Effect of Superior Placement of the Hip Center on Abductor Muscle Strength in Total Hip Arthroplasty

The Journal of Arthroplasty Vol. 24 No. 2 2009 Effect of Superior Placement of the Hip Center on Abductor Muscle Strength in Total Hip Arthroplasty Takahiko Kiyama, MD, Masatoshi Naito, MD, PhD, Hiroshi Shitama, MD, and Akira Maeyama, MD Abstract: We evaluated 100 limbs in 50 patients who had undergone unilateral primary total hip arthroplasty with a normal contralateral hip. The 50 patients were divided into 2 groups by postoperative acetabular cup position, specifically by inferior and superior placement (inferior and superior groups). Hip abductor muscle strength was evaluated qualitatively by the modified Trendelenburg test and quantitatively by handheld dynamometer. The ratio of normalized strength of the reconstruction side to that of the nonoperated side was calculated (strength ratio). The modified Trendelenburg test was positive in 5 of 23 patients in the inferior group and 11 of 27 in the superior group (P b.05). The strength ratio of the superior group was decreased by 7.7% in comparison with that of the inferior group (P b.01). Key words: total hip arthroplasty, abductor muscle strength, acetabular component, superior placement, Trendelenburg sign. 2009 Elsevier Inc. All rights reserved. In primary total hip arthroplasty (THA) for acetabular dysplasia, the ideal placement of the socket remains controversial [1]. There are a number of situations in which loss of bone stock may indicate superior placement of the hip center to provide secure fixation of the acetabular component. A superior placement of the hip center has several problems. First, the iliac bone at the level of the false acetabulum is very thin and cannot support the cup of the prosthesis. Furthermore, the leg length discrepancy (LLD) would remain unchanged, at least in the unilateral case. Finally, the abductor muscles of the hip in this position are lax and insufficient, resulting in a Trendelenburg sign. From the Department of Orthopaedic Surgery, Fukuoka University School of Medicine, Fukuoka, Japan. Submitted July 1, 2008; accepted August 28, 2008. No benefits or funds were received in support of the study. Reprint requests: Takahiko Kiyama, MD, Department of Orthopaedic Surgery, Fukuoka University School of Medicine, 7-45-1, Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan. 2009 Elsevier Inc. All rights reserved. 0883-5403/08/2402-0012$36.00/0 doi:10.1016/j.arth.2008.08.012 Several basic studies have been performed with regard to the effects of a superior-placed hip center on the function of the abductor muscles. Johnston et al [2], using a 2-dimensional biomechanical analysis, reported that displacement of the center of the hip 20 mm lateral, 20 mm proximal, and 10 mm posterior from the anatomical position resulted in a 22% increase in the resultant joint moment. This also resulted in a 20% increase in the joint contact force and a 116% increase in the force of the abductor muscle of the hip that is needed for normal gait. In a clinical study, Asayama et al [3] reported that inferomedial cup positioning appeared to have an advantage for postoperative abductor function. However, object patients in their study did not have dysplastic hip; and they used different surgical approaches to the hip. It remains controversial about where the acetabular component should be placed in patients who have subluxated hip due to acetabular dysplasia. The purpose of this study was to investigate clinical results and abductor muscle strength in special reference to the acetabular cup position in patients who had undergone primary THA because 240

of advanced osteoarthritis secondary to dysplasia of the unilateral hip. Superior Placement of the Hip Center in THA Kiyama et al 241 Patients and Methods A total of 688 patients underwent primary THA at our institution between 1997 and 2004. All patients were Asian. Preoperative diagnosis for inclusion of this study was advanced osteoarthritis secondary to dysplasia of the hip. Hips with severe subluxation or dislocation such as type 3 or 4 according to the criteria of Crowe et al [4] were excluded from this study. Patients with neurologic disorders or any functional and morphologic disorders of the knee or spine were not included in this study. Patients who had radiographic abnormalities or functional disorders in contralateral hips were also excluded. In the remaining 90 patients, we found 30 patients who had a superior placement of the hip