15 years and the 5-year survival of the wide-platform 5-mm-diameter implant replacing posterior teeth. The standard diameter had a 91.6% implant survival, while the 5-mm implant had a 76.3% rate of survival. Ivanoff, et al25 stated the higher failure rate may be due to an early learning curve, the implant being used in poor bone quality, and the use of the wide-diameter implant as a rescue implant when the standard diameter did not reach stability or failed. 

NATURAL TEETH

The natural tooth roots may serve as an indicator for implant size requirements in terms of width for prosthetic loads. When comparing implant diameter to natural tooth roots, 3.0- to 3.5-mm-diameter implants may be used in the mandibular incisor regions and in the area of the maxillary lateral incisors; 4-mm-diameter implants may be used in the areas of the maxillary anteriors, premolars of both arches, and the mandibular canines; and 5- or 6-mm-diameter implants may be used in the molar areas of both arches (Figure 10).  When larger diameter implants cannot be used in the molar region, two 4-mm-diameter implants for each molar should be considered.

SUMMARY

The wide-diameter implant has surgical, loading, and prosthetic advantages. The treatment plan should include the ideal width of the implant prior to implant surgery. The ideal width is primarily based upon prosthetic load and aesthetic requirements of the restoration (when within the aesthetic zone).

References

1. Adell R, Lekholm U, Rockler B, et al. A 15-year study of osseointegrated implants in the treatment of the edentulous jaw. Int J Oral Surg. 1981;10:387-416.
2. Misch CE. Stress factors influence on treatment planning. In: Misch CE, ed. Dental Implant Prosthetics. St Louis, Mo: Mosby; 2005.
3. Misch CE. Consideration of biomechanical stress in treatment with dental implants. Dent Today. May 2006;25:80-85.
4. Brunski JB. Biomechanics of oral implants: future research directions. J Dent Educ. 1988;52:775-787.
5. Bidez MW, Misch CE. Force transfer in implant dentistry: basic concepts and principles. J Oral Implantol. 1992;18:264-274.
6. Langer B, Langer L, Herrmann I, et al. The wide fixture: a solution for special bone situations and a rescue for the compromised implant. Part 1. Int J Oral Maxillofac Implants. 1993;8:400-408.
7. Jividen GJ Jr. Immediate placement of wide-diameter implants in the premaxilla. Dent Implantol Update. 1998;9:89-92.
8. Naert I, Koutsikakis G, Duyck J, et al. Biologic outcome of implant-supported restorations in the treatment of partial edentulism. Part I: a longitudinal clinical evaluation. Clin Oral Implants Res. 2002;13:381-389.
9. Misch CE. Early crestal bone loss etiology and its effect on treatment planning for implants. Postgrad Dent. 1995;2:3-17.
10. Frost HM. Mechanical adaptation. Frost?s mechanostat theory. In: Martin RB, Burr DB, eds. Structure, Function and Adaptation of Compact Bone. New York, NY: Raven Press; 1989:179-181.
11. Misch CE, Suzuki JB, Misch-Dietsh FM, et al. A positive correlation between occlusal trauma and peri-implant bone loss: literature support. Implant Dent. 2005;14:108-116.
12. Petrie CS, Williams JL. Comparative evaluation of implant designs: influence of diameter, length, and taper on strains in the alveolar crest. A three-dimensional finite-element analysis. Clin Oral Implants Res. 2005;16:486-494.
13. Aparicio C, Orozco P. Use of 5-mm-diameter implants: Periotest values related to a clinical and radiographic evaluation. Clin Oral Implants Res. 1998;9:398-406.
14. Misch CE, Bidez MW. A scientific rationale for dental implant design. In: Misch CE, ed. Contemporary Implant Dentistry. 2nd ed. St Louis, Mo: Mosby; 1999:329-343.
15. Graves SL, Jansen CE, Siddiqui AA, et al. Wide diameter implants: indications,  considerations and preliminary results over a two-year period. Aust Prosthodont J. 1994;8:31-37.
16. Jarvis WC. Biomechanical advantages of wide-diameter implants. Compend Contin Educ Dent. 1997;18:687-694.
17. Boggan RS, Strong ST, Misch CE, et al. Influence of hex geometry and prosthetic table width on static and fatigue strength of dental implants. J Prosthet Dent. 1999;82:436-440.
18. Cho SC, Small PN, Elian N, et al. Screw loosening for standard and wide diameter implants in partially edentulous cases: 3- to 7-year longitudinal data. Implant Dent. 2004;13:245-250.
19. Krennmair G, Waldenberger O. Clinical analysis of wide-diameter frialit-2 implants. Int J Oral Maxillofac Implants. 2004;19:710-715.
20. Griffin TJ, Cheung WS. The use of short, wide implants in posterior areas with reduced bone height: a retrospective investigation. J Prosthet Dent. 2004;92:139-144.
21. Anner R, Better H, Chaushu G. The clinical effectiveness of 6 mm diameter implants. J Periodontol. 2005;76:1013-1015.
22. Misch CE, Steigenga J, Barboza E, et al. Short dental implants in posterior partial edentulism. A multicenter retrospective 6-year case series study. J Periodontol. In press.
23. Eckert SE, Meraw SJ, Weaver AL, et al. Early experience with Wide-Platform Mk II implants. Part I: Implant survival. Part II: Evaluation of risk factors involving implant survival. Int J Oral Maxillofac Implants. 2001;16:208-216.
24. Attard NJ, Zarb GA. Implant prosthodontic management of partially edentulous patients missing posterior teeth: the Toronto experience. J Prosthet Dent. 2003;89:352-359.
25. Ivanoff CJ, Grondahl K, Sennerby L, et al. Influence of variations in implant diameters: a 3- to 5-year retrospective clinical report. Int J Oral Maxillofac Implants. 1999;14:173-180.

Dr. Misch is the co-chairman of the International Congress of Oral Implantologists and author of more than 200 clinical articles. He has trained more than 2,500 doctors in his hands-on, yearly forum of education on implant dentistry. He can be reached at (248) 642-3199.

Disclosure: Dr. Misch is co-inventor of the BioHorizons Dental Implant System, is a paid consultant of BioHorizons, and is also on the board of directors.