MR imaging of the hip is frequently used in symptomatic patients before hip preservation surgery; it is used as a decision-making tool and as a planning tool. The MRI can confirm the preliminary working diagnosis, identify other possible sources of pain, and highlight anatomic areas that are not routinely viewed during surgery. In addition, MR imaging is capable of illustrating normal and abnormal bony morphology of the femur and pelvis; and in the case that arthrography is used, diagnostic injection can be administrated concurrently. This article highlights a surgeon’s perspective on the use of MR imaging in the patient with nonarthritic hip pain.
Key Points
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MR imaging of the hip is frequently used in symptomatic patients before hip preservation surgeries; it is used as a decision-making tool and as a planning tool.
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High-quality MR imaging with a detailed report is very important to the hip surgeon, to help confirm the correct diagnosis, help see areas not usually visualized during hip arthroscopy, assist with surgical planning, and provide patients with the correct information to help manage their expectations.
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Examination can confirm the preliminary working diagnosis, identify other possible sources of pain, and highlight anatomic areas that are not routinely viewed during surgery.
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In addition, MR imaging is capable of illustrating normal and abnormal bony morphology of the femur and pelvis; and in the case that arthrography is used, diagnostic injection can be administrated concurrently.
Introduction
MR imaging is one of the best noninvasive examination tools of the hip; nonetheless, some may argue it is not sine qua non before hip preservation surgery. However, MR imaging is a very useful tool to the surgeon from many perspectives. MR imaging helps to confirm the diagnosis; to rule out other causes of pain; to assist with surgical planning; to visualize areas that are either not well seen at arthroscopy or located in the periarticular regions, which are not routinely evaluated without specific indication; to plan surgery; and to manage patient expectations ( Box 1 ).
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Confirm the diagnosis
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Local anesthetics for diagnosis of the hip joint as the source of pain
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Quantify pathology and morphology
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Visualize periarticular structures that are not seen during routine hip arthroscopy: gluteus medius, iliopsoas tendon, peripheral compartment
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Assess areas that are hard to fully visualize during surgery: inferior acetabulum, inferior and central femoral head, posterior and medial femoral neck
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Diagnose bony pathologies: edema, AVN, subchondral cysts, tumors
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Decision-making tool
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Surgical planning tool
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Set patients’ expectations from the surgery
MR imaging is a complimentary tool in the patient’s clinical assessment process. Whenever a patient is referred for MR imaging with hip pain, plain radiographs of the hip and pelvis and comprehensive physical examination have already been completed. Unfortunately, the physical examination of the hip does not allow for a specific diagnosis, even in experienced hands, compared with other joints. However, by the time MR imaging is ordered, the physician already has a preliminary working diagnosis, and MR imaging is used to confirm this diagnosis and to exclude other entities, particularly extra-articular pathologies, such as stress fractures that are not apparent on radiographs ( Fig. 1 ).
MR arthrography has been shown to be more sensitive than plain MR imaging at detecting various lesions around the hip. In addition, administration of local anesthetics with the gadolinium injection may provide additional confirmation of the diagnosis. Importantly, decrease in the pain level after the injection confirms the hip as the source of pain. However, in some cases the gadolinium itself can cause pain as a side effect; thus, the examination has the opposite effect, with increase in pain level. In those cases, if there is any doubt in diagnosis, a second dedicated intra-articular anesthetic injection is advised.
MR imaging may also give the surgeon data for quantifying the pathology and morphology of the hip and pelvis (eg, femoral neck version, femoral head coverage, and alpha angle). MR imaging may give information on pathology that is not readily seen during hip arthroscopy, such as bony edema, subchondral cysts, or intramuscular ganglia. MR imaging can also assess periarticular areas that are not routinely visible during routine hip arthroscopic surgery; for example, gluteus medius tears, iliopsoas bursitis or tendinopathy, and the peripheral compartment may be demonstrated. Moreover, MR imaging may point out areas of interest to be explored during the surgery; such areas as the posteroinferior acetabulum and the inferocentral femoral head are hard to view during hip arthroscopy and may require special attention, thus MR imaging may help in surgical planning.
MR imaging may be useful in preoperative planning or setting patients’ postoperative expectations. Chondral damage and subchondral cysts may be identified preoperatively using MR imaging. In the presence of limited full-thickness chondral damage, where a microfracture procedure may be indicated, the patient may be counseled to expect a longer rehabilitation on crutches because of microfracture procedure, and in case of a large chondral cyst bone grafting may be planned. MR imaging findings may also alter or postpone a planned surgery; for example, extensive chondral damage may lead to the decision to replace the hip, whereas low-grade avascular necrosis (AVN) postpones arthroscopic surgery until it is resolved or the procedure changed to core decompression.
