Mechanical Modulation of Fracture Healing and Implications for Skeletal Tissue Engineering - Engineered Bone: Tissue Engineering Intelligence Unit - page 80

suggesting a mechanosensory mechanism. 30 This could regulate inter-fragmentary displace-
ment in a “stroke control” feed-back loop within a window of applied loads. Park et al (1998), 64
also investigated the effect of an oblique osteotomy compared to a transverse in a rabbit model
using both locked and telescoping fixators. They also reported on the relationship between
magnitude of inter-fragmentary motion recoded in vivo and the pattern of healing. With a
transverse osteotomy and a telescoping fixator there was a high level of inter-fragmentary dis-
placement initially but this reduced over the first week, whereas with an oblique osteotomy the
level of inter-fragmentary displacement was high throughout the early stages of healing The
oblique osteotomy group showed not only a greater inter-fragmentary movement but also a
greater amount of periosteal callus and higher mechanical properties than the other groups.
The authors concluded that the sliding related shear promoted greater cartilage differentiation
and callus stimulation than axial motion or the locked configurations. This effect is difficult to
relate to the micro-mechanics at the fracture site but does indicate a role for early cyclical
deformation in enhancing callus differentiation and size to restore mechanical integrity of the
fractured bone. Issues still remain to determine if these effects are the type of strain or merely
reflecting magnitudes of deformation. The loads acting through the fracture or osteotomy site
will also influence the magnitude of inter-fragmentary strain. These are influenced by the load
applied to the limb and the load sharing of the fixation device.
The properties of the fixation device are constant, the loads applied by the patient being
regulated be many factors including the fixation device stiffness, but also many undefined
influences ranging from psychological reaction to the fracture and the fixation device through
the direct and indirect biomechanical effects of the combined fracture and the fixation system,
to systemic effects such as smoking, concurrent pharmacological treatment for pain and oedema.
Thus the control of the inter-fragmentary motion is complex and the influence of this mechanical
movement on the progression of healing may not be optimal for these reasons. A potential
solution to this problem is to provide an applied dynamisation of the healing fracture.
Application of Cyclical Mechanical Stimulation to Modulate Bone Repair
In the development of external fixation for the repair of fractures and control of osteotomies
a number of advantages have been identified in relation to a variable control of fracture site
mechanics. These are to some extent offset by the disadvantage of the percutaneous fixation
pins and associated care issues. However, the ability to both control and modify the stiffness
provides the means to both monitor and influence the progression of healing in individual
patients.
In many cases of fracture there are concurrent injuries that impede ambulation and conse-
quent loading of a fracture. In such cases mechanical stimulation can be applied via an external
fixator.
A very rigid fixator will act as a load sharing device and strain protect the fracture. However,
a low stiffness fixator while reducing strain protection and providing mechanical stimulation to
the fracture site will also increase the stresses at the pin/bone interface. Using the information
from studies in intact bone, that short periods of cyclical deformation provide a potent osteo-
genic stimulus, a similar mechanical stimulus was applied to a healing osteotomy using a pneu-
matic actuator on a rigid external fixator. 34 The application of short periods of inter-fragmentary
cyclical deformation was shown to modulate the progression of healing of a standard
mid-diaphyseal tibial osteotomy. The magnitude of the initial deformation in relation to the
osteotomy gap can provide either an enhancing or inhibitory effect compared to a nonstimulated
control. An initial deformation in a 3 mm gap of 0.5 mm increased the rate of increase in both
mineralisation of the osteotomy gap and in stiffness index of the healing osteotomy, whereas an
initial 2 mm displacement induced an inhibition in the time related increases of these param-
eters, although the larger initial displacements induced a greater proliferation of callus (Fig. 4).
A similar observation that callus proliferation is related to the magnitude of inter-fragmentary
displacement was made by Wolf et al (1998). 79 These authors also implied an optimal