I. Femoroacetabular impingement (FAI) syndrome has been defined, via the Warwick Agreement International Consensus statement, as a motion-related clinical disorder of the hip with a triad of ¹ :

1. 
   

   Symptoms.

   
   
2. 
   

   Clinical signs.

   
   
3. 
   

   Imaging findings.

II. FAI syndrome represents symptomatic premature contact between the proximal femur and the acetabulum.

III. Similarly, extra-articular impingement refers to other osseous or soft-tissue sources of abnormal contact, with symptoms, clinical signs, and imaging findings consistent with the two impinging structures around the hip.

IV. Several different sources of extra-articular impingement have been described:

1. 
   

   Subspine (anteroinferior iliac spine [AIIS]) impingement.

   
   
2. 
   

   Ischiofemoral impingement.

   
   
3. 
   

   Iliopsoas impingement.

   
   
4. 
   

   Trochanteric–pelvic impingement.

I. Subspine impingement:

1. 
   

   Contact between AIIS and distal anterior femoral neck (or anterior edge of the greater trochanter).

   
   
2. 
   

   AIIS is origin for direct head of rectus femoris and iliocapsularis ² :

   
   
   
   
   1. 
      

      Composed of two facets, divided by horizontal AIIS ridge ( Fig. 17.1 ) ³ :

      
      
      
      
      1. 
         

         Superior: occupied entirely by direct head rectus femoris (teardrop-shaped attachment, tapered proximally).

         
         
      2. 
         

         Inferior: occupied by iliocapsularis origin (iliocapsularis lies immediately superficial, and adherent, to anteromedial iliofemoral ligament). ⁴

II. Ischiofemoral impingement:

1. 
   

   Contact between the lesser trochanter and the ischium ⁵ in the ischiofemoral interval (narrowest distance between the apex of the medial cortex of the lesser trochanter and the lateral cortex of the ischial tuberosity) ⁶ :

   
   
   
   
   1. 
      

      Lesser trochanter is insertion of the iliopsoas tendon ( Fig. 17.2 ).

      
      
   2. 
      

      Ischium is origin of the hamstring (superolateral semimembranosus and central–inferomedial conjoint tendon semitendinosus and biceps femoris long head).

      

      

      
      
   3. 
      

      Up to 84% of cadaveric specimens may demonstrate ischiofemoral impingement in 10-degree extension, 10-degree adduction, and 29-degree external rotation. ⁵

   
   
2. 
   

   Leads to compression of the quadratus femoris and the sciatic nerve ( Fig. 17.3 ).

   
   
3. 
   

   May also occur between the posterior edge of the greater trochanter and the lateral ischium in extreme hip external rotation. ⁷

   
   
4. 
   

   Deep gluteal space borders ⁸ :

   
   
   
   
   1. 
      

      Posterior: gluteus maximus.

      

      
      
   2. 
      

      Anterior: posterior column of the acetabulum, posterior hip capsule, proximal posterior femur.

      
      
   3. 
      

      Lateral: linea aspera and gluteal tuberosity.

      
      
   4. 
      

      Medial: sacrotuberous ligament, falciform fascia.

      
      
   5. 
      

      Superior: inferior margin of the sciatic notch.

      
      
   6. 
      

      Inferior: proximal origin of the hamstring at ischial tuberosity.

      
      
   7. 
      

      Contains sciatic nerve, piriformis, blood vessel containing fibrous bands, gluteal muscles, hamstring tendons, gemelli–obturator internus complex:

      
      
      
      
      1. 
         

         Sciatic nerve excursion 28 mm during hip flexion. ⁹

III. Iliopsoas impingement:

1. 
   

   Contact between iliopsoas and anterior hip (3 o’clock position on acetabular clockface with 3 o’clock denoting the superior margin of the psoas-u) with compression on the labrum. ³ , ¹⁰

   
   
2. 
   

