JBJS | Article

The Journal of Bone & Joint Surgery, Volume 83, Issue 6

Articles | June 01, 2001

Hydroxyapatite-Coated Acetabular Components : Histological and Histomorphometric Analysis of Six Cups Retrieved at Autopsy Between Three and Seven Years After Successful Implantation

Alfons Tonino, MD, PhD; Cees Oosterbos, MD; Ali Rahmy, MD; Michel Thèrin, MD; Christina Doyle, PhD

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Investigation performed at the Department of Orthopaedics, De Wever Hospital, Heerlen, The Netherlands
Alfons Tonino, MD, PhD
Cees Oosterbos, MD
Ali Rahmy, MD
Department of Orthopaedics, De Wever Hospital, PO Box 4446, 6401 CX Heerlen, The Netherlands. E-mail address for A. Tonino: a.tonino@inter.nl.net

Michel Thèrin, MD
R & D Department, Sofradim, 116, Avenue du Formans, 01600 Trévoux, France. E-mail address: m.therin@sofradim.com

Christina Doyle, PhD
Howmedica International, Ash House, Fairfield Avenue, Staines, Middlesex TW18 4AN, England
One or more of the authors has received or will receive benefits for personal or professional use from a commercial party related directly or indirectly to the subject of this article. In addition, benefits have been or will be directed to a research fund, foundation, educational institution, or other nonprofit organization with which one or more of the authors is associated. No funds were received in support of this study.

A commentary is available with the electronic versions of this article, on our web site (www.jbjs.org) and on our CD-ROM (call 781-449-9780, ext. 140, to order).

The Journal of Bone & Joint Surgery. 2001; 83:817-825

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Abstract

Background: Important questions remain regarding the use of hydroxyapatite-coated acetabular components in total hip arthroplasty. What is the relation of resorption of the hydroxyapatite coating to enduring fixation? Will unresorbed or dislodged hydroxyapatite particles cause adverse tissue reactions? Retrieval studies of clinically well-functioning acetabular components should help to answer these questions.

Methods: We examined six clinically successful hydroxyapatite-coated cementless acetabular components that were retrieved at autopsy between 3.3 and 6.6 years after implantation. All components were of the same design. The prostheses and the surrounding bone were prepared for qualitative histological and quantitative histomorphometric analysis. The percentage of bone growth onto the implant, the relative bone area around the implant, the extent of residual hydroxyapatite coating, and the coating thickness were measured.

Results: All of the cups showed bone ongrowth, with a mean bone-implant contact (and standard deviation) of 36.5% ± 13.5%. The contact area was the same in all three zones delineated by DeLee and Charnley. The extent and thickness of the hydroxyapatite layer were much reduced in the specimens from older patients and in those associated with a longer duration of implantation. Degradation of the hydroxyapatite coating by osteoclasts was observed. We did not observe loose hydroxyapatite granules far from the coating, nor did we note any adverse tissue reaction to these granules. In contrast, polyethylene debris was noted in approximately half of the empty screw-holes.

Conclusions: Cell-mediated hydroxyapatite resorption seems to be the main reason for loss of hydroxyapatite coating. The area of bone ongrowth was within a certain range (20% to 50%) of the measured surfaces, and it was independent of the amount of hydroxyapatite residue. The hydroxyapatite coating showed a slow rate of resorption with time, without any adverse tissue reactions.

Figures in this Article

Introduction

Introduction | Materials and Methods | Results | Discussion | References

The primary fixation mode of cementless acetabular components is mechanical and is dependent on a physical interlock between the cup and the reamed acetabulum. Secondary fixation is biological and is achieved with osseointegration at the implant-bone interface by means of bone growth onto or into the substrate. The fixation surface of cementless metal-backed sockets typically consists of either a porous coating of beads or fiber metal, a titanium plasma-sprayed surface, various sintered surface textures, or a bioactive ceramic coating such as hydroxyapatite or tricalcium phosphate. When hydroxyapatite is used as an intermediary, bone apposition occurs without the formation of a fibrous interface^1-6. For long-term stability, it is essential that this direct bond between the implant and the bone be maintained, even after complete resorption of the hydroxyapatite. The results of recent clinical studies have suggested that hydroxyapatite-coated implants can provide long-term stability^7-10. Histomorphometric analysis of hydroxyapatite-coated femoral stems has shown that the percentage of bone ongrowth was almost constant, regardless of the amount of hydroxyapatite residue¹¹.

