Bone Healing

X-ray of a bone fracture in the

process of healing.

Bone healing or fracture healing is a proliferativephysiological process, in which the body facilitates repair of Bone fractures.

Physiology and process of healing

In the process of fracture healing, several phases of recovery facilitate the proliferation and protection of the areas surrounding fractures and dislocations. The length of the process is relevant to the extent of the injury, and usual margins of two to three weeks are given for the reparation of the majority of upper bodily fractures; anywhere above four weeks given for lower bodily injury.

The process of the entire regeneration of the bone can depend upon the angle of dislocation or fracture, and dislocated bones are generally pushed back into place via relocation with or without anaesthetic. While the bone formation usually spans the entire duration of the healing process, in some instances, bone marrow within the fracture having healed two or fewer weeks before the final remodeling phase.

While immobilization and surgery may facilitate healing, a fracture ultimately heals through physiological processes. The healing process is mainly determined by the periosteum (the connective tissue membrane covering the bone). The periosteum is the primary source of precursor cells which develop into chondroblasts and osteoblasts that are essential to the healing of bone. The bone marrow (when present), endosteum, small blood vessels, and fibroblasts are secondary sources of precursor cells.

Phases of fracture healing

There are three major phases of fracture healing, two of which can be further sub-divided to make a total of five phases;

  • 1. Reactive Phase
  • i. Fracture and inflammatory phase
  • ii. Granulation tissue formation
  • 2. Reparative Phase
  • iii. Callus formation
  • iv. Lamellar bone deposition
  • 3. Remodeling Phase
  • v. Remodeling to original bone contour
Reactive Phase

After fracture, the first change seen by light and electron microscopy is the presence of blood cells within the tissues which are adjacent to the injury site. Soon after fracture, the blood vessels constrict, stopping any further bleeding.[1] Within a few hours after fracture, the extravascular blood cells, known as a "hematoma", form a blood clot. All of the cells within the blood clot degenerate and die.[2] Some of the cells outside of the blood clot, but adjacent to the injury site, also degenerate and die.[3] Within this same area, the fibroblasts survive and replicate. They form a loose aggregate of cells, interspersed with small blood vessels, known as granulation tissue.[4]

Reparative Phase

Days after fracture, the cells of the periosteum replicate and transform. The periosteal cells proximal to the fracture gap develop into chondroblasts and form hyaline cartilage. The periosteal cells distal to the fracture gap develop into osteoblasts and form woven bone. The fibroblasts within the granulation tissue also develop into chondroblasts and form hyaline cartilage.[5] These two new tissues grow in size until they unite with their counterparts from other pieces of the fracture. This process forms the fracture callus.[6] Eventually, the fracture gap is bridged by the hyaline cartilage and woven bone, restoring some of its original strength.

The next phase is the replacement of the hyaline cartilage and woven bone with "lamellar bone". The replacement process is known as "endochondral ossification" with respect to the hyaline cartilage and "bony substitution" with respect to the woven bone. Substitution of the woven bone with lamellar bone precedes the substitution of the hyaline cartilage with lamellar bone. The lamellar bone begins forming soon after the collagen matrix of either tissue becomes mineralized. At this point, "vascular channels" with many accompanying osteoblasts penetrate the mineralized matrix. The osteoblasts form new lamellar bone upon the recently exposed surface of the mineralized matrix. This new lamellar bone is in the form of "trabecular bone".[7] Eventually, all of the woven bone and cartilage of the original fracture callus is replaced by trabecular bone, restoring much, if not all, of the bone's original strength.

Remodeling Phase

The remodeling process substitutes the trabecular bone with "compact bone". The trabecular bone is first resorbed by osteoclasts, creating a shallow resorption pit known as a "Howship's lacuna". Then osteoblasts deposit compact bone within the resorption pit. Eventually, the fracture callus is remodelled into a new shape which closely duplicates the bone's original shape and strength.[8]

Inadequate healing or formation

Inadequate bone healing is known as an "incomplete" form of bone healing, in which the regeneration of bone through natural processes is impeded due to other factors, such as malnutrition or immune disorders, which may prevent the reparation of bone due to the lack of nutrient intake, such as that seen in the case of osteomalacia and osteoporosis.

Similarly, factors such as the intake of carcinogens, such as nicotine or exposure to radiation may lead to the malformation or incomplete healing of bones, which can further facilitate the formation of newer fractures, due to the already weakened site of injury being more easily affected by impact or strain, as well pseudarthrosis, undesired mobility in what appears to have become a new joint.

Medical Treatments

In terms of medical treatments and procedures, several options are available which facilitate faster reparation of bone if the specific patient has an aforementioned bone disorder. The use of Bone morphogenetic proteins is incurred in small amounts, and is also used in clinical practice, alongside immobilizing surgical procedures involving vertebroplasty or percutaneouskyphoplasty in the case of bone malformation, and stimulate the growth of bone in areas which require "strengthening", such as in the case of spinal fusion.

Osseointegration

Osseointegration is the pattern of growth exhibited by bone tissue during assimilation of surgically-implanted devices, prostheses or bone grafts to be used as either replacement parts (e.g., hip) or as anchors (e.g., endosseous dental implants).

References

  • Brighton, Carl T. and Robert M. Hunt (1986), "Histochemical localization of calcium in the fracture callus with potassium pyroantimonate: possible role of chondrocyte mitochondrial calcium in callus calcification", Journal of Bone and Joint Surgery, 68-A (5): 703-715
  • Brighton, Carl T. and Robert M. Hunt (1991), "Early histologic and ultrastructural changes in medullary fracture callus", Journal of Bone and Joint Surgery, 73-A (6): 832-847
  • Brighton, Carl T. and Robert M. Hunt (1997), "Early histologic and ultrastructural changes in microvessels of periosteal callus", Journal of Orthopaedic Trauma, 11 (4): 244-253
  • Ham, Arthur W. and William R. Harris (1972), "Repair and transplantation of bone", The biochemistry and physiology of bone, New York: Academic Press, p. 337-399

Types of Bone Fractures

In orthopedicmedicine, fractures are classified as closed or open (compound) and simple or multi-fragmentary (formerly comminuted).

