Objective　To make clear the range of firearm wound in the maxillofacial region, the optical repair time and the characteristics of accompanied indirect brain damage, and to offer the principle of emergency treatment and the early repair of war wound.
　　Methods　With the aid of the standard Sweden model, 200 dogs were used in the experiment. Varies tissues around the primary canal were harvested chronologically, in different zone and different tissue, for histopathological examination.
　　Results　The necrotic range of various tissues in the maxillofacial region was less than that in the extremities. In the maxillofacial region, there was a significant temporary cavity following the passing of bullet, which caused indirect brain damages.
　　Conclusion　These findings are helpful to the treatment of war wound in the maxillofacial region. Early bone transplantation using microvascular anastomosis in the treatment of gunshot wound in the maxillofacial region is recommendable.

　　Firearm wound in maxillofacial region is one of the major injuries in modern war. The proportion, according to the statistics in the recent regional wars, has climbed over 10%. Since the maxillofacial region is close to craniocerebral region, the emergency treatment and early repair of its war injuries are so complicated that it is urged to be investigated intensively. Since 1987, a series of experimental studies in our department have been carried out for the early repair of wounds and to raise the success rate of the treatment.

METHODS

　　Two hundred and one mongrels, 2-year-old, were used in the experiment. With the aid of the standard Sweden model, the middle points of the mandibular angle of the dogs were shot 6 meters away with a sphere which was 1.03 g in weight, 5.56 mm in diameter and 1300-1500 m/s of muzzle velocity. The impact velocity of the sphere was recorded with a laser velometer (Model TC-150, Shanghai). The energy and it's exhausting rate were calculated by a computer (IBM compatible 386-66, Beijing) at the same time. The temporary changes in tissues were taken by pause X-ray camera of 1/1000 second. This model was easy to be duplicated. Varies tissues around the primary canal were harvested chronologically, in different zone and different tissue, for histopathological examination.

RESULTS AND DISCUSSION

Mass observation and temporary cavitation
　　In the injuries in the maxillofacial region induced by high velocity sphere, the injury entrance was larger in area and the tissue was hurt more severely than that in the exit side. The sphere is so small in mass and with such high velocity that when it comes bounce against the tissue and meets huge resistance, the velocity slows down sharply and most energy is delivered and consumed at the entrance. That is so-called splash effect.

　　It was also found in the experiment that there was temporary cavity existing indeed in the maxillofacial region.1 Since the local tissues absorbed the energy and were accelerated violently forward and outward, the diameter of temporary cavity appears more than ten times of the sphere, and the cavity expand far from the primary wound canal (Fig. 1). It is the momentum that causes the tissues to move continuously after the passage of the sphere.

Histopathological and histochemical observation
　　By means of histological, transmission electron microscope (TEM), blood biochemical examinations, etc, it was found that necrotic area of skin and oral mucosa was no more

Fig. 1. Microsecond radiograph of the maxillofacial region of an anesthetized dog, showing a temporary cavity caused by a sphere bullet traveling.than 2 mm from the canal edge. The necrotic area of muscle could be up to 8 mm. Due to its high content of fluid, the muscle was affected more seriously than any other tissues. Within the 9 mm to 16 mm away from the primary canal, the survival and necrotic muscles were interwined together (Fig. 2).2　Though the content of adenosine triphosphatase (ATPase) in the muscles decreased quickly immediately after injury, it showed a trend of recovery after primary debridement, suggesting that they might have survived the shot.
　　All the small blood vessels in and around the temporary cavity were pressed, distorted, and displaced. The main changes in the blood vessels around the primary canal included microthrombus, peeling of intima, break of inner-elastic lamina, infiltration of white blood cell (Fig. 3), etc. Such changes could be seen 3 cm away from the canal.3,4

Fig. 2. Muscle patch necrosis 1 cm from the canal 2 hours after wound (HE, original magnification ×40).
Fig. 3. Inner-elastic lamina broken 1 cm away from the canal 3 days after wound (HE, original magnification ×40).

　　When temporary cavity came into being, the nerves around the canal were pushed away and concussed, displaced about 2 cm and elongated about 18% in length. Nerve damages included swelling of myelin sheath of nerve trunk (Fig. 4), sub-epineurial bleeding, twist or break of part of neural fibers, obscured lamina structure, and degeneration of mitochondria. The amplitude of invoked action potential decreased, the latency elongated and the nerve conduction velocity slowed down. Fortunately, these dysfunctional changes due to indirect injuries could gradually return to normal along with the time after debridement and reduction of pressure.5 It was demonstrated that the debridement and reducing pressure were favorable to the recovery of blood circulation and nerve function (Fig. 5).

　　For the gunshot fracture of the mandible in dogs, bony necrosis appeared 3 to 7 days after injury because the pathological changes developed slowly. The scope of bony necrosis was about 5 mm away from the fractured end. Though the

Fig. 4. The parts of facial nerve fiber broken 1 cm away from the canal 3 days after wound.

energy absorption in the bone was much more than that in the soft tissue, the quantity to be trimmed for the mandible was more than that for the skin, while less than that for the muscle (Fig. 6).

Fig. 5. The changes of amplitude of invoked action potential of facial nerve before (left panel) and after (right panel) wound.

