8. Bone and bone grafts
Structure and Physiology of Bone
1. Notwithstanding the high quantity of inorganic and inert calcium compounds [mainly hydroxy apatite (35% of a dry bone)] as its content, the bone is essentially a living connective tissue, irrespective of whether it develops in a cartilage (long bones) or in a membrane (as in the maxillofacial skeleton and the skull).
2. In the long bones, though the precursor for development is cartilage, the bone itself behaves quite differently from cartilaginous tissue, in that it reflects a host of physiological as well as the pathophysiological changes that the rest of the body undergoes. In that respect it is far more living than a cartilage. The same holds true for bones that develop in a membrane though probably not to the same magnitude as in the long bones.
3. The bone on its outer surface is covered by periosteum. This is made of dense fibrous tissue on the outside which gives anchor to tendons and muscles, and a softer tissue inside which contains a rich network of blood vessels and lymphatics which nourish the underlying bone. Blood vessels penetrate horizontally through canals and then these vessels join a vertical system of blood vessels also in canals. This vertical system in turn is surrounded by smaller canaliculi which carry smaller blood vessels. A bone marrow cavity when present and which (in a sense) is separated from the rest of the bone by an endosteum also contains a rich network of blood vessels and has connections with the bone.
4. Along the longitudinal system of blood vessels lie the osteons which can be called the individual units of a bone. The bone itself is usually divided into two types – compact or cortical and coarse or cancellous. The latter is less dense, its trabeculae are spaced apart as compared to the compact bone but in essence the two are not two distinct entities.
5. The formation as well as the mineralization of bone is the function of a line of cells called osteoblasts. Absorption and consequent demineralization is performed by osteoclasts which probably arise from a monocytic precursor cell related to the haemopoietic system and is therefore present in the bone marrow. The two types of cells act in conjunction when bone is remodeled but when growth alone is preponderant the osteoblasts are more active. When bone is being deposited, an active osteoblast might get trapped within its own bony cocoon and is called an osteocyte, though in the event that this its surrounding is damaged or gets reabsorbed it reverts back to its osteoblastic phase.
6. Fractures: Plastic surgeons frequently have to deal with fractures in the hand and cranio-facial area. They are also often involved in helping orthopaedic surgeons when fractures of the long bones are associated with loss of soft tissue cover.
7. When dealing with fractures which need to be fixed, it is now well established that the opposing fractured fragments must be in very good opposition with minimum interference with their blood supply. Whether the fixation should be very rigid or slightly elastic is at the present time disputed but the following points need to be emphasized:
a. Intramedullary fixation has the drawback of not being able to prevent a rotational movement. A nail in the form of a bouquet which opens after its introduction in the medullary cavity is a recent solution to the problem.
b. Surface plating overcomes this problem of rotational strain but plating may involve stripping or damage of the periosteum, a vital vascular structure which helps healing. A new type of plate, shaped like the arches of a bridge with multiple limited contacts to the bone is now replacing the conventional flat plate.
c. External fixation, using an apparatus with pins across the bone and plates to hold the pins and thereby the bones together, can be cumbersome and needs frequent adjustments as the opposing bone surfaces reabsorb, remodel and readapt to healing. This drawback has now been overcome by an ingenious mechanical advance which ensures a rigid fixation between the pins and the external plates, not allowing any changes at the fracture site.
d. Tension band wiring also is less traumatic but fixation may not be precise.
e. At all times the plastic surgeon must be conscious that most of the blood supply of a bone is extrinsic in origin and therefore a proper cover of the bone with soft tissue and skin must be achieved either prior to, during the time that the bone is fixed or soon afterwards.
f. An accurate and stable approximation of the bone ends, with minimum interference with the periosteum and adequate soft tissue cover will allow for both endosteal and periosteal healing and should lead to a primary union (a term not frequently used in healing of fractures but which should be the aim of treatment). R.K. Sharma from the post-graduate institute in Chandigarh adds “this point needs to be readdressed; in the recent past we do not necessarily aim at primary bone healing but rather secondary healing utilizing functional fixation particularly for Craniomaxillofacial fractures.” He also elaborates on this principle at the end of the chapter.
8. Secondary healing occurs when callus forms around the fracture site and is not uncommon when the fracture is communited and accurate approximation of the bone ends is not possible. Under these circumstances the least traumatic of the above treatment choices probably is an external pin fixation.
