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Load Dispersion Effects of Acetabular Reinforcement Devices Used in Revision Total Hip Arthroplasty
The Journal of Arthroplasty Vol. 26 No. 7 2011
Load Dispersion Effects of Acetabular Reinforcement Devices Used in Revision Total Hip Arthroplasty A Simulation Study Using Finite Element Analysis Keiichi Kawanabe, MD, PhD,*y Haruhiko Akiyama, MD, PhD,y Koji Goto, MD, PhD,y Sumihiko Maeno,z and Takashi Nakamura, MD, PhDy
Abstract: Several types of acetabular reinforcement devices are used to prevent the collapse of grafted bone in revision total hip arthroplasty. However, it remains unclear how the stress is reduced by different devices. We used finite element analysis to evaluate 4 types of acetabular reinforcement devices: Kerboull-type device, Burch-Schneider anti-protrusio cage, Mueller ring, and Ganz ring. The control was a socket fixed with bone cement without any reinforcement devices. The stress distribution on the inner surface of each socket was calculated by binarization image processing. For all 4 reinforcement devices, the stress was reduced to less than one-half of that in the control. All the devices were useful for preventing the collapse of bulk bone grafts applied to load-bearing defects. Keywords: load dispersion effect, acetabular reinforcement device, revision total hip arthroplasty, finite element analysis. © 2011 Elsevier Inc. All rights reserved.
Acetabular bone loss in revision total hip arthroplasty (THA) is a major reconstructive challenge, and the problem of how to reconstruct acetabular deficiencies has become increasingly important. There are 3 ways to compensate for bone loss: placement of the socket at a high hip center [1], prosthetic augmentation, and bone grafting. Eccentric acetabular components [2] and oversized sockets [3] are among the prosthetic options. However, most orthopedists would agree that bone grafting is preferable to the prosthetic options after taking further revision surgery into account [4]. Femoral head bulk allografts are usually used for large bone defects in revision THA, and there are several methods for fixing bulk allografts, such as the use of noncemented acetabular sockets and cemented sockets
From the *Department of Orthopaedic Surgery, Kobe City Medical Center General Hospital, Kobe, Japan; yDepartment of Orthopaedic Surgery, Kyoto University, Kyoto, Japan; and zJapan Medical Materials Co. Ltd., Osaka, Japan. Submitted January 17, 2011; accepted April 14, 2011. The Conflict of Interest statement associated with this article can be found at doi:10.1016/j.arth.2011.04.019. Reprint requests: Keiichi Kawanabe, MD, PhD, Department of Orthopaedic Surgery, Kobe City Medical Center General Hospital, 4-6 Minatojima-nakamachi, Chuo-ku, Kobe 650-0046, Japan. © 2011 Elsevier Inc. All rights reserved. 0883-5403/2607-0015$36.00/0 doi:10.1016/j.arth.2011.04.019
with or without acetabular reinforcement devices. Pollock et al [5] and Hooten et al [6] reported poor results of massive allografts in total hip revision surgery using non-cemented porous coated acetabular components. Revision THA with bulk allografts using cemented sockets without a reinforcement device was reported to have a higher long-term failure rate [7], even after impaction grafting [8]. Garbuz et al [9] recommended the use of an acetabular reinforcement ring with a bulk allograft and had an overall success rate of 88% at a mean follow-up of 7.5 years, compared with a success rate of 44% in the absence of a reinforcement ring. It appears to be advisable to use an acetabular reinforcement device to protect bulk bone grafts in load-bearing defects [4,10]. However, there are several types of reinforcement devices available for revision THA, including the Burch-Schneider anti-protrusio cage (APC) [11], Ganz reinforcement ring (G) [12], Mueller support ring (M) [13], Kerboull device [10], and Kerboull-type device (KT) [14]. The reported failure rates of these devices vary from 0% to 24%. Although all reinforcement devices definitely reduce the stress on the grafted bone, the extents of the stress reduction and possible differences in the dispersion effects among the devices remain unclear. We examined the stress intensity and dispersion with 4 types of reinforcement devices by means of finite element analysis.
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Fig. 1. Photographs of the acetabular reinforcement devices. A, Kerboull-type device. B, Burch-Schneider anti-protrusio cage. C, Mueller support ring. D, Ganz reinforcement ring.
Materials and Methods Acetabular Reinforcement Devices Four types of acetabular reinforcement devices were evaluated in this study, namely, the KT (Japan Medical Materials Co Ltd, Osaka, Japan) (Fig. 1A), APC (Protek AG, Bern, Switzerland) (Fig. 1B), M (Protek AG) (Fig. 1C), and G (Protek AG) (Fig. 1D). A socket with no reinforcement devices was used as a control.
Creation of the Experimental Models The femoral head was 22 mm in diameter, and the acetabulum was 52 mm in diameter. The outer diameter of the control socket was 50 mm, and that of the sockets used with the 4 types of devices was 44 mm. All the devices had an inner diameter of 48 mm and a thickness of 2 mm. The thickness of the bone cement layer was 2 mm.
