Clinical Practice Jaw clenching effects in relation to two extreme occlusal features: patterns of diagnoses in a TMD patient population Daniele Manfredini 1 , Michele Vano 2 , Redento Peretta 3 , Luca Guarda-Nardini 3 1 Assistant Professor, Temporomandibular Disorders Clinic, Department of Maxillofacial Surgery, University of Padova, Italy, 2 Assistant Professor, Department of Surgical Pathology, Medicine, Molecular and Critical Area, University of Pisa, Italy, 3 Assistant Professor, Temporomandibular Disorders Clinic, Department of Maxillofacial Surgery, University of Padova, Italy, 4 Luca Guarda-Nardini, Head, Temporomandibular Disorders Clinic, Department of Maxillofacial Surgery, University of Padova, Italy Objective: The aim of this investigation was to compare the pattern of temporomandibular disorder (TMD) diagnoses in clenching patients with different occlusal features, the null hypothesis being that no between- group differences exist. Materials and methods: Two groups of subjects receiving a jaw clenching diagnosis and having large overjet or anterior open bite (Group A; N545, 75.5% females, mean age: 38.1615.9 years) or normal occlusion (Group B; N569, 71% females, mean age: 34.6613.8 years) were recruited among a TMD patient population and were given Research Diagnostic Criteria for TMD (RDC/TMD) axis I diagnoses, namely, group I muscle disorders, group II disc displacements, and group III arthralgia/osteoarthritis/osteoarthrosis. Major findings: The distribution of RDC/TMD single and combined group diagnoses was significantly different between the two groups (P,0.05), with Group A subjects showing a higher prevalence of multiple diagnoses (60% versus 43.3%), as well as a higher prevalence of combined RDC/TMD axis I group II and III diagnoses (37.8% versus 20.2%). All TMD signs and symptoms were more frequent in the patients with large overjet or anterior open bite with respect to the patients with normal occlusion. Conclusion: In a TMD patient population, jaw clenching may have different consequences in subjects with large overjet or anterior open bite with respect to subjects featuring normal occlusion. Keywords: Temporomandibular disorders, Clenching, Occlusion, Bruxism Introduction In recent years, increasing attention has been given to bruxism, with focus on its definition, etiology, epidemiology, diagnosis, and consequences on natural tooth and dental implants. 1–8 Also, the relationship of bruxism with temporomandibular disorder (TMD) has been much debated, due to the often contrasting findings of observational investigations assessing the association between the two disorders and studies on experimental tooth clenching and/or grinding. 9,10 Researches should be performed to unravel several aspects of the TMD–bruxism relationship, especially in light of the potentially different effects of the various motor activities characterizing bruxism, namely, clenching and grinding, as well as the possibly different individual predisposition to develop symptoms based on anatomo-skeletal fea- tures. Studies on the role of the occlusal features in the etiopathogenesis of TMD suggested that the importance of dental occlusion as a risk factor for TMD is lower than believed in the past, and that an association exists only with some gross occlusal abnormalities. 11 Among those, a large horizontal overlap, namely, a large overjet, and an anterior open occlusal relationship, namely, an anterior open bite, seem to be the two occlusal features that were found to be associated with TMD in almost all studies on TMD and occlusion. 12–14 Also, occlusion has been considered the battleground on which muscle forces are exerted and through which they are transferred to the temporomandibular joint (TMJ). 15 With these premises, it should be interesting to assess if patients with the above occlusal features are Correspondence to: D Manfredini, Viale XX Settembre 298, 54033 Marina di Carrara (MS), Italy. Email: daniele.manfredini@tin.it ß W. S. Maney & Son Ltd 2014 DOI 10.1179/0886963413Z.0000000009 CRANIOH: The Journal of Craniomandibular & Sleep Practice 2014 VOL. 32 NO.1 45