MANAGEMENT OF BONE FRACTURES
in order to understanding how fractures be healed and be management we should be first know the pathophysiology of fracture healing and known the methods of treatment of bone fractures by using external and internal fixation and their advantages and complications as follow
PATHOPHYSIOLOGY OF FRACTURE HEALING
The haematoma around the fracture is invaded with small
capillaries while the macrophages remove the haematoma itself. At the same time connective tissue is laid down. The connective tissue slowly organises. This pattern of layers of organised tissue appear, first as a collar arising from the periosteum close to the end of each broken bone. The collars appear to grow towards the collar on the other bone. Eventually, the spurs of
callus meet and bridge the fracture site. They become increasingly thick, and strong fibrocartilage stabilises the fracture. This period, which in the adult occurs over the first few weeks after the fracture, is described as the fracture becoming sticky. It may still be possible to angulate the fracture but it is no longer possible to translate the fracture (move it from side to side). Meanwhile. in the fracture cleft Itself, osteoclasts continue to resorb haematoma and other dead tissue and to eat away the broken bone ends. This can result in the fracture becoming more obvious on radiographs over the first few weeks and, indeed, can make visible fractures that were initially invisible (e.g. the scaphoid). The callus of fibrous cartilage around the fracture cleft becomes calcified and then ossified (so that it is visible on radiographs). Ossification starts not at the bone ends but in the centre of the fracture cleft, where oxygen levels may be very low. Cartilage may be laid down initially rather than bone. This cartilage is then replaced by bone (endochondral ossification). It is not clear whether the callus is derived from the haematoma or from the periosteum, but it is clear that movement stimulates the production of a callus.
When the fracture can no longer be angulated with normal
loads, and it is not painful to try, the fracture is said to be clinically united. on radiographs, when the Strands of ossified callus can be seen to be stretching continuously from one bone end to another the fracture is said til be radologically united. in neither case is the fracture at full strength yet, but at this stage limited activity can be undertaken safe. Finally, the callus forms a fat cuff of woven bone from one bone end to the other. This callus is at least as strong as the bone around it, because it has widened the diameter of the tube and this confers extra strength. This stage is called consolidation Over the next months the woven bone is replaced by Haversian cortical bone which remodels over the following years, until it is almost impossible to see where the fracture was in the bone,
PRINCIPLES OF MANAGEMENT
Early treatment neurovascular problems
The principle of management of fractures is to deal with life and limb saving problems first. This means paying attention to ABC (airway, breathing and circulation) and to the neurovascular status of the limb before dealing with the fracture Itself. If there vascular compromise and the limb is distorted, It is always worth straightening the limb as far as possible in case it is simply the pressure of the displaced bone that is causing the problem .
Reduction of fractures and dislocations
Some fractures may not need reduction, especially if the minimal malunion which results will cause no cosmetic or functional problem. An impacted stable fracture that is only slightly displaced may actually be made worse by reduction. The position may be improved but the fracture will become unstable. This may make future management much more difficult .
Holding a fracture
Once a fracture has been reduced it needs to be held until it has
united the bone ends have joined together.
Once the fracture is stabilised. the patient may need help with rehabilitation to ensure that life resume as fully and as independently as possible .
Planning fracture management
• Care of soft tissues
• Reduction of fracture
• Holding of position • Rehabilitation of patient
If a fracture is allowed to heal in a displaced position the fracture will unite, but it may go on to malunion. This may be unacceptable because it is ugly (deformity) or because it interferes with function of the limb. Malunion is not usually painful.
Remodelling In children
Fractures in children remodel as the skeleton grows. Some deformity can therefore be accepted because this will correct itself over the following months. Grade 2 epiphyseal fractures in children are easy to reduce but may then slip back out of a satisfactory position. reduction carries a risk of causing growth plate damage (especially if it is performed more than 2 or 3 days after the fracture). In these circumstances it may be better to accept the malunion resulting from rather than risk an epiphyseal arrest while trying to produce a perfect reduction. In this case the enemy of good is perfect .
