Imaging evaluation of the hip after arthroscopic surgery for femoroacetabular impingement
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Arthroscopic surgery for femoroacetabular impingement (FAI) is increasingly frequently performed. Initial reports were that complications were very low, but as experience has increased, a number of long-term complications, in addition to factors related to poor clinical outcomes, have been identified. This review describes the normal and abnormal postoperative imaging appearance of the hip after arthroscopy for FAI. Abnormalities discussed include incomplete resection or over-resection of the impingement lesion, heterotopic ossification, cartilage damage, chondrolysis, instability and dislocation, recurrent labral tear, adhesions, psoas atrophy, infection, and avascular necrosis.
KeywordsFemoroacetabular impingement Hip arthroscopy complications Hip instability Psoas atrophy
Femoroacetabular impingement (FAI) is a clinical syndrome of hip pain and limited motion that many hip specialists believe to be due to morphological abnormalities of the femoral head, acetabulum, or both. First described by Ganz and colleagues, FAI has been identified as a cause of pain, labral tears, juxtalabral cartilage damage, and premature osteoarthritis [1, 2].
Arthroscopy of the hip is increasingly performed for treatment of labral tears and FAI. Among privately insured patients between the ages of 18 and 64, the rate of surgery increased from 3.6 per 100,000 in 2005 to 16.7 per 100,000 in 2013 . There are a number of known short-term and long-term complications of the surgery, including pain due to under-resection of the cam lesion, fracture, heterotopic ossification, cartilage damage, joint instability, adhesions, psoas abnormalities, neuropraxia, and osteoarthritis. This review is designed to familiarize radiologists with the normal and abnormal imaging appearance of the hip following arthroscopic surgery for FAI.
Imaging evaluation of impingement morphology
Radiographs, CT, and MRI with or without arthrography are all used to detect morphological abnormalities of the proximal femur (cam lesion) or of the acetabulum (pincer lesion) or both (mixed-type impingement). Radiographs include anteroposterior (AP), frog leg lateral, and false profile views. CT scan includes standard axial, sagittal, and coronal reformatted views. Three-dimensional CT images are often useful in characterizing the extent of morphological changes. It cannot be overemphasized that patients may have morphological findings associated with impingement without having the clinical syndrome [4, 5]. Radiologists must be careful in their reports to describe morphological findings only, rather than to imply that the clinical syndrome is present.
Arthroscopic technique for FAI
Hip arthroscopy is technically demanding, and a precise surgical technique is key in preventing complications . The hip joint is first distracted by a traction device. Access to the hip is gained via at least two of several standard arthroscopic portals (usually anterior and anterolateral). A vertical capsulotomy may be performed to improve access to the joint, in addition to a horizontal capsulotomy adjacent to the margin of the anterior acetabulum. Once access to the joint is obtained, fluoroscopy is used to locate the cam lesion. The area to be resected may be marked by cautery to allow accurate debridement. A burr is then used to remove the bony prominence. Acetabular labral tears may be repaired, or the labrum may be resected. The surgeon can treat mild pincer impingement by detaching the labrum, trimming the bone, and reattaching the labrum. This procedure is commonly known as a “rim trim.” More severe femoral over-coverage is treated with periacetabular pelvic osteotomy, which lies outside the scope of this article. Chondral lesions may be treated with debridement or a variety of repair techniques .
Normal postoperative imaging appearance of the hip
Postoperative radiographs, computed tomography (CT), and magnetic resonance imaging (MR) after arthroscopic resection of a cam lesion show a sharply angled concave contour (Fig. 1c, d) at the femoral head–neck junction [8, 9]. This angular contour change may be mistaken by the unwary for osteophyte formation. The acetabular “rim trim” may not be visible on radiographs, although a subtle change in contour is sometimes visible. It will be better visualized on CT (Fig. 2) and MRI.
