Effects of a low-velocity fluvial environment on saw mark evidence in bone.

Saws are common tools used in postmortem dismemberment to alter human remains. Dismemberment may coincide with other concealment methods such as disposal in aquatic environments, so forensic anthropologists must recognize how taphonomy affects saw mark preservation. This study focuses on exposure of saw cut bone to a fluvial environment. Cross sections from pig humeri cut by a 7 TPI saw were evaluated on all cut surfaces for tooth hop (TH), exit chipping, and breakaway spurs. Pre-fluvial exposure, features were measured by two observers using a stereomicroscope while a one-dimensional profilometer was used to define surface roughness. Bones were randomly assigned to control or experimental groups and within each divided by duration of exposure (24, 48, 72, and 96 h). Control groups (four cross sections) were exposed to water in a 3-meter-long hydraulic channel; experimental groups (20 cross sections) were exposed to water and sediment in a 12-meter-long sediment-recirculating hydraulic channel. Velocity was maintained at 0.45 m/s. All measurements were repeated post-exposure. Observer A recorded 268 TH (mean: 3.70 ± 0.34 mm); post-exposure, count increased by 16.79% (mean: 3.71 ± 0.29 mm). Observer B recorded 247 TH (mean: 3.46 ± 0.42 mm); post-exposure, count decreased by 29.15% (mean: 3.36 ± 0.33 mm). All TPI estimates calculated include the 7 TPI saw. Exit chipping lost flakiness (in both channels), but was still visible. Breakaway spurs were not visibly altered. Surface staining was limited to bones exposed to sediment. Roughness was significantly lower in experimental groups post-fluvial exposure (W = 5705, p < 0.05). Overall, this fluvial environment did not obliterate saw mark evidence from bone.

Tooth hop variability in human and nonhuman bone: Effect on the estimation of saw blade TPI.

Forensic research has demonstrated that tooth hop (TH) is a valuable measurement from saw-cut bones as it can be used to estimate teeth-per-inch (TPI) of a saw used in postmortem dismemberment cases. However, error rates for TPI estimation are still under development and knowledge of how bone tissue affects TH measurements remains unclear. The purpose of this research was to investigate the effects of tissue variability through the use of different taxa on the accuracy and precision of TH measurements in the bone to estimate TPI of the blade. A total of 1766 TH measurements were analyzed from human, pig, and deer long bones cut by two 7 TPI saw blades of different tooth type. Fifty distance-between-teeth measurements before and after sawing were collected directly from each blade for comparison to bone-measured TH to assess potential effects of tooth wear on TH variability. ANOVA and F tests were used to compare mean TH and variance, respectively, by saw-species (i.e., crosscut-deer, rip-deer) and species groups (i.e., all deer, all pig), with significance determined at the p < 0.05 level. TH measurements were converted to usable TPI ranges, which would typically be presented in a forensic report. It is concluded that significant differences in TH (mm) do not necessarily reflect significant differences in associated TPI ranges of suspect blades. Forensic reports should report mean TPI ± 1.5-2.5 TPI while providing a sample size indicating number of TH measured rather than just number of cuts or cut surfaces examined.