Properties of titanium dental implants produced by electro-discharge compaction

Clinical Maierials

11 (1994) 203-209

Eke&r Science Limited Printed in Great Britain 0267-6605(95)00019-4

ELSEVIER

of Titani wharge C

Department

of Materials

Science and Engineering,

Implants

University

of Kentucky,

Lexington,

Kentucky

405060046,

USA

(Received 1 July 1994; accepted 11 August 1995)

Abstract: Highly porous dental implants of a cylindrical shape were fabricated from commercial grade titanium REP-atomized powders by EDC. They were intended to have the maximum porosity for maximum osseointegration, but to maintain the minimum mechanical properties functionally required. Two level full factorial experiments were conducted with respect to the sample weight, capacitance, input energy and electrode configuration. All samples were X-rayed prior to tests to ascertain the presence, extent and distribution of internal macroscopic voids. Torque and compression tests were conducted to evaluate the yield or ultimate strengths (34490cm N and 205- 502 MPa, respectively). These results indicate that macroscopic void-free, highly porous implants can be fabricated.

1 INTRODUCTION

techniques, With the porous surfaced implants, the fabrication process, including machining of the core material and sintering beads to it, is costly, timeconsuming, and still results in reduced mechanical properties,‘r-l3 regardless of an effort to retain the positive aspects of a porous surface from the oxide film at the sintered interface. Okazaki et al. l4 fabricated porous-surfaced prototype implants utilizing an ‘electro-discharge compaction’ (EDC) technique. A discharge of a high voltage, high density current from a capacitor bank through a column of titanium beads in a Pyrex tube produced a cylindrical implant that has the central solid core surrounded by a porous surface layer of beads, which are connected by necks formed between them. Implants were evaluated for core size, neck size, pore size, and grain microstructure, and a very strong positive correlation was found between input energy and torque. Most significant was the fact that the microstructure of the beads and neck regions remained unchanged during the EDC process. They concluded that the mechanical properties may not be different from those of the stock material. Lifland et de15 evaluated the mechanical properties of E implants and obtained a compressive yield strength

This paper reports the fabrication of highly porous implants by EDC (electro-discharge compaction) and the quantitative evaluation of their mechanical properties. The greatest degree of scientific and clinical success with endosseous implants was obtained from a series of animal and human clinical trials by Branemark et ~11.l.~using a threaded titanium implant. The term ‘osseointegration’ was used by Branemark to describe the connection of bone to these implants. Subsequently, many modifications of Branemark’s design were developed in an attempt to achieve better and/or faster osseointegration, so that they provide enhanced mechanical or physiological interlocking in the jawbone.3>4 Of particular interest is a new system that uses titanium beads sintered onto a solid titanium core to furnish a porous surface layer.5-9 Maniatopoulos et al.” noted that the porous surface design provided better osseointegration than threaded designs because of its greater surface area. All of the aforementioned implant designs suffer from one or more deficiencies. For example, the threaded implants require precise machining, which is costly and time-consuming, and their success is very sensitive to surgical 203

