Ultrasound Therapy in Physiotherapy: Frequency & Uses

Introduction to Therapeutic Ultrasound

Therapeutic ultrasound is a widely used electrophysical modality in physical rehabilitation. Unlike diagnostic ultrasound, which is used to capture internal images of the body, therapeutic ultrasound is designed to apply energy to deep tissues to promote healing, manage pain, and reduce muscle spasms. It works by converting high-frequency electrical energy into mechanical sound waves (vibrations) using a piezoelectric crystal housed in the ultrasound transducer head.

In a professional physiotherapy and pain management program, ultrasound therapy is utilized to prep tight joint capsules, ligaments, and tendons prior to manual stretching and active exercises.

The Role of Frequency: 1 MHz vs. 3 MHz

The frequency of the ultrasound wave determines the depth of penetration and the rate of absorption in the body. Choosing the correct frequency is the first step in clinical application:

• 1 MHz (Megahertz): This frequency vibrates 1 million times per second. It has a longer wavelength and penetrates deeply, reaching tissues at a depth of 3 to 5 cm. It is used to treat deep-seated structures such as the hip joint capsule, piriformis muscle, deep hamstring fibers, and large muscle bellies.
• 3 MHz: This frequency vibrates 3 million times per second. It has a shorter wavelength and is absorbed much faster by the tissues, concentrating its energy superficially at a depth of 1 to 2 cm. It is ideal for superficial tissues such as the lateral epicondyle (tennis elbow), patellar tendon (jumper's knee), Achilles tendon, and wrist ligaments.

Continuous vs. Pulsed Modes: Thermal and Mechanical Effects

Therapeutic ultrasound can be delivered in two primary modes, each producing different physiological effects:

1. Continuous Mode (Thermal Effects)

In this mode, the sound waves are delivered constantly without interruption (100% duty cycle). The continuous mechanical vibration frictionally heats the target tissue.

• Clinical Benefits: Increases the extensibility of collagen fibers (perfect for tight joint capsules), raises local blood flow to promote healing, reduces joint stiffness, and relaxes muscle spasms.
• Best Used For: Chronic joint contractures, old scar tissue, and chronic muscle spasms.
• Warning: Must not be used during acute inflammatory phases, as adding heat will worsen swelling and irritation.

2. Pulsed Mode (Non-Thermal/Mechanical Effects)

In this mode, the sound waves are delivered in short pulses, interrupted by brief rest periods (commonly at a 20% or 50% duty cycle). This prevents heat accumulation, producing purely mechanical, cellular-level effects.

• Clinical Benefits: Stimulates two key processes: cavitation (the expansion and contraction of gas bubbles in tissue fluid) and acoustic streaming (the movement of fluid along cell membranes). These actions increase cell membrane permeability, stimulating fibroblasts to synthesize collagen and accelerate the cellular repair process of injured tissues.
• Best Used For: Acute ankle sprains, acute tendinopathies, muscle tears, and early-stage tissue healing.

Comparison Table: Ultrasound Frequencies & Modes

| Parameter | 1 MHz Continuous | 1 MHz Pulsed (20% Duty Cycle) | 3 MHz Continuous | 3 MHz Pulsed (20% Duty Cycle) | | :--- | :--- | :--- | :--- | :--- | | Depth of Action | Deep (3 - 5 cm) | Deep (3 - 5 cm) | Superficial (1 - 2 cm) | Superficial (1 - 2 cm) | | Primary Effect | Deep tissue heating | Deep mechanical cellular repair | Superficial tissue heating | Superficial mechanical repair | | Best Clinical Use | Deep joint stiffness (hips, shoulders), old scar tissue | Deep muscle tears, deep acute ligament sprains | Superficial joint stiffness (wrist, elbow), chronic tendinitis | Tennis elbow, Achilles tendinopathy, acute ankle sprains | | Heat Generation | Yes (moderate-to-high) | No (purely mechanical) | Yes (rapid surface heating) | No (purely mechanical) |

Contraindications and Safety Guidelines

While therapeutic ultrasound is safe, therapists must adhere to strict safety guidelines. The transducer head must be kept in constant motion, using an ultrasound gel to conduct the sound waves. Holding the head still can focus the energy and cause a painful periosteal burn (heating of the bone lining).

Absolute contraindications include:

• Pregnancy: Never apply ultrasound over the abdomen, pelvis, or lower back of a pregnant patient.
• Epiphyseal Plates: Do not apply over the growing ends of bones in children, as it can disrupt bone growth.
• Metal/Joint Replacements: While pulsed mode is safe, continuous mode should not be used directly over cement or plastic joint implants.
• Cancer: Do not apply over known tumor sites, as the increased blood flow could stimulate cell growth.
• Thrombophlebitis: Do not apply over blood clots to avoid dislodging a thrombus.