BIOMECHANICS OF EDENTULOUS STATE IN PROSTHODONTICS

The edentulous state represents a compromise in the integrity of the masticatory system. It is frequently accompanied by adverse functional & esthetic sequelae, which are varyingly perceived by the affected patient. Perceptions of the edentulous state may range from feelings of inconvenience due to total loss of teeth. Consequently, the required treatment addresses a range of biomechanical problems that involve a wide range of individual tolerences & perceptions.

1. BIOMECHANICS OF THE EDENTULOUS STATE DR SANKAR MADHAVAN

2. INTRODUCTION The edentulous state represents a compromise in the integrity of the masticatory system. It is frequently accompanied by adverse functional & esthetic sequelae, which are varyingly perceived by the affected pt. Perceptions of the edentulous state may range from feelings of inconvenience due to total loss of teeth. Consequently, the required treatment addresses a range of biomechanical problems that involve a wide range of individual tolerences & perceptions .

3. SUPPORT MECHANISM Natural dentition Complete denture

4. SUPPORT MECHANISM FOR NATURAL DENTITION Masticatory system is made up of closely related morphological, functional, behavioral components. Their interactions are affected by changes in the mechanism of support for a dentition when natural teeth are replaced by artificial ones. It is involved in trituration of food. Direct responsibility for this task falls on teeth & supporting tissues

5. Teeth function properly only if adequately supported. This support is provided by periodontium. periodontium connective tissue * hard * soft teeth periodontium bone of jaws Periodontium cementum dentin PERIODONTIUM

6. Periodontium regarded as functional unit . periodontium provides Resilient suspensory apparatus to forces. Allows teeth to adjust to their position under stress. periodontium  covered by epithelium. * hard connective tissue  cementum  bone * soft C.T  periodontal ligament  lamina propria of gingiva.

7. Periodontium is attached to dentin by cementum and the jaw bone by the alveolar process. The pdl and lamina propria maintain continuity between these 2 hard tissue components. PERIODONTAL LIGAMENT : Means by which force exherted on tooth is transmitted to the bone that supports it. Functions: Support. Positional adjustment of tooth. Sensory perception. The pt. Who needs CD therapy is deprived of periodontal support, & the entire mechanism of functional load transmission to the supporting tissues is altered.

8. Occlusal forces exherted on teeth are controlled by neuromuscular mechanisms of masticatory system. Reflex mechanisms with receptors in the muscles, tendons, joints & periodontal structures regulate mandibular movements. Through normal function, the periodontal structures in a healthy dentition undergo characteristic mechanical stress.

9. FORCES ACTING ON TEETH : vertical  * mastication –short duration *deglutition –long duration ( 500 times/day ) horizontal  *tongue *circumoral musculature. During activity  tongue > cheek,lips During rest  tongue = cheek,lips.

10. BITING FORCES : During mastication transmitted through bolus to opposing teeth (whether the teeth make contact or not) These forces  increase steadily  reach a peak  abruptly return to zero. Depends on: Consistency of food Chewing sequence Dental status

11. DIRECTION OF FORCES : Principally  to occlusal plane But the forward angulation of most natural teeth leads to introduction of a horizontal component. This tends to tilt the teeth medially, buccally or lingually. Upper incisors may be displaced labially with each biting thrust causing proximal wear facets.

12. TOOTH CONTACT : Normally, occurs during  * functional movements (chewing, deglutition) * parafunctional (clenching, grinding) *All the forces exherted onto the tooth seem to be well within the tolerence level of healthy periodontal tissues.

