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Neck fracture of femoral stems with a sharp slot at the neck: biomechanical analysis
J Orthop Sci (2015) 20:881–887 DOI 10.1007/s00776-015-0745-1
ORIGINAL ARTICLE
Neck fracture of femoral stems with a sharp slot at the neck: biomechanical analysis Kensei Yoshimoto1 · Yasuharu Nakashima1 · Akihiro Nakamura2 · Taro Mawatari1 · Mitsugu Todo3 · Daisuke Hara1 · Yukihide Iwamoto1
Received: 25 March 2015 / Accepted: 12 June 2015 / Published online: 25 July 2015 © The Japanese Orthopaedic Association 2015
Abstract Background Fracture of the femoral stem in total hip arthroplasty (THA) is a rare complication. We have encountered 2 cases of neck fractures of the femoral stem occurring 9 and 12 years after THA. Morphological and biomechanical analysis were performed to investigate the mechanism of these fractures. Method A titanium alloy femoral stem having a slot with sharp corners (R = 0.2 mm) at the neck had been implanted in both cases. Fracture surfaces were examined by use of scanning electron microscopy (SEM). Stress concentration was simulated by using a finite element method (FEM) to compare slots with sharp (R = 0.2 mm) and smooth (R = 2 mm) corners. Results Study of the retrieved stems revealed that neck fractures had occurred at the distal end of the slot in both cases. SEM revealed numerous fine fissures extending from the anterolateral edge, striations on the middle of the fracture surface, and dimples on the posteromedial surface, suggesting that the fractures had occurred from the anterolateral aspect toward the posteromedial aspect because of metallic fatigue. FEM analysis showed that mechanical stress was concentrated at the distal and anterolateral
* Yasuharu Nakashima [email protected]‑u.ac.jp 1
Department of Orthopedic Surgery, Graduate School of Medical Sciences, Kyushu University, 3‑1‑1 Maidashi, Higashi‑ku, Fukuoka 812‑8582, Japan
2
Kyocera Medical Corporation, 3‑3‑31 Miyahara, Yodogawa‑ku, Osaka 532‑0003, Osaka, Japan
3
Division of Renewable Energy Dynamics, Research Institute for Applied Mechanics, Kyushu University, Fukuoka, Japan
corners of the slot. Under 3500-N loading force, the stress at the sharp corner was 556 MPa, which was approximately twofold that at the smooth corner and exceeded the fatigue strength of titanium alloy. Conclusion These findings showed that the sharp corner of slot increased stress concentrations at the anterolateral aspect and led to the neck fractures.
Introduction Fracture of stainless steel femoral stems was a major complication in the early history of total hip arthroplasty (THA), with reported prevalence ranging from 0.23 % [1] to as high as 11 % [2]. With the development of femoral stems made of cobalt–chromium–molybdenum alloy or titanium alloys, femoral stem fracture has become a rare complication. Recently, there have been several reports of femoral stem neck fracture as a result of additional manufacturing processes involved while creating the neck. To prevent impingement and improve the range of motion, a stem neck with slots on the anterior and posterior aspects was produced. This resulted in a smaller neck radius, which in turn increased the tensile stress transmitted through the stem neck. In one study, this was found to cause 9 neck fractures [3]. Material machining, for example laser etching, reportedly further reduced the strength of the stem and resulted in several fractures [4]. Modular hip systems have been gaining popularity, because they restore normal hip biomechanics with the ability to adjust offset. However, in several studies these systems have been associated with neck fractures caused by crevice corrosion [5, 6]. Specifically, additional machining of the stem neck resulted in a decrease of its mechanical strength.
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We have encountered 2 cases of neck fractures. These fractured stems had a slot with a sharp corner at the neck and fracture had occurred at the distal corner of the slot. In this study, fracture surfaces were fully examined by use of scanning electron microscopy (SEM), and the effect of the sharp corner of the slot was analyzed by use of a finite element method (FEM).
Materials and methods Case presentation This study was approved by the institutional review board at our institution (IRB number 25–11). Informed consent was obtained from each patient. Case 1
Fig. 1 Radiographs showed that, in both cases, the fractures occurred at the distal end of the neck of the femoral stem. a Case 1. b Case 2
In 2000, a 45-year-old man underwent right THA for traumatic osteoarthritis. His height was 178 cm, his weight 78 kg, and his body mass index (BMI) 24.8 kg/m2. We implanted a cementless 54-mm acetabular shell with a highly cross-linked polyethylene (HXPE) liner, a proximally coated collared cementless stem, and a zirconia 22-mm femoral head (Kyocera, Osaka, Japan). Postoperative radiographs showed that the implant position was neutral. Nine years and 4 months after the operation the patient felt severe pain in the right hip while squatting and could not walk. Radiographic examination revealed a neck fracture of the femoral stem without loosening (Fig. 1a). Revision THA was performed. This case was included in our previous report of this THA system [7]. Case 2 In 1997, a 49-year-old man underwent the right side unipolar hip arthroplasty for avascular necrosis of the femoral head. His height was 170 cm, his weight 70 kg, and his BMI 24.2 kg/m2. We implanted a 52-mm outer head and a proximally coated collared cementless stem (Kyocera). Two years and 8 months after the first operation, proximal migration of the outer head resulted in right THA revision by use of a cementless 52-mm acetabular shell with an HXPE liner and a metal 26-mm femoral head (Kyocera). The femoral stem was not revised. Ten years and 2 months after the THA revision, the patient experienced severe pain in the right hip while closing a door and could not stand. Radiographic examination revealed a neck fracture of the femoral stem without loosening (Fig. 1b). Revision THA was performed.
