The Lower Free End Saddle (distal extension saddle)

Transcription

1 Giles Perryer 1997 I The Lower Free End Saddle (distal extension saddle) Free end saddle dentures move Excessive movement of the denture can cause pain, tissue damage, and complaints of instability and looseness Patients will complain immediately about pain and instability, but tissue damage tends to be long term and irreversible, painless until it is too late You could try to prevent a free end saddle denture from moving by ensuring the saddle area is continually under maximum loading (which will cause ischaemia) and by gripping the abutment teeth with clasps and guide planes so tightly as to allow no freedom of movement (which will put unacceptable torquing forces on them) This will not earn you the admiration of your patients, colleagues, or examiners The art of designing a free end saddle denture involves understanding all the movements that the denture can make, and designing them into the denture so that they are hardly noticed by the patient, and cause minimal damage to the mouth Downward Movement of the Saddle The free end saddle will move forwards and downwards with occlusal load, as it rotates about the abutment rest seat Reducing Rotational Movement of the Saddle The largest downwards movement will be at the posterior end of the saddle, where the mucosa will be maximally compressed As you get closer to the rest- seat axis, the movement will be more horizontal If you are very close to the axis, the movement will be completely horizontal and the mucosa may get puckered up against the abutment Ideally, the entire saddle bearing mucosa will be evenly compressed, spreading the load equally over its whole area This ideal is not possible, but the further away the rest-seat axis is from the saddle, the closer to ideal you will get To reduce rotational movement of the saddle Put the abutment rest seat mesially, to keep it as far from the saddle as possible

2 2 Reducing Downwards Movement Of The Saddle Assuming an even mucosal loading, the mucosa will get compressed under the saddle and, as tissue fluid is squeezed out, the saddle will sink The greater the pressure, the more it will sink At a gross level, too much mucosal displacement will cause pain or ridge resorbtion More subtly, as the saddles sink, the anterior part of the denture will rise, the denture rocking about the rest-seats This rocking is very irritating if there are anterior components such as indirect retainers, and will lead to immediate complaints (Additionally, if clasps are positioned so that their tips rise during this movement, they will torque the teeth ) Reducing the Force To ensure minimum mucosal displacement, the pressure on the mucosa must be reduced (no matter how even it is) Pressure = Force / Area So, pressure can be reduced by reducing the force, or increasing the area Share the force with a tooth Put a rest on a tooth close to the saddle (but keep it mesial!) Consider leaving off the second molars If you keep occlusal forces near to the rest-seat axis, the leverage forces will be lighter, casing less saddle movement The abutment will pick up the extra force Use Narrow Teeth Small sharp teeth will cut into food with less force needed than broad ones, causing less pressure on the mucosa Thus the saddle will sink less Increasing the Effective Saddle Area Reduce the pressure by increasing the saddle area Ensure the saddle is extended distally to cover the pear-shaped pad (half way up the retromolar pad), and fully extended over the buccal shelf, and into the sulcus Avoid the buccal fraenum Consider the Altered Cast Technique to increase the effective area With this technique, the saddle is shaped to fit the mucosa when it is under load, ensuring an even spread offorce when it is most needed The more evenly the force is spread, the less the saddle will sink

3 Upward Movement of the Saddle 3 Guide Planes Sticky foods will move the saddle upwards We assume the path of displacement is at right angles to the occlusal plane If the denture is made so it can not be moved in this direction, by using guide planes, then it will resist displacement However, the displacing forces have to be absorbed somewhere, so the abutment teeth will get torqued You must find a balance between unacceptable torquing by guide planes, and unacceptably easy displacement To minimise upward displacement of the saddle, use angled guide planes, but keep them short to minimise abutment torquing Indirect Retainers These help resist upwards movement of the saddle, assuming an effective clasp The nearer the clasp is to the Indirect Retainer, the greater the upward leverage on the clasped tooth Put the Indirect Retainers as far anterior to the clasped tooth as possible, to reduce leverage on the clasped tooth If the ideal tooth for an indirect retainer is steeply sloped (as are most anterior teeth), the IR component may slide down the lingual face You could put the IR component on the incisal edge, but this is unsightly Where no suitable rest exists for Indirect Retention, create one with adhesive and composite Drilling a seat for IR should be a last resort Upward Movement of the Clasp Tips When a denture is at rest, the clasp tips sit passively in their undercuts Let's assume a very stiff buccal clasp Should the tip rise, it will push the tooth lingually Lateral tooth movements cannot be tolerated, so this is prevented with a lingual bracing arm Now if the clasp rises, the tooth is pulled out ofits socket, which is considered preferable, but still pretty bad Clearly, the answer is to either arrange the denture so that upward clasp movement is minimal, or that extrusive forces are gentle over an acceptable range of upward movement Ensure the most common clasp tip movement is downwards by engaging undercuts distal to the rest seat When the saddle sinks, the clasp tips will sink Put the clasp tip just fractionally under the survey line so it doesn't have to swing out much If a lot of clasp movement is likely because of a non-ideal tip position (ie deep in an undercut), you should increase the clasp flexibility You can use a more flexible material (wrought stainless steel) or make the clasp arm longer ( eg by using a gingivally approaching clasp ) In deep undercuts, use a wrought clasp or a longer clasp

4 4 Coping with the bruxist Loss of posterior teeth in a bruxist can cause occlusal problems, such as forward posturing and attrition of the front teeth However, if the back teeth are replaced with a denture, bruxing on the denture can cause mucosal pain If you then ease the denture occlusally so it is 'clear', attrition will continue on the front teeth, causing more collapse, until the denture is 'back in the bite' and the saddle mucosa gets sore again The cycle continues until you realise it is not the much blamed friable mucosa If a patient returns every few months for easing, you should suspect parafunction The altered cast technique (supposedly) allows for the different compressibility of different parts of the mucosa, and ensures that an even pressure is applied everywhere when force is applied The Altered Cast Technique An impression of the saddle area is taken while it is under light pressure The stiff bits of mucosa (eg over the ