- Email: [email protected]
A quantitative study of bone and soft tissues in cementless porous-coated acetabular components retrieved at autopsy
The Journal of Arthroplasty Vol. 8 No. 2 1993
A Q u a n t i t a t i v e Study of B o n e and Soft Tissues in C e m e n t l e s s P o r o u s - c o a t e d Acetabular C o m p o n e n t s R e t r i e v e d at A u t o p s y Laurent
E. P i d h o r z , M D , R o b e r t D a l e R. S u m n e r ,
M. Urban, Joshua
J. J a c o b s ,
PhD, and Jorge O. Galante,
MD,
MD
Abstract: The authors examined 11 cementless acetabular components of one design retrieved at autopsy and made observations concerning tissue ingrowth and local tissue reaction, radiographic-histologic correlation, and the distribution of particulate wear debris. The cups were hemispherical in design with a commercially pure titanium fiber: metal porous coating. All of the prosthes.es were implanted with screws. The implants were in place for an average of 4 i months (range, 5'weeks to 75 months). Ten of the cups had bone ingrowth, with the average volume fraction being 12.1 • 8.2%. There were no differences in the amount of bone ingrowth when the component was partitioned into ninc anatomic regions. However, th,~re was more bone adjacent to screw holes through which screws were inserted compared with empty screw holes. As the number of radiolucent zones increased on the clinical radiographs less bone ingrowth was observed histologically. The amount of metal debris in holes with screws and holes without screws was similar. In the longest term cases, polyethylene debris was noted within empty screw holes, but no granulomatous reactions or osteolytie processes were observed. Key words: bone ingrowth, porous-coated implant, acctabular component, total hip arthroplasty, polyethylene, titanium.
Cementless acetabt, lar reconstruction with porous-coated c o m p o n e n t s has been used with increasing frequency in recent years. Excellent clinical resuits have been reported at the m i d - t e r m follow-up evaluation with a n u m b e r of such devices, xg,t~ ~5.~8 There are sonic concerns, however, that relate to their long-term performance. The durability of fixation is an issue that is related to the histological features of the interface and particularly to the type and extent of tissue ingrowth present. In addition, the potential for the production of metallic and polymeric debris resulting from fretting and/or w e a r of tile artictdar liner or stabilizing screws has given rise
to the concern that adverse tissue reactions m a y occur at the b o n e - p r o s t h e s i s interface. Recently, acetabular osteolysis has been reported in association with porous-coated c o m p o n e n t s in 27.8% of 115 asyml~tomatic patients 6 years following cementless primary total hip arthrop]asty (THA). 20steolysis has also been associated with excessive wear of the polyethylene bearing surface and onset of late pain, necessitating revision in 2.8% of 249 patients followed from 5 to 7 years. 24 Histologic studies of cementless acetabular components have been limited largely to specimens retrieved at revision surgery and have focused primarily on bone ingrowth. 6,7,22 Most reports have indicated that the a m o u n t of b o n e ingrowth was less anal its distribution different than expected based on early canine studies. L~3 However, specimens obtained at revision surgery m a y not necessarily reflect tile status of the devices in patients w h o are function-
From the Department of Orthopedic Surgery, Rttsh-Presbyterian-St. Luke's Medical Center, Chicago, Illinois. Supported by NIH grant AR39310 and Zimmer Inc. Reprint requests: Mr. Robert Urban, Department of Orthopedic Surgery, Rush-Presbyterian-St. Luke's Medical Center, i 653 West Congress Parkway, Chicago, IL 60612.
213
214
The Journal of Arthroplasty Vol. 8 No. 2 April 1993
ing normally and are asymptomatic. More relevant information can be obtained by the appropriate study of specimens retrieved at the time of autopsy. 5 The purpose of this study was to report the histological findings in a group of 11 acetabular c o m p o nents and surrounding tissues retrieved at autopsy. The acetabular components were all implanted without b o n e - c e m e n t , were of the same design, and were obtained from patients w h o had well functioning prostheses at the time of their demise.
Materials and Methods Patient Population Eleven cementless acetabular c o m p o n e n t s were retrieved at autopsy from eight patients w h o had previously undergone primary THA. In every case, the acetabu!ar c o m p o n e n t was a Harris-Galante I cup (Zimmei', Inc. Warsaw, IN). These were hemispherical components made of commercially pure titanium, with a porous layer on the full outside surface. The porous surface consisted of a 1.65 m m laye r of fiber metal made of commercially pure titanium wire, with a diameter of 250 ~ m and a m e a n pore size of 400 I.tm. The components contained an ultrahigh molecular weight polyethylene liner press fit into the metal shell. Seven of the implants (5 patients) came from our hospital (Rush-Presbyterian-St. Luke's, Chicago, IL) and four implants (3 patients) came from other institutions. Clinical information, obtained from the patients' medical records, included age, sex, the diagnosis at insertion, time in situ, Harris hip score, and cause of death (Table 1). The average age of the patients at the time of surgery was 56 years (range, 3 3 . 5 - 8 2 . 5 years). There were four w o m e n and four men. Six left and five right acetabular c o m p o n e n t s were obtained, and
three patients had bilateral THAs. The m e a n time in situ was 41 m o n t h s (range, 5 weeks to 75 months) and the m e a n time to last clinical examination was 35 months. Seven of the cups had been in place for more than 45 m o n t h s and the other 4 had been in place for 8 months or less. Femoral reconstruction consisted of a conventional length cementless titanium alloy fiber-metal prosthesis (Harris-Galante, Zimmer) in nine cases, a long-stem cementless titanium alloy fiber-metal prosthesis (BIAS, Zimmer) in one case (patient 6), and a cemented cobalt alloy prosthesis (Precoat, Zimmer) in one case (patient 4). In all of these c o m p o n e n t s the prosthetic head was made from a cobalt-chromium m o l y b d e n u m alloy. In all but one case, the surgical procedure was done by a direct lateral approach without trochanteric o s t e o t m o m y (Hardinge). For the procedures done at our institution, the components were inserted without impaction into an acetabular bed reamed to size. The initial fixation was achieved by 3-,5 cancellous, 4.5 or 5.1 m m screws of Ti6AI4V placed through the holes of the cup. The outer diameter of the c o m p o n e n t s ranged from 46 to 62 mm. At our hospital, the postoperative weight-bearing protocol varied depending u p o n the femoral reconstruction. For cementless femoral components, full weight bearing was not allowed until 6 weeks after surgery. These patients were advised to use crutches for 3 m o n t h s followed by a cane for 3 months. For the cemented femoral component, full weight bearing was allowed within the first postoperative week, and supports were generally used for 6 weeks. Harris hip scores were available only on cases from our institution. Five of these 7 hips had Harris hip scores in excess of 80 consistent with good to excellent clinical results. One patient with a bilateral THA (patient 8) had hip scores of 47 and 52. These low scores reflected diffuse bone pain with multiple pathologic fractures as a result of renal osteodystro-
T a b l e 1. Clinical Data in Patients With Cementless Porous-coated Acetabular Components
Patient No. l 2 2 3 4 5 5 6 7 8 8
Implant
Time in situ (too)
Side
1 2 3 4 5 6 7 8 9* tO 11
1 4 6 8 46 51 69 54 71 63 75
R R L L R L R L L L R
Last Harris Hip
Sex
Age at THA
Indication
F M M b,l F F F M F M M
75 39 39 54 83 65 64 67 60 35 34
OA AVN AVN RA OA AVN AVN OA Q_A. AVN AVN
Cause of Death
Score
Septicemia Pneumonia Metastatic CA, unknown primary Metastatic breast CA Metastaticlung CA Metastatic lung CA Metastalicbreast CA Renal failure
-95 97 97 83 92 52 47
OA, osteoarthritis; AVN, avascular necrosis; RA, rheumatoid arthritis; CA, carcinoma. *Trochanteric osteotomy fixed by wires.
Porous-coated Acetabular Components Retrieved at Autopsy
phy rather than specific symptoms related to his hip arthroplasties, which were functioning well at the time of the last examination 3 months prior to his death. This patient ultimately succumbed to the complications of renal failure and was the only case in this series with k n o w n metabolic bone disease. For the four implants obtained from other institutions (patients 1, 2, and 3), the patient's physician indicated satisfactory progress with their arthroplasty. These implants had been in place from 1 to 8 months. No patient w h o died as a result of cancer had metastatic disease in the periacetabular bone. No clinical or histologic evidence of infection in the periprosthetic tissues was found. Radiographs were available for review in 10 cups. For all the seven long-term implants, serial radiographs were available. The interface was divided into five zones modified after DeLee and Charuley, 8 with I zones 1 and 2 corresponding to zone A and zones 3 and 4 corresporiding to zone B. The presence or absence of a radiolucency at each zone was noted as well as the thickness and progression of radiolucent lines. No implant demonstrated radiographic migration according to criteria previously described. ~8
Methods of Analysis The components with surrounding bone and soft tissue were removed by osteotomy of the ischium, pubis, and ilium and were immediately immersed in 10% neutral buffered formalin. Before processing, contact anteroposterior and lateral radiographs and photographs were made. The polyethylene liner was removed by levering from the rim of the acetabular c o m p o n e n t without disturbing the underlying shell or fixation screws. The fixation screws were left in place. Each specimen was sectioned perpendicular to the opening of the cup from anterior to posterior into five to seven blocks of 10 m m thickness using a diam o n d wheel cut-offsaw. After dehydration in graduated alcohol solutions and xylene, the undecalcified blocks were embedded in methyl methacrylate m o n o m e r and polymer. W h e n polymerized, a thin section was cut from each block and ground to approximately 100 lzm and stained with toluidine blue and basic fuchsin for light microscopy. Quantitative data from five of these sections (2 anterior, 1 central, and 2 posterior) were gathered for each cup except for the 46 m m cup (implant 9), which had four stained sections. In addition, one anterior, one central, and one posterior section from each cup were carbon-coated for scanning electron microscopy. In order to express the quantity of different tissues within and around the cup, we defined the parameter, extent, as the fraction of 1 m m fields with a particular tissue type. A grid in the microscope eyepiece
9 Pidhorz et al.
215
Level I Periphery Levee 2 Interface
Level 3 Coating
Level 4 Substrate
Fig. 1. Method for determining the quantity of different tissues within and around the prosthesis. A grid was superimposed on microscopic fields at x 200 magnification. The extent of each tissue type was defined by the fraction of 1 mm fields containing that tissue. This was recorded for each of tile four levels shown.