Introduction
MR imaging is one of the best noninvasive examination tools of the hip; nonetheless, some may argue it is not sine qua non before hip preservation surgery. However, MR imaging is a very useful tool to the surgeon from many perspectives. MR imaging helps to confirm the diagnosis; to rule out other causes of pain; to assist with surgical planning; to visualize areas that are either not well seen at arthroscopy or located in the periarticular regions, which are not routinely evaluated without specific indication; to plan surgery; and to manage patient expectations ( Box 1 ).
- •
Confirm the diagnosis
- •
Local anesthetics for diagnosis of the hip joint as the source of pain
- •
Quantify pathology and morphology
- •
Visualize periarticular structures that are not seen during routine hip arthroscopy: gluteus medius, iliopsoas tendon, peripheral compartment
- •
Assess areas that are hard to fully visualize during surgery: inferior acetabulum, inferior and central femoral head, posterior and medial femoral neck
- •
Diagnose bony pathologies: edema, AVN, subchondral cysts, tumors
- •
Decision-making tool
- •
Surgical planning tool
- •
Set patients’ expectations from the surgery
MR imaging is a complimentary tool in the patient’s clinical assessment process. Whenever a patient is referred for MR imaging with hip pain, plain radiographs of the hip and pelvis and comprehensive physical examination have already been completed. Unfortunately, the physical examination of the hip does not allow for a specific diagnosis, even in experienced hands, compared with other joints. However, by the time MR imaging is ordered, the physician already has a preliminary working diagnosis, and MR imaging is used to confirm this diagnosis and to exclude other entities, particularly extra-articular pathologies, such as stress fractures that are not apparent on radiographs ( Fig. 1 ).
MR arthrography has been shown to be more sensitive than plain MR imaging at detecting various lesions around the hip. In addition, administration of local anesthetics with the gadolinium injection may provide additional confirmation of the diagnosis. Importantly, decrease in the pain level after the injection confirms the hip as the source of pain. However, in some cases the gadolinium itself can cause pain as a side effect; thus, the examination has the opposite effect, with increase in pain level. In those cases, if there is any doubt in diagnosis, a second dedicated intra-articular anesthetic injection is advised.
MR imaging may also give the surgeon data for quantifying the pathology and morphology of the hip and pelvis (eg, femoral neck version, femoral head coverage, and alpha angle). MR imaging may give information on pathology that is not readily seen during hip arthroscopy, such as bony edema, subchondral cysts, or intramuscular ganglia. MR imaging can also assess periarticular areas that are not routinely visible during routine hip arthroscopic surgery; for example, gluteus medius tears, iliopsoas bursitis or tendinopathy, and the peripheral compartment may be demonstrated. Moreover, MR imaging may point out areas of interest to be explored during the surgery; such areas as the posteroinferior acetabulum and the inferocentral femoral head are hard to view during hip arthroscopy and may require special attention, thus MR imaging may help in surgical planning.
MR imaging may be useful in preoperative planning or setting patients’ postoperative expectations. Chondral damage and subchondral cysts may be identified preoperatively using MR imaging. In the presence of limited full-thickness chondral damage, where a microfracture procedure may be indicated, the patient may be counseled to expect a longer rehabilitation on crutches because of microfracture procedure, and in case of a large chondral cyst bone grafting may be planned. MR imaging findings may also alter or postpone a planned surgery; for example, extensive chondral damage may lead to the decision to replace the hip, whereas low-grade avascular necrosis (AVN) postpones arthroscopic surgery until it is resolved or the procedure changed to core decompression.
Acetabular morphology and bony pathology
The acetabular morphology cannot be assessed from inside the joint during surgery, thus any decision regarding acetabular procedures should be taken beforehand. The first and basic tool is plain pelvic radiography, which gives information about the arthritic status; acetabular inclination (Tonnis angle); lateral acetabular coverage (center edge angle of Wiberg); acetabular depth; and gross estimation of the acetabular version by the crossover sign and the size of the ischial prominence. In that scenario, the role of MR imaging is to give a comprehensive look of the acetabulum and hemipelvis and help to assess and quantify the bony morphology and pathologies of the acetabulum.
