Analysis of the prevalence of different topographical characteristics of the residual ridge in mandibular free-end arches Análise da prevalência de diferentes características topográficas do rebordo residual em arcos mandibulares com extremidades livres

This study observed the prevalence of different types of residual ridge inclination in free-ends of mandibles and reported possible correlative factors that may affect resorption. For this purpose, periapical radiographs and individual data collected from a sample of 64 hemiarches were used. Two radiographs were taken of each freeend, and tracing was employed to determine the angles formed by the resorption configuration in the area of the 1 mandibular molar. The following conclusions were drawn: 1) the great majority of alveolar ridges were distally descending; 2) the average angle was wider for users of mandibular removable partial dentures; 3) the results obtained suggest that the type of opposing maxillary arch affects the inclination of mandibular ridges; 4) greater inclination was observed when the 2 bicuspids of the mandible were the abutment teeth; 5) no significant correlation was established between age, sex and residual ridge resorption. DESCRIPTORS: Denture, partial, removable; Radiography. RESUMO: Esta pesquisa constatou a prevalência dos tipos de rebordos residuais no sentido ântero-posterior em extremidades livres inferiores, além de correlacionar alguns fatores que possam influenciar as suas reabsorções. Para isso, utilizamos radiografias periapicais e dados individuais colhidos na amostra de 64 hemiarcos; foram obtidas 2 radiografias de cada área de extremidade livre e, a partir de traçados, determinamos os ângulos formados pela reabsorção na altura do 1o molar inferior. Podemos constatar que: 1o) a grande maioria de inclinação encontrada foi de rebordos descendentes para distal; 2o) a média de angulação foi maior para usuários de prótese parcial removível inferior; 3o) os resultados sugeriram existir influência do tipo de arco antagônico superior na inclinação dos rebordos inferiores; 4o) observou-se aumento de inclinação quando os segundos pré-molares inferiores eram os dentes adjacentes ao espaço protético; e 5o) não foram encontradas correlações significantes entre idade, sexo e reabsorção do rebordo residual. DESCRITORES: Prótese parcial removível; Radiografia. * Adjunct Professor, Department of Dentistry, University of Brasília. ** Chair Professor, Department of Dental Prosthesis, University of São Paulo. *** Senior Lecturer, Department of Dental Prosthesis, City of São Paulo University. Prosthesis Guedes CG, Zanetti AL, Feltrin PP. Analysis of the prevalence of different topographical characteristics of the residual ridge in mandibular free-end arches. Braz Oral Res 2004;18(1):29-34. 30 sists of thick, dense or porous cortical bone at the crest of the ridge and fine trabecular bone inside, in addition to another type of bone of lower density displaying thin, porous cortical bone at the ridge and encasing a fine trabecular bone. Nevertheless, biomechanical studies show that mandibular removable partial dentures (RPDs) in action form a system of levers on the fibromucosal tissue in which the abutment tooth acts as a rotational fulcrum, while the angle of inclination of the ridge forms a slanted plane system, which increases proportionally to the inclination of the residual ridge and may cause deflections at the point where bite force is applied. Although the form of residual ridge is affected by the type of fibromucosal support in each individual, this is the real support for the RPD. Studying the angle is therefore relevant to aid prosthodontists in designing prosthetic devices and also to supply valuable data for research on the biomechanical behaviour of dentures subjected to bite force. METHODOLOGY Selection of sample Patients were selected from clinics run by the Discipline of Dentistry, University of Brasília, based on the presence of unior bilateral free-ends when the abutment teeth were mandibular cuspids or 2 bicuspids, whether or not the patients wore mandibular RPDs. All the individuals examined reported good health and had no systemic disturbances when the selection took place. A total of 64 hemiarches were classified by the consecutive method, divided into two groups of 32 hemiarches for each situation, of both genders and from different age groups. Cases in which the 1 bicuspid was the abutment tooth were excluded from the sample, since