The type of electromagnetic field (EMF) your doctor is suggesting doesn't produce radiation and can't be compared directly to power lines. They are both forms of energy on a scale or continuum called the electomagnetic spectrum. When used to enhance bone growth, EMF is delivered in a pulsed form. This means the wave is turned on and off, so you aren't exposed to a steady dose of EMF.
Pulsed EMF still has biologic effects even if it's not strong enough to cause ionizing radiation. The waves produce electric currents in tissues, causing a heating effect. Increased heat in an area brings more blood to cool it off. Improved circulation may be what enhances bone formation and solid fusion.