In vivo Implant Fixation of Carbon Fiber-Reinforced PEEK Hip Prostheses in an Ovine Model Ichiro Nakahara,1 Masaki Takao,2 Shunichi Bandoh,3 Nicky Bertollo,4 William R Walsh,4 Nobuhiko Sugano2,5 1
Department of Orthopaedic Surgery, Osaka National Hospital, Osaka, Japan, 2Department of Orthopaedic Surgery, Osaka University Graduate School of Medicine, Suita, Japan, 3B.I.TEC.Co., Ltd., Kakamigahara, Japan, 4Surgical and Orthopaedic Research Laboratories, Prince of Wales Hospital, University of New South Wales, Sydney, Australia, 5Department of Orthopaedic Medical Engineering, Osaka University Graduate School of Medicine, Suita, Japan Received 29 April 2012; accepted 24 September 2012 Published online 23 October 2012 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/jor.22251
ABSTRACT: Carbon fiber-reinforced polyetheretherketone (CFR/PEEK) is theoretically suitable as a material for use in hip prostheses, offering excellent biocompatibility, mechanical properties, and the absence of metal ions. To evaluate in vivo fixation methods of CFR/ PEEK hip prostheses in bone, we examined radiographic and histological results for cementless or cemented CFR/PEEK hip prostheses in an ovine model with implantation up to 52 weeks. CFR/PEEK cups and stems with rough-textured surfaces plus hydroxyapatite (HA) coatings for cementless fixation and CFR/PEEK cups and stems without HA coating for cement fixation were manufactured based on ovine computed tomography (CT) data. Unilateral total hip arthroplasty was performed using cementless or cemented CFR/PEEK hip prostheses. Five cementless cups and stems and six cemented cups and stems were evaluated. On the femoral side, all cementless stems demonstrated bony ongrowth fixation and all cemented stems demonstrated stable fixation without any gaps at both the bonecement and cement-stem interfaces. All cementless cases and four of the six cemented cases showed minimal stress shielding. On the acetabular side, two of the five cementless cups demonstrated bony ongrowth fixation. Our results suggest that both cementless and cemented CFR/PEEK stems work well for fixation. Cup fixation may be difficult for both cementless and cemented types in this ovine model, but bone ongrowth fixation on the cup was first seen in two cementless cases. Cementless fixation can be achieved using HA-coated CFR/PEEK implants, even under load-bearing conditions. ß 2012 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 31:485–492, 2013 Keywords: Carbon-fiber-reinforced polyetheretherketone; cement; cementless; implant fixation; total hip
Interest in the use of carbon fiber-reinforced polyetheretherketone (CFR/PEEK) for orthopedic implants stems from its excellent mechanical properties and high chemical stability, as well as in vitro evidence of biocompatibility with direct cellular contact.1,2 In comparison with clinically used metallic implants, CFR/ PEEK implants can be designed with more appropriate strength, toughness, or stiffness through control of the reinforcing fibers3 and can provide better fatigue resistance.4 Clinically, CFR/PEEK has been applied successfully for spinal cages.5–8 As CFR/PEEK can be designed with an elastic modulus similar to the surrounding cortical bone, bone resorption of the proximal femur caused by stress shielding can be potentially minimized in hip arthroplasty.9–12 The high fatigue resistance and toughness of CFR/PEEK could minimize the risk of implant fracture.13–15 Moreover, CFR/PEEK prostheses cause no artifacts on computed tomography (CT) or MR imaging, potentially allowing more lucid interpretation of the periprosthetic interface.16 Concerns over metal allergy are also eliminated17 and no trouble can be expected at security gates with metal detectors.18 When considering the clinical introduction of CFR/ PEEK hip prostheses, achievement of in vivo stable fixation with surrounding bone is of primary importance Grant sponsor: The Japan Science and Technology Agency. Correspondence to: Nobuhiko Sugano (T: 81-6-6879-3271; F: 81-66879-3272; E-mail:
[email protected]) ß 2012 Orthopaedic Research Society. Published by Wiley Periodicals, Inc.
for the long-term success of the prosthesis. However, such fixation has not yet been confirmed with cementless or cemented fixation. We manufactured both cementless and cemented types of CFR/PEEK hip prostheses and performed implantation in an ovine model. The purpose of our study was to radiographically and histologically investigate in vivo implant fixation of cementless and cemented CFR/PEEK hip prostheses up to 52 weeks after implantation.
