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Eight- to Ten-Year Clinical and Radiographic Outcome of a Porous Tantalum Monoblock Acetabular Component
The Journal of Arthroplasty Vol. 24 No. 5 2009
Eight- to Ten-Year Clinical and Radiographic Outcome of a Porous Tantalum Monoblock Acetabular Component George Macheras, MD,*1 Konstantinos Kateros, MD, y Athanassios Kostakos, MD,* Stefanos Koutsostathis, MD,* Dimitrios Danomaras, MD,* and Panayiotis J. Papagelopoulos, MD, DSc z1
Abstract: In a prospective study, the authors used a porous tantalum monoblock acetabular component for primary total hip arthroplasty between November 1997 and June 1999. A total of 156 consecutive primary total hip arthroplasty were done in 143 patients younger than 75 years. A total of 151 hips had a follow-up time from 8 to 10 years. The average preoperative total Harris hip score of 44.0 ± 13.8 increased to 97.0 ± 6.2 at the latest follow-up. The average preoperative Oxford hip score of 43.3 ± 6.5 improved to 13.9 ± 2.3 at the latest follow-up. Radiographic evaluation including the Ein-Bild-Röntgen-Analyse (EBRA) digital system showed no radiographic evidence of gross polyethylene wear, progressive radiolucencies, osteolytic lesions, acetabular fracture, or component subsidence. There were 7 (4.5%) postoperative complications all unrelated to the acetabular component. Key words: porous tantalum, monoblock acetabular cup, total hip arthroplasty, osteolysis, polyethylene wear. © 2009 Elsevier Inc. All rights reserved.
Successful acetabular component implantation in primary total hip arthroplasty (THA) requires correct component position, immediate component macrofixation, component stability, and an optimum surface for biologic on-growth surface preparations. Host bone morphometry involving an intact acetabular rim for press-fit stability is necessary for immediate component macrofixation [1]. The por-
osity of the acetabular component and the porous size is an important factor for quick and safe bone ingrowth [2]. The avoidance of acetabular component failure involves both manufacturing and surgical/technical variables. These include cup to liner conformity, locking mechanism integrity, and polyethylene thickness, which will vary by component design geometry [3,4]. The optimum placement of a noncemented press-fit acetabular component within the so-called acetabular arch, to capture the peripheral, has been described [5]. Gaining optimum peripheral press-fit stability and maximizing component macrofixation allows for subsequent biologic fixation (ingrowth/on-growth) and longterm acetabular component durability. Osteolysis produced by biologic reaction to polyethylene and metallic debris is the major failure mode of cemented or uncemented acetabular fixation [3-11]. The advantages of a monoblock acetabular uncemented component have been
From the *Department of Orthopaedics, IKA Hospital, Athens, Greece; ySecond Department of Orthopaedics, Athens University Medical School, Athens, Hellenic Republic, Greece; and zFirst Department of Orthopaedics, Athens University Medical School, ATTIKON University Hospital, Athens, Hellenic Republic, Greece. Submitted August 12, 2007; accepted June 16, 2008. No benefits or funds were received in support of this study. Reprint requests: Panayiotis J. Papagelopoulos, MD, DSc, First Department of Orthopaedics, Athens University Medical School, 4 Christovassili Street 15451, Neon Psychikon, Athens, Greece. 1 The first and last authors contributed equally to this work. © 2009 Elsevier Inc. All rights reserved. 0883-5403/08/2405-0008$36.00/0 doi:10.1016/j.arth.2008.06.020
705
706 The Journal of Arthroplasty Vol. 24 No. 5 August 2009 reported. First, there is no extra-articular back surface polyethylene wear. Second, locking rings that may generate metallic debris are eliminated. Third, screw holes, which decrease the surface area for ingrowth, are not required, and pelvic entrance points for wear debris are eliminated. Fourth, an elliptical configuration allows better cooptation of the shell to the dome of the acetabulum [11]. Tantalum metal for orthopedic use was initially introduced in 1997, with a subsequent rapid evolution of orthopedic applications [11-16]. The use of tantalum metal for the acetabular component in THA was developed to enhance the fixation properties, diminish polyethylene wear, decrease aseptic loosening, and avoid the potential pathways. Its porosity is 80% of the total volume and the porous size 550 μm, which is optimal for bone ingrowth and on-growth. The purpose of this prospective study was to monitor and report the 8- to 10-year prospective clinical and radiographic results of the use of a porous tantalum monoblock acetabular component for primary THA.
