Neutral Zone in Complete Denture - PDF

Neutral zone in CD 5th year Prosthodontics Lect. Dr. Mustafa Saadi Ali Neutral zone in complete denture Definition Neutral zone: is 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 t...

Neutral zone in CD 5th year Prosthodontics Lect. Dr. Mustafa Saadi Ali Neutral zone in complete denture Definition Neutral zone: is 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. Neutral zone concept In completely edentulous patients, there is a void in the oral cavity called “potential denture space”. The boundaries of the potential denture space are: Superiorly: maxilla and soft palate. Inferiorly: mandible and floor of the mouth. Medially: tongue. Laterally: muscles and tissues of cheek and lips. The stability of complete dentures is influenced by the surrounding neuromuscular system in the oral cavity. Oral functions, such as speech, mastication, swallowing, smiling, and laughing, involve the synergistic actions of the tongue, lips, cheeks, and floor of the mouth that are very complex and highly individual. Neuromuscular control is the key for the stability of dentures. Size and position of denture teeth and the contours of polished surface play a crucial role in denture’s stability as they are subjected to destabilizing forces from the tongue, lips, and cheeks if they interfere with the function of oral structures. Neutral zone is that area in the mouth where, during function, the forces of tongue pressing outward are neutralized by the forces of the cheeks and lips pressing 1 Neutral zone in CD 5th year Prosthodontics Lect. Dr. Mustafa Saadi Ali inwards. Since these forces are developed through muscular contraction during chewing, speaking, swallowing, … etc., they vary in magnitude and direction in different individuals and in different periods of life. The way these forces are directed against the denture will either stabilize or dislodge them. Our objective is to utilize this information to position the teeth and the external surface (polished surface) so that the force the musculature exerts will have a seating effect. This can be only accomplished by a knowledge of neutral zone. The neutral zone concept is based upon that for each individual, there is a denture space which is a specific area where the function of the musculature will not unseat the denture and where the forces generated by the tongue are neutralized by the forces generated by lips and cheeks. So, the idea for the neutral zone technique is to locate this space. Objectives of neutral zone technique 1. Rehabilitation of complete denture patient. 2. Achieve maximum prosthesis stability, comfort, and function. 3. Arrange the denture teeth and contour the complete denture polished surfaces within the neutral zone. 4. Minimize the ongoing diminution of the residual alveolar ridges. Indications of neutral zone technique 1. Severely atrophic mandibular ridge. 2. Patients with prominent and highly attached mentalis muscle and patients with lateral spreading of tongue as a result of poor transition from dentate to edentulous state and severe resorption. 3. Patients with poor or diminished neuromuscular control such as those with a neuromuscular disease, history of stroke, Parkinson’s disease, or patients with impaired motor innervation to oral (motor nerve damage to the tongue) and facial muscles as a result of brain surgery. 4. Patients with atypical shape or consistency of oral and perioral structures. For example, patients who have scleroderma, marginal or segmental mandibulectomy, partial glossectomy, or trauma to the oral and facial structures. 5. To locate optimal position for implants in cases of implant-supported or implant-retained overdentures, which enhances the overall outcome of treatment. 2 Neutral zone in CD 5th year Prosthodontics Lect. Dr. Mustafa Saadi Ali Advantages of neutral zone technique 1. Improved stability and retention of complete dentures (particularly lower complete dentures). 2. Correct positioning of posterior teeth allows for sufficient tongue space. 3. Reduced food trapping adjacent to the molar teeth. 4. Improved esthetics due to facial support. Disadvantages of neutral zone technique 1. Increased clinical time. 2. Increased laboratory costs. Muscles involved in the neutral zone Muscles of the cheek 1. Masseter: affects distobuccal border of mandibular denture. 2. Buccinator: muscle fibers form a continuous band; hence the size of the arch is limited by the strength of the contractile force and the length of the muscles when they are contracted. Common practice of centralization or lingualization of occlusion creates a space between the cheek and teeth and external surface of the denture which prevents the buccinator from performing its proper function in two ways: First: Food accumulation and it becomes more difficult to the cheek to place the food back to the occlusal surface of the teeth. 