13. MASTICATION Actual chewing time / meal 4 meals /day 1 chewing stroke/ sec Duration of each stroke Total chewing forces/day DEGLUTITION Meals: Duration of 1 deglutition During chewing, 3 deglutitions/min Between meals: Daytime: 25/hr (16hr) Night time: 10/hr (8hr) TOTAL: 450 sec 1800 sec 1800 strokes 0.3 sec 540 sec( 9 min) 1 sec 30 sec( 0.5min) 400sec(6.6) 80 sec(1.3 min) 1050sec 17.5 min

14. SUPPORT MECHANISM FOR COMPLETE DENTURE The unsuitability of the tissues supporting CD for load bearing function must be immediately recognised because the mucous membrane is forced to serve an identical purpose as pdl. 45cm sq area of pdl is available in each arch. Mean-denture bearing area: Edentulous maxilla  22.96cm sq (>) Edentulous mandible  12.25cm sq This denture bearing area becomes progressively smaller as the residual ridge resorbs. Any disturbance in the normal metabolic process may lower the upper limit of mucosal tolerence & initiate inflammation.

15. MASTICATORY LOADS: Natural teeth  44lb (20 kg) Complete denture  13-16 lb (6-8 kg) Maximal bite force CD < natural dentition ( 5-6 times) The forces required for chewing vary with the type of food being chewed . Patients with prostheses frequently limit the loading of supporting tissues by selecting food that doesn ’ t require masticatory effort exceeding their tissue tolerence.

16. RESIDUAL RIDGE : Denture bearing mucosa Sub mucosa & periosteum residual alveolar bone The RRR is > in early postextraction period than the later period. Inspite of all the changes of residual alveolar ridge the rest position of the mandible remains fairly stable throughout life.

17. Alveolar process (contains teeth) Loss of teeth, alveoli that contained the roots of the teeth fill in with new bone. This alveolar process becomes residual ridge ( denture foundation area). ALVEOLAR PROCESS  RESIDUAL RIDGE

18. ALVEOLAR PROCESS RESIDUAL RIDGE

19. DIFFERENCE: ALVEOLAR BONE SUPPORTING NATURAL TEETH EDENTULOUS RESIDUAL RIDGE * Recieves tensile loads *Recieves vertical, diagonal & horizontal loads applied by denture. * Loads received through large area of pdl. * Loads received through surface area much smaller than total area of pdl of all natural teeth that had been present.

20. Wearing dentures is almost invariably accompanied by an undesirable & irreversible bone loss . 2 concepts concerning the ineviteble loss of residual bone: A direct consequence of loss of periodontal structures , variable progressive bone reduction occurs. The other maintains that residual bone loss is not a necessary consequence of tooth removal but depends on a series of poorly understood factors. FACT…

21. RETENTION The quality inherent in the prosthesis which resists the force of gravity,adhessiveness of foods, & the forces associated with the opening of the jaws. (GPT). Retention is the ability of the denture to withstand displacement against path of insertion. Dentures are held in position constantly by means of retention. Retention gives psycological comfort, & prevents dislodgement during speech/ eating.

22. 2 PHYSICAL FACTORS: Maximal extension of denture base. Intimate contact of denture base & mucosa . ANATOMICAL FACTORS : Size of denture bearing area.( mx > md.) Quality of denture bearing area.

23. Border seal : positive contact of the entire perimeter of the denture base to resilient tissues. Relief areas  decreases retention Peripheral seal  increases retention MECHANICAL FACTORS:

24. Undercuts : unilateral  aid in retention. Bilateral  non desirable, to be removed surgically UNDERCUTS

25. MUSCULAR FACTORS To increase retention & stability. Buccinator, orbicularis oris, intrinsic & extrinsic muscles of tongue – key muscles Design of labial, buccal, lingual polished surfaces of denture & Form of dental arch *are considered in balancing the forces generated by the tongue & perioral musculature.

26. ill fitting dentures lead to adverse psycological effects on some patients. Nervous influences  affect salivary secretions  affects retention. *Muscular stabilization of denture  Reduction in actual physical forces used in retaining dentures. PSYCOLOGICAL EFFECT ON RETENTION :

27. STABILITY The quality of the prosthesis to be firm,steady or constant ,to resist displacement by functional horizontal or rotational stresses. FACTORS AFFECTING STABILITY : The relationship of the denture base to the underlying tissues. Relationship of external surface & periphery to surrounding orofacial musculature . Relationship of opposing occlusal surfaces.

