Ultrasound (US) is well suited for use in clinical practice given its affordability, portability, and lack of radiation exposure for image acquisition. These features allow clinicians from almost any specialty the ability to perform image-guided evaluations and interventions.[1][2][3][4][5] Radiologists are uniquely suited to perform these types of evaluations and procedures given their extensive training, knowledge of human anatomy, and understanding of varied anatomical appearances on multiple imaging modalities.[6]

Part of a radiologist’s training comprises an understanding of medical physics, which includes how medical images are obtained.  In ultrasound, the sonographer or physician utilizes a transducer to interrogate the area of interest. Within the transducer are piezoelectric crystals that vibrate when exposed to an alternating current.  These vibrations generate sound waves that are transmitted into the patient’s soft tissues via a layer of ultrasound gel.  The sound waves then interact with the tissue and are ultimately reflected back to the transducer.  Once the sound waves return to the transducer, they are converted back into an electric current. The computer then calculates the time interval between when the sound wave was initially generated and when it was received back at the transducer to determine the location (depth) of the tissue, which reflected the sound wave.

When planning a procedure, one must consider the type of transducer which will yield the highest quality images.  There are a variety of transducers that can be used with varied frequency and shape. In general, higher frequency transducers are utilized for imaging more superficial structures as they offer better resolution. However, the higher frequency sound wave is more easily attenuated and thus has decreased ability to penetrate tissue, which limits their use in the evaluation of deeper structures.  For deeper structures or larger patients, one might choose a lower frequency or curved transducer. When imaging small body parts, such as a finger, a small transducer (hockey stick) may be helpful.

Image guidance for procedures includes the use of ultrasound, fluoroscopy, computed tomography (CT), or in some cases, MRI. In general, ultrasound is particularly helpful in the guidance of soft tissue procedures such as therapeutic tendon sheath injection, soft tissue biopsy, as well as cyst or abscess aspiration/drainage.[1][7] Ultrasound, CT, and fluoroscopy guidance can be used for both joint aspiration and therapeutic injection. The chosen modality may depend on availability, user expertise, the body part to be treated, and patient body habitus. Advantages to US include lack of ionizing radiation and direct real-time visualization of the needle and surrounding soft tissues during the procedure. During fluoroscopic guidance, the needle is intermittently imaged, and it’s position judged only in relation to the associated osseous structures. The involved soft tissue structures must be inferred by knowledge of anatomy. Also, fluoroscopy often requires the use of a contrast agent to verify needle placement as opposed to US, which allows for direct visualization of the needle position. CT is useful for biopsy of osseous structures or therapeutic injection of deep anatomic structures, which can be difficult to visualize with US or palpate on physical examination.[7][8][9][10]

The long head biceps muscle has a proximal insertion at the shoulder where the tendon is intra-articular, originating on the supraglenoid tubercle and superior glenoid labrum. It exits the glenohumeral joint at the level of the humeral head near the rotator cuff attachments on the humerus, entering the bicipital groove. It is held within the proximal bicipital groove by the coracohumeral and transverse humeral ligaments as well as fibers of the subscapularis tendon.

In most patients, there are distinct attachments of the long and short head of the biceps tendon on the radial tuberosity. The distal biceps tendons traverse the elbow joint and attach to the radial tuberosity. The long head normally constitutes the more proximal insertion on the radial tuberosity and the short head more distal. Therefore it is possible to have an isolated tear of either the long or short head.

At the distal biceps myotendinous junction, there is a broad aponeurosis known as the lacertus fibrosus (also called the bicipital aponeurosis), which circumferentially covers the flexor muscle attachments as well as the median nerve and brachial artery. This aponeurosis originates at the distal biceps tendon and inserts onto the radial tuberosity and proximal ulna.  It is held in place by multiple strong fascial connections and serves as a protective covering for the neurovascular structures which course beneath. In most cases, the lacertus fibrosus prevents a torn biceps tendon from retracting more than a few centimeters proximal to the elbow joint, however, if the lacertus fibrosus is also injured the tendon retraction may be more severe leading to a more difficult surgical repair.

