Mechanical Modulation of Fracture Healing and Implications for Skeletal Tissue Engineering - Engineered Bone: Tissue Engineering Intelligence Unit - page 76

include the femoral neck, distal radius and vertebral bodies. It is though there may be a genetic
predisposition to these osteoporotic fractures, relating to a collagen gene polymorphisms. 70
With ageing population femoral neck fractures will increase in number and represent a greater
burden for Healthcare systems than even current predictions indicate. Thus the prevention and
treatment of these fractures is an important area of research. The biological process of bone
repair in osteoporotic fractures is poorly documented. However, in this condition the rate of
bone remodelling is increased although the net mass of bone is decreased. 13
Fracture Repair
Direct Bone Healing
Following a fracture the fragments must be stabilised for the repair process to initiate the
restoration of functional and morphological integrity. The method of treatment and
post-operative patient activity determines the level of interfragmentary movement. In the late
1960s and early seventies knowledge of both the biology and biomechanics of fracture treat-
ment and repair were advanced considerably by the AO Research Group. The concept of com-
pression plating to provide rigid internal fixation and minimal interfragmentary movement
was advocated to induce direct bone healing by osteonal remodelling across the fracture site. 69
The pattern of bone repair under these conditions was studied experimentally and shown to
comprise osteonal remodelling across the fracture site at locations where the matrix of each
fragment was compressed together. Where small gaps occurred, these were filled initially with
woven bone that subsequently underwent osteonal remodelling. Osseointegration of the frag-
ments was achieved by extensive secondary osteonal remodelling of the fracture site over a
prolonged time period of months to years. In this type of repair process with minimal move-
ment between fragments there is no evidence of external periosteal callus, however, internal
endosteal callus within the medullary callus may be seen. The pattern of repair was initially
termed “primary” fracture healing but more recently is termed direct bone repair.
The presence of a rigid fixation device, such as a plate, inevitably leads to a reduction in
loading of the underlying bone, as a consequence of load sharing between the bone and the
plate, and also affects the periosteal blood supply. The consequence of mechanical load protec-
tion is a reduction in strain levels of the cortical bone beneath the fracture plate with a conse-
quent bone resorption. 2 The site of bone loss in a tubular diaphyseal region of a bone to
maintain bending strength with minimal material would be to thin the cortical wall by en-
dosteal resorption. The tubular structure of diaphyseal bone provides resistance to bending and
torsional loading. Thus the removal of endosteal bone allows a minimal bone mass for a maxi-
mum resistance to bending. The effect of the presence of a plate on the formation of intra-cortical
porosity was also initially attributed to the mechanical strain protection. 45 However, more
recently this has been shown to be a temporary phenomenon and attributed to changes in
haemodynamics as a consequence of the application of the plate to the periosteal surface of the
bone. 67 The development of plates with reduced points of contact with the periosteum has
reduced the initial intra-cortical porosity and pathed the way for fixation systems that do not
compromise periosteal blood supply. 68 The location of a plate in relation to the pattern of
loading of a bone is also important in providing optimal mechanical conditions at the fracture
site. Again the interaction between the biomechanics of the skeleton and positioning of frac-
ture plates formed part of the philosophy advocated by the AO Group. Application of plates to
the tension surface of the bone allows compression to be increased across the fracture site
during functional loading. Thus bones that are loaded in bending are ideally suited for the use
of compression plates to induce direct bone healing. The major disadvantage of this pattern of
healing is the time taken to remodel the fracture site and provide restoration of mechanical
integrity without the need to retain the fixation device. The advantage is a restoration of ana-
tomical integrity with minimal callus formation. The use of direct bone healing is ideal for
intra-articular fractures where precise reduction is required with no callus formation.