KEY NOTES CHAPTER 1: GENERAL PRINCIPLES - Wound healing and skin grafts Flashcards
0
Q
Tell me about haemostasis.
A
Haemostasis (immediate)
- thromboxanes and PG from damaged cells cause vasoconstriction.
- platelets bind to exposed collagen forming platelet plug
- platelets degranulate and bind to fibrinogen.
- membrane glycoprotein IIb/IIIa is modified (blocked by clopidogrel).
- PAF, vWF and TXA2 stimulate conversion of fibrinogen to fibrin.
Thrombus propagates
- initially white thrombus (platelets only).
- red thrombus (red blood cells also trapped).
1
Q
What are the phases of wound healing?
A
- Haemostasis
- Inflammation
- Proliferation
- Remodelling
2
Q
Tell me about inflammation
A
Inflammation (2-3 days post-injury)
- stimulated by physical injury, Ab-Ag reaction or infection.
- Platelets release growth factors (PDGF) and pro-inflammatory factors e.g. serotonin, bradykinin, PG, TXA2, histamine which increase cell proliferation and migration.
- endothelial cells swell, vasodilate and cause egress of PMNs and monocytes.
T lymphocytes
- migrate into wound.
- secrete cytokines e.g. epidermal growth factor and basic fibroblast growth factor (bFGF).
- mediate cell immunity and antibody production.
3
Q
Tell me about proliferation
A
Proliferation (from day 2-3 for 2-4wks)
- monocytes mature to macrophages, release PDGF & TGF-B, chemoattractant to fibroblast.
- fibroblasts enter wound,
secrete GAGs to produce ground substance .
- produce collagen (type III initially) and elastin.
- some fibroblasts become myofibroblasts and affect wound contraction.
- angiogenesis occurs.
4
Q
Tell me about remodelling
A
Remodelling (From 2-4wks to 1+years).
- Type III collagen is replaced by type I (stronger).
- Haphazard collagen fibres are arranged in more organised manner.
- tensile strength 50% at 3 months, 80% after remodelling.
- scar initially red due to dense capillary network, apoptosis leads to a pale scar.
5
Q
What is abnormal scarring?
A
Keloid and hypertrophic scarring
6
Q
Tell me about keloid scarring hypertrophic scarring?
A
- limited to original wound margins, commoner than keloids.
- usually starts ~ 8wks.
- rapid growth for ~6mths, then gradually regresses (may be years).
- common around shoulders, neck, presternal area, knees, ankles.
7
Q
Tell me about keloid scarring
A
- extend beyond original wound margins
dark skin more prone, family history
may suddenly develop anytime after injury. - persist, don’t tend to regress.
- can be painful, hypersensitive.
- common on anterior chest, shoulders, earlobes, upper arms, cheek.
- excision alone has high risk of recurrence.
- microscopy: type I and III collagen bundles are poorly organised with few myofibroblasts.
- expression of PCNA (proliferation cell nuclear antigen) and p53 upregulated.
8
Q
How does epithelialisation occur?
A
- Mobilisation.
- Migration.
- Mitosis.
- Cellular differentiation.
9
Q
What occurs during mobilisation?
A
- Marginal epithelial cells elongate, flatten and form pseudopodia.
- Cells detach from neighbour and basement membrane.
10
Q
What occurs during migration?
A
- decreased contact inhibition promotes cell migration.
- meanwhile marginal epi cells proliferate to replace them.
- when cells meet opposite migrating epi cells, contact inhibition is reinstituted and migration ceases.
11
Q
What occurs during mitosis?
A
Epithelial cells
- proliferate.
- secrete proteins to form new BM.
- desmosomes and hemidesmosomes re-establish themselves and anchor the cells to BM and each other.
- becomes new stratum germinativum.
12
Q
What occurs during cellular differentiation?
A
- normal structure of stratified squamous epithelium is re-established.
13
Q
Tell me about collagen.
A
- 30% of total body protein
- amino acids lysine and proline are hydroxylated by enzymes (require Vit C)
- procollogen (in cell)
- tropocollagen (outside cell)
- collagen - 3 polypetide chains wound in left handed helix, 2 chains wound in right handed coil to form basic tropocollagen unit
14
Q
What inhibits collagen production?
