Fracture Healing

General principles

Fracture healing is always an interplay of biology and mechanics.

Bone heals by regeneration NOT by scar tissue formation.

Strain – a fracture will heal with different types of tissue depending upon the mechanical stress it is under because different tissues can tolerate different amounts of pressure (i.e. for a bone of 30mm diameter, granulation tissue will tolerate 40° angulation, fibrocartilage 5° , and bone 0.7° ). After a fracture, the bone is initially pliable then becomes increasingly non-compliant and stiff.

Strength of external callus: as external in diameter increases, the ability of the lever arm to resist bending increases

bending strength of the bone = (radius of bone)³

Terminology

Fracture: any break in the continuity of bone

Complete: NO continuity of the bone (fracture completely through the bone)

Incomplete: some continuity exists (fracture not completely through the bone)

Closed (simple): fracture is encased within the skin (no skin breaks)

Open (Compound – an old term): fracture breaks through the skin

Transverse – clean through the bone (perpendicular break)

Oblique – Any angle that Is deviated from the axial plane (45° angle break)

Spiral – fracture through multiple planes (spiral break)

Comminuted – the bone is splintered (multiple breaks)

Displaced – ends of the bone of the fracture are not aligned

Pathologic fracture – break occurs in a bone altered by a disease process

Stress fracture – slowly developing fracture

Angulation – alignment of bone after fracture (angle/percent of displacement) Þ results from muscle pull across the fracture

Reduction – process through which alignment of displaced fractures is restored; shortening of bone when displaced into surrounding muscle

Fractures are the most common non-lethal injuries in children

Primary Fracture Healing – Fractures healing under STABLE conditions

Surgical stabilization provides proper alignment of displaced fractures (reduction) and complete immobilization (via traction or casting). Under these conditions there is typically NO endochondral (EC) bone formation (via cartilage) and only intramembraneous (IM) bone formation (DIRECT osteoid deposition) within the fracture. This shortens the healing time.
Stages:

Contact healing: provides simultaneous union and reconstruction of bone by osteonal ("Haversian") remodeling. There is direct bone deposition by osteoblasts without a cartilagenous scaffold producing woven bone (IM bone formation) which eventually turns to lamellar bone under the influence of functional stresses.

Gap healing: If gaps , 1mm and strain < 2%, then lamellar bone will fill gap first (in contrast to cartilage). This if followed by osteonal remodeling (refer below)

Secondary Fracture Healing (Callus healing)– Fractures healing under UNSTABLE conditions

Fractures that are displaced, comminuted, or inadequately immobilized heal with elements of both endochondral (EC) and intramembraneous (IM) bone fomation. Bones heal via a Callus which is cartiliganeous, uncalcified tissue which provides stability between the ends of the bones. Upon this scaffold endochondral bone formation takes place.

Histology: Mixed Spicules – Mixed calcified cartilage and lamellar bone because of endochondral and intramembraneous bone formation

Stages:

Hematoma - following a fracture ruptured blood vessels form blood clots that fill the fracture and provides a fibrin mesh.

Inflammation – ingrowth of fibroblasts and blood vessels (formation of granulation tissue). Influx of inflammatory cells which activate osteoprogenitor cells in the periosteum, medullary cavity, and surrounding soft tissues.

Procallus – represents undifferentiated mesenchymal cells that via chemotaxis are attracted to the fracture site and eventually differentiate into chondroblasts and osteoprogenitor cells (osteoblasts)

Soft callus – Chondroblasts lay down cartilage replacing the granulation tissue (the composition of the tissue is therefore loose fibrous connective tissue intermixed with increasing collagen – forms best when strain to the fracture < 17%)

Hard tissue callus - osteoprogenitor cells in the endosteum and periosteum are activated and lay down woven bone within and around the soft callus. (Composition – woven bone and firgro-cartilaginous tissue)

Remodeling – as the callus matures (woven bone to lamellar bone) the portions of the bone that are not physically stressed are resorbed. The callus is then reduced in size and the shape and outline are reestablished. This allows angular deformities to be corrected in almost any plane (mostly children, limited in adults).

After remodeling no evidence of a fracture is left.

Factors that affect fracture healing

Growth: nothing will help speed up healing other than proper reduction and immobilization

Drinking milk – will not help since serum Ca is tightly controlled via PTH

Smoking, ETOH – known to impede the healing process

Bone Morphogenic Proteins (BMP’s) – in the TGF-b family. Can induce endochondral ossification.

Growth Plate Fractures

Salter and Harris Classification: Five classes

Class 1: Break across the diaphysis and through the plate (lowest incidence of growth plate arrest but difficult to differentiate between class 5)

Class 2: Break across the metaphysis

Class 3: Break through the epiphysis and articular surface

Class 4: Break through the metaphysis and the epiphysis

Class 5: Direct crushing of the growth plate (difficult to diagnose: worst prognosticator of viable growth plate)

Classes represent a spectrum of the viability of the growth plate (1 being the best and 5 the worst)

Treatment: alignment, immobilization

Remodeling Þ depends on the age of the patient (children have the best outcome)

Fracture Stabilization

Protects vascular repair process

Stabilizes soft tissue

Prevents infection

ß bleeding

ß pain

simplifies transport and care, allows for mobilization.

prompt stabilization improves outcomes of fracture treatment and the general medical condition of the multiple trauma patient.

Emergencies of fractures

24hrs – head injury, hemorrhage

1rst wk – ARDS

late – Multiple system organ failure, sepsis

Medical Complications

Associated complications are those attributable to forced recumbancy

Adult Respiratory Distress Syndrome (ARDS): complex sequlae of shock, sepsis, and trauma leading to a final common pathway of diffuse damage to the alveolar capillary walls.