   Iliopsoas tendon is a confluence of the psoas and iliacus muscles:

   
   
   
   
   1. 
      

      Psoas major muscle originates from transverse processes and bodies of T12–L5 and their intervertebral disks.

      
      
   2. 
      

      Iliacus muscle originates from the iliac fossa, overlying the anterior sacroiliac joint, and the lateral sacrum.

      
      
   3. 
      

      Iliopsoas is composed of 40% tendon and 60% muscle at the level of the labrum. ¹¹ , ¹²

   
   
3. 
   

   Iliopsoas tendon may be multibanded (single tendon, bifid, trifid) ¹³ :

   
   
   
   
   1. 
      

      Single banded: 28.3%.

      
      
   2. 
      

      Double banded: 64.2%.

      
      
   3. 
      

      Triple banded: 7.5%.

   
   
4. 
   

   Iliopsoas tendon is an anterior stabilizer from 0 to 15 degrees of hip flexion ¹⁴ :

   
   
   
   
   1. 
      

      As the iliopsoas crosses the superior pubic ramus (at an angle of 35–45 degrees), the relative anterior position increases its leverage for hip flexion ( Fig. 17.4 ):

      
      
      
      
      1. 
         

         As the hip flexes, the iliopsoas loses contact with the femoral head at approximately 14 degrees (7–19 degrees) and loses contact with the iliopectineal eminence at 54 degrees (42–67 degrees). ¹⁴

         
         
      2. 
         

         Increased lesser trochanteric retroversion (further posterior relative position of the lesser trochanter) observed in patients with symptomatic iliopsoas impingement. ¹⁵

         

   
   
5. 
   

   Iliopsoas contraction (or contracture) causes anterior pelvic tilt.

   
   
6. 
   

   Psoas tunnel is the groove in which the iliopsoas tendon passes, medial to the AIIS and iliopectineal eminence ¹⁶ :

   
   
   
   
   1. 
      

      For suture anchor placement during labral repair between 2 and 4 o’clock positions, there is concern for perforation through the anteromedial cortex of the acetabular dome: the psoas tunnel. ¹⁶ , ¹⁷

IV. Trochanteric–pelvic impingement:

1. 
   

   Contact between the greater trochanter and ilium with the hip abducted and extended:

   
   
   
   
   1. 
      

      Repetitive impingement will cause both compressive and tensile injuries to the abductor (gluteus medius, minimus) insertion.

      
      
   2. 
      

      Repetitive impingement will cause a levering effect with greater femoral head translation (vs. rotation).

   
   
2. 
   

   Abductor (gluteus medius and minimus) tension is decreased.

I. Subspine impingement:

1. 
   

   Four diagnostic criteria ¹⁸ :

   
   
   
   
   1. 
      

      Anterior hip pain aggravated by hip flexion, kicking, and sprinting.

      
      
   2. 
      

      Positive subspine impingement test (straight sagittal plane flexion, with limited terminal hip flexion motion) ¹⁹ and tender AIIS:

      
      
      
      
      1. 
         

         Greater degrees of flexion loss with increasing AIIS type ¹⁹ :

         
         
         
         
         • 
           

           i. Type I: 120 ± 12 degrees.

           
           
         • 
           

           ii. Type II: 107 ± 10 degrees.

           
           
         • 
           

           iii. Type III: 93 ± 20 degrees.

         
         
      2. 
         

         Greater degrees of internal rotation loss at 90 degrees of hip flexion with increasing AIIS type ¹⁹ :

         
         
         
         
         • 
           

           i. Type I: 21 ± 10 degrees.

           
           
         • 
           

           ii. Type II: 11 ± 9 degrees.

           
           
         • 
           

           iii. Type III: 8 ± 9 degrees.

      
      
   3. 
      

      Negative response to intra-articular injection.

      
      
   4. 
      

      Prominent AIIS (type II or III) on radiographs and/or CT.

   
   
2. 
   

