Introduction to the limbs

Question 1

Bones: explain and contrast the basic mechanisms of intramembranous and endochondral bone development

Answer 1

Appositional growth = surface growth.

• Skeleton starts to form at 6 weeks and continues until 25 years of age.

There are 2 types of ossification:

1. Intramembranous (pictured left):

• In existing vascular connective tissue.
• Bone matrix deposited around collagen.
• Mineralises to form woven bone and remodels to form lamellar bone.

2. Endochondral:

• Within existing cartilage models.
• Cartilage calcifies and chondrocytes die.
• Osteoclasts cut channels for sprouting vessels.
• Osteoblasts enter bone with vessels to build bone around them.

Long bones must support forces whilst growing so the shaft must ossify first followed by the epiphyses.

Ossification and growth then continues at the cartilage plate with cessation of growth occurring when the cartilage plate is overrun with ossification.

Endochondral Ossification and Bone Elongation

1. Cartilage model forms and grows.

2. Primary ossification centre forms.

3. Medullary cavity forms and develops.

4. Secondary ossification centre forms.

5. Epiphyseal plate forms which is a cartilage plate that enables bone elongation growth.
   a. Note that in young children, they will have what appears to be breaks in the bones on X-ray but these are just epiphyseal plates.
   b. These epiphyseal plates ossify in the 2nd year but remain cartilaginous until after puberty.
   * Bone can do all this due to – large blood supply, osteocyte/osteoblast interactions and bone creation, osteoclast destruction.

Question 2

outline the properties of bone, and recall the different types of bone organisation in relation to function;

Answer 2

Bone

Functions of bone:

1. support body
2. support muscle action
3. protection of viscera
4. site of blood cell formation
5. mineral storage pool

Bones must have:

1. Cable-like flexibility and resistance to tension – framework is collagen.
2. Pillar-like stiffness and resistance to compression – impregnation of collagen with hydroxyapatite.

There are 2 types of bone:

1. Woven (immature) bone – only found in repairing fractures or in disease of the bone (i.e. Paget’s disease).
2. Lamellar (mature) bone – shows concentric rings of cells as opposed to woven bone that does not.

Structure of lamellar bone:

• Outer hard layer of compact bone (cortical bone).
• Inner layer of interlacing struts (cancellous/spongy/trabecular bone).

Lamellar Bone – Outer layer.

• Lamellar bone is arranged in concentric circles called “Osteons” which are osteocytes and lacuna arranged surrounding blood vessels.
• Inner trabecular bone is also arranged in this way but the concentric rings are found in structures called trabeculae- doesn’t resist the same amount of compression.

Osteons

• central canal- nerves and vessels
• concentric layers with calcium deposits - they form gap junctions to form cell

The “long bone” is the femur:

• Epiphysis = head.
• Metaphysis is in between these.
• Diaphysis = shaft.
• Nutrient foramen- vessels enter and leave
• Hollow center with bone marrow (medullary cavity to prevent weight)

Blood Supply

• Fracturing bones can cause a lot of bleeding.
• Note the epiphyseal line which divides the epiphysis and the diaphysis.
• Nutrient foramina in bone allow vessels to pass into the bone.

Periosteum and Cells

• The bones are covered by an outer periosteum.
• There is a fibrous and cellular layer.
• These have key roles in – bone growth and repair, vascular effects and have a good sensory nerve innervation (fractures hurt).
• Cells found in the bones are shown on the left.

Question 3

Recall the mechanisms of appositional growth of bones, long-bone elongation and bone remodeling.

Answer 3

Adaptability of Bone

• Can grow without compromising its support functions
• Increases or decreases bulk and density in response to the pattern of use
• Can alter its external and internal shape in response to pattern of use – remodelling
• Can repair when fractured

Keys to Growth and Remodelling

• Bone has a large blood supply – cells are never far from nutrients and O2
• Osteocytes maintain matrix but can activate osteoblasts for new bone building
• Osteoclasts are giant cells specialised for destruction of bone matrix

Appositional Bone Growth (Bone Diameter Increase)

• Apposition = addition to exterior at periosteum.
• Osteoblasts/clasts create ridges/grooves on bone surface that blood vessels align in.
• Osteoblasts build new osteons around the vessels and the osteoclasts remove bone from the endosteal (inner) surface.

Question 4

Understand the basic mechanisms involved in bone fracture healing

Answer 4

Fractures and Calcium Control

Bleeding is an important part of the process. The haematoma becomes infiltrated by fibrous matrix and invaded by cartilage/bone progenitors.

• Initially a haematoma forms and then a fibrocartilaginous callus forms.
• The fibrocartilaginous callus -> bony callus which is then remodelled into new bone.
• The fracture repair phase involves woven bone formation.
• Ca2+ high – calcitonin released by parafollicular cells à osteoclasts inhibited and uptake of Ca2+ into bone promoted.
• Ca2+ low – PTH released by chief cells of parathyroid -> osteoclasts stimulated and Ca2+ is resorbed in the kidneys.

Question 5

Limbs development: describe the early process in limb development that results in the differences in flexion/extension movements between the upper and lower limbs

Answer 5

Development

• The skeleton is made of the axial (ribcage and spine) and appendicular (limbs) skeleton.

Nerve supply:

• The upper limbs are supplied by the brachial plexus (C5-T1).
• The lower limbs are supplied by L2-S3.

Flexion/extension:

o Upper limbs – flexors are anterior (e.g. biceps brachii), extensors are posterior (e.g. triceps brachii).

o Lower limbs – flexors are posterior, extensors are anterior (limbs rotate internally, hence other way around).