28. Denture base to tissues :  mandibular lingual flange . * lingual slope 90 degrees to occlusal plane. * resists horizontal forces. *post. Flange to extend more inferiorly. *extent of contact depends on the functional mobility of FOM.  residual ridge anatomy * large, square broad ridges  > resistance to lateral forces. *square or tapered arch  > rotational forces.

29. Periphery to orofacial musculature: *Buccal,labial flanges  concave * contouring of denture base to permit modiolus to function freely. Opposing occlusal surface:  Tooth position * arrange as close to natural position. *occ. Plane of mandible too high  dec. stability  Ridge relations *prognathic/ retro  dec. stability.

30. SUPPORT Foundation area on which a dental prosthesis rests. It is the resistance to vertical movement of the denture base towards the ridge. TYPES: Initial denture support. Long term support. Support is based on relationship b/w denture base & supporting tissues. Tissues must be able to tolerate functional stresses.

31. Primary components of human dental occlusion : Dentition Neuromuscular system Craniofacial structures. FUNCTIONAL CONSIDERATIONS OCCLUSION

32. DEVELOPMENT & ADAPTATION OF OCCLUSION DEVELOPING DENTITION HEALTHY ADULT DENTITION DETERIORATING ADULT DENTITION EDENTULOUS STATE *Extensive sensory input. *Development of motor skills *Dental,alveolar, craniofacial adaptability *Dental adaptation *Bone adaptation *Functional adaptation *Partial edentulism *Periodontal disease *Dimnished dental reflex adaptation. *Residual ridge reduction. *Compromised reflexes *Increased parafunctional mov. *Maladaptive denture experience

33. MASTICATION is the rhythmic separation & apposition of the jaws and involves biophysical & biochemical processes, including the use of all lips,teeth,cheek, palate & all the oral structures to prepare food for swallowing. Control of masticatory movements requires sensory information b ’ coz deviation from normal path can injure soft tissues.

34. During mastication the tongue& cheek muscles play an essential role in keeping the food bolus b/w the occlusal surfaces of the teeth Hence artificial teeth is to be placed within the confines of a functional balance of the musculature involved in controlling the food bolus b/w the occlusal surfaces of the teeth.

35. MASTICATION  other functions: Indirectly excites salivary & gastric secretions. Mixes food with saliva. Facilitates swallowing. Digestion of carbohydrates by amylase.

36. It has been concluded that masticatory efficiency as low as 25% is sufficient for complete digestion of foods. Maximal bite force  denture wearers 5-6 times< in edentulous pts.

37. FACT….! Edentulous patients are clearly handicapped in masticatory function, and even clinically satisfactory CD are a poor substitute for natural teeth. The quality of prosthetic service may have a direct bearing on the denture wearer ’ s masticatory performance.

38. Chewing  premolar & molar region Tough consistency foods  premolars *This is apparent even in pts. with mandibular partial dentures opposing C.D

39. Difference b/w pts.with natural teeth & CD Mucosal mechanism of support as opposed to support by the periodontium. The movements of dentures during mastication. Progressive changes in maxillo-mandibular relations & eventual migration of dentures. Different physical stimuli to the sensor motor systems.

40. Denture bearing tissues are constantly exposed to frictional contact of the overlying denture bases. Dentures move during mastication  due to dislodging forces of the surrounding musculature. movements  displacing, lifting, sliding, tilting,or rotating of denture. UNSEATING OF DENTURE

41. Opposing tooth contacts occur with both natural & artificial teeth during function & parafunction when the patient is both awake & asleep. Hence tissue displacement beneath the denture base results in tilting of the denture & tooth contacts on the non-chewing side + Occlusal pressure on the dentures displaces soft tissues of the basal seat & allows the dentures to move closer to the supporting bone. This change of position under pressure induces a change in the relationship to each other.

