MPT Electrophysiology and Wound Care 2 — Questions and Answers

Question 1: TENS achieves pain relief via the gate control theory primarily through which mechanism?

Activating small unmyelinated C-fibers to inhibit pain
Activating large-diameter A-beta mechanoreceptor fibers to inhibit nociceptive transmission at the dorsal horn (Correct answer)
Stimulating the motor cortex directly via the spinothalamic tract
Releasing substance P from dorsal horn interneurons

Correct answer: Activating large-diameter A-beta mechanoreceptor fibers to inhibit nociceptive transmission at the dorsal horn

Conventional TENS activates large A-beta sensory fibers which activate inhibitory interneurons in the dorsal horn, gating (inhibiting) nociceptive transmission from smaller A-delta and C-fibers.

Melzack and Wall's gate control theory proposes that activation of large-diameter (A-beta) mechanoreceptor afferents activates inhibitory interneurons in the substantia gelatinosa of the dorsal horn, which reduce transmission from nociceptive A-delta and C-fibers. Conventional TENS (high frequency 80-150 Hz, low intensity) preferentially recruits A-beta fibers, producing rapid-onset, short-duration pain relief. Acupuncture-like TENS (low frequency 1-10 Hz, high intensity) recruits A-delta fibers and stimulates endorphin release. Both modes are used clinically by PTs for acute and chronic pain management.

Question 2: Therapeutic ultrasound at 3 MHz is preferred over 1 MHz for treatment of which tissue location?

Deep muscles and joints (3-5 cm depth)
Superficial tendons and bursae (less than 2 cm depth) (Correct answer)
Bone fractures
Spinal cord stimulation

Correct answer: Superficial tendons and bursae (less than 2 cm depth)

3 MHz ultrasound is absorbed in superficial tissues (less than 2 cm) due to higher frequency causing faster energy attenuation, making it ideal for treating superficial structures like tendons and bursae.

Ultrasound frequency determines tissue penetration depth. 1 MHz penetrates to approximately 3-5 cm (useful for hip and shoulder deep muscles). 3 MHz is attenuated quickly and effective for depths of 1-2 cm (Achilles tendon, lateral epicondyle, superficial bursae). Treatment intensity is 0.5-2.0 W/cm squared. Pulsed mode (20% duty cycle) is used for non-thermal effects (cavitation, microstreaming promoting cell permeability) and continuous mode for thermal effects. Contraindications include active infection, malignancy, over the gravid uterus, and over open epiphyses in children.

Question 3: A pressure injury that appears as a full-thickness wound with visible fat but no bone, tendon, or muscle is classified as:

Stage 1
Stage 2
Stage 3 (Correct answer)
Stage 4

Correct answer: Stage 3

Stage 3 pressure injuries involve full-thickness skin loss with visible subcutaneous fat, but bone, tendon, or muscle is not exposed or palpable.

The NPUAP staging system: Stage 1 — intact skin with non-blanchable erythema. Stage 2 — partial-thickness loss, shallow open ulcer or intact/ruptured blister. Stage 3 — full-thickness loss, subcutaneous fat visible, no exposed bone, tendon, or muscle. Stage 4 — full-thickness loss with exposed bone, tendon, or muscle. Unstageable — full-thickness base covered by slough or eschar. Suspected Deep Tissue Injury — purple or maroon discoloration over intact skin. PTs document staging accurately for care planning, billing, and wound care team communication.

Question 4: Which parameter of NMES most directly determines the depth of muscle recruitment?

Frequency (pulse rate)
Pulse duration (pulse width)
Intensity (amplitude) (Correct answer)
Waveform shape

Correct answer: Intensity (amplitude)

Intensity (amplitude) most directly determines the depth and breadth of motor unit recruitment. Increasing amplitude depolarizes more deeply located motor nerve fibers, recruiting deeper motor units.

In NMES, intensity (mA or V) is the primary determinant of recruitment depth because it controls the electrical field gradient. Higher intensity creates larger current density that spreads deeper into tissue, recruiting more and deeper motor units. Pulse duration determines which nerve fiber types are recruited (longer pulses recruit smaller or denervated fibers). Frequency (Hz) determines contraction type (twitches vs. tetanic). Waveform affects comfort and charge balance. PTs increase intensity gradually while monitoring for comfort and visible or palpable muscle contraction.

Question 5: Autolytic debridement of a wound is achieved by which mechanism?

Application of sharp instruments to remove necrotic tissue
Use of maggot therapy (biosurgical debridement)
Maintaining a moist wound environment with occlusive dressings, allowing endogenous enzymes to liquefy necrotic tissue (Correct answer)
Application of enzymatic collagenase ointments

Correct answer: Maintaining a moist wound environment with occlusive dressings, allowing endogenous enzymes to liquefy necrotic tissue

Autolytic debridement uses the body's own proteolytic enzymes and macrophages, maintained at the wound surface by moisture-retentive dressings, to selectively liquefy and remove necrotic tissue.

Autolytic debridement is the most selective and least traumatic form of debridement, using endogenous wound fluid enzymes (proteases, lipases, collagenase from macrophages) to break down necrotic tissue while preserving healthy tissue. Moisture-retentive dressings (hydrocolloids, hydrogels, transparent films) maintain the moist environment. It is slower than sharp debridement but appropriate for patients who cannot tolerate sharp debridement. Contraindication: infected wounds (moisture may promote bacterial growth). PTs and PTAs select debridement methods based on wound type, tissue presence, infection status, and patient tolerance.

Question 6: Iontophoresis using dexamethasone (negative pole/cathode) is most commonly used to treat which condition?

Chronic venous insufficiency
Inflammatory conditions such as lateral epicondylitis, plantar fasciitis, and bursitis (Correct answer)
Open wounds for antimicrobial effect
Muscle spasm via motor nerve stimulation

Correct answer: Inflammatory conditions such as lateral epicondylitis, plantar fasciitis, and bursitis

Dexamethasone is negatively charged and is repelled by the cathode (negative pole), driving the anti-inflammatory medication into superficial inflammatory conditions like tendinopathies and bursitis.

Iontophoresis uses direct current to transport ionized medication across the skin without needles. The principle that like repels like determines electrode placement: negatively charged dexamethasone sodium phosphate is placed at the cathode. Dexamethasone is used for inflammatory conditions (epicondylitis, bursitis, plantar fasciitis). Acetic acid (cathode) is used for calcium deposits. Lidocaine (anode) for pain. Parameters: current density less than 0.5 mA/cm squared, total charge 40-80 mA-min. Skin must be intact; contraindications include metal implants under treatment area and hypersensitivity to medication.

MPT Electrophysiology and Wound Care 2

TENS achieves pain relief via the gate control theory primarily through which mechanism?