Understanding Biomechanical Forces (Skliar)

MedicalContinuing Education Continuing Medical Education

Goals and Understanding Objectives 1) To familiarize podiatrists with a basic review and/or Biomechanical introduction to biomechanical forces.

Forces 2) To be able to associate biomechanical effects on patients and their mechanical problems.

3) To be able to treat biome- Here’s a chanical problems mechani- cally and surgically in accord refresher on with established mechanical 173 the basics of principles. this science. 4) To have an understanding of biomechanical forces that By J. David Skliar, influence the use of surgical DPM implants and to be able to min- imize destructive forces during and after surgical procedures.

Welcome to Podiatry Management’s CME Instructional program. Our journal has been approved as a sponsor of Con- tinuing Medical Education by the Council on Podiatric Medical Education. You may enroll: 1) on a per issue basis (at $23.00 per topic) or 2) per year, for the special rate of $179 (you save $51). You may submit the answer sheet, along with the other information requested, via mail, fax, or phone. You can also take this and other exams on the Internet at www.podiatrym.com/cme. If you correctly answer seventy (70%) of the questions correctly, you will receive a certificate attesting to your earned credits. You will also receive a record of any incorrectly answered questions. If you score less than 70%, you can retake the test at no additional cost. A list of states currently honoring CPME approved credits is listed on pg. 180. Other than those entities currently accepting CPME-approved credit, Podiatry Management cannot guarantee that these CME credits will be acceptable by any state licensing agency, hospital, managed care organization or other entity. PM will, however, use its best efforts to ensure the widest acceptance of this program possible. This instructional CME program is designed to supplement, NOT replace, existing CME seminars. The goal of this program is to advance the knowledge of practicing podiatrists. We will endeavor to publish high quality manuscripts by noted authors and researchers. If you have any questions or comments about this program, you can write or call us at: Podiatry Management, P.O. Box 490, East Islip, NY 11730, (631) 563-1604 or e-mail us at [email protected]. Following this article, an answer sheet and full set of instructions are provided (pg. 180).—Editor

force is defined as that opposite force exists which cancels Loads which can cause accel- the first one. In biomechanics, we A load is any force applied to the eration of matter, or differentiate two categories of forc- outside of a structure, and thus sus- the resistance of mat- es. Those that come from outside a tained by the structure. The body’s ter to acceleration. If structure are called loads. Those that weight on the foot bones and the re- Aa force causes no visible accelera- are generated within a structure are active force of the ground pushing up tion, it must be because an equal but called stresses (Figure 1). Continued on page 174 www.podiatrym.com MARCH 2014 | PODIATRY MANAGEMENT CME Continuing

Medical EducationBiomechanical (from page 173) sistance of the intermolec- ular bonds in a substance on the foot structures are examples to deformation by exter- of loads. The pull of a muscle on a nal loads. Note, we are bone is a load on the bone. Howev- talking about mechanical er, it is equally valid to consider the stress, which is unrelat- bone’s resistance to the muscle as a ed to physiological stress load on the muscle. (Figure 3). 1) Tension Stress: A Specification of Stresses and Loads force in matter which re- There are two ways in which sists its being pulled apart forces can be expressed. or stretched. When a mus-

Figure 1: The force applied to the bars is the load. The force resisting the shearing force is the stress. The forces generated in a substance in response to the load applied to it are known as stresses.

1) Total Force: The total weight cle contracts, its tendon of forces regardless of the area, such develops tension stress at 174 as body weight. This is employed in its bony insertion. determining joint reactive forces. 2) Compression Stress: 2) Unit Force: This is the force A force in matter which re- per unit area, such as pounds per sists its being pushed to- square inch. This specification is gether. It is also referred to most significant in dealing with bio- as pressure. Body weight mechanical forces (Figure 2). on the foot bones produces compression stress. Figure 2: Total loads are the same in both halves, but unit loads Stress 3) Shear Stress: A force are greater on the right side. In accordance with Newton’s in matter which resists its third law, when a force is exerted, it being torn (scissored) apart. The re- Strain causes an equal and opposite force sistance of a substance to motion Strain is the deformation of an on the substance acted upon. The (friction) is a shear stress. Friction object when a load is applied to it. forces generated in a substance in makes the underlying surface a point All deformations are a combination response to the load applied to it of firm application. Without it, there of the three principal strains, analo- are known as stresses. It is the re- would be no locomotion. Continued on page 175

Figure 3: The three principal stresses. Figure 4: The three principal strains.

