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1362748638 5 chapter5


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1362748638 5 chapter5

  1. 1. 26 Chapter V BIOMECHANICS AND FOOT PRESSURES A NOTE FOR UNDERSTANDING THE ISSUES INVOLVED:  FOOT PRESSURES IN RELATION TO FOOT INJURY  PHYSICS OF PRESSURE AND UNITS OF MEASUREMENT  CONCEPT OF SHEAR FORCES  DYNAMICS OF PRESSURE CHANGES  MEASURING PRESSURES  FACTORS CAUSING INCREASED PRESSURE Insensate feet2 resulting in repetitive unrecognized trauma and abnormal distribution of pressure on feet emerge as the principal factors in causing foot ulcers. External injuries were discussed in the pathogenesis of foot ulcers earlier in this monograph. Infection per se is a very small part of ulcer development. Vasculopathy certainly interferes with healing but the causation of ulcer is attributable to the first three reasons alluded to.8, 9, 10 Excessive mechanical pressure with loss of protective sensation seems to be the most important factor. Diabetic patients with a history of neuropathic plantar ulceration have abnormally high pressures under the foot. The peaks of pressure occur most frequently under the metatarsal heads and correlate with the sites of ulceration. However, patients with high plantar pressures in the presence of protective sensation, as seen for example in rheumatoid disease, do not develop foot ulcers since they do not have insensate feet.5 Hence majority of the foot ulcers that occur do so on the insensate plantar forefoot subjected to repeated trauma and abnormal pressures.11,12, 13, 14, 15,16,17 Dorsal injuries are most often the result of footwear pressing on those areas. There is no accurate idea about how correct the footwear currently marketed in India as diabetic footwear is. But the chances are that at least some varieties may be suboptimal and cause to add these injuries to the diabetic foot. It is now known why persons with diabetes have higher pressures under the feet. We also need to know something about the concept of shear forces adding to the biomechanical causes resulting in the foot ulcers. The tissue properties also play a part in it. The remedial measures will be discussed in other sections of the monograph. The Physics: The pressure is any mechanical force divided by the area over which it is applied. The force is measured as Dynes, or Newton. In the context of diabetic foot it is the patients weight while standing divided by the area of both feet actually bearing the pressure i.e. weight. The weight in a normal foot is equally divided on each foot, on the metatarsal heads, the lateral arch and the heel. The average area of the adult foot
  2. 2. 27 is 175 Sq. Centimeter. Out of which, normally 100 to 125 sq Centimeters comes in contact with the flat surface while bare foot walking. Considering one's weight of 60 KG and area of contact to be 100 sq cm. the pressure he will exert while standing on one foot normally will be 0.6 KG per sq cm. which is equivalent to 61 kilo Pascal. But at the time of heel landing or take off with a push where only a small portion of the foot is in contact it can be much higher and will be higher with faster speed. The concept of shear forces: Shear forces can be understood as the force exerted on the feet while walking on the rollers. Two types of shears are described, vertical shear and the anteroposterior and lateral shears. These shears are a fraction of the total pressure exerted by the weight of the patients. Changes in the pressure dynamics: The heads of the metatarsals, the lateral arch and the heel distribute the weight among them. While walking the 1st metatarsal head could bear pressures much higher than the other metatarsal heads. If for any reason the shape of the foot changes and all these points no longer remain in contact with the floor the pressure distribution will be uneven, and only those points in contact will bear all the pressure as the weight of the patient remains unchanged. It is also then understandable that if the pressure rises beyond the point the skin can take without breaking down, necrosis and tissue breakdown will occur forming an ulcer. Therefore direct measurement of the planter pressures is essential. Bony prominences do not always show high pressures and undeformed feet do not always come up with low pressures. Dynamic changes in foot pressures: As the heel strikes the ground and the foot progressively comes in contact with the floor, the heel lifts and the forefoot bears all the pressures there is a continuous change in the quantum of pressure each part in contact