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Musculoskeletal Imaging |
1 From the Departments of Radiology (C.W.A.P., J.H., M.Z.) and Orthopedic Surgery (H.P.N., C.D.), University Hospital Balgrist, Forchstrasse 340, CH-8008 Zurich, Switzerland. Received March 1, 2004; revision requested May 11; revision received July 18; accepted August 18. Address correspondence to C.W.A.P. (e-mail: christian@pfirrmann.ch).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS DISCUSSIONREFERENCES |
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MATERIALS AND METHODS: The institutional review board approved the study, and all patients provided informed consent. Two musculoskeletal radiologists blinded to clinical information analyzed triplanar MR images of the greater trochanter obtained in 25 patients without and 39 patients with trochanteric pain and abductor weakness after THA. Tendon defects, diameter, signal intensity, and ossification; fatty atrophy; and bursal fluid collections were assessed. In 14 symptomatic patients, MR imaging and surgical findings were correlated. Differences in the frequencies of findings between the two groups were tested for significance by using 
2 analysis.
RESULTS: Tendon defects were uncommon in asymptomatic patients and significantly more frequent in symptomatic patients: Two asymptomatic versus 22 symptomatic patients had gluteus minimus defects (P < .001); four asymptomatic versus 24 symptomatic patients, lateral gluteus medius defects (P < .001); and no asymptomatic versus seven symptomatic patients, posterior gluteus medius defects (P = .025). In both patient groups, tendon signal intensity changes were frequent, with the exception of those in the posterior gluteus medius tendon, which demonstrated these changes more frequently in symptomatic patients (in 23 vs five asymptomatic patients, P = .002). Tendon diameter changes were frequent in both groups but significantly (P = .001 to P = .009) more frequent in symptomatic patients (all tendon parts). Fatty atrophy was evident in the anterior two-thirds of the gluteus minimus muscle in both groups, without significant differences. In the posterosuperior third of the gluteus minimus muscle, however, differences in fatty atrophy between the two groups were significant (P = .026). Fatty atrophy of the gluteus medius muscle was present in symptomatic patients only, with significant differences among all muscle parts. Bursal fluid collections were more frequent in symptomatic patients (n = 24) than in asymptomatic patients (n = 8, P = .021). The MR imaging–based diagnosis was confirmed in all 14 patients who underwent revision surgery.
CONCLUSION: Abductor tendon defects and fatty atrophy of the gluteus medius muscle and the posterior part of the gluteus minimus muscle are uncommon in asymptomatic patients after THA.
© RSNA, 2005
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS DISCUSSIONREFERENCES |
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Magnetic resonance (MR) imaging is the modality of choice for imaging soft-tissue abnormalities of the musculoskeletal system. MR imaging was initially considered to be of limited value owing to the presence of artifacts caused by the metallic implants. Later, optimized sequences began to be used to image soft-tissue abnormalities in the presence of metallic implants (8–12). More recently, MR imaging has been shown to be a valuable diagnostic tool in patients who have undergone total hip replacement (10,13). However, these findings are based on the experiences in studies involving only a few symptomatic patients (13). To our knowledge, the normal MR imaging appearance of the greater trochanter after hip replacement in asymptomatic patients has not been described.
Thus, the purpose of our study was to prospectively evaluate MR imaging findings of the abductor tendons and muscles in asymptomatic and symptomatic patients after THA performed with a lateral transgluteal approach.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS DISCUSSIONREFERENCES |
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Asymptomatic Patients
Between November 2002 and May 2003, 25 consecutive asymptomatic patients (14 men aged 39–81 years [mean, 60.4 years], 11 women aged 33–88 years [mean, 60.2 years]) who were asymptomatic after THA were prospectively included in the study protocol. All of these patients had undergone surgery because of osteoarthritis of the hip. All asymptomatic patients were clinically examined by one of two experienced hip surgeons (C.D. or H.P.N., with 12 and 18 years experience, respectively) during the routinely scheduled 1-year follow-up examination. Only patients fulfilling the following criteria were included: (a) It was 1 year since they had undergone standardized lateral transgluteal THA (14); (b) they had no hip pain at rest, during physical activity, or with local palpation of the greater trochanter; (d) they were capable of normal abduction force on the hip side on which surgery was performed compared with their abduction force capability on the contralateral side; and (e) they were not limping.
