BONE-GRAFTS 

Bone-grafts are both osteoinductive and osteoconductive, i.e. they are able to stimulate osteogenesis, and they also provide linkage across defects and a scaffold upon which new bone can form. Osteogenesis is brought about partly by the activity of cells surviving on the surface of the graft, but mainly by the stimulation of osteoprogenitor cells in the host bed - an effect that is due to the presence of bone morphogenetic protein in the graft matrix. Cancellous grafts are more rapidly incorporated into host bone than cortical grafts, but sometimes the greater strength of cortical bone is needed to provide structural integrity.

TYPES: 1- Autografts (autogenous grafts)

In these, bone is transferred from one place to another in the same individual. This is the most commonly used form of bone-grafting, but it depends on there being sufficient donor bone of the sort required and a recipient site with a clean vascular bed. Most of the transplanted bone dies, but it continues to act as a scaffold, which is gradually replaced by a process of creeping substitution. Cancellous Autografts can be obtained from the thicker portions of the ilium, the greater trochanter, the proximal metaphysis of the tibia, the lower radius, the olecranon, or from an excised femoral head. Cortical grafts can harvested from any convenient long bone or from the iliac crest; they usually need to be fixed with screws, sometimes reinforced by a plate, and can be placed on the host bone, or inlaid, or slid along the long axis of the bone.

The ideal vascularised autograft is one with an intact blood supply. Bone is transferred complete with its blood vessels, which are anastomosed to vessels at the recipient site. The technique is difficult and time consuming, requiring microsurgical skill. Available donor sites include the iliac crest (complete with one of the circumflex arteries), the fibula (with the peroneal artery) and the radial shaft. Vascularized grafts remain completely viable and become incorporated by a process analogous to fracture healing.

2- Allografts (homografts)

With these, bone is transferred from one individual (alive or dead) to another of the same species. The bone is harvested and stored until needed. The method is particularly useful when large defects have to be filled.

Fresh allografts, though dead, are not immunologically inert. They induce an inflammatory response in the host and this may lead to rejection. However, the antigenicity can be reduced by freezing or freeze-drying, or by ionizing radiation. The process of incorporation (when it occurs) is similar to that with auto grafts but slower and less -complete. Demineralization is another way of reducing antigenicity and it may also enhance the osteoinductive properties of the graft.

Allografts are plentiful and can be stored for long periods. However, sterility must be ensured. This can be done by exposure to ethylene oxide or by ionizing radiation, but their physical properties and potential for osteoinduction may be altered by doses that are high graft must then be harvested under sterile conditions and the donor must be cleared for (A)malignancy, (B)venereal disease, (C)hepatitis and (D)human immunodeficiency virus (HIV).

Other types of graft 3- Xenografts

are obtained from another mammalian species, such as pigs or cows. After treatment for antigenicity, they should, theoretically, behave like allografts, but in practice they are much less effective unless host marrow is added to the graft.

4- 'Artificial bone‘ made of hydroxyapatite composites can be used in

the same way to fill a cavity or bridge a small gap. Bioactive bone cements (injectable calcium phosphate preparations) offer a simple alternative, e.g. for replacing bone loss in metaphyseal fractures.

Applications Cancellous grafts are used for filling cavities,

augmenting healing and promoting arthrodesis. Cortical or corticocancellous grafts are needed where bone has been lost as a result of trauma or has been removed because it contained a tumour. When reinforced by metallic implants, large gaps can be filled. Vascularized grafts tend to be used only in exceptional circumstances, such as treating large bone defects.

DISTRACTION HISTOGENESIS AND LIMB RECONSTRUCTION

 

Present-day limb reconstruction is founded on the principle that new-bone formation is stimulated in response to gradual increases in tension. This was originally discovered by Gavril Ilizarov in Russia and the application of this principle to bone reconstruction is widely referred to as the Ilizarov method.

Distraction histogenesis Callotasis  Callus distraction, or callotasis, is perhaps the single

most important application of the tension-stress principle. It is used for limb lengthening or the filling of large defects in bone, through either bone transport or other strategies. The basis of the technique is to produce a careful fracture through the bone, followed by a short wait (5-10 days) before the young callus is gradually distracted by traction on the bone via a circular or unilateral external fixator" Distraction proceeds at 1 mm a day, with small (usually 0.25 mm) increments spaced out evenly. New callus can be seen on the x-ray after 3 weeks; in optimum conditions, it forms an even column in the gap between the bone fragments (this is called the regenerate) . If the distraction rate is too fast, or the osteotomy performed poorly, the regenerate may be thin with an hourglass appearance; conversely,

if distraction is too slow, it may appear bulbous or, worse still, may consolidate prematurely, thereby preventing any further lengthening. When the desired length is reached, a second waiting period follows which allows the regenerate callus to consolidate and harden. Weight-bearing is permitted throughout this period. When cortices of even thickness appear in the regenerate, the fixator can be removed. Throughout treatment, physiotherapy is important to preserve joint movement and avoid contractures. Patients should be warned that bone lengthening takes months rather than weeks and carries a risk of complications, such as pin-track infection, angulatory deformity, re-fracture and non-union. Ilizarov techniques should be employed only by surgeons who have undergone training in this method.