The purpose of the present study was to document the extent and pattern of bone apposition associated with six clinically successful hydroxyapatite-coated acetabular cups that were retrieved at autopsy between three and seven years after insertion. In addition, the tissue reactions to titanium, polyethylene, and hydroxyapatite particles were studied.

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Fig. 1:: Hemispherical hydroxyapatite-coated cup with twelve screw-holes. Two spikes are applied prior to cup insertion.

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Fig. 2:: Case 6. The retrieved acetabulum and cup. Three segments were cut, corresponding to the three zones delineated by DeLee and Charnley. Five sections were cut from each segment for qualitative histological and quantitative histomorphometric analysis.

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Fig. 3:: Case 6. One of the five sections of the cup, corresponding to zone II of DeLee and Charnley. The histomorphometric analysis was performed on four areas for each section. Areas 34 through 37 are shown. The central hole to the right of area 35 is covered by a dense collagenous membrane (stained blue). The screw-hole to the left of area 35 appears to be covered by a thin bridge of bone that may have inhibited the migration of polyethylene particles and granulomatous tissue.

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Fig. 4-A:: Figs. 4-A and 4-B Numerous bridges of lamellar trabecular bone (b) are seen directed perpendicular to the substrate (s). The bone marrow (bm) appears normal, and the hydroxyapatite coating (ha) is thicker where it is covered with bone (¥28). Fig. 4-A Case 1. Zone I of DeLee and Charnley.

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Fig. 4-B:: Case 4. Zone II of DeLee and Charnley.

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Fig. 5:: Case 5. Zone II of DeLee and Charnley. Even after nearly total hydroxyapatite resorption, bone tissue (b) is focally noted directly on the surface of the metal substrate (s). The bone marrow (bm), which appears normal, is also seen directly on the substrate (¥28).

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Fig. 6:: Case 2. Zone I of DeLee and Charnley. Actual degradation of the hydroxyapatite coating (ha) by osteoclasts (arrows) is visible. The hydroxyapatite coating is thick and regular where it is in contact with bone (b). The bone marrow (bm) appears normal (¥45).

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Fig. 7:: Case 4. Zone I of DeLee and Charnley. The proximal screw-hole (a) shows cystic osteolysis, while the distal screw-hole (b) is closed by an osseous bridge containing the polyethylene particles.

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Fig. 8:: Case 3. Zone I of DeLee and Charnley. The granulation tissue (G) is induced by polyethylene particles. The large areas of bone resorption are slowly producing debonding of the cup.

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TABLE I: Data on the Six Cases

*THA = total hip arthroplasty. †HA = hydroxyapatite. ‡+++ = abundant particles, ++ = many particles, + = few particles, and — = no particles. §PE = polyethylene. (Adapted from: Tonino AJ, Thèrin M, Doyle C: Hydroxyapatite-coated femoral stems. Histology and histomorphometry around five components retrieved at post mortem. J Bone Joint Surg Br. 1999;81:148-54. Reprinted with permission.)

	Case 1	Case 2	Case 3	Case 4	Case 5	Case 6
Gender, age at op. (yr)	F, 63	M, 65	F, 81	M, 58	F, 66	F, 86
Diagnosis	Osteoarthritis	Osteoarthritis	Osteoarthritis	Osteoarthritis	Osteoarthritis	Osteoarthritis
Duration of implantation (yr)	3.3	5.2	5.4	5.7	6.2	6.6
Weight (kg)	85	65	52	50	75	65
Height (m)	1.39	1.69	1.60	1.52	1.63	1.60
Merle d’Aubigné score³⁴(points)	18	18	18	15	18	18
Harris hip score³⁵(points)	100	100	97	90	100	95
Cause of death	Cardiac arrest	Cerebral hemorrhage	Cardiac arrest	Cardiac arrest	Aortic aneurysm	Pancreatitis
Previous operation	—	Femoral osteotomy	—	Contralateral THA*	—	—
Clinical notes	Not active	Active	Active	Not active, Down syndrome	Active	Active
Alignment of cup	45Â°	49Â°	45Â°	55Â°	45Â°	45Â°
Alignment of stem	Neutral	Neutral	4Â° valgus	Neutral	2Â° varus	Neutral
Femoral fill	Complete	Almost complete	Poor	Complete	Poor	Poor
Bone ongrowth (%)/extent of residual HA coating† (%)¹²
Zone I	47/28	36/19	?9/3	60/12	41/7	40/0
Zone II	25/24	32/17	23/5	50/12	35/1	39/0
Zone III	59/26	38/24	27/3	38/14	40/0	14/0
Mean	44/26	35/20	21/4	49/13	39/3	31/0
Bone area (%)/thickness of residual HA coating† (m)¹²
Zone I	27/34	26/27	?6/16	20/32	22/25	18/0
Zone II	13/40	17/28	17/17	11/30	14/19	12/0
Zone III	29/31	16/25	15/15	17/36	28/17	11/0
Mean	23/35	20/27	13/16	16/33	21/20	14/0
Particles‡
Capsule
PE§	—	+	+++	—	—	+
Metal	—	++	—	—	—	—
HA†	—	—	—	—	—	—
Sections
PE§	—	+	+++	+	+	+
Metal	—	+	—	—	—	—
HA†	—	—	—	—	—	—
Bone-implant contact (%)
Femur	44	56	31	51	46	22
Acetabulum	44	35	21	49	39	31
Bone area (%)
Femur	26	12	14	20	11	14
Acetabulum	23	20	13	16	21	14
Extent of residual HA coating† (%)
Femur	56	21	?5	65	?8	?3
Acetabulum	26	20	?4	13	?3	?0