   Frequently observed in one of five possible clinical scenarios:

   
   
   
   
   1. 
      

      Old rectus femoris avulsion/AIIS avulsion as adolescent. ¹⁸ , ²⁰ , ²¹

      
      
   2. 
      

      Hypermobile dancers. ²⁰ , ²² – ²⁴

      
      
   3. 
      

      Acetabular retroversion. ²⁰ , ²⁵

      
      
   4. 
      

      Post-periacetabular osteotomy (post-PAO) overcorrection.

      
      
   5. 
      

      Valgus neck, anteverted femur. ²⁶

II. Ischiofemoral impingement:

1. 
   

   Patients typically complain of chronic, insidious onset, atraumatic, deep posterior buttock pain (100%), especially with sitting (88%), with radiation distally, with or without sciatica ²⁷ :

   
   
   
   
   1. 
      

      Shortened stride during gait is frequently observed (avoidance of hip extension).

      
      
   2. 
      

      If Trendelenburg gait with abductor weakness, there is relative limb adduction in extension, exacerbating pain due to lesser trochanter impingement on the lateral ischium.

      
      
   3. 
      

      Patients may have a variety of relevant past orthopaedic history issues:

      
      
      
      
      1. 
         

         Ischial tuberosity avulsion.

         
         
      2. 
         

         Medialized total hip replacement (or low offset).

         
         
      3. 
         

         Previous peritrochanteric hip fracture.

         
         
      4. 
         

         Prior valgus proximal femoral osteotomy.

         
         
      5. 
         

         Legg–Calvé–Perthes disease.

      
      
   4. 
      

      Patients tend to be older than cam and pincer FAI syndrome patients (~47 years of age), greater female distribution (~82%), with a long duration of symptoms (~30 months). ²⁷

   
   
2. 
   

   Physical examination should assess for tenderness of the ischiofemoral space and the presence/absence of Tinel’s sign of the sciatic nerve:

   
   
   
   
   1. 
      

      Ischiofemoral impingement test involves hip extension, adduction, and maximal external rotation: reproduction of the symptoms accounting for the chief complaint of pain’s location is a positive test.

      
      
   2. 
      

      Long-stride walking test: pain reproduction and forced stride shortening due to pain when attempting to take long strides.

      
      
   3. 
      

      The active piriformis (sensitivity 0.78; specificity 0.80; positive likelihood ratio 3.9; diagnostic odds ratio 14.4) and seated piriformis stretch test (sensitivity 0.52; specificity 0.90; positive likelihood ratio 5.22; diagnostic odds ratio 9.8) combined (sensitivity 0.91; specificity 0.80; positive likelihood ratio 4.57; diagnostic odds ratio 42.0) are the optimal tests to identify sciatic nerve entrapment in deep gluteal space. ²⁸

   
   
3. 
   

   Diagnostic injection (ultrasound or CT guided) into ischiofemoral space with local anesthetic with or without steroid is useful adjunct in accurate diagnosis of ischiofemoral impingement. ²⁹

III. Iliopsoas impingement:

1. 
   

   Patients typically complain of deep anterior hip or groin pain (with a “C” sign or “between the fingers” sign), with focal iliopsoas tenderness (body habitus permitting), sitting pain, and infrequently with internal snapping:

   
   
   
   
   1. 
      

      Audible “pop” is usually iliopsoas.

      
      
   2. 
      

      Visible lateral “pop” (patient will report “my hip is dislocating”) is usually iliotibial band.

   
   
2. 
   

   Patients tend to be athletic, younger, greater female distribution.

   
   
3. 
   

   Physical examination reveals a similar examination to that of FAI syndrome with positive anterior impingement maneuver:

   
   
   
   
   1. 
      

      Positive Stinchfield, positive hip extension pain, positive Ludloff, positive iliopsoas test, ³⁰ positive FABER (flexion, abduction, and external rotation).

IV. Trochanteric–pelvic impingement:

1. 
   