Outcomes of hip arthroscopy
The literature shows a wide variation in reported outcomes of hip arthroscopy. A 2013 meta-analysis of 92 studies with more than 6,000 patients found that the reported rate of early major complications was 0.58% . The reported major complications included deep infection, skin damage, pulmonary embolus, intra-abdominal fluid extravasation resulting in abdominal compartment syndrome, femoral or femoral circumflex vessel injury, avascular necrosis, femoral neck fracture, dislocation, and death. Minor complications occurred at a rate of 7.5%, and the most common of these were iatrogenic chondrolabral injury and temporary perineal or femoral neuropraxia. A 2016 review of 258 patients found a significantly higher complication rate: 14.34% in the first year after surgery. Major complications occurred in 1.2%, and included femoral neck fracture, septic arthritis, and avascular necrosis of the femoral head . This complication rate is significantly higher than the 4.7% overall knee arthroscopy complication rate reported in over 92,000 cases performed by orthopedic surgeons sitting for part II of the American Board of Orthopedic Surgery .
One useful measure of the long-term outcomes of arthroscopic treatment of FAI is the rate at which a second surgery is performed on the affected hip. The second surgery may be revision arthroscopic surgery, open hip surgery for FAI, or conversion to total hip arthroplasty. The data from several studies indicate a significant rate of repeat surgery after arthroscopic surgery for FAI [10, 11, 13, 14, 15, 16, 17]. A 2013 meta-analysis found a rate of second surgery after arthroscopic treatment for FAI of 6.3% . The most common second surgery was conversion to total hip arthroplasty. A 2015 meta-analysis found that although functional hip scores improved overall when measured 1 year after index surgery, 14.6% of patients underwent either repeat hip preservation surgery or hip arthroplasty . Most repeat surgeries were performed within 2 years of the index surgery . Repeat surgery often disclosed more than one abnormality. Findings included incomplete resection of the impinging lesion, labral and articular cartilage abnormalities, adhesions, osteoarthritis, and instability [10, 11, 13, 14, 15, 16, 17].
Some poor outcomes of hip arthroscopy for FAI are undoubtedly related to patient selection . Patients who have pre-existing osteoarthritis (joint space <2 mm) or are over the age of 50 have worse outcomes of arthroscopy, and a significantly higher rate of conversion to total hip arthroplasty [3, 18, 19, 20]. Patients undergoing hip arthroscopy at the ages of 55–64 years have been reported to have a cumulative risk of conversion to total hip arthroplasty of 35% at 5 years .
Another outcome measure is patient satisfaction with the results of hip arthroscopy. Only one study could be found that specifically addressed this issue. The authors performed preoperative questionnaires regarding hip function and pain on 86 patients undergoing arthroscopic or mini-open surgery for FAI. They then approached the cohort 12 months postoperatively and asked them to complete new questionnaires . Fourteen percent of the original cohort refused to participate in the 12-month postoperative evaluation because of their degree of unhappiness with the surgery. More than 50% of those who responded stated that they did not have their expectations met for postoperative hip pain, sport, and general physical capacity. It is difficult to know whether the findings of this study reflect technically poor outcomes, poor communication between patients and surgeons, or unrealistic expectations for pain relief.
Imaging findings of complications of FAI surgery
Many complications of FAI surgery can be seen on imaging studies: heterotopic ossification, incomplete resection or over-resection of the impingement lesion, cartilage damage, rapid osteoarthritis, instability, dislocation, recurrent labral tear, anchor displacement, adhesions, psoas atrophy, infection, and avascular necrosis. Infection and avascular necrosis are not included in this review, as their appearance in this setting is not different than in other settings, and is commonly known. At our institution, small field-of-view images of the affected hip are supplemented with large field-of-view images to evaluate the entire pelvis, to detect causes of hip pain that lie outside the area of surgery.
Incomplete resection of the impingement lesion
The radiographs and MRI should be scrutinized carefully for residual deformity (Fig. 3). Postoperative radiographs may show a persistent bone prominence, but the patient may still have a successful outcome. As correlation with symptoms is variable, the radiographic report should be descriptive rather than implying the cause of symptoms.
