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The skeletal system: Bone growth and healing

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Brendan Docherty

PGCE, MSc, RN, is clinical stream manager, cardiology and critical care, South Eastern Sydney and Illawarra Area Health Service, Australia

This article, the second in a four-part series, covers bone growth and healing, known as osteology (Fig 1) and explores some of the related nursing and medical considerations.

Bone growth

There are two types of bone growth: intramembranous and endochondral. The former is membrane-formed (no cartilage) and has numerous small blood vessels, while the latter is cartilage-formed and does not have a vascular component within its initial growth (Tortora and Grabowski, 2003). However, at the end of the formation process most bones contain both a compact outer surface and a matrix interior. The main stages of the two processes are outlined briefly below (Martini, 2005) but are described in further detail by Tortora and Grabowski (2003).

Intramembranous growth

Pluripotent cells migrate to the position of bone formation;

The cells condense and align, secreting an organic compound of extracellular matrix, which is formed in long strands (osteoid);

Osteoblasts line the osteoid and begin to deposit calcium salts (mineralisation), forming the bone matrix;

Once the organic strands are mineralised they are termed trabecula. Consecutive growth rings called lamella are added to the trabecula to increase thickness;

Trabecula within the developing bone contact one another to form a lattice structure;

Bones that do not completely fill in and contain lattice structures are called primary cancellous bones;

Those that fill in are called compact bones.

Endochondral growth

Pluripotent cells migrate to the position of bone formation;

The cells are induced to become cartilage, which forms in the shape of the ensuing bone;

The cartilage secretes collagen and grows (interstitial growth);

The cartilage is replaced with bone and blood vessels invade the cartilage, which brings in additional cell types and vascularisation occurs;

The outer layer of pluripotent cells, which support appositional growth over areas replaced with bone, is now called the periosteum;

Osteoblasts secrete additional collagen, which is then calcified creating strands of trabecula;

Trabecula extend by appositional growth via the osteoblasts.

Bone continuously renews itself, producing new cells and removing old ones in a cyclical process. Bone-making cells (osteoblasts) maintain the balance of calcium in the blood and bone but when this balance is altered (for example in osteoporosis) the removal of bone is higher than the production, making the bone thin and brittle (McRae, 2006).

The calcium balance is a critical element of both bone growth and repair, and is affected by vitamin D levels in the body, renal and intestinal function, parathyroid gland functioning, and adrenal hormones (Tortora and Grabowski, 2003). Long growth occurs at the shaft, thus increasing the length of the bone. Growth stops when the hyaline cartilage stops reproducing itself and fully converts to bone.

Common fractures

One of the most common types of fracture, particularly in older people or those with osteoporosis, occurs near the head of the humerus (proximal). The ball of the upper arm is fractured but due to the shoulder anatomy it is impossible to use a plaster cast to immobilise the bone. However, most of these fractures heal well with the use of a sling and minimal movement to facilitate the healing process.

If the fracture is displaced or complex (or there is damage to the rotator cuff muscles of the shoulders), surgery may be necessary to pin, screw or plate the head back to the shaft of the bone.

Other common fractures are classified as:

Hairline fractures - these have minimal trauma to the bone and often only affect the outer bone layer;

Simple (Fig 2a) - one of the more straightforward types of fracture as there is only bone damage with little or no soft tissue damage;

Greenstick (Fig 2b) - incomplete fractures in which only one side of the bone has been broken. These are mostly seen in children and healing can be quick;

Comminuted (Fig 2c) - more than two fragments of bone have broken off. These fractures are highly unstable;

Compound (Fig 2d) - the bone breaks through the skin. These fractures are highly traumatic in nature and involve a high risk of bleeding and infection.

Bone healing (osteology)

Fractures heal through the process of osteology (Fig 1), which can be aided by bone immobilisation, restricted movement or surgery.

The chances of healing are mainly determined by the capacity and damage to the periosteum (the connective tissue membrane covering the bone). This is the origin of the fibroblasts that participate in bone healing - the more intact the periosteum is, the more likely healing will be facilitated (Tortora and Grabowski, 2003). The healing process takes months to complete (McRae, 2006). After a fracture the following takes place:

0-6 hours - a haematoma forms originating from the ruptured blood vessels in the bone;

6-48 hours - cytokines are released as part of the inflammatory process and this causes fibroblasts to migrate to the haematoma to develop granulation tissue;

2-7 days - granulation tissue develops, becomes denser and more stable and joins with infiltrating cartilage tissue (this process is undertaken by pluripotent cells - see part one of this series). Damaged bone is resorbed by osteoclasts, and the haematoma by macrophages;

Weeks - the structure surrounding the fracture area becomes harder, and is then classified as a callus. This woven bone is initially remodelled into lamellar bone;

Months - with time the bone is remodelled, and the callus becomes smaller as the bone is reconstructed, including removal of uneven or protruding surfaces.

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