- Email: [email protected]
Strict component positioning is necessary in hip resurfacing
J Orthop Sci (2013) 18:290–297 DOI 10.1007/s00776-012-0351-4
ORIGINAL ARTICLE
Strict component positioning is necessary in hip resurfacing Yoshitomo Kajino • Tamon Kabata • Toru Maeda • Shintaro Iwai • Kazunari Kuroda Kenji Fujita • Hiroyuki Tsuchiya
•
Received: 19 August 2012 / Accepted: 20 December 2012 / Published online: 12 January 2013 Ó The Japanese Orthopaedic Association 2013
Abstract Background Hip resurfacing arthroplasty has some advantages, including improved metal-on-metal articulation, a lower dislocation rate and preserved femoral bone. This procedure is a surgical option for younger and more active patients with osteoarthritis and osteonecrosis of the femoral head. Although there have been some reports about the efficacy of this technique, others report serious complications caused by metal debris. Additionally, femoral neck preservation adversely decreases the head–neck ratio and results in postoperative impingement. Methods We evaluated the range of motion after hip resurfacing with various component orientations and optimal component orientations to avoid postoperative impingement using computer simulations in 10 male patients with osteonecrosis. Results The mean ranges of motion in flexion, extension, abduction, adduction and internal rotation at 90° of flexion were 92.4° ± 13.8°, 25.7° ± 13.8°, 38.0° ± 11.1°, 29.1° ± 10.0° and 20.9° ± 11.5°, respectively. The oscillation angle in flexion and extension motion was 118.1° ± 10.3°. More than 100° of flexion was acquired in 79 of 240 simulations (32.9 %), and more than 20° extension was acquired in 142 simulations (59.2 %). Combined anteversion was significantly correlated with maximal flexion and
Electronic supplementary material The online version of this article (doi:10.1007/s00776-012-0351-4) contains supplementary material, which is available to authorized users. Y. Kajino T. Kabata (&) T. Maeda S. Iwai K. Kuroda K. Fujita H. Tsuchiya Department of Orthopaedic Surgery, Graduate School of Medical Science, Kanazawa University, 13-1 Takaramachi, Kanazawa, Ishikawa 920-8641, Japan e-mail: [email protected]
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extension angles. The component safe zone to fulfill the range of motion criteria varied among patients, and 4 of 10 patients had no safe zone. Conclusions Postoperative impingement occurs relatively frequently in hip resurfacing because of preservation of the femoral neck and component malpositioning. The safe zone of the acetabular component to avoid postoperative impingement is very narrow. Greater care should be taken regarding patient selection, rigorous preoperative planning and accurate component positioning.
Introduction The advantages of improved metal-on-metal bearing articulation, a lower dislocation rate, the natural feeling of a large femoral head and preservation of the femoral bone have recently made metal-on-metal hip resurfacing arthroplasty (HRA) a surgical option in patients with osteoarthritis and osteonecrosis of the femoral head (ONFH), especially in younger and more active patients [1–6]. Moderate midterm results of this technique have been reported. McMinn et al. [6] reported 96 % survivorship at 13 years in all ages, and Amstutz and Le Duff [7] reported that the 5-year survivorship was 95.2 %. On the other hand, complications with metal debris have been reported, and the surgical indication for this procedure needs to be reconsidered. In comparison with conventional total hip arthroplasty (THA), the lower head-neck ratio due to the native wide femoral neck may be associated with a risk of postoperative impingement accompanied by component malposition [3, 8, 9]. Radiography after HRA sometimes shows indentation and osteophytes in the anterior femoral neck (Fig. 1). This finding strongly suggests the occurrence of impingement between the preserved
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Fig. 1 Impingement signs around the femoral neck, such as the formation of the indentation (arrowhead) and osteophytes (arrow), were observed in a postoperative lateral radiograph (left) and at 4-year
follow-up (right) of a 41-year-old man. Postoperative ROM of his right hip joint was 115° in flexion, 15° in extension, 45° in abduction, 25° in adduction, 45° in external rotation and 25° in internal rotation
femoral neck and the rim of the acetabular component [10]. There have been many clinical studies and simulations of THA regarding the safe zone of the acetabular component’s orientation, and mathematical formulas have been reported to avoid both postoperative impingement and dislocation in daily activities [11, 12]. However, the optimal component position in HRA is still unclear. We hypothesized that in HRA a lower head–neck ratio frequently yields postoperative impingement, decreases the range of motion (ROM) and makes the safe zone of the acetabular component very narrow.
Table 1 Patients’ demographic data Patient characteristics (n = 10) Age
50.8 ± 6.0 (45–62)
Side (right/left)
4/6
Height (cm)
170.8 ± 7.3 (161–182)
Weight (kg)
68.5 ± 8.4 (55–87)
Body mass index
23.5 ± 2.9 (18.5–28.2)
All values are mean ± standard deviation (range)
Coordinate systems of the pelvis and femur Materials and methods Patients and image data processing To investigate our hypothesis, we performed computer simulation analysis in 10 male patients treated with ONFH at our institution. The patients’ demographics are shown in Table 1 and Supplementary Table 1. There were no previous operations or anatomical deformities such as dysplasia, and preoperative radiographs showed no sign of femoroacetabular impingement [13]. Informed consent was obtained from all patients, and the research protocol was approved by the hospital investigational review board. Computed tomography (CT) scans from the pelvis to the femoral condyle were obtained preoperatively. These data were transferred to the three-dimensional (3D) software Mimics (version 13.1; Materialise, Leuven, Belgium), by which patient-specific 3D hip models and segmentation were achieved. The computer-assisted design (CAD) of Birmingham hip resurfacing (BHR; Smith & Nephew, Memphis, TN, USA) was used as a component.