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Fig. 2 A cross-section of the femoral neck. The size of the slot was 6.65 mm × 6.65 mm and the depth of the slot was 1.1 mm. The slot had a sharp corner angle that was 0.2 mm in radius
Femoral stem design Identical cementless femoral stems were implanted in both cases. These stems were made from Ti–6Al–4V and were collared. The proximal one-third of the stem was roughened by titanium arc spray and coated with hydroxyapatite [7, 8]. At the stem neck, the slot was machined to enable an instrument to grip the neck for stem removal. The slot had a sharp corner angle (R = 0.2 mm). The size of the slot was 6.65 mm × 6.65 mm and its depth was 1.1 mm (Fig. 2).
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Neck fracture of femoral stems with a sharp slot at the neck: biomechanical analysis
Surface analysis The 2 fractured stems were examined visually by use of a digital microscope (VHX-200; Keyence, Osaka, Japan) to characterize the fracture surfaces. Further investigation with SEM (S-3400N; Hitachi, Tokyo, Japan) was performed to assess the fracture mechanism. FEM analysis We hypothesized that the sharp corner of the slot was involved in these fractures. Stress distribution was analyzed with 2 software products, Ansys Workbench Ver.13 (Ansys, Houston, TX, USA) and Solid Edge Ver.20 (Siemens PLM Software, Plano, TX, USA). Two slot designs were evaluated. The first had a sharp (R = 0.2 mm) corner angle equal to that of the slot on the fractured stem. The other had a smooth (R = 2 mm) corner angle which was equal to that on the next-generation stem. We constructed 3-dimensional models based on ISO7206-6 (1992), and the stems were implanted in PMMA bone cement with 10° of varus and 9° of flex position (Fig. 3). ISO7206-6 was followed for the elastic modulus and the Poisson’s ratio of the materials. Details are listed in Table 1. These fractured stems could be attached with −4, 0 and +4 mm balls. As the stress at the neck was higher with longer neck (+4 mm ball), the superior margin of the neck was extended to the position of the center of the +4 mm ball. To reduce computation time, the distal section of the stem was resected, because this section only minimally affected the results of the analysis (Fig. 3). Finite element meshes were generated by using tetrahedral 10-node elements (C3D10) (Fig. 4). The mesh size used at each site (Table 2) was that obtained when the analysis result converged by less than 5 % as the mesh size was gradually reduced. In accordance with ISO7206-6 (1992), both designs were exposed to a downward 2300-N load at the superior margin of the neck, and the maximum principal stress was measured. Because stress to the hip is reportedly four or five times the body weight [9, 10], further FEM analysis was conducted with a more severe loading stress of 3500 N.
Fig. 3 FEM analysis model based on ISO7206-6. Two slot designs were evaluated. The first had a sharp (R = 0.2 mm) corner angle equal to that of the slot on the fractured stem. The other had smooth (R = 2 mm) corner angles to enable the effect of slot shape to be assessed. The stem was implanted in PMMA bone cement with 10° of varus and 9° of flex position. Because we assumed that a +4 mm ball was located at the neck, the superior margin of the neck was extended to the position of the center of the +4 mm ball. The stem was exposed to a downward loading force at the superior margin of the neck
Table 1 The material properties for the 3-dimensional model
Young’s modulus (GPa) Poisson’s ratio
Ti-6Al-4V
PMMA
110.6
2.65
0.326
0.455
Results Examination of the retrieved femoral stems (Fig. 5) revealed that the neck fractures occurred at the distal end of the slot in both cases. SEM images of the fracture surfaces revealed numerous fine fissures extending from the anterolateral edge toward the posteromedial aspect of the neck (Fig. 6a). Striations were observed in the middle of the surface (Fig. 6b). Dimples were observed on the posteromedial surface (Fig. 6c).
FEM analysis revealed high levels of stress at the anterolateral aspect of the femoral neck. Maximum principal stress was concentrated at the anterolateral aspect of the distal portion of the corner (Fig. 7; Table 3). For a 2300-N load, the maximum stress was 365 MPa at the sharp corner and 195 MPa at the smooth corner whereas for a 3500-N load, the maximum stress was 566 MPa at the sharp corner and 296 MPa at the smooth corner.