center after THA. Of the 30 patients, 3 patients were excluded because of evidence of radiographic acetabular component loosening. From the 60 other patients who did not have a superior placement of the hip center, 23 without radiographic abnormalities such as an acetabular or a femoral stem loosening were selected using a table of random numbers to avoid selection bias and served as controls. A total of 100 limbs in 50 patients who had undergone unilateral THA by a single surgeon were included in this study. There were 5 men and 45 women. The average age at the time of operation was 59.8 years (range, 41-78 years). The average postoperative follow-up period was 4.1 years (range, 2-8 years). The preoperative radiographs were assessed to classify the hips according to the criteria of Crowe et al [4]. Thirtysix hips were classified as type 1, and 14 hips were classified as type 2. The 50 patients were classified by the position of the acetabular component according to the system of Pagnano et al [5]. In this system, 4 zones were created by the intersection of a horizontal line and a vertical line through a point 1 cm superior and lateral to the approximate femoral head center. Zone 1 lies inferior and medial; zone 2, superior and medial; zone 3, superior and lateral; and zone 4, inferior and lateral (Fig. 1). We defined zone 1 or zone 4 as an inferior positioning of the hip center (inferior group), and zone 2 or zone 3 as a superior positioning of the hip center (superior group). There were 23 patients in the inferior group and 27 in the superior group. All 50 THAs were undergone via a posterolateral approach with capsule and piriformis tendon repair. All patients Fig. 1. Diagram showing the 4-zone system used to assess the initial position of the acetabular cup. The zones are created by the intersection of a horizontal and a vertical line through a point 1 cm superior and 1 cm lateral to the approximate femoral head center. were primary operations, and none of the acetabular or femoral components were cemented. Clinical Assessment All patients had clinical evaluation according to the Harris hip score. We performed the modified Trendelenburg test, as described by Hardcastle and Nade [6], to qualitatively evaluate abductor function at preoperative and the latest follow-up visit. The evaluation was performed by a single author (TK) who was blinded to the type of groups. The patient was asked to stand on 1 leg, flexing the other leg at the knee, while keeping the hip in extension. The investigator carefully observed the patient's ability to keep this position for 30 seconds. The test was evaluated as a negative sign when the pelvis on the nonstance side could be elevated as high as hip abduction on the stance side would allow and this position could be kept for 30 seconds. The test was evaluated as a positive sign when this could not be done. When the pelvis could be lifted on command but could not be kept in that position for 30 seconds, it was also scored as a positive sign. For the quantitative evaluation of postoperative muscle strength in abduction, the anterior superior iliac spine and both thighs were immobilized with specialized bands to prevent false motion of the pelvis. The isometric muscle strength was measured for 3 seconds using a Microfet Handheld dynamometer (Hoggan Health Industries, South Draper, UT) fixed to a custom-designed table [7]. The pad of the resistance arm was centered over the distal lateral femur at a standardized point 80% of the

242 The Journal of Arthroplasty Vol. 24 No. 2 February 2009 length of the greater trochanter to the lateral femoral condyle [3]. The nonoperated limb was tested first. The measurements of all hip muscle strengths were performed by a single author (TK) who was blinded to the type of groups. The abductor muscle strength was measured 3 times, and the mean was calculated from the 3 measurements. The ratio of normalized isometric hip abductor strength of the reconstructed side to that of the nonoperated side was then calculated (strength ratio) [8]. Radiographic Evaluation A standard anteroposterior radiograph of the pelvis was taken in the supine position. The limbs were kept in a neutral position during the radiographic examination. The height of the center of the hip, used to measure proximal displacement, was defined as the vertical distance along a line extending from the center of the femoral head perpendicular to the interteardrop line (A) (Fig. 2). The horizontal location of the center of the hip, used