Method of Simulation Material Constant The femoral head was made of alumina ceramic, and the socket was made of ultra-high-molecular-weight polyethylene. The bone cement was polymethyl methacrylate, and all the acetabular reinforcement devices were made of titanium. The acetabulum consisted of cancellous bone. The elasticity moduli of the materials (MPa) are 404 000, 1040, 2700, 106 000, and 1000, respectively [15,16]. Magnitude and Load Angle On the assumption that a human with a body weight of 80 kg was standing on one leg, the load was assumed to be 200 kgf, which was about 2.5 times higher than the body weight. The load angle was defined as the angle between the load axis from the femoral head to the
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socket and the horizontal axis. The load angles chosen for analysis were 78° ± 12° (66° and 90°). The abduction angle and the anterior opening angle of the socket were 45° and 0°, respectively [17].
The number of black pixels represented the dimensions of the stress. The total number of pixels on the inner side of the socket was 54 400.
Condition of Fixation There was bond fixation between the socket and the cement, the cement and the acetabular reinforcement device, and the acetabular reinforcement device and the acetabular bone. The acetabular bone itself was fixed and immobile. Control. The femoral head was allowed to move only in the direction of the load.
Stress Distribution
Kerboull-Type Device. The palette of the device was fixed to the acetabular bone with 3 screws. The hook was fixed to the acetabular notch but able to rotate around the notch. The femoral head was allowed to move only in the direction of the load. Mueller Support Ring. The device was fixed to the acetabular bone with 3 screws through the most cranial holes, and the remaining screw holes were left empty. The femoral head was allowed to move only in the direction of the load. Ganz Reinforcement Ring. The device was fixed to the acetabular bone with 2 screws. The hook was fixed to the acetabular notch but able to rotate around the notch. The femoral head was allowed to move only in the direction of the load.
Results Control The stress distribution on the inner side of the control socket at a load angle of 78° is shown in Fig. 2A. The other components are not displayed. The maximum stress was set at 1.8 MPa. The highest stress area was located in the upper half of the socket. The stress on the socket was higher when the direction of the load was perpendicular. The number of black pixels was 24 937 when the load angle was 78° (Fig. 2B). Kerboull-Type Device The stress distribution on the inner side of the device at a load angle of 78° is shown in Fig. 3A. The femoral head, socket, and cement are not displayed. The maximum stress was set at 20 MPa. High stress areas were observed in part of the hook and the bend of the palette of the device, but not on the inner side of the socket (Fig. 3B). The number of black pixels in the inner side of the socket was 11 668 (Fig. 3C).
Burch-Schneider Anti-Protrusio Cage. The device was fixed to the acetabular bone with 2 screws and to the ischial tuberosity with 2 screws. The femoral head was allowed to move only in the direction of the load. Simulation of the Stress Distribution and Dimensions Inner Side of the Acetabular Reinforcement Device The stress distribution on the inner side of the device was calculated. The stress was represented by Von Mises stress (MPa). Inner Side of the Socket The stress distribution on the inner side of the socket was calculated. The stress was represented by Von Mises stress (MPa). Binarization of the Stress Distribution The binarization image showed the binarized stress distribution on the inner side of the socket. Areas where the stress was 1.2 MPa or greater were displayed with black pixels, and areas where the stress was less than 1.2 MPa were displayed with white pixels. Since the highest stress on the inner side of the socket, 2.4 MPa, was observed for the control socket when the load angle was 90°, we chose 1.2 MPa as a threshold so that the stress would be above the threshold in about one-half of the area of the inner side of the socket under that condition.
Fig. 2. Stress distribution calculated by finite element analysis. The stress was represented by Von Mises stress (MPa). A, Stress distribution on the inner side of the socket without any devices (control) at a load angle of 78°. The maximum stress was set at 1.8 MPa. B, Binarization image (threshold of 1.2 MPa) of the stress on the inner side of the socket in the control. The number of black pixels was 24 937.
1064 The Journal of Arthroplasty Vol. 26 No. 7 October 2011 Mueller Support Ring High stress areas were located around the screw holes of the device. On the inner side of the socket, the stress was distributed in the central part. The stress on the socket did not change as the load angle increased. Burch-Schneider Anti-Protrusio Cage The stress distribution on the inner side of the device was similar to that of M. The stress on the socket did not change as the load angle increased. Binarization of the Stress Distribution Fig. 4 shows the number of pixels for the 4 devices and the control when the load angles were 66°, 78°, and 90°. The control showed the largest number of pixels for all loading directions, and the number of pixels increased slightly from 23 803 to 27 391 as the load angle increased from 66° to 90°. The KT and G showed similar trends, and the stress increased by approximately threefold as the load angle increased from 66° to 90°. On the other hand, M and APC showed similar trends, and the stress decreased slightly as the load angle increased from 66° to 90°. The proportions of pixels for the socket with KT relative to the control socket were 20%, 46.8%, and 59.1% at load angles of 66°, 78°, and 90°, respectively. Both KT and G had a hook and showed similar tendencies for the stress to increase as the load angle increased. The M and APC did not have a hook and showed similar tendencies for the stress to decrease slightly as the load angle increased.
