Biomaterials and Biomechanics in Dental Implant Design · 2008. 11. 19. · dental implant,...

JOMI on CD-ROM, 1988 Feb (85-97 ): Biomaterials and Biomechanics in Dental Implant … Copyrights © 1997 Quinte…

Biomaterials and Biomechanics in Dental Implant DesignJohn B. Brunski, PhD

This article seeks to give the clinician insight into the design process and biomaterial/biomechanical aspects of endosseous implant design. Specific facets considered relate to materials, implant shape, special surface coatings, shock-absorbers, and the implant-tissue interface.(Int J Oral Maxillofac Implants 1988;3:85-97)

Key words: biomaterials, biomechanics, design, forces, interface, stress transfer

Bioengineering is critical in dental implant design. The object of this article is to summarize knowledge about biomaterials and biomechanics of dental implants to help a clinician confront the following questions: Why are there so many different implant biomaterials and shapes? Should implants have special coatings on the surface, and if so why? Should an implant have some sort of built-in shock absorber? Is there a certain optimal implant-tissue interface, and if so, in what respect is it optimal?

The goal is to give the clinician insight into the design process and biomaterial and biomechanical aspects of implant design. A clinician who thinks about dental implants from the design perspective will demand answers to the following questions: What are the objectives for a particular dental implant system? What is the rationale for trying to achieve the objectives in the way proposed? Are the objectives achieved in practice?

Attention will be restricted to endosseous (endosteal) dental implants.

Design processDesign means to create according to a plan.1 The word design indicates a process, not an end product such as the particular shape or material of a dental implant. Shape and material are only two of the many considerations in the multivariable design problem for dental implants.

The design process is a generic approach to problem solving and consists of these steps1:

1. Identification of a need

2. Definition of the problem (and sub-problems) to be solved

3. Search for necessary background information and data


4. Formulation of objectives and criteria

5. Consideration of alternative solutions to the problem

6. Analyses and evaluations of alternative solutions

7. Decision-making and optimization

Design has some identifying characteristics. A complicated design problem will usually be broken down into sub-problems, so these can be addressed separately and then considered together in reaching final solutions. Often, design must go forward even when there is missing or unknown information. In design, judgments about the quality of a solution are made by measuring performance against the stated goals, not the other way around.

Finally, design is often iterative. There may be a need to design and redesign several times to optimize performance with respect to goals. There may be no perfect solution to a design problem, but instead a compromise solution representing the best solution under conflicting constraints.

Engineering designIn applying the design process to dental implants, it is easy to identify the need (step 1) for an implant. It is also easy to define the problem (step 2). What can be difficult is translating these generalities into specifics. Exactly how will the problem be solved? Initial questions are: What are the "masticatory functions" that will be restored (step 3)? How will design criteria for implants evolve from this (step 4)? What are alternative approaches to achieving the goals (step 5)?

It is critical that steps 1 to 4 be followed. If these steps are clear, then steps 5, 6, and 7 outline a path toward creating, evaluating, and optimizing a proposed solution to the problem. But if steps 1 to 4 are missing or ambiguous (as they are for some current dental implants), then it becomes difficult if not impossible, to understand the whole process.

The importance of clearly stating the problem to be solved; the design goals; the rationale for solving the problem via a certain approach; and the evidence that the chosen approach does, or does not, satisfy the design objectives can't be overemphasized.

Trial and error and intuition have legitimate places in design, but if these are the only elements in the process, then this should be acknowledged. There is nothing intrinsically wrong about a trial-and-error solution to a design problem. However, it usually indicates that the designer isn't sure why the solution works. This also means that the designer may not be sure when it will not work. Even if something "works," it is useful to know why.

How do biomaterials and biomechanics fit into dental implant design? These


subjects represent only two of many subproblems in the entire design effort. Any implant must be constructed from a biomaterial. The biological performance of the chosen biomaterial will be of concern. Dental implants must function biomechanically, so biomechanical issues will arise. Implantation surgery, postoperative care, periodontal health, patient physiology, costs to the patient, and other aspects also are key subproblems in implant design, but these go beyond the scope of this review.

SubproblemsBiomechanics is the application of engineering mechanics (statics, dynamics, strength of materials, and stress analysis) to the solution of biological problems. Biomechanics pertains to dentistry because the teeth and jaw perform biomechanical activities during mastication.2 Biomaterials deals with the effects of an implanted material on the body and vice versa.

Biomechanical and biomaterial subproblems are depicted in Fig 1. First, any dental implant, regardless of its biomaterial or shape (Fig 2), will be exposed to intraoral forces and moments. These loadings may be appreciable and the implant must withstand these loadings without being damaged.

Second, the implant has to be supported within the jaw by some method which will involve biomaterial and shape factors.

Third, the implant will transmit loading to the interfacial tissues around the implant, which then must tolerate them without adverse tissue response. The problem is selecting the material and shape of the implant so the implant functions properly.

BackgroundA design problem cannot begin to be solved without background data or information. For implants, prior research has provided some—but not all—of the biomechanical and biomaterial data for design.

Following is a synopsis of background research pertaining to implant design.

In vivo loadings. Vertical components of chewing forces have been reported for patients with natural teeth and for patients wearing conventional and implant-supported dentures (Table 1).

The normal human dentition is capable of exerting large forces. Axial components are in the range of 200 to 2,440 N, and lateral force components are of the order of 30 N (a newton is approximately the weight of one apple; 1 N = 0.2249 lb). For dentures supported by dental implants (fixtures) workers in Sweden8 have measured vertical closure forces of 42 to 412 N.

Implant design must distinguish between closure forces and vector components of forces and moments on individual implants supporting a bridge (Fig 3). While


closure forces are useful, individual loading components on implants are required for detailed design analyses of implants and surrounding interfacial tissues.

Unfortunately, except in animal studies,14 no direct measurements are available for loading components on dental implants in vivo, although data may be forthcoming.15 Without these data, in vivo forces must be estimated on dental implants, and the estimates used for stress analyses of implants or interfacial tissues. These analyses will only be as good as the input information, which is approximate at this time.

Implant properties. Implants should not fracture, yield, fatigue, wear, or otherwise fail during in vivo use. Failure prevention necessitates testing and stress analyses of the implants and tissues. Assuming there is accurate background data on typical implant loading (which is limited, as previously noted), the problem is to select adequate intrinsic and structural mechanical properties of implants.