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Fig. 4 Meshes configured with tetrahedral 10-node elements (C3D10) were generated to analyze stress distribution. The mesh size was that obtained when the analysis result converged by less than 5 % as the mesh size was gradually reduced
Table 2 Mesh size R = 0.2 (mm)
R = 2 (mm)
Stem PMMA bone cement Slot of the neck
1 2 0.2
1 2 0.2
Corner of the slot
0.02
–
Discussion We report 2 cases of neck fractures of the femoral stem. In both cases, fractures had occurred at the distal part of the sharp corner, from the anterolateral to the posteromedial aspect. The presence of numerous striations suggested metal fatigue of the Ti–6Al–4V was the main cause of fracture. FEM analysis showed that loading stress was concentrated at the anterolateral aspect of the distal part of the slot, and that the sharp corner resulted in greater stress concentration than the smooth one. These findings indicated that excessive stress at the sharp corner was the primary cause of the neck fractures. There were several limitations to our study. We evaluated only 2 cases, and FEM analysis was performed for a vertical static load only. In practice, the femoral stem is exposed to stress caused by a variety of dynamic motions, for example walking, stair climbing, squatting, and twisting of the legs. Further investigation should include study of the effect of multiple dynamic loads. Furthermore, in Case 2 the taper of the femoral neck was possibly scratched
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Fig. 5 Examination of the retrieved stems revealed that the fractures had occurred at the distal part of the sharp corner of the slot. a Case 1. b Case 2
during the initial revision THA. During the isolated acetabular revision surgery, forceful retraction of the femoral stem by use of retractors or bone hooks placed around the stem neck may cause damage to the neck, disrupt the passivation layer, and enhance corrosion. In comparison with stainless steel femoral stems, fractures of femoral stems made of titanium alloy or cobalt– chromium–molybdenum are rare [11, 12]. Furthermore, most stem fractures reportedly occurs at the middle third of the implant. Neck fracture of femoral stem, as seen in
Neck fracture of femoral stems with a sharp slot at the neck: biomechanical analysis
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Fig. 6 SEM image of the fractured surface of Case 1. The cross section of the fractured neck was examined. a Anterolateral part of the fractured surface. Fissures extended from the anterolateral edge
toward the center (yellow arrows). b Middle part of the fractured surface. Striations were observed. c Posteromedial part of the fractured surface. Dimples were observed
this study, is rare [13, 14]. Specifically, a review of the literature indicated that neck fractures were because of manufacturing processes involving the neck. With monoblock prostheses used in the past, defects created while welding the neck to the head had the potential to cause metal fatigue and fracture [15–17]. More recently it has been suggested that several machining processes used to create the neck, intended to prevent impingement and improve the range of motion, cause neck fracture. Vatani et al. [3] reported 9 fractures at the stem neck because of abnormal transmission of forces through the narrowest segment of the neck. Laser etched markings on the neck have also resulted in 2 neck fractures [4, 18]. Grivas et al. [4] mentioned that the process of laser etching induced localized changes in the metallic microstructure and creation of a local stress concentrator. Crevice corrosion and fretting corrosion at the head–neck junction, both of which may cause stem neck fracture, have been reported in several papers [12, 19–22]. Femoral neck fracture, crevice corrosion, and fretting corrosion at the neck–stem junction have also been reported for modular hip systems consisting of a head, neck, and stem [5, 6, 23–25]. Grupp et al. [26] estimated a fracture prevalence of 1.4 % for modular stems.
In our cases, numerous fine fissures extending from the anterolateral fracture surface toward the posteromedial surface indicated that the femoral neck fracture initially occurred at the anterolateral aspect of the neck and propagated posteromedially. Striations on the middle of the fracture surface were indicative of metal fatigue whereas dimples on the posteromedial surface were indicative of static fracture. Continuous loading stress might have caused metal fatigue and subsequent fracture. FEM analysis revealed that the sharp corner caused excess stress concentrations at the anterolateral and distal portions of the slot. Sharp notches are well known to cause excess stress concentration in metal materials [27]. The fatigue strength of Ti–6Al–4V is approximately 400 MPa [28]. In this case the maximum stress at the sharp corner was 365 MPa with 2300-N loading force based on ISO7206-6 (1992). However, several studies have shown that the femoral neck is exposed to 4 or 5 times the body weight during walking or running [9, 10]. This would result in a 3500-N loading force on the femoral stem; in this case, the stress at the sharp corner would be 566 MPa. This stress concentration at the sharp corner was excessively high. As the stress at the smooth corner was equivalent to that previously reported under similar experimental conditions
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Fig. 7 Stress distribution calculated by use of FEM analysis. Stress was concentrated at the anterolateral aspect of the distal part of the corner of the slot under both 2300 and 3500-N loading force conditions
Table 3 Comparison of stress distribution Loading force 2300 N Maximum stress Spot exposed maximum stress 3500 N Maximum stress Spot exposed maximum stress
R = 0.2 mm
R = 2 mm
365 MPa Distal corner of the slot
195 MPa Distal corner of the slot
556 MPa
296 MPa
Distal corner of the slot
Distal corner of the slot
[29], the sharp corner possibly led to this high stress concentration. These results indicated that the neck of the fractured stem might be exposed to stress above the fatigue strength of Ti–6Al–4V. Overall, it seems the sharp corner was exposed to continuous loading stress that caused metal fatigue and subsequent fracture. According to the manufacturer, 12,248 stems with slots with sharp corners were implanted between 1995 and 2002 in Japan, and 71 of the stems (0.58 %) fractured at the neck. In our institution, 518 stems were implanted in that period, with 2 fractures (0.38 %). All the fractures occurred
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at the same site, the distal corner of the slot. Therefore, we assumed that the mechanism of the fractures was similar. Other stems with a slot at the neck include the Bi-Metric (Biomet, Warsaw, IN, USA) and VerSys (Zimmer, Warsaw, IN, USA) stems. However, these stems have relatively rounded slot corners compared with our fractured stems, and there have been no reports of neck fracture at the slot. These findings, also, indicate that stress concentration at sharp corner led to neck fracture. Neck fracture may be caused not only by material factors, but also by patient or surgery-related factors. Rand and Chao [16] suggested that risk factors include increased activity, varus positioning of the stem, and increased body weight. Vatani et al. [3] showed that the main independent risk factors for fracture of the femoral stem were younger age and heavier weight. Both of our cases were male and each had a body weight above 70 kg, and Case 1 was 55 years old when he underwent the first THA. These factors might have contributed to the fractures reported here. After the occurrence of neck fracture, manufacturing processes were refined. A slot with rounded corner was incorporated in the next-generation stem (Chamfered Perfix; Kyocera, Osaka, Japan); 12,923 of these new stems have been implanted, with no reports of neck fractures.