Materials and Methods Between November 1997 and June 1999, 156 consecutive primary THAs were done in 143 patients younger than 75 years. There were 96 female (64.36%) and 47 male patients (35.64%). The mean age of females was 61.1 ± 11.3 years (range, 32-75 years) and of males, 57.2 ± 14.1 years (range, 24-75 years). Osteoarthritis was the primary underlying diagnosis in 116 hips. Other causes of the end-stage hip disease include developmental dysplasia of the hip in 26 hips, avascular necrosis in 11 hips, and rheumatoid arthritis (RA) in 3 hips. A press-fit porous tantalum monoblock acetabular component (Trabecular Metal Monoblock Acetabular Component System; Zimmer Inc, Warsaw, Ind) was used in all cases. The component has 2 distinct profiles: solid trabecular metal (TM) backing without peripheral screws and solid TM backing with peripheral screw holes for adjunctive fixation. All the procedures were performed by the senior surgeon using a posterior surgical approach. The acetabular component was inserted using press-fit technique, taking particular care to prevent soft tissue interposition between the implant and the acetabulum bone during implantation. The acetabulum was prepared with hemispherical reamers. The diameter of the final reamer matches the polar diameter of the acetabular component and is 2 mm less than the equatorial diameter.
No bone grafting was used in any of these acetabular reconstructions. Intraoperatively, the initial stability of the acetabular prosthesis was accessed manually and was considered satisfactory in all cases. There were 5 cases of developmental dysplasia of the hip in which peripheral screws were used to gain additional acetabular component stability: one with one screw and 4 where 2 screws were used. The Continuum T Cast Hip Stem F-115 (Implex, Allendale, NJ) was used in all cases. On the second postoperative day, the patients were mobilized with partial weight bearing for 6 weeks thereafter, followed by full weight bearing. All patients were evaluated clinically at 6, 12, and 24 weeks and 12 months and then at 2, 5, 8, and 10 years. Clinical measurement included the Harris hip score (HHS) and the Oxford hip score (OHS) [17,18]. At same time intervals, all patients had radiologic evaluation using standard anteroposterior pelvic radiographs and the lateral of the operated hip. Patient radiographs with evidence of periacetabular dome gaps were digitized and assessed using software that incorporates the EBRA digital measurement system [19]. The EBRA system allows the user to compare image position in postoperative radiographs over long periods, excluding those with positioning errors that may influence the assessment. Using the EBRA system, we prospectively estimated the periacetabular dome gap filling [20], the acetabular cup migration, and the polyethylene wear.
Results Five patients (5 hips) were excluded from this study. Two of them died from reasons unrelated to their arthroplasty 8 and 9.5 years postoperatively. In another patient, the acetabular component was revised at 50 months for recurrent dislocation (Fig. 1A,B). In another 2 patients, the acetabular component was revised because of late hematogenous infection 6 and 7.5 years postoperatively. In the retrieved components, we observed an excellent bone coverage and penetration in the porous surface, and despite the infection, the components were stable at the time of removal. Both patients underwent successful 2-stage reimplantation with the use of a TM cup at the second stage. The minimum follow-up time was 8 years, and the maximum follow-up time was 10 years. The average preoperative total HHS was 44.0 ± 13.8 (4-86.75) and improved to 95.2 ± 4.8 (81-100) (P b .05) at 1 year and to 97.0 ± 6.2 (58.85-100) (P b .05) at the latest follow-up evaluation. Similarly, the average preoperative HHS pain component was
Porous Tantalum Monoblock Acetabular Macheras et al
Fig. 1. (A) Acetabular component revised at 50 months for recurrent dislocation. (B) Upon visual inspection, the revised acetabular component had extensive bone-tocomponent apposition.
14.0 ± 6.7 (0-44) and increased to 42.4 ± 2.0 (40-44) (P b .05) at 1 year and remained constant through the latest follow-up (43.6 ± 1.6, 30-44; P b .05). The average OHS improved from a preoperative score of 43.3 ± 6.5 to 15.2 ± 2.3 at 1 year postoperatively and 13.9 ± 2.3 at the latest follow-up evaluation. Preoperative range of motion (ROM) included an average flexion of 72.6° ± 21.3° (range, 0°-130°), which improved at the latest follow-up to 106.9° ± 11.1° (range, 80°-130°) (P b .05). Preoperative abduction was 15.0° ± 3.0° (range, 0°-30°) and was improved at the latest follow-up to 44° ± 2.5° (range, 35°-55°) (P b .05). All scores and ROM results are summarized in Table 1. Immediate postoperative radiographs revealed well-fixed and positioned components as per the surgical guidelines for successful implantation. Acetabular component inclination in the initial postoperative radiograph was measured from 39.76° to 50.30° (mean, 45.38°; SE, 0.47°). In the initial postoperative radiographs, periacetabular dome gaps were observed in 25 hips (16%). Of the cases with initial acetabular dome gaps, radiographic evidence of progressive gap filling was observed in all cases by 24 weeks (Fig. 2A-B). There was no further radiographic evidence of acetabular dome gaps at the 10-year follow-up. Also, there was no radiographic evidence of gross polyethylene wear, backside wear, progressive
707
Fig. 2. Postoperative radiographic evidence of dome gap (A) and progressive gap filling at 12 weeks postoperatively (B).