In the simulation model, the authors defined the following coordinate systems of the pelvis and femur to measure the range of hip motion (Fig. 2). The X-axis of the pelvis (Xp) was perpendicular to the anterior pelvic plane (APP) defined by both the bilateral anterior superior iliac spine (ASIS) and the pubic tubercle, and its positive direction is anterior. The Z-axis (Zp) passed through both ASISs with its positive direction pointing right. The Y-axis (Yp) was mutually perpendicular to both the Xp and Zp axes, pointing upward. For the femur, the X-axis (Xf) was perpendicular to the retrocondylar plane defined by both the most posterior point of the proximal femur and the femoral condyles, pointing to the anterior. The Y-axis (Yf) was parallel to the retrocondylar plane, passing through the center of the femoral head and knee center (i.e., the mechanical axis). The Z-axis (Zf) was mutually perpendicular to both the Xf and Yf axes with the positive direction pointing right. The neutral hip position was defined in which all axes in both pelvic and femoral coordinate systems were parallel, and the ROM of the hip joint was determined as the relative angle between the two coordinate systems.
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Fig. 2 Definition of the pelvic and femoral coordinate systems. The neutral hip position was defined where all axes in both the pelvic (Xp, Yp, Zp) and femoral coordinate systems (Xf, Yf, Zf) were parallel.
The ROM of the hip joint was determined as the relative angle between the two coordinate systems
Component implantation
ROM analysis and evaluation of the safe zone
The size of the acetabular component was determined according to the diameter and depth of the original acetabulum using 3D templating software (CT-based Hip, version 1.0; Stryker Navigation, Freiburg, Germany). Virtual implantation of hip resurfacing and ROM analysis were performed using Solid Edge CAD software (version 20; Siemens PLM Software, Plano, TX, USA) by which bony and prosthetic impingement was automatically detected when each element was touching (Fig. 3). The acetabular component was implanted at the site of the original acetabular contact with the lateral wall of the teardrop [5]. The acetabular component angle was described using the radiographic definition according to the report by Murray [14]. In the analysis of the combined anteversion, the anatomical definition was used to make the measuring plane uniform. On the femoral side, the component size was determined by the diameter of the native femoral head without superior neck notching. The femoral component was implanted so that the center of the native femoral head was reproduced and the coronal stem shaft angle of the femoral component was 140° [15, 16]. In the lateral view, the stem axis was parallel to the native femoral neck.
First, the maximal ROM of the hip with various orientations of the acetabular component was evaluated by measuring five arcs: flexion/extension in neutral abduction and rotation, abduction/adduction in neutral flexion and rotation, and internal rotation at 90° of flexion including both bony and prosthetic impingement. Inclination of the acetabular component was changed from 20° to 40° (10° each), and anteversion was changed from 0° to 35° (5° each). Twenty-four simulations were performed in each direction. Resection of the bone and osteophytes around the acetabular component was not considered in this study. Additionally, correlations were performed between the combined acetabular and femoral anteversion and maximal flexion and extension angles in 40° of inclination. Finally, we defined the minimum criteria of hip motion required in daily living according to previous reports [17–19]: i.e., 100° in flexion, 20° in extension, 30° in abduction, 20° in adduction, 30° in external rotation and 60° in internal rotation. The orientations of the acetabular component to fulfill these criteria without bone-to-implant impingement were investigated. The lower and upper borders for inclination were intentionally set to 30° and 50°, respectively, according to a previous report [11].
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Fig. 3 Virtual implantation of the component and the evaluation of the impingement (case 6) by the Solid Edge CAD software (version 20; Siemens PLM Software, Plano, TX, USA). Frontal view (left) and lateral view (right). Inclination and anteversion of the acetabular component were 40° and 15°, respectively. Anterior impingement occurred in 107° of flexion
Statistical analysis Descriptive data are shown as mean ± standard deviation (SD). Statistical analyses were performed using SPSS version 19.0 (SPSS Inc., Chicago, IL, USA), and p \ 0.05 was taken to indicate statistical significance.
