Charles Kim. Step 1 Kennedy classification

Transcription

1 Step 3 Survey cast Step 2 Anatomical landmarks Step 1 Kennedy classification Classification criteria -Class 1: bilateral edentulous areas posterior to the remaining teeth -Class 2: unilateral edentulous area posterior to the remaining teeth -Class 3: unilateral edentulous area bounded anteriorly/posteriorly by teeth -Class 4: single edentulous area anterior to the remaining teeth and crossing the midline Applegate s modifications to the Kennedy classification -Classification should be done after all useless teeth are extracted -A missing 3 rd molar doesn t count in the classification unless it will be used as an abutment -A missing 2 nd molar doesn t count if it is not to be replaced (like if opposing tooth is also missing) -The most posterior edentulous area always determines the classification -Edentulous areas not included in the classification are called modification spaces -Class 4 arches cannot have a modification space because any edentulous spaces will be posterior to it, and hence take that classification -On the cast: landmarks should be outlined in green pencil Mandibular arch -Retromolar pad -Marks the distal border of type 1+2 PRDPs -Useful because it landmarks a site that does not resorb over time -Does not resorb due to all its muscle insertions -Occlusal plane is set at 1/2~2/3 the height of the pad -Lingual, labial and buccal frena -Lingual tori Maxillary arch -Tuberosities -Functionally same as retromolar pads -Should be covered by the denture base in class 1+2 -Hamular/pterygomaxillary notch -Most distal extension of the denture base -Connecting the left/right notches yields the vibrating line -Vibrating line -Border between soft and hard palate -Denture bases should stop just anterior to this line -Labial and buccal frena -Buccal or palatal tori Positioning the survey arm relative to the cast -Start with the cast s occlusal plane perpendicular to the survey arm (AKA spindle) -Adjust the position of the spindle to evenly distribute undercuts mesio-distally and bucco-lingually -Example: 15 present, 16 missing, 17 present. If the 17 mesial has a huge undercut when surveyed but the 15 has no undercut, then the cast must be tilted to shrink the 17 undercut and increase the 15 undercut -Favourable undercuts are shallow, and must be maximised -Undercut gauges (screw-on tip with a small disk on the end in the microsurveyor kit) can be used to see how much undercut there is on each tooth surface -Example: if you are planning to do wrought wire clasps on the anterior tooth, then attach a 0.5mm undercut gauge to the microsurveyor spindle and try to see if this undercut can be achieved -The survey arm position determines the PRDP s path of insertion, so it is important to note what direction the survey arm is when the cast is surveyed Survey arm perpendicular to cast Resulting PRDP will be inserted straight down, perpendicular to occlusal plane Survey arm tilted toward cast Resulting PRDP will be inserted downwards in an anterior posterior direction Survey arm tilted away from cast Avoid. PRDP cannot be inserted posteriorly to anteriorly -Other factors determining tilt of survey arm: satisfies appropriate guide planes and satisfies esthetic goals -If the anterior alveolar ridge is slanted anteriorly, the path of insertion will need to accommodate for it Mark heights of contour on the cast -Height of contour = the most protrusive part of the tooth/soft tissue. Anything below the HOC is considered an undercut -Once the survey arm is locked and path of insertion determined, expose the lead and run it on all the teeth crowns to mark their heights of contour. This must be done on all surfaces (including lingual) -Also, mark the height of contour for the soft tissues. This determines the inferior border of the PRDP -Very shallow soft tissue undercuts (going below the HOC) may also be favourable, for retention purposes -Note: HOC can be modified by the dentist. It can be lowered by shaving with a bur or raised by adding composite or crowning the tooth with a pre-surveyed crown. This is done when the HOC is too low or high for a clasp to fit properly Recording survey arm tilt on the cast -If the cast was removed, there must be a way to re-establish the same tilt when the cast is re-mounted on the surveyor -Scribing: mark 2 vertical lines on the back + lateral side of the cast in lead or red pencil. Keep these lines as far apart as possible and re-scribe the lines to make it clearer -Preferred method at UBC -Tripoding: 3 crossed circles in lead or red pencil drawn on the palatal/lingual surface of the cast. These 3 points represent the horizontal plane of the survey arm s tilt. Not preferred as they can be easily erased