   Patients typically are bimodally distributed:

   
   
   
   
   1. 
      

      Young hypermobile female patients who frequently perform high-flexibility sports (ballet, gymnastics, rhythmic gymnastics, figure skating, yoga, cheerleading). ³¹ , ³²

      
      
   2. 
      

      Older female patients with chronic lateral peritrochanteric pain.

   
   
2. 
   

   Pain is located deep anterior, lateral, and posterior with provocative activities, including excessive abduction (in variable degrees of rotation determining the location of impingement).

   
   
3. 
   

   A limp (with Trendelenburg gait and/or sign) is frequently present.

   
   
4. 
   

   Physical examination should scrutinize range of motion, especially the amount of abduction (in several different degrees of internal/external rotation), and abductor muscle strength (manual muscle testing, hand-held dynamometry) ³³ :

   
   
   
   
   1. 
      

      Typically less abduction in internal rotation (vs. external). ³²

      
      
   2. 
      

      Pain within 30 seconds of single leg stance is highly specific (100%, positive likelihood ratio ~12) of gluteal tendinopathy. ³⁴

      
      
   3. 
      

      No pain on palpation of greater trochanter highly sensitively (80%) rules out gluteal tendinopathy. ³⁴

   
   
5. 
   

   Beighton score should be assessed for joint hypermobility syndrome, as it has been shown to significantly influence hip motion. ³⁵

I. Subspine impingement:

1. 
   

   Plain radiographs:

   
   
   
   
   1. 
      

      False profile is useful for evaluation of AIIS morphology.

      
      
   2. 
      

      Anteroposterior (AP) pelvis may illustrate prominent AIIS (crossover sign):

      
      
      
      
      1. 
         

         Caution: only 50% of subjects with a positive crossover sign actually have a retroverted acetabulum on CT (the other 50% have a type II or III AIIS). ²⁵

   
   
2. 
   

   CT scan is optimal imaging modality for visualization of AIIS morphology.

   
   
3. 
   

   Three types of AIIS morphology ¹⁹ :

   
   
   
   
   1. 
      

      Type I: smooth section of the ilium (without bony prominences) between the most caudal aspect of the AIIS and the most cranial aspect of the acetabular rim.

      
      
   2. 
      

      Type II: AIIS sits at the level of the acetabular rim, appearing as “rooflike” prominence over the anterior hip (at the level of the sourcil on the AP radiograph).

      
      
   3. 
      

      Type III: AIIS extends distally to the anterosuperior rim (extends distal to the sourcil on the AP radiograph; Fig. 17.5 ).

II. Ischiofemoral impingement:

1. 
   

   Definitions (Torriani’s classification ³⁶ , ³⁷ ; Fig. 17.6 ):

   
   
   
   
   1. 
      

      Ischiofemoral space: smallest distance between the lateral cortex of the ischial tuberosity and the medial cortex of the lesser trochanter:

      
      
      
      
      1. 
         

         Greater than 17 mm is normal.

      
      
   2. 
      

      Quadratus femoris space: smallest space for passage of the quadratus femoris muscle defined by the superolateral surface of the hamstring tendons and the posteromedial surface of the iliopsoas tendon or the lesser trochanter:

      
      
      
      
      1. 
         

         Greater than 8 mm is normal.

         

   
   
2. 
   

   FNVLTV (femoral neck version lesser trochanteric version) angle is a measure of the angle between the femoral neck version and the lesser trochanter ( Fig. 17.7 ):

   
   
   
   
   1. 
      

      FNVLTV angle is significantly increased in patients with symptomatic ischiofemoral impingement.

      
      
   2. 
      

      Femoral neck version is significantly increased in patients with symptomatic ischiofemoral impingement.

   
   
3. 
   

   Statics measurements from axial MRI significantly overestimate ischiofemoral space (distance) in comparison to dynamic measurements from dual fluoroscopy during walking, hip adduction, extension, and external rotation. ³⁸

   
   
4. 
   

