For patients with hip symptoms, MRI is of great significance for early diagnosis and treatment, and can provide anatomical details of the hip joint, including information about the bones, articular cartilage, and surrounding soft tissues that make up the joint. This article also summarizes the MRI imaging manifestations of common hip joint diseases.
The hip joint is a ball-and-socket joint composed of the femoral head and acetabulum. Except for the fovea, the femoral head is covered with transparent cartilage that extends to the junction of the femoral head and neck. The acetabulum side articular surface is crescent-shaped, and the articular cartilage covers its anterior, superior, and posterior regions. The pelvis and proximal femur are the reserve sites of red bone marrow, so the bone marrow signals of the pelvis and proximal femur are usually uneven (Figure 1).
Figure 1 MRI image of normal acetabular labrum. Coronal T2WI can clearly show the joint capsule (↑) and the transitional part of the labrum cartilage (*)
Key points for MRI diagnosis of common lesions:
1. OA: OA is the most common disease of the hip joint and is the result of degeneration of articular cartilage, subchondral bone and soft tissue. MRI helps to identify hip OA early and can be used for follow-up evaluation. In general, hip OA is diagnosed by a physician and reported as mild, moderate and severe. Other more detailed MRI-based hip scoring systems are mostly used for research rather than routine clinical reporting. Common related imaging features that need to be focused on in hip OA should include: bone morphology of the acetabulum and femoral head and neck, bone hyperplasia and osteophytes, cartilage and subchondral cysts and edema, acetabular labral injury, joint effusion, synovitis and loose bodies. It should be noted that the thinning of articular cartilage can be assessed by visual inspection by the physician without measurement (Figure 2).
Figure 2 MRI images of bilateral hip osteoarthritis. Coronal T2WI showed bilateral hip joint narrowing, bone hyperplasia, and patchy bone marrow edema under the articular surface of the right femoral head
2.Acetabular labrum injury: Acetabular labrum injury may be caused by trauma or by degeneration, wear and tear due to joint activity and excessive motion. DDH, FAI and other hip joint morphological abnormalities are prone to labrum injury. Most acetabular labrum tears occur in the anterior or anterosuperior area of the hip joint. Isolated posterior labrum tears are uncommon. The location of acetabular labrum tears should be described using the Blankenbaker clock face or traditional anatomical descriptions, such as anterior, anterosuperior, etc. Labrum tears can be divided into partial tears and bone-labrum junction tears, and both types can occur at the same time. If localized high signal is seen in the labrum, it is diagnosed as labrum injury, and the changes in labrum morphology (thinning, hyperplasia), the nature of labrum injury (degeneration, wear) and the degree (mild, moderate or severe) should be reported. If a linear high signal is observed from the labrum articular side through the labrum base or into the labrum substance, it is considered to be a tear (Figure 3), and the location, type, length and presence of labrum separation should be mentioned. If there is an intra-labral or para-labral cyst, the size and location should be described.
Figure 3 MRI image of acetabular labrum tear. Coronal T2WI shows lateral superior acetabular labrum tear (short↑) with paraglenoid cyst formation (long↑)
3.FAI: FAI is caused by the contact and collision between the proximal femur and the acetabular rim, which can lead to lesions of the acetabulum, labrum and cartilage, produce pain and dysfunction, and can lead to early-onset OA. The pathogenesis of FAI may be caused by excessive coverage due to a deep acetabulum (Pincer type, also known as clamp type) and/or insufficient concavity at the junction of the femoral head and neck (Cam type, also known as cam type). The two anatomical abnormalities often occur at the same time (mixed type). The imaging management and evaluation of FAI should first be performed with an anteroposterior pelvic X-ray. Symptomatic patients should undergo frog position examination and CT examination to fully evaluate the morphology of the acetabulum and femoral head and neck. MRI is also an important means of comprehensively evaluating FAI, mainly focusing on evaluating the degree and range of damage to the labrum and cartilage. The anterior labrum is the main site of injury in FAI (Figure 4). It should be noted that the diagnosis of FAI can only be made when typical imaging features and clinical symptoms occur at the same time.
Figure 4 MRI image of hip impingement. Coronal T2WI shows acetabular superior labrum tear (short↑) accompanied by bone marrow edema at the junction of the femoral head and neck (long↑)
4:DDH: DDH is a disease involving abnormal development of the femoral head and acetabulum (Figure 5). It is mainly caused by the relatively shallow acetabulum, which leads to insufficient coverage of the femoral head, resulting in abnormal joint stress distribution, joint instability, cartilage damage and secondary hip OA. Radiologists should be familiar with the acetabular bone, cartilage, ligaments and soft tissue structure of infants and young children. When using MRI to evaluate DDH, the retroversion of the dysplastic acetabulum and the degree of femoral head coverage should be evaluated. Delayed ossification of the femoral head can be evaluated by contrasting the two hips. The main advantage of MRI is the ability to visualize the cartilaginous acetabulum and determine its coverage of the femoral head, which is more accurate than X-rays in showing acetabular coverage. Because the center of the femoral head is difficult to determine in young children, the bone acetabular index and cartilaginous acetabular index can be used on MRI to evaluate DDH. It should be noted that in mild DDH, the angle measurement results can be ignored.
