http://informahealthcare.com/mor ISSN 1439-7595 (print), 1439-7609 (online) Mod Rheumatol, 2014; Early Online: 1–7 © 2014 Japan College of Rheumatology DOI: 10.3109/14397595.2014.988863
ORIGINAL ARTICLE
Effects of hip joint center location and femoral offset on abductor muscle strength after total hip arthroplasty Taro Tezuka, Yutaka Inaba, Naomi Kobayashi, Hiroyuki Ike, So Kubota, Masaki Kawamura, and Tomoyuki Saito
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Department of Orthopaedic Surgery, Yokohama City University, Yokohama, Japan Abstract
Keywords
Objectives. The purposes of this study were 1) to examine the changes in the hip joint center (HJC) position and the femoral offset (FO) after total hip arthroplasty (THA) and 2) to investigate the effects of the HJC and FO on isometric abductor muscle strength. Methods. We evaluated 51 patients who underwent unilateral primary THA. The FO, and horizontal and vertical distances from the HJC to the tip of the teardrop were measured and isometric hip abductor muscle strength was measured. Results. The HJC of the affected side moved medially postoperatively compared with that of the unaffected side (p ⬍ 0.05), and the FO was reconstructed similarly to the unaffected side. There were significant negative correlations between the changes in the horizontal distance from the HJC and FO to the tip of the teardrop. An increase in the FO and infero-medial cup position optimized hip abductor muscle strength. Conclusion. The HJC was reconstructed medially and superiorly, and the change in the FO after THA was influenced by the change in the horizontal distance of the HJC. Multiple regression analysis revealed that the medial and inferior HJC and increase in the FO constitute an effective procedure for restoring abductor strength.
Hip abductor strength, Hip joint center, Femoral offset, Total hip arthroplasty
Introduction Total hip arthroplasty (THA) is one of the most effective methods for relieving arthritic pain caused by destructive hip disease, and the importance of restoring normal hip biomechanics following THA has long been recognized as a primary goal for ambulation. In particular, restoring the abductor moment arm to preserve normal gait after THA is essential. However, anatomical reconstruction of the hip joint center (HJC) is not always easy [1,2]. In gait analysis, the location of the HJC is one of the key factors for rehabilitation after THA [3]. Therefore, the purposes of this study were 1) to examine the changes in the HJC position and the femoral offset (FO) after THA and 2) to investigate the effects of the HJC and FO on isometric abductor muscle strength.
Patients and methods The study was approved by the authors’ institutional review board, and informed consent was obtained from all patients. Hips of fifty-one patients treated with unilateral THA between 2008 and 2009 that were followed up for 1 year were enrolled. All cases were affected unilaterally, and the contralateral hips were diseasefree with normal radiological findings. The patients included 43 women and 8 men, and the mean age at surgery was 62 ⫾ 8.9 (range, 42–80) years. The diagnoses were osteoarthritis in 46 hips (88%), osteonecrosis of the femoral head in 3 (8%), and rheumatoid arthritis in 2 (4%). Anthropometric data for the study population
History Received 27 August 2014 Accepted 14 November 2014 Published online 18 December 2014
included a mean height of 153 ⫾ 7.6 (range, 142–174) cm, a mean body weight of 56 ⫾ 11 (range, 37–90) kg, and a mean body mass index (BMI) of 24 ⫾ 4.8 (range, 16–41) kg/m2. The femoral prostheses used in the current study included 19 SL-Plus stems (Smith and Nephew, Memphis, TN, USA), 12 Accolade stems (Stryker Orthopedics, Mahwah, NJ, USA) with a 132 or 127 neck–shaft angle, 14 VerSys Fiber Metal Midcoat (Zimmer, Warsaw, IN, USA), 2 VerSys Heritage stems (Zimmer), 2 CentPillar stems (Stryker Orthopedics), and 1 Exeter stem (Zimmer). The FOs varied for each type of prosthesis as a result of the surgeons’ attempt to optimize each patient’s kinematics based on his/her preoperative radiographs. All acetabular components were cementless, and in cases of acetabular dysplasia of the hip, we did not use isolated femoral heads for the bulk of the structural autogenous bone graft. Femoral head sizes of the 51 hips included 26 mm (12 hips), 28 mm (26 hips), 32 mm (3 hips), and 36 mm (10 hips). The mini-anterolateral approach was used in 17 cases, and the mini-incision direct lateral approach was used in the remaining 34 cases. All hip surgeries, except 3, involved the use of the cementless stem using the Stryker Navigation System II Cart with Computed Tomography-based Hip Navigation software, version 1.1 (Stryker Orthopedics) or the Stealth Station Navigation System Treon Plus with Universal HIP software (Medtronic Inc., Minneapolis, MN, USA). Clinical assessment
Correspondence to: Yutaka Inaba, MD, PhD, Department of Orthopaedic Surgery, Yokohama City University, 3-9 Fukuura, Kanazawa, Yokohama, Kanagawa, Japan 236-0004, Tel: ⫹ 81-45-787-2655. Fax: ⫹ 81-45-781-7922. E-mail: [email protected]
Each patient underwent clinical evaluation using the Harris Hip Score (HHS) [4], including a careful visual assessment during natural gait for evidence of a limp. The visual gait assessment was administered under natural walking conditions at a point when the
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patients were not aware of the observation. Any visual evidence of a lateral imbalance in the pelvic movement during gait was scored as a limp and was classified according to the HHS.
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Isometric abductor muscle strength Isometric hip abductor strength measurements were performed on all patients using an isokinetic/isometric dynamometer (microFET; Hoggan Health Industries, Inc., Draper, UT, USA). During the test, the participant lay in a side-lying position with the test leg up and the resistance arm of the dynamometer secured to his/her lateral thigh. The pad of the resistance arm was centered over the distal lateral femur at a standardized point of 80% of the length of the greater trochanter to the lateral femoral condyle. The test leg was positioned straight (0 of hip and knee flexion and 0 of hip rotation), whereas the nontest leg was positioned in approximately 30 of hip flexion and 30 of knee flexion against the table for comfort and stability. The participant was asked to perform three maximal isometric contractions for 5 seconds with 30 seconds of rest between the repetitions on each side. The ratio of the isometric hip abductor strength of the reconstructed side to that of the nonoperative side was calculated (strength ratio) preoperatively and at 1 year postoperatively. The measurement of abductor muscle strength was performed by well-trained hip surgeons at our institution. Radiographic assessment A standard anteroposterior radiograph of the pelvis was taken before and after surgery. During the radiographic examination, the patient lay supine with the lower extremities in a neutral position. The legs were positioned at 15 internal rotation, and the coccyx was centered on the pubic symphysis. In the radiographic evaluation, the HJC location was defined as the geometrical center of the femoral head, which was modeled as a sphere. The horizontal distance between the HJC and the tip of the teardrop, which is parallel to the inter-teardrop line, and the vertical distance between the HJC and the inter-teardrop line were measured. Each radiographic parameter was measured on both sides before and after surgery. The FO was defined as the perpendicular distance from the center line of the femur to the HJC (Figure 1). We defined the changes in the FO in the affected hip between pre- and post-surgery as ΔFO. Furthermore, Δhorizontal distance and Δvertical distance were defined in the same manner. The magnification was corrected on postoperative radiographs using the acetabular component size and the femoral head diameter. On preoperative radiographs, the correction of the magnification was accomplished by comparing the preoperative and postoperative radiographic femoral shaft diameters at a fixed distance of 10 cm distal to the proximal corner of the lesser trochanter. Radiographic measurements were taken three times after an interval of at least 2 weeks by a single author. The average values of the radiographic measurements were calculated. Intraobserver reliabilities of the radiographic measurements were assessed using intraclass correlation coefficients. Table 1 shows the intraclass correlation coefficients for each measurement. The degree of subluxation of the femoral head was classified according to the method reported by Crowe et al. [5].