2 Step 5 Guide planes Step 4 Rest seats -On the cast: Rest seats should be outlined in red pencil -Indentations made on the tooth surface to receive a ledge on the PRDP -Allows the PRDP to hook on to hard tooth structure and prevent settling into soft tissues -If possible, rest seats should be made on abutment teeth adjacent to edentulous spaces -Recommended burs: football or round shaped diamond/carbide burs -Fluoride varnish on newly cut enamel and etch/bond/restore on exposed dentin -Try not to use anesthesia, but dentin sensitivity may occur Occlusal rest Cingulum rest Incisal rest -All premolars and molars -Try to avoid on amalgam due to its poor tensile strength. Crown instead -Spoon shaped, deeper axially than at marginal ridge -Right angle to tooth s long axis -B/L width: 1/2 of tooth -M/D width: 1/3~1/2 of tooth -Thickness is 1~1.5mm -Most preferred type of rest -Optimal support -Low morbidity of tooth -Maxillary canine -Maxillary central with a big cingulum -Any incisor with a surveyed crown -1.5mm deep -1~1.5mm wide (B/L) -2~3mm long (M/D) -Semilunar shape -Mandibular anteriors -Maxillary laterals -1~1.5mm deep -1~1.5mm wide (M/D) -1.5~2mm above gingival embrasure -Wrap the rest with a semilunar preparation buccolingually -Less optimal support and has a risk of -Poor esthetics. Solutions: move the enamel perforation rest to premolars OR crown anterior -Better esthetics than incisal rest tooth + put a cingulum rest on -If 2 occlusal rests are indicated on teeth adjacent to each other (example: 16 MO + 15 DO), make sure to factor in the occlusal clearance so the clasp can exit through the buccal/lingual embrasures -On the cast: guide planes are drawn in red on the mesial/distal face of the tooth straight occluso-gingivally Guide planes -Teeth crowns are naturally bulbous, and will not normally form a straight path of insertion -Guide planes are adjustments made to a tooth surface to be as parallel as possible to the path of insertion. It forces the PRDP to only be able to be applied/removed in 1 direction better retention and stability -Normal crowns have big undercuts, which can trap food/plaque guide planes shrink these undercuts -Done typically next to edentulous spaces -Plane should only be 2~4mm occluso-gingivally (or less), to reduce stress on the abutment tooth -Plane should extend from the buccal line angle to the lingual line angle, following the natural curve of the tooth -Recommended burs: parallel or tapered diamond/carbide burs to cut and 12 bladed finishing bur to smooth -Fluoride varnish on newly cut enamel and etch/bond/restore on exposed dentin -Try not to use anesthesia, but dentin sensitivity may occur -If a tooth is too tilted, severe reduction will be required to form a parallel guide plane. In these cases, a surveyed crown (as mentioned before) is more favourable -Summary: improves retention + stability + occlusion + esthetics, protects the gingiva, and minimizes food/plaque traps -Risks: distal extension teeth, lone standing teeth, and endo+crowned premolars are at risk of excessive torqueing forces -Solution: do conservative guide planes (small reduction), or avoid guide planes altogether -Limit guide planes to only 1 per tooth (this applies to all teeth) Guide plate (AKA proximal plate) -Metal part of the PRDP that engages the guide plane -Should avoid the gingival margin if possible, due to possible irritation -The guide plate should extend 1~2mm onto the alveolar ridge for cleansability

3 Step 6 Direct retention -Function: resists vertical dislodgement along the path of insertion Types of direct retainers -Intracoronal (left): consists of the patrix (A) and matrix (B). Will not be studied further -Extracoronal attachments (no pic): also relies on patrices and matricies, but extracoronally and on the lingual side -Extracoronal clasp assemblies (right): relies on a retentive arm and a reciprocal arm. Retentive arms come around the occluso-facial side in suprabulge assemblies (top right pic). Retentive arms come around the gingiva-facial side in infrabulge assemblies (bottom right pic) Clasp assemblies -The assembly that brings the previous items (rests + guide plates) together -Optimally, clasp assemblies are located adjacent to edentulous areas -4 components: rest, guide