- Email: [email protected]
A Mechanical Analysis of Femoral Resurfacing Implantation for Osteonecrosis of the Femoral Head
The Journal of Arthroplasty Vol. 25 No. 8 2010
A Mechanical Analysis of Femoral Resurfacing Implantation for Osteonecrosis of the Femoral Head Daigo Sakagoshi, MD,* Tamon Kabata, MD, PhD,* Yuichiro Umemoto,y Jiro Sakamoto,y and Katsuro Tomita, MD, PhD*
Abstract: Hip resurfacing is becoming a popular procedure for treating osteonecrosis of the femoral head. However, the biomechanical changes that occur after femoral resurfacing have not been fully investigated with respect to the individual extent of the necrosis. In this study, we evaluated biomechanical changes at various extents of necrosis and implant alignments using the finite element analysis method. We established 3 patterns of necrosis by depth from the surface of femoral head and 5 stem angles. For these models, we evaluated biomechanical changes associated with the extent of necrosis and the stem alignment. Our results indicate that stress distribution near the bone-cement interface increased with expansion of the necrosis. The maximum stress on the prosthesis was decreased with stem angles ranging from 130° to140°. The peak stress of cement increased as the stem angle became varus. This study indicates that resurfacing arthroplasty will have adverse biomechanical effects when there is a large extent of osteonecrosis and excessive varus or valgus implantation of the prosthesis. Keywords: hip, resurfacing, finite element analysis, osteonecrosis, femoral head. © 2010 Elsevier Inc. All rights reserved.
Femoral head resurfacing is morphologically similar to the normal proximal femoral anatomy. It has several advantages over conventional total hip arthroplasty; these include minimal bone resection, easier revision, and maintenance of physiological stress within the proximal femur [1]. Earlier hip resurfacing implants suffered from poor manufacturing quality, resulting in osteolysis and aseptic loosening [2]. However, current metal-on-metal hip resurfacing arthroplasty seems to have solved the problems of poor materials and manufacture, producing clinical results comparable to conventional total hip arthroplasty. Hip resurfacing has become a popular procedure for young active patients with avascular necrosis of the femoral head [3-5]. Hip resurfacing has some particular complications. Some of these, for example, are metal sensitivity, aseptic lymphocytic vasculitis–associated lesions [6], increased wear as the result of poor cup alignment [7], loosening From the *Department of Orthopaedics Surgery, School of Medichine, Kanazawa University, Kanazawa, Japan; and yDepartment of Human and Mechanical Systems Engineering, Kanazawa University, Kanazawa, Japan. Submitted March 26, 2009; accepted September 12, 2009. No benefits or funds were received in support of the study. Reprint requests: Tamon Kabata, MD, PhD, 13-1 Takaramachi, Kanazawa 920-0934, Japan. © 2010 Elsevier Inc. All rights reserved. 0883-5403/2508-0017$36.00/0 doi:10.1016/j.arth.2009.09.002
of the femoral component [8], and femoral neck fracture [9,10]. Loosening of the femoral component and femoral neck fractures were reported to be the most common complications of this procedure and reasons for revision [9]. Both of these complications may be attributed to poor bone stock of the femoral head, arising from large cystic lesions and osteonecrosis. Amstutz reported that large femoral cystic lesions lead to a high failure late within 5 years [8]. Osteonecrosis of the femoral head may cause poor bone stock, and the surgical indication of this procedure when there is wide extended osteonecrosis is controversial. However, it is not yet clear exactly how much osteonecrosis would permit this procedure and how much would be a contraindication. The aims of this study are to analyze the resurfaced femoral head using finite element models and, in particular, to examine the influence of the extent of avascular necrosis and metaphysical stem shaft angles within the femoral head.
Materials and Methods A solid model of a femur was generated from computed tomography (CT) scans (Light Speed Ultra16; GE Medical Systems, Tokyo) of a normal hip from a 28-year-old Japanese woman. For each CT slice, the outer contour of the femur was defined and lofted to
1282
FEM of Resurfacing for Osteonecrosis Sakagoshi et al
form a 3-dimensional solid model of the femur using MECHANICAL FINDER 2.0 (Research Center of Computational Mechanics Inc, Tokyo). We used the proximal 190-mm compartment of the femur. The femur was then virtually implanted with a suitably sized femoral component (Conserve, 42-mm diameter; Wright Medical Technology Inc, Memphis, Tenn). In the implanted femur model, the prosthesis was considered as having a cemented inner surface with a thickness of 1 mm, as proposed by Amstutz et al [9]. In real surgery, the cement would be interdigitated with the cancellous bone of the reamed femoral head, but for simplicity in this study, the models had a 1-mm-thick cement mantle. The implant-cement, cement-bone, and implant-bone interfaces were assumed to be rigidly bonded. We set 3 patterns of necrosis according to depth from the surface of the femoral head [11]. Extension of necrosis to a quarter of the femoral head diameter is type A, to a half is type B, and to three fourths is type C. The shape of the osteonecrosis was determined as the crossover region between the femoral head and a ball with the same diameter as the femoral head. Assuming the common location of femoral head necrosis, the occurrence point of the necrosis was established at the surface of the femoral head according to the “gravity of necrosis” concept reported by Nishii (Fig. 1) [12]. Assuming the center of the femoral head as the coordinate center, the occurrence point of the necrosis was established at 55.7° medial from the vertical direction and 45.6° frontal from the horizontal direction. We modeled 5 types of stem shaft angles against the femoral shaft axis from 125° to 145° by 5°. Thus, including the 3 models for extent of necrosis, we had a total of 15 pattern models in this study (Fig. 2). We replaced all the necrosis areas with cement. The models were meshed using liner tetrahedoral elements with a 3-mm element edge length by MSC. Patran 2001(MSC Software Corporation, USA). The meshes consisted of 45,788 to 47,318 tetrahedral elements. Material properties were applied to the bone using the software ADVENTURE CTBone (ADVENTURE PROJECT, http://adventure.q.t.u-tokyo.ac.jp/jp/alliances/). This program could interpolate Hounsfield units from the CT scans and determine apparent densities. The Young modulus of each element could then be calculated [13]. The conversion equation from Hounsfield units to bone density (p; g/cm3) was as follows:
1283
Fig. 1. Location of the necrosis was established at the surface of femoral head according to the concept of gravity of necrosis.
be about 17 GPa [17]. Considering the maximum value of bone density of this model, the formula devised by Carter [14] was most suitable for this study.
q = ðHU + 1:4246Þ 0:001=1:0580 : ð1 b HUÞ q = 1:0 108 : ðHU V 1Þ The maximum bone density of this model was calculated as 1.6 g/cm3. There are several formulas for calculating the material properties of the bone [14-16]. The Young modulus of the cortex bone was assumed to
Fig. 2. Three types of expansion of necrosis area from one forth to three forths and 5 types of stem shaft angles from 125° to 145° were established.
