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Application of Bone Fractography to a Medical Examiner Sample A Case Series.

Anthropologists evaluate bone fracture patterns to assess trauma type as well as other aspects of the traumatic event, such as force direction, magnitude, and timing (Cohen et al. 2016, 2017; Hart 2005; Isa et al. 2017; Reber & Simmons 2015). Traditionally, anthropologists have focused on bone fragment shape and overall fracture pattern to interpret skeletal trauma (i.e., fracture pattern analysis). Recently, however, the science of fractography was shown to possibly help anthropologists interpret skeletal fractures. Fractography is the study of fracture surface features and their relationship to crack propagation. Fracture surface features of a brittle material can reveal the location of crack initiation and direction of crack propagation (Hull 1999). Similar features have been observed in bone fracture surfaces, and a recent study evaluated the diagnostic value and expression variation of these features (Christensen et al. 2018).

Prior to reviewing recent fractography studies of bone trauma, a brief overview of bone biomechanics is warranted. Force is defined as an external load, and stress as the internal resistance to the force. When a straight object, such as a long bone shaft, is loaded with a bending force, the bone simultaneously undergoes tensile stress (experienced on the convex surface) and compressive stress (experienced on the concave surface). At the interface between the tensile stress and compressive stress is the neutral axis (Wulpi 2000). Under bending forces, bone typically first fails at the point of greatest tension. The fracture travels through the bone until it reaches the neutral axis. At this point the bone fails under shear stress (a load causing deformation along an angle to the imposed stress) until reaching the point of greatest compression (Harkess et al. 1975). This bone failure often presents as a partial transverse fracture that bifurcates into two angled fractures creating a triangular wedge of bone (see case 2, below) (Carter & Hayes 1977; Cohen et al. 2016, 2017; Currey 1984; DeLand 2013; Harkess et al. 1975; Isa et al. 2017; Reber & Simmons 2015). This fracture pattern is often referred to as a "butterfly fracture" or "wedge fracture." The apex of the wedge segment is positioned toward the side of the bone that experienced tension, and impact direction can be inferred based on the location and orientation of the wedge.

Christensen et aL (2018) recently applied fractography to skeletal fracture surfaces and identified several fractographic features (Table 1) that indicated the site of fracture initiation and direction of fracture propagation. In that study, twelve human femora that were experimentally fractured using controlled three-point bending were examined. The location of impact was the posterior surface for six femora and the anterior surface for six femora, but the impact location was not known to the assessors at the time of the analysis. The assessors consisted of three forensic anthropologists who are experts in human skeletal analysis but who had no previous experience with fractography, and four forensic fractographers who are experts in fractographic analysis of materials in forensic contexts such as glass, ceramic, metal, and plastic but with no previous experience examining bone. The assessors independently examined each fracture surface and documented fracture surface features. Based on the presence and orientation of the fractographic features, there was 100% agreement betv,reen all assessors for all specimens on the direction of crack propagation, which correctly corresponded to the known impact locations. Despite the correspondence between the direction of crack propagation and known direction of impact, the researchers caution that the two should not be conflated; direction of fracture propagation and direction of impact (external load) are two separate components of bone failure and may not be the same. Further, the researchers found that features may be more readily identified on bones with greater cortical area, and by assessors with more experience. Ultimately, they concluded that fractography is a reliable, inexpensive, and user-friendly method for assessing crack propagation in bone.

To date, bone fractography studies have been based on fractures created under controlled experimental conditions, with unidirectional force application, and utilizing bones that produce maximum fracture surface area. In anthropological casework, fractures are created under far more complex conditions and involve bones and bone areas other than those with the greatest cortical area. For example, when a pedestrian is struck by a vehicle, the impact force is largely unidirectional, but the bones are also experiencing axial loading from standing and possible rotational force from twisting toward or away from the vehicle. These complexities may make the application of fractography to forensic anthropological casework more challenging.