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TABLE I: Data on the Six Cases

	Case 1	Case 2	Case 3	Case 4	Case 5	Case 6
Gender, age at op. (yr)	F, 63	M, 65	F, 81	M, 58	F, 66	F, 86
Diagnosis	Osteoarthritis	Osteoarthritis	Osteoarthritis	Osteoarthritis	Osteoarthritis	Osteoarthritis
Duration of implantation (yr)	3.3	5.2	5.4	5.7	6.2	6.6
Weight (kg)	85	65	52	50	75	65
Height (m)	1.39	1.69	1.60	1.52	1.63	1.60
Merle d’Aubigné score³⁴(points)	18	18	18	15	18	18
Harris hip score³⁵(points)	100	100	97	90	100	95
Cause of death	Cardiac arrest	Cerebral hemorrhage	Cardiac arrest	Cardiac arrest	Aortic aneurysm	Pancreatitis
Previous operation	—	Femoral osteotomy	—	Contralateral THA*	—	—
Clinical notes	Not active	Active	Active	Not active, Down syndrome	Active	Active
Alignment of cup	45Â°	49Â°	45Â°	55Â°	45Â°	45Â°
Alignment of stem	Neutral	Neutral	4Â° valgus	Neutral	2Â° varus	Neutral
Femoral fill	Complete	Almost complete	Poor	Complete	Poor	Poor
Bone ongrowth (%)/extent of residual HA coating† (%)¹²
Zone I	47/28	36/19	?9/3	60/12	41/7	40/0
Zone II	25/24	32/17	23/5	50/12	35/1	39/0
Zone III	59/26	38/24	27/3	38/14	40/0	14/0
Mean	44/26	35/20	21/4	49/13	39/3	31/0
Bone area (%)/thickness of residual HA coating† (m)¹²
Zone I	27/34	26/27	?6/16	20/32	22/25	18/0
Zone II	13/40	17/28	17/17	11/30	14/19	12/0
Zone III	29/31	16/25	15/15	17/36	28/17	11/0
Mean	23/35	20/27	13/16	16/33	21/20	14/0
Particles‡
Capsule
PE§	—	+	+++	—	—	+
Metal	—	++	—	—	—	—
HA†	—	—	—	—	—	—
Sections
PE§	—	+	+++	+	+	+
Metal	—	+	—	—	—	—
HA†	—	—	—	—	—	—
Bone-implant contact (%)
Femur	44	56	31	51	46	22
Acetabulum	44	35	21	49	39	31
Bone area (%)
Femur	26	12	14	20	11	14
Acetabulum	23	20	13	16	21	14
Extent of residual HA coating† (%)
Femur	56	21	?5	65	?8	?3
Acetabulum	26	20	?4	13	?3	?0

Materials and Methods

Introduction | Materials and Methods | Results | Discussion | References

Six total hip arthroplasties were performed through a Hardinge approach by the same surgeon (A.T.). The femoral stems (ABG; Howmedica International, Staines, Middlesex, England) were made of titanium alloy (Ti-6Al-4V), and the proximal third of each stem was coated with hydroxyapatite on a macro-relief surface. The hemispherical metal cups were made of the same alloy, were totally coated with hydroxyapatite, and had twelve uniform screw-holes (Fig. 1). Cup sizes ranged from 46 to 58 mm. Two spikes were used for initial rotational stability; no screws were used to augment fixation. The femoral heads were made of cobalt-chromium and were 28 mm in diameter.