Intrinsic properties pertain to the material and not its shape. They include a material's elastic moduli, yield point, ultimate tensile strength, compressive strength, fatigue strength, and hardness. (For corrosion behavior, intrinsic properties could also be defined.) Values can be found in textbooks and literature, or they may be directly measured via standard test methods.16-19 Caution is advised in using handbook values, because manufacturing processes can cause significant property differences between raw material and the finished product.

Structural mechanical properties embody both the intrinsic material property and the geometrical shape of the device being considered. For example, the deformability of a beam in bending depends on the product EI (flexural rigidity), where E is Young's modulus of elasticity and I is the second moment of area of the beam's cross-section. The deflections of a cantilevered dental bridge could be inappropriate even when the bridge is made of a strong, high-modulus (E) dental alloy because its deflection under load will depend on both modulus and dimensions. There are handbooks and articles on proper structural design that can be applied to implant design.20-22

Biomechanical properties. When considering an implant design, it would also be helpful, if not essential, to have data on:

• The percentage of an implant's surface that will actually be supported by hard versus soft interfacial tissues

• The mechanical properties of the interfacial tissues

• The extent to which the implant will rely on mechanical support from trabecular versus cortical bone

• The response of interfacial tissues to the imposed mechanical conditions arising from in vivo loads on the implant


• The presence or absence of significant attachment or bonding of interfacial tissues to implant

Data exist on the mechanical behavior of both trabecular and compact bone, including elastic behavior, viscoelastic properties, anisotropy, yielding, fracture, fatigue behavior, and related aspects.23 However, little is known about the biomechanical properties of bone adjacent to a dental implant.

Those who advocate fibro-osseous integration wish to have fibrous tissue at the interface.24 It would be appropriate to study mechanical properties of this tissue also.

For interfacial bone, it is not clear if properties measured on standard bone samples should be extrapolated to bone found adjacent to implants, where there can be variations in porosity, maturity and degree of mineralization. In the case of interfacial fibrous tissue, there is little information on its properties.25,26

It is clear that interfacial bonding— whether at the endosseous or permucosal interface—can make a difference in the interfacial load transfer problem. The results of finite element stress analyses of implants in bone depend on whether interfacial bonding is assumed in the computer models27 (Fig 4).

Interfacial load transfer. In dentistry and orthopedics it has long been appreciated that bone is a living tissue capable of responding to its biomechanical environment (Wolff's Law). However, recent studies have only begun to unravel the nature of this law and some of its possible mechanisms at bone-implant interfaces. A recent review article gives references on this topic in connection with implant design28

It is suspected that mechanical factors such as relative motion (sometimes called micromotion) stress, strain, and related factors at the interface can influence tissue response to implants. Relative motion is meant to describe the situation in which an implant moves or displaces with respect to interfacial tissues (Fig 4).

Studies (Table 2) indicate that relative motion correlates with fibrous tissue formation around implants in bone, especially if this motion occurs during the early healing stages after implantation. Brånemark's osseointegration system is predicated on a two-stage implantation method that discourages relative motion of fixtures with respect to interfacial tissues during early postoperative healing, leading to a close adaptation of bone (rather than soft tissue) to fixture surface.

Finally, there can be long-term bone reactions at interfaces in response to biomechanical loading. Brånemark's long-term results show "laminalization" of trabeculae around a loaded fixture.40 Likewise, biomechanically-mediated bone remodeling has been of interest in orthopedics.41

In both orthopedics and dentistry, the search continues for a quantitative understanding of the biomechanics of bone remodeling in relation to implant design


and load transfer at the interface.

Implant materials. When material is implanted into a surgically prepared site in bone, there are numerous critical issues relating to biomaterial-biological interactions (Fig 5). From the biological side, the dentist needs to consider water, ions, macromolecules, cells, and tissue responses—inflammatory response, wound healing, and cell attachment—in the presence of a biomaterial surface.

On the biomaterial side, the structure and properties of the biomaterial's surface, release of corrosion products, surface energy, and oxide stability should be considered. Recent textbooks and articles discuss these topics in further detail.42-47

NIDR publications have also summarized biomaterials research as it relates to dental implants.48,49 A continuing challenge for each dental implant designer is to clearly define design goals in specific biomaterial terms.

Objectives of various implant systemsThe text by Linkow and Cherchève50 and a recent article51 provide a history of the evolution of dental implants. Implants have been made from metallic, ceramic, and polymeric biomaterials. The myriad of early modalities included both one- and two-stage configurations.

For example, Greenfield's buried two-stage latticed cage was patented about 1909 and predates more recent two-stage systems. Other early shapes for implants included pins; screws; root forms; single and double spirals; screws with holes (vent-plants); sapphire-coated screws; numerous varieties of blade-vents; and two-stage designs such as Cherchève's "sleepaway" implant, screws with protruding pins, and designs with expansion mechanisms analogous to those of a hollow wall anchor.

More recently, there has been the osseointegrated system of Brånemark, followed by analogous systems of various shapes, sizes, and biomaterials (Fig 2).

The early evolution of dental implants reveals the main design strengths to be creativity, cleverness, boldness, and pioneering spirit. However, as a rule, the main weakness has been incomplete understanding and execution of the steps in engineering design.

Sometimes designers have blurred the difference between design rationale, which may be theoretical, and design evaluation, which requires experimental evidence. Also, there has been a tendency toward teleological interpretation of evidence. That is, after observing limited experimental results, the investigator claims to understand and see evidence of nature's purpose in these results.

The following sections summarize and discuss the main design rationales, objectives, alternative strategies, and results for a variety of dental implants. The emphasis is on biomechanical and biomaterial subproblems. No clinical comparisons


are made. It will be evident that a main theme relates to the question of how best to fix a dental implant in its interfacial tissues.

Pseudoperiodontal ligament. Rationale. An influential early design rationale was the "pseudoperiodontal ligament," discussed by Linkow and Cherchève in 197050 and more recently by James52 as well as Weiss in connection with fibro-osseous integration.24 Linkow and Cherchève asserted that "a good implant design . . . must either complement or supplement the natural biomechanical forces of the site," and "the operative procedure. . . must be so precise as to cause as little trauma and destruction to the site as possible."

