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Clin Orthop Relat Res. 2009 November; 467(11): 2872–2879.
Published online 2009 March 28. doi:  10.1007/s11999-009-0796-8
PMCID: PMC2758968

Inferior Survival of Hydroxyapatite versus Titanium-coated Cups at 15 Years

Maiken Stilling, MD,corresponding author Ole Rahbek, MD, PhD, and Kjeld Søballe, MD, DMSc

Abstract

Hydroxyapatite (HA) particles have long been suspected to disintegrate from implant surfaces, become entrapped in joint spaces of orthopaedic bearing couples, and start a cascade leading to progressive polyethylene (PE) wear, increased osteolysis, and aseptic loosening. We compared cup revision at 15 years’ followup in a randomized group of patients with 26 cementless THA components with titanium (Ti) versus first-generation HA coating. We also assessed radiographic PE wear and osteolysis to the 12-year followup or end point revision at a minimum of 5 years (mean, 10.9 years; range, 5–12.6 years). Two Ti-coated cups (17%) and eight HA-coated cups (57%) were revised at 15 years’ followup. Femoral head penetration rate was 0.46 mm/year (standard deviation, 0.26) with the HA-coated cups (n = 12) and 0.38 mm/year (standard deviation, 0.14) with the Ti-coated cups (n = 10); we observed a wide variance of linear wear with the HA-coated cups. We also observed a positive association between high wear rate and revision, and between a high volume of osteolysis and revision. Our findings suggest inferior survival of medium-thickness spray-dried HA-coated cups with individual cases of excessive PE wear and premature cup failure. These findings apply to first-generation modular cups and may not apply to other cup designs and new HA-coating technologies.

Level of Evidence: Level III, therapeutic study. See Guidelines for Authors for a complete description of levels of evidence.

Introduction

Wear of the PE component is widely regarded as the primary factor limiting longevity of THA [16]. If the origins of premature or accelerated wear can be identified, then it may be possible to alter or improve the design and enhance performance and survival of THAs [27]. HA coatings reportedly provide bony ingrowth and improved initial implant stability [41, 44]. Rahbek et al. suggested enhanced fixation and reduction in radiolucencies are related to a superior sealing effect of the HA coating reducing wear particle migration into the bone-implant interface [37, 38]. The sealing effect of HA coating is believed to reduce macrophage-induced osteolysis and prolong the lifespan of uncemented implants [14, 38, 41].

Reliable clinical fixation, osseointegration, and longer-term revision rates less than 1% for HA-coated femoral components have been reported [12, 13, 30, 36, 39, 47]. Two studies of HA-coated versus nonHA-coated acetabular components showed equal or improved fixation and reduced periprosthetic radiolucencies [14, 41], but reports of PE wear rates reveal values higher than expected (range, 0.15–0.32 mm/year) with HA-coated cups and medium-term revision rates between 13% and 40% [7, 13, 21, 29, 40]. In some situations, such as those in which the HA coating was applied directly to a smooth implant surface and subsequently flaked off, high rates of early failure might be readily explained [13, 29]. The major overall concern is disintegration of the HA coating in vivo, resulting in loss of fixation, formation of particulate HA debris, and abrasive third-body wear between the articulating surfaces of the prosthetic components [3, 48]. Supporting this concern, HA particles have been detected on the PE surface of retrieved components [4, 9], and loose HA particles may increase production of excessive PE particles, leading to increased PE wear and liner revision, premature periprosthetic osteolysis, and aseptic implant loosening [4, 33]. In general, release of HA particles from the implant surface could generate a clinical problem with few, if any, early warning signs [6, 33].

We therefore asked (1) whether there was a difference in cup survival rates with or without HA coating, and whether any difference in survival rates was associated with (2) the amount and rate of wear and (3) the amount of osteolysis.