Neck fracture of femoral stems with a sharp slot at the neck: biomechanical analysis
Conclusion In conclusion, the sharp corner at the neck slot was the primary cause of neck fractures in our cases. Mechanical stress was concentrated at the anterolateral and distal corners of the slot, and continuous excessive stress caused metal fatigue. Several manufacturing processes used to create femoral stem necks may contribute to neck fracture. Acknowledgement This work was supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (no. 24592268) and in part by Kyocera Co., Ltd. Compliance with ethical standards Conflict of interest The authors declare that they have no conflict of interest.
References 1. Charnley J. Fracture of femoral prostheses in total hip replacement. A clinical study. Clin Orthop Relat Res. 1975;111:105–20. 2. Martens M, Aernoudt E, de Meester P, Ducheyne P, Mulier JC, de Langh R, Kestelijn P. Factors in the mechanical failure of the femoral component in total hip prosthesis: report of six fatigue fractures of the femoral stem and results of experimental loading tests. Acta Orthop Scand. 1974;45(5):693–710. 3. Vatani N, Comando D, Acuña J, Prieto D, Caviglia H. Faulty design increases the risk of neck fracture in hip prosthesis. Acta Orthop Scand. 2002;73(5):513–7. 4. Grivas TB, Savvidou OD, Psarakis SA, Bernard PF, Triantafyllopoulos G, Kovanis I, Alexandropoulos P. Neck fracture of a cementless forged titanium alloy femoral stem following total hip arthroplasty: a case report and review of the literature. J Med Case Rep. 2007;6(1):174–80. 5. Dangles CJ, Altstetter CJ. Failure of the modular neck in a total hip arthroplasty. J Arthroplasty. 2010 Oct; 25(7):1169.e5-7. 6. Wright G, Sporer S, Urban R, Jacobs J. Fracture of a modular femoral neck after total hip arthroplasty: a case report. J Bone Joint Surg Am. 2010;92(6):1518–21. 7. Nakashima Y, Sato T, Yamamoto T, Motomura G, Ohishi M, Hamai S, Akiyama M, Hirata M, Hara D, Iwamoto Y. Results at a minimum of 10 years of follow-up for AMS and PerFix HA-coated cementless total hip arthroplasty: impact of cross-linked polyethylene on implant longevity. J Orthop Sci. 2013;18(6):962–8. 8. Nakashima Y, Hayashi K, Inadome T, Uenoyama K, Hara T, Kanemaru T, Sugioka Y, Noda I. Hydroxyapatite-coating on titanium arc sprayed titanium implants. J Biomed Mater Res. 1997;35(3):287–98. 9. Duda GN, Schneider E, Chao EY. Internal forces and moments in the femur during walking. J Biomech. 1997;30(9):933–41. 10. Edwards WB, Gillette JC, Thomas JM, Derrick TR. Internal femoral forces and moments during running: implications for stress fracture development. Clin Biomech (Bristol, Avon). 2008 Dec; 23(10):1269-1278.