The hydroxyapatite coating was applied with a plasma-spray torch under a vacuum onto a sublayer of titanium to improve adhesion. The coating had a hydroxyapatite content of more than 90%, a porosity of less than 10%, and a calcium-to-phosphate ratio of 10:6. The crystallinity was 100% before coating and more than 75% after coating. The grain size was 20 to 50 m, and the strength of the tensile bond was 62 to 65 MPa. The thickness of the hydroxyapatite layer was 60 ± 30 m. The roughness of the cup was 9 ± 2.5 m after sandblasting, 8.5 ± 2.5 m after titanium spraying, and 5 ± 1 m after hydroxyapatite coating. All patients had an uneventful total hip arthroplasty, and all died of causes unrelated to the procedure.

Specimen Preparation

All six patients had given written consent for prosthetic retrieval at autopsy. The prostheses and the surrounding bone were collected post mortem and were immersed in buffered formalin for seven days and then in 70% ethanol for twenty-four hours. Photographs and radiographs of the samples were made. Three 1.0-cm-thick gross segments were cut, corresponding to the three zones delineated by DeLee and Charnley¹²(Fig. 2). Each segment was embedded en bloc in a polymethylmethacrylate resin, and five 20-m sections were cut from each zone with the technique of Donath and Breuner¹³. The sections were stained for qualitative histological examination (paragon staining, a combination of basic fuchsin and toluidine blue) and quantitative histomorphometric analysis. A biopsy specimen of the joint pseudocapsule was taken, embedded in paraffin, and prepared for light microscopy with use of Masson trichrome staining.

Specimen Analysis

We used a Polyvar microscope (Reichert-Jung, Vienna, Austria) for qualitative analysis and an Axioskop microscope (Carl Zeiss, Munich, Germany) equipped with a color-image-analyzing system (Samba; Samba Technologies, Grenoble, France) for quantitative analysis. The quantitative histomorphometric evaluation of the surrounding bone tissue (bone-implant contact and bone area) was performed on four areas for each section of the cup (Fig. 3). The surfaces of these areas ranged from 10 to 12 mm²; they were scanned, reduced, and stored before reconstruction of the image. Sixteen microscopic fields were necessary to scan the entire surface of each area. Each pixel of the reduced image represented 61 m²of the section. The implant, bone, and lacunae, including all soft tissues, were successively identified, and their respective surfaces and contacts with the implant were measured. For each of the 360 areas, the extent of bone apposition to the hydroxyapatite, the extent of bone apposition to the titanium, and the extent of residual hydroxyapatite were measured linearly and were expressed as a percentage of the total linear implant surface as visualized in the microscopic fields. Likewise, the relative bone area was expressed as a percentage of the total area of the section. Also, the thickness of the remaining coating was measured and was expressed in micrometers. Bone-implant contact, or the extent of bone apposition, was defined as the direct growth of bone onto the hydroxyapatite coating or onto the titanium surface without hydroxyapatite and was identical to the amount of osseointegration¹¹. Bone area was defined as the percentage of the surface of the measured field that was covered by bone.

The means and standard deviations were calculated for each section. Because of partial polyethylene-induced debonding, the values for one cup (Case 3) were excluded from the calculation of correlations. We performed statistical analysis with the Spearman rank-order test, the Mann-Whitney rank-sum tests, and the paired t test. A p value of less than 0.01 was considered significant. The pathologist (M.T.) was blinded to the clinical results. The area of adjacent bone and the linear extents of bone apposition and residual hydroxyapatite coating for the femoral components from the first five patients were reported previously¹¹. The data on all six patients are included for comparison in Table I.

Results

Introduction | Materials and Methods | Results | Discussion | References

The data on the patients and the results of the analyses are shown in Table I. All of the cups were stably fixed in the acetabulum at the time of the autopsy. A thick, white-yellow pseudocapsule connected the rim of the acetabulum with the proximal aspect of the femur.

Radiographic Findings

All six cups had had radiographic evidence of stable fixation throughout the follow-up period. The alignment was between 45Â° and 55Â° of abduction on the immediate postoperative radiograph, without any subsequent migration or change in the position of the cup. No radiolucent lines, osteolysis, or other bone reactions had been detected during the follow-up period. However, the postmortem radiograph of the acetabulum and cup that were retrieved from one cadaver (Case 3) showed some osteolysis at the rim of the cup, especially in zone I of DeLee and Charnley. This patient had had a slight asymmetrical tracking of the femoral head on the five-year follow-up radiograph as well as polyethylene particulate-induced debonding of the most proximal part of the femoral stem¹¹.