Materials and Methods

In 1991 and 1992, we prospectively enrolled 27 patients (28 hips) scheduled for primary THA and randomly allocated them to receive either a Ti- (n = 13) or an HA-coated (n = 15) implant for a femoral stem migration study [47]; one patient had bilateral surgery with one Ti hip and one HA hip. A followup of those patients is reported here. The study was approved by the local ethical committee and informed consent was obtained from all participants. We included patients with osteoarthritis of the hip and age older than 18 years and younger than 67 years, and excluded patients with congenital hip disorders, osteoporosis (ie, those under medical treatment), metabolic bone disorders, rheumatoid arthritis, malignant disease, and femoral neck fractures. All patients meeting the inclusion and exclusion criteria were offered participation in the migration study until allocation of 28 hips was reached. No patient offered enrollment refused to participate. All components (femoral stems and acetabular cups) were similar except for the coating (which for a given patient was the same for the stem and the cup). One group had implants with porous Ti coating with HA and the other group had implants with a similar porous Ti coating without HA. A sample size calculation based on an expected clinically relevant difference of 40% in revision between the groups suggested 12 patients per group with a power of 49 (alpha = 0.05). The difference in proportion was based on the existing literature of similar hemispheric cups with longer-term followup [26, 29]. Distributions of gender, age, weight, cup size, liner thickness, average followup, and hip side were similar between the patient groups (Table 1). The patients were randomized on the day of surgery. Three patients from the original group of 27 patients (28 hips) were lost in each group (Fig. 1). One patient who entered the study with bilateral surgery (one Ti hip and one HA hip), died 1 year after surgery of causes unrelated to the THA. According to patient records, neither of the patient’s THAs was revised and the patient was excluded from analysis in this study (survival, wear, osteolysis). Twenty-six patients remained for determination of survival based on revision at 15 years. Radiographs of 22 of the 26 patients were available to measure linear wear (accessible postoperative and followup radiographs), and 25 patients had radiographs (last followup) available to quantify osteolysis (Fig. 1). The minimum duration of radiographic followup was 5 years (mean, 10.9 years; range, 5.0–12.6 years) for the HA group and 5.6 years (mean, 11.1 years; range, 5.6–12.5 years) for the Ti group. The patients who did not have revision surgery or were deceased at 12 years’ followup did have a standard radiographic followup. We assessed the rate of revision THAs among the 26 patients (26 hips) in the groups by reading through patient records where a description of the revision surgery was noted. The information for all 26 patients was crosschecked with the Danish Hip Arthroplasty Registry, a nationwide clinical database on primary THAs, revisions, and postoperative complications in patients in Denmark since the beginning of 1995. The patients’ information in the registry was the same as in our records.

Table 1
Patient demographics (mean, range)
Fig. 1
The diagram shows the patients available for evaluation of revision, PE wear, and osteolysis in the two groups; the numbers and reasons for drop outs; and the time and reasons for cup revision (HA = Hydroxyapatite; Ti = Titanium; ...

One surgeon (CB) performed all operations by the posterolateral surgical approach. The cups were fixed with two to three titanium screws. The femoral component was a solid Ti6A14 V alloy core Universal® Bi-Metric® design (Biomet Inc, Warsaw, IN) with a collarless straight stem and a circumferential Ti plasma spray porous coating to the proximal quarter. The acetabular metal shell was a hemispheric Ti plasma spray porous Universal® design (Biomet Inc) with a rim flair, holes for optional bone screw supplemental fixation, and a Hexloc® locking mechanism (Biomet Inc) for the liner (Fig. 2). The femoral heads (Biomet Inc) were 28-mm chrome-cobalt alloy. The liner was 10° face GUR 415 bar extruded ultrahigh molecular weight PE (UHMWPE) in all cases. A postextrusion thermal cycle (annealing), known to slightly increase crystallinity, density, and rigidity, was conducted before machining to maintain the PE bar in stable shape. To reduce oxidation, the PE was packaged in an inert argon environment. The overall mechanical properties for this PE reportedly are superior (Table 2) to the recommended standard of the American Society of Testing and Materials [2]. We used six different PE thicknesses (range, 3.39–11.46 mm). The Ti-coated components had a pore size of 300 μm. The HA-coated components had a similar Ti porous coating with an additional 50- to 75-μm layer of spray-dried synthetic HA deposited by plasma spraying. The crystallinity was 90%. The surface roughness (Ra) was 41 μm for the HA components and 47 μm for the Ti components [47].