was used to divide the stained sections into 1 m m field s (Fig. 1). At a magnification of • 200, the tissue within each field was categorized either as bone, marrow, cartilage, fibrous, or necrotic tissue. This was done at four locations: a 2 m m peripheral zone (level 1), the interface between the outer surface of the porous coating and the host bone (level 2), within the porous coating (level 3), and the interface between the porous coating and the substrate of the cup (level 4). The extent of each tissue type at each level was defined as the fraction of 1 m m fields positive for that tissue. The m a x i m u m possible value for each tissue type was 100%. W h e n an interfacial m e m b r a n e was present at the interface (level 2), its thickness was measured. An average of 286 1 m m fields was examined for each specimen, 32 fields for each of the 9 anatomic regions (anterosuperior, anteromid, anteroinferior, central-superior, central-mid, central-inferior, posterosuperior, posteromid, posteroinferior). From these data, the regional distribution of bone ingrowth was evaluated by: (1) comparing ingrowth among the nine anatomic regions, (2) comparing the area within 5 m m of the rim of the cup to the remainder of the cup, and (3) comparing the a m o u n t of bone ingrowth within 3 m m of screw holes containing screws to screw holes left empty. In addition, the data for the central section were also tabulated in five zones corresponding to the zones used in the radiographic analysis. The volume fraction of bone ingrowth was determined with the aid of backscattered scanning electron microscopy and an image analyzer. 2~ This vari-
216
The Journal of Arthroplasty Vol. 8 No. 2 April 1993
able was defined as the fraction of void space within the porous coating occupied by mineralized bone. The m a x i m u m possible value was 100%. The entire porous coating from three sections (anterior, central, and posterior) was analyzed. The average total area analyzed was 322 m m 2. In addition, to determine the density of the periaeetabular bone bed, the volume fraction of the cancellous bone outside of the porous coating was also measured using the central section. For this measurement, three sites (superior, mid, and inferior) were chosen so that the area measured was as far removed from the implant as possible. An average of 66 m m 2 of tissue per cup was assessed. Particulate debris and associated cellular response were studied in the stained sections by regular and polarized light microscopy. At the screw holes, the a m o u n t of metallic and polymeric debris and the degree of the histiocyte and foreign-body giant cell reaction Were graded, 0 t h r o u g h 3, according to the technique of Mirra for metal particles and m o n o n u clear histiocytes.~9 This was done for the area within the hole and for the porous coating a r o u n d the hole. The scores for holes with screws were compare d to those of holes left empty. Twenty-three h o l e s w i t h screws and 31 holes without screws were evaluated. Also, the presence or absence of debris and cellular infiltrate were noted at the interface between the outer surface of the porous coating and the host bone and at the cup rim. Means, standard deviations, and ranges were calculated for all of the variables. Repeated measures analyses of variance were used to test for regional differences in the distribution of bone ingrowth. Pearson product m o m e n t and Spearman rank order correlation analyses were performed to assess the strength of association a m o n g variables. For the analysis of debris and cellular response, the Wilcoxon malched-pairs signed-ranks test was used to compare holes with screws to those without. Because of the small sample size and the potential multivariate nature of the data, it was not possible to test for the statistical influence of age, sex, length of implantation, or n u m b e r of fixation screws on the a m o u n t and distribution of bone ingrowth.
Table 2. Volume Fraction and Extent of Bone Ingrowth Time Implant
Bone ingrowth was present in 10 of the 11 cups (Table 2). The mean volume fraction of bone ingrowth was 12.1 _ 8.2% and the m e a n extent of bone at the interface (level 2) was 29.7 • 20.1%. These two measures of b o n y fixation were strongly correlated (r = 0.91; P < .001).
(mo)
1
1
2 3 4 5 6 7
4 6 8 46 51 69
4.8 13.7 14.5 0.8 4.6 15.2 16.6
8
54
1.5
9 10 11
71 63 75
27.6 16.9 17.4
Extent (%) 12.7 21.7 18.3 0.0 3.4 21.5 26.3 7.9 55.0 18.7 44.7
The a m o u n t of bone did not differ a m o n g the nine defined regions. There was, however, more bone at the interface (29.7 • 20.1%) than within the coating (20.9 __ 16.6%) or at the substrate (5.4 - 8.2%; P < .001). The a m o u n t of bone within 5 m m of the rim did not vary significantly from the remainder of t h e cup, although there was a tendency for more bone ingrowth to be near the rim (29.1 +- 26.7%) than elsewhere (19.0 • 15.2 %). There was approximately 50% more b o n e ingrowth adjacent to holes with screws than to holes left empty (P < .05, Table 3). Bone ingrowth in the 1-month specimen consisted of slender trabeculae of w o v e n bone oriented randomly within the pores (Fig. 2). In the 4-~8-month cups the ingrown bone was composed of thickened, lamellar trabeculae with occasional Haversian-like structures (Fig. 3). The components in situ for longer than 4 years also demonstrated lamellar trabeculae, but at the rim of the cup and surrounding the screw holes the ingrown bone often completely filled the pores and Haversian structures were c o m m o n (Fig. 4). The patient with renal osteodystrophy d e m o n strated ingrown trabeculae, which were sometimes
Table 3. Percent Extent of Bone Ingrowth Within 3 mm of Holes With Screws and Holes Without Screws (Mean +_ SD)
Results Bone Ingrowth
Volume Fraction (%)
in situ
Interface Coating Substrate
Screws
No Screws
47.2 ~- 33.8 34.7 • 34.0 4.4 • 14.1
33.4 - 27.7 19.5 +_ 16.0 2.1 • 5.1
The data were analyzed by repeated measures analyses of vari.a~acewith depth (interface vs. coating vs. substrate) and presence/ absence of a screw as the.within subjects factors. Depth was a significant factor (P < .001) as was presence/absence of a screw (P < .05). There were no significant interactions between depth and presence/absence of a screw.
Porous-coated Acetabular Components Retrieved at Autopsy
9 Pidhorz et al.
217
Fig. 2. Section of an acetabular component retrieved 1 month after implantation. Note the ingrowth of narrow trabeculae of woven bone (original magnification • 88).
extremely thin with an irregular surface and almost entirely covered by a thin, pale, pink layer of osteoid (Fig. 5). The volume fraction of periacetabular cancellous bone was 12.0 _ 6.5%, but was not correlated with the a m o u n t of bone ingrowth even though their mean values were nearly identical.
Fig. 3. Section of an acetabular component retrieved 6 months after implantation. This field demonstrates ingrowth of lamellar bone with thick trabeculae and occasional Haversian systems (original magnification • 16).
Radiographic Analysis and Correlation With Bone Ingrowth Of the seven long-term cases, three had no radiolucencies, one had a radiolucency in one zone, one had radiolucencies in two zones, and two had radiolucencies in three zones. None of these cases had radio-
218
The Journal of Arthroplasty Vol. 8 No. 2 April 1993
Fig. 4. Section of an acetabular component retrieved 63 months after implantation. In this longer-term retrieval, note the Haversian systems with extensive bone ingrowth (original magnification x 160).
lucencies in more than three zones. All radiolucencies were less than 1 m m and nonprogressive, except for one hip in the patient with renal osteodystrophy (patient 8), that demonstrated a progressive radiolucency in one zone between 1 and 2 m m in thickness. The n u m b e r of zones with radiolucencies was inversely related to the extent of bone at the interface
Fig. 5. Section of an acetabular component retrieved 75 months after implantation. This patient had renal osteodystrophy and demonstrated foci of thin irregular trabeculae surrounded by marrow elements (original magnification • 160). Most trabeculae were covered by a thin osteoid layer, which does not reproduce well in black-and-white photomicrographs.
(r = - 0 . 8 5 ; P < .05), and bone ingrowth within the coating (r = - 0 . 7 6 ; P < .05) and at the substrate (r = - 0 . 7 8 ; P < .05) In addition,in direct comparison between the radiographic analysis and the central histologic section, the zones that had no radiolucencies had t w i c e as m u c h bone ingrowth at the interface and three times as m u c h within the porous
Porous-coated Acetabular Components Retrieved at Autopsy
Table 4. Percent Extent of Bone Ingrowth in Zones With Lucency and Zones Without Lucency (Mean -+ SD)
Interface Coating * P < . 0 5 ; 'f P =
No Lucency
Lucency
43.3 - 30.3 32.4 • 31.7
19.3"I- • 31.5 10.9" • 19.9
9 Pidhorz et al.
219
In most of the areas without bone ingrowth, however, the interface (level 2) consisted of loose to moderately dense fibrous tissue, often containing dilated vessels (Fig. 8). The membranes Varied greatly in thickness, depending u p o n the a m o u n t of bone ingrowth. The cups with less than 10% v o l u m e fraction of bone ingrowth (implants I, 4,: 5, and 8) had thicker membranes (460 +_ 67 lira) than the cups with more than 10% bone ingrowth (182 _+ 71 Ftm; P < .001). Few lymphocytes or plasma cells were present in the interface membranes or within the porous coating except for three patients (1, 3, 4). The patient with rheumatoid disease (patient 3) had a mild, diffuse lymphocytic infiltrate. The patients w h o died at 1 m o n t h and 46 m o n t h s after surgery (patients 1 and 4) had moderate, focal accumulations of lymphocytes. Polymorphonuclear leukocytes were rarely observed. In all cases, all of the fixation screws appeared to be well fixed. In the case harvested at 1 month, the surface of the threads were surrounded by thin spicules Of bone that conformed to the threads. In the o t h e r 10 cases, the screw threads were covered by thickened trabeculae or dense plates of lamellar bone. (Fig. 9).