1284 The Journal of Arthroplasty Vol. 25 No. 8 December 2010
Fig. 3. The loading conditions of this study replicated the stance phase of level gait. The models were loaded with 1500 N applied to the femoral head and 1000 N acting as the abductor muscle (Ipavec et al [19]).
The apparent density was related to the Young modulus by the equation: E = 0:001 : ðq V 1:0 108 Þ E = 2875p3 : ð1:0 108 b qÞ Where E is the Young modulus (MPa) and p is the apparent density (g/cm3). The Poisson ratio for bone was assumed to be 0.4. The polymethyl methacrylate (PMMA) bone cement mantle and cobalt-chrome implant were modeled as linear elastic materials with a Poisson ratio of 0.19 and 0.3, and Young modulus of 2000 and 200 000 MPa [18]. The models were physiologically loaded to replicate peak loads during the stance phase of level gait, in accordance with the findings of Ipavec [19]. In this study, the models were loaded with 1500 N applied to the femoral head and 1000 N acting as the abductor muscle. Both forces were distributed over an area to
avoid the effect of point loading. The contact force to the femoral head was distributed as a sine curve, acting at 13° from the vertical in the frontal plane and 40° in the axial plane. The abductor force was acting at 20° from the vertical in the frontal plane and 40° in the axial plane (Fig. 3). The models were rigidly constrained at the distal surface. The finite element models were analyzed using the software ADVENTURE system (ADVENTURE PROJECT, http://adventure.q.t.u-tokyo.ac.jp/). The von Mises stress of ductile materials; the implant, bone, and von Mises stress of the brittle material; and PMMA for each model were compared.
Result Expanding the necrosis area, which was replaced by cement, altered the strain distribution within the femoral head. As can be seen Fig. 4, which presents coronal plane sections of each model, strain concentration was not observed in the “type A” femoral head. However, in the “type B” and “type C” femoral heads, we see increased strain near the bone and cement interface corresponding to the expansion of the necrosis replaced by a cement mantle. The strain concentration is particularly evident in the type C model, which has wide cement mantle replacement on the resurfaced femoral head. The von Mises stress distributions of the implant are shown in Fig. 5. Stress concentration was observed at the stem shaft regardless of extent of necrosis when the shaft was implanted at angles of 125° or 145°. The peak value of the implants' von Mises stress is shown in Fig. 6A. At 130° and 135°, the peak values of von Mises stress
Fig. 4. The figures show the strain distributions in the coronal plane sections. As the necrosis area that was replaced by cement expand, the stress concentration increase at the bone-cement interface.
FEM of Resurfacing for Osteonecrosis Sakagoshi et al
1285
Fig. 5. The figures show the von Mises stress of implants.
were lower (less than 80 MPa) and recognized at the shell rim of the implant. By contrast, at implantation angles of 125°, 140°, or 145°, the peak stress arose at the stem shaft. The peak stress of the stem shaft is shown in Fig. 6B. In this area, the peak stress was lowest at 130° but increased as the stem angle became varus or valgus. However, for all variables examined, the maximum peak stress values in implants were well below the yield stress of cobalt-chrome alloys, which was assumed to be 445 to 517 MPa [20]. The peak stress at the anterior and inferior shell rims is shown in Fig. 6C-D. The change of stress seems to be indistinct at the anterior edge of the implant shell, but particularly at the inferior edge, the peak stress increases as the stem angle becomes varus. Fig. 7 presents the maximum principal (tensile) stress of the cement. Stress concentrations were recognized at the anterior and inferior edges of the cement mantle. The peak stress value of the cement is shown in Fig. 8A. For all the models, the peak value was less than 35 MPa, which is below the yield stress of PMMA, assumed to be 50.1 MPa [21]. The peak stress at the inferior edge of the cement mantle is shown in Fig. 8B. Compared to the varus stem, the valgus stem reduced the magnitude of the peak stress in the cement near the inferior shell rim.