3 Neutral zone in CD 5th year Prosthodontics Lect. Dr. Mustafa Saadi Ali Second: The space prevents the buccinator from neutralization the lateral forces of the tongue during function. Lingualized occlusion Muscles of the lips 1. Orbicularis oris: during function as in chewing, smiling, and swallowing, it exerts force against teeth and denture flanges which counteracted by the tongue. 2. Levator anguli oris (Caninus): this together with other muscles, pulls the upper lip upward and in sucking and swallowing pull the lip forward, thus exerting forces on teeth and labial flanges. 3. Zygomatico major: pulls the angle of the mouth upward and backward. 4. Risorius: retract the corner of the mouth. 5. Mentalis: turns the lower lip outward and on contraction makes the lower labial vestibule shallow. 6. Triangularis: contracts during sucking and exert pressure on teeth and denture flange. 7. Modiolus: is a dense, compact, mobile, fibromuscular tissue mass of facial muscles that alters the position of the angle of the mouth. It is formed by the interlacing of a number of muscles: Orbicilaris oris. Buccinator. Levator anguli oris. Depressor anguli oris. Zygomaticus major. Risorius. Platysma. Levator labii superioris (Quadratus labii superioris). 4 Neutral zone in CD 5th year Prosthodontics Lect. Dr. Mustafa Saadi Ali Because of strength and variability of movement of the area, modiolus is very important in stability of lower denture. Proper positioning of teeth and contouring and narrowing of external surface of premolar area should be done otherwise the modiolus will constantly unseat the lower denture. Muscles of the tongue 1. Intrinsic muscles: they are confined to the tongue and not attached to the bone. They produce change in shape of the tongue. 2. Extrinsic muscles: these muscles attached to the bones and soft palate. They are: genioglossus, styloglossus, hyoglossus, and palatoglossus. They are responsible for tongue movement and change in shape. The tongue is capable of changing shape and position during function as in mastication, swallowing, and speech. During function it will be in constant 5 Neutral zone in CD 5th year Prosthodontics Lect. Dr. Mustafa Saadi Ali contact with lingual surface of lower teeth, lingual flange of lower denture, and palatal surface of upper denture. The common practice of lingualization is one of the greatest influencing factors of lower denture instability because it violates the neutral zone and encroaches on the tongue space. Influence of muscles on dental arches During childhood, the teeth erupt under the influence of muscular environment created by forces exerted by tongue, cheeks, and lips, in addition to genetic factors. These forces have a definite influence upon the position of the erupted teeth, arch form, and occlusion. Generally, muscular activity and habits which develop during childhood continue throughout life and after teeth loss. It is important to position artificial teeth in the arch form compatible with these muscular forces. As the impression surface area decreases (due to alveolar ridge resorption), the retention and stability of the denture decrease. Consequently, retention and stability become more dependent on the correct positioning of the teeth and contours of the polished surfaces of the dentures. 6 Neutral zone in CD 5th year Prosthodontics Lect. Dr. Mustafa Saadi Ali The polished surfaces should be so contoured that the horizontally directed forces applied by the peri-denture muscles should act to seat the denture. The artificial teeth should not be placed on the crest of the ridge or buccally or lingually to it, rather these should be placed as dictated by musculature. After the teeth have been lost, muscle function greatly influences any complete dentures that are placed in the mouth. It is therefore, extremely important that the teeth be placed in the mouth within the arch form that falls within the area that is compatible with muscular forces. Neutral zone always moves according to the periods of edentulism and tonicity of the perioral musculatures and tongue. Arrangement of the teeth in neutral zone, increasing the impression surface area of the denture and reduction in the volume of the denture, provides good retention, stability and comfort to the patients. 7 Neutral zone in CD 5th year Prosthodontics Lect. Dr. Mustafa Saadi Ali Influence of forces on denture surfaces The greater the ridge loss, the lesser the area of denture base and the lesser the influence of impression surface areas of the denture on its stability and retention, and the more external surface area which is needed to be contoured properly to overcome this situation. The forces on external surfaces (polished surfaces) are constantly changing in magnitude and direction during function and remain constant at rest. In order to construct denture that function properly, we must develop fit and contour of external surface just as accurately and meticulously as the fit and contour of impression and occlusal surfaces. The influence of the lip on lower denture stability becomes more critical as resorption of the ridge increases or as the patient becomes older. Neutral zone technique recording The neutral zone technique can be performed in: Impression stage. Jaw relation stage. Try-in stage. Finished or previous denture. Recording neutral zone in the final impression stage These are the steps for recording the neutral zone in the final impression stage: 1) Primary impression of upper and lower arches and pouring of primary casts. 