implant biomechanics jpd

KNUDSON, WILLIAMS, AND KEMPLE

Fig. 2. Verification jig made on master cast. Fig. 3. Verification jig placed intraorally to determine accuracy of master cast. Note that each transfer coping has a positive seat on each abutment cylinder.

Inc., Anaheim, Calif.) material is applied and activated by ter cast. In addition, the jig can verify the accuracy of sub- the catalyst. The verification jig can be used intraorally to sequent casts poured from the same impression. The verifi- confirm the accuracy of the master cast at the next appoint- cation jig can eliminate the making of a framework on an ment before making maxillomandibular jaw relation records inaccurate cast. (Fig. 3).

SUMMARY DR. Rom& C. KNUDSON WILFORD HALL MEDICAL CENTEREGDP

This article presents a simplified technique for the fabri- cation of a verification jig to check the accuracy of the mas-

LACIUAND AFB, TX 78236

Basic biomechanics of dental implants in prosthetic dentistry

E-J. Richter, Dr.Med.Dent., Dipl.-Ing.* Aachsn University Medical Center, Aachen, West Germany

A discussion of loads applied to implants must include the clinical consideration that not only rigid implant types are used, such as the Ttibingen immediate implant and the TPS screw implant without shock absorber, but also systems with inherent resilience integrated in the implant design, such as the IMC and Flexiroot implants. The common goal of all of these implant systems is to achieve a stable anchorage of the implant body in the bone tissue, that is, contact osteogenesis or osteointdgra- tion. In the implant-to-bone interface region there is an implant mobility resulting from the elasticity of the bone. The question of whether additional implant- integrated elastic elements are necessary to simulate the periodontal attachment is controversial. (J PROSTHET D~~~1989;81:602-9.)

A discussion of loads applied to implants and the reactions in bone must include the clinical consideration that not only rigid implant types such as the Frialit (Tuebingen type, Friedrichsfeld, West Germany) implant for immediate extraction sites, Linkow (Oratronics Inc., New York, N.Y.)

*Department of Prosthcdontics and Dental Materials.

implants, and the titanium plasma-coated screw (TPS) (Park Dental Research, New York, N.Y.) without “shock absorber” are used, but also systems with the resilience integrated in the implant design such as the intramobile cylinder implant (IMZ) (Interpore-IMZ, Irvine, Calif.) and Flexiroot (F.A.I.R., Inc., Bala Cynwyd, Pa.) implants. The common goal of all implant systems is to achieve a stable anchorage of the implant in the bone tissue (osteointegra-

602 MAY 1988 VOLUME 61 NUMBER 6

BASIC BIOMECHANICS OF DENTAL IMPLANTS

sbml

t

4’ I II

Fig. 1. Schematic drawing of tooth mobility with characteristic parameters. s = Elastic movement; F = load.

I

ds

dt

g = Load rate

Fig. 2. Load-rate 2 0

determines level from which sec-

ondary phase of movement starts.

tion). In the implant-bone interface region there is implant mobility resulting from the elasticity of the bone. The question is controversial whether additional implant-integrated elastic elements are necessary to simulate a periodontal ligament.

Clinical results show that in the edentulous lower jaw implant, resilience seems to be of subordinate importance, whether there is a fixed denture with a chewing surface of acrylic resin teeth on Branemark (Nobelpharma, Weston, Mass.) implants, or a removable complete denture, bar- guided according to the concepts of the TPS or IMZ system. However, in the posterior region the situation is more com- plicated. The success of a fixed partial denture on a tooth and an implant depends on the interaction of the elastic mandible, mobile teeth, and directly loaded implants placed in the chewing center. For these reasons there is a need to “brake” the load and to integrate shock-absorbing elements, using special implant designs to avoid stress and overloading of the mandible, the teeth, and the implants.

Frequently, craterlike regions of bone destruction around an implant may be seen after a relatively short time in func- tion. The reason may be the loading of the implants, espe- cially traumatic overloading.

Resilience

0, =lQOOONlmm

ANNI 10

Fig. 3. Force movement ratio of osteointegrated implant.

7

r

I

z, z2

“IF72 ’ // ///

“‘fR2

& z1 / /// yr /////// Fig. 4. Load distribution on normal fixed partial denture on natural teeth (Zi and Zs).