Discussion Early hip resurfacing implants suffered from poor manufacturing quality of the bearing surface and materials. However, the current generation of metalon-metal resurfacing arthroplasty, using improved manufacturing techniques, has shown promising results in clinical trials [3-5]. This procedure is regaining its popularity, especially as a treatment option for young
and active patients with avascular necrosis of the femoral head, which is difficult to treat with joint preserving surgery (eg, proximal femoral osteotomy). However, concerns remain regarding the risk of postoperative femoral neck fracture [10] and loosening of the femoral component [6], and these pathological mechanisms have not been not clearly explained. Furthermore, the surgical indication criteria of this procedure for expanded femoral head necrosis are still unclear [22]. The aim of this study is to implement finite element techniques in the investigation of the effects of implant orientation and the expanse of necrosis replaced by the cement mantle. In particular, we wanted to determine if variable implant angles and expanse of necrosis replaced by cement affect the load transfer distribution from the implant to the adjacent cement and bone, and we also wanted to evaluate the stress of the implant and the cement mantle. Finite elements analysis was performed to analyze the various hip arthroplasties. In the past studies, constant material properties were given to the cortical and cancellous bones. In the analysis of femoral component of the conventional total hip arthroplasty, material properties seem to have few influences at the results. This is because the prosthesis is mainly supported by cortical bone after cutting of the femoral neck, which consists of inhomogeneous cancellous bone trabeculae. Contrary to this, femoral resurfacing arthroplasty is supported by inhomogeneous cancellous bone. For a physiological analysis, the model should have the heterogeneous material properties of the trabecular bone of the femoral head. We used the ADVENTURE system, which permits configuration of the material
1286 The Journal of Arthroplasty Vol. 25 No. 8 December 2010 properties [13]. Our study provides high-resolution stress and strain fields around the femur, incorporating the inhomogeneous CT-based distribution of bone properties obtained from a young donor. This represents
the target population for hip resurfacing arthroplasty. Although there have been some finite element studies regarding hip resurfacing arthroplasty [23-27], in which the effects of cement mantle or implant orientation were discussed, we could not find any report that refers to the effects and indications of expanded avascular necrosis of the femoral head. The size and location of femoral head necrosis vary for each patient. Nishii [12] evaluated lesion size and location using magnetic resonance imaging on 65 hips in a consecutive series of 47 patients with osteonecrosis and found no radiological evidence of collapse. He reported the center of gravity as the most common location of osteonecrosis in the femoral head. This described that location of the gravity center of osteonecrosis was used in this study. We created models for 3 patterns of osteonecrosis. This study found differences of stress distribution in the femoral head among these models. In the model with little necrosis replaced by cement (type A), the stress was transferred through the stem to the inferior femoral neck. Only a small amount of stress was generated in the resurfaced femoral head and led to the stress shielding. This result corresponds to the findings of previous studies [18,25,28], suggesting that the stiff arthroplasty protects the proximal section of the femoral bone from the load. Increased stress concentration near the bonecement interface was observed in the model with an expanded cement mantle in the resurfaced femoral head (type C). We think that hip resurfacing should not be considered for remarkable expanded osteonecrosis like as type C model, and we think that the adaptive limit of this procedure is between types A and C. The volume of the osteonecrois of the type B model would become reduced after cylindrical reaming of femoral head. With filling the remained bone defect with cement, hip resurfacing might be performed for the type B model. At the type B model, some stress concentration was recognized in the resurfaced femoral head. The larger load portion taken by the thicker cement mantle is then spread out over the area of cancellous bone at the bone and cement interface. In this part, cancellous bone is important as a role of anchor of cement. At the wide expanded necrosis model as type C, excessive cement mantle protruding over a half of femoral head diameter Fig. 6. (A) The peak value of the implant's von Mises stress. At 130° and 135°, the peak values were lower and recognized at the shell rim of the implant. At 125°, 140°, or 145°, the peak stress arose at the stem shaft. (B) The peak stress of the part of stem shaft. The peak stress was lowest at 130° but increased according to the stem angle becoming varus or valgus. (C) The peak stress of the part of the anterior edge of implant shell. The change of stress seemed to be indistinctive. (D) The peak stress of the part of the inferior edge of implant shell. The peak stress increased corresponding to the stem angle becoming varus.
FEM of Resurfacing for Osteonecrosis Sakagoshi et al
1287
Fig. 7. The figures show the maximum principal (tensile) stress of cement. The stress concentrations were recognized at the anterior edge and the inferior edge of cement mantle.
in the resurfaced femoral head had possibilities to make larger stress against the cancellous bone adjacent to the cement mantle. The tolerance of cancellous bone as
Fig. 8. (A) The peak stress values of cement are shown. They were below the yield stress of PMMA. (B) The peak stress of inferior edge of cement mantle. The valgus stem reduced the magnitude of the peak stress compared to the varus stem.
anchor should be a concern in that situation. Unfortunately, we could not refer about the yield stress of cancellous bone adjacent to the cement mantle. Morlock [29] analyzed 55 retrieved implants of failed resurfacing arthroplasties. In that study, only 31% of failed femoral heads had been cemented in accordance with the manufacturer's guideline; the thickness of cement mantles or penetrations often exceeded 5 mm. The authors recommended that excessive cementing into the femoral head should be avoided. Considering the result of our study and Morlock's recommendation, we should be careful about considering resurfacing in the case of patients who have widely expanded osteonecrosis that might be replaced by cement on the femoral head. Implant stem fracture is one of the rare complications of hip resurfacing arthroplasty. There are a few case reports in the literatures [30-32]. Although the cause of this complication is not apparent, osteonecrosis occurring because of surgical disruption of the vascular supply, progressive bone resorption, and cemented stem were thought to be risk factors. In all cases of stem fractures reported previously, the stems were firmly cemented to the centralizing canal. In this study, the condition of implant-bone interface was bonded, and this might be likened to a stem fixed by cement. We found stress concentration at the stem shaft when it was implanted at varus or excessively valgus angles. However, the peak stress at the stem shaft was well below the yield stress of implants. Probably, this rare complication can be attributed to a combination of factors, not just implant angle alone, such as momentary dynamic force against the femoral head, metal fatigue caused by repeated stress, and the aforementioned risk factors. Considering the results of this study, varus or excessively
1288 The Journal of Arthroplasty Vol. 25 No. 8 December 2010 valgus implantation should be avoided to prevent stress concentration at the stem shaft. Stress concentrations in the cement were observed at the anterior and inferior edges in each model. Regardless of the extent of necrosis replaced by cement, the stress values at the inferior edge increased as the stem shaft angle became varus. It has been shown in clinical trials that the varus stem orientation may be related to failures in the resurfaced femoral head. Amustutz [9] reported a significantly higher incidence of revision for femoral loosening in hips with more varus orientation, and Shimnin [10] described varus placement in 42 (84%) of 50 cases of femoral neck fractures. Some finite element studies [24,27] of the relation between varus implant orientation and neck fracture also support the experience of clinical trials. Long [27] found that a valgus stem orientation reduced the local stress, which occurred within the inferior aspect of the cement mantle near the shell rim. This result was similar to ours. Our results also suggest that a varus stem orientation should be avoided. There are several limitations to this study. This model is not validated and is theoretical. Some notable elements that are lacking are as follows. First, we modeled the interface conditions of bone and cement and implant as bonded. Although clinically the bone and cement interface is assumed to be bonded, sliding might occur at the stem and bone interface with some friction. In addition, in this study, the necrosis area was totally replaced by cement, but clinically, we do not always completely remove the necrotic bone. Finally, the loading condition of this study was assumed to be a level gait. If more demanding loads were applied, such as active sports, running, or heavy weight, the stress concentration would be much greater. It is unclear from the results of this study whether active sports can be permitted after hip resurfacing. Further work will be required to elucidate the biomechanical consequences considering the previously mentioned factors.