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11. Magnissalis EA, Zinelis S, Karachalios T, Hartofilakidis G. Failure analysis of two Ti-alloy hip arthroplasty femoral stems fractured in vivo. J Biomed Mater Res B Appl Biomater. 2003;66(1):299–305. 12. Gilbert JL, Buckley CA, Jacobs JJ, Bertin KC, Zernich MR. Intergranular corrosion-fatigue failure of cobalt-alloy femoral stems. A failure analysis of two implants. J Bone Joint Surg Am. 1994;76(1):110–5. 13. Lee EW, Kim HT. Early fatigue failures of cemented, forged, cobalt chromium femoral stems at the neck shoulder junction: a case report. J Arthroplasty. 2001;16(2):236–8. 14. Heck DA, Partridge CM, Reuben JD, Lanzer WL, Lewis CG, Keating EM. Prosthetic component failures in hip arthroplasty surgery. J Arthroplasty. 1995;10(5):575–80. 15. Burstein AH, Wright TM. Neck fractures of femoral prostheses. A report of two cases. J Bone Joint Surg Am. 1985;67(3):497–9. 16. Rand JA, Chao EY. Femoral implant neck fracture following total hip arthroplasty. A report of three cases. Clin Orthop Relat Res. 1987;221:255–9. 17. Aspenberg P, Kolmert L, Person L, Onnerfalt R. Fracture of hip prostheses due to inadequate welding. Acta Orhop Scand. 1987;58(5):479–82. 18. Jang B, Kanawati A, Brazil D, Bruce W. Laser etching causing fatigue fracture at the neck-shoulder junction of an uncemented femoral stem: A case report. J Orthop. 2013;10(2):95–8. 19. Collier JP, Mayor MB, Jensen RE, Surprenant VA, Surprenant HP, McNamar JL, Belec L. Mechanisms of failure of modular prostheses. Clin Orthop Relat Res. 1992;285:129–39. 20. Botti TP, Gent J, Martell JM, Manning DW. Trunion Fracture of a Fully Porous-Coated Femoral Stem. Case report. J Arthroplasty. 2005;20(7):943–5. 21. Morgan-Hough CV, Tavakkolizadeh A, Purkayastha S. Fatigue failure of the femoral component of a cementless total hip arthroplasty. J Arthoplasty. 2004;19(5):658–60. 22. Unnanuntana A, Chen DX, Unnanuntana A, Wright TM. Trunnion fracture of the Anatomic Medullary Locking A Plus femoral component. J Arthroplasty 2011;26(3):504.e13–16. 23. Skendzel JG, Blaha JD, Urquhart AG. Total hip arthroplasty modular neck failure. J Arthroplasty. 2011; 26(2):338.e1–4. 24. Sotereanos NG, Sauber TJ, Tupis TT. Modular femoral neck fracture after primary total hip arthroplasty. J Arthroplasty. 2013; 28(1):196.e7–9. 25. Ellman MB, Levine BR. Fracture of the modular femoral neck component in total hip arthroplasty. J Arthroplasty. 2013 Jan; 28(1):196.e1-5. 26. Grupp TM, Weik T, Bloemer W, Knaebel HP. Modular titanium alloy neck adapter failures in hip replacement–failure mode analysis and influence of implant material. BMC Musculoskelet Disord. 2010;4(11):3–14. 27. Takahashi K, Sato E. Influence of Surface Treatments on Fatigue Strength of Ti6Al4 V Alloy. Material Transactions. 2010;51:694–8. 28. Welsch G, Boyer R, Collings EW. Materials Properties Handbook: Titanium Alloys. ASM International: Almere; 1993. p. 553. 29. Oshkour AA, Abu Osman NA, Yau YH, Tarlochan F, Abas WA. Design of new generation femoral prostheses using functionally graded materials: a finite element analysis. Proc Inst Mech Eng H. 2013;227(1):3–17.
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ORIGINAL ARTICLE
Neck fracture of femoral stems with a sharp slot at the neck: biomechanical analysis Kensei Yoshimoto1 · Yasuharu Nakashima1 · Akihiro Nakamura2 · Taro Mawatari1 · Mitsugu Todo3 · Daisuke Hara1 · Yukihide Iwamoto1
Received: 25 March 2015 / Accepted: 12 June 2015 / Published online: 25 July 2015 © The Japanese Orthopaedic Association 2015
Abstract Background Fracture of the femoral stem in total hip arthroplasty (THA) is a rare complication. We have encountered 2 cases of neck fractures of the femoral stem occurring 9 and 12 years after THA. Morphological and biomechanical analysis were performed to investigate the mechanism of these fractures. Method A titanium alloy femoral stem having a slot with sharp corners (R = 0.2 mm) at the neck had been implanted in both cases. Fracture surfaces were examined by use of scanning electron microscopy (SEM). Stress concentration was simulated by using a finite element method (FEM) to compare slots with sharp (R = 0.2 mm) and smooth (R = 2 mm) corners. Results Study of the retrieved stems revealed that neck fractures had occurred at the distal end of the slot in both cases. SEM revealed numerous fine fissures extending from the anterolateral edge, striations on the middle of the fracture surface, and dimples on the posteromedial surface, suggesting that the fractures had occurred from the anterolateral aspect toward the posteromedial aspect because of metallic fatigue. FEM analysis showed that mechanical stress was concentrated at the distal and anterolateral
* Yasuharu Nakashima [email protected]‑u.ac.jp 1
Department of Orthopedic Surgery, Graduate School of Medical Sciences, Kyushu University, 3‑1‑1 Maidashi, Higashi‑ku, Fukuoka 812‑8582, Japan
2
Kyocera Medical Corporation, 3‑3‑31 Miyahara, Yodogawa‑ku, Osaka 532‑0003, Osaka, Japan
3
Division of Renewable Energy Dynamics, Research Institute for Applied Mechanics, Kyushu University, Fukuoka, Japan
corners of the slot. Under 3500-N loading force, the stress at the sharp corner was 556 MPa, which was approximately twofold that at the smooth corner and exceeded the fatigue strength of titanium alloy. Conclusion These findings showed that the sharp corner of slot increased stress concentrations at the anterolateral aspect and led to the neck fractures.