Development

• The reason the flexors and extensors are the other way around in the lower limbs is due to the lower limb rotation that occurs during development.
• In addition, the dermatomes have twisted leading to twisted and oblique fields opposed to the straighter fields in the upper limbs.

Question 6

Organisation of the limbs: describe the skeletal and regional organisation of the upper and lower limbs, and describe the organisation the neuromuscular compartments of the limbs, and their main innervation, blood supply and functions

Answer 6

Compartments

• The limbs are divided into compartments that each have a distinct function- they have the same nerve and blood supply as well.

Upper limb compartments include – pectoral girdle muscles, intrinsic shoulder muscles, anterior arm muscles, posterior arm muscles, anterior forearm muscles, posterior forearm muscles, intrinsic hand muscles.

Lower limb compartments include – hip abductors/extensors/flexors, anterior/medial/posterior thigh muscles, anterior/lateral/posterior leg muscles and intrinsic foot muscles.

Blood Supply of the Limbs

Upper limbs:

• Arterial supply – aorta -> subclavian artery -> axillary artery -> brachial arteries (pulse point) -> ulnar and radial arteries (pulse point) -> hand arches (palmar)-> metacarpal and digital arteries.
• Venous drainage – superficial and deep systems – dorsal venous arch (sup.) -> cephalic (lateral, sup.) vein and basilic (medial, sup.) vein -> Venae comitantes (deep) -> Axillary vein (deep) -> subclavian vein, à superior vena cava.

NOTE – cubital fossa superficial veins are often used for phlebotomy – median cubital vein.

· This vein links the basilic and cephalic veins (but it’s not always present).

• o Lymphatic system – cubital lymph nodes can drain to the axillary pectoral lymph nodes.

Lower limbs:

• Arterial supply – aorta -> common iliac arteries (int. & ext.) -> external iliac artery -> femoral artery (pulse point) -> Popliteal artery (pulse point) – Posterior tibial (pulse point), anterior tibial, peroneal (pulse point) & dorsalis pedis (pulse point) arteries.
• The femoral artery is more anterior whilst the popliteal artery is more posterior.

-Venous drainage – superficial and deep systems:

1. DEEP – Ant. & Post. Tibial Venae Comitantes -> popliteal vein -> femoral vein -> external iliac.
2. SUPERFICIAL – Venous arches -> long saphenous vein -> short saphenous vein.

· The great saphenous vein -> femoral vein at the groin.

· The small saphenous vein -> popliteal vein at the popliteal fossa.

o The saphenous veins are consistent (other veins vary).

Question 7

Important limb clinical issues: understand the basics of important clinical issues relating to limb anatomy including arterial pulses, venepuncture and phlebotomy, venous graft harvest, deep-vein thrombosis, compartment syndrome, musculoskeletal trauma and pathology, neurological pathology.

Answer 7

Important Clinical Points

1. The femoral triangle is the region where the femoral artery can be accessed – e.g. to carry out angiograms and angioplasties.
   
   • The pulse can be felt here as well.
2. Perforating veins are veins that connect superficial and deep veins and these have a valve that only allow flow from superficial à deep.
   
   • If the valve is compromised = varicose veins.
3. Elastic surgical socks promote more vigorous deep venous return as patients are often immobilised for long periods of time in surgery. This is to prevent DVT.
4. Compartment Syndrome

Muscle groups are in confined compartments separated by fascia.

Compartment syndrome – ischaemia caused by trauma-induced increased pressure in a confined compartment.

• Commonly affects – anterior, posterior and lateral compartments of the leg.
• Normal pressure = 25mmHg, but you only need 50-60mmHg to collapse the small vessels (and cause ischaemia) but the pulse is still present (120/80).
• Acute = trauma associated.
• Chronic = exercise-induced.
• Treatment – emergency fasciotomy.

ΙM Injections into Buttock

• Muscles have a greater blood supply and thus uptake of the drug is often faster than from SC and sub-dermal injections.
• The buttock is often contraindicated due to proximity to major vessels and nerves and the variation in adipose tissue over the muscles.

Question 8

Describe the basic organisation of the nerve plexuses of the limbs and their importance in understanding the consequences of spinal and peripheral nerve damage

Answer 8

Spinal Nerves

• Spinal nerves – 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 (or 2) coccygeal. (8, 12, 5, 5, 1).

Organisation of spinal nerves:

• C1-4 – neck.
• C5-T1 – upper limbs (brachial plexus).
• T2-L1 – trunk. Μπορω να εχω νευρικο κλονισμό δηλαδη επειδη εχω κωλαρα? -Βαλια
• L2-S3 – lower limbs.
• S2-Cx2 – perineum.

Spinal Nerves

• Nerves to the lower limbs emerge from the lumbosacral plexus.

1. Femoral nerve: anterior compartment of thigh.
2. Obturator nerve: medial (adductor) compartment of thigh.
3. Sciatic nerve à remaining compartments of thigh.

• Innervation can be segmental or peripheral.
  
  • Flexor = decrease angle of joint (good way to remember).
• Segmental supply – muscles are supplied by two adjacent segments.
  
  • Opposing muscles nerves (flex vs. extend) are 1-2 segments above or below each other.
  • Muscles more distal in the limb have nerves originating more caudal in the spine (closer to tail).

Root Injury E.G. Prolapsed disc at L5/S1.

• Motor – loss of eversion (lateral face).
• Sensory – loss of sensation to the outer border of foot.
• Reflex – loss of ankle jerk (S1).
• Autonomic – minimal.

Peripheral Nerve Injury

• Motor – foot drop.
• Sensory – dorsum of foot (at least).
• Reflex – none.
• Autonomic – minimal.