.068.
coating (P < .05) than the zones in which a radiolucency was present (Table 4).
Extent of Marrow, Cartilage, and Fibrous Tissues The extent of the tissue types at the four levels studied is s h o w n in Table 5. As expected, the 2-ram zone peripheral t o the c o m p o n e n t s (level 1) consisted primarily 6f bone and marrow. At the interface (level 2) as well as within the pores of the coating (level 3), there were some fields in which m a r r o w was present without bone. In these areas, the normal matrix of medullary m a r r o w pene= trated deeply between the metal fibers w i t h o u t an intervening fibrous membrane, thus giving the appearance of integration with the surrounding acetabular tissues in implants 6, 7, 10, and 11 (51, 69, 63, and 75 months, respectively). In contrast, foci of fibrocartilage were present at the interface (level 2) in l0 of the cups (extent, 0 . 4 - 6 . 3 % ) , but were seldom seen deep within the porous coating {Fig. 6) and never adjacent to screws. The a m o u n t of fibrocartilage at the interface was greatest in the implant with the least bone ingrowth (implant 4) and no fibrocartilage was found in the implant that had the greatest a m o u n t of bone ingrowth (implant 9). However, there was no correlation between the a m o u n t of fibrocartilage at the interface and either the volume fraction or extent of bone ingrowth. Residual articular cartilage was identified at the interface of implants 2 and 8. In both cases, the porous coating beneath these areas was devoid of bone ingrowth (Fig. 7).
Particulate Debris and Cellular Response at the Screw Holes Metallic debris was observed at the holes in the cup whether or not a screw was present (Fig. 10). This was seen in all of the cases except for implant 2 where metal debris was absent from the holes ,~vithout screws. Overall, the a m o u n t of debris and histiocytic response was small and localized. No granutome formations or evidence of bone Iysis were seen. The m e a n scores for metallic debris were less than 2 for debris in the hole and less than 1 in the adjacent porous coating (Table 6). There was no difference in the a m o u n t of metal debris or histiocytes as a function of the presence or absence of a screw, nor was
Table 5. Percent Extent of Tissue Type by Level (Mean -+ SD) Level l Periphery Bone Marrow Cartilage Fibrous Necrosis Other*
81.9 7,8 0.1 5.1 0.2 4.9
• + •
8.2 8.9 0.2 4.6 0.4 2.2
* screws, screw holes, metal fibers.
Level 2 Interface 29.7 9.6 3.6 45.6 1.2 10.3
_ • _ ~
20.1 1.4 3.4 28.3 2.9 4.0
Level 3 Coating 20.9 6.9 0.3 53.3 1.3 17.2
_ • _ • •
16.6 9.6 3.4 25.6 3.0 5.1
Level 4 Substrate 5.4 2.3 0.0 54.3 1.2 36.8
_ 8.2 _ 4.5 -+_ 13.2 • 2.8 • 6.0
220
The Journal of Arthroplasty Vol. 8 No. 2 April 1993
Fig. 6. Section of acetabular component retrieved 63 months after implantation. This case had 16.9% volume fraction of bone ingrowth, but in this field demonstrated fibrocartilage (original magnification • 88).
there a Correlation b e t w e e n length of implantation and either metal debris or histiocytic response. The metallic particles were similar in a p p e a r a n c e at holes with or without screws. The particles were irregular in shape and approximately 1 - 3 0 ~ m in diameter. They were found either loose within fibrous tissue, within histiocytes, or within apparent
F-i"P
lymphatic channels (Fig. 11). Occasionally, there were shards of metal up to 300 lzm in length. Metal debris scores were correlated with the histiocyte scores for holes w i t h o u t screws (r = .86; P < .01) and holes with screws (r = .88; P < .001). There was no relationship b e t w e e n the a m o u n t of debris and the a m o u n t of b o n e ingrowth in the cup. Poly-
...... Ill
Fig. 7. Section of an acetabular component retrieved 4 months after implantation. Note an area of retained articular cartilage between the porous coating and the acetabular bone (original magnification • 16).
Porous-coated Acetabular Components Retrieved at Autopsy
Fig. 8. Section of an acetabular component retrieved 54 months after implantation. In this field, fibrous ingrowth was demonstrated. The inteffacial membrane contained moderately dense connective tissue without histiocytes, plasma cells, or lymphocytes. Note the vascularity of this tissue and the reactive b o n e front (original magnificationi x 120).
,,
Fig. 9. Section of an acetabular component retrieved 71 months after implantation. This field demonstrated condensation of bone around the threads of a fixation screw with extensive bone ingrowth within the porous coating immediately adjacent to the screw (original magnification x 8).
9
,.
9 Pidhorz et al.
221
222
The Journal of Arthroplasty Vol. 8 No. 2 April 1993
Fig. 10. Section of an acetabular component from the same device as Figure 9. Note the metal debris within an unfilled screw hole and adjacent porous coating (original magnification • 32).
ethylene debris at the screw holes was rare, except for implants 6, 7, and 9 (51, 69, and 71 months, respectively) where polyethylene particles within foreign-body giant cells were observed only in holes without screws.
Particulate Debris at the Interface and Cup Rim Histiocytes were present at the rim of the cup in implants 7 and 9. In one case, this was due to a small gap between the fiber metal and the substrate where a small a m o u n t of metallic particles was found within the histiocytes. In the other case, fibers of the pseudocapsule penetrated the porous coating of the
Table 6. Mean Scores for Metal Particles and Histiocytes at Holes and Adjacent Coating With and Without Screws Screws
Metal particles in hole Metal particles in coating Histiocytes in hole Histiocytes in" coating
No S c r e w s
Mean
SD
Mean
SD
1.5
(0.4)
1.3
(0.7)
0.8
(0.6)
0.7
(0.7)
1.4 0.7
(0.6) (0.5)
1.4 0.8
(0.8) (0.7)
Metal particles per high p o w e r field ( • 500): 0 = None, 1 = 1-19, 2 = 2 0 - 4 9 9 , 3 = 500 or more. Hisfiocytes per high p o w e r field ( • 500):0 = None, I = 1 - 5 , 2 = 6 - 4 9 , 3 = 50 or more. ~9
superior rim at an area without bony coverage. At this site, histiocytes containing fine polyethylene debris were observed within the porous coating. Away from the rim in the interface membranes, fine metal~ lic debris was only seen within a few scattered histiocytes in six cups (implants 1, 3, 5, 7, 8, and 9). There was no indication in the one implant with a cemented femoral c o m p o n e n t that cement debris had migrated to the interface, the cup rim, or screw holes.
Discussion The systematic study of joint arthroplasty components and their surrounding tissues retrieved at autopsy provides unique information with regard to the performances of these devices. While observations made from clinical studies, from revision/retrieval studies, as well as from isolated case reports also provide useful information, only autopsy retrieval studies allow radiographic, histologic, and clinical correlations from patients with well-functioning prostheses. It is important to realize that while autopsy retrievals do provide a unique source of information, the subjects m a y have been chronically ill for some time before their ultimate demise. Thus, the presence of metaboli'c bone disease or other systemic disorders (ie, metastatic cancer, malnutrition, sepsis, ischemia, hypoxemia) can influence the local histologic appearance. Nonetheless, previous autopsy retrieval
Porous-coated Acetabular Components Retrieved at Autopsy
9 Pidhorz et al.
223
Fig. 11. Section of an acetabular component from the same device as Figure 9. Fine metallic debris was apparent within histiocytes and within the fibrous tissue within an unfilled screw hole (original magnification • 200). 9
t
;
~
~-
.
