in vivo wear of ti6al4v femoral heads: a retrieval study

IIZ vivo wear of Ti6A14V femoral heads: A retrieval study

T. E. McGovern? J. J. J. Jacobs? R. M. Graham,' and M. LaBergel,' Departments of *Bioengineering and 2Meckanical Engineering, Clemson University, Clenzson, South Carolina 29634; and 3Departmenf of Orfkopaedic Surgery, Rusk-Presbyterian-St. Luke's Medical Center, Chicago, Illinois 60612

The surface characteristics of sixteen "monobloc" titanium- 6% aluminum-4% vanadium (Ti6A14V) femoral components (two of the 6-Ti-28 type and 14 of the 6-Ti-32 type) retrieved after periods of 78-131 months following loosening of the femoral component, as well as two unimplanted controls, were studied. The femoral heads were examined by a combination of noncontact light profilometry, scanning electron microscopy, and energy-dispersive X-ray analysis. No consistent correlations were found between classical surface roughness parameters (average, root mean square, peak-to- valley roughness, and radius of curvature) and any clinical

parameter studied (patient gender, weight, and height; primary diagnosis; implantation time; or calculated force applied on the hip joint). This extensive quantitative topographic analysis suggests that wear mechanisms in vivo are complex and that wear of titanium alloy femoral heads is partly attributed to a combination of an imperfect nature of the surface before implantation, removal of the oxide layer causing abrasion of the alloy, subsequent deformation of the bearing surface including polishing, and, to a very small degree, patient parameters. 0 1996 John Wiley & Sons, Inc.

INTRODUCTION

The clinical success of titanium-6% aluminum-4% vanadium (Ti6A14V) alloy is due to its outstanding mechanical properties, corrosion resistance, and satis- factory local tissue re~ponse. '-~ The reaction of tissue adjacent to titanium or to its alloy Ti6A14V is benign, and direct bone ingrowth or osseointegration with titanium implants has been observed ~linically.~ Because of its high compliance compared with other metals used for total hip replacements, this alloy has the theo- retical advantage (over stainless-steel and cobalt-base alloys) of reducing the incidence of proximal bone re- sorption due to stress shielding5

Despite the numerous advantages of the Ti6A14V alloy as an implant material, its suitability as a bearing material for total hip replacements (THR) was called into question early on.6 Galante et al.6 as well as more recent workers7,* report in vitro examples of excessive wear against ultrahigh-molecular-weight polyethylene (UHMWPE), particularly when foreign particles such as fragments of bone or polymethylmethacrylate debris are deliberately introduced into the articulating interface. Despite the assertion of Galante et al. (refer-

*To whom correspondence should be addressed at the Department of Bioengineering, 301 Rhodes Research Center, P.O. Box 340905, Clemson University, Clemson, SC 29634- 0905.

ence 6, p. 30) that "Titanium is unsuitable as the material for the femoral head in a human hip joint pros- thesis," several Ti6A14V femoral components were developed and used. In recent years the use of this alloy as an articulating surface has decreased drastically, primarily because of the observation of extensive discoloration (blackening) of tissue adjacent to scratched femoral heads at retrieval.' A series of clinical retrieval and experimental wear studies have reinforced earlier conclusions that this alloy by itself is not suitable for in vivo bearing application^.'^-'^

Ti6A14V alloy is more susceptible to wear than Co- Cr alloy when articulating against UHMWPE.15 Ac- cording to McKellop et a1.,I6 Ti6A14V is more susceptible to severe abrasive wear by particles of acrylic bone cement than Co-Cr alloys. However, macroscopic ex- amination by Clarke et al.15 of seven retrieved femoral components, by Agins et a1.I0 of nine femoral components, and by Witt and Sward7 of 444 femoral components failed to show evidence of abnormal wear after up to 57 months of implantation.

Such wear of Ti6A14V alloy femoral heads would be expected to produce excessive UHMWPE wear,I8 which could be estimated by radiographic means. However, two recent long-term clinical follow-up s t ~ d i e s ' ' ~ ~ ~ of Ti6A14V femoral heads articulating with UHMWPE acetabular cups failed to demonstrate patterns of excessive wear in still-functioning THRs for periods of up to 1Ols and 1719 years, respectively. While other clinical series contain occasional examples of ex-

Journal of Biomedical Materials Research, Vol. 32, 447-457 (1996) 0 1996 John Wiley & Sons, Inc. CCC 0021-9304/96/030447-11

McGOVERN ET AL. 448

cessive wear (although not resulting in clinical failure), radiographic estimates of linear wear rates were 0.078- 0.107 m m / ~ e a r ’ ~ and 0.11 ? 0.15 mm/year,2’ respectively; the total wear in the latter study did not significantly exceed that found by the same techniques for conventional stainless-steel/UHMWPE THRs (Charnley) up to 14 years postimplantation.

In light of the continuing controversy associated with the clinical use of the Ti6A14V/UHMWPE wear pair, this study was designed to characterize the surface of retrieved Ti6A14V alloy THR femoral heads in an attempt to evaluate the effect of clinical variables including implantation time, patient height, and patient weight on topographic parameters such as average (R,), root mean square (RMS), and peak-to-valley (RJ roughness, and radius of curvature (r,) of anatomically relevant regions on the femoral head. This study was designed with the assumption that noncontact three-dimensional surface mapping (rather than linear) profilometry techniques combined with scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDXA) techniques would provide an under- standing of the origins of metallic surface roughness, and consequently of the tribologic performance of the Ti6A14V/UHMWPE wear pair.

