FOCUS ON: Musculoskeletal Imaging
Review
Atypical Hip Impingement
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OBJECTIVE. This article summarizes how atypical forms of hip impingement can be assessed with radiographs, CT, and MRI.
CONCLUSION. Subspine impingement, ischiofemoral impingement, and iliopsoas impingement are atypical forms of hip impingement and are less common than classic femoroacetabular impingement. Additional forms of atypical impingement, such as abnormal femoral antetorsion, abnormal pelvic and acetabular tilt, and extreme hip motion, can occur combined with classic femoroacetabular impingement or as separate entities.
Keywords: atypical hip impingement, femoral antetorsion, iliopsoas impingement, ischiofemoral impingement, subspine impingement
In the past decade, a large body of work has been published on the cam and pincer types of femoroacetabular impingement (FAI) [1–4]. However, a number of recent studies have focused on atypical hip impingement, encompassing lesser-known forms of intraarticular impingement and extraarticular causes of hip impingement [5–9]. Symptoms of atypical hip impingement are often nonspecific, and imaging can play an important role in diagnosing atypical hip impingement. Some types of atypical hip impingement have been reported only in small study groups, and further validation of clinical and radiologic diagnostic criteria are needed. This review gives an overview of the various forms of atypical hip impingement and describes the radiologic findings that allow differentiating atypical hip impingement from classic FAI and other hip disorders.
| Subspine Impingement |   |
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The anterior inferior iliac spine (AIIS) is located just above the anterosuperior portion of the acetabular rim and is the origin of the straight head of the rectus femoris tendon. Avulsion injuries of the AIIS are commonly seen in adolescent athletes and are also known as sprinter's fractures [10]. They are usually caused by repetitive traction injuries of the straight head of the rectus femoris in patients who play soccer or other sports that involve kicking [5]. Rapid high-energy knee flexion combined with hip extension has been proposed as a possible pathologic mechanism for this injury [11]. Inferior displacement of the apophysis can lead to mal-union, which results in an enlarged AIIS or a bony protrusion that has been dubbed “pelvic digit” or “iliac rib” and is characterized by a bony protrusion extending inferiorly, sometimes caudad of the femoral head apex or the acetabular rim [5, 12]. There is a strong male predominance for this injury, and patients are typically 14–30 years old [5, 13].
Although most patients with an avulsion injury of the AIIS have a good response to conservative treatment, in some patients, there is a mechanical conflict between the enlarged bony protrusion at the AIIS and the anterior portion of the femoral neck (Fig. 1), which is called “iliac spine impingement,” “AIIS impingement,” or “subspine impingement” [5]. These patients typically present with anterior hip or groin pain aggravated by hip flexion, and pain is reported when the AIIS is palpated at physical examination [5]. Furthermore, a limited range of motion is found for hip flexion. There is no or only partial pain relief and a persistent limitation of hip flexion after intraarticular administration of a local anesthetic [5].
 View larger version (130K) | Fig. 1 —15-year-old male soccer player with subspine impingement of right hip visible on anteroposterior pelvic radiograph. After old avulsion injury of apophysis, right anterior inferior iliac spine is protruding inferiorly (arrow), to level slightly below apex of femoral head. Patient complained of anterior hip pain during hip flexion, and palpation of anterior inferior iliac spine reproduced pain. Note that, on left side anterior inferior iliac spine is slightly enlarged after prior similar injury, but osseous protrusion (asterisk) is much smaller. |
On radiography, an avulsion fracture of the AIIS can be detected immediately after the injury, but the subtle findings are commonly missed, according to reports in the literature [13, 14]. Over time, a displaced avulsion fracture may develop into a prominent bony AIIS deformity, which can be more accurately visualized at CT or MRI, and is often point shaped [5]. In other cases, calcified deposits are found within the proximal portion of the straight head of the rectus femoris tendon [15]. Three-dimensional CT datasets may be used for preoperative dynamic simulations, to detect a direct osseous impingement between the AIIS deformity and the femoral neck at maximum hip flexion [15]. If herniation pit cysts are present in the proximal femur, they may be located more distally in patients with subspine impingement compared with the classic herniation pits in FAI [15].