THEORETICAL BASIC ASPECTS

With the help of mechanical calculations (statics, firm- ness, and dynamics) it is possible to define the load of a restoration that is fixed on an implant and a tooth and to reg- ister the reactions in bone. There is only one possible kind of load on an implant or a restoration, which is a force that causes reacting forces and bending momentums* in the bone. Because teeth and implants are not freely movable in the jaw, a balance of forces and bending momentums is es- sential. A load from outside onto a system causes inner stress in the system and stress reactions in the bone anchorage that are of the same value but in opposite directions. Action is like reaction.

After loss of the molars in a lower jaw and insertion of an implant, a modern fixed, conditionally removable fixed par-

*A momentum is the product of force and length of a lever-arm.

THE JOURNAL OF PROSTHETIC DENTISTRY

RICHTER

Fig. 6. Load distribution on cantilever fixed partial denture on natural teeth (Zr and 22).

soft resilience : strong resilience :

large yielding little yielding

resilience

DI ’ 02 Fig. 6. Mechanical principles of normal fixed partial denture fixed on tooth (2) and implant (1,).

tial denture to the last premolar could be provided. The es- sential biomechanical factor is the different functional sup- port of the two abutments.

INFLUENCE OF VERTICAL FORCES

A tooth has a two-stage mobility under physiologic stress (Fig. 1). The resilience (the quotient of force [F] and elastic

n ~1, n=f (D, I

M = F(l-l/n).1 Fig. 7. Load distribution of normal fixed partial denture with different abutments (Z). Load (F) on more yielding abutment Z causes momentum at less yielding abutment I.

movement [s]) differs about the factor of 10 for areas I and II. To be specific, the initial phase of the movement of a tooth is influenced by the load-rate in addition to the amount of load. The higher the load-rate, the deeper the level from which the secondary phase of movement starts (Fig. Z).l

The mobility of an osteointegrated implant is less than that of a tooth and because of the missing periodontal ligament there is no initial increase (Fig. 3).2 The resilience of approximately 10,000 N/mm is approximately 10 to 100 times higher compared with a tooth.

By using implants as an abutment it is possible to make normal fixed prostheses-a successful therapy in prosth- odontics. But the different anchorage of tooth and implant is important. The abutments of a normal fixed partial denture and a cantilever fixed partial denture on natural dentition have the same kind of anchorage.

The mechanical principles are shown in Figs. 4 and 5. Both restorations have the same length, a vertical force is placed in the middle or at one end. The springs Zi and Z2 may char- acterize the effect of the periodontal ligaments. The stress in the abutment teeth can be determined by using the rules of mechanical balance. Fig. 4 shows that the load (F) is equally distributed between both abutment teeth for a normal fixed partial denture; however, if the force (F) is placed at the left end of the fixed partial denture, all of the load is carried by the left tooth, the same as in the middle loaded cantilever fixed partial denture (Fig. 5). When the outer load (F) is placed on the end of the lever-arm, the distal abutment has to carry twice the force (F) and the mesial tooth is stressed extrusively.

The situation of a fixed partial denture supported by a tooth and an implant is shown in Fig. 6. The different anchorages are expressed by the stiffness of the springs, sig- nified by the width of the lines. The resilience of the implant abutment (DI) is less than that of the tooth (Dx).

604 MAY 1989 VOLUME 61 NUMBER 6


with F-1N: D,=lOON/mm

16

11

12

10

8

6

4

2

O,[Nlmml

Fig. 8. Reacting momentum (MI) in bony implant anchorage in dependence of stiffness (Dz) of tooth by variation of resilience (Dr) of implant. (Technical data: length of fixed partial denture, 16 mm; cross-section, 6 X 6 mm; elastic modulus, lo5 N/mm2; load on dental abutment, 1 N.)

Because of high yielding of the soft spring, the ligament, there must be bending on the right spring, the implant, and the surrounding bone. That bending means that only a part of the muscle power “flows” through the tooth, the other part flows as a bending momentum through the implant into the bone (Fig. 7). Only part of the force (F) is carried by the tooth, and a reacting momentum (M) results in the bone implant anchorage. This means one can find an unfavorable load distribution to the less yielding abutment: exoneration of the tooth and bending of the implant.

