Understanding Bone healing process is important in treatment of fractures in many ways that will be explained here, it is also helpful in expecting the time by which the fracture heals.
First we need to know what is needed for the fracture to heal, this include:
We have three types of bone healing
If there is small amount of tension (below 2%) on the fracture (the fracture ends held very stable) => primary bone healing occurs; such as when using plates and screws (type of internal fixation)
If the tension is between 2-10 % (there is a little movement between the fracture ends due to external forces) => secondary bone healing will occur such as when using a cast, intramedullary nail or external fixation
Secondary bone healing is on 5 stages:
Initially when the fracture occur, it would lead to bleeding coming from the bone and soft tissue at the fracture site. This ultimately lead to collection of blood known as hematoma
first stage of bone healing: hematoma formation
After the hematoma forms, inflammatory process start
Last until fibrous tissue, cartilage or bone formation begins (1-7 days) after fracture
second stage of bone healing: inflammation
Callus is a temporary bony material, cartilaginous material, fibrous connective tissue and woven bone that replaces the hematoma
Woven bone is bone tissue characterized by haphazard arrangement of collagen fibers and will be remodeled into cortical bone in the upcoming stages
Soft callus: soft bony material (collagen type 2= cartilage) so it is mainly formed by cartilage, secreted by mesenchymal stem cells that are gathered to injury site because of the inflammation process
Soft callus forms After 2 weeks post fracture. The fragments can’t move freely now, they are stable in length but they can angulate. The stiffer the immobilization of the fracture the less amount of callus formed
third stage of bone healing: soft callus formation
Mesenchymal stem cells are undifferentiated and they differentiate to osteoblasts and chondroblasts. Important sources of mesenchymal stem cells in fractures include periosteum, bone marrow and surrounding muscles. Gathered during the inflammation process
Hard callus: hard bony material (woven bone which is type 1 collagen) so it is mainly bone material, secreted by osteoblasts and chondroblasts. Starts forming after the formation of the soft callus and ends when the fragments are firmly united by 3-4 months post fracture. Forms in the periphery of the fracture and progressively moves centrally
forth stage of bone healing: hard callus formation
The woven bone is slowly replaced by lamellar bone. The medullary bone canal is continuous in this stage. Last from few months to several years post fracture
fifth stage of bone healing: remodeling
Bone ends are put in close contact. When fracture surfaces are in intimate contact (less than 0.1 mm distance) with absolute stability => osteoclasts will form cutting cones that traverse the fracture line => capillaries occupy the newly formed cavities with osteoblasts => form lamellar bone from osteons. The intermediate stages are skipped (no callus what so ever)
Primary bone healing process called cutting cone remodeling, Haversian remodeling
Gap healing occurs when there is small gaps between fracture surfaces (< 1mm) => gaps invaded by new capillaries and osteoblast cells that form => woven bone => remodeled to lamellar bone
Adults:
Pediatric patients: 4-6 weeks in most fracture
This article is a part from the Orthopedics trauma basic principles course, this course also includes these topics: