Abstract
Inaccuracy of ischiofemoral space (IFS) measurement may result in radiographic misdiagnosis of ischiofemoral impingement, as well as insufficient or excessive osseous resection when surgery is indicated. This study compared the IFS measured in magnetic resonance imaging (MRI) performed in distinct health services for the same patient. Sixty-five patients (95 hips) who had hip MRI performed at an outside institution (noncontrolled MRI) followed by a hip MRI with lower extremity positioning reproducing the standing position (controlled MRI) were studied. For each hip, the IFS measured in the noncontrolled MRI was compared to the IFS measured in the controlled MRI. The categorization of a hip as presenting decreased IFS (≤17 mm) or normal IFS (>17 mm) changed in 19% of the hips when comparing the noncontrolled MRI to the controlled MRI. From the 32 hips (34%) with a difference ≥4 mm in the IFS, the predominant positioning change was hip flexion/extension in 47%, hip rotation in 44%, and hip abduction/adduction in 9%. In conclusion, a difference >4 mm in the IFS was observed in 1 out of every 3 hips when comparing noncontrolled MRI with controlled MRI reproducing the lower limb positioning in the standing position.
Keywords: Hip impingement, hip magnetic resonance imaging, ischiofemoral impingement, lesser trochanter ischial impingement, quadratus femoris muscle
Impingement between the lesser trochanter and the ischium is a cause of hip pain and limitation in hip extension.1–5 The ischiofemoral space (IFS) is the distance between the lesser trochanter and the ischium, with values ≤17 mm traditionally indicating ischiofemoral impingement.6 Variation in hip flexion/extension, abduction/adduction, and internal/external rotation during the acquisition of imaging studies is a potential source of variation in IFS.7 Inaccuracy of the IFS measurement may result in radiographic misdiagnosis of ischiofemoral impingement, as well as insufficient or excessive osseous resection in patients surgically treated. This study compared the IFS measured in magnetic resonance imaging (MRI) studies performed in distinct health services for the same patient. The hypothesis is that variability in the IFS is a frequent finding.
METHODS
This study was carried out in a tertiary care academic orthopedic facility and was approved by the institutional review board. A retrospective review was performed in 341 consecutive patients who had a first office visit between October 2018 and September 2019. These individuals were screened for the following inclusion criteria: availability of hip MRI performed in another institution prior to the first visit (noncontrolled MRI) and hip MRI performed in our institution following the first office visit (controlled MRI). The exclusion criteria were total hip arthroplasty, hip osteotomy performed in the period between the noncontrolled MRI and controlled MRI, divergence of 2 mm or more on the calibration of the measurement tool between the noncontrolled MRI and controlled MRI, and absence of axial cuts at the lesser trochanter level in the noncontrolled MRI (Figure 1). The controlled MRIs were performed reproducing the lower limb positioning observed in a standing position (Figure 2).
Figure 1.
Diagram of patient selection.
Figure 2.
(a), (b) Patient positioning prior to MRI. The distances between the right and left knees, ankles, and great toes are measured in a standing position. The foot progression angle is also observed. (c) Without any pillow behind the knees, the patient is positioned supine with reproduction of the foot progression angle and the distances between right and left knees, ankles, and great toes measured in the standing position. The positioning is maintained with a spacer of proper size associated with taping.
A fellowship-trained hip surgeon screened the images and performed the imaging measurements. To rule out differences in calibration of the MRI between the noncontrolled and controlled MRIs, the diameter of the femoral head was concomitantly measured at the same level images of the functional and noncontrolled MRI. Divergence >2 mm on the ruler measurement tool between the noncontrolled MRI and controlled MRIs was considered an exclusion criterion. The mean difference between the femoral head diameter measured in the noncontrolled MRI and the diameter in the controlled MRI was 0.6 mm (95% confidence interval, 0.5 to 0.7 mm).
The axial MRI image with the smallest distance between the lesser trochanter and ischium was selected in the noncontrolled MRI.6 In sequence, the controlled MRI was assessed for the same axial cut level based on the format of the ischium, inferior pubic ramus, lesser trochanter, and iliopsoas tendon. The axial images of the noncontrolled and controlled MRI were placed simultaneously in the same screen for the measurements, utilizing the same reference points for the measurement (Figure 3). The observer was blinded to the IFS measured in the noncontrolled MRI while measuring the IFS in the controlled MRI and vice versa. The IFS was measured as the smallest distance between the lateral cortex of the ischial tuberosity and medial cortex of the lesser trochanter.6 For each hip, the IFS measured in the noncontrolled MRI was compared to the IFS measured in the controlled MRI.
Figure 3.
Axial MRI of the right hip demonstrating the comparison of the ischiofemoral space measured in an outside institution MRI (19 mm) with the institution MRI (10 mm).
