Bone development and Healing

Osteoblast

Development and repair

Osteocyte

Maintain bone structure

Osteoclast

Breakdown bone matrix, allowing remodelling

Bone marrow

Tissue where haematopoiesis occurs

Primarily in long bones

Long bone epiphysis

Proximal (top) end and distal (bottom) of bone

Intramembranous Ossification

Bone develops within two layers of fibrous connective tissue, may result in flattened bone

Flat bones i.e. skull, ribs, scapula and pelvis

Endochondrial Ossification

Bone develops within cartilage framework, replacing cartilage with bone as animal skeletally matures

Bones e.g. long bones femur, humerus, short bones - carpal bones

Intramembranous Ossification stages

Skeletal anatomy of foetus begins as soft fibrous tissue, allow growth development.

Once bone has been deposited, structure fixed in size, small adjustments due to remodelling

As bone fills space between 2 parallel membranes, shape characteristically flat appearance

Endochondrial Ossification

Dynamic, continuing months after animal born, affected by lifestyle, environment and nutrition and genetics

Endochondrial Ossification stages 1-3

1. Cartilage framework - foetus initially produces hyaline cartliage skeleton. Temporary skeleton complete prior birth

2. Primary Ossification - (long bones) primary centre of ossification forms with diaphysis. Osteoblasts replace cartilage with bone. Blood vessels infiltrate outer layers of cartilage framework, periosteum develops

3. Bone modelling - Amount of bone increases within main shaft, separating long bone into diaphysis (ossification site) and epiphyses at ends of long bone (initially no ossification). Structure of bone develops, more bone deposited within outer edges (cortical bone), blood vessels infiltrate into centre of bone (medullary cavity)

Endochondrial Ossification stages 4-6

1. Secondary Ossification - Occur within proximal and distal ends of bone - epiphyses. As with diaphysis, bone cells replace cartilage. Dense bone created at the edges (cortical bone), no medullary cavity formation. Cancellous (spongy) bone develops instead, becoming important site for bone marrow

2. Epiphyseal (Growth) Plates - As primary secondary site of ossification expan, eventually remains only a band of cartilage between diaphysis and each of epiphyses - epiphyseal plate. Bone continues to be deposited within cartliage, allowing animal to grow

3. Skeletal maturity - Growth ends when epiphyseal plates fused, becoming completely bone. Distal growth plate fuses first, then proximal plate. Age at which close depends on factors e.g. nutrition, although mainly influenced by genetics

Breed

Affects development of bone

Great Dane 18-24 months to skeletally mature, epiphyseal plates remaining open for extended period

Chihuahua 6m

Chondrodysplatic e.g. dachshund have growth plates calcifying earlier than other breeds - result shortened bones, curved appearance

Bone

Bone is a dynamic tissue, blood supply and exchange of nutrients occuring as with other tissues. Natural cell death requires continual replacement of bone within skeletal system

Bone density

Can naturally alter to postural changes

Place more weight on one hip siting/standing, developing posture that places more strain on some joints

Often seen after injury, e.g. stifle ligament rupture (cruciate), dog uses on hindlimb to previous injured

Cause muscle strain, discomfort, physical changes can occur in dependent limb (bearing weight), increased bone deposition in joints in response to increased pressure

Bone Healing

Endochondrial Ossification adapted to ensure bone repairs to function as load-bearing structure, capable of support animal weight

Bone healing stages

1. Haematoma - After impact, bone fragments cause damage to surrounding tissue - haematoma formation - collection of blood outside blood vessels

2. Inflammation - Haematoma replaced by granulation tissue, osteoblasts and stem cells. Migrate into fracture gap (esp. around periosteum), new blood vessels migrate to area and medullary cavity

3. Callus - Tissue formed. Consists fibrous tissue, cartilage, immature bone, increasing density of tissue in area and stabilising to fracture site

4. Bone Infiltration - As bone heals, amount of fibrous tissue decreases, bone and cartilage increase to provide greater stability. Chondroclasts resorb cartilage within callus, osteoblasts line surfaces with new bone to create mineralised matrix

5. Clinical union - Bone fragements become rigidly united by callus. Typically 12-16 weeks in adult cat/dogs with fractures in young animals healing more quickly

6. Remodelling - When bone stabilises, callus remodels, replaced by new, more organised bone. Can take years

7. Incomplete healing

Incomplete healing

Imperfect fracture heal - non-unions.

May be associated with limb length discrepancy, infection and stiff joints

Fractures can heal in incorrect position with shortening, angular deviation or rotation. Malunion

If bone fragment does not ‘glue’ back, small fragment can turn into sequestrum, acts as foreign body