Earlier works by authors dealing with the problem of the application of the stereo-optical (stereometric) method, have shown good correlation between mathematical measurements under laboratory conditions [11-15].
In this regard, further research using stereooptic methods will certainly reveal many more important details and facts regarding the treatment of partially-edentulous jaws with remaining dental tissues.
Strains and deformations of remaining teeth in the casts investigated in the present study were differently distributed.
In the control incisor without protective coping, the test did not reach Stage 3 because during Stage 2 the tooth cracked on our 3D printed model. The material then exceeded the tensile strength, which according to the literature is 65 MPa. For this reason, the final deformation of the incisor with and without a coping i.e. cap at a maximum load of 1000 N could not be compared, because the incisor without the coping did not withstand the specified load. Instead, those from stage 1 (Tab. 1) were used to compare deformation values. The cross-section of the incisor without the coping was smaller, so at an identical force it would have a significantly higher voltage value, thus it would be expected to fracture first. This is evidenced by the results obtained; the incisor covered with a coping withstood a load of 1000 N while the incisor without a coping did not. By comparing the results in Stage 1 for the incisor with and without the coping, we demonstrated that a greater deformity is present in the incisor without the coping.
A major strain field was present around abutment teeth, as reported by previous authors [10]. A lower intensity compared to the major strain field was observed towards the alveolar ridge. The difference in the obtained values may be attributed to the effect of the highest stress on the junction of different structures–the dentinal-cement border, and also the region below the gingival sulcus with the composition of the dentin-cement-alveolar bone junction of the root of the remaining tooth in the alveola. Major strain values for the casts of the partially-edentulous lower jaw showed greater strain of the occlusal portion of the premolars either without or with a covering coping. For the incisor without a coping the major von Misses strains were in a region of remaining tooth substance towards the incisal areas. In contrast, for the incisor with a coping the major von Misses strains were in the gingival areas, towards the gingival sulcus. In addition, loading of incisors, canines and premolars of the experimental casts caused only limited stress transfer to distant portions underneath tooth structures. However, our data show that the bone underlying the remaining teeth could have undergone higher resorption in the buccal region, which is in agreement with the findings of previous authors [16].
Maximum displacements were noted for the experimental incisor with a coping and for the control canine (Fig. 3, 5). It would certainly have been noted for the control incisor too, but for the fact that the control incisor cracked earlier, at stage 1 with a load of 500 N, removing the possibility of additional displacements (Tab. 1).
According to the results of the present study, the risk of bone overload appears to be highest around the dentin–cementum junction in the cervical parts of remaining teeth, confirming that the majority of occlusal loading is transferred to the crestal bone. The biomechanical distribution of stress occurs primarily where the coping is in contact with the crown and enamel (dentin) of remaining tooth substance, therefore the stress distributions along the coping lengths should reflect their design. Although these differences were not extremely marked, they may be an important contributing factor to negative clinical effects and crestal bone loss if associated with other aggravating factors such as coping overloading, non-axial loading, insufficient number of remaining teeth and copings, and poor bone quality [17].
As for the question of selection of the type of partially-edentulous cast for the experiment, this could be discussed from the perspective of the problem of choice of design as well as the topography of the cast. This experiment was performed on a Kennedy class 1 partially-edentulous mouth i.e. B3 (and C2) classification of Eichner, because this type of partially-edentulous jaw is very frequently represented in clinical practice [1,2]. Moreover it remains to be discussed why the remaining teeth were present only on one side of the mandible, taking into consideration the well-known consequent problem that remaining tooth substance can undergo ‘additional loading’ as a consequence of the edentulous side of the mandible. This was done due to the objective fact that for many of the patients in everyday clinical practice, overdentures represent the ultimate and transient solution in a situation when quite a number of teeth have been missing due to extractions from the alveolae [1,2].
The optical stereometric method would have revealed whether it would be better for the remaining tooth substance to be covered by a casted coping or better to be left uncovered. Based on the results of this study, we conclude that the better option is for the remaining teeth to be covered by cast (or ceramic) copings. This is because the remaining tooth substance without coping protection could crack even at a low applied force of 500 N, while on the other hand, tooth substances covered and protected by copings, because of the form of the copings and the hardness of the selected material (alloy), will not start cracking until the applied force reaches 800N or more.
The design of the copings on the experimental mandibular cast was also very important but was also original in this study. Firstly, the coping shapes used in this study were such that the form was adapted to minimal dental removals in order to prepare abutments for coping acceptance. In this sense, and secondly, a special design was created of ‘semi-anatomical’ forms in which the curves were pointed towards the tips of the cusps and nodules and in accordance with the original morphology of the occlusal surface, which is a specific option in relation to the previously applied forms in clinical practice [1,2]. These forms also resulted in specific distributions of forces which were mostly present on the surfaces of the incisor copings and even the canines according to the cervical portions, while on the other hand, the forces applied to the premolar remained concentrated on the occlusal plateau (Fig. 5, 6) [1,2].
Conversely, increasing the strength of the overdenture by reinforcement decreased the deformation of the denture base and decentralized stress to the contralateral side, thereby decreasing stress on the working side, which seems to be true in the present study too, based on the present observations [18].
Fracture of abutment teeth is frequently reported. This possibility is especially likely in situations where the remaining dental substances are only present on one side of the jaw, and even more so if they are at the corner, i.e. at the border of the intercanine and transcanine sectors. Reasons for this could be the unfavorable position of remaining tooth substance in the mandibular arch, inadequate tooth preparation for copings or overloading. However, fracture of the abutment teeth has been reported as a motive for tooth loss much less frequently than caries or periodontal breakdown [19].