MATERIALS AND METHODS Animal Model Sixteen skeletally mature sheep were used following ethical approval. Animals were 18 months old and weighed between 51 and 65 kg. Unilateral total hip replacement was performed using either a cementless or cemented prosthesis. Based on fixation patterns of the acetabular cup and femoral stem, the animals were classified into four groups (n ¼ 4 each; Table 1). Implants Acetabular cups and femoral stems were designed based on CT data of pelvi and femora obtained from 10 sheeps of the same age. Uni-directional (UD)-type CFR/PEEK composites and/or fabric-type CFR/PEEK composites were used to make the implants by gluing with injected PEEK compounds including chopped carbon fibers. The mechanical properties of the UD-type of CFR/PEEK, the fabric-type of CFR/PEEK, the PEEK compounds, titanium alloy (Ti6Al4V), and cobaltchromium (CoCr) based alloy are shown in Table 2. Both cementless and cemented cups were monoblocks with an outer diameter of 28 mm and an inner diameter of 22 mm. The cups consisted of two layers: An outside layer of fabric-type CFR/PEEK composites to support the load and JOURNAL OF ORTHOPAEDIC RESEARCH MARCH 2013
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Table 1. The Study Protocol No. evaluated Type 1 2 3 4
Cup
Stem
No. operated on
12 w
26 w
52 w
Cement Cement Cementless Cementless
Cement Cementless Cement Cementless
4 4 4 4
1 1 1 1
1 1 1 0
1 1 1 1
Table 2. Mechanical Properties of the CFR/PEEK Composites and Titanium Alloy CFR/PEEK composites
Carbon content Elastic modulus 1 Elastic modulus 2
UD type
Fabric type
PEEK compounds
Ti6Al4V
CoCr
65% (volumetric fraction) 150 GPa (fiber direction) 9.8 GPa (transverse direction)
50% (volumetric fraction) 58.5 GPa (fiber direction) 58.5 GPa (fiber direction)
30% (weight fraction) 19 GPa
126 GPa
234 GPa
share stress and an inside layer of PEEK compounds for the bearing surface. On the outside surface, quadrangular pyramid-shaped protuberances 0.5 mm high at 1 mm intervals were molded to enhance initial stability using PEEK compounds (Fig. 1a). Only the surfaces of cementless cups were further coated with an hydroxyapatite (HA) layer of 75-mm thickness as follows. HA granules were dredged on the polymer surface and pressed with 0.1 MPa at 3808C. After this procedure, the surface was blasted with alumina beads to expose the granules. The surface was then treated with nitrogen (N2) plasma to improve the hydrophilic properties. Next, the implant was dipped in 40% a-tricalcium phosphate (a-TCP) solution, dehydrated in air at 408C for 4 h, and subsequently treated with water vapor at 608C for 24 h. This method transformed the a-TCP into HA (Fig. 1b).19 The cementless femoral stem had a collared straight tapered structure with two sizes, while the cemented stem had a smooth surface with one size. The main structure consisted
Figure 1. (a) The cemented CFR/PEEK cup. (b) The cementless CFR/PEEK cup. (c) The cementless CFR/PEEK stem. (d) The cemented CFR/PEEK stem. JOURNAL OF ORTHOPAEDIC RESEARCH MARCH 2013
of two parts: A main spar and an outer skin. These were glued together with injected PEEK compounds (Fig. 2). The structure was hollow inside to decrease stiffness. In cementless stems, the main spar was made of UD-type CFR/PEEK composites to support the femoral head and transfer load to the outer skin, while the outer skin was made of fabric-type CFR/PEEK composites so that fiber directions were 458 to
Figure 2. Inner structure of the CFR/PEEK stem.