Conclusions Finite element analysis of hip resurfacing methods was performed against femoral head necrosis models with various extents of necrosis areas and several implant angles. This study suggests that hip resurfacing for patients in whom osteonecrosis extends widely should be considered very carefully; increased stress concentration near the bone-cement interface may occur when all the necrotic bone is replaced by cement. Further, excessive varus or valgus implantation of the prosthesis has potentially adverse biomechanical effects for implants and the cement mantle.
References 1. McMinn D. Metal on metal surface replacement of the hip. Clin Orthop 1996;329S:89.
2. Freeman MAR. ICLH surface replacement of the hip: an analysis of the first 10 years. J Bone Joint Surg 1983; 65:405. 3. Amstutz HC. Metal-on-metal hybrid surface arthroplasty: two to six-year follow-up study. J Bone Joint Surg Am 2004;86-A:28. 4. Beaule PE. Metal-on-metal surface arthroplasty with a cemented femoral component: a 7-10 year follow-up study. J Arthroplasty 2004;19:17. 5. Daniel J. Metal-on-metal resurfacing of the hip in patients under the age of 55 years with osteoarthritis. J Bone Joint Surg Br 2004;86:177. 6. Amstutz HC. Metal-on-metal hybrid surface arthroplasty. Surgical technique. J Bone Joint Surg (Am) 2006;88(Suppl 1 Pt2):234. 7. Morlock MM. Modes of implant failure after hip resurfacing: morphological and wear analysis of 267 retrieval specimens. J Bone Joint Surg (Am) 2008;90 (Suppl 3):89. 8. Counsell A. A groin mass caused by metal particle debris after hip resurfacing. Acta Orthop Berg 2008;74:870. 9. Amstutz HC. Fractures of the neck of the femur after surface arthroplasty of the hip. J Bone Joint Surg (Am) 2004;86-A:1874. 10. Shimmin A. Femoral neck fractures following Birmingham hip resurfacing. J Bone Joint Surg 2005;87-B:463. 11. Shimizu K. Prediction of collapse with magnetic resonance imaging of avascular necrosis of the femoral head. J Bone Joint Surg 1994;76A-2:215. 12. Nishii T. Significance of lesion size and location in the prediction of collapse of osteonecrosis of the femoral head: a new three-dimensional quantification using magnetic resonance imaging. J Orthop Res 2002;20:130. 13. Tawara D. Large scale analysis of finite-element method considering inhomogeneousness of bone. J Jpn Soc Mech Eng 2002;2:147 [in Japanese]. 14. Carter DR. The compressive behavior of bone as a twophase porous structure. J Bone Joint Surg 1977;59-A:954. 15. Keyak JH. Prediction of femoral fracture load using automated finite element modeling. J Biomech 1998;31:125. 16. Nanzawa I. A biomechanical study on the senile intracapsular fractures of the femoral neck—spaclation concerning the fracture lines and etiological factors. J Jpn Orthop Assoc 1981;55:167 [in Japanese]. 17. Nishii T. Mechanical evaluation around unstable femoral components with 3D-finite element analysis—a comparison with clinical, periprosthetic bone density examination. J Jpn Soc Clin Biomech 1996;17:11 [in Japanese]. 18. Watanabe Y. Biomechanical study of the resurfacing hip arthroplasty: finite element analysis of the femoral component. J Arthroplasty 2000;15:505. 19. Ipavec M. Mathematical modeling of stress in the hip during gait. J Biomech 1999;32:1229. 20. Ratner BD. Biomaterial science: an introduction to materials in medicine. New York: Academic Press; 1996. 21. Harper EJ. Tensile characteristics of ten commercial acrylic bone cements. J Biomed Mater Res 2000;53:605. 22. Schmalzried TP. Total resurfacing for osteonecrosis of the hip. Clin Orthop Relat Res 2004;429:151. 23. Ong KL. Biomechanics of the Birmingham hip resurfacing arthroplasty. J Bone Joint Surg (Br) 2006;88-B:1110.
FEM of Resurfacing for Osteonecrosis Sakagoshi et al 24. Radcliffe IAJ. Investigation into the effect of varus-valgus orientation on load transfer in the resurfaced femoral head: a multi-femur finite element analysis. Clin Biomech 2007;22:780. 25. Radcliffe IAJ. Investigation into the effect of cementing techniques on load transfer in the resurfaced femoral head: a multi-femur finite element analysis. Clin Biomech 2007;22:422. 26. Little JP. Changes in femur stress after hip resurfacing arthroplasty: response to physiological loads. Clin Biomech 2007;22:440. 27. Long JP. Surgical variables affect the mechanics of a hip resurfacing system. Clin Orthop Relat Res 2006;453:115.
1289
28. Huiskes R. Interface stress in the resurfaced hip-finite element analysis of load transmission in the femoral head. Acta Orthop Scand 1985;56:474. 29. Morlock MM. Biomechanical, morphological, and histological analysis of early failures in hip resurfacing arthroplasty. Proc Inst Mech Eng J Eng Med 2006;220:333. 30. Gabriel JL. Stem fracture after hemiresurfacing for femoral head osteonecrosis. J Arthroplasty 2003;18:96. 31. Stem E. Stem fracture of conserve hemiarthroplasty. J Arthroplasty 2004;19:923. 32. Bowman NK. Fracture of the stem of the femoral component after resurfacing arthroplasty of the hip. J Bone Joint Surg (Br) 2006;88:1652.
