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1362748605 3 chapter3


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1362748605 3 chapter3

  1. 1. 9 Chapter III SURGICAL ANATOMY AND FUNCTIONAL CORRELATIONS  BONES, JOINTS, LIGAMENTS, ARCHES AND ARTERIES  FUNCTIONS OF ARCHES  BASIC FOOT DYNAMICS The human foot: The human foot is a complex functional unit. During the evolution of human being the method of locomotion has changed from quadrupedal to orthograde (erect and bipedal.) During the evolutionary development the basic function of foot having gripping ability in quadrupeds changed to ability of support. The human foot during the evolution has undergone developmental changes and these are based on the requirement to adjust the line passing through the centre of gravity to a very small area of the supporting surface. Once the gravitational stress was adjusted to the small area of support then the ability to continue to support the body by balanced action of the muscles against the fluctuation of the line of gravity became the basic function of the foot. The bipedal alternating gait of the human beings during the evolutionary development changed the articulations of the foot. The opposability of the big toe that was basic requirement for gripping ability was abolished and the certain changes in the tarsal bones ensured better supporting ability of the human foot. The mechanical marvel: The human foot is a mechanical marvel. Each foot consists of 29 joints, 26 bones, and 42 muscles. In a lifetime this phenomenal machine walks between 75,000 to 100,000 miles a distance equivalent to 3 to 4 times around the world and is exposed to significant pressures with each step. The muscles and the neurovascular structures, which descend from the leg into the foot, pass under the retinaculum in the foot. The dorsal structures like tendons, vessels and nerves are encased in a loose retinaculum while the medial structures are encased in a tight retinaculum. (Fig 1) This is clinically significant. The infection of the medial tendons like Flexor Halusis Longus tend to spread rapidly to the leg while those of extensor tendons tend to get localised over the dorsum of the foot. The plantar skin: The plantar skin is 4-5 mm thick and is thickest over the heel and the distal metatarsals. The skin at the birth is very delicate. However the process of walking makes the skin thicker and tougher. The skin of the sole is attached to the plantar aponeurosis and is creased. The plantar skin slides only about 1 cm. The dorsal skin slides over 2 to 3 cms. These factors help the human beings to walk more easily. The sole of the foot is a specialised structure and has the ability to spread the weight from the small area of bones to the large area of the skin. It is an anatomical fact that the barefoot walking makes the skin thicker and tougher with increased ability to bear
  2. 2. 10 the stress. It is said that the constant use of footwear reduces this ability of the NORMAL foot to bear stress and strain and adapt to the various shear forces. However it is the irony of the nature that this anatomical fact is counterproductive in case of insensitive foot like the one in diabetes. (Fig 2) Sweating and the plantar skin: The skin of the sole has no hair follicles but has numerous sweat glands. In diabetes, due to the progressive sudomotor dysfunction, ie abnormalities of the sweat secretion, the sweat glands reduce and the sweating reduces. The clinical significance is that the patient needs to be educated to observe the reduction in the sweating. The feet that sweat do not ulcerate as the skin remains moist and prevents the formation of fissures or cracks. The Heel: Heel of the foot is a specialized structure. It has thick skin with subcutaneous septae, which holds the skin to the fascia. The subcutaneous fat - heel pad of fat, occupies the space between the septae. This acts as shock absorber when the heel strikes the floor in the cycle of locomotion. However this also prevents the infection from being eradicated completely as the infections remains loculated. The foot size: The foot size does not change (Fig 3) after certain age in adulthood except in certain rare circumstances like acromegaly or gigantism. The perception that foot size changes as the neuropathy advances is incorrect. What changes is the shape of the foot due to advancing neuropathy. Again this has important clinical application. As the neuropathy advances the foot arches collapse and the site of the 1st metatarsophalangeal (MTP) joint in relation to the insole of the footwear changes. The appearance is that the foot size has increased. Another peculiarity of the human foot is that it cannot be studied in an animal model. The human foot is a complex structure and therefore it is not possible to duplicate it in animal model. This puts some restrictions on the research modalities. Ligaments of the foot: Ligaments of the foot are many. (Fig 4) The basic function served by these ligaments is to hold the foot as one functional unit during locomotion and to prevent the unnecessary movement. The ligaments also serve an important function in weight distribution. The collapse of the foot in diabetic neuropathy occurs when these ligaments loose their elasticity and the mobility of the joints increases leading to the destabilisation of the foot. Arches of the foot: Arches of the foot (Fig 5) are an important component of the human foot. The foot has to act as a pliable platform to support the body weight in upright posture. It also has to act as a lever for forward propulsion during walking, running and jumping. The arches are segmented and concave to sustain the stress. The arches are present