This case review assesses the applicability of forensic fractography of bone to forensic casework through the evaluation of surface features of four blunt-trauma cases received by the District of Columbia Office of the Chief Medical Examiner (OCME). The objectives of the case review were to determine (1) whether fractographic features are readily identifiable in more complex trauma scenarios, and (2) whether the fractographic analysis corresponds to the traditional anthropological fracture pattern analysis. The 0Cfv1E serves an urban population of approximately 650,000 individuals and autopsies more than 700 cases a year. Approximately 25% of the deaths examined by OCME result from blunt trauma. The office employs a full-time forensic anthropologist, and the forensic pathologists regularly request anthropological fracture analyses in cases of blunt trauma. Typically, the bones under question are removed during autopsy, chemically processed to remove soft tissue, reconstructed, and examined grossly and microscopically. Trauma interpretation is based on fracture pattern analysis, soft-tissue injury patterns, and radiographs taken prior to autopsy. After analysis, the bones are archived. The archived bone segments, autopsy reports, autopsy photographs, and radiographs from several blunt-trauma cases affecting limb bones served as ideal specimens to evaluate fractographic features on bone surfaces.

Method

For the fractographic examination, the examination approach recommended by Christensen et al. (2018) was used. One surface of each fracture was coated with dual-contrast fingerprint powder, examined under low-power microscopy using indirect light, and photographed. For each fracture, apparent fractographic features were noted, and direction of fracture propagation and impact direction were interpreted. The results were then compared to the direction of force identified during the original analysis, which is supported by both anthropological examination and autopsy findings including soft-tissue injuries and radiographs.

Case Review Results

Case 1

A 6-year-old male was struck by a vehicle after running into a roadway. He was transported to the local children's hospital with CPR in progress but was pronounced dead approximately 30 minutes after arrival. A segment of the fractured left femur was retained during autopsy for an anthropology consultation.

The fractography assessment of the proximal fracture surface revealed a prominent cantilever curl positioned at the lateral edge, arrest ridges oriented in the anterior posterior plane, and bone hackle oriented in the medial to lateral plane (Fig. 1). These features indicate that the fracture initiated on the medial surface, propagating from medial to lateral. The left thigh was interpreted to be struck on the lateral surface and the force was directed from left to right (lateral to medial).

The initial anthropological assessment and autopsy findings indicated that the left leg was impacted on the lateral surface in a lateral to medial direction, consistent with the fractography assessment. During the initial anthropological assessment, two butterfly fractures were noted on the left femoral shaft segment (Fig. 2). The more proximal positioned fracture was incomplete; the more distal positioned fracture was complete. For both fractures, the apex of the fracture wedge was positioned toward the medial aspect of the bone, indicating the location of tension and initial failure. During the autopsy, a 1-inch abrasion was observed on the left lateral buttocks and a 1.5-inch abrasion on the left knee. Subcutaneous hemorrhage was pronounced in the lateral musculature of the left thigh.

Case 2

A 58-year-old male was witnessed to fall backward off a 30-foot wall and landed in a mulched planting bed of small trees and shrubs. He was transported to the hospital with CPR in progress, but died from his injuries approximately 10 minutes after arrival. A segment of the fractured right femur was retained during autopsy for an anthropology consultation.

The fractography assessment of the proximal fracture surface revealed a prominent cantilever curl positioned at the anterior surface and arrest ridges oriented in the mediolateral plane (Fig. 3). Subtle bone hackle oriented in an anterior to posterior plane was also observed. The location of the cantilever curl and orientation of the arrest ridges are consistent with the fracture propagating from posterior to anterior, with a corresponding force application in the anterior to posterior direction.

The initial anthropological assessment and autopsy findings indicated that the right leg was impacted on the anterior surface in an anterior to posterior direction, consistent with the fractography assessment. During the initial anthropological assessment, a complete, complex, butterfly pattern fracture is present on the retained femoral shaft segment (Fig. 4). The apex of the fracture wedge is positioned toward the posterior surface of the bone, indicating the location of tension and initial failure. The anterior surface of the bone is marked with chipping along the fracture margin, indicating the location of compressive stress and failure. During the autopsy, abrasions and lacerations were noted on the right forearm, right lateral knee, right lateral ankle, right upper back, left toes, and the back of the head. The radiographs showed that at the fracture site the distal portion of the femoral shaft was displaced posteriorly to the proximal portion.