A case series has shown that concomitant cam type of FAI was present in eight of 10 patients with surgically proven subspine impingement [5]. Some patients initially undergo surgery for classic FAI, and subspine impingement is recognized and treated only if postoperative results are not satisfactory [15].
| Ischiofemoral Impingement | |
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An impingement between the ischium and the proximal femur was first reported in three patients with either total hip arthroplasty or proximal femoral osteotomy [16]. Recent reports have focused on this disorder in patients without prior surgery [6, 17, 18]. Ischiofemoral impingement is characterized by a narrowed space between the ischial tuberosity and the lesser trochanter, with intermittent compression of the quadratus femoris muscle between the ischium and the lesser trochanter, and associated edema or fatty replacement of the quadratus femoris muscle [6, 19] (Figs. 2 and 3).
 View larger version (79K) | Fig. 2 —Schematic figure of right hip in oblique posterior view showing pathologic mechanism for development of ischiofemoral impingement. Quadratus femoris muscle is shown (red striped shape), with origin at lateral border of ischial tuberosity and insertion at intertrochanteric crest of femur. During hip movements, quadratus femoris is intermittently compressed when lesser trochanter advances toward ischium (arrow indicates movement of lesser trochanter). (Drawing by Sutter R) |
 View larger version (214K) | Fig. 3A —59-year-old woman with ischiofemoral impingement of right hip. A, On transverse T1-weighted MRI (A) and transverse STIR image (B), quadratus femoris muscle (asterisks) shows some atrophy (A) and diffuse edema (B) due to intermittent compression between lesser trochanter (arrows) and ischial tuberosity (pound signs). Note proximity of hamstring tendon origin (arrowheads) to quadratus femoris muscle. |
 View larger version (172K) | Fig. 3B —59-year-old woman with ischiofemoral impingement of right hip. B, On transverse T1-weighted MRI (A) and transverse STIR image (B), quadratus femoris muscle (asterisks) shows some atrophy (A) and diffuse edema (B) due to intermittent compression between lesser trochanter (arrows) and ischial tuberosity (pound signs). Note proximity of hamstring tendon origin (arrowheads) to quadratus femoris muscle. |
Ischiofemoral impingement is substantially more common in women than in men, and bilateral involvement is reported in about one third of patients [6, 19]. The affected patients are usually older than patients with other types of impingement, with a mean age of 51–53 years (range, 14–77 years) [6, 19]. Patients typically present with nonspecific pain in the hip, groin, or buttock. Pain can radiate distally to the lower extremity, probably because of an irritation of the adjacent sciatic nerve [6, 19]. There is no specific test for ischiofemoral impingement at physical examination, and it is often confused with intraarticular hip pain; thus, the diagnosis is largely dependent on MRI [19].
In patients with ischiofemoral impingement, the distance between the ischial tuberosity and the lesser trochanter (also known as the ischiofemoral distance) is reduced, with mean (± SD) measurements of 13 ± 5 mm in patients versus 23 ± 8 mm in control subjects [6]. Some caution is advised when measuring this distance in daily practice, because the measurements are dependent on the degree of internal or external rotation of the hip during image acquisition. So far, to our knowledge, no prospective study exists on ischiofemoral impingement; therefore, inconsistencies in patient positioning cannot be excluded in the available studies [6, 19].
The quadratus femoris muscle is one of several external rotators of the hip. It arises from the lateral border of the ischial tuberosity, anterosuperior to the hamstring origins, and inserts at the intertrochanteric crest on the posterior aspect of the proximal femur.
