Methods
The study was approved by the local ethics committee at the Hannover Medical School, Germany (EK/2009).
Experimental Set-up
The firing tests were performed with bullets of different calibers, i.e. .22-caliber Magnum bullets, .222-caliber Winchester bullets and 6.5 × 57 mm bullets (all RWS, Rottweil, Germany). All bullets were full metal jacketed. The projectiles had different weights, but comparable similar initial velocities (v0 945-970 m/s).
The experimental set-up used a firing apparatus with a test chamber and a rifle support. Before each shot, 5.0 g of barium titanate (Aldrich, Steinheim, Germany) with a particle size of 3-6 μm were placed inside the chamber. Three air pressure valves were attached to the chamber (at the bottom and on the left and right sides) and connected to an air compressor. The porcine hind limbs and the gelatin blocks were mounted on a support frame that was positioned directly behind the chamber in such a way that the gelatin blocks and hind limbs could be placed in the line of fire.
Specimens
The gelatin blocks that were used in our experiments consisted of 20% porcine gelatin (Merck, Darmstadt, Germany) and water. The size of the blocks was 120 mm by 120 mm by 180 mm. The hind limbs were obtained from freshly slaughtered pigs with a total weight of approximately 65 kg. They were separated from the body in the median plane in order to facilitate their handling during the tests.
The experimental set-up described above was used to fire .22-caliber Magnum bullets into gelatin blocks (n = 6) and hind limbs (n = 6). The other two types of bullets (.222-caliber Winchester and 6.57 × 57) were fired only into porcine hind limbs (n = 6 each).
Procedure
One shot was fired into each gelatin block and hind limb. At the time of firing, the gelatin blocks had a temperature ranging between +8°C and +10°C. They were mounted on the support frame in such a way that the bullet passed right through the block. Before each shot, 5.0 g of barium titanate were placed inside the test chamber. An air-permeable filter paper was inserted in the chamber to separate the interior of the chamber from the barrel of the rifle. A gelatin block was placed directly against the opposite side of the chamber. Shortly before a shot was fired, the compressor was used to expose the chamber to a blast of compressed air (0.5 bar) and to generate a cloud of barium titanate particles inside the chamber. A shot was then fired from the rifle that was mounted on the rifle support.
Barium titanate was again placed inside the chamber and the porcine hind limbs were positioned against the chamber in such a way that they tightly sealed the chamber. The chamber was again exposed to a blast of compressed air and a shot was fired from each of the three rifles into the hind limbs.
After each shot, 16-slice computed tomography (CT) scans (GE Medical Systems, Lightspeed, United States) of the gelatin blocks and hind limbs were obtained. All scans were performed at 120 kV and 200 mA.
Data Evaluation
The data obtained were made available in a digital format (DICOM) and transferred to a personal computer for further analysis. Voxim software (Voxim, IVS Solution, Germany) was used for statistical analysis. For this purpose, the gelatin blocks were virtually cut into 20-mm-thick vertical slices and evaluated. Once the center of the gelatin block was identified, the mean diameter of the permanent cavity was calculated. The distances from eight sites on the wall of the permanent cavity to the center of the block were measured radially and the results were averaged for each vertical slice.
Likewise, the distances from eight sites on the wall of the temporary cavity to the center of the block were measured radially and the results were averaged for each vertical slice. In order to determine the depth of infiltration in the gelatin blocks, the distances between barium titanate particles detected inside the temporary wound cavity and the center of the gelatin block at eight sites was measured. The results were averaged for each vertical slice.
Similarly, the permanent and temporary cavities and the distribution of barium titanate particles inside the temporary wound cavity were assessed in the hind limbs. For this purpose, the path of the bullet was determined precisely on the basis of radiological parameters such as the presence of gas and the entrance and exit wounds. Two-cm-thick CT slices were obtained perpendicular to the bullet path and analyzed using the aforementioned parameters.
Statistical Analysis
Results are expressed as means ± SEM. Differences between groups were evaluated with an analysis of variance (ANOVA) on ranks. Differences within groups were analyzed by one-way repeated measures ANOVA. Student-Newman-Keuls or Dunn's post-hoc tests were used to isolate specific differences. A p-value < 0.05 was considered significant.