Case 3

A 19-year-old male was struck by a vehicle. The individual was transported to the hospital and died approximately 20 hours after arrival. A segment of the fractured right femur was retained during autopsy for an anthropology consultation.

The fractography assessment of the distal fracture surface revealed a cantilever curl along the posterior surface, bone hackle organized in the anterior to posterior plane, and arrest ridges oriented in the mediolateral plane (Fig. 5). These features were consistent with the fracture propagating from anterior to posterior and with the force applied posterior to anterior.

The initial anthropological assessment and autopsy findings indicated that the right leg was impacted on the posterior surface in a posterior to anterior direction, consistent with the fractography assessment. During the initial anthropological assessment, a complete, complex butterfly pattern fracture was noted on the retained shaft segment (Fig. 6). The apex of the fracture wedge was positioned toward the anterior surface of the bone, indicating the location of tension and initial failure. The posterior surface of the bone was marked with chipping along the fracture margin, indicating the location of compressive stress and failure. During autopsy, abrasions and contusions were observed on the left face, both forearms and hands, anterior surface of the left lower leg, medial surface of the right lower leg, and posterolateral surface of the right thigh.

Case 4

A 63-year-old male was a victim of a fatal assault with a blunt object(s), possibly a baseball bat and/or curtain rod. The right distal ulna, proximal fibula, and proximal tibia were fractured. The distal ulna was fractured in two locations. Segments of the distal ulna, proximal tibia, and proximal fibula were retained during autopsy for an anthropology consultation.

The fractography assessment revealed a cantilever curl on the posterolateral fracture margin of the proximal ulnar fracture surface (Fig. 7). The feature indicated an anteromedial fracture initiation, with force applied to the posterolateral surface. No other fractographic features were observed, which was attributed in part to the small amount of cortical bone present. The more distal ulnar fracture was reconstructed with adhesive and unavailable for fractographic analysis. No fractographic features were observed on either the tibia (Fig. 8) or fibula due to the thin cortical bone of these specimens.

During the initial anthropological assessment, two parry fractures were noted on the distal right ulnar shaft and present as a butterfly and a transverse pattern fracture (Fig. 9). For the butterfly pattern fracture, the apex of the wedge segment was positioned toward the anteromedial surface of the bone, indicating the location of tension and initial failure. For the transverse fracture, chipping of the cortical bone was noted along the posterolateral surface of the fracture margin, indicating the location of compressive stress and failure. The fracture patterns indicated a posterolateral to anteromedial directed force and were consistent with the fractography findings that the fracture initiated on the anteromedial surface of the bone. Additionally, the right distal tibia was fractured immediately below the tibial tuberosity (Fig. 10). The fracture pattern was a partial butterfly, with the bone failing in tension along the anterior surface consistent with a posterior to anterior directed force. The right proximal fibula was comminuted. The direction of force could not be interpreted from the fracture type. During autopsy, extensive contusions and abrasions were observed on the decedent's body, including along the lateral right forearm and the posterior right upper calf.

Discussion and Conclusion

In all four cases, the direction of fracture propagation identified through fractographic analysis was consistent with the direction of force inferred from the fracture pattern analysis and autopsy findings. In one of the four cases, fractographic analysis of two bones was inconclusive due to the thin cortical bone of the affected proximal tibia and filbula.

Cantilever curl and arrest ridges were the most apparent features in these cases, and were informative to the direction of fracture propagation, and therefore direction of force. In fact, in case 4, a cantilever curl was the only feature identified in the fractography analysis, and provided information that was consistent with autopsy and anthropological findings. Visualization of features was enhanced by the application of contrast medium and low-power microscopy, but most features were observed easily with the unaided eye. Interestingly, the cantilever curl observed in cases 1 and 2 was displaced in the direction of the fracture propagation, illustrating plastic deformation commonly associated with blunt trauma. Bone hackle was observed in cases 1 and 3 and was informative to the direction of fracture propagation, but was far more subtle than cantilever curl and arrest ridges.