A Mechanical Analysis of Femoral Resurfacing Implantation for Osteonecrosis of the Femoral Head Daigo Sakagoshi, MD,* Tamon Kabata, MD, PhD,* Yuichiro Umemoto,y Jiro Sakamoto,y and Katsuro Tomita, MD, PhD*
Abstract: Hip resurfacing is becoming a popular procedure for treating osteonecrosis of the femoral head. However, the biomechanical changes that occur after femoral resurfacing have not been fully investigated with respect to the individual extent of the necrosis. In this study, we evaluated biomechanical changes at various extents of necrosis and implant alignments using the finite element analysis method. We established 3 patterns of necrosis by depth from the surface of femoral head and 5 stem angles. For these models, we evaluated biomechanical changes associated with the extent of necrosis and the stem alignment. Our results indicate that stress distribution near the bone-cement interface increased with expansion of the necrosis. The maximum stress on the prosthesis was decreased with stem angles ranging from 130° to140°. The peak stress of cement increased as the stem angle became varus. This study indicates that resurfacing arthroplasty will have adverse biomechanical effects when there is a large extent of osteonecrosis and excessive varus or valgus implantation of the prosthesis. Keywords: hip, resurfacing, finite element analysis, osteonecrosis, femoral head. © 2010 Elsevier Inc. All rights reserved.
Femoral head resurfacing is morphologically similar to the normal proximal femoral anatomy. It has several advantages over conventional total hip arthroplasty; these include minimal bone resection, easier revision, and maintenance of physiological stress within the proximal femur [1]. Earlier hip resurfacing implants suffered from poor manufacturing quality, resulting in osteolysis and aseptic loosening [2]. However, current metal-on-metal hip resurfacing arthroplasty seems to have solved the problems of poor materials and manufacture, producing clinical results comparable to conventional total hip arthroplasty. Hip resurfacing has become a popular procedure for young active patients with avascular necrosis of the femoral head [3-5]. Hip resurfacing has some particular complications. Some of these, for example, are metal sensitivity, aseptic lymphocytic vasculitis–associated lesions [6], increased wear as the result of poor cup alignment [7], loosening From the *Department of Orthopaedics Surgery, School of Medichine, Kanazawa University, Kanazawa, Japan; and yDepartment of Human and Mechanical Systems Engineering, Kanazawa University, Kanazawa, Japan. Submitted March 26, 2009; accepted September 12, 2009. No benefits or funds were received in support of the study. Reprint requests: Tamon Kabata, MD, PhD, 13-1 Takaramachi, Kanazawa 920-0934, Japan. © 2010 Elsevier Inc. All rights reserved. 0883-5403/2508-0017$36.00/0 doi:10.1016/j.arth.2009.09.002
of the femoral component [8], and femoral neck fracture [9,10]. Loosening of the femoral component and femoral neck fractures were reported to be the most common complications of this procedure and reasons for revision [9]. Both of these complications may be attributed to poor bone stock of the femoral head, arising from large cystic lesions and osteonecrosis. Amstutz reported that large femoral cystic lesions lead to a high failure late within 5 years [8]. Osteonecrosis of the femoral head may cause poor bone stock, and the surgical indication of this procedure when there is wide extended osteonecrosis is controversial. However, it is not yet clear exactly how much osteonecrosis would permit this procedure and how much would be a contraindication. The aims of this study are to analyze the resurfaced femoral head using finite element models and, in particular, to examine the influence of the extent of avascular necrosis and metaphysical stem shaft angles within the femoral head.
Materials and Methods A solid model of a femur was generated from computed tomography (CT) scans (Light Speed Ultra16; GE Medical Systems, Tokyo) of a normal hip from a 28-year-old Japanese woman. For each CT slice, the outer contour of the femur was defined and lofted to
1282
FEM of Resurfacing for Osteonecrosis Sakagoshi et al
form a 3-dimensional solid model of the femur using MECHANICAL FINDER 2.0 (Research Center of Computational Mechanics Inc, Tokyo). We used the proximal 190-mm compartment of the femur. The femur was then virtually implanted with a suitably sized femoral component (Conserve, 42-mm diameter; Wright Medical Technology Inc, Memphis, Tenn). In the implanted femur model, the prosthesis was considered as having a cemented inner surface with a thickness of 1 mm, as proposed by Amstutz et al [9]. In real surgery, the cement would be interdigitated with the cancellous bone of the reamed femoral head, but for simplicity in this study, the models had a 1-mm-thick cement mantle. The implant-cement, cement-bone, and implant-bone interfaces were assumed to be rigidly bonded. We set 3 patterns of necrosis according to depth from the surface of the femoral head [11]. Extension of necrosis to a quarter of the femoral head diameter is type A, to a half is type B, and to three fourths is type C. The shape of the osteonecrosis was determined as the crossover region between the femoral head and a ball with the same diameter as the femoral head. Assuming the common location of femoral head necrosis, the occurrence point of the necrosis was established at the surface of the femoral head according to the “gravity of necrosis” concept reported by Nishii (Fig. 1) [12]. Assuming the center of the femoral head as the coordinate center, the occurrence point of the necrosis was established at 55.7° medial from the vertical direction and 45.6° frontal from the horizontal direction. We modeled 5 types of stem shaft angles against the femoral shaft axis from 125° to 145° by 5°. Thus, including the 3 models for extent of necrosis, we had a total of 15 pattern models in this study (Fig. 2). We replaced all the necrosis areas with cement. The models were meshed using liner tetrahedoral elements with a 3-mm element edge length by MSC. Patran 2001(MSC Software Corporation, USA). The meshes consisted of 45,788 to 47,318 tetrahedral elements. Material properties were applied to the bone using the software ADVENTURE CTBone (ADVENTURE PROJECT, http://adventure.q.t.u-tokyo.ac.jp/jp/alliances/). This program could interpolate Hounsfield units from the CT scans and determine apparent densities. The Young modulus of each element could then be calculated [13]. The conversion equation from Hounsfield units to bone density (p; g/cm3) was as follows:
1283
Fig. 1. Location of the necrosis was established at the surface of femoral head according to the concept of gravity of necrosis.
be about 17 GPa [17]. Considering the maximum value of bone density of this model, the formula devised by Carter [14] was most suitable for this study.
q = ðHU + 1:4246Þ 0:001=1:0580 : ð1 b HUÞ q = 1:0 108 : ðHU V 1Þ The maximum bone density of this model was calculated as 1.6 g/cm3. There are several formulas for calculating the material properties of the bone [14-16]. The Young modulus of the cortex bone was assumed to
Fig. 2. Three types of expansion of necrosis area from one forth to three forths and 5 types of stem shaft angles from 125° to 145° were established.