Stable impacted fractures
If the fracture is stable and impacted, then it will heal quickly With a minimum need for protection. Disimpaction (separation of the fragments) and reduction will automatically make the fracture unstable.the fracture will then need more sophisticated methods for holding in the elderly, a rapid return to independent existence may be more important than cosmesis . A stable distal radial (Colles fracture in elderly patient who is only just managing to cope with independent existence may be best left unreduced and managed in a removable splint for comfort slightly displaced,within 2 weeks it will be almost painless and can be used for everyday actvities. Reduction would require a plaster for at least 4 weeks, during which time the patient might forever lose his or her abiliry to cope independently.
Indications for leaving a fracture unreduced
• In children for whom remodelling will correct the position
• Elderly patients for whom function is more important than
Reducing a fracture involves trying to return the bones to as to near their original position as possible . Reduction can be performed open, in which case the fracture is exposed surgically so that the fragments can be reduced under direct vision. If a fracture is reduced closed,then the accuracy of the reduction can be checked only on a radiograph. The main advantage of closed reduction is that the soft tissues and blood supply do not need to be disrupted or further than occurred at the time of the trauma if a fracture is closed and can be reduced closed then it should be. If it is already open, then the opportunity should be used when excising the wound to make sure that the reduction is precise
• Minimises damage to blood suppl
• Relies on soft tissue attachments to reduce the fragments
• Is rarely adequate tor intra-articular fractures
• Is difficult to perform in babies when the bones cannot be seen on radiogrophs
• Is not usually an emergency operation
• Needs careful planning and preparation of the patient, like all surgery
Emergency reduction should be undertaken immediately .and Without analgesia if the circulation or the skin of the limb compromised .
Principles of closed reduction
Closed reduction is always preferable to open reduction provided that a satisfactory reduction can be obtained and held. closed reduction relies on the attachments of the bone to soft tissue (periosteum and/or ligaments) to reduce the fragments and to hold them. lntra-articular fractures. where fragments may not have any soft-tissue attachments, cannot usually be reduced closed. These fractures also need accurate reduction if traumatic secondary arthritis is to be avoided, and so are best performed open. In children. the bones may not be clearly visible on radiographs because they have not yet fully ossified, especially around the elbow. Open reduction may be needed to be certain that reduction has been achieved
Patients need to be free of pain when fractures are being reduce so a general anaesthetic will be required if a regional block is not possible. If there is no neurovascular compromise, this is not an urgent operations so time should be allowed for the patient,s
stomach to empty before a general anaesthetic is administered if it is late in the day it may be best to send the patient home with a splint and analgesia so that the procedure on a fully staffed list the following morning if there is any chance that the reduction will not be successful and that the reduction and that the fracture will need to to be opened then this should be planned in advance another good reason to leave the case untill the following morning you will also need to plan how you are going to check that the fracture has indeed reduced satisfactorily so you will need to look either image intensifier or plain radiographs and the patient will need to be positioned so that imaging in two planes is possible you will also need to plan how to hold the fracture and what facilities such as plaster or traction pins and frames you will need
Value of the periosteum
when a bone fractures the periosteum remains largely intact especially on the concave side of the fracture this strong membrane is not visible on the radiographs and so its value in guiding the fracture to a stable reduction may not always be fully apprecicated . Impacted fractures which arc also partially displaced need disimpaction before the displacement can be corrected disimpaction is carried out by applying steady distraction to fracture until you feel the bone ends separate. The force applied should be no more than 4 or 5 kg as otherwise there is a danger especially i9n the elderly of degloving the limb (pulling off the skin and soft tissue) if the fracture does not initially disimpact, then the fracture should be bent further than it is already angular exaggerating the deformity this manoeuvre should disengage the jammed ends without damaging the periosteal bridge the limb will lengthen slightly, and the fracture will become floppy Traction should then be continued for another couple of minutes to drive oedema out of the tissues around the fracture. This allow the soft tissues to extend to their normal length and make the reduction easier,
Engaging the bone ends
The intact periosteum on the concave side of the fracture now block reduction unless the tension is taken off it. This is done by angulaung the fracture even further than before, and sliding the fractured end of the distal fragment up the cortex of the proximal fragment until it slips over the broken edge of the proximal fragment. As soon as this occurs the fracture can be rolled into place with the Jagged ends of the fracture interdigitating like gear wheels. When the fracture comes to anatomical alignment,the intact periosteum on what was the concave side will become and prevent overcorrection of the fracture. Providing that any lateral pressure exerted on the fracture is in the direction of overcorrection the fracture will remain stable, splinted by the periosteum.
