Surgery – Magnetic field applied to body for therapy – Electromagnetic coil
Reexamination Certificate
2000-06-07
2002-09-03
Lacyk, John P. (Department: 3736)
Surgery
Magnetic field applied to body for therapy
Electromagnetic coil
Reexamination Certificate
active
06443883
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to medical appliances for biomedical therapeutic applications, including osteogenesis, based on application of pulsed electromagnetic fields (PEMF), and more particularly to PEMF's developed in a multi-coil, multi-functional PEMF therapeutic device which optimizes penetration of focussed electromagnetic fields at a treatment site for bone and soft tissue therapy.
BACKGROUND OF THE INVENTION
Electricity is common in living things. In the human body, it provides the basis for thoughts, senses, movement and the rhythm of the heart. As has been learned over approximately the last 50 years, it may also play a crucial role in the functioning of the skeletal system. It is now known that bones carry electric potentials that occur when the bones are at rest. These “bioelectric” potentials are an inherent property of living bone. They are a product of cellular metabolism, thus they disappear when cellular death occurs. It has been shown that active growth plates are electronegative, while the mid-shaft is not. When a fracture occurs, that site also becomes negative and is accompanied by an increase in negativity over the farthest growth plate from the fracture. These intriguing findings lead one to believe that this negative electrical state may be a signal for bone growth.
Bone becomes stronger when subjected to mechanical stress, such as walking, running, weight lifting or hard physical labor. These mechanical stresses are termed weight bearing or bone loading by rehabilitation specialists. When under stress, bone tissue deposits more of the mineral salts that lend strength to bone. When the same stress is removed, bone-resorbing cells (called osteoclasts) go to work and tear down the unnecessary bone. This is why a bone seems to shrink in size when it has been in a cast for some time. This would also partly explain the space osteoporosis that develops in astronauts during long space flights, due to the lack of bone loading in microgravity.
The principles of bone growth and fracture healing follow a process according to Wolff's Law, named after the Orthopedic Surgeon J. Wolff, who discovered this phenomenon in the late nineteenth century. Wolff's Law states that “every change in the form and function of bones or of their function alone is followed by certain definite changes in their configuration in accordance with mathematical laws.” This principle states that a bone responds to stress by growing into whatever shape best meets the demands the body makes of it. When a bone is bent, one side is compressed and the other is stretched. When it is bent consistently in one direction, extra bone grows to strengthen the compressed side, and some is absorbed from the stretched side. This law can explain how weight bearing, athletics and the activities of daily living influence the bone structure of a tennis or baseball player, body builder, etc. The Wolff's Law phenomenon of bone reorganization occurs because there is a stimulus to the periosteum to grow new bone at a surface where there is compressional stress, while dissolving bone where there is tensional stress.
An understanding of Wolff's Law wasn't reached until the early 1950s. Research done by I. Yasuda in Japan showed naturally occurring stress generated potentials (SGP's) in bone. This shows that mechanical stress has an effect on the electrical forces in bone. He also found that when a bone is stressed it carries an electropositive charge on the convex (stretched) side, while the concave (compressed) side has an electronegative charge. Bones are made of piezoelectric crystals (calcium apatite) with electrical potential. By mechanically bending a piezoelectric crystal hard enough to deform it slightly, a pulse of current is generated through it. In effect, the pressure “pops” electrons out of their places in the crystal lattice. They migrate down the compression, so the charge on the inside curve of a bent crystal is negative. The potential quickly disappears if the stress is sustained, but when it is released, an equal and opposite positive pulse appears as the electrons rebound before settling back into place.
This finding was a major step in explaining the mechanisms behind Wolff's Law, showing that bone will remodel via deposition of new bone at areas of compression and via resorption of bone at areas of tension. After 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 areas.
It has also been discovered that when a bone fractures, the entire bone becomes electronegative with a peak electro-negativity at the fracture site. This is the same type of direct current that powers a low voltage battery. Since this discovery, the field of electro-biology came into prominence as a science where researchers devote their time to studying the effects of electrotherapy to promote bone growth. Areas of growth in bone have been shown to be electronegative, thereby indicating that 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).
Marino and Becker (1970) associated the piezoelectric effect and growth control in bone with a mathematical formula. They demonstrated that on loading, bone will generate a bound surface charge distribution, p(xt) which is nulled by ion current in the permeating interstitial fluid. This process was monitored on a macroscopic level by measuring voltage. A symmetric biphasic pulse is seen, thereby proving the link between mechanical and electromagnetic energy in bone.
To mimic nature's own natural healing mechanisms with electrotherapy, currents of electromagnetism (pulsating electromagnetic fields or PEMEs) are sometimes applied to bones that fail to heal properly. Electromagnetic coils are placed outside the surface of the cast creating weak electromagnetic fields directed to stimulate the fracture site. Like the piezoelectric effect, it is believed that PEMFs stimulate reproduction of bone cells responsible for producing osteogenesis.
In osteology, a callous is defined as bony an cartilaginous material forming a connecting bridge across a bone fracture during repair. Within one to two days after injury, a provisional callous forms, enveloping the fracture site. Bone-forming cells in the periosteum (the bone layer where new bone is produced) proliferate rapidly, forming collars around the ends of the fracture, which grow toward each other to unite the fragments. The definitive callous form slowly as the cartilage becomes ossified. Two to three weeks after injury, strong bony extensions (trabeculae) join the fractured bone ends, and the organized aspect of bone gradually recurs. The callous is resorbed over a period of months.
Bone growth stimulation treatment has seen some success using two main types of biophysical treatment, each of which has been shown to be beneficial in stimulating bone growth. These two types are mechanical signals and electrical signals. These biophysical signals already occur naturally in the human body. However, it is unclear which specific components of the biophysical forces acting on the bone are actually osteogenic and which are just byproducts of bone loading.
The clinical applications of mechanical signals in osteogenesis can be seen as follows. Since past research has found that bone is sensitive to biophysical stimuli induced at low frequencies, the possible role for mechanical stimulation of bone has been further investigated. It is known from past research that a range of frequencies has been shown to persist between 10 and 50 Hz in living bone. Therefore, studies were done by McLeod et al. (McLeod and Rubin, 1992) to see if low amplitude mechanical energy induced at the optimal frequency range could in fact induce an osteogenic response. It wa
Ostrow Alvin S.
Tannenbaum Joseph
Edward Langer, Pat.Atty.
Lacyk John P.
Medical Bracing Systems, Ltd.
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