   Static measurements:

   
   
   
   
   1. 
      

      Plain radiographs: AP pelvis may illustrate a narrowed ischiofemoral space, decreased femoral offset, coxa valga (neck–shaft angle >135 degrees ³⁹ ):

      
      
      
      
      1. 
         

         Important to rule out other more common diagnoses (e.g., osteoarthritis, FAI syndrome, hamstring tendon pathology).

      
      
   2. 
      

      CT: best bony evaluation for ischiofemoral space.

      

      

      
      
   3. 
      

      MRI:

      
      
      
      
      1. 
         

         Quadratus femoris edema (Torriani’s classification) ³⁶ :

         
         
         
         
         • 
           

           i. Grade 1: mild.

           
           
         • 
           

           ii. Grade 2: moderate.

           
           
         • 
           

           iii. Grade 3: severe.

      
      
   4. 
      

      No significant difference in ischiofemoral space dimensions exist using static ultrasound versus MRI in healthy volunteers. ⁴⁰

   
   
5. 
   

   Dynamic measurements:

   
   
   
   
   1. 
      

      Dual fluoroscopy ³⁸ :

      
      
      
      
      1. 
         

         Ischiofemoral space decreases during hip external rotation (minimum), adduction, and extension.

      
      
   2. 
      

      Ultrasound:

      
      
      
      
      1. 
         

         Ischiofemoral space increases maximally in abduction and internal rotation (mean 5.2 cm) and decreases maximally in adduction and external rotation (3.1 cm) in healthy volunteers. ⁶

      
      
   3. 
      

      Full range-of-motion (FROM) MRI:

      
      
      
      
      1. 
         

         In zero degrees of hip flexion, external rotation up to 60 degrees may yield greater trochanteric impingement on the lateral ischium, with compression of the quadratus femoris between—distinguish from lesser trochanter versus ischium ischiofemoral impingement and greater trochanteric tip versus pelvis trochanteric-pelvic impingement.

   
   
6. 
   

   Patients with symptomatic ischiofemoral impingement demonstrate significantly smaller ischiofemoral space and quadratus femoris space in comparison to controls (14.9 vs. 26.0 mm and 9.6 vs. 16.0 mm, respectively). ⁴¹

III. Iliopsoas impingement:

1. 
   

   MRI (with arthrography) should be scrutinized for ⁴² :

   
   
   
   
   1. 
      

      Increased signal intensity between iliopsoas and capsule at the 3 o’clock position,

      
      
   2. 
      

      Edema within the iliopsoas tendon or adjacent capsule.

      
      
   3. 
      

      Labral tear at the 3 o’clock position.

      
      
   4. 
      

      Irregularity of the deep margin of the iliopsoas tendon.

      
      
   5. 
      

      Dimensions of iliopsoas tendon.

      
      
   6. 
      

      Location of iliopsoas tendon as it passes over the labrum.

IV. Trochanteric–pelvic impingement:

1. 
   

   Plain radiographs should be scrutinized for:

   
   
   
   
   1. 
      

      Neck–shaft angle (coxa vara defined as angle <120 degrees or trochanteric tip >7 mm above the head center ³⁹ ).

      
      
   2. 
      

      Femoral offset (distance from the center of the femoral head rotation to a line bisecting the center of the long axis of the femur).

      
      
   3. 
      

      Trochanteric height (also known as center–trochanteric distance) relative to the femoral head center).

      
      
   4. 
      

      “Splits” radiograph may demonstrate impingement above the superior rim at approximately 12 o’clock position with the limb maximally internally rotated, while permissive external rotation may demonstrate impingement behind the posterior rim at approximately 9 o’clock position ( Fig. 17.8 ) ³² :

      
      
      
      
      1. 
         

         This may cause a levering of the femoral head out of the acetabulum (translation) with loss of the suction seal and a vacuum sign in approximately 36% of patients. ³¹