The significance of bone mineral density in the depiction of memento mori.

Abstract

To the Editor: Ile Sainte Marie is a small island off the east coast of Madagascar, which in the 17th century gained a reputation as a pirate stronghold. Perhaps its most famous resident was Captain William Kidd, who preyed on the local maritime routes that were full of ships laden with wealthy cargo returning from Southern Asia to Western Europe around the coast of Africa. According to local folklore, located outside the island's capital Ambodifotatra, there is a small pirate graveyard, unique in that the tombs contain depictions of the famous “skull and crossbones” that have become the universal symbol of piracy, through their flag, the Jolly Roger. The question as to why the skull and crossbones (rather than other bones) became the symbol of piracy and, indeed, death prompted the authors to review the academic literature to establish if there was any scientific reasoning behind the longevity of these bones. First, it must be acknowledged that beyond pirate legend, across the spectrum of human cultures, the skull and crossbones have been a symbol of the brevity of life. Indeed, in the Middle Ages, artists incorporated it in many paintings and sculptures as a memento mori or reminder of death. Perhaps the skull as the skeletal focus of emotion through our facial expressions encompasses what it means to be alive. However, it undergoes a transformation when stripped of flesh, becoming the most potent symbol of death in our visual vocabulary. The skull and crossbones symbol can be found on many tombs in graveyards around the world from Ireland to New England, USA, as an insignia of death. From the catacombs of Peru to the ossuaries of Europe, there is physical evidence that the skull and crossbones in particular survive beyond the spectre of death. Cortical bone, which comprises more than 80% of the adult skeleton, is the major structural determinant of bone strength.1 The appendicular skeleton (eg, femur, tibia) is designed to resist bending forces and therefore is composed of predominantly cortical bone. The axial skeleton (eg, vertebrae, sternum) is designed to resist compressive forces and therefore has a higher ratio of cancellous bone. The exception is the cranium, which despite being part of the axial skeleton is primarily composed of cortical bone.2 Studies performed using phosphorus/hydroxyproline ratios to examine the difference in bone mineralization across the human skeleton have shown significantly higher mineralization values in bones with higher proportion of cortical to cancellous tissue.3 Cancellous bone, rather than cortical bone, which is relatively spared, is the preferred source of additional calcium to maintain calcium homeostasis during periods of relative systemic calcium deficiency, as can occur with reduced intestinal calcium absorption, increased urinary calcium excretion, or when calcium requirements are high, such as when breast feeding.4 Therefore, the higher mineralization of predominantly cortical bones likely accounts for the appendicular skeleton “surviving” longer than the more cancellous-rich bones of the axial skeleton. After death, demineralization of bones is a multifactorial process involving the actual burial process as well as the release of organic acids from soft tissue breakdown and the ground environment (water, temperature, acidity, and microorganisms).5 Thus, the memento mori symbol of the skull and crossbones (cranium and femur / tibia), which together have the highest bone mineral density are perhaps the bones that should last the longest after death and hence are the most visually potent reminder of our mortality.