Results ROM analysis The results of ROM analysis are shown in Fig. 4. Maximal flexion increased as anteversion of the acetabular component increased. There was a complementary decrease in extension. Maximal extension motion increased slightly in combination with the increase in inclination. The differences between cases in abduction were small, and abduction improved with increasing inclination. Abduction showed the greatest impairment in some cases in combination with increasing anteversion. In most cases in adduction, the limiting factor of ROM was bony impingement between the lesser trochanter and ischium (filled markers). Bony impingement between the femoral neck and the rim of the native acetabulum was observed in other motions. Internal rotation at 90° of flexion increased as anteversion of the acetabular component increased in most cases. In some simulations, there was contact between the femoral neck and the acetabular component, even in the neutral position of the hip, which made it impossible to implant the component (no marker). The mean ranges of motion in flexion, extension, abduction, adduction and internal rotation at 90° of flexion were 92.4° ± 13.8°, 25.7° ± 13.8°, 38.0° ± 11.1°, 29.1° ± 10.0° and 20.9° ± 11.5°, respectively. The oscillation angle in flexion and extension was 118.1° ± 10.3°. Flexion of more than 100° was achieved in only 79 simulations (32.9 %), and more than 20° in extension was achieved in 142 simulations (59.2 %). Combined acetabular and femoral anteversion was significantly correlated with
both maximum flexion and extension angles (R = 0.861, p \ 0.01 and R = -0.789, p \ 0.01, respectively) (Fig. 5). Safe zone of the acetabular component The orientations of the acetabular component that fulfilled the criteria of ROM in daily activities are shown in Fig. 6. The area of the safe zone varied markedly between cases. In 4 of 10 cases (cases 5, 7, 8, 10), there was no optimal orientation (safe zone) of the acetabular component that achieved our criteria. There was no significant difference in patient morphology between groups with or without a safe zone (Supplementary Table 2). However, the sample size was small, and further analysis is necessary.
Discussion Hip resurfacing arthroplasty (HRA) is an alternative to conventional THA in young active patients, and its midterm results have been reported to be moderate [4–7, 15]. Although the advantages of HRA compared with THA include improved metal-on-metal bearing articulation, a lower dislocation rate, joint stability and the natural feeling of an anatomically large head, one of the most attractive points is the preservation of femoral bone stock, which makes future revision easier [1, 2, 4, 5, 20]. However, unlike the narrow, tapered and circular neck in THA, the native femoral neck preserved in HRA is wide and oval [5, 21]. It has been reported that the head-neck ratio after HRA ranges from 1.3 to 1.8 compared with [1:2 in THA [20, 21], and the head-neck ratio around the native neck is not uniform [8]. A smaller head-neck ratio leads to the possibility of impingement between the preserved femoral neck and acetabular component, and a decrease in postoperative ROM [2, 3]. Some authors have reported improved ROM after HRA, but this is controversial [2–4, 18, 19, 22]. Incavo et al. [20] analyzed the ROM after HRA using intact cadavers and reported that maximal motion after hip
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Fig. 4 ROM analyzed in various component orientations. a 20°, b 30° and c 40° of inclination. Filled markers indicate bone-to-bone impingement. Left to right angle in flexion, extension, abduction, adduction and internal rotation at 90° of flexion
resurfacing was significantly decreased compared with the 32-mm head THA because of impingement in flexion and internal rotation at 90° of flexion. However, Le Duff et al. [22] compared the postoperative ROM bilaterally in patients who had undergone both hip resurfacing and conventional THA and reported no significant difference between the two procedures. The optimal component orientation after HRA to avoid postoperative impingement is not known. In conventional THA, simulation models and mathematical formulas have
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been developed to determine the optimal component position to prevent postoperative impingement and dislocation [11]. However, it is impossible to apply the same concepts to HRA because of the aforementioned shape of the native femoral neck. Therefore, we performed a computer simulation study using CT data from patients to investigate the optimal component orientation in HRA. We selected ONFH patients because the femur in these cases is characterized by a normal head-neck ratio with no mismatch in size between the acetabular and femoral components compared
Safe zone of hip resurfacing
Fig. 5 The correlation between combined acetabular and femoral anteversion and maximum flexion angle (a) and extension angle (b) in acetabular inclination of 40°
to patients with osteoarthritis [1]. Our results with the ROM simulation showed marked variation among patients, with frequent postoperative impingement after HRA. These results suggest that many factors may affect postoperative ROM after HRA. Impingement after HRA can affect not only postoperative ROM, but also results in loosening of the component, pain, release of metal debris and femoral neck fracture [2, 13, 21, 23–25]. Metal ion release and pseudotumor generation are the most serious complications after HRA. There have been several clinical reports regarding the relationships between component orientation and both pseudotumor generation and blood metal ion concentrations. Grammatopoulos et al. [24] recommended implantation of the BHR acetabular component at an inclination of 45° ± 10° and anteversion of 20° ± 10° to minimize the risk of pseudotumor formation. Langton et al. [23] also found a significant correlation between high metal ion concentrations and a steeper and markedly anteverted acetabular component, and they suggested that the BHR acetabular component should be implanted with a target inclination of 45° ± 5° and anteversion of 10°–20°. These findings taken together with the results of the present study indicate that the requirements for optimal component
