Skeletal embryology and limb growth

Question 1

When do the upper limb bub appear?

Answer 1

• 4 weeks of age
• the lower limb buds appear a few days later

Question 2

What does a limb consist of?

Answer 2

• Mesoderm covered by a thin surface of ectoderm
• the mesoderm** forms somite and lateral mesodermal plate**

Question 3

What does the lateral mesoderm form?

Answer 3

• Bone and connective tissue

Question 4

What does the somite form?

Answer 4

• Muscle

Question 5

What gene controls mass and local growth in a distal direction?

Answer 5

• Homeobox (HOX) gene

Question 6

What mediates the activity of the zone of proliferating activity in the ectoderm?

Answer 6

• The sonic the hedgehog gene

Question 7

What directs the development of the limb in a anterior to posterior direction?

Answer 7

• The sonic the hedgehog gene

Question 8

What does the zone of proliferating activity produce?

Answer 8

• Apical ectodermal ridge from which the digits form

Question 9

How does embyrological limb development continue?

Answer 9

• The mesenchyme condenses
• chondrocytes then differentitiate within mesenchyme = chondrifcation
• first occurs in diaphsysis of humerus and femur
• new chondrocytes produce a matrix of glycosaminogylcans
• the cartilagninous mesenchyme ( the cartilage anlage) then undergoes resorption and cavitation between future segements= segmentation -> primitve joint

Question 10

How is the joint formed?

Answer 10

• The primary ossification centre develops with hypertrophy of the middiaphyseal chondrocytes and invasion of blood vessels which form the primitive nutrient artery system.
• Initally intramembranous ossification occurs circumferentially from the periosteum around the primary ossification centre
• However in the epiphyseal regions, the cartilage cells become hypertrophic and after vascular invasion the process of enchondral ossification commences
• by 12th week ossification centres are present in all bones

Question 11

What happens to the limb at 9 wks?

Answer 11

• The limb rotates thru 90 degrees laterally and medially
• results in lateral thumb and medial hallux

Question 12

When do the secondary ossification centres develop?

Answer 12

• At the end of fetal development in the distal femur- after 36 weeks
• a separate blood system has grown into epiphysis
• these vascualrised chondrocytes hypertrophy to form a central mass
• the central mass forms a spherical physis over its entire surface, which later reshapes in to a hemispherical outer surface and a discoid non-physeal surface towards the metaphysis

Question 13

When does the capital femoral epiphysis secondary ossification centre appear?

Answer 13

• After 3 months before 1 year

Question 14

What effect did thalidomide have on the developing limbs?

Answer 14

• Antiangiogenic effecft that interupted the development of the vascularization of the ossification centres -> phocomelia

Question 15

What is the classiifcation for congential limb deformities?

Answer 15

• Swanson 1976
• International society of hand surgeons

Question 16

Can you describe Swanson’s classification for congential limb deformities?

Answer 16

• Failure of formation = ​fibular hemimelia
• Failure of separation = ​Tarsal coalition
• Hypoplasia = hypoplastic digit
• **Overgrowth = **​macrodactyly
• Duplication = ​polydactyly
• **Constriction ring syndrome = **​Aminotic bands
• Dysplasias or syndromes = achondroplasia

Question 17

In longitudinal deformities what is in seen?

Answer 17

• Abnormality is produced along one side of the whole limb
• Preaxial ( radial ) or postaxial ( ulna)

Question 18

In transverse deformities in the limbs what is in seen?

Answer 18

• Proximal to the level of insult there is normal growth
• Distal to the insult growth and development are affected variably

Question 19

In the lower limb a defect in the cartilage anlage ( primary ossification centre) may lead to what?

Answer 19

• Proximal focal femoral deficiency
• Autosomal dominant
• assoc with Sonic the hedhehog gene
• 50% bilateral
• spectrum of disease
  
  • ​absent hip
  • cervical pseudoarthrosis
  • absent /shortened femur
• Aitken classified
• Class A femoral head present, acetabulum normal
• Class B femoral head present, acetbulum mildy dysplasic
• Class C femoral head absent, severly acetabular dysplasia
• Class D femoral head absent, acetbular absent
• assoc with ACL deficiency, fibular hemimelia, coxa vara, knee contractures

Question 20

Name a post -axial lower limb deficiency?

Answer 20

• Fibular hemimelia
  
  • complete absent or fibular short
  • most common congential Long bone abnormality
  • sonic hedgehog gene involved
  • anteromedial bowing
  • absent lateral rays
  • tarsal coalition 50% -> ball and socket ankle joint and ankle instability
  • femoral abnormalities - PFFD/coxa vara
  • Genu valgum due to hypoplasic lateral femoral condyle
  • absent cructiate ligaments

Question 21

Name a preaxial lower limb abnormality?

