Biomechanics in implantology

implants

1. BIOMECHANICS IN IMPLANTOLOGY Presented by Dr. Anuja Gunjal MDS II 22/12/2017 1

2. CONTENTS Introduction Loads applied to dental implants Mass, force and weight Forces and components of forces Three types of forces Stress Stress-strain relationship Biting forces 2

3. Force delivery and failure mechanism A scientific rationale for dental implant design Summary References 3

4. Introduction The discipline of bioengineering, which applies engineering principles to living systems, has unfolded the new era in diagnosis, treatment planning and rehabilitation in patient care. One aspect of this field, biomechanics concerns the response of biological tissues to applied loads. 4

5. The fundamental concepts and principles of dental biomechanics are for long-term success of dental implants and restorative procedures. 5

6. Definition (GPT9) Biomechanics:- 1. the application of mechanical laws to living structures, specifically the locomotor systems of the body; 2 . the study of biology from the functional viewpoint; 3 . an application of the principles of engineering design as implemented in living organisms; 6

7. Loads Applied To Dental Implants 7 In function – occlusal loads Such loads may vary in magnitude, frequency, duration depending on patient parafunctional habits . Absence of function – Perioral forces Horizontal loads

8. These may be of greater magnitude with parafunctional oral habits. Thus basic units in mechanics help to understand such physiologic and nonphysiologic load, and can determine which t/t renders more risk. 8

9. MASS, FORCE AND WEIGHT Mass – A property of matter, is the degree of gravitational attraction the body of matter experiences . Unit – kgs : ( lbm ) FORCE (SIR ISAAC NEWTON 1687) Newton’s II law of motion Acceleration of a body is inversely proportional to its mass and directly proportional to the force that caused the acceleration F = ma Where a = 9.8 m/s2 Thus mass (m) is a determining factor in establishing magnitude of a static load . 9

10. WEIGHT Is simply a term for the gravitational force acting on an object at a specified location. 10

11. Forces and force components: Magnitude, duration, direction, type and magnification ‘Vector quantities’ Load direction – dramatic influence A force applied to a dental implant rarely is directed absolutely longitudinally along a single axis. 11

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13. Three dominant clinical axes exist in implant dentistry. Mesiodistal, faciolingual and Occluso-apical and they commonly result from single occlusal contact. The process by which three-dimensional forces are broken down into their component parts is referred to as vector resolution 13

14. Along with the direction of force it is also important to specify the point of action of a vector Vectors are usually written in bold faces (F) or with an arrow above F and magnitude is written as simply F. 14

15. F = 44.5 N at pt B Analysis - vector resolution Co-ordinate system Angles that the F vector makes with co-ordinate axes, resolution of F into its 3 components is possible i.e. Fx , Fy & Fz F = F 2 x + F 2 y + F 2 z Cos 2  x + Cos 2  y + Cos 2  z = 1 Lateral as well as vertical components are acting at the same time Not || to direction of long axis of implant

16. If more than one force is acting on some object then the resulting force is the vectorial sum of all the forces acting on the body Vector addition : More than one force FR = F1 + F2 + F3 16

17. MOMENT / TORQUE action which tend to rotate a body Eg : In addition to axial force, there is a moment on the implant which is equal to magnitude of force times (multiplied by) the perpendicular distance (d) between the line of action of the F and center of the implant 17

18. Three types of forces Compressive Tend to push masses towards each other Maintains integrity of bone – implant interface Accommodated best by implant system Cortical bone strongest in compression Cements, retention screws, implant components and bone – implant interfaces- accommodate compressive forces Dominant in all implant prosthetic occlusion

19. Tensile Shear   Pull object apart Sliding Both of these forces distract / disrupt the bone implant surface. Shear forces are most destructive and cortical bone is weakest in shear

20. Cylinder implants are at highest risk for harmful shear loads at the implant to tissue interface. So require a coating to manage it by having more wider and uniform bone attachment. Threaded / Finned dental implants impart a combination of all three force types at the interface under the action of a single occlusal load. 20

21. STRESS The manner in which a force is distributed over a surface is referred as mechanical stress  = F/A •The internal stress have strong influence on longevity of the implants. So, goal of t/t planning should be to minimize and evenly distribute mechanical stress in the implant system and contiguous bone.

22. The magnitude of stress depends on 1) Force magnitude 2) Cross sectional area over which the force is dissipated. Force magnitude :- Magnitude of force can be reduced by reducing the magnifiers of force as Crown height Cantilever length Night guards to decreases nocturnal parafunction. Occlusal material that decreases impact force. Overdentures that can be removed at night.

23. 23 Functional cross sectional area Functional cross sectional area is the area that participates significantly in load bearing and stress dissipation. This area may be optimized by :- Increasing the number of implants for a given edentulous site. Selecting an implant geometry that has been designed to maximize functional cross sectional area.