Histological Findings

The pseudocapsule was mostly composed of a thick and dense fibrous tissue; was focally infiltrated by an inflammatory component containing macrophages, lymphocytes, and a few giant cells; and was covered by a pluri-stratified synovial-like layer. Metallic debris could only be detected in the specimen from one patient (Case 2); this debris was found both close to the surface and more deeply within the membrane. A modest number of polyethylene particles, with an associated inflammatory reaction that was predominated by mononuclear infiltrates, were detected in the specimens from two patients (Cases 2 and 6). However, in the specimen from another patient (Case 3), abundant polyethylene particles were detected in association with an inflammatory reaction.

Bone Ongrowth

All acetabular implants were surrounded by numerous bridges of lamellar trabecular bone, mostly directed perpendicular to the implant (Figs. 4-A and 4-B). The hydroxyapatite coating appeared thin, irregular, and almost completely resorbed in areas where bone marrow reached the interface. Even after total or nearly total resorption of the hydroxyapatite (Cases 5 and 6), bone tissue and bone-marrow cells could be focally noted directly on the surface of the metal substrate, without interposition of any connective tissue (Fig. 5). The surrounding bone marrow was histologically normal in all of the sections. Macroscopically, the highest percentages of bone-implant contact and bone area were observed at the rim of the cups and on the top of the spikes.

Particles

Three types of particles were observed: metal, hydroxyapatite, and polyethylene (Table I). Metal particles were rarely seen and were only noted near the outer surface of the cup in two of the ninety sections, both from the same patient (Case 2). The tissue reaction to metal was very moderate, with small amounts of macrophages sometimes observed in the bone-marrow tissue. Hydroxyapatite granules were sporadically observed in the bone marrow, but they were only found close to the coated surface and were not associated with any inflammatory reaction. When bone growth onto the coating had occurred, no loose hydroxyapatite particles were noted. In one of the 360 measured areas, actual degradation of the hydroxyapatite coating by osteoclasts (Fig. 6) and phagocytosis by macrophages were seen. No signs of dissolution of the hydroxyapatite coating, abrasion, or delamination could be detected in any section.

In most screw-holes, numerous birefringent polyethylene particles were seen under polarized light. However, in approximately half of the holes a dense collagenous membrane or an osseous bridge closed the screw-hole and seemed to contain the particles (Fig. 3). The other holes were associated with more-or-less pronounced bone-resorption activity, with signs of early debonding and even early cystic osteolysis (Fig. 7).

In the specimen from one patient (Case 3), abundant polyethylene particles with an associated inflammatory tissue reaction were present, especially in zone I of DeLee and Charnley (Fig. 8), where osteolysis with very diminished bone area and bone-implant contact was most prominent. The other two zones had far less debonding, with normal values for bone area.

Histomorphometric Findings

The mean percentage of bone-implant contact was 36.5% ± 13.5%. No significant difference was noted among the bone-implant contact ratios for the three zones of DeLee and Charnley. The predominant areas of bone-implant contact were near the rim of the acetabular cup and around the spikes (Fig. 3). With the limited numbers available, the mean percentage of bone-implant contact for the acetabulum did not differ significantly from that for the metaphyseal part of the femur in any patient (Table I). However, the value for the femur was higher in all patients but two (Cases 1 and 6). Also, the bone-area value for the acetabulum was similar to that for the metaphyseal part of the femur in every patient (p = 0.5539, alpha = 0.050). The two oldest patients (Cases 3 and 6) had the lowest values for bone-implant contact in both the acetabulum and the femur as well as the lowest values for bone area in the acetabulum.

Hydroxyapatite Coating

No significant differences were noted with regard to the extent of hydroxyapatite residue in the three zones of DeLee and Charnley. The two cups with the longest duration of implantation (Cases 5 and 6) showed little or no hydroxyapatite residue. Overall, the extent of the residual hydroxyapatite coating on the cup was always less than that in the metaphyseal part of the femur. This points to a higher rate of bone remodeling on the acetabular side. The appearance of the coating was not uniform. In the specimens from the first four patients (Cases 1 through 4), the coating was thick and regular in the areas covered by bone (Figs. 4-A and 4-B) and it was thin or irregular or partly or fully absorbed in the areas covered by bone marrow. However, such features were not observed in the specimens from the last two patients (Cases 5 and 6), in which hardly any hydroxyapatite could be detected even in the areas of normal bone-implant contact (Fig. 5). Nevertheless, in these last two specimens the percentage of bone ongrowth measured 31% and 39% of the cup surface (Table I).