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Biomechanical (from page 174) used in orthotics), but it is consid- when the force is removed. In pa- ered 100% elasticity even though thology, its ability to stretch is in- gous to the stresses. it lacks resilience. Such materials creased (lowered elastic modulus), 1) Tension Strain: Elongation of are termed viscoelastic (Figure 5). and its elastic limit is reduced, re- an object to which tension stress is They are characterized by a partial sulting in permanent deformation, applied (Figure 4). instantaneous deformity with the e.g., osteomalacia. 2) Compression Strain: Short- application of a load followed by a Young’s Modulus (Elastic Modulus) This is a measure of the stiffness, When material is loaded, the strain will be or rigidity, of a material. It is math- proportional to the stress for a while. Eventually, the ematically arrived at by dividing the stress by the resulting strain (defor- strain begins to increase faster than the stress. mation). The larger the modulus, the stiffer the material. Note that strength (toughness, the resistance to frac- ening or squashing of an object time-related continued deformation. ture) and stiffness are not the same. to which compression stress is Upon removal of the load, the For example, wet bone has an elastic applied. process is reversed with a partial modulus of 2 x 106 psi and an ulti- 3) Shear Strain: Displacement or instantaneous rebound followed by mate tensile strength of 12,000 psi. delamination of an object to which a a time-related return to the original Glass has an elastic modulus of 10 shear stress is applied. shape. The instantaneous deforma- x 106 (5 times the resistance to de-

175

Figure 5: Viscoelastic Stress-Strain Relations Figure 6: Solids Stress-Strain Relationship oA: Instantaneous deformation A: Proportional limit. OA: Proportional range. B: Elastic limit. AB: Time-related deformation 0B: Elastic range. C: Yield point. D: Ultimate strength. E. Rupture point. BC: Instantaneous return CE: Plastic range. YIX: Slope for Young’s modulus. CD: Time-related return

Stress-Strain Relationship tion can be likened to the action of a formation, or stiffness, as bone) and 1) Solids: a) Elasticity: All mate- spring, and materials which do so are an ultimate tensile strength of only rials have some elastic properties. It known as Hookian Bodies. The grad- 5,000 psi (about 2.5 times weaker is the ability to return to its original ual deformation can be likened to a than bone). size and shape after having been dash pot. In mechanics, such materi- Some materials have the same deformed, following removal of the als are known as Newtonian bodies. moduli in compression and tension. load. If the return to the original Thus, any biologic material that dis- These are homogeneous solids like state is instantaneous, the mate- plays both Newtonian and Hookian plastics and metals. Biological struc- rial is said to be resilient, (e.g., a properties is termed viscoelastic. tural materials like bone, cartilage, new tennis ball or a billiard ball). Tissues other than bone (soft tis- and tendon, have a “grain” and thus In some materials, the return to the sues) possess viscoelasticity. Bone, have different moduli in different original state may be time-delayed although slightly elastic, does not directions. (e.g., some plastic polyethylenes always return to its original form Continued on page 176 www.podiatrym.com MARCH 2014 | PODIATRY MANAGEMENT CME Continuing

Medical EducationBiomechanical (from page 175) joints to protect them from wear and seizure. Proportional Limit When material is loaded, the Stress Gradient strain will be proportional to the When a tension load is applied stress for a while. Eventually, the to an object, the lines of stress are in strain begins to increase faster than an opposite direction and all stress the stress. That is the point known lines are parallel to each other. If as the proportional limit. The pro- a hole is placed in the object, the portional range (from start to propor- stress lines will branch out around tional limit) is the area of the stress- the hole causing the lines to become strain curve from which Young’s concentrated in that area. This con- modulus is determined (Figure 6). centration of stress lines is known as the stress gradient (Figure 7). If the Elastic Limit hole is less than 20% of the diame- This is the ultimate point to Figure 7: Stress gradient caused by a defect in ter, it will have minimal effect on the which a material can be deformed the structure. structural strength of the part. Holes and still return to original shape with greater than 20% of the diameter of a removal of the load. point. It is any additional load nec- bone will lead to bone failure. essary to further deform a substance If a bone is to be drilled for Yield Point after its yield point. screws or a plate, multiple holes This is the point on a stress-strain should be kept twice the diameter curve from which progressive de- Rupture Strength of the hole from each other. If this is 176 formation occurs without increasing Following a material’s yield not possible, it is best to stagger the the load on the part. This is the start point, it continues to deform so that holes. The purpose of a pin or screw of the plastic range. Some materials less external load is necessary to pro- in a bone hole is not to keep the (like plastics) will have large plastic ranges, i.e., ductility, and may not even reach a fracture point. Materials Fatigue damage that have a yield point take impact well and are said to be tough. Materi- takes time to develop. Early stages are almost als that lack a yield point take impact poorly and are called brittle. impossible to detect.