radiolucencies, osteolytic lesions, acetabular fracture, or component subsidence. At the latest follow-up, radiologic evaluation showed an increased bone density and remodeling and thickening of trabecular bone around the acetabular implant (Fig. 3A-B). One case had an acetabular radiolucent line at all 3 Charnley zones at 1 and 2 years but none at 5 years. This may be due to either patient positioning or gap filling. In the case with acetabular dome gaps, the acetabular component was deemed radiographically stable and the patient had no complaints of pain. In these cases, the EBRA study showed no difference in the total migration compared with the average total migration of the acetabular components without acetabular dome gaps. Early complications included intraoperative femoral fracture in one hip (0.7%) treated with cerclage wires, one superficial wound infection treated with local debridement and antibiotics, and one wound hematoma treated with wound drainage. Deep vein thrombosis was seen in 2 hips (one operated side and one contralateral side), and these were successfully treated with low molecular weight heparin (LMWH). Pulmonary embolism occurred in one patient (one hip, b1%), which was successfully treated with LMWH. One early dislocation in one
Table 1. Preoperative and Latest Follow-up of HHS, OHS, and ROM in 151 Primary THA Using the Porous Tantalum Monoblock Acetabular Component
HHS (total) HHS (pain) OHS ROM (flexion) ROM (abduction)
Preoperative
Latest follow-up
44.0 ± 14.0 ± 43.3 ± 72.6° ± 15.0° ±
97.0 ± 43.6 ± 13.9 ± 106.9° ± 44° ±
13.8 6.7 6.5 13.8° 3.0°
6.2 * 1.6 * 2.3 * 11.1° * 2.5° *
* Statistically significant change (P b .05) when compared with preoperative measurements.
Fig. 3. Preoperative (A) and 10-year follow-up (B) anteroposterior radiograph of the right hip after THA using the porous tantalum monoblock acetabular component.
708 The Journal of Arthroplasty Vol. 24 No. 5 August 2009 hip (b1%) was treated by closed reduction without further incident. Of the 151 hips, no acetabular component was revised or needed revision at the last follow-up because of aseptic loosening, or mechanical failure of the femoral component occurred in one hip at 8 years postoperatively (b1%) and required only femoral component revision; the acetabular component was stable without evidence of polyethylene wear.
Discussion Acetabular component success in THA has evolved with the changes in component design characteristics as well as surgical techniques. Initial uncemented acetabular component designs involving various material combinations, component conformity, and implantation techniques yielded only average results. More recently, intermediate results of uncemented acetabular components have shown significant decreases in failure rates attributed to aseptic loosening [8-10]. However, radiographic evidence of radiolucencies adjacent to the acetabular component still exists and may be indicative of early aseptic loosening and component failure [8,10]. Common uncemented acetabular components have design characteristics that include low volumetric porosity, low coefficient of friction, and surface preparations that involve bonding to a solid substrate. There is a nonreported series with longterm results from a component with high volume of porosity, elimination of backside wear, high friction coefficiency, and elasticity modulus close to the subchondral bone as in our series. The Norwegian Arthroplasty Registry reported on a total of 5021 THAs in which the cumulative acetabular component revision rate for aseptic loosening was 3.2% at 5 years with an increase to 7.1% at 6 years across multiple component designs [9]. Latimer and Lachewicz [7], in an intermediate study of porous-coated acetabular components, reported no acetabular component failure, with the end point being revision, but radiographic evidence of periacetabular radiolucencies in 2 zones in 4% of the cases and in one zone in 25% of the cases is reported. The design considerations and the use of tantalum metal for the acetabular component used in our study have design goals of immediate stable component macrofixation and an environment for optimum biologic microfixation. The trabecular metal monoblock acetabular component used in this study incorporated direct fusion and compression of the
