Skeletal analysis

DEFINITION: Examinations by forensic anthropologists of bones that are subjects of criminal investigations.

SIGNIFICANCE: Because skeletons and teeth are the hardest and most durable parts of human bodies, they are often the only parts that investigators recover after victims of crimes have been dead for long periods of time. Forensic anthropologists are trained to identify skeletal remains and interpret the unique evidence they contain; they can often help identify homicide victims and reconstruct the circumstances of both their lives and their deaths.

Forensic anthropology is the application of physical anthropological knowledge to evidence used in the legal process. The field is one of the recognized subdisciplines of the American Academy of forensic Science. Practitioners in the field are frequently called upon to identify skeletal, badly decomposed, or otherwise unidentified human remains. Often, their first task is to apply techniques developed within physical anthropology to determine whether unidentified remains are human. After confirming that remains are, in fact, those of human beings, they try to answer other questions about the identities of the decedents, the manner of their deaths, and whether they were victims of foul play.

Forensic anthropologists frequently work alongside forensic pathologists, odontologists, and other investigators. They not only help identify decedents but also help determine their manner of death and how long they have been dead—the so-called postmortem interval. Although these professionals may initially be called upon to assist in locating and recovering suspicious human remains, they do most of their work on skeletons after they are recovered. Using the evidence they find in bones, they build decedents’ biological profiles, which include their sex, age, ancestry, stature, and unique identifying features, such as former injuries.

Human Skeletons

The bone matter in skeletons is made up of two primary materials: a hard mineral salt known as hydroxyapatite and a more flexible organic material, collagen. Hydroxyapatite gives bones their structural support and torsion strength, and collagen gives them their elasticity and tensile strength. Bones in living bodies are also living matter. They are innervated and receive nutrients from the bodies’ blood supplies. Like other living tissues, they produce waste from metabolic processes associated with their growth and development.

In addition to their structural support function, the muscles, ligaments, and tendons that are connected to the skeleton permit the body’s animation. Skeletons also house the bone marrow that produces the vital red blood cells. Because living bones are replaced at a rate of about 10 percent per year and are constantly being re-formed by the mechanical forces exerted on them, they effectively record information about the body after an individual’s death. The analytical techniques that forensic anthropologists use help bones speak for the dead.

In 2023, scientists at Iowa’s Des Moines University Osteopathic Medical Center developed an online database known as OsteoID that can help forensic scientists differentiate human and animal bones more easily. The database contained detailed measurements from more than 18,800 human and animal bones.

Methods of Identifying Bones

Determining whether recovered bones belong to human beings requires a detailed knowledge not only of the morphology of all the bones in human skeletons but also of all types of animal bones with which human bones might be confused. Because there are only two sexes, one might expect that determination of a given decedent’s sex should be correct approximately 50 percent of the time. However, given the critical functional differences between male pelvises and female pelvises, which are modified by childbirth, as well as general sexual differences in the muscle masses attached to bones, forensic experts should be able to identify the sex of unknown decedents from skeletal evidence in more than 95 percent of cases they investigate.

Methods of determining age from bones are more complex than those used for determining sex. Many criteria are employed to estimate the age of a decedent from a skeleton. Moreover, matching the estimated age at time of death of a body with the age of a long-missing person can be further complicated by uncertainty about the missing person’s age at time of death. This latter complication is especially relevant to identifying the skeletal remains of children, whose bones change more quickly than those of adults. If the bones of what appears to have been a three-year-old child were recovered several years after a two-year-old child disappeared, those remains might belong to the missing child, but making such a match would pose special difficulties.

Age is best determined through the employment of multiple criteria. Among infants and children, for example, the numbers of bones, the presence or absence of specific bones, and the degree of bone development can all be important in estimating age at time of death. Infant humans have approximately four hundred bones; however, as the children grow older, the shapes and numbers of their bones change. Some of the bones fuse to others at growth plates, and the predictable timing of such events can be used to estimate age at time of death. The emergence of deciduous, or “baby,” teeth and their gradual replacement by permanent teeth are also developmental events that occur at predictable ages.

The ages of middle-aged adults and older persons can be estimated based on changes in the appearance of the pubic bones of the pelvis, suture closures among their skull bones, the presence and degree of development of arthritis, and combinations of a host of degenerative changes that normally occur in human bones as individuals age. It is more difficult to estimate the ages of adult decedents than young decedents, however, because as human beings age, many other variables can affect their skeletal development. These variables include individual genetics, dietary differences, and exposure to disease and physical traumas.

Determining the ancestry of decedents from their skeletal remains is the most difficult aspect of creating biological profiles. Skulls must be present and in reasonably good condition to enable a host of measurements and observations to be collected. The data that are collected are then compared with measurements and other information gathered from large numbers of people of known ancestry and stored in databases. The results of multiple discriminant statistical procedures, along with researchers’ experience and expertise in assessing the wide range of human skeletal variation, assist in identifying the ancestry of unknown decedents.

Estimating the stature of decedents from their skeletal remains is comparatively easy and involves the application of only basic mathematics. Forensic anthropologists simply measure the longest bones recovered in a set of skeletal remains and multiply the lengths by figures listed in the most relevant regression tables. The main challenge is in choosing which formulas to use from among the many available. Allowing for differences in sex, age, and ancestry, different formulas are used for the various long bones of the body.

Other unique skeletal features of decedents used to help identify remains include dental work, orthopedic prostheses, and records of fractures. In some cases, the itself or other injuries sustained around the time of death leave evidence on skeletons. Examples include perimortem injuries, evidence of sharp force or blunt force traumas, bullet wounds, and manual strangulation. For example, the recovered skull of a person known to have been shot through the head is likely to contain persuasive evidence of the decedent’s cause of death.

Bibliography

Burns, Karen R. Forensic Anthropology Training Manual. Upper Saddle River, N.J.: Prentice Hall, 2006.

Gill, George W., and Stanley Rhine. Skeletal Attribution of Race: Methods for Forensic Anthropology. Albuquerque: Maxwell Museum of Anthropology, University of New Mexico, 2004.

Katzenberg, M. Anne, and Shelley R. Saunders, eds. The Biological Anthropology of the Human Skeleton. New York: John Wiley & Sons, 2000.

Krogman, Wilton Marion. “A Guide to the Identification of Human Skeletal Material.” FBI Law Enforcement Bulletin 8, no. 8 (1939): 3-31.

Krogman, Wilton Marion, and Mehmet Yasar Iscan. The Human Skeleton in Forensic Medicine. 2d ed. Springfield, Ill.: Charles C Thomas, 1986.

Martini, Frederic H., Michael J. Timmons, and Robert B. Tallitsch. Human Anatomy. 6th ed. San Francisco: Pearson/Benjamin Cummings, 2008.

"OsteoID: A New Forensic Tool to Help Identify the Species of Skeletal Remains." National Institute of Justice, 15 Nov. 2023, nij.ojp.gov/topics/articles/osteoid-new-forensic-tool-help-identify-species-skeletal-remains. Accessed 18 Aug. 2024.

Scheuer, Louise, and Sue Black. The Juvenile Skeleton. Burlington, Mass.: Elsevier Academic Press, 2004.

Schwartz, Jeffrey H. Skeleton Keys: An Introduction to Human Skeletal Morphology, Development, and Analysis. New York: Oxford University Press, 2007.

White, Tim D., and Pieter A. Folkens. The Human Bone Manual. Burlington, Mass.: Elsevier Academic Press, 2005.