How Do They Work?

In the 1950's in Japan, Yasuda did a number of scientific studies on the treatment of fractures.  He discovered naturally occurring stress generated potentials in bone ^2,11,19.  He found that when a bone is stressed it will carry an electropositive charge on the convex side, while the concave side has an electronegative charge.  This helps to explain Wolff's Law by showing that bone will remodel via deposition of new bone at areas of compression and via resorption of bone at areas of tension².  With further examination, it was confirmed that areas of active growth in living bone, such as epiphyseal plates and repairing areas, were electronegative when compared with less active areas¹¹.   It has also been discovered that when a bone fractures, the entire bone becomes electronegative with a peak electronegativity at the fracture site.  This is the same type of direct current that powers a battery.  Since this discovery, many researchers have devoted their time to using electricity to promote bone growth ^1,2,6,10,11.  Areas of growth in bone have been shown to be electronegative, therefore, osteoblasts are activated by negative charges.  By implanting weak electrical current directly into the bone, research has demonstrated that bone formation is increased around the cathode (negative electrode) and decreased around the anode (positive electrode).  Researchers found the optimal bone growth took place with a current between 5 adn 20 microamperes.  They noted that levels below 5 microamperes did not enhance growth, while levels above 20 microamperes caused cell necrosis and bone death ².  From this research, bone growth stimulators have been developed as a rehabilitative tool for complete bone repair.
 

Types of Bone Growth Stimulators

There are three different techniques used with bone growth stimulators; a semi-invasive direct current, invasive direct current, and pulsing electromagnetic fields (PEMF) ^2,6,11.  Because each of these techniques have been proven to work, the decision is left up to the patient and the doctor as to which type will yield greater benefits.

• Semi-invasive Direct Current:  With the patient under local anesthesia, a cathode (can be more than one) is inserted percutaneously and transcortically into the fracture site, while an anode is placed directly on the skin.  The electrodes are then connected to a power pack that delivers 20 microamperes and is embedded within the cast.  This unit is designed to be applied for 12 weeks^2,11.

• Invasive Direct Current:  This technique requires that the cathode, anode and battery supply be surgically implanted into the fracture site.  The cathodes are made of stainless steel or titanium and with the exception of the 1-cm bare tip, they are coated with Teflon.  The cathode is coiled equally in a slot spanning the fracture site.  The battery is placed in  muscle plane with an external monitor available for verification of output.  After the fracture has healed, another surgery is required to remove the anode and the battery pack.  The cathode wire, however, remains imbedded in the fracture site^2,11.

• Pulsing Electromagnetic Fields (PEMF):  This method uses paired copper coils that are placed on either side of the fracture site.  They are attached to the cast via Velcro strips.  The electrical power needed to generate therapeutic electric field amplitudes requires a 110-volt source.  Therefore, the unit must be plugged into a standard electrical wall outlet.  Ten hours of treatment per day are necessary.  It is preferable that they are done consecutively, though the treatment period may be cumulative as long as no one exposure time is less than one hour.  The voltage applied is nonhazardous even if the coils are inadvertantly immersed in water.  PEMF's trigger calcification of the fibrocartilage within the fracture gap.  This should be worn until radiographs show bone stress lines gradually bridging the fracture gap to produce cortical continuity ^2,11.

Indications

Pulsed electromagnetic fields are indicated in nonunions, delayed unions, failed arthrodeses, and congenital pseudarthroses.  PEMF's also work well for avascular necrosis, infection, internal/external fixation, osteoperosis, osteogenesis imperfecta, or certain other pathologic fractures ².  Bone growth stimulators are not used as an initial treatment because fractures generally heal on their own with immobilization.  As stated previously, they have traditionally been reserved for the fractures that do not heal sufficiently.
 

Contraindications and Precautions

Noninvasive stimulators are contraindicated in nonunion fractures with synovial pseudarthrosis, fracture gaps greater than 1cm or greater than half the diameter of the bone, patients with implanted magnetic fixation devices, and patients with demand-type pacemakers who may develop inhibition of pacemaker output from electromagnetic stimulation ^2,11.  Implantable stimulators have no known contraindications or adverse effects.  However, it is not recommended that they be used with active osteomyelitis or if there is a pathologic fracture from a malignant tumor.

The long-term effects of use of bone growth stimulators near epiphyses is unknown.  Use on pregnant patients has not been evaluated and, therefore, is not recommended.  Electrical stimulation will not correct angular deformities, articular surface disruption, or osseous shortening ².