Blunt-Force Trauma: Radiating, Concentric and Depressed Fractures

Radiating fractures are linear fractures that extend outward from the point of impact in a pattern analogous to the spokes of a wheel. They are produced by the tensile stress that develops on the inner (endocranial) surface of the skull as the outer surface is compressed. The inner-table failure initiates a crack that propagates outward (from inner table toward outer table) and simultaneously extends radially away from the impact point, following the path of maximum tensile stress.

In a moderate-energy impact on the parietal bone (the most common site in homicide-related blunt-force cranial trauma), the typical pattern is three to five radiating fractures extending outward from the impact point in multiple directions, with the exact directions influenced by local structural features: suture lines (which represent planes of relative weakness and tend to arrest or redirect fractures), areas of differential thickness (fractures deflect toward thinner regions), and the geometry of the diploe (the cancellous bone between the inner and outer tables, which absorbs some energy).

Radiating fractures that cross a suture line rather than being arrested by it are diagnostically significant: they indicate that the energy of the impact was high enough to overcome the arrest effect of the suture. A fracture arrested at a suture indicates a moderate-energy impact; a fracture that crosses multiple sutures indicates high energy. This arrest vs. cross-suture criterion was documented by Gurdjian and Lissner in the 1940s and has been confirmed in modern cadaveric and finite-element modelling studies at the UK Cranfield University forensic biomechanics group and at the University of Tennessee.

The direction of propagation of a radiating fracture (inward from outer surface to inner surface, or outward from inner surface to outer surface) can sometimes be determined by examining the fracture edges: an inside-out fracture produces a bevelling pattern on the outer table similar to the exit bevelling in a gunshot wound (see the ballistic trauma topic). This feature is used to distinguish a direct skull impact from a contre-coup fracture (fracture at the site opposite the impact, produced by the skull's deformation and rebound).

Concentric fractures are roughly circular or arc-shaped fractures that encircle the impact point at some distance from it, intersecting with the radiating fractures. They are produced by the bending of the skull plate around the impact zone: as the skull flexes inward under the applied force, the outer table of the area surrounding the impact is tensioned (because the outer table is on the convex side of the bending region at some distance from the focal impact). This outer-table tension initiates circumferential cracking at a ring-shaped zone of maximum outer-table stress.

The relationship between radiating and concentric fractures in terms of timing is consistent: radiating fractures initiate before concentric fractures in any single impact event, because the inner-table tension (which drives the radiating crack) develops earlier in the loading sequence than the outer-table tension of the surrounding bending zone (which drives the concentric crack). Concentric fractures therefore always post-date the radiating fractures of the same impact. This sequence relationship is the foundation of Hering's principle, discussed in the next section.

In a severe impact with high energy transfer, the radiating and concentric fractures may produce a pattern sometimes described as "buttercup" or "spider-web": a central depressed zone (if the impact was focal), surrounded by radiating cracks, in turn intersected by one or more concentric arcs. The complexity of this pattern increases with impact energy and decreases with the presence of soft tissue at impact (which distributes the force and reduces the peak stress concentration at the bone surface).

Concentric fractures are most clearly developed on the frontal and parietal bones, where the skull plate is relatively flat and can flex as a plate under bending load. On curved regions (the occipital squama, the temporal bone), the curvature itself provides some structural resistance to bending, so concentric fracture patterns are less well-developed and may be absent even in severe impacts.

A depressed cranial fracture is produced when a focal impact transfers enough energy to deform the skull surface inward beyond its elastic limit, creating a permanent depression. The outer table in the area of direct contact is driven inward, and in the simplest case the depressed fragment (the piece of skull driven inward) retains the approximate geometry of the striking surface at its deepest point.

The diagnostic significance of the depressed fracture geometry rests on the concept of the bearing surface: the area of the implement that was actually in contact with the skull at the moment of maximum force transfer. A circular depressed fracture with a diameter of 15 to 25 mm is consistent with a hammerhead face or a ball-peen hammer; a rectangular depression with a squared edge is consistent with a wrench or a crowbar end; a narrow linear depression suggests a rod or pipe edge. These estimates of implement geometry from fracture geometry are subject to significant uncertainty because the final depression shape is influenced by the overlying scalp and hair (which distribute the force), the thickness of the skull at the contact site, and the rate of loading.

A key distinction in casework is between a pure depressed fracture (a single depression with no or few radiating cracks) and a comminuted depressed fracture (a central depression surrounded by radiating and concentric cracks that fragment the surrounding bone). Pure depressed fractures tend to result from moderate-energy focal impacts; comminuted patterns result from higher-energy impacts. The severity of fragmentation provides a rough ordinal scale for energy: a single depressed fragment with smooth margins indicates lower energy than a central crater with multiple radiating cracks and secondary fragments.

In mass-disaster and skeletal remains casework, depressed fractures may not retain the fragment in its depressed position if the cranium has been disrupted postmortem. In such cases, the depression can be inferred from the geometry of the fracture margins: the bevelling of the inner and outer tables at the fracture edge (the outer table is under-cut toward the depressed zone; the inner table is over-cut) can identify the direction of the applied force even when the fragment is missing.