On MRI, edema of the quadratus femoris muscle is visible in patients with ischiofemoral impingement, although only to a minor or moderate degree in most patients [6, 19]. Some patients present with fatty infiltration of the quadratus femoris muscle, sometimes combined with muscle atrophy [6, 19]. There are contradictory data in the literature about whether ischiofemoral impingement can be associated with a quadratus femoris tear [6, 19]. Finally, a bursalike structure or mild edema around the iliopsoas tendon has been reported in some patients with ischiofemoral impingement [6].
The cause of ischiofemoral impingement is often congenital, but several acquired forms of ischiofemoral narrowing have been reported (e.g., as a result of fractures of the proximal femur, in patients with intertrochanteric osteotomy, or in older patients with superior and medial migration of the femur due to hip osteoarthritis) [6, 16, 19]. The female predominance of ischiofemoral impingement might be due to the different osseous configuration of the female pelvis compared with the male anatomy [6, 19].
Differential diagnoses, such as a strain or tear of the quadratus femoris muscle or delayed-onset muscle soreness, must be ruled out by performing a precise clinical history. One morphologic feature that might help in differentiating these entities at MRI is the fact that edema from a tear or strain most commonly occurs at the musculotendinous junction, whereas the muscle edema in ischiofemoral impingement is diffuse [20]. Although some authors recommend surgical decompression of the quadratus femoris muscle with resection of the lesser trochanter, most reports favor conservative treatment or CT-guided injection of steroids [6, 16, 21].
| Iliopsoas Impingement | |
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In patients with classic FAI, most labral tears occur at the anterosuperior position and are caused by osseous abnormalities of the acetabulum and the proximal femur [22, 23]. In a recent arthroscopic report, Domb and colleagues [7] described a distinct pattern of labral pathologic abnormality, with localized labral damage anteriorly that did not extend to the anterosuperior portion of the acetabulum and that was directly adjacent to the iliopsoas tendon. The study population consisted of 23 female and two male patients with this pattern of labral injury but without radiologic evidence of dysplasia or FAI. In all patients, the diagnosis was confirmed at arthroscopy [7].
The authors proposed that this pattern of labral injury arises because of a previously unrecognized pathologic mechanism and coined the term “iliopsoas impingement” for this disorder (Fig. 4). Several theoretic pathologic mechanisms are suggested: Iliopsoas impingement might by induced by a tight or inflamed iliopsoas tendon that causes impingement during hip extension, or it might be due to repetitive traction injury caused by an iliopsoas tendon that is scarred or adherent to the capsule-labrum complex [7]. All patients complained of anterior hip pain, and 80% of the patients were involved in regular sports activities [7]. The impingement test was positive in all patients during physical examination, and nonspecific focal tenderness was found over the iliopsoas at the level of the anterior portion of the joint [7]. All patients were arthroscopically treated with psoas tenotomy at the level of the labrum and with either labral resection or repair, with good results at 1-year follow-up in all patients but one, for whom no change in symptoms was described [7].
 View larger version (142K) | Fig. 4 —43-year-old woman with groin pain. Anterior labral tear (arrowhead) is seen directly adjacent to iliopsoas tendon (arrows) on sagittal intermediate-weighted fat-saturated MRI of left hip. Because no radiographic hallmarks of femoroacetabular impingement were present in this patient (not shown), combination of findings is suggestive of iliopsoas impingement. |
The work by Domb and colleagues [7] confirmed the findings of an earlier series of revision hip arthroscopies, where a tight iliopsoas tendon adjacent to a torn or inflamed anterior labrum was identified as the reason for failed primary hip arthroscopy in seven patients [24]. The concept of the iliopsoas impingement is further strengthened by an anatomic study with eight cadaveric hips that highlights the close anatomic proximity of the musculotendinous junction of the iliopsoas to the anterior capsulolabral complex [25].