The measured bone and coating parameters were matched according to age at death and duration of implantation. A possible relationship between the bone apposition percentages and the bone area according to age was noted. A slow but progressive resorption of the hydroxyapatite coating according to the duration of implantation was obvious.

Discussion

Introduction | Materials and Methods | Results | Discussion | References

To our knowledge, the present study is the first to examine the long-term histological and histomorphometric results associated with hydroxyapatite-coated acetabular cups that were implanted in humans by the same surgeon at the same institution and that were reviewed by the same pathologist. The weakness of the study is the small number of specimens, which limited our ability to draw definite conclusions about possible correlations between measured bone and coating parameters. The results of the histological and histomorphometric analysis of the femoral stems from the first five patients have already been published¹¹.

Five of the six specimens showed the complete lack of a fibrous membrane, suggesting that the implant was mechanically stable. Even in the remaining patient (Case 3), in whom a polyethylene-laden granulation-tissue reaction had caused partial debonding, the cup was clinically successful and presumably was still mechanically stable. The amount of bone growth associated with these hydroxyapatite-coated cups was comparable with that associated with retrieved porous-coated acetabular components, which sometimes have no bone growth into the porous coating at all^14-17. The mean amount of bone-implant contact in the present study was 36.5% ± 13.5%; this is somewhat higher than that noted by Pidhorz et al.¹⁶, who reported a mean value of 29.7% in a study of eleven porous-coated acetabular cups retrieved at autopsy at a mean of forty-one months after implantation.

In the two specimens from patients in whom the hydroxyapatite coating had nearly or completely disappeared (Cases 5 and 6), the percentage of bone ongrowth still measured 31% and 39% of the cup surface. Both of these patients had been active and had had the implant in situ for more than six years. These findings agree with those of Overgaard et al.¹⁸. In a canine cortical-bone model involving weight-bearing hydroxyapatite-coated implants, those authors found that 36% ± 6.0% of a completely resorbed hydroxyapatite coating was replaced by bone in direct contact with the implant. Conversely, in a canine trabecular-bone model involving non-weight-bearing implants, Overgaard et al.¹⁹ observed that only one-fifth of the surface with complete resorption of the hydroxyapatite coating was replaced by newly formed bone. The amount of bone ongrowth seems to be related to weight-bearing, surface finish, and various other factors.

In the present study, the mean amount of bone ongrowth in the proximal aspect of the femur occurred over a somewhat broader range (22% to 56%). This result is in agreement with the range of 20% to 67% reported by Carlsson et al.²⁰ in association with hydroxyapatite-coated and grit-blasted titanium implants with a mean roughness of 3.1 m that were inserted for three months in the proximal aspect of arthritic human tibiae. However, those authors also reported that smooth titanium implants with a mean roughness of 0.9 m that were inserted in the same location for the same duration were mostly encapsulated by fibrous tissue. Therefore, not only loading but also the roughness of both the hydroxyapatite coating and the substrate can have a profound impact on the amount of bone ongrowth. Furthermore, animal experiments have shown that stability of surgical fit, implantation site, bone type, and time of implantation can also play a role in the quality and percentage of bone ongrowth^21,22.

Only the duration of implantation and the age, weight, level of activity, and bone stock differed among our patients. Nevertheless, it was still surprising to find that the amount of bone-implant contact was in a narrow range, regardless of the age of the patient or the duration of implantation. Moreover, whereas implant fixation is surely maintained by the metal surface once the hydroxyapatite coating is completely resorbed, it is clear from our study that a minimum of 20% of bone ongrowth is still sufficient to maintain reliable fixation of an implant.

In contrast with the findings of other studies^23-25, in which hydroxyapatite debris was associated with an inflammatory response or loose hydroxyapatite granules were found to have generated third-body wear, we did not detect loose hydroxyapatite granules away from the coating or in the retrieved polyethylene inserts²⁶. Direct degradation of the hydroxyapatite layer by osteoclasts and subsequent resorption by macrophages was seen only once.

These observations suggest that both the quality of the hydroxyapatite coating and the process of applying it to the substrate are of paramount importance. Morscher et al.²⁵ reported poor results in association with cups that had coatings as thick as 300 m. Hot-pressing of hydroxyapatite granules with grain sizes of 125 to 250 m is not a reliable coating method. Furthermore, previous experiments have demonstrated that thicker hydroxyapatite coatings have considerably poorer mechanical properties and an increased risk of hydroxyapatite abrasion or delamination^27,28. The application of a thin (approximately 60-m-thick) hydroxyapatite coating with a plasma-spray torch under a vacuum results in the most reliable adhesion to the substrate and the least chance of delamination.