Ultimate Strength This is the maximum stress a ma- duce additional strain. Elongation parts together, but rather to remove terial will sustain. With brittle ma- will continue until the material ulti- the stress from the fracture site. terials, this is usually at or near the mately breaks. Materials may differ yield point. With ductile materials in their rupture strength according to Stress Risers the ultimate strength may be sub- the type of stress applied. Surface scratches, even mi- stantially higher than the rupture The rupture strength of typical croscopic ones, sharp angles, and wet bone is 12,000 psi in grooves weaken structures and are tensile strength, 15,000 called stress risers. They cause an psi in compressive increase in stress concentrations that strength, and 8,000 psi start and cause the propagation of in shear strength. This surface cracks (Figure 8). The abil- difference can be cor- ity of a scratch or groove to cause related to the site of frac- a local increase in surface stress is ture of a bone. called the notch effect. The sharper the curve at the root of the defect, Fluids the more the structure is weakened. Rounding the edges of holes drilled Viscosity in bone for screws reduces some of This is a measure the notch effect. of resistance of fluid to Figure 8: The number below each surface notch is the fraction of flow. Flow is a form of Factors Influencing Bone Failures the original strength of the material that is left. The sharper the shearing strain. Low vis- curve, the more the structure is weakened. This is the notch effect. cosity fluids flow quick- Bone Diameter A notch is a stress riser that locally concentrates stresses above the ly. High viscosity fluids As the cross-sectional area of a surrounding average value. are better lubricants for Continued on page 177

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Biomechanical (from page 176) balanced so that the structure stays still. bone decreases, stresses increase. Uniaxial Loads If a load is centered ex- Sex actly over a structure, thus There is no apparent acting in line with the axis difference in the behavior of the structure, it is known and properties of human as a uniaxial load. Stress- bone on the basis of sex. Figure 10: A beam loaded in static bending. C: compression. T: tension. es would be generated per- There are differences, how- S: shear. fectly evenly throughout ever, in males and females the material in the structure in the frequency of types of bone plastic arteries, sutures, etc., or any (Figure 9). injuries. device implanted in the body is due Compression Stress and Strain: to fatigue failures. Unlike inorganic These forces develop greatest when Age Because of the frequency of tibial injury in the sport of skiing, that bone has been intently studied. It Bending generates unequal stresses in material, the was found that the strength proper- greatest stresses occurring at surfaces, and the least at ties of the tibia in individuals over the age of 70 were approximately some region inside the structure. half those at age 20. In those under 17 years of age, strength limits of 177 bones were also about half those implants, living structures rarely parallel to the line of action of the of adults. This is attributed to the fail in fatigue. This is because they load. are self-repairing, not because Tension Stress and strain: They they have an inherent resistance develop greatest at right angles to the to fatigue. Fatigue failures of load direction, and at equal magni- bone are usually secondary to tude to the compression stress. something that impairs the repair Shear Stress and strain: These system. develop along a line of action that Fatigue damage takes time to is 45 degrees to the lines of greatest develop. Early stages are almost compression and tension. The shear impossible to detect. Factors that stress is half the magnitude of the increase the possibility of bone compression or tension stress. fatigue are the condition of the The above forces are true for surface (scratches), changes in structurally homogeneous materials. surface contour (functional adap- Biological substances—bone, tendon, tation of bone), and stress risers. Continued on page 178

External Loads In a study of the influence of force on the body in skiing, it was de- Figure 9: Stresses and strains loaded vertically in termined that the acceler- uniaxial compression. C: compression. T: tension. S: shear. ation at the leg increased geometrically with in- smaller cross-section of the tibia in crease in ski velocity, where- children. Between 20 and 60 years, as the acceleration at the hip the strength properties of the tibia and head only increased lin- remain constant. early. Moreover, the acceler- ations at the tibia are about Fatigue seven times larger in skiing Breakage caused by repeated than in running. Figure 11: Horizontal shearing stress in a structure loaded loading and unloading within the in bending can be shown by bending a stack of boards. Top: normal elastic limits of a substance Static Forces no load. Bottom: Loaded in static bending. Note that if this is called fatigue. All solids are sus- These involve stress- were a solid structure like bone, the various layers would try ceptible to fatigue failure. Much of es and strains in structures to slide on each other. The resistance to sliding would devel- the breakage of plates, pins, screws, in which external loads are op a horizontal shearing stress. www.podiatrym.com MARCH 2014 | PODIATRY MANAGEMENT CME Continuing