polyethylene into the metal shell. This has the advantage of eliminating backside wear as a source of polyethylene debris and of decreasing potential pathways for debris to the periacetabular regions of the pelvis. This should result in a decreased incidence of aseptic loosening. The design offers more physiologic acetabular bone loading because of the implant elasticity. The porous tantalum monoblock acetabular component has unique manufacturing and mechanical properties with an unusually large and interconnecting porous surface, which corresponds to 75% to 80% of its total volume and an overall geometry, shape, and size similar to those of cancellous bone [12]. The friction coefficient of porous tantalum on bone is approximately twice that of other porous surfaced biomaterials. The average pore diameter of the porous tantalum shell is 550 μm, and the polyethylene liner is compression molded into the porous tantalum shell to a depth of 1 to 2 mm, leaving 2 to 3 mm of porous tantalum for tissue ingrowth. This type of manufacturing process allows for a 48-mm acetabular component to incorporate a minimum total polyethylene thickness of 8.5 mm for a 28-mm head, whereas a 40-mm acetabular component allows a minimum total polyethylene thickness of 8 mm for a 22-mm head. Radiographic review of our data has shown early series of periacetabular dome gaps in 25 hips (16.7%). However, at 24 weeks of follow-up, there was a gap filling, and no further radiographic evidence of periacetabular gaps was noted [20]. From the evidence of our reported radiographic findings, we believe with certainty that the osteoconductive properties of tantalum metal allows for the resolution of most line-to-line component to host bone differences found after primary THA. Similarly, Gruen et al [21] reported radiographic evidence of periacetabular gaps using the same acetabular component with complete gap filling at 2 years. The gaps seem to be related to the surgeon's ability to seat the acetabular component completely. There are potential disadvantages to using a monoblock cup [11]. First, fixation of the monoblock component may not be rigid when the cup is inserted in small acetabula (46-50 mm). An increase in medial reaming often allows better seating of the cup. If movement and failure of bony fixation of the cup remain, peripheral screws can be used. Second, because screw holes are not present, dome contact cannot be visualized. A hemispherical trial can be used to gauge the depth to which the cup must be seated. It is important to observe the depth of the last acetabular reamer and to seat the cup to
Porous Tantalum Monoblock Acetabular Macheras et al
the depth of acetabular reaming. When inserting the elliptical socket, it is necessary to seat the cup into the acetabular periphery so that it is circumferentially centered before impacting the cup medially. This seating prevents the rim of the cup impinging on the acetabular mouth and migrating superiorly when the cup is impacted. When the cup has been seated into the socket, further impaction is directed to achieve the proper abduction and anteversion angle. In a radiographic review of 661 elliptical monoblock titanium cups, an incidence of dome gaps of greater than 1.5 mm has been reported in 5.1% of the cases [11]. The advantages of a monoblock elliptical cup are reduction in metallic and polyethylene debris. There is maximal area for ingrowth, and press-fit fixation is excellent. This cup has been used in all patients younger than 75 years routinely. The only reason we did not use this cup in elderly people is the cost. At this time, the concept of an elliptical monoblock acetabular component is theoretically attractive, and results have been encouraging. In conclusion, the use of this uncemented acetabular component in primary THA has shown excellent midterm clinical and radiographic outcome. The revision rate for aseptic loosening in the 8 to 10 years of follow-up was 0. No radiolucencies, no polyethylene wear, and no acetabular cup migration were observed. The radiographic findings, with absence of backside wear, absence of osteolysis, and the appearance of more physiologic morphology of the adjacent bone, confirm the effectiveness of this implant and support the theoretical advantages of tantalum metal. Further review of the current patient population is warranted to determine the long-term durability of this acetabular composite.