42. PARAFUNCTIONAL CONSIDERATIONS Hamful to teeth or other components of masticatory system. Additional loading of denture bearing area. Clenching  *common. *soreness of denture bearing area.

43. CLENCHING

44. CAUSES *psycosocial factors. *medical conditions. *Sleep disorders. *behavior disorders. Intra oral conditions *pain. *oral lesions. *xerostomia. *occlusal discomfort. Bruxism  *increase in tonic activity of jaw muscles.

45. Tongue thrusting  *thrusting tongue forward * causes sore tongue. A strong response of lower lip & mentalis muscle has been observed in CD wearers with impaired retention & stability of lower denture. Thus, tentative occlusal contacts triggers development of habitual non-functional occlusion.

46. Blood flow: Pressure  soreness  interruption of blood flow  upsets metabolism of tissue involved. Parafunction can be included as a possible significant prosthetic variable that contributes to the magnitude of ridge reduction.

47. FORCE GENERTATED MASTICATION PARAFUNCTION DIRECTION MAINLY VERTICAL FREQUENTLY HORIZONTAL & VERTICAL DURATION & MAGNITUDE INTERMITTENT & LIGHT DIURNAL ONLY PROLONGED, EXCESSIVE , BOTH DIURANL & NOCTURNAL DURING FUNCTION & PARAFUNCTION

48. CLENCHING MASTICATION

49. CHANGES IN MORPHOLOGICAL FACE HEIGHT Skeletal growth terminates  20-25 yrs. Also continues into adult life  changes in adult facial skeleton Morphological face height increases  intact dentition Decreases  attrition/ abrasion Further reduction  edentulous/ CD wearers

50. ATTRITION ABRASION

51. Maxillomandibular changes : Occurs slowly  years Due to bone remodelling. Involves articular surface of TMJ (proliferative capacity) Any change in morphological face height/ loss of teeth  transmitted to TMJs. But the articular surfaces undergo continious remodelling to maintain height

52. Reduction in residual ridge in CD Reduction in VD Reduction in total facial height Mandibular prognathism

53. Mean reduction of height Mandibular process in anterior region resorbs 4times > maxillary process. Resorption patterns Maxilla: *outer cortical plate  thinner *teeth flare  downward & outward. *resorption  upward & inward. Mandible: *outer cortical plate  thickest *teeth inclined forwards  anteriors *vertical or lingual  posteriors width of mandible is greatest at inferior border *resorption  lingual & inferior  anterior  buccal  posteriors.

54. VERTICAL DIMENSION Refers to length of the face. Maintained by *occlusion * balanced tonic contraction. Measured by *VD of physiologic rest position *VD of occlusion.

55. VD at rest: The length of the face when the mandible is in rest position (GPT). Position of mandible in relation to maxilla when oral musculature are in a state of tonic equilibrium. Reference point when recording VD at occlusion.

56. Registration of jaw in physiological rest position gives an indication of appropriate VD of occlusion. The inter occlusal distance is the distance or gap existing b/w upper & lower teeth when mand. Is in physiological rest position. Usually 2-4mm in the 1 st premolar region.

57. VD at occlusion The length of the face when the teeth( occlusal rims, central bearing points, or any other stops) are in contact and the mandible is in centric relation or the teeth are in centric relation ( GPT). The VD of occlusion is established by the vertical height of the two dentures when teeth are in contact. It is established for edentulous patients so that their denture teeth will come into contact at an appropriate height.

58. INCREASED VD Leads to *resorption * clicking noise *facial distortion *swallowing difficulty.

59. DECREASED VD Leads to * damage to TMJ *limited tongue space *more facial distortion *chin closer to nose *lips loose fullness. *facial muscles tonicity lost *angular chelitis

60. REST POSITION OCCLUSION

61. CENTRIC RELATION Reference/starting point in jaw relations. The most posterior position of the mandible relative to the maxillae at the established vertical dimension.( most unstrained position) AP bone-bone relation,that of the maxilla& mand. The most retruded position of idle condyles in the glenoid fossa. (McCollum) Unconscious swallowing is carried out with the mandible at or near the centric relation position.

