Neutral zone in complete dentures

seminar on neutral zones in complete dentures

1. The neutral zone in complete dentures BY PAAVANA II MDS

2. CONTENTS INTRODUCTION DEFINITION MUSCLE FORCES IN THE DENTAL ARCH CHANGES IN THE EDENTULOUS MOUTH DENTURE SURFACE AND ITS ROLE RECORDING OF NEUTRAL ZONE CASE REPORT S REFERENCES

3. INTRODUCTION Complete dentures are primarily mechanical devices which are fabricated in harmony with normal neuromuscular function.

4. When all of the remaining natural teeth are removed, a void exists within the oral cavity that may be called the potential denture space. The denture space is bounded by the soft tissues in the oral cavity. Within the denture space, there is an area that has been termed as a NEUTRAL ZONE .

5. Forces are developed through muscle contraction during the various functions of chewing, swallowing and speaking, they vary in magnitude and direction in different individuals and in different periods of life . The way these forces are directed against the dentures will either help to stabilize them or will tend to dislodge them . The neutral zone is that area in the mouth where the forces of the tongue pressing outward are neutralized by the forces of the cheeks and lips pressing inward.

6. In the literature, NEUTRAL ZONE has been called as Dead Zone ( Fish, 1933 ) Potential Denture Space (Robert, 1960) Stable Zone ( Brill & Tryde,1965 ) Biometric Denture Space (Watt & Mc Gregor,1986) Zone of Least Interference ( Wright, 1991)

7. Various theories have been put forward to enhance stability of the mandibular denture . Lammie et al(1956) suggested that mandibular posterior teeth should be arranged over the buccal shelf area to provide increased tongue space and to facilitate development of facial polished surfaces.

8. Wright (1961) believed that mandibular posterior teeth should be arranged directly over the centre of the stress bearing area. This location may or may not correlate with the crest of the edentulous ridge . Campbell (1980) stated that mandibular posterior teeth should be arranged slightly lingual to the crest of the ridge while maxillary posterior teeth should be arranged slightly buccal to the edentulous ridge . Lang (1983) and Sharry suggested that posterior denture teeth should be arranged directly over the crest of the ridge.

9. DEFINITION The potential space between the lips and cheeks on one side and the tongue on the other; that area or position where the forces between the tongue and cheeks or lips are equal.(GPT – 9)

10. INFLUENCE OF MUSCLE FORCES IN THE DENTAL ARCH Teeth erupt into the mouth under the influence of muscular, environmental and genetic factor. This environment which is created by the forces between the tongue, cheeks and lips has a definite influence on the position of the erupting teeth, the resultant arch form and occlusion .

11. When the teeth are erupting into the oral cavity during childhood and adolescence, the muscular activity and habits that develop continues through life. Even after the teeth are lost, the forces created by these habits and actions still persist and will have a great influence on any complete or extensive partial removable prosthesis that is placed into the mouth. It is therefore extremely important that the teeth be placed in that part of the mouth and with an arch form that falls within the area formed by muscular forces.

12. Fish in 1933 highlighted the importance of the muscular function of the tongue, cheeks and lips as being critical factors for denture stability. The soft tissues that form the internal and external boundaries of the denture space greatly affect and influence the stability of the dentures and help to determine the -Peripheral borders -Tooth position -External contours of the dentures.

13. MUSCLES INVOLVED IN NEUTRAL ZONE

14. MUSCLES OF CHEEKS BUCCINATOR ORIGIN   Alveolar processes of   maxilla  and  mandible and   temporomandibular joint INSERTION Fibers of the  orbicularis oris ARTERIAL SUPPLY Buccal artery NERVE SUPPLY Buccal branch of the facial nerve (VII cranial nerve)

15. SIGNIFICANCE Buccinator presses against the dental arches when it contracts. During chewing and swallowing the muscle rhythmically contracts with muscles of mastication. It assists in placing the food between the teeth and returning the food to occlusal table which has escaped into the vestibule .

16. MUSCLES OF LIPS AND MODIOLUS Modiolus – “hub of wheel” in latin Forms a distinct conical prominence just distal to corner of the mouth

17. MUSCLES INVOLVED IN MODIOLUS Orbicularis oris Zygomaticus major Levator labii superioris Levator anguli oris Buccinator Triangularis Risorius Mentalis

18. SIGNIFICANCE As situated at the corner of the mouth it is in a strategic position to unseat the lower denture and sometimes the upper denture too. This may occur if the arch form is too wide and restricts the movement of the modiolus .