o
J
|o
9
studies on cemented femoral and acetabular components have enhanced our understanding of the longterm tissue reaction to methyl methacrylate and polyethylene and have suggested possible mechanisms of aseptic loosening. 4"~1,17,23,26 This study documented bone ingrowth in 10 of 11 acetabular components. The mean volume fraction of bone ingrowth of 12.1% was more than that previously reported. 6"~'22 While there are many factors that could account for the differences, this study included only autopsy retrievals, whereas the earlier reports included mostly prostheses retrieved at revision surgery due to failure of the reconstruction. Considerable variability in the amount of bone ingrowth was found within this study population. Of the four patients in this study with a volume fraction of bone ingrowth less than 10%, one was observed to have extensive amounts of articular cartilage at the bone-prosthesis interface, suggesting inadequate preparation of the acetabulum at the time of implantation. A second patient had a history of remote hip sepsis and was noted to have extensive necrosis of the periacetabular bone and pseudocapsule, and a third case was retrieved only 1 month after implantation. In six of the remaining seven cases, the volume fraction of bone ingrowth was consistent, ranging from 13.7% to 17.4%, with the seventh case having 27.6%. Undoubtedly, as with various experimental studies, a number of factors, which influence the host biology and mechanical stability (eg, extent of reaming and impaction, and the use of prophylactic agents
for heterotopic ossification), can be expected to influence the amount and pattern of bone ingrowth. 2~ As more autopsy specimens become available, the role of these factors as well as the development of biologic fixation over time will be elucidated. In spite of the variability in bone ingrowth in this study, all of the components demonstrated adequate fixation as judged by the clinical course and the findings of stable devices at autopsy. In these components the screws were obviously well incorporated within the host skeleton and must have provided some of this stability. This raises the question as to how much, if any, bone ingrowth is needed for adequate fixation in the presence of well-fixed adjuvant screws. In a group of 121 consecutively operated hips with a follow-up period between 55 and 79 months using cups of the same design reported in this study, all were found to be radiographically stable with no evidence of migration and none were revised fo r aseptic loosening. ~8 In the long term, it remains to be seen whether fixation will continue to be adequate, and what the effects of the newer surgical techniques of underreaming and avoiding adjuvant screw fixation will be. The finding that the mean volume fraction of cancellous bone distant from the prosthesis was on the order of the mean volume fraction of bone ingrowth (approximately 12%) suggests that a "goal" of 100% volume fraction of bone ingrowth may be neither feasible nor desirable. In the optimum situation, the implantation of a prosthetic device should minimize
224
The Journal of Arthroplasty Vol. 8 No. 2 April 1993
the perturbation of the local biomechanical milieu. Therefore, maintenance or reestablishment of local trabecular morphology within and surrounding the porous coating, while also providing for rigid implant fixation, would seem to be the desired endpoint of bony ingrowth into porous surfaces. Unfortunately, the trabecular volume fraction in these patients before implantation and before the onset of the joint pathology is not known. The main objective tool that the clinician has to monitor the performance of joint arthroplasty prostheses is the radiograph. For a hemispherical device, the bone-prosthesis interface is truly imaged in only one plane (where the incident x-ray beam is tangential to the periphery of the device). Despite this limitation, we found an inverse correlation between the amount of bone ingrowth and the number of radiolucent zones at the interface, indicating that the more extensive the radiolucencies are on clinical I radiographs, the more extensive is ingrowth with fibrous tissue. Considerable controversy exists concerning the use of s,~rews for adjuvant fixation of cementless acetabular components. 14,16,25The disadvantages of stabilizing screws include slightly prolonged operative time, the risk to intrapelvic neurovascular and visceral structures during insertion, the risk of fretting of the screw heads against the metal substrate or polyethylene liner, and the risk that the screw holes could serve as pathways for the migration of particulate wear debris and thereby compromise the bone-implant interface. The mechanical advantage of screw fixation is the improvement in immediate postoperative component stability and the potential for enhanced fixation by bone ingrowth. In this study, the screws influenced the pattern of bone ingrowth--50% more bone was seen in regions adjacent to filled screw holes in comparison to nonfilled holes. In addition, the screws themselves appeared to be very well fixed and may have made substantial contributions to the short- and intermediate-term stability of the implants. These observations raise the possibility that the screws may serve as stress concentrators and, therefore, could influence the pattern of bone remodeling and, perhaps, the long-term stability of the implant. The finding that the amount of metal debris was similar in holes with screws and those without screws suggests that screw/substrate fretting may not be the sole or even the main source of metal particle generation. Other potential sources include the polyethylene-facing surface of the substrate, the interface between the porous coating and the substrate, the porous coating itself, and the junction of the modular head and neck. Clearly, the composition of metal
particles needs to be characterized to identify the source(s) of the metallic debris. The role of unfilled screw holes as potential conduits for migration of wear debris was suggested by the presence of polyethylene debris within foreignbody giant cells in such holes in three longer term cases. However, none of the components examined here demonstrated granuloma formation nor was there histologic or radiographic evidence of osteolysis. Nevertheless, it is possible that osteolysis will develop in the long term if the particulate burden becomes excessive. The origin of this polyethylene debris is not known and needs to be identified. Invasion of a polyethylene-laden bone resorbing membrane from the articular margins of the implant was not observed in this study, suggesting that the polyethylene observed within empty screw holes may well have been generated from the substrate-facing surface of the liner.
Conclusions This autopsy retrieval study has shown that under the appropriate conditions substantial bone ingrowth in cementless porous-coated acetabular components can be achieved, although the optimal amount of bone ingrowth has not been defined. The degree of bone ingrowth was inversely correlated with the number of zones with radiolucencies at the bone-prosthesis interface. The use of screws enhanced bone ingrowth in the vicinity of the screws and may have made substantial direct contributions to the stability of the implants. Unfilled screw holes may potentially act as conduits for migration of wear debris to the bone-prosthesis interface, but in the cases studied no granulomatous reactions or osteolyric processes were observed. This issue needs to be further investigated in longer-term retrieval studies.
Acknowledgments Technical assistance was provided by Deborah Hall, Alberta Smith, Lisa DeLaurentis, and Susan Hitchingham. The authors would like to thank Drs. Blakey, Gitelis, Hunter, Kyzer, Rosenberg, and Sheinkop.
References 1. Amstutz HC, Kim WC, O'Carroll PF, Kabo JM: Canine porous resurfacing.hip arthroplasty: long-term results. Clin Orthop 207:270, 1986 2. Beauchesne RP, Kukita Y, Knezevich S, Suthers K:
Porous-coated Acetabular Components Retrieved at Autopsy
3.
4. 5.
6.
7.
8.
9.
10. 11.
12.
13.
14.
Roentgenographic evaluation of the AML porous coated acetabular component: a six year minimum follow-up study. Presented at the 59th Annual Meeting of the American Academy of Orthopaedic Surgeons, Washington, DC, February 1992 Callaghan J J, Dysart SH, Savory CG: The uncemented porous-coated anatomic total hip prosthesis: two-year results of a prospective consecutive series. J Bone Joint Surg 70A:337, 1988 Charnley J: Low friction arthroplasty of the hip: theory and practice. Springer-Verlag, Berlin, 1979 Collier JP, Bauer T, Bloebaum RD et al: Results of implant retrieval from post-mortem specimens in patients with well-functioning long term THR. Clin Orthop 274:97, 1992 Collier JP, Mayor MB, Chae JC et al: Macroscopic and microscopic evidence of prosthetic fixation with porous-coated materials, p. 97. In Tullos HS (ed): Instructional course lectures: American Academy of Orthopaedic Surgeons. CV Mosby, St. Louis, 1991 Cook SD: Clinical radiographic and histologic evaluation of retrieved human noncement porous coated implants. J Long-Term Effects of Medical Implants I: 11, 1991 De Lee JG, Charnley J: Radiological demarcation of cemented sockets in total hip replacement. Clin Orthop 121:20; 1976 Dodge BM, Fitzrandolph R, Collins DN: Noncemented porous-coated anatomic total hip arthroplasty. Clin Orthop 269:16, 1991 Engh CA, Griffin WL, Marx CL: Cementless acetabular components. J Bone Joint Surg 72B:53, 1990 Fornassier V, Wright J, Seligman J: The histomorphologic and morphometric study of asymptomatic hip arthroplasty: a postmortem study. Clin Orthop 271: 272, 1991 Haddad RJ, Cook SD, Brinker MR: A comparison of three varieties of non cemented porous-coated hip replacement. J Bone Joint Surg 72B:2, 1990 Jasty M, Harris WH: Observations on factors controlling bony ingrowth into weight-bearing, porous, canine total hip replacements, p. 175. In Fitzgerald R Jr (ed): Non-cemented total hip arthrop]asty. Raven Press, New York, 1988 Keating EM, Ritter MA, Farris PM: Structures at risk from medially placed acetabular screws. J Bone Joint Surg 72A:509, 1990
9 Pidhorz et al.
225
15. Kim Y-O, Kim VEM: Results of the Harris-Galante cementless hip prosthesis. J Bone Joint Surg 74B:83, 1992 16. Kirkpatrick JS, Callaghan J J, Vandemark RM, Goldner RD: The relationship of the intrapelvic vasculature to the acetabulum: implications in screw-fixation acetabular components. Clin Orthop 258:183, 1990 17. Maloney WJ, Jasty M, Burke DW et al: Biomechanical and histologic investigation of cemented total hip arthroplasties: a study of autopsy-retrieved femurs after in vivo cycling. Clin Orthop 249:129, 1989 18. Martell JM, Pierson RH III, Jacobs JJ et al: Primary total hip reconstruction with a cementless titanium fiber coated prosthesis. J Bone Joint Surg (in press) 19. Mirra JM, Amstutz HC, Matos M, Gold R: The pathology of the joint tissues and its clinical relevance in prosthesis failure. Clin Orthop 117:221, 1976 20. Sumner DR, Bryan JM, Urban RM, Kuszak JR: Measuring the volume fraction of bone ingrowth: a comparison of three techniques. J Orthop Res 8:448, 1990 21. Sumner DR, Galante JO: Bone ingrowth, p. 151. In Evarts CMcC (ed): Surgery of the musculoskeletal system. 2nd ed. Churchill Livingstone, New York, 1990 22. Sumner DR, Jasty M, Turner TM et al: Bone ingrowth in,porous-coated cementless acetabular components retrieved from human patients. Presented at the 33rd Annual Meeting of Orthopedic Research Society, San , Francisco, CA, January 1987 23. Schmalzried TP, Dwang LD, Jasty Met al: The mechanism of loosening of cemented acetabular components in total hip arthroplasty: analysis of specimens retrieved at autopsy. Ciin Orthop 274:60, 1992 24. Stulberg BN, Buly RL, Howard PL et al: Porous coated anatomic acetabular failure: incidence and modes of failure in uncemented total hip arthroplasty. Presented at the 59th Annual Meeting of the American Academy of Orthopaedic Surgeons, Washington, DC, February 1992 25. Wasielewski RC, Cooperstein LA, Kruger MP, Rubash HE: Acetabular anatomy and the transacetabular fixation of screws in total hip arthroplasty. J Bone Joint Surg 72A:501, 1990 26. Willert H-G, Ludwig J, Semlitsch M: Reaction of bone to methacrylate after hip arthroplasty: a long-term gross, light microscopic and scanning electron microscopic study. J Bone Joint Surg 56A: 1368, 1974
A Q u a n t i t a t i v e Study of B o n e and Soft Tissues in C e m e n t l e s s P o r o u s - c o a t e d Acetabular C o m p o n e n t s R e t r i e v e d at A u t o p s y Laurent
E. P i d h o r z , M D , R o b e r t D a l e R. S u m n e r ,
M. Urban, Joshua
J. J a c o b s ,
PhD, and Jorge O. Galante,
MD,
MD
Abstract: The authors examined 11 cementless acetabular components of one design retrieved at autopsy and made observations concerning tissue ingrowth and local tissue reaction, radiographic-histologic correlation, and the distribution of particulate wear debris. The cups were hemispherical in design with a commercially pure titanium fiber: metal porous coating. All of the prosthes.es were implanted with screws. The implants were in place for an average of 4 i months (range, 5'weeks to 75 months). Ten of the cups had bone ingrowth, with the average volume fraction being 12.1 • 8.2%. There were no differences in the amount of bone ingrowth when the component was partitioned into ninc anatomic regions. However, th,~re was more bone adjacent to screw holes through which screws were inserted compared with empty screw holes. As the number of radiolucent zones increased on the clinical radiographs less bone ingrowth was observed histologically. The amount of metal debris in holes with screws and holes without screws was similar. In the longest term cases, polyethylene debris was noted within empty screw holes, but no granulomatous reactions or osteolytie processes were observed. Key words: bone ingrowth, porous-coated implant, acctabular component, total hip arthroplasty, polyethylene, titanium.