plate, flexible retentive arm (usually buccal), rigid reciprocal/bracing arm (usually lingual) -Functions of clasp assemblies -Support (resistance to force towards tissues) is derived from rigid rests -Retention (resistance to force away to tissues) is derived from guide plates, retentive arms, and reciprocal arms -Stability (resistance to lateral and rotational movement) is derived from guide plates and flexible retentive arms -Other considerations with clasp assemblies -Encirclement: the clasp assembly should have at least 3 contact points encircling > 180 of the tooth -Passivity: direct retention should only be active when dislodging forces are applied to the PRDP -Occlusion: the clasp assembly should not interfere with occlusion What factors determine how much retention the retentive arm provides? -Flexibility of the retentive arm: if the retentive arm is too flexible, then the PRDP will easily pop out. Flexibility depends on several factors: -Length of the arm: flexure is proportional to (length) 3 -Cross sectional form: round arms can bend in any dimension, so it is more flexible than a half-round arm -Diameter: flexure is indirectly proportional to diameter of the arm -Taper: to allow flexure where needed, the arm should taper to ½ its thickness near the tip -Alloy/material: wrought wire is more flexible than cast clasps -Amount of undercut engaged: the retentive arm must be under the height of contour to provide retention. Generally, 1 mm of combined undercut from all the teeth is enough to retain a PRDP. As mentioned before, undercuts can be modified (increased/decreased) using enameloplasty, composite resin, or surveyed crowns -Type of retentive arm used: many types exist (wrought wire circlet, cast circlet, cast ring clasp, cast I bar, cast Y bar) Comparing wrought wire and cast metal in the retentive arm Wrought wire Cast clasp 19g (1.02mm) stainless steel or gold alloy. Higher Chrome cobalt alloy, same as rest of the Material gauges are OK, but higher risk of distortion PRDP framework Flexibility High (cross section = round) Low (cross section = half circle) Resistance to fracture High Low Adjustability High Low Use -Class 1/2 PRDP s to control the potential for excess forces on abutment teeth -Also used on the indirect retention tooth of class 1/2 PRDP s -Only in suprabulge clasp assemblies -Class 3/4 PRDP s because risk of overloading the teeth are low, so a less flexible clasp is OK -Supra/infrabulge clasp assemblies Undercut needed 0.50 mm 0.25 mm How to draw -Clasp drawn in blue pencil -Clasp drawn in red pencil - WW written below the tooth Other info -Called a combination clasp -Less expensive -Soldered to the PRDP framework -Less likely to distort

4 Step 7 Reciprocation/bracing Step 6 Direct retention cont. Suprabulge clasp assembly -Main components of a suprabuldge clasp include: the shoulder, midsection, and terminus -Shoulder is the widest and is the rigid part of the arm -Midsection is slightly tapered and more flexible than the shoulder -Terminus is the most tapered and is the most flexible. This is the only 1/3 that engages the undercut -Note: at UBC, ring clasps will have the retentive arm on the facial, not lingual (opposite of the image). Also, the lingual surface may be plated for less food trapping and tissue trauma -The clasp must be at least 1mm over the gingiva at all points. It must not touch the free gingival margin Infrabulge clasp assembly -Main components of an infrabulge clasp include: approach arm and terminus -Approach arm goes horizontally up to the tooth, then goes vertically (crossing the gingiva at 90 ), and ends just apically to the survey line. None of the approach arm is in contact with the tooth -Terminus is the part that contacts the tooth surface, apical to the undercut -Not recommended at UBC unless unavoidable -I bars are usually enough, and T bars are generally unnecessary -Cannot be used very often due to soft tissue undercuts -RPI (rest, proximal plate, I bar) clasp system: proposed alternative to the wrought wire combination clasp in class 1 and 2s -Rest placed on the mesial side to prevent distal movement during loading -Proximal plate placed on distal because of the distal free end -I bar placed on mid-buccal or mesio-buccal area -No clinical evidence to justify its use, but may be come esthetic advantages -Not preferred compared to the simpler combination clasp -On the cast: clasp is outlined in red pencil on the tooth surface Reciprocal arms -Unlike the retentive arm, it does not