19. ORBICULARIS ORIS ORIGIN Maxilla and   mandible INSERTION Skin around the lips ARTERIAL SUPPLY Inferior labial artery and   superior labial artery . NERVE SUPPLY Cranial nerve VII ,  buccal branch SIGNIFICANCE- It is active when the lips are pressed against the teeth. Like buccinator , orbicularis oris rhythmically contracts during chewing, sucking and swallowing

20. ZYGOMATICUS MAJOR ORIGIN Anterior  of  zygomatic INSERTION Modiolus of   mouth ARTERIAL SUPPLY Facial artery NERVE SUPPLY Zygomatic and   buccal  branches of the  facial nerve SIGNIFICANCE- It pulls the angle of the mouth upward and backward.

21.  LEVATOR ANGULI ORIS (CANINUS) ORIGIN Maxilla INSERTION Modiolus ARTERY Facial artery NERVE Buccal branches  of the  facial nerve SIGNIFICANCE - In swallowing, helps to pull the lips forward, thus exerting forces on the teeth and alveolar process.

22. MENTALIS ORIGIN Anterior   mandible INSERTION Chin NERVE Mandibular branch of   facial nerve

23. SIGNIFICANCE The bottom of the sulcus is lifted when the muscle contracts thereby the depth and the width of the oral vestibule can be decreased considerable . The denture must be relieved over, and contoured around them. Extensions beyond the crest will interfere with the mentalis muscle movement and lead to denture instability.

24. INCISIVE LABII INFERIORIS Origin Oblique line of the  mandible , between the  symphysis and the  mental foramen Insertion Integument of the  lower lip ,  Orbicularis oris   fibers on both sides. Nerve Facial nerve  - Mandibular branch Actions Depression of the lower lip

25. SIGNIFICANCE C ontraction of the muscle can reduce the denture space. * In action it pulls the modioli forward and tenses the buccinator thereby applying pressure on the polished surface .

26. MUSCLES OF TONGUE The extrinsic muscles have their origin external to the tongue, but their course terminates within it. Their contraction causes the tongue to move in relation to other oral structures. The intrinsic muscles lie completely within the tongue, and their activities sustain or alter tongue form.

27. SIGNIFICANCE The tongue is capable of many varied shapes and positions during speech, mastication, and swallowing and in all of these functions is in constant contact with the lingual surface of the teeth, the lingual flange of the lower denture and the palatal surface of the upper denture. Because of this contact, the tongue is a dominant factor in establishing the neutral zone and therefore in the stability or lack of stability of the lower denture.

28. CHANGES IN EDENTULOUS MOUTH MAXILLA AND MANDIBLE Neither of the alveolar ridges resorb uniformly. Mandibular residual alveolar ridges tend to resorb more from the lingual while maxillary residual alveolar ridges resorb more from the buccal . Usually, the longer a patient is edentulous, the greater is this interridge facial/lingual and facial/palatal dimensional disparity.

29. LIPS AND MODIOLI Collapse of lips Reduced prominence of philtrum and vermilion border Drooping of corners of mouth Modioli become sagging, less active

30. MUSCLE ATTACHMENT As the mandibular ridge resorbs the crest falls below the level of the mentalis. As a result mentalis tends to fold over and rests on the ridge. It pushes the neutral zone posteriorly. The freni occupy a more superior position on the ridge. In some cases the buccinator attachment over the external oblique ridge is lost after severe resorption .

31. TONGUE It is estimated that tongue size increases by approximately 10% in the edentulous patient. The position of the neutral zone in relation to the alveolar ridge was found to be highly affected by the period of edentulousness . The longer the edentulousness ,the more buccally / labially located was the neutral zone

32. DENTURE SURFACES Sir Wilfred Fish (1948) described a denture having three surfaces, with each surface playing an independent and important role in the overall fit, stability, and comfort of the denture.

33. INFLUENCE OF FORCES ON DENTURE SURFACES The greater the ridge loss, the lesser the area of the denture base and lesser the influence of the impression surface area will have on the stability and retention of the denture . Where more of the ridge has been lost, the more the denture stability and retention is dependent on the polished surface than on the impression surface.

