# Musculoskeletal Ultrasound Fundamentals for the Physiatrist

## Introduction

**Musculoskeletal ultrasound (MSKUS)** has become an essential point-of-care imaging and procedural guidance tool in physical medicine and rehabilitation. It offers real-time, dynamic imaging without radiation, is portable, and provides superior resolution for superficial soft tissue structures. MSKUS competency is now a core requirement in PM&R residency training.

## Physics and Instrumentation

### Basic Ultrasound Physics
Sound waves generated by **piezoelectric crystals** in the transducer. Frequency range for MSKUS: **5-18 MHz** (higher frequency = better resolution but less penetration). Pulse-echo principle: Sound transmitted, reflected at tissue interfaces, and received. **Acoustic impedance** differences between tissues create image contrast.

### Transducer Selection
**Linear transducer (high-frequency, 10-18 MHz)**: Primary probe for MSKUS; superficial structures. **Curvilinear transducer (5-8 MHz)**: Deeper structures (hip, spine). **Hockey-stick transducer**: Small parts; hands, feet, pediatric applications. Higher frequency probes provide better resolution but limited depth penetration.

### Image Optimization
**Depth**: Adjust to visualize target structure with minimal excess. **Gain**: Overall brightness of image; adjust for tissue contrast. **Time gain compensation (TGC)**: Adjust gain at specific depths. **Focus**: Position focal zone at depth of interest. **Frequency**: Increase for superficial targets; decrease for deeper structures. ![Ultrasound transducer types and their applications in musculoskeletal imaging](images/us-transducer-types.jpg)

## Tissue Echogenicity

### Normal Tissue Appearance
**Bone**: Hyperechoic cortical surface with posterior acoustic shadowing. **Tendon**: Fibrillar hyperechoic pattern; changes with angle (anisotropy). **Muscle**: Hypoechoic with hyperechoic fascial septae (pennate pattern). **Nerve**: Honeycomb or fascicular pattern on short-axis; hypoechoic fascicles within hyperechoic epineurium.

**Cartilage**: Hypoechoic homogeneous layer over bone. **Fluid**: Anechoic (black); compressible. **Fat**: Hyperechoic; variable echogenicity.

| Tissue | Echogenicity | Key US Features |
|--------|-------------|-----------------|
| Bone cortex | Hyperechoic | Bright line with posterior acoustic shadowing |
| Tendon | Hyperechoic (fibrillar) | Fibrillar pattern; affected by anisotropy |
| Muscle | Hypoechoic | Hyperechoic fascial septae; pennate pattern |
| Nerve | Mixed (fascicular) | Honeycomb pattern on short-axis |
| Cartilage | Hypoechoic | Homogeneous layer over bone |
| Fluid | Anechoic | Black; compressible |
| Fat | Hyperechoic | Variable echogenicity |

### Key Artifacts
**Anisotropy**: Tendon appears hypoechoic when ultrasound beam is not perpendicular; most common MSKUS pitfall. **Acoustic shadowing**: Signal loss deep to highly reflective surfaces (bone, calcification). **Posterior acoustic enhancement**: Increased brightness deep to fluid-filled structures. **Reverberation artifact**: Parallel reflections creating false echoes.

## Scanning Technique

### Standard Approach
**Two orthogonal planes**: Short-axis (transverse) and long-axis (longitudinal). Systematic scanning protocol for each joint/region. Dynamic assessment: Active and passive motion evaluation. Contralateral comparison for reference. Document pathology with measurements and labels.

### Probe Positioning
Light pressure to avoid compressing structures. Generous coupling gel application. Perpendicular orientation to target structure (minimize anisotropy). Slow, methodical sweeping through region of interest.

## Common Pathology Identification

### Tendon Pathology
**Tendinosis**: Thickened, hypoechoic tendon with loss of fibrillar pattern. **Tendon tear (partial)**: Focal hypoechoic or anechoic defect within tendon. **Tendon tear (complete)**: Full-thickness discontinuity with retraction. **Tenosynovitis**: Fluid surrounding tendon within sheath. **Calcific tendinopathy**: Hyperechoic foci with or without shadowing.

### Joint Pathology
**Effusion**: Anechoic or hypoechoic fluid within joint capsule. **Synovitis**: Thickened, hyperemic synovium on Doppler. **Cartilage defects**: Focal thinning or irregularity. **Loose bodies**: Echogenic foci within joint.

### Nerve Pathology
**Nerve entrapment**: Cross-sectional area (CSA) enlargement proximal to entrapment. **Median nerve at carpal tunnel**: CSA >10 mm2 suggests carpal tunnel syndrome. **Neuroma**: Fusiform enlargement of nerve. Loss of fascicular pattern in severe compression.

### Muscle Pathology
**Strain/tear**: Hypoechoic or anechoic disruption of muscle fibers. **Hematoma**: Heterogeneous collection; evolves over time. **Atrophy**: Decreased muscle bulk with increased echogenicity (fatty infiltration).

![Normal ultrasound anatomy of tendon, muscle, and nerve on short-axis and long-axis views](images/mskus-normal-anatomy.jpg)

## Power Doppler

Detects blood flow within tissues. **Increased vascularity** indicates active inflammation (synovitis, tenosynovitis). Used to differentiate active inflammation from chronic changes. Settings: Low wall filter, low PRF, maximum gain without artifact. Avoid excessive transducer pressure (compresses vessels).

## Documentation and Reporting

Include patient demographics and clinical indication. Label anatomical structures and pathology. Measure size of abnormalities in two planes. Compare with contralateral side when relevant. Document Doppler findings. Clinical correlation and recommendations.

![Power Doppler demonstrating active synovitis with increased vascularity in inflammatory arthritis](images/power-doppler-synovitis.jpg)

## Key Clinical Pearls

1. **Anisotropy** is the most common pitfall in MSKUS; always maintain perpendicular probe orientation to tendons and rock/tilt the probe to confirm findings. 2. A cross-sectional area greater than 10 mm2 of the median nerve at the carpal tunnel inlet is the widely accepted threshold for carpal tunnel syndrome diagnosis. 3. Dynamic scanning with active patient movement is a unique advantage of ultrasound over MRI, allowing real-time assessment of tendon subluxation, impingement, and nerve instability. 4. Always compare with the contralateral side, as normal anatomic variation is common and bilateral comparison reduces false-positive diagnoses.

## References

1. Jacobson JA. *Fundamentals of Musculoskeletal Ultrasound*. 3rd ed. Elsevier; 2018.
2. Smith J, Finnoff JT. Diagnostic and interventional musculoskeletal ultrasound: part 1. Fundamentals. *PM&R*. 2009;1(1):64-75.
3. Cartwright MS, Hobson-Webb LD, Boon AJ, et al. Evidence-based guideline: neuromuscular ultrasound for the diagnosis of carpal tunnel syndrome. *Muscle & Nerve*. 2012;46(2):287-293. 4. Bianchi S, Martinoli C. *Ultrasound of the Musculoskeletal System*. Springer; 2007.