MSK

Introduction to MSK Ultrasound

• MSK (Musculoskeletal) Ultrasound involves the dynamic evaluation of tendons.
• Clinical experience is crucial for accuracy in this specialty.
• In specific scenarios, MSK ultrasound can exceed the diagnostic accuracy of MRI and CT, particularly when movement is incorporated into the examination.
• Note: This material is not included on the Sonography Canada Exam.

Tendon Anatomy and Physiology

• Tendons consist of bundles of collagen fibers.
• Their primary function is to attach muscle to bone, specifically at sites called tuberosities.
• They facilitate joint movement, including flexion and extension.
• Tendons possess a relatively poor blood supply, which contributes to poor healing capabilities.
• Raised ridges are typically present at the specific point of attachment to the bone.

Sonographic Appearance of Tendons

• Short Axis (SAX):
  • Appears as hyperechoic, finely punctate foci (representing collagen fibers).
  • The connective tissue interspersed within appears hypoechoic.
  • The overall shape can be round, oval, or flattened.
• Long Axis (LAX):
  • Appears as a linear band composed of hyperechoic strands.
  • Exhibits a characteristic "fibrillar pattern," where hyperechoic collagen strands are interspersed with hypoechoic connective tissue.

Anisotropy and Scanning Artifacts

• Tendon echogenicity is highly dependent on the angle of insonation.
• Anisotropy: A loss of echogenicity that occurs when the ultrasound beam is not perpendicular to the tendon. This typically happens when the angle is $> 10 \text{ degrees}$ off perpendicular.
• Anisotropy is a common artifact that may be mistaken for pathology, such as a tear or inflammation.
• To mitigate this, the beam must remain perpendicular to the tendon. For curved tendons, the sonographer must change the probe position or attempt to straighten the tendon through movement.

Fibrous Sheaths and Bursae

• Fibrous Sheaths:
  • Synovial Sheaths: These wrap around the tendon to reduce friction and secrete lubricant fluid. On ultrasound, they appear as a thin, hypoechoic area around the tendon.
  • Paratenon/Peritenon: Loose connective tissue found surrounding tendons that lack a synovial sheath, such as the Achilles and patellar tendons.
• Bursa:
  • A flattened, synovial-lined sac or pouch located at points of high friction.
  • They produce synovial fluid.
  • Sonographically, a bursa is a hypoechoic, flattened structure that is often difficult to identify when healthy and normal.

Sonography of Nerves, Ligaments, and Other Structures

• Nerves:
  • LAX: Displays a "railroad" appearance, characterized by hypoechoic nerve fibers divided by hyperechoic connective tissue lines.
  • SAX: Displays a "honeycomb" pattern, consisting of circular hypoechoic nerve fibers surrounded by hyperechoic connective tissue. This is commonly observed in the wrist.
• Ligaments:
  • Composed of collagen and serve to attach bone to bone, providing stability and strength.
  • Sonographically similar to tendons but are generally smaller and more difficult to visualize.
• Bone:
  • Ultrasound assessment of bone is limited to the cortical (superficial) surface.
  • Appears as a bright, hyperechoic line with strong posterior shadowing.
• Cartilage:
  • Acts as a shock absorber; hyaline cartilage is located at the terminal ends of bones.
  • Sonographically, it is hypoechoic, well-defined, smooth, and non-compressible.
• Muscle:
  • Skeletal striated muscle can contract and extend.
  • Appears as hypoechoic tissue containing hyperechoic fiber striations.

The Shoulder: Rotator Cuff Anatomy

• The shoulder examination requires a high-resolution linear array transducer ($18 \, \text{MHz}$).
• The patient is typically seated on a rotating stool.
• The rotator cuff consists of four technical tendons:
  • Subscapularis.
  • Supraspinatus.
  • Infraspinatus.
  • Teres minor.

Specific Shoulder Tendon Scanning Techniques

• Biceps Tendon:
  • Functions as a flexor and extensor; sits in the bicipital groove between the greater tuberosity (GT) and lesser tuberosity (LT), anterior to the humerus.
  • Technique: Arm in a neutral position with the elbow bent at $90 \, \text{degrees}$. In SAX, document the proximal tendon in the groove. In LAX, document the tendon proximally and slightly more distally.
• Subscapularis Tendon:
  • Extends from the underside of the scapula and attaches to the lesser tuberosity.
  • Technique: Arm in external rotation. The probe is transverse on the patient, providing a LAX view of the tendon. Images are taken at the coracoid and the insertion point on the LT. A dynamic assessment is essential.
• Supraspinatus Tendon:
  • Responsible for humerus abduction and stabilization; superior to the humeral head and attaches to the greater tuberosity. It is the most commonly torn rotator cuff tendon.
  • Technique: Arm is hyperextended and internally rotated (achieved by placing the hand behind the back with the palm out, or in the back pocket with the elbow pulled back). Imaged in both LAX and SAX; in LAX, it often resembles a "beak."
• Infraspinatus and Teres Minor:
  • Infraspinatus acts as an external rotator and abductor; sits lateral and posterior to the shoulder, extending from the scapula to the GT.
  • Teres minor acts as an adductor and inserts on the GT.
  • Technique: Patient places the arm across the chest (the "Pledge" position). Both are typically imaged in LAX only.

