PATHOPHYSIOLOGY OF FRACTURE HEALING

Speaker-Raghavendra MSModerator-Dr Marulasiddappa G

INTRODUCTION

Fracture is a break in the continuity of bone or periosteum

The healing of fracture is in many ways similar to soft tissue healing except that the end result is mineralized mesenchymal tissue i.e BONE

Fracture healing starts as soon as bone breaks and continues for many months

HYSTORY In 17th century Albrecht Haller, observed invading

capillary buds in fracture callus and thought that blood vessels are responsible for callus formation

John Hunter, a pupil of Haller, described the morphologic sequence of fracture healing.

In 1873, Kolliker observed the role of multinucleated giant cells, osteoclast to be responsible for bone resorption.

In1939, Gluksman suggested pressure and shearing stresses are possible stimuli for fracture healing.

In 1961, Tonna and Cronkie demonstrated the role of local mesenchymal cells in fracture repair.

Components of bone formation

Bone marrow Periosteum Cortex Soft tissue

PERIOSTEUM

Is a membrane that lines the outer surface of all bones except at the joints of long

bones.Is made up of : Outer FIBROUS layer : made up of white

connective and elastic tissue. Inner CAMBIUM layer : which has a looser

composition, is more vascular and contains cells with osteogenic potency.

FUNCTIONS OF PERIOSTEUM

1. Anchors tendons and ligaments to bone.2. Acts as a limiting membrane.3. Participates in growth (appositional) and

repair through the activities of the osteoprogenitor cells .

4. Periosteum helps in fracture healing by forming periosteal callus.

5. It also lessen the displacement of the # and helps in reduction.

6.  Allows passage of blood vessels, lymphatics and nerves into and out of the bone.

OSTEO PROGENITOR CELLS

OSTEOBLASTS Are basophilic, cuboidal to pyramidal in shape ,

associated with bone formation, these cells are located where new bone is forming, eg: in the periosteum.

Osteoblasts often appear stratified as in an epithelium.

The nucleus is large with a single prominent nucleolus.

Osteoblasts contain the enzyme alkaline phosphatase used to calcify the osseous matrix.

They synthesize type 1 collagen, osteocalcin (bone Gla protein) and osteonectin

OSTEOCLASTS

Giant, multinuclear cells which vary greatly in shape.

They are found on the surfaces of osseous tissue usually in shallow depressions called Howship’s lacunae.

The cytoplasm is slightly basophilic and contains lysosomal vacuoles.

Under E.M. the cell surface facing the osseous matrix shows numerous cytoplasmic projections and microvilli described as a ruffled border.

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RUFFLED BORDER

MICROSCOPIC PICTURE ELECTRON MICROSCOPY

OSTEOCLASTS

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Basically, an osteoblast that has been enclosed within the bony matrix in a space called the lacuna.

The cytoplasm of the osteocyte is faintly basophilic containing fat droplets and granules of glycogen with single dark stained nucleus.

In developing bone, the cytoplasmic processes from one osteocyte make contact with the processes (i.e.: cannaliculi) from adjoining osteocytes. In mature bone, the processes are withdrawn almost completely.

In mature bone the empty canaliculi remain as passage ways for the diffusion of nutrients and wastes between bone and blood.

4.OSTEOCYTES

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Are flattened epithelium in adult skeleton found on resting surfaces.

Plays active role in differentiation of progenitor cells

Controls osteoclasts, mineral hemostasis and may secrete collagenase.

Lines – endosteal surface of marrow cavity - periosteal surface

- vascular channels within osteons.

5.BONE LINING CELLS

Modes of bone formation

Endoochondral Intramembranous ossification Oppositional new bone formation Osteonal migration (creeping substitution)

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Mechanism by which a long bone grows in width.

Osteoblasts differentiate directly from pre osteoblasts and lay down seams of osteoid.

Does NOT involve cartilage anlage.

INTRAMEMBRANOUS BONE FORMATION(PERIOSTEAL)

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INTRAMEMBRANOUS BONE FORMATION

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Mechanism by which a long bone grows in length.

