Preclinical Fixed Prosthodontics I Principles of Tooth Reduction PDF

Course Title: Preclinical Fixed Prosthodontics I Course Code: PRO121 Topic of Lecture : Principles of Tooth Reduction for Extra-coronal Restorations Lecturer: Dr. Marwa Wahsh Post :Professor of Fixed Prosthodontics F A C U L T Y O F D e n t i s t r y T h e F u t u r e...

Course Title: Preclinical Fixed Prosthodontics I Course Code: PRO121 Topic of Lecture : Principles of Tooth Reduction for Extra-coronal Restorations Lecturer: Dr. Marwa Wahsh Post :Professor of Fixed Prosthodontics F A C U L T Y O F D e n t i s t r y T h e F u t u r e S t a r t s H e r e F a l l 2 0 2 4 Successful tooth preparation and subsequent restoration depend on three categories: Biologic considerations: which affect the health of the oral tissues Mechanical considerations: which affect the integrity and the durability of the restoration Esthetic considerations: which affect the patient appearance Principles of tooth reduction of extra- coronal restorations Reduction varies according to: 1. Tooth size and form. 2. Tooth position and alignment in the dental arch. 3. Type of restoration indicated. Path of insertion It is an imaginary line along which the restoration will be placed onto or removed from the preparation OR The direction through which the restoration could be precisely seated on the prepared tooth Classification of path of insertion 1. For single restoration: A. Line of insertion: Parallelism between the opposing axial walls is indicated. Any deviation from this line will not allow the seating of the restoration. Classification of path of insertion 1. For single restoration: B. Range of insertion: Converging angle between two opposing walls. The restoration can be seated within the limits of the converging walls.  Classification of path of insertion 2. For a bridge (Two or more abutments): A. Common line of insertion: Only one line of insertion for the bridge is allowed and deviation from that line does not allow seating. Classification of path of insertion 2. For a bridge (Two or more abutments): B. Common range of insertion: Within the limits of the convergence of the walls, the restoration can be precisely seated on the corresponding preparations. Factors affecting path of insertion 1. Reduction of axial walls 2. Rounding of axial line angles 3. Proximal grooves 4. Ledges and/or indentations 5. Pinholes 1. Reduction of the opposing axial surfaces: The reduced opposing axial surfaces may be parallel to each other or slightly converging towards the occlusal surface  Buccal and lingual walls The tip of the stone is held parallel to the long axis of the tooth to remove the tooth bulge, regardless of the original divergence below the height of contour.  Path of insertion Slight taper of the opposing walls towards the occlusal surface allow complete seating of the restoration during cementation. 2. Rounding the axial line angles: The reduced opposing axial surfaces will leave the prepared tooth with four cervical undercuts at the axial line angles. 2. Rounding the axial line angles: Establish proper path of insertion Obtain continuous finish line in the gingival crevice  Path of insertion Using the whole thickness of the tapered stone with round end o prepare chamfer finish line leads to the formation of enamel lip (undermined or unsupported enamel) which can fracture later leading to inaccurate A. Tilting the tip of the stone away from the tooth margin. leads to the formation of undercut. B. Tilting the tip of the stone towards the tooth leads to overconvergence of the wall. PATH OF INSERTION Undercut Line of insertion Overconvergence  Limited path of placement So slight occlusal convergence (Taper) of the axial walls is recommended Within 2 - 6° for each wall, with a total occlusal convergence of 4-12° Undercut (reverse taper) Path of insertion evaluation The path of insertion is viewed from the top view with one eye opened at a distance of 30 cm. Binocular vision (using both eyes) should be avoided as a preparation with undercut can appear to have standard taper PATH OF INSERTION EVALUATION  Path of insertion Limited path of insertion: The diameter at the cervical area is slightly bigger than that at the occlusal area Range of insertion: The greater difference in diameter between cervical and occlusal areas indicate occlusal over convergence Common path of insertion Common path of insertion To check the common path of insertion between two abutment teeth, the mirror should be adjusted to see one preparation from the top view to have a path of insertion, then the mirror is moved firmly without changing its angle till the other preparation is centered. Axial grooves 3. Alignment of axial grooves  Alignment of grooves To prepare the proximal grooves for ¾ crown the stone or bur parallel to the long axis of the tooth. The two grooves are prepared parallel to each other.  3. ALIGNMENT OF AXIAL GROOVES Buccal and lingual walls of the groove should be slightly divergent  3. ALIGNMENT OF AXIAL GROOVES Lingual wall of the groove in relation to the prepared lingual surface should be slightly converging. Buccal wall of the groove in relation to the prepared lingual surface will not affect the path of insetion 3. ALIGNMENT OF AXIAL GROOVES The axial wall of the groove The axial walls of the 2 grooves should be slightly converging occlusally. 4. ALIGNMENT OF LEDGES AND INDENTATIONS The direction of ledges create no problem regarding the path of insertion 5. ALIGNMENT OF PINHOLES A. Proximal view: Pinholes are prepared with slight occlusal divergence to allow seating and removal of the restoration. 5. ALIGNMENT OF PINHOLES B. Facial view Incisal convergence or divergence during preparation of the grooves will make the seating impossible. All pinholes should be prepared parallel to each other. Intra-coronal indirect restorations Proximal boxes are prepared with slight occlusal divergence  Posts are prepared with slight occlusal divergence