Moreover, Linkow and Cherchève proposed that the part of a dental implant in bone "should be irregularly shaped so bone could grow into and through the irregularities." They stated that "a band of collagenous connective tissue forms between the implant and its surrounding bone. . . which will firmly wrap around and bind itself to the implant."

They also claimed that this collagenous tissue was a pseudoperiodontal ligament that could behave as follows: (a) the fibers connect to bone in a way reminiscent of Sharpey's fibers; (b) the fibers can pull on the bone when the implant is loaded; and ( c) this pulling on the bone recreates the tension on bone necessary for its healthy and continued growth.

Critique. A difficulty with this rationale is that the argument is teleological. There is little detailed experimental evidence that interfacial collagenous tissue actually functions in the ways proposed. It is alleged that dental implants with interfacial fibrous tissue do function properly in humans and there must therefore be functional capabilities of fibrous tissue around such implants. However, these are frequently anecdotal clinical reports with no attempts to quantify any functional capabilities of this fibrous tissue.

Theories can be proposed to suggest how fibrous tissue may be functioning. However, data are still needed on the structure, function, properties, and formation of this tissue in relation to implant success.

Actually, the evidence is strong that fibrous tissue forms as a result of relative motion of implant and healing bone, regardless of biomaterial used (Table 2). This suggests that fibrous tissue is a byproduct of mechanical interference with bone healing around the implant. Whether it also functions as a pseudoperiodontal or suspensory ligament for a dental implant is a separate question that has not been fully answered, according to some researchers.

Ten Cate,53 for example, disputes the analogy between fibrous tissue and normal periodontium on the basis of its cellular makeup and biological origin. Therefore, in terms of design, the problem remains for advocates of fibrous tissue to explain the rationale and objectives for this interfacial tissue, and, most importantly, to properly


demonstrate that these objectives are achieved in practice.

Macro-irregularities. Rationale. Macro-irregularities in an implant include macroscopic threads, fenestrations, pores, grooves, steps, threads, or other surface irregularities that are visible. The idea is to create mechanical interlocking between implant and bone at the macro level.

The concept has a long history, as evident from the 1909 "buried cage" of Greenfield up through the many vented screws and blades mentioned earlier.

Porous implant systems have also been devised by researchers. To make a porous implant, beads of material are typically sintered or fused together onto the surface of an implant core, which is frequently a cylindrical shape. Metallic, ceramic, and polymeric porous materials have all been employed.54-63

A variation on this theme is the fiber metal (fibermesh) system of Weiss and Rostoker,64 in which a fine meshwork of titanium alloy is sintered onto a titanium core to create a macroporous surface morphology.

Because Brånemark's fixtures and other similar shapes (Fig 2) have grooves, steps, and ledges for bone ingrowth, it can be argued that these systems are also based on macro-interlocking. However, they will be discussed later under the separate heading of osseointegration.

Critique. Concerning this design rationale, there are five key findings. First, bone can grow into the pores of porous systems, provided the pores are interconnected and have a diameter of about 100 microns, since this allows proper vascularity and mineralization.65,66 This result is seen in metallic, ceramic, and polymeric systems.

A second key finding is that bone ingrowth will not occur if there is relative motion during post-implantation bone healing.67,68 Fibrous tissue ingrowth will occur instead (the threshold of relative motion beyond which bone ingrowth will not occur has not been established). It follows that a two-stage implantation procedure is advisable in porous dental implant systems to minimize interfacial fibrous tissue formation.

Third, a two-stage implantation method alone may not guarantee stability during early healing, since stability also depends on the initial fit and stability of the implant in the bone site. Porous implants are frequently intended to be press-fit into a carefully prepared bone site. Even this does not guarantee that they will remain immobile during subsequent bone healing, which will include a resorptive phase that can expand the size of the prepared site and loosen the implant.

Also, although the buried portion of a two-stage implant is not directly loaded, it might still be indirectly loaded by jaw deformations,69 which in turn can lead to relative motion and fibrous tissue. Buried, screw-shaped fixtures of the Brånemark


type may provide greater resistance to relative motion than press-fit shapes.

Alternatively, the hollow basket70 and single crystal sapphire (Al2O3) screw71 are one-stage implants that apparently afford enough stability during the early healing—probably because of their geometry—to allow bone ingrowth into surface asperities and holes.

Fourth, interfacial shear strengths for porous designs have been measured and give some insight into mechanical integrity and loading limits of the bone-implant interface. Tests of porous Ti-6AI-4V and polyethylene63 show interfacial shear strength values of 2 to 10 MPa, depending on implantation time. Based on the dimensions of the implants (approximately 4 mm diameter, 8 mm long), these stresses convert to failure loads of 176 to 1,407 N. These may be interpreted as upper bounds for safe axial biting loads on implants of this geometry. However, if bite force data in Table 1 are realistic, these porous implants would seem to be underdesigned and liable to fail by overload.

Finally, with porous systems in orthopedics there has been concern about adverse interfacial bone remodeling (stress protection atrophy), possibly related to nonphysiological interfacial stress distribution caused by porous implants.72 It is uncertain if this is also a problem with porous dental implants. Until more is known about the physiological desirability of one stress field over another, this subject is difficult to discuss with respect to implant design.

Micro-irregularities. Rationale. Here, the surface irregularities are at the microscopic level, possibly in conjunction with macro-irregularities. This would afford the possibility of microscopic interlocking of bone and implant, which might enhance the load transmitting capabilities of the interface.

For instance, the Tubingen dental implant is a tapered stepped cylinder of aluminum oxide ceramic (Al2O3) with rounded edges so pressure points can be avoided in these bone contact zones.73 The shape is intended to require force transfer to bone mainly at right angles to the transfer surface. The Tubingen implant also has shallow, circular depressions (about 0.9 mm in diameter) repeated over the entire surface, which are designed to encourage osteonal bone apposition at the micro level.

Carbon and carbon-coated dental blade-vent implants can also be made to have a microporous surface texture, with soft undulations of about 10-micron depth. A blade-type implant consisting of a graphite substrate and a 0.03-inch coating of low-temperature isotropic (LTI) pyrolitic carbon, was manufactured, tested,74 and implanted in baboons.75

The rationale for use of this material was its general biocompatible performance in blood-contacting applications, its possible prevention of corrosion if used as a coating on macroporous metallic implants, and the surface microtexture of the LTI


material.