Importance of keeping the periosteum as intact as possible
• Minimise damage to blood supply to bone
• Helps to guide fragments to perfect reduction
• Prevent overcorrection of the fracture
• However it is less valuable in reducing intra-articular fractures
Open reduction of fractures
Exposure of a fracture should allow adequate access to see as much of the fracture as necessary while minimizing damage to soft tissues .It should also minimize damage to the periosteum. which will be providing the bulk of the blood supply to the broken bone fragments. If that blood supply is lost. then the fracture cannot unite. The incision will have to take into account any wounds already present and should be extensile (able to be extended if necessary). If a plate is to be put on the bone the incision should be planned to enable the plate to be put on the side of the bone which will be in tension. If there is skin and soft-tissue loss then incisions should be planned with a plastic surgeon to ensure that skin and soft-tissue cover of the bone and fixation can be obtained at the end of the operation. Fractures that are contaminated and those which are open (which must be treated as contaminated) are an emergency. but not a life-threatening one. Every hour that goes by increases the risk of the fracture becoming infected. so surgery needs to be performed as soon as the anaesthetist feels that it is safe.
• Allows wound to be cleaned and fragments to be reduced exactly
• Risks damage to the blood supply of the bone
• Incision must be extensile
• Soft-tissue cover must be possible
PRINCIPLES OF HOLDING FRACTURES
There are two main ways in which a fracture can be held which make a profound difference to the way in which the fracture heals
Rigid fixation blocks the normal callus formation of bone healing.The bone appears to be unaware that there is a fracture if there is no movement at the fracture site. As the bone undergoes normal physiological remodelling. the fracture cleft is gradually obliterated by new bone. This takes about a year. During that time the fixation must share the loads normally taken by the bone. Most implants fatigue under the repetitive load imposed by the human body.and will soon fail if the bone does not heal and take over its original function. Fracture healing is therefore a race against time: the bone must unite before the implant fails or the construct will collapse.
• Non-rigid means of fixing (such as plaster of Paris) allow limited movement and loading of the fracture site. The aim is to allow movement and load to stimulate callus formation without allowing the fracture to redisplace.This delicate balancing act depends on the quality of the fixation. the type of fracture and the compliance of the patient .
Rigid versus non-rigid fixation
• Rigid fixation allows immediate loading but does not stimulate callous formation
• Non.rigid fixation risks loss of reduction but stimulates rapid callus formation
If the fixation of the fracture is not completely rigid then some callus will form rapidly. but the patient may be able to resume near-normal function because the fracture is held stable if not immobile by the fixation. This partial rigidity therefore offers the best of both worlds. with rapid biological healing combined with the benefits of early mobilisation of the patient.
Types of fixation
Fixation can be divided into external and internal fixations. Implants that are fitted directly on to or put down the inside of the bone and are then covered with soft tissues and skin are classified as internal fixation. Those where the mechanical strength of the construct is outside the skin are defined as external fixation.
Types of Internal fixation
Screws can be used to hold plates on to bone or can he used in their own right to hold bone fragments together. In orthopaedics, screws have been standardised to an agreed set of diameters. The threads of the screws also come in two standard forms. one for cortical and the other for cancellous bone. The size of these thread, and their pitch (the distance between each thread) are specifically designed to give the best possible grip in healthy human bone. The drills which create the holes for these screws are also standerdised allow as snug a fit of the screws as possible without putting under load on the bone Taps' are also supplied which cut the grooves in the bone to take the threads of the screws tables are available in every orthopaedic theatre to show which drill should be used for which screw .
Plates and screws
• Sizes of screw and plates are now standardised
• Maximum grip is obtained without risking crocking the bone
• Nevertheless, plates must be plated on the tension side of the bone
if a screw is to be used to compress two bone fragments together it is important that the thread of the screw should grip only the distal fragment in which the tip of the screw is embedded. as the screw is tightened. the shoulder of the screw (the part that tapers in under the head) presses down on the proximal fragment and compresses the two fragments together. If the thread of the screw engages with the proximal fragment the screw can actually hold the fragment apart. There are techniques used to ensure that the fragments are drawn together as the screw is tightened. First. a screw can be used which has no proximal thread. just a smooth shaft this known as lag screw. An alternative strategy is to use a fully threaded screw but to drill the hole in the proximal fragment to a slightly larger size so that the screw threads cannot engage with the wall of the hole. This is called lagging the drill hole and serves the same purpose as using a lag screw.