Figure 5 MRI image of hip dysplasia. Coronal T1WI shows that the acetabulum and femoral head joint surface are poorly aligned, and the coverage rate of the right acetabulum on the femoral head is significantly reduced.
5.Avascular necrosis of the femoral head: Since avascular necrosis of the femoral head is often bilateral, MRI of both hip joints should be selected during imaging examination, and special attention should be paid to assessing the symmetry of the bilateral hip joints. Typical avascular necrosis of the femoral head is manifested as a crescent-shaped subchondral lesion on MRI, surrounded by a low signal edge, and a characteristic “double line sign” on T2WI (Figure 6). The location and size of the osteonecrosis, the estimated percentage of the femoral head volume affected (<15%, 15%~30%, >30%), and the stage of the lesion should be described. Since femoral head collapse represents more advanced disease and irreversible damage, whether the femoral head collapses should be described (<2mm is mild, ≥2mm is moderate to severe), and if secondary hip OA and femoral head fragmentation are present, they should also be described. The main signals of osteonecrosis can be used to assess the stage. The fat signal indicates hyperacute or healing lesions, the bone marrow edema signal indicates acute or subacute lesions, and the osteosclerosis signal indicates that the lesion is in the chronic stage.
Figure 6 MRI image of avascular necrosis of the femoral head. Transverse T2WI shows two strip-shaped high signals in both femoral heads, high inside and low outside, parallel to each other, which is a double line sign
6.Muscle tendon injury: Muscle strain accounts for a high proportion of all acute sports injuries. The gluteus minimus and gluteus medius tendons are the most commonly injured gluteal tendons. Most tears are at the insertion point, and partial tears are more common than complete tears. MRI is recommended to better understand the location and extent of the lesion, predict recovery time and clinical outcomes. In the past, acute muscle injuries were usually divided into strains (grade I), partial tears (grade II) and complete tears (grade III). Although there are currently a variety of muscle injury grading and classification systems, such as the BAMIC classification, which can be used to grade injuries, their clinical application value remains to be clarified (Figure 7).
Figure 7 MRI image of hamstring tendon injury
In the cross-sectional T2WI at the level of the ischial tuberosity, it can be observed that the semimembranosus tendon (long ↑) running anterolaterally and the biceps femoris-semitendinosus combined tendon (short ↑) running lateral to the ischial tuberosity are thickened with high signal
7.Sciatic femoral impingement syndrome: Sciatic femoral impingement syndrome is caused by the impingement of the quadratus femoris muscle between the lesser trochanter and the ischial tuberosity, resulting in deep buttock pain due to the narrowing of the sciatic femoral space. In symptomatic patients, if the sciatic femoral space is abnormally narrow and the quadratus femoris muscle is edematous with or without atrophy, it indicates the possibility of sciatic femoral impingement syndrome. There is still controversy about the critical value for determining the narrowing of the sciatic femoral space, which may be due to anatomical differences and different measurement methods in different geographical populations, but it is generally believed that the probability of sciatic femoral impingement is high when the sciatic femoral space is <15mm and/or the quadratus femoris distance is <10mm (Figure 8). If quadratus femoris edema is seen in the absence of sciatic femoral space narrowing, sciatic femoral impingement should not be considered, and causes such as quadratus femoris strain, tear or denervation should be considered.
Figure 8 MRI image of ischiofemoral impingement. Transverse T2WI showed narrowing of the femoral ischial space, with a gap of 6.4 mm, accompanied by quadratus femoris edema (↑)
8.Postoperative evaluation of hip joint: Hip arthroscopy, labral surgery, osteochondroplasty (Figure 9) and hip replacement are widely used in patients with hip joint injuries. After labral repair, if obvious fluid signals are observed entering the labrum and/or the labral cartilage junction, residual tear or recurrent labral tear can be diagnosed. During follow-up after hip replacement, MRI is the most accurate imaging method for evaluating periprosthetic osteolysis and synovitis, and can also evaluate the tendon and neurovascular structures related to the surgical area. Imaging evaluation after hip replacement should take into account common postoperative complications, mainly including periprosthetic osteolysis, prosthesis loosening and displacement, postoperative infection, joint effusion, synovitis, occult incomplete fracture, heterotopic ossification, and degeneration and tear of surrounding muscles and tendons. Other hip surgery methods, such as internal fixation of femoral neck fractures, should pay attention to clarify whether postoperative complications such as avascular necrosis of the femoral head are combined.
Figure 9 MRI images after hip impingement surgery. Coronal T2WI shows the acetabular nail channel (long ↑) and local bone defect of the femoral head and neck (short ↑), and surrounding soft tissue edema
References:
Chinese Medical Association Radiology Branch Bone and Joint Group. Expert consensus on hip MRI examination and diagnosis [J]. Chinese Journal of Radiology, 2024, 58(1): 18-24. DOI:10.3760/cma.j.cn112149-20231012-00286.
Statement: This article is derived from professional journals and books, and is compiled by the editor of Orthopedics Online. If there are any copyright issues, please contact us.
Post time: May-30-2024