Also, experiments have explored the use of grit-blasting and other texturing methods for enhancing fixation properties such as stiffness and strength of the interface for carbon-coated and noncoated implants.76-78

Critique. The interface observed for micro-irregular surfaces tends to be close, if not direct bone-implant apposition, suggesting that the goal of microscopic interlocking could indeed be achieved. Thomas, Anderson, Cook, and co-workers76 used a dog femoral transcortical plug model in which 6-mm diameter × 18-mm long cylinders of various materials and surface textures were inserted in dog femora and then pushed out via a special mechanical test rig after various implantation times.

The testing provided data on interface shear strength and interface shear stiffness as a function of implant material and surface texture. The former is the shear stress (in MPa) at maximum load during a push-out test and the latter is the ratio between shear stress and displacement (units GPa/m).

Data from this model76-78 indicate that the interface shear strength for LTI pyrolitic carbon samples—and a number of other materials and surface textures—is about one order of magnitude less (range: 1.56 to 4.48 MPa) than values for macro-irregular, porous systems (roughly 2 to 16 MPa). These stress values help define how large a force (axial) can be taken by the implant before the interface fails.

For a typical LTI carbon cylinder in the transcortical dog model (where the bone-implant contact area is about 80 mm2), the push-out force is of the order of 150 N. This force is low with respect to anticipated axial biting forces (Table 1). Therefore, more surface area would be required to increase the failure load for a dental implant based on this geometry and fixation rationale.

In a recent study by Cook79 using hydroxylapatite (HA) coated onto Ti-6Al-4V cylinders in the same transcortical plug model, HA-coated samples did have larger interfacial shear strengths (6.07 to 7.27 MPa) than uncoated pure titanium samples (0.93 to 1.21 MPa) with implantation times from 5 to 32 weeks.

Assuming approximately the same interfacial areas of bone-implant contact for all samples, this result means that HA samples have larger failure forces (about 500 N) than uncoated samples, but these values are still low compared to possible axial components of in vivo forces.

It is also noteworthy that the pushout interfacial strength for HA-coated cylinders was limited by the bond between HA and metal substrate rather than HA and bone (90 percent of the cases at the 32-week implantation time failed by separation of the Ti-6Al-4V cylinder from its HA coating). Further research is needed to establish the biomechanical integrity of the coating in the presence of the body environment and fatigue-type loading.


Regardless of whether failure occurs at the bone-HA interface or the HA-substrate interface, the failure forces remain low compared to anticipated axial biting force components in vivo.

In terms of interface shear stiffness, there is little difference among materials with macro- versus micro-irregular surfaces. Stiffness values are in the range of 5 to 60 GPa/m.76 Stiffness values for HA-coated samples (33.7 to 55.45 GPa/m) exceeded those for uncoated samples in the most recent experiment of Cook (5.66 to 11.77 GPa/m), but did not exceed the values for uncoated materials (28.7 to 59.8 GPa/m) reported in earlier experiments.76 The clinical design significance of different stiffness values is not immediately obvious from these studies alone.

Bioactive materials. Rationale. Some implant materials are bioinert; others are surface-active or bioactive.80 These terms are misleading, as any implanted biomaterial must provoke biological activity and is thus bioactive. However, bioactive is meant to imply the design objective that biomaterials should enhance or stimulate new bone formation and promote bone-implant attachment.80

The rationale for bioactive implants in bone comes from observations that implant loosening is the leading cause of implant failure, and that failure usually follows movement at the tissue-implant interface.80 The idea is to have an interfacial bond to prevent this movement.

Ceramics such as bioglass,80 tricalcium phosphates,81 hydroxylapatite,82-84 plus metals or ceramics coated with these substances85-89 represent alternative means for achieving this objective.

Critique. From a design standpoint, there are strengths and weaknesses of this approach, starting with the rationale itself. The assertion80 that failure usually follows movement at the interface requires clarification, since the nature of this movement is not detailed.

For example, it is known that bioglass dental implants can develop interfacial fibrous tissue instead of bone bioglass bonding if they are loaded during the early healing period.34 This is due to relative motion in much the same way as with other implant shapes and materials (Table 2, Fig 5). Therefore, use of bioglass material alone does not prevent movement (relative motion) of this type and subsequent fibrous tissue formation.

Alternatively, bone-implant bonding may be a valid way to limit or prevent interfacial movement of the type discussed by Sir John Charnley,87 a pioneer in early hip joint replacement. He noted the possibility of relative motion between surfaces of two substances of different elastic moduli occurring after some sort of initial bone-implant apposition has been formed.

Assuming that it is possible to produce bone in close apposition to implant


materials, Charnley pointed out that the only mechanical difference between glued (bonded) and interlocking surfaces is the inability of simple mechanical interlocking to resist perpendicular (tensile) forces tending to pull the surfaces apart.

It is implicit in Charnley's remarks that relative motion would cause fibrous tissue formation and implant failure. Bioactive materials may be beneficial in producing a bone-implant bond capable of resisting tensile forces across an interface.

One problem is that the extent to which tensile interfacial forces are present depends on the specifics of the situation. Implant loading, implant geometry, and mechanical properties of bulk implant and bone should be considered. Finite element models of screw-shaped implants27 suggest that it is possible to have regions of tensile separation at the interface if there is no bonding assumed in the model (Fig 4).

Brånemark points to the importance of having intimate adaptation and attachment of bone to the implant surface to resist tensile and shear loadings.88 Other investigators believe it possible to design implants so the majority of load transmission will be compressive,89 obviating the necessity for a bond to resist tensile or shear loads. Two Dutch research teams90,91 are using hydroxylapatite dental implants, intending that bone will bond along the surface and thus resist tensile and shear loading. The best design rationale with respect to this issue awaits continuing evaluation of both the bond strengths and the overall physiological acceptability of the intraosseous stress fields produced by the various shapes of implants.

