Three-dimensional polyethylene wear of a press-fit titanium prosthesis

The Journal of Arthroplasty Vol. 12 No. 3 1997

T h r e e - d i m e n s i o n a l P o l y e t h y l e n e Wear of a Press-fit T i t a n i u m P r o s t h e s i s Factors Influencing Generation of Polyethylene Debris P e t e r A. D e v a n e , M B C h B , J. G e o f f r e y H o r n e , M B C h B , K e r r y n M a r t i n , M B C h B , Geoffrey Coldham, MBChB, and Brett Krause, MBChB

Abstract: Between 1985 and 1990, 108 consecutive Harris-Galante I (Zimmer, Warsaw, IN) total hip arthroplasties were performed by four surgeons at a single hospital. At the time of follow-up evaluation, 80 hips were available for review. The mean rate of linear wear was 0.15 mm/y, mean rate of three-dimensional femoral head displacement was 0.21 rarely, and mean rate of volumetric wear was 121 mm~/y. Eight hips (10%) in this series had radiologic osteolysis around either the femoral or acetabular component. A significantly greater volumetric wear rate was found in patients who were younger, those with a higher activity level, those who received a 32-mm-diameter femoral head, and those with vertical orientation of their acetabular component. No relationship could be made with patient weight, gender, Harris hip score, or cup diameter. Key words: hip, arthroplasty, polyethylene wear, osteolysis, activity level.

Since its first introduction in 1961 by Sir J o h n Charnley [1], total hip arthroplasty (THA) has u n d e r g o n e considerable modifications of design, fixation, and materials [2]. The Harris-Galante I (Zimmer, Warsaw, IN), a titanium cementless prosthesis, was introduced during the early 1980s in an attempt to overcome some of the problems believed to be caused by cemented fixation [3]. Early clinical results of the Harris-Galante prosthesis appeared satisfactory [4,5], but press-fit THAs suffer from the problem of osteolysis, the localized loss of bone stock at the bone-prosthesis interface [6-8]. The reported incidence of osteolysis in the Harris-Galante prosthesis varies from 1% [9] to 29% [10]. Particulate debris from the prosthesis, particularly polyethylene particles from the plastic

liner [11,121 , has been implicated as the major cause of osteolysis [13,14]. With current imaging modalities, only m e a s u r e m e n t of particulate debris generated from the primary bearing surface (articulating surface formed by the femoral head and acetabular cup) is possible f151. Current techniques assessing polyethylene wear depend on measurement of femoral head displacem e n t made from two anteroposterior radiographs [16]. Studies of total hip prostheses use this twodimensional measurement, termed linear wear, to calculate the theoretic volume of debris generated by a change in femoral head position over time [17]. The three-dimensional technique is a new computerized m e t h o d for polyethylene wear measurement w h e r e b y digitized points from anteroposterior and lateral radiographs of metal-backed acetabular components are combined to form a three-dimensional model of the prosthesis in the computer [181. Measurements made using the three dimensional technique include orientation of the acetabular component, two- and three-dimensional

From the Department of Surgery, Wellington School of Medicine, Wellington, New Zealand. Reprint requests: Peter A. Devane, Department of Surgery, Wellington School of Medicine, EO. Box 7343, Wellington, South, New Zealand.

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Polyethylene Wear

m e a s u r e m e n t of femoral head displacement over time, and calculation of the m i n i m u m volume of polyethylene debris generated from the primary bearing surface. The three dimensional technique has a validated accuracy of _+ 0.15 m m for femoral head displacement m e a s u r e m e n t within a metalbacked acetabular component, whereas accuracy of volumetric wear is within 7% of the true volume removed, if a cylindrical wear tract is assumed [19]. The purpose of this study was to measure the three-dimensional rate of polyethylene wear in the Harris-Galante I prosthesis. Correlation was made b e t w e e n volumetric polyethylene wear and various clinical, prosthetic, and radiologic factors such as patient weight, femoral head size, and osteolysis.