Eduard Hering's 1853 observation, extended by subsequent cadaveric and clinical research, states: a radiating fracture propagating from a second impact will be arrested when it reaches a fracture line already produced by a prior impact. The mechanism is stress relief: the first impact creates a fracture line that releases the elastic strain energy in the surrounding bone, creating a zone of reduced tensile stress at the fracture margin. When a radiating fracture from a second impact approaches this zone, it reaches the boundary of the released-stress area and cannot propagate further; it terminates at the pre-existing fracture line.

This arrest-at-prior-fracture criterion is the operational basis for determining the sequence of impacts in multiple-blow blunt-force cases. The analysis proceeds as follows. The analyst maps all fracture lines on the cranium, noting which fracture lines terminate at (are arrested by) other fracture lines. A fracture line that is arrested at another fracture line is younger than the fracture that arrested it. By mapping all arrest relationships, the analyst constructs a directed sequence: fracture A is older than fracture B if B terminates at A; fracture B is older than fracture C if C terminates at B; and so on. The resulting sequence gives the order of the impacts.

Hering's principle has been validated in cadaveric experiments (Berryman and Symes 1998; Courville 1942; Gurdjian et al. 1950), in finite-element modelling (Motherway et al. 2009 at University College Dublin), and in casework review across multiple jurisdictions. It is now accepted as the standard methodology for impact sequence determination in UK Crown Court proceedings (per the Forensic Science Regulator's Codes), US federal court proceedings under Daubert (where the underlying cadaveric validation literature establishes the scientific foundation), and in Indian casework under the BNSS expert-evidence framework where the basis of an opinion must be disclosed.

The limitations are equally important to understand. Hering's principle operates only when the fracture lines are clearly distinct and the arrest relationship can be unambiguously read. In highly comminuted injuries with dozens of overlapping fractures, the arrest relationships may be too complex to resolve reliably. In such cases, the anthropologist should report the sequence as indeterminate for some pairs of fractures even if others can be reliably ordered. The principle also applies only to fractures produced within a short time window (while the bone retains its elastic properties and before blood, fluid, or postmortem change alters the fracture margin); it cannot be reliably applied to fractures produced days apart.

Berryman and Symes (1998) described the wedge fracture as the characteristic three-dimensional fracture geometry produced by a focal blunt-force impact on a flat bone such as the skull vault. The wedge fracture consists of: (1) an outer-table defect at the impact site, which may or may not be depressed; (2) a set of radiating fractures extending outward from the impact in the plane of the bone surface; and (3) a conical or wedge-shaped fracture that extends from the impact site through the diploe to a wider base on the inner table. This inner-table cone is wider than the outer-table defect, analogous in geometry (though opposite in direction) to the internal bevel of an entrance gunshot wound.

The inner-table cone is produced by the compressive load directly under the impact site being transmitted through the diploe as a cone of compression: the outer table is compressed inward (point of the cone), and the force spreads as it travels through the diploe, producing a wider failure zone at the inner table (base of the cone). This geometry was confirmed in finite-element models (Horgan et al. 2004) and in forensic casework review at the University of Tennessee and at the University of Sheffield (UK).

The Berryman-Symes framework is used to assess whether a fracture pattern is consistent with a single focal impact or multiple impacts, and to estimate the direction of the force relative to the skull surface. A near-perpendicular impact produces a roughly circular or regular polygonal inner-table cone; an oblique impact produces an elongated, asymmetric cone with one axis longer than the other in the direction of the oblique component of the force.

The framework has been applied in high-profile US casework. In the Phil Spector trial (California, 2007, retried 2009), the prosecution forensic pathologist and a consulting forensic anthropologist testified about the gunshot wound geometry on Lana Clarkson's skull, including the entrance bevel and the consistency of the wound track with a contact shot. The Casey Anthony trial (Florida 2011) involved expert testimony about the absence or presence of perimortem skeletal trauma, with anthropological analysis addressing whether observed fractures were perimortem (at or near the time of death) or postmortem (after death and skeletonisation), a distinction that rests on the same fracture-margin criteria used in the Berryman-Symes framework.

The distinction between focal and diffuse loading is the first interpretive branch in any blunt-force fracture analysis. Focal loading is produced by implements with a small bearing surface relative to the skull surface: a ball-peen hammer, a brick corner, a tyre iron end, a golf club head, a rock with a pointed projection. Diffuse loading is produced by broad-surface impacts: a fall onto a flat floor, a blow from a flat board, a vehicle bumper impact, contact with a broad ground surface in a pedestrian run-over.

In focal loading, the energy is concentrated at the impact site, the stress gradient from the impact point outward is steep, and the fracture pattern radiates clearly from a discrete epicentre. A depressed defect is common. The implement's bearing surface area can often be estimated from the depressed defect geometry, and comparison with candidate implements is feasible.