A
- colchicine, penicillamine, steroids and Vit C and iron deficiency inhibits collagen formation.
- cortisol: stimulates collagen degradation
15
Q
Name some common collagen types.
A
- 28 types of collagen (diff cross-linkages and hydroxyproline and hydroxylysine).
Type I: mature skin, bone tendon (90% total body collagen).
Type II: hyaline cartilage and cornea.
Type III: healing tissue (esp fetal wounds).
Type IV: BM.
Type V: BM, hair and placenta.
Normal skin I:III = 5:1.
16
Q
What is the function of a macrophage?
A
- Derived from mononuclear leukocytes.
- Debrides tissue, removes micro-organisms.
- Co-ordinates angiogenesis and fibroblast activity by releasing growth factors: PDGF, FGF 1&2, TNF-a, TGF-b.
- Orchestrator of wound healing
17
Q
What is the function of myofibroblasts?
A
- contains alpha-smooth muscle actin
- responsible for wound contraction
- increased numbers in fascia of Dupuytren’s disease
18
Q
What secretes TGF-beta? What is it’s role in wound healing?
A
- Macrophages, fibroblasts, platelets, keratinocytes, endothelial cells.
Plays a central role in wound healing: - Chemoattractant for fibroblasts and macrophages.
- Induction of angiogenesis.
- Stimulation of extracellular matrix deposition.
- Keratinocyte proliferation.
3 isoforms identified
1&2 - promote wound healing and scarring (unregulated in keloids).
3 - decreases wound healing and scarring (possible factor for deceased inflammation and improved scarring in fetal wound healing)
19
Q
How are factors affecting healing classified?
A
Systemic
- Congenital
- Acquired
Local
20
Q
Systemic factors: congenital
A
Pseudoxanthoma elasticum Ehler-Danlos syndrome Cutis laxa Progeria Werner syndrome Epidermolysis bullosa
21
Q
Systemic factors: acquired
A
Nutrition - Vitamins A, C (collagen synthesis), Vitamin E, zinc, copper, selenium (cofactors for enzymes), hypoalbuminaemia.
Endocrine abnormalities - diabetes, hypothyroidism.
Pharmacological - steroids, NSAIDs, anti TNF-alpha drugs (RA), cytotoxics.
Age - mitosis rate decreases.
Smoking - nicotine (vasoconstriction), CO (decreased O2 carriage by Hb), hydrogen cyanide.
22
Q
Local factors
A
Infection
Radiation - endothelial cell, capillary and arteriole, lymphatic damage. Fibroblasts secrete less collagen and ECM.
Blood supply - decreased by low pO2, low Hb, low O2 transfer from Hb, poor tissue perfusion. Decreased tissue O2 reduces collagen formation, ECM deposition, angiogenesis and epithelialisation.
Trauma - neoepidermis disrupted.
Neural supply - possibly related to levels of chemoattractant neuropeptides in wound.
23
Q
What do you know about fetal wound healing?
A
Foetus’ first 6 months - heals by regeneration, not repair, therefore no scarring.
- Reduced inflammation.
- Reduced platelet aggregation and degranulation.
- Reduced angiogenesis.
- More rapid epithelialisation.
- No myofibroblasts, no wound contraction.
- Type III > I collagen.
- Wound contains more water and hyaluronic acid.
- More TGF-B3 than 1&2
24
Tell me about skin grafts
Full or split thickness (of epidermis and dermis).
Donor sites differ.
Primary contraction - immediate recoil after harvest.
Secondary contracture - after skin graft has taken, and is due to myofibroblasts.
25
What are the reasons for graft failure?
1. Haematoma.
2. Infection (if >100,000 organisms / gram, less if Group A B-haemolytic Strep, as streptokinase and hyaluronidase prevent adhesion)
3. Seroma.
4. Shear.
5. Inappropriate bed (cartilage, tendon, bone).
6. Technical error.
26
How do skin grafts take?
1. Adherence (fibrin) - immediate.
2. Serum imbibition (absorption of fluid and nutrients, graft swells) - D2-4.
3. Revascularisation - D4.
- Inosculation- direct anastomosis between vessels in graft and those in recipient tissue.