1284 The Journal of Arthroplasty Vol. 25 No. 8 December 2010
Fig. 3. The loading conditions of this study replicated the stance phase of level gait. The models were loaded with 1500 N applied to the femoral head and 1000 N acting as the abductor muscle (Ipavec et al [19]).
The apparent density was related to the Young modulus by the equation: E = 0:001 : ðq V 1:0 108 Þ E = 2875p3 : ð1:0 108 b qÞ Where E is the Young modulus (MPa) and p is the apparent density (g/cm3). The Poisson ratio for bone was assumed to be 0.4. The polymethyl methacrylate (PMMA) bone cement mantle and cobalt-chrome implant were modeled as linear elastic materials with a Poisson ratio of 0.19 and 0.3, and Young modulus of 2000 and 200 000 MPa [18]. The models were physiologically loaded to replicate peak loads during the stance phase of level gait, in accordance with the findings of Ipavec [19]. In this study, the models were loaded with 1500 N applied to the femoral head and 1000 N acting as the abductor muscle. Both forces were distributed over an area to
avoid the effect of point loading. The contact force to the femoral head was distributed as a sine curve, acting at 13° from the vertical in the frontal plane and 40° in the axial plane. The abductor force was acting at 20° from the vertical in the frontal plane and 40° in the axial plane (Fig. 3). The models were rigidly constrained at the distal surface. The finite element models were analyzed using the software ADVENTURE system (ADVENTURE PROJECT, http://adventure.q.t.u-tokyo.ac.jp/). The von Mises stress of ductile materials; the implant, bone, and von Mises stress of the brittle material; and PMMA for each model were compared.
Result Expanding the necrosis area, which was replaced by cement, altered the strain distribution within the femoral head. As can be seen Fig. 4, which presents coronal plane sections of each model, strain concentration was not observed in the “type A” femoral head. However, in the “type B” and “type C” femoral heads, we see increased strain near the bone and cement interface corresponding to the expansion of the necrosis replaced by a cement mantle. The strain concentration is particularly evident in the type C model, which has wide cement mantle replacement on the resurfaced femoral head. The von Mises stress distributions of the implant are shown in Fig. 5. Stress concentration was observed at the stem shaft regardless of extent of necrosis when the shaft was implanted at angles of 125° or 145°. The peak value of the implants' von Mises stress is shown in Fig. 6A. At 130° and 135°, the peak values of von Mises stress
Fig. 4. The figures show the strain distributions in the coronal plane sections. As the necrosis area that was replaced by cement expand, the stress concentration increase at the bone-cement interface.
FEM of Resurfacing for Osteonecrosis Sakagoshi et al
1285
Fig. 5. The figures show the von Mises stress of implants.
were lower (less than 80 MPa) and recognized at the shell rim of the implant. By contrast, at implantation angles of 125°, 140°, or 145°, the peak stress arose at the stem shaft. The peak stress of the stem shaft is shown in Fig. 6B. In this area, the peak stress was lowest at 130° but increased as the stem angle became varus or valgus. However, for all variables examined, the maximum peak stress values in implants were well below the yield stress of cobalt-chrome alloys, which was assumed to be 445 to 517 MPa [20]. The peak stress at the anterior and inferior shell rims is shown in Fig. 6C-D. The change of stress seems to be indistinct at the anterior edge of the implant shell, but particularly at the inferior edge, the peak stress increases as the stem angle becomes varus. Fig. 7 presents the maximum principal (tensile) stress of the cement. Stress concentrations were recognized at the anterior and inferior edges of the cement mantle. The peak stress value of the cement is shown in Fig. 8A. For all the models, the peak value was less than 35 MPa, which is below the yield stress of PMMA, assumed to be 50.1 MPa [21]. The peak stress at the inferior edge of the cement mantle is shown in Fig. 8B. Compared to the varus stem, the valgus stem reduced the magnitude of the peak stress in the cement near the inferior shell rim.