Figure 1. The horizontal distance between the HJC and the tip of the teardrop, which is parallel to the inter-teardrop line, and the vertical distance between the HJC and the inter-teardrop line were measured. The femoral offset was defined as the perpendicular distance from the center line of the femur to the HJC.
Statistical analysis The Mann–Whitney U test was used to examine the significance of the differences in the HHS. The paired Student’s t-test was used to evaluate the differences in the isometric abductor muscle strength and strength ratio. One-way analysis of variance and Student–Newman–Keuls test were performed to compare each radiographic measurement. To evaluate the relationship between the HJC location and the FO, Pearson’s correlation coefficients were calculated to analyze the relationships between the ΔFO and Δhorizontal distance, Δhorizontal distance and Δvertical distance, and Δvertical distance and ΔFO. To evaluate the effect of the radiographic measurements and abductor muscle strength on the HHS, the Pearson’s correlation coefficient was used to examine the relationship between each of the radiographic measurements or the abductor muscle strength ratio and the HHS. To evaluate the effect of patients’ baseline characteristics on abductor muscle strength, Pearson’s correlation coefficient was used to examine the relationship between the abductor muscle strength ratio and the following factors: sex, age, BMI, subluxation of the femoral head according to Crowe’s classification, and surgical approach. Pearson’s correlation coefficient was also used to examine the relationship between abductor muscle strength ratio and surgical approach. Additionally, we performed simple regression analysis to evaluate the association between the abductor strength ratio and the
Table 1. The intraclass correlation coefficients for each radiographic measurement.
Intraclass correlation coefficient
Horizontal distance Unaffected Pre- Postside
Vertical distance Unaffected Pre- Postside
Femoral offset Pre-
Post-
Unaffected side
0.92
0.9
0.91
0.92
0.91
0.93
0.93
0.94
0.94
pre-: preoperative value of the affected side; post-: postoperative value of the affected side; unaffected: values of the unaffected side
Hip joint center location and femoral offset on abductor muscle 3
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DOI 10.3109/14397595.2014.988863
Figure 2. Isometric abductor muscle strength ratio before and at 1 year after THA. The abductor muscle strength ratio improved from 0.71 ⫾ 0.20 before surgery to 0.85 ⫾ 0.15 at 1 year postoperatively (PO).
radiographic measurements after THA. We used multiple regression analysis to identify independent determinants of the ratio of hip abductor muscle strength. The radiographic measurements were entered simultaneously. The adjusted coefficient of multiple determination (adjusted R2) was used to indicate how much variability in the ratio of hip abductor muscle strength was accounted for by the independent variables. Standardized regression coefficients (β) and associated p values were determined to assess statistical significance (p ⬍ 0.05). All statistical analyses were performed using SPSS, version 21.0 (IBM Corp., Armonk, NY, USA).
Results Clinical assessment The average HHS improved from 58 ⫾ 16 points before surgery to 93 ⫾ 6.7 points postoperatively (p ⬍ 0.01). The average preoperative HHS for pain, function, deformity, and motion improved from 21 ⫾ 9.2, 28 ⫾ 8.9, 3.8 ⫾ 0.5, and 4.2 ⫾ 1.1, respectively, to 42 ⫾ 2.4, 41 ⫾ 6.0, 3.9 ⫾ 0.2, and 4.8 ⫾ 0.5, respectively (p ⬍ 0.01, p ⬍ 0.01, p ⫽ 0.13, p ⬍ 0.01, respectively). In assessment at 1 year postoperatively, 12 patients had a limp; 11 were classified as slight and 1 as moderate. Twenty-six patients used a cane; 14 patients needed a cane for long walks, and 12 needed a cane most of the time. None of the patients needed to use a crutch. The mean HHS limp score of the patients who needed a cane for long walks, and the patients who needed a cane most of the time were 8.0 ⫾ 1.6 points and 7.0 ⫾ 1.5 points, respectively. The remaining 25 patients who did not need a cane had a limp score of 10 ⫾ 0.3 points. To evaluate the relationship between limp score and cane use, we performed one-way analysis of variance and Student–Newman– Keuls test. There was a significant difference between patients who
Figure 3. The results of radiographic measurements before and after THA. The horizontal distance of the HJC of the unaffected side was located laterally compared with the unaffected side (p ⬍ 0.01), and it moved medially after surgery compared with before surgery (p ⬍ 0.01). The vertical distance was superior before surgery (p ⬍ 0.01) compared with that on the unaffected side, and this superior position remained postoperatively. The femoral offset of the affected side was shorter than that of the unaffected side before surgery (p ⬍ 0.01) and was reconstructed to a similar degree as the unaffected side.
needed a cane for long walks and patients who did not need a cane (p ⬍ 0.01); there was also a significant difference between patients who needed a cane most of the time and patients who did not need a cane (p ⬍ 0.01). However, there was no significant difference between patients who needed a cane most of the time and patients who needed a cane for long walks. Isometric abductor muscle strength The mean abductor muscle strength ratio improved from 0.71 ⫾ 0.20 before surgery to 0.85 ⫾ 0.15 at 1 year postoperatively (p ⬍ 0.01) (Figure 2). The mean abductor muscle strength ratio in patients with a limp at 1 year after THA was 0.73 ⫾ 0.13, and the mean abductor muscle strength ratio in patients without a limp was 0.86 ⫾ 0.13 (p ⫽ 0.02). We performed simple regression analysis between abductor muscle strength ratio and pain score of HHS. Preoperatively, there was a weak correlation between the abductor muscle strength ratio and HHS pain score (r ⫽ 0.310, p ⫽ 0.020), such that more severe pain was associated with a lower abductor muscle strength. However, there was no significant relationship 1 year after THA (r ⫽ 0.132, p ⫽ 0.361). Radiographic assessment Of the 51 hips, 44 were classified as Crowe’s group I, 5 as group II, and 2 as group III; no cases were classified as group IV. Pearson’s
Table 2. Pearson’s correlation coefficients for the relationship between the changes in each of the radiographic measurements. Δhorizontal distance Δvertical distance ΔFO FO: femoral offset ** p ⬍ 0.01
Correlation coefficient p Correlation coefficient p Correlation coefficient p
Δhorizontal distance 1
Δvertical distance 0.264 0.057 1
ΔFO ⫺ 0.594** 0.001 ⫺ 0.156 0.275 1
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Table 3. Pearson’s correlation coefficients between the abductor muscle strength ratio before THA and the radiographic measurements.
Strength ratio Correlation coefficient before THA p
Horizontal distance 0.004 0.976
Vertical distance ⫺ 0.018
FO 0.180
0.899
0.220
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THA: total hip arthroplasty; FO: femoral offset
correlation coefficient analysis showed that there was no significant relationship between Crowe’s classification and abductor muscle strength ratio (r ⫽ ⫺ 0.23, p ⫽ 0.11). The results of measurements on the pelvic radiographs are shown in Figure 3. The horizontal distance of the HJC of the unaffected side was 34.7 ⫾ 5.4 mm and that of the affected side changed from 42.5 ⫾ 9.4 mm before surgery to 32.2 ⫾ 5.9 mm postoperatively. The vertical distance of the HJC of the unaffected side was 17.7 ⫾ 4.2 mm and that of the affected side changed from 25.4 ⫾ 7.9 mm before surgery to 26.0 ⫾ 7.1 mm postoperatively. Compared with the HJC on the unaffected side, the HJC on the affected side was superior before surgery (p ⬍ 0.01), and the superior placement persisted postoperatively. The FO of the unaffected side was 35.6 ⫾ 7.3 mm and that of the affected side changed from 27.4 ⫾ 10.3 mm before surgery to 36.3 ⫾ 7.6 mm postoperatively. The FO was well reconstructed, and no significant difference was observed between the unaffected and affected sides postoperatively. A negative coefficient was derived for the relationship between the Δhorizontal distance and the ΔFO (r -0.594, p ⬍ 0.01) (Table 2). There was no significant correlation between the radiographic measurements and the abductor muscle strength ratio before THA (Table 3). Table 4 shows Pearson’s correlation coefficient for the relationship between abductor muscle strength and baseline characteristic factors, and surgical approach. There was no significant correlation between the abductor muscle strength ratio and the baseline characteristic factors of patients. There was also no significant correlation between surgical approach and abductor muscle strength.