A recent study compared preoperative MR arthrography findings in 23 patients (19 female patients and four male patients) with iliopsoas impingement diagnosed at arthroscopy, with findings in 24 patients for whom no iliopsoas impingement was detected [8]. Of the various diagnostic criteria that were assessed, only the presence of a labral tear at the anterior position showed promise as a possible indicator of iliopsoas impingement: reader 1 detected an anterior labral tear in 20 of 23 patients and reader 2 detected an anterior labral tear in 18 of 23 patients in the iliopsoas impingement group, whereas in the nonimpingement group, such tears were detected in 13 of 24 patients for reader 1 and in 10 of 24 patients for reader 2 (p = 0.024 and 0.017 for reader 1 and 2, respectively) [8]. The available data are inconsistent on whether patients with iliopsoas impingement have substantial pain relief after intraarticular hip injections or iliopsoas bursa injections, so these injections cannot be used for narrowing down the diagnosis in patients with suspected iliopsoas impingement [7, 8].
The average age of patients with iliopsoas impingement is 25–35 years (range, 15–57 years) [7, 8]. It has been suggested that the strong female predominance for iliopsoas impingement might be explained by a narrower iliopsoas tendon in female individuals (10.2 mm) compared with male individuals with iliopsoas impingement (11.9 mm) and female individuals without iliopsoas impingement (11.0 mm) [8]. Further research is necessary to examine possible pathologic mechanisms and to establish a causal relationship between abnormalities of the iliopsoas tendon and focal anterior labral damage.
| Abnormal Femoral Antetorsion | |
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Femoral antetorsion is characterized by the angle between the femoral neck axis and the femoral condyles and is synonymous with femoral anteversion [26, 27]. Tönnis and Heinecke [26, 28] suspected two decades ago that abnormal femoral antetorsion might play a role in the development of osteoarthritis. However, it has only recently been confirmed that reduced femoral antetorsion is associated with the cam type of FAI in an MRI study with 126 individuals [27] and in 3D simulations of CT data of 30 individuals [29].
Femoral antetorsion is decreased in patients with the cam type of FAI (10.0° ± 9°) when compared with asymptomatic volunteers (12.7° ± 10°) [27], but with a wide scatter of antetorsion values in both groups [27]. In some patients with the cam type of FAI, the femoral antetorsion does not contribute to mechanical impingement, but there are patients with a substantially reduced femoral antetorsion or a femoral retrotorsion, where mechanical impingement at internal rotation is markedly increased because of this torsion abnormality [27].
A decreased or increased femoral antetorsion occurs independently of a decreased or increased acetabular version, as shown by Tönnis and Heinecke [26]: With radiographic measurements, they were able to group 147 patients according to several combinations of femoral torsion and acetabular version. Thirty-one percent of hips had a decreased femoral torsion combined with a decreased acetabular version, whereas 20% had a decreased femoral torsion combined with a normal acetabular version, and 21% had a decreased femoral torsion combined with an increased acetabular version [26].
Different combinations of normal and abnormal femoral torsion and acetabular version exist, and, in some cases, symptoms and signs of FAI may be present in patients with no or only minor cam and pincer deformities but with a reduced femoral antetorsion [27, 30] (Fig. 5).
 View larger version (166K) | Fig. 5A —23-year-old woman with femoral retrotorsion (−12°). Patient complained of hip pain, and impingement test was positive at physical examination, but there were no radiologic features of femoroacetabular impingement (not shown). A, Transverse T2-weighted MRI scans show proximal (A) and distal (B) right femur. Proximal reference line (white line, A) for femoral torsion measurement is drawn between center (small circle, A) of femoral head (large circle, A) and center of femoral neck at its base. This line is drawn simultaneously on two separate transverse sections over proximal femur. Distal reference line (red line, B) connects dorsal border of femoral condyles. Femoral torsion is defined as angle between white and red reference lines, as indicated. |
 View larger version (102K) | Fig. 5B —23-year-old woman with femoral retrotorsion (−12°). Patient complained of hip pain, and impingement test was positive at physical examination, but there were no radiologic features of femoroacetabular impingement (not shown). B, Transverse T2-weighted MRI scans show proximal (A) and distal (B) right femur. Proximal reference line (white line, A) for femoral torsion measurement is drawn between center (small circle, A) of femoral head (large circle, A) and center of femoral neck at its base. This line is drawn simultaneously on two separate transverse sections over proximal femur. Distal reference line (red line, B) connects dorsal border of femoral condyles. Femoral torsion is defined as angle between white and red reference lines, as indicated. |
In patients with the pincer type of FAI, the mean femoral antetorsion is typically increased (18.3° ± 10°), but again with a large spread of antetorsion values [27]. The risk of posterior mechanical impingement is increased in patients with a large femoral antetorsion, especially if there is also an increased acetabular anteversion [26, 27].