Fig. 2
A Universal® acetabular metal shell with plasma porous spray coating and Hexloc® locking mechanism for the PE liner are shown. (Photograph courtesy of and reprinted with permission of Biomet Denmark ApS, Horsens, Denmark.)
Table 2
Properties of UHMWPE*

We collected all available hard-copy postoperative anteroposterior (AP) radiographs and latest followup radiographs. Intermediate followup radiographs at 3 months, 6 months, 1 year, and 5 years also were available for 22 patients with adequate followup. The postoperative (baseline) radiographs were obtained after partial weightbearing and mobilization within a few days after surgery. Five to six AP radiographs were available for all patients. The radiographs were digitized with a high-resolution optical scanner (Mustek® P3600 A3 pro, Mustek Systems, Inc, Irvine, CA) to 100% scale and 300 dpi. Cross-table lateral radiographs also were available, but we chose not to include them for analysis owing to their lesser quality. Femoral head penetration was measured with a computer-assisted method (PolyWare™ Digital Version 5.10; Draftware Developers, Inc, Vevay, IN) [20] on AP radiographs of the hip. This shadow-casting method [15] uses a digital edge detection algorithm to fit circles to the peripheries of the femoral head and acetabular cup. Based on some assumptions this computer-assisted technology applies over the radiograph a three-dimensional solid model of the acetabular component and femoral head based on back-projection of the radiographs [19]. Linear wear was measured in the plane of the AP radiograph by the software with an accuracy of approximately 0.15 mm [19].

The quality of the digitized AP radiographs generally was good and we rarely manually overruled the automatic circle fitting. PE wear analysis was performed retrospectively in intervals among the six followups by one observer (CS), who was blinded to the groups. Precision of wear measurement analysis (intraobserver error) with the computer-assisted method was evaluated for the one observer (CS) by double analysis 1 month apart for 11 patients who had 12 years of followup. The mean absolute difference with the first and second analyses was 0.08 mm (standard deviation [SD], 0.28 mm).

Radiographic assessment of osteolysis was performed blinded by an experienced orthopaedic consultant (KS) with a subspecialty in hip arthroplasty. Twenty-five end-point radiographs were available for evaluation of osteolysis. Osteolytic zones were marked by the surgeon in India ink directly on the radiograph (Fig. 3). The radiographs then were digitized again and osteolysis quantified by a function in the computer-assisted method (PolyWare™) developed by Devane et al. [20]. The osteolytic areas traced on the AP radiograph were automatically scaled for the known size of the femoral head and converted to a volumetric measurement (cubic millimeters). The software (PolyWare™) used the assumption that the lesion was the same size on the AP and the cross-table lateral radiographs. Precision of quantitative osteolysis with the computer-assisted method also was evaluated for one observer (LLA) by double analysis 1 month apart for 25 patients. The mean absolute difference with the first and second analyses was 44.8 mm3 (SD, 62.89 mm3).

Fig. 3
A 12-year followup AP radiograph shows a large osteolytic lesion in the acetabulum marked in India ink. PE wear is evident because the femoral head is decentralized. Clinically, the patient had pain and discomfort. Revision surgery revealed a loose cup, ...

We compared survival rates of the cups by a log-rank test at 15 years’ followup. According to a Shapiro-Wilk test [1], head penetration and wear rates were normally distributed when converted to log scale. Similarities or differences in variances of the log scale wear data for the two groups were tested by an F test. The log mean values of the two groups were compared by a two-sample t test with unequal variances (head penetration) and a two-sample t test with equal variance (wear rates). Head penetration (mm) of the longest followup per patient (minimum 5 years, maximum 12.6 years) and head penetration rates (mm/year) are presented on a normal scale for interpretational reasons. Mean osteolysis between the groups was compared by Mann-Whitney U-test. The associations between wear rate and revision and between osteolysis and revision also were assessed by a Mann-Whitney U-test. The Intercooled Stata® 9.0 (StataCorp LP, College Station, TX) package was used for statistical computations.

Results

At last followup, more (p = 0.045) HA cups had been revised than Ti cups: eight of 14 HA cups (57%) and two of 12 Ti cups (17%) (Fig. 4). At revision surgery, aseptic loosening, massive acetabular osteolysis, and metallosis were clinically evident for all revised cups, except for one Ti cup that was revised after the patient experienced a traumatic fall at 5.6 years. All of the cups (Ti and HA) revised within the 12-year followup had wear rates greater than 0.4 mm/year or osteolysis greater than 25,000 mm3 (Fig. 5).

Fig. 4
A Kaplan-Meier survival graph shows poor survival (p = 0.045) of the HA-coated cup (n = 14) compared with the Ti-coated cup (n = 12).
Fig. 5
The graph shows an association between revised cups (solid symbols) and a wear rate greater than 0.4 mm/year (dotted horizontal line) or osteolysis greater than 25,000 mm3 (dotted vertical line). The one Ti cup (open triangle) with a wear ...