go below the height of contour -It is made from cast metal and is rigid. It resists tipping force generated as the retentive arm pushes over the height of contour when the PRDP is placed/removed. Also gives the PRDP lateral stability -Arm is usually placed on the lingual side Reciprocation VS bracing -The ideal reciprocal arm will touch the tooth when the retentive arm touches the tooth. This is ideal because the reciprocal arm can immediately resist the tipping forces generated by the retentive arm. This is proper reciprocation -In real life, it is not always possible. Sometimes, the reciprocal arm will only touch the tooth after the retentive arm has engaged the height of contour. This is less ideal and is referred to as bracing Adjustments made by the dentist -Lingual side will be reduced to reduce the HOC to the middle 1/3 of the crown. This will allow more reciprocation -When viewed occlusally, the reduction should follow the curvature of the tooth Other methods of reciprocation -Lingual plates and minor connectors can act as reciprocators -In a cingulum rest clasp assembly, the cingulum rest acts as the reciprocator -Note: more than ½ the tooth s circumference needs to be bound

5 Step 9 Major/minor connector Step 8 Indirect retention -On the cast: drawn in red pencil Fulcrum lines -Only relevant to class 1, 2, and sometimes class 4 arches -Class 3 arches are fully supported by teeth no fulcrum line -Soft tissue can act as support for PRDP s. Because soft tissues are not entirely rigid, they allow some wiggle room -During loading and unloading, the PRDP will rotate about an axis called the fulcrum line -The fulcrum line can be drawn by connecting the 2 most distal rests on an arch -Diagram on the right shows the fulcrum line in red Forces around the fulcrum line Force towards ridge Force away from ridge Example -Normal chewing forces -Sticky foods pulling on PRDP Diagram Forces exerted on -Yellow line in diagram -Green line in diagram dentition -Distal tipping force on tooth -Mesial tipping force on tooth How to minimize -Well adapted distal extension base -Add a rest to serve as indirect retention trauma on abutment -Minimal movement around fulcrum -Place a rest on the left arrow How to design an indirect retention unit in class 1/2 Kennedy arches -Draw the fulcrum line at the midpoint of the line, draw a perpendicular line towards the anterior direction the tooth that intersects with the line is the ideal area for indirect retention -However, adding an incisal rest to an anterior tooth will result in poor esthetics -Instead, the rest can be moved to a nearby canine for a cingulum rest or a premolar for an occlusal rest (preferred) -Remember: cingulum rest on a mandibular canine is not acceptable due to its thin enamel -A lingual plate with a rest may also be used for indirect retention -If the class 1/2 arch has a modification space in the anterior region, a rest in that modification space can be used as for indirect retention as well. How to design an indirect retention unit in class 4 Kennedy arches -No need for true indirect retention -Instead, a direct retainer with a rest is placed as far distally as possible to give a similar function Are wrought wires used bilaterally? -Case: patient has a class 2 mod 1 arch -Patient s left side obviously needs the wrought wire because the distal extension base will torque the abutment tooth -Patient s right side has full tooth support on the mesial and distal. Only one tooth is missing. Do they still need a wrought wire or is cast clasp OK? -They still need a wrought wire on this side because drawing a fulcrum line on the most distal rests shows rotation is still possible -On the cast: outlined in red pencil Major/minor connectors -Major: connects the PRDP from one side of the arch to the other -Minor: connects all parts (rests, clasps, retainers, denture bases) of the PRDP to the major connector -Factors to consider: must be rigid, comfortable, esthetic, minimize phonetic changes, and allow hygiene access -Should not impinge on any soft tissues like gingiva, frena, or unattached mucosa -Edges should be rounded and smooth Minor connectors -Must be at least 2mm thick and uniform. Slight taper is OK too -There should be at least 6mm between individual minor connectors. If not, then it is better to plate the surface instead -Preferably placed in interproximal embrasure spaces to disguise its thickness -Can function as a reciprocating arm opposite to the retainer clasp -Allow relief near the gingiva and soft tissues to prevent trauma -Connects to the major connector at a right angle, but the intersection is gradual and smooth