MATERIALS AND METHODS

Sixteen Ti6A14V alloy femoral components of THRs (eight right and eight left; 14 of the 6-Ti-32 type and two of the 6-Ti-28 type; Zimmer USA, Warsaw, IN) were obtained following surgical revision for pain and/or loosening. These components were implanted for periods ranging from 78 to 131 months in 55-80- year-old patients (nine male and seven female) (Table I). All femoral components were fixed to the femur with polymethylmethacrylate (PMMA) bone cement. In 15 of 16 cases, the acetabular component was fixed with PMMA bone cement. In one case, a cementless fiber-metal acetabular component (Harris-Galante; Zimmer) was implanted in conjunction with a 6-Ti-28 femoral component in a patient with an aseptically loose THR (case no. 5). The latter case represents the only one in which the component was implanted in a patient who had a previous THR. One of the 15 sock- ets (case no. 10) that was implanted with cement had a metal backing; the remaining 14 consisted of UHMWPE only. At the time of revision surgery 11 patients underwent a revision of both the acetabular and femoral components, while five had a revision of the femur only. The specimens were retrieved at Rush- Presbyterian-St. Luke’s Medical Center during THR revision procedures, cleaned using a standard hospital cleaning protocol, and ethylene oxide gas sterilized prior to evaluation. The specimens were kept in airtight

bags to prevent surface alteration prior to the analysis, and extreme care was taken to prevent handling of the femoral heads during surface analysis. Two unimplanted components (6-Ti-32) were obtained from the manufacturer for comparison.

The femoral heads were examined macroscopically prior to a qualitative and quantitative surface analysis. Four anatomically relevant regions of each femoral head were examined: anterior, posterior, medial, and lateral. The surface finish of the femoral heads was then characterized with a noncontact profilometer (Topo- 3D’“; Wyko Corp., Tucson, AZ) using a X20 Mirau magnification head (cutoff area of 500 X 500 pm). The R,, RMS, and R,,, roughness [Fig. 1(A)] were measured at 10 different locations for each region at 1-mm intervals [Fig. l(B)]. A total of 10 values of each surface parameter were taken at each location using a position- inglclamping system. The relative sphericity or Y, of the femoral head was also determined indirectly, by mathematical reconstruction, in each region. R,, Rpv, and Y, values were measured with an accuracy of 0.7, 2.13, and 0.13% at one location for no movement, and with an accuracy of 5.6, 4.36, and 2.3% for the same location after repositioning, respectively. The surface of each region was examined under SEM geol-JSM- IC848; 15 kV) on each femoral head. Backscattering imaging and EDXA of the metallic surfaces were also performed to evaluate surface change.

A general linear model was developed in SAS (Statis- tical Analysis Software, Cary, NC) to compare the surface parameters’ values for male and female patients, and right and left hips as a function of anatomic location. This model consisted of analysis of variance [(ANOVA), a = 0.051, least-squares means tests, and Student’s t tests (a = 0.05). Using this model, the femoral heads were compared individually to the control and then divided into groups based on a roughness scale using the Fisher’s least significant difference (LSD) method (a = 0.05). The LSD method uses a nonconservative approach to dividing population means into groups from pairwise comparisons among a set of population means. The LSD method simplified the comparison of surface roughness values (R,, Rpv, and Y,) of retrieved femoral components to control femoral components. [R, was found to correlate linearly with RMS (R’ 2 0.98). Thus, although RMS values are given later for completeness, no further analyses involving RMS were performed.] The grouping was performed after observing no statistically significant difference between right and left hips and between male and female patients for all topographic parameters as a function of anatomic regions.

The least-squares linear regression method was used to determine any correlation between R,, Rpu, and r, values and implantation time, patient height and/or weight, and calculated normalized force values on the femoral head. Paul2’,” showed that the hip joint reac-

IN VIVO WEAR OF Ti6A14V FEMORAL HEADS 449

TABLE I Patient Information at Retrieval Time of Ti6A14V Femoral Components

Age Primary Component Height Weight Implantation Patient Hip Sex (years) Diagnosis Revised (cm) (kg) Time (mo)

1 Right M 55" OA B 180 93 103 2 Right M 74 OA/AVN B 178 82 64

4 Left F 61 RA B 155 58 93 5+* Left F 69* Aseptic F 136 55 87

6 Left F 71 OA B 155 60 82 7 Right F 59 OA F 155 82 109 8 Left M 66 OA F 175 100 110 9 Left F 75 OA B 171 78 155

100 Left M 70 OA B 170 84 108 11 Left M 62 AVN B 165 78 110 12 Left M 77 OA B 175 79 80 13 Right F 71 RA B 160 47 117 14 Right M 80 OA B 182 63 78 15 Right F 61 CDH B 160 63 123 16 Right M 74 RA F 178 107 131

3 Right M 58 PTA F 184 75 93

loosening

AVN: avascular necrosis; CDH: congenital dislocation of the hip; OA: osteoarthritis; PTA: post-traumatic arthritis;

*Zimmer Model 6-Ti-28. +Specimen retrieved from a patient who had a previous THR. $Cementless titanium acetabular component. §Metal-backed acetabular component.

RA: rheumatoid arthritis; M. male; F: female; B: both acetabular and femoral components; F: femoral component only.

tion force increases linearly with ML2, where M is the body weight (kilograms) and L is the double stride

varies according to the following relationships for a free walking speed of 75 m/min:

length (meters). Stride length during free walking increases linearly with height, with different relation- shim for men and women (reflecting different gait

S L (women) = 53.8 + 0.44 H S L (men) = 61.0 + 0.37 H

(R2 = 0.988) (R2 = 0.996),

meihanic~) .~~ The stride length (SL) fvor each genvder where H is the height of the subject in meters and R

4 LCO * - -________-_-- - - - - -

I

@I I

Roughness Average = Ra = Lir./y(xlJdx L-0

Root M ~ d n Square = RMS = ;&a yz (XJ dx

(A) R, = Maxvnum value of the p e a k to - valley distance wthm L. (mdth cut off) (B)

Figure 1. (A) Schematic of a given surface profile illustrating the center line (OX), the peak-to-valley roughness (Rpu), and the radius of curvature within a roughness width cutoff Lco. The roughness average (RJ, root mean square (RMS), and Rpv of each femoral head were measured at 10 different locations (black dots) for each anatomic region (medial, lateral, anterior, and posterior) at approximately 1-mm intervals (B).

450 McGOVERN ET AL.

is the correlation factor.23 Data were also stratified based on the primary diagnosis for total hip replacement, focusing on osteoarthritis (OA) and rheumatoid arthritis (RA).