This small case series shows promise for the application of fractography to skeletal fracture analysis in forensic anthropological casework. Previous fractography studies of bone trauma were based on controlled, experimentally induced fractures created under unidirectional force. Fractures occurring during life events and examined in forensic contexts are typically more complex and may involve the simultaneous application of forces of different types and directions. The results of this case series show that several fractographic features are observable and informative even when fracture patterns are complex.

References

Carter DR, Hayes WC. Compacl bone fatigue damage-I. Residual slrength and stiffness. Journal of Biomechanics 1977; 10(5-6):325-337

Christensen AM, Hefner JT, Smith MA, Webb JB, Bottrell MC, Fenton TW. Forensic fractography of bone: A new approach to skeletal trauma analysis. Forensic Anthropology 2018; 1(1)32 51.

Cohen H, Kugel C, May H, Medlej B, Stein D, Slon V, et al. The impact velocity and bone fracture pattern: Forensic perspective. Forensic Science International 2016;266:54-62.

Cohen H, Kugel C, May H, Medlej B, Stein D, Slon V, et al. The influence of impact direction and axial loading on the bone fracture pattern. Forensic Science International 2017;277: 197-206.

Currey JD. The A4echanical Adaptations of Bones. Princeton: Princeton University Press; 1984.

DeLand TS. Studies on the Development and Fracture Mechanics of Cortical Bone. [master's thesis]. East Lansing: Michigan State University; 2013.

Harkess JW, Ramsey WC, Ahmadi B. Principles of fractures and dislocalions. In: Rockwood CA, Green DP, eds. Fractures in Adults. Vol 1. 2nd ed. Philadelphia: Lippincott; 1975:1-76.

Hart GO. Fraclure pattern interpretation in the skull: Differentiating blunt force from ballistics trauma using concentric fractures. Journal of Forensic Sciences 2005;50(6):1276-81.

Hull D. Fracfography: Observing, Measuring and Interpreting Fracture Surface Topography. Cambridge, MA: Cambridge University Press; 1999.

Isa MI, Fenton TW, Deland T, Haut RC. Assessing impact direction in 3-point bending of human femora: Incomplete butterfly fractures and fracture surfaces. Journal of Forensic Sciences 2017;63(1):38-46.

Reber SL, Simmons T. Interpreting injury mechanisms of blunt force trauma from butterfly fracture formation. Journal of Forensic Sciences 2015;60(6): 1401-11.

Wulpi DJ. Understanding How Components Fail. 2nd ed. Materials Park, OH: ASM International, 2000.

Jennifer C. Love (a*) * Angi M. Christensen (b)

(a) District of Columbia Office of the Chief Medical Examiner, Washington, DC 20024, USA

(b) FBI Laboratory, Quantico, VA 22135, USA

(*) Correspondence to: Jennifer C. Love, District of Columbia Office of the Chief Medical Examiner, 401 E St. SW, Washington, DC 20024, USA

E-mail: jennifer.love@dc.gov

Disclaimer: The views expressed are those of the authors and do not necessarily reflect the official policy or position of the FBI. Names of commercial manufacturers are provided for identification purposes only, and inclusion does not imply endorsement of the manufacturer or its products or services by the FBI.

Received 31 January 2018; Revised 8 April 2018; Accepted 6 May 2018

DOI: 10.5744/fa.2018.0024
TABLE 1--Fractography Definitions (modified from
Christensen et al. 2018).

Term             Definition

Bone mirror      Relatively featureless region of a fractured bone
                 surface near the fracture initiation site
Bone hackle      Angular or rounded ridges aligned in the direction of
                 propagation resulting from increasing crack speed and
                 instability
Arrest ridges    Large raised ridges or peaks perpendicular to the
                 direction of crack propagation resulting from drastic
                 changes in crack propagation velocity
Cantilever curl  A curved lip that occurs just before total fracture of
                 a bone loaded in bending
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Title Annotation:TECHNICAL NOTE
Author:Love, Jennifer C.; Christensen, Angi M.
Publication:Forensic Anthropology
Article Type:Report
Date:Sep 22, 2018
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