References 1. Morscher EW. Current status of acetabular fixation in primary total hip arthroplasty. Clin Orthop Relat Res 1992:172. 2. Bobyn JD, Pilliar RM, Cameron HU, et al. The optimum pore size for the fixation of porous-surfaced metal implants by the ingrowth of bone. Clin Orthop Relat Res 1980:263. 3. Tradonsky S, Postak PD, Froimson AI, et al. A comparison of the disassociation strength of modular acetabular components. Clin Orthop Relat Res 1993:154. 4. Lieberman JR, Kay RM, Hamlet WP, et al. Wear of the polyethylene liner-metallic shell interface in modular acetabular components. An in vitro analysis. J Arthroplastly 1996;11:602. 5. Maloney WJ, Galante JO, Anderson M, et al. Fixation, polyethylene wear, and pelvic osteolysis in primary
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total hip replacement. Clin Orthop Relat Res 1999:157. Blumenfeld TJ, Bargar WL. Early aseptic loosening of a modern acetabular component secondary to a change in manufacturing. J Arthroplasty 2006;21: 689. Latimer HA, Lachewicz PF. Porous-coated acetabular components with screw fixation. Five to ten-year results. J Bone Joint Surg Am 1996;78-A:975. Sochard DH. Relationship of acetabular wear to osteolysis and loosening in total hip arthroplasty. Clin Orthop Relat Res 1999;363:135. The Norwegian Arthroplasty Registry Group. The Norwegian Arthroplasty Register. 2005. Ref Type: Generic. Müller U, Gautier E, Roeder C, et al. The relationship between cup design and the radiological signs of aseptic loosening in total hip arthroplasty. J Bone Joint Surg Br 2003;85-B:31. Sculco T. The acetabular component: an elliptical monoblock alternative. J Arthroplasty 2002;17 (Suppl 1):4. Bobyn JD, Stackpool G, Hacking SA, et al. Characteristics of bone ingrowth and interface mechanics of a new porous tantalum material. J Bone Joint Surg Br 1999;81-B:907. Sporer SM, Paprosky WG. The use of a trabecular metal acetabular component and trabecular metal augment for severe acetabular defects. J Arthroplasty 2006;21(6 Suppl 2):83. Mulier M, Rys B, Moke L. Hedrocel trabecular metal monoblock acetabular cups: mid-term results. Acta Orthop Belg 2006;72:326. Christie MJ. Clinical applications of Trabecular Metal. Am J Orthop 2002;31:219. Komarasamy B, Vadivelu R, Bruce A, et al. Clinical and radiological outcome following total hip arthroplasty with an uncemented trabecular metal monoblock acetabular cup. Acta Orthop Belg 2006;72:320. Harris WH. Traumatic arthritis of the hip after dislocation and acetabular fractures: treatment by mold arthroplasty. An end-result study using a new method of result evaluation. J Bone Joint Surg Am 1969;51:737. Gosens T, Hoefnagels NH, de Vet RC, et al. The “Oxford Hip Score”: the translation and validation of a questionnaire into Dutch to evaluate the results of total hip arthroplasty. Acta Orthop 2005;76:204. Krismer M, Bauer R, Tschupik J, et al. EBRA: a method to measure migration of acetabular components. J Biomech 1995;28:1225. Macheras GA, Papagelopoulos PJ, Kateros K, et al. Radiological evaluation of the metal-bone interface of a porous tantalum monoblock acetabular component. J Bone Joint Surg Br 2006;88:304. Gruen TA, Poggie RA, Lewallen DG, et al. Radiographic evaluation of a monoblock acetabular component. A multi-center study with 2- to 5-year followup. J Arthroplasty 2005;20:369.
Eight- to Ten-Year Clinical and Radiographic Outcome of a Porous Tantalum Monoblock Acetabular Component George Macheras, MD,*1 Konstantinos Kateros, MD, y Athanassios Kostakos, MD,* Stefanos Koutsostathis, MD,* Dimitrios Danomaras, MD,* and Panayiotis J. Papagelopoulos, MD, DSc z1
Abstract: In a prospective study, the authors used a porous tantalum monoblock acetabular component for primary total hip arthroplasty between November 1997 and June 1999. A total of 156 consecutive primary total hip arthroplasty were done in 143 patients younger than 75 years. A total of 151 hips had a follow-up time from 8 to 10 years. The average preoperative total Harris hip score of 44.0 ± 13.8 increased to 97.0 ± 6.2 at the latest follow-up. The average preoperative Oxford hip score of 43.3 ± 6.5 improved to 13.9 ± 2.3 at the latest follow-up. Radiographic evaluation including the Ein-Bild-Röntgen-Analyse (EBRA) digital system showed no radiographic evidence of gross polyethylene wear, progressive radiolucencies, osteolytic lesions, acetabular fracture, or component subsidence. There were 7 (4.5%) postoperative complications all unrelated to the acetabular component. Key words: porous tantalum, monoblock acetabular cup, total hip arthroplasty, osteolysis, polyethylene wear. © 2009 Elsevier Inc. All rights reserved.