34. The forces on the polished surface are constantly changing in magnitude and direction during swallowing, speaking, and mastication. It is only when the mouth is completely at rest that the forces are constant. When the occlusal surfaces of the teeth are not in contact, the stability of the denture is determined by the fit of the impression surface and the direction and amount of forces transmitted through the polished surfaces. In order to construct dentures that function properly not only in chewing but also in speaking and swallowing, we must develop the fit and contour of the polished surface just as accurately and meticulously as the fit and contour of the impression and the occlusal surfaces.

35. The influence of the lip on lower denture stability becomes more critical as resorption of the ridge increases or as the patient becomes older.

36. NEUTRALIZATION OF FORCES The theory of the neutralization of forces is that to stabilize dentures; and the rationale involved was one of the major contributions made by Dr. Russell Tench (1952) and his co- workers. The lips, cheek and tongue in the passive and functioning state exert forces on the natural teeth. In natural dentition, arch integrity and tooth position are maintained when all the forces generated by the musculature are neutralized. Any changes in the forces generated by the musculature because of increased size, altered muscle function, or abnormal habit patterns will upset the equilibrium and result in alteration of tooth position and the arch form.

37. RECORDING OF NEUTRAL ZONE

38. DIFFERENT METHODS AFTER IMPRESSION AFTER TENTATIVE JAW RELATION DURING TRY IN PROCEDURES SWALLOWING METHOD PHONATION METHOD

39. NEUTRAL-ZONE APPROACH, REVERSED SEQUENCE IN DENTURE CONSTRUCTION PRIMARY IMPRESSION

40. CONSTRUCTION OF DENTURE BASES

41. TRYIN OF THE DENTURE BASES

42. MATERIALS USED Many materials have been suggested for shaping the neutral zone: Modeling plastic impression compound ( Tench ) Soft wax (Buchman & Gelb, Lott & Levin , and Russell) A polymer of dimethyl siloxane filled with calcium silicate( Heath) Tissue conditioners (Gahan & Walmsley)

43. CLINICAL PROCEDURES IN LOCATING THE NEUTRAL ZONE Tench et al used modelling impression compound for the first time to record neutral zone. A water bath, preheated to the proper temperature, is used to soften the material, which is then kneaded and worked until it is uniformly soft.

44. ADAPTING COMPOUND ON TO THE TRAY

45. LOCATING THE NEUTRAL ZONE FOR THE MANDIBULAR ARCH To locate the neutral zone for the lower arch the patient's lips are lubricated with petrolatum jelly. The tray with the softened modeling compound is carefully seated into the mouth. The patient is instructed to swallow and then purse the lips as in sucking.

46. Molded compound rim

47. TESTING THE STABILITY OF THE LOWER OCCLUSION RIM The lower occlusion rim is placed back into the patient's mouth and checked for stability by having the patient open wide, wet the lips with the tongue, count from one to ten , and say exaggerated " ohs ," " ahs ," and " ees ." If these movements raise the rim, the lack of stability must be caused by an improper molding of the compound.

48. LOCATING THE NEUTRAL ZONE FOR THE UPPER ARCH The modeling compound is attached to the upper tray. It is flamed, tempered and molded into the shape of an rim.

49. The upper rim is placed into the oral cavity and patient is instructed to suck and swallow. Usually excess compound will seen extending below the relaxed upper lip. A line is scribed about 2 mm below the upper lip at rest and the compound is trimmed to this line.

50. NEUTRAL ZONE FOR MANDIBULAR ARCH

51. SCHIESSER’S TECHNIQUE NEUTRAL ZONE FOR MANDIBULAR ARCH

52. ADMIXED TECHNIQUE

53. SECONDARY IMPRESSIONS To achieve optimum success in complete denture prosthesis, the dentures should be both retentive and stable. The retention of a denture is mainly dependent on the accuracy of impression and fit of denture base to the tissues. Closed or open mouth impression technique is used More accurate functional border molding with minimal displacement of soft tissues Even distribution of pressure and impression material with less likelihood of excessive pressure in one area or another

54. FABRICATION OF A TONGUE, LIP, AND CHEEK MATRICES To be sure that the teeth will been set within the neutral zone , matrices are constructed around the occlusion rims. The matrices may be made of plaster, stone, or modeling compound.

55. Prior to construction of the matrices, the casts must be indexed so that the matrices will fit back into their proper position. Several circular holes are made on the labial and buccal surfaces of the cast and a cross is made in the tongue area of the lower model.