Furthermore, simple regression analysis showed that there was no significant relationship between femoral head size and the abductor muscle strength ratio (r ⫽ 0.021, p ⫽ 0.465). Table 5 shows the parameters that affected the HHS, according to Pearson’s correlation coefficient. There was a weak positive correlation between the abductor muscle strength ratio and the total HHS score (r ⫽ 0.390, p ⫽ 0.005), and we also found a positive correlation between the abductor muscle strength ratio and the HHS function score (r ⫽ 0.373, p ⫽ 0.008). There was a weak positive correlation between the FO and the total HHS score (r ⫽ 0.289, p ⫽ 0.048). Figure 4 shows the simple linear regression between the abductor muscle strength ratio and the radiographic measurements postoperatively. There was a negative correlation between the horizontal distance from the HJC to the tip of the teardrop and the strength ratio (r ⫽ ⫺0.359; p ⫽ 0.010) (Figure 4a), and there was a negative correlation between the vertical distance from the HJC to the tip of the teardrop and the strength ratio (r ⫽ ⫺ 0.374; p ⫽ 0.007) (Figure 4b). Additionally, there was a positive correlation between the FO and the strength ratio (r ⫽ 0.330; p ⫽ 0.018) (Figure 4c). Multiple regression analysis showed that the horizontal distance (β ⫽ ⫺ 0.314; p ⫽ 0.016), vertical distance (β ⫽ ⫺ 0.325, p ⫽ 0.014), and FO (β ⫽ 0.299, p ⫽ 0.021) were significantly associated with the abductor muscles strength ratio. The final model accounted for 28% (adjusted R2 ⫽ 0.28) (Table 6). Figure 5 shows an 80-year-old man who presented to our department with a history of coxalgia on his left hip which gradually deteriorated. Plain radiography of the hip showed an osteoarthritis, and the HJC was situated laterally and was superior compared with the unaffected HJC. The FO was slightly shortened compared with unaffected side (Figure 5a). The strength ratio of abductor muscle was 0.77. We performed THA on the patient with mini direct lateral approach using VerSys Fiber Metal Midcoats Stem and Trilogy Acetabular Cup (Zimmer International, Warsaw, IN, USA). Plain radiography showed that the horizontal distance moved medially and the FO was fully reconstructed compared with the unaffected side. Superior displacement remained; however, we attempted to compensate with that using a long prosthetic neck (Figure 5b). The strength ratio of abductor muscle after surgery was 0.98.
Table 4. Pearson’s correlation coefficient for the relationship between the abductor muscle strength ratio and patients’ demographics, pain score or surgical approach. Strength ratio before THA Strength ratio at 1 year after THA
Correlation coefficient p Correlation coefficient p
Sex (vs male) 0.048 0.740 ⫺ 0.002 0.991
Age 0.275 0.056 ⫺ 0.044
BMI 0.175 0.250 ⫺ 0.356
0.766
0.164
Pain 0.318* 0.023 0.132
Approach N.A.
0.361
0.104 0.473
pain: pain score of Harris Hip Score N.A.: not available * p ⬍ 0.01
Table 5. Pearson’s correlation coefficients between the HHS and the following factors: abductor muscle strength ratio at 1 year after THA, horizontal distance from the HJC to the tip of the teardrop, vertical distance from the HJC to the tip of the teardrop, and the FO. HHS Pain Function Deformity 0.132 0.162 Strength ratio at 1 year after THA Correlation coefficient 0.373** 0.390** p 0.005 0.361 0.008 0.262 Horizontal distance Correlation coefficient ⫺ 0.108 0.205 ⫺ 0.178 ⫺ 0.037 p 0.457 0.217 0.801 0.153 Vertical distance Correlation coefficient ⫺ 0.067 0.252 ⫺ 0.162 ⫺ 0.222 p 0.642 0.077 0.260 0.121 0.155 0.249 0.229 FO Correlation coefficient 0.289* p 0.042 0.281 0.082 0.110 HHS: Harris Hip Score; ROM: range of motion; THA: total hip arthroplasty; FO: femoral offset *p ⬍ 0.05; **p ⬍ 0.01.
ROM 0.006 0.967 ⫺ 0.192 0.181 ⫺ 0.078 0.593 ⫺ 0.004 0.978
Hip joint center location and femoral offset on abductor muscle 5
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Figure 4. Simple linear regression between the abductor muscle strength ratio and the radiographic measurements. (a) A negative correlation of the strength ratio with the horizontal distance from the HJC to the tip of the teardrop is observed in 51 reconstructed hips (r ⫽ ⫺ 0.359; p ⫽ 0.010). (b) A negative correlation of the strength ratio with the vertical distance from the HJC to the tip of the teardrop is observed in 51 reconstructed hips (r ⫽ ⫺ 0.374; p ⫽ 0.007). (c) A positive correlation of the strength ratio with the FO is observed in 51 reconstructed hips (r ⫽ 0.330; p ⫽ 0.018).
Discussion The locations of the HJC and the FO following THA are crucial radiographic parameters that affect the joint reaction force and the abductor muscle strength by changing the moment arm of the muscle strength, and the joint reaction force affects the amount of polyethylene wear, which also affects the longevity of the prosthesis and the clinical result [6,7]. Several authors have reported that the FO and the position of the reconstructed hip center correlated with the strength of the hip abductors [8]; however, they did not mention the relationship between the FO and the HJC after THA. Therefore, we investigated the multiple effects of radiographic parameters, including the FO and the HJC location on abductor muscle strength after THA. In the present study, the postoperative HHS was generally satisfactory, and the strength ratio of the abductor muscle significantly improved from 0.73 preoperatively to 0.85 postoperatively. In this study, the pain score of HHS before THA affected the abductor muscle strength ratio. After THA, there was no significant correlation between pain score and abductor muscle strength, which meant that severe pain caused the incompetence of abductor muscle. Abductor muscle strength after THA showed a positive correlation with the function item of the HHS and also had a statistically significant effect on the clinical result. In this study, the Table 6. The results of multiple regression analysis for evaluating the association between the abductor muscle strength ratio and radiographic parameters.
Horizontal distance Vertical distance FO
Regression coefficient
Standardized regression coefficients (β)
⫺ 0.010 ⫺ 0.008 0.007
⫺ 0.314 ⫺ 0.325 0.299
VIF: variance inflation factor; FO: femoral offset.
