Porotic hyperostosis is one of the most common skeletal conditions observed in archaeological remains. It is a term that refers to porous lesions occurring on bones of either or both the cranium and/or post-cranium. When these same lesions specifically appear on the orbital roof or eye sockets, it is called cribra orbitalia. Despite differing locations of occurrence, and thus nomenclature, porotic hyperostosis is a term that is meant to encompass the overall porotic skeletal condition.
However, despite that this common and generally indiscriminate condition affects various elements of the skeleton, lesions do have a fairly particular pattern of occurrence specifically on the cranium, where lesions tend to concentrate at parietal, frontal and sometimes occipital bones as well as at the roof of the eye orbits (cribra orbitalia). In cases where there are separate occurrences of lesions, either on the vault or reserved only to the orbital roof, some have questioned whether this is due to different etiologies. Complicating the matter, sometimes porotic lesions are observed on both aspects to a varying degree, leading some to suggest that the appearance of lesions is phasic, explaining their eventual appearance together where one simply expresses before the other.
The leading explanation for the expression of porotic lesions on the cranium is hyperplasia of the cranial diploe or spongy bone of the cranium. Hyperplasia is the increase in production of cells, which may lead to the expansion of the tissue, in this case, the diploe. For instance, the overproduction of red blood cells in the cranial diploe causes the tissue to expand where, in contrast, the compact outer cranial bone may be resorbed, leading to bone loss and porosity (i.e. porotic hyperostosis). Since an overproduction of red blood cells is usually the response to most anemic conditions, anemia is the generally accepted explanation for porotic hyperostosis. As such, the expectation for the cranial characteristic for individuals with this condition, given the effect of tissue hyperplasia, is an expanded or thick diploic (spongy bone) layer and a thin porotic outer cranial bone layer.
Our study on cranial thickness, where we used medical CT scans to measure each of the three cranial layers (outer, diploic, and inner table) in individuals without a porotic condition and on those lacking porotic lesions on the cranial vault but having lesions on the orbital roof (cribra orbitalia), showed that the cranial characteristic was the opposite of the expected thick cranial diploe and thin outer layer observed in cases of porotic hyperostosis of the vault. That is, individuals with cribra orbitalia had significantly thinner diploic bone and thicker outer cranial layers at particular locations across the vault than in individuals who did not have a pathological porotic condition.
With such strong evidence that cranial diploic thickening and a porotic outer cranial table occurs in anemic individuals, we explored if there were any anemic conditions that may cause diploic hypoplasia (reduction in cell production), and thus the opposite of hyperplasia, that could explain the reduction in cranial thickness in individuals with porotic orbital lesions.
Indeed, there are several anemias with this characteristic, including anemias of chronic disease, renal failure, and endocrine disorders as well as, aplastic anemia, and protein deficiency anemia. Of these, we found that anemia of chronic disease is the most likely cause given the physiological characteristics as well as both the incidence and prevalence of this anemia, which most closely matches the common occurrence observed in skeletal remains.
However, if anemia can be excluded as a cause, there is convincing evidence that scurvy, caused by a deficiency in vitamin C and which may lead to subperiosteal hematomas and inflammation in the orbital roof, is also a likely possibility contributing to the appearance of cribra orbitalia. Lastly, since our study found that hypoplasia, evident by a significantly thin diploe, occurs in individuals with cribra orbialia and that hyperplasia, or thickening of the diploe, occurs in porotic hyperostosis of the vault, the two different physiological responses and separate locations of occurrence may indicate two separate etiologies.
The study, New evidence suggesting a dissociated etiology for cribra orbitalia and porotic hyperostosis was recently published in the American Journal of Physical Anthropology.
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