The analysis of the specimens from our first four patients confirmed our previous findings¹¹ and those of Overgaard et al.²⁶ that less resorption of the coating was seen when bone was present at the coating surface. In contrast, when bone marrow was present at the interface nearly all of the hydroxyapatite was found to have been resorbed. Two theories have been suggested to explain this phenomenon^4,26: (1) direct disintegration of the hydroxyapatite into the extracellular fluid (a non-cell-mediated process) and (2) cell-mediated hydroxyapatite resorption (a process in which hydroxyapatite is disintegrated, by osteoclastic enzymes, into smaller granules that are then phagocytosed and broken down by diverse types of cells)¹⁸. We believe that there are more convincing arguments for the theory of cell-mediated hydroxyapatite resorption through bone-remodeling. Our oldest patients (Cases 3, 5, and 6), with presumably the highest rates of bone resorption, showed the lowest amounts of hydroxyapatite residue.

The fate of the hydroxyapatite coating in the intermediate and long term and its relationship to long-term implant fixation continue to be of great interest. In an experimental study involving human trabecular bone, Overgaard et al.²⁶ estimated the hydroxyapatite resorption rate to be approximately 20% of the coating thickness per year. They suggested that, in the clinical situation, resorption may be accelerated because of weight-bearing, micromotion, and eventual access of joint fluid into the extended joint space. We are aware of only one study¹¹, involving an analysis of femoral components retrieved from humans at autopsy, in which the percentage of implant fixation by bone was found to be independent of the amount of hydroxyapatite residue. This observation is consistent with the results of the current study. On the one hand, these findings confirm the view of many authors that a hydroxyapatite coating offers early, reliable, and augmented bone ongrowth with improved fixation^{1-11,19-22,26-33}. On the other hand, we believe that it is the geometric design and the substrate surface texture that mainly determine the longevity of the prosthesis when all of the hydroxyapatite is resorbed.

In conclusion, we believe that a plasma-sprayed hydroxyapatite coating with high crystallinity and a thickness of approximately 60 m enhances the rapid biological fixation of the implant by means of bone ongrowth while allowing only slow resorption. Resorption depends on the rate of bone-remodeling, which is mainly related to patient characteristics such as level of activity, age, and duration of implantation. Higher bone turnover in the acetabulum than in the proximal aspect of the femur might explain why there was less hydroxyapatite residue in the acetabulum. An important finding of the present study was the absence of foreign-body reaction, inflammatory response, and delamination in association with this ceramic coating. Despite total hydroxyapatite resorption, the percentage of bone-implant contact remained stable and within a range that seems sufficient for long-term stability. The important role of the texture of the substrate in this respect cannot be overemphasized.

Note: The authors thank the editorial board of the British volume of The Journal of Bone and Joint Surgery for granting permission to use some formerly published data. The authors also thank Mr. J. Ten Kate for statistical analysis of the presented data. The histological and histomorphometric analysis was performed at Biomatech SA, Chasse sur Rhone, France.

References

Introduction | Materials and Methods | Results | Discussion | References

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Fig. 1:: Hemispherical hydroxyapatite-coated cup with twelve screw-holes. Two spikes are applied prior to cup insertion.

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Fig. 4-B:: Case 4. Zone II of DeLee and Charnley.

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Fig. 8:: Case 3. Zone I of DeLee and Charnley. The granulation tissue (G) is induced by polyethylene particles. The large areas of bone resorption are slowly producing debonding of the cup.

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TABLE I: Data on the Six Cases