with the floor. When finally the forefoot metatarsal heads dorsiflex and the foot is lifted off the grounds it is the first metatarsal that bears the greatest pressure. That is also the reason why majority of the forefoot ulcers occur below the first metatarsal head. Hallux rigidus causes very high pressures as it does not dorsiflex at take off. Units of pressure measurement: The literature of foot pressure dynamics use a number of terms briefly explained below. The most frequent is Kilo Pascal. A weight of 1.02 kg per square centimeter is equal to 100 Kilo Pascal. To get an idea of what is 1 kg / square centimeter; one could look at the tyre pressures in our cars. Car tyre pressure is normally 28 to 30 PSI. That is a weight of 30 pounds is applied per squire inch of the tube. It is also nearly equivalent to 2 kg per square centimeter of the tube. It also is the same as 2 atmospheric pressures which is easy to understand. One Atmospheric pressure is equal to 14.7 pounds per sq. inch is nearly equal to 100 Kilo Pascal. The Truck tyre pressure is nearly 80 PSI. The pressure used to inflate the balloon for Angioplasty is up to three atmospheric pressures. Physiologically speaking, at 6 Kilo Pascal the capillary flow is obliterated. At a pressure of 15 Kilo Pascal the normal arterial blood pressure of 120 mm of Hg is obliterated. The peak plantar pressure generated while walking in normal feet is about 600 Kilopascals. The breakdown pressure of the normal skin is between 500 to
  3. 3. 28 700 Kilo Pascal. In a normal speed walking Rosenbaum has shown the pressures at the first metatarsal head to be about 299 kilopascals.18 The pressures are related to the speed of walking. The higher the speed greater is the pressure. The longer the stride the higher is the pressure, including the shear stresses. (Other terms the readers may come across in the literature are Newtons, Dynes and Joules. Joules is Measure of Work, ie equal to lifting One KG by One Meter is One Joule. That is Consumption of 0.4 Calories. Calorie of food is actually Kilo calorie that is 1000 Calories. It is equivalent of One watt bulb burning for 1 second. This could also be understood as the Force applied over Time. The force time product is also called or understood as Power. Dyne is Force, which in the patient context is weight, which is mass in grams multiplied by the force of gravity (981 cm / second / second). One Newton is equivalent of 1 kg weight. It means that a mass of 1 kg is being acted upon by the gravity which is 9.81 meters / second /second. It means that this is a force capable of accelerating a kg of mass as a free falling body at the rate of 9.81 meters / second / second.) Additional factors causing increased pressures on the diabetic foot: Pressure necrosis and ulceration is the most expensive of all the complications. The expense, both to the patients and the community, of hospital treatment for this common complication is well documented.1 Foot ulceration generally preventable, and relatively simple interventions can reduce amputations by up to 80%.3 Based on many studies linking high barefoot pressure to ulceration, it makes sense to measure barefoot plantar pressure in all patients with loss of protective sensation. It was suggested that a pressure of 750 Kpa provides the best level of discrimination between low-risk and high-risk patients.4 Diabetic patients with a history of neuropathic plantar ulceration have abnormally high pressures under the foot. The peaks of pressure occur most frequently under the metatarsal heads and correlate with the sites of ulceration. However, patients with high plantar pressures in the presence of protective sensation, as seen for example in rheumatoid disease, do not develop foot ulcer.5 Limited Joint Mobility (LJM) is associated with an increased plantar pressure and a greater history of foot ulceration. Glycosylation of collagen in tendons and ligaments results in limited motion of joints. The movement of the subtalar joint and the big toe was significantly reduced in the diabetic foot, because any reduction in mobility of this joint may cause an increase in plantar pressures during walking. In the presence of LJM, the foot is unable to provide its shock-absorbing mechanism and may lose its ability to maintain normal plantar pressures. The presence of LJM can result in abnormally high intrinsic plantar pressure and lead to ulceration, the final mechanism involves prolonged repetitive moderate stress.6 LJM and increased foot pressure appears to be important determinants of foot ulceration in diabetic patients.
