The Fracture Healing Process
When fractures occur the process of healing starts immediately. The length of the process will
depend on several things: The severity and extent of the injury, angle and type or classification of
fracture, articular involvement, and the location of the fracture within the body. Upper extremity
fractures typically take less time to heal than do lower extremity fractures. Operative and non-
operative treatment can also vary healing times. So, while fracture healing maybe unpredictable, the
process the body uses to repair the bone is always constant.
While medical treatments, whether internal or external(casting or surgical), may facilitate
healing, fracture healing or bone repair is a physiological process. The healing process is mainly
determined by the periosteum and whether its blood supply has been compromised or maintained. The
periosteum supplies precursor cells which transform into chondroblasts and osteoblasts. These are
primarily essential to healing of the bone. Bone marrow,which produces mesenchymal stem cells,
endosteum, small blood vessels, and fibroblasts are secondary sources of precursor cells.
Phases of Fracture Healing
There are three major phases of bone healing and two of these can be sub-divided to make a
total of five phase. The first phase is the reactive phase. This phase is sub-divided and includes the
fracture and inflammatory phase and the granulation tissue formation phase. The second phase is the
reparative phase. This phase is also sub-divided and includes callus formation and lamellar bone
deposition. The third phase is the final and is the remodeling phase in which bone remodels itself to
resemble its original shape and strength.
The reactive phase includes the trauma of the fracture and the first few hours following the
initial injury. During this time granulation tissue starts to form from the replication of fibroblasts.
These fibroblasts survive in spite of the fact other cells and blood cells are degenerating and dying in
and around the hematoma that is forming.
In the days following the injury, replication and transformation of periosteal cells continues.
Those cells proximal to the fracture develop into chondroblasts and form hyaline cartilage. The cells
distal to the fracture develop into osteoblasts. Osteoblasts produce osteoid which is composed mainly
of Type 1 collagen, they are also responsible for mineralization of the osteoid matrix. Although
osteoblasts implies an immature cell type, osteoblasts are in fact the mature bone cells entirely
responsible for generating bone tissue or osteocytes. As the osteoids form woven bone and the
fibroblasts develop into chondroblasts and form into hyaline cartilage, these two new tissues grow and
join together to form the fracture callus.
After the callus restores some of the bones original strength, the next phase begins with the
replacement of the two new tissues with lamellar bone. This is known as endochondral ossification.
The lamellar starts forming soon after the collagen matrix of either tissue becomes mineralized. As the
osteoblasts penetrate the mineralized matrix they form lamellar bone called cancellous bone.
Eventually the fracture callus is replaced by cancellous bone and more of the bones original strength is
Remodeling is the final phase of bone healing and involves the body's use of osteoclasts. Osteoclasts
are a type of bone cell that removes bone tissue by removing the mineralized matrix. The process
know as resorption happens when the cancellous bone is first absorbed leaving a shallow pit,
“Howship's lacuna”, while osteoblasts deposit cortical bone in its place. This process remodels the
bone into a new shape leaving it to closely resemble the bones original shape and strength.