Answer 21

• Tibial hemimelia
• less common
• Autosomal dominant inheritance = Genetic counselling
• medial ray toe deformities
• anterolateral bowing
• prominent fibular
• assoc polydactyly/ ulnar aplasia/ cleft hand ( ectrodactyly)
• Jones classification
  
  • complete absence of tibia
  • partial absence divided into
    
    • proximal
    • distal
    • ankle diastasis
• Tx based on knee stability
• no active knee extension/ absence = amputation
• prox tib and extensor mechanism intact= tibiofibular synpstosis with syme amputation
• ankle diastasis = syme amputation

Question 22

Is there any association with a preaxial ( radial ) upper limb abnormality?

Answer 22

• relatively common abnormality
• other congential abnormalities = VACTERL
• vertebral
• anal atresia
• cardiac anomlies
• tracheo-oesphageal fistula
• renal disorders
• limb abnormalities

Question 23

What is radio-ulnar synostosis a type in swanson’s classification?

Answer 23

• failure of separation
• synostosis in proximal half as the radius and ulna separates from distal to proximal
• Bilateral in 60%
• pts frequently have duplication in sex chromosome
• no pronationor supination
• pronation limited ( 50% pts have >50^o of pronation)
• Tx observe if unilateral but if fuse 10-20^o pronation
• if bilateral dominant hand fuse in 30-40^o then non dominant supination 20-30^o

Question 24

What are packaging disorders?

Answer 24

• disorders in which the limb that have been formed normally are moulded into abnormal position in the third trimester
• include
• plagiocephaly
• torticollis
• infantile skeletal skew and calcaneovalgus feet
• normally self correcting and

Question 25

How does the bone grow longitudinally?

Answer 25

* By **enchondral ossification** at the physis

Question 26

How does the bone grow circumferentially?

Answer 26

* **Zone of Ranvier** at the **level of the physis** * **Appositional ossification** in the **osteogenic layer** of the **periosteum** along the diaphysis

Question 27

What does the zone of Ranvier contain?

Answer 27

* Osteoblasts * fibroblasts * chondrocytes

Question 28

What is the perichondial ring of LaCroix?

Answer 28

* Is a sting fibrous structure that secures the epiphysis to the metaphysis

Question 29

Describe the layers of the physis?

Answer 29

* **Resting zone= **low O2 tension. Small chondrocytes * **Proliferative =** chondrocytes increase, High O2 tension * **Hypertrophic zone= chondrocytes enlarge 5-10x** * **zone of maturation** * **zone of degeneration** * **Zone of calcification**

Question 30

Name the physis and appearance in the developing elbow?

Answer 30

* Capitellum 2 yrs * Radial head 4yrs * Internal ( medial ) epicondyle 6yrs * Trochlear 8yrs * Olecranon 10 yrs * Lateral epicondyle 12 yrs

Question 31

Where does 2/3rds of lower limb growth come from?

Answer 31

* the **knee** with 70% around distal femur * estimate **distal femur 9mm/year** of growth and **prox tibia 6mm/year** * proximal femur and distal tibia account for only a few mm (3-4mm) each per year

Question 32

Where do fractures around the physis occur?

Answer 32

* Thru the hypertrophic zone * zone of calcification

Question 33

Describe the salter harris classification?

Answer 33

* Type 1 - thru physis * type 2 away from physis with Thurtson-holland metaphsyeal fx * type 3 towards physis/ intra-articular * type 4- thru both sides of physis * type 5 - crush * type 6 - added by Mercer Rang = injury to perichondral ring

Question 34

How can growth disturbance can occur?

Answer 34

* From a bride of bone forming across the physeal cartilage * Can be **Peripheral, Central or linear** * may lead to **LLD or angular deformity** * **if the bony bridge is \>40% of growth plate then non resectible**

Question 35

Why will remodelling occur at the physis?

Answer 35

* Due to forces acting in the plane of motion of the joint according to Wolfe's law which states that * bone remodels according to the applied stresses

Question 36

Who classified skeletal dysplasia?

Answer 36

* **Ruben** * divided in location of the defect adn whether there is an increased or decreased activity

Question 37

Can you describe Ruben's classification of skeletal dysplasia's?

Answer 37

* **Epiphysis** * Hyperplasia - Trevor's disease * Hypoplasis - SED, MED, pseudoachondroplasia * **Physis** * Hyperplasia - Enchondromatosis- ollier's * Hypoplasia - Achondroplasia * **Metaphysis** * Hyperplasia - Hereditary multiple exostoses * Hypoplasia- Osteopetrosis * Diaphysis * Hyperplasia- Diaphyseal dysplasia * Hypoplasia- osteogenesis imperfecta

Question 38

How does rickets affect the growth plate?

Answer 38

* **Failure of calcification of the chondrocytes in the provisional zone of calcification in the physis** * there is normal matrix but it fails to calcify * this in the followed by disordered invasion by blood vessels * radiographically this results in **widening, thickened physis** and hazy appearance of **metaphysis** with **flaring and cupping** due to **persistence of cartilage** in the metaphysis