Chondrodiatasis

 In children, bone lengthening can be achieved by

distracting the physis (growth plate). No osteotomy is needed and the distraction rate is slower, usually 0.25 mm twice daily. Although a wide, even column of regenerate is usually seen, the fate of the physis is sealed: it frequently closes after the process, and for this reason the technique is best reserved for children close to the end of growth.

Bone transport  

The principle of callotasis is used not only for limb lengthening bur also as a means of treating non-union and filling defects in bone. Bone transport allows a defect (or gap) to be filled in gradually creating a 'floating' segment of bone through corticotomy either proximal or distal to the defect, and slowly moving the isolated segment of bone across the gap. As the segment is transported from the corticotomy site to the new docking site, leaves a trail of regenerate new bone behind it. An external fixator provides stability during this process.

DEFORMITIES OF THE FOOT 

The normal position of the foot is plantigrade - i.e. when the patient stands, the sole is at right angles to the leg. Equinus (like a horse's foot) means .that the hindfoot is fixed in plantarflexion (pointing downwards), Plantaris looks similar, but the ankle is neutral and only the forefoot is plantarflexed. Calcaneus is fixed dorsiflexion at the ankle. A dorsiflexion deformity in the mid-foot produces a rocker-bottom foot.

Normally the medial border of the foot, even when weight-bearing, forms a longitudinal arch. The arrangement of the metatarsals also produces an anterior or transverse arch in the forefoot. Flattening of the longitudinal arch is referred to as a planus deformity or flat-foot; and a dropped metatarsal arch as anterior flat-foot. An excessively high arch produces a cavus deformity.

 Common deformities of the toes are lateral

deviation of the big toe (hallux valgus), proximal interphalangeal flexion of one of the lesser toes (hammer-toe) and flexion of both interphalangeal joints of several toes (claw-toes).

CONGENITAL TALIPES EQUINOVARUS (IDIOPATHIC CLUB-FOOT)

 In this deformity the heel is in equinus (pointing

downwards), the entire hindfoot in varus (tilted towards the midline) and the mid-foot and forefoot adducted and supinated (twisted medially and the sole turned upwards). It is relatively common; the incidence is 1 or 2 per 1000 birth and boys are affected twice as often as girls. The condition is bilateral in one-third of cases. Similar deformities are seen in neurological disorders e.g., myelomeningocele, and in arthrogryposis.

The skin and soft tissues of the calf and the medial side of the foot are short and underdeveloped. If the condition is not corrected early, secondary growth changes occur in the bones and these are permanent. Even with treatment, the foot is liable to be short and the calf may remain thin.

Clinical features  The deformity is usually obvious at birth; the foot is

both turned and twisted inwards so that the sole faces posteromedially. The heel is usually small and high, and deep creases appear posteriorly and medially. In a normal baby the foot can be dorsiflexed and everted until the toes almost touch the front of the leg. In club-foot this manoeuvre meets with varying degrees of resistance and in severe cases the deformity is fixed.

The infant must always be examined for associated disorders such as congenital hip dislocation and spina bifida.

 X-rays The tarsal bones are incompletely ossified at this

age and the anatomy is therefore difficult to define. However, the shape and position of the tarsal ossific centres are helpful in assessing progress after treatment.

Treatment  The aim of treatment is to produce and maintain a

plantigrade, supple foot that will function well. There are several methods of treatment, but relapse is common, especially in babies with associated neuromuscular disorders.

 Conservative treatment  Treatment should begin early, preferably within a day

or two of birth. This consists of repeated manipulation and adhesive strapping or application of plaster-of-Paris casts, which will maintain the correction. If adhesive strapping is used, parent, are taught how to do the manipulation and they can then carry out gentle stretches on a regular basis with the strapping still in place. Treatment is supervised by a physiotherapist, who alters the strapping as correction is gradually obtained. Plaster-of-Paris casting requires serial changes and manipulations in a clinic setting. Sometimes surgical release of the Achilles tendon is needed to complete the correction.

Operative treatment

Resistant cases will need surgery. The objectives are (a) the complete release of joint tethers (capsular and ligamentous contractures and fibrotic bands) and (b) lengthening of tendons so that the foot can be positioned normally without undue tension. A detailed knowledge of the pathological anatomy is a sine qua non. After operative correction, the foot is immobilized in its corrected position in a plaster cast. Kirschner wires are sometimes inserted across the intertarsal and ankle joints to augment the hold. The wires and cast are removed at 6-8 weeks, after which hobble boots (Dennis Browne) or a customized orthosis are used to maintain the correction.

LATE OR RELAPSED CLUB-FOOT Late presenters often have severe deformities

with secondary bony changes, and the relapsed club-foot is complicated by scarring from previous surgery. A revision .of the soft-tissue releases may be considered; this can be combined with shortening of the lateral side of the foot by calcaneocuboid fusion or cuboid enucleation (Dilwyn Evans). Alternatively, gradual correction by means of a circular external fixator (the Ilizarov method) has proved effective in treating difficult relapsed cases and severe deformities; the early results are encouraging.