To obtain numerical details, a computer program (PI- NUS-RZ, Cubus AG, Zurich, Switzerland) was used. In this calculation the fixed partial denture is 16 mm long and the dental abutment is stressed with an occlusal load of 1 new- ton (N). The reacting momentum (MI) of the implant is de- pendent on the stiffness (Dz) of the tooth while the resilience (Dr) of the implant abutment is varied as shown in Fig. 8. In the region of normal tooth mobility (the hatched zone) there is still a momentum of a quarter of the maximum value. With the use of an advantageous technical construction, this momentum could be minimized.

On the other side, a load directly applied on the less yielding abutment in the center of the implant axis does not influence the highly elastic tooth (Fig. 9). There will not be a momentum in the bone around the tooth socket.

The same yielding of teeth and implants as well as the antagonist teeth in centric occlusion is therefore of enormous importance. The distinguishing marks of a harmonious system are occluding units that can yield equally according to their position in the dental arch. In natural dentition, each of two opposing teeth will yield approximately 15 pm if the jaws approach 30 Mm after initial contact when the muscles

z Dz iL

Fig. 9. No load distribution on normal fixed partial denture with different abutments (2, I): load (F) on stiffer abutment I causes only small momentum at abutment Z with high yieldability.

flex. As shown in Fig. 10, just that part of the muscle-force will be transmitted that equals 5 N. If there is an implant instead of a tooth and if the conditions of harmonious yielding of 30 pm should be fulfilled, a load of 16 N must be es- tablished (Fig. 11). That means that (1) the implant and the antagonist tooth are stressed much higher than in the natural dentition and (2) the antagonist tooth must equalize the missing yieldability of the implant.

By diminishing the occlusal contacts, one is trying to deactivate the occlusal situation. This means that an occlusion gold foil of approximately 10 pm in thickness would not hold if the teeth just contact each other and no muscle force is exerted, But as Fig. 12 shows there is only a partial reduction in the transmitted load.

THE JOURNAL OF PROSTHETIC DENTISTRY 605

RICHTER

Tooth

F IN1

lo-

Tooth 15-

Stat lSN)= 30pn

20-

51 [pm1

Fig. 10. Force-movement ratio of two antagonistic teeth. (Upper part: way of upper tooth; lower part: way of lower tooth.) After initial contact both teeth yield approximately 15 pm and transmit load of 5 N.

25

Tooth 20

Implant

Sl tpml

1 10 16

FIN1

Fig. 11. Force movement ratio of tooth (upper jaw). In centric occlusion with harmonic resilience for all antagonistic units (30 pm) 16 N are transmitted.

The schematic drawings describe the clinical situations with reservations. The changing of bone tissue for instance is not taken into consideration, but the drawings explain the theoretical biomechanics. To avoid a traumatic implant load, possible procedures to deactivate the implant stress could be detailed by using these diagrams.

Fir&possibility. Diminishing the occlusal contacts almost totally causes only a parallel shifting of the implant characteristic inferiorly (Fig. 13) because the anchorage of the implant in bone and the implant design itself is not changed.

Se&w& possibility. Fig. 14 shows that integration of a “soft” cushioning element as a spring will change the implant

606

characteristic so that the system might behave like a tooth. Thirdpossibility. By using a special buffering element,

the characteristic of an implant can better simulate the two- stage mobility of a tooth (compare Fig. 15 with Fig. 10). This implant design has the advantage of continuous even sup- port of the occluding units with a normal load by using the effect of the same yielding of the abutments. The implant characteristic should be steeper than that of the tooth to avoid overloading the implant.

INFLUENCE OF HORIZONTAL FORCES

An implant is located in the jaw like a tooth, so that horizontal forces will cause similar reactions in the bone (Fig.

MAY 1989 VOLUME 61 NUMBER 6


Tooth

-- !

a

-i- .-

Fig. 12. Force-movement ratio of tooth (upper jaw) and implant (lower jaw). To come to total resilience of 30 pm, load of 8.5 N still is needed.

Tooth $3 3513

Stot3= a l ha + %+ 513 10 -

eJ3 = 43

Star ISNI a 3Opm

xl -- ----

Implant tm~if~d)

&a

Fig. 13. Effects of procedures to avoid overloading of implant. Diminishing occlusal contacts almost totally causes parallel shifting of implant characteristic inferiorly.