The head penetration rate for the HA group at a mean of 10.6 years (0.46 mm/year; SD, 0.26; range, 0.16–0.90 mm/year) was similar to (p = 0.33) that of the Ti group (0.38 mm/year; SD, 0.14; range, 0.20–0.72 mm/year) at a mean of 11.1 years. Mean head penetration in the HA group at a mean of 10.6 years (4.8 mm; SD, 2.6; range, 1.97–10.56 mm) also was similar to (p = 0.25) that of the Ti group (3.8 mm; SD, 0.9; range, 2.51–5.36 mm) at a mean of 11.1 years. Wear of the PE continued in both study groups throughout the followup (Fig. 6). During the first 6 months, the PE wear was quite large in both groups, illustrating a combination of wear and bedding-in. After 1 year, linear head penetration continued in both study groups but at a less steep curve. Reflected by variance, the distribution of wear was wider (p = 0.017) in the HA group (SD, 2.6; range, 1.97–10.56 mm) than in the Ti group (SD, 0.9; range, 2.51–5.36 mm). This indicates the HA coating affected femoral head penetration with some of the investigated components because some patients in the group had large head penetration with subsequent component failure and revision whereas other patients had low head penetration and normal component longevity. We observed a greater (p = 0.0001) wear rate in patients who had revision surgery than in patients who did not have revision surgery.

Fig. 6
The graph shows a mean linear PE wear throughout the 12-year followup period in the HA and Ti groups. The steep wear curve during the first year of followup is a combination of wear and creep.

We observed a higher (p = 0.003) volume of osteolysis in patients who had revision surgery. Osteolytic lesions (Fig. 3) were visible on the plain radiographs in all 25 patients at the latest available followup. The mean measure of osteolysis was 9320 mm3 (SD, 8838; range, 178–29,028 mm3) in the HA group and 7531 mm3 (SD, 10,915; range, 1091–34,698 mm3) in the Ti group (p = 0.30).

Discussion

The osteoconductive properties of HA-coated titanium implants are well documented [46]. However, theories of coating degradation resulting in particulate HA-induced third-body wear [5, 8, 33] have long been discussed. This study was initiated to evaluate the potential disadvantages of HA-coated implants for THA including (1) survival, (2) increased wear, and (3) increased osteolysis.

Limitations of the study include the small number of patients in this randomized patient group, which originally was powered for radiostereometric migration analysis of the femoral stems [47], and seemingly the small sample size likely explains the lack of statistical significance with comparison of wear and osteolysis (ie, a Type 2 error). Although the groups were randomized, there were an equal number of each gender in the HA group, but more females than males in the Ti group, and this potentially could have contributed to a higher wear rate in the HA group. Hard-copy radiographs of four patients (two with Ti cups, two with HA cups) were lost which could introduce some selection bias. We compared the mean wear of the two groups from the postoperative baseline until the last radiograph available (revision, death, or the 12-year followup) at a minimum of 5 years. It would be desirable to compare all patients at the same and longest followup time; however, excluding revisions from a wear analysis potentially removes the worst cases of wear and thus we chose to include all available patients to the last available radiograph. The level of activity was not controlled for and this could have influenced wear. The computerized methods for plain radiographs are not as accurate as retrieval analysis or radiostereometric analysis and limitations have been described [23]. However, the method used in this study was validated for clinical evaluation of longer-term followups and series of high wear [27]. We only used AP images, as the cross-table lateral radiographs were of varying projection and quality, and thus we consciously restricted wear analysis to two-dimensional results. Two-dimensional and three-dimensional measurements of femoral head penetration in the PE liner correlate well, and for most patients (95%) PE wear can be estimated from AP radiographs alone [50].

We report high 15-year cup revision rates of 57% and 17% for HA- and Ti-coated cups, respectively (Fig. 4). A recent study from the Finnish Arthroplasty Registry suggests the most common reason for revision of the Universal® cup (combined data for the Ti- and HA-coated cups) is wear leading to liner exchange, with a 13-year revision rate of 26% [24]. Other studies of the same acetabular component reveal revision rates between 13% and 26% at means of 7 and 10 years followup [28, 32]. Numerous authors have described a larger main group of well-performing cups with a smaller group of cup failures attributable to osteolysis, wear, aseptic loosening, and PE fracture (Table 3). Apparently, there is no clear association between worn failing implants and the clinical performance [29, 33], and thus expansile (cystic) osteolysis and loosening may progress without warning signs [5, 33, 42].