  3. 3. 11 from birth but are masked in infants by fat in the soles. There are longitudinal and transverse arches. Medial longitudinal arch: Medial longitudinal arch is higher, mobile and resilient. The bones of calcaneum, talus, navicular-cuneiform and the inner three metatarsals form the line of the arch. The main joint of this arch is talo-calcano-navicular. This arch is built more to absorb the shear forces. Lateral longitudinal arch: Lateral longitudinal arch is low, has limited mobility and is built to transmit weight of the body. The line of the lateral arch is formed by the bones calcaneum, cuboid and lateral two metatarsals. The longitudinal arches are maintained by bony factors, intersegmental ties, tie beams and sling supports. Each factor has clinical significance. The bone supports can derange due to osteolysis with neuropathy, tie beams can become loose due to glycosilation, and the sling supports can be damaged due to tenosynovitis. The transverse arches: The transverse arches are two in number. The anterior transverse arch is formed by the heads of five metatarsal bones and is complete as the 1st and 5th metatarsal heads meet the ground. Tarsal and metatarsal bones form the posterior transverse arch. It is incomplete unless both the feet are brought together. (Fig. 6) The functions of the arches: The functions of the arches are important in relation to diabetic foot. The arches bear the weight, help in walking and running, act as shock absorbers, and protect soft tissues and blood vessels due to concavity of the arches. The distribution of the weight is shown in (Fig 7). It is a computer generated image of pressure scans. The pressure is seen as borne by the heel, the metatarsal heads and the ball of the great toe as red in color. The collapse of arches due to the progressive neuropathy in diabetes causes damage to the soft tissues and results in typical Charcot's foot with large plantar ulcer. It is important to realize the basic anatomical alteration in the structure to prevent the ulceration of the foot. The collapse of the foot due to the damage to the arches needs to be prevented or at least halted to preserve the functional foot. This can be done if it is understood how the arches are maintained. Arteries of the foot: Arteries of the foot are important factors in the processes of etiopathogenesis of the diabetic foot. The posterior tibial is the dominant artery in the foot. It contributes major part of the plantar arch. The arteries of the leg that derive from popliteal artery which contribute the blood supply to the foot are anterior tibial and the peroneal tibial trunk, which divides into tibio-peroneal trunk which divides into peroneal and posterior tibial artery.
  4. 4. 12 The basic foot dynamics: To understand the factors, responsible for causation of the plantar ulcers in diabetes, it is necessary to understand the basic foot dynamics. As foot touches the ground, heel strikes the ground at the calcaneum which takes all the weight. As the foot becomes flat on the ground the talus bears the weight, which is divided between anterior and posterior pillars of the arch. The head of the talus is pushed downwards and is supported by the spring ligament. The outer border of the foot along with 5th metatarsal takes the weight. As the foot comes fully to the ground the weight is borne by the calcaneum, the base of 5th metatarsal and the heads of all 5 metatarsal bones. At the onset of second cycle as the foot comes off the ground the gastrocnemeus and the soleus lifts the calcaneum upwards. This exerts the bowstring effect through the muscles and other supports of the arch. Now only the heads of the metatarsals bear the weight of the body. Finally the contraction of the flexors of the toes gives the final thrust, which pushes the body forwards. At this point the weight on the foot is taken off. This basic cycle is repeated with every step. The whole mechanism is so finely tuned with mechanical action, coupled with sensory inputs that the tremendous mechanical complexity of the normal walking is never realized. It must be understood that the forefoot bears the maximum stress during the cycle of walking and majority of foot ulcers are in forefoot. The pressure borne by the forefoot during the cycle of walking is up to 600 Kilo Pascal's (kPa). This is high pressure. At 15 kPa the arteries carrying blood at a pressure of 120 mm of Hg are blocked while at 6 kPa the capillary circulation is blocked. However with every cycle of walking there is instant recovery from ischemia and anoxia. (A Note on Pressure Measurement on Foot: Among the sophisticated, computerized methods quantitatively measuring foot pressures there is variance in measurements. All systems use pressure transducer sensors. The value of the pressure is directly related to the density of the sensor in a given area. The greater the density higher is the pressure measured. Hence while pressure data is given especially in respect to criticality for skin breakdown it will be worthwhile to compare it with some other quantitative method with reference to sensor density. We will write about it in more detail in following chapters). Reference: 1. Bowker, John H and Pfeifer, Michael A (2001) Levin and O'Neal's. The Diabetic Foot. 6th Edition. Mosby St.Louis.