2) Construction of individual (special) trays. The special trays are fabricated without handles and similar to acrylic record bases with retentive wires to retain the material during recording neutral zone area. 8 Neutral zone in CD 5th year Prosthodontics Lect. Dr. Mustafa Saadi Ali 3) Modeling compound is used to fabricate occlusion rims, establishment of occlusal plane. These rims are then molded intra orally according to the muscle function (recording of neutral zone). The patient is instructed to perform functional movements such as licking lips, sucking, puckering, smiling, grinning, swallowing, pronouncing some words, or combination of these. 4) Establishing a tentative occlusal vertical dimension (OVD) and centric relation (CR). 5) Obtain the final impression using ZOE impression material with closed mouth technique. 6) Final determination of the OVD and CR. 7) Pouring the master casts, forming the plaster index (for the neutral zone area), mounting of the casts on the articulator, and setting up of the teeth guided by the paster index. 8) Wax try-in of the dentures and verification of the tooth position intraorally. 9 Neutral zone in CD 5th year Prosthodontics Lect. Dr. Mustafa Saadi Ali Recording neutral zone in the jaw relation stage 1) Primary impression with impression compound. 2) Final impression with ZOE. 3) Fabrication of lower acrylic record base with metal wire loops. The upper occlusal rim is fabricated wit acrylic base and wax. 4) Occlusal pillars built with green stick material on the wire loops to establish the occlusal height (OVD). 5) Jaw relation record. 6) Tissue conditioner is mixed and placed on the lower record base and the patient is instructed perform certain oral movement including: sucking, grinning, whistling, pursing of the lips, and swallowing. The tissue conditioner is molded with mouth movement. 10 Neutral zone in CD 5th year Prosthodontics Lect. Dr. Mustafa Saadi Ali 7) The record base is returned to the cast and plaster index is formed. 8) Setting up of teeth according to the obtained neutral zone. Recording neutral zone in the try-in stage 1) Apply Vaseline on the teeth of the trial denture before making impression. 2) Impression material (usually light body silicone) is applied on buccal and lingual surfaces of waxed up denture. 11 Neutral zone in CD 5th year Prosthodontics Lect. Dr. Mustafa Saadi Ali 3) Patient performs oral function. 4) Inspect the impression on polished area including lingual surface. 5) Carefully carve the material over tooth surfaces with carver. Recording neutral zone in the finished denture First, the complete denture should be properly examined to check for fitness to neutral zone technique. 1) Coat the polished surface of the denture with low viscosity silicone impression material. 2) Ask the patient to perform functional movements while the material sets. 3) Inspect the denture and adjust any heavy muscle contact. 4) Form the index for the neutral zone. 5) Remove the old denture teeth with some denture base. 6) Place wax in the neutral zone and arrange the teeth accordingly. 7) Process and finish the denture. 12 Neutral zone in CD 5th year Prosthodontics Lect. Dr. Mustafa Saadi Ali This technique can be performed for a segment of the denture that needs modification. It is done by removing the teeth and some denture base for the segment. Apply adhesive for the silicone impression material. Then continue with the same steps above. Comparison between conventional mandibular denture and mandibular denture made by neutral zone technique According to the setting of the teeth in relation to the crest of the ridge: Teeth Conventional method Neutral zone technique Anterior Slightly labial Slightly lingual Premolar area Exactly on the crest Slightly lingual Posterior Lingually Buccally According to denture surface area: The neutral zone denture tissue surface area is higher than the conventional denture. According to denture volume: The neutral zone denture volume is lower than the volume of the conventional denture. Limitations for the success of neutral zone technique 1) Viscosity of the material used. The more viscous, the more difficult for the muscle to mold. 2) Geriatric patients could suffer from the procedure due to loss of their muscular tone. 3) Proper stability and retention of the record bases are required, so as the comfort, to perform the functional movements properly. 4) The resultant neutral zone is often narrow and might be lingually placed, this might affect the functional movement of the tongue and phonetics. 5) The technique does not offer any guidelines for the selection of teeth. 13