Fig. 5.1
(a) Normal AP view of the pelvis. The curves formed by joint surface of the femoral head and the acetabulum are parallel. The femurs are internally rotated as indicated by the fact that the lesser trochanter (white arrow) is barely visible. (b) Normal frog leg lateral view of the hip. (c) Normal cross-table lateral view of the hip
An organized approach to the evaluation of the radiograph increases the likelihood of detection of pathology. The interpreter must remember to include the sacroiliac joints, lumbar spine, pubic symphysis, obturator foramen, and soft tissues in their scan pattern and not to focus on just the femoral head and hip joints .
5.2.2 Ultrasound
The availability of ultrasound (US) scanners in sports medicine practice is rapidly expanding allowing both diagnositic imaging and guidance for therapeutic interventions. The technique requires that the operator be knowledgeable and the learning curve can be quite steep. US is best used in the assessment of soft tissue disorders such as muscle and tendon pathology. For example, the diagnosis of a snapping iliopsoas tendon is rapidly and easily confirmed with ultrasound.
5.2.3 Computed Tomography
Computed tomography (CT) is most commonly used to evaluate suspected or known fractures. However, the development of helical CT in modern scanners has led to the ability to perform reconstructed images in multiple planes. Complex oblique images are now obtainable on-the-fly on many workstations. Some investigators have taken advantage of this capability and have shown that CT arthrography can yield useful information in patients that are not suitable for MR evaluation regardless of the reason.
It is extremely important that referring clinicians understand that radiation dose is not insignificant with this modality. Techniques not requiring radiation such as MR should be considered before CT particularly in young patients .
5.2.4 Magnetic Resonance Imaging
The introduction of clinical magnetic resonance imaging (MR) has revolutionized the field of musculoskeletal imaging and has improved our understanding of injuries to the athlete. Although expensive and time-consuming, MR offers a global evaluation of the hip including assessment of bone, cartilage, muscle, and tendons. The two most critical aspects leading to successful diagnosis of pathology are the quality of the examination and the experience of the interpreter.
Not all MR examinations or scanners are equal and it is extremely important that the referring clinician has an open line of communication with the radiologist to ensure that the appropriate examination is done with the appropriate equipment. One consideration is often whether the patient should be scanned on an open low field magnet or a closed high field system. In general, if the question is fracture or muscle injury, either a low field or high field system will yield equivalent results. However, if the question pertains to intraarticular pathology such as cartilage defects or labral tears, then the examination should be performed on a high field system (ideally 3 Tesla) with a small field of view .
5.2.5 Magnetic Resonance Arthrography
Magnetic resonance arthrography (MRA) is the preferred examination for the assessment of cartilage pathology in the hip joint [3]. Some investigators feel that high quality, small field of view imaging is sufficient for detection of pathology but most radiologists prefer MRA. Diagnoses can be rendered more confidently when the joint is distended by contrast. Although MRA may be superior to non-contrast examination, it does have some negative aspects. The examination is typically performed following the direct administration of contrast into the joint which requires an image guided introduction of a needle into the joint and this can be painful for some patients. One distinct advantage of direct arthrography is that additional diagnostic information can be obtained if anesthetic is injected into the joint at the same time as contrast. Relief of pain with provocative maneuvers following the intraarticular injection of anesthetic can go a long way towards confirming an intraarticular source of pain. Some radiologists advocate the use of indirect arthrography which involves intravenous injection of contrast with the imaging of the joint following exercise and a brief delay. While this technique is less painful, it does not distend the joint and is associated with rare but potential allergic reaction to gadolinium contrast.
A summary of recommended examinations is provided in Table 5.1.
Table 5.1
Recommended radiology studies
Diagnosis | X-ray | CT | US | MR | MRA |
|---|---|---|---|---|---|
Stress fracture | +++ | – | – | +++ | – |
Acute fracture | +++ | ++ | – | ++ | – |
Arthritis | +++ | – | – | +++ | – |
Femoroacetabular impingement | +++ | ++ | – | ++ | +++ |
Labral tear | – | – | – | + | +++ |
Tendon injury | + | – | ++ | +++ | – |
Muscle injury | + | + | ++ | +++ | – |
5.3 Radiologic Diagnosis of Hip Pain in Athletes
5.3.1 Osseous
Bone is an amazing organ system that develops based on a combination of genetic and physical influences. However, despite its remarkable mechanical and physiologic properties, bone may fail in response to excessive acute or chronic repetitive forces.