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orientation after HRA are extremely strict and that a more anteverted acetabular component allows greater ROM in flexion. However, this leads to anterior edge loading, which can cause considerable release of metal ions [23, 24]. In a clinical situation, avoiding tragic complications related to wear properties is more important than a better ROM. Our final simulation regarding the optimal component orientation after HRA was similar to that in THA [11, 18]. According to the previous study [17–19], the authors defined the original ROM criteria of the hip joint, except lumbar motion, for Japanese daily activities such as seiza and squatting. The intended ROM criteria of this current study were less strict than in previous studies of THA. Nevertheless, only 6 of 10 patients achieved these criteria. These observations indicated that extensive impingement occurred in HRA when the acetabular component was malpositioned. In addition, the safe zone of the acetabular component in HRA varied among patients, and it was difficult to solve the simple mathematical formulas recommended in THA. In the current study, there was no significant difference between groups with or without safe zones in the patients’ morphological parameters, such as femoral neck anteversion, anterior head-neck offset, CCD angle or head diameter. Further analysis is necessary. If the safe zone cannot be achieved because of the shape of the proximal femur, the surgeon should consider the HRA procedure to be contraindicated. There were several limitations to this study. First, the relatively small cohort size made it difficult to apply our results to all patients. Second, resection of the osteophytes around the acetabular component was not considered, and this could yield an underestimate of the range of motion. Also, the clinical impact of bony impingement after HRA remains unclear. Third, the femoral component was implanted to reproduce the center of the native femoral head. The center of the femoral head was sometimes displaced from the axis of the femoral neck and often translated posteriorly [5]. Therefore, this may have caused a decrease in anterior head-neck offset of the femoral head. It may be possible to improve motion by adjusting the head-neck offset intraoperatively. Vendittoli et al. [8] reported that translation of the femoral component improved the ROM in the direction of the translation but reduced the arc of motion in the opposite direction. They suggested that translating the femoral component was only desirable in specific cases. We used the APP coordinate system and retrocondylar plane. However, there are several coordinate systems for investigating ROM of the hip joint [18]. In addition, it is unclear whether the true physiological neutral position of the hip joint varies among patients. Mobility of the lumbar spine and pelvic functional position should also be considered clinically [19]. Finally, other factors could influence the ROM
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Fig. 6 Safe zone of the acetabular component after hip resurfacing. The meshed area indicates the safe zone that fulfilled the criteria of ROM in daily activities. No safe zone was obtained in cases 5, 7, 8 and 10
clinically after HRA, including muscle tension, implant size and design [2, 3, 8, 20, 21].
Conclusions Postoperative impingement seems to occur relatively frequently in cases of HRA in which the native femoral neck
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is preserved and the safe zone of the acetabular component is relatively narrow. Although this was a computer simulation study, further investigations such as motion analysis are necessary. To avoid postoperative complications, greater care should be taken in HRA than in conventional THA with regard to rigorous preoperative planning, patient selection and accurate component positioning using surgical tools such as navigation systems.
Safe zone of hip resurfacing Conflict of interest All authors have no conflict of interest in relation to this study.
References 1. Vendittoli PA, Lavigne M, Girard J, Roy AG. A randomised study comparing resection of acetabular bone at resurfacing and total hip replacement. J Bone Joint Surg Br. 2006;88:997–1002. 2. Bengs BC, Sangiorgio SN, Ebramzadeh E. Less range of motion with resurfacing arthroplasty than with total hip arthroplasty: in vitro examination of 8 designs. Acta Orthop. 2008;79:755–62. 3. Kluess D, Zietz C, Lindner T, Mittelmeier W, Schmitz KP, Bader R. Limited range of motion of hip resurfacing arthroplasty due to unfavorable ratio of prosthetic head size and femoral neck diameter. Acta Orthop. 2008;79:748–54. 4. Quesada MJ, Marker DR, Mont MA. Metal-on-metal hip resurfacing. J Arthroplasty. 2008;23(Suppl):69–73. 5. Vail TP. Hip resurfacing. J Am Acad Orthop Surg. 2011;19(4): 236–44. 6. McMinn DJ, Daniel J, Ziaee H, Pradhan C. Indications and results of hip resurfacing. Int Orthop. 2011;35:231–7. 7. Amstutz HC, Le Duff MJ. Eleven years of experience with metalon-metal hybrid hip resurfacing. J Arthroplasty. 2008;23(Suppl 1): 36–43. 8. Vendittoli PA, Ganapathi M, Nun˜o N, Plamondon D, Lavigne M. Factors affecting hip range of motion in surface replacement arthroplasty. Clin Biomech (Bristol, Avon). 2007;22:1004–12. 9. Malviya A, Lingard EA, Malik A, Bowman R, Holland JP. Hip flexion after Birmingham hip resurfacing. J Arthroplasty. 2010; 25:387–91. 10. Gruen TA, Le Duff MJ, Wisk LE, Amstutz HC. Prevalence and clinical relevance of radiographic signs of impingement in metalon-metal hybrid hip resurfacing. J Bone Joint Surg Am. 2011;93: 1519–26. 11. Widmer KH, Zurfluh B. Compliant positioning of total hip components for optimal range of motion. J Orthop Res. 2004;22: 815–21. 12. Lewinnek GE, Lewis JL, Tarr R, Compere CL, Zimmerman JR. Dislocations after total hip replacement arthroplasties. J Bone Joint Surg Am. 1978;60:217–20.
