Ballistic Study Of High-Speed Fragments And Its Biomechanical Mechanism Of Temporomandibular Joint Injury

High-speed and ultra-high-speed fragments produced by high-energy explosive weapons have become the most important cause of war injury.Oral and maxillofacial region has long been a high incidence area for fragment injury due to its special anatomical location and difficult to effectively protect.The main types of fragments produced by modern warfare are high-speed,high-energy and small-quality prefabricated fragments.The mechanism of the injury mainly includes direct injury,pressure wave and instantaneous cavity effect.The two major factors that affect the ability of fragmentation injury are the kinetic energy and the energy transfer efficiency of the fragments.Among them,the main influencing factor of the former is the flying speed of the fragments,and the main influencing factor of the latter is the shape factor of the fragments.Temporomandibular joint is the only movable joint in the maxillofacial region,maintain the mandibular movement,chewing,swallowing,speaking and other important functions.When the mandible impacted by external forces,can lead to condylar fractures,joint fissure,joint disc perforation,joint capsule tear,ligament rupture,synovial tear and other injuries,subsequently followed by TMD,osteoarthritis,joint rigidity,and even cranial fossa fracture,seriously impact on the oral and maxillofacial function and the facial appearance.As the mandible as an entire,both sides of the TMJ were connected rigidly,once the mandible was impacted,additionally to direct combat side of the joints will be injured,the contralateral joints can also occur indirect injury.In this trail,the most advanced two-stage light gas gun was used as experimental loading platform,and the mechanism of organism damage contributed by such fragments was studied at the two levels of ballistics and the biomechanical effect of TMJ injury.And provide theoretical support for the study and clinical treatment of high-speed fragment injury in the maxillofacial region.Method:In the part of ballistics study,we selected the customized soap blocks as target material,and used the two-stage light gas gun to transmit the cylindrical fragments to carry out the orthogonal impact experiment.According to the purpose of the experiment,respectively,set the flight speed and the length to diameter ratio of the fragment as the controlling factors.In experiment 1,the length to diameter ratio of all fragments was 1,and the flying speed was taken as the method of group.In the experiment two,the flying speed of all the fragments was 1100m/s,and the length to diameter ratio was used as the basis.After loading,the ballistic inlet diameter,the maximum diameter,the depth of the trajectory and the volume of the cavity formed by the fragment in the target were measured.In the part of biomechanical research part of this experiment,the experimental condition was determined based on the result of the experiment one and experiment two,subsequently used the two light light gun loaded steel cylindrical fragments on the right mandibular angle of fresh pig cadaver heads.Three acceleration sensors were arranged in the lower part of the mandibular angle and the middle of the forehead.Two strain gauges were arranged in the bilateral condylar neck.The sensors and gauges were connected to the oscilloscope via a wire to record the corresponding values of acceleration and strain during the impact.Result:Experiment 1: The ballistic shape of the fragments with different velocities under the length-diameter ratio of 1:1 was similar,which the inlet diameter was large and gradually reduced,and the maximum diameter of the trajectory appeared at the entrance or near the entrance.The greater the velocity was,the greater differences between the diameter,depth,and the volume of the ballistic trajectory,.Although the fragments with different velocities formed trajectory of the cavity with large diameter differences in their initial section,in the latter part of the narrowing was not obvious,and at the end of the trajectory was almost the same with the increase of the fragment velocity,the inlet diameter,the maximum diameter and the ballistic volume of the trajectory were increased consequently,while the difference of the ballistic depth between the medium speed group and the high speed group was quite small.Experiment 2: The ballistic trajectory of different length diameter ratios cylindrical fragments were different,comparing with the ballistic entrance of length diameter ratio 1 fragments,the trajectory of the small length diameter ratio fragment was larger and the constriction is more obvious,and the ballistic entrance of the large length diameter ratio fragment were smaller,and it became narrow after the first stage of small scale expansion.The fragment with a length to diameter ratio of 1 is in a cone shape,which is the largest at the entrance,and then narrowed uniformly,to the end of a long tail;the fragment with a length diameter ratio 2 experienced a short period of expansion at the initial stage of penetration,and then began to shrink,the end of which was similar to the fragment with a length diameter ratio of 1;the fragments with a length diameter ratio of 0.5 was rapidly narrowed at the initial stage,and then continues to penetrate in a short distance.With the fragment length diameter ratio increases,the trajectory of entrance diameter maximal diameter decreases,ballistic depth increased,however,the difference of the ballistic depth between fragments with large length diameter ratio and small length diameter ratio was relatively small;the variation of ballistic cavity volume with the length diameter ratio was not obvious,and the volume of the ballistic cavity of the middle length diameter ratio group was smaller than that of the other two groups.Experiment 3: The lateral force of the direct impacted side of all groups was significantly greater than that of the contralateral side;the difference of the lateral acceleration of the TMJ is greater than that of the contralateral side,and the lateral damage of the TMJ is more than that of the contralateral side;with the increase of the fragment velocity,the ratio of the lateral acceleration differences between the impacted side and the contralateral side increased correspondingly,so,the higher velocity fragment is more effective than the contralateral side;with the increase of fragment velocity,the value of the acceleration difference between the two sides of condyle increases correspondingly,so the higher the fragment velocity is,the more obvious the lateral stress is in the side to side propagation.Experiment 4: The axial force of the impacted side’s condylar neck was about 2~3 times of that of the contralateral side,and the ratio was less than the ratio of the lateral force;relative to the lateral force,the axial force on the attenuation of condylar neck is only less than 50%;it is shown that the axial compressive damage of theTMJ and the strain value of the impact force of the articular disc and glenoid fossa are larger than that of the contralateral side;therefore,the risk and severity of the fracture of the impacted side’s TMJ,the contusion of the articular disc and the fracture of the middle cranial fossa were greater than those of the contralateral side;with the increase of the fragment velocity,the strain value of the bilateral condylar neck increases correspondingly,so that the higher velocity fragment can aggravate the injury of bilateral TMJ;with the increase of the fragment velocity,the increase of the impacted side’s strain value is greater than that of the projectile side,therefore,the higher the fragment velocity,the greater the risk of the occurrence of TMJ injury on the contralateral side is greater than that of the projectile side..Conclusion:1.Two-stage light gas gun as the maxillofacial high-speed fragment injury experimental loading platform has good reliability and stability,combined with the auxiliary measuring equipment can be used as a standardized experimental platform in the battle-related injury research;2.The fragment velocity has a greater impact on the inlet diameter and ballistic volume of the formed trajectory,and has little effect on the ballistic depth.The fragment of the higher velocity fragment is wider,the volume of the cavity is larger,and the constriction of the trajectory is faster.The end of all trajectories is similar;3.The length diameter ratio of the fragment has a greater influence on the inlet diameter and the depth of the ballistic trajectory.The smaller the diameter ratio is,the wider the inlet diameter,the shallower the trajectory and the faster constriction of the trajectory is.The maximum trajectory diameter of the large length-diameter ratio fragment appears a certain distance below the entrance;4.The horizontal stress on the impact side of the temporomandibular joint is significantly greater than that of the contralateral side,and the gap between them was increased with the increase of fragment velocity.5.The axial compressive stress of the temporomandibular joint was 2 times that of the contralateral side,but the difference decreased with the increase of fragment velocity.