Orthodontic tooth movement is achieved by applying a continuous mechanical load (orthodontic force) on the tooth, primarily affecting the periodontal ligament (PDL) and the alveolar bone. This force remodels the PDL and the bone, which enables tooth transposition. Since the bony response is mediated by the PDL, tooth movement is primarily a PDL phenomenon. The molecular and cellular responses of PDL cells to the mechanical force, which ultimately regulates tissues response, have still not been fully identified.

The PDL separates the tooth from the surrounding bone. The major component of the tooth’s extracellular matrix (ECM) is a network of collagen fibers running from the cementum of the tooth to the surrounding alveolar bone. The most abundant cells in the PDL are fibroblasts and undifferentiated mesenchymal cells.

The orthodontic force on the tooth is immediately sensed by the PDL and the bone as these  structures are exposed to both pressure and tension force.

The immediate effect of this force is extensive damage to the PDL and to the PDL blood vessel network. All the ECM components are injured and cells within the PDL, which were not irreversibly damaged, will respond to the tissue inflammation while being under continuous mechanical force.

To date, no orthodontic appliance could successfully monitor the magnitude of the orthodontic force. This meant that the immediate effect of the force on collapsed blood vessels was inevitable. Consequently, a necrotic area (hyalinized area and later undermining bone resorption) within the PDL developed and tooth movement would only be possible after the area was repaired. This cycle of PDL damage and repair is a the main reason for prolonged treatment.

Several in-vitro cell studies, in-vivo animal studies and experience treating humans clearly supported and evidenced that fully controlled pulsating orthodontic force can shorten the treatment period. The cause for this is assumed to be the fact that damage to capillary vessels within the PDL is prevented. As a result, continuous frontal bone resorption is initiated, bypassing the hyalinization process, to achieve faster tooth movement.

The pulsating force acts in a way that is aligned with human physiology and within the force range, which keeps blood vessels open and un-damaged.

 

 

References:

 

Shapiro E. applied pulsating forces to human maxillary molars.
The rate of movement as well as the total movement of the pulsed tooth was greater than the control tooth.

Shapiro E. et al. . Orthodontic movement using pulsating force induced Piezoelectricity. AJO 1979;76: 59-66.

Oates found that the premolar which received continuous force moved 0.6 mm..
The premolar which received the pulsating force moved 0.9 mm, during a 12 day experimental period.

Oates JC. et al. Pulsating forces in orthodontic treatment AJO. 1978;74: 557-86.

Hayashi H. examined the effect of intermittent force (8 hours per day) versus continuous force on the amount of molar movement in rats.
The results showed that 8 hours of intermitted force (33.3% relative to continuous) gave 70% of tooth movement.
It is assumed that if we raise the time to 50% we can approach 100% of tooth movement.

Hayashi H. et al. Intermittent 8-hour activation in orthodontic molar movement AJODO 2004 ;125:302-9.

Igarashi's study on rats showed that an activation of force for 12 hours with 12 hours of rest produces similar results to those achieved when force is activated for 24 hours per day.

Igarashi K. et al. Diurnal variation in tooth movement in response to orthodontic force in rats. AJODO. 1998;114:8-14.

Pulsating orthodontic appliance - Craven Kurz
"As a result, the use of the pulsating orthodontic appliance of the present invention results in faster movement of the tooth; reduction of root resorption during orthodontic movement; reduction of horizontal bone loss during bone reconstruction; reduced discomfort from heavy orthodontic pressures; and reduction in tooth extrusion from their boney sockets when pressurized".

U.S Patent, Oct 1980 & June 1981.

"It is suggested that the Pulsed Electro Magnetic Field -induced vibration may enhance the effect of mechanical and magnetic forces on tooth movement"

Darendeliler MA. et al.Effects of pulsed electromagnetic field vibration on tooth movement induced by magnetic and mechanical forces: a preliminary study.
Australian Dental Journal 2007;52:282-7.

"Although it's a result only from the in vitro study of one single cell type (osteoblast), and only indicated by one single factor (ALP), it still bring us an enlightenment of attempting an invention of an orthodontic appliance that can produce fluctuating forces which might lead to more rapid tooth movement"

Zhihe Zhao et al. The adaptive response of periodontal ligament to orthodontic force loading - A combined biomechanical and biological study. Clinical Biomechanics 2008;23:S59-S66.