Osteoblasts line a cartilage precursor.

The chondrocytes hypertrophy, degenerate and calcify (area of low oxygen tension).

Vascular invasion of the cartilage occurs followed by ossification (increasing oxygen tension).

ENDOCHONDRAL BONE FORMATION

Endochondral Bone Formation

Picture courtesy Gwen Childs, PhD.

Creeping substitution

The process of bone remodelling by osteoclastic resorption and creation of new vascular channals with osteoblastic bone formation resulting in new haversian systems

Seen in bone healing after bone grafting

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Primarily a

mechanism to

remodel bone.

Osteoclasts at the

front of the cutting

cone remove bone.

Trailing osteoblasts

lay down new bone.

CUTTING CONES

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Stages of fracture healing

•Stage of hematoma formation•Stage of inflammation and cellular proliferation

Reactive phase

•Stage of callus formation•stage of consolidation

Reparative phase

•Stage of remodelingRemodeling phase

1975, Cruess and Dumont

1989, FROST

Stage of hematoma formation

Torn vessels will bleed and form hematoma

A mass of clotted blood(hematoma) formed around the fracture site

Stage of inflammation and cellular proliferation(0-7 days)

Hematoma is invaded inflammatory cells & they degrade necrotic tissue

Release of TGF-β,PDGF by macrophages

Procuring osteoprogenitor cells

Stage of callus formation

new capillaries organize fracture hematoma into granulation tissue – procallus

Fibroblasts and osteogenic cells invade procallus.

Make collagen fibers which connect ends together

Chondroblasts begin to produce fibrocatilage

This is called soft callus(7days-6wks)

Stage of consolidation

Osteoblasts lay more boney trabacule

Fibrocartilagenous frame work is calcified to form boney callus(6-12wks)

Now fracture is painless and allows weight bearing

Stage of remodeling

Globular callus is slowly remodeled over years

More trabacule are layed in the line of stress and non stressed area is resorbed

Hence bone will take original shape

Woolfs law

A bone will adapt to mechanical stress and strain by changing size, shape and structure

Types for Bone Healing

Direct (primary) bone healing Indirect (secondary) bone

healing

Direct Bone Healing

Mechanism of bone healing seen when there is no motion at the fracture site (i.e. absolute stability)

Does not involve formation of fracture callus

Osteoblasts originate from endothelial and perivascular cells

Components of Direct Bone Healing

Contact Healing Direct contact between the fracture ends allows

healing to be with lamellar bone immediately Gap Healing

Gaps less than 200-500 microns are primarily filled with woven bone that is subsequently remodeled into lamellar bone

Larger gaps are healed by indirect bone healing (partially filled with fibrous tissue that undergoes secondary ossification)

Direct Bone Healing

Indirect Bone Healing Mechanism for healing in

fractures that have some motion, but not enough to disrupt the healing process.

Bridging periosteal (soft) callus and medullary (hard) callus re-establish structural continuity

Callus subsequently undergoes endochondral ossification

Process fairly rapid - weeks

Local Regulation of Bone Healing

Growth factors Transforming growth factor Bone morphogenetic proteins Fibroblast growth factors Platelet-derived growth factors Insulin-like growth factors Cytokines Interleukin-1,-4,-6,-11, macrophage and

granulocyte/macrophage (GM) colony-stimulating factors (CSFs) and Tumor Necrosis Factor

Prostaglandins/Leukotrienes Hormones Growth factor antagonists

Transforming Growth Factor

Super-family of growth factors (~34 members)

Acts on serine/threonine kinase cell wall receptors

Promotes proliferation and differentiation of mesenchymal precursors for osteoblasts, osteoclasts and chondrocytes