- Revascularisation - new vessel ingrowth from recipient tissue along graft's vascular channels.
- Neovascularisation - new vessel ingrowth from recipient tissue along new channels in graft.
4. Remodelling
- histological architecture of graft returns to that of normal skin.
27
Draw a cross-section of bone
Label
- Periosteum
- Cortical bone
+ Osteon: Haversian canal, lamellae, canaliculi.
- Cancellous bone
+ Trabecullae
28
Outline the structure of bone.
* Outer cortical layer and inner cancellous layer.
* Cancellous bone consists of loosely woven trabeculae of organic and inorganic bone.
• Cortical bone consists of:
- Multiple osteons (columnar bone units) and a central
Haversian canal that contains a central blood vessel.
- Transverse Volkmann nutrient canals connecting adjacent osteons.
* Bone is laid down in concentric layers around each Haversian canal.
* Osteocytes are scattered throughout osteons, each within its own space (lacuna).
29
What is bone composed of and how does it develop embryologically?
• Derived from mesenchyme.
• Composed of organic matrix (osteoid), mineralised by hydroxyapatite (calcium
salt).
• Embryologically, bones form by:
1. Intramembranous ossification
∘ Deposition of bone within a vascularised membranous template.
∘ e.g. flat bones of face, calvarium and ribs.
2. Endochondral ossification
∘ Develops from a cartilage precursor, or anlage.
∘ e.g.: all long bones and iliac crest.
30
What is the blood supply to bone?
1. Periosteal vessels at muscle attachment.
2. Apophyseal vessels at tendon and ligament attachment.
3. Nutrient arteries supplying medullary cavity (endosteal supply).
4. Epiphyseal vessels supplying growth plates.
31
How do bones heal?
1. Haematoma formation
2. Inflammation
- Fracture haematoma is gradually replaced by granulation tissue.
- Osteoclasts remove necrotic bone.
3. Cellular proliferation
- Stem cell recruitment.
- Periosteal proliferation occurs on outer aspect of cortex.
- Endosteal proliferation occurs on inner aspect of cortex.
4. Callus formation
- Immature woven bone produced by osteoblasts and hyaline cartilage produced by chondroblasts.
- This soft callus (osteoid) is mineralised with hydroxyapatite to form hard callus (mature woven bone).
5. Remodelling
- Woven bone slowly replaced by lamellar bone, until cortical structure and medullary cavity are restored.
• Osteoblasts form new bone by producing osteoid.
• Osteoclasts are responsible for bone resorption.
• Osteocytes are osteoblasts that have become trapped within lacunae in bone matrix.
• Osteoid is unmineralised, organic component of bone.
32
What is primary bone healing?
• Healing without callus formation, when bone ends are directly fixed with absolute stability.
• Fracture haematoma is removed during surgery.
• Bone 'tricked' into thinking it was never fractured.
• Inflammatory and proliferative phases of healing do not occur.
• Rather, it is a process of osteonal bone remodelling:
∘ Osteoclasts 'drill' across fracture site from one cortex to other, blood vessels and osteoblasts cross fracture and new Haversian systems and bone architecture are established.
33
What is secondary bone healing?
* This is healing by callus formation.
* Occurs if fragments are not rigidly fixed, or if a gap exists between bone ends.
* It cannot occur if there is no fracture haematoma.
34
What are the complications of fractures?
* Delayed union
* Non-union
* Malunion - rotation, angulation, shortening.
* Infection
* Avascular necrosis (AVN)
* Damage to adjacent structures.
35
How can bone graft materials be classified?
1. Biological
- Autograft, allograft, xenograft
2. Engineered biological
- Growth factors, recombinant BMPs, stem cells, platelet-rich plasma concentrate (PRPC)
3. Synthetic
- Metals, ceramics, polymers.
• Gold standard is autologous.
36
How do autologous bone grafts heal?
Incorporation
• Adherence of graft to host tissue.
• Maximised in immobilised, well-vascularised tissue.
Osseoconduction
• Bone graft acts as a scaffold along which vessels and osteoprogenitor cells travel.
• Creeping substitution - old bone is resorbed as new is deposited.
Osseoinduction
• Differentiation of mesenchymal cells within recipient local tissue into osteocytes. (Osteoclasts, osteoblasts and osteocytes within bone graft are not capable of mitosis).
• Controlled by BMPs.
Osteogenesis
• Formation of new bone by surviving cells within bone graft.
• Vascularised bone grafts incorporate rapidly this way, without creeping substitution.
• Minimal osteogenesis in non-vascularised bone grafts.
37
What factors influence the survival of bone grafts?
1. Systemic factors
2. Intrinsic graft factors
3. Graft placement.
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Systemic factors
• Age
• Nutrition
• Immunosuppression
• Drugs
• Diabetes
• Smoking
• Obesity.
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Intrinsic graft factors
• Grafts with periosteum included undergo less resorption.
• Membranous bone undergoes less resorption than endochondral bone.
• Cancellous grafts revascularise easier than cortical grafts.
Graft placement factors
(a) Ortho / Heterotopic
• Orthotopic - graft placed into a position normally occupied by bone (less resorption).
• Heterotopic - graft in a position not normally occupied by bone.
(b) Quality of recipient bed
• Radiotherapy, scarring and infection adversely affect graft survival.
(c) Graft fixation
• Rigid fixation survive better than mobile.
(d) Site of graft placement
• Better survival in areas where bone is normally laid down (depository sites) e.g. zygoma and mandible in children.
38
What is the structure of cartilage?
• Derived from condensed mesenchyme.
• Differentiates into chondroblasts that secrete ECM.
• Chondroblasts trapped in lacunae within matrix become chondrocytes.
• Matrix contains type II collagen, elastin and ground substance (GAGs).
• Cartilage is classified according to relative proportions of these 3 components into:
1. Hyaline cartilage
2. Fibrocartilage
3. Elastic cartilage.
Cartilage has no blood, nerve or lymph supply, and has little reparative ability.
39
Where is cartilage harvested from?
∘ Ear conchal bowl
∘ Nasal septum (via lateral rhinotomy incision)
∘ Costal cartilage.
40
Describe the anatomy of a nerve.
* Nerve cells (neurons) consist of a cell body.
* Outgoing impulses are carried axons.
* Impulses are received either on cell body or dendrites.
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Layers (inside to out)
• Axon (nerve fibre)
• Endoneurium
• Fascicles
• Perineurium.
• Intraneural epineurium (+blood vessels).
• Outer epineurium
• Peripheral nerve.
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41
What is the function of a nerve?
* Schwann cells produce multilaminated myelin sheath of myelinated nerves.
* Unmyelinated nerves are ensheathed by a Schwann cell-derived double basement membrane.
* Schwann cells of myelinated nerves abut at nodes of Ranvier.
* Nerve conduction involves passage of an action potential along a nerve.
* Saltatory conduction: impulse in myelinated nerves jumps between adjacent nodes of Ranvier.
42
How are nerve fibres classified based on their diameter?
Group A - Myelinated, large-diameter, high-conduction velocity nerves.
- Group A-α fibres: motor and proprioception.
- Group A-β fibres: pressure and proprioception.
- Group A-γ fibres: motor to muscle spindles.
- Group A-δ fibres: pain, touch, temperature.
Group B - Myelinated, small-diameter, low-velocity fibres.
• Preganglionic autonomic nerves.
Group C
• Unmyelinated, small-diameter, low-velocity fibres
• Postganglionic autonomic nerves
• Dorsal root nerves for pain, temperature, touch, pressure and itch.
43
How is nerve function classified?
MRC grading of nerve function.
44
What happens after a nerve is injured?
• Degeneration proximally → nearest node of Ranvier.
• Distally → axons and myelin undergo Wallerian degeneration (phagocytosis by macrophages and Schwann cells).
∘ Remaining basement membranes form endoneurial tubes which guide regenerating axons to their targets.
• Neurotropism = selective, directional growth of fibres towards end organs, mediated by nerve growth factors and cell-cell interactions:
1. Proximal axon sprouts many new daughter axons, forming a growth cone.
2. Fibres growing in inappropriate direction atrophy.
3. Those growing in correct direction survive.
• Neurotrophism = non-selective, non-directional growth of nerve fibres.
• Neurotrophic factors are almost all produced by Schwann cells:
e.g. Growth factors: Nerve growth factor, ciliary neurotrophic factor, insulin-like growth factor.
e.g. Extracellular matrix components: Fibronectin, laminin, neural cell adhesion molecule, N-cadherin.
45
What is Seddon's classification of nerve injury?
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Seddon classification:
1 Neurapraxia (1st)
2 Axonotmesis (2nd-4th)
3 Neurotmesis (5th)
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46
What is Sunderland's classification of nerve injury?
1st-degree injury
• Axon in continuity but conduction is impaired.
• Recovery should be complete.
2nd-degree injury
• Axonal injury, Wallerian degeneration distally.
• Endoneurium and connective tissue layers intact, recovery is good.
3rd-degree injury
• Axon and endoneurium divided.
• Perineurium and epineurium intact.
• Recovery reasonable.
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4th-degree injury
• Complete division of all intraneural structures.
• Epineurium intact.
• Recovery of some function is expected.
• May result in neuroma-in-continuity.
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5th-degree injury
• Nerve trunk completely divided.
• Early surgical repair.
47
Quick summary of Sunderland's classification!
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1st - axon squashed
2nd - axon divided, endo intact
3rd - endo divided
4th - peri divided
5th - epi divided
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48
What are the principles of nerve repair?
Tensionless repair, trim nerve, magnification, 9/0 S&T epieneural repair.
Fascicular identification by:
- Matching of anatomical structures
- Electrical stimulation (motor nerves only respond for 72hrs post injury)
- Knowledge of internal nerve topography
49
Tell me about nerve grafts.
Sural: behind lat malleolus
Lateral antebrachial cutaneous nerve: adjacent to cephalic vein alongside ulnar border of brachioradialis.
Medial antebrachial cutaneous nerve (20cm): in groove between triceps and biceps, alongside basilic vein
Terminal branch of PIN: base of 4th ext compartment.
50
What are the principles of nerve grafting?
Nerves can be mobilised prox and dist, or transposed to reduce tension on repair.
Can be vascularised / healthy wound bed.
If cable graft, stagger levels of repairs.
Other sources: allograft (give immunosuppression (tacrolimus) until Tinel's sign is in distal nerve.
51
.
• Tendons are composed of dense, metabolically active connective tissue.
∘ Collagen is predominantly type I, with small amounts of types III and IV.
∘ Tenocytes
∘ Synovial cells
∘ Fibroblasts.
52
What layers surround a tendon?
* Endotendon encloses tendon bundles. Continuous perimysium and periosteum.
* Epitenon = outer layer of synovial tendons.
* Paratenon is a loose adventitial layer that surrounds extra-synovial tendons.
53
Where do tendons receive their blood supply?
1 Musculotendinous junction
2 Bony insertion
3 Mesenteric vincular vessels.
54
What are the mechanisms of tendon healing?
Extrinsic healing
• Dependent on fibrous attachments forming between tendon sheath and tendon.
Intrinsic healing
• Dependent on:
∘ Blood flow through long and short vinculae.
∘ Diffusion of nutrients from synovial fluid.
55
What research suggests intrinsic healing takes place, making EAM possible?
• Lunborg showed tendons heal when wrapped in a semipermeable membrane (permits passage of nutrients, but not cells) and placed in knee joint of a rabbit.
56
What are the phases of tendon healing?
Inflammation
• Inflammatory cells infiltrate the wound.
• Secrete growth factors that attract fibroblasts.
Proliferation
• Fibroblasts are responsible for tissue proliferation.
• They secrete type III collagen and GAGs.
• Collagen is initially arranged randomly; consequently, the tendon lacks strength.
Remodelling ~ 3 weeks
• Type III collagen is replaced by type I.
• The tendon remodels into an organised structure.
• Early motion limits fibrous attachments between tendon and sheath and promotes intrinsic healing over extrinsic.
∘ Mobilised tendons are stronger than immobilised tendons.
