Introduction
Frozen shoulder, clinically known as adhesive capsulitis, is a painful condition characterized by progressive stiffness and restricted range of motion in the glenohumeral joint. The joint capsule becomes inflamed, thickened, and scarred, making simple movements like reaching overhead or behind the back nearly impossible. Treatment focuses on relieving pain and restoring joint elasticity. Ultrasound therapy for frozen shoulder is a widely utilized electrophysical modality that penetrates deep into the joint capsule, preparing it for subsequent manual mobilization and stretching.
The Physics and Physiology of Therapeutic Ultrasound
Therapeutic ultrasound uses high-frequency sound waves (usually 1 MHz or 3 MHz) generated by the piezoelectric effect in a transducer head. These sound waves travel through a coupling gel into the body's tissues, producing two distinct types of effects:
- Thermal Effects (Continuous Mode): The sound waves cause molecules in deep tissues to vibrate, generating friction and heat. In frozen shoulder, continuous ultrasound increases the temperature of the joint capsule and surrounding tendons. This heat makes collagen-rich tissues more stretchable (extensible), reduces muscle spasms, and increases localized blood flow.
- Non-Thermal Effects (Pulsed Mode): By pulsing the sound waves, therapists avoid heat generation while still promoting cellular healing. This occurs through:
* Cavitation: The formation and oscillation of microscopic gas bubbles in the tissues, which alters cell membrane permeability.
* Acoustic Streaming: The physical movement of fluids along cell membranes, stimulating fibroblast activity and accelerating collagen synthesis and repair.
Stages of Frozen Shoulder and the Role of Ultrasound
Adhesive capsulitis progresses through three clinical stages. The therapeutic application of ultrasound must align with these phases:
* Freezing Stage (Painful Phase): Characterized by severe pain and gradual loss of motion. Ultrasound is applied in pulsed mode to reduce inflammation and manage pain without generating heat.
* Frozen Stage (Stiff Phase): Pain decreases slightly, but joint stiffness is profound. Ultrasound is applied in continuous mode to deliver deep heat, softening the thickened capsule before stretching exercises.
* Thawing Stage (Recovery Phase): Range of motion slowly returns. Continuous ultrasound is paired with aggressive manual therapy and joint mobilizations to break down remaining adhesions.
Ultrasound Operating Modes Comparison Table
| Parameter | Continuous Ultrasound | Pulsed Ultrasound |
|---|---|---|
| **Duty Cycle** | 100% (constant sound waves) | 20% or 50% (intermittent waves) |
| **Primary Effect** | Deep heating, increased collagen extensibility | Mechanical agitation, cellular biostimulation |
| **Indications** | Frozen stage stiffness, chronic capsular scarring | Freezing stage acute pain, active inflammation |
| **Tissue Temperature** | Increases significantly (40°C - 45°C) | No significant increase |
Clinical Application and Protocols
When treating adhesive capsulitis, clinicians adhere to the following protocols:
* Frequency Selection: 1 MHz is used because it penetrates deep (up to 3-5 cm), reaching the glenohumeral joint capsule. (3 MHz is reserved for superficial tissues like the elbow or wrist).
* Technique: The transducer head must be kept in constant motion (slow circular or stroking movements) to prevent "hot spots" caused by standing waves, which can cause bone aches.
* Immediate Mobilization: The therapeutic window is brief. Physiotherapists must perform joint mobilizations and passive stretching immediately following the ultrasound treatment, while the joint capsule remains warm and pliable.
Contraindications and Safety Precautions
Do not use ultrasound therapy over:
* Pacemakers or electrical implants.
* Areas with active malignancies or local infections.
* Epiphyseal plates in growing children.
* Areas with deep vein thrombosis (DVT) or acute vascular issues.