Stimulates both enchondral and intramembranous bone formation Induces synthesis of cartilage-specific

proteoglycans and type II collagen Stimulates collagen synthesis by osteoblasts

Bone Morphogenetic Proteins

These are included in the TGF-β family Except BMP-1

Sixteen different BMP’s have been identified

BMP2-7,9 are osteoinductive BMP2,6, & 9 may be the most potent in

osteoblastic differentiation Involved in progenitor cell transformation to

pre-osteoblasts Work through the intracellular Smad

pathway Follow a dose/response ratio

Bone Morphogenetic Proteins Osteoinductive proteins initially isolated from

demineralized bone matrix Induce cell differentiation

BMP-3 (osteogenin) is an extremely potent inducer of mesenchymal tissue differentiation into bone

Promote endochondral ossification BMP-2 and BMP-7 induce endochondral bone

formation in segmental defects Regulate extracellular matrix production

BMP-1 is an enzyme that cleaves the carboxy termini of procollagens I, II and III

Timing and Function of Growth Factors

Table from Dimitriou, et al., Injury, 2005

Clinical Use of BMP’s Used at doses between 10x & 1000x

native levels Negligible risk of excessive bone

formation rhBMP-2 used in “fresh” open fractures to

enhance healing and reduce need for secondary procedures after unreamed IM nailing

It is found that application of rhBMP-2 decreases infection rate in Type IIIA & B open fractures.

BMP-7 approved for use in recalcitrant nonunions in patients for whom autografting is not a good option (i.e. medically unstable, previous harvesting of all iliac crest sites, etc.)

BMP Future Directions BMP-2

Increased fusion rate in spinal fusion BMP-7 equally effective as ICBG in

nonunions (small series: need larger studies)

Must be applied locally because of rapid systemic clearance

BMP Antagonists May have important role in bone

formation Noggin

Extra-cellular inhibitor Competes with BMP-2 for receptors

BMP-13 found to limit differentiation of mesenchymal stromal cells Inhibits osteogenic differentiation

Fibroblast Growth Factors

Both acidic (FGF-1) and basic (FGF-2) forms

Increase proliferation of chondrocytes and osteoblasts

Enhance callus formation FGF-2 stimulates angiogenesis

Platelet-Derived Growth Factor

A dimer of the products of two genes, PDGF-A and PDGF-B PDGF-BB and PDGF-AB are the predominant

forms found in the circulation Stimulates bone cell growth Mitogen for cells of mesenchymal origin Increases type I collagen synthesis by

increasing the number of osteoblasts PDGF-BB stimulates bone resorption by

increasing the number of osteoclasts

Insulin-like Growth Factor Two types: IGF-I and IGF-II

Synthesized by multiple tissues IGF-I production in the liver is

stimulated by Growth Hormone Stimulates bone collagen and matrix

synthesis Stimulates replication of osteoblasts Inhibits bone collagen degradation

Cytokines Interleukin-1,-4,-6,-11, macrophage and

granulocyte/macrophage (GM) colony-stimulating factors (CSFs) and Tumor Necrosis Factor

Stimulate bone resorption IL-1 is the most potent

IL-1 and IL-6 synthesis is decreased by estrogen May be mechanism for post-menopausal bone

resorption Peak during 1st 24 hours then again during

remodeling Regulate endochondral bone formation

Specific Factor Stimulation of Osteoblasts and Osteoclasts

Cytokine Bone Formation Bone ResorptionIL-1 + +++TNF-α + +++TNF-β + +++TGF-α -- +++TGF-β ++ ++PDGF ++ ++IGF-1 +++ 0IGF-2 +++ 0FGF +++ 0

Prostaglandins / Leukotrienes

Effect on bone resorption is species dependent and their overall effects in humans unknown

Prostaglandins of the E series Stimulate osteoblastic bone formation Inhibit activity of isolated osteoclasts