Introduction Fracture of stainless steel femoral stems was a major complication in the early history of total hip arthroplasty (THA), with reported prevalence ranging from 0.23 % [1] to as high as 11 % [2]. With the development of femoral stems made of cobalt–chromium–molybdenum alloy or titanium alloys, femoral stem fracture has become a rare complication. Recently, there have been several reports of femoral stem neck fracture as a result of additional manufacturing processes involved while creating the neck. To prevent impingement and improve the range of motion, a stem neck with slots on the anterior and posterior aspects was produced. This resulted in a smaller neck radius, which in turn increased the tensile stress transmitted through the stem neck. In one study, this was found to cause 9 neck fractures [3]. Material machining, for example laser etching, reportedly further reduced the strength of the stem and resulted in several fractures [4]. Modular hip systems have been gaining popularity, because they restore normal hip biomechanics with the ability to adjust offset. However, in several studies these systems have been associated with neck fractures caused by crevice corrosion [5, 6]. Specifically, additional machining of the stem neck resulted in a decrease of its mechanical strength.
13
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K. Yoshimoto et al.
We have encountered 2 cases of neck fractures. These fractured stems had a slot with a sharp corner at the neck and fracture had occurred at the distal corner of the slot. In this study, fracture surfaces were fully examined by use of scanning electron microscopy (SEM), and the effect of the sharp corner of the slot was analyzed by use of a finite element method (FEM).
Materials and methods Case presentation This study was approved by the institutional review board at our institution (IRB number 25–11). Informed consent was obtained from each patient. Case 1
Fig. 1 Radiographs showed that, in both cases, the fractures occurred at the distal end of the neck of the femoral stem. a Case 1. b Case 2
In 2000, a 45-year-old man underwent right THA for traumatic osteoarthritis. His height was 178 cm, his weight 78 kg, and his body mass index (BMI) 24.8 kg/m2. We implanted a cementless 54-mm acetabular shell with a highly cross-linked polyethylene (HXPE) liner, a proximally coated collared cementless stem, and a zirconia 22-mm femoral head (Kyocera, Osaka, Japan). Postoperative radiographs showed that the implant position was neutral. Nine years and 4 months after the operation the patient felt severe pain in the right hip while squatting and could not walk. Radiographic examination revealed a neck fracture of the femoral stem without loosening (Fig. 1a). Revision THA was performed. This case was included in our previous report of this THA system [7]. Case 2 In 1997, a 49-year-old man underwent the right side unipolar hip arthroplasty for avascular necrosis of the femoral head. His height was 170 cm, his weight 70 kg, and his BMI 24.2 kg/m2. We implanted a 52-mm outer head and a proximally coated collared cementless stem (Kyocera). Two years and 8 months after the first operation, proximal migration of the outer head resulted in right THA revision by use of a cementless 52-mm acetabular shell with an HXPE liner and a metal 26-mm femoral head (Kyocera). The femoral stem was not revised. Ten years and 2 months after the THA revision, the patient experienced severe pain in the right hip while closing a door and could not stand. Radiographic examination revealed a neck fracture of the femoral stem without loosening (Fig. 1b). Revision THA was performed.
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Fig. 2 A cross-section of the femoral neck. The size of the slot was 6.65 mm × 6.65 mm and the depth of the slot was 1.1 mm. The slot had a sharp corner angle that was 0.2 mm in radius
Femoral stem design Identical cementless femoral stems were implanted in both cases. These stems were made from Ti–6Al–4V and were collared. The proximal one-third of the stem was roughened by titanium arc spray and coated with hydroxyapatite [7, 8]. At the stem neck, the slot was machined to enable an instrument to grip the neck for stem removal. The slot had a sharp corner angle (R = 0.2 mm). The size of the slot was 6.65 mm × 6.65 mm and its depth was 1.1 mm (Fig. 2).
