Principles of Bone Growth

General Principles of Bone Growth

• Bone growth initiates with a cartilaginous model about 6 weeks post-fertilization.
• Endochondral ossification transforms cartilage into bone.

Primary Ossification

• Blood enters the cartilage, activating osteoblasts for bone formation.
• Begins in the Diaphysis, forming the medullary cavity while the Epiphysis remains cartilage.

Secondary Ossification

• The Epiphyses undergo a similar ossification process to the Diaphysis, but a section inferior to the Epiphyses remains cartilage.
• A cartilaginous growth plate persists, allowing continuous bone growth.
• Bone layers are added under and above the growth plate until puberty, when the plate ossifies.

Bone Growth in Width

• Appositional growth leads to bone widening, layers of bone removed.
• Osteoblast activity creates circumferential lamellae as osteoclasts shape the bone and expand the medullary cavity.

Microanatomy of Tissues in the Joints

• Bones connect at joints.
• Joints hold bones together, involving bone ends and soft tissues, enabling controlled movements.

Cartilage

• Consists of chondrocytes stored in lacunae with collagen fibers in ground substance.
• Avascular; nutrients diffuse through the ECM during joint loading.

Hyaline Cartilage

• Effective at resisting compression.
• High water content and sparse collagen fibers.
• Creates a smooth, frictionless surface for bone articulation, helping avoid pain.
• It degrades with age.

Fibrocartilage

• Resists both compression and tension.
• Bundles of collagen fibers align with stress directions like pulling to increase strength.
• It is valuable at joints experiencing compression and tension.
• Functions as a buffer to distribute force over a wide area.
• Deepens articular surfaces.

Dense Fibrous Connective Tissue (DFCT)

• Fibroblasts create fibers.
• Uses collagen and sometimes elastin fibers for muscle tension resistance.
• It is dense and tightly packed
• Low vascularity delays healing.

Ligaments

• Connect bone to bone.
• Contain collagen and elastin to resist tension, allow stress and recoil.
• Restrict movement away from their location.

Tendons

• Connect muscle to bone.
• Less elastin than ligaments.
• Facilitate and control movements.
• Transmit muscle contraction to bone.

Bony Congruence

• Represents the combined surfaces that articulate at a joint
• Less congruence requires more soft tissue support.

Fibrous Joints

• Limit movement, providing stability (e.g., cranial sutures).
• The ankle joint prevents rotational movements.

Tissue

• Cells grouped together based on specific structure and function (e.g., DFCT).

Structure

• Formed of a tissue (e.g., ligament).

Cartilaginous Joints

• Provide some movement.
• Made of fibrocartilage.
• Varies per bone.

Synovial Joints

• Consist of many tissues and structures.
• Enable lots of movement.
• Some bones are entirely connected by fibrocartilage (e.g., intervertebral joint).