p 0.016 0.012 0.021
VIF 1.012 1.001 1.013
mean abductor muscle strength ratio of patients with a limp at 1 year after THA was 0.73 ⫾ 0.13, and the mean abductor muscle strength ratio of patients without a limp was 0.86 ⫾ 0.13. Asayama et al. [9] reported that a strength ratio of ⬎ 0.88 cannot result in a positive Trendelenburg sign, which coincided with the results of this study. The HJC was located laterally and superiorly before surgery compared with the unaffected hip due to the loss of cartilage and dysplasia of the acetabulum, which can cause the femoral head to slide and migrate away from the normal hip center. There was no significant relationship between radiographic measurements and the abductor muscle strength ratio before THA, which may be due to other factors (e.g., contracture of the hip or coxalgia affected the abductor muscle strength). After surgery, the HJC was reconstructed medially in comparison to the unaffected side, but the superior placement remained. Most studies concurred that the true acetabulum is the optimal position for the infero-medial cup and results in the best outcome [10–12]. In our series, we attempted to recreate the original articulation using the navigation system; however, the anatomical reconstruction of the hip is not always easy in cases with developmental dysplasia of the hip or large bone defects. In such cases, we reluctantly reconstruct the hip center superiorly to obtain an adequate press fit without autogenous bone graft implantation and attempt to compensate for muscle tension or a leg length discrepancy with the neck length or the FO. Iglic et al. [13] reported that their mathematical model showed that a superior HJC decreases the strength of the hip abductor muscle strength. Delp and Maloney [14] also reported that a 2-cm superior replacement of the HJC decreases abduction force and moment arm; however, they [12] also reported that a superomedially positioned cup, along with restoration of the leg length via a long prosthetic neck, led to force-generating capacities of the abductors by ⬍ 10% and maintained 166% of moment, which is needed for standing on single leg. In the current study, there was 1 hip that showed superior replacement of the HJC by ⬎ 2 cm compared
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Figure 5. An 80-year-old man with left hip osteoarthritis. On radiography before surgery, the HJC was located laterally and superiorly compared with the unaffected HJC. (a) The FO was slightly shortened compared with the unaffected side, and the strength ratio of the abductor muscle was 0.77. (b) Radiography after THA showing that the HJC moved medially, and the FO was reconstructed compared with the unaffected side. Slight superior displacement of the HJC remained. The strength ratio of the abductor muscle postoperatively was 0.98.
with the unaffected side, and the patient had a limp after THA. Furthermore, 4 hips showed superior replacement of 1.5–2.0 cm, and 2 of these patients (50%) had a limp. In the remaining 46 hips with superior replacement of ⬍ 1.5 cm, 9 patients (20%) had a limp. These results suggested that the superior replacement caused limping after THA; however, other factors such as horizontal distance of the HJC or the FO may have some effect on the presence of limping. In the present study, the FO was reconstructed similarly to the unaffected side, and a significant negative correlation was observed between the changes in the horizontal distance of the HJC and that of the FO. This finding was the result of the compensation performed by surgeons using a high-offset stem, long prosthetic neck, or stems with a smaller neck–shaft angle. Thus, several factors may affect the abductor muscle strength and limping, which correlate with each other. Therefore, we investigated multiple relationships among the abductor muscle strength and radiographic measurements, while several other studies have only reported on the relationship between the abductor muscle strength and the radiographic measurements after THA [8,9,15–17]. In this study, multiple regression analysis revealed that the horizontal distance, vertical distance, and FO were associated with abductor muscle strength to the same degree, and the smaller the horizontal distance was, the better the abductor muscle strength. The statistical analysis also revealed that the larger the FO was, the better the abductor muscle strength was after THA. The FO before surgery was shorter than that of the unaffected side and was similarly reconstructed to the unaffected side. Furthermore, there was a strong negative correlation between the ΔHD and the ΔFO, meaning that the sum of the HD and FO after surgery was equal to the sum of the HD and FO of the unaffected side. These findings indicate that among patients with the same sum of HD and FO, patients with a smaller HD and larger FO were at a greater advantage with regard to abductor muscle strength. The current study has several limitations. First, 5 different types of stems were used in the patients included. Ideally, a uniform stem type should be used in this type of study. We chose the type of femoral prosthesis according to the patients’ geometry, and the size and bone quality of the femur. For example, cemented stems were used for patients with an osteoporotic femur, while the type of cementless stem used was determined according to the patient’s geometry and the size of the femur. One-way analysis of variance and Student–Newman–Keuls test were used to examine the effect of stem type on femoral offset, and revealed that there was no significant difference between stem type and femoral offset. Therefore, we believe that the effect of stem type on the femoral offset was weak. Second, the patient’s subcutaneous fat volume and abductor muscle volume were not accounted for in our analysis of abductor muscle strength. It is plausible that failure to account for these
factors might explain the relatively small adjusted R2 of multiple regression; the residual 72% might be accounted for by abductor muscle volume or other factors.
Conclusion The average postoperative HHS and strength ratio of the abductor muscle were generally satisfactory, and the FO and abductor muscle strength ratio influenced the clinical results. The HJC was reconstructed medially and superiorly, and the change in the FO after THA was influenced by the change in the horizontal distance of the HJC. Multiple regression analysis revealed that the medial and inferior HJC and increase in the FO was an effective procedure for restoring abductor strength.
Acknowledgments We wish to thank Dr. Yohei Yukizawa, Dr.Choe Hyonmin, Dr. Hiroshi Fujimaki, Dr. Yasuhide Hirata, and Dr. Masaki Kawamura for their valuable contribution to the current study.
Conflict of interest The authors declare no conflicts of interest. The authors alone are responsible for the content and writing of the paper.
References 1. Jerosch J, Steinbeck J, Stechmann J, Guth V. Influence of a high hip center on abductor muscle function. Arch Orthop Trauma Surg. 1997;116(6–7):385–9. 2. Bourne RB, Rorabeck CH. Soft tissue balancing: the hip. J Arthroplasty. 2002;17(4 Suppl 1):17–22. 3. Ehrig RM, Taylor WR, Duda GN, Heller MO. A survey of formal methods for determining the centre of rotation of ball joints. J Biomech. 2006;39(15):2798–809. 4. Harris WH. Traumatic arthritis of the hip after dislocation and acetabular fractures: treatment by mold arthroplasty. An end-result study using a new method of result evaluation. J Bone Joint Surg Am. 1969;51(4):737–55. 5. Crowe JF, Mani VJ, Ranawat CS. Total hip replacement in congenital dislocation and dysplasia of the hip. J Bone Joint Surg Am. 1979; 61(1):15–23. 6. Cereatti A, Donati M, Camomilla V, Margheritini F, Cappozzo A. Hip joint centre location: an ex vivo study. J Biomech. 2009;42(7): 818–23. 7. Erceg M. The influence of femoral head shift on hip biomechanics: additional parameters accounted. Int Orthop. 2009;33(1):95–100. 8. McGrory BJ, Morrey BF, Cahalan TD, An KN, Cabanela ME. Effect of femoral offset on range of motion and abductor muscle strength after total hip arthroplasty. J Bone Joint Surg Br. 1995;77(6): 865–9. 9. Asayama I, Chamnongkich S, Simpson KJ, Kinsey TL, Mahoney OM. Reconstructed hip joint position and abductor muscle strength after total hip arthroplasty. J Arthroplasty. 2005;20(4):414–20.
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10. Linde F, Jensen J. Socket loosening in arthroplasty for congenital dislocation of the hip. Acta Orthop Scand. 1988;59(3):254–7. 11. 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(4):587–92. 12. Delp SL, Wixson RL, Komattu AV, Kocmond JH. How superior placement of the joint center in hip arthroplasty affects the abductor muscles. Clin Orthop Relat Res. 1996;(328):137–46. 13. Iglic A, Antolic V, Srakar F. Biomechanical analysis of various operative hip joint rotation center shifts. Arch Orthop Trauma Surg. 1993;112(3):124–6.
Hip joint center location and femoral offset on abductor muscle 7 14. Delp SL, Maloney W. Effects of hip center location on the momentgenerating capacity of the muscles. J Biomech. 1993;26(4–5):485–99. 15. Kamada S, Naito M, Nakamura Y, Kiyama T. Hip abductor muscle strength after total hip arthroplasty with short stems. Arch Orthop Trauma Surg 2011;131(12):1723–9. 16. Kiyama T, Naito M, Shitama H, Maeyama A. Effect of superior placement of the hip center on abductor muscle strength in total hip arthroplasty. J Arthroplasty. 2009;24(2):240–5. 17. Dastane M, Dorr LD, Tarwala R, Wan Z. Hip offset in total hip arthroplasty: quantitative measurement with navigation. Clin Orthop Relat Res 2011;469(2):429–36.