Dysplasia
In general, the degree of acetabular dysplasia, the age of the patient, and the chondral status help to determine the surgical intervention. Young patients with moderate to severe dysplasia and preserved cartilage are advised to undergo periacetabular osteotomy with concomitant hip arthroscopy to treat any intra-articular damage. Mild dysplasia can be treated with arthroscopy alone; however, the surgeon must be careful to avoid iatrogenic labral damage while creating the first portal because of labral hypertrophy; moreover, in the presence of a crossover sign, acetabular rim trimming should be avoided as much as possible to reduce the risk for iatrogenic instability.
Femoral Head Overcoverage and Acetabular Version
Deep acetabulum (profunda or protrusio) and increased acetabular retroversion are considered causes of pincer-type impingement. In that case the surgical treatment is acetabular rim trimming of the anterolateral rim to eliminate the impingement.
Subchondral Cysts and Bone Marrow Edema
Acetabular bone marrow edema with or without subchondral cysts has been described in association of femoroacetabular impingement (FAI) and chondral damage. Both may be visible at the point of abutment on the acetabular side or on the femoral neck, and may help confirm the diagnosis of FAI.
Anterior Inferior Iliac Spine Impingement
The anterior inferior iliac spine is the origin of the direct head of the rectus femoris tendon. In some cases, with excessive distal or anterior extension, it may result in impingement against the distal femoral neck. This can be developmental in nature because of repetitive stress by contraction of the rectus femoris, prior anterior inferior iliac spine avulsions, or pelvic osteotomies. MR imaging may help define the anatomy of the anterior inferior iliac spine, help confirm the diagnosis, and help guide treatment. Arthroscopic surgical decompression of the anterior inferior iliac spine has been described as a treatment option ( Box 2 ).
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Dysplasia
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Femoral head over coverage
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Acetabular version
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Subchondral cysts and bone marrow edema
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Anterior inferior iliac spine impingement
Femoral morphology and bony pathology
Femoral Neck Anteversion
The femoral neck version is measured between the femoral neck and the axis that crosses the distal femoral condyles. Increased femoral neck anteversion may increase the pressure in the front of the hip joint, causing iliopsoas impingement and anterior ligament laxity. Furthermore, femoral anteversion has been implicated in FAI, with reduced anteversion associated with cam-type impingement, and increased anteversion with pincer-type impingement. Although there is a correlation between increased femoral anteversion and increased internal rotation of the hip, and it is possible to estimate it on pelvic radiographs, those methods are not sufficiently accurate for surgical decision making. Computer tomography (CT) and MR imaging can be used to evaluate the femoral anteversion more accurately. Strong correlations between the two modalities with correlation coefficient of 0.77 to 0.80 have been shown; however, in two studies the CT measurements exceeded the MR imaging measurements on average of 7 to 11 degrees. There have been reports of a higher rate of failed iliopsoas lengthening surgery in the treatment of internal snapping hip in patients with increased femoral anteversion. Thus, in situations where patients have increased femoral anteversion as measured on MR imaging and symptomatic FAI or internal snapping hip, the surgeon should take caution about performing hip arthroscopy and consider other alternatives, such as a derotational osteotomy of the femur.
Alpha Angle
The alpha angle was described by Notzli and colleagues in 2002 as a measurement on oblique axial MR imaging. It is a simple method to quantify the concavity of the femoral head-neck junction, and it was shown to correlate with anterior hip impingement. The alpha angle can also be measured on plain radiographs, and was found to correlate well with MR imaging measurements. An increased alpha angle is considered a positive sign of cam-type impingement; however, cam impingement may also occur in the presence of a normal alpha angle. Furthermore, normal alpha angle values have fluctuated since the original report, with angles of more than 50 or 55 degrees being considered abnormal. However, outcomes do not correlate with the restoration of the alpha angle, and alpha angle measurements have a high interobserver variability. Additionally, as originally described, the alpha angle only measured anterior femoral head-neck offset. There is a spectrum of cam pathology, with loss of offset anteriorly, laterally, anterolaterally, or a combination; thus, it was suggested to use radial MR imaging or three-dimensional CT to measure the alpha angle in different planes. Unfortunately, in the United States it has become a conventional measure that needs to be reported as an indication for FAI surgery. Surgical reshaping of the femoral head-neck junction (femoral osteoplasty) is used to eliminate cam impingement.