Materials and Methods B e t w e e n J a n u a r y 1986 and December I990, 108 consecutive press-fit titanium THAs were performed in 99 patients by four senior surgeons at a single hospital. The Harris-Galante I acetabular and femoral c o m p o n e n t was used in all cases. Inclusion criteria for this study were the availability of goodquality anteroposterior and lateral radiographs taken within 6 weeks of surgery; the presence of an intact Harris-Galante acetabular c o m p o n e n t at the time of follow-up evaluation; and the patient's ability to present for clinical and radiologic assessment. Of the 108 procedures performed, 6 patients (6 hips) had died, there had been 5 revisions of the acetabular component, and 16 patients (17 hips) were ineligible for the study. Of these 16 patients, 8 no longer resided in the area and were not available for clinical or radiologic assessment and 8 were unable to be contacted. Thus, complete clinical and radiologic evaluation was available for 80 hips in 72 patients, all with Harris-Galante THAs. Of these 80 hips, 8 had required a revision of the press-fit HarrisGalante femoral c o m p o n e n t to a cemented femoral stern with a modular cobalt-chrome head of the same size. Wear data for these hips were analyzed separately and compared with those obtained from the other 72 hips. The preoperative diagnosis was osteoarthritis in 58 patients (80%), r h e u m a t o i d arthritis in 10 (14%), and avascular necrosis of the hip in 4 patients (6%). No attempt was made to analyze patients with either r h e u m a t o i d arthritis or avascular necrosis separately because of their relatively small numbers. The Harris-Galante I femoral c o m p o n e n t is manufactured from titanium alloy, has the option of collarless or a medial collar, and comes in multiple

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sizes. Initial stability and e n c o u r a g e m e n t of bone ingrowth are achieved by two commercially pure titanium fibermesh pads attached to the anterior and posterior aspects of the proximal stem. Modular 28- and 32-mm-diameter cobalt-chrome femoral heads, available in three neck lengths and attached to the Morse taper of the femoral component, articulate with the acetabular cup. The hemispheric acetabular shell is coated with titanium fibermesh to obtain initial stability and encourage bony ingrowth. Multiple holes allow supplementary fixation with screws, and a modular liner of ultrahighmolecular-weight polyethylene is inserted after placement of the metal shell. The operation was performed using a posterior approach in 50 patients (69%) and a Hardinge direct lateral approach in 22 patients (31%). The acetabular and femoral c o m p o n e n t s were reamed to the same size as the prosthesis used. All acetabular c o m p o n e n t s had supplementary fixation with two or three titanium screws. Prophylactic antibiotic cover with a second-generation cephalosporin was continued for 48 hours after surgery, and antithromboembolic stockings were used. Postoperative m a n a g e m e n t consisted of 7 to 10 days of hospitalization and 6 weeks of partial weight bearing using crutches. Seventy-two patients (80 hips) were contacted, and a Harris hip score and clinical examination were performed by one person (K.M.). In addition to the Harris hip score, the patient's activity level was graded on a scale of 1 to 5 (Table 1). Standard anteroposterior and lateral radiographs of the patient's hip arthroplasty were taken at that time. Radiographs were assessed by three observers, blinded to each other's results, as to the presence, location, and size of osteolytic areas. For the purposes of this study, radiographic osteolysis was defined as focal well-marginated areas of bone loss occurring at the interface b e t w e e n the bone and the prosthesis. Initial postoperative and final radiographs were digitized using the three-dimensional technique and m e a s u r e m e n t s were made of acetabular orientation, three-dimensional femoral Table 1. Patient Activity Level

Type of Activity Employed in strenuous manual labor plays contact sport or competitive tennis Employed in light job (desk), noncontact sport (golf) or social tennis Leisure activities, gardening, swimming Semisedentary, household chores Sedentary, dependent on outside assistance

Grade 5 4 3 2 1

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The Journal of Arthroplasty Vol. 12 No. 3 April 1997

head displacement, and m i n i m u m v o l u m e of polyethylene produced. Statistical correlation was made between polyethylene wear m e a s u r e m e n t s and various clinical and radiologic features using Student's t-test and analysis of variance (CSS, Statsoft, Tulsa, OK).