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the longitudinal stem axis to allow bearing of torsional loads. This design could provide proper shear-loading without peaky hot stress spots to the medullary canal. A previous simulation study revealed that shear stress along the longitudinal axis is high in the proximal area of the CFR/PEEK stem, while stress concentration occurs at the tip of the distal area of a traditional Ti6Al4V stem.20 On the surface, protuberances 0.5-mm high at intervals of 0.5 mm were molded using PEEK compounds for scratch fixation against the medullary canal; longitudinal fins 0.5 mm high were also molded at the anterior and posterior metaphyseal portions to enhance rotational stability. On the surface, a 75-mm HA layer was attached using the technique described above (Fig. 1c). In the cemented stems, the main spar was also made of UDtype CFR/PEEK composites, but the outer skin consisted of two layers, including an outside layer with fabric-type CFR/ PEEK composites and an inside layer of UD-type CFR/PEEK composites to increase stiffness. Cemented stems did not have any surface bumps or HA coating (Fig. 1d). Surgical Protocol Surgery was performed on the right hind limb under general anesthesia. The animal was placed in the lateral position, and the limb was disinfected with iodine after shaving the wool, then draped. The operation was performed via an anterolateral approach. After the joint was dislocated, the femoral head was resected at a subcapital level with an inclination of 458 to the long axis of the femoral shaft using a bone saw. On the acetabular side, after exposure of the acetabular edge by resecting the limbus, reaming was started using a reamer with a 26 mm diameter. For cementless fixation, the acetabulum was further reamed up to 27 mm. The cementless CFR/PEEK cup was implanted in a press-fit manner. For cemented fixation, the acetabulum was further reamed up to 28 mm. Multiple holes for cement anchors were drilled into the acetabulum using a 3-mm steel bar, and the acetabulum was lavaged and dried prior to cementation. PMMA cement (Antibiotic Simplex1 P; Howmedica, Limerick, Ireland) was manually filled into the acetabulum and the cemented CFR/PEEK cup pre-mounted with the cement was inserted into the acetabulum and held with pressure until the cement cured. On the femoral side, a pilot hole into the canal was opened using a hand drill and reamed distally into the canal at intervals of 0.5 mm until the reaming machined the endosteal walls of the proximal diaphysis. For cementless implantation, broaches, in two sizes with shapes identical to the final implants, were used to further prepare the canal. Broaching was performed to prepare a 0.25-mm undersized canal for press-fit. The cementless CFR/PEEK stem was then implanted by gentle tapping. For cemented implantation, broaching was performed using a single broach to prepare a 0.5-mm oversized canal, and then the canal was lavaged and dried. PMMA bone cement was manually filled into the canal and the CFR/PEEK stem pre-mounted with the PMMA bone cement was inserted and held with pressure until the cement cured. A 22-mm alumina ceramic modular head was inserted, and the joint was relocated. Stability was checked, and the wound closed in a layer-to-layer manner. Postoperative plain AP and lateral radiographs were taken to check for implant position and the occurrence of periprosthetic fracture. Animals were allowed full weight-bearing as tolerated after
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recovery from anesthesia. Each animal was evaluated daily for general health and gait abnormalities. Post-Operative Evaluations Fixation was evaluated at 12, 26, or 52 weeks, after excluding sheep with early complications indicated by severe limping. Bilateral pelvi and femora from each animal were harvested and photographed. Implant stability was evaluated by manual assessment at explanation. The joint was manually loaded in axial and mediolateral directions; any implant motion relative to the bone was noted. Macroscopic findings of delamination, cracking, or particle migration from the implant were recorded. Plain AP and lateral radiographs of bilateral acetabula and femora were taken using a Faxitron machine (Faxitron, Wheeling, IL) with digital plates. Presence of a periprosthetic radiolucent line, cortical hypertrophy, reactive line, or remodeling were evaluated by comparing the implanted to the contralateral side and the location was recorded according to modified DeLee and Charnley21 zones for the acetabular side and Gruen zones for the femoral side (Fig. 3).22 Osteopenia of the proximal femur due to stress shielding by comparing the implanted to the contralateral femur was recorded according to the criteria of Engh et al.23: 1st degree, rounding of the calcar; 2nd degree, loss of cortical density in Gruen zones 1 and 7; 3rd degree, loss of cortical density in zones 1, 2, 6, and 7; 4th degree, loss of cortical density in zones 1–3 and 5–7. Acetabula and femora were also examined using CT (Asteion; Toshiba Medical Systems, Tochigi, Japan) with 0.5mm axial slices or a micro-CT system (Inveon CT; Siemens Medical Solutions, Knoxville, TN) with 100-mm axial slices. Three orthogonal planes were reconstructed using 3D viewer software (3D Template; Japan Medical Materials, Osaka). CT data were examined in the axial, coronal, and sagittal planes for signs of bony ongrowth, osteopenia, radiolucent line, and geometrical changes to the bone. To quantify bone apposition on the cementless cups, multiple radial planes through the cup axis were reconstructed by rotating planes around the axis at 158 increments from a plane including the deepest point of the acetabular notch (Fig. 4a). To quantify
Figure 3. Radiographic zonal analysis scheme for the acetabular (a) and femoral (b) sides. JOURNAL OF ORTHOPAEDIC RESEARCH MARCH 2013
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Figure 4. (a) Radial planes through the cup axis at 158 increments from a plane including the deepest point of acetabular notch. (b) Cross-sectional planes perpendicular to the longitudinal stem axis at 3-mm increments from a plane including the most proximal medial cortex.
apposition on the cementless stems, multiple cross-sectional planes perpendicular to the longitudinal stem axis were reconstructed at 3 mm increments from a plane including the most proximal medial cortex (Fig. 4b). On the acetabular side, lengths of bone ongrowth surface and the HA-coated surface were measured on each reconstructed plane, and percentage of total bone ongrowth surface to the total HA-coated surface was calculated by the outcomes measured from 12 reconstructed planes. On the femoral side, after dividing the HA-coated surface into proximal (zones 1 and 7 in Fig. 3b), middle (zones 2 and 6 in Fig. 3b), and distal portions (zones 3 and 5 in Fig. 3b), the percentage was also calculated in the same way. Acetabula and femora were fixed in phosphate-buffered formalin, dehydrated in an ethanol series and embedded in PMMA. Acetabula were sectioned perpendicular to the opening plane of the cup from superior to inferior in 1-mm thicknesses at three locations (10-mm anterior from the cup center, cup center level, and 10-mm posterior from the cup center). Femora were sectioned perpendicular to the long axis of the stem in 1-mm thickness at four locations (10, 30, 50, and 70 mm from the level of the neck cut). The same processing was performed for the contralateral (left) acetabula and femora. Sections were radiographed using the Faxitron machine and a 100-mm slab from each section block was obtained for histology. Stevenel’s Blue stain was used for evaluation under light microscopy. Cut sections were also polished and examined under environmental scanning electron microscopy (ESEM).
RESULTS All animals recovered from surgery uneventfully. No radiographic findings of periprosthetic fracture or other complications were seen. Five sheeps with severe limping were euthanized at 4 weeks and excluded from the study. All five showed periprosthetic femur fracture. The remaining 11 animals were used to examine fixation of the cups (cementless, n ¼ 5; cemented, n ¼ 6) and stems (cementless, n ¼ 5; cemented, n ¼ 6). These animals were fully weightbearing with normal gait. Sheeps were euthanized at JOURNAL OF ORTHOPAEDIC RESEARCH MARCH 2013
12 (cementless cup and stem, n ¼ 2; cemented cup and stem, n ¼ 2), 26 (cementless cup and stem, n ¼ 1; cemented cup and stem, n ¼ 2), and 52 weeks (cementless cup and stem, n ¼ 2; cemented cup and stem, n ¼ 2) (Table 1). At dissection, all five cementless and all six cemented stems were well-fixed on manual examination without evidence of delamination, cracking, or particulate migration from the stems on visual inspection. Two cementless cups were also well-fixed on manual examination. The remaining three cups and six cemented cups were not well-fixed. In one cementless and five cemented cups, micromotion was observed; however, the cups stayed in the acetabulum during manual examination. Two cementless cups and one cemented cup were easily dislodged