• Ensures that bone fragments are drown together as the screw is tightened
Plate, come in several sizes. each designed to be used with a standard set of screws. They are designed to fit on to the curved surface of bone and to be held there by screws. The plates can be used In several ways and there are specific plate designed for each function .
Use of plates
• They can butress,compress or neutralise
• In all cases their strength is in tension
• They are not good at resisting bending
Buttress plates Buttress plates prevent one fragment of bone slip ping on another. They arc especially useful in oblique fractures in load bearing bones, when they will stabilise what is a very unstable fracture configuration.
Dynamic compression plates (DCP) Dynamic compression plates have oval screw holes in them with tapered walls. If the screw holes are drilled into the bone at one end of these holes (there are drill guides to assist in doing this) then the plate slides along the bone as the screw is tightened home. If the plate has already been firmly fixed to the other fragment then the slip can be used to compress the fragments of bone tightly together. This has the benefit of stabilising the construct by increasing the area of contact. it also appears to stimulate healing by putting the bone edges in close apposition.
:-Neutralisation plates Neutralisation plates are used to prevent bone ends from being distracted. They can therefore be used to resist angular forces by being placed on the side of a bone that goes into tension when load is applied (the side that opens when the fracture bends). Plates with screws are excellent at resisting
tension. and this is how they are used in neutralisation. Plates have very little resistance to bending and so should never be put on the side of the bone that is in compression and which will go into concave angulation when load is applied.
Wires are much less traumatic than plates and screws. They can be used temporarily to hold fragments reduced while plates and screws are applied. They can also be used to resist shear where loads are not great. They are especially useful m children's fractures, when plates and screws could damage the epiphyseal plate. Wires can cross the growth plate without causing
long-term effects, and if left protruding from the skin can be removed when the fracture is secure without the need for a further surgery .
The value of wires
• Can be introduced and removed percutaneously
• Safe to cross an epiphyseal plate
• Can be used as a guide for cannulated screws
Kapanji wires These are a technique which can be used in fractures in which Impaction may have left a defect that leaves the fracture unstable when reduced. After the fracture has been disimpacted and reduced, wires are introduced into the fracture cleft on the side of the defect. As soon as the tip of the wire is in the medulla the wire is tilted so that its tip travels proximally and embeds on the Inside of the far cortex. One or more wires placed in this way substitute for the missing cortex and work with the intact periosteum on the other side to create a stable reduction.
Figure-of-eight wiring This allows a strong wire suture to be woven over the cortex of bone which is held in tension. The device is not prominent and so fits well subcutaneously and is commonly used on the olecranon and on the patella .
Implants driven down the medulla of a long bone sutter from a significant mechanical disadvantage because they must be narrower than the bone into which they are introduced, The resistance of an implant to bending and twisting b proportional to the square of its diameter, Nevertheless, the medulla can provide a natural guide for the implant, and introducing the nail into one end of the bone (under image intensifier control) minimises the risk of infection from opening the fracture, .and preserves the periosteal blood supply. Nails are now available for the humerus and tibia as well as the femur. In recent years the scope of intramedullary nails has been increased by the introduction of the locking nail. This system has hole through the nail at each end. using jigs or an image intensifier screw can be passed through the bone, the hole in the nail and out through the opposite cortex of the bone. This produces a construction that holds the bone rigidly .and is especially resistant to twisting it follow an intramedullary nail to be used for a far greater range of long bone fractures including those in the metaphysis some of the newer nails can now be passed down the medulla without requiring any reaming in advance the unreamed nails this makes the operation quicker and reduces the trauma to the patient .
In summary, internal fixation can allow accurate reduction of fractures, and allows strong and stable fixation, so that the patient can rapidly return to everyday activities, with the minimum of inconvenience.