6 Step 9 Major/minor connector cont. Major connectors Maxilla Clearance -Connector should be 6mm away from gingival margin -Use a lingual plate if it is not possible -Borders running M-D should be parallel to the gingival margins of the teeth Dimensions -Should be flat (1~2mm) minimal speech disturbance -Should be at least 8mm wide for rigidity Types -Palatal strap: for class 1,2,3 with no missing anteriors Principles Tori -A/P palatal strap: for missing/compromised anteriors. Most common connector. Try to plate the anterior strap to minimize speech disturbance -U shaped: rarely used, preferred in presence of a torus. Very prone to distortion, even with minimum thickness of 15mm -Full coverage palatal plate: used if most teeth are missing, ridge is low/missing. Most obtrusive -Palatal bar: avoid -Minimize any palatal coverage anterior to the molars -Anterior strap should either be a lingual plate or be avoided. If it is unavoidable, the borders of the strap should end on the posterior slope of a ruga to blend the prosthesis with tissue contours -Midline and gingiva should be crossed at right angles -Posterior border of the PRDP should clear the vibrating line by at least 2mm -PRDP should be flush with the palate, no relief -Beaded finish lines (elevations of 1mm) are used to prevent food getting stuck under prosthesis. Created by scoring the cast -Should be avoided or surgically removed as they are sites with very thin mucosa -If unavoidable, plate over it with relief Mandible -Connector should be 3mm away from gingival margin. However, more clearance is needed if probing depths are greater than 3mm -Use a lingual plate if it is not possible -Borders running M-D should be parallel to the gingival margins of the teeth -Bar: should be 5mm wide and 2~3 mm thick -Since bar is 5mm wide and clearance is 3mm, 8+mm is needed for a bar to be viable -Lingual bar: used in most class 1,2,3 with no anteriors missing and adequate height -Lingual plate: used in mandibles with missing/weak anteriors. Plate runs along middle 1/3 of teeth crowns. Dimensions are still 5mm thick. Plate needs to be supported on both sides by incisal rests or occlusal rests no further back than the mesial of the first premolars. Plate can be scalloped (top) or stepped back (bottom) for esthetics -Sublingual bar: used when lingual bar can t fit and anteriors have large diastemas. Bar fits in floor of mouth. Challenging to execute -Cingulum bar: too thick to be tolerable. Avoid. -Kennedy (double) bar: difficult, irritates tongue, food trap. Avoid. -Labial bar: avoid unless teeth have severe lingual inclination -Should extend from distal abutment on one side to the distal abutment on the other side -Inferior border is placed at or slightly above the functional vestibule -Anterior part of the connector requires some relief to prevent damaging tissues during normal PRDP movement. If the lingual tissues are vertical, minimal relief is needed (0.2~0.5mm). More relief is needed if lingual tissues are less sloped -Cross section of the connector is a half-pear shape. The thickest part is closest to the floor -Should be surgically removed because avoiding it in the mandible is rarely possible

7 Step 10 Denture base retention -On the cast: drawn in red pencil. Each type of base has a different pattern associated with it Denture base principles -Type of minor connector used to join the denture base and artificial teeth to the major connector -The bases must have sharp edges and undercut-butt joints for acrylic resin retention -Acrylic easily fractures if it is thin, so having 90 degree edges are ideal -The major connector can be extended posteriorly to these bases -The major connector forms the lingual side of the base -Allows less metal on the ridge more room to fit artificial teeth on the ridge -This is done in cases with minimal interproximal space or interarch (<4mm) space -For open web and mesh bases, 1mm of relief must be given so that acrylic can be packed underneath it. The bare metal does not touch the soft tissues -For other bases, the tissues do touch the bare metal Denture base types Base Diagram Design principles Open construction or Open web Mesh Reinforced acrylic pontic -Buccal side and lingual side have longitudinal struts -Lingual strut can be an extension of the major connector -Transverse struts connect the 2 longitudinal struts -Ideally, 1 