Fig. 6. The complete bone necrosis 0.5 cm away from the canal 7 days after wound (HE, original magnification ×40).
　　As the skull and brain adjoin to the maxillofacial region, the traumatic tension generated by gunshot could be conducted directly to the base of skull. The brain in the skull was abruptly vibrated due to the acceleration effect on the head, resulting in extensive brain contusion. Moreover, hematoma could also happen. The indirect injuries occurred in the brain stem with mandibular fracture, especially comminuted fracture on the ramus. Six of 11 animals suffered this kind of damage. The main sign was subdural hematoma or hemorrhage, especially at the prone side. It was suggested that there was a close relationship between maxillofacial injury and some unreasonable death in clinic. Therefore, doctors should pay much attention to this kind of complicated wound when serious maxillofacial gunshot wound occurs.

　　Isoenzyme of the cerebral myokinase (CK-BB) in the cerebrospinal fluid and lipid peroxide (LPO) in the blood and tissues were also evaluated before and after grave injuries in the maxillofacial region of the dogs. It was found that CK-BB? and LPO increased significantly with brain contusion and bleeding at the base of skull. Test of CK-BB and LPO could be helpful for the diagnosis of coincided brain damage.6,7

　　In the indirect visceral wounds in the chest and abdomen by shock wave resulting from high speed sphere in the maxillofacial region, point or patch hemorrhages were also found in the lung in 42.1% and in the heart in 31.6% dogs, though these damages were not so severe as to threaten the life. However, this possible damage should not be neglected when a simple maxillofacial gunshot wound is complicated with severe symptoms, such as shock, adult respiratory distress syndrome (ARDS), dissensinated intravascular coagulation (DIC), multi-organ failure and inflammation.8

Primary reconstruction
　　The authors tested the practicability to repair the small vessels primarily around the permanent wound canal. 30 dogs were divided into 3 groups, i.e. the group undergoing immediate repair after injury, the groups undergoing repair 3 days and 7 days after injury. By immediate repair of the arteries and veins, the patent rates were compared among the three groups. The results showed that the vessels 3 cm away from the primary canal could be used 3 days after injury. The patent rate could reach 90% in 7 days' group which was superior to any other groups. It would be better, as the experiment suggested, to use the vessels 3 cm away from the primary canal and be best to perform the primary repair of the firearm wound defect 7days after injury.

　　Furthermore, for the primary reconstruction, the bullets of 5.56 mm or 7.62 mm in diameter and 800 m/s in muzzle velocity were used to shot through the bucca of the dogs. Redebridement was carried out 72 hours after primary debridement. The inner wound was closed either hooking in (in 5 dogs) or with sliding flap (in 3 dogs). Then a saphenal flap was harvested to repair the outside defect of the cheek by microvascular anastomosis. There were 6 flaps survived and 2 failed. It was proved that early repair for maxillofacial war wound could be practicable if only the primary canal was properly treated.

　　For bone reconstruction, routine debridement was carried out 6 hours later, which included trimming 5 mm tissue at the edge of the broken bone, especially in the lingual side, clearing the fragments and blot, and closing the wound open to the oral cavity. One week later after primary debridement, the wound was redebrided to the normal bone and soft tissue, and washed down for repairing. Twenty one dogs in the experimental group were repaired by microvascular anastomosed bone transplantation combined with implants, and 18 dogs in the control group were repaired with iliac bone grafts. The results showed that in the experimental group, 19 cases undergoing transplantation got a total success and 2 failed, while 15 cases in the control group which accepted grafts failed because of inflammation. The success rates were 90% and 17% in the experimental and control group, respectively. The former was superior to the latter. The appreciation by X-ray, angiography, tetracycline fluorescence and histologic examination got the same conclusion. It was proved that early bone transplantation with microvascular anastomosis in the treatment of gunshot wound was recommendable.9

CONCLUSION

　　According to our experiments, the following conclusion can be reached. 1) In maxillofacial region, there is also a temporary cavity in a twinkling after high energy bullet wound as in the other parts of the body. The pressure wave generated by the missile expands very fast and extrudes, concusses and tears tissues around the track canal, which makes the injury more serious and complicated. The negative pressure can cause severe infection; 2) The injury severity of maxillofacial wound is quite different from the injury in the other parts of the body. Suggested range of debridement should be less than 5 mm away from the primary canal for the vessels, bone and muscle, and less than 2 mm for the skin and mucosa. The injury severity of tissues can be reduced by debridement; the early, the better. Because early debridement can improve local circulation of tissues and prepare a better recipient bed for the early reconstruction; 3) Coincided craniocerebral indirect injury has been confirmed when the maxillofacial region is struck by high velocity projectors, although the incidence is lower than that of chest and lung wound; 4) Early reconstruction should start at the 7th day after primary debridement. The routine procedure should be: a) to debride as soon as possible after injury; b) to fix the fracture; c) to close the wound, especially the wound open to the oral cavity; d) to redebride 7 days after primary debridement; e) to perform one-stage reconstruction by vascularized tissue transplantation. The site for vascular anastomosis should be 3 cm away from the primary canal to prevent thrombosis.

REFERENCES