Symptomatic Patients
Thirty-nine consecutive symptomatic patients (19 men aged 30–80 years [mean, 62.7 years], 20 women aged 39–79 years [mean, 64.3 years]) were prospectively included in the study during a period of 28 months—January 2001 to May 2003. All 39 patients were referred by the same two surgeons (H.P.N., C.D.) from our outpatient hip clinic for MR imaging of the greater trochanter for the evaluation of trochanteric pain, limping, and/or abduction weakness after total hip replacement. Thirty-four (87%) of these 39 patients presented with trochanter pain, and 36 (92%) presented with abductor weakness. Total hip replacement had been performed because of osteoarthritis of the hip in all 39 patients.
MR Imaging
MR imaging was performed in the asymptomatic and symptomatic patient groups by using a 1.5-T system (Symphony; Siemens Medical Solutions, Erlangen, Germany) according to a standardized protocol. A flexible wraparound receive-only surface coil was used. Coronal T2-weighted fast spin-echo (3910/75 [repetition time msec/echo time msec], echo train length of nine, 4-mm section thickness, 220 x 100-mm field of view, 512 x 256 matrix), sagittal T1-weighted spin-echo (707/23, 6-mm section thickness, 180 x 80-mm field of view, 512 x 256 matrix), transverse short inversion time inversion-recovery (5550/34/150 [repetition time msec/echo time msec/inversion time msec], 4-mm section thickness, 180 x 100-mm field of view, 512 x 256 matrix), and transverse T1-weighted spin-echo (669/18, 3-mm section thickness, 140 x 80-mm field of view, 512 x 256 matrix) MR images were obtained. The frequency-encoding gradient was always parallel to the long axis of the prosthesis (craniocaudal direction). Only postsurgical hips were imaged.
Image Analysis
All MR images were independently analyzed by two experienced musculoskeletal radiologists (C.W.A.P. and M.Z., with 6 and 10 years experience, respectively). The asymptomatic and symptomatic patient images were reviewed randomly. The readers were blinded to the clinical symptoms and as to whether the patients were in the asymptomatic or symptomatic group. After the independent readings, interobserver differences were resolved by consensus.
The gluteus minimus tendon (ie, anterior part of the abductor apparatus), the lateral part of the gluteus medius tendon, and the posterior part of the gluteus medius tendon were analyzed separately. Tendon tears or detachments were defined as hyperintense signals extending to both surfaces of the tendon on MR images obtained with fluid-sensitive (ie, T2-weighted and short inversion time inversion-recovery) sequences. Osseous detachment of the tendon was recorded. The tendon diameter was qualitatively rated as normal, thinned, or thickened with respect to the radiologist’s knowledge of normal tendon diameter. Tendon signal intensity was graded on the T1-weighted MR images as normal (hypointense) or abnormal (increased signal intensity) compared with the signal intensity of a normal tendon.
Bursal fluid collections were rated as absent or present at fluid-sensitive MR imaging. The extent of the bursal fluid collection was measured electronically in all three dimensions by using a picture archiving and communication system workstation. The volume (V) of the bursal fluid was calculated by using the formula for elliptical volume: V = 0.52 x H x W x D, where H is the height, W is the width, and D is the depth. Ossifications within the abductor tendon were documented to be present or absent. An area of ossification within the abductor mechanism was diagnosed only when it was depicted (with all sequences) as having a hypointense cortical rim with a centrally located area of fat signal intensity representing bone marrow.
Fatty atrophy of the abductor muscles was assessed on the transverse T1-weighted MR images by using the following grading system: Grade 0 indicated that no intramuscular fat was present; grade 1, that some fat streaks were present; grade 2, that fat was evident, but there was less fat than muscle tissue; grade 3, that there were equal amounts of fat and muscle tissue; and grade 4, that there was more fat than muscle tissue. This grading system corresponds to a classification system that is commonly used to categorize the rotator cuff muscles (15). Fatty atrophy was assessed on transverse T1-weighted MR images at two different levels: at one-third and at two-thirds of the distance between the iliac crest and the tip of the greater trochanter. At each of these levels, the anterior, middle, and posterior portions of the gluteus medius and gluteus minimus muscles were evaluated separately.
On a sagittal T1-weighted MR image obtained through the most lateral part of the greater trochanter, the muscle tissue of the gluteus medius muscle reached the bone outline of the greater trochanter like a fan (Fig 1). We termed this finding the fan sign and considered it to be positive in the presence of a defect in the muscle tissue, which, when present, was usually at the surgical site.