Following surface analysis, femoral heads randomly selected were each sonicated for 15 min in high- performance liquid chromatography (HPLC) water and washed with 150 mL of boiling xylene. The concen- trate was resuspended and washed with HPLC-grade methanol. This solution was filtered using a 0.2-pm pore-size, gold-coated Teflon filter (Spectra Tech, Inc., Stamford, CT). The particulate was then analyzed with Fourier-transform infrared spectroscopy to characterize the chemical nature of material attached onto the surface of the femoral heads using a Nicolet Magna 550 FTIR Spectrophotometer (Nicolet Analytical In- struments, Madison, WI) equipped with an NIC-PLAN FTIR microscope. The precipitate was framed in the IR microscope and 64 scans were collected in the re- flectance mode at a resolution of 4 cm-l. The resultant spectra were compared with reference library spectra of UHMWPE.

RESULTS

Macroscopically, the surface finish of the highly pol- ished control (unimplanted) specimens appeared to be regular. Under SEM, surface defects (small depressions, pitting, and polishing marks) were observed on both specimens, randomly located in all four regions (Fig. 2). Despite these defects, the small range of R, (-31 to 50 nm) combined with small standard errors (9-17%) suggest that a highly reproducible polishing technique was used during manufacture (Tables II-

Figure 2. Scanning electron micrograph of a control femoral head showing surface defects such as small depressions, pitting, and asperities randomly located, irrespective of anatomically relevant regions (Jeol-JSM-IC848; 15 kV).

IV). An overall radius of curvature of 15.6 2 1.0 mm (type 6-Ti-32) was measured for the controls, consistent with the manufacturer’s specifications.

However, as shown in Figure 3(a-c), widely different topographic patterns associated with comparable roughness average measurements strongly indicate that R, values do not adequately characterize these surfaces. The surface profilometer used in this study is limited to measuring asperities <1-2 pm in height. Therefore, some defects could not be analyzed and quantified with the topographic analysis. For this rea- son, the topographic analysis did not include some of the large scratches observed macroscopically. This might partly explain why no statistically significant differences ( p < 0.05) were found for roughness parameters of the control specimens as a function of anatomically relevant regions (anterior, posterior, medial, and lateral) even though different scratch patterns were observed on the retrieved specimens.

Macroscopically, all retrieved femoral heads showed a mapping of dull regions with no preferential organi- zation and region, and no apparent correlation between area and implantation time, patient weight, or height. Randomly distributed burnished areas and a mapping of discoloration or change in color were also observed macroscopically on the bearing surfaces. SEM emphasized the effect of implantation on the bearing surfaces. Scratching and pitting were observed on all specimens irrespective of implantation parameters [Fig. 4(a, b)]. Debris were observed to be embedded within scratches by both SEM and backscattering imaging. EDX analysis was performed to characterize the elemental composition of the retrieved femoral heads. The spectrum of the altered surfaces was typical of the Ti6A14V alloy. The presence of polyethylene was observed on the retrieved femoral heads with FTIR with characteristic absorbency bands at 2920 and 2850 cm-’ associated with symmetric and asymmetric car- bon-hydrogen stretching and 1470 cm-’ associated with CH2 bending. Therefore, this debris proved to be nonmetallic on EDX and was assumed to be UHMWPE as confirmed with FTIR. Greater amounts of aluminum were measured on the control (9%) than on the retrieved surfaces (4%). Titanium was the predominant metallic ion detected along with aluminum and vanadium.

The profile of the control specimens was consider- ably modified following implantation. Profiles different from the control surfaces were observed in several specimens irrespective of anatomic regions. Tables I1 and I11 present the R, and R,, values for all retrieved specimens as a function of anatomic region. Surface profile was used in the past as an indication of wear or material removal for femoral components. How- ever, the topographic parameters when taken individually are only a representation of surface change from a baseline profile and do not provide conclusive evi-

IN VWO WEAR OF Ti6A14V FEMORAL HEADS 451

TABLE I1 Mean Average Roughness (nm) of Retrieved ( n = 16) and Control (n = 2) Femoral Heads

Anatomic Region

ID No. Medial Lateral Anterior Posterior

1 148 2 33 327 i 66 287 i 52 52.0 ? 14.2 2 427 i 63 39.6 i- 11.7 148 i 53 104 i- 22 3 49.1 f 23.5 51.4 i 17.4 80.5 t 43.5 58.0 i 16.3 4 37.1 i 8.8 74.6 i 15.5 41.9 f 4.6 56.0 2 19.5 5 98.9 t 28.3 29.2 t 1.8 116 i 28 470 2 119 6 28.6 f 12.1 45.2 t 11.0 121 t 22 27.5 t 1.0

113 t 31 7 66.2 i 21.1 103 t 20 137 i- 31 8 110 f 16 93.9 i 27.0 31.1 i 2.4 73.6 t 17.5 9 80.7 t 55.7 56.2 2 9.4 443 i 70 113 t 32

290 t 69 10 228 t 56 68.8 i 25.0 316 i- 36 11 76.1 f 16.9 50.9 t 16.3 288 i 52 42.9 i 15.4

272 t 68 12 59 t 11.3 34.0 i- 12.1 55.9 t 16.1 13 39.4 i- 11.0 32.4 i 2.2 29.4 i 1.0 320 i- 29 14 82.6 i 24.7 105 i- 56 158 2 28 178 i 49 15 50.6 f 3.1 51.2 i 10.2 41.8 t 9.5 63.4 t 25.1 16 35.2 t 2.1 36.8 i 13.8 188 i 44 44.7 t 5.1

Mean 101 t 53.5 75.0 i- 38.1 155 i 64 142 t 68 Con 49.9 i 3.6 31.1 i 2.0 38.9 i- 4.8 41.1 i 2.7 Mean/control 2.02" 2.41* 3.98" 3.45"

Con: control values. *Significance at p < 0.05.

dence of material removal and wear. The noncontact profilometer used in this study is not able to measure surfaces with apparent scratches. Large indentations and scratches could not be measured, and appear as missing data.