9. The treatment of fractures of large long bones in the rest of the body are not the province of the plastic surgeon. But as an ally of the orthopedic surgeon in the treatment of compound fractures it is the responsibility of the plastic surgeon to oversee the wound from the phase of excision of necrotic tissue, to confirm, in a couple of days that no new necrosis has occurred and then achieve wound closure by way of suturing, local flaps or distant two staged flaps. The last are now on their way out in most metropolitan plastic surgery centres in India and are now replaced by free flaps. It must however be stressed that if a free microvascular flap is not feasible and a local flap is not possible, if a fracture is stable or adequately fixed, a distant two stage flap will improve the local milieu considerably and favourably alter the prognosis for the union of the fracture. Good examples are an abdominal flap for a compound fracture of the forearm or a cross leg flap for the leg or the foot when net tissue deficit is present in the injured part.
10. Generally speaking bone grafting is resorted to when the osteogenic capacity of a bony defect is such that it will not allow the defect to heal on its own. A bone graft can be purely cancellous with a proportionately high osteoblastic activity and which revascularises easily or can be corticocancellous where support is needed and which is invariably fixed in one form or another to facilitate the revascularization of its cancellous surface for the survival of the whole graft which also forms connections with leashes of blood vessels to the periosteal layer from its surrounding tissue. Examples of where bone grafting helps are: a fracture with an irretrievable loss of bone, one where the opposite ends of the fractured bones are badly communited and are avulsed of their periosteal layer together with its soft tissue, when the fracture is pathological for example in tuberculosis or through a secondary malignant deposit where the original disease is being controlled. In addition there are areas in some bones which are notorious for a poor blood supply. For example, when treating a fracture of the lower 1/3 of the tibia a plastic surgeon should persuade his orthopedic colleague to perform a bone graft as he replaces the skin and soft tissue cover. Some fractures of the scaphoid frequently require bone grafting for the same reason.
11. In terms of vascularisation, a bone graft occupies a middle territory when compared to a cartilage graft at one end and a skin graft at the other end. While the former is somewhat impervious to its biological environment because its metabolism is anaerobic, a skin graft’s survival depends on its quick revascularization and is therefore very fragile when it comes to its take. Bone grafts with their mineral content are comparatively solid and when placed with their cancellous surface facing a rich vascular network will survive for days prior to beginning their osteoblastic activity. The periosteo-cortical component of such a graft gives it a certain stability which allows the cancellous part on its surface to vascularise from its bed. In the end the subperiosteal vessels themselves also get linked to the adjoining area as the process of consolidation can begin.
12. In the same way that immobility is crucial to the take of a skin graft or a cartilage graft, a bone graft too must be fixed firmly for it to thrive. Any movement will jeopardize its revascularization. Rigid fixation is possible if the bone graft is of significant size. If the defect is of a smaller dimension then the two opposite ends of the bones which are being treated can be properly spaced and fixed and the space between them can be closely packed with cancellous bone grafts before skin cover is achieved without leaving any dead space, ensuring fixity.
13. How a free bone graft vascularises, lives, contributes its osteogenic activity, creates bone and then remodels itself to become one with the host site is not yet fully known. Whether the original bone graft survives by demineralising itself to gain neovascularity for its osteoblasts and then remodels itself or if on the other hand, demineralization is a process to leave behind only a scaffold into which osteoblasts migrate to form new bone has not been fully understood. Suffice to say that in the absence of infection and with good conditions in the recipient bed, a bone graft will survive and then remodel itself in a manner that will exactly resemble the bony architecture of the treated part. Split rib grafts, when properly fixed over a large defect in the skull with good cover, heal so as to become almost identical to original skull bone. However, with the advent of power driven cutting tools, and in a reasonable sized defect, an outer cortical bone graft of the exact required dimensions from another part of the skull, with its cancellous surface facing the dura, is now an excellent option.