297 13. Tannast M, Siebenrock KA, Anderson SE. Femoroacetabular impingement. AJR Am J Roentgenol. 2007;188:1540–52. 14. Murray DW. The definition and measurement of acetabular orientation. J Bone Joint Surg Br. 1993;75:228–32. 15. Davis ET, Olsen M, Zdero R, Waddell JP, Schemitsch EH. Femoral neck fracture following hip resurfacing. J Bone Joint Surg Br. 2008;90:1522–7. 16. Sakagoshi D, Kabata T, Umemoto Y, Sakamoto J, Tomita K. A mechanical analysis of femoral resurfacing implantation for osteonecrosis of the femoral head. J Arthroplasty. 2010;25:1282–9. 17. Yamamura M, Miki H, Nakamura N, Murai M, Yoshikawa H, Sugano N. Open-configuration MRI study of femoro-acetabular impingement. J Orthop Res. 2007;25:1582–8. 18. Miki H, Yamanashi W, Nishii T, Sato Y, Yoshikawa H, Sugano N. Anatomic hip range of motion after implantation during total hip arthroplasty as measured by a navigation system. J Arthroplasty. 2007;22:946–52. 19. Hagio K, Sugano N, Nishii T, Miki H, Otake Y, Hattori A, Suzuki N, Yonenobu K, Yoshikawa H, Ochi T. A novel system of fourdimensional motion analysis after total hip arthroplasty. J Orthop Res. 2004;22:665–70. 20. Incavo SJ, Thompson MT, Gold JE, Patel RV, Icenogle KD, Noble PC. Which procedure better restores intact hip range of motion. J Arthroplasty. 2011;26:391–7. 21. Lavigne M, Rama KR, Roy A, Vendittoli PA. Painful impingement of the hip joint after total hip resurfacing. J Arthroplasty. 2008;23:1074–9. 22. Le Duff MJ, Wisk LE, Amstutz HC. Range of motion after stemmed total hip arthroplasty and hip resurfacing. Bull NYU Hosp Jt Dis. 2009;67:177–81. 23. Langton DJ, Sprowson AP, Joyce TJ, Reed M, Carluke I, Partington P, Nargol AV. Blood metal ion concentrations after hip resurfacing arthroplasty. J Bone Joint Surg Br. 2009;91:1287–95. 24. Grammatopoulos G, Pandit H, Glyn-Jones S, McLardy-Smith P, Gundle R, Whitwell D, Gill HS, Murray DW. Optimal acetabular orientation for hip resurfacing. J Bone Joint Surg Br. 2010;92: 1072–8. 25. Beaule´ PE, Harvey N, Zaragoza E, Le Duff MJ, Dorey FJ. The femoral head/neck offset and hip resurfacing. J Bone Joint Surg Br. 2007;89:9–15.
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ORIGINAL ARTICLE
Strict component positioning is necessary in hip resurfacing Yoshitomo Kajino • Tamon Kabata • Toru Maeda • Shintaro Iwai • Kazunari Kuroda Kenji Fujita • Hiroyuki Tsuchiya
•
Received: 19 August 2012 / Accepted: 20 December 2012 / Published online: 12 January 2013 Ó The Japanese Orthopaedic Association 2013
Abstract Background Hip resurfacing arthroplasty has some advantages, including improved metal-on-metal articulation, a lower dislocation rate and preserved femoral bone. This procedure is a surgical option for younger and more active patients with osteoarthritis and osteonecrosis of the femoral head. Although there have been some reports about the efficacy of this technique, others report serious complications caused by metal debris. Additionally, femoral neck preservation adversely decreases the head–neck ratio and results in postoperative impingement. Methods We evaluated the range of motion after hip resurfacing with various component orientations and optimal component orientations to avoid postoperative impingement using computer simulations in 10 male patients with osteonecrosis. Results The mean ranges of motion in flexion, extension, abduction, adduction and internal rotation at 90° of flexion were 92.4° ± 13.8°, 25.7° ± 13.8°, 38.0° ± 11.1°, 29.1° ± 10.0° and 20.9° ± 11.5°, respectively. The oscillation angle in flexion and extension motion was 118.1° ± 10.3°. More than 100° of flexion was acquired in 79 of 240 simulations (32.9 %), and more than 20° extension was acquired in 142 simulations (59.2 %). Combined anteversion was significantly correlated with maximal flexion and
Electronic supplementary material The online version of this article (doi:10.1007/s00776-012-0351-4) contains supplementary material, which is available to authorized users. Y. Kajino T. Kabata (&) T. Maeda S. Iwai K. Kuroda K. Fujita H. Tsuchiya Department of Orthopaedic Surgery, Graduate School of Medical Science, Kanazawa University, 13-1 Takaramachi, Kanazawa, Ishikawa 920-8641, Japan e-mail: [email protected]
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extension angles. The component safe zone to fulfill the range of motion criteria varied among patients, and 4 of 10 patients had no safe zone. Conclusions Postoperative impingement occurs relatively frequently in hip resurfacing because of preservation of the femoral neck and component malpositioning. The safe zone of the acetabular component to avoid postoperative impingement is very narrow. Greater care should be taken regarding patient selection, rigorous preoperative planning and accurate component positioning.