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Neck fracture of femoral stems with a sharp slot at the neck: biomechanical analysis
Surface analysis The 2 fractured stems were examined visually by use of a digital microscope (VHX-200; Keyence, Osaka, Japan) to characterize the fracture surfaces. Further investigation with SEM (S-3400N; Hitachi, Tokyo, Japan) was performed to assess the fracture mechanism. FEM analysis We hypothesized that the sharp corner of the slot was involved in these fractures. Stress distribution was analyzed with 2 software products, Ansys Workbench Ver.13 (Ansys, Houston, TX, USA) and Solid Edge Ver.20 (Siemens PLM Software, Plano, TX, USA). Two slot designs were evaluated. The first had a sharp (R = 0.2 mm) corner angle equal to that of the slot on the fractured stem. The other had a smooth (R = 2 mm) corner angle which was equal to that on the next-generation stem. We constructed 3-dimensional models based on ISO7206-6 (1992), and the stems were implanted in PMMA bone cement with 10° of varus and 9° of flex position (Fig. 3). ISO7206-6 was followed for the elastic modulus and the Poisson’s ratio of the materials. Details are listed in Table 1. These fractured stems could be attached with −4, 0 and +4 mm balls. As the stress at the neck was higher with longer neck (+4 mm ball), the superior margin of the neck was extended to the position of the center of the +4 mm ball. To reduce computation time, the distal section of the stem was resected, because this section only minimally affected the results of the analysis (Fig. 3). Finite element meshes were generated by using tetrahedral 10-node elements (C3D10) (Fig. 4). The mesh size used at each site (Table 2) was that obtained when the analysis result converged by less than 5 % as the mesh size was gradually reduced. In accordance with ISO7206-6 (1992), both designs were exposed to a downward 2300-N load at the superior margin of the neck, and the maximum principal stress was measured. Because stress to the hip is reportedly four or five times the body weight [9, 10], further FEM analysis was conducted with a more severe loading stress of 3500 N.
Fig. 3 FEM analysis model based on ISO7206-6. Two slot designs were evaluated. The first had a sharp (R = 0.2 mm) corner angle equal to that of the slot on the fractured stem. The other had smooth (R = 2 mm) corner angles to enable the effect of slot shape to be assessed. The stem was implanted in PMMA bone cement with 10° of varus and 9° of flex position. Because we assumed that a +4 mm ball was located at the neck, the superior margin of the neck was extended to the position of the center of the +4 mm ball. The stem was exposed to a downward loading force at the superior margin of the neck
Table 1 The material properties for the 3-dimensional model
Young’s modulus (GPa) Poisson’s ratio
Ti-6Al-4V
PMMA
110.6
2.65
0.326
0.455
Results Examination of the retrieved femoral stems (Fig. 5) revealed that the neck fractures occurred at the distal end of the slot in both cases. SEM images of the fracture surfaces revealed numerous fine fissures extending from the anterolateral edge toward the posteromedial aspect of the neck (Fig. 6a). Striations were observed in the middle of the surface (Fig. 6b). Dimples were observed on the posteromedial surface (Fig. 6c).
FEM analysis revealed high levels of stress at the anterolateral aspect of the femoral neck. Maximum principal stress was concentrated at the anterolateral aspect of the distal portion of the corner (Fig. 7; Table 3). For a 2300-N load, the maximum stress was 365 MPa at the sharp corner and 195 MPa at the smooth corner whereas for a 3500-N load, the maximum stress was 566 MPa at the sharp corner and 296 MPa at the smooth corner.
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Fig. 4 Meshes configured with tetrahedral 10-node elements (C3D10) were generated to analyze stress distribution. The mesh size was that obtained when the analysis result converged by less than 5 % as the mesh size was gradually reduced
Table 2 Mesh size R = 0.2 (mm)
R = 2 (mm)
Stem PMMA bone cement Slot of the neck
1 2 0.2
1 2 0.2
Corner of the slot
0.02
–
Discussion We report 2 cases of neck fractures of the femoral stem. In both cases, fractures had occurred at the distal part of the sharp corner, from the anterolateral to the posteromedial aspect. The presence of numerous striations suggested metal fatigue of the Ti–6Al–4V was the main cause of fracture. FEM analysis showed that loading stress was concentrated at the anterolateral aspect of the distal part of the slot, and that the sharp corner resulted in greater stress concentration than the smooth one. These findings indicated that excessive stress at the sharp corner was the primary cause of the neck fractures. There were several limitations to our study. We evaluated only 2 cases, and FEM analysis was performed for a vertical static load only. In practice, the femoral stem is exposed to stress caused by a variety of dynamic motions, for example walking, stair climbing, squatting, and twisting of the legs. Further investigation should include study of the effect of multiple dynamic loads. Furthermore, in Case 2 the taper of the femoral neck was possibly scratched
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Fig. 5 Examination of the retrieved stems revealed that the fractures had occurred at the distal part of the sharp corner of the slot. a Case 1. b Case 2
during the initial revision THA. During the isolated acetabular revision surgery, forceful retraction of the femoral stem by use of retractors or bone hooks placed around the stem neck may cause damage to the neck, disrupt the passivation layer, and enhance corrosion. In comparison with stainless steel femoral stems, fractures of femoral stems made of titanium alloy or cobalt– chromium–molybdenum are rare [11, 12]. Furthermore, most stem fractures reportedly occurs at the middle third of the implant. Neck fracture of femoral stem, as seen in
Neck fracture of femoral stems with a sharp slot at the neck: biomechanical analysis
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Fig. 6 SEM image of the fractured surface of Case 1. The cross section of the fractured neck was examined. a Anterolateral part of the fractured surface. Fissures extended from the anterolateral edge
toward the center (yellow arrows). b Middle part of the fractured surface. Striations were observed. c Posteromedial part of the fractured surface. Dimples were observed
this study, is rare [13, 14]. Specifically, a review of the literature indicated that neck fractures were because of manufacturing processes involving the neck. With monoblock prostheses used in the past, defects created while welding the neck to the head had the potential to cause metal fatigue and fracture [15–17]. More recently it has been suggested that several machining processes used to create the neck, intended to prevent impingement and improve the range of motion, cause neck fracture. Vatani et al. [3] reported 9 fractures at the stem neck because of abnormal transmission of forces through the narrowest segment of the neck. Laser etched markings on the neck have also resulted in 2 neck fractures [4, 18]. Grivas et al. [4] mentioned that the process of laser etching induced localized changes in the metallic microstructure and creation of a local stress concentrator. Crevice corrosion and fretting corrosion at the head–neck junction, both of which may cause stem neck fracture, have been reported in several papers [12, 19–22]. Femoral neck fracture, crevice corrosion, and fretting corrosion at the neck–stem junction have also been reported for modular hip systems consisting of a head, neck, and stem [5, 6, 23–25]. Grupp et al. [26] estimated a fracture prevalence of 1.4 % for modular stems.
