Therapeutic ultrasound is a treatment that uses sound waves to heat deep tissues, reduce pain, and promote healing in muscles, tendons, and ligaments. Unlike the ultrasound used to create images during pregnancy or diagnose injuries, therapeutic ultrasound delivers energy into tissue at intensities designed to produce biological changes. It’s one of the most widely used tools in physical therapy clinics, applied primarily to musculoskeletal problems like tendinitis, ankle sprains, back pain, and knee osteoarthritis.
How It Differs From Diagnostic Ultrasound
When most people hear “ultrasound,” they picture the imaging tool used to check on a developing baby or examine an organ. Diagnostic ultrasound sends sound waves into the body and listens for the echoes to build a picture. The energy levels are kept extremely low because the goal is observation, not interaction with tissue.
Therapeutic ultrasound works on a fundamentally different principle. It intentionally deposits energy into tissue to trigger physical and biological effects. The intensities typically range from 0.5 to 2 watts per square centimeter, far higher than imaging levels. A clinician applies a handheld transducer (sometimes called a “wand”) to the skin, moving it slowly over the treatment area while ultrasound gel ensures the sound waves pass efficiently from the device into the body.
Thermal and Non-Thermal Effects
Therapeutic ultrasound produces two broad categories of effects, and both appear to contribute to its clinical benefits.
The thermal effect is the more intuitive one: sound waves vibrate molecules in deep tissue, generating heat. This deep heating can raise tissue temperature to around 40°C (104°F), which increases blood flow, improves the elasticity of collagen-rich structures like tendons and joint capsules, and can reduce pain at tendon or ligament insertion points. Continuous-mode ultrasound is typically used when deep heating is the goal, especially for chronic pain conditions.
The non-thermal effects are subtler but potentially just as important. As sound waves travel through tissue, they create two phenomena. The first, acoustic streaming, is a mechanical pressure that pushes fluid along and around cell membranes, displacing ions and small molecules. The second, cavitation, involves microscopic gas bubbles in tissue fluid that expand and contract in rhythm with the sound wave’s pressure changes. This pulsation can alter cell membrane function and disrupt normal cellular activity in a way that appears to stimulate a repair response. Early research suggests ultrasound may briefly “stress” cells, triggering increased protein production as the cells recover, essentially jumpstarting the healing process.
Frequencies and Tissue Depth
Therapeutic ultrasound devices typically operate at one of two frequencies: 1 MHz or 3 MHz. The choice depends on how deep the target tissue sits beneath the skin.
At 1 MHz, the sound waves penetrate deeper, reaching tissues approximately 2.3 to 5 centimeters below the surface. This makes it the standard choice for problems in deeper structures like hip muscles or tissues beneath thick layers of soft tissue.
At 3 MHz, the energy is absorbed more superficially, generally within 0.8 to 1.6 centimeters of depth. This frequency is well suited for conditions close to the skin surface, such as plantar fasciitis (the bottom of the foot), patellar tendinitis (just below the kneecap), and lateral epicondylitis (tennis elbow). Interestingly, research has shown that 3 MHz ultrasound can heat tissues deeper than originally theorized. In one study, 3 MHz ultrasound produced vigorous heating at 2.5 centimeters deep within about 3.4 minutes, while 1 MHz ultrasound failed to reach the same heating threshold at that depth. This challenges the traditional textbook rule that 3 MHz is strictly for shallow treatment.
Tissues that fall in the medium-depth range, roughly 1.6 to 2.5 centimeters, present something of a gray area. Clinicians use their judgment about which frequency will deliver the best results for a given patient and body region.
What a Treatment Session Looks Like
A typical session is short. Treatment times range from about 4 to 10 minutes per area, depending on the condition and the size of the region being treated. For degenerative joint conditions like knee osteoarthritis, sessions commonly last 5 to 10 minutes at intensities between 0.5 and 2 watts per square centimeter.
During treatment, you’ll feel a gentle warmth in the area, though pulsed-mode treatments (which deliver sound waves in bursts rather than continuously) produce little to no noticeable heat. The therapist applies a water-based gel to your skin and moves the transducer head in slow, overlapping circles. The constant motion prevents hot spots from forming in the tissue. The treatment area is generally about twice the size of the transducer’s face.