Knee and Ankle MSK Sonography

• Knee:
  • Quadriceps Tendon: Formed by the junction of all four quadriceps muscles; inserts onto the base of the patella. It is concave when the leg is extended, so the leg should be flexed during scanning. Appears oval in SAX.
  • Patellar Tendon: Extends from the apex of the patella to the tibial tuberosity. Dimensions are approximately $5 \text{ – } 6 \, \text{cm}$ in length, $2 \text{ – } 2.5 \, \text{cm}$ in width, and $4 \text{ – } 5 \, \text{mm}$ in AP thickness. It is technically a ligament.
• Ankle (Achilles Tendon):
  • Formed by the fusion of the gastrocnemius and soleus muscle aponeuroses; inserts on the posterior calcaneus (heel bone).
  • Technique: Patient in a prone position with the foot hanging off the table. Dorsiflexion helps straighten the tendon.
  • Dimensions: Typically $1.2 \text{ – } 1.5 \, \text{cm}$ wide and $0.5 \text{ – } 0.7 \, \text{cm}$ AP.
  • Kager's Fatty Triangle: An area of variable echogenicity located anterior to the distal half of the tendon.

The Hand and Wrist: Carpal Tunnel

• Most pathology in the hand/wrist results from overuse or compression, with Carpal Tunnel Syndrome (CTS) being the most common.
• Carpal Tunnel: The space between carpal bones and the flexor retinaculum (ligament) containing tendons, the median nerve, muscles, and vessels.
• Median Nerve: Passes through the carpal tunnel, anterior to the flexor tendon of the second finger.
• Technique: Scanned in transverse/SAX with the forearm on a flat surface and the wrist in supination. The ulnar artery serves as the medial landmark.

General MSK Pathology and Tendon Tears

• General Pathology: Includes tears, inflammation (tendonitis/peritendinitis), ganglion cysts, Baker's cysts, CTS, tumors, and foreign bodies.
• Factors Contributing to Tears: Age, calcifications, chronic use of corticosteroids (which inhibit collagen synthesis), and systemic diseases such as lupus, gout, or diabetes.
• Complete vs. Incomplete Tears:
  • Complete: Full disruption of the tendon; results in a gap that may fill with fluid or allow the deltoid to sit directly on the humeral head. The tendon may retract out of the field of view.
  • Incomplete: Focal defect in the tendon or at the attachment point; often presents with symptoms similar to tendonitis.
• Sonographic Signs of Tears:
  • Hypoechoic defects and focal thinning.
  • Architectural distortion and fluid-filled gaps.
  • Cartilage-interface sign: A thin, hyperechoic line appearing at the interface between cartilage and an abnormally hypoechoic tendon.
  • Geyser sign: Fluid escaping through the AC joint and resting on top of the shoulder.
  • Rim rent: A small incomplete tear appearing as a linear or comma-shaped hyperechoic line.

Inflammation and Bursitis

• Tendonitis: Inflammation of the tendon (focal or diffuse) often caused by overuse or sports. Sonographically characterized by thickening, decreased echogenicity, ill-defined margins, and hyperemia (increased blood flow). Chronic cases may show calcifications.
• Peritendinitis: Inflammation of the peritenon, common in the Achilles.
• Tenosynovitis: Inflammation of a synovial tendon sheath.
• Bursitis: Inflammation of the bursa due to trauma or chronic microtrauma; appears as a sonolucent (fluid) collection with ill-defined walls.

Cystic Masses and Tumors

• Ganglion Cysts:
  • Benign soft tissue cystic masses attached to a joint or tendon sheath, common in the wrist.
  • Appear as oval fluid collections with acoustic enhancement; chronic cysts may appear hypoechoic and solid.
• Popliteal (Baker's) Cyst:
  • A synovial cyst in the posteromedial knee caused by a dilated gastrocnemio-semimembranous bursa.
  • Associated with rheumatoid arthritis; can rupture or mimic DVT symptoms.
• Benign Tumors:
  • Giant Cell Tumors: Benign, lobulated hypoechoic masses of the tendon sheath.
  • Osteochondroma: Benign cartilaginous tumor; can develop in a Baker's cyst, appearing hyperechoic with posterior shadowing.
  • Lipoma: Painless, mobile, and compressible adipose tissue; usually hyperechoic.

Carpal Tunnel Syndrome (CTS) and Foreign Bodies

• CTS Imaging Signs:
  • Encroachment of the median nerve due to decreased tunnel size or increased volume.
  • A nerve that is $3 \times$ greater in one axis than the other at $90 \, \text{degrees}$.
  • Loss of the "honeycomb" appearance in SAX.
  • Sudden change in contour in LAX.
  • Increased cross-sectional area compared to the unaffected side.
• Foreign Bodies:
  • Ultrasound identifies non-radio-opaque objects (like wooden splinters) that X-rays miss.
  • Metal: Appears hyperechoic with a characteristic "comet tail" artifact.
  • Soft tissue is examined for secondary inflammation or abscess formation.