In our cases, numerous fine fissures extending from the anterolateral fracture surface toward the posteromedial surface indicated that the femoral neck fracture initially occurred at the anterolateral aspect of the neck and propagated posteromedially. Striations on the middle of the fracture surface were indicative of metal fatigue whereas dimples on the posteromedial surface were indicative of static fracture. Continuous loading stress might have caused metal fatigue and subsequent fracture. FEM analysis revealed that the sharp corner caused excess stress concentrations at the anterolateral and distal portions of the slot. Sharp notches are well known to cause excess stress concentration in metal materials [27]. The fatigue strength of Ti–6Al–4V is approximately 400 MPa [28]. In this case the maximum stress at the sharp corner was 365 MPa with 2300-N loading force based on ISO7206-6 (1992). However, several studies have shown that the femoral neck is exposed to 4 or 5 times the body weight during walking or running [9, 10]. This would result in a 3500-N loading force on the femoral stem; in this case, the stress at the sharp corner would be 566 MPa. This stress concentration at the sharp corner was excessively high. As the stress at the smooth corner was equivalent to that previously reported under similar experimental conditions
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Fig. 7 Stress distribution calculated by use of FEM analysis. Stress was concentrated at the anterolateral aspect of the distal part of the corner of the slot under both 2300 and 3500-N loading force conditions
Table 3 Comparison of stress distribution Loading force 2300 N Maximum stress Spot exposed maximum stress 3500 N Maximum stress Spot exposed maximum stress
R = 0.2 mm
R = 2 mm
365 MPa Distal corner of the slot
195 MPa Distal corner of the slot
556 MPa
296 MPa
Distal corner of the slot
Distal corner of the slot
[29], the sharp corner possibly led to this high stress concentration. These results indicated that the neck of the fractured stem might be exposed to stress above the fatigue strength of Ti–6Al–4V. Overall, it seems the sharp corner was exposed to continuous loading stress that caused metal fatigue and subsequent fracture. According to the manufacturer, 12,248 stems with slots with sharp corners were implanted between 1995 and 2002 in Japan, and 71 of the stems (0.58 %) fractured at the neck. In our institution, 518 stems were implanted in that period, with 2 fractures (0.38 %). All the fractures occurred
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at the same site, the distal corner of the slot. Therefore, we assumed that the mechanism of the fractures was similar. Other stems with a slot at the neck include the Bi-Metric (Biomet, Warsaw, IN, USA) and VerSys (Zimmer, Warsaw, IN, USA) stems. However, these stems have relatively rounded slot corners compared with our fractured stems, and there have been no reports of neck fracture at the slot. These findings, also, indicate that stress concentration at sharp corner led to neck fracture. Neck fracture may be caused not only by material factors, but also by patient or surgery-related factors. Rand and Chao [16] suggested that risk factors include increased activity, varus positioning of the stem, and increased body weight. Vatani et al. [3] showed that the main independent risk factors for fracture of the femoral stem were younger age and heavier weight. Both of our cases were male and each had a body weight above 70 kg, and Case 1 was 55 years old when he underwent the first THA. These factors might have contributed to the fractures reported here. After the occurrence of neck fracture, manufacturing processes were refined. A slot with rounded corner was incorporated in the next-generation stem (Chamfered Perfix; Kyocera, Osaka, Japan); 12,923 of these new stems have been implanted, with no reports of neck fractures.
Neck fracture of femoral stems with a sharp slot at the neck: biomechanical analysis
Conclusion In conclusion, the sharp corner at the neck slot was the primary cause of neck fractures in our cases. Mechanical stress was concentrated at the anterolateral and distal corners of the slot, and continuous excessive stress caused metal fatigue. Several manufacturing processes used to create femoral stem necks may contribute to neck fracture. Acknowledgement This work was supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (no. 24592268) and in part by Kyocera Co., Ltd. Compliance with ethical standards Conflict of interest The authors declare that they have no conflict of interest.
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