Advantages of intramedullary nails
• Now available for all major long bones
• Can be put in closed and unreamed
• Locking screws gives great stability
• Periosteal blood supply is preserved
• Patient can be mobilised early
Disadvantages and complications of internal fixation
The disadvantages of internal fixation are those of damage to soft tissues, especially blood supply. The rigidity of fixation slows the natural healing process, even though it allows earlier mobilisation of the patient. Internal fixation is technically demanding, requires a large range of implants and instruments, and is best performed in ultra clean theatres as infection is a disaster, Internal fixation requires careful preplanning and the best surgery is performed if the fractures are drawn out on stencils first, and the problems of reduction and obtaining mechanical stability planned in advance. This includes size and type of plates and position of screws. Only in this way can the operation be performed quickly and cleanly (minimising the risk of tissue damage and infection) so that the strongest fixation is obtained.
Internal fixation is best performed under a tourniquet, if possible. in order to obtain a blood-free view. There are complications inherent in using a tourniquet. such as cuff damage to nerves as a result of inflation to an excessive pressure. and problems of reperfusion injury if the cuff is left inflated for too long.
Exposure of the fracture may damage the soft-tissue attachments to the bone and produce avascular fragments, which will delay or even prevent fracture union. Soft-tissue dissection should therefore be kept to a minimum but must be adequate to obtain a clear view and access. All incisions should be designed so that they can be extended safely if necessary - extensile exposure.
The risk of infection can be minimised by cleaning out open
fractures and leaving them open. with the fractures stabilised until it is certain that all dead and contaminated tissue has been removed. Only when they are clean should they be closed (delayed primary closure). When internal fixation is used, infection is minimised by performing quick, tidy and well-planned surgery, and by adhering to strict theatre discipline on theatre sterility. Surgery should be covered by three doses of a broad-spectrum antibiotic which has good activity against Staphylococcus (the most common infective organism) and Streptococcus (the second most common).
Internal fixation can also leave unsightly scars, and thus should be planned to minimise cosmetic deformity without compromising access.
Drills and screws can damage nerves and vessels. Drill guards should always be used to prevent soft tissues being inadvertently dragged into a spinning drill. When the drill is cutting into the far cortex, the hand that the surgeon is using to hold the drill should have a straight finger resting on the limb through which the drill is passing. Only light pressure should be applied to the drill so that when the drill then comes out through the far cortex it will not suddenly penetrate deep inn the soft tissues on the far side of the bone. where it might perforate a nerve or vessel .
Complications of Internal fixation
• Damage to soft tissues and blood supply
• Risk of introducing infection
•Callus formation is inhibited
Removal of internal fixation
Implants for internal fixation are made of surgical-grade stainless steel and should not corrode. Nevertheless, the alloys contain transitional metal such as chromium and vanadium, whose salts are allergenic. toxic and may even be carcinogenic. Despite this, there is little evidence that metalware left in patients for long periods causes any chemical or even allergic problems. Children should have metalware removed if it is likely to compromise growth. It should be removed as early as possible because periosteal bone grows rapidly over the plates and makes their removal difficult. Internal fixation also shields the bone around it from load. and so may cause local osteoporosis. The load passing down the bone may then peak at the end of a plate (a stress raiser) and cause a fracture. Internal fixation of a fracture next to an old plate already embedded in the bone is very difficult to manage. Despite this, it is now normal practice to leave plates and even intramedullary nails in the patient unless they are causing pain or there is another specific reason for the patient to receive another general anaesthetic for another procedure. in which case plates can be removed at the same time .
Reasons for removing metalwork
• Plates may load shield, producing osteoporosis
• Salts of stainless steel may be toxic in long term
An alternative way to holding a fracture is to insert pins and wires into the bone on each side of the fracture, and to attach these to an external frame that provides the structural integrity. Fixators can be as simple as a set of pins incorporated into a plaster through single- and double-bar fixators or as complex as ring fixators holding the bone through tension wires .
There is a trade-off between cost, ease of fitting, adjustability rigidity and convenience to the patient . The choice of fixator will depend on what is available and the use to which it is to be put. The llizarov fixator tensions wires onto an external ring. Wires are easy and safe to introduce, tend not to get infected and are then very strong in tension.
Advantages of external fixation
• Minimally invasive
• Can be used when soft tissue cover is compromised
• Allows early mobilisation
• Can be adjusted later
Uses of an external fixator
Emergency use of the external fixator
Fixators are used for two main reasons in an emergency.