Results

Table 3. Direction of Femoral Head Displacement Displacement (mm) Direction

Mean + SD

Medial Superior Forward (n = 18) Back (n = 62)

0.65 _+0.88 0.46 _+0.47 0.48 _+0.56 0.47_+ 0.55

Range

-2.45 to 3.54 -0.62 to 1.97 0.03 to 2.2 0.01 to 2.76

Clinical Results Demographic data for the patients in this study are listed in Table 2. Twenty-two 2 8 - m m - d i a m e t e r femoral heads and fifty-eight 3 2 - m m - d i a m e t e r femoral heads were implanted. The most c o m m o n acetabular cup size used (20 of 80 hips) was 54 m m (range, 4 8 - 6 4 ram). A large variation existed in polyethylene liner thickness (range, 2.36-11.36 mm). The m e a n thickness of polyethylene was calculated as 6.27 m m (SD, 2.18 mm). Forty-seven patients were graded as activity level 3, 4, or 5, and 25 patients were graded as activity level 1 or 2. According to the Harris hip score, 48 hips (60%) had a good or excellent result (80-100 points) and 17 hips (21%) had a fair result (70-80 points). There had been eight revisions of the femoral c o m p o n e n t in eight patients, including one for recurrent dislocation, one for immediate loosening due to undersizing, and six for slowly progressive migration or subsidence causing pain. In all of these patients, the acetabular c o m p o n e n t was judged by the operating surgeon at the time of surgery to be well fixed to the b o n y pelvis and show no signs of macroscopic polyethylene wear.

Three-dimensional m e a s u r e m e n t of femoral head displacement was 0.21 m m / y (range, 0.00-0.54 m m / y ) . Direction of femoral head displacement shows a p r e d o m i n a n c e of head displacem e n t superiorly and medially (Table 3). All direction m e a s u r e m e n t s were made from a standard frontal view of the acetabular cup [18]. A m e a n of 121 m m 3 (range, - i 0 to 372 mm3; SD, 8I mm3/y) of polyethylene debris is produced each year from the primary bearing surface in this prosthesis. None of the eight patients w h o u n d e r w e n t a revision of their femoral c o m p o n e n t had an exchange of the polyethylene liner. The m e a n linear wear rate in this group of patients was 0.18 mm/y, m e a n three-dimensional head displacement rate was 0.26 m m / y and m e a n volumetric wear rate was 114 mm3/y. None of these three m e a s u r e m e n t s was significantly different from rates observed in the other 72 hips in w h i c h the original Harris-Galante femoral c o m p o n e n t remained in situ (Student's t-test). As patients w h o had an isolated revision of the femoral c o m p o n e n t did not have an increased rate of polyethylene wear rate, they were included in the study.

Radiographic Results No acetabular cup s h o w e d radiographic signs of migration or loosening. The m e a n acetabular tilt (measured as degrees from the vertical axis) was 52.5 ° (range, 30.0°-70.8°), and the m e a n anteversion was 10.7 ° (range, -4.2 ° to 37.4°). Two-dimensional linear wear rate measured in the plane of the anteroposterior radiograph was 0.15 m m / y (range, 0.00-0.48 m m / y ) .

Table 2. Patient Demographic Data

Age (y) Weight (kg) Follow-up period (y) Gender

Mean _+SD

Range

54.0 + 1 0 . 1 79.6 _+1 6 . 4 5.78 _+1 . 2 8 40 men

23.7-74.8 49.0-114.0 4.00-8.55 32 women

Fig. 1. Osteolysis around a 4.5-mm titanium cancellous bone screw used to obtain initial stability of the metal shell. All osteolytic lesions around screws (6) in this series were small.

Polyethylene Wear

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Fig. 2. (A) Initial postoperative radiograph of a 55-yearold woman graded as activity level 3. (B) Six years after surgery, resorption of the calcar of the femoral component is clearly seen. The patient had no thigh pain and a Harris hip score of 96 points. Volumetric wear was measured as 741 mm3, giving a volumetric wear rate of 114 mm3/y.

Eight hips had radiologic osteolysis, but in all cases the lesions were less than 2 cm in diameter. Six lesions appeared a r o u n d screws used for fixation of the acetabular cup and two a r o u n d the distal portion of the femoral stem (Fig. 1). Three hips had bone loss a r o u n d the calcar (Fig. 2). As the exact etiology of these calcar lesions is not proven, they were analyzed separately. Three hips were noted to have subsidence of the femoral c o m p o nent on the long-term follow-up anteroposterior radiograph (Fig. 3).