Discussion No previous studies have confirmed the dispersion effects of acetabular reinforcement devices used in revision THA. In this study, the stress on the inner surface of the socket was 35–47% of the control stress when the 4 types of reinforcement devices were used at a load angle of 78°, which corresponds to the direction of load for standing in a neutral position. It is suggested that
Fig. 3. A, Stress distribution on the inner side of the KT at a load angle of 78°. The maximum stress was set at 20 MPa. B, Stress distribution on the inner side of the socket used with KT. The maximum stress was set at 1.8 MPa. C, Binarization image (threshold of 1.2 MPa) of the stress on the inner side of the socket used with KT. The number of black pixels was 11 668.
Ganz Reinforcement Ring The stress distribution on the inner side of the device was similar to that of KT. However, G showed higher stress areas on the inner side of the socket than KT.
Fig. 4. Relationships between the stress on the inner side of the sockets (number of pixels) used with the 4 devices and the control and the load angles.
Load Dispersion Effects of Acetabular Reinforcement Devices Kawanabe et al
these acetabular reinforcement devices can protect structural grafts from high stresses during the prolonged period of maturation and incorporation. In this simulation study, the control showed slightly increased stress, and KT and G showed approximately three-fold increases in stress, while APC and M showed slight decreases in stress, as the load angle increased from 66° to 90°. In the case of the control socket with no devices, the stress increased as the load angle increased to 90°. The reason for this increase is considered to be a decrease in the contact area between the socket and the femoral head as the load angle increased [18]. The APC and M each had a portion of high stiffness below the part of the screw fixation and were difficult to bend, such that the load was not largely transmitted to the acetabulum. In contrast, KT and G each had a flexible portion below the part of the screw fixation and were able to decrease the amount of load transfer compared with the control. However, they could not change the tendency for the stress to increase as the load angle increased. The clinical results for these 4 types of acetabular reinforcement devices are variable. The survival rate of reconstructions using the Kerboull device with bulk allografts was 92.1% at 13 years [10], while the survival rate of KT was 100% at a mean follow-up of 5 years [14]. However, bulk allografts showed good results compared with morselized allografts when used with the Kerboull device [21]. Strong bulk allografts must be used for large bone defects with KT because it showed higher stresses than APC and M in this study. Zehntner and Ganz [19] reviewed the use of M with morselized allografts. After a 7-year follow-up, there was an 18% failure rate, with 44% of the sockets showing migration of more than 2 mm. They reported more migration with morselized grafts, which were believed to be incapable of securely supporting an acetabular component and concluded that segmental and combined defects (according to the classification of the American Academy of Orthopaedic Surgeons [20]) may require additional fixation with plates and screws. Gerber et al [12] reported that the Kaplan-Meier survivorship rate was 81% after 10 years when G with morselized allografts was used for reconstruction of acetabular defects in revision THA. However, they concluded that this method may not be the preferred approach for reconstructing a segmental defect or pelvic discontinuity. These reports support the use of structural allograft having a good mechanical strength combined with acetabular reinforcement devices in the presence of large bone defects to achieve the goal of revision hip arthroplasty [9,10,12,21]. On the other hand, a few methods of revision without allograft were reported. Placement of the socket above the normal hip center of rotation has been recommended by Harris [22] to get a good contact with native bone and porous-coated acetabular component. How-
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ever, femoral component loosening rate of 13% to 25% [23,24] and hip instability of 25% were reported in this technique [24]. Eccentric porous-coated acetabular component has been proposed recently and Sutherland [25] reported that instability was a major problem, leading to mechanical failure in 26% of revised acetabulum and an 11% revision rate at less than 4 years. These revision techniques without allograft showed instability and do not restoration of large bone defect, which is especially important if additional revision surgery is expected [10]. One question that remains is the mechanism of continued fixation as a patient walk on a cage as it transmits force to allograft. The fixation between the socket and the cage is bone cement, and between the cage and the native bone are screws. Incorporation of allograft was studied using single photon emission computed tomography by Shin et al [26]. The study showed the evidence of incorporation at earliest 13 months after operation, but a thin layer of living bone overlying dead bone even after 3 years. Much of allograft except surface might be remained intact over the long term without incorporation and entry of vessels. Therefore, allograft used in the loading site is required a high mechanical strength. Failures reported by Kerboull et al [10] were related to partial or complete resorption of the graft. Poor intrinsic mechanical properties of the graft and insufficient quality of bone stock restoration could account for this phenomenon. In conclusion, the stress on the inner surface of the socket in four reinforcement devices was reduced and 35% to 47% of the control at the direction load for standing in a neutral position. These acetabular reinforcement devices are useful when a large bone defect is compensated with a structural bone allograft in revision THA surgery.