the intention was to investigate the relation between the sagittal length of the edentulous segment and the degree of inclination. All the individuals selected were informed about the research and signed a declaration of consent, duly approved by the Ethics & Research Committee. Casts and tooth arrangement Plaster casts were obtained for all the patients, on which an acrylic resin denture base was made and artificial teeth were arranged. The occlusal plane was used as a reference in all the casts, standardised by tracing an imaginary line through the incisal angle of the cuspid to the end of the 2 third of the retromolar pad, as shown in Figure 1. Base points were created by setting 3 mm ball-bearings at the end of the 2 third of the retromolar pad, fixed in the internal part of the resin base. The central cavity of the 1 artificial molar was used as the base reference point for measuring this angle; another ball-bearing was set under this area. This region was chosen because it receives a greater magnitude of bite force, and is more likely to have greater resorption since the 1 permanent molars are the first to be extracted from the mandible. Radiographic shots The radiographic shots were taken on ultrarapid periapical film (Kodak Ektaspeed Plus, EP21P, type 2, Rochester, New York, USA), with a 0.7 second exposure time. The area radiographed extended from the cuspid to the retromolar pad, and the artificial tooth-base appliance was positioned in the mouth. Since this is a considerable area to be radiographed, occlusal film (Kodak Ektaspeed occlusal radiographic film, Rochester, New York, USA) was used, cut lengthwise in a darkroom and spliced using dark isolating tape. The radiographic technique employed was the parallel technique with the aid of a film-holder/ positioner (Jon brand radiographic positioner, São Paulo, Brazil) and a long-cone locator. For each hemiarch 2 radiographic shots were taken, as recommended by El Basty, Eid (1985). Since full occlusal or incisal surfaces were required, cases in which the remaining teeth did not possess adequate clinical crowns or had crowns diverging from the occlusal plane were eliminated from the sample. Tooth extrusions in the maxillary arch FIGURE 1 Sagittal view of mandible arch plaster cast with artificial teeth mounted. Note the occlusal plane obtained by the points of reference (tip of canine cusp and end of 2 third of the retromolar pad). Guedes CG, Zanetti AL, Feltrin PP. Analysis of the prevalence of different topographical characteristics of the residual ridge in mandibular free-end arches. Braz Oral Res 2004;18(1):29-34. 31 ence plane, joining the lowest point of the sphere placed at the 1 molar level to the highest point of the edentulous segment (line DE) (Figure 2c). A line was then traced from point C to point E to form the angle DCE, thus measuring the ridge inclination angle (Figure 2d). RESULTS AND DISCUSSION From the results obtained, it can be observed that among the 64 hemiarches examined, there were only 3 cases of distally ascending ridges and 1 case of a horizontal (i.e. 0° inclination) ridge, with respective prevalences of 4.68% and 1.56%. Distally descending ridges presented a general average inclination of 17.95° (19.8° when 2 bicuspids were the abutment teeth and 16.10° when cuspids were the abutment teeth). These data help elucidate certain aspects were likewise corrected by abrasion or prosthetic means to prevent them from interfering in the positioning of the artificial denture base for the radiographic shots in occlusion. The radiographs were developed by the visual method and assessed by a single observer in a dark environment. They were subsequently fixed on tracing paper to obtain the tracings. Tracings A straight line AB was traced corresponding to the occlusal reference plane (Figure 2a). Using a ruler and a square, a line was traced parallel to the AB segment, using the distal alveolar crest of the abutment tooth as a reference to determine the individual’s current bone loss (point C) (Figure 2b). A line was traced perpendicular to the referFIGURE 2 a) radiographic image with tracing representing the occlusal plane (line segment AB); b) line segment parallel to the occlusal plane taking the distal alveolar crest of the abutment tooth (point C) as the point of reference; c) line traced perpendicular to the occlusal plane parallel (line segment DE); d) tracing of line CE to form angle DCE corresponding to the ridge inclination angle. b a