For 45S5 bioglass, Hench and Wilson estimate bond strengths to be 117 MPa,80 but push-out experiments in animals (similar to those conducted by Cook) show values of 3 MPa, which is little more than that for smooth titanium ( 1 MPa), and less than that for hydroxylapatite-coated titanium (6 to 7 MPa).

Evidence indicates that bioactive implant materials may form interfaces that resist tensile and shear forces. However, precise data on the bond strength and its limitations remain to be fully documented, according to some researchers.

Finally, there has been a continuing concern about the mechanical properties of bioactive materials themselves, since most are relatively brittle ceramics. Dutch workers have seen fatigue fractures of HA implants in vivo, and are now using prestressed cylinders to reduce the likelihood of such fractures.91

Osseointegration. Rationale. Brånemark92 coined the term osseointegration (OI) to mean a direct structural and functional connection between ordered living bone and the surface of a load-carrying implant. Brånemark's objective was to avoid getting fibrous tissue. He sought direct bone-implant apposition instead.

The rationale was that if the deeper connective tissues were allowed to heal properly around a dental implant, then the implant would be firmly fixed in bone and


better able to support forces and allow formation of a proper permucosal interface.

To achieve OI, he devised, developed, and tested an implant material (pure Ti), geometry (screw shape), surgical protocol (slow speed bone cutting, two-stage implantation), and appropriate prosthodontic techniques. He and his team evaluated the system using biomechanical, biomaterial, biological, and clinical testing.93

Other workers have based their implant designs on the concept of obtaining close bone apposition into screw threads or similar geometry. Not all of the designs have involved two-stage implants; the hollow basket,70 and the single crystal sapphire (Al2O3) implant71 are examples of one-stage implants in which the observed bone-implant interface resembles, at least at the light microscopic level, the close bone apposition seen in Brånemark-type fixtures.

Critique. In terms of the design process, the development of the OI system also shows strengths and weaknesses.

The main strengths have been the published goals and rationale for design features, followed by appropriate evaluations via theory and experiments. The selection of a two-stage implant, the gentle surgical technique, and the screw-shaped fixture geometry were devised to maximize stability of the fixture in bone during postoperative healing, which discourages fibrous tissue formation.

Biomechanical evaluations of the OI system have included measurements of closure forces in patients with OI fixtures8 discussions of the mechanics of load transfer at the fixture-bone interface, and theoretical discussions of load sharing among multiple fixtures.88

Also, biomechanical aspects of prosthodontic design have been discussed.22 Biomaterial evaluations have used optical and electron microscopy to characterize the nature of the bone-implant interface,94 and surface analytical studies95 to relate surface properties and tissue response.

However, even the OI system, which is arguably the most extensively documented of all dental (if not orthopedic) systems, has some weaknesses.

• What is the exact structural definition of OI? Over what percentage of the total surface area of a fixture must there be direct bone contact for OI to exist? (The percentage may depend on the relative amounts of cortical versus cancellous bone at implantation sites—Fig 6.) Must there be a bond between bone and the fixture in order for OI to exist, and if so, what is the nature and strength of the bond?

• What is the detailed functional definition of OI? What rules govern the alleged mechanically related bone remodeling adjacent to fixtures? Are existing fixtures optimized with respect to load transfer in cancellous versus cortical bone? What are the loading limits in vivo?

• Are fixtures stiffer than normal teeth, and if so, does this have clinical


significance when fixtures are used with natural tooth abutments?

• Is the present fixture geometry the only one that will work? Is the current shape optimized, and if so, with respect to what criteria? Can fixtures be loaded earlier without problems at the interface?

Intramobile elements. Rationale. The design of at least two dental implant systems (IMZ and Flexiroot) has been based on theoretical and clinical concerns about the in vivo, biomechanical behavior of osseointegrated dental implants, particularly if these implants are used in conjunction with natural teeth.

IMZ literature96 suggests that it may be necessary to integrate flexible and shock-absorbing elements into the implants to imitate the flexibility of natural teeth and to bring about improved force transfer to bone. Also, computer models suggest improvements in intrabony stress fields when there is an intramobile element (IME) in an implant.97 On the other hand, Flexiroot literature asserts that there may be "excessive mobility" in the IMZ system, which "creates the opposite problems encountered by the rigid Brånemark system. . . if it is connected to the natural dentition, the natural dentition will bear an overwhelming portion of the occlusal strains and stresses."98 Flexiroot therefore favors a design based on limited mobility.

Critique. One of the problems in assessing these systems is a lack of experimental data. In theory, simplified biomechanical models can illustrate how stiffness of implants and teeth might play a role in load partitioning among bridge supports (Fig 7). The results of such a model, however, will depend entirely on the assumed biomechanical behavior of the teeth and implant abutments, which at present are not known.

Experimental evaluation of both IMZ and Flexiroot systems would require careful measurements of both tooth and implant mobility in all relevant directions, but so far only limited data are available.

Tooth mobility has been studied for years, but implant mobility has not. Likewise, the question of shock absorption by implants has been theoretically discussed,88 but experimental data are few. Although some of the design rationales may prove to have merit, it remains for the designers to justify them more fully in terms of clinical need and experimental performance.

ConclusionFor nearly all implant systems, the major objective is long-term fixation of implants to bone. To achieve this goal, designers of implant systems must confront biomaterial and biomechanical subproblems, including in vivo forces on implants, load transmission to the interface, and interfacial tissue response. These subproblems have been discussed along with design rationales for the different dental implant systems that have evolved.

The purpose of this discussion has been to encourage clinicians to think about


dental implants in terms of the design process, which offers a framework to build an understanding of the burgeoning dental implant field. In confronting the numerous implant systems on the market, the clinician will do well to seek clear statements of objectives, rationale, and measures of performance with respect to goals.


1. Eide AR, Jenison RD, Mashaw LH, Northrup LL: Engineering Fundamentals and Problem Solving, ed 2. New York, McGraw-Hill Inc, 1986.

2. Brunski JB: Tooth and jaw, biomechanics of, in Webster J (ed): Encyclopedia of Medical Devices and Instrumentation. New York, John Wiley & Sons Inc, 1988, pp 2776-2788.

3. Craig RG (ed): Restorative Dental Materials ed 6. St. Louis, CV Mosby Co, 1980, pp 60-6;.

4. Meng TR, Rugh JD: Biting force on overdenture and conventional denture patients. J Dent Res 1983;62:249, abstr No. 716.