Discussion Early hip resurfacing implants suffered from poor manufacturing quality of the bearing surface and materials. However, the current generation of metalon-metal resurfacing arthroplasty, using improved manufacturing techniques, has shown promising results in clinical trials [3-5]. This procedure is regaining its popularity, especially as a treatment option for young
and active patients with avascular necrosis of the femoral head, which is difficult to treat with joint preserving surgery (eg, proximal femoral osteotomy). However, concerns remain regarding the risk of postoperative femoral neck fracture [10] and loosening of the femoral component [6], and these pathological mechanisms have not been not clearly explained. Furthermore, the surgical indication criteria of this procedure for expanded femoral head necrosis are still unclear [22]. The aim of this study is to implement finite element techniques in the investigation of the effects of implant orientation and the expanse of necrosis replaced by the cement mantle. In particular, we wanted to determine if variable implant angles and expanse of necrosis replaced by cement affect the load transfer distribution from the implant to the adjacent cement and bone, and we also wanted to evaluate the stress of the implant and the cement mantle. Finite elements analysis was performed to analyze the various hip arthroplasties. In the past studies, constant material properties were given to the cortical and cancellous bones. In the analysis of femoral component of the conventional total hip arthroplasty, material properties seem to have few influences at the results. This is because the prosthesis is mainly supported by cortical bone after cutting of the femoral neck, which consists of inhomogeneous cancellous bone trabeculae. Contrary to this, femoral resurfacing arthroplasty is supported by inhomogeneous cancellous bone. For a physiological analysis, the model should have the heterogeneous material properties of the trabecular bone of the femoral head. We used the ADVENTURE system, which permits configuration of the material
1286 The Journal of Arthroplasty Vol. 25 No. 8 December 2010 properties [13]. Our study provides high-resolution stress and strain fields around the femur, incorporating the inhomogeneous CT-based distribution of bone properties obtained from a young donor. This represents
the target population for hip resurfacing arthroplasty. Although there have been some finite element studies regarding hip resurfacing arthroplasty [23-27], in which the effects of cement mantle or implant orientation were discussed, we could not find any report that refers to the effects and indications of expanded avascular necrosis of the femoral head. The size and location of femoral head necrosis vary for each patient. Nishii [12] evaluated lesion size and location using magnetic resonance imaging on 65 hips in a consecutive series of 47 patients with osteonecrosis and found no radiological evidence of collapse. He reported the center of gravity as the most common location of osteonecrosis in the femoral head. This described that location of the gravity center of osteonecrosis was used in this study. We created models for 3 patterns of osteonecrosis. This study found differences of stress distribution in the femoral head among these models. In the model with little necrosis replaced by cement (type A), the stress was transferred through the stem to the inferior femoral neck. Only a small amount of stress was generated in the resurfaced femoral head and led to the stress shielding. This result corresponds to the findings of previous studies [18,25,28], suggesting that the stiff arthroplasty protects the proximal section of the femoral bone from the load. Increased stress concentration near the bonecement interface was observed in the model with an expanded cement mantle in the resurfaced femoral head (type C). We think that hip resurfacing should not be considered for remarkable expanded osteonecrosis like as type C model, and we think that the adaptive limit of this procedure is between types A and C. The volume of the osteonecrois of the type B model would become reduced after cylindrical reaming of femoral head. With filling the remained bone defect with cement, hip resurfacing might be performed for the type B model. At the type B model, some stress concentration was recognized in the resurfaced femoral head. The larger load portion taken by the thicker cement mantle is then spread out over the area of cancellous bone at the bone and cement interface. In this part, cancellous bone is important as a role of anchor of cement. At the wide expanded necrosis model as type C, excessive cement mantle protruding over a half of femoral head diameter Fig. 6. (A) The peak value of the implant's von Mises stress. At 130° and 135°, the peak values were lower and recognized at the shell rim of the implant. At 125°, 140°, or 145°, the peak stress arose at the stem shaft. (B) The peak stress of the part of stem shaft. The peak stress was lowest at 130° but increased according to the stem angle becoming varus or valgus. (C) The peak stress of the part of the anterior edge of implant shell. The change of stress seemed to be indistinctive. (D) The peak stress of the part of the inferior edge of implant shell. The peak stress increased corresponding to the stem angle becoming varus.
FEM of Resurfacing for Osteonecrosis Sakagoshi et al
1287
Fig. 7. The figures show the maximum principal (tensile) stress of cement. The stress concentrations were recognized at the anterior edge and the inferior edge of cement mantle.
in the resurfaced femoral head had possibilities to make larger stress against the cancellous bone adjacent to the cement mantle. The tolerance of cancellous bone as
Fig. 8. (A) The peak stress values of cement are shown. They were below the yield stress of PMMA. (B) The peak stress of inferior edge of cement mantle. The valgus stem reduced the magnitude of the peak stress compared to the varus stem.
anchor should be a concern in that situation. Unfortunately, we could not refer about the yield stress of cancellous bone adjacent to the cement mantle. Morlock [29] analyzed 55 retrieved implants of failed resurfacing arthroplasties. In that study, only 31% of failed femoral heads had been cemented in accordance with the manufacturer's guideline; the thickness of cement mantles or penetrations often exceeded 5 mm. The authors recommended that excessive cementing into the femoral head should be avoided. Considering the result of our study and Morlock's recommendation, we should be careful about considering resurfacing in the case of patients who have widely expanded osteonecrosis that might be replaced by cement on the femoral head. Implant stem fracture is one of the rare complications of hip resurfacing arthroplasty. There are a few case reports in the literatures [30-32]. Although the cause of this complication is not apparent, osteonecrosis occurring because of surgical disruption of the vascular supply, progressive bone resorption, and cemented stem were thought to be risk factors. In all cases of stem fractures reported previously, the stems were firmly cemented to the centralizing canal. In this study, the condition of implant-bone interface was bonded, and this might be likened to a stem fixed by cement. We found stress concentration at the stem shaft when it was implanted at varus or excessively valgus angles. However, the peak stress at the stem shaft was well below the yield stress of implants. Probably, this rare complication can be attributed to a combination of factors, not just implant angle alone, such as momentary dynamic force against the femoral head, metal fatigue caused by repeated stress, and the aforementioned risk factors. Considering the results of this study, varus or excessively
1288 The Journal of Arthroplasty Vol. 25 No. 8 December 2010 valgus implantation should be avoided to prevent stress concentration at the stem shaft. Stress concentrations in the cement were observed at the anterior and inferior edges in each model. Regardless of the extent of necrosis replaced by cement, the stress values at the inferior edge increased as the stem shaft angle became varus. It has been shown in clinical trials that the varus stem orientation may be related to failures in the resurfaced femoral head. Amustutz [9] reported a significantly higher incidence of revision for femoral loosening in hips with more varus orientation, and Shimnin [10] described varus placement in 42 (84%) of 50 cases of femoral neck fractures. Some finite element studies [24,27] of the relation between varus implant orientation and neck fracture also support the experience of clinical trials. Long [27] found that a valgus stem orientation reduced the local stress, which occurred within the inferior aspect of the cement mantle near the shell rim. This result was similar to ours. Our results also suggest that a varus stem orientation should be avoided. There are several limitations to this study. This model is not validated and is theoretical. Some notable elements that are lacking are as follows. First, we modeled the interface conditions of bone and cement and implant as bonded. Although clinically the bone and cement interface is assumed to be bonded, sliding might occur at the stem and bone interface with some friction. In addition, in this study, the necrosis area was totally replaced by cement, but clinically, we do not always completely remove the necrotic bone. Finally, the loading condition of this study was assumed to be a level gait. If more demanding loads were applied, such as active sports, running, or heavy weight, the stress concentration would be much greater. It is unclear from the results of this study whether active sports can be permitted after hip resurfacing. Further work will be required to elucidate the biomechanical consequences considering the previously mentioned factors.