Fig. 3. (A) Initial postoperative radiograph of a 55-yearold woman graded as activity level 2. (B) Eight years after surgery, radiographic subsidence of the femoral component is identified by comparison of the shoulder of the prosthesis with the tip of the greater trochanter. The patient had moderate thigh pain (5 of 10 on a visual analog scale) and a Harris hip score of 85 points. Volumetric wear was measured as 174 mm 3, giving a relatively low volumetric wear rate of 20.4 mm3/y.

Comparative Results There was a highly significant correlation between the duration of follow-up period and volumetric wear, with those patients followed for a longer period of time having a greater volume of polyethylene debris generated than those patients with a shorter follow-up period (Fig. 4A). There was also a statistically significant positive correlation between length of follow-up period and rate of polyethylene wear, with those patients having a longer follow-up period also having a greater wear rate (Fig. 4B).

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The Journal of Arthroplasty Vol. 12 No. 3 April 1997 r=0.478; p=O.O00 Volume = -0.448 + 0.203 * Followup

Fig. 4. (A) Regression analysis of follow-up period in years versus total volumetric wear in cubic centimeters (r = .478). Extremely strong correlation is found (P = .00), suggesting that polyethylene volumetric wear increases with time (volume = -0.448 + 0.203 • follow-up period). (B) Regression analysis of follow-up period in years versus rate of polyethylene wear in cubic centimeters per year (r = .243). Although correlation of polyethylene wear with volume rate is not as strong as with total volume (P = .030) a positive relationship is found. In the Harris-Galante titanium prosthesis, polyethylene wear rate appears to increase with time (volume rate = 0.033 + 0.015 • follow-up period) This may lead to increasing problems of osteolysis with longer follow-up periods.

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Polyethylene Wear

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r=-0.P.59; p=O.O'gO Volume rate = 0.232 - 0.002 * Ale

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rate, there w a s a trend (r = -.128, P -- .256) toward thinner liners being associated with a greater volumetric wear rate than thick liners (Fig. 8). P o l y e t h y l e n e w e a r w a s correlated w i t h orientation of the acetabular cup (tilt and a n t e v e r s i o n ) to a n a l y z e the effect of these variables o n either the direction of femoral head displacement or v o l u metric wear. No significant correlation w a s f o u n d b e t w e e n p o l y e t h y l e n e w e a r and the individual

c o m p o n e n t s (medially, superiorly, or front/back) of femoral head displacement. Acetabular tilt, recorded as degrees from the vertical axis, did, h o w e v e r , s h o w a significant correlation w i t h total femoral head displacement. M o r e vertically oriented cups had a greater m e a s u r e d femoral head displacement (Fig. 9). No correlation w a s f o u n d b e t w e e n acetabular a n t e v e r s i o n and v o l u m e t r i c wear.

r=0.30P.; p=0.008 Volume rate = 0.050 + 0.0?A * Activity

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262

The Journal of Arthroplasty Vol. 12 No. 3 April 1997 .8 220

T Fig. 7. Comparison of polyethylene wear in 28- and 32mm-diameter femoral heads. Linear wear rate was not significantly different in the two groups. Three-dimensional volumetric wear rate in 32-ram-diameter heads was significantly higher than three-dimensional volumetric wear rate in 28-mmdiameter heads (P = .006). Volume, cross-hatched bars; three-dimensional displacement, wide-striped bar; twodimensional displacement, narrow-striped bar.