to measure lateral displacement, was defined as the horizontal distance along the interteardrop line extending lateral or medial from the inferior point of the teardrop to the perpendicular line dropped from the center of the femoral head (B). The distance between the centers of rotation of the bilateral femoral heads (C), femoral offset (C), distance between the centers of rotation of the bilateral femoral heads (D), and LLD (E) were measured from the radiograph. The femoral offset Table 1. Data on the Patients Inferior Superior P Value No. of patients 23 27 Sex (male-female) 2:21 3:24.27 Age at surgery (y)* 58.8 ± 8.3 60.3 ± 7.8.88 Duration of 4.3 ± 1.6 3.9 ± 1.3.21 follow-up (y)* Crowe classification 17:6 19:8.83 (type 1 type 2) LLD (mm)* 17.5 ± 7.01 16.8 ± 5.1.72 * The values are shown as the mean ± standard deviation. ratio (%FO) was calculated by dividing the femoral offset by the distance between the centers of rotation of the bilateral femoral heads and multiplying by 100 [9]. The radiographic measurements were made 3 times by 2 authors (TK, HS) who were blinded to the clinical results, and the average values were calculated. Statistical Analysis One-factor analysis of the variance, the Scheffe method, and the Pearson correlation coefficient were used for statistical analyses, with the significance level at P equals.05. A simple linear regression analysis showed a 95% confidence level with n = 27 THA limbs in the superior group for detecting correlation between the horizontal distance, the vertical distance, and the strength ratio, accepting an r value of more than 0.361 and a P value of less than.05. Results Patient demographics and surgical date are presented in Table 1. No significant differences were seen in patient demographics, Crowe classification, and preoperative LLD between the 2 groups. The mean prosthetic neck length was 2.9 ± 3.2 mm in the inferior group and 6.7 ± 2.9 mm in the superior group. The prosthetic neck length of the superior group was longer than that of the inferior group (P b.01). Fig. 2. Locus of measurements of the center of the hip on an anteroposterior radiograph of the pelvis. A = height of the center of the hip, B = horizontal location of the center of the hip, C = distance between the rotation centers of the bilateral femoral head, D = femoral offset, and E = LLD. Clinical Assessment Preoperative average Harris hip scores for pain, gait, and activity were 16.9 points (0-30), 21.1 points (11-30), and 10.4 points (10-14) in the inferior group and 17.7 points (0-30), 20.8 points (0-30), and 10.1 points (5-14) in the superior group, respectively. No significant differences were seen in preoperative pain, gait, and activity scores between

Superior Placement of the Hip Center in THA Kiyama et al 243 the 2 groups (P =.85, P =.80, and P =.85, respectively). The number of patients who needed medication stronger than aspirin for pain was 19 of 23 patients (82%) in the inferior group and 20 of 27 patients (74%) in the superior group. Postoperative average Harris hip score was 94.0 points (87-99) in the inferior group and 91.6 points (84-98) in the superior group. Although the average value of the inferior group tended to be higher than that of the superior group, no significant difference was seen in postoperative Harris hip score between the 2 groups (P =.28). Preoperative qualitative evaluation of abductor function using delayed Trendelenburg test showed that all 50 patients had a positive delayed Trendelenburg sign. The qualitative evaluation of abductor function after THA revealed that 5 of 23 patients (27.7%) had a positive Trendelenburg sign in the inferior group. Eleven of 27 patients (40.7%) had a positive Trendelenburg sign in the superior group. A significant difference was seen in the postoperative Trendelenburg sign between the 2 groups (P b.05). The average strength ratio was 87.8% ± 9.22% (range, 71%-108%) in the inferior group and 80.1% ± 9.01% (range, 63%-99%) in the superior group. The strength ratio in the inferior group was significantly higher compared with that in the superior group (P b.01). In the 50 THA limbs, 34 limbs had a negative delayed Trendelenburg sign; and 16 limbs had a positive delayed Trendelenburg sign. The strength ratio was significantly higher for the THA limbs that exhibited a negative delayed Trendelenburg sign (mean, 89.7% ± 7.93%) compared with that for the THA limbs with a positive delayed Trendelenburg sign (mean, 72.8% ± 5.07%) (P b.001). Preoperatively, 36 hips were classified as