Cementless acetabt, lar reconstruction with porous-coated c o m p o n e n t s has been used with increasing frequency in recent years. Excellent clinical resuits have been reported at the m i d - t e r m follow-up evaluation with a n u m b e r of such devices, xg,t~ ~5.~8 There are sonic concerns, however, that relate to their long-term performance. The durability of fixation is an issue that is related to the histological features of the interface and particularly to the type and extent of tissue ingrowth present. In addition, the potential for the production of metallic and polymeric debris resulting from fretting and/or w e a r of tile artictdar liner or stabilizing screws has given rise
to the concern that adverse tissue reactions m a y occur at the b o n e - p r o s t h e s i s interface. Recently, acetabular osteolysis has been reported in association with porous-coated c o m p o n e n t s in 27.8% of 115 asyml~tomatic patients 6 years following cementless primary total hip arthrop]asty (THA). 20steolysis has also been associated with excessive wear of the polyethylene bearing surface and onset of late pain, necessitating revision in 2.8% of 249 patients followed from 5 to 7 years. 24 Histologic studies of cementless acetabular components have been limited largely to specimens retrieved at revision surgery and have focused primarily on bone ingrowth. 6,7,22 Most reports have indicated that the a m o u n t of b o n e ingrowth was less anal its distribution different than expected based on early canine studies. L~3 However, specimens obtained at revision surgery m a y not necessarily reflect tile status of the devices in patients w h o are function-
From the Department of Orthopedic Surgery, Rttsh-Presbyterian-St. Luke's Medical Center, Chicago, Illinois. Supported by NIH grant AR39310 and Zimmer Inc. Reprint requests: Mr. Robert Urban, Department of Orthopedic Surgery, Rush-Presbyterian-St. Luke's Medical Center, i 653 West Congress Parkway, Chicago, IL 60612.
213
214
The Journal of Arthroplasty Vol. 8 No. 2 April 1993
ing normally and are asymptomatic. More relevant information can be obtained by the appropriate study of specimens retrieved at the time of autopsy. 5 The purpose of this study was to report the histological findings in a group of 11 acetabular c o m p o nents and surrounding tissues retrieved at autopsy. The acetabular components were all implanted without b o n e - c e m e n t , were of the same design, and were obtained from patients w h o had well functioning prostheses at the time of their demise.
Materials and Methods Patient Population Eleven cementless acetabular c o m p o n e n t s were retrieved at autopsy from eight patients w h o had previously undergone primary THA. In every case, the acetabu!ar c o m p o n e n t was a Harris-Galante I cup (Zimmei', Inc. Warsaw, IN). These were hemispherical components made of commercially pure titanium, with a porous layer on the full outside surface. The porous surface consisted of a 1.65 m m laye r of fiber metal made of commercially pure titanium wire, with a diameter of 250 ~ m and a m e a n pore size of 400 I.tm. The components contained an ultrahigh molecular weight polyethylene liner press fit into the metal shell. Seven of the implants (5 patients) came from our hospital (Rush-Presbyterian-St. Luke's, Chicago, IL) and four implants (3 patients) came from other institutions. Clinical information, obtained from the patients' medical records, included age, sex, the diagnosis at insertion, time in situ, Harris hip score, and cause of death (Table 1). The average age of the patients at the time of surgery was 56 years (range, 3 3 . 5 - 8 2 . 5 years). There were four w o m e n and four men. Six left and five right acetabular c o m p o n e n t s were obtained, and
three patients had bilateral THAs. The m e a n time in situ was 41 m o n t h s (range, 5 weeks to 75 months) and the m e a n time to last clinical examination was 35 months. Seven of the cups had been in place for more than 45 m o n t h s and the other 4 had been in place for 8 months or less. Femoral reconstruction consisted of a conventional length cementless titanium alloy fiber-metal prosthesis (Harris-Galante, Zimmer) in nine cases, a long-stem cementless titanium alloy fiber-metal prosthesis (BIAS, Zimmer) in one case (patient 6), and a cemented cobalt alloy prosthesis (Precoat, Zimmer) in one case (patient 4). In all of these c o m p o n e n t s the prosthetic head was made from a cobalt-chromium m o l y b d e n u m alloy. In all but one case, the surgical procedure was done by a direct lateral approach without trochanteric o s t e o t m o m y (Hardinge). For the procedures done at our institution, the components were inserted without impaction into an acetabular bed reamed to size. The initial fixation was achieved by 3-,5 cancellous, 4.5 or 5.1 m m screws of Ti6AI4V placed through the holes of the cup. The outer diameter of the c o m p o n e n t s ranged from 46 to 62 mm. At our hospital, the postoperative weight-bearing protocol varied depending u p o n the femoral reconstruction. For cementless femoral components, full weight bearing was not allowed until 6 weeks after surgery. These patients were advised to use crutches for 3 m o n t h s followed by a cane for 3 months. For the cemented femoral component, full weight bearing was allowed within the first postoperative week, and supports were generally used for 6 weeks. Harris hip scores were available only on cases from our institution. Five of these 7 hips had Harris hip scores in excess of 80 consistent with good to excellent clinical results. One patient with a bilateral THA (patient 8) had hip scores of 47 and 52. These low scores reflected diffuse bone pain with multiple pathologic fractures as a result of renal osteodystro-
T a b l e 1. Clinical Data in Patients With Cementless Porous-coated Acetabular Components
Patient No. l 2 2 3 4 5 5 6 7 8 8
Implant
Time in situ (too)
Side
1 2 3 4 5 6 7 8 9* tO 11
1 4 6 8 46 51 69 54 71 63 75
R R L L R L R L L L R
Last Harris Hip
Sex
Age at THA
Indication
F M M b,l F F F M F M M
75 39 39 54 83 65 64 67 60 35 34
OA AVN AVN RA OA AVN AVN OA Q_A. AVN AVN
Cause of Death
Score
Septicemia Pneumonia Metastatic CA, unknown primary Metastatic breast CA Metastaticlung CA Metastatic lung CA Metastalicbreast CA Renal failure
-95 97 97 83 92 52 47
OA, osteoarthritis; AVN, avascular necrosis; RA, rheumatoid arthritis; CA, carcinoma. *Trochanteric osteotomy fixed by wires.
Porous-coated Acetabular Components Retrieved at Autopsy
phy rather than specific symptoms related to his hip arthroplasties, which were functioning well at the time of the last examination 3 months prior to his death. This patient ultimately succumbed to the complications of renal failure and was the only case in this series with k n o w n metabolic bone disease. For the four implants obtained from other institutions (patients 1, 2, and 3), the patient's physician indicated satisfactory progress with their arthroplasty. These implants had been in place from 1 to 8 months. No patient w h o died as a result of cancer had metastatic disease in the periacetabular bone. No clinical or histologic evidence of infection in the periprosthetic tissues was found. Radiographs were available for review in 10 cups. For all the seven long-term implants, serial radiographs were available. The interface was divided into five zones modified after DeLee and Charuley, 8 with I zones 1 and 2 corresponding to zone A and zones 3 and 4 corresporiding to zone B. The presence or absence of a radiolucency at each zone was noted as well as the thickness and progression of radiolucent lines. No implant demonstrated radiographic migration according to criteria previously described. ~8
Methods of Analysis The components with surrounding bone and soft tissue were removed by osteotomy of the ischium, pubis, and ilium and were immediately immersed in 10% neutral buffered formalin. Before processing, contact anteroposterior and lateral radiographs and photographs were made. The polyethylene liner was removed by levering from the rim of the acetabular c o m p o n e n t without disturbing the underlying shell or fixation screws. The fixation screws were left in place. Each specimen was sectioned perpendicular to the opening of the cup from anterior to posterior into five to seven blocks of 10 m m thickness using a diam o n d wheel cut-offsaw. After dehydration in graduated alcohol solutions and xylene, the undecalcified blocks were embedded in methyl methacrylate m o n o m e r and polymer. W h e n polymerized, a thin section was cut from each block and ground to approximately 100 lzm and stained with toluidine blue and basic fuchsin for light microscopy. Quantitative data from five of these sections (2 anterior, 1 central, and 2 posterior) were gathered for each cup except for the 46 m m cup (implant 9), which had four stained sections. In addition, one anterior, one central, and one posterior section from each cup were carbon-coated for scanning electron microscopy. In order to express the quantity of different tissues within and around the cup, we defined the parameter, extent, as the fraction of 1 m m fields with a particular tissue type. A grid in the microscope eyepiece
9 Pidhorz et al.
215
Level I Periphery Levee 2 Interface
Level 3 Coating
Level 4 Substrate
Fig. 1. Method for determining the quantity of different tissues within and around the prosthesis. A grid was superimposed on microscopic fields at x 200 magnification. The extent of each tissue type was defined by the fraction of 1 mm fields containing that tissue. This was recorded for each of tile four levels shown.