Leukotrienes Stimulate osteoblastic bone formation Enhance the capacity of isolated osteoclasts to

form resorption pits

Hormones Estrogen

Stimulates fracture healing through receptor mediated mechanism

Modulates release of a specific inhibitor of IL-1 Thyroid hormones

Thyroxine and triiodothyronine stimulate osteoclastic bone resorption

Glucocorticoids Inhibit calcium absorption from the gut

causing increased PTH and therefore increased osteoclastic bone resorption

Hormones (cont.) Parathyroid Hormone

Intermittent exposure stimulates Osteoblasts Increased bone formation

Growth Hormone Mediated through IGF-1 (Somatomedin-

C) Increases callus formation and fracture

strength

Vascular Factors Metalloproteinases

Degrade cartilage and bones to allow invasion of vessels

Angiogenic factors Vascular-endothelial growth factors

Mediate neo-angiogenesis & endothelial-cell specific mitogens

Angiopoietin (1&2) Regulate formation of larger vessels

and branches

Factors affecting fracture healing

Systemic Factors A. Age B. Activity level including 1. General immobilization 2. Space flight C. Nutritional status D. Hormonal factors 1. Growth hormone 2. Corticosteroids

(microvascular osteonecrosis)

3. Others (thyroid, estrogen, androgen, calcitonin,

parathyroid hormone, prostaglandins)

E. Diseases: diabetes, anemia, neuropathies, tabes

F. Vitamin deficiencies, A, C, D, K G. Drugs: nonsteroidal

antiinflammatory drugs (NSAIDs), anticoagulants, factor XIII,

calcium channel blockers (verapamil), cytotoxins,

diphosphonates, phenytoin, sodium fluoride, tetracycline H. Other substances (nicotine,

alcohol) I. Hyperoxia J. Systemic growth factors K. Environmental temperature L. Central nervous system trauma

Local FactorsA. Factors independent of injury, treatment, or complications 1. Type of bone 2. Abnormal bone a. Radiation necrosis b. Infection c. Tumors and other pathological conditions 3. DenervationB. Factors depending on injury 1. Degree of local damage a. Compound fracture b. Comminution of fracture c. Velocity of injury d. Low circulatory levels of vitamin K1 2. Extent of disruption of vascular supply to bone, its fragments (macrovascular osteonecrosis), or soft tissues; severity of injury 3. Type and location of fracture (one or two bones, e.g., tibia and fibula or tibia alone) 4. Loss of bone 5. Soft tissue interposition 6. Local growth factors

C. Factors depending on treatment 1. Extent of surgical trauma (blood supply, heat) 2. Implant-induced altered blood flow 3. Degree and kind of rigidity of internal or external fixation and the influence of timing 4. Degree, duration, and direction of load-induced deformation of bone and soft tissues 5. Extent of contact between fragments (gap, displacement, overdistraction) 6. Factors stimulating posttraumatic osteogenesis (bone grafts, bone morphogenetic protein, electrical stimulation, surgical technique, intermittent venous stasis [Bier]) D. Factors associated with complications 1. Infection 2. Venous stasis 3. Metal allergy

Electromagnetic Field Electromagnetic (EM) devices are based

on Wolff’s Law that bone responds to mechanical stress: In vitro bone deformation produces piezoelectric currents and streaming potentials.

Exogenous EM fields may stimulate bone growth and repair by the same mechanism

Clinical efficacy very controversial No studies have shown PEMF to be effective in

“gap healing” or pseudarthrosis

Types of EM Devices

Microamperes Direct electrical current Capacitively coupled electric fields Pulsed electromagnetic fields (PEMF)

PEMF Approved by the FDA for the treatment of

non-unions Efficacy of bone stimulation appears to

be frequency dependant Extremely low frequency (ELF) sinusoidal

electric fields in the physiologic range are most effective (15 to 30 Hz range)

Specifically, PEMF signals in the 20 to 30 Hz range (postural muscle activity) appear more effective than those below 10 Hz (walking)

Ultrasound

Low-intensity ultrasound is approved by the FDA for stimulating healing of fresh fractures

Modulates signal transduction, increases gene expression, increases blood flow, enhances bone remodeling and increases callus torsional strength in animal models

Ultrasound

Human clinical trials show a decreased time of healing in fresh fractures treated nonoperatively Four level 1 studies show a decrease in

healing time up to 38% Has also been shown to decrease the

healing time in smokers potentially reversing the ill effects of smoking