In diffuse loading, the energy is spread over a larger area, the stress gradient is more gentle, and the fracture pattern may lack a clear radiating epicentre. Instead, diffuse loading tends to produce a broader pattern of eggshell outer-table cracking (superficial fracturing of the outer table over a wide area) or, in higher-energy diffuse impacts (vehicle crashes, falls from height), may produce widespread communition without a clear focal point.

The medicolegal importance of this distinction is direct: a focal fracture pattern is consistent with an intentional blow with a hand-held implement; a diffuse fracture pattern is consistent with a fall, a vehicular impact, or a broad-surface strike. Falls from height are a common alternative explanation offered in court for cranial fracture patterns, and the focal vs diffuse distinction is the principal tool for evaluating that alternative. A discrete, geometrically regular depressed fracture with radiating cracks and a clear epicentre is not consistent with a flat-surface fall, because a fall onto a flat floor distributes the impact energy broadly; it can, however, be consistent with a fall onto a raised protrusion (a door-frame corner, a kerb edge, a stone step edge) that produces focal loading despite the fall mechanism.

The 2014 Sunanda Pushkar case in Delhi brought these interpretive questions into Indian public debate. The forensic pathology report cited blunt-force trauma, and subsequent debate included questions about whether the injuries were consistent with an intentional blow or a fall, a question that focal vs diffuse fracture pattern analysis directly addresses. The case also highlighted the need for a trained forensic anthropologist alongside the forensic pathologist in complex blunt-force cases, a gap that Indian CFSL and AIIMS forensic medicine units have begun addressing since 2015.

The Sunanda Pushkar case (India, 2014) illustrated the consequence of ambiguous blunt-force findings when expert opinion diverges. The initial post-mortem report and subsequent reviews disagreed on whether the injuries were consistent with homicidal blunt force or with a medical or accidental aetiology; this disagreement reached the Delhi High Court and the Supreme Court of India in subsequent proceedings. Under the BNSS framework governing expert evidence in India, the basis of an expert opinion must be disclosed and is subject to cross-examination, mirroring the requirement in the UK (Part 35 Civil Procedure Rules, applied in criminal proceedings via Practice Direction 35) and in the US (Federal Rules of Evidence Rule 702, as filtered through Daubert).

The Casey Anthony trial (Florida 2011) involved the absence of blunt-force skeletal trauma in a case where the child's death was alleged to have involved intentional harm. The anthropological testimony addressed the difficulty of assessing perimortem vs postmortem skeletal damage when remains are extensively decomposed and scattered; the fracture-margin criteria (sharp edges with bone colour matching the bone interior indicate perimortem fracture; weathered, rounded edges with a colour different from the cut surface indicate postmortem fracture) were central to the expert evidence.

The Phil Spector retrial (California 2009) involved detailed examination of the skeletal wound on Lana Clarkson's palate and mandible, with competing expert analyses of the wound direction and the consistency of the wound track with a self-inflicted contact shot vs a shot delivered from outside the oral cavity. The testimony illustrates how osteological analysis of skull fracture geometry directly intersects with the forensic pathology opinion in gunshot cases.

In India, the Nithari serial killing case (Noida, 2006 to 2007) involved recovery and identification of disarticulated skeletal remains from the house of Moninder Singh Pandher, where forensic anthropologists and pathologists from AIIMS Delhi worked alongside the CBI laboratory. The case prompted the first systematic review of forensic anthropology capability at AIIMS Delhi and the Central Forensic Science Laboratory, and led to the designation of forensic anthropology as a specialist function at CFSL Hyderabad by 2010.

1. Scene and recovery documentation
   Photograph the cranium in situ before any movement. Record orientation, associated soft tissue or scalp, and the location of any loose fragments. All fragments must be retained and their recovery location mapped.
2. Initial macroscopic assessment
   Articulate or semi-articulate the cranium. Identify each fracture line, noting origin, terminus, and any arrest relationships. Photograph under raking illumination. Distinguish perimortem from postmortem fractures using margin colour and edge sharpness.
3. Fracture mapping and epicentre identification
   Map all fracture lines on a standard cranial diagram. Identify the apparent epicentre of each fracture system (the point from which radiating cracks appear to originate). Note suture arrest vs suture crossing for each radiating fracture.
4. Sequence determination using Hering's principle
   For each pair of intersecting fracture lines, identify which line is arrested at the other. The arrested line is younger. Build the directed sequence graph. Report indeterminate pairs where the arrest relationship is ambiguous.
5. Focal vs diffuse assessment and implement class estimate
   Assess the fracture pattern for focal vs diffuse loading. If focal, measure the depressed defect dimensions and estimate the bearing-surface geometry. Document whether the depressed defect geometry is consistent with or inconsistent with candidate implement classes.
6. Report and testimony preparation
   The report states the level of conclusion: consistent-with vs inconsistent-with for implement class; the directed sequence for impact order where determinable; and the uncertainty envelope around each conclusion. All measurements and photographs are appended as exhibits.