Bone repair and self-extendible endoprostheses
The self-generative powers of bones are not always enough for the repair in-vitro, which may have been caused by extensive shattering of bone structures as in automobile accidents, poor circulation in old bones and certain birth defects.
Bone tissue is deposited in regions of negative electrical charge and gets absorbed in regions of positive charge, but it is still unknown how electricity promotes are healing, giving us two possible theories;
1) Negative fields prevent parathyroid hormone from stimulating the osteoclast cells at the fracture site.
2) Fields induce production of growth chemicals that stimulate the osteoblasts.
With electrical stimulation of fracture sites, increase of the speed and completeness of healing in slowly healing fractures are obtained. Comparing to ultrasound, which speed the repair of fresh fractures, a daily exposure to pulsed low-power ultrasound waves reduces the healing time of shin bones by 35 to 45 percent. All done by stimulated cartilage cells, making a fibrocartilage callus. Two parts of split bones failing to join (non-union fracture), stands for major troublesome injuries which traditionally are treated with grafts, thus sections of bone are taken from the hip and inserted into the gap. However, this requires several painful grafting sessions, and 30 percent of the grafts fail to heal.
The potential improvement will be free vascular fibular graft technique which uses some pieces of fibula to replace missing bone. It grafts normal blood vessels along with the bone sections, giving a subsequent remodelling; good replica of the pre-injury bone. Still, problems of bone implants have been less satisfactory for children with growing bones. Using self-extendible endoprostheses for knee replacement patients, the problems are partly solves. Automatic elongation on continual basis is the main technique for the telescopic sleeve inserted in-vitro of knees. Overlengthening of the prosthesis is prevented by tension in the surrounding tissue which increases after each elongation and then gradually decreases as the soft tissues grow. Presently several types of artificial bone materials are available, which can serve as scaffolding on which new bone can grow. Crushed cadaver bones and synthetic materials, bone substitutes, can fill the gaps in non-union defects. However, cadaver bone is a foreign tissue that the immune system may reject and the body also fails to replace it with new bone. Furthermore, the cadaver bone contains disease organisms.
Norian SRS, a bone cement made of calcium phosphate, provides immediate structural support to fractured or osteoporotic sites. Mixed at the time of surgery, N-SRS paste is then injected into damaged bone areas, creating an internal “cast”. The paste hardens in minutes and cures into a substance with greater compressive strength than spongy bone. The crystalline structure continues to provide support as it is gradually remodelled and replaced by host bone. Its biomechanical forces are stressed due to compression and bending at the central shaft. The MG-1000, made from mildly heated hydroxyapatite (chemically synthesized), gives bioceramic products a big lift as they are stronger than N-SRS. By moulding and carving into long pieces, it essentially works as an artificial bone graft.
Sina Nedadahandeh
Investigative Orthopaedics (2007)