The most commonly reported reason for failure of FAI surgery is under-resection of the impinging bone (53–90 %) [5, 6]. Successful correction of FAI requires removal of the entire impinging area. Residual prominences following surgery can cause persistent symptoms even when much of the impinging location of the femoral head-neck junction has been resected. The most commonly reported location of residual FAI is the posterosuperior or lateral location along the femoral head-neck junction when an interportal capsulotomy is used [3]. It has been proposed that the substantial learning curve to perform hip arthroscopy could be culpable for this technical failure and need for revision surgery. Additionally, lack of recognition or ability to address acetabular over-coverage has been reported as another cause of undertreatment of FAI.

Philippon et al. found that 36 of 37 patients had persistent radiographic evidence of FAI in a series of revision hip arthroscopy cases between 2005 and 2006 [14]. Following correction, the modified Harris Hip Scores (mHHS) improved from 56 preoperatively to 77 postoperatively. Kelly et al. found that 79 % of early revision hip arthroscopies between 2003 and 2007 were due to under-addressed or untreated FAI [15]. Likewise, Clohisy et al. reported that 68 % of 60 revisions following primary hip arthroscopy were for residual or untreated FAI [16]. This series differs from the previously mentioned studies because all revision procedures – both open and arthroscopic – were included. In combination, these series report that residual FAI was the most common reason for early revision procedures; however, it is unknown if this trend has continued in recent years as more attention is placed on the underlying bony morphology responsible for labral tears and chondral injuries [17]. For example, in the series by Clohisy et al., osseous deformity was only addressed in 17 of 60 hips at the time of initial FAI surgery, which differs significantly from current practice. Further, as more surgeons are aware of FAI and more surgeons are more comfortable performing FAI surgery, it may be that lack of resection or under-resection of FAI bony anatomy is becoming a less prevalent cause of failed FAI surgery.

Open or arthroscopic methods can be used to address the underlying asphericity of residual bony impingement, but diligent preoperative planning is necessary to identify the location of the lesion. Three-dimensional CT- or MRI-based software can be extremely helpful to understand bony anatomy following previous FAI surgery. Static 2D radiographs show only shadows of the femoral head-neck anatomy corresponding to an orthogonal position from the direction of the X-ray beam. For example, an AP radiograph shows the 12:00 position, 45° Dunn shows the 1:30 position, and lateral views show the 3:00 position; however, the area of maximal deformity could occur anywhere between these positions. 3D reconstructions help map the bony topography and allow visualization of areas that can be missed on standard radiographic views. Figure 18.1 shows the AP, cross-table lateral, and 3D CT image of a patient who had persistent symptoms of FAI following incomplete resection; an easily identifiable transition between the resected and residual C deformity occurs at the 1:30 position extending anteriorly to 3:00. Milone et al. found that alpha angles were underestimated on plain radiographs by an average of 8.2° compared to 3D CT scan [18]. Software advances have recently enabled independent visualization of the femoral and acetabular sides, along with dynamic modeling of joint motion to locate potential sources of impingement. 3D CT evaluations should be considered part of the standard workup for failed FAI surgery to identify concerning areas for residual impingement. Reconstructed MRI images can be used in lieu of CT scans for younger patients where the lifetime risk of cancer from radiation exposure is higher.

Although under-resection has been commonly reported as a source of failure, overaggressive bony resection can have its own set of detrimental consequences. Excessive deepening of the head-neck junction risks disruption of the labral suction seal that can reduce stability of the joint (Fig. 18.2). Biomechanical studies show an increased risk of femoral neck fracture if greater than one-third of the femoral neck is resected. Acetabuloplasty can change the contact pressures in the acetabulum resulting in edge loading and early wear mimicking developmental dysplasia of the hip. In extreme cases, instability and dislocation events are possible [19]. Correction of excessive acetabuloplasty may necessitate an open approach to the hip with osteochondral allograft or periacetabular osteotomy to restore normal contact pressures and joint stability. With time, joint damage can be unsalvageable necessitating total hip arthroplasty (Fig. 18.3).

Extra-articular impingement refers to locations besides the proximal femur with the acetabular rim where hip motion may be symptomatically impeded. The most common location occurs when the femur impinges on the anterior inferior iliac spine (AIIS), but can also occur between the femur and the ischial tuberosity as well as with the greater trochanter colliding with the acetabulum. Extra-articular impingement is rare, occurring in only 4 % of patients with hip pain, but frequently coexists with CAM morphology and traditional FAI [7]. Patients with documented findings of extra-articular impingement are more likely to be young, female, and those who have undergone previous surgery. These patient demographics overlap substantially with the typical patient who has capsular laxity and may result in markedly increased hip motion allowing extra-articular impingement. It has been hypothesized that extra-articular impingement could create a potential fulcrum that exacerbates capsular stretching and pain via mechanoreceptors within the hip capsule [20].

Patients who should be suspected of having subspinous impingement are those who have a low-lying AIIS and pain with hip hyperflexion in neutral rotation. Pathologic AIIS morphology, particularly when the AIIS extends below the level of the acetabular rim, are often caused by a malunited rectus femoris avulsion injury, though elongation of the AIIS may also be seen, and thought to be the result of sprinting and kicking activities as a child, resulting in overgrowth. A history of remote trauma from a previous hip strain can occasionally be elicited. In patients with suspected subspinous impingement, a false profile view or 3D CT scans should be obtained to evaluate AIIS morphology. Intraoperatively, the AIIS can be identified by tracing the direct head of the rectus back to its bony origin from either an extracapsular or intracapsular approach. An iliac oblique intraoperative fluorospot places the AIIS on profile and can be used to ensure adequate resection of the impinging area. Decompression of the AIIS was found to predict improvements in patient-reported outcomes in revision hip arthroscopy cases by Larson et al. [3].

Ischiofemoral impingement occurs when the quadratus femoris musculature is compressed between the lesser tuberosity and the ischium producing chronic groin or buttock pain. Pain is reproduced on physical examination by extension, external rotation, and adduction. Radiographs may reveal decreased femoral offset or evidence of altered bony anatomy such as previous ischial avulsion injury. MRI often shows increased signal within and around the quadratus on fluid-sensitive imaging indicative of compression injury. A diagnostic injection with local anesthetic using ultrasound or CT guidance is confirmatory for the diagnosis. Resection of the impinging bone can be performed endoscopically or through an open posterior approach.

The capsule is an important stabilizer of the hip joint that works in concert with static bony restraints, dynamic muscular forces, and the suction seal of the labrum to maintain the congruency of the joint throughout physiologic range of motion [21]. Hip instability results from an inability to maintain a concentric joint without undue stress [22]. Symptoms from instability range from pain only to hip joint unsteadiness or rarely joint dislocation [9]. The concept of hip instability is still early and somewhat controversial. Recent research confirms this evolving concept is a clinical entity. The term microinstability is used to denote pain without frank subluxation or dislocation.