Gender did not significantly contribute to modifying the surface profile from the baseline data. No statistically significant differences have been observed be-

tween male and female patients for all topographic parameters, and between right and left hips, after cor- rection for anatomic anterior-posterior orientation. The R, values of the retrieved specimens were significantly ( p < 0.05) higher than the control specimens in 37% of cases for the lateral region, 56% for the medial and posterior regions, and 75% of cases for the anterior region. On average, by region, the R, of retrieved speci-

TABLE I11 Mean Peak-to-Valley Roughness (nm) of Retrieved (n = 16) and Control (n = 2) Femoral Heads

Anatomic Region


1 2 3 4 5 6 7 8 9

10 11 12 13 14 15 16

Mean Con Mean/control

976 t 791 3194 i- 371 566 i- 202 473 f 131 986 t 118 388 t 136 623 i 83

1023 i 146 946 i 192

1683 i- 553 924 t 249 712 i 185 410 i 125 762 ? 179 452 t 44 340 i- 66 904 i- 372 536 i 94

1.70*

3036 i- 803 752 t 197 901 i- 523 674 i- 217 308 t 40.0 574 i- 115

1170 t 240 747 i 121 542 i- 161 776 i 84.5 120 i 85.7 481 i 101 488 t 92.6

1012 i 481 606 i- 91.4 488 t 133 820 f 335 307 i 18

2.67*

2399 i 377 1414 t 284 985 i 310 419 i 63.0

1057 i 182 1775 i 446 1298 i- 304 276 f 10.3

3684 i 598 2543 t 339 2076 t 337 595 t 145 217 t 19.1

1424 i- 182 474 i 223

1748 t 382 1398 i- 508 336 i 34

4.16"

752 i 128 952 i 116 767 i 266 687 i 88.0

4281 t 887 217 i- 10.4

1320 f 446 744 t 108

1203 i 244 2308 t 714 511 t 104

4516 f 725 2248 i 339 1717 t 682 487 t 185 465 f 256

1449 t 694 318 i 22

4.57"

Con: control values. *Significance at p < 0.05.

452

TABLE 1V Mean Radius of Curvature (nm) of Retrieved and Control Femoral Heads

McGOVERN ET AL.

Anatomic Region


6-Ti-28 (n = 2) 1 19.9 t- 2.0 15.0 t 1.1 18.3 t 2.6 15.9 i- 0.3

19.5 t 0.7 5 14.9 t 0.1 13.8 2 0.8 13.2 t- 1.0 Mean 17.3 14.4 15.8 17.7 Nom 14 14 14 14 6-Ti-32 (n = 14)

2 15.8 t- 0.1 16.1 t 1.4 17.6 t- 2.9 16.0 2 0.7 3 16.2 i- 0.4 16.3 t 0.3 16.2 i- 0.8 15.7 i- 0.3

15.7 t 0.1 4 17.1 t 0.8 16.4 i- 0.3 16.8 -C 0.6 6 15.7 i- 0.2 16.2 t 0.3 16.9 i- 0.6 16.0 t 0.1 7 15.7 t 0.8 15.6 t 0.5 16.2 t 1.56 15.5 t 1.1 8 14.7 t 0.6 14.9 t 0.7 17.1 t 0.2 16.0 2 0.6 9 15.9 2 0.2 15.3 2 0.7 18.3 t 0.9 16.5 t- 1.4

10 15.7 t 0.7 16.4 ? 2.2 18.5 t 2.0 16.2 t 2.4 15.6 t 0.3 11 15.9 t 0.4 15.5 t- 0.6 19.3 i- 3.5

12 15.8 2 0.3 15.9 t- 0.5 15.9 t 0.5 16.5 t 2.3 13 15.8 t 0.2 15.7 t 0.2 16.1 t 0.1 20.0 t 2.7

16.6 t 1.0 15.5 t- 1.5 14 14.7 t 0.9 16.6 t 0.8 15 15.8 t 0.3 15.3 t 0.2 15.7 t 0.2 16.4 t 0.2

16.2 t 0.3 16 16.7 ? 0.2 16.2 t 0.1 14.8 t 1.6 Mean 16.0 t 0.6 15.7 t 0.4 16.7 t 0.8 16.3 t 0.7 Nom 16 16 16 16 Con 15.7 t 0.1 15.5 t 0.1 16.6 t 0.2 14.6 t 0.3 Mean/control 1.02* 1.01* 1.01* 1.12,

Con: control value; Nom: nominal value. *Significance at p < 0.05.

mens were 2.02-3.98 times greater than those for controls ( p < 0.05). The R,, values of retrieved specimens were significantly higher ( p < 0.05) than the controls in 75% of cases in the posterior and anterior regions, compared to 40% of the cases in the medial and lateral regions. On average, by region, the R,, of retrieved specimens were 1.7-4.57 times greater than those for controls ( p < 0.05). Even though evident changes were observed for the r, after retrieval, these changes were found to be statistically different: mainly higher ( p < 0.05) from the control values in only 18% of the cases for all regions (Table IV).

On an individual basis, 62% of the retrieved specimens demonstrated R, and R,, values significantly different ( p < 0.05) as a function of anatomic region. All anatomic regions were significantly different for R,. Higher Ra and R,,, values were measured on the anterior region. Overall, the anterior region of the femoral heads appeared to be more affected than the other anatomically relevant regions.

The average r,s of all four regions were found to be statistically identical in 25% of the specimens which were all of type 6-Ti-32. The Y, of the anterior region was significantly higher than the medial, lateral, and posterior regions in 71, 83, and 57% of the specimens, respectively, indicating a possible relationship between anatomic region and y,. Overall, it was observed that the Y, of both roughened and smoothened regions had changed from its presumed original value. For

the profilometer used in this study (Topo3D), the Y,

represents the radius of best-fit portion of a sphere. No correlation could be established for paired ante-

rior and posterior regions, or medial and lateral regions for roughness values. The superposition of dimensional profiles of control and retrieved specimens emphasized surface change after implantation.