24. Deformation and Strain o A load applied to implant can cause deformation of implant and surrounding tissue. Surrounding tissue can react to it by remodeling. o Deformation and stiffness characteristics of implant material may influence interfacial tissue, ease of implant manufacture and clinical longevities. o Concept of strain is believed to be a key mediator of bone activity. 24

25. Under act of tensile force FN the straight bar originally LO is elongated by amount  l. The strain = deformation per unit length. 25

26. STRESS – STRAIN RELATIONSHIP If any elastic body is subjected experimentally to an applied load, a load-versus-deformation curve can be generated. Dividing the load values (Force) by the surface area over which they act and the charge in the length by the original length produces a classic stress-strain curve.

27. 27 Such curve provides for the prediction of how much strain will be experienced in a given material under the action of an applied load. The slope of the linear portion of this curve is referred to as modulus of elasticity. Closer the modulus of elasticity of the implant to surrounding biologic tissues, the less the chances of relative motion at bone-implant interface.

28. The cortical bone is at least 5 times more flexible than titanium. Once a implant system ( i.e specific biomaterial) is selected , the only way for a clinician to control the strain is by controlling applied stress or change the density of bone around the implant . 28

29. The density of bone is related not only to the bone strength, but also the modulus of elasticity (stiffness). The stiffer the bone, the more rigid; the softer the bone, more flexible. It is more important to decrease stress in softer bone, for two primary reasons: To reduce the resultant tissue strains resulting from elasticity difference. Because softer bone exhibits a lower ultimate strength. 29

30. Hooke ’s law is the name give to the relationship between stress and stain: 6 = E  where 6 = normal stress (Pa or Psi), E = modulus of elasticity (Pa or Psi), and  = normal strain ( unitless ). For shear stress and shear strain, where the constant of proportionality is the modulus of rigidity (G) expressed by t = G g Where t = shear stress (Pa or psi), G = modulus of rigidity (Pa or psi), and g= shear strain ( unitless ). 30

31. BITING FORCES Normal human :- Axial component of biting forces : (100 – 2400 N) / (27 – 550 lbs ) It tends to increase as one moves distally Lateral component - 20 N (approx .) subjects with prosthesis in the first mandibular molar region. Net chewing time per meal = 450 sec Chewing forces will act on teeth for = 9 min/day If includes swallowing = 17.5 min/day Further be increased by parafunction Provides minimum time /day that teeth (implants) are bearing load due to mastication and related events

32. IMPACT LOADS When two bodies collide in a very small interval of time (fractions of a second), relatively large reaction forces develop. Such a collision is described as impact . The higher the impact load, the greater the risk of implant and bridge failure and bone fracture. For decreasing it: 32

33. Skalak has suggested the need for using acrylic teeth in conjunction with osteointegrated fixtures to partially mitigate high impact loads, which might damage bony tissues adjacent to the implant. Weiss has proposed that a fibrous tissue-to-implant interface provides for physiologic shock absorption in a similar fashion to that exhibited by a functioning periodontal ligament. Misch advocates an acrylic provisional restoration with a progressive occlusal loading to improve the bone-to-implant interface before the final restoration. 33

34. STIFFNESS OF TOOTH AND IMPLANT Prosthesis supported by teeth and implants. Neither Rangert nor Skalak model specifically deal with differencing mobility A way to approach this problem is Displacement in any direction Unidirectional force but displacement in many direction Secondary effect Application of constant force Increase in displacement slowly with time  Creep Not significant with implants Intrusive tooth displacement is not always Linear – usually bilinear Net stiffness > natural tooth

35. FORCE DELIVERY AND FAILURE MECHANISMS The duration of a force may affect the ultimate outcome of an implant system . Relatively low magnitude forces, applied repetitively over a long time, may result in fatigue failure of an implant and/or prosthesis. Stress concentrations and, ultimately, failure may develop if insufficient cross-sectional area is present to adequately dissipate high magnitude forces. 35

36. MOMENT LOADS If a force is applied some distance away from a weak link in an implant or prosthesis, bending or torsional failure may result from moment loads The moment is defined as a vector (M) (vectors are described in terms of both magnitude and direction), whose magnitude equals the product of the force magnitude multiplied by the perpendicular distance, i.e., M = F  D 36

37. Torques or bending moments imposed on implants as a consequence of, for example, excessively long cantilever bridge or bar sections, may result in: 1.      interface breakdown, 2.      bone resorption, 3.      prosthetic screw loosening, and/or 4.      bar/bridge fracture. 37

38. Clinical Moment Arms and Crestal Bone Loss A total of six moments may develop about the three clinical co-ordinate axes. Mesiodistal axis – Lingual / Facial movement Faciolingual axis – Occlusal / Apical movement Vertical Axis – lingual Transverse / Facial Transverse movement. 38

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40. Such moment loads induce micro rotations and stress concentrations at the crest of the alveolar ridge at the implant-to tissue interface, which lead inevitably to crestal bone loss. Three clinical moment arms exist :- Occlusal height Cantilever length Occlusal width 40

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42. Minimizations of each of these moment arms is necessary to prevent- 1.      Unretained restorations, 2.      Fracture of components, 3.      Crestal bone loss and/or 4. Complete implant system failure. 42

