Mechanical Modulation of Fracture Healing and Implications for Skeletal Tissue Engineering - Engineered Bone: Tissue Engineering Intelligence Unit - page 81

Figure 4. Effects of short periods of micro-movement on callus distribution in bone healing.
inter-fragmentary displacement of 0.5 mm in a similar model but could not show a statistically
significant effect. They concluded that externally applied mechanical stimulation could have a
role in stimulation of healing in patients prevented from normal weight bearing, avoiding a
delayed union.
The Characteristics of Applied Mechanical Stimulation
These studies using in vivo models demonstrate how sensitive the repair process is to local
mechanical conditions. However, the relevance of the role of mechanical environment in rela-
tion to human clinical fractures has also been demonstrated. The application of a mechanical
stimulus increased the rate of healing with respect to an objective end point of fracture stiff-
ness. 43,71 Using similar methods it is possible to both stimulate and monitor the progression of
fracture repair in terms of restoration of mechanical integrity.
The process of bone repair, however, can be enhanced or inhibited by very subtle changes in
the type and timing of the mechanical stimulation. Thus, there is potential for an optimal level
and type of stimulus. The experimental studies can provide guidelines for clinical fracture
management. However, each patient may require “fine tuning” of these principles. One major
advantage of the system of external fixation is the ability to modify the system stiffness in
response to the healing progress in that particular individual. Unfortunately, this system of
fracture management is often regarded as labour intensive in the short term and may also have
lower patient compliance than systems such as intra-medullary nailing.
Subsequent studies have indicated the characteristics of the stimulus that can be used to
modulate bone healing.
These studies have shown significant effects in modulation of the healing process as a con-
sequence of relatively small movements between fracture fragments. Recently, it has been shown
that even sub-physiological strains can be potent osteogenic signals in intact bone. The applica-
tion of these to healing osteotomies has also shown the potential to enhance indirect bone
repair. Using the ovine tibial mid-diaphyseal osteotomy model with an external fixator, a 30
Hz cyclical stimulation was applied to a 3 mm osteotomy gap for a short period each day. The
magnitude of the applied displacement was initially only 250 microns, while the
inter-fragmentary displacement induced by walking was of the order of 1 mm. The effect of