ORIGINAL ARTICLE
Effects of hip joint center location and femoral offset on abductor muscle strength after total hip arthroplasty Taro Tezuka, Yutaka Inaba, Naomi Kobayashi, Hiroyuki Ike, So Kubota, Masaki Kawamura, and Tomoyuki Saito
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Department of Orthopaedic Surgery, Yokohama City University, Yokohama, Japan Abstract
Keywords
Objectives. The purposes of this study were 1) to examine the changes in the hip joint center (HJC) position and the femoral offset (FO) after total hip arthroplasty (THA) and 2) to investigate the effects of the HJC and FO on isometric abductor muscle strength. Methods. We evaluated 51 patients who underwent unilateral primary THA. The FO, and horizontal and vertical distances from the HJC to the tip of the teardrop were measured and isometric hip abductor muscle strength was measured. Results. The HJC of the affected side moved medially postoperatively compared with that of the unaffected side (p ⬍ 0.05), and the FO was reconstructed similarly to the unaffected side. There were significant negative correlations between the changes in the horizontal distance from the HJC and FO to the tip of the teardrop. An increase in the FO and infero-medial cup position optimized hip abductor muscle strength. Conclusion. The HJC was reconstructed medially and superiorly, and the change in the FO after THA was influenced by the change in the horizontal distance of the HJC. Multiple regression analysis revealed that the medial and inferior HJC and increase in the FO constitute an effective procedure for restoring abductor strength.
Hip abductor strength, Hip joint center, Femoral offset, Total hip arthroplasty
Introduction Total hip arthroplasty (THA) is one of the most effective methods for relieving arthritic pain caused by destructive hip disease, and the importance of restoring normal hip biomechanics following THA has long been recognized as a primary goal for ambulation. In particular, restoring the abductor moment arm to preserve normal gait after THA is essential. However, anatomical reconstruction of the hip joint center (HJC) is not always easy [1,2]. In gait analysis, the location of the HJC is one of the key factors for rehabilitation after THA [3]. Therefore, the purposes of this study were 1) to examine the changes in the HJC position and the femoral offset (FO) after THA and 2) to investigate the effects of the HJC and FO on isometric abductor muscle strength.
Patients and methods The study was approved by the authors’ institutional review board, and informed consent was obtained from all patients. Hips of fifty-one patients treated with unilateral THA between 2008 and 2009 that were followed up for 1 year were enrolled. All cases were affected unilaterally, and the contralateral hips were diseasefree with normal radiological findings. The patients included 43 women and 8 men, and the mean age at surgery was 62 ⫾ 8.9 (range, 42–80) years. The diagnoses were osteoarthritis in 46 hips (88%), osteonecrosis of the femoral head in 3 (8%), and rheumatoid arthritis in 2 (4%). Anthropometric data for the study population
History Received 27 August 2014 Accepted 14 November 2014 Published online 18 December 2014
included a mean height of 153 ⫾ 7.6 (range, 142–174) cm, a mean body weight of 56 ⫾ 11 (range, 37–90) kg, and a mean body mass index (BMI) of 24 ⫾ 4.8 (range, 16–41) kg/m2. The femoral prostheses used in the current study included 19 SL-Plus stems (Smith and Nephew, Memphis, TN, USA), 12 Accolade stems (Stryker Orthopedics, Mahwah, NJ, USA) with a 132 or 127 neck–shaft angle, 14 VerSys Fiber Metal Midcoat (Zimmer, Warsaw, IN, USA), 2 VerSys Heritage stems (Zimmer), 2 CentPillar stems (Stryker Orthopedics), and 1 Exeter stem (Zimmer). The FOs varied for each type of prosthesis as a result of the surgeons’ attempt to optimize each patient’s kinematics based on his/her preoperative radiographs. All acetabular components were cementless, and in cases of acetabular dysplasia of the hip, we did not use isolated femoral heads for the bulk of the structural autogenous bone graft. Femoral head sizes of the 51 hips included 26 mm (12 hips), 28 mm (26 hips), 32 mm (3 hips), and 36 mm (10 hips). The mini-anterolateral approach was used in 17 cases, and the mini-incision direct lateral approach was used in the remaining 34 cases. All hip surgeries, except 3, involved the use of the cementless stem using the Stryker Navigation System II Cart with Computed Tomography-based Hip Navigation software, version 1.1 (Stryker Orthopedics) or the Stealth Station Navigation System Treon Plus with Universal HIP software (Medtronic Inc., Minneapolis, MN, USA). Clinical assessment
Correspondence to: Yutaka Inaba, MD, PhD, Department of Orthopaedic Surgery, Yokohama City University, 3-9 Fukuura, Kanazawa, Yokohama, Kanagawa, Japan 236-0004, Tel: ⫹ 81-45-787-2655. Fax: ⫹ 81-45-781-7922. E-mail: [email protected]
Each patient underwent clinical evaluation using the Harris Hip Score (HHS) [4], including a careful visual assessment during natural gait for evidence of a limp. The visual gait assessment was administered under natural walking conditions at a point when the
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patients were not aware of the observation. Any visual evidence of a lateral imbalance in the pelvic movement during gait was scored as a limp and was classified according to the HHS.
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Isometric abductor muscle strength Isometric hip abductor strength measurements were performed on all patients using an isokinetic/isometric dynamometer (microFET; Hoggan Health Industries, Inc., Draper, UT, USA). During the test, the participant lay in a side-lying position with the test leg up and the resistance arm of the dynamometer secured to his/her lateral thigh. The pad of the resistance arm was centered over the distal lateral femur at a standardized point of 80% of the length of the greater trochanter to the lateral femoral condyle. The test leg was positioned straight (0 of hip and knee flexion and 0 of hip rotation), whereas the nontest leg was positioned in approximately 30 of hip flexion and 30 of knee flexion against the table for comfort and stability. The participant was asked to perform three maximal isometric contractions for 5 seconds with 30 seconds of rest between the repetitions on each side. The ratio of the isometric hip abductor strength of the reconstructed side to that of the nonoperative side was calculated (strength ratio) preoperatively and at 1 year postoperatively. The measurement of abductor muscle strength was performed by well-trained hip surgeons at our institution. Radiographic assessment A standard anteroposterior radiograph of the pelvis was taken before and after surgery. During the radiographic examination, the patient lay supine with the lower extremities in a neutral position. The legs were positioned at 15 internal rotation, and the coccyx was centered on the pubic symphysis. In the radiographic evaluation, the HJC location was defined as the geometrical center of the femoral head, which was modeled as a sphere. The horizontal distance between the HJC and the tip of the teardrop, which is parallel to the inter-teardrop line, and the vertical distance between the HJC and the inter-teardrop line were measured. Each radiographic parameter was measured on both sides before and after surgery. The FO was defined as the perpendicular distance from the center line of the femur to the HJC (Figure 1). We defined the changes in the FO in the affected hip between pre- and post-surgery as ΔFO. Furthermore, Δhorizontal distance and Δvertical distance were defined in the same manner. The magnification was corrected on postoperative radiographs using the acetabular component size and the femoral head diameter. On preoperative radiographs, the correction of the magnification was accomplished by comparing the preoperative and postoperative radiographic femoral shaft diameters at a fixed distance of 10 cm distal to the proximal corner of the lesser trochanter. Radiographic measurements were taken three times after an interval of at least 2 weeks by a single author. The average values of the radiographic measurements were calculated. Intraobserver reliabilities of the radiographic measurements were assessed using intraclass correlation coefficients. Table 1 shows the intraclass correlation coefficients for each measurement. The degree of subluxation of the femoral head was classified according to the method reported by Crowe et al. [5].