56. ARRANGEMENT OF TEETH One of the major advantages of the neutral concept is that the position of the anterior as well as the posterior teeth is determined for the dentist and technician by the patient's neuromuscular function. With the neutral zone concept, the labiolingual position of the teeth is limited by the boundaries of the neutral zone. This greatly simplifies the problem as to where to position anterior teeth.

57. The following are a step by step sequence for arrangement of anterior and posterior teeth Mandibular anterior teeth are set to the fit of the labial matrix and to the labial limit of the neutral zone. The maxillary anterior teeth are set against the labial limits of the maxillary martix . The mandibular posterior teeth are set against the tongue matrix and against the template occlusally . The maxillary posterior teeth are set to the buccal limits of the neutral zone

58. Maxillary posterior teeth will have to be rearranged to assure maximum contact with the lower posterior teeth. The maxillary and mandibular posterior teeth are checked for the buccal and lingual relationship to one another. In order to avoid an edge to edge relationship which might lead to cheek biting, the mandibular posterior teeth may be moved buccally within the neutral zone, resulting in a cross bite relationship

59. EXTERNAL IMPRESSIONS With the neutral zone approach, there is another extremely important step to be completed during the trial denture try-in , that is, the making of external impressions on the labial, buccal, and lingual surfaces of the dentures. These will determine the thickness, contours, and shape of the polished surfaces of the denture.

60. By means of external impressions, a physiologic molding is made so that the external surfaces are functionally compatible with muscle function. Materials for external impressions are zinc oxide eugenol, conditioning materials or light body addition silicon impression material. Another important reason for using this procedure is that it tends to minimize the accumulation of food on the external surface of the denture.

61. Patient performs oral functions including chewing to determine the thickness, contour and shape of the polished surfaces

62. The material flown over the tooth surfaces must be removed carefully with a carver

63. The laboratory procedures for investing, packing, and processing of dentures when using the neutral zone technique is generally the same as used for conventional dentures.

64. CASE REPORT Improvised Neutral Zone Technique in a Completely Edentulous Patient with an Atrophic Mandibular Ridge and Neuromuscular Incoordination: A Clinical Tip Saravanakumar P, Thangarajan ST, Mani U.Cureus . 2017 Apr;9(4 ). CHIEF COMPLAINT- A 64-year-old man reported to the Department of Prosthodontics, Faculty of Dental Sciences, Sri Ramachandra University, Chennai, India, with the chief complaint of an unstable loose mandibular denture.

65. MEDICAL HISTORY - D iabetic , hypertensive, and under medication. The patient presented with a history of neuromuscular incoordination for the past four years . He also complained of difficulty moving his jaws, normally being a complete denture wearer for the past seven years, leading to difficulty in chewing and speech, primarily due to loose lower dentures . ORAL EXAMINATION The maxillary residual alveolar ridge was rounded and well formed, but the mandibular residual ridge was unfavorable due to a high degree of resorption Highly Resorbed Mandibular Ridge

66. The treatment approach for this patient was to construct a mandibular denture using the conventional neutral zone technique and to use improvised procedures to minimize the chairside visits for the patient.

67. CLINICAL VISIT 1

68. CLINICAL VISIT 2

69. FUNCTIONAL MOVEMENTS RECORDED WITH ADMIX MATERIAL

70. ARTICULATED OCCLUSAL RIMS

71. INDEX MADE WITH IMPRESSION PLASTER

72. CLINICAL VISIT 3 A wax try-in was performed to evaluate mandibular record base stability, aesthetics, and intraoral occlusion. The patient successfully performed all the movements mentioned earlier. The trial dentures were processed with heat-cure acrylic resin . The denture was polished so that the customized contours remained unaltered.

73. CLINICAL VISIST 4 The mandibular denture was again evaluated with the plaster index prior to denture insertion. The denture was inserted and verified for retention, stability, and occlusion. The patient was comfortable with the complete denture prosthesis. Periodic recall visits were scheduled to verify the retention, comfort, and function. Processed denture verified with index

74. Morphologic comparison of two neutral zone impression techniques: A pilot study Makzoumé JE. The Journal of prosthetic dentistry. 2004 Dec 1;92(6):563-8 . AIM-The purpose of this pilot study was to compare the outline form of the phonetic and swallowing neutral zone impression techniques for the same subjects.