Introduction The advantages of improved metal-on-metal bearing articulation, a lower dislocation rate, the natural feeling of a large femoral head and preservation of the femoral bone have recently made metal-on-metal hip resurfacing arthroplasty (HRA) a surgical option in patients with osteoarthritis and osteonecrosis of the femoral head (ONFH), especially in younger and more active patients [1–6]. Moderate midterm results of this technique have been reported. McMinn et al. [6] reported 96 % survivorship at 13 years in all ages, and Amstutz and Le Duff [7] reported that the 5-year survivorship was 95.2 %. On the other hand, complications with metal debris have been reported, and the surgical indication for this procedure needs to be reconsidered. In comparison with conventional total hip arthroplasty (THA), the lower head-neck ratio due to the native wide femoral neck may be associated with a risk of postoperative impingement accompanied by component malposition [3, 8, 9]. Radiography after HRA sometimes shows indentation and osteophytes in the anterior femoral neck (Fig. 1). This finding strongly suggests the occurrence of impingement between the preserved
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Fig. 1 Impingement signs around the femoral neck, such as the formation of the indentation (arrowhead) and osteophytes (arrow), were observed in a postoperative lateral radiograph (left) and at 4-year
follow-up (right) of a 41-year-old man. Postoperative ROM of his right hip joint was 115° in flexion, 15° in extension, 45° in abduction, 25° in adduction, 45° in external rotation and 25° in internal rotation
femoral neck and the rim of the acetabular component [10]. There have been many clinical studies and simulations of THA regarding the safe zone of the acetabular component’s orientation, and mathematical formulas have been reported to avoid both postoperative impingement and dislocation in daily activities [11, 12]. However, the optimal component position in HRA is still unclear. We hypothesized that in HRA a lower head–neck ratio frequently yields postoperative impingement, decreases the range of motion (ROM) and makes the safe zone of the acetabular component very narrow.
Table 1 Patients’ demographic data Patient characteristics (n = 10) Age
50.8 ± 6.0 (45–62)
Side (right/left)
4/6
Height (cm)
170.8 ± 7.3 (161–182)
Weight (kg)
68.5 ± 8.4 (55–87)
Body mass index
23.5 ± 2.9 (18.5–28.2)
All values are mean ± standard deviation (range)
Coordinate systems of the pelvis and femur Materials and methods Patients and image data processing To investigate our hypothesis, we performed computer simulation analysis in 10 male patients treated with ONFH at our institution. The patients’ demographics are shown in Table 1 and Supplementary Table 1. There were no previous operations or anatomical deformities such as dysplasia, and preoperative radiographs showed no sign of femoroacetabular impingement [13]. Informed consent was obtained from all patients, and the research protocol was approved by the hospital investigational review board. Computed tomography (CT) scans from the pelvis to the femoral condyle were obtained preoperatively. These data were transferred to the three-dimensional (3D) software Mimics (version 13.1; Materialise, Leuven, Belgium), by which patient-specific 3D hip models and segmentation were achieved. The computer-assisted design (CAD) of Birmingham hip resurfacing (BHR; Smith & Nephew, Memphis, TN, USA) was used as a component.
In the simulation model, the authors defined the following coordinate systems of the pelvis and femur to measure the range of hip motion (Fig. 2). The X-axis of the pelvis (Xp) was perpendicular to the anterior pelvic plane (APP) defined by both the bilateral anterior superior iliac spine (ASIS) and the pubic tubercle, and its positive direction is anterior. The Z-axis (Zp) passed through both ASISs with its positive direction pointing right. The Y-axis (Yp) was mutually perpendicular to both the Xp and Zp axes, pointing upward. For the femur, the X-axis (Xf) was perpendicular to the retrocondylar plane defined by both the most posterior point of the proximal femur and the femoral condyles, pointing to the anterior. The Y-axis (Yf) was parallel to the retrocondylar plane, passing through the center of the femoral head and knee center (i.e., the mechanical axis). The Z-axis (Zf) was mutually perpendicular to both the Xf and Yf axes with the positive direction pointing right. The neutral hip position was defined in which all axes in both pelvic and femoral coordinate systems were parallel, and the ROM of the hip joint was determined as the relative angle between the two coordinate systems.
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Fig. 2 Definition of the pelvic and femoral coordinate systems. The neutral hip position was defined where all axes in both the pelvic (Xp, Yp, Zp) and femoral coordinate systems (Xf, Yf, Zf) were parallel.
The ROM of the hip joint was determined as the relative angle between the two coordinate systems
Component implantation
ROM analysis and evaluation of the safe zone
The size of the acetabular component was determined according to the diameter and depth of the original acetabulum using 3D templating software (CT-based Hip, version 1.0; Stryker Navigation, Freiburg, Germany). Virtual implantation of hip resurfacing and ROM analysis were performed using Solid Edge CAD software (version 20; Siemens PLM Software, Plano, TX, USA) by which bony and prosthetic impingement was automatically detected when each element was touching (Fig. 3). The acetabular component was implanted at the site of the original acetabular contact with the lateral wall of the teardrop [5]. The acetabular component angle was described using the radiographic definition according to the report by Murray [14]. In the analysis of the combined anteversion, the anatomical definition was used to make the measuring plane uniform. On the femoral side, the component size was determined by the diameter of the native femoral head without superior neck notching. The femoral component was implanted so that the center of the native femoral head was reproduced and the coronal stem shaft angle of the femoral component was 140° [15, 16]. In the lateral view, the stem axis was parallel to the native femoral neck.