Successful acetabular component implantation in primary total hip arthroplasty (THA) requires correct component position, immediate component macrofixation, component stability, and an optimum surface for biologic on-growth surface preparations. Host bone morphometry involving an intact acetabular rim for press-fit stability is necessary for immediate component macrofixation [1]. The por-
osity of the acetabular component and the porous size is an important factor for quick and safe bone ingrowth [2]. The avoidance of acetabular component failure involves both manufacturing and surgical/technical variables. These include cup to liner conformity, locking mechanism integrity, and polyethylene thickness, which will vary by component design geometry [3,4]. The optimum placement of a noncemented press-fit acetabular component within the so-called acetabular arch, to capture the peripheral, has been described [5]. Gaining optimum peripheral press-fit stability and maximizing component macrofixation allows for subsequent biologic fixation (ingrowth/on-growth) and longterm acetabular component durability. Osteolysis produced by biologic reaction to polyethylene and metallic debris is the major failure mode of cemented or uncemented acetabular fixation [3-11]. The advantages of a monoblock acetabular uncemented component have been
From the *Department of Orthopaedics, IKA Hospital, Athens, Greece; ySecond Department of Orthopaedics, Athens University Medical School, Athens, Hellenic Republic, Greece; and zFirst Department of Orthopaedics, Athens University Medical School, ATTIKON University Hospital, Athens, Hellenic Republic, Greece. Submitted August 12, 2007; accepted June 16, 2008. No benefits or funds were received in support of this study. Reprint requests: Panayiotis J. Papagelopoulos, MD, DSc, First Department of Orthopaedics, Athens University Medical School, 4 Christovassili Street 15451, Neon Psychikon, Athens, Greece. 1 The first and last authors contributed equally to this work. © 2009 Elsevier Inc. All rights reserved. 0883-5403/08/2405-0008$36.00/0 doi:10.1016/j.arth.2008.06.020
705
706 The Journal of Arthroplasty Vol. 24 No. 5 August 2009 reported. First, there is no extra-articular back surface polyethylene wear. Second, locking rings that may generate metallic debris are eliminated. Third, screw holes, which decrease the surface area for ingrowth, are not required, and pelvic entrance points for wear debris are eliminated. Fourth, an elliptical configuration allows better cooptation of the shell to the dome of the acetabulum [11]. Tantalum metal for orthopedic use was initially introduced in 1997, with a subsequent rapid evolution of orthopedic applications [11-16]. The use of tantalum metal for the acetabular component in THA was developed to enhance the fixation properties, diminish polyethylene wear, decrease aseptic loosening, and avoid the potential pathways. Its porosity is 80% of the total volume and the porous size 550 μm, which is optimal for bone ingrowth and on-growth. The purpose of this prospective study was to monitor and report the 8- to 10-year prospective clinical and radiographic results of the use of a porous tantalum monoblock acetabular component for primary THA.
Materials and Methods Between November 1997 and June 1999, 156 consecutive primary THAs were done in 143 patients younger than 75 years. There were 96 female (64.36%) and 47 male patients (35.64%). The mean age of females was 61.1 ± 11.3 years (range, 32-75 years) and of males, 57.2 ± 14.1 years (range, 24-75 years). Osteoarthritis was the primary underlying diagnosis in 116 hips. Other causes of the end-stage hip disease include developmental dysplasia of the hip in 26 hips, avascular necrosis in 11 hips, and rheumatoid arthritis (RA) in 3 hips. A press-fit porous tantalum monoblock acetabular component (Trabecular Metal Monoblock Acetabular Component System; Zimmer Inc, Warsaw, Ind) was used in all cases. The component has 2 distinct profiles: solid trabecular metal (TM) backing without peripheral screws and solid TM backing with peripheral screw holes for adjunctive fixation. All the procedures were performed by the senior surgeon using a posterior surgical approach. The acetabular component was inserted using press-fit technique, taking particular care to prevent soft tissue interposition between the implant and the acetabulum bone during implantation. The acetabulum was prepared with hemispherical reamers. The diameter of the final reamer matches the polar diameter of the acetabular component and is 2 mm less than the equatorial diameter.
No bone grafting was used in any of these acetabular reconstructions. Intraoperatively, the initial stability of the acetabular prosthesis was accessed manually and was considered satisfactory in all cases. There were 5 cases of developmental dysplasia of the hip in which peripheral screws were used to gain additional acetabular component stability: one with one screw and 4 where 2 screws were used. The Continuum T Cast Hip Stem F-115 (Implex, Allendale, NJ) was used in all cases. On the second postoperative day, the patients were mobilized with partial weight bearing for 6 weeks thereafter, followed by full weight bearing. All patients were evaluated clinically at 6, 12, and 24 weeks and 12 months and then at 2, 5, 8, and 10 years. Clinical measurement included the Harris hip score (HHS) and the Oxford hip score (OHS) [17,18]. At same time intervals, all patients had radiologic evaluation using standard anteroposterior pelvic radiographs and the lateral of the operated hip. Patient radiographs with evidence of periacetabular dome gaps were digitized and assessed using software that incorporates the EBRA digital measurement system [19]. The EBRA system allows the user to compare image position in postoperative radiographs over long periods, excluding those with positioning errors that may influence the assessment. Using the EBRA system, we prospectively estimated the periacetabular dome gap filling [20], the acetabular cup migration, and the polyethylene wear.
Results Five patients (5 hips) were excluded from this study. Two of them died from reasons unrelated to their arthroplasty 8 and 9.5 years postoperatively. In another patient, the acetabular component was revised at 50 months for recurrent dislocation (Fig. 1A,B). In another 2 patients, the acetabular component was revised because of late hematogenous infection 6 and 7.5 years postoperatively. In the retrieved components, we observed an excellent bone coverage and penetration in the porous surface, and despite the infection, the components were stable at the time of removal. Both patients underwent successful 2-stage reimplantation with the use of a TM cup at the second stage. The minimum follow-up time was 8 years, and the maximum follow-up time was 10 years. The average preoperative total HHS was 44.0 ± 13.8 (4-86.75) and improved to 95.2 ± 4.8 (81-100) (P b .05) at 1 year and to 97.0 ± 6.2 (58.85-100) (P b .05) at the latest follow-up evaluation. Similarly, the average preoperative HHS pain component was
Porous Tantalum Monoblock Acetabular Macheras et al
Fig. 1. (A) Acetabular component revised at 50 months for recurrent dislocation. (B) Upon visual inspection, the revised acetabular component had extensive bone-tocomponent apposition.