type 1 and 14 hips were classified as type 2 according to the Crowe classification. Although the average value of strength ratio in type 1 (mean, 84.6% ± 9.38%) tended to be higher than that of type 2 (mean, 81.6% ± 11.18%), no significant difference was seen between the 2 groups (P =.26). Table 2. Result of the Radiographic Measurement Inferior Superior P Value Vertical distance (mm)* 21.4 ± 6.2 33.3 ± 7.3 b.0001 Horizontal distance (mm)* 32.5 ± 5.1 34.7 ± 8.2.35 Femoral offset (mm)* 39.1 ± 6.8 40.7 ± 6.1.32 %FO (%)* 19.5 ± 3.1 19.9 ± 3.3.51 LLD (mm)* 3.9 ± 3.0 4.5 ± 2.6.37 * The values are shown as the mean ± standard deviation. Fig. 3. A, Simple linear regression showing the negative correlation of the strength ratio with the horizontal distance in the superior group (r = 0.65, P b.0001). B, Simple linear regression showing the negative correlation of the strength ratio with the vertical distance in superior group (r = 0.391, P b.05). Radiological Evaluation The factors measured from the pelvic radiographs are shown in Table 2. We did not detect a statistically significant difference in femoral offset, %FO, and postoperative LLD between the 2 groups. In the superior group, the Pearson correlation coefficient analysis revealed significant associations of strength ratio with horizontal distance (r = 0.65, P b.0001) and weak association of strength ratio with vertical distance (r = 0.391, P b.05) (Fig. 3). Discussion Most experienced hip surgeons agree that the optimal location for the center of rotation of a THA is the anatomical position. In difficult primary THAs, often with dysplasia, and in revision arthroplasty, bone grafting may be used to bring the acetabular component down to an anatomical position. Unfortunately, long-term follow-up studies have shown

244 The Journal of Arthroplasty Vol. 24 No. 2 February 2009 that this technique may be associated with a high incidence of collapse of the graft [10]. To avoid complications associated with bone grafting, some elevation of the hip center has been advocated [1]. However, superior placement of the acetabular cup results in shortening of the leg, decreased abductor muscle tension, and increased risk of dislocation [11]. We note several limitations to our study. First, it involved only a small number of hips (50) and only 50 total surgical hips divided into 2 groups. Second, both Crowe type 1 and type 2 patients were included in this study. Although the difference in Crow classification was not significant between the 2 groups, the difference of preoperative dysplastic condition may have affected outcomes. Finally, we could not measure the preoperative abductor muscle strength because most of our patients had severe coxalgia and needed medication for pain before THA. However, preoperative Harris hip scores for gait and activity were similar between the 2 groups. We do not think that there was any significant difference in preoperative abductor function between the 2 groups. Jerosch et al [12] evaluated the influence of a superior placement of the hip center on abductor muscle function in 20 specimens from cadavers using a computer model to compare muscle force and muscle length before and after implantation of a high hip center. Their analysis suggested that the effect of the simultaneous change of the abductor muscle lever arms was an increase in necessary muscle strength for pelvic stabilization from 140% to 250% compared with the original estimated strength before implantation. To assess whether a superior placement of the hip center affects postoperative abductor function, we compared the clinical outcomes and abductor muscle strength in our patients with superior placement or not superior placement of the hip center. In our clinical study, the abductor muscle strength of superior positioning of the hip center was decreased by 7.7% in comparison with that of inferior positioning of the hip center. The result about positive influence being given to an abductor function by inferior cup placement supported a report of Asayama et al [3]. In addition, even if there was difference in surgical approach and the underlying disease, it was suggested that positive influence was given to a postoperative abductor function by inferior cup placement. Doehring et al [13] did an experimental and analytical study to determine the effect of superior and superolateral relocation of the hip center on the forces about the hip. They reported that although Fig. 4. Radiographs showing superolateral placement of the hip