Ch 73 - Osteochondrosis

Question 1

What are the main processes of endochondral ossification?

Answer 1

• Matrix mineralisation
• Chondrocyte death
• Vascularisation
• Ossification

Question 2

What are the three main phases of osteochondrosis?

Answer 2

• Osteochondrosis latens - early, microscopic lesion
• Osteochondrosis manifesta - subclinical lesions which are macroscopically and radiographically apparent
• Osteochondrosis dissecans - Attached or loose cartilage flaps are present and typically result in clinincal signs

Question 3

How much longitudinal bone growth is formed from the growth plates and from the epiphysis?

Answer 3

• Growth plate: 75 - 80%
• Epiphysis: 20 - 25%

Question 4

What are the widely accepted location of OCD?

Answer 4

• Humeral head
• Medial aspect of humeral condyle
• Lateral or medial femoral condyle
• Medial or lateral trochlear ridge of the talus

Question 5

What inhibits circumferential expansion of the growth plate?

Answer 5

Perichondral ring of Lacroix

Question 6

What are the 4 zones of the growth plate?

Answer 6

• Resting zone (epiphyseal side)
• Proliferative zone
• Hypertrophic zone
• Mineralisation zone

Question 7

Which is the only vascularized zone of the physis?

Answer 7

The resting zone
- Penetrated by chondro-epiphyseal blood vessels within cartilage canals

Question 8

Describe the chondrocyte appearance in each of the physeal zones:

Answer 8

• 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

Question 9

What effect does growth hormone have on the resting zone?

Answer 9

• Promotes differentiation into daughter cells capable of making Insulin-like growth factor 1 (IGF-1) which stimulates clonal expansion of chondrocytes

Question 10

What substances are involved in the local feed-back loop of chondrocyte proliferation?

Answer 10

• 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.

Question 11

How does physeal mineralised cartilage undergo endochondral ossification?

Answer 11

• 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

Question 12

What are the 2 layers of the articular-epiphyseal complex?

Answer 12

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

Question 13

What is chondroification?

Answer 13

Endochondral ossification - the process of regression of cartilage canals as the ossification from the center of the epiphysis reaches newly formed epiphyseal cartilage.

Question 14

What remains once the process of epiphyseal ossification is finished?

Answer 14

A thin layer of avascular articular cartilage and a crtilage disc between the epiphysis and metaphysis (growth plate)

Question 15

How has microtrauma been speculated to cause OCD?

Answer 15

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

Question 16

What is the grading scheme for OCD lesions in the proximal humerus?

Answer 16

• Grade I: Cartilage surface is normal, slightly thickened with miniscule subchondral defect
• Grade II: Surface mottled, more thickened, small subchondral cleft
• Grade III: Discoid elevation of cartilage surface, large subchondral cleft, sclerotic subchondral bone
• Grade IV: Partially detached flap or seperated flap and joint mouse

Question 17

What is the main difference in the manifestation of articular OCD vs growth place osteochondrosis?

Answer 17

• Articular is associated with cartilage necrosis
• Growth plate is associated with persistence of hypertrophic chondrocytes (thought to result from damage to resting zone and metaphyseal blood supply)

Question 18

What is thought to be the cause of clinical signs with a OCD lesion?

Answer 18

• Joint fluid coming into contact with subchondral bone may elute necrotic cartilage particles or inflammatory mediators and provoke a synovitis
• Inflammatory mediators within synovial fluid can stimulate nociceptors in subchondral bone
• Altered loading

Question 19

What are palliative techniques for OCD?

Answer 19

Debridement and lavage

Question 20

List the main reparative techniques for surgical treatment of OCD

Answer 20

• Curettage
• Spongialisation (complete debridement of subchondral bone, not really recommended)
• Abrasion arthroplasty
• Forage (aka osteostixis, subchondral drilling) using K-wire or microdrill burr
• Microfracture using small picks or awls. 3-4mm between each hole

Question 21

List the main restorative techniques for surgical treatment of OCD

Answer 21

• Fragment reattachment (common in humans, not really done in dogs)
• Osteochondral transplant using single large plugs or mosaicplasty (OATS®, Arthrex). Clinical improvement is expected but a degree of lameness tends to persist
• Synthetic osteochondral resurfacing (SynACART®, Arthrex) Long-term function excellent, osseous integration 83% with no periarticular reaction or damage on 2nd look arthroscopy

Question 22

What are four risk factors for OCD?

Answer 22

Heredity
Rapid growth
Diet
Trauma
(IMPORTANT)

Question 23

Most long bone growth in dogs occurs between what ages?

Answer 23

12-26 weeks of age (IMPORTANT)