Using Fisher’s LSD method (a = 0.05), the retrieved femoral heads were divided into five groups irrespective of implant type. Grouping was based on a roughness average scale using R, and R,,, values as a function of anatomic regions (Table V). Group I consisted of hip nos. 3,4,6, and 15; group I1 of hips 11,12,13, and 16; group I11 of hips 2, 5, 9; group IV of hips 7, 8, and 14; and group V of hips 1 and 10. Table V shows the average and 95% confidence interval for all groups. Only groups IV and V showed significantly different R, and R,,, values from the controls ( p < 0.05). More specifically, medial and anterior regions in group IV, and medial, lateral, and anterior regions in group V were statistically larger than the controls. Specimen grouping as a function of profile roughness emphasized a predominant surface change in the medial and anterior regions.

Patients were also divided into three groups as a function of implantation time: group A (85 t 7 months), group B (110 2 4 months), and group C (124 -+ 7 months). Surface profiles were significantly different among these groups ( p < 0.05). No significant differ-


Figure 3. Typical three-dimensional profiles of the medial (A), lateral (B), and anterior regions (C) of a control Ti6A14V alloy femoral head showing that for similar R, values, the overall appearance of the surface topography can be different. (A: R, = 46.3 nm, RMS = 59.3 nm, R,, = 640 nm, r, = 15.9 mm; B: R, = 38.1 nm, RMS = 37.6 mn, R,, = 296 nm, Y, = 15.3 mm; C: R, = 38.1 nm, RMS = 47.6 nm, R,, = 279 nm, r, = 15.6 mm) (Topo-3DTM; X20 Mirau magnification head; 500 X 500-pm cutoff area).

ences were observed between the roughness parameters of the retrieved groups and the controls, partly because of large standard deviations as a function of implantation time. Roughness measurements were in-

Figure 4. Different surface changes were observed on the retrieved specimens using scanning electron microscopy (A). Nondirectional scratches (B) were noticed on all retrieved femoral heads irrespective of anatomic regions. All micrographs were obtained with specimen no. 8 (Jeol-JSM- IC848; 15 kV).

conclusive in stating that Ti6A14V alloy surfaces are overall highly significantly modified after implantation.

A linear regression analysis was performed to evaluate a possible correlation between dependent variables X,, Rpv, rc, and patient parameters, including implantation time, patient weight and height, and predicted/ calculated force on the femoral head. A statistical analysis (F-test variance analysis) was used to verify whether patient parameters would improve the ability to estimate the dependent variables. It was only found that: 1) changes in the r, of the posterior region are predictable functions of height and weight ( p < 0.05), and 2) the Y, of the lateral region was also influenced by implantation time. In both cases, the r, decreased as a function of increasing implantation time, height, and weight. However, only weak correlations (R2 ranging from 0.001 to 0.288) were observed between roughness and patient parameters (Table VI).

Patients were also grouped by diagnosis (OA vs. RA), since disease state affects synovial fluid composi- tionZ4 and thus, perhaps, wear. No statistically significant correlations were observed between R,, Rpvr and Y, of specimens retrieved from RA- and OA-grouped patients with a 95% confidence level.

454 McGOVERN ET AL.

TABLE V Mean Average (Ra) of Grouped Femoral Components, Anatomic Regions

Ra ~

Group (n) Medial Lateral Anterior Posterior

41.4 2 15.3 55.6 t 18.4 71.4 t 49.9 51.2 t 22.1 38.5 t 9.2 140.4 t 116.3 169.9 t 126.0 52.4 t 23.7 29.3 t 13.6 90.1 t 83.5 199.1 t 97.3 179.0 t 139.2

100.7 i 8.1 108.5 i 72.3 121.7 t 55.6 86.4 -t- 17.9 197.9 t 161.7 301.2 t 21.9 170.9 i 110.1 187.8 t 61.0

1 (4) 2 (4) 3 (3) 4 (3) 5 (2)

Control (2) 49.9 2 3.6 31.1 2 2.0 38.9 i 4.8 41.1 t 2.7

Stride lengths and hip-joint force were calculated using the models proposed by Skinnerz3 and Pau1,2’ respectively (Table VII). It should be noted that the activity of the patients at THR revision is unknown, and therefore the force extrapolated from the height of the individuals provides a gross estimation of an apparent hip-joint force. Weak correlations between R,, Rpv, and Y, and calculated hip-joint force were observed ranging from 0.06 to 0.29 (Rz).

DISCUSSION

Wear of metallic surfaces of implants has commonly been analyzed qualitatively with microscopy and quantitatively with profilometry. Topographic charac- terizations of femoral heads after retrieval were per-

formed by several investigators in attempts to determine the amount of material removed during service or to correlate the effect of metallic surface profile with the wear rate of the UHMWPE counterface. Such comparative surface profile studies before implantation and after removal use contact profilometry to measure the average roughness (R,) and indirectly the RMS and R,, roughness of the bearing surfaces. However, the stylus used for contact profilometry typically has a tip diameter of 21 pm, which does not detect surface asperities or cavities of lesser dimensions. More re- cently, Dwyer et al.,25 Black et a1.,26 and McKellop et al.27 used noncontact profilometry, either interferometry or laser, to characterize metallic retrieved prostheses. However, as pointed out by McKellop et al.,24 such techniques provide only limited information on the overall condition of the surface and cannot distinguish among material transfer, creep, surface deformation,

TABLE VI Linear Regression Parameters and Correlation Factors between Radius of Curvature (r,) and Patient Parameters

Anatomic Region r, [Slope (mo/mm)] r, [Intercept (mm)] r, (R2) ~~

Implantation time Medial 9.34E-03 15.08 0.029 Lateral -1.36E-01* 30.71 0.159 Anterior 7.28E-02 11.40 0.041 Posterior 1.75E-02 14.66 0.075

Anatomic Region r, [Slope (cm/mm)] r, [Intercept (mm)] r, (R2)

Height Medial Lateral Anterior Posterior

1.22E-02 -7.59E-02 5.56E-02

-2.87E-02‘

13.99 5.04 9.38

16.83

0.045 0.044 0.021 0.002

Anatomic Region r, [Slope (kg/mm)l r, [Intercept (mm)] r, (R2) ~~

Weight Medial 1.64E-02 14.89 0.068 Lateral 4.79E-02 13.95 0.015 Anterior 4.83E-01 14.94 0.013 Posterior -1.81E-02* 17.68 0.060

Anatomic Region r, [Slope (kN/mm)] r, [Intercept (mm)] r, (R2)

Hip-joint force Medial Lateral Anterior Posterior

1.116 0.173

1.837 1.308

13.93 15.34 14.21 20.09

0.115 0.009 0.102 0.288

*Statistically significant F value ( p < 0.05).