204

M. 1. L$md,

K. Qkazaki

of 270-530 MPa, and an ultimate compressive strength of 390-600 MPa. The compressive strength was twice what has been reported by Yue et aLI3 for sintered titanium implants. The encouraging results of EDC-fabricated prototype implants, in particular their excellent mechanical properties, led us to the fabrication of highly porous implants by EDC, intended for the maximum osseointegration, while maintaining the minimum mechanical properties functionally required. 2 MATERIAL AND EXPERIMENTAL METHODS Commercially pure titanium spherical poyders (230&300pm diameter), prepared by a plasma rotating electrode process, were obtained from Fukuda Metal Foil & Powder Co., Ltd., Japan. The samples were weighed to either 0.500 or 0,600 g with a precision of &to*001g, and placed in a Pyrex tube of 4mm inner diameter with a lower copper electrode of 4mm outer diameter. The tube was vibrated for 180 s to increase packing. In case of the concave electrodes, the upper electrode was inserted to gently compress the sample, and the entire assembly was inverted and vibrated for an additional 180 s. The assembly was then placed in the EDC unit and a clamping force (approximately 0.3 N) via spring loaded electrode holders was maintained during discharge. To evaluate and optimize the fabrication of highly porous implants, a two level full factorial experiment with replication was designed. The fabrication parameters investigated were: input energy (397 and 562 J), capacitance (480 and 720pF), sample weight (0.500 and O-600g), and electrode tip configuration (flat and concave). Based on previous work by Lifland et a1.,‘5 the low and high levels of the fabrication parameters were established (Table 1). The concave electrode tip configuration was incorporated to offset the surface versus bulk flow of current during a discharge. In fabricating low density (highly porous) implants, the integrity of the surface bead bonding has not been as high as the interior bead bonding. It was

Fig. 1. Sketch of torque

test fixture.

postulated that, if the distance between electrodes at the outer periphery was less than that at the center, a more uniform current could be obtained, resulting in more uniform bonding of the beads. The concavity of the electrodes was produced by drilling the ends of electrodes with a standard 118” point, 4mm diameter drill to 1.20 mm depth. The evaluation criteria were torque strength, compressive strength and internal integrity. The most common method of attaching the supergingival devices is by a 2 x 0.4mm titanium screw. To ascertain the torque strength of this type of screw, 20 samples were torque-tested. A chi-square test of the screw torque data supports the hypothesis of normality (a = 0.05) and the average strength (2,) and standard deviation (S,) were found to be 2, = 39.2 cm N and ,S’,= 1.24cm N. To ensure that the implant would have a higher torque strength than the screw, the minimum torque requirement for the implant was established to be the average screw torque strength plus six times Load

Push Rod riGuideRods

en

/ 1

Table 1.

Parameter Input energy (joules) Capacitance (bf) Sample weight (1 g) Electrode tip

Low 562 480 0,500 flat

High 397 720 0.600 concave Fig. 2. Sketch of compression

test fixture

Properties

of titanium dental implants

the standard deviation (46.7cm N). A Sturtevant torque test fixture, model TTF-14 was used for the present experiment (Fig. 1). The compressive strength of the implants was measured on an Instron universal testing machine, with a crosshead speed of 0.03 mm/min. The cylindrical samples of 4mm OD and 6 mm length were placed between guided parallel blocks in a customdesigned test fixture (Fig. 2). Since the maximum compressive stress exerted in adult human molars has been reported to be 20MPa,i6 this strength was taken as the minimum required for the implant. The run order was randomized to avoid or minimize confounding the test variables with extraneous effects. Using more than one replicate provided an estimate of the pure error and increased the precision of the effect estimates. The full two-level factorial design permitted estimation of all main effects and all interaction effects. Table 1 shows the experimental design in run order, and its associated observations order. Design-Ease softwarei was used to set up and evaluate this design. A 24 factorial with two replicates required 32 runs. Table 2. Experimental Run obs

Ord

21 I 16 17 15 28 9 30 12 2 6 23 31 11 32 29 8 24 25 14 27 7 LO 18 26 13 5 22 4 3 19 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

205

The primary method of evaluating the internal integrity of the EDC samples was to place them in a plastic holder, with the X-ray film directly beneath, and irradiate the entire sample perpendicular to their long axis, using a standard dental Xray unit. This non-destructive method allowed for subsequent mechanical testing. The secondary internal evaluation was made by cutting the sample in half, parallel to the long axis of the cylinder, and examining the metallographically-prepared crosssection. The X-ray results were classified in terms of existence of internal macroscopic voids and/or cracks, using a scale of zero (0) to ten (lo), with 10 showing no internal defects.