References 1. Kwong LM, Jasty MJ, Harris WH. High failure rate of bulk femoral head allografts in total hip acetabular reconstruction at 10 years. J Arthroplasty 1993;8:341. 2. Sutherland CJ. Treatment of type III acetabular deficiencies in revision total hip arthroplasty without structural bone- graft. J Arthroplasty 1996;11:91. 3. MacKenzie JR, Callaghan JJ, Pedersen DR, et al. Areas of contact and extent of gaps with implantation of oversized acetabular components in total hip arthroplasty. Clin Orthop Relat Res 1994;298:127. 4. Gill TJ, Sledge JB, Müller ME. The management of severe acetabular bone loss using structural allograft and acetabular reinforcement devices. J Arthroplasty 2000;15:1. 5. Pollock FH, Whiteside LA. The fate of massive allografts in total hip acetabular revision surgery. J Arthroplasty 1992; 7:271. 6. Hooten JP, Engh Jr CA, Engh CA. Failure of structural acetabular allografts in cementless revision hip arthroplasty. J Bone Joint Surg Br 1994;76:419.
1066 The Journal of Arthroplasty Vol. 26 No. 7 October 2011 7. Lee BP, Cabanela ME, Wallrichs SL, et al. Bone-graft augmentation for acetabular deficiencies in total hip arthroplasty. Results of long-term follow-up evaluation. J Arthroplasty 1997;12:503. 8. Van Haaren EH, Heyligers IC, Alexander FGM, et al. High rate of failure of impaction grafting in large acetabular defects. J Bone Joint Surg Br 2007;89:296. 9. Garbuz D, Morsi E, Gross AE. Revision of the acetabular component of a total hip arthroplasty with a massive structural allograft: study with a minimum five-year follow-up. J Bone Joint Surg Am 1996;78:693. 10. Kerboull M, Hamadouche M, Kerboull L. The Kerboull acetabular reinforcement device in major acetabular reconstructions. Clin Orthop Relat Res 2000;378:155. 11. Berry DJ, Müller M. Revision arthroplasty using an antiprotrusio cage for massive acetabular bone deficiencies. J Bone Joint Surg Br 1992;74:711. 12. Gerber A, Pisan M, Zurakowski D, et al. Ganz reinforcement ring for reconstruction of acetabular defects in revision total hip arthroplasty. J Bone Joint Surg Am 2003;85:2358. 13. Schlegel UJ, Bitsch RG, Pritsch M, et al. Mueller reinforcement rings in acetabular revision. Outcome in 164 hips followed for 2-17 years. Acta Orthop 2006;77:234. 14. Tanaka C, Shikata J, Ikenaga M, et al. Acetabular reconstruction using a Kerboull-type acetabular reinforcement device and hydroxyapatite granules: a 3- to 8-year follow-up study. J Arthroplasty 2003;18:719. 15. Asada K. Chapter 7 artificial joint and biomaterials. In: Shimadzu A, Asada K, editors. Biomechanics in orthopedics, Vol. 347. Tokyo: Kanehara Press; 1997. p. 347. [in Japanese]. 16. Hobatho MC, Rho JY, Ashman RB. Anatomic variation of human cancellous bone mechanical properties in vitro. In: Lowet G, et al, editor. Bone research in biomechanics. Amsterdam: IOS Press; 1997. p. 157.
17. Hayashi K. Chapter 3 fundamentals of biomechanics. In: Japan Society of Mechanical Engineers, editor. Summary of biomechanics, 46. Tokyo: Ohmu Co. Ltd.; 1993. p. 46. [in Japanese]. 18. Korhonen RK, Koistinen A, Konttinen YT, et al. The effect of geometry and abduction angle on the stresses in cemented UHMWPE acetabular cups finite element simulations and experimental tests. Biomed Eng Online 2005;4:32. 19. Zehntner MK, Ganz R. Midterm results (5.5-10 years) of acetabular allograft reconstruction with the acetabular reinforcement ring during total hip revision. J Arthroplasty 1994;9:469. 20. D'Antonio JA, Capello WN, Borden LS. Classification and management of acetabular abnormalities in total hip arthroplasty. Clin Orthop 1989;243:126. 21. Kawanabe K, Akiyama H, Onishi E, et al. Revision total hip replacement using the Kerboull acetabular reinforcement device with morselised or bulk graft: results at a mean follow-up of 8.7 years. J Bone Joint Surg Br 2007;89:26. 22. Harris WH. Management of the deficient acetabulum using cementless fixation without bone grafting. Orthop Clin North Am 1993;25:663. 23. Kelly SS. High hip center in revision arthroplasty. J Arthroplasty 1994;9:503. 24. Schutzer SF, Harris WH. High placement of porous-coated acetabular component in complex total hip arthroplasty. J Arthroplasty 1994;9:359. 25. Sutherland CJ. Early experience with eccentric acetabular components in revision total hip arthroplasty. Am J Orthop 1996;25:284. 26. Shin CH, Chen CH, Tsai MF, et al. Incorporation of allograft for acetabular reconstruction. Single photon emission CT in 21 hip arthroplasties followed for 2.5-5 years. Acta Orthop Scand 1994;65:589.