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Finally, n o correlation could be m a d e b e t w e e n v o l u m e t r i c w e a r a n d osteolysis (8 hips), calcar r e s o r p t i o n (3 hips), or f e m o r a l subsidence (3 hips).

t h a t d e p e n d o n initial stability for p r o m o t i o n of b o n y i n g r o w t h [20]. G o o d initial stability is a c h i e v e d w h e n t h e H a r r i s - G a l a n t e prosthesis is i m p l a n t e d w i t h a d d i t i o n a l s c r e w f i x a t i o n [21]. W h e n r a d i o g r a p h s of t h e H a r r i s - G a l a n t e w e r e analyzed using radiostereophotogrammetry, a v e r y a c c u r a t e t e c h n i q u e for t h e m e a s u r e m e n t of p r o s t h e t i c m i g r a t i o n [221, a l o w i n c i d e n c e of early

Discussion

The s e a r c h for i m p r o v e d m e t h o d s of f i x a t i o n has led to t h e d e v e l o p m e n t of press-fit p r o s t h e s e s

r = - 0 . 0 9 8 ; p=0.384 Volume rate = 0.144 - 0.038 * TJ~er thick

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Fig. 8. Regression analysis of liner thickness in centimeters versus polyethylene volume in cubic centimeters (r = -.098). Although not statistically significant, there was a trend toward less wear with increased polyethylene thickness (P --- .384; volume rate = 0.144 - 0.036 • liner thickness).

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r=-.244; p--.029 Head displaeenzent = 0.230 - 0.002 *

Fig. 9. Regression analysis of acetabular tilt in degrees from the vertical versus three-dimensional femoral head displacement in centimeters (r = -.244). Vertical orientation of the acetabular cup increases femoral head displacement (P = .029; head displacement = 0.230 0.002 • tilt).

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subsidence is seen [23]. This m a y a c c o u n t for the good early and m e d i u m clinical results seen with the Harris-Galante prosthesis [24,25]. No predictors of failure of the Harris-Galante prosthesis have b e e n identified [26,27], but osteolysis at the b o n e - p r o s t h e s i s interface is a c o m m o n finding, occurring at a m e a n of 39 m o n t h s in a series of 20 hips f o u n d to have femoral cortical osteolysis [7[. Eighty-six p e r c e n t of hips in that series had radiologic progression, and t h r e e of these c o m p o n e n t s had b e e n j u d g e d radiographically stable w h e n the endosteal erosion had first b e e n identified. There is n o w o v e r w h e l m i n g e v i d e n c e that particulate material g e n e r a t e d from the prosthesis, particularly p o l y e t h y l e n e debris g e n e r a t e d from the articulating surface, is responsible for osteolysis [28]. M e a s u r e m e n t of femoral head displacement from a single anteroposterior radiograph, initially developed by Charnley et al, [29,30], has a reported accuracy of _+ 0.5 mm. Modifications of this linear wear m e a s u r e m e n t technique have been made by various authors [16,31], but to our knowledge, m e a s u r e m e n t of linear wear in the Harris-Galante prosthesis has not been reported. Application of the three-dimensional technique to a series of 14l Porous Coated Anatomic (PCA, Howmedica, Rutherford, NJ) THAs revealed a m e a n three-dimensional femoral head displacem e n t of 0.264 m m / y and a m e a n volumetric wear rate of 97 mm3/y [32]. There was a strong correla-

tion b e t w e e n increased volumetric wear of the PCA prosthesis and osteolysis. The m e a n three-dimensional femoral head displacement rate of the Harris-Galante prostheses was 0.21 m m / y and the m e a n volumetric wear was 121 mm/y. Despite the rate of femoral head displacement being slightly less in the titanium Harris-Galante prosthesis compared with the vitallium PCA prosthesis [32], the rate of volumetric wear is higher at 121 mm3/y, as direction of wear in the Harris-Galante prosthesis is m o r e medial than superior, creating a larger v o l u m e of polyethylene debris (Table 3). Those patients receiving a Harris-Galante prosthesis followed for a longer period not only had a greater total volumetric wear; they had a significantly greater volumetric wear rate, suggesting that volumetric wear of the titanium Harris-Galante prosthesis accelerates with time. This finding was not evident with the PCA prosthesis, w h e r e wear rate was completely i n d e p e n d e n t of follow-up period [32]. One possible explanation is that third-body wear from titanium interposed b e t w e e n the two sides of the bearing surface plays an important role in accelerating polyethylene wear in the Harris-Galante prosthesis. This series is not an attempt to analyze survivorship or clinical results of the Harris-Galante prostheses, which are already d o c u m e n t e d [4,24,25], but rather to measure and correlate t h r e e - d i m e n sional polyethylene wear of an intact press-fit titanium acetabular c o m p o n e n t . A relationship