Patients who undergo imaging for FAI should also be evaluated for abnormal femoral antetorsion before surgery, because an abnormally decreased or increased femoral antetorsion might result in a different treatment plan [27]. Several slightly different anatomic definitions of femoral antetorsion are available, so the implementation of values from the literature into clinical practice has to be done in respect to the corresponding definition [27, 29, 31, 32]. Commonly, femoral antetorsion is measured on transverse CT or MRI over the proximal and distal femur: The proximal reference line is drawn between the center of the femoral head and the center of the femoral neck at its base, and the distal reference line consists of the line connecting the dorsal aspects of the femoral condyles [27, 32] (Fig. 5). A positive angle between these two reference lines is termed “antetorsion,” and the term “retrotorsion” is used for negative angles.
The anatomically correct definition of antetorsion corresponds to measurements on transverse images over the proximal and distal femur [31]. If the proximal reference line is drawn on a transverse oblique image parallel to the femoral neck, the resulting antetorsion value will differ from the anatomically correct value, unless trigonometric corrections are performed [31].
| Abnormal Pelvic and Acetabular Tilt | |
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Pelvic tilt influences the extent of focal acetabular overcoverage detectable on anteroposterior radiographs [33–35]. Furthermore, a close relationship has been described between the sagittal balance of the spine, the pelvic tilt, and hip impingement [36–38].
The degree of acetabular overcoverage is usually determined in reference to the pelvis in neutral position [33], but the pelvis is often not positioned neutrally in the standing position and during gait [36, 39–41]. The pelvic tilt increases or reduces the degree of acetabular overcoverage (Fig. 6); therefore, an abnormal pelvic tilt may promote the development of FAI or may even result in atypical impingement in patients without the classic hallmarks of FAI [36, 40]. Pelvic tilt can be assessed on weight bearing radiographs or weight bearing biplanar radiographs of the pelvis and lower extremities, and different methods have been described for determining the amount of pelvic tilt [42–45].
 View larger version (91K) | Fig. 6A —Lateral view of 3D model of pelvis based on weight-bearing biplanar radiographic data for two different patients. A, Images are shown for 18-year-old woman (A) and 39-year-old man (B), illustrating concept of pelvic tilt. Pelvic tilt can be described by angle between vertical line and line connecting femoral head center (white points) and midpoint (gray points) of cranial endplate (red lines) of first sacral body. Increased forward pelvic tilt increases anterior acetabular overcoverage (illustrated in this model by green bands) in weight-bearing position, and may thus promote development of femoroacetabular impingement or of atypical impingement without classic hallmarks of femoroacetabular impingement. Forward pelvic tilt and anterior acetabular overcoverage are larger in (B) than in (A) in this example. Both patients underwent biplanar radiographs (EOS system) of lower extremities and pelvis during their workup for unrelated knee problems. |
 View larger version (86K) | Fig. 6B —Lateral view of 3D model of pelvis based on weight-bearing biplanar radiographic data for two different patients. B, Images are shown for 18-year-old woman (A) and 39-year-old man (B), illustrating concept of pelvic tilt. Pelvic tilt can be described by angle between vertical line and line connecting femoral head center (white points) and midpoint (gray points) of cranial endplate (red lines) of first sacral body. Increased forward pelvic tilt increases anterior acetabular overcoverage (illustrated in this model by green bands) in weight-bearing position, and may thus promote development of femoroacetabular impingement or of atypical impingement without classic hallmarks of femoroacetabular impingement. Forward pelvic tilt and anterior acetabular overcoverage are larger in (B) than in (A) in this example. Both patients underwent biplanar radiographs (EOS system) of lower extremities and pelvis during their workup for unrelated knee problems. |
So far, to our knowledge, there are no conclusive data available that determine the role of pelvic tilt in patients with FAI. However, it has been suggested that correcting an increased lumbar lordosis, strengthening of core muscles, or physical therapy could be part of a nonsurgical treatment in mild cases of patients with FAI by normalizing pelvic tilt [41, 46–48].
