Chapter 73 - Osteochondrosis

Question 1

What percentage of all dogs presented for orthopedic problems are diagnosed with OCD?

What % of dogs presenting for ortho problems at under 1 year of age are diagnosed with OCD?

Answer 1

3.7% of ALL orthopedic dogs

9% of orthopedic dogs under 1 year.

Question 2

Male dogs are more commonly affected with OCD at all locations except where?

Answer 2

Females are more common for OCD of the talus (tarsus).

Question 3

In most long bones, growth plates (physes) contribute ____% of final bone length whereas epiphyses contribute approximately ____%?

Answer 3

Growth plates contribute MOST 75-80% of final bone length (IMPORTANT)

Epiphyses contribute approx. 20-25%

Question 4

At what age does the medial aspect of humeral condyle (a center of ossification) fuse to the lateral condyle?

Answer 4

6 weeks

Question 5

What is saltation and stasis?

Which bones of the dog are known to experience it?

Answer 5

In human children, short periods of active growth alternate with longer periods with no measurable growth.
Saltation is growth and stasis is obvious.

Saltatory growth has been demonstrated in the dog radius, ulna and tibia.

Question 6

What is the Hueter-Volkmann law and what does it describe?

Answer 6

Growth is retarded by increased mechanical compression and accelerated by reduced loading.

This fits with studies in lambs where 90% of growth-plate related bone lengthening occurred while they were lying down and nearly none when standing.

Question 7

What are the (4) zones of the physis?

Answer 7

Resting (epiphyseal side)
Proliferative
Hypertrophic
Mineralization

Question 8

In the outer layer of the articular-epiphyseal complex, what does the presence of the tidemark indicate?

Answer 8

The tidemark is a microscopically visible line that represents the separation of the zone of calcified cartilage from the radial zone.

Emergence of the tidemark indicates that cartilage maturation is complete.

Question 9

Olsson and Reiland described two types of OCD. Most of our understanding is based on study of Type 1 lesions.

What are the two types, and where are they found?

Answer 9

Type 1: Occurs at the center of the affected articular surface, away from vascular attachments.
- Sites include the caudal humeral head, medial humeral condyle, lateral and/or medial femoral condyles (IMPORTANT)

Type 2: Occurs at joint margin and retains vascular attachment.
- Sites include medial or lateral trochlear ridges of talus (which attach to joint capsule and/or ligaments) and MCP of ulna (which is vascularized through joint capsule or the annular ligament)

Question 10

There is a grading scheme for OCD of the proximal humerus that grades from I to IV. What does a lesion of each type look like?

Answer 10

Type I: Cartilage surface normal, cartilage slightly thickened, miniscule subchondral bone defect
Type II: Cartilage surface mottled, more thickened, small “cleft” of subchondral bone
Type III: Discoid elevation of cartilage surface. Underlying bone sclerotic
Type IV: Typically a partially or fully detached flap and joint mice

Question 11

What are the three stages of OCD?

Answer 11

Oseochondrosis Latens
Osteochondrosis Manifesta
Osteochondrosis/itis dissecans

Question 12

Describe the lesion of osteochondrosis latens?

Answer 12

Early, microscopic

Question 13

Describe the lesion of Osteochondrosis manifesta?

Answer 13

Subclinical lesion that is both macroscopically and radiologically apparent.

Question 14

Describe the lesion of Osteochondrosis dissecans?

Why is this also called Osteochondritis, even though it isn’t inflammatory in pathology?

Answer 14

An attached or loose cartilage flap is present. These dogs are usually clinically affected.

Because the flap causes synovitis, this is often referred to conventionally as an -itis even if the pathology is not truly inflammatory.

Question 15

What is the incidence of OCD in cats?

Answer 15

0.001%

There are only a few cases in the literature, considered very uncommon.

Question 16

What prevents circumferential expansion of the growth plate?

Answer 16

The perichondral ring of Lacroix.

Question 17

The articular-epiphyseal complex, at the cartilaginous end of developing bone, has two layers.
The inner layer is like a physis. The outer layer is immature cartilage and is avascular. It has four zones - what are they?

Answer 17

Superficial zone
Transitional zone
Radial zone
(Tidemark)
Zone of calcified cartilage

Question 18

OCD is considered a polygenetic trait, but there is a strong breed and bloodline predisposition. What is the reported range of heritability of OCD (in %)?