3 RESULTS Table 2 summarizes the results of the torque and compression tests, as well as the results of X-ray examination, along with experimental variables. Table 3 summarizes the analysis of variance (ANOVA) for the experimental design in terms of

design and results Electrode

Flat Flat Concave Flat Concave Concave Flat Flat Concave Flat Flat Concave Concave Concave Concave Flat Concave Concave Flat Flat Concave Concave Flat Fiat Flat Flat Flat Flat Concave Concave Concave Concave

Weight (grams)

Energy (joules)

Capacitance (micro-farad)

Torque (cm N)

Compressive (MPa)

X-ray (IO = best)

0.600 0,500 0.600 0.500 0.600 0.500 0.500 0.600 0.500 0.500 0.600 0.600 0.600 0.500 0.600 0.600 0.600 0.600 0.500 0,600 0.500 0.600 0.500 0.500 0.500 0,600 0.600 0.600 0.500 0.500 0.500 0.500

397 397 562 397 562 562 562 562 562 397 397 397 562 562 562 562 397 397 562 562 562 397 562 397 562 562 397 397 397 397 397 397

720 480 480 720 480 720 4x0 720 480 480 480 720 720 480 720 720 480 720 720 480 720 480 480 720 720 480 480 720 480 480 720 720

41.8 50.9 28.3 46.4 72.4 79.2 90.5 67.9 72.4 44.1 41.8 40.7 45.2 81.4 19.2 70.1 41.8 33.9 90.5 62.2 79.2 38.5 62.2 49.8 90.5 65.6 40.7 49.8 39.6 49.8 41.8 49.8

205.3 300.9 290.3 309.7 254.9 431.8 286.7 424.8 407.1 166.4 203.5 219.5 315.0 269~0 272.6 219.5 237.2 194.7 502.6 336.3 460.2 224.8 435.4 258.4 269.0 329.2 212.4 304.4 269.0 332.7 226.5 315.0

10 4

5 10

4 4 9 4 10 4 5 4 4

10

Al. 1. LiJliand, K. Qkazaki

206 Table 3. ANOVA

for selected model

VAR A B C D

VARIABLE Electrode Weight Energy Capacitance

I. Response: X-ray SOURCE MODEL RESIDUAL PURE ERROR COR TOTAL ROOT MSE DEP MEAN C.V. R-SQUARED II. Response: SOURCE

Model selected for Factorial: UNITS

-1 LEVEL Flat 0.500 397.000 480.000

grams joules micro-farad

SUM OF SQUARES 1~875000 3~000000 3~000000 4.875000 0.433013 0.187500 230.94% 0.3846

DF

SUM OF SQUARES 8267.39 2170.10 2170.10 10437.49 11.6461 57.4338 20.28% 0.7921

DF

SUM OF SQUARES 130827.4 92544.5 92544.5 223371.9 76.0528 296.4000 25.66% 0.5857

DF

15 16 16 31

+1 LEVEL Concave 0.600 562.000 720.000

MEAN SQUARE 0.125000 0.187500 0.187500

F VALUE 0.6667

PROB

MEAN SQUARE 551.16 135.63 135.63

F VALUE 4.064

PROB

MEAN SQUARE 8721.8 5784.0 5784.0

F VALUE l-508

PROB >F

>F

0.7809

Torque

MODEL RESIDUAL PURE ERROR COR TOTAL ROOT MSE DEP MEAN C.V. R-SQUARED III. Response: SOURCE MODEL RESIDUAL PURE ERROR COR TOTAL ROOT MSE DEP MEAN C.V. R-SQUARED

15 16 16 31

>F

0.0042

Compressive

15 16 16 31

the model with respect to torque, compression, and X-ray criteria. The average torque strength for the flat electrodes was greater than that of the concave electrodes at both energy levels (Fig. 3). The average compressive strength varied slightly with the electrode configuration but increased as the energy level increased (Fig. 4). The cube plots (Figs 5 and 6) illustrate the effects of the experimental variables on the torsional strength. The input energy was the most significant positive factor, and the electrode tip configuration the least significant. The two levels of capacitance (480 ,uF and 720 pF) offered a very slight improvement at the higher level. The weight also showed a slight effect, indicating that the lower the weight, the higher the torque. It should be noted that, with a constant diameter of the samples, a larger weight corresponded to a longer sample, i.e. more beads and, thus, a greater electrical resistance of the sample. From an analysis of