First, the maximal ROM of the hip with various orientations of the acetabular component was evaluated by measuring five arcs: flexion/extension in neutral abduction and rotation, abduction/adduction in neutral flexion and rotation, and internal rotation at 90° of flexion including both bony and prosthetic impingement. Inclination of the acetabular component was changed from 20° to 40° (10° each), and anteversion was changed from 0° to 35° (5° each). Twenty-four simulations were performed in each direction. Resection of the bone and osteophytes around the acetabular component was not considered in this study. Additionally, correlations were performed between the combined acetabular and femoral anteversion and maximal flexion and extension angles in 40° of inclination. Finally, we defined the minimum criteria of hip motion required in daily living according to previous reports [17–19]: i.e., 100° in flexion, 20° in extension, 30° in abduction, 20° in adduction, 30° in external rotation and 60° in internal rotation. The orientations of the acetabular component to fulfill these criteria without bone-to-implant impingement were investigated. The lower and upper borders for inclination were intentionally set to 30° and 50°, respectively, according to a previous report [11].
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Fig. 3 Virtual implantation of the component and the evaluation of the impingement (case 6) by the Solid Edge CAD software (version 20; Siemens PLM Software, Plano, TX, USA). Frontal view (left) and lateral view (right). Inclination and anteversion of the acetabular component were 40° and 15°, respectively. Anterior impingement occurred in 107° of flexion
Statistical analysis Descriptive data are shown as mean ± standard deviation (SD). Statistical analyses were performed using SPSS version 19.0 (SPSS Inc., Chicago, IL, USA), and p \ 0.05 was taken to indicate statistical significance.
Results ROM analysis The results of ROM analysis are shown in Fig. 4. Maximal flexion increased as anteversion of the acetabular component increased. There was a complementary decrease in extension. Maximal extension motion increased slightly in combination with the increase in inclination. The differences between cases in abduction were small, and abduction improved with increasing inclination. Abduction showed the greatest impairment in some cases in combination with increasing anteversion. In most cases in adduction, the limiting factor of ROM was bony impingement between the lesser trochanter and ischium (filled markers). Bony impingement between the femoral neck and the rim of the native acetabulum was observed in other motions. Internal rotation at 90° of flexion increased as anteversion of the acetabular component increased in most cases. In some simulations, there was contact between the femoral neck and the acetabular component, even in the neutral position of the hip, which made it impossible to implant the component (no marker). The mean ranges of motion in flexion, extension, abduction, adduction and internal rotation at 90° of flexion were 92.4° ± 13.8°, 25.7° ± 13.8°, 38.0° ± 11.1°, 29.1° ± 10.0° and 20.9° ± 11.5°, respectively. The oscillation angle in flexion and extension was 118.1° ± 10.3°. Flexion of more than 100° was achieved in only 79 simulations (32.9 %), and more than 20° in extension was achieved in 142 simulations (59.2 %). Combined acetabular and femoral anteversion was significantly correlated with
both maximum flexion and extension angles (R = 0.861, p \ 0.01 and R = -0.789, p \ 0.01, respectively) (Fig. 5). Safe zone of the acetabular component The orientations of the acetabular component that fulfilled the criteria of ROM in daily activities are shown in Fig. 6. The area of the safe zone varied markedly between cases. In 4 of 10 cases (cases 5, 7, 8, 10), there was no optimal orientation (safe zone) of the acetabular component that achieved our criteria. There was no significant difference in patient morphology between groups with or without a safe zone (Supplementary Table 2). However, the sample size was small, and further analysis is necessary.