Medical EducationBiomechanical (from page 177) the amount of material composing sistance of the tibia is about three it resists bending, better than does times the twisting resistance. The and cartilage—are grained materials a solid rod of the same kind and forward bending elastic threshold and as such respond variably, depen- amount of material. and plastic limit are directly pro- dent upon the type of stress consid- portional to the cross section of the ered. Bone, for example, has greater Torque (torsion) tibia shaft. The backward bending resistance to compression than to Torque is a twist or rotation-like fracture limit is 15% less and the distention. bending. It results in unequal stress- elastic threshold 10% less than in

Eccentric Loads Uniaxial loads, although ideal, are rarely encountered in the body. In real structures, loading conditions Usually, there is some eccentricity are usually combinations of the three primary to the line of action of the load with respect to the line of resis- types of loads. tance by the structure. As a result, bending forces occur which have to be added to those due to the es in material. It is produced by two forward bending. This is probably compression. couples of forces acting in parallel related to the fact that the anteri- planes at right angles to the axis, or aspect of the tibia is narrower Bending and working in opposite directions. than the posterior surface, and also Bending generates unequal Tension and compression are zero there is a slight forward bend nor- stresses in material, the greatest when the forces are parallel, oppo- mally in the tibia. 178 stresses occurring at surfaces, and site, and at right angles to the bone. the least at some re- Tension and com- Combined Loads gion inside the struc- pression stresses are In real structures, loading con- ture. Tension stress maximal when the ditions are usually combinations of and strain develop two forces are par- the three primary types of loads. on the convex sur- allel with the long Bending and torque both produce face. Compression axis of the bone. maximal stresses at surface fibers. In stress and strain This is very signifi- fact, much larger surface stresses are develop on the con- cant in functional produced by combined bending and cave and horizontal adaptation of bone, torque than any other combination of plane. The verti- and explains why de- two loads. cal forces are equal vices such as a De- This is usually the result of ath- throughout the sur- nis-Browne Bar are letic injuries, especially skiing. Ulti- face; the horizontal ineffectual in correct- mate strengths and elasticity of bone forces of shear are ing a femoral or tibi- are decreased about 12-15 % when greatest at the cen- al torsion. there is combined bending and tral part (neutral twisting. PM plane) of the struc- Cantilever ture, and least at the Bending References outer surfaces (Fig- This is a situation Frost, H.M., An Introduction To Bio- ure 10). wherein a bending mechanics, Charles Thomas, Springfield, The horizontal force is put on an 1967 shearing stress and erect beam. There Biomechanical Studies of the Musculo-Sketal System,Ed. F. Gaynor strain may be visu- is a uniaxial com- Figure 12: Cantilever bending and the Evans, Charles Thomas, Springfield, alized by bending a distribution of compression (converging pression load on 1961 stack of boards. When arrows) and tension (diverging arrows) the beam equal to a beam made in this stress produced. the load, causing a way is bent, the similar compression Dr. Skliar was Associate Professor in Bio- boards at the end displace (Figure stress, distributed evenly (as shown mechanics at N.Y. 11). In solid beams and bones, this by the arrows inside the beam). College of Podiatry slippage cannot occur, but tries to, There is also a bending force creating and Barry University. and the resistance to this strain is the tension stress on the external side He is the author of the textbook “Biome- horizontal shearing stress. (convex), subtracting from the com- chanics and Podiatric Failure (fracture) occurs pri- pression there (Figure 12). Orthopedics” and is marily on the convex surface. Long The forces on the tibia in ski- past president of the bones are essentially tubular in ing are an example of cantilever American College of structure. A tube in proportion to bending. The forward bending re- Foot Orthopedists.

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See answer sheet on page 181.