Tooth

10

Implant (modified)

Fu, = h,

Sto+ l4,25Nl= Mpm

FWI 4.25 10 1

a

Fig. 14. Effects of procedures to avoid overloading of implant. Integration of “soft” cushioning element. Implant characteristic is similar to secondary phase of tooth mobility.


RICHTER

Tooth *,3 f 33

Stot3 = %3+ 513

Fu = 53

Stat IL.5 Nl = 30pm

F IN1

Implant (modified 1

Fig. 15. Effects of procedures to avoid overloading of implant. Imitation of two-stage yieldability without reduction of occlusal contacts-ideal adaptation of normal tooth mobility.

- C Strain

(&g] plane

Model a Model b Model c

Fig. 16. Reactions in bone caused by horizontal force (F). Model a: stress (qualitatively) in cortical and spongy bone; model b: simplification of model a; model c: similar to model b, but reacting stress expressed as reacting forces FK and Fs.

Fig. 17. Mechanical principles to explain loading of implant and cantilever fixed partial denture. (F = force.)

16). Model a considers the stress in cortical and spongy bone, model b is a simplification, and if the bone reactions are not interpreted as stress but as forces, model c has to be used.

If model c is turned to the left side and the arrangement of forces is compared with that of the cantilever fixed par-

Fig. 18. Eccentric vertical load causes same reacting forces: in bone as direct horizontal load to implant. (F = force; M = momentum.)

tial denture (Fig. 5), the same situation exists (Fig. 17). In each instance, the reacting force nearest to the free lever-arm has the highest amount (Fig, 17,F~). This is the reason that prosthodontists have a cautious approach to cantilever fixed partial dentures.

Consequently horizontal loads to implants cause high stress in cortical bone. This confirms finite element calculations.3*4 Mechanically it is unfavorable because the margin of the bone has to react as an implant-supporting element. The development of craterlike bone destruction is combined with a transfer of the load-supporting region to the better conditioned inner parts of the bone, but clinically periodontal problems often arise. A narrow and plain chewing surface is best to avoid strong horizontal loads to implants, with occlusal contacts within the implant diameter and free articulating movements without bruxism.

MAY 1989 VOLUME 61 NUMBER 6


Fig. 19. Periodontal space is wider in apical and marginal region than in central part.

An eccentric vertical load causes the same reacting forces in the bone as a direct horizontal load to the implant (Fig. 18). The discussion of loads applied to implants should include the characteristics of the periodontal ligament. In the apical and marginal region the periodontal ligament is

slightly wider than in the central part (Fig. 19). There is no overloading of the bone in the critical zones because there is more space and the ligament itself could be displaced. The eflkiency of this perfect construction is evident in the success of conventional treatment with fixed prostheses.

SUMMARY

Although it is possible to describe the biomechanics of implants by use of mechanical principles, only relative con- clusions are possible because the limiting level for stress in bone is unknown.

Implants with definite resilience integrated in the implant design can diminish stress in bone so that the goal of improving implants should be to avoid bending of the implant and to achieve a mobility that is almost equal to that of the natural teeth.

REFERENCES

1. Richter E-J. Die badeutung der versuchsbedingungen im wissenschaftli- ehen experiment, dargestellt am beispiel der zahnbeweglichkeit. Dtsch Zahnantl 2 1985;40:404-9.

2. Spiekermann H. Implantatprothetik. In: Voss R, Meiners H, eds. Fortechritte der xabnaerztlichen prothetik und werkstoffkunde. 2nd ed. Munich: Hanser, 1984;189-218.

3. Soltesz U, Siegele D. Einfluss der steifigkeit des implant&materials auf die im knochen erxeugten spannungen. Dtsch Zahnarstl 2 1984;39:183-6.

4. Borchers L, Reichart P. Three-dimensional stress distribution around a dentai implant at different stages of interface development. J Dent Res 1983;62:155-9.

Reprint requests to: DR E-J. RCHTFZ UNNERSY OF AACHEN DEPARTMENT OF PR~~o~~~~ AND DENTAL MATERIALS 5100 AACHEN, PAUWELS~~E WEST GERMANY


implant biomechanics jpd

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