Discussion Hip resurfacing arthroplasty (HRA) is an alternative to conventional THA in young active patients, and its midterm results have been reported to be moderate [4–7, 15]. Although the advantages of HRA compared with THA include improved metal-on-metal bearing articulation, a lower dislocation rate, joint stability and the natural feeling of an anatomically large head, one of the most attractive points is the preservation of femoral bone stock, which makes future revision easier [1, 2, 4, 5, 20]. However, unlike the narrow, tapered and circular neck in THA, the native femoral neck preserved in HRA is wide and oval [5, 21]. It has been reported that the head-neck ratio after HRA ranges from 1.3 to 1.8 compared with [1:2 in THA [20, 21], and the head-neck ratio around the native neck is not uniform [8]. A smaller head-neck ratio leads to the possibility of impingement between the preserved femoral neck and acetabular component, and a decrease in postoperative ROM [2, 3]. Some authors have reported improved ROM after HRA, but this is controversial [2–4, 18, 19, 22]. Incavo et al. [20] analyzed the ROM after HRA using intact cadavers and reported that maximal motion after hip
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Fig. 4 ROM analyzed in various component orientations. a 20°, b 30° and c 40° of inclination. Filled markers indicate bone-to-bone impingement. Left to right angle in flexion, extension, abduction, adduction and internal rotation at 90° of flexion
resurfacing was significantly decreased compared with the 32-mm head THA because of impingement in flexion and internal rotation at 90° of flexion. However, Le Duff et al. [22] compared the postoperative ROM bilaterally in patients who had undergone both hip resurfacing and conventional THA and reported no significant difference between the two procedures. The optimal component orientation after HRA to avoid postoperative impingement is not known. In conventional THA, simulation models and mathematical formulas have
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been developed to determine the optimal component position to prevent postoperative impingement and dislocation [11]. However, it is impossible to apply the same concepts to HRA because of the aforementioned shape of the native femoral neck. Therefore, we performed a computer simulation study using CT data from patients to investigate the optimal component orientation in HRA. We selected ONFH patients because the femur in these cases is characterized by a normal head-neck ratio with no mismatch in size between the acetabular and femoral components compared
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Fig. 5 The correlation between combined acetabular and femoral anteversion and maximum flexion angle (a) and extension angle (b) in acetabular inclination of 40°
to patients with osteoarthritis [1]. Our results with the ROM simulation showed marked variation among patients, with frequent postoperative impingement after HRA. These results suggest that many factors may affect postoperative ROM after HRA. Impingement after HRA can affect not only postoperative ROM, but also results in loosening of the component, pain, release of metal debris and femoral neck fracture [2, 13, 21, 23–25]. Metal ion release and pseudotumor generation are the most serious complications after HRA. There have been several clinical reports regarding the relationships between component orientation and both pseudotumor generation and blood metal ion concentrations. Grammatopoulos et al. [24] recommended implantation of the BHR acetabular component at an inclination of 45° ± 10° and anteversion of 20° ± 10° to minimize the risk of pseudotumor formation. Langton et al. [23] also found a significant correlation between high metal ion concentrations and a steeper and markedly anteverted acetabular component, and they suggested that the BHR acetabular component should be implanted with a target inclination of 45° ± 5° and anteversion of 10°–20°. These findings taken together with the results of the present study indicate that the requirements for optimal component
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orientation after HRA are extremely strict and that a more anteverted acetabular component allows greater ROM in flexion. However, this leads to anterior edge loading, which can cause considerable release of metal ions [23, 24]. In a clinical situation, avoiding tragic complications related to wear properties is more important than a better ROM. Our final simulation regarding the optimal component orientation after HRA was similar to that in THA [11, 18]. According to the previous study [17–19], the authors defined the original ROM criteria of the hip joint, except lumbar motion, for Japanese daily activities such as seiza and squatting. The intended ROM criteria of this current study were less strict than in previous studies of THA. Nevertheless, only 6 of 10 patients achieved these criteria. These observations indicated that extensive impingement occurred in HRA when the acetabular component was malpositioned. In addition, the safe zone of the acetabular component in HRA varied among patients, and it was difficult to solve the simple mathematical formulas recommended in THA. In the current study, there was no significant difference between groups with or without safe zones in the patients’ morphological parameters, such as femoral neck anteversion, anterior head-neck offset, CCD angle or head diameter. Further analysis is necessary. If the safe zone cannot be achieved because of the shape of the proximal femur, the surgeon should consider the HRA procedure to be contraindicated. There were several limitations to this study. First, the relatively small cohort size made it difficult to apply our results to all patients. Second, resection of the osteophytes around the acetabular component was not considered, and this could yield an underestimate of the range of motion. Also, the clinical impact of bony impingement after HRA remains unclear. Third, the femoral component was implanted to reproduce the center of the native femoral head. The center of the femoral head was sometimes displaced from the axis of the femoral neck and often translated posteriorly [5]. Therefore, this may have caused a decrease in anterior head-neck offset of the femoral head. It may be possible to improve motion by adjusting the head-neck offset intraoperatively. Vendittoli et al. [8] reported that translation of the femoral component improved the ROM in the direction of the translation but reduced the arc of motion in the opposite direction. They suggested that translating the femoral component was only desirable in specific cases. We used the APP coordinate system and retrocondylar plane. However, there are several coordinate systems for investigating ROM of the hip joint [18]. In addition, it is unclear whether the true physiological neutral position of the hip joint varies among patients. Mobility of the lumbar spine and pelvic functional position should also be considered clinically [19]. Finally, other factors could influence the ROM
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Fig. 6 Safe zone of the acetabular component after hip resurfacing. The meshed area indicates the safe zone that fulfilled the criteria of ROM in daily activities. No safe zone was obtained in cases 5, 7, 8 and 10
clinically after HRA, including muscle tension, implant size and design [2, 3, 8, 20, 21].
Conclusions Postoperative impingement seems to occur relatively frequently in cases of HRA in which the native femoral neck
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is preserved and the safe zone of the acetabular component is relatively narrow. Although this was a computer simulation study, further investigations such as motion analysis are necessary. To avoid postoperative complications, greater care should be taken in HRA than in conventional THA with regard to rigorous preoperative planning, patient selection and accurate component positioning using surgical tools such as navigation systems.
Safe zone of hip resurfacing Conflict of interest All authors have no conflict of interest in relation to this study.
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