TABLE VII Hip-Joint Force as a Function of Stride Length (L) , Height (H), and Weight (W) of Patients

Hip No. Gender Height (cm) Stride Length (m) Weight (kg) WL2 (kg*m2) Force (kN)

1 2 3 4 5 6 7 8 9

10 11 12 13 14 15 16

M M M F F F F M F M M M F M F M

180 1 78 184 155 136 155 155 175 171 170 165 175 160 182 160 178

1.28 1.27 1.29 1.22 1.14 1.22 1.22 1.26 1.29 1.24 1.22 1.26 1.24 1.28 1.24 1.27

93 82 75 58 55 60 82

100 78 86 78 79 47 63 63

106

152.37 132.26 124.81 86.33 71.48 89.30

122.05 158.76 129.80 132.23 116.10 125.42 72.27

103.22 96.87

170.97

2.43 2.16 2.06 1.55 1.35 1.59 2.03 2.52 2.13 2.16 1.95 2.07 1.36 1.78 1.69 2.68

polishing, or material removal as causative factors of surface features.

Surface waviness, amplitude, and magnitude of the asperities, represented as Rpv, asperity density, and asperity geometry, are parameters that provide an indication of the response of the surface to compressive loading. Contact stresses will be initially distributed over the real contact area determined by the number of highest peaks and their geometry. Based on the surface profile (not surface roughness data) of control specimens showing asperities with very small width or diameter, and assuming that the bearing counterface is regular, both the anterior and posterior regions would initially have smaller real contact areas than the medial and lateral regions and be more susceptible to stress-induced surface change. This is consistent with the data of Afoke et a1.** as well as R~de11,2~ who experi- mentally identified the anterosuperior surface as an area of high pressure in normal hip joints.

The defects observed with SEM are stress risers that most likely contributed to the initial local failure of the metallic surface under loading. Wroblewski et al.30 emphasized the importance of the quality of the femoral head surface finish to minimize the wear of the polyethylene counterpart. This statement is also valid for the wear of the femoral component itself.

All man-made materials operated in tangential motion, including total hip replacement surfaces, are subject to physical alteration, and therefore, to wear. In tribosystems consisting of a polymeric and a metallic counterface, as in conventional THR, alteration of the metallic surface can result from abrasion by a third body, delamination, pitting due to cyclic and recipro- cating loading, and transfer of the polymeric material onto the metal surface.

While the quality of the surface finish of the femoral heads prior to implantation is an initial contributing factor to their wear behavior in use, the original surface

profile of the metallic component can be modified over time by addition or removal of material or by deformation of the surface. Microscopically, this is reflected in the generally higher R, and R,, values in the retrieved specimens found in this study. The generally higher values in the anterior and posterior regions reflect a directional orientation associated with the A-P movement being greater than M-L movement in hip articula- t i ~ n . ~ * Directional and nondirectional scratches of different length were noticed on the bearing surfaces as previously reported by Salvati et al.,I3 McKellop et a1.,I6 and Black et al.”

Data showed possible transfer of polyethylene onto retrieved Ti6A14V alloy surfaces as also observed by other authors,3l~~~ despite McKellop’s7 observation that the presence of proteins prevents such transfer in vitro onto stainless steel.

The source of the high surface concentrations of aluminum is uncertain. Witt and Swann17 also identified a greater amount of aluminum (14%) on nonabraded Ti6A14V surfaces associated with embedded particles of aluminum oxide grit. Aluminum-bearing silica particles have been previously found embedded in retrieved UHMWPE counterfa~es,2~,~~ and the latter authors suggested that they represent fragments of catalyst carriers. However, because we observed greater amounts of aluminum on unimplanted (9%) than retrieved (4%) femoral heads, it seems more likely that it represents an impurity introduced during pro- cessing.

Four main mechanisms of wear are normally observed on surfaces in tangential motion: abrasion, ad- hesion, corrosion, and fatigue wear (as reflected in part by delamination). For a tribosystem consisting of a hard surface and a softer surface as in this retrieval study, it can be expected that the harder surface will be modified by material removal through abrasion. If fact, previous studies by others emphasized that bulk

456 McGOVERN ET AL.

Ti6A14V femoral heads are primarily abraded by re- leased oxide layer debris, which is harder than the alloy i t~e l f . ' ~ ,~~ Through a breakdown of the oxide film and high stresses at the interface between the oxide and the bulk alloy, oxide particles are detached from the continuous layer and are responsible for third-body abrasive wear. Because Ti6A14V alloy is known to re- plenish its protective oxide layer in vivo, such wear is a progressive process once initiated. In this respect, quantitative relationships for abrasive wear rate correlate the volume of material removal to the load applied on the surface. Load and surface velocity are two variables that influence wear rates and wear mechanism. In the hip joint, the highest loads correspond to posi- tions of low surface velocity. When surface velocities are maximum, the loads are lowest.35 Paulz2 proposed a linear relationship between hip joint force (F) and ML2, where M is the subject body mass and L the double stride length. However, we failed to find a strong relationship between this parameter and any wear parameter, supporting the data of Buchanan et al.," who noted no dependence of Ti6A14V alloy wear in vitro on calculated nominal stress.

Other investigators have evaluated the effect of implantation on retrieved Ti6A14V alloy femoral surfaces for periods ranging from 11 to 57 months,'O 6 to 47 months,"h 13 to 46 rn~nths, '~ 34 to 35 rnonths,I2 and 12 to 134 rnonths.l6 This study evaluated femoral heads after 64-131 months of implantation.