5. Ralph WJ: The effects of dental treatment on biting force. J Prosthet Dent 1979;41:143.

6. Colaizzi FA, Javid NS, Michael CG, Gibbs CJ: Biting force, EMG, and jaw movements in denture wearers. J Dent Res 1984;63:329, abstr No. 1424.

7. Haraldson T, Karlsson U, Carlsson GE: Bite force and oral function in complete denture wearers. J Oral Rehabil 1979;6:41.

8. Carlsson GE, Haraldson T: Functional response, in Brånemark P-I, Zarb G, Albrektsson T (eds): Tissue-Integrated Prostheses. Chicago, Quintessence Publ Co Inc, 1986, pp 155-163.

9. Graf H: Occlusal forces during function, in Occlusion: Research on Form and Function. Ann Arbor, University of Michigan, 1975, p 90.

10. Harrison A, Lewis TT: The development of abrasion testing machine. J Biomed Mater Res 1975;9:341.

11. Ahlgren J: Mechanism of mastication. Acta Odontol Scand 1966;24(suppl 44):100-104.

12. Graf H: Bruxism. Dent Clin North Am 1969;13:659.

13. Carlsson GE: Bite force and chewing efficiency. Front Oral Physiol 1974;1:265.

14. Brunski JB, Hipp JA: In vivo forces on endosteal implants: A measurement system and biomechanical considerations. J Prosthet Dent 1984;51:82.

15. Wenz J, Berthoud C, El Wakad M, Hipp JA, Brunski JB: An osseointegrated dental implant instrumented to sense bite forces. J Dent Res 1986;65:304, abstr No. 1215.

16. Swanson SAV, Freeman MAR (eds): The scientific basis of total joint replacement. New York, John Wiley & Sons Inc, 1977.

17. Park JB: Biomaterials Science and Engineering. New York, Plenum Publishing


Corp, 1984.

18. Cochran GVB: A Primer of Orthopaedic Biomechanics. New York, Churchill Livingstone Inc, 1982.

19. Van Vlack LH: Materials Science and Engineering, ed 4. Reading, Mass, Addison-Wesley 1980.

20. Williams DF, Roaf R: Implants in Surgery. Philadelphia, WB Saunders Co, 1973.

21. Wainwright SA, Biggs WD, Currey JD, Gosline JM: Mechanical Design in Organisms. Princeton, NJ, Princeton University Press, 1982.

22. Glantz P-O: Aspects of prosthodontic design, in Brånemark P-I, Zarb G, Albrektsson T (eds): Tissue-Integrated Prostheses. Chicago, Quintessence Publ Co Inc, 1985, pp 329-332.

23. Cowin SC, Van Buskirk WC, Ashman RB: Properties of bone, in Skalak R, Chien S (eds): Handbook of Bioengineering. New York, McGraw-Hill Inc, 1987, pp 2.1-2.27.

24. Weiss CM: Tissue integration of dental endosseous implants: Description and comparative analysis of the fibro-osseous integration and osseous integration systems. Oral Implantol 1986,12:169-214.

25. Brunski JB, Schock RB: Mechanical behavior of a fibrous tissue interface of an endosseous dental implant. Trans 11th Annual Meeting of the Society for Biomaterials, Clemson, SC, 1987, p 41.

26. Hori RY, Lewis JL: Mechanical properties of the fibrous tissue found at the bone-cement interface following total joint replacement. J Biomed Mater Res 1982;16:921.

27. Hipp JA, Brunski JB, Shephard MS, Cochran GVB: Finite element models for implants in bone: Interfacial assumptions, in Schneider E, Perren SA (eds): Biomechanics: Current Interdisciplinary Research. Dordrecht, The Netherlands, Martinus Nijhoff Publishers, 1985, pp 447-452.

28. Brunski JB: The influence of force, motion, and related quantitites of the response of bone to implants in Fitzgerald RH (ed): Non-Cemented Total Hip Arthroplasty. New York, Rave Press, 1988, pp 7-21.

29. Hodosh M, Povar M, Shklar G: The polymer dental implant concept. J Prosthet Dent 1969;22:371.

30. Linkow LI, Glassman PE, Asnis ST: Macroscopic and microscopic studies of endosteal blade-vent implants (six-month dog study). Oral Implantol 1977;3:281.

31. Piliero SJ, Schnitman PA, Pentel L, Dennison TA: Histopathology of oral endosteal metallic implants in dogs. J Dent Res 1973;52:1117.


32. Gourley IM, Richards LW, Cordy DR: Titanium endosteal dental implants in the mandibles of beagle dogs. A two-year study. J Prosthet Dent 1976,36:550.

33. Listgarten MA, Lai CH: Ultrastructure of the intact interface between an endosseous epoxy resin dental implant and the host tissues. J Biol Buccale 1975;3:13.

34. Ethridge EC: The effect of different materials on oral tissue compatibility. J Metals 1976 28:14.

35. Brånemark P-I, Briene U, Adell R, Hansson BO, Lindstrom J, Ohlsson A: Intra-osseous anchorage of dental prostheses I. Experimental studies. Scand J Plast Reconstr Surg 1969;3:81.

36. Albrektsson T, Brånemark P-I, Hansson H-A, Kasemo B, Larsson K Lundstrom I, McQueen DH, Skalak R: The interface zone of inorganic implants in vivo. Ann Biomed Eng 1983;11:1.

37. Hassler CR, McCoy LG, Downes R, Russel O: Ceramic tooth implants in baboons. J Dent Res 1977;56:A117.

38. Karagianes MT, Westerman RE, Rasmussen JJ, Lodmell AN: Development and evaluation of porous dental implants in miniature swine. J Dent Res 1976;55:85.

39. Brunski JB, Moccia AF, Pollack SR, Korostoff E, Trachtenberg DI: The influence of functional use of endosseous dental implants on the tissue-implant interface. Part I Histological aspects. J Dent Res 1979;58:1953.

40. Adell R: Long-term treatment results, in Brånemark P-I, Zarb G, Albrektsson T (eds): Tissue-Integrated Prostheses. Chicago, Quintessence Publ Co Inc, 1985, p 175.

