Part 4 - ‘The Implications of Variable Anatomical Traits of the Foot’ Greg Quinn, FCPodS, Podiatric Surgeon “The materials of action are variable, but the use we make of them should be constant”. Epictetus
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umans have evolved as upright bipedal apes and this requires our species to share particular physical traits that allow us to do this e.g. the sub-talar joint translates leg rotation into the foot during contact pronation. This ability is passed on because we inherit the many genes that govern development from our parents. The complex interplay of developmental growth factors continues for life as our feet are used to deliver their purpose i.e. to carry us forward in an efficient and pain free manner. As growth continues, we hope to ideally acquire an ability to deliver a foot function that matches the perfect exploitation of our inherited traits. There are three things that can work against this: Firstly, we may inherit physical traits in the foot that make this process more difficult to achieve; secondly we require an adaptability of our muscles, ligaments, bones and joints that can alter their make-up to match our individual movement patterns; and thirdly, our environment may alter to challenge the foots ability to function normally. Any of these three issues can result in anatomical or functional differences that stress the tissues of the foot, induce malfunction and ultimately produce the signs and symptoms that go with it.
Genetic factors It is widely appreciated that to one extent or another, familial resemblance can be very striking between generations and this is true for any aspect of our biology. Morphology, physiology and even behaviours can and have been genetically linked and this is also true for our feet. Whilst detailed genetic mapping studies are not yet available, foot problems have been recently implicated as an inherited trait1. Higher arched feet will demonstrate a more limited range of ankle joint extension and are more likely to have less range of motion at the sub-talar joint2. Conversely, low arched feet are more likely to demonstrate greater eversion of the heel as they pronate. Furthermore, the relative length of the first metatarsal is linked to population differences (themselves genetic in origin) and this is significant in that longer 1st toes are more prone to hallux valgus and hallux rigidus. The calcaneo-cuboid joint that is vital to the time sensitive change in stability of the mid-tarsus, also demonstrates differences between ape species and individual humans, which strongly implies differences in the timing, duration and specificity of genetically controlled bone growth. Changes that induce a less stable configuration of the opposing joint surfaces will produce a less stable arch, and this is unhelpful if the arch and heel are to lift together as the foot approaches the propulsive phase of ideal walking (see part 3).
Adaptive plasticity This term does not refer to the physical plasticity of the foot’s structures but rather to the ability of the foot to change its physiology to reduce the impact of potentially damaging forces. All biological systems have some measure of adaptive plasticity, either physical or physiological, and this ability is very useful to survival in a changing environment. E.g. the plantar skin has a variable ability to respond to high ground reaction forces to produce hyperkeratosis. However this protective ability whilst helpful, presents a potential problem: If the forces continue to be
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high, cornification of the skin can proceed at a faster rate, ultimately producing parakeratosis at the point of highest pressure i.e. plantar corn instead of protective callus. This can give rise to an underlying inflammatory response and pain. These are local physiological expressions of an inherent genetically controlled process and gives rise to a broader issue: adaptive plasticity is something of a double-edged sword. All musculo-skeletal tissues possess adaptive plasticity. They are physiologically in tune with the forces generated within and outside of the body. When the internal or external environment changes beyond the range of physiological normal values, the body adapts to minimise this effect. The sub-talar joint has been shown to be adaptively plastic during our early lives, changing shape to respond to increasing body weight3. When muscles are stimulated through continued exertion over a period of time, muscle size and strength increase. When not employed, they may atrophy and weaken and so save on valuable resources that can be used elsewhere. As forces change across the 1st MTP joint, the sub-articular bone will be strengthened or weakened according to Wolff’s law. This adaptively plastic response can create an abnormal joint angle i.e. hallux valgus. However, should forces be too high or occur over too short a time period for the physiological response to occur, then traumatic injury will occur e.g. metatarsal stress fracture. Within the musculo-skeletal system, pathological change is perhaps too often regarded as a separate change or different entity from what is regarded as physiologically normal. In actuality, there is only a quantitative change in expression of the body’s ability to respond to forces. At a particular and often arbitrary point, biological function is compromised and beyond that an individual body’s ability to cope is exhausted and pain results. The foot will demonstrate key common anatomical structures that create an ability to deliver a common purpose. Therefore, it is our shared genetic and adaptive responses that characterise our feet. These processes not only give rise to what we have in common, but also explain our individuality. It is not what position our bones and joints must be in at a particular point in time e.g. sub-talar neutral that matters, but the avoidance of exceeding the limits of tolerable forces and the body’s response that counts. I referred to three things that work against the ideal foot. The last of these, the environment and how it can challenge the function of the foot will form the basis of the 5th and final part of my article in the next issue of the journal. References 1. Hannan. Hallux valgus and Pes Cavus are highly heritable in older men and women: The Framingham Foot Study. American College of Rheumatology Annual Scientific Meeting, Atlanta, 2010. 2. Bruckner J: Variations in the Human Subtalar Joint. J Orthop Sports Phys Ther 8(10): 489-494, 1987. 3. Hellier CA, Jeffrey N: Morphological Plasticity in the Juvenile Talus. J Foot Ankle Surg 12: 139-147, 2006.
Greg Quinn, Sheffield gregquinn.podsurgeon@gmail.com