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b e t w e e n polyethylene wear of the Harris-Galante prosthesis and gender, weight, or Harris hip score could not be established, a finding observed in wear analysis of other prostheses [33-36]. As found in other studies [37,381, y o u n g e r patients have a higher rate of volumetric wear. In a retrospective study of 79 patients followed for 10.3 years [39], patient activity level has been s h o w n to be related to polyethylene wear. Our study supports the finding that increased activity of patients leads to increased polyethylene wear. Livermore et al. found that 3 2 - m m femoral heads generated a greater v o l u m e of polyethylene debris, they r e c o m m e n d e d use of 26- or 28-mmdiameter femoral heads as an alternative [i6]. Our study agrees with this finding, and we no longer use 32-ram femoral heads. Anteversion of the acetabular cup did not appear to influence polyethylene wear in any specific direction, but this m a y be due to our inability to measure anteversion of the femoral c o m p o n e n t in the femur. Orientation of the femoral c o m p o n e n t m a y have at least as m u c h influence on wear in the front-to-back direction of the acetabular liner as does anteversion of the acetabular cup. Increasing tilt (angle in degrees from the vertical axis) of the cup, however, does correlate with a decrease in femoral head displacement. There are conflicting reports of this association, with Sarmiento et al, reporting that increasing tilt leads to decreased cup containment, which in turn leads to increased wear {40]. Other authors, however, report that increasing tilt causes less wear [41,42]. Although we found that a vertical cup correlated with increased femoral head displacement, this m a y be due to the effect of creep. Vertical orientation of the acetabular cup uncovers the femoral head laterally, causing a concent,ration of stress on the polyethylene in the superior portion of the acetabular c o m p o n e n t . Over time, this m a y cause creep of the acetabular liner in a superior direction. The three-dimensional technique is unable to distinguish b e t w e e n polyethylene wear (removal of material from the prosthesis in the form of particulate debris) and polyethylene creep (plastic deformation of the liner), and interprets all creep as polyethylene wear, giving an artificially high estimate of the a m o u n t of polyethylene debris generated. For this reason, acetabular tilt is correlated with femoral head displacement rather t h a n volumetric wear. Although polyethylene wear is one cause of calcar resorption [43], other factors such as stress shielding [44], c o m p o n e n t subsidence [45], and periosteal soft tissue stripping also play a role,

explaining w h y no correlation was f o u n d b e t w e e n polyethylene wear and calcar resorption. A low incidence (10%) of osteolysis was found in this series, and no massive lesions (> 2 cm) were found. Unlike for the vitallium PCA prosthesis [32], no correlation was observed b e t w e e n polyethylene wear of the titanium Harris-Galante prosthesis and osteolysis. It is possible that particulate debris from sources other than the primary bearing surface (which cannot be measured) plays a greater role in osteolysis a r o u n d a titanium prosthesis, explaining the lack of correlation b e t w e e n wear rate and osteolysis in the Harris-Galante prosthesis. These other sources m a y include the Morse taper, w h e r e it is established that a higher rate of corrosion exists w h e n mixed-alloy c o m p o n e n t s (cobalt alloy heads on titanium alloy stems) are used [461, and the interface b e t w e e n the metal shell and the polyethylene liner. The effect of polyethylene liner m o v e m e n t in an unpolished shell with multiple screw holes both filled and unfilled with screws m a y increase debris production, and the locking mechanism of the Harris-Galante acetabular comp o n e n t has since been modified. This u n m e a s u r e d debris may also cause increased third-body wear, explaining the higher wear rate seen in a titanium prosthesis compared with the wear rate of a vitallium prosthesis.

Conclusion We found a measured rate of volumetric polyethylene wear in the Harris-Galante prosthesis of 121 mm3/y. Increased wear was seen in y o u n g e r patients and those with a higher activity level. Rate of wear appeared to increase with follow-up period. Thirty-two-millimeter-diameter femoral heads and vertical orientation of the acetabular cup increased polyethylene wear. The incidence of osteolysis was low (10%), with no massive lesions observed. No correlation b e t w e e n patients with osteolysis and polyethylene wear was seen.

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