For November 1999 issue

SCIENCE IN CRIME DETECTION

Reconstructing face from the skull

‑Dr. Anil Aggrawal

Many persons may not believe it, but we can often make a perfect face of a person merely from his skull. This is an important part of our practice. Many times, the police brings us merely the skull found in a forest and a nallah, and want to know us who that person was. If we can make the exact face of the person from that skull, the police can publish that photograph in the newspapers, and the person’s relatives can come forward to claim the skull. They can help in the police investigation as to when their relation disappeared, whom they suspect and so on. So in this installment, I am going to tell about this strange art, which I am sure, many readers would find interesting.

Three-dimensional facial reconstruction

The basis of the soft tissue reconstruction is the use of average soft tissue thicknesses over various points on the face. We already know how thick the facial skin is over each point of the skull bone. Unfortunately, the amount of data that has been published on soft tissue thicknesses related to the skull is quite limited. The limitations include relatively small numbers of subjects in all studies, few comparisons of soft tissue thicknesses in different races and few studies detailing sex and age differences. Comparison of the studies is difficult because of the varying methods of measurement, the varying points at which measurements are taken, the different racial groups studied and the differing status of the subjects (e.g. alive versus dead, fixed versus unfixed tissues). Additional problems are encountered with the ear and nose, which have a cartilaginous frame rather than direct bony support. A considerable amount of individual variation is possible in these structures as well as in the hairstyle, length and colour, and their reconstruction is probably the most variable and least accurate.

Another significant problem is that the number of reconstructions of childrens' faces reported in the literature is minimal. The soft tissue thicknesses usually appearing in forensic textbooks are based on those of adults, and their value in reconstructions of a child's face is questionable. Hodson and colleagues in 1985 measured soft tissue thicknesses in a small number of children using an ultrasound technique and found that tissue thickness tended to decrease with age in children. Differences between adult and child tissue thicknesses were clearly demonstrated by Gerasimov in 1971. With the advent of new techniques like magnetic resonance imaging and ultrasound, more objective and reproducible measurement of soft tissue thicknesses on large numbers of subjects may be possible and would clearly be desirable. However, the importance of the accuracy of soft tissue thicknesses may be overestimated given that, ultimately, a two-dimensional image of the three dimensional reconstruction is generally used for identification purposes.

The anatomical landmarks from a table (giving thickness of facial tissues at various points on the skull) are marked on the duplicate skull using wooden or plastic markers. These are cut to the average soft tissue thickness for a given point and glued to the skull. Coloured plastic or the coloured ends of safety matches are useful for this purpose as the coloured ends are visible through a thin layer of clay. Reconstructions building the soft tissue thickness in bulk without regard to the underlying soft tissue anatomy have been performed, but in this article, a technique in which muscles are built up in an anatomical manner is described. The various muscles of the face such as the temporalis, masseter, buccinator and occipitofrontalis are initially built up, followed by orbicularis oris and orbicularis oculi (these last two are the muscles of the eye).

(Editor: Please reproduce figures 1-4 here)

The soft tissues of the neck are built up in bulk (size is greatly helped if a shirt with a collar is found on the body), although the bulge of the major neck muscle (technically known as the sternocleidomastoid muscles) and the manubrial notch are usually highlighted. At this stage, it is often useful to reconstruct the nose and lips before forming the other muscles.

The lips are approximately as wide as the interpupillary distance (distance between the pupils of the two eyes), but considerable variation of form occurs with age, sex, race, occlusion and loss of vertical dimension. In young adults, the lip tends not to cover the upper incisors entirely, but it may in older age, when considerable tooth attrition or inadequate dentures are present. Class II division 2 occlusions are associated with a short upper lip and deep mental sulcus, whereas class III occlusions are associated with protrusion of the lower lip. The philtrum of the lip is affected by the length of the lip, the lip support given by the teeth and the shape of the base of the nose. The philtrum is usually placed in the midline of the face, but this may not correspond to the midline of the teeth.

(Editor: Please reproduce figure 5 here)

The nose is difficult to reconstruct because of the limited underlying bone and the wide individual variation possible. The profile of the nose is arbitrarily determined by projecting two lines from the skull midline, as suggested by Gerasimov in 1971. The first is a continuation of the inclination of the nasal bones, and the second is a horizontal line from the anterior nasal spine. On average, Gatliff found (in 1984) the width of the nose to be 1.67 times that of the anterior nasal aperture (this is a hole in the skull, where the nose should be), but Schultz found that racial and individual variations were considerable. The nose is positioned in the midline unless obvious bony asymmetry is present.

The muscles of facial expression are then added, followed by completion of the completion of the soft tissue around the eyes. The eyelids normally cover part of the iris and extend from 2 mm lateral to the medial orbital rim to 4 mm medial to the lateral orbital rim. The orientation of the palpebral fissure (the line between the closed eyelids) varies between races; for example, Asians typically have a palpebral fissure that passes inferiorly from the lateral to the medial aspect. At this stage, the tissues are built up to within 1 mm of the soft tissue thickness markers and the ears are then reconstructed. The latter are arbitrarily positioned so that the ear canal is positioned at the external acoustic meatus and set obliquely with a 15 degree posterior inclination. Caldwell noted that virtually all authors experienced in facial reconstruction found ears 'complicated' to reproduce. Finally, fleshing of the face is carried out to the tips of the depth markers, and characterization may be added if indicated. For example, an aged appearance is simulated by the addition of wrinkles and the accentuation of skin folds. Similarly, further racial characterization may also be carried out if indicated.

(Editor: Please reproduce figure 6 and 7 here)

Duplication of the reconstruction

Duplication of the clay reconstruction is also advisable as a plaster model is considerably more durable than a clay model. The clay reconstruction is duplicated using a split-mould technique similar to that used to duplicate the original skull. The only significant difference is that the clay reconstruction is supported by towels, and sheets of wax are used to form the 5 cm table around the head.

(Editor: Please reproduce figure 8,9 and10 here)

On the plaster reconstruction additional identifying material may be added, for example hair, jewelry and spectacles if these were found with the body. Alternatively, a two-dimensional image of the reconstruction may be carried out and additional alterations made with the use of computer software. This is often a more efficient method of finishing the reconstruction.

(Editor: Please reproduce figure 11 here)

This in short is the way we reconstruct the face from the skull. I hope this important aspect of crime detection technique is now clear to the readers.

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Captions of figures

Figures 001 to 004: These figures (figures 001-004) show the transition from duplicate skull with soft tissue thickness markers to reconstruction of the underlying muscles, nose and lips.

figures 005-007: Completion of the reconstruction involves simulation of the skin (fleshing out), placement of the ears and final characterization of the face.

Figure 008: Duplication of the reconstruction is similar to duplication of the original skull. However, the reconstruction is supported by towels, and a wax table is built around to support the first half of the split mould.

Figure 009: Impression of the clay reconstruction.

Figure 010: Plaster casts of the clay reconstruction.

figure 011: Complete three-dimensional plaster model

(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)