In this article, we will describe the structure mechanics behind fracture healing. The findings have been summarized by our orthopedic implants team. In order to understand things from a mechanical viewpoint, the process of fracture healing must be defined in terms of the macroscopic structural features in callus development.

The mechanical behavior of the whole callus is described by the local, biological and tissue-related events occurring in the individual region of the fracture callus. “Each of these events has their own temporal and spatial distribution and combined together they contribute to the mechanical behavior of the whole callus”, quotes renowned ortho surgical implants specialists.

 Before we proceed further, let us first understand the stages of callus development based on structural development.

On a structural basis, the first stage for callus development includes a period of instability. This instability is usually caused by the disruption of soft tissues and the bone resulting from the early injury. At this point, the motion of the fragments is wholly regulated by the elastic support from the surrounding soft tissues (Soft tissue includes tendons, ligaments, fascia, skin, fibrous tissues, fat, etc.). The instability that occurs during the initial stage of callus development is characterized by a gross motion of the fracture fragments. This motion is related to the degree of soft tissue damage.

The soft tissue structures that control the fragments and increase bony stability can get tightened due to the displacement of the fragments or due to any kind of external pressure from the surrounding soft tissues.  No bone or organized soft tissue has time to form in the bridging region and further, a hematoma (A localized bleeding outside of blood vessels) fills the central area. The peripheral area is enveloped by the disrupted soft tissue. Top orthopedic implants and instruments specialists quote that in such situations the stability can only be provided by the founding soft tissues.

During this stage of healing, pain is one of the most important factors in controlling fracture site motion. Top spinal Implants manufacturers in India research report states that if the pain is not present to limit the level of activity and tissue then strain (in the unstable phase) and tissue damage may continue and nonunion (permanent failure of healing after a broken bone fracture) may develop

Stage II of callus formation is the soft callus stage. During this stage, the callus behaves in a rubbery compliant manner and controls the bone fragments. In this stage, before bone formation occurs, there appears to be a form-function relationship in the early peripheral soft tissue. This form-function relationship is supposedly needed for tensile stress resistance in the periphery. This tensile stress resistance is needed to provide bending and torsional stability with the central region thus aiding in compressive stress resistance.

In the early phase, the relatively compliant soft tissues in the peripheral region are responsible for the tension band mechanism. This is termed the stage of "soft callus". As per our orthopedic surgical instruments experts, this soft callus stage is responsible for early stabilization.

The adjacent regions have increased in diameter due to subperiosteal (situated or occurring beneath the periosteum) new bone formation. In the central region, soft tissues and cartilage persist since it has no blood supply: The fluid like nature and compacting resistance of these tissues may provide a hydraulic-like resistance mechanism (relating to a liquid moving in a confined space under pressure) in these tissues called dilatational tissue strains. This mechanism is confined to the peripheral region.

Such a structure could transmit loads like an inter-vertebral disc between two rigid vertebral bodies surrounded by a tough, tensile, resistant annulus (any body part that is shaped like a ring). The pressure on the tissues is minimal for even heavy loading conditions. This is because of the large callus size. Evidence found in the ortho surgical implants study suggests that in the soft tissue callus stage, the ends of the fracture fragments are repeatedly moving. Further analysis suggests that this happens as a result of the loading provided by the functional activity.

Read more:

Biomechanics of Fracture Stability