from the acetabulum. Cement fracture was observed in the loosened cemented cup. On radiographic examination, radiolucent lines around the cups or reactive lines parallel to the cup surfaces were not detected in any zones in all three unremoved cementless cups on manual assessment. In the five unremoved cemented cups on manual assessment, no radiolucent lines at the bone-cement or cement-stem interfaces were observed in any zones in two cases, but a radiolucent line was seen at the bonecement interface in zone I in one case and in zones I and II in two cases (Fig. 5). For the femora, spot weldlike sclerotic lines were identified at the stem and cancellous bone interface in all zones in all five cementless cases. No radiolucent line around the stem, reactive line parallel to the stem surface, or cortical hypertrophy was observed in any zones in the five cementless cases or in any zones in all six cemented cases. All five cases with cementless stems showed 1st degree stress shielding, while four cases showed 1st degree stress shielding and two cases showed 2nd degree stress shielding with cemented stems (Fig. 6). On CT examination of cementless cases, formation of a neo-cortex on the surface was observed in two cups and in all five stems with stable manual examination (Fig. 7). One cementless cup displaying micromotion was entirely surrounded by a radiolucent zone. Figure 8 shows the ratios of bone contacted surface to the total HA-coated surface. The average ratio was 15.2% in cementless cups and 79.9% in the proximal, 85.0% in the middle, and 80.6% in the distal stems. Although a radiolucent gap around the entire bonecement interface was identified in the five cemented cups (Fig. 9a), radiolucent gaps were not observed at either the bone-cement or cement-implant interface in the six cemented stems (Fig. 9b). Histology confirmed findings of neo-cortex formation on the implant surface in cementless implants (Fig. 10a,b). In the cementless cup surrounded by a radiolucent zone on the CT image, neo-cortex formation on the surface was not observed; instead a fibrous tissue interposition was observed in all sections. Confirming the CT findings, fibrous tissue interposition was
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Figure 5. AP and lateral radiographs of acetabula with cementless stem (a) and cemented cup (b). A radiolucent line at the bonecement interface was evident in zones I and II in the cemented cup.
not detected at either the bone-cement or cementimplant interface in the six cemented stems (Fig. 10c,d).
DISCUSSION This is the first report to investigate preferable in vivo implant fixation of CFR/PEEK prostheses. All stems showed a stable interface, regardless of cementless or cemented fixation. Two of five cementless cups also achieved stable fixation through osseointegration. A small number of previous reports investigated in vivo fixation of cementless carbon fiber-reinforced polymer (CFRP) stems. However, steady osseointegration on the surface was not accomplished.24,25 One report demonstrated osseointegration on cementless CFR/PEEK stems in an animal hemiarthroplasty model, but the success rate was not high enough to warrant further trials.26
Attachment of a bioactive HA layer onto a bioinert material surface has been studied extensively for metal implants, and the results confirmed rapid and stable adhesion of bone.27–29 With CFR/PEEK materials, HA coating was attempted,30,31 and one study using nonload-bearing conditions showed improved bone adhesion with HA-coated CFR/PEEK.19 In our study, a 75-mm HA layer was attached, and the coated stems with a roughened surface demonstrated successful fixation, even under load-bearing conditions. However, three of five cementless cups did not achieve stable fixation. This failure could be attributable to two factors. First, we did not consider deformation of the bearing surface in the manufacturing and implantation processes. Such deformation would reduce bearing clearance and result in higher frictional torque,32 preventing successful fixation. Second, the
Figure 6. AP and lateral radiographs of femora with cementless stem (a) and cemented stem (b). Remodeling was evident in all zones except zone 4 with the cementless stem. Bone resorption due to stress shielding was 1st degree in the cementless stem and 2nd degree in the cemented stem. JOURNAL OF ORTHOPAEDIC RESEARCH MARCH 2013
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Figure 8. Percentages of bone ongrowth surface to the HAcoated surface in two cementless cups and five cementless stems.