14.0 ± 6.7 (0-44) and increased to 42.4 ± 2.0 (40-44) (P b .05) at 1 year and remained constant through the latest follow-up (43.6 ± 1.6, 30-44; P b .05). The average OHS improved from a preoperative score of 43.3 ± 6.5 to 15.2 ± 2.3 at 1 year postoperatively and 13.9 ± 2.3 at the latest follow-up evaluation. Preoperative range of motion (ROM) included an average flexion of 72.6° ± 21.3° (range, 0°-130°), which improved at the latest follow-up to 106.9° ± 11.1° (range, 80°-130°) (P b .05). Preoperative abduction was 15.0° ± 3.0° (range, 0°-30°) and was improved at the latest follow-up to 44° ± 2.5° (range, 35°-55°) (P b .05). All scores and ROM results are summarized in Table 1. Immediate postoperative radiographs revealed well-fixed and positioned components as per the surgical guidelines for successful implantation. Acetabular component inclination in the initial postoperative radiograph was measured from 39.76° to 50.30° (mean, 45.38°; SE, 0.47°). In the initial postoperative radiographs, periacetabular dome gaps were observed in 25 hips (16%). Of the cases with initial acetabular dome gaps, radiographic evidence of progressive gap filling was observed in all cases by 24 weeks (Fig. 2A-B). There was no further radiographic evidence of acetabular dome gaps at the 10-year follow-up. Also, there was no radiographic evidence of gross polyethylene wear, backside wear, progressive
707
Fig. 2. Postoperative radiographic evidence of dome gap (A) and progressive gap filling at 12 weeks postoperatively (B).
radiolucencies, osteolytic lesions, acetabular fracture, or component subsidence. At the latest follow-up, radiologic evaluation showed an increased bone density and remodeling and thickening of trabecular bone around the acetabular implant (Fig. 3A-B). One case had an acetabular radiolucent line at all 3 Charnley zones at 1 and 2 years but none at 5 years. This may be due to either patient positioning or gap filling. In the case with acetabular dome gaps, the acetabular component was deemed radiographically stable and the patient had no complaints of pain. In these cases, the EBRA study showed no difference in the total migration compared with the average total migration of the acetabular components without acetabular dome gaps. Early complications included intraoperative femoral fracture in one hip (0.7%) treated with cerclage wires, one superficial wound infection treated with local debridement and antibiotics, and one wound hematoma treated with wound drainage. Deep vein thrombosis was seen in 2 hips (one operated side and one contralateral side), and these were successfully treated with low molecular weight heparin (LMWH). Pulmonary embolism occurred in one patient (one hip, b1%), which was successfully treated with LMWH. One early dislocation in one
Table 1. Preoperative and Latest Follow-up of HHS, OHS, and ROM in 151 Primary THA Using the Porous Tantalum Monoblock Acetabular Component
HHS (total) HHS (pain) OHS ROM (flexion) ROM (abduction)
Preoperative
Latest follow-up
44.0 ± 14.0 ± 43.3 ± 72.6° ± 15.0° ±
97.0 ± 43.6 ± 13.9 ± 106.9° ± 44° ±
13.8 6.7 6.5 13.8° 3.0°
6.2 * 1.6 * 2.3 * 11.1° * 2.5° *
* Statistically significant change (P b .05) when compared with preoperative measurements.
Fig. 3. Preoperative (A) and 10-year follow-up (B) anteroposterior radiograph of the right hip after THA using the porous tantalum monoblock acetabular component.
708 The Journal of Arthroplasty Vol. 24 No. 5 August 2009 hip (b1%) was treated by closed reduction without further incident. Of the 151 hips, no acetabular component was revised or needed revision at the last follow-up because of aseptic loosening, or mechanical failure of the femoral component occurred in one hip at 8 years postoperatively (b1%) and required only femoral component revision; the acetabular component was stable without evidence of polyethylene wear.