Figure 1. The horizontal distance between the HJC and the tip of the teardrop, which is parallel to the inter-teardrop line, and the vertical distance between the HJC and the inter-teardrop line were measured. The femoral offset was defined as the perpendicular distance from the center line of the femur to the HJC.
Statistical analysis The Mann–Whitney U test was used to examine the significance of the differences in the HHS. The paired Student’s t-test was used to evaluate the differences in the isometric abductor muscle strength and strength ratio. One-way analysis of variance and Student–Newman–Keuls test were performed to compare each radiographic measurement. To evaluate the relationship between the HJC location and the FO, Pearson’s correlation coefficients were calculated to analyze the relationships between the ΔFO and Δhorizontal distance, Δhorizontal distance and Δvertical distance, and Δvertical distance and ΔFO. To evaluate the effect of the radiographic measurements and abductor muscle strength on the HHS, the Pearson’s correlation coefficient was used to examine the relationship between each of the radiographic measurements or the abductor muscle strength ratio and the HHS. To evaluate the effect of patients’ baseline characteristics on abductor muscle strength, Pearson’s correlation coefficient was used to examine the relationship between the abductor muscle strength ratio and the following factors: sex, age, BMI, subluxation of the femoral head according to Crowe’s classification, and surgical approach. Pearson’s correlation coefficient was also used to examine the relationship between abductor muscle strength ratio and surgical approach. Additionally, we performed simple regression analysis to evaluate the association between the abductor strength ratio and the
Table 1. The intraclass correlation coefficients for each radiographic measurement.
Intraclass correlation coefficient
Horizontal distance Unaffected Pre- Postside
Vertical distance Unaffected Pre- Postside
Femoral offset Pre-
Post-
Unaffected side
0.92
0.9
0.91
0.92
0.91
0.93
0.93
0.94
0.94
pre-: preoperative value of the affected side; post-: postoperative value of the affected side; unaffected: values of the unaffected side
Hip joint center location and femoral offset on abductor muscle 3
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DOI 10.3109/14397595.2014.988863
Figure 2. Isometric abductor muscle strength ratio before and at 1 year after THA. The abductor muscle strength ratio improved from 0.71 ⫾ 0.20 before surgery to 0.85 ⫾ 0.15 at 1 year postoperatively (PO).
radiographic measurements after THA. We used multiple regression analysis to identify independent determinants of the ratio of hip abductor muscle strength. The radiographic measurements were entered simultaneously. The adjusted coefficient of multiple determination (adjusted R2) was used to indicate how much variability in the ratio of hip abductor muscle strength was accounted for by the independent variables. Standardized regression coefficients (β) and associated p values were determined to assess statistical significance (p ⬍ 0.05). All statistical analyses were performed using SPSS, version 21.0 (IBM Corp., Armonk, NY, USA).
Results Clinical assessment The average HHS improved from 58 ⫾ 16 points before surgery to 93 ⫾ 6.7 points postoperatively (p ⬍ 0.01). The average preoperative HHS for pain, function, deformity, and motion improved from 21 ⫾ 9.2, 28 ⫾ 8.9, 3.8 ⫾ 0.5, and 4.2 ⫾ 1.1, respectively, to 42 ⫾ 2.4, 41 ⫾ 6.0, 3.9 ⫾ 0.2, and 4.8 ⫾ 0.5, respectively (p ⬍ 0.01, p ⬍ 0.01, p ⫽ 0.13, p ⬍ 0.01, respectively). In assessment at 1 year postoperatively, 12 patients had a limp; 11 were classified as slight and 1 as moderate. Twenty-six patients used a cane; 14 patients needed a cane for long walks, and 12 needed a cane most of the time. None of the patients needed to use a crutch. The mean HHS limp score of the patients who needed a cane for long walks, and the patients who needed a cane most of the time were 8.0 ⫾ 1.6 points and 7.0 ⫾ 1.5 points, respectively. The remaining 25 patients who did not need a cane had a limp score of 10 ⫾ 0.3 points. To evaluate the relationship between limp score and cane use, we performed one-way analysis of variance and Student–Newman– Keuls test. There was a significant difference between patients who
Figure 3. The results of radiographic measurements before and after THA. The horizontal distance of the HJC of the unaffected side was located laterally compared with the unaffected side (p ⬍ 0.01), and it moved medially after surgery compared with before surgery (p ⬍ 0.01). The vertical distance was superior before surgery (p ⬍ 0.01) compared with that on the unaffected side, and this superior position remained postoperatively. The femoral offset of the affected side was shorter than that of the unaffected side before surgery (p ⬍ 0.01) and was reconstructed to a similar degree as the unaffected side.
needed a cane for long walks and patients who did not need a cane (p ⬍ 0.01); there was also a significant difference between patients who needed a cane most of the time and patients who did not need a cane (p ⬍ 0.01). However, there was no significant difference between patients who needed a cane most of the time and patients who needed a cane for long walks. Isometric abductor muscle strength The mean abductor muscle strength ratio improved from 0.71 ⫾ 0.20 before surgery to 0.85 ⫾ 0.15 at 1 year postoperatively (p ⬍ 0.01) (Figure 2). The mean abductor muscle strength ratio in patients with a limp at 1 year after THA was 0.73 ⫾ 0.13, and the mean abductor muscle strength ratio in patients without a limp was 0.86 ⫾ 0.13 (p ⫽ 0.02). We performed simple regression analysis between abductor muscle strength ratio and pain score of HHS. Preoperatively, there was a weak correlation between the abductor muscle strength ratio and HHS pain score (r ⫽ 0.310, p ⫽ 0.020), such that more severe pain was associated with a lower abductor muscle strength. However, there was no significant relationship 1 year after THA (r ⫽ 0.132, p ⫽ 0.361). Radiographic assessment Of the 51 hips, 44 were classified as Crowe’s group I, 5 as group II, and 2 as group III; no cases were classified as group IV. Pearson’s
Table 2. Pearson’s correlation coefficients for the relationship between the changes in each of the radiographic measurements. Δhorizontal distance Δvertical distance ΔFO FO: femoral offset ** p ⬍ 0.01
Correlation coefficient p Correlation coefficient p Correlation coefficient p
Δhorizontal distance 1
Δvertical distance 0.264 0.057 1
ΔFO ⫺ 0.594** 0.001 ⫺ 0.156 0.275 1
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Table 3. Pearson’s correlation coefficients between the abductor muscle strength ratio before THA and the radiographic measurements.
Strength ratio Correlation coefficient before THA p
Horizontal distance 0.004 0.976
Vertical distance ⫺ 0.018
FO 0.180
0.899
0.220
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THA: total hip arthroplasty; FO: femoral offset
correlation coefficient analysis showed that there was no significant relationship between Crowe’s classification and abductor muscle strength ratio (r ⫽ ⫺ 0.23, p ⫽ 0.11). The results of measurements on the pelvic radiographs are shown in Figure 3. The horizontal distance of the HJC of the unaffected side was 34.7 ⫾ 5.4 mm and that of the affected side changed from 42.5 ⫾ 9.4 mm before surgery to 32.2 ⫾ 5.9 mm postoperatively. The vertical distance of the HJC of the unaffected side was 17.7 ⫾ 4.2 mm and that of the affected side changed from 25.4 ⫾ 7.9 mm before surgery to 26.0 ⫾ 7.1 mm postoperatively. Compared with the HJC on the unaffected side, the HJC on the affected side was superior before surgery (p ⬍ 0.01), and the superior placement persisted postoperatively. The FO of the unaffected side was 35.6 ⫾ 7.3 mm and that of the affected side changed from 27.4 ⫾ 10.3 mm before surgery to 36.3 ⫾ 7.6 mm postoperatively. The FO was well reconstructed, and no significant difference was observed between the unaffected and affected sides postoperatively. A negative coefficient was derived for the relationship between the Δhorizontal distance and the ΔFO (r -0.594, p ⬍ 0.01) (Table 2). There was no significant correlation between the radiographic measurements and the abductor muscle strength ratio before THA (Table 3). Table 4 shows Pearson’s correlation coefficient for the relationship between abductor muscle strength and baseline characteristic factors, and surgical approach. There was no significant correlation between the abductor muscle strength ratio and the baseline characteristic factors of patients. There was also no significant correlation between surgical approach and abductor muscle strength.