41. Cowin SC, Lanyon LE, Rodan G: Functional adaptation in bone tissue. Calc Tiss Intl 1984;36(suppl 1):S1-S155.

42. Black J: The Biological Performance of Materials. New York, Marcel Dekker Inc, 1981.

43. Smith DC, Williams DF (eds): Biocompatibility of dental materials, in CRC Series in Biocompatibility. Boca Raton, Fla, CRC Press, 1982 (Vols I-IV).

44. Andrade J: Surface analysis of materials for medical devices and diagnostic products. Med Dev and Diag Ind 1980;2:22.

45. Baier RE: Conditioning surfaces to suit the biomedical environment. Recent progress. J Biomech Eng 1982;104:257.

46. Kasemo B, Lausmaa J: Metal selection and surface characteristics, in Brånemark P-I, Zarb G, Albrektsson T (eds): Tissue-Integrated Prostheses. Chicago,


Quintessence Publ Co Inc, 1985, p 99.

47. Ivarsson B, Lundstrom I: Physical characterization of protein absorption on metal and metaloxide surfaces, in CRC Critical Reviews in Biocompatibility. Boca Raton, Fla, CRC Press, 1985;2:1 .

48. Reese JA, Valega TM (eds): Restorative Dental Materials: An Overview. Chicago, Quintessence Publ Co Inc, 1985.

49. Schnitman PA, Shulman LB: Recommendations of the consensus development conference on dental implants. J Amer Dent Assoc 1979;98:373.

50. Linkow LI, Cherchève R: Theories and Techniques of Oral Implantology. St Louis, CV Mosby Co, 1970.

51. Driskell T: The history of dental implants. J Calif Dent Assoc, October 1987, pp 16-25.

52. James RA: The support system and the pergingival defense mechanism of oral implants. Oral Implantol 1975;6:270.

53. Ten Cate AR: The gingival junction, in Brånemark P-I, Zarb G, Albrektsson T (eds): Tissue-Integrated Prostheses. Chicago, Quintessence Publ Co Inc, 1985, p 145.

54. Peterson L, McKinney C, Pennel B, Klawitter J, Weinstein A: Clinical, radiographical, and histological evaluation of porous rooted cobalt-chromium alloy dental implants. J Dent Res 1980;59:99.

55. Cook SD, Weinstein AM, Klawitter J: Analysis of a porous Co-Cr-Mo alloy dental implant. J Dent Res 1982;61:25.

56. Young FA, Kresch CA, Spector M: Porous titanium endosseous dental implants in rhesus monkeys: microradiographic and histological evaluation. J Biomed Mater Res 1979;13:843.

57. Spector M, Young FA, Marchinak F, Kresch CH: Porous coatings for artificial tooth roots, in Kawahara H (ed): Implantology and Biomaterials in Stomatology. Japan Society of Implant Dentistry, 1980, p 180.

58. Spector M, Michno MJ, Smarook WH, Kwiatkowski GT: A high-modulus polymer for porous orthopaedic implants: Biomechanical compatibility of porous implants. J Biomed Mater Res 1978;12:665.

59. Peterson L, Pennel B, McKinney R, Klawitter J, Weinstein A: Clinical, radiographical, and histological evaluation of porous rooted, polymethylmethacrylate dental implant: Preliminary studies. J Dent Res 1979;58:489.

60. Hodosh M, Shklar B, Povar M: The porous vitreous carbon


polymethylmethacrylate tooth implant: Preliminary studies. J Prosthet Dent 1974;32:326.

61. Klawitter J, Weinstein AM, Cooke FW, Peterson LJ, Pennel BM, McKinney RV: An evaluation of porous alumina ceramic dental implants. J Dent Res 1977;56:1768.

62. Pilliar RM: Powder metal-made orthopaedic implants with porous surface for fixation by tissue ingrowth. Clin Orthop Rel Res 1983;176:42.

63. Young FA, Kresch CH, Spector M: Mechanical properties of the bone-implant interface for porous titanium and porous polyethylene dental implants, in Hastings GW, Williams DF (eds): Mechanical Properties of Biomaterials. England, John Wiley and Sons Ltd, 1980, p 407.

64. Weiss M, Rostoker W: Multiple uses of a new metallic endosseous implant. J Dent Res 1980;59(special issue):B918.

65. Bobyn JD, Pilliar RM, Cameron HU, Weatherly GC The optimum pore size for the fixation of porous surfaced metal implants by the ingrowth of bone. Clin Orthop Rel Res 1980;150:263.

66. Klawitter J, Hulbert SF: Application of porous ceramics for the attachment of load-bearing internal orthopaedic applications. J Biomed Mater Res 1971;2:161.

67. Cameron HU, Pilliar RM, Weatherly GC: The effect of movement on the bonding of porous metal to bone. J Biomed Mater Res 1973;7:301.

68. Schatzker JG, Horne JG, Sumner-Smith G: The effects of movement on the holding power of screws ill bone. Clin Orthop Rel Res 1975;111:257.

69. Hylander WL: Patterns of stress and strain in the macaque mandible, in Carlos DS (ed): Craniofacial Biology. Monograph No. 10, Craniofacial Growth Series, Center for Human Growth and Development, 1981, p 1.

70. Schroeder A, Zypen E, Stich H, Sutter F: The reactions of bone, connective tissue and epithelium to endosteal implant with titanium sprayed surfaces. J Maxillofac Surg 1981;9:15.

71. McKinney RV, Steflik DE, Koth DL: The biologic response to the single-crystal sapphire endosteal dental implant: Scanning electron microscopic observations. J Prosthet Dent 1984;51:372.

72. Lewis JL, Lew WD: Bioengineering of total joint replacement, in Skalak R, Chien S (eds): Handbook of Bioengineering. New York, McGraw-Hill Inc, 1987.

73. Schulte W: The intra-osseous Al2O3 (Frialit) Tuebingen implant: Developmental status after eight years (I). Quintessence Int 1984;15:1-18.

74. Shim HS: The mechanical behavior of LTI carbon dental implants. Biomat Med


Dev Art Org 1976;4:181.

75. Kent JN, Bockros JC: Pyrolitic carbon and carbon-coated metallic dental implants. Dent Clin North Am 1980;24:465.

76. Anderson RC, Cook SD, Weinstein AM, Haddad RJ: An evaluation of skeletal attachment to LTI pyrolitic carbon, porous titanium, and carbon-coated porous titanium implants. Clin Orthop Rel Res 1984;1982:242.