In hips with a difference ≥4 mm in the IFS comparing the noncontrolled MRI and controlled MRI, the change in hip positioning was assessed by measuring the anteriorization/posteriorization and lateralization/medialization, as follows: 1) the axial images of the noncontrolled and controlled MRI were placed simultaneously in the same screen and 2) the anteriorization/posteriorization and lateralization/medialization of the hip were determined utilizing the center of the femoral canal and ischium tuberosity as reference points for both the noncontrolled MRI and controlled MRI (Figure 4a, 4b). Calculation using sine and cosine trigonometrical functions was performed to determine the contribution of hip internal rotation to lateralization and anteriorization of the femoral canal and the contribution of hip external rotation to medialization and posteriorization of the femoral canal (Figure 4c, 4d). Flexion, extension, abduction, adduction, and internal or external rotation were considered to contribute to the change in the IFS when they anteriorized or posteriorized the femoral canal >2 mm (Table 1). To determine the predominant cause of change on the IFS, the amount and direction of change provoked by hip rotation was subtracted or added to the changes related to abduction/adduction or flexion/extension.
Figure 4.
Determination of the hip positioning utilizing axial MRI of the right hip. (a) The red line represents the lateralization of the femur and the blue line represents the anteriorization. The yellow line is the reference line linking the right and left ischial tuberosities. The center of the femoral canal is found (circle). The red line is parallel to the reference line and runs from the center of the femoral canal to the level of anterolateral corner of the ischial tuberosity, where it meets the blue line. (b) MRI of the same hip with controlled positioning of the lower limbs demonstrating decreased anteriorization and lateralization, with consequent approximation between the lesser trochanter and ischial tuberosity. (c, d) The contribution of the hip rotation to the anteriorization and lateralization of the femur was calculated utilizing the trigonometrical functions of sine and cosine for the angle between the green and white lines. The white line runs from the projected center of the femoral head (arrow) to the center of the femoral canal. The green line and black lines are the catheti, and the white line is the hypotenuse of a right triangle. The difference between the length of the green line in (c) and the length of the green line in (d) represents the contribution of the hip rotation to the change in lateralization. The difference between the length of the black line in (c) and the length of the black line in (d) represents the contribution of the hip rotation to the change in anteriorization.
Table 1.
Change in femoral positioning and ischiofemoral space according to hip motion
| Hip motion | Femoral position change | Effect on ischiofemoral space |
|---|---|---|
| Flexion | Anteriorization | Increase |
| Extension | Posteriorization | Decrease |
| Abduction | Lateralization | Increase |
| Adduction | Medialization | Decrease |
| Internal rotation | Anteriorization and lateralization | Increase |
| External rotation | Posteriorization and medialization | Decrease |
Statistical analysis was performed with SPSS software. The normality of data distribution was determined utilizing the Kolmogorov-Smirnov test. Paired t tests were utilized to establish the significance of any noted differences, and P values <0.05 were considered significant. Pearson’s coefficients were calculated to examine correlations between variables.
The IFS measurements were assessed for intra- and interrater reliability in the controlled MRI of 20 hips randomly chosen from the 95 included hips. The intrarater reliability was determined based on a second measurement performed by the main investigator at least 10 days after the original measurement. The interrater reliability was determined by comparing the measurement of the main investigator to the measurements of a senior orthopedic surgery resident. Precision of measurement by a single observer (intrarater reliability) and between observers (interrater reliability) was determined by calculating the 95% confidence interval between the repeated measurements and their average. The intra- and interrater intraclass correlation coefficients for IFS were 0.966 and 0.946, respectively.
RESULTS
Sixty-five consecutive patients (95 hips) were studied, including 52 women and 13 men with an average age of 46 years (range, 16–78). Considering all 95 hips, the mean ± SD IFS was 22 ± 7 mm in the noncontrolled MRI and 21 ± 6 mm in the controlled MRI. A difference ≥4 mm was observed in 32 of 95 hips (34%) comparing the IFS of noncontrolled MRI with the IFS of controlled MRI (Table 2). Of the 32 hips with a difference ≥4 mm, 11 hips had increased IFS from the noncontrolled MRI to the controlled MRI, and 21 hips had decreased IFS from the noncontrolled MRI to the controlled MRI. The categorization of a hip as presenting decreased (≤17 mm) or normal (>17 mm) IFS changed in 18 hips (19%) when comparing the noncontrolled MRI to the controlled MRI. From the 19 hips with decreased IFS in noncontrolled MRI, 26% had normal IFS in the controlled MRI. From the 76 hips with normal IFS in noncontrolled MRI, 17% had decreased IFS in the controlled MRI (Figure 5).
Table 2.
