Internal Fixation of Bone Fracture

The challenge of fractures through the ages has been how to manage the severe pain, immediate disability and long term sequelae of these acutely presenting emergencies, making broken bones clear priorities due to the large number of incidences. The variety of fracture treatments includes traction, joint replacement, immobilisation, amputation and internal fixation. Open fractures with significant soft tissue injury and damage were and remain at risk of infection which was commonly treated with amputation in the past. Lister, who pioneered immunisation, developed the ideas of the open reduction of patella fractures and their internal fixation.

In the 1880s and 1890s the use of plates, screws and wires was introduced but was compromised by infection, implant design, allergy to the metals and a poor understanding of the biology underlying fracture healing. The techniques and principles of fracture fixation developed in the 1950s and more recent scientific advancement in mechanical and biological understanding of fractures and their healing have led to modern methods of assessing, managing and fixing fractures.

The maintenance of an adequate blood supply is vital for healing and a fracture disrupts both blood supply through the bone and that of the membrane surrounding the bone, the periosteum. The four stages of bone repair are inflammation, soft callus formation, hard callus formation and remodelling. Clinical signs of inflammation are pain, swelling, redness and heat, with a haematoma forming from the considerable bleeding which occurs at the fracture site. Inflammatory cells migrate in, which stimulate formation of new blood vessels and general cell multiplication.

The Biology of Bone Fracture Repair

The inflammatory phase is followed by the haematoma around the fracture site being infiltrated by fibrous tissue and cells which secrete cartilage, called chondroblasts. This material is more stable than the blood clot and begins the process of gradual stabilisation. Steadily the soft callus is converted into rigid bone, the hard callus phase, by conversion of the cartilage to bone and bone formation below the periosteum. Once the connection between the fracture fragments is more solid the fracture is said to have united and then it proceeds to remodelling where it becomes mature or lacunar bone.

Indirect fracture repair or secondary bone union is the process whereby fibrous bone is changed into mature lamellar bone, the typical way that fractures heal. In secondary healing the formation of callus occurs in a fracture which is not rigidly fixed and which has some displacement. The healing bone biology can be altered by reducing the fracture closely, i.e. getting the fragments in close contact and then fixing it internally. Removing the stresses applied to the fracture is achieved by the close approximation and stabilisation of the break and this can lead to the missing out of the callus stages in a direct healing across the break. As long as inappropriate levels of force are not applied to the site whilst healing the process completes, a process named primary bone union or direct bone healing.

The surgeon’s decisions about which form of internal fixation to use for a particular fracture determines the method of fracture healing which occurs at the operated site. If a high level of stability is provided, with little or no movement at the fracture, then primary or direct bone healing will occur with remodelling. If a lower degree of stability and more potential movement is present at the fracture the healing will be secondary or indirect bone repair.

Types of Fixation – Pins and Wires

Fractures are fixed by a variety of devices which include pins, wires, nails, plates and screws, depending on the location and severity of the fracture and the type of fixation provided. Pins and wires are the simplest methods of fracture fixation and the commonest ones are named after the people who developed them. Kirschner or K-wires are narrow wires varying in diameter from 0.6 to 3.0 millimetres and Steinmann pins vary from 3 to 6 millimetres in diameter. K-wires are not stiff and can be easily bent as a typical wire can, so they are mostly an addition to other methods of fracture stabilisation. These techniques can initially fix a fracture in preparation for more definitive techniques later, with minimal soft tissue and bone damage occurring.