FRACTURES: Mechanisms & Stages of Healing
A fracture is a disruption in the continuity of bone that occurs when mechanical forces exceed the bone’s structural capacity. Fractures range from microscopic stress injuries to complete displacement of bone fragments and are influenced by bone strength, loading patterns, and the speed and direction of applied force. Understanding fracture pathophysiology explains why some fractures occur with minimal trauma, why healing varies widely between individuals, and why complications such as non-union or delayed healing develop despite appropriate immobilisation.
What You Need to Know
Fractures occur when the mechanical forces applied to bone exceed its structural capacity to absorb and distribute load. Normal bone strength depends on the interaction between mineral content, which provides rigidity, and collagen, which allows limited deformation under stress. Fractures may result from a single high-energy force, repetitive loading that causes cumulative microdamage, or reduced bone strength that lowers the threshold for failure. Conditions such as osteoporosis, osteomalacia, malignancy and prolonged corticosteroid exposure weaken bone architecture, meaning relatively minor trauma can result in fracture.
Following fracture, healing begins immediately through a coordinated biological response that progresses through overlapping stages rather than discrete steps. An initial inflammatory phase establishes a local environment that supports repair, followed by cellular proliferation and callus formation that stabilise the fracture site. Over time, immature bone is gradually remodelled into organised lamellar bone that restores strength and load-bearing capacity. This process is energy-dependent and sensitive to both local and systemic influences.
Several core factors determine whether fracture healing proceeds normally or becomes delayed:
Adequate blood supply to deliver oxygen, nutrients and repair cells to the fracture site
Mechanical stability that limits excessive movement while allowing controlled load transmission
Sufficient metabolic and hormonal support, including calcium, vitamin D and overall nutritional status
When any of these elements are compromised, healing may be delayed or incomplete, increasing the risk of non-union, malunion or chronic pain. Fracture healing therefore reflects not only the severity of the initial injury, but also the biological environment in which repair occurs.
Beyond the Basics
Mechanisms of fracture formation
Fractures develop when the forces applied to bone exceed its ability to absorb and distribute stress. In acute trauma, this threshold is surpassed suddenly, as occurs with falls from height or high-speed collisions, producing immediate structural failure. In contrast, repetitive low-energy loading causes microscopic cracks within the bone matrix. Under normal conditions, these microcracks are repaired through ongoing bone remodelling. When loading continues without sufficient recovery time, microdamage accumulates faster than repair can occur, leading to stress fractures even when bone density is normal.
The pattern of a fracture reflects how force is transmitted through the bone. Compression forces shorten and compact bone, producing impaction fractures that may retain some inherent stability. Torsional forces twist the bone along its axis, creating spiral fractures that often have greater surface area but less stability. Bending forces place one side of the bone under tension and the other under compression, resulting in transverse or oblique fractures. These patterns influence how fragments move under load and determine the mechanical environment in which healing must occur.
Vascular disruption and local injury
Bone relies on an extensive vascular network to maintain cellular viability and support repair. When a fracture occurs, blood vessels within the periosteum, cortex and bone marrow are torn, causing bleeding and formation of a fracture haematoma. This haematoma is biologically active rather than passive. It concentrates platelets, inflammatory mediators and growth factors that signal the start of healing and provide a provisional scaffold for incoming repair cells.
The extent of vascular injury strongly influences healing capacity. Fractures that preserve periosteal blood supply generally heal more reliably than those that disrupt it extensively. Certain anatomical sites are particularly vulnerable because their blood supply is limited or enters the bone at a single point. In fractures of the femoral neck or scaphoid, vascular disruption can prevent delivery of oxygen and osteogenic cells to the fracture site, increasing the risk of delayed union or avascular necrosis, where bone tissue dies due to prolonged ischaemia.
Stages of bone healing
The inflammatory phase begins immediately after fracture and establishes the biological conditions required for repair. Bleeding and tissue injury activate platelets and immune cells, which release cytokines and growth factors that initiate cell recruitment. Fibroblasts, chondroblasts and osteoprogenitor cells migrate into the fracture site through newly permeable blood vessels. This phase explains the early clinical features of pain, swelling, warmth and reduced movement, which arise from vascular changes and inflammatory mediator release rather than from structural instability alone.