75. Material and methods Nine denture wearers with advanced mandibular ridge resorption were included in this study. For each subject 2 trays were prepared in autopolymerizing acrylic resin . One method used phonetics and tissue conditioner to shape the neutral zone; the second method used swallowing and modeling plastic impression compound.

76. The resulting neutral zone impressions were leveled to the same occlusal height by gently grinding the occlusal surface on sandpaper until it corresponded with landmarks noted on the cast . The impression was inverted onto graph paper, and the contour was outlined with a lead pencil . One impression was made for each subject, for each technique. The buccal contours of both neutral zones coincided at the median line.

77. The maximum distance between the zones was measured in a buccolingual direction in the anterior, premolar, and molar regions bilaterally. When the location of the phonetic neutral zone in relation compared to the swallowing neutral zone was buccally oriented, a plus score was given. When the phonetic neutral zone was lingually located, a minus score was given. When the 2 lines coincided, a score of 0 was given. Measurements were made from direct readings on the graph paper.

78. Tracings of phonetic neutral zone and swallowing neutral zone for 9 subjects. Solid line represents swallowing neutral zone. Dashed line represents phonetic neutral zone.

79. Conclusion. Within the limits of this study, the phonetic neutral zone appears to be narrower posteriorly compared to the swallowing neutral zone, thus limiting premolar and molar positioning.

80. Arrangement of artificial teeth in the neutral zone after surgical reconstruction of the mandible: A clinical report Kokubo , Y., Fukushima, S., Sato, J., & Seto , K. (2002). The Journal of Prosthetic Dentistry, 88(2), 125–127.

81. CLINICAL REPORT A 57-year-old woman was referred to the oral and maxillofacial clinic of Tsurumi University Hospital (Yokohama, Japan) for examination of gingival swelling in the mandible. After being diagnosed with squamous cell carcinoma, the patient underwent partial resection of the right mandible and one-fourth resection of the tongue and floor of the oral cavity. Immediate surgical reconstruction was performed with vascularized iliac crest bone fixed with a reconstruction plate

82. One year and 7 months after the reconstruction, five 3.75 X 18-mm dental implants were placed in the grafted bone of the mandible At the same time, the reconstruction plate was removed. The implants were exposed after a healing time of 15 months, and standard abutments were connected 2 weeks after exposure

83. Panoramic radiograph of dental implants placed in grafted bone.

84. Intraoral view

85. After impressions and soft tissue working casts were made, the casts were surveyed and the design of the final prosthesis was determined. Registration of the maxillo -mandibular relation was performed, the casts were mounted in an articulator, composite replacement teeth were selected and arranged, and jaw relations and esthetics were evaluated intraorally. The composite artificial teeth were adjusted to accommodate the position of the implants and the opposing maxillary dental arch. The prosthesis was completed and attached to the implants with gold screws

86. Implant-supported prosthesis in position.

87. After 2 weeks, the patient returned to the clinic with the complaint of tongue and cheek biting in the molar region on the right side. The tongue and cheek had been severely injured from biting, so an attempt was made to modify the artificial teeth through grinding with pressure-indicating material (Fit-checker; GC Corp, Tokyo, Japan) as the disclosing agent. Unfortunately , this modification did not eliminate the problem . The artificial teeth in the molar region were removed from the denture base, and a waiting period followed until the tongue and cheek healed.

88. A new wax occlusion rim was formed on the denture base. The denture base with the softened wax occlusion rim was attached to the implants with implant screws . Muscle trimming with tongue and cheek movements was performed to establish the neutral zone position of the wax occlusion rim . The final detail of the neutral zone was determined with tissue-conditioning material (Soft conditioner; GC Corp)

89. In the laboratory, the prosthesis with the neutral zone wax occlusion rim was fixed on the working cast. Buccal and lingual matrices were formed with silicone impression material ( Coltoflax ; Coltene , Altsatten , Switzerland) to make the neutral zone reproducible on the working cast . Using the matrices as a guide, new artificial teeth were arranged in the zone and the prosthesis was reprocessed. The teeth were rearranged to ensure that they did not interfere with cheek and tongue movements.

90. After this procedure, no tongue biting was noted, and the patient expressed satisfaction with the implant-retained prosthesis.

91. CONCLUSION Neutral zone is an alternative technique for the construction of lower complete dentures on highly atrophic ridges. The aim of the neutral zone is to construct a denture in muscle balance, as muscular control will be the main stabilising and retentive factor during function. The technique is relatively simple but there is increased chair side time and laboratory costs.

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