# Curve of Spee and Second Mandibular Premolar Agenesis--Present Knowledge and Future Perspectives
## 4 Discussion  
A stable occlusion with the highest number of required interactions of the masticatory system is the foundation for clinical success. This study was conducted to discover the effects of a persisting primary second mandibular molar on the COS depth, considering that a severe COS is accompanied by a wide range of orthodontic problems, such as a deep bite [21]. As already found in few studies, there was no relevant difference in COS depth between the oral left and right side [3,22,23]. Therefore, in the present study mean values of measurements from both sides were examined for the control group. Our findings showed that the COS is deepened in the presence of a deciduous tooth (ppM2). Thus, clinicians should keep in mind that neighboring teeth may tip towards the primary tooth in consequence of its lower marginal ridges, which deepens the COS. As the first primary molar is determined as the most affected tooth affected by ankylosis [24], the presence of a deep COS might be the first sign which must be followed in a timely manner to prevent further complications. Furthermore, patients with tooth agenesis not only show tooth related changes which can be orthodonically treated, but also morphometrical changes in the mandible. [PERSON] et al. [25] concluded that in consequence of a missing lower second premolar, a reduced mandible length and shape differences might be expected. The study compared the morphology of the mandible between patients with agenesis of the second lower premolar and patients without agenesis. In the treatment of patients with an agenesis, the orthodontist has to deal with a reduced dimension of the mandible due to the absence of sagittal growth. In consequence, the reduced sagittal dimension of the mandible may lead to an increased OJ, though our study collective misses to prove. As a deep COS may lead to masticatory problems or temporomandibular joint (TMJ) disorders [26,27], professional management of a ppM2 is crucial for dentists, as well as for orthodontists. [PERSON] et al. [9] reported that the depth of the COS (also obtained by digital methods) was approximately 1.8 mm to 1.9 mm, which is similar to the COS measurements in the control group of the present study (1.6 mm). According to previous researchers [9,23,28], in our study population there are no significant differences in the COS depth between males and females. The current study population consisted of patients with fully erupted teeth, as existing literature showed that there is no significant change in the COS depth between adolescence and adults [29,30]. However, in our study population, the COS depth decreased with age in the control group and deepened in patients with agenesis. One possible explanation is that the COS may be pathologically altered in situations resulting from tipping of neighboring teeth when there is no sufficient restauration on the deciduous one. In contrast, a recent study from [PERSON] et al. [31] stated that the depth of the COS is maintained throughout life. However, orthodontists should be aware of individual occlusal adjustments with age, ending in a customized COS. Thus, if this individual COS got changed, the masticatory system and oral health of the patient may get damaged [32]. In our study population, COS is deepest in class II subjects (5.33 mm) and most levelled in class III subjects (0.3 mm), which is in accordance with other studies [13,23,33,34]. This also supports the hypothesis that arch length is associated with the COS depth [10,11], as a class III malocclusion is related to an exceeded lower arch length resulting in an anterior cross bite [35]. Furthermore, measurements of OJ did not show statistically significant dependencies relating to a ppM2 or angle-classification. It might be assumed that COS depth is not an issue when it comes to OJ measuring.