Discussion Acetabular component success in THA has evolved with the changes in component design characteristics as well as surgical techniques. Initial uncemented acetabular component designs involving various material combinations, component conformity, and implantation techniques yielded only average results. More recently, intermediate results of uncemented acetabular components have shown significant decreases in failure rates attributed to aseptic loosening [8-10]. However, radiographic evidence of radiolucencies adjacent to the acetabular component still exists and may be indicative of early aseptic loosening and component failure [8,10]. Common uncemented acetabular components have design characteristics that include low volumetric porosity, low coefficient of friction, and surface preparations that involve bonding to a solid substrate. There is a nonreported series with longterm results from a component with high volume of porosity, elimination of backside wear, high friction coefficiency, and elasticity modulus close to the subchondral bone as in our series. The Norwegian Arthroplasty Registry reported on a total of 5021 THAs in which the cumulative acetabular component revision rate for aseptic loosening was 3.2% at 5 years with an increase to 7.1% at 6 years across multiple component designs [9]. Latimer and Lachewicz [7], in an intermediate study of porous-coated acetabular components, reported no acetabular component failure, with the end point being revision, but radiographic evidence of periacetabular radiolucencies in 2 zones in 4% of the cases and in one zone in 25% of the cases is reported. The design considerations and the use of tantalum metal for the acetabular component used in our study have design goals of immediate stable component macrofixation and an environment for optimum biologic microfixation. The trabecular metal monoblock acetabular component used in this study incorporated direct fusion and compression of the
polyethylene into the metal shell. This has the advantage of eliminating backside wear as a source of polyethylene debris and of decreasing potential pathways for debris to the periacetabular regions of the pelvis. This should result in a decreased incidence of aseptic loosening. The design offers more physiologic acetabular bone loading because of the implant elasticity. The porous tantalum monoblock acetabular component has unique manufacturing and mechanical properties with an unusually large and interconnecting porous surface, which corresponds to 75% to 80% of its total volume and an overall geometry, shape, and size similar to those of cancellous bone [12]. The friction coefficient of porous tantalum on bone is approximately twice that of other porous surfaced biomaterials. The average pore diameter of the porous tantalum shell is 550 μm, and the polyethylene liner is compression molded into the porous tantalum shell to a depth of 1 to 2 mm, leaving 2 to 3 mm of porous tantalum for tissue ingrowth. This type of manufacturing process allows for a 48-mm acetabular component to incorporate a minimum total polyethylene thickness of 8.5 mm for a 28-mm head, whereas a 40-mm acetabular component allows a minimum total polyethylene thickness of 8 mm for a 22-mm head. Radiographic review of our data has shown early series of periacetabular dome gaps in 25 hips (16.7%). However, at 24 weeks of follow-up, there was a gap filling, and no further radiographic evidence of periacetabular gaps was noted [20]. From the evidence of our reported radiographic findings, we believe with certainty that the osteoconductive properties of tantalum metal allows for the resolution of most line-to-line component to host bone differences found after primary THA. Similarly, Gruen et al [21] reported radiographic evidence of periacetabular gaps using the same acetabular component with complete gap filling at 2 years. The gaps seem to be related to the surgeon's ability to seat the acetabular component completely. There are potential disadvantages to using a monoblock cup [11]. First, fixation of the monoblock component may not be rigid when the cup is inserted in small acetabula (46-50 mm). An increase in medial reaming often allows better seating of the cup. If movement and failure of bony fixation of the cup remain, peripheral screws can be used. Second, because screw holes are not present, dome contact cannot be visualized. A hemispherical trial can be used to gauge the depth to which the cup must be seated. It is important to observe the depth of the last acetabular reamer and to seat the cup to
Porous Tantalum Monoblock Acetabular Macheras et al
the depth of acetabular reaming. When inserting the elliptical socket, it is necessary to seat the cup into the acetabular periphery so that it is circumferentially centered before impacting the cup medially. This seating prevents the rim of the cup impinging on the acetabular mouth and migrating superiorly when the cup is impacted. When the cup has been seated into the socket, further impaction is directed to achieve the proper abduction and anteversion angle. In a radiographic review of 661 elliptical monoblock titanium cups, an incidence of dome gaps of greater than 1.5 mm has been reported in 5.1% of the cases [11]. The advantages of a monoblock elliptical cup are reduction in metallic and polyethylene debris. There is maximal area for ingrowth, and press-fit fixation is excellent. This cup has been used in all patients younger than 75 years routinely. The only reason we did not use this cup in elderly people is the cost. At this time, the concept of an elliptical monoblock acetabular component is theoretically attractive, and results have been encouraging. In conclusion, the use of this uncemented acetabular component in primary THA has shown excellent midterm clinical and radiographic outcome. The revision rate for aseptic loosening in the 8 to 10 years of follow-up was 0. No radiolucencies, no polyethylene wear, and no acetabular cup migration were observed. The radiographic findings, with absence of backside wear, absence of osteolysis, and the appearance of more physiologic morphology of the adjacent bone, confirm the effectiveness of this implant and support the theoretical advantages of tantalum metal. Further review of the current patient population is warranted to determine the long-term durability of this acetabular composite.
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