	Case 1	Case 2	Case 3	Case 4	Case 5	Case 6
Gender, age at op. (yr)	F, 63	M, 65	F, 81	M, 58	F, 66	F, 86
Diagnosis	Osteoarthritis	Osteoarthritis	Osteoarthritis	Osteoarthritis	Osteoarthritis	Osteoarthritis
Duration of implantation (yr)	3.3	5.2	5.4	5.7	6.2	6.6
Weight (kg)	85	65	52	50	75	65
Height (m)	1.39	1.69	1.60	1.52	1.63	1.60
Merle d’Aubigné score³⁴(points)	18	18	18	15	18	18
Harris hip score³⁵(points)	100	100	97	90	100	95
Cause of death	Cardiac arrest	Cerebral hemorrhage	Cardiac arrest	Cardiac arrest	Aortic aneurysm	Pancreatitis
Previous operation	—	Femoral osteotomy	—	Contralateral THA*	—	—
Clinical notes	Not active	Active	Active	Not active, Down syndrome	Active	Active
Alignment of cup	45Â°	49Â°	45Â°	55Â°	45Â°	45Â°
Alignment of stem	Neutral	Neutral	4Â° valgus	Neutral	2Â° varus	Neutral
Femoral fill	Complete	Almost complete	Poor	Complete	Poor	Poor
Bone ongrowth (%)/extent of residual HA coating† (%)¹²
Zone I	47/28	36/19	?9/3	60/12	41/7	40/0
Zone II	25/24	32/17	23/5	50/12	35/1	39/0
Zone III	59/26	38/24	27/3	38/14	40/0	14/0
Mean	44/26	35/20	21/4	49/13	39/3	31/0
Bone area (%)/thickness of residual HA coating† (m)¹²
Zone I	27/34	26/27	?6/16	20/32	22/25	18/0
Zone II	13/40	17/28	17/17	11/30	14/19	12/0
Zone III	29/31	16/25	15/15	17/36	28/17	11/0
Mean	23/35	20/27	13/16	16/33	21/20	14/0
Particles‡
Capsule
PE§	—	+	+++	—	—	+
Metal	—	++	—	—	—	—
HA†	—	—	—	—	—	—
Sections
PE§	—	+	+++	+	+	+
Metal	—	+	—	—	—	—
HA†	—	—	—	—	—	—
Bone-implant contact (%)
Femur	44	56	31	51	46	22
Acetabulum	44	35	21	49	39	31
Bone area (%)
Femur	26	12	14	20	11	14
Acetabulum	23	20	13	16	21	14
Extent of residual HA coating† (%)
Femur	56	21	?5	65	?8	?3
Acetabulum	26	20	?4	13	?3	?0

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TABLE I: Data on the Six Cases

	Case 1	Case 2	Case 3	Case 4	Case 5	Case 6
Gender, age at op. (yr)	F, 63	M, 65	F, 81	M, 58	F, 66	F, 86
Diagnosis	Osteoarthritis	Osteoarthritis	Osteoarthritis	Osteoarthritis	Osteoarthritis	Osteoarthritis
Duration of implantation (yr)	3.3	5.2	5.4	5.7	6.2	6.6
Weight (kg)	85	65	52	50	75	65
Height (m)	1.39	1.69	1.60	1.52	1.63	1.60
Merle d’Aubigné score³⁴(points)	18	18	18	15	18	18
Harris hip score³⁵(points)	100	100	97	90	100	95
Cause of death	Cardiac arrest	Cerebral hemorrhage	Cardiac arrest	Cardiac arrest	Aortic aneurysm	Pancreatitis
Previous operation	—	Femoral osteotomy	—	Contralateral THA*	—	—
Clinical notes	Not active	Active	Active	Not active, Down syndrome	Active	Active
Alignment of cup	45Â°	49Â°	45Â°	55Â°	45Â°	45Â°
Alignment of stem	Neutral	Neutral	4Â° valgus	Neutral	2Â° varus	Neutral
Femoral fill	Complete	Almost complete	Poor	Complete	Poor	Poor
Bone ongrowth (%)/extent of residual HA coating† (%)¹²
Zone I	47/28	36/19	?9/3	60/12	41/7	40/0
Zone II	25/24	32/17	23/5	50/12	35/1	39/0
Zone III	59/26	38/24	27/3	38/14	40/0	14/0
Mean	44/26	35/20	21/4	49/13	39/3	31/0
Bone area (%)/thickness of residual HA coating† (m)¹²
Zone I	27/34	26/27	?6/16	20/32	22/25	18/0
Zone II	13/40	17/28	17/17	11/30	14/19	12/0
Zone III	29/31	16/25	15/15	17/36	28/17	11/0
Mean	23/35	20/27	13/16	16/33	21/20	14/0
Particles‡
Capsule
PE§	—	+	+++	—	—	+
Metal	—	++	—	—	—	—
HA†	—	—	—	—	—	—
Sections
PE§	—	+	+++	+	+	+
Metal	—	+	—	—	—	—
HA†	—	—	—	—	—	—
Bone-implant contact (%)
Femur	44	56	31	51	46	22
Acetabulum	44	35	21	49	39	31
Bone area (%)
Femur	26	12	14	20	11	14
Acetabulum	23	20	13	16	21	14
Extent of residual HA coating† (%)
Femur	56	21	?5	65	?8	?3
Acetabulum	26	20	?4	13	?3	?0