0.2119

Torque vs Electrode ConJguration

Flat (397 j)

Fig. 3. Average

Flat (562 j) Concovr (397 j) Electrode Configuration

torque strength

versus electrode

Concave (562 j)

configuration.

Properties

of titanium

dental

207

implants

Strength vs

Compressive

Ektrode

CTonfguration 283.2

327.5

392.0

Weight

/

//

0

Concovr (391 j)

Flor (397 j)

Elecrrode

Fig.

4.

Average

Flat (562 j)

’

285.8

*’ 258.9

Concave (S62 j)

Configurarion

compressive strength configuration.

versus

Electrode

electrode

Fig. 7. Cube plot of input variables

/ I

66.45

versus compressive

strength

(MPa).

56.27

I

/

.600*In

38.74

I I

43.54

Ai

/

Weight /’

/

,/”

/

’ 83.41 /AL----------

’

,/i7.78

300 gnl.

45.24

L

concave

Flat

358.4

+

Weight

7.20lrf

3973 I//

Electrode Fig.

plot of input

5. Cube

variables (cm N)

versus

torque

L

strength

Concave

Flat

Capacitance

480 PI

Electrode 49.76

57.40

A

.600 gnr

4

/ /” S2.59

Fig. 8. Cube plot of input variables

versus compressive

45.24

4.25

4.00

69.27

Weight

Ii

.500 gm.

strength

(MPa).

,,/’

60.79

El.92

Electrode Fig. 6. Cube plot of input

Weight variables

versus torque

(cm N). /’

means, the sample weight and input energy had significant effects on the torque strength. The cube plots of compressive strength (Figs 7 and 8) emphasized the effects of input energy. Sample weight showed a negative effect. The electrode tip

,/’

562j

/’

/’ ,400gm.

Energy 7.00

l/4:25

concove

FlUf

397j

Electrode Fig. 9. Cube plot of input

variables

versus X-ray anaiysis.

M. I. L$and,

208

4.75

6.25

.I500 gm

1

5.00

7,

5.25

5.50 ’

Weight

/’ ,500 gm.

K. Okuzaki

A_?

/’

/’ /’ /’

75 C0WXV.E

Fld

480 d

Electrode

Fig. 10. Cube plot of input variables versus X-ray analysis.

Fig. 12. Microphotograph firming the internal

of sample four from Fig. 10, convoids (original magnification x 22).

configuration had a slightly positive effect. The higher capacitance augmented the strength. An analysis of means, showed that only the input energy was significant at the 99% level. The internal integrity of the implants as interpreted via X-ray revealed the only significant (99% level) effect was that of the weight-energy interaction. The cube plots (Figs 9 and 10) showed an interesting effect of electrode configuration, i.e. the concave electrode appeared to enhance the internal integrity at the lower energy level, but at the higher energy level, the effect was reversed. The correlation coefficient (0.385 = R2) between the input energy and the X-ray evaluation of internal voids was not encouraging. The metallographic evaluation of the internal existence of voids and/or cracks correlated very closely with the X-ray data (Figs 11 and 12).

properties has been demonstrated. The data summarized in Table 2 indicated that torque strength in excess of that of the attachment screw (46.7cm N) was obtainable. At the higher energy level (562 J), and low weight (0.500 g), the minimum torque was 62.2 cm N with a maximum of 90.5 cm N and an average of 79.4cm N. The value of capacitance and electrode tip configuration are of minor significance. The compressive strength values were all in excess of the functional requirements. The minimum value of 166.4 MPa was eight times the maximum reported bite stress. The maximum compressive strength was also obtained using the low weight, high energy conditions. The major concern was the lack of consistent internal integrity, i.e. voids. The data indicated a random occurrence of internally sound samples. The presence of the internal voids, in light of the excellent mechanical properties and the fact of osseointegration (bone will fill the voids open to the surface), should be of minor importance to the function of the implant.

