Osteochondrosis (Chapter 73) Flashcards
What are the main processes of endochondral ossification?
- Matrix mineralisation
- Chondrocyte death
- Vascularisation
- Ossification
What are the three main phases of osteochondrosis?
- Osteochondrosis latens - early, microscopic lesion
- Osteochondrosis manifesta - subclinical lesions which are macroscopically and radiographically apparent
- Osteochondrosis dessicans - Attached or loose cartilage flaps are present and typically result in clinincal signs
How much longitudinal bone growth is formed from the growth plates and from the epiphysis?
- Growth plate: 75 - 80%
- Epiphysis: 20 - 25%
What are the widely accepted location of OCD?
- Humeral head
- Medial aspect of humeral condyle
- Lateral or medial femoral condyle
- Medial or lateral trochlear ridge of the talus
What inhibits circumferential expansion of the growth plate?
Perichondral ring of Lacroix
What are the 4 zones of the growth plate?
- Resting zone (epiphyseal side)
- Proliferative zone
- Hypertrophic zone
- Mineralisation zone
Which is the only vascularized zone of the physis?
The resting zone
- Penetrated by chondro-epiphyseal blood vessels within cartilage canals
Describe the chondrocyte appearance in each of the physeal zones:
- Resting zone: Small, scattered, randomly organized. Primarily slowly dividing stem cells
- Proliferative zone: Flat, relatively small, organized into COLUMNS. They divide, slowly enlarge and produce matrix
- Hypertrophic zone: Spheroid and relatively large. Rapidly swell and continue synthesis of matrix
- Mineralization zone: Newly formed matrix mineralizes and chondrocytes undergo apoptosis
What effect does growth hormone have on the resting zone?
- Promotes differentiation into daughter cells capable of making Insulin-like growth factor 1 (IGF-1) which stimulates clonal expansion of chondrocytes
What substances are involved in the local feed-back loop of chondrocyte proliferation?
- PTHrP
- IHH (Indian hedgehog)
- TGF-B
Controls the irreversible differentiation of proliferative chondrocytes into hypertrophic chondrocytes. BMP, thyroid hormone and others are also needed for this phenotypic change to occur.
How does physeal mineralised cartilage undergo endochondral ossification?
- Clasts remove the tranverse matrix septa seperating the apoptotic chondrocytes from the metaphyseal vasculature
- Clasts form void lacunae in a mineralised matrix scaffold
- Vasculature from the metaphysis and osteoprogenitor cells invade the lacunae under control of VEGF produced by hypertrophic zone chondrocytes
- Osteoprogenitor cells differentiate into osteoblasts and produce woven bone which is then replaced by lamellar bone
What are the 2 layers of the articular-epiphyseal complex?
Relatively thin outer layer
- Specialised immature articular cartilage, avascular and takes no part in endochondral ossification
- Developes into mature cartilage with 4 zones: Superficial, transitional, radial and calcified cartilage
- Noncalcified radial zone seperated from calcified cartilage by the tidemark. This indicates completion of maturation process
Inner layer functionally similar to the growth plate with 2 main differences:
- Visually disorganised withouh ordered zonal and columnar arrangement of chondrocytes
- Abundant vasculature from the perichondral plexus and course through cartilage canals, forming glomeruli
- Most proliferation occurs at periphery whereas conversion into bone occurs at the center
What is chondroification?
Emdochondral ossification - the process of regression of cartilage canals as the ossification from the center of the epiphysis reaches newly formed epiphyseal cartilage.
What remains once the process of epiphyseal ossification is finished?
A thin layer of avascular articular cartilage and a crtilage disc between the epiphysis and metaphysis (growth plate)
How has microtrauma been speculated to cause OCD?
Microtrauma can cause damage to cartilage canal vessels at the chondroosseous junction and subsequent necrosis of cartilage canals leading to areas of cartilage ischaemia and necrosis
- Infarcted cartilage focally prevents endochondral ossification however cartilage proliferation continues, resulting in focal thickening
- Thickened cartilage may be less resistant to mechanical stress and may be metabolically deprved and degenerate
- Weakened cartilage may deform, fissures can form and may propage along tidemark (osteochondral junction)
- If fissures extend to the joint surface, an OCD lesion develops