Session frequency varies by condition. In clinical studies on knee osteoarthritis, patients received treatments five times per week for two weeks. In practice, many treatment plans involve two to three sessions per week over several weeks, often combined with exercise, stretching, or manual therapy.
Common Conditions Treated
Therapeutic ultrasound is most frequently applied to soft tissue and joint problems. The conditions with the strongest track record include myofascial pain syndrome (tight, painful knots in muscle), chronic back pain, hip pain, acute ankle sprains, and knee osteoarthritis. It’s also commonly used on tendon and ligament injuries, where research shows low-intensity ultrasound can improve tendon healing by increasing tensile strength and improving collagen alignment. For skeletal muscle and ligament injuries, studies have found increased cell proliferation during the regeneration phase and improvements in the tissue’s ability to bear load and absorb energy.
The evidence is generally more consistent for chronic musculoskeletal conditions than for acute injuries. When used for chronic pain, continuous-mode ultrasound targeting deep tissues at tendon or ligament attachment points often provides meaningful pain relief.
Phonophoresis: Driving Medication Through Skin
One specialized application of therapeutic ultrasound is phonophoresis (also called sonophoresis), which uses sound waves to push topical medications through the skin and into underlying tissue. The technique dates back to the 1950s, when researchers first demonstrated that ultrasound could enhance the skin penetration of cortisol, a steroid used to reduce inflammation.
In practice, the medication is mixed into the coupling gel applied to the skin during treatment. As the ultrasound waves pass through the skin, they temporarily increase its permeability, allowing the drug to reach deeper tissues than it would from surface application alone. The most commonly delivered medications are corticosteroids for inflammation and lidocaine for local pain relief. Some research has shown that combining ultrasound with chemical penetration enhancers can further boost drug delivery, though the standard approach in most clinics is simply to use a medicated gel during a regular treatment session.
When Therapeutic Ultrasound Is Not Appropriate
Therapeutic ultrasound is generally safe when applied by a trained clinician, but certain situations call for avoiding it entirely. It should not be used over areas with active cancer or tumors, since increased blood flow and cellular stimulation could theoretically promote tumor growth. It’s also avoided over the eyes, the brain, the heart, a pregnant uterus, and areas with active infection.
In children and adolescents, ultrasound is not applied over growth plates because of the risk of disrupting bone development. People with impaired sensation in the treatment area (from nerve damage or conditions like diabetic neuropathy) face a higher risk of thermal burns because they can’t feel when the tissue is overheating. Metal implants in the treatment zone and areas with compromised blood supply are also situations where clinicians exercise caution or avoid treatment.
At high intensities, ultrasound can cause blood vessel spasm and hemorrhage when it generates excessive cavitation in tissue. This is primarily a concern with high-intensity focused ultrasound (HIFU), a much more powerful form used in surgical applications, not the low-to-moderate intensity devices found in rehabilitation settings.
How Strong Is the Evidence?
The evidence for therapeutic ultrasound is mixed, which is worth understanding if you’re considering it as part of your treatment. Systematic reviews support its use for promoting soft tissue healing and improving outcomes after musculoskeletal injuries and post-operative recovery. The biological mechanisms are well-documented: it does heat deep tissue, it does stimulate cellular activity, and laboratory studies consistently show improved tendon strength, collagen organization, and tissue biomechanics.
Where the picture gets murkier is in clinical trials on pain relief, where results vary across studies. Some of this inconsistency likely stems from differences in how ultrasound is applied: different frequencies, intensities, treatment durations, and conditions being treated. There’s also a long-standing observation among clinicians that ultrasound’s benefits seem to exceed what heating alone would explain, especially when it’s delivered at non-thermal settings. The non-thermal mechanisms (acoustic streaming, cavitation, and direct cellular effects) are real, but harder to standardize and study in controlled trials.
In practice, therapeutic ultrasound is rarely used as a standalone treatment. It’s most effective as one component of a broader rehabilitation program that includes targeted exercise, manual therapy, and activity modification. If your physical therapist includes it in your treatment plan, it’s typically because the combination of deep heating and tissue stimulation complements the other interventions rather than replacing them.