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Statistical Analyses
Differences in the frequencies of findings between the two patient groups were tested for significance by using 2 analysis. The Mann-Whitney U test was used to assess significant differences in fat content among the abductor muscles in both groups.
Statistics were calculated for interobserver agreement. According to the method of Landis and Koch (16), agreement was rated as follows: Values of 0–0.20 indicated slight agreement; values of 0.21–0.40, fair agreement; values of 0.41–0.60, moderate agreement; values of 0.61–0.80, substantial agreement; and values of 0.81–0.99, excellent agreement. A value of 1.00 indicated absolute agreement.
A computer software package (SPSS, version 10.0.7; SPSS, Chicago, Ill) was used to perform all statistical calculations. P < .05 was considered to indicate a significant difference.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS DISCUSSIONREFERENCES |
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2 analysis). No significant differences in side were seen: In the asymptomatic group, 13 hips on the right side and 12 hips on the left side were assessed. In the symptomatic group, 21 hips on the right side and 18 hips on the left side were assessed (P = .129, 2 analysis). Age distribution was not significantly different between the two groups: Mean ages for the asymptomatic and symptomatic groups were 60.3 and 63.5 years, respectively (P = .353, Mann-Whitney U test).
MR Imaging
The frequency of MR imaging findings of the abductor tendons in the asymptomatic and symptomatic patients are summarized in Table 1. Tendon defects were uncommon in asymptomatic patients and significantly more frequent in symptomatic patients: Gluteus minimus defects were identified in two (8%) of the 25 asymptomatic patients versus 22 (56%) of the 39 symptomatic patients (P < .001). Lateral gluteus medius tendon defects were identified in 16% (four of 25) of asymptomatic patients versus 62% (24 of 39) of symptomatic patients (P < .001) (Fig 2). Posterior gluteus medius tendon defects were identified in no asymptomatic patients versus 18% (seven of 39) of symptomatic patients (P = .025). In one case, the gluteus minimus tendon showed a osseous detachment from the greater trochanter (Fig 3).
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, 0.577–0.862). Agreement was lower for assessment of tendon diameter (, 0.474–0.559) and signal intensity changes (, 0.341–0.581).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS DISCUSSIONREFERENCES |
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A wide spectrum of abnormalities affecting the soft tissues of the hip has been noted in patients with trochanteric pain. Besides the frequently encountered trochanteric bursitis, there may also be so-called rotator cuff tears of the hip (18,19). Bunker and co-workers (19) described the typical appearance of this tear as a circular or oval gluteus minimus tendon defect that extends posteriorly into the lateral part of the gluteus medius tendon. MR imaging has been reported to be useful for the diagnosis of either tendinosis or tendon tears of the abductors (20,21). So far, little is known about MR imaging changes in the abductor tendons after total hip replacement (13).
The three basic surgical approaches used most commonly for THA are the lateral, posterior, and transtrochanteric methods (22). Complications related to each of these approaches have been reported and include joint dislocation, heterotopic ossification, neurovascular damage, postoperative limp, implant malalignment, and trochanteric nonunion (with the transtrochanteric approach) (3,4,7). The lateral approach necessitates splitting the middle of the gluteus medius muscle and releasing its anterior tendinous portion from the greater trochanter and has been reported to enable ease of access into the hip joint, optimal joint visualization, protection of the neurovascular structures of the hip, and achievement of predictable results in terms of postoperative hip function (14,23,24).
However, in a study to assess the postoperative integrity of the gluteus medius tendon after 97 consecutive lateral THAs, metal markers were placed in the gluteal aponeurosis—one on each side of the suture line (4). Radiographs were obtained immediately, 2 weeks, 2 months, and 1 year after surgery. A division between the markers developed in about half of the patients, but gross divisions were rare. Trendelenburg gait was significantly increased in only those patients who had divisions larger than 2.5 cm; this finding indicates that a moderate gluteal elongation may be readily compensated for (4). In our series, 8% of the asymptomatic patients had a defect in the gluteus minimus tendon and 16% of these patients had a defect in the gluteus medius tendon. None of these patients were limping at presentation. The frequency of tendon defects was significantly higher in the symptomatic patients: 56% had gluteus minimus tendon defects, and 62% had gluteus medius tendon defects.
