Mechanical Modulation of Fracture Healing and Implications for Skeletal Tissue Engineering - Engineered Bone: Tissue Engineering Intelligence Unit - page 79

Figure 3. The material properties of tissues occurring during the process of indirect bone repair.
bridging fixation that induced indirect bone healing as the most effective fixation in stimulat-
ing repair. 38
Fracture Geometry
The geometry of the fracture or osteotomy site may influence the distribution of strains
that occur with a given method of fixation. The absolute size of the fracture gap may be critical
in a number of ways. Perren indicated that in rigid fixation of fractures with good reduction
any small gaps could generate very high levels of strain if the fixation was not absolutely rigid.
This was tested experimentally 10 where lower strain field was found to facilitate bone union.
The process of repair involves the synthesis of connective tissue matrices, ultimately lamel-
lar bone. Each of the tissues that differentiates in this process has a range of material properties
(Fig. 3). The ultimate strain for each of connective tissue has a specific value, for example bone
can only withstand a strain of 2% before damage occurs, yet fibrous tissue can tolerate around
12-15%. Thus if the mechanical environment at the fracture site generates an inter-fragmentary
strain of 12% it is impossible for bone to exist unless the load is reduced so that with tissue
differentiation the level of strain decreases with the increased stiffness of the inter-fragmentary
tissues.
Another aspect of the fracture site geometry is the level of shear strain. Traditionally it has
been thought that shear per se is disadvantageous to bone repair. However, when a load is
applied to a structure or material strains are induced. The principal tensile and compressive
strains are orthogonal to each other and a maximum shear strain occurs at 45 degrees to these
principal strains, thus shear strains do exist. Some workers have used oblique osteotomies to
investigate the effects of increased shear when compared to a transverse osteotomy, 1 in this
model there was a reduction in early weight bearing on the oblique osteotomies and the me-
chanical properties of the healed osteotomies were greater in the transverse osteotomies. Inter-
estingly, in this external fixation model the pin loosening was higher in the oblique osteotomies
despite the initial reduction in weight bearing. The amount of inter-fragmentary displacement
was not measured in this study, however, the stiffness of fixation in the oblique osteotomies was
only 45% of that in the transverse osteotomy constructs. The stiffness of the fixation construct
has been shown to influence both the initial weight bearing and the progression of healing,