Table 3
Studies of uncemented HA coated hemispherical acetabular components

The higher variation of femoral head penetration in the HA group as opposed to the Ti group (SD 0.26 mm/year and 0.14 mm/year, respectively) supports the concerns expressed in other studies with individual cases of high PE wear with HA-coated cups leading to implant failure [7, 8, 11, 21, 29, 42]. The cases of high PE wear in our study could be related to HA particles separating from the coating leading to increased backside wear and articulate wear on the PE, a mechanism described by Bauer, Morscher et al., and Rokkum et al. [4, 33, 43]. Disintegration of the HA coating is influenced by numerous factors, including the purity of HA, manufacturing, crystallinity, porosity, method of coating, the thickness of the HA coating, the surface on which HA is deposited (smooth or rough), the adhesive strength of HA to the substrate [33], and the speed of resorption [8, 35, 45]. Calcium phosphate coatings have been applied plasma-sprayed with different quality and coating thicknesses ranging from 300 μm [33, 43], over 150 μm [42], to 40 μm [41]. Thinner plasma-sprayed HA coatings (50 μm) have been recommended, as they give a stronger fixation and reduce the risk of HA fracture [44]. Today plasma-sprayed HA deposition has been replaced by much thinner (approximately 5 μm) and electrochemically deposited HA coatings that ensure even distribution and quick resorption [17], but medium- and long-term results are yet to be seen.

The femoral head penetration rates of the Ti and HA groups are high compared with other studies (Table 3) reporting wear of intermediate or long-term followup. The majority of time-dependent deformation (creep or bedding-in) occurs within the first postoperative year [26, 49]. We chose to include the first year because wear along with creep does occur in this period, and possibly the amount and speed of creep are different with different PEs and sterilization methods. Another explanation for the high wear we see is the inferior quality of the UHMWPE liner in combination with a poor PE locking mechanism (Hexloc®, Biomet Inc) [10]. Also, the screw hole-designed acetabular cups introduce an exit for PE wear debris directly to the bone and an entrance for HA particles to accelerate backside wear [21] that cannot be distinguished from articulate wear [31].

An osteolysis threshold of a PE wear rate of 0.1 mm/year has been described [22], and numerous reports of wear with HA-coated cups (Table 3) have much higher annual wear rates. We found a femoral head penetration rate greater than 0.4 mm/year was associated with cup failure and revision. PE particles liberated in the wear process induce a biologic reaction, resulting in osteolysis [16]. Massive osteolysis leads to loosening and we saw a tendency for cups with osteolysis greater than 25,000 mm3 to become loose and necessitate revision (Fig. 5).

A midterm RSA study reported similar fixation of a press-fit hemispherical porous-coated titanium alloy cup with and without HA-coating [41], but studies reporting wear, osteolysis, and revision reveal a high failure rate with various HA-coated cups (Table 3). For some cups, there is a plausible explanation of failure, eg, in cases of HA coating applied to smooth surfaces [13, 29, 42], very thick coatings [33, 42], the presence of screw holes [7, 25], a poor locking mechanism [32], and conventional PE. However, for other HA-coated cup designs, the reasons for the wear rate, osteolysis, and revision are not entirely transparent [7, 21, 25, 34] and raise concern regarding continued use of HA coating for acetabular cups. Generalizability of our results based on discontinued components is unclear. We therefore suggest further investigations to clarify the performance (survival, wear, osteolysis) of newer HA coating technologies, such as thinner HA coatings and electrochemical HA coatings, cross-linked PEs, and newer implant designs. We advise close followup of patients with HA-coated cups similar to those used in our study.

Acknowledgments

We thank Professor Cody Bünger for performing all surgeries in the 1990s, mechanical engineer Claus Stilling for independently analyzing linear wear measurements, and biomedical scientist Lone Løvgren Andersen for electronic registration of the osteolytic lesions.

Footnotes

Each author certifies that he or she has no commercial associations (eg, consultancies, stock ownership, equity interest, patent/licensing arrangements, etc) that might pose a conflict of interest in connection with the submitted article.

Each author certifies that his or her institution has approved the human protocol for this investigation, that all investigations were conducted in conformity with ethical principles of research, and that informed consent for participation in the study was obtained.

This study was performed at Aarhus University Hospital.

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