2 International Journal of Dentistry Table 1: Number of subjects observed at diﬀerent followup examinations and age, gender, and number of occlusal units at baseline observation. Number (%) of subjects observed at follow-up SDA + RPD SDA SDA previous RPD Complete dentition Baseline 25 (100) 55 (100) 19 (100) 72 (100) 3-year 24 (96) 47 (85) 19 (100) 59 (82) 6-year 21 (84) 41 (74) 18 (95) 53 (74) 9-year 17 (68) 30 (55) 12 (63) 41 (57) Mean age (SD) 44.1 (8.4) 40.1 (12.4) 41.7 (9.1) 36.2 (9.8) % Female 68 61 63 49 Mean No (SD) of occlusal units 3.5 (0.8) 3.9 (0.5) 3.4 (0.6) 12 (0.0) SDA = shortened dental arch, RPD = mandibular distal extension removable partial denture. temporomandibular disorders compared to subjects with complete dentitions [9]. This study aims to investigate whether mandibular distal extension removable partial dentures designed for moderate shortened dental arches are beneﬁcial or not to the patients regarding some clinically relevant parameters on occlusion and temporomandibular function. Metal frame removable partial dentures may generate occlusal interferences by the rests and clasps especially in distal extension types when reduction of the alveolar bone under the saddles is progressing [10, 11]. Although there is no strong evidence for cause-and-eﬀect relation- ships, occlusal interferences have been regarded as a cause of bruxism [12–15] (i.e. clenching or grinding habits). Occlusal interferences and bruxism have been described provoking further disorders in temporomandibular function [16, 17]. According to this line of reasoning, removable partial dentures have been associated with increased risk for bruxism, and increased risk for signs and symptoms related to temporomandibular disorders [10, 11]. It is assumed that bruxism habits can be veriﬁed by occlusal tooth wear as incisal and occlusal wear correlates with bruxism [18]. Occlusal tooth wear has also been associated with the number of teeth, gender, and age [17]. An indicator of increased incisal tooth wear in the anterior region might be a decrease of the vertical overlap. As distal extension removable partial dentures intend to restore posterior support, this decrease of vertical overlap should be even more marked compared to shortened dental arches without these dentures. In line with the above arguments regarding occlusion and function, we hypothesized that subjects with shortened dental arches with distal extension removable partial den- tures (i) more frequently report awareness of bruxism, (ii) have intensiﬁed occlusal tooth wear, and (iii) have more signs and symptoms related to temporomandibular disorders as compared with subjects with shortened dental arches without extension dentures and subjects with complete dentitions. Furthermore, we hypothesized that subjects with extension dentures (iv) have smaller vertical and horizontal overlap of the anterior teeth. 2. Material and Methods 2.1. Sample and Observations. For this prospective observa- tional cohort study convenience samples were composed of Table 2: Number of subjects according to the duration (yr) the shortened dental arch existed at baseline observation. <5 ≥5 and <10 SDA + RPD∗ 3 SDA 13 SDA previous RPD 3 Total SDA 19 6 14 4 24 Duration (yrs) ≥10 and <15 10 15 6 31 ≥15 Total 6 25 13 55 6 19 25 99 ∗At baseline 9 subjects were wearing an RPD for <5 yrs; 8 subjects ≥5 and <10 yrs; 6 subjects ≥10 and <15 yrs; 2 subjects ≥15 yrs SDA: shortened dental arch, RPD: mandibular distal extension removable partial denture. subjects attending the Nijmegen Dental School Clinic. Three moderate shortened dental arch groups were constructed: (1) subjects with mandibular distal extension removable partial dentures (“SDA + RPD group”); (2) subjects without extension dentures (“SDA group”); (3) subjects without distal extension removable partial dentures but who were wearing this type of denture in the past (“SDA previous RPD group”). As a reference a group of subjects with complete dentitions was constructed (“CD group”). A shortened dental arch was classiﬁed as moderate if comprising 3 to 5 occlusal units (OUs) with a minimum of 1 OU at each side (left/right) (Table 1). An OU was deﬁned as a pair of occluding natural premolars (1 OU); a pair of occluding natural molars was considered to be 2 OUs. All subjects of the “SDA + RPD group” had a distal extension removable metal frame in the lower jaw; three of them had also this type of denture in the upper jaw. Of the 25 mandibular dentures, 19 were bilateral, and 6 were unilateral distal extension dentures. All dentures (and any replacement denture made during the followup) were conventional removable metal frame dentures without precision attachments and made following the Dental School Clinic protocols. Already during baseline observation, most subjects with shortened dental arches had that condition for a long period (see Table 2). Also the removable partial dentures in the “SDA + RPD group” were worn for a long period (see Table 2). Informed consent of the subjects was attained according to the guidelines of the University of Nijmegen. They were interviewed and examined by one calibrated observer at baseline up to 9 years, with 3-year intervals (Table 1).