Load Dispersion Effects of Acetabular Reinforcement Devices Used in Revision Total Hip Arthroplasty A Simulation Study Using Finite Element Analysis Keiichi Kawanabe, MD, PhD,*y Haruhiko Akiyama, MD, PhD,y Koji Goto, MD, PhD,y Sumihiko Maeno,z and Takashi Nakamura, MD, PhDy
Abstract: Several types of acetabular reinforcement devices are used to prevent the collapse of grafted bone in revision total hip arthroplasty. However, it remains unclear how the stress is reduced by different devices. We used finite element analysis to evaluate 4 types of acetabular reinforcement devices: Kerboull-type device, Burch-Schneider anti-protrusio cage, Mueller ring, and Ganz ring. The control was a socket fixed with bone cement without any reinforcement devices. The stress distribution on the inner surface of each socket was calculated by binarization image processing. For all 4 reinforcement devices, the stress was reduced to less than one-half of that in the control. All the devices were useful for preventing the collapse of bulk bone grafts applied to load-bearing defects. Keywords: load dispersion effect, acetabular reinforcement device, revision total hip arthroplasty, finite element analysis. © 2011 Elsevier Inc. All rights reserved.
Acetabular bone loss in revision total hip arthroplasty (THA) is a major reconstructive challenge, and the problem of how to reconstruct acetabular deficiencies has become increasingly important. There are 3 ways to compensate for bone loss: placement of the socket at a high hip center [1], prosthetic augmentation, and bone grafting. Eccentric acetabular components [2] and oversized sockets [3] are among the prosthetic options. However, most orthopedists would agree that bone grafting is preferable to the prosthetic options after taking further revision surgery into account [4]. Femoral head bulk allografts are usually used for large bone defects in revision THA, and there are several methods for fixing bulk allografts, such as the use of noncemented acetabular sockets and cemented sockets
From the *Department of Orthopaedic Surgery, Kobe City Medical Center General Hospital, Kobe, Japan; yDepartment of Orthopaedic Surgery, Kyoto University, Kyoto, Japan; and zJapan Medical Materials Co. Ltd., Osaka, Japan. Submitted January 17, 2011; accepted April 14, 2011. The Conflict of Interest statement associated with this article can be found at doi:10.1016/j.arth.2011.04.019. Reprint requests: Keiichi Kawanabe, MD, PhD, Department of Orthopaedic Surgery, Kobe City Medical Center General Hospital, 4-6 Minatojima-nakamachi, Chuo-ku, Kobe 650-0046, Japan. © 2011 Elsevier Inc. All rights reserved. 0883-5403/2607-0015$36.00/0 doi:10.1016/j.arth.2011.04.019
with or without acetabular reinforcement devices. Pollock et al [5] and Hooten et al [6] reported poor results of massive allografts in total hip revision surgery using non-cemented porous coated acetabular components. Revision THA with bulk allografts using cemented sockets without a reinforcement device was reported to have a higher long-term failure rate [7], even after impaction grafting [8]. Garbuz et al [9] recommended the use of an acetabular reinforcement ring with a bulk allograft and had an overall success rate of 88% at a mean follow-up of 7.5 years, compared with a success rate of 44% in the absence of a reinforcement ring. It appears to be advisable to use an acetabular reinforcement device to protect bulk bone grafts in load-bearing defects [4,10]. However, there are several types of reinforcement devices available for revision THA, including the Burch-Schneider anti-protrusio cage (APC) [11], Ganz reinforcement ring (G) [12], Mueller support ring (M) [13], Kerboull device [10], and Kerboull-type device (KT) [14]. The reported failure rates of these devices vary from 0% to 24%. Although all reinforcement devices definitely reduce the stress on the grafted bone, the extents of the stress reduction and possible differences in the dispersion effects among the devices remain unclear. We examined the stress intensity and dispersion with 4 types of reinforcement devices by means of finite element analysis.
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Fig. 1. Photographs of the acetabular reinforcement devices. A, Kerboull-type device. B, Burch-Schneider anti-protrusio cage. C, Mueller support ring. D, Ganz reinforcement ring.
Materials and Methods Acetabular Reinforcement Devices Four types of acetabular reinforcement devices were evaluated in this study, namely, the KT (Japan Medical Materials Co Ltd, Osaka, Japan) (Fig. 1A), APC (Protek AG, Bern, Switzerland) (Fig. 1B), M (Protek AG) (Fig. 1C), and G (Protek AG) (Fig. 1D). A socket with no reinforcement devices was used as a control.
Creation of the Experimental Models The femoral head was 22 mm in diameter, and the acetabulum was 52 mm in diameter. The outer diameter of the control socket was 50 mm, and that of the sockets used with the 4 types of devices was 44 mm. All the devices had an inner diameter of 48 mm and a thickness of 2 mm. The thickness of the bone cement layer was 2 mm.
