Therapeutic Ultrasound for Knee: How It Works, Benefits, and Conditions Treated

How Does Ultrasound Work on Knee?

Therapeutic ultrasound utilizes high-frequency sound waves to penetrate soft tissues around the knee, creating both thermal and non-thermal effects that can promote healing, reduce pain, and decrease inflammation.

What is Therapeutic Ultrasound?

Therapeutic ultrasound is a non-invasive modality commonly used in physical therapy and rehabilitation to address various musculoskeletal conditions, including those affecting the knee joint. Unlike diagnostic ultrasound, which creates images, therapeutic ultrasound delivers energy to tissues to elicit a physiological response designed to promote healing and reduce symptoms.

The Science Behind Ultrasound Application

At its core, therapeutic ultrasound operates on the principle of converting electrical energy into sound waves.

• Transducer: The device used to apply ultrasound is called a transducer or applicator head. This head contains a piezoelectric crystal.
• Piezoelectric Effect: When an alternating electrical current is applied to the piezoelectric crystal, it vibrates rapidly, generating high-frequency sound waves (typically 1-3 MHz for therapeutic purposes).
• Sound Wave Propagation: These sound waves are then transmitted into the body's tissues. A coupling gel is applied to the skin to eliminate air pockets, which would otherwise block the sound waves, ensuring efficient transmission into the knee area.

How Ultrasound Interacts with Knee Tissues

As the sound waves travel through the knee, their energy is absorbed by the various tissues, leading to different effects.

• Absorption: Tissues with higher collagen content, such as ligaments, tendons, fascia, and joint capsules (all prevalent around the knee), tend to absorb ultrasound energy more efficiently than tissues with high water content, like fat or blood. This selective absorption allows for targeted treatment of specific structures within the knee.
• Reflection: Some sound waves are reflected when they encounter interfaces between different tissue densities (e.g., bone and soft tissue). This is why the transducer is constantly moved to prevent "hot spots" from excessive energy concentration.
• Refraction: Sound waves can also bend or change direction as they pass from one tissue type to another.

Therapeutic Effects of Ultrasound on the Knee

The interaction of ultrasound waves with knee tissues produces two primary categories of therapeutic effects:

Thermal Effects

When ultrasound energy is absorbed by tissues, it causes the molecules within those tissues to vibrate, generating heat.

• Deep Tissue Heating: Unlike superficial heating pads, ultrasound can effectively heat tissues several centimeters deep, making it beneficial for structures like the joint capsule, deep ligaments, and tendons around the knee.
• Increased Tissue Extensibility: Heating can increase the elasticity of collagen fibers, which is useful for stretching tight structures, such as a stiff joint capsule after injury or surgery.
• Increased Blood Flow: Heat causes vasodilation, improving circulation to the treated area, which can bring more oxygen and nutrients essential for healing and help remove waste products.
• Pain Reduction: Increased blood flow and direct effects on nerve conduction can contribute to pain relief.

Non-Thermal (Mechanical) Effects

These effects occur even at lower intensities where significant heat is not generated, or at higher intensities alongside thermal effects.

• Cavitation: The oscillation of sound waves causes microscopic gas bubbles within the tissue fluids to expand and contract. Stable cavitation involves the rhythmic compression and expansion of these bubbles, while unstable cavitation (avoided in therapy) involves their collapse. Stable cavitation is thought to enhance cellular activity.
• Acoustic Streaming: This refers to the unidirectional flow of fluid around the vibrating cells. It can alter cell membrane permeability, facilitating the movement of ions and nutrients across cell membranes.
• Microstreaming: This is a small-scale eddying of fluids around the vibrating gas bubbles. It creates mechanical stress on cell membranes, which can stimulate cellular activity, including protein synthesis and enzyme activity.
• Enhanced Cellular Activity: These mechanical effects can stimulate various cellular processes critical for tissue repair, such as fibroblast activity (which produces collagen), mast cell degranulation (releasing histamine and other healing mediators), and macrophage activity (important for clearing debris).
• Reduction of Swelling and Inflammation: By influencing cell membrane permeability and fluid dynamics, ultrasound may help reduce localized swelling and the inflammatory response.
• Scar Tissue Remodeling: The mechanical forces can help break down disorganized scar tissue and promote the alignment of new collagen fibers, leading to stronger, more flexible repaired tissue.

Common Knee Conditions Treated with Ultrasound

Therapeutic ultrasound is often incorporated into treatment plans for various knee-related issues:

• Osteoarthritis of the Knee: Can help reduce pain and improve joint mobility.
• Patellar Tendinopathy (Jumper's Knee) and Quadriceps Tendinopathy: Aids in reducing pain, inflammation, and promoting tendon healing.
• Ligament Sprains (e.g., MCL, LCL): Can assist in the healing process and reduce swelling.
• Bursitis (e.g., Prepatellar Bursitis, Pes Anserine Bursitis): Helps manage inflammation and pain.
• Post-Surgical Rehabilitation: Used to reduce pain, swelling, and promote tissue healing after procedures like ACL reconstruction or meniscus repair.
• Muscle Strains: Facilitates recovery by reducing pain and inflammation.

Application and Procedure for Knee Ultrasound

During a typical ultrasound treatment for the knee:

• Patient Positioning: The patient will be positioned comfortably to expose the knee area.
• Gel Application: A specialized ultrasound gel is applied generously to the skin over the target area of the knee.
• Transducer Movement: The therapist moves the ultrasound transducer head in small, continuous circles or strokes over the skin, ensuring full contact and even distribution of the sound waves.
• Intensity and Duration: The intensity (measured in W/cm²) and duration (typically 5-10 minutes) of the treatment are carefully selected based on the specific condition, its stage (acute vs. chronic), and the depth of the target tissue. Pulsed ultrasound is often used for acute conditions to maximize non-thermal effects and minimize heat, while continuous ultrasound is used for chronic conditions where thermal effects are desired.

Evidence and Efficacy

While therapeutic ultrasound has been a staple in rehabilitation for decades, the scientific evidence supporting its efficacy for all conditions is mixed and continues to be researched. Some studies demonstrate clear benefits, particularly for pain reduction and soft tissue healing in specific conditions, while others show limited or no significant difference compared to placebo. It is widely accepted that ultrasound is most effective when used as part of a comprehensive rehabilitation program that includes exercise, manual therapy, and patient education, rather than as a standalone treatment.

Limitations and Contraindications

Despite its benefits, therapeutic ultrasound is not suitable for everyone or every condition.

• Malignancy: Should not be used over cancerous areas.
• Pregnancy: Avoid over the abdomen or lower back during pregnancy.
• Implants: Caution is advised over areas with pacemakers, joint replacements (especially cemented ones), or other electronic implants.
• Acute Infections: Not typically used over active infections.
• Growth Plates: Avoid direct application over open epiphyseal growth plates in children.
• Vascular Conditions: Avoid over areas with deep vein thrombosis (DVT) or severe atherosclerosis.
• Sensory Impairment: Patients with impaired sensation may not feel if the treatment is too intense.

Conclusion

Therapeutic ultrasound offers a valuable adjunct to rehabilitation for a variety of knee conditions. By harnessing the power of high-frequency sound waves, it can induce both thermal and mechanical effects within deep tissues, promoting healing, reducing pain, decreasing inflammation, and improving tissue extensibility. While its precise mechanisms and efficacy continue to be refined through research, when applied by a knowledgeable practitioner as part of a holistic treatment plan, ultrasound can significantly contribute to the recovery and functional improvement of the knee joint.