For DECEMber 1999 issue

SCIENCE IN CRIME DETECTION

Duplicating the skull for forensic identification

‑Dr. Anil Aggrawal

Three-dimensional reconstruction of the face from a skull may be carried out to assist in the identification of human remains but is only used when more reliable methods have failed or are impossible. As a technique used for identification, it has definite limitations, but it still has a valuable role in forensic identification. The three-dimensional reconstruction image may later be altered, via computer software or other means, enabling further characterization and adjustment in a more efficient and rapid manner than with traditional three-dimensional reconstruction methods.

Variations of many aspects of the reconstruction method are possible, but a successful reconstruction can only be carried out if the person is familiar with the normal human anatomy of the skull and attached soft tissues, dental occlusion, sculpting and split-mould techniques of duplication. The steps involved in a reconstruction may include collection of preliminary data, duplication of the skull, reconstruction of the soft tissues and duplication of the reconstruction.

Preliminary collection of data

Although the skull is the basis of the reconstruction, other valuable physical remains may be available. Occasionally, reconstruction of only part of the skull soft tissues is required, and close inspection of the soft tissue remaining by the person carrying out the reconstruction is obviously then vital. Jewelry found with the deceased, such as earrings and necklaces, may be incorporated in to the final reconstruction, as may hats, glasses or other accessories. The finding of hair with the human remains is obviously invaluable in choosing hair colour and style in the reconstruction. Details of race, sex, age and build may be obtained by a thorough examination of skeletal remains, teeth, clothing and jewelry. This information is usually provided by other specialists such as police, forensic pathologists, anthropologists and odontologists, either directly or in the form of written reports, and should be avidly sought for every reconstruction.

Close examination of the skull may reveal details that affect the final reconstruction. Clearly, if the facial skeleton is incomplete or not intact, a preliminary reconstruction of the bony skeleton will be required. Occasionally, bony pathology will be present, which may influence the soft tissue reconstruction and lead to asymmetry. More commonly, the prominence of certain bony landmarks, such as the supraorbital ridges (areas on the skull corresponding to eye brows) and zygomas (cheek bones), will affect the general appearance of the eyes. The ruggedness of muscle attachments, profile of the jaws and bony face, and symmetry of the nasal bones may all give clues to the gross appearance. The dentition and/or dentures are closely examined as they give clues to articulation of the lower jaw with the upper jaw. It is possible that loss of the vertical dimension of the face owing to excessive wear of the occlusal surfaces, abnormal posturing of the mandible and soft tissue support of the lips may have occurred.

Duplication of the skull

Although the reconstruction may be carried out directly on the skull, it is often preferable to duplicate the skull and carry out the reconstruction on a plaster model. Advantages of using a duplicate include minimizing damage to the skull and having the skull available as a reference during the reconstruction. It may even be essential if the skull is available only for a short time or if multiple reconstructions of the single skull are to be made.

Uncommonly, some of the soft tissues covering the skull remain intact but not those of the face. In such cases the skull with its attached soft tissue is duplicated. However, usually only the bony skull and minimal or no other soft tissue remains are present. in this cases, the skull is cleaned and the remaining soft tissue removed. If the skull is incomplete, damaged or fragmented, these defects need to be repaired, for example with wax.

(Editor: Please reproduce figure 001 here. Caption to figure 001: Skull dried and cleaned for duplication)

The degree of accuracy with which this can be done varies with the site of the damage and decreases with increasing size of the defect.

Re-articulation of the mandible is relatively simple if natural or artificial teeth are present. The mandible is positioned so that the teeth in the upper and lower jaw are opposed to one another and the upper ends of the lower jaw bone (known technically as mandibular condyles) fit snugly in the sockets provided for them in the base of the skull. The mandible is then held in place by the judicious use of wax.

(Editor: Please reproduce figure 002 here. Caption to figure 002: Repair of a substantial part of the calvarium has been required on this skull. Sizeable repairs to the facial skeleton may considerably reduce the accuracy)

In an edentulous person (i.e. a person who does not have any teeth) in whom no dentures are found, average measurements are used to establish the vertical dimension between the mandible and maxilla, and occlusal wax rims similar to those used in the construction of dentures are used. A similar technique is used when there are an insufficient number of teeth to articulate the lower jaw with the upper jaw. The wearing of dentures can dramatically affect facial appearance and emphasizes the importance of accurate positioning of the mandible.

