August 1, 2006
by Carl E. Misch, BS, DDS, MDS.
The initial treatment plan for implant dentistry should include the ideal implant size, based primarily upon biomechanic and esthetic 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 esthetic restoration within biomechanical guidelines.
The size of an implant, in the past, 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 limited by the mandibular canal and maxillary sinus. The width of the implant would primarily be one diameter implant (3.75mm) used in all situations.
A biomechanical approach to dental implant treatment plans has been proposed by Misch over the years to decrease the most common complications — those related to stress.2 The prosthesis is first planned, including whether the restoration is fixed or removable, how many teeth are replaced, and the esthetic considerations for the restoration. The force factors of the patient are then evaluated in order to evaluate the magnitude and type of forces applied to the restoration. Consideration of parafunction, crown height and masticatory dynamics (sex, size and age) are especially noted. The bone density is then assessed, in the regions of potential implant abutments. Softer bone types are especially noted.
The ideal treatment plan then considers the key implant positions and the number of implants necessary to support a restoration with the amount of patient force factors and the bone density in the prospective implant sites. For example, the key implant positions eliminate cantilevers. 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 abutments transfer greater stresses to the implant-bone interface and should be countered by increasing the number of implants. The next consideration in this ideal treatment plan sequence is the implant size.3
Over the past three decades of endosteal implant history, implants have gradually increased in width. The Branemark implant in the 80s first presented a primary implant body diameter of 3.75mm.1 Over the years, many manufacturers have provided a wide range of diameter implant diameters. The increase in implant diameter has surgical, loading and prosthetic considerations.
WIDE DIAMETER SURGICAL ADVANTAGE
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.4 This eliminates the need for bone grafting, the time 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.5 Since the diameter of most teeth is greater than 4mm, a larger diameter implant leaves less of a defect space between the alveolus and the wide implant body (Table 1).
Dental implants function to transfer loads to surrounding biologic tissues.6 Biomechanical load management is dependent on two factors: the character of the applied force and the functional surface area over which the load is dissipated.7 The size of the implant directly affects the functional surface area which distributes a load transferred to the prosthesis.
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 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 very relevant and is 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.
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 3.5-fold reduction in stress when wider diameter implants (up to 6mm) were compared to narrow diameters (3.5mm) (Fig. 1). A study by Aparicio and Orozco used Periotest values to clinically confirm less stress transferred to the implant-bone complex with wide diameter implants.13 They observed PTV’s from 5mm diameter wide implants in the maxilla and mandible were 1.1 and 0.6 units lower than for 3.75mm 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 in the posterior regions of the mouth are all conditions which increase force and would benefit from a wider diameter implant (Fig. 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.7 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 of greater force (Fig. 3). The logical method to increase functional surface area in this region is by increasing the diameter of the implant, since the opposing 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 millimeter implant diameter increase may increase the functional surface area by 30 to 200 percent, dependent upon the implant design (re: cylinder vs. th
read)14(Fig. 4 & Table 2)
The prosthetic advantages of the wide diameter implant include an improved emergence profile for the crown (Figs. 5 & 6). The greater the implant diameter, the closer the emergence profile to that of a natural tooth, especially in the posterior region of the jaws. This contour may improve the esthetics of the restoration. The wider crown contour may also decrease the interproximal space and decrease 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, Jarvis emphasized the biomechanical advantage of wide diameter implants, particularly in reducing the magnitude of stress delivered to the various parts of the implant.16 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 4mm diameter implant is 16 times stronger than a 2mm diameter and 16 times less strong than an 8mm diameter implant. Hence, when forces are greater than usual, a larger diameter implant will decrease the risk of fracture.
In 1999, Boggan et al showed the force on an abutment screw is reduced with a larger diameter implant.17 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. For example, in a clinical article by Cho et al. in 2004, wide diameter implants had 5.8 percent screw loosening compared to 14.5 percent for standard diameter implants18 (Table 3).
Clinical reports indicate improved implant survival with wide diameter implants.19 Griffin and Cheung reported on short, wide implants in posterior areas with reduced bone height for 168 HA-coated implants 6mm in diameter and 8mm long in 167 patients.20 The overall cumulative survival rate for up to 68 months (mean 34.9 months) after loading was 100 percent. Anner et al, in 2005, reported a 100 percent survival rate in 45 implants with a mean loading period of 2 years with a 6mm wide, tapered HA-coated implant.21 Graves et al., in 1994, reported 96 percent survival over a two year period with 268 wide implants in 196 patients.15 All failures occurred before Stage II surgery, due to non-integration of the implant.
In 2006, Misch, et al compared 4.0mm and 5.0mm implants 7 and 9mm long in the posterior maxilla and mandible22 (Figs. 7 & 8). The 5 year retrospective report found 100 percent implant success for the 5.0mm implant, while the 4.0mm implant had a 98 percent survival. Hence, these reports seem to find the larger diameter implants has a similar or improved implant survival compared to the standard 3.75mm diameter implant body.
DISADVANTAGES OF WIDE IMPLANTS
The disadvantages of a wide bodied implant are discussed in reports which indicate a higher failure rate. For example, Eckert et al. in 2001, found the implant loss of 19 percent in the mandible and 29 percent in the maxilla, out of 85 wide-platform MK II implants in 63 patients.23 In 2003, Attard and Zarb compared the success rate of the standard diameter 3.75mm at 15 years and the 5 year survival of the wide-platform 5mm diameter implant replacing posterior teeth.24 The standard diameter had a 91.6 percent implant survival, while the 5mm implant had a 76.3 percent rate. Ivanoff et al. stated the higher failure rate may be due to an early learning curve, the implant 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.25
The natural tooth roots may serve as an indicator for implant size requirements in width for prosthetic loads. In this light, the mandibular incisors regions and the maxillary laterals incisors may use 3.0 to 3.5mm diameter implants, the maxillary anteriors, premolars of both arches, the mandibular canine may use 4mm diameter implants and the molars 5 or 6mm diameter implants in both arches (Fig. 9). When larger diameter implants can not be used in the molar region, two 4mm diameters implants for each molar should be considered.
The wide diameter implant has surgical, loading and prosthetic advantages. The treatment plan should include the ideal width of the implant, prior to the implant surgery. The ideal width is primarily based upon prosthetic load and esthetic requirements of the restoration (when within the esthetic zone).
Dr. Misch is Clinical Professor and Director of Oral Implantology, Temple University, Philadelphia, PA and Director of Misch International Implant Institute, Beverly Hills, MI.
Oral Health welcomes this original article.
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udinal data. Implant Dent 13 (3), 245-250, 2004.
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* “Rescue” implant for initial fixation
* Immediate reimplant after failure
* Tooth extraction/immediate insertion
* Greater force conditions
* Posterior regions
* Greater crown height
* Angled forces
* Short implants
* Emergence Profile
* Oral hygiene
* Less screw loosening
* Less fracture