Ultrasound has uses in a wide range of musculoskeletal procedures including joint or bursa aspiration to evaluate for infection or crystalline deposition disease, joint or bursa injection such as an arthrogram or steroid injection, tendon sheath injection, tendinosis therapy such as dry needling, platelet-rich plasma (PRP) injection, and percutaneous biopsy. Injections into a joint, bursa, or tendon sheath can be both therapeutic (pain relief) and diagnostic by allowing the referring provider to gain knowledge of the patient’s pathology based on their pain response post-treatment.[11][12]

Shoulder pain can be complex and multifactorial. Clinicians use a combination of physical examination, history, and imaging to establish a differential diagnosis.[13][14] In some cases, MRI imaging may identify multiple abnormal findings, and injections performed to help elucidate the clinical significance of the imaging findings. For example, the MRI examination may identify a superior labral tear and biceps tendinosis in addition to rotator cuff tear, but the physical examination is most consistent with rotator cuff pathology. The orthopedist may request a biceps tendon injection to help determine if a biceps tenodesis at the time of rotator cuff repair is indicated.[6][15][16]

Absolute contraindications to corticosteroid injections include local infection and bacteremia. If the injection is intraarticular, absolute contraindications would also include septic arthrosis and intraarticular fracture. In the setting of biceps tendon sheath injection, it should be noted that the biceps tendon sheath communicates with the glenohumeral joint.  Therefore, medication injected into the biceps tendon sheath will also extend to the joint. If the clinical indication is to assess for septic arthrosis or tenosynovitis, one must carefully evaluate the overlying soft tissues for signs of cellulitis.  It is crucial to avoid any potential contamination of the joint or tendon sheath, which could occur by placing the needle through infected soft tissue before accessing the joint. Relative contraindications include local or juxta-articular osteoporosis, coagulopathy, or recent injection or multiple frequent injections, defined as three times over the last year or within the prior six weeks.[17]

There are several varieties of steroids available for use on the market today, including methylprednisolone acetate, triamcinolone, betamethasone acetate with betamethasone sodium phosphate and dexamethasone sodium phosphate, each of which offers their strengths and weakness. The most commonly used steroids in the United States include methylprednisolone, triamcinolone hexacetonide, triamcinolone acetonide, and betamethasone. [18]

Of note, the equivalent potency of betamethasone and dexamethasone sodium phosphate is approximately five times that of the others. Solubility also varies between the steroid formulations. For example, the acetate portion of betamethasone is insoluble, while the sodium phosphate portion of betamethasone and dexamethasone sodium phosphate is freely water-soluble. Particle size varies from 0.5 micrometers dexamethasone to over 500 micrometers methylprednisolone acetate, and triamcinolone has extensive particle aggregation, which requires meticulous mixing of the solution before injection.

Dosage varies based on the chosen steroid and location treated with small joints (metacarpophalangeal, facet, acromioclavicular), medium-sized joints (elbow, wrist), and larger joints (knee, shoulder, hip, ankle) receiving respectively increased amounts of steroid. For example, while a large joint might require 40 to 80mg of methylprednisolone acetate, that same joint would only require 8 to 15 mg of dexamethasone sodium phosphate. Similarly, a small joint might require 4 to 10 mg of methylprednisolone acetate, while that same joint would only require 0.8 to 2 mg of dexamethasone sodium phosphate. Some examples of steroid mixtures used for biceps tendon sheath injection at our institution include 3.75 mg dexamethasone mixed with 2 ml 1% lidocaine or 1ml betamethasone (6 mg/ml) mixed with 2 ml 1% lidocaine.[19]

Before injecting the steroid, a local anesthetic is provided to the skin and superficial soft tissues using 1% lidocaine. Although we typically use around 5 to 7 mL for local anesthetic, it should be noted that the maximum safe dosage for lidocaine in the United States is 300 mg, which equates to 30 mL of 1% lidocaine.[18] Bupivacaine is an alternate medication that can be used for anesthesia.  Its long duration of action is advantageous, but it has been shown to be the most toxic to chondrocytes of the local anesthetics. It should never be used for intraarticular injections.[11][12][18][20][21]