  4. 4. 29 The forefoot to rear foot plantar pressure ratio (F/R), gives the clinician more information about the plantar pressure than mere knowledge of the peak pressure. Since it is believed that there is an imbalance in pressure distribution between the rear foot and forefoot in diabetic patients with neuropathy, it becomes important to determine the F / R ratio which provides a measure of the peak plantar pressure under the forefoot and rear foot. An association between F / R ratio and LJM was determined7 that concluded that, in South Indian diabetic patients, an F/R greater than 2 was found to be associated with neuropathy, LJM and ulceration. Footwear and appropriately fitted insoles may drastically relieve the areas of high foot pressure and decrease the mechanical stress of walking. Orthoses and footwear, designed to distribute the pressure evenly under the feet are needed to compensate joint mobility in the feet. References: 1. Shobhana R, Rama Rao P, Lavanya A, Vijay V, Ramachandran A. Cost burden to diabetic patients with foot complications- a study from southern India. JAPI 2000;48:12. 2. Ellenberg M. Diabetic neuropathic ulcer. J Mt Sinai Hosp 1968;35:585-594. 3. Boulton AJM. Why bother educating the multidisciplinary team and the patient: the example of prevention of lower extremity amputations in diabetes. Patient Educ Couns 1995;26:183-188. 4. Armstrong DG, Peters EJG, Athanosiou KA, Lavery LA. Is there a critical level of plantar foot pressure to identify patients at risk for neuropathic foot ulceration. J Foot Ankle Surg 1998; 37:303-307. 5. Masson EA, Hay EM, Stockley I. Abnormal foot pressure alone may not cause ulceration. Diab Med 1989;6:426-428. 6. Vijay V, Snehalatha C, Sivagami M, Ramachandran A. Association of limited joint mobility and high plantar pressure in diabetic foot ulceration in Asian Indians. Diab Res and Clin Pract 2003; 60:57-61. 7. Viswanathan V, Sivagami M, Seena R, Snehalatha C, Ramachandran A, Veves A. Increased forefoot to rear foot plantar pressure ratio in South Indian patients with diabetic foot ulceration. Diab Med 2004;21:393-399. 8. Cavanagh PR, Simoneau GG, Ulbrecht JS: Ulceration, unsteadiness, and uncertainty: The biomechanical consequences of diabetes mellitus. J Biomech 26, (Suppl1), 23- 40, 1993. 9. Frykberg RG: Biomechanical considerations of the diabetic foot. Lower Extremity 2:207-214, 1995. 10. Payne CB: Biomechanics of the foot in diabetes mellitus. Some theoretical considerations. J Am Podiatr Med Assoc, 88:285-289, 1998. 11. Barrett JP, Mooney V: Neuropathy and diabetic pressure lesions. Orthop Clin North Am, 4:43-47, 1973.
  5. 5. 30 12. Boulton AJM: The pathogenesis of diabetic foot problems: An overview. Diabet Med 13 (Suppl 1): S12-S16, 1996. 13. Brand PW: Repetitive stress in the development of diabetic foot ulcers. In Levin ME, O'Neal LW (eds): The Diabetic Foot, 4th ed. St.Louis: CV Mosby 1988, pp 83-90. 14. Smith L, Plehwe W, McGill M, et al: Foot bearing pressure in patients with unilateral diabetic foot ulcers. Diabet Med 6:573-575, 1989. 15. Stess RM, Jensen SR, Mirmiran R: The role of dynamic plantar pressures in diabetic foot ulcers. Diabetes Care 20:855-858, 1997. 16. Stokes IAF, Faris IB, Hutton WC: The neuropathic ulcer and loads on the foot in diabetic patients. Acta Orthop Scand 46:839-847, 1975. 17. Veves A, Murray H, Young MJ, Boulton AJM: Do high foot pressures lead to foot ulcerations? A prospective study [abstract]. Diabetologia, 34, (Suppl 2): A40, 1991. 18. Rosenbaum D, Hautmann S, Gold M, Claes L: Effects of walking speed on plantar pressure patterns and hind foot angular motion. Gait Posture 2: 191-197, 1994. REMINDER: WHY DIABETES PATIENTS GET FOOT PROBLEMS? Loss of sensation (Neuropathy) Reduced blood supply (Vasculopathy) Injury and Infection