It is important to appreciate here that skull has many normal “holes” in it. For instance there are ear holes on the two sides of the skull. These are technically known as external acoustic meatus. Similarly there are holes for the nose. There is a hole at the base of the skull from which a major nerve bundle – the spinal cord – leaves the skull. It is important to “block out” these “holes” before any attempt to duplicate the skull is carried out. Other prominent holes such as those underneath the cheek bones (zygomatic arches) are also blocked out. Modeling clay is a useful material for blocking out these “holes” or “undercuts” as it is pliable, non-toxic, inert and easily washed away after duplication has been completed. The orbits are fitted with plaster or prosthetic eyes at this stage to remove the undercut of the bony orbit. Again, modeling clay is used to hold the eyes in position. The eyes are centered in a line between the midpoints of the upper and lower borders of the “hole” meant for the eyes, and should be just visible when viewed from above.

(Editor: Please reproduce figure 003 here. Caption to figure 003: The prosthetic eye is positioned midway between the superior and inferior orbital margins on the skull prior to duplication)

A split-cast technique is employed to duplicate the skull. The skull is supported by modeling clay so that the face is upright and the plane passing through the mental sulcus, body of the mandible, zygomatic arch, external acoustic meatus and parietal bones is horizontal. Some of these are technical terms and the average reader may not want to worry himself with these terms. What is important is that he must realize that the skull is positioned in a particular way. The position is such, as if the skull were looking in the sky in front of him. This would become clearer on referring to figure 004 here.

(Editor: Please reproduce figure 004 here. Caption to figure 004: The initial step in the production of the splitcast mould involves embedding the skull in modeling clay. A table of clay 5 cm wide is built around the skull and notched at regular intervals. (This will result in accurate joining of the two halves of the completed cast)

A horizontal table of clay 5 cm wide is added around the entire skull and notched at regular intervals (to facilitate later accurate positioning of both halves of the completed cast). A suitable quantity of impression material (e.g. 450g alginate powder) is mixed and applied to the skull, initially by rubbing it on to the surface by hand and then by pouring it over the skull. The impression material needs to be strong, flexible and slow-setting. Once set, thin plastic film is applied to the outside of the impression material (to act as a separating medium). An outer plaster supporting layer is then poured. Before the plaster is set, a shallow groove is made in it to facilitate tying of both halves of the mould together at later stages. When the plaster has set, the supporting clay is removed and the second half of the mould constructed in a manner similar to that of the first half. The two halves of the mould are then carefully removed from the skull.

(Editor: Please reproduce figure 005 here. Captions to figure 005: The skull being devested. The completed split cast is carefully separated to avoid tearing the impression material.)

A suitable quantity of plaster is mixed and manually rubbed over the inner aspect of each half of the mould. These are then filled. Both halves are repositioned together accurately (using location notches) and tied together (using string in scribed groove), and the plaster is allowed to set. The duplicate plaster skull is then carefully uncovered and checked for accuracy; further duplicates can then be made if required.

(Editor: Please reproduce figure 006 here. Captions to figure 006: The original skull should be compared with the duplicate model. Used carefully, several models can be made from the single split cast.)

For convenience and to minimize the risk of damage, the duplicate plaster skull is mounted on a revolving workstand with a metal bracket long enough to allow space to reconstruct the neck. A hole is drilled in the region of the foramen magnum (This is a big hole at the base of the skull) to a distance of about 4 cm, and the metal bracket is inserted and fixed in place with a new mix of plaster. When the plaster has set, the exposed portion of the metal bracket is fixed to the turntable with the duplicate skull's centreline vertical and Frankfort plane horizontal.

A little bit about Frankfort plane here. This is an imaginary plane very much talked about in reference to the skull. This plane is especially talked about in the practice of dentistry. If you look at your right eye and the right ear hole carefully, you can know about this plane easily. Draw an imaginary line through the lower border of your eye and the upper border of the ear hole, and you will get the so-called Frankfort plane. In the skull, the technician draws an imaginary line through the lower border of the bony orbit of the eye, and the upper border of the ear hole, and he gets the Frankfort plane.

When the skull is fully cast, it should be positioned in such a way that the Frankfort plane is horizontal. This ensures, that the skull assumes a normal position.

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(Dr. Anil Aggrawal, M.D. is a Professor of Forensic Medicine at the Maulana Azad Medical College, New Delhi. His E-mail is dr_anil@hotmail.com, and his website is http://members.tripod.com/~Prof_Anil_Aggrawal/index.html)