62. Centric relation: Retruded position. Most retruded position. Records that coincide with CR can be made at varying vertical positions with teeth/ occlusal rims.

63. According to possell,the border movements of the mandible are reproducible & all other movements take place within the framework of the borders. TERMINAL HINGE MOVEMENT Mandible retruded  hinge recorded in saggital plane at incisor point.

64. SIGNIFICANCE OF CR It is a definite learned position. Pt. can voluntarily & reflexly return to this position. Can be recorded & repeated. In mounting the cast on an articulator , the AP relation of maxi. & mand. casts becomes a definite entity. This position can be verified ,as other records can be made in the mouth. CR is a reference point in recording maxillomandibular relationships & a starting point for the development of occlusion. It is a point of return.

65. Erupting teeth are guided into occlusion by the surrounding musculature. Contacts of inclined planes of teeth aid in alignment of the erupting dentition. Position of mandible determined by its location in space during the act of unconscious swallowing. DEVELOPMENT

66. Occlusion of CD is designed to harmonize with primitive and unconditioned reflex of patients unconscious swallow. Tooth contacts & mandibular bracing against maxilla occurs during swallowing in CD pts. Hence, CD occlusions should be compatible with forces developed during deglutition.

67. CENTRIC OCCLUSION Reference starting point in designing of artificial occlusion( cant be registered accurately in ed.) During deglutition, mandible is close to,in centric relation, or the position of maximum mandibular retrusion relative to the maxillae at the established VD of occlusion. Most functional natural tooth contacts occur in a mandibular position anterior to centric relation, position referred to as centric occlusion .

68. CENTRIC OCCLUSION is the relation of opposing occlusal surfaces that provides the maximum intercuspation. Tooth-tooth relation. ECCENTRIC RELATION is contacting of teeth when jaws are in any other relation than CR.

69. ESTHETIC CHANGES Loss of tooth can adversely affect a person ’ s appearance. Morphological changes associated with edentulous state: Deepening of nasolabial groove. Loss of labiodental angle. Horizontal labial angle. Narrowing of lips. Inc.in columella-philtral angle. Prognathic appearance. Recently, plastic surgical interventions, facial cosmetics have been popularized for correction.

70. MORPHOLOGICAL CHANGES

71. BEHAVIORAL & ADAPTIVE RESPONSES Pts. Ability & willingness to accept & learn to use the dentures ultimately determine the success of clinical treatment. Presence of dentures in an edentulous mouth( foreign objects) ellicts different stimuli to sensorymotor system, inturn influencing the cyclic masticatory stroke pattern. Both exteroceptors & proprioceptors are affected by the prosthesis.

72. LEARNING  acquisition of new activity or change of an existing one. MUSCULAR SKILL  capacity to co-ordinate muscular activity to execute movement. HABITUATION  gradual diminution of responses to continued or repeated stimuli .

73. contact of prosthesis with oral cavity tactile stimuli arises Ignored later Each response is related to a memory trace (storage of information  habituation) Difficulty to store information..old age Hence older people have difficulties becoming comfortable with dentures.

74. Stimuli must be specific & identical to achieve habituation. This prevents transfer of habituation evoked by an old familiar denture to a new denture. This gives rise to new range of stimuli causing adaptation problems. Learning & co-ordination diminishes with age.(atrophy of cerebral cortex) Certainly, patient motivation dictates the speed with which adaptation to dentures takes place.

75. CONCLUSION The edentulous state represents a compromise in the integrity of the masticatory system.It is frequently accompanied by adverse functional & esthetic sequelae, which are varyingly perceived by the affected patient. Irrespective of future population needs, the psycological & biomechanical consequences of tooth loss must never be overlooked.