Recently, the acetabular tilt, which describes the forward rotation of the entire ace-tabular cup around its central axis, has been investigated [49]. Köhnlein and colleagues [49] propose that acetabular and pelvic tilt should be separately analyzed for accurate preoperative planning before acetabular rim resection is performed in patients with FAI. In their analysis of 33 pelvic bones, the acetabular opening plane was characterized by an anteversion of 21° ± 5°, by an inclination of 48° ± 4°, and by an acetabular forward tilt of 19° ± 6°, with no differences between sexes [49]. The authors propose that, apart from decreased acetabular version, which is known from the classic pincer or mixed types of FAI, a decreased acetabular inclination also can lead to the pincer type of FAI and that a decreased acetabular tilt might have the same effect because the constant osseous rim prominences of the acetabulum are located more inferiorly with decreased acetabular tilt [49]. However, to our knowledge, there are no published data available on how acetabular tilt can be reliably assessed with imaging, and the clinical implications of an abnormal acetabular tilt have yet to be confirmed.
| Extreme Hip Motion | |
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Classic FAI is common in young athletes [50–52], but there have been several reports about atypical impingement associated with extreme hip motion in ballet dancing, martial arts, and other sports [9, 53–55]. In such patients, a “pincerlike” impingement is caused by excessive hip motion, without the presence of the acetabular pathomorphology that is seen in the classic pincer type of impingement [56]. Recently, Kolo and colleagues [9] examined 30 female professional ballet dancers with MRI: Acetabular cartilage lesions greater than 5 mm were found in 29% of hip joints in the ballet dancers, with most defects detected at the superior position, whereas only 7% of 14 asymptomatic nondancers had such cartilage lesions. Also, labral tears were much more common in the dancers (54 lesions) than in the control subjects (10 lesions). The prevalence of classic FAI was low for the dancers, with only one individual featuring the cam type and none with the pincer type. The non-typical distribution of the lesions of the acetabular cartilage and labrum might be explained by hip joint subluxation during the extreme hip movements, which constitutes the pincer-like impingement mechanism already described [9, 53]. During these extreme positions, a linear contact occurred between the superior or posterosuperior acetabular rim and the femoral head-neck junction of the ballet dancers, as detected by MRI in the splits position [9, 57].
Data from a study of 41 athletes with labral damage who underwent arthroscopy strengthen the extreme motion hypothesis: Although most cases (32 patients) were due to FAI, in 10 cases without FAI, a labral degeneration was found at arthroscopy, which was likely associated with repetitive trauma during sports activities [54]. Interestingly, 34% of the athletes included in the study practiced martial arts that feature extreme hip motion [54], so a similar pathologic mechanism might be present in these athletes as the pathologic mechanism described for the ballet dancers.
| Conclusion | |
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Subspine impingement, ischiofemoral impingement, and iliopsoas impingement are atypical forms of hip impingement and are less common than classic FAI. Other forms of atypical impingement, such as abnormal femoral antetorsion, abnormal pelvic and acetabular tilt, and extreme hip motion, can occur combined with classic FAI or as separate entities. Knowing the radiologic findings as well as the clinical background of the different variants of atypical hip impingement is useful for reaching the correct diagnosis, especially in patients who present with nonspecific hip or groin symptoms. Because some forms of atypical hip impingement have been reported in small study groups, further research data will be beneficial for validating the clinical criteria and radiologic findings associated with atypical hip impingement.
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