Furthermore, simple regression analysis showed that there was no significant relationship between femoral head size and the abductor muscle strength ratio (r ⫽ 0.021, p ⫽ 0.465). Table 5 shows the parameters that affected the HHS, according to Pearson’s correlation coefficient. There was a weak positive correlation between the abductor muscle strength ratio and the total HHS score (r ⫽ 0.390, p ⫽ 0.005), and we also found a positive correlation between the abductor muscle strength ratio and the HHS function score (r ⫽ 0.373, p ⫽ 0.008). There was a weak positive correlation between the FO and the total HHS score (r ⫽ 0.289, p ⫽ 0.048). Figure 4 shows the simple linear regression between the abductor muscle strength ratio and the radiographic measurements postoperatively. There was a negative correlation between the horizontal distance from the HJC to the tip of the teardrop and the strength ratio (r ⫽ ⫺0.359; p ⫽ 0.010) (Figure 4a), and there was a negative correlation between the vertical distance from the HJC to the tip of the teardrop and the strength ratio (r ⫽ ⫺ 0.374; p ⫽ 0.007) (Figure 4b). Additionally, there was a positive correlation between the FO and the strength ratio (r ⫽ 0.330; p ⫽ 0.018) (Figure 4c). Multiple regression analysis showed that the horizontal distance (β ⫽ ⫺ 0.314; p ⫽ 0.016), vertical distance (β ⫽ ⫺ 0.325, p ⫽ 0.014), and FO (β ⫽ 0.299, p ⫽ 0.021) were significantly associated with the abductor muscles strength ratio. The final model accounted for 28% (adjusted R2 ⫽ 0.28) (Table 6). Figure 5 shows an 80-year-old man who presented to our department with a history of coxalgia on his left hip which gradually deteriorated. Plain radiography of the hip showed an osteoarthritis, and the HJC was situated laterally and was superior compared with the unaffected HJC. The FO was slightly shortened compared with unaffected side (Figure 5a). The strength ratio of abductor muscle was 0.77. We performed THA on the patient with mini direct lateral approach using VerSys Fiber Metal Midcoats Stem and Trilogy Acetabular Cup (Zimmer International, Warsaw, IN, USA). Plain radiography showed that the horizontal distance moved medially and the FO was fully reconstructed compared with the unaffected side. Superior displacement remained; however, we attempted to compensate with that using a long prosthetic neck (Figure 5b). The strength ratio of abductor muscle after surgery was 0.98.
Table 4. Pearson’s correlation coefficient for the relationship between the abductor muscle strength ratio and patients’ demographics, pain score or surgical approach. Strength ratio before THA Strength ratio at 1 year after THA
Correlation coefficient p Correlation coefficient p
Sex (vs male) 0.048 0.740 ⫺ 0.002 0.991
Age 0.275 0.056 ⫺ 0.044
BMI 0.175 0.250 ⫺ 0.356
0.766
0.164
Pain 0.318* 0.023 0.132
Approach N.A.
0.361
0.104 0.473
pain: pain score of Harris Hip Score N.A.: not available * p ⬍ 0.01
Table 5. Pearson’s correlation coefficients between the HHS and the following factors: abductor muscle strength ratio at 1 year after THA, horizontal distance from the HJC to the tip of the teardrop, vertical distance from the HJC to the tip of the teardrop, and the FO. HHS Pain Function Deformity 0.132 0.162 Strength ratio at 1 year after THA Correlation coefficient 0.373** 0.390** p 0.005 0.361 0.008 0.262 Horizontal distance Correlation coefficient ⫺ 0.108 0.205 ⫺ 0.178 ⫺ 0.037 p 0.457 0.217 0.801 0.153 Vertical distance Correlation coefficient ⫺ 0.067 0.252 ⫺ 0.162 ⫺ 0.222 p 0.642 0.077 0.260 0.121 0.155 0.249 0.229 FO Correlation coefficient 0.289* p 0.042 0.281 0.082 0.110 HHS: Harris Hip Score; ROM: range of motion; THA: total hip arthroplasty; FO: femoral offset *p ⬍ 0.05; **p ⬍ 0.01.
ROM 0.006 0.967 ⫺ 0.192 0.181 ⫺ 0.078 0.593 ⫺ 0.004 0.978
Hip joint center location and femoral offset on abductor muscle 5
DOI 10.3109/14397595.2014.988863
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Figure 4. Simple linear regression between the abductor muscle strength ratio and the radiographic measurements. (a) A negative correlation of the strength ratio with the horizontal distance from the HJC to the tip of the teardrop is observed in 51 reconstructed hips (r ⫽ ⫺ 0.359; p ⫽ 0.010). (b) A negative correlation of the strength ratio with the vertical distance from the HJC to the tip of the teardrop is observed in 51 reconstructed hips (r ⫽ ⫺ 0.374; p ⫽ 0.007). (c) A positive correlation of the strength ratio with the FO is observed in 51 reconstructed hips (r ⫽ 0.330; p ⫽ 0.018).
Discussion The locations of the HJC and the FO following THA are crucial radiographic parameters that affect the joint reaction force and the abductor muscle strength by changing the moment arm of the muscle strength, and the joint reaction force affects the amount of polyethylene wear, which also affects the longevity of the prosthesis and the clinical result [6,7]. Several authors have reported that the FO and the position of the reconstructed hip center correlated with the strength of the hip abductors [8]; however, they did not mention the relationship between the FO and the HJC after THA. Therefore, we investigated the multiple effects of radiographic parameters, including the FO and the HJC location on abductor muscle strength after THA. In the present study, the postoperative HHS was generally satisfactory, and the strength ratio of the abductor muscle significantly improved from 0.73 preoperatively to 0.85 postoperatively. In this study, the pain score of HHS before THA affected the abductor muscle strength ratio. After THA, there was no significant correlation between pain score and abductor muscle strength, which meant that severe pain caused the incompetence of abductor muscle. Abductor muscle strength after THA showed a positive correlation with the function item of the HHS and also had a statistically significant effect on the clinical result. In this study, the Table 6. The results of multiple regression analysis for evaluating the association between the abductor muscle strength ratio and radiographic parameters.
Horizontal distance Vertical distance FO
Regression coefficient
Standardized regression coefficients (β)
⫺ 0.010 ⫺ 0.008 0.007
⫺ 0.314 ⫺ 0.325 0.299
VIF: variance inflation factor; FO: femoral offset.