center. superolateral displacement of the hip center resulted in higher forces across the hip center because of an increase in the abductor moment, only superior displacement did not significantly alter the total joint force magnitudes or directions. Our data showed correlations between the horizontal distance and strength ratio in the superior group. When superior placement of the hip center is necessary in THA, the hip may not be left in a lateral position because doing so decreases the postoperative abductor muscle strength (Fig. 4). However, the surgeon should do medialization of the acetabular component carefully in patients who have acetabular deformity and poor bone quality. The stability of fixation clearly must take priority. It is possible to preserve the moment generating capacity of the abductor muscles after superior positioning of the hip center by increasing prosthetic neck length. Gore et al [14] evaluated the strength of the abductors of the hip in patients who had had a THA with a relatively high hip center and displacement of the greater trochanter. They found little difference in the strength of the abductors when high placement of the acetabular component was compensated for by an increase in the length of the femoral neck. However, in our study, although the prosthetic neck length of the superior group was longer than that of the inferior group, increasing

Superior Placement of the Hip Center in THA Kiyama et al 245 prosthetic neck length did not counteract the adverse effects on postoperative abductor muscle strength. Delp et al [15] reported that increasing neck length increased muscle length but changed the moment arm very little after superolateral placement using a computer model. In conclusion, in our clinical study, we found that postoperative abductor function in the case of superior placement of the hip center was inferior to that of inferior placement of the hip center. When superior placement of the hip center is necessary in THA for advanced osteoarthritis secondary to dysplasia of the hip, if possible, the hip may not be left in a lateral position because doing so decreases the postoperative abductor muscle strength. References 1. Russotti GM, Harris WH. Proximal placement of the acetabular component in total hip arthroplasty: a long-term follow-up study. J Bone Joint Surg Am 1991;73:587. 2. Johnston RC, Brand RA, Crowninshield RD. Reconstruction of the hip: a mathematical approach to determine optimum geometric relationships. J Bone Joint Surg Am 1979;61:639. 3. Asayama I, Chamnongkich S, Simpson KJ, et al. Reconstructed hip joint position and abductor muscle strength after total hip arthroplasty. J Arthroplasty 2005;20:414. 4. Crowe JF, Mani VJ, Ranawat CS. Total hip replacement in congenital dislocation and dysplasia of the hip. J Bone Joint Surg Am 1979;61:15. 5. Pagnano MW, Hanssen AD, Lewallen DG, et al. The effect of superior placement of the acetabular component on the rate of loosening after total hip arthroplasty. J Bone Joint Surg Am 1996;78:1004. 6. Hardcastle P, Nade S. The significance of the Trendelenburg test. J Bone Joint Surg Br 1985;67:741. 7. Wiles CM, Karni Y. The measurement of muscle strength in patients with peripheral neuromuscular disorders. J Neurol Neurosurg Psychiatry 1983;46: 1006. 8. Yamaguchi T, Naito M, Asayama I, et al. The effect of posterolateral reconstruction on range of motion and muscle strength in total hip arthroplasty. J Arthroplasty 2003;18:347. 9. Asayama I, Naito M, Fujisawa M, et al. Relationship between radiographic measurements of reconstructed hip joint position and the Trendelenburg sign. J Arthroplasty 2002;17:747. 10. Mulroy RD, Harris WH. Failure of acetabular autogenous grafts in total hip arthroplasty. J Bone Joint Surg Am 1990;72:1536. 11. Tanzer M. Role and results of the high hip center. Orthop Clin North Am 1998;29:241. 12. Jerosch M, Steinbeck JJ, Stechmann JJ, et al. Influence of a high hip center on abductor muscle function. Arch Orthop Trauma Surg 1997;116:385. 13. Doehring TC, Rubash HE, Shelley FJ, et al. Effect of superior and superolateral relocations of the hip center on hip joint forces: an experimental and analytical analysis. J Arthroplasty 1996;11:693. 14. Gore DR, Murray MP, Gardenr GM, et al. Roentgenographic measurements after Muller total hip replacement. J Bone Joint Surg Am 1977;59:948. 15. Delp SL, Wixson RL, Komattu AV, et al. How superior placement of the joint center in hip arthroplasty affects the abductor muscles. Clin Orthop 1996;328:137.