Method of Simulation Material Constant The femoral head was made of alumina ceramic, and the socket was made of ultra-high-molecular-weight polyethylene. The bone cement was polymethyl methacrylate, and all the acetabular reinforcement devices were made of titanium. The acetabulum consisted of cancellous bone. The elasticity moduli of the materials (MPa) are 404 000, 1040, 2700, 106 000, and 1000, respectively [15,16]. Magnitude and Load Angle On the assumption that a human with a body weight of 80 kg was standing on one leg, the load was assumed to be 200 kgf, which was about 2.5 times higher than the body weight. The load angle was defined as the angle between the load axis from the femoral head to the
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socket and the horizontal axis. The load angles chosen for analysis were 78° ± 12° (66° and 90°). The abduction angle and the anterior opening angle of the socket were 45° and 0°, respectively [17].
The number of black pixels represented the dimensions of the stress. The total number of pixels on the inner side of the socket was 54 400.
Condition of Fixation There was bond fixation between the socket and the cement, the cement and the acetabular reinforcement device, and the acetabular reinforcement device and the acetabular bone. The acetabular bone itself was fixed and immobile. Control. The femoral head was allowed to move only in the direction of the load.
Stress Distribution
Kerboull-Type Device. The palette of the device was fixed to the acetabular bone with 3 screws. The hook was fixed to the acetabular notch but able to rotate around the notch. The femoral head was allowed to move only in the direction of the load. Mueller Support Ring. The device was fixed to the acetabular bone with 3 screws through the most cranial holes, and the remaining screw holes were left empty. The femoral head was allowed to move only in the direction of the load. Ganz Reinforcement Ring. The device was fixed to the acetabular bone with 2 screws. The hook was fixed to the acetabular notch but able to rotate around the notch. The femoral head was allowed to move only in the direction of the load.
Results Control The stress distribution on the inner side of the control socket at a load angle of 78° is shown in Fig. 2A. The other components are not displayed. The maximum stress was set at 1.8 MPa. The highest stress area was located in the upper half of the socket. The stress on the socket was higher when the direction of the load was perpendicular. The number of black pixels was 24 937 when the load angle was 78° (Fig. 2B). Kerboull-Type Device The stress distribution on the inner side of the device at a load angle of 78° is shown in Fig. 3A. The femoral head, socket, and cement are not displayed. The maximum stress was set at 20 MPa. High stress areas were observed in part of the hook and the bend of the palette of the device, but not on the inner side of the socket (Fig. 3B). The number of black pixels in the inner side of the socket was 11 668 (Fig. 3C).
Burch-Schneider Anti-Protrusio Cage. The device was fixed to the acetabular bone with 2 screws and to the ischial tuberosity with 2 screws. The femoral head was allowed to move only in the direction of the load. Simulation of the Stress Distribution and Dimensions Inner Side of the Acetabular Reinforcement Device The stress distribution on the inner side of the device was calculated. The stress was represented by Von Mises stress (MPa). Inner Side of the Socket The stress distribution on the inner side of the socket was calculated. The stress was represented by Von Mises stress (MPa). Binarization of the Stress Distribution The binarization image showed the binarized stress distribution on the inner side of the socket. Areas where the stress was 1.2 MPa or greater were displayed with black pixels, and areas where the stress was less than 1.2 MPa were displayed with white pixels. Since the highest stress on the inner side of the socket, 2.4 MPa, was observed for the control socket when the load angle was 90°, we chose 1.2 MPa as a threshold so that the stress would be above the threshold in about one-half of the area of the inner side of the socket under that condition.
Fig. 2. Stress distribution calculated by finite element analysis. The stress was represented by Von Mises stress (MPa). A, Stress distribution on the inner side of the socket without any devices (control) at a load angle of 78°. The maximum stress was set at 1.8 MPa. B, Binarization image (threshold of 1.2 MPa) of the stress on the inner side of the socket in the control. The number of black pixels was 24 937.
1064 The Journal of Arthroplasty Vol. 26 No. 7 October 2011 Mueller Support Ring High stress areas were located around the screw holes of the device. On the inner side of the socket, the stress was distributed in the central part. The stress on the socket did not change as the load angle increased. Burch-Schneider Anti-Protrusio Cage The stress distribution on the inner side of the device was similar to that of M. The stress on the socket did not change as the load angle increased. Binarization of the Stress Distribution Fig. 4 shows the number of pixels for the 4 devices and the control when the load angles were 66°, 78°, and 90°. The control showed the largest number of pixels for all loading directions, and the number of pixels increased slightly from 23 803 to 27 391 as the load angle increased from 66° to 90°. The KT and G showed similar trends, and the stress increased by approximately threefold as the load angle increased from 66° to 90°. On the other hand, M and APC showed similar trends, and the stress decreased slightly as the load angle increased from 66° to 90°. The proportions of pixels for the socket with KT relative to the control socket were 20%, 46.8%, and 59.1% at load angles of 66°, 78°, and 90°, respectively. Both KT and G had a hook and showed similar tendencies for the stress to increase as the load angle increased. The M and APC did not have a hook and showed similar tendencies for the stress to decrease slightly as the load angle increased.