was used to divide the stained sections into 1 m m field s (Fig. 1). At a magnification of • 200, the tissue within each field was categorized either as bone, marrow, cartilage, fibrous, or necrotic tissue. This was done at four locations: a 2 m m peripheral zone (level 1), the interface between the outer surface of the porous coating and the host bone (level 2), within the porous coating (level 3), and the interface between the porous coating and the substrate of the cup (level 4). The extent of each tissue type at each level was defined as the fraction of 1 m m fields positive for that tissue. The m a x i m u m possible value for each tissue type was 100%. W h e n an interfacial m e m b r a n e was present at the interface (level 2), its thickness was measured. An average of 286 1 m m fields was examined for each specimen, 32 fields for each of the 9 anatomic regions (anterosuperior, anteromid, anteroinferior, central-superior, central-mid, central-inferior, posterosuperior, posteromid, posteroinferior). From these data, the regional distribution of bone ingrowth was evaluated by: (1) comparing ingrowth among the nine anatomic regions, (2) comparing the area within 5 m m of the rim of the cup to the remainder of the cup, and (3) comparing the a m o u n t of bone ingrowth within 3 m m of screw holes containing screws to screw holes left empty. In addition, the data for the central section were also tabulated in five zones corresponding to the zones used in the radiographic analysis. The volume fraction of bone ingrowth was determined with the aid of backscattered scanning electron microscopy and an image analyzer. 2~ This vari-
216
The Journal of Arthroplasty Vol. 8 No. 2 April 1993
able was defined as the fraction of void space within the porous coating occupied by mineralized bone. The m a x i m u m possible value was 100%. The entire porous coating from three sections (anterior, central, and posterior) was analyzed. The average total area analyzed was 322 m m 2. In addition, to determine the density of the periaeetabular bone bed, the volume fraction of the cancellous bone outside of the porous coating was also measured using the central section. For this measurement, three sites (superior, mid, and inferior) were chosen so that the area measured was as far removed from the implant as possible. An average of 66 m m 2 of tissue per cup was assessed. Particulate debris and associated cellular response were studied in the stained sections by regular and polarized light microscopy. At the screw holes, the a m o u n t of metallic and polymeric debris and the degree of the histiocyte and foreign-body giant cell reaction Were graded, 0 t h r o u g h 3, according to the technique of Mirra for metal particles and m o n o n u clear histiocytes.~9 This was done for the area within the hole and for the porous coating a r o u n d the hole. The scores for holes with screws were compare d to those of holes left empty. Twenty-three h o l e s w i t h screws and 31 holes without screws were evaluated. Also, the presence or absence of debris and cellular infiltrate were noted at the interface between the outer surface of the porous coating and the host bone and at the cup rim. Means, standard deviations, and ranges were calculated for all of the variables. Repeated measures analyses of variance were used to test for regional differences in the distribution of bone ingrowth. Pearson product m o m e n t and Spearman rank order correlation analyses were performed to assess the strength of association a m o n g variables. For the analysis of debris and cellular response, the Wilcoxon malched-pairs signed-ranks test was used to compare holes with screws to those without. Because of the small sample size and the potential multivariate nature of the data, it was not possible to test for the statistical influence of age, sex, length of implantation, or n u m b e r of fixation screws on the a m o u n t and distribution of bone ingrowth.
Table 2. Volume Fraction and Extent of Bone Ingrowth Time Implant
Bone ingrowth was present in 10 of the 11 cups (Table 2). The mean volume fraction of bone ingrowth was 12.1 _ 8.2% and the m e a n extent of bone at the interface (level 2) was 29.7 • 20.1%. These two measures of b o n y fixation were strongly correlated (r = 0.91; P < .001).
(mo)
1
1
2 3 4 5 6 7
4 6 8 46 51 69
4.8 13.7 14.5 0.8 4.6 15.2 16.6
8
54
1.5
9 10 11
71 63 75
27.6 16.9 17.4
Extent (%) 12.7 21.7 18.3 0.0 3.4 21.5 26.3 7.9 55.0 18.7 44.7
The a m o u n t of bone did not differ a m o n g the nine defined regions. There was, however, more bone at the interface (29.7 • 20.1%) than within the coating (20.9 __ 16.6%) or at the substrate (5.4 - 8.2%; P < .001). The a m o u n t of bone within 5 m m of the rim did not vary significantly from the remainder of t h e cup, although there was a tendency for more bone ingrowth to be near the rim (29.1 +- 26.7%) than elsewhere (19.0 • 15.2 %). There was approximately 50% more b o n e ingrowth adjacent to holes with screws than to holes left empty (P < .05, Table 3). Bone ingrowth in the 1-month specimen consisted of slender trabeculae of w o v e n bone oriented randomly within the pores (Fig. 2). In the 4-~8-month cups the ingrown bone was composed of thickened, lamellar trabeculae with occasional Haversian-like structures (Fig. 3). The components in situ for longer than 4 years also demonstrated lamellar trabeculae, but at the rim of the cup and surrounding the screw holes the ingrown bone often completely filled the pores and Haversian structures were c o m m o n (Fig. 4). The patient with renal osteodystrophy d e m o n strated ingrown trabeculae, which were sometimes
Table 3. Percent Extent of Bone Ingrowth Within 3 mm of Holes With Screws and Holes Without Screws (Mean +_ SD)
Results Bone Ingrowth
Volume Fraction (%)
in situ
Interface Coating Substrate
Screws
No Screws
47.2 ~- 33.8 34.7 • 34.0 4.4 • 14.1
33.4 - 27.7 19.5 +_ 16.0 2.1 • 5.1
The data were analyzed by repeated measures analyses of vari.a~acewith depth (interface vs. coating vs. substrate) and presence/ absence of a screw as the.within subjects factors. Depth was a significant factor (P < .001) as was presence/absence of a screw (P < .05). There were no significant interactions between depth and presence/absence of a screw.
Porous-coated Acetabular Components Retrieved at Autopsy
9 Pidhorz et al.
217
Fig. 2. Section of an acetabular component retrieved 1 month after implantation. Note the ingrowth of narrow trabeculae of woven bone (original magnification • 88).
extremely thin with an irregular surface and almost entirely covered by a thin, pale, pink layer of osteoid (Fig. 5). The volume fraction of periacetabular cancellous bone was 12.0 _ 6.5%, but was not correlated with the a m o u n t of bone ingrowth even though their mean values were nearly identical.
Fig. 3. Section of an acetabular component retrieved 6 months after implantation. This field demonstrates ingrowth of lamellar bone with thick trabeculae and occasional Haversian systems (original magnification • 16).
Radiographic Analysis and Correlation With Bone Ingrowth Of the seven long-term cases, three had no radiolucencies, one had a radiolucency in one zone, one had radiolucencies in two zones, and two had radiolucencies in three zones. None of these cases had radio-
218
The Journal of Arthroplasty Vol. 8 No. 2 April 1993
Fig. 4. Section of an acetabular component retrieved 63 months after implantation. In this longer-term retrieval, note the Haversian systems with extensive bone ingrowth (original magnification x 160).
lucencies in more than three zones. All radiolucencies were less than 1 m m and nonprogressive, except for one hip in the patient with renal osteodystrophy (patient 8), that demonstrated a progressive radiolucency in one zone between 1 and 2 m m in thickness. The n u m b e r of zones with radiolucencies was inversely related to the extent of bone at the interface
Fig. 5. Section of an acetabular component retrieved 75 months after implantation. This patient had renal osteodystrophy and demonstrated foci of thin irregular trabeculae surrounded by marrow elements (original magnification • 160). Most trabeculae were covered by a thin osteoid layer, which does not reproduce well in black-and-white photomicrographs.
(r = - 0 . 8 5 ; P < .05), and bone ingrowth within the coating (r = - 0 . 7 6 ; P < .05) and at the substrate (r = - 0 . 7 8 ; P < .05) In addition,in direct comparison between the radiographic analysis and the central histologic section, the zones that had no radiolucencies had t w i c e as m u c h bone ingrowth at the interface and three times as m u c h within the porous
Porous-coated Acetabular Components Retrieved at Autopsy
Table 4. Percent Extent of Bone Ingrowth in Zones With Lucency and Zones Without Lucency (Mean -+ SD)
Interface Coating * P < . 0 5 ; 'f P =
No Lucency
Lucency
43.3 - 30.3 32.4 • 31.7
19.3"I- • 31.5 10.9" • 19.9
9 Pidhorz et al.
219
In most of the areas without bone ingrowth, however, the interface (level 2) consisted of loose to moderately dense fibrous tissue, often containing dilated vessels (Fig. 8). The membranes Varied greatly in thickness, depending u p o n the a m o u n t of bone ingrowth. The cups with less than 10% v o l u m e fraction of bone ingrowth (implants I, 4,: 5, and 8) had thicker membranes (460 +_ 67 lira) than the cups with more than 10% bone ingrowth (182 _+ 71 Ftm; P < .001). Few lymphocytes or plasma cells were present in the interface membranes or within the porous coating except for three patients (1, 3, 4). The patient with rheumatoid disease (patient 3) had a mild, diffuse lymphocytic infiltrate. The patients w h o died at 1 m o n t h and 46 m o n t h s after surgery (patients 1 and 4) had moderate, focal accumulations of lymphocytes. Polymorphonuclear leukocytes were rarely observed. In all cases, all of the fixation screws appeared to be well fixed. In the case harvested at 1 month, the surface of the threads were surrounded by thin spicules Of bone that conformed to the threads. In the o t h e r 10 cases, the screw threads were covered by thickened trabeculae or dense plates of lamellar bone. (Fig. 9).