Herniation Cysts
Small cystic lesions at the anterosuperior portion of the femoral neck, also known as herniation pits, are a common imaging finding of FAI ( Fig. 2 ). Herniation pits were first described by Pitt in 1982, and were believed to be inclusion cysts that were normal variants and of no significance. Subsequently, Leunig and colleagues showed that 33% of patients undergoing surgery for FAI demonstrated herniations pits, and these occurred in the location of the impingement. These herniation cysts usually do not require special treatment; most are excised during cam decompression, confirming the location of the impingement. Some surgeons have recommended bone grafting of large herniation pits. Although these can be frequently seen on plain radiographs, MR imaging is more sensitive at identifying these cysts; notation of cyst location may help guide the surgeon to the location of FAI.
AVN and Transient Idiopathic Osteoporosis
AVN and transient idiopathic osteoporosis are pathologies that often require MR imaging for diagnosis. Both may be overlooked in the presence of substantial FAI morphology or physical examination that suggest labral tear. Arthroscopic surgery is to be undertaken with caution in these situations, and only if the pain is confirmed to be within the joint. There is some concern that traction required for hip arthroscopy may affect blood flow to the femoral head. If AVN is already present, there is a concern that traction will exacerbate the condition ( Box 3 ).
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Femoral neck anteversion
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Cam-type impingement and alpha angle
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Cysts on the femoral neck as a sign of impingement
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Bony edema on the femoral neck as a sign of impingement
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AVN
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Transient idiopathic osteoporosis
Intra-articular finding
Labral Tears
Labral tears are known as a source of hip pain, and are the most common indication for hip arthroscopy. In 2001, Seldes and colleagues described two types of labral tears: type 1, detachment of the labrum from the articular cartilage surface ( Fig. 3 ); and type 2, intrasubstance tears of the labrum ( Fig. 4 ). Type 1 tears are thought to be caused by shear forces within the joint, whereas type 2 tears occur secondary to compression forces on the labrum. Because of the lack of intrasubstance blood supply of the acetabular labrum, most surgeons consider type 1 tears reparable (because of the blood supply from the bony acetabular rim) and type 2 tears irreparable (because of lack of intrasubstance blood supply). In addition, the location of the acetabular labral tear can suggest its source: anterior tears may be caused by iliopsoas impingement or hip instability, anterolateral tears caused by FAI, and a posterior tear caused by instability or contrecoup lesion in FAI. The viability of the labrum as seen during surgery determines whether the labrum can be refixated (usually the type 1 lesions, also known as labral-chondral separations) or has to be debrided (type 2 lesions). It has been reported that refixation of the labrum is associated with improved clinical outcome compared with debridement. Labral and perilabral cysts are indicative of labral tears, even if the tear is not clearly seen ( Fig. 5 ).
Ligamentum Teres Tears
Ligamentum teres (LT) tears are a well-known source of hip pain and may be traumatic or atraumatic in nature. The function of the LT after the age of 14 years is unclear, although it may play a role in hip stability. LT tear is the third most common intra-articular pathology; however, it is infrequently mentioned or described in the MR imaging report. In 1997, Gray and Villar described three types of LT tears: grade 1, full-thickness traumatic tear; grade 2, partial tear; and grade 3, degenerative tear. Other studies have separated LT tears into partial or full-thickness tears. Botser and colleagues further divided the partial LT tear into low- and high-grade tears. The incidence of LT tears among patients with hip arthroscopy has varied. Some studies reported low incidence ranging from 4% to 17%, whereas other studies reported a much higher rate of LT tears ranging from 51% to 65%. Some surgeons do not routinely evaluate the LT during hip arthroscopy; thus, reporting of LT pathology on MR imaging may be helpful to some surgeons. The common surgical treatment of LT tears is debridement using radiofrequency or a shaver device and recently reconstruction of the LT has been reported.
Cartilage Damage
Plain radiographs only identify chondral damage when it is advanced, resulting in joint space narrowing. However, with increasing interest in hip preservation, and the treatment of prearthritic or nonarthritic hip pathology, earlier detection of chondral damage before joint space narrowing is evident is required. MR imaging is a sensitive tool for detecting chondral damage ( Fig. 6 ). However, there are limitations of detecting early chondral injury: hip cartilage is thin, and the hip joint is deep, resulting in low signal-to-noise ratio. Also, the spectrum of chondral injury seen in the hip is quite unique. Based on the pathophysiology of FAI, there is softening of the anterolateral articular cartilage, followed by delamination of the articular cartilage from the acetabular bone, and subsequent chondral flap formation, which can lead to detachment. Identification of “cartilage at risk” before softening is a key and allows for intervention before delamination occurs. This is particularly important because current treatments for chondral loss are limited and clinical outcome is worse when there is chondral damage.