  • Closed fractures are those in which the skin is intact, while open (compound) fractures involve wounds that communicate with the fracture and may expose bone to contamination. Open injuries carry an elevated risk of infection; they require antibiotic treatment and usually urgent surgical treatment (debridement). This involves removal of all dirt, contamination, and dead tissue.
  • Simple fractures are fractures that occur along one line, splitting the bone into two pieces, while multi-fragmentary fractures involve the bone splitting into multiple pieces. A simple, closed fracture is much easier to treat and has a much better prognosis than an open, contaminated fracture. Other considerations in fracture care are displacement (fracture gap) and angulation. If angulation or displacement is large, reduction (manipulation) of the bone may be required and, in adults, frequently requires surgical care. These injuries may take longer to heal than injuries without displacement. Another type of bone fracture is a compression fracture. An example of a compression fracture is when the front portion of a vertebra in the spine collapses due to osteoporosis, a medical condition which causes bones to become brittle and susceptible to fracture (with or without trauma).

Other types of fracture are:

  • Complete Fracture- A fracture in which bone fragments separate completely.
  • Incomplete Fracture- A fracture in which the bone fragments are still partially joined.
  • Linear Fracture- A fracture that is parallel to the bone's long axis.
  • Transverse Fracture- A fracture that is at a right angle to the bone's long axis.
  • Oblique Fracture- A fracture that is diagonal to a bone's long axis.
  • Compression Fracture-A fracture that usually occurs in the vertebrae.
  • Spiral Fracture- A fracture where at least one part of the bone has been twisted.
  • Comminuted Fracture- A fracture causing many fragments.
  • Compacted Fracture- A fracture caused when bone fragments are driven into each other
  • Open Fracture- A fracture when the bone reaches the skin
  • Bug fracture- A fracture when the bone is in place, but the fracture has the appearance of a crushed insect.

Special considerations for children

In children, whose bones are still developing, there are risks of either a growth plate injury or a greenstick fracture.

  • A greenstick fracture occurs because the bone is not as brittle as it would be in an adult, and thus does not completely fracture, but rather exhibits bowing without complete disruption of the bone's cortex.
  • Growth plate injuries, as in Salter-Harris fractures, require careful treatment and accurate reduction to make sure that the bone continues to grow normally.
  • Plastic deformation of the bone, in which the bone permanently bends but does not break, is also possible in children. These injuries may require an osteotomy (bone cut) to realign the bone if it is fixed and cannot be realigned by closed methods.

Treatment

First aid for fractures includes stabilizing the break with a splint in order to prevent movement of the injured part, which could sever blood vessels and cause further tissue damage. Waxed cardboard splints are inexpensive, lightweight, waterproof and strong. Compound fractures are treated as open wounds in addition to fractures.

At the hospital, closed fractures are diagnosed by taking an X-ray photograph of the injury.

Since bone healing is a natural process which will most often occur, fracture treatment aims to ensure the best possible function of the injured part after healing. Bone fractures are typically treated by restoring the fractured pieces of bone to their natural positions (if necessary), and maintaining those positions while the bone heals. To put them back into the natural positions, the doctor often "snaps" the bones back into place. This process is extremely painful without anesthesia. The pain is about as bad as breaking the bone itself. To this end, a fractured limb is usually immobilized with a plaster or fiberglasscast which holds the bones in position and immobilizes the joints above and below the fracture. If being treated with surgery, surgical nails, screws, plates and wires are used to hold the fractured bone together more directly. Alternatively, fractured bones may be treated by the Ilizarov method which is a form of external fixator.

Occasionally smaller bones, such as toes, may be treated without the cast, by buddy wrapping them, which serves a similar function to making a cast. By allowing only limited movement, fixation helps preserve anatomical alignment while enabling callus formation, towards the target of achieving union.

Surgical methods of treating fractures have their own risks and benefits, but usually surgery is done only if conservative treatment has failed or is very likely to fail. With some fractures such as hip fractures (usually caused by osteoporosis), surgery is offered routinely, because the complications of non-operative treatment include deep vein thrombosis (DVT) and pulmonary embolism, which are more dangerous than surgery. When a joint surface is damaged by a fracture, surgery is also commonly recommended to make an accurate anatomical reduction and restore the smoothness of the joint.

Infection is especially dangerous in bones, due to their limited blood flow. Bone tissue is predominantly extracellular matrix, rather than living cells, and the few blood vessels needed to support this low metabolism are only able to bring a limited number of immune cells to an injury to fight infection. For this reason, open fractures and osteotomies call for very careful antiseptic procedures and prophylactic antibiotics.

Sometimes bones are reinforced with metal, but these fracture implants must be designed and installed with care. Stress shielding occurs when plates or screws carry too large of a portion of the bone's load, causing atrophy. This problem is reduced, but not eliminated, by the use of low-modulus materials, including titanium and its alloys. The heat generated by the friction of installing hardware can easily accumulate and damage bone tissue, reducing the strength of the connections. If different metals are installed in contact with one another (i.e., a titanium plate with cobalt-chromium alloy or stainless steel screws), galvanic corrosion will result. The metal ions produced can damage the bone locally and may cause systemic effects as well.

References

Ham, Arthur W. and William R. Harris (1972), "Repair and transplantation of bones", The biochemistry and physiology of bone, New York: Academic Press, p. 337-399