.068.
coating (P < .05) than the zones in which a radiolucency was present (Table 4).
Extent of Marrow, Cartilage, and Fibrous Tissues The extent of the tissue types at the four levels studied is s h o w n in Table 5. As expected, the 2-ram zone peripheral t o the c o m p o n e n t s (level 1) consisted primarily 6f bone and marrow. At the interface (level 2) as well as within the pores of the coating (level 3), there were some fields in which m a r r o w was present without bone. In these areas, the normal matrix of medullary m a r r o w pene= trated deeply between the metal fibers w i t h o u t an intervening fibrous membrane, thus giving the appearance of integration with the surrounding acetabular tissues in implants 6, 7, 10, and 11 (51, 69, 63, and 75 months, respectively). In contrast, foci of fibrocartilage were present at the interface (level 2) in l0 of the cups (extent, 0 . 4 - 6 . 3 % ) , but were seldom seen deep within the porous coating {Fig. 6) and never adjacent to screws. The a m o u n t of fibrocartilage at the interface was greatest in the implant with the least bone ingrowth (implant 4) and no fibrocartilage was found in the implant that had the greatest a m o u n t of bone ingrowth (implant 9). However, there was no correlation between the a m o u n t of fibrocartilage at the interface and either the volume fraction or extent of bone ingrowth. Residual articular cartilage was identified at the interface of implants 2 and 8. In both cases, the porous coating beneath these areas was devoid of bone ingrowth (Fig. 7).
Particulate Debris and Cellular Response at the Screw Holes Metallic debris was observed at the holes in the cup whether or not a screw was present (Fig. 10). This was seen in all of the cases except for implant 2 where metal debris was absent from the holes ,~vithout screws. Overall, the a m o u n t of debris and histiocytic response was small and localized. No granutome formations or evidence of bone Iysis were seen. The m e a n scores for metallic debris were less than 2 for debris in the hole and less than 1 in the adjacent porous coating (Table 6). There was no difference in the a m o u n t of metal debris or histiocytes as a function of the presence or absence of a screw, nor was
Table 5. Percent Extent of Tissue Type by Level (Mean -+ SD) Level l Periphery Bone Marrow Cartilage Fibrous Necrosis Other*
81.9 7,8 0.1 5.1 0.2 4.9
• + •
8.2 8.9 0.2 4.6 0.4 2.2
* screws, screw holes, metal fibers.
Level 2 Interface 29.7 9.6 3.6 45.6 1.2 10.3
_ • _ ~
20.1 1.4 3.4 28.3 2.9 4.0
Level 3 Coating 20.9 6.9 0.3 53.3 1.3 17.2
_ • _ • •
16.6 9.6 3.4 25.6 3.0 5.1
Level 4 Substrate 5.4 2.3 0.0 54.3 1.2 36.8
_ 8.2 _ 4.5 -+_ 13.2 • 2.8 • 6.0
220
The Journal of Arthroplasty Vol. 8 No. 2 April 1993
Fig. 6. Section of acetabular component retrieved 63 months after implantation. This case had 16.9% volume fraction of bone ingrowth, but in this field demonstrated fibrocartilage (original magnification • 88).
there a Correlation b e t w e e n length of implantation and either metal debris or histiocytic response. The metallic particles were similar in a p p e a r a n c e at holes with or without screws. The particles were irregular in shape and approximately 1 - 3 0 ~ m in diameter. They were found either loose within fibrous tissue, within histiocytes, or within apparent
F-i"P
lymphatic channels (Fig. 11). Occasionally, there were shards of metal up to 300 lzm in length. Metal debris scores were correlated with the histiocyte scores for holes w i t h o u t screws (r = .86; P < .01) and holes with screws (r = .88; P < .001). There was no relationship b e t w e e n the a m o u n t of debris and the a m o u n t of b o n e ingrowth in the cup. Poly-
...... Ill
Fig. 7. Section of an acetabular component retrieved 4 months after implantation. Note an area of retained articular cartilage between the porous coating and the acetabular bone (original magnification • 16).
Porous-coated Acetabular Components Retrieved at Autopsy
Fig. 8. Section of an acetabular component retrieved 54 months after implantation. In this field, fibrous ingrowth was demonstrated. The inteffacial membrane contained moderately dense connective tissue without histiocytes, plasma cells, or lymphocytes. Note the vascularity of this tissue and the reactive b o n e front (original magnificationi x 120).
,,
Fig. 9. Section of an acetabular component retrieved 71 months after implantation. This field demonstrated condensation of bone around the threads of a fixation screw with extensive bone ingrowth within the porous coating immediately adjacent to the screw (original magnification x 8).
9
,.
9 Pidhorz et al.
221
222
The Journal of Arthroplasty Vol. 8 No. 2 April 1993
Fig. 10. Section of an acetabular component from the same device as Figure 9. Note the metal debris within an unfilled screw hole and adjacent porous coating (original magnification • 32).
ethylene debris at the screw holes was rare, except for implants 6, 7, and 9 (51, 69, and 71 months, respectively) where polyethylene particles within foreign-body giant cells were observed only in holes without screws.
Particulate Debris at the Interface and Cup Rim Histiocytes were present at the rim of the cup in implants 7 and 9. In one case, this was due to a small gap between the fiber metal and the substrate where a small a m o u n t of metallic particles was found within the histiocytes. In the other case, fibers of the pseudocapsule penetrated the porous coating of the
Table 6. Mean Scores for Metal Particles and Histiocytes at Holes and Adjacent Coating With and Without Screws Screws
Metal particles in hole Metal particles in coating Histiocytes in hole Histiocytes in" coating
No S c r e w s
Mean
SD
Mean
SD
1.5
(0.4)
1.3
(0.7)
0.8
(0.6)
0.7
(0.7)
1.4 0.7
(0.6) (0.5)
1.4 0.8
(0.8) (0.7)
Metal particles per high p o w e r field ( • 500): 0 = None, 1 = 1-19, 2 = 2 0 - 4 9 9 , 3 = 500 or more. Hisfiocytes per high p o w e r field ( • 500):0 = None, I = 1 - 5 , 2 = 6 - 4 9 , 3 = 50 or more. ~9
superior rim at an area without bony coverage. At this site, histiocytes containing fine polyethylene debris were observed within the porous coating. Away from the rim in the interface membranes, fine metal~ lic debris was only seen within a few scattered histiocytes in six cups (implants 1, 3, 5, 7, 8, and 9). There was no indication in the one implant with a cemented femoral c o m p o n e n t that cement debris had migrated to the interface, the cup rim, or screw holes.
Discussion The systematic study of joint arthroplasty components and their surrounding tissues retrieved at autopsy provides unique information with regard to the performances of these devices. While observations made from clinical studies, from revision/retrieval studies, as well as from isolated case reports also provide useful information, only autopsy retrieval studies allow radiographic, histologic, and clinical correlations from patients with well-functioning prostheses. It is important to realize that while autopsy retrievals do provide a unique source of information, the subjects m a y have been chronically ill for some time before their ultimate demise. Thus, the presence of metaboli'c bone disease or other systemic disorders (ie, metastatic cancer, malnutrition, sepsis, ischemia, hypoxemia) can influence the local histologic appearance. Nonetheless, previous autopsy retrieval
Porous-coated Acetabular Components Retrieved at Autopsy
9 Pidhorz et al.
223
Fig. 11. Section of an acetabular component from the same device as Figure 9. Fine metallic debris was apparent within histiocytes and within the fibrous tissue within an unfilled screw hole (original magnification • 200). 9
t
;
~
~-
.
