Bone Biology

This chapter introduces readers to the basics of understanding bone’s functions, which include calcium homeostasis and enabling movement, bone’s components, such as the collagen, and bone’s organization, such as the Haversian system found in cortical bone. The focus of this chapter is on explaining concepts of bone remodeling, which is thought to prevent fractures and other bone damage, and repair, which can take place at macro-levels and micro-levels. Wolff’s Law of bone remodeling, which was initially focused on trabecular bone changes, is discussed in terms of bone’s response to forces that result in strains and stresses. Finally, diarthrodial joint remodeling and repair are discussed; cartilage cells were once thought to be static, yet now they are known to also respond to stresses.

Conclusions

This chapter concludes with the major theme that ran through the previous chapters; how much of each of these skeletal activity reconstruction features are a result of environmental influences (i.e., activities) and how much of the variation in these features are a result of genes. Biological confounds, which are largely genetic, have been found in all of the skeletal features covered in the previous chapters. For example, evolutionary body type rules (i.e., Bergmann’s and Allen’s Rules) affect measures of cross-sectional geometries. Plus, age is known to increase entheseal change scores. Furthermore, twin studies have revealed hereditary etiologies for osteoarthritis and Schmorl’s nodes. Yet, not all of the variance is genetic and, thus, the question remains whether skeletal indicators of activity can still be used to reconstruct activity patterns. Methods that avoid circular reasoning and aim to use only skeletal features with predictive validity should be the ultimate goal for those studying skeletal remains. If skeletal indicators of activity cannot be used to reconstruct what people did in the past, then perhaps these skeletal features can help in other ways, such as improving age estimates or drawing better conclusions about biological relatedness.

Osteoarthritis

In this chapter, osteoarthritis etiology, which is often thought to be the result of wear and tear on joints, is examined using a multidisciplinary approach. Bioarchaeologists look for porosity, eburnation, and osteophytic lipping on joints to identify osteoarthritis. Osteoarthritis is ubiquitous in bioarchaeological skeletal remains, but it also occurs in 70 percent of people over 65 years old in living populations. Anthropologists use occupational research to surmise that osteoarthritis is caused by overuse of joints, but medical researchers have found that osteoarthritis of certain joints (e.g., knee) have high heritability rates. Plus, age is the best predictor for who has osteoarthritis. Yet osteoarthritis patterns between past populations and modern clinical samples differ; for example, earlier populations have an earlier onset than in modern populations.

Entheseal Changes

This chapter focuses on locations called entheses where muscles attach to bones and whether muscle use can create changes at these locations. Entheseal changes, which have also been called musculoskeletal stress markers, have been utilized to reconstruct activity patterns in a wide range of studies. Some entheseal change researchers suggest that the muscle use causes microtrauma at the entheses; others suggest that bone remodeling (rather than repair) sufficiently explains the localized changes found at entheses. Yet entheseal changes also correlate with non-activity variables; age is the best predictor of entheseal changes. It seems that fibrocartilaginous entheses, which attach muscle to bone via a fibrous tendon, are less prone to non-activity confounds than the more often examined fibrous entheses, which involve large muscles that attach directly onto bone.

Activity Indicator Facets

This chapter covers activity indicator facets, which are extra facets, facet extensions, or fossae near joints that have been associated with habitual activities. Many activity indicator facets are used to determine postural behaviors, such as kneeling and squatting. Load-carrying behaviors have also been assumed to result in accessory facets, especially at the sacro-iliac joint. Activity indicator facets, such as the Poirier’s facets, Allen’s facets, and Baastrup’s kissing spines, have become less frequently used by bioarchaeologists in recent years. Now, activity indicator facets are mainly found in archaeological site descriptions in which the activities are reconstructed using the site’s artifacts too. The waning interest in activity indicator facets may be due to the lack of clinical, sports, and animal research to support the link between the facets and activities.

Stress Fractures

Stress fractures (or fatigue fractures) are covered in this chapter. Stress fractures are assumed to occur due to microcracks from repetitive forces that accumulate to result in a complete bone break, but these fractures can result from a single traumatic event. The most common location for stress fractures is the vertebral column; two common vertebral stress fractures include spondylolysis and clay-shoveler’s fractures. Spondylolysis fractures, which occur on the lumbar vertebrae, are the most often reported stress fractures. Non-activity causes of stress fractures and the closely related stress hernias, called Schmorl’s nodes, have been found in twin studies; these fractures and hernias seem to be correlated with normal anatomical variation found in the vertebral column. These anatomical variations are likely determined by genes.

Cross-Sectional Geometries

Cross-sectional geometries are examined in chapter 2; long bones can be envisioned as beams that experience biomechanical stresses, such as bending, torsion, and compressive forces, to which they remodel to prevent from breaking. Using cross-sectional measures, such as moments of inertia that represent bone’s ability to resist bending and torsional stresses, researchers can determine the direction of stresses and, therefore, the activities that cause these stresses. One common activity examined is mobility, which leads to anteroposteriorly oriented femoral cross-sections due to bending stresses. Asymmetry studies on tennis players have helped confirm that cross-sectional geometries change in response to activities. Yet animal studies, such as those on baboons, have been key in helping bioarchaeologists understand the non-activity pressures, such as age, sex, and genes, on cross-sectional geometries.