4 CONCLUSIONS

REFERENCES

Fig. 11. X-rays of EDC samples used to evaluate internal integrity. Light areas are internal voids and horizontal light areas indicate cracks.

The ability of the EDC process to produce highly porous dental implants with sufficient mechanical

1. Branemark, Strom,

J.,

P., Kansson, B., Adell, Hallen, 0. & Ohman,

R., Breine, LJ., LinA., Osseointegrated

Properties of titanium dental implants implants in the treatment of the edentulous jaw. Scnn. J. Plast. Reconstr. Surg. II Suppl., 16 (1977). 2. Adell, R., Lekholm, U., Rockier, B. & Branemark, P., A 15

year study of osseointegrated implants in the treatment of the edentulous jaw. ht. J. Oral Surg., 10 (1981) 387-99. 3. Council on dental materials, instruments and equipment: dental endosseous implants. JADA, 113 (1986) 949950. 4. English, C., Cylindrical implants. Calif Dent. J., 16 (1988) 17-38. 5. Spector, M., Fleming, W. & Kreutner, A., Bone growth into porous high density polyethylene. J. Biomed. Mater. Res., 7 (1976) 595. 6. Klawitter, J.; Bogwell, J., Weinstein, A., Sauer, B. & Pruitt,

J. R., An evaluation of bone growth into porous high density polyethylene. J. Biomed. Mater. Res., 10 (1976) 3 11. 7. Young, F., Spector, M. & Kresch, C.; Porous titanium endosseous dental implants in Rhesus monkeys: Microradiography and histological evaluation. J. Biomed. Mater. Res., 13 (1979) 843-56. 8. Kariagienes, J., Westerman,

R. & Hamilton, A., Investigation of long term performance of porous metal dental implants in non-human primates. J. Oral Implant., 10 (1982) 189-207. 9. Pilliar, R., Porous-surface metallic implants for orthopedic applications. J. Biomed. Mater. Res., 21:Al (1987) l-33.

209

10. Maniatopoulos, C., Pilliar, R. & Smith, D., Threaded vs. porous surfaced designs for implant stabilization in bone endodontic implant model. J. Biomed. Mater. Res., 20 (1986) 1309-33.

11. Cook, S., Georgette, F., Skinner, M. & Haddad, R., Fatigue properties of carbon and porous tihAl-4V Alloy. J. Biomed. Mater. Res., 18 (1984) 497-512. 12. Asaoka, K., Kuwayama, N., Okuno, 0. & Miara, I., Mechanical properties and biomechanical compatibility of porous titanium for dental implants. J. Biomed. Mater. Res., 19 (1985) 699-13. 13. Yue, S., Pilliar, R. & Weatherly, G., The fatigue strength of porous coated Ti 6%A14%V implant alloy. J. Biomed. Mater. Res., 18 (1984) 1043-58.

14. Okazaki, K.: Lee, W., Kim, D. & Kopczyk, R., Physical characteristic of Ti-6Al-4V implants fabricated by electrodischarge compaction. J. Biomed. Mater. Res., 25 (1991) l-13. 15. Lifland, M. & Okazaki, K., Mechanical properties of a Tip

6Al-4V dental implant produced by electro-discharge compaction. Clin. Mater., 14 (1993) 13-9. 16. Brunski, J., Biomechanics of oral implants: Gurute research D. sections. J. Dent. Edw., 52(12) (1988). 17. Design-Ease Software, ver 2.04, Stat-Ease, Inc., Minn., Mn., 1991.