o
J
|o
9
studies on cemented femoral and acetabular components have enhanced our understanding of the longterm tissue reaction to methyl methacrylate and polyethylene and have suggested possible mechanisms of aseptic loosening. 4"~1,17,23,26 This study documented bone ingrowth in 10 of 11 acetabular components. The mean volume fraction of bone ingrowth of 12.1% was more than that previously reported. 6"~'22 While there are many factors that could account for the differences, this study included only autopsy retrievals, whereas the earlier reports included mostly prostheses retrieved at revision surgery due to failure of the reconstruction. Considerable variability in the amount of bone ingrowth was found within this study population. Of the four patients in this study with a volume fraction of bone ingrowth less than 10%, one was observed to have extensive amounts of articular cartilage at the bone-prosthesis interface, suggesting inadequate preparation of the acetabulum at the time of implantation. A second patient had a history of remote hip sepsis and was noted to have extensive necrosis of the periacetabular bone and pseudocapsule, and a third case was retrieved only 1 month after implantation. In six of the remaining seven cases, the volume fraction of bone ingrowth was consistent, ranging from 13.7% to 17.4%, with the seventh case having 27.6%. Undoubtedly, as with various experimental studies, a number of factors, which influence the host biology and mechanical stability (eg, extent of reaming and impaction, and the use of prophylactic agents
for heterotopic ossification), can be expected to influence the amount and pattern of bone ingrowth. 2~ As more autopsy specimens become available, the role of these factors as well as the development of biologic fixation over time will be elucidated. In spite of the variability in bone ingrowth in this study, all of the components demonstrated adequate fixation as judged by the clinical course and the findings of stable devices at autopsy. In these components the screws were obviously well incorporated within the host skeleton and must have provided some of this stability. This raises the question as to how much, if any, bone ingrowth is needed for adequate fixation in the presence of well-fixed adjuvant screws. In a group of 121 consecutively operated hips with a follow-up period between 55 and 79 months using cups of the same design reported in this study, all were found to be radiographically stable with no evidence of migration and none were revised fo r aseptic loosening. ~8 In the long term, it remains to be seen whether fixation will continue to be adequate, and what the effects of the newer surgical techniques of underreaming and avoiding adjuvant screw fixation will be. The finding that the mean volume fraction of cancellous bone distant from the prosthesis was on the order of the mean volume fraction of bone ingrowth (approximately 12%) suggests that a "goal" of 100% volume fraction of bone ingrowth may be neither feasible nor desirable. In the optimum situation, the implantation of a prosthetic device should minimize
224
The Journal of Arthroplasty Vol. 8 No. 2 April 1993
the perturbation of the local biomechanical milieu. Therefore, maintenance or reestablishment of local trabecular morphology within and surrounding the porous coating, while also providing for rigid implant fixation, would seem to be the desired endpoint of bony ingrowth into porous surfaces. Unfortunately, the trabecular volume fraction in these patients before implantation and before the onset of the joint pathology is not known. The main objective tool that the clinician has to monitor the performance of joint arthroplasty prostheses is the radiograph. For a hemispherical device, the bone-prosthesis interface is truly imaged in only one plane (where the incident x-ray beam is tangential to the periphery of the device). Despite this limitation, we found an inverse correlation between the amount of bone ingrowth and the number of radiolucent zones at the interface, indicating that the more extensive the radiolucencies are on clinical I radiographs, the more extensive is ingrowth with fibrous tissue. Considerable controversy exists concerning the use of s,~rews for adjuvant fixation of cementless acetabular components. 14,16,25The disadvantages of stabilizing screws include slightly prolonged operative time, the risk to intrapelvic neurovascular and visceral structures during insertion, the risk of fretting of the screw heads against the metal substrate or polyethylene liner, and the risk that the screw holes could serve as pathways for the migration of particulate wear debris and thereby compromise the bone-implant interface. The mechanical advantage of screw fixation is the improvement in immediate postoperative component stability and the potential for enhanced fixation by bone ingrowth. In this study, the screws influenced the pattern of bone ingrowth--50% more bone was seen in regions adjacent to filled screw holes in comparison to nonfilled holes. In addition, the screws themselves appeared to be very well fixed and may have made substantial contributions to the short- and intermediate-term stability of the implants. These observations raise the possibility that the screws may serve as stress concentrators and, therefore, could influence the pattern of bone remodeling and, perhaps, the long-term stability of the implant. The finding that the amount of metal debris was similar in holes with screws and those without screws suggests that screw/substrate fretting may not be the sole or even the main source of metal particle generation. Other potential sources include the polyethylene-facing surface of the substrate, the interface between the porous coating and the substrate, the porous coating itself, and the junction of the modular head and neck. Clearly, the composition of metal
particles needs to be characterized to identify the source(s) of the metallic debris. The role of unfilled screw holes as potential conduits for migration of wear debris was suggested by the presence of polyethylene debris within foreignbody giant cells in such holes in three longer term cases. However, none of the components examined here demonstrated granuloma formation nor was there histologic or radiographic evidence of osteolysis. Nevertheless, it is possible that osteolysis will develop in the long term if the particulate burden becomes excessive. The origin of this polyethylene debris is not known and needs to be identified. Invasion of a polyethylene-laden bone resorbing membrane from the articular margins of the implant was not observed in this study, suggesting that the polyethylene observed within empty screw holes may well have been generated from the substrate-facing surface of the liner.
Conclusions This autopsy retrieval study has shown that under the appropriate conditions substantial bone ingrowth in cementless porous-coated acetabular components can be achieved, although the optimal amount of bone ingrowth has not been defined. The degree of bone ingrowth was inversely correlated with the number of zones with radiolucencies at the bone-prosthesis interface. The use of screws enhanced bone ingrowth in the vicinity of the screws and may have made substantial direct contributions to the stability of the implants. Unfilled screw holes may potentially act as conduits for migration of wear debris to the bone-prosthesis interface, but in the cases studied no granulomatous reactions or osteolyric processes were observed. This issue needs to be further investigated in longer-term retrieval studies.
Acknowledgments Technical assistance was provided by Deborah Hall, Alberta Smith, Lisa DeLaurentis, and Susan Hitchingham. The authors would like to thank Drs. Blakey, Gitelis, Hunter, Kyzer, Rosenberg, and Sheinkop.
References 1. Amstutz HC, Kim WC, O'Carroll PF, Kabo JM: Canine porous resurfacing.hip arthroplasty: long-term results. Clin Orthop 207:270, 1986 2. Beauchesne RP, Kukita Y, Knezevich S, Suthers K:
Porous-coated Acetabular Components Retrieved at Autopsy
3.
4. 5.
6.
7.
8.
9.
10. 11.
12.
13.
14.
Roentgenographic evaluation of the AML porous coated acetabular component: a six year minimum follow-up study. Presented at the 59th Annual Meeting of the American Academy of Orthopaedic Surgeons, Washington, DC, February 1992 Callaghan J J, Dysart SH, Savory CG: The uncemented porous-coated anatomic total hip prosthesis: two-year results of a prospective consecutive series. J Bone Joint Surg 70A:337, 1988 Charnley J: Low friction arthroplasty of the hip: theory and practice. Springer-Verlag, Berlin, 1979 Collier JP, Bauer T, Bloebaum RD et al: Results of implant retrieval from post-mortem specimens in patients with well-functioning long term THR. Clin Orthop 274:97, 1992 Collier JP, Mayor MB, Chae JC et al: Macroscopic and microscopic evidence of prosthetic fixation with porous-coated materials, p. 97. In Tullos HS (ed): Instructional course lectures: American Academy of Orthopaedic Surgeons. CV Mosby, St. Louis, 1991 Cook SD: Clinical radiographic and histologic evaluation of retrieved human noncement porous coated implants. J Long-Term Effects of Medical Implants I: 11, 1991 De Lee JG, Charnley J: Radiological demarcation of cemented sockets in total hip replacement. Clin Orthop 121:20; 1976 Dodge BM, Fitzrandolph R, Collins DN: Noncemented porous-coated anatomic total hip arthroplasty. Clin Orthop 269:16, 1991 Engh CA, Griffin WL, Marx CL: Cementless acetabular components. J Bone Joint Surg 72B:53, 1990 Fornassier V, Wright J, Seligman J: The histomorphologic and morphometric study of asymptomatic hip arthroplasty: a postmortem study. Clin Orthop 271: 272, 1991 Haddad RJ, Cook SD, Brinker MR: A comparison of three varieties of non cemented porous-coated hip replacement. J Bone Joint Surg 72B:2, 1990 Jasty M, Harris WH: Observations on factors controlling bony ingrowth into weight-bearing, porous, canine total hip replacements, p. 175. In Fitzgerald R Jr (ed): Non-cemented total hip arthrop]asty. Raven Press, New York, 1988 Keating EM, Ritter MA, Farris PM: Structures at risk from medially placed acetabular screws. J Bone Joint Surg 72A:509, 1990
9 Pidhorz et al.
225
15. Kim Y-O, Kim VEM: Results of the Harris-Galante cementless hip prosthesis. J Bone Joint Surg 74B:83, 1992 16. Kirkpatrick JS, Callaghan J J, Vandemark RM, Goldner RD: The relationship of the intrapelvic vasculature to the acetabulum: implications in screw-fixation acetabular components. Clin Orthop 258:183, 1990 17. Maloney WJ, Jasty M, Burke DW et al: Biomechanical and histologic investigation of cemented total hip arthroplasties: a study of autopsy-retrieved femurs after in vivo cycling. Clin Orthop 249:129, 1989 18. Martell JM, Pierson RH III, Jacobs JJ et al: Primary total hip reconstruction with a cementless titanium fiber coated prosthesis. J Bone Joint Surg (in press) 19. Mirra JM, Amstutz HC, Matos M, Gold R: The pathology of the joint tissues and its clinical relevance in prosthesis failure. Clin Orthop 117:221, 1976 20. Sumner DR, Bryan JM, Urban RM, Kuszak JR: Measuring the volume fraction of bone ingrowth: a comparison of three techniques. J Orthop Res 8:448, 1990 21. Sumner DR, Galante JO: Bone ingrowth, p. 151. In Evarts CMcC (ed): Surgery of the musculoskeletal system. 2nd ed. Churchill Livingstone, New York, 1990 22. Sumner DR, Jasty M, Turner TM et al: Bone ingrowth in,porous-coated cementless acetabular components retrieved from human patients. Presented at the 33rd Annual Meeting of Orthopedic Research Society, San , Francisco, CA, January 1987 23. Schmalzried TP, Dwang LD, Jasty Met al: The mechanism of loosening of cemented acetabular components in total hip arthroplasty: analysis of specimens retrieved at autopsy. Ciin Orthop 274:60, 1992 24. Stulberg BN, Buly RL, Howard PL et al: Porous coated anatomic acetabular failure: incidence and modes of failure in uncemented total hip arthroplasty. Presented at the 59th Annual Meeting of the American Academy of Orthopaedic Surgeons, Washington, DC, February 1992 25. Wasielewski RC, Cooperstein LA, Kruger MP, Rubash HE: Acetabular anatomy and the transacetabular fixation of screws in total hip arthroplasty. J Bone Joint Surg 72A:501, 1990 26. Willert H-G, Ludwig J, Semlitsch M: Reaction of bone to methacrylate after hip arthroplasty: a long-term gross, light microscopic and scanning electron microscopic study. J Bone Joint Surg 56A: 1368, 1974