Witt and Swann17 reported no apparent wear of Ti6A14V alloy femoral heads, but did not provide veri- fication of their observations. Concerns about abnormal wear of Ti6A14V alloy expressed by Zych et al.36 were not supported by the qualitative postretrieval data. McKellop et a1.16 presented a model of the clinical wear performance for hip prostheses with titanium alloy bearing against polyethylene, with emphasis on the role of bone cement particles that cause excessive abrasion of the surface. McKellop et al.I4 attributed evidence of abrasive wear on retrieved Ti6A14V alloy femoral heads to the presence of bone cement particles in the joint space. However, Cates et al.19 and Ebramza- deh et al.?O found wear rates in uiuo similar to those observed for cemented THRs, despite the possible presence of bone cement particles in the capsular tis- sues. For noncemented THR, McKellop et con- cluded that the wear properties of well-fixed titanium alloy prostheses are comparable with those of Co-Cr alloy and stainless-steel components.

C 0 N C L U S I 0 N S

In this study, all components were retrieved following loosening of the femoral component, the acetabular component, or both. This extensive quantitative topo-

graphic analysis shows that wear of titanium alloy femoral heads is partly attributed to a combination of an imperfect nature of the surface before implantation, removal of the oxide layer causing abrasion of the alloy, subsequent deformation of the bearing surface including polishing, and, to a very small degree, patient parameters. Taking R, or R,,, as an indicator of wear, retrieved femoral heads are two to four times rougher than implanted ones, with a tendency to show greater roughness in the anterior and posterior regions. However, this roughness cannot be associated in a simple fashion with patient parameters or implantation time. One explanation for this finding is the enor- mous variability in patient activity as measured by a digital pedometer.37

This retrospective study was performed at revision time, a single postoperative point in time, for a hetero- geneous group of patients. Even though difficult to attain, serial measurements on a single joint as a function of time related to the level of stability of the implant might provide a better indication of the wear process of Ti6A14V heads. Improvements to the surface properties of this alloy by ion implantati~n~*-~I or other hardening techniques may well be able to eliminate the occasional high wear rates seen both in vitro and in vivo, most probably associated with third-body abrasive wear.

The authors thank Dr. L. W. Grimes for his valuable partici- pation in the experimental analysis. Dr. M. J. Drews and K. Ivey are acknowledged for their contribution in the FTIR analysis. This study was supported by NIH Grant R01 AR39310, the Hunter Endowment, and NSF/REU (Research Education for Undergraduates).

References

1.

2.

3.

4.

5.

6.

1. Lemons, K. M. W. Niemann, and A. B. Wiess, "Bio- compatibility studies on surgical grade titanium-, cobalt- and iron-based alloys," J. Bioined. Muter. Res., 7,

R. J. Solar, S. R. Pollack, and E. Korostoff, "In vitro corrosion testing of titanium surgical implant alloys: An approach to understand titanium release from implants," I. Biomed. Muter. Res., 13, 217-250 (1979). D. I. Bardos, "Titanium and titanium alloys," in Concise Encyclopedia of Medical and Dental Materials, D. Williams (ed.), Pergamon Press, Oxford, 1990, pp. 360-365. T. Albrektsson, P. I. Branemark, H. A. Hansson, and J. Lindstrom, "Osseointegrated titanium implants," Acta Ortkop. Scand., 52, 155-170 (1981). R. R. Tarr, I. C. Clarke, T. A. Gruen, and A. Sarmiento, "Comparison of loading behavior of femoral stems of Ti6A14V and cobalt chromium alloys: A three dimensional finite element analysis," in Titanium Alloys in Surgical Implants, H. A. Luckey and FredKubli, Jr. (eds.), American Society for Testing and Materials, Philadel- phia, 1983, pp. 88-101. J. 0. Galante and W. Rostocker, "Wear in totalhip prostheses: An experimental evaluation of candidate materials," Acta Ortkop. Scand. (Supp1.)145, 6-46 (1973).

549-553 (1976).


7.

8.

9.

10.

11.

12.

13.

14.

15.

16.

17.

18.

19.

20.

21.

22.

23.

24.

H. McKellop, I. C. Clarke, K. Markolf, and H. Amstutz, "Friction and wear properties of polymer, metal and ceramic prosthetic joint materials evaluated in a multi- channel screening device," J. Biomed. Mater. Res., 15,