transverse strut placed where the neck of an artificial tooth will be leads to best esthetics -Most common type -Preferred for missing adjacent teeth with resorption -Excellent denture flange retention and easily relined -Can be used with other retentive elements like a braided post -Drawback: difficult to properly pack resin into holes -Preferred for missing max. anteriors with minimally resorbed ridge -Not meant for large denture flanges -Difficult to reline Beaded metal surface Braided post/nail head help me ^ -Preferred for areas with minimal interarch space (<5mm) since relief is not needed -Acrylic prone to fracture/separate under heavy load -Better hygiene (metal thickens at borders, creating a seal against food traps) and thermal sensation -Difficult to reline -Preferred for single tooth support with a well healed ridge -Not designed for large denture flanges -Difficult to reline For distal extension bases -It is hard to pack resin under the base because there is no tooth support. When resin is packed, the base bends -For easier processing, a tissue stop (3x3 mm piece of metal under the base) is required in distal extension bases -The tissue stop will be in contact with the ridge, and lifts the denture base 1mm -Tissue stops should be located on a flat area of the ridge, in front of the retromolar pad/hamular notch -Also, step backs are required: Maxilla Mandible -Major connector is extended to hamular notch -Denture base is stepped back sharply 2~3 mm -Provides adequate bulk for smooth transition to processed acrylic resin base -Major connector is extended to distal abutment tooth -2mm beyond the tooth, base is stepped back sharply 2~3mm -Provides adequate bulk for smooth transition to processed acrylic resin base

8 Step 11 Special considerations Written directions example -Rest seats: specify tooth numbers and location of rest as needed (e.g. Occlusal: 26 MO, 14 DO; Cingulum 23) -Guide planes: -Direct retention: clasp arm type with list of location and depth of retentive undercut and tooth number (e.g. Cast Clasp into: 0.25mm DB u/c 26, 0.25mm MB u/c 24; 19g WW Clasp into 0.5mm MB u/c 14) -Reciprocation/bracing: clasp arm type (or linguoplating) with list of location and tooth number (e.g. Cast Clasp: L 26, L 24; Linguoplate L 14) -Indirect retention: specify type of rest with list of tooth numbers and location of rest as needed (e.g. Incisal: 33 DI) -Major connector: specify type, location as needed (e.g. lingual bar, or A-P strap as linguoplate) -Denture base retention: specify type and list location (e.g. mesh with tissue stop at 15-17, and 25-26; nail head at 13; RAP 11, 21) -Artificial teeth: specify type and location (e.g. Acrylic teeth at 17, 16, 15, 13, 11, 21, 25, 26, 27) Special consideration example on a clinical plan -Adhesive metal rest seat to be bonded for 11 cingulum rest -Lower height of contour 37 MF, 35 DF, 44 DF (to optimize clasp flexibility and eliminate occlusal interference w/ direct retainer) and 37, 35, and 44 Lingual (to improve reciprocation) -Others: Reduce extruded 16 for restoration of occlusal plane; Do surveyed crown on structurally weak 14; Replace defective amalgam restoration 46 MO -Note: It is also helpful to mark on study cast any required tooth adjustments (e.g. for guide plane preps or areas to lower height of contour) to be made using a different colour of pencil (e.g. green) to serve as a chairside reminder Special consideration example on a lab prescription -Place normal gingival relief in area of gingival sulcus, or place normal relief under lingual bar, or place extra relief over a torus (if covering it) (the example of gingival relief would be for the areas where the metal extends from the Guide Plate on to mucosa for ~2mm to obtain the smoothest least porous surface possible and prevent a sharp edge of metal at the sulcus) -Keep internal and external finish lines sharp (this example would be to obtain the smoothest junction between metal and acrylic; useful for most prescriptions especially when you haven t worked with the technician for long) -Place 1mm deep beading at soft-tissue periphery of palatal strap -A retentive arm is not to be used for tooth 14 (periodontal reasons), only a DO occlusal rest and proximal plate at the distal are being used Pages not covered in PRDP module manual: 1~9 (Table of contents and random facts) 11 (Phase 1/2/3 treatment planning for PRDP s) 24 (How to make custom trays) 50~56 (Clinical follow up and more info about custom trays) 57~64 (Sources + paper)