77. Thomas KA, Cook SD, Skinner HB, Weinstein AM, Yapp R, Haubold A: Design variables affecting bone-biomaterial interface mechanics. Trans 9th Annual Meeting of the Society for Biomaterials, 1983, p 118.

78. Thomas KA, Cook SD, Renz EA, Anderson RC, Haddad RJ, Haubold AD, Yapp R: The effect of surface treatments on the mechanics of LTI pyrolitic carbon implants. J Biomed Mater Res 1985;19:145.

79. Cook SD, Kay JF, Thomas KA, Jarcho M: Interface mechanics and histology of titanium and hydroxylapatite-coated titanium for dental implant applications. Int J Oral Maxillofac Implants 1987;2:15-22.

80. Hench LL, Wilson J: Surface-active biomaterials. Science 1984,226:630.

81. Driskell TD, Hassler CR, Tennery VJ, McCoy LG, Clarke WJ: Calcium phosphate resorbable ceramics: A potential alternative to bone grafting. r Dent Res 1973;123, IADR abstr No. 259.

82. Jarcho M, Kay JF, Gumaer KI, Doremus RH, Drobeck HP: Tissue, cellular, and subcellular events at a bone-ceramic hydroxylapatite interface. J Bioeng 1977;1:79.

83. Kay JF, Jarcho M, Logan G, Embry J, Stinner C: Physical and chemical characteristics of hydroxylapatite coatings on metal. J Dent Res 1986;65, AADR abstr No. 472.

84. Kay JF, Jarcho M, Logan G, Liu ST: The structure and properties of hydroxylapatite coatings on metal. Trans 12th Annual Meeting of the Society for Biomaterials, 1986.

85. Ducheyne P, Hench LL, Kagan A, Martens M, Bursens A, Mulier IC: Effect of hydroxylapatite impregnation on skeletal bonding of porous coated implants. k Biomed Mater Res 1980;14:225.

86. Weinstein AM, Klawitter J, Cook SD: Implant-bone interface characteristics of bioglass dental implants. J Biomed Mater Res 1980;14:23.

87. Charnley J: Acrylic Cement in Orthopaedic Surgery. New York, Churchill Livingstone Inc, 1970.

88. Skalak R: Aspects of biomechanical considerations, in Brånemark P-I, Zarb G,


Albrektsson T (eds): Tissue Integrated Prostheses. Chicago, Quintessence Publ Co Inc, 1985, p 117.

89. Heimke G, Griss P, Jentschura, Werner E: Bioinert and bioactive ceramics in orthopaedic surgery, in Hastings GW, Williams DF (eds): Mechanical Properties of Biomaterials. London, John Wiley and Sons Ltd, 1980, p 207.

90. De Putter C, De Groot K, Sillevis Smitt PAE: Implants of dense hydroxylapatite in prosthetic dentistry, in Lee AJC, Albrektsson T, Brånemark P-I (eds): Clinical Applications of Biomaterials. New York, John Wiley & Sons Inc, 1982, p 123.

91. De Putter C, De Lange GL, De Groot K, Sillevis Smitt PAE: Prestressed permucosal dental implants of dense hydroxylapatite, in Ducheyne P, Van der Perre G, Aubert AE (eds): Biomaterials and Biomechanics, Amsterdam, Elsevier Science Publishers BV, 1983, p 439.

92. Brånemark P-I, Hansson BO, Adell R, Breine U, Lindstrom J, Hallen O, Ohman A: Osseointegrated implants in the treatment of the edentulous jaw: Experience from a 10-year period. Scand J Plast Reconstr Surg 1977;(suppl 16).

93. Brånemark P-I, Zarb G, Albrektsson T (eds): Tissue Integrated Prostheses. Chicago, Quintessence Publ Co Inc, 1985.

94. Albrektsson T, Jacobsson M: Bone-metal interface in osseointegration. J Prosthet Dent 1987;57:597

95. McQueen D, Sundgren J-E, Ivarsson B, Lundstrom B, af Ekenstam B, Svensson A, Brånemark P-I, Albrektsson T: Auger electron spectroscopic studies of titanium implants, in Lee AJC, Albrektsson T, Brånemark P-I (eds): Clinical Applications of Biomaterials. New York, John Wiley & Sons Inc, 1982, p 179.

96. Kirsch A: Implants in the case of a reduced mandibular dentition, in Tetsch P (ed): Enossale Implantationen in der Zahnheilkunde. Munich, Carl Hanser Verlag, 1984, Chap 8.

97. Siegele D, Soltesz U: Implantate mit intramobilen einsatzen als bruckenpfeiler. Z Zahnarztl Implantol 1986;2:117-124.

98. Haris AG, Mozsary PG: A new concept: CODAR (complete osteointegrated dento-alveolar replacement) and a corresponding dental implant design (Flexiroot). Oral Implantol 1986;12(4):630-660 .


Fig. 1 Biomechanical and biomaterial subproblems are part of dental implant design. They involve in vivo forces and moments, interfacial load transmission, and interfacial tissue attachment and response.

Fig.Charles English, VA Hospital, Augusta, Ga).


Fig.supporting implants and teeth. The interfacial tissues, in turn, have a biomechanical function in that they must support the abutments.


Fig.


Fig. 5 When any biomaterial is implanted into tissue, there are a number of possible biological reactions that must be addressed by implant designers. (Reprinted with permission from Andrade.44)

Fig. 6 Scanning electron microscopic view of the bone "crypt" left behind after a titanium fixture (3.5 mm OD) was removed from a canine mandible after approximately 7 months. It reveals a larger percentage of bone adjacent to titanium near the cortical plates of the mandible (left and right borders of photo), and a lesser percentage distally.


Fig. 7 A simplified biomechanical model illustrating the relevance of the "stiffness" of implants versus natural teeth. If abutments and their interfaces are represented as simple elastic springs, then the fraction of the applied load P taken by each abutment is controlled by the relative values of k1 k2, and k3.

Biomaterials and Biomechanics in Dental Implant Design · 2008. 11. 19. · dental implant,...

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