H. McKellop and I. C. Clarke, "Wear of artificial joint materials in laboratory tests," Acta Orthop. Scand., 59,

J. T. Scales, "Black staining around titanium prostheses-an orthopaedic enigma," J. Bone Joint Surg., 738,

H. J. Agins, N. W. Alcock, M. Bansal, E. A. Salvati, P. D. Wilson, Jr., P. M. Pellicci, and P. G. Bullough, "Metallic wear in failed titanium-alloy total hip replacements: A histological and quantitative analysis," J . Bone Joint Surg., 70A, 347-356 (1988). J. Black, H. Sherk, J. Bonini, W. R. Rostoker, F. Schajo- wicz, and J. 0. Galante, "Metallosis associated with a stable titanium-alloy femoral component in total hip replacement," J. Bone Joint Surg., 72A, 126-130 (1990). A. V. Lombardi, T. H. Mallory, B. K. Vaughn, and P. Drouillard, "Aseptic loosening in total hip arthroplasty secondary to osteolysis induced by wear debris from titanium-alloy modular femoral heads," J. Bone Joint Surg., 71A, 1337-1342 (1989). E. A. Salvati, F. Betts, and S. B. Doty, "Particulate metallic debris in cemented total hip arthroplasty," Clin. Or- thop. Rel. Res., 293, 160-173 (1993). R. A. Buchanan, E. D. Rigney, Jr., and J. M. Williams, "Wear-accelerated corrosion of Ti6A14V and nitrogen- ion-implanted Ti6A14V: Mechanisms and influence of fixed-stress magnitude," J. Biomed. Mater. Res., 21,367- 377 (1987). I.C. Clarke, H.A. McKellop, P. McGuire, R. Okuda, and A. Sarmienta, "Wear of Ti6A14V implant alloy and ultra high molecular weight polyethylene combina- tions," in Titanium Alloys in Surgical Implants, H. A. Luckey and Fred Kubli, Jr. (eds.), American Society for Testing and Materials, Philadelphia, 1983, pp. 136-147. H. A. McKellop, A. Sarmiento, C. P. Schwinn, and E. Ebramzadeh, "ln uiuo wear of titanium alloy hip prostheses,'' ]. Bone Joint Surg., 72A, 512-517 (1990). J. Witt and M. Swam, "Metal wear and tissue response in failed titanium alloy total hip replacements," J. Bone Joint Surg., 738, 559-562 (1991). J. Fisher, "Surface damage to femoral head prostheses [letter]," J. Bone Joint Surg., 768, 852 (1994). H. E. Cates, P. M. Faris, E. M. Keating, and M. A. Ritter, "Polyethylene wear in cemented metal-backed acetabular cups," J. Bone Joint Surg., 758, 249-253 (1993). E. Ebramzadeh, H. McKellop, A. Sarmiento, P. Nor- mand, and S. Elkins, "Wear of titanium alloy and stainless steel prostheses in-vivo," in Proceedings of the Sympo- sium on Medical Applications of Titanium and Its Alloys: The Materials and Biological Issues, J. Lemons and S. A. Brown (eds.), American Society for Testing and Mate- rial, Philadelphia, 1996. J. P. Paul, "Forces transmitted by joints in the human body," Proc. Inst. Mech. Eng., 181, 8-15 (1967). J. P. Paul, "Loading on normal hip and knee joints and on joint replacements.," in Artificial Hip and Knee Joint Technology, M. Schaldach and D. Hohmann (eds.), Springer Verlag, Berlin, 1976, pp. 53-70. S. Skinner, "Development of gait," in Human Walking, 2nd ed., J. Rose and J. G. Gamble (eds.), Williams and Wilkins, Baltimore, MD, 1994, pp. 125-138. R. W. Ropes, Synovial Fluid Changes in Joint Disease, Vol. 1, Harvard University Press, Cambridge, MA, 1953,

619-653 (1981).

349-352 (1988).

534-536 (1991).

pp. 1-77.

25.

26.

27.

28.

29.

30.

31.

32.

33.

34.

35.

36.

37.

38.

39.

40.

41.

Rec Acc

K. A. Dwyer, L. D. T. Topoleski, D. J. Bauk, R. Nakielny, and C. A. Engh, "Wear in TKA: An initial surface damage analysis of retrieved femoral components," Trans. SOC. Biomater., 16, 22 (1992). J. Black, C. R. Dillon, J. Yang, R. M. Urban, and J. J. Jacobs, "Surface characterization of retrieved cemented Ti6A14V femoral hip replacement components," Trans. SOC. Biomater., 16, 21 (1992). H. A. McKellop, P. Campbell, S. H. Park, and B. Lu, "Comparison of surface roughness on new and retrieved metal and ceramic total hip prostheses," Trans. SOC. Biomater., 16, 85 (1993) N. Y. P. Afoke, P. D. Byers, and W. C. Hutton, "Contact pressures in the human hip joint," J. Bone Joint Surg.,

N. Rydell, "Biomechanics of the hip joint," Clin. Orthop. Rel. Res., 92, 6-15 (1973). 8. M. Wroblewski, P. J. McCullagh, and P. D. Siney, "Quality of the surface finish of the head of the femoral component and the wear rate of the socket in long-term results of the Charnley low-friction arthroplasty," Proc. Inst. Mech. Eng., 206, 181-183 (1992). D. A. Miller and R. D. Ainsworth, "A comparative evaluation of the wear of ultra-high molecular weight polyethylene abraded by Ti-6A1-4V'" Wear, 28, 207-216 (1 974). A. Nedelec-Pichat, "Etude du Couple Metal-Polymere pour Application Orthopediques: Influence du Traite- ment de Surface par Implantation Ionique," Doctoral thesis, Ecole National Superieure des Mines de Paris et de L'Ecole national Superieure des Mines de Saint- Etiene, Saint-Etienne, France, 1990. H. S. Dobbs, J. T. Scales, and K. W. J. Wright, "Silicon containing particles in ultra high molecular weight polyethylene," J. Bioeng., 1, 113-114 (1977). E. S. Machlin and W. R. Yankee, "Friction of clean metals and oxides with special reference to titanium,"

A. Unsworth, "Tribology of human and artificial joints," Proc. Inst. Mech. Eng., 205, 163-172 (1991). G. Zych, L. L. Latta, and W. Mnaymneh, "Preliminary clinical analysis of mechanical performance of the STH titanium alloy total hip replacement," in Titanium Alloys in Surgical Implants, H. A. Luckey and Fred Kubli, Jr. (eds.), American Society for Testing and Materials, Phil- adelphia, 1983, pp. 151-172. A. Belcher, T. P. Schmalzried, and H. C. Amstutz, "Quantitative assessment of activity in joint replacement patients," in Transactions of 40th Annual Meeting of the Orthopaedic Research Society, Vol. 19, 1994, p. 680. S. R. Shepard and N. P. Suh, "The effect of ion implantation on friction and wear of metals," J. Lub. Technol.,

R. G. Vardiman and R. A. Kant, "The improvement of fatigue life in Ti-6A1-4V by ion implantation," J. Appl.

R. A. Buchanan, E. D. Rigney, Jr., and J. M. Williams, "Ion implantation of surgical Ti-6Al-4V for improved resistance to wear-accelerated corrosion," J. Biomed. Mater. Res., 21, 355-366 (1987). H. A. McKellop and T. V. Rostlund, "The wear behavior of ion-implanted Ti-6A1-4V against UHMW polyethylene," J. Biomed. Mater. Res., 24, 1413-1425 (1990).

698,536-541 (1987).

J. Appl. Phys., 25, 576-581 (1954).

104, 29-38 (1982).

Phys., 53, 690-694 (1982).

:eived March 17, 1995 :epted February 7, 1996

in vivo wear of ti6al4v femoral heads: a retrieval study

Documents