1) Body weight on the foot bones mined on a stress-strain rela- 12) The purpose of a pin or is considered to be a: tion from zero stress to its: screw in bone is to: A) Load A) proportional limit A) Keep the parts together B) Stress B) elastic limit B) Reduce stress at the frac- C) Strain C) yield point ture site D) none of the above D) rupture point C) Stimulate new bone growth D) none of the above 2) Forces generated in a sub- 8) The larger the modulus of stance without deforming it are elasticity: 13) Stress risers in biologic mate- termed: A) the stiffer the material rials can be caused by: A) loads B) the greater its tensile A) Surface scratches B) stresses strength B) sharp angles C) strains C) the higher is its rupture C) grooves D) unit forces point D) all of the above D) none of the above 3) Friction is equated with: 14) Stress gradients can be min- A) Tension stress 9) Rupture strength of a biolog- imized when placing multiple 179 B) Shear stress ical material is: screws into a bone: C) Compression stress A) weakest in tensile stress A) by placing the screw D) Compression strain B) intermediate in shear holes a minimum of 1X the stress diameter of the hole from 4) Delamination of an object is C) greatest in compressive each other. the result of: stress B) by placing the screw A) Tension stress D) not related to type of ap- holes a minimum of 2X the B) Compression strain plied stress diameter of the hole from C) Shear strain each other. D) Shear stress 10) A hole placed into a bone C) and not staggering the will not effectively lead to bone holes. 5) Perfect elasticity is exhibit- failure if the hole is no more D) None of the above. ed if, after removal of the load, than __% of the diameter of the the object returns to its original bone. 15) Which factors usually cause shape: A) 75% increased bone fractures: A) instantaneously B) 50% A) Large bone diameter. B) in a delayed manner C) 40% B) Being an adult male. C) viscoelastically D) 20% C) Age under 17 years. D) all of the above. D) None of the above. 11) All are correct except: 6) Elastic Modulus refers to a A) Viscosity is a measure of 16) The rupture strength of typi- material’s: resistance of fluid to flow. cal wet bone is: A) Stiffness B) Flow is a form of tension A) 2,000 psi in tensile strength B) Toughness stress. B) 4,000 psi in tensile strength C) Elastic limit C) Low viscosity fluids flow C) 12,000 psi in tensile D) Rupture strength quickly. strength D) High viscosity fluids are D) 24,000 psi in tensile 7) Young’s modulus is deter- better lubricants. strength Continued on page 180 www.podiatrym.com MARCH 2014 | PODIATRY MANAGEMENT $ CME EXAMINATION PM’s Continuing

Medical Education CPME Program 17) The rupture strength of typical wet bone is: Welcome to the innovative Continuing Education A) 5,000 psi in compressive strength Program brought to you by Podiatry Management B) 15,000 psi in compressive strength Magazine. Our journal has been approved as a C) 25,000 psi in compressive strength sponsor of Continuing Medical Education by the D) 35,000 psi in compressive strength Council on Podiatric Medical Education.

18) Regarding Torque (torsion), which of the Now it’s even easier and more convenient to following is correct? enroll in PM’s CE program! A) Tension and compression are zero You can now enroll at any time during the year when the forces are parallel, opposite, and and submit eligible exams at any time during your at right angles to a bone. enrollment period. B) Tension and compression are maxi- PM enrollees are entitled to submit ten exams mized when the forces are parallel with published during their consecutive, twelve–month the long axis of the bone. enrollment period. Your enrollment period begins C) Because of torque effect on long bones, with the month payment is received. For example, bars are ineffectual in correcting a femoral if your payment is received on September 1, 2006, 180 or tibial torsion. your enrollment is valid through August 31, 2007. D) All of the above. if you’re not enrolled, you may also submit any exam(s) published in PM magazine within the past 19) Cantilever bending results from a force twelve months. CME articles and examination placed on an erect beam at a distance from the questions from past issues of Podiatry Manage- beam: ment can be found on the Internet at http:// A) which causes only a tension stress on www.podiatrym.com/cme. Each lesson is ap- the beam. proved for 1.5 hours continuing education contact B) which causes only a compression stress hours. Please read the testing, grading and payment on the beam. instructions to decide which method of participa- C) which causes a compression stress and tion is best for you. a bending force causing tension stress on Please call (631) 563-1604 if you have any ques- the external side. tions. A personal operator will be happy to assist you. D) which makes the convex side more Each of the 10 lessons will count as 1.5 credits; prone to fracture. thus a maximum of 15 CME credits may be earned during any 12-month period. You may select any 10 20) Regarding bone fracture: in a 24-month period. A) Most bone fractures are the result of the three primary types of loads. The Podiatry Management Magazine CME B) Bending and torque produce maximal program is approved by the Council on Podiatric stresses at surface fibers. Education in all states where credits in instructional C) Combined bending and torque are media are accepted. This article is approved for usually the result of athletic injuries, 1.5 Continuing Education Contact Hours (or 0.15 especially skiing. CEU’s) for each examination successfully completed. D) All of the above.

Home Study CME credits now See answer sheet on page 181. accepted in PennsylvaniaContinued on page 180

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EXAM #3/14 Understanding Biomechanical Forces (Skliar)

Circle: 1. A B c D 11. a B c D 2. A B c D 12. a B c D 3. A B c D 13. a B c D 4. A B c D 14. a B c D 5. A B c D 15. a B c D 6. A B c D 16. a B c D 7. A B c D 17. a B c D 8. A B c D 18. a B c D 9. A B c D 19. a B c D 182 10. a B c D 20. a B c D

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