Bruce Latimer

Editorial: The Perils of Being Bipedal

Humans are unique among mammals in that we walk in a most peculiar way, on our extended hind limbs. Indeed, as far as we know no other mammal species, save our immediate ancestors, has ever even experimented with this bizarre method of locomoting. This is perhaps not altogether surprising in light of the fact that the fastest human on earth cannot beat a rabbit in a hundred meter dash. Humans are notoriously slow runners. They also are prone to numerous musculoskeletal injuries that do not plague other mammals. This article will address this latter issue. That is, the role that human walking plays in the etiology of several human conditions. The evolutionary shift to upright human-like walking is quite ancient with fossil evidence suggesting that it occurred somewhere between 6 and 8 million years before present (MYBP). Walking upright is, thus, not only rare and an adaptation of long standing, it likely is the “breakthrough” adaptation of the lineage that eventually led to modern Homo sapiens. Associated with the transition to bipedality are a host of obvious musculoskeletal modifications rendering the adaptation itself relatively easy to recognize even from rather scant paleontological evidence. This means that the evolution of bipedality can be closely followed through the existing fossil record. The end result is that the modern human postcranial skeleton differs markedly from our closest biological cousin, the chimpanzee. Most of the major differences we see between the postcranial skeletons of contemporary apes and humans are directly related to the reorganizations made to accommodate the kinematic and kinetic consequences of force transmission during locomotion. In standing habitually upright, humans have redirected the gravity vector some 90◦ from where it was in our prevertically oriented ancestor. This alteration has had profound anatomical implications. It has, moreover, created a novel mechanical environment which has, in turn, led to numerous problems, or maladies that are similarly unique to humans. The following will examine some of these conditions, particularly in the human foot. We will also consider the possible relationship between habitual bipedality and the systemic condition known as age-related osteopenia or osteoporosis, a condition that naturally occurs, among mammals, only in humans. Indeed, it is unfortunate but true that if we live long enough, nearly all of us will suffer from being bipedal. Not just aching feet, sprained ankles, or arthritic knees and hips, but a whole host of conditions that are as unique to our species as is our peculiar way of walking. For example, only our species regularly endures such common maladies as fractured hips, bunions, hernias (inguinal and femoral), fallen arches, torn menisci, shin splints, herniated intervertebral discs, fractured vertebrae, spondylolysis, scoliosis, and kyphosis—just to name a few. Why among mammals, including our cousins, the chimpanzee and the gorilla, do we alone suffer these debilitating problems? The answer can be found in evolution, the process that allows us to understand how we developed into what we are today. Before progressing it is valuable to briefly discuss the approach taken here. The question of modern human musculoskeletal anatomy is seen from the point of view of comparative anatomy, especially that of higher primates. It is important to keep in mind that comparing and contrasting the anatomy of modern African apes and humans is to focus on the end points of evolution. In order to address the stages that led to what we see today and to reconstruct the selective agents responsible, it is necessary to examine the fossil record. Therefore, we will also examine the evolutionary history of our own species, looking for clues that may shed light on our present condition, especially contemporary problems whose etiology is related to two-legged, upright walking. As noted, the skeletal modifications related to the transition to bipedality are relatively easy to recognize and we can analyze these modifications with respect to the probable mechanical role each change had during the emergence from a nonbipedal ancestor. Any dramatic skeletal features found in humans or our ancestors but not present in chimpanzees or gorillas are likely to be important in the overall functioning of the animal. This method of analyzing contemporary clinical conditions from an evolutionary perspective has been called an adaptive or evolutionary approach.10

Comparison of Helical Tomotherapy versus Conventional Radiation to Deliver Craniospinal Radiation

The purpose of this study was to investigate whether helical tomotherapy would better dose-limit growing vertebral ring apophyses during craniospinal radiation as compared to conventional techniques. Four pediatric patients with M0 medulloblastoma received tomotherapy craniospinal radiation (23.4 Gy, 1.8 Gy/fx) by continuous helical delivery of 6 MV photons. Weekly blood counts were monitored. For comparison, conventional craniospinal radiation plans were generated. To assist in tomotherapy planning, a cross-sectional growth study of 52 children and young adults was completed to evaluate spine growth and maturation. Vertebral ring apophyses first fused along the posterolateral body-pedicle synostosis, proceeding circumferentially toward the anterior vertebral body such that the cervical and lumbar vertebrae fused early and mid-thoracic vertebrae fused late. For the four pediatric patients, tomotherapy resulted between 2% and 14% vertebral volume exceeding 23 Gy. Conventional craniospinal radiation predicted between 33% and 44% exceeding 23 Gy. Cumulative body radiation doses exceeding 4 Gy were between 50% and 57% for tomotherapy and between 25% and 37% for conventional craniospinal radiation. Tomotherapy radiation reduced neutrophil, platelet, and erythrocyte hemoglobin levels during treatment. Tomotherapy provides improved dose avoidance to growing vertebrae as compared to conventional craniospinal radiation. However, the long-term effects of tomotherapy dose avoidance on spine growth and large volume low dose radiation in children are not yet known.