5.3.2 Stress Fracture
Stress fractures are a result of excessive force being applied to bone in a chronic repetitive fashion. The normal physiologic response to new stress to a bone is for remodeling to occur but if the osteoblastic response is outpaced by removal of bone by osteoclasts, the bone can mechanically fail. Stress fracture should be considered in the differential diagnosis of any patient that has recently changed the intensity of their physical activity. In addition to poor physical conditioning, risk factors for development of stress fracture include female gender, Caucasian race, smoking, steroid use, tall/thin physique, and low sex hormones [4, 5]. The basicervical portion of the femoral neck is the classic location for stress fracture to occur in the hip or pelvis but other described locations include the pubic rami, sacrum, superior acetabulum [6], medial femoral diaphysis [7], and even the femoral head [8].
Radiographs are usually the first line modality used to evaluate stress fractures . The appearance of a stress injury to bone will be dependent on the temporal nature of the injury and the bone involved. The initial radiographic examination may be completely normal because 30–50 % of bone must be resorbed for a lucency to be appreciated on radiography. It is very important to make sure that the femur is properly positioned for the radiographic examination [2]. The femur should be internally rotated approximately 10–15° to ensure that the femoral neck is adequately evaluated (Fig. 5.2).
Fig. 5.2
Twenty-five-year-old runner with hip pain and completed stress fracture. (a) AP view of the hip in external rotation. The lesser trochanter (white arrow) is in profile indicating external rotation which foreshortens the femoral neck. (b) AP view of the hip in internal rotation. The fracture that was not appreciated in external rotation is now easily seen (black arrowheads). Note the linear lucency laterally and the condensation of trabeculae medially
The classic radiographic findings that may be appreciated include sclerosis and periosteal reaction [9]. The initial radiographs may be normal. Sclerosis is usually in a linear pattern transverse to the long axis of the involved bone. On occasion, this sclerotic reaction and periosteal reaction can be exuberant leading to concern for tumor. The linear nature of the abnormality and the patient’s history will usually be sufficient to dispel concerns about neoplasm. It is not uncommon for subtle findings to be missed particularly when the abnormality is in an unusual location or at a site that may be obscured by bowel gas such as the sacrum. A high degree of suspicion and careful side-to-side comparison of the AP radiograph of the pelvis will increase the likelihood of detection of a stress injury (Fig. 5.3).
Fig. 5.3
Twenty-two-year-old active duty military woman with right hip pain after recent increase in miles run per week. AP view of the pelvis shows subtle band of sclerosis at the inferior basicervical portion of the femoral neck (black arrows) indicating stress fracture
MR of the hip should be ordered when a stress fracture is clinically likely and the radiograph is normal. The importance of early detection has been emphasized because the prognosis is poorer if the patient progresses to a complete or displaced fracture. The MR examination should evaluate the entire pelvis with both T1 and a fluid sensitive sequence such as a fat suppressed T2 weighted or short-tau inversion recovery (STIR) sequence.
Stress fractures present as a linear band of signal replacing normal bone marrow [10]. On T1 weighted sequences, the normal bright signal of fat will be replaced by a linear band of low signal. On fat suppressed sequences, the normal low signal fat is replaced by bright signal that may or may not surround a thin band of low signal. The MR presentation is usually very characteristic (Fig. 5.4).
Fig. 5.4
Twenty-one-year-old active duty female with left hip pain after changing physical training regimen. (a) AP view of the pelvis shows subtle sclerosis in the left femoral neck (white arrowheads). (b) Coronal T2 fat saturation sequence clearly demonstrates diffuse edema of the femoral neck (asterisk) and medial callus formation (white arrow)
Two recently described presentations of stress injuries deserve special mention, namely, thigh splints [7] and subchondral stress fracture of the femoral head [8]. Patients with thigh splints present with either medial sided thigh or groin pain. This injury occurs secondary to abnormal forces at the insertion of the adductor muscles on the medial diaphysis of the femur. Radiographically, thigh splints present as solid periosteal reaction along the medial aspect of the femur. The MR correlate for this plain film finding is high signal on fluid sensitive sequences located medial to the cortex of the diaphysis of the femur. This subtle MR finding can be easily overlooked.