43. Occlusal Height Occlusal height serves as the moment arm for force components directed along the faciolingual axis for: Working or balancing occlusal contacts, Tongue thrusts, In “passive” loading by cheek and oral musculature, as well as force components directed along the mesiodistal axis. 43

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45. Cantilever Length Large moments may develop from vertical axis force components with cantilever extensions or offset loads from rigidly fixed implants. A lingual force component may also induce a twisting moment about the implant neck axis if applied through a cantilever length 45

46. Force applied directly over the implant does not induce a moment load or torque. When a full-arrived prosthesis with cantilever segment supported by anterior 4 or 6 implants. Infinite no. of loading cycles can be maintained at low stress levels. The stress level below which an implant biomaterial can be loaded indefinitely is referred to as its endurance limit. T1 alloy exhibits high endurance limit than CpTi . 46

47. Because stress = force / area, both aspects are considered . The magnitude and direction of force are determined by- parafunction , crown height, masticatory dynamics, gender , age , and arch location. 47

48. The functional surface area is determined by: the number of implants, width, length, design, and bone density, which determines area of contact and bone strength. 48

49. The greater the A-P distance between the center of the most anterior implant(S) to the most distal aspect of the posterior implants, the smaller the resultant loads on the implant system form cantilevered forces because of the stabilizing effect of the anteroposterior distance. According to Misch , the length of this distal cantilever is determined by the amount of stress applied to the system. 49

50. Clinical experiences suggest that the distal cantilever should not extend 2.5 times the A-P distance under ideal conditions (parafunction absent, ideal implants, etc.). The maxilla has less dense bone than the mandible, and more often has an anterior cantilever with the prosthesis. 50

51. As a result, more distal implants may be required in the maxilla to increase the A-P distance for the anterior or posterior cantilever than in the mandible, and sinus augmentation may be required to permit posterior placement of the implant. 51

52. Occlusal width Wide occlusal tables increase the moment arm for any offset occlusal loads. Faciolingual tipping (rotation) can be significantly reduced by narrowing the occlusal tables and/or adjusting the occlusion to provide more centric contacts. 52

53. PRELOAD Preload is the axial force in the neck of the screw, which is between the first mating thread and the head of the screw. It improves both the locking effect and the fatigue strength of the screw. To be effective the level of preload must be less than the elastic limit or proof load, i.e. the maximum load at which no permanent deformation occurs, of the screw material and greater than any force applied by biting action. 53

54. The relationship between applied torque and preload depends on several factors: They include: • Screw geometry • Material properties, particularly stiffness • Surface texture and condition of the mating surfaces • Degree of lubrication • Rate of tightening • Integrity of joint Hence , screw threads are never used for clamping purposes, and locking devices and/or thread adhesive is usually used. 54

55. Fatigue Failure Fatigue failure is characterized by dynamic, cyclic loading conditions. Four “fatigue factors” significantly influence the likelihood of fatigue failure in implant dentistry: Biomaterial Geometry of the structure. Force magnitude Number of Cycles. 55

56. Biomaterial Fatigue behavior of biomaterials is characterized graphically in what is referred to as an S-n curve ( a plot of applied stress versus number of loading cycles). 56

57. If an implant is subjected to an extremely high stress, only a few cycles of loading can be tolerated before fracture will occur. Alternatively , an infinite number of loading cycles can be maintained at low stress levels. The stress level below which an implant biomaterial can be loaded indefinitely is referred to as its endurance limit . 57

58. Geometry: Influences the degree to which it can resists bonding and torsional loads and ultimately fatigue fracture. Implants rarely display fatigue fracture under axial compressive loads compared to lateral loads. The geometry also includes the thickness of metal or implant . 58

59. Fatigue fracture is related to fourth power of the thickness difference. Often a weak link in an implant body design is affected by the difference in the inner and outer diameter of the screw and the abutment screw space in the implant. 59

60. Force magnitude : Reduction of applied load (stress) If an applied load (stress) can be reduced, the likelihood of fatigue is reduced. Magnitude can be reduced by : Arch position i.e Higher loads on posterior compared to anterior maximum / mat. Elimination of moment loads. Optimize geometry for functional area. Increase the number of implants used. 60

61. Loading cycles :- Fatigue failure is reduced to the extent if the number of loading cycles can be reduced. Aggressive strategies to eliminate parafunctional and reduced occlusal contacts some to protect against fatigue failure. 61

62. Summary The most common complication in implant-related reconstruction are related to biomechanical conditions. Prosthesis failure may result from all of the foregoing or bending fracture resistance. In addition, the manifestations of biomechanical loads on dental implants controls the long term health of the bone-to implant interface. Knowledge of basic biomechanical principles is thus required for the dentist 62

63. References Contemporary Implant Dentistry IInd Edition. Carl E. Misch . Dental Implant Prosthetics. Carl E. Misch . Skalak R. Biomechanical considerations in osseointegrated prostheses. J Prosthet Dent. 1983 Jun;49(6):843-8. 63