14. In fractures of the rest of the maxillofacial skeleton bone grafting is generally not undertaken (for example the maxilla or the zygomatic bone) because of the abundance of blood supply and the rarity with which large areas of skin necrose, though a school advises that cancellous bone chips are useful when fractures of these bones are fixed either with plates or wires. In unfavourable fractures of the mandible, where the line of fracture gets distracted with movement and in other parts of the mandible where non union can occur because of intrusion of soft tissues, or when a significant amount of bone is lost, cortico-cancellous bone grafts are used and the fractures are fixed with a fracture plate, which is larger than the mini plate which suffices for standard fractures. In realigning the ununited mandibular fracture, care is taken to achieve occlusion of teeth by fashioning models and then splints to achieve accurate opposition. Bone is also excised during surgery for congenital defects or excision of tumours. In some congenital defects the gap so created is left alone because the aim is to reduce the intracranial pressure caused by unnatural, early fusion of sutures. In other situations a properly fixed cortico-cancellous bone graft is used. In situations involving big tumours where major portions of the bone are excised, a free microvascular skin muscle bone transfer is ideal and because such major surgery is done in a large centre, such a procedure can usually be undertaken. However, large blocks of bone in such circumstances transferred without a proper vascular attachment either in the flap itself or with a microvascular anastomosis will shrink or get extensively absorbed, for e.g. sizable section of a rib to a mandibular defect will disappear in a vast majority of cases.
15. Bone grafting to the bridge of the nose for a saddle deformity is usually cortico cancellous. (rib, iliac crest or calvarial) and is placed after raising the periosteum over the nasal bone (blindly) and placing the cancellous surface over the denuded nasal bone. The bone is fixed either by an external plaster or can also be fixed with a screw to the frontal bone. The same is true for the contour defects of the maxilla or the zygoma. Reconstruction of the “blow out” orbital fracture with a slice of thin cortico cancellous bone is one of the most gratifying uses of bone grafting in maxillofacial surgery. Except in the floor of the orbit, in all bone graftings to the facial skeleton for contour defects, some over correction is needed to allow reabsorption of up to 15%.
16. A variety of non biological commercially available materials have been in use for contour defects over the last two decades. They will be narrated in a separate chapter later as a part of these short notes.
17. Mostly through evidence gained from bones of animals in a laboratory setting, a substance known as bone morphological protein has shown great promise in hastening the healing of iatrogenically created bone defects. This model is difficult to create in a human being because of ethical considerations. Also the kind of bone defects that are encountered in clinical practice with or without infection or contamination cannot be replicated in a laboratory. Be that as may, BMP now appears to be popular (at least empirically) for healing in bones and has been used singly or together with hydroxy-apatite paste in bone defects. The story of BMP is still evolving and its cost might be prohibitive for average Indians in a charitable hospital at the present time.
18. Stem cells are also now on the horizon and here the future might indeed be very exciting because with these cells the key to growth, healing, as well as neogenesis might be at hand.
Comment by Dr. Rameshkumar Sharma (Chandigarh)
There are two ways by which fractures heal: primary (direct) fracture healing and secondary (indirect) fracture healing. Primary fracture healing occurs without intermediate callous formation. Both cancellous and cortical bone will heal primarily when rigid fixation assures minimal interfragmental movement. Unlike secondary healing, primary fracture healing does not have distinct remedial stages. Secondary fracture healing commonly occurs when severe injury or comminuted high-energy fractures limit fixation. Secondary fracture healing can be divided into four stages: (1) inflammatory phase with hematoma formation; (2) proliferation phase with soft callous formation; (3) modeling phase with hard callous formation; and (4) remodeling phase with sustained bone turnover. Since both primary and secondary healing can result in successful fracture repair, it should be emphasized that primary fracture healing is not equated with early healing and secondary healing is not a delayed healing and is not necessarily inferior in quality.
The plastic surgeons are generally called upon to manage maxillofacial trauma. In such patients, the rigid fixation is promoted by the AO/ASIF group. In this concept, compression, tension, torsion, and shearing forces, which develop under functional loading, are neutralized by thick solid plates fixed along the lower border of the mandible by bicortical screws. In contrast, the Champy’s method of semi rigid fixation uses one easily bendable monocortical miniplate along an ideal osteosynthesis line; the developing forces are neutralized by masticatory forces that produce a natural strain of compression along the inferior border of the mandible. The dimensions of the plates can be kept small as the miniplates only have to cope with tensile stress. An ideal line for osteosynthesis has been found by experimental work of Champy — a line of maximum tensile stress running from the oblique line along the base of the alveolar ridge to the mental foramen. Here one single miniplate should be sufficient. Additional torque in the region between these foramina required a second (more basal) plate. As discussed earlier, the current principles of management of Craniomaxillofacial fractures employ semi-rigid fixation and therefore result in secondary healing, This allows some “manipulations” for “fine tuning” the occlusion.
September 30, 2011
8. Bone and bone grafts