Stress fracture in subchondral bone of the femoral head is a relatively newly described condition that may be confused with avascular necrosis (AVN). This diagnosis should be considered when findings mimicking AVN are seen in the femoral head of an athlete without risk factors for osteonecrosis. A correct diagnosis of subchondral stress fracture in these patients leads to the correct treatment (rest) and the prognosis is generally much better than that of AVN .
5.3.3 Acute Fracture
Acute hip or pelvis fracture is uncommon as a result of acute trauma in athletics even in high-energy contact sports. Children, however, are an exception to this rule because of the inherent weakness at the physis and apophyses of growing bone [11]. Radiography with AP view of the pelvis and orthogonal view of the symptomatic hip is usually sufficient to render a diagnosis. Careful side-to-side comparison between the affected and asymptomatic side helps establishing otherwise diagnoses (Fig. 5.5).
Fig. 5.5
Thirteen-year-old boy with left hip pain after being tackled while playing football with fractured left acetabulum. An AP view of the pelvis shows subtle asymmetry in the triradiate cartilage, which is wider on the left (white arrowheads) than on the right (white arrow) indicating a Salter I fracture of this growth center
Slipped capital femoral epiphysis (SCFE) is an important diagnostic consideration in the pre-adolescent or adolescent athlete. These patients are approximately 11 years of age with risk factors including obesity and black race. SCFE can present bilaterally although it is not common to see both hips symptomatically affected at the same time. Children present with either an acute or chronic history of groin pain. Radiographs are usually abnormal (Fig. 5.6). The involved physis is usually widened and the proximal femoral epiphysis is usually displaced inferiorly and medially relative to the femoral diaphysis [12]. The displacement of the epiphysis is sometimes appreciated on the AP view of the hip if the femoral head is located below a line drawn along the superior border of the femoral neck (Kline’s line). This line should normally intersect the superior lateral aspect of the femoral head. A lateral view will confirm the displacement of the femoral epiphysis. MR may be useful in the situation when a radiograph is normal in a patient with hip pain at risk for slipped femoral epiphysis. The MR will reveal widening of the physis with increased signal on fluid sensitive sequences in the adjacent metaphysis.
Fig. 5.6
Thirteen-year-old boy with right hip pain and slipped capital femoral epiphysis . (a) An AP view of the pelvis shows subtle widening of the right physis (hollow white arrowheads) in comparison with the left hip. Note the femoral head is below the white dotted line (Klein’s line) drawn parallel to the superior aspect of the femoral neck indicating the proximal femoral epiphysis is inferiorly displaced. (b) Frog leg lateral view confirms posterior displacement of the femoral head
The apophyses about the hip and pelvis are at risk for avulsion and these injuries are being diagnosed with increased frequency as children are exposed to highly competitive athletic activities that require sudden or violent muscle contraction such as soccer, hockey, gymnastics, and sprinting sports [11]. The apophyses at risk include the anterior superior iliac spine (ASIS) (sartorius origin), anterior inferior iliac spine (AIIS) (rectus femoris origin), ischial tuberosity (hamstring origin), ilium (oblique insertion), and lesser trochanter (iliopsoas insertion). Diagnosis is usually readily established with an AP radiograph (Fig. 5.7) although injuries in the ASIS and AIIS may be better appreciated on view of the pelvis obliqued to the side of injury. Avulsion of an apophysis can be easily missed on MR [13] and may be better appreciated on US or CT if confirmation is required in the setting of a “normal” radiograph .
Fig. 5.7
Sixteen-year-old woman with acute onset of pain in left hip and avulsion of the anterior superior iliac spine. (a) AP view of the pelvis shows avulsion of the anterior superior iliac spine (white arrow). (b) Avulsion injury is better appreciated on a left posterior oblique view of the left hip (white arrowhead)
5.4 Articular Pathology
There is an ever-increasing body of knowledge directed at the early detection of intraarticular injuries of the hip. Although the hip is an intrinsically stable joint, it should be no surprise that cartilage structure of the hip is at risk for tears since contact forces are 3–5 times body weight or higher in athletes. It is very important to remember to evaluate the sacroiliac joints and the pubic symphysis for conditions such as sacroiliitis or osteitis pubis. Disease in both of these joints may present as hip or groin pain in the athlete.
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