Over-resection of the impingement lesion
If the surgeon is overly aggressive in removing a cam lesion, the femoral neck may be at an increased risk for fracture. However, there are no data on how much resection increases fracture risk. Risk of stress fracture of the femoral neck has been estimated at 0.07%, with an increased risk in patients who did not follow postoperative weight-bearing restrictions, and in women over the age of 50 . If the pincer lesion is overly resected, the patient may develop anterior instability .
Obliteration of the paralabral sulcus is very common in asymptomatic patients  and should be considered a normal postoperative finding.
Absence of the labrum on MRI in a postoperative hip may indicate attrition of a repaired labrum, or a resected labrum. As labral resection is an accepted surgical option, it is prudent for the radiologist to review the surgical report in a postoperative patient. A small study with a 10-year follow-up found that patients with labral resection vs labral repair had no difference in pain with impingement maneuver . In this study, revision arthroscopic surgery was performed in 24% of patients who had been treated with labral repairs, but in 54% of patients who had undergone labral resection. Rates of conversion to THA were equal in both groups.
The hip joint owes much of its anterior stability to the iliofemoral ligament (ligament of Bigelow). The iliofemoral ligament protects the hip against hyperextension, external rotation, and anterior displacement . It arises from the inferior margin of the anterior inferior iliac spine, immediately inferior to the straight head of the rectus femoris, and has two limbs that diverge in an inverted V shape to insert on the anterior intertrochanteric ridge, one medially and one laterally. Arthroscopic portals and capsulotomy may compromise the integrity of the iliofemoral ligament [32, 33]. Many arthroscopists close the capsule at the conclusion of the surgery to improve joint stability . With or without capsule closure, the iliofemoral ligament may be compromised, leading to anterior subluxation and sometimes to dislocation.
Signs and symptoms of anterior hip instability are nonspecific. Patients complain of anterior pain, and sometimes of a “popping” or a sensation of “giving way”. They may show apprehension when the hip is extended.
Another possibility to consider is that unrecognized hip instability may have been present before hip arthroscopy. Hip instability in the absence of previous surgery was long considered to be rare, but is increasingly recognized [32, 35]. FAI has also been suggested as a cause of posterior hip instability [36, 37].
Osteoarthritis may be seen to progress rapidly after hip arthroscopy. This may have multiple causes. Direct trauma by arthroscopic instruments to the cartilage is one cause. Another cause of rapidly-developing osteoarthritis is iatrogenic instability, and a third is progression of previous osteoarthritis.
Arthroscopic release of the psoas tendon is a common procedure performed during arthroscopy to alleviate iliopsoas impingement . The psoas tendon is readily visible on MRI at the level of the hip as a black, round structure lying posterior to the iliacus muscle. A recent study found that compared with patients who did not undergo release, patients who had undergone release had significant psoas atrophy on MRI evaluation, which corresponded to measurable muscle weakness .
Psoas weakness may occur, even when an arthroscopic release is not performed. A study of 8 patients who underwent arthroscopy without psoas release found that they had persistent psoas weakness 2.5 years after surgery . The strength of other muscle groups was normal.
Infection and avascular necrosis
These complications are fortunately rare, but should always be on the radiologist’s checklist in evaluating any musculoskeletal imaging study.
Failure due to incorrect initial diagnosis
If a patient has persistent pain after surgery for FAI, the possibility that the initial pain was due to a different cause than FAI should be considered. Cam and pincer type morphology are very common and may be asymptomatic [4, 5]. Pain due to osteoarthritis, athletic pubalgia, muscle/tendon tears, ischiofemoral impingement, or iliopsoas impingement may be erroneously ascribed to FAI. A new problem may also emerge following successful arthroscopy. The radiologist interpreting a preoperative or postoperative study should be alert for other abnormalities which may cause hip pain.
The radiologist plays an important role in the evaluation of the hip after arthroscopy. A checklist of possible abnormalities is useful to ensure complete evaluation and accurate diagnosis.
Compliance with ethical standards
Conflicts of interest
The author has no conflicts of interest to disclose.
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