Fatty atrophy of the gluteus minimus muscle seems to be very frequent after THA, even in asymptomatic patients. In contrast, fatty atrophy of the gluteus medius muscle was seen almost exclusively in the symptomatic patients. Fatty atrophy of the muscle is an important predictor of the success of a tendon reconstruction. It is known from experiences with shoulder joints that once fatty degeneration of the muscle tissue has developed owing to a tendon tear, fatty atrophy rarely resolves after tendon reconstruction (25). Muscle atrophy is an important differential diagnosis relative to simple tendon tear in patients with limping, and, therefore, careful attention should be paid to muscle atrophy.
Another cause of Trendelenburg gait or limping is damage to the superior gluteal nerve after lateral-approach hip surgery. In a prospective study involving 81 consecutive patients who underwent lateral-approach surgery, the abductor muscles of the hip were assessed electrophysiologically and clinically. In nine patients, complete denervation occurred. Persistent damage to the nerve was associated with a positive Trendelenburg test result (26).
The results of repeat surgery for repair of abductor tendon tears that occurred as a complication of primary THA seem to be promising. In a study (27) to analyze the results obtained for nine patients after they underwent revision of the abductor tendons, limping was markedly decreased in five of these patients. The need for ambulatory aids also was reduced in five of these patients. However, with regard to pain reduction, the procedure was less successful; this result suggests that the best indications for repair are symptoms of marked abductor weakness. It seems that substantial preoperative pain is less likely to be decreased (27).
Fluid collections around the greater trochanter were frequently found in both the asymptomatic and the symptomatic patients in this study. Fluid collections around other joints after surgery seem to be a common finding in asymptomatic patients also and should not be misinterpreted as a sign of bursitis (28). However, in our series, fluid collections larger than 4 mL were seen only in the symptomatic patients.
Traditionally, MR imaging has had a very limited role in the examination of patients after arthroplasty, primarily because of susceptibility artifacts related to the metallic implants. Imaging evaluation of the painful hip after arthroplasty typically has been limited to conventional radiography, arthrography, and bone scintigraphy; however, the findings of such examinations are often focused on the bones and the orthopedic hardware. More recently, ultrasonography has been shown to be promising for evaluating the soft tissue around the greater trochanter (29). Modifications of conventional MR imaging sequences can be used to reduce the artifacts generated by implants, including total knee arthroplasty (11), THA (10), and shoulder arthroplasty (12) prostheses. Optimized image quality can be achieved with spin-echo MR imaging by using a high bandwidth (at least 130 Hz per pixel), a high-spatial-resolution matrix (512 x 512), sequences with multiple refocusing pulses, and a frequency-encoding axis parallel to the long axis of the prosthesis. The degree of distortion is reduced by using this optimized technique (30). Imaging of the greater trochanter and the abductors with acceptable image quality in the setting of total hip replacement was possible in all patients in the symptomatic and asymptomatic groups. We believe that MR imaging can yield important information for the differential diagnosis of trochanteric pain and limping in patients after hip arthroplasty (13).
There were limitations to our study that should be considered. No preoperative MR imaging of the hip was performed in our study; therefore, some of the findings may have been present before surgery. For example, fatty atrophy of the abductor muscles may have been present before surgery. All surgical revisions were based on clinical and MR imaging findings. Only those patients who received a positive diagnosis of abductor tendon tear at MR imaging underwent surgery. Therefore, the number of false-negative cases could not be assessed.
It is important to recognize that although many MR imaging findings such as altered signal intensity and abductor tendon diameter, bursal fluid collections, and fatty atrophy of the anterior gluteus minimus muscle are more frequent in symptomatic patients, they are also frequently found in asymptomatic patients after lateral transgluteal THA. However, defects of the abductor tendons and fatty atrophy of the gluteus medius muscle and the posterior part of the gluteus minimus muscle are uncommon in asymptomatic patients after THA and therefore appear to be clinically relevant.
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FOOTNOTES |
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Authors stated no financial relationship to disclose.
Author contributions: Guarantors of integrity of entire study, C.W.A.P., M.Z.; study concepts, C.W.A.P., H.P.N., J.H.; study design, C.W.A.P., C.D., M.Z.; literature research, C.W.A.P.; clinical studies, H.P.N., C.D.; data acquisition, C.D., H.P.N.; data analysis/interpretation, all authors; statistical analysis, C.W.A.P., J.H.; manuscript preparation, C.W.A.P., J.H., M.Z.; manuscript definition of intellectual content, H.P.N., C.D., C.W.A.P.; manuscript editing, M.Z.; manuscript revision/review, J.H., M.Z., C.D.; manuscript final version approval, all authors
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS DISCUSSIONREFERENCES |
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