Conclusions Finite element analysis of hip resurfacing methods was performed against femoral head necrosis models with various extents of necrosis areas and several implant angles. This study suggests that hip resurfacing for patients in whom osteonecrosis extends widely should be considered very carefully; increased stress concentration near the bone-cement interface may occur when all the necrotic bone is replaced by cement. Further, excessive varus or valgus implantation of the prosthesis has potentially adverse biomechanical effects for implants and the cement mantle.
References 1. McMinn D. Metal on metal surface replacement of the hip. Clin Orthop 1996;329S:89.
2. Freeman MAR. ICLH surface replacement of the hip: an analysis of the first 10 years. J Bone Joint Surg 1983; 65:405. 3. Amstutz HC. Metal-on-metal hybrid surface arthroplasty: two to six-year follow-up study. J Bone Joint Surg Am 2004;86-A:28. 4. Beaule PE. Metal-on-metal surface arthroplasty with a cemented femoral component: a 7-10 year follow-up study. J Arthroplasty 2004;19:17. 5. Daniel J. Metal-on-metal resurfacing of the hip in patients under the age of 55 years with osteoarthritis. J Bone Joint Surg Br 2004;86:177. 6. Amstutz HC. Metal-on-metal hybrid surface arthroplasty. Surgical technique. J Bone Joint Surg (Am) 2006;88(Suppl 1 Pt2):234. 7. Morlock MM. Modes of implant failure after hip resurfacing: morphological and wear analysis of 267 retrieval specimens. J Bone Joint Surg (Am) 2008;90 (Suppl 3):89. 8. Counsell A. A groin mass caused by metal particle debris after hip resurfacing. Acta Orthop Berg 2008;74:870. 9. Amstutz HC. Fractures of the neck of the femur after surface arthroplasty of the hip. J Bone Joint Surg (Am) 2004;86-A:1874. 10. Shimmin A. Femoral neck fractures following Birmingham hip resurfacing. J Bone Joint Surg 2005;87-B:463. 11. Shimizu K. Prediction of collapse with magnetic resonance imaging of avascular necrosis of the femoral head. J Bone Joint Surg 1994;76A-2:215. 12. Nishii T. Significance of lesion size and location in the prediction of collapse of osteonecrosis of the femoral head: a new three-dimensional quantification using magnetic resonance imaging. J Orthop Res 2002;20:130. 13. Tawara D. Large scale analysis of finite-element method considering inhomogeneousness of bone. J Jpn Soc Mech Eng 2002;2:147 [in Japanese]. 14. Carter DR. The compressive behavior of bone as a twophase porous structure. J Bone Joint Surg 1977;59-A:954. 15. Keyak JH. Prediction of femoral fracture load using automated finite element modeling. J Biomech 1998;31:125. 16. Nanzawa I. A biomechanical study on the senile intracapsular fractures of the femoral neck—spaclation concerning the fracture lines and etiological factors. J Jpn Orthop Assoc 1981;55:167 [in Japanese]. 17. Nishii T. Mechanical evaluation around unstable femoral components with 3D-finite element analysis—a comparison with clinical, periprosthetic bone density examination. J Jpn Soc Clin Biomech 1996;17:11 [in Japanese]. 18. Watanabe Y. Biomechanical study of the resurfacing hip arthroplasty: finite element analysis of the femoral component. J Arthroplasty 2000;15:505. 19. Ipavec M. Mathematical modeling of stress in the hip during gait. J Biomech 1999;32:1229. 20. Ratner BD. Biomaterial science: an introduction to materials in medicine. New York: Academic Press; 1996. 21. Harper EJ. Tensile characteristics of ten commercial acrylic bone cements. J Biomed Mater Res 2000;53:605. 22. Schmalzried TP. Total resurfacing for osteonecrosis of the hip. Clin Orthop Relat Res 2004;429:151. 23. Ong KL. Biomechanics of the Birmingham hip resurfacing arthroplasty. J Bone Joint Surg (Br) 2006;88-B:1110.
FEM of Resurfacing for Osteonecrosis Sakagoshi et al 24. Radcliffe IAJ. Investigation into the effect of varus-valgus orientation on load transfer in the resurfaced femoral head: a multi-femur finite element analysis. Clin Biomech 2007;22:780. 25. Radcliffe IAJ. Investigation into the effect of cementing techniques on load transfer in the resurfaced femoral head: a multi-femur finite element analysis. Clin Biomech 2007;22:422. 26. Little JP. Changes in femur stress after hip resurfacing arthroplasty: response to physiological loads. Clin Biomech 2007;22:440. 27. Long JP. Surgical variables affect the mechanics of a hip resurfacing system. Clin Orthop Relat Res 2006;453:115.
1289
28. Huiskes R. Interface stress in the resurfaced hip-finite element analysis of load transmission in the femoral head. Acta Orthop Scand 1985;56:474. 29. Morlock MM. Biomechanical, morphological, and histological analysis of early failures in hip resurfacing arthroplasty. Proc Inst Mech Eng J Eng Med 2006;220:333. 30. Gabriel JL. Stem fracture after hemiresurfacing for femoral head osteonecrosis. J Arthroplasty 2003;18:96. 31. Stem E. Stem fracture of conserve hemiarthroplasty. J Arthroplasty 2004;19:923. 32. Bowman NK. Fracture of the stem of the femoral component after resurfacing arthroplasty of the hip. J Bone Joint Surg (Br) 2006;88:1652.