Figure 7. CT images of the cementless acetabular cup (a) and axial (b) and coronal (c) views of the cementless femoral stem. Neo-cortex formation on the implants was evident.
acetabular bone bed was sclerotic with little cancellous bone. Such bone is at a disadvantage for fixation. Nonetheless, successful fixation was still achieved in 40% of cementless cases. Further improvements in implant design are needed to enhance mechanical stability, but this result demonstrates osseointegration using cementless CFR/PEEK cups. Bone ongrowth ratios on the cementless cup surfaces were 13.6 and 16.9%, respectively, which were slightly smaller than a previous report that showed 32% of the porous coated surface achieved bone ingrowth in cementless metal cups retrieved at autopsy.33 Conversely, the mean ratios of bone ongrowth on the proximal, middle, distal cementless CFR/PEEK stem surface were 79.9, 85.0, and 80.6%, respectively; a previous report on cementless extensively coated metal stems retrieved at autopsy showed bone
ingrowth for a mean of 37% of the coated surface.34 Differences between sheeps and humans or differences in surface coatings or implant design might have contributed to these differences. Bone ongrowth occurred equally at the proximal, middle, and distal portions in our study, while the ingrowth area at the distal portion was 2.5-times higher than that at the proximal portion in cementless metal stems.34 This remodeling pattern suggests that stress concentration at the distal side of the stem could be prevented with CFR/PEEK stems. Achievement of stable fixation in polymeric prostheses could be expected, because polyethylene cups for cemented fixation have been used successfully in clinical situations. In our study, the cemented stem was designed with higher stiffness compared with the cementless stem, since previous reports showed poor results for cemented stems with lower stiffness.35 Consequently, the CFR/PEEK stem was fixed stably in all cases without any gaps at the bone-cement and cement-stem interfaces.
Figure 9. CT images of the cemented acetabular cup (a) and cemented femoral stem (b). JOURNAL OF ORTHOPAEDIC RESEARCH MARCH 2013
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Figure 10. (Original magnification) (a) Micrograph (40) of a cementless case with direct bony contact (B) on the implant (I) surface. (b) Histology under ESEM of a cementless case with direct contact between the bone (B) and HA layer (H) on the implant (I). (c) Micrograph (4) of a cemented case without any interposition of fibrous tissue at the bone (B)-cement (C) and cement-implant (I) interface. d) Histology under ESEM of a cemented case without any gap at the bone (B)-cement (C) interface.
None of the six cemented cups achieved stable fixation. Although we could not detect any radiolucent lines around the cups on postoperative radiographs immediately after surgery, gaps might exist at the time of initial implantation due to poor cementing technique. Alternatively, gaps might develop over time if the bone-cement interface cannot tolerate shear stresses in vivo and the interface fails. Many factors should be reconsidered, including cement technique, animal model, and frictional torque from the bearing surface to achieve successful cement fixation. In terms of the cement-implant interface, however, five of the six cases showed a stable interface. Achieving rigid fixation at the distal part of traditional metal stems causes concerns for proximal bone resorption caused by stress shielding12 or stem fracture due to stress concentration.13,14 Previous finite element and in vitro studies indicated that reduction of bone resorption was expected using isoelastic CFR/ PEEK stems,10,36 but evaluation of bone resorption after in vivo implantation is necessary. In our study, five cementless and four cemented cases showed 1st degree stress shielding and two cemented cases showed 2nd degree stress shielding. Although no control data were obtained using metal stems with higher stiffness, the fact that most cases showed 1st degree and no case showed 3rd degree stress shielding suggests that more physiological load transfer from stem to femur without stress concentration can be expected with
CFR/PEEK stems. More bone resorption in two cases with cemented stems might be partially attributable to higher stiffness of the cemented stem compared with the cementless stem. Some limitations must be considered. The number of cases was too small to allow statistical analysis between the two fixation types. The in vivo implantation period was also short. Moreover, the results apply to an ovine model and may not translate to other animal models or to humans. Despite these limitations, this represents the first report to demonstrate stable in vivo fixation of CFR/PEEK prostheses in both cementless and cemented cases even under load-bearing condition. Our results should encourage the development of CFR/PEEK hip prostheses.
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