During the reparative phase, the body focuses on restoring continuity across the fracture. Granulation tissue forms first, followed by development of a soft callus composed of collagen and cartilage. This callus stabilises the fracture by limiting movement while remaining flexible enough to tolerate some load. As new blood vessels grow into the callus, oxygen tension increases and cartilage is progressively replaced by woven bone through endochondral ossification. Mechanical conditions during this phase are critical, as excessive motion disrupts callus formation, while controlled loading stimulates bone production and alignment.
The remodelling phase transforms mechanically weak woven bone into strong, organised lamellar bone. Osteoclasts resorb excess or poorly aligned bone, while osteoblasts deposit new bone along lines of mechanical stress. This gradual reshaping restores bone strength and adapts internal architecture to functional demands. Remodelling may continue for months or years and explains why radiographic appearances evolve long after pain and function have improved.
Factors influencing fracture healing
Successful healing depends on the interaction between local mechanical conditions and systemic biological support. Adequate alignment and stability allow repair tissue to mature without disruption, while preserved blood supply supports ongoing cellular activity. Extensive soft tissue injury compromises both mechanical support and vascular delivery, increasing healing time.
Systemic factors influence the efficiency of each healing phase. Ageing reduces cellular responsiveness, while poor nutrition limits collagen synthesis and mineralisation. Smoking impairs blood flow and oxygen delivery, directly affecting callus formation. Endocrine and metabolic disorders such as diabetes, chronic kidney disease and vitamin D deficiency disrupt cellular signalling and bone turnover. Corticosteroids suppress inflammation and osteoblast activity, delaying progression from early repair to bone formation.
Complications of fracture healing
When the biological or mechanical environment is suboptimal, healing may be altered or fail entirely. Delayed union occurs when normal repair processes proceed slowly, often due to inadequate stability or compromised blood supply. Non-union reflects failure of the bone ends to bridge, commonly associated with persistent motion, infection or severe vascular impairment. Malunion results when healing occurs in poor alignment, altering joint mechanics and increasing long-term functional impairment.
Additional complications arise when tissue viability is compromised. Open fractures carry a higher risk of infection due to direct contamination and impaired local immunity. Avascular necrosis develops when blood supply is insufficient to sustain bone tissue, leading to collapse and secondary joint degeneration. These outcomes highlight that fracture complications arise from disruption of coordinated biological repair, not from mechanical injury alone.
Clinical Connections
Fractures typically present with pain, swelling, deformity and reduced function, but clinical severity does not always align with imaging findings. High-energy fractures may appear dramatic on radiographs yet preserve neurovascular integrity, while stress fractures or occult fractures can produce persistent, localised pain with little visible change on initial imaging. In these cases, pain triggered by weight-bearing or repetitive use arises from microstructural failure within bone, and symptoms often appear before cortical disruption becomes visible.
Several clinical features help differentiate fracture patterns and guide assessment:
Focal pain that is reproducible with loading suggests structural compromise even when plain radiographs appear normal
Progressive pain with activity and relief at rest raises concern for stress fracture rather than isolated soft tissue injury
Marked swelling, deformity or neurovascular change indicates instability or associated tissue injury requiring urgent evaluation
Diagnosis relies on clinical assessment supported by imaging. Plain radiographs are used initially, while MRI or bone scintigraphy is used when fractures are suspected but not visible, particularly in stress fractures and injuries involving the scaphoid or hip. Advanced imaging detects marrow oedema and microfracture before cortical changes develop, allowing earlier identification of structurally significant injury.
Management aligns with the biological requirements of bone repair. Immobilisation and alignment limit excessive movement that would disrupt callus formation, while adequate nutrition supports collagen production and mineral deposition. Once sufficient stability is achieved, gradual reloading promotes bone formation by stimulating osteoblast activity and directing remodelling along functional stress lines. Controlled mobilisation improves long-term strength and function when introduced at the appropriate stage.
Surgical intervention is indicated when mechanical stability or blood supply cannot be restored conservatively. Internal fixation provides a stable environment for repair (referred colloquially as “pins and screws or plates'“, and procedures that protect or re-establish vascular supply are critical in fractures with high risk of avascular necrosis. These decisions are guided by the interaction between biomechanics and tissue viability rather than radiographic appearance alone.
Concept Check
Why does reduced bone strength increase fracture risk even with minimal trauma?
How does vascular disruption influence fracture healing?
Why is controlled mechanical stability important during the reparative phase?
How do systemic conditions such as diabetes impair bone healing?
Why can fracture healing take months despite early symptom improvement?