Discussion No previous studies have confirmed the dispersion effects of acetabular reinforcement devices used in revision THA. In this study, the stress on the inner surface of the socket was 35–47% of the control stress when the 4 types of reinforcement devices were used at a load angle of 78°, which corresponds to the direction of load for standing in a neutral position. It is suggested that
Fig. 3. A, Stress distribution on the inner side of the KT at a load angle of 78°. The maximum stress was set at 20 MPa. B, Stress distribution on the inner side of the socket used with KT. The maximum stress was set at 1.8 MPa. C, Binarization image (threshold of 1.2 MPa) of the stress on the inner side of the socket used with KT. The number of black pixels was 11 668.
Ganz Reinforcement Ring The stress distribution on the inner side of the device was similar to that of KT. However, G showed higher stress areas on the inner side of the socket than KT.
Fig. 4. Relationships between the stress on the inner side of the sockets (number of pixels) used with the 4 devices and the control and the load angles.
Load Dispersion Effects of Acetabular Reinforcement Devices Kawanabe et al
these acetabular reinforcement devices can protect structural grafts from high stresses during the prolonged period of maturation and incorporation. In this simulation study, the control showed slightly increased stress, and KT and G showed approximately three-fold increases in stress, while APC and M showed slight decreases in stress, as the load angle increased from 66° to 90°. In the case of the control socket with no devices, the stress increased as the load angle increased to 90°. The reason for this increase is considered to be a decrease in the contact area between the socket and the femoral head as the load angle increased [18]. The APC and M each had a portion of high stiffness below the part of the screw fixation and were difficult to bend, such that the load was not largely transmitted to the acetabulum. In contrast, KT and G each had a flexible portion below the part of the screw fixation and were able to decrease the amount of load transfer compared with the control. However, they could not change the tendency for the stress to increase as the load angle increased. The clinical results for these 4 types of acetabular reinforcement devices are variable. The survival rate of reconstructions using the Kerboull device with bulk allografts was 92.1% at 13 years [10], while the survival rate of KT was 100% at a mean follow-up of 5 years [14]. However, bulk allografts showed good results compared with morselized allografts when used with the Kerboull device [21]. Strong bulk allografts must be used for large bone defects with KT because it showed higher stresses than APC and M in this study. Zehntner and Ganz [19] reviewed the use of M with morselized allografts. After a 7-year follow-up, there was an 18% failure rate, with 44% of the sockets showing migration of more than 2 mm. They reported more migration with morselized grafts, which were believed to be incapable of securely supporting an acetabular component and concluded that segmental and combined defects (according to the classification of the American Academy of Orthopaedic Surgeons [20]) may require additional fixation with plates and screws. Gerber et al [12] reported that the Kaplan-Meier survivorship rate was 81% after 10 years when G with morselized allografts was used for reconstruction of acetabular defects in revision THA. However, they concluded that this method may not be the preferred approach for reconstructing a segmental defect or pelvic discontinuity. These reports support the use of structural allograft having a good mechanical strength combined with acetabular reinforcement devices in the presence of large bone defects to achieve the goal of revision hip arthroplasty [9,10,12,21]. On the other hand, a few methods of revision without allograft were reported. Placement of the socket above the normal hip center of rotation has been recommended by Harris [22] to get a good contact with native bone and porous-coated acetabular component. How-
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ever, femoral component loosening rate of 13% to 25% [23,24] and hip instability of 25% were reported in this technique [24]. Eccentric porous-coated acetabular component has been proposed recently and Sutherland [25] reported that instability was a major problem, leading to mechanical failure in 26% of revised acetabulum and an 11% revision rate at less than 4 years. These revision techniques without allograft showed instability and do not restoration of large bone defect, which is especially important if additional revision surgery is expected [10]. One question that remains is the mechanism of continued fixation as a patient walk on a cage as it transmits force to allograft. The fixation between the socket and the cage is bone cement, and between the cage and the native bone are screws. Incorporation of allograft was studied using single photon emission computed tomography by Shin et al [26]. The study showed the evidence of incorporation at earliest 13 months after operation, but a thin layer of living bone overlying dead bone even after 3 years. Much of allograft except surface might be remained intact over the long term without incorporation and entry of vessels. Therefore, allograft used in the loading site is required a high mechanical strength. Failures reported by Kerboull et al [10] were related to partial or complete resorption of the graft. Poor intrinsic mechanical properties of the graft and insufficient quality of bone stock restoration could account for this phenomenon. In conclusion, the stress on the inner surface of the socket in four reinforcement devices was reduced and 35% to 47% of the control at the direction load for standing in a neutral position. These acetabular reinforcement devices are useful when a large bone defect is compensated with a structural bone allograft in revision THA surgery.
