INDIRECT FRACTURES TO BONES BY BALLISTIC INJURY

Abstract

INTRODUCTION Trauma due to gunshot wounds is a significant cause of morbidity with 115,000 missile injuries annually in the United States [1]. The study of wound ballistics provides the ability to identify wounding mechanisms and patterns. Two distinct wound cavities have been identified with the passage of a bullet through soft tissues. A temporary cavity which represents the cavitation as the bullet passes through the tissues. After the pulsations have subsided, a permanent cavity remains that is representative of the tissue that is permanently damaged. The size of the permanent cavity has been correlated to the extent of energy transfer [2]. Studies conducted to characterize fractures produced without direct contact by the bullets, commonly known as indirect bone fractures, have provided conflicting results. Two mechanisms of injury have been postulated for long bones. The first theory suggests that fracture occurs after the passage of a projectile, caused in part by the temporary cavity [3]. The second theory proposes that the pressure wave that precedes the bullet in tissue causes the fracture, not the expansion of the temporary cavity [4]. The goal of the current research is to use strain gage technology, in conjunction with high speed video analysis, to establish the temporal relationship between the fracture of the bone and passing of the bullet. It is hypothesized that the bone fractures due to the temporary cavity and not due to a shock wave that precedes the bullet. METHODS Prior to commencement of the study, approval was garnered from Wayne State University’s Human Investigation Committee. A total of three post-mortem human specimens were used for the study. Femurs and tibias were removed from the specimens. A total of six bones were used; four femurs and two tibias. After all soft tissue was removed, strain gage rosettes 031RB (Vishay Micro-Measurements, Shelton, CT) were attached to the center of the bone using M-Bond 200 (Vishay Micro-Measurements, Shelton, CT). 10% Ordinance gelatin was mixed and poured into three cylindrical molds, measuring 5.7 inches in diameter and 7.0 inches tall, and three rectangular molds, measuring 5x5x10 inches. The instrumented bones were then implanted into molds. Two different rounds were tested: 9 mm and 5.56 mm. Two 9 mm rounds were fired at a velocity of about 900 feet per second (fps) at both a rectangular and cylindrical block. A total of four 5.56 mm rounds were fired at approximately 3,000 fps; two at cylindrical blocks and two rectangular blocks. All rounds were targeted at approximately 9 mm from the edge of the bone. High speed video of the impact event was collected at 20,000 frames per second using a MotionXtraTM HG-100K camera (Redlake, Tucson, AZ). Video synchronized strain data was collected using TDAS Pro (Diversified Technologies, Seal Beach, CA) at 20 kHz. RESULTS AND DISCUSSION The blocks impacted with the lower velocity 9 mm did not result in fracture of the bones. However, those impacted with the higher velocity, 5.56 mm round produced fracture in all four tests. The exact time of fracture was determined by analyzing the strain output. As depicted in Figure 1, the time of fracture is indicated by the point where the strain data show an abrupt change. This is in contrast to the testing with the 9 mm round where the strain data demonstrates the pulsations of the temporary cavity. Figure 1 clearly shows the time of fracture occurred during the expansion of the temporary cavity.