Pelvis They can be used to stabilise an unstable pelvic fracture to try to reduce life-threatening haemorrhage from the pelvic veins. Closing and stabilising an open pelvis fracture may reduce bleeding by reducing movement of the pelvic veins this may stabilize clots and reduce haemorrhage. Closing the pelvis may increase the intrapelvic pressure and tamponade the veins to reduce bleeding. a bar fixator attached to pins inserted into the pelvic wings will need to be used. The bar should be set as low as possible to give enough room over the abdomen should a laparotomy be needed
if a limb has an unstable fracture and has lost its blood supply the skeleton needs to be stabilised before the vascular repair can be performed one option is to insert a stent and provide a temporary blood supply to the limb while a definitive orthopaedic fixation is performed an alternative is to use an external fixator that can be applied quickly to stabilise the
fracture so that the vascular surgeon can start work with the minimum of delay. The disadvantage of this approach is that an external fixator may not be the optimal way of stabilising that particular fracture, but once it has been applied the risk of infection from the pin tracks makes a conversion to a plate or an intramedullary nail potentially risky.
Non-emergency use of the external fixator
Soft-tissue damage If there is extensive damage to the soft tissues then it may not be possible to achieve good cover of the bone. If bone is contaminated and/or exposed internal fixation may not be advisable. in these circumstances an external fixator may offer the best option. The position of the pins can be planned with the plastic surgeons to enable them to rotate flaps without the fixator or the pins getting in the way.
Leg lengthening and correction of deformity Over the last decade one of the great advances in orthopaedics has been the discovery that bones can be lengthened gradually callostasis. Segments of bone can be moved across defects and, if the periosteum is left as intact as possible, new bone will be laid down in the defect - bone transport.
In order for the pins of the fixator to be able to move through the soft tissues as the bones move they need to be very thin, and it is now routine to use wires which gain their rigidity by being tensioned on a ring (the Ilizarov technique). The key to the technique is to move the bone so slowly that new bone can be laid down in its track, but not so slowly that the bone unites and prevents any further distraction. The fixation pins must be positioned to avoid damaging vital structures as they carve through the soft tissues. Care must also be taken to avoid overstretching nerves and vessels, and to avoid contractures caused by ligaments, tendons and muscles failing to extend in concert with the bone.
HOW YOU CAN DETERMINING UNION OF BONE FRACTURES
Types of bone union
• Clinically united pain free to pressure not full strength
• Radiologically united bone cross the fracture cleft
• Consolidatiaon osteoblastic activity has returned to near normal or full strength
A bone is clinically united when putting load on the fracture produces no detectable movement and no pain. The fracture site will not yet he as strong as the bone around it, hut it is united.
This is not the same as clinical union. It occurs when the callus around the fracture can be seen to pass from one broken bone end to the other without a gap between. The fracture across the medulla of the bone may still he visible, but the callus around the bone is continuous. The bone should now be able to cope with normal loads but will not be as strong as the bone around it. From a management point of view, it is the time when movement and loading of the limb should be increased to build up muscle power, mobility and proprioception. If the patient plays sport or works in a job involving heavy labour he or show should not return to this unless the bone is protected or until the fracture has consolidated .
Consolidation takes much longer than union, and is defined as the time when the process of fracture healing is complete and the strength of the bone has risen to normal levels or even beyond. The formation of callus around a fracture creates a strong cuff. The diameter of this cuff is greater than the diameter of the bone itself, and so a consolidated fracture can be stronger than the original bone.
When a fracture occurs, there will be damage to soft tissues. Muscles may be bruised or torn. Ligaments may be ruptured. joints filled with blood and nerves and blood vessels damaged The original philosophy in orthopaedics was that the key to management of fractures was immobilisation, and of injury was rest. This has now all changed. Fractures are stabilised to allow mobilisation of the limb and the patient. Rigid fixation of the fractures actually inhibits callus formation and slows healing. However, stabilisation of the fractures allows the patient to start moving the soft tissues to promote healing and reduce stiffness . It also allows the patient to return to a normal independent life sooner. Physiotherapy is a key element in the rehabilitation of trauma cases.
it allows early mobilisation of the limb while ensuring that loads are not so excessive that the fixation will fail
provides instruction and advice to the patient on their own rehabilitation builds the patient,s confidence
retrains proprioception so that the feedback loops between sensors of joint position and tendon load start to coordinate with motor' nerves serving the muscles.
Mobilisation after fracture
• Mobilisation reduces stiffness, improves muscle power and blood supply and Stabilisation of fractures allows early mobilisation