Shortwave Diathermy
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Direct answer
27.12 megahertz, wavelength about 11 metres — those two numbers identify shortwave diathermy, the classic deep-heating workhorse of physiotherapy departments. An oscillating electromagnetic field heats tissue at depth — commonly quoted as 3 to 5 centimetres and beyond — through ionic movement and dielectric losses, delivered either as a capacitive field between two condenser-plate electrodes or an inductive field from a coil or drum electrode. Continuous-mode SWD raises tissue temperature for analgesia, circulatory benefit and extensibility before stretching or mobilisation; pulsed SWD delivers the same field in bursts with minimal heating, reserved for acute lesions and oedema.
What you must remember
- The allocated frequency is 27.12 MHz (wavelength roughly 11 metres) in the industrial-scientific-medical band; "diathermy" literally means heating through.
- Electrode methods: capacitive (condenser field between plates, spaced from skin by towels and air gap) preferentially heats fat and superficial tissue; inductive (drum or coil) fields preferentially heat muscle — the standard teaching being "inductothermy for muscle".
- Continuous SWD produces thermal effects — analgesia, hyperaemia, reduced muscle spasm, increased collagen extensibility — dosed by the patient's report of mild, comfortable warmth for 15–20 minutes.
- Pulsed SWD (pulsed electromagnetic field) delivers short bursts many times per second with low mean power, used for pain and oedema in acute and subacute lesions where heating is unwanted.
- Condenser plate geometry concentrates the field: the smaller electrode sits over the thinner part, and equal plates with equal spacing give even heating.
- Contraindications are absolute and heavily examined: cardiac pacemakers and implanted electronics, metal implants in the field, pregnancy, malignancy, over the eyes and gonads, damp or sweaty clothing, sensory loss, acute inflammation (for thermal doses) and unhealed wounds.
- Safety discipline: check sensation before the first treatment, remove metal and wet clothing, keep the patient at a safe distance from the machine, and never leave the room with the machine on.
A continuous SWD session for chronic knee osteoarthritis
Screen first: any implanted metal, pacemaker, pregnancy, sensory impairment — one yes cancels the plan. The knee is chronic, stiff and achy without acute effusion, so continuous thermal SWD is appropriate before exercise.
Position the patient long-sitting with a towel roll under the knee. Choose the drum electrode over the anterior knee, or condenser pads straddling the joint with two to three centimetres of towel-and-air spacing; drum placement is quicker and better tolerated. Switch to continuous mode, raise the output over the first two minutes until the patient reports mild, clearly comfortable warmth — "just noticeable" is the target, never a burning edge — then settle the timer for 15 to 20 minutes.
The modality is only the overture. In the last five minutes prepare the exercise: when the machine stops, move immediately into quadriceps work, patellar mobilisation and sustained flexion stretches while the capsule is warm, because extensibility gained with heat is the actual therapeutic product. Document the dose as "continuous SWD, mild warmth, 18 minutes, anterior knee" and re-examine next visit. Had the same knee presented hot, swollen and acutely inflamed, the same machine would be set to pulsed mode at low mean power instead — the decision hinges on acuteness, not on the machine.
Where students slip
Metal and moisture are the two kill factors candidates forget: rings, coins, damp underclothing and sweaty skin create local hot spots and burns. The capacitive-versus-inductive tissue preference is a fixed MCQ (fat under condenser fields, muscle under inductive), and students routinely swap continuous and pulsed indications. The dosage question is answered by sensation — mild warmth, with sharp heat a stop signal — and the operating-distance rule keeps the therapist clear of stray field exposure during long treatment lists.
Frequently asked questions
What frequency and wavelength does shortwave diathermy use?
27.12 MHz, a wavelength of approximately 11 metres in the allocated medical band.
Which SWD method preferentially heats muscle?
The inductive method — drum or coil electrodes — producing eddy currents in the water-rich muscle.
Why are metal implants and damp clothing excluded from the field?
Both concentrate the electromagnetic field into local hot spots, causing deep or skin burns.
How does pulsed SWD differ from continuous?
Pulsed delivers bursts with low mean power for athermal effects in acute lesions; continuous delivers steady energy for deep heating.
What sensation indicates a correctly dosed continuous treatment?
Mild, comfortable warmth; sharp or burning heat means the output is too high and must be reduced or stopped.