8 The Hip: Diagnostic Imaging
Ultrasound examination has always had a relatively limited role in the assessment of hip pathology due to restrictions with regard to the deeper joints such as the shoulder and hip from which little detail of internal structure may be determined. However, despite these limitations much hip pathology is well detected provided that the clinician is familiar with the normal anatomy of the hip. Examination includes assessment of the soft tissues, tendons, ligaments, and muscles, and also of the bone and joint where there is acoustic access allowing adequate visualization.
Ultrasound is of particular use in the evaluation of the periarticular soft tissues and in the detection of both intra and periarticular synovial effusions and collections, and if required provides an easy and non traumatic guidance for needle aspiration for diagnostic purposes or therapeutic intervention. Given the complex anatomy around the hip joint, the examiner should make accurate differential diagnoses based on careful history taking and objective findings in order to focus on the relative appropriate structures of the hip prior to scanning. The hip is usually divided into four quadrants during scanning, the anterior, medial, lateral, and posterior aspects and accurate clinical information allows the examiner to focus on particular quadrants. It would not be a normal practice to examine all four quadrants in all patients.
An ultrasound examination of the hip should be conducted with a large footprint linear probe of a medium to low frequency (9-12 MHz). In the larger patient, a low frequency curvilinear probe may be of benefit.
8.1 Diagnostic Imaging of the Hip: Introduction
Examination of the hip will be dependent on the specific structure and pathology suspected from a thorough clinical examination. Based on this examination it would be normal to scan one or two specific structures. In addition to static scanning dynamic imaging should be included particularly when imaging tendons and ligaments to fully assess the patency of these structures. It should be noted that examination of the hip can be problematic particularly in the muscular or obese patient given the anatomical position of the joint. The use of relatively low-frequency ultrasound should be used where necessary to maximize image quality.
Imaging includes the following:
Hip joint including the femoral head, neck, capsule, and anterior synovial recess.
Iliopsoas muscle, tendon, and bursa.
Anterior inferior iliac spine (AIIS) and the tendon and muscle of rectus femoris.
Anterior superior iliac spine (ASIS) and the tendons and muscles of sartorius and tensor fascia lata.
Lateral femoral cutaneous nerve and inguinal ligament.
Medial region—supine in frog-leg position
Adductor tendons and muscles.
Gluteus maximus, tensor fascia lata, and the fascia lata.
Gluteus medius muscle and tendon.
Gluteus minimus muscle and tendon.
Greater trochanter and bursa (if pathological).
Hamstring muscles and tendon.
Ischial tuberosity and bursa (if pathological).
Anterior Hip Joint: Longitudinal Scan
The hip joint may only be effectively visualized from its anterior aspect which also allows imaging of the anterior femoral recess and the iliopsoas tendon and bursa (if pathological).
The patient is positioned in supine. A small pillow placed under the knee allows the hip to rest in a few degrees flexion which can facilitate scanning. To visualize the anterior aspect of the hip joint and the anterior femoral recess, a large footprint probe is required. In addition, given the depth of the joint particularly in the larger patient, a low-frequency probe should be used (Fig. 8‑1 , Fig. 8‑2 , Fig. 8‑3 ).
Anterior Inferior Iliac Spine: Longitudinal Scan
The patient is positioned in supine. A small pillow placed under the knee allows the hip to rest in a few degrees flexion which may facilitate scanning. The probe is placed over the AIIS in the sagittal plane to image both the AIIS and the tendon of rectus femoris. The use of a large footprint probe allows for better visualization of this area. In the larger patient, a low-frequency curvilinear probe should be used (Fig. 8‑4 , Fig. 8‑5 ).
Anterior Inferior Iliac Spine: Transverse Scan
The transducer is placed over the AIIS in the transverse plane to image the origin of the direct tendon of rectus femoris. The transducer is then moved in a caudal direction maintaining alignment in the transverse plane to image first the musculotendinous junction of the rectus femoris and then the muscle belly itself which can be found positioned between tensor fasciae lata, sartorius, and iliopsoas. More distally the muscle of rectus femoris may be seen to overlay the vastus intermedius muscle belly (Fig. 8‑6 , Fig. 8‑7 , Fig. 8‑8 , Fig. 8‑9 ).