References 1. Kwong LM, Jasty MJ, Harris WH. High failure rate of bulk femoral head allografts in total hip acetabular reconstruction at 10 years. J Arthroplasty 1993;8:341. 2. Sutherland CJ. Treatment of type III acetabular deficiencies in revision total hip arthroplasty without structural bone- graft. J Arthroplasty 1996;11:91. 3. MacKenzie JR, Callaghan JJ, Pedersen DR, et al. Areas of contact and extent of gaps with implantation of oversized acetabular components in total hip arthroplasty. Clin Orthop Relat Res 1994;298:127. 4. Gill TJ, Sledge JB, Müller ME. The management of severe acetabular bone loss using structural allograft and acetabular reinforcement devices. J Arthroplasty 2000;15:1. 5. Pollock FH, Whiteside LA. The fate of massive allografts in total hip acetabular revision surgery. J Arthroplasty 1992; 7:271. 6. Hooten JP, Engh Jr CA, Engh CA. Failure of structural acetabular allografts in cementless revision hip arthroplasty. J Bone Joint Surg Br 1994;76:419.
1066 The Journal of Arthroplasty Vol. 26 No. 7 October 2011 7. Lee BP, Cabanela ME, Wallrichs SL, et al. Bone-graft augmentation for acetabular deficiencies in total hip arthroplasty. Results of long-term follow-up evaluation. J Arthroplasty 1997;12:503. 8. Van Haaren EH, Heyligers IC, Alexander FGM, et al. High rate of failure of impaction grafting in large acetabular defects. J Bone Joint Surg Br 2007;89:296. 9. Garbuz D, Morsi E, Gross AE. Revision of the acetabular component of a total hip arthroplasty with a massive structural allograft: study with a minimum five-year follow-up. J Bone Joint Surg Am 1996;78:693. 10. Kerboull M, Hamadouche M, Kerboull L. The Kerboull acetabular reinforcement device in major acetabular reconstructions. Clin Orthop Relat Res 2000;378:155. 11. Berry DJ, Müller M. Revision arthroplasty using an antiprotrusio cage for massive acetabular bone deficiencies. J Bone Joint Surg Br 1992;74:711. 12. Gerber A, Pisan M, Zurakowski D, et al. Ganz reinforcement ring for reconstruction of acetabular defects in revision total hip arthroplasty. J Bone Joint Surg Am 2003;85:2358. 13. Schlegel UJ, Bitsch RG, Pritsch M, et al. Mueller reinforcement rings in acetabular revision. Outcome in 164 hips followed for 2-17 years. Acta Orthop 2006;77:234. 14. Tanaka C, Shikata J, Ikenaga M, et al. Acetabular reconstruction using a Kerboull-type acetabular reinforcement device and hydroxyapatite granules: a 3- to 8-year follow-up study. J Arthroplasty 2003;18:719. 15. Asada K. Chapter 7 artificial joint and biomaterials. In: Shimadzu A, Asada K, editors. Biomechanics in orthopedics, Vol. 347. Tokyo: Kanehara Press; 1997. p. 347. [in Japanese]. 16. Hobatho MC, Rho JY, Ashman RB. Anatomic variation of human cancellous bone mechanical properties in vitro. In: Lowet G, et al, editor. Bone research in biomechanics. Amsterdam: IOS Press; 1997. p. 157.
17. Hayashi K. Chapter 3 fundamentals of biomechanics. In: Japan Society of Mechanical Engineers, editor. Summary of biomechanics, 46. Tokyo: Ohmu Co. Ltd.; 1993. p. 46. [in Japanese]. 18. Korhonen RK, Koistinen A, Konttinen YT, et al. The effect of geometry and abduction angle on the stresses in cemented UHMWPE acetabular cups finite element simulations and experimental tests. Biomed Eng Online 2005;4:32. 19. Zehntner MK, Ganz R. Midterm results (5.5-10 years) of acetabular allograft reconstruction with the acetabular reinforcement ring during total hip revision. J Arthroplasty 1994;9:469. 20. D'Antonio JA, Capello WN, Borden LS. Classification and management of acetabular abnormalities in total hip arthroplasty. Clin Orthop 1989;243:126. 21. Kawanabe K, Akiyama H, Onishi E, et al. Revision total hip replacement using the Kerboull acetabular reinforcement device with morselised or bulk graft: results at a mean follow-up of 8.7 years. J Bone Joint Surg Br 2007;89:26. 22. Harris WH. Management of the deficient acetabulum using cementless fixation without bone grafting. Orthop Clin North Am 1993;25:663. 23. Kelly SS. High hip center in revision arthroplasty. J Arthroplasty 1994;9:503. 24. Schutzer SF, Harris WH. High placement of porous-coated acetabular component in complex total hip arthroplasty. J Arthroplasty 1994;9:359. 25. Sutherland CJ. Early experience with eccentric acetabular components in revision total hip arthroplasty. Am J Orthop 1996;25:284. 26. Shin CH, Chen CH, Tsai MF, et al. Incorporation of allograft for acetabular reconstruction. Single photon emission CT in 21 hip arthroplasties followed for 2.5-5 years. Acta Orthop Scand 1994;65:589.