Injury and healing

• The three different mechanisms of bone fracture are trauma, stress and pathological
• The two types of trauma are low energy or high energy trauma
• Stress refers to abnormal stress on normal bone
• Repetitive applications of forces on a particular bone lead to overuse of bones and increased stress of bone, lead to weakening through microfractures and ultimately a stress fracture.
• Stress fractures can occur as a combination of abormal stress but with underlying abnormality of bone e.g. anorexia, osteoporosis and amenorrhea
• The female athlete triad consists of amennorhoea (loss of periods and low oestrogen), osteoporosis (loss of bone mineral denisity, porous structure, susceptible to fracture) and disordered eating (Ca2+ deficiency)
• A pathological fracture describes normal stress on abnormal bone. This can cause an insufficiency fracture if the bone is already weak due to underlying abnormality.
• Conditions that can lead to insufficiency fractures include osteopenia and osteoporosis, malignancy, Vitamin D deficiency (leads to osteomalacia in adults and rickets in children), osteomyelitis (bone infection), osteogenesis imperfecta (collagen deficiency) and Paget's disease
• Osteoporosis/ osteopenia is loss of bone density. Osteoclast activity is more than osteoblast activity. It is associated with fragility fractures and low energy trauma fractures.
• Primary osteoporosis is related to old age
• Secondary osteoporosis is caused by hypogonadism (low oestrogen), glucocorticoid excess (inhibits insulin growth factor 1, which reduces osteoblast function) and alcoholism (increases PTH which leaches Ca2+ from the bone and excess alcohol can kill osteoblasts)
• Senile osteoporosis occurs at age >70
• Post menopausal osteoporosis occurs in women aged 50-70
• In osteoporosis the bone has fewer trabecular, thinning of cortical bone and widening of Haversian canals
• Vitamin D facilitates calcium, magnesium and phosphate absorption. Deficiency leads to defect in osteoid matrix mineralisation.
• Causes of vitamin D deficiency include malabsorption or dietary insufficiency, inadequate sun exposure, liver diseases, renal disease and receptor defects.
• The pathogenesis of congenital OI is decreased type I collagen due to decreased secretion and production of abnormal collagen. Leads to insufficient osteoid production. Effects bones, hearing, heart and sight.
• Collagen is an ECM protein secreted by fibroblasts and osteoblasts and organised into insoluble fibres, comprising the ECM surrounding cells.
• OI shows an autosomal dominant or recessive inheritance pattern
• Paget's diseases is excessive bone breakdown and disorganised remodelling leading to deformity, pain, fracture or arthritis. It occurs due to too much/ too little osteoblast or osteoclast activity. It comes from genetic and acquired factors. It can become an osteosarcoma in chronic cases.
• The four examples of bone cancer are osteosarcoma (cancers in osteoblasts), chondrosarcoma (cancer in chondrocytes) Ewing sarcoma and lymphoma.
• Secondary bone cancers are metastatic bone tumours from other tissues. They can be blastic (bone forming) or lytic (bone eating) depending on which cancer they came from. Prostate is blasting, breast is both and kidney, thyroid and lung are lytic.
• The ways we can describe fractures are soft tissue integrity, bony fragments and movement.
• Soft tissue integrity means an open or closed fracture
• An open fracture is where at least one end of the bone penetrates the skin; presenting potential risk of infection
• A closed fracture is where the skin remains intact
• Bony fragments are further classified by greenstick, simple and multi fragmentary
• Greenstick means bone fragments are bent and not fully broken
• Simple means two fragments with a fracture in between
• Multifragmentary (comminuted) means that the bone is broken into several pieces
• Movement of a fracture is classified into displaced and undisplaced
• Displaced means bone fragments have moved apart a lot
• Undisplaced means bone fragments hardly moved apart
• The process of fracture healing has four basic steps: Bleeding, Inflammation, New tissue formation and Remodelling.
• In fracture healing first, a heamatoma forms (bleeding between bone ends). Inflammation occurs- cytokines are released and there's granulation (connective/ fibrotic) tissue deposited and blood vessel formation. In repair, chondroblasts make soft callus (type 2 collagen) which is converted to hard callus (type 1 collagen, similar to bone) facilitated by osteoblast activity. In remodelling, callus responds to activity, external forces, functional demands and growth- osteoblast heavily involved. Excess bone is removed.
• Inflammatory stage takes hours to days. Repair takes days to weeks. Remodelling takes months to years.
• Wolff's law states that bone grows and remodels in response to the forces that are placed on it.
• Primary bone healing is intramembranous healing. Mesenchymal stem cell goes straight to osteoblast and there's a direct formation of woven bone. Happens when you have a stable fracture and ends of the bone are really close together
• Secondary bone healing is endochondral healing which results in more callus forming and involves responses in the periosteum and external soft tissues. Occurs when you have a relatively stable fracture. Mesenchymal stem cell goes to chondral precursor which produces bone cells.
• Primary bone healing is faster