p 0.016 0.012 0.021
VIF 1.012 1.001 1.013
mean abductor muscle strength ratio of patients with a limp at 1 year after THA was 0.73 ⫾ 0.13, and the mean abductor muscle strength ratio of patients without a limp was 0.86 ⫾ 0.13. Asayama et al. [9] reported that a strength ratio of ⬎ 0.88 cannot result in a positive Trendelenburg sign, which coincided with the results of this study. The HJC was located laterally and superiorly before surgery compared with the unaffected hip due to the loss of cartilage and dysplasia of the acetabulum, which can cause the femoral head to slide and migrate away from the normal hip center. There was no significant relationship between radiographic measurements and the abductor muscle strength ratio before THA, which may be due to other factors (e.g., contracture of the hip or coxalgia affected the abductor muscle strength). After surgery, the HJC was reconstructed medially in comparison to the unaffected side, but the superior placement remained. Most studies concurred that the true acetabulum is the optimal position for the infero-medial cup and results in the best outcome [10–12]. In our series, we attempted to recreate the original articulation using the navigation system; however, the anatomical reconstruction of the hip is not always easy in cases with developmental dysplasia of the hip or large bone defects. In such cases, we reluctantly reconstruct the hip center superiorly to obtain an adequate press fit without autogenous bone graft implantation and attempt to compensate for muscle tension or a leg length discrepancy with the neck length or the FO. Iglic et al. [13] reported that their mathematical model showed that a superior HJC decreases the strength of the hip abductor muscle strength. Delp and Maloney [14] also reported that a 2-cm superior replacement of the HJC decreases abduction force and moment arm; however, they [12] also reported that a superomedially positioned cup, along with restoration of the leg length via a long prosthetic neck, led to force-generating capacities of the abductors by ⬍ 10% and maintained 166% of moment, which is needed for standing on single leg. In the current study, there was 1 hip that showed superior replacement of the HJC by ⬎ 2 cm compared
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Figure 5. An 80-year-old man with left hip osteoarthritis. On radiography before surgery, the HJC was located laterally and superiorly compared with the unaffected HJC. (a) The FO was slightly shortened compared with the unaffected side, and the strength ratio of the abductor muscle was 0.77. (b) Radiography after THA showing that the HJC moved medially, and the FO was reconstructed compared with the unaffected side. Slight superior displacement of the HJC remained. The strength ratio of the abductor muscle postoperatively was 0.98.
with the unaffected side, and the patient had a limp after THA. Furthermore, 4 hips showed superior replacement of 1.5–2.0 cm, and 2 of these patients (50%) had a limp. In the remaining 46 hips with superior replacement of ⬍ 1.5 cm, 9 patients (20%) had a limp. These results suggested that the superior replacement caused limping after THA; however, other factors such as horizontal distance of the HJC or the FO may have some effect on the presence of limping. In the present study, the FO was reconstructed similarly to the unaffected side, and a significant negative correlation was observed between the changes in the horizontal distance of the HJC and that of the FO. This finding was the result of the compensation performed by surgeons using a high-offset stem, long prosthetic neck, or stems with a smaller neck–shaft angle. Thus, several factors may affect the abductor muscle strength and limping, which correlate with each other. Therefore, we investigated multiple relationships among the abductor muscle strength and radiographic measurements, while several other studies have only reported on the relationship between the abductor muscle strength and the radiographic measurements after THA [8,9,15–17]. In this study, multiple regression analysis revealed that the horizontal distance, vertical distance, and FO were associated with abductor muscle strength to the same degree, and the smaller the horizontal distance was, the better the abductor muscle strength. The statistical analysis also revealed that the larger the FO was, the better the abductor muscle strength was after THA. The FO before surgery was shorter than that of the unaffected side and was similarly reconstructed to the unaffected side. Furthermore, there was a strong negative correlation between the ΔHD and the ΔFO, meaning that the sum of the HD and FO after surgery was equal to the sum of the HD and FO of the unaffected side. These findings indicate that among patients with the same sum of HD and FO, patients with a smaller HD and larger FO were at a greater advantage with regard to abductor muscle strength. The current study has several limitations. First, 5 different types of stems were used in the patients included. Ideally, a uniform stem type should be used in this type of study. We chose the type of femoral prosthesis according to the patients’ geometry, and the size and bone quality of the femur. For example, cemented stems were used for patients with an osteoporotic femur, while the type of cementless stem used was determined according to the patient’s geometry and the size of the femur. One-way analysis of variance and Student–Newman–Keuls test were used to examine the effect of stem type on femoral offset, and revealed that there was no significant difference between stem type and femoral offset. Therefore, we believe that the effect of stem type on the femoral offset was weak. Second, the patient’s subcutaneous fat volume and abductor muscle volume were not accounted for in our analysis of abductor muscle strength. It is plausible that failure to account for these
factors might explain the relatively small adjusted R2 of multiple regression; the residual 72% might be accounted for by abductor muscle volume or other factors.
Conclusion The average postoperative HHS and strength ratio of the abductor muscle were generally satisfactory, and the FO and abductor muscle strength ratio influenced the clinical results. The HJC was reconstructed medially and superiorly, and the change in the FO after THA was influenced by the change in the horizontal distance of the HJC. Multiple regression analysis revealed that the medial and inferior HJC and increase in the FO was an effective procedure for restoring abductor strength.
Acknowledgments We wish to thank Dr. Yohei Yukizawa, Dr.Choe Hyonmin, Dr. Hiroshi Fujimaki, Dr. Yasuhide Hirata, and Dr. Masaki Kawamura for their valuable contribution to the current study.
Conflict of interest The authors declare no conflicts of interest. The authors alone are responsible for the content and writing of the paper.
References 1. Jerosch J, Steinbeck J, Stechmann J, Guth V. Influence of a high hip center on abductor muscle function. Arch Orthop Trauma Surg. 1997;116(6–7):385–9. 2. Bourne RB, Rorabeck CH. Soft tissue balancing: the hip. J Arthroplasty. 2002;17(4 Suppl 1):17–22. 3. Ehrig RM, Taylor WR, Duda GN, Heller MO. A survey of formal methods for determining the centre of rotation of ball joints. J Biomech. 2006;39(15):2798–809. 4. Harris WH. Traumatic arthritis of the hip after dislocation and acetabular fractures: treatment by mold arthroplasty. An end-result study using a new method of result evaluation. J Bone Joint Surg Am. 1969;51(4):737–55. 5. Crowe JF, Mani VJ, Ranawat CS. Total hip replacement in congenital dislocation and dysplasia of the hip. J Bone Joint Surg Am. 1979; 61(1):15–23. 6. Cereatti A, Donati M, Camomilla V, Margheritini F, Cappozzo A. Hip joint centre location: an ex vivo study. J Biomech. 2009;42(7): 818–23. 7. Erceg M. The influence of femoral head shift on hip biomechanics: additional parameters accounted. Int Orthop. 2009;33(1):95–100. 8. McGrory BJ, Morrey BF, Cahalan TD, An KN, Cabanela ME. Effect of femoral offset on range of motion and abductor muscle strength after total hip arthroplasty. J Bone Joint Surg Br. 1995;77(6): 865–9. 9. Asayama I, Chamnongkich S, Simpson KJ, Kinsey TL, Mahoney OM. Reconstructed hip joint position and abductor muscle strength after total hip arthroplasty. J Arthroplasty. 2005;20(4):414–20.
DOI 10.3109/14397595.2014.988863
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10. Linde F, Jensen J. Socket loosening in arthroplasty for congenital dislocation of the hip. Acta Orthop Scand. 1988;59(3):254–7. 11. 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(4):587–92. 12. Delp SL, Wixson RL, Komattu AV, Kocmond JH. How superior placement of the joint center in hip arthroplasty affects the abductor muscles. Clin Orthop Relat Res. 1996;(328):137–46. 13. Iglic A, Antolic V, Srakar F. Biomechanical analysis of various operative hip joint rotation center shifts. Arch Orthop Trauma Surg. 1993;112(3):124–6.
Hip joint center location and femoral offset on abductor muscle 7 14. Delp SL, Maloney W. Effects of hip center location on the momentgenerating capacity of the muscles. J Biomech. 1993;26(4–5):485–99. 15. Kamada S, Naito M, Nakamura Y, Kiyama T. Hip abductor muscle strength after total hip arthroplasty with short stems. Arch Orthop Trauma Surg 2011;131(12):1723–9. 16. Kiyama T, Naito M, Shitama H, Maeyama A. Effect of superior placement of the hip center on abductor muscle strength in total hip arthroplasty. J Arthroplasty. 2009;24(2):240–5. 17. Dastane M, Dorr LD, Tarwala R, Wan Z. Hip offset in total hip arthroplasty: quantitative measurement with navigation. Clin Orthop Relat Res 2011;469(2):429–36.