Stratification of the difference in ischiofemoral space comparing an outside MRI with institutional MRI
| Difference in IFS (mm) | Number of hips (% of total) |
|---|---|
| 0 to 1.9 | 40 (42%) |
| 2 to 3.9 | 23 (24%) |
| 4 to 5.9 | 16 (17%) |
| ≥6 | 16 (17%) |
Figure 5.
Distribution of the hips in decreased ischiofemoral space (IFS) and normal IFS comparing outside institution MRI (left) to institution MRI (right).
From the 32 hips with a difference ≥4 mm in the IFS comparing the noncontrolled MRI and controlled MRI, the predominant positioning change was hip flexion/extension in 47%, hip rotation in 44%, and hip abduction/adduction in 9%. A combination of positioning changes in two or three planes of motion was observed in 53% of the hips. No correlation was observed between the body mass index and the difference in the IFS from the controlled MRI to the noncontrolled MRI (r = −0.097, P = 0.37).
DISCUSSION
The most important finding from this study is that a difference >4 mm in the IFS was observed in 1 out of every 3 hips when comparing MRI studies from distinct institutions. Change on the imaging diagnosis was observed in 1.9 out of every 10 patients when only considering the IFS and comparing controlled MRI with MRI performed in other institutions without controlled positioning of the lower limbs.
Variation in the degree of hip flexion was the most common cause of change in the IFS. Excessive hip flexion, leading to increased IFS, often results from placement of a pillow under the knees to provide comfort to the patient during the acquisition of MRI. The practice of holding the knees together during the MRI acquisition can also lead to adduction of the lower limbs and decrease the IFS. Femoral torsion also influences the IFS considering individual hips, and decreased IFS is observed in association with increased femoral antetorsion (Figure 6).8,9 In hips with increased femoral antetorsion, internal rotation improves the femoroacetabular coupling and also increases the functional offset by bringing the lesser trochanter laterally.
Figure 6.
Variation in the ischiofemoral space provoked by rotation in a patient with increased femoral antetorsion. (a) MRI without control of the lower limb positioning. (b) Axial pelvis and knee cuts demonstrate increased femoral torsion. (c) In the positioning reproducing the standing position, the lower limb has an attitude in internal rotation to improve the femoroacetabular coupling.
The positioning of the patient during the imaging acquisition is variable among authors describing the normal IFS in different populations.6,8,10,11 Considering that the interpretation of the IFS assessed in hip MRI by clinicians is dependent on the lower limb positioning during MRI acquisition, including this information on the exam report would be helpful in the diagnosis and surgical planning for ischiofemoral impingement. In addition to decreased IFS, the physical examination and additional imaging findings are essential to determine whether the impingement between the lesser trochanter and ischium has clinical repercussion. The ischiofemoral impingement test, long-stride walking test, and hip-spine extension test are included in the physical examination to diagnose ischiofemoral impingement.1,4,12 Helpful findings to diagnose ischiofemoral impingement in standing radiographs include the presence of asymmetric IFS and sclerosis and cystic changes at the ischial tuberosity. The presence of signal changes at the quadratus femoris muscle and adjacent hamstring tendons is another finding supporting the diagnosis of ischiofemoral impingement (Figure 7).6,13
Figure 7.
Secondary changes in the hamstring origin caused by ischiofemoral impingement. (a) Axial MRI demonstrating ischiofemoral impingement of the right hip leading to deformation of the hamstring’s tendon origin (red dashed line) in adaptation to the conflict with the lesser trochanter (yellow dashed line). (b) MRI of the left hip of another patient demonstrating a cyst at the hamstring origin (yellow arrow) and fatty infiltration over the quadratus femoris muscle (red arrow) caused by ischiofemoral impingement. (c) Fatty infiltration at the quadratus femoris muscle (yellow arrow). (d) Quadratus femoris muscle hyperintensity (yellow arrow) and hamstring origin avulsion (red arrow) from the ischial tuberosity secondary to ischiofemoral impingement.
There are limitations to this study. First, a comparison of the signal changes in the quadratus femoris muscle between the controlled and noncontrolled MRI was not possible due to differences in the technique for acquisition of the fluid-sensitive sequences. Second, the measurements obtained in the present study are based on a static exam, and the IFS changes significantly along the gait cycle, particularly in patients with abductor weakness.14
In conclusion, a difference >4 mm in the IFS was observed in one of every three hips when comparing MRI obtained with variable lower limb positioning to MRI reproducing the lower limb positioning in the standing position. The physical examination, radiographs, and quadratus femoris muscle changes on MRI should be considered in addition to the IFS for the diagnosis and surgical treatment of ischiofemoral impingement.
Conflict of Interest
H.D.M. reports personal fees from Smith & Nephew, outside the submitted work. M.H, R.L.M., and S.J.N. report no conflict of interest.
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