Answer 18

10-45%

Question 19

Most long bone growth in dogs occurs between what ages?

Answer 19

12-26 weeks of age (IMPORTANT)

Question 20

What are four risk factors for OCD?

Answer 20

Heredity
Rapid growth
Diet
Trauma
(IMPORTANT)

Question 21

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

Answer 21

• 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 22

How has microtrauma been speculated to cause OCD?

Answer 22

Microtrauma can cause damage to cartilage canal vessels at the chondroosseous junction and subsequent necrosis of cartilage canals leading to areas of cartilage ischemia and necrosis.

Question 23

What remains once the process of epiphyseal ossification is finished?

Answer 23

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

Question 24

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

Answer 24

• 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. (IMPORTANT)

Question 25

What effect does growth hormone have on the resting zone of cartilage?

Answer 25

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

Question 26

Describe the chondrocyte appearance in each of the physeal zones:

Answer 26

- 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 27

Which is the only vascularized zone of the physis?

Answer 27

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

Question 28

What are the widely accepted locations of OCD?

Answer 28

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

Question 29

What are the main processes of endochondral ossification?

Answer 29

- Matrix mineralization - Chondrocyte death - Vascularization - Ossification

Question 30

What is the primary pathophysiologic event in osteochondrosis?

Answer 30

Failure of endochondral ossification due to disruption of epiphyseal cartilage canal blood supply.

Question 31

Which stage of osteochondrosis is characterized by subclinical cartilage necrosis without radiographic change?

Answer 31

Osteochondrosis latens.

Question 32

What defines osteochondrosis manifesta?

Answer 32

Persistence of a focal cartilage thickening visible radiographically without cartilage flap formation.

Question 33

What pathologic event distinguishes osteochondrosis dissecans (OCD)?

Answer 33

Formation of a cartilage flap due to mechanical failure of necrotic cartilage.

Question 34

Why are rapidly growing large-breed dogs predisposed to osteochondrosis?

Answer 34

Rapid growth increases cartilage thickness and metabolic demand, predisposing to vascular failure.

Question 35

Which joint is most commonly affected by OCD in dogs?

Answer 35

The caudal humeral head (shoulder).

Question 36

What is the role of cartilage canals in developing epiphyses?

Answer 36

They provide vascular supply to deep cartilage prior to ossification.

Question 37

How does excessive dietary energy contribute to osteochondrosis?

Answer 37

It accelerates growth and increases mechanical stress on immature cartilage.

Question 38

Why does retained necrotic cartilage predispose to flap formation?

Answer 38

It lacks structural integrity and fails under repetitive load.

Question 39

What imaging modality is most sensitive for early OCD lesion detection?

Answer 39

Magnetic resonance imaging (MRI).

Question 40

Why is arthroscopy preferred for OCD flap removal?

Answer 40

It allows minimally invasive debridement with superior joint visualization.

Question 41

What biomechanical factor contributes to lesion predilection at convex joint surfaces?

Answer 41

Higher compressive and shear forces across convex articular cartilage.

Question 42

Why are subchondral bone cysts sometimes associated with osteochondrosis?

Answer 42

Persistent necrotic cartilage weakens subchondral bone, leading to cystic change.

Question 43

Which endocrine imbalance has been experimentally linked to altered endochondral ossification?

Answer 43

Excess growth hormone or thyroid dysfunction.

Question 44

Why may bilateral lesions be present in osteochondrosis?

Answer 44

The condition is developmental and influenced by systemic factors.

Question 45

What histologic feature characterizes osteochondrosis latens?

Answer 45

Focal chondrocyte necrosis with retained cartilage matrix.

Question 46

Why does surgical curettage stimulate healing in OCD?

Answer 46

Removal of necrotic cartilage exposes vascular subchondral bone to promote fibrocartilage repair.

Question 47

What is the long-term complication of untreated OCD?

Answer 47

Development of secondary osteoarthritis.

Question 48

Why are cats less commonly affected by osteochondrosis than dogs?

Answer 48

Differences in growth rate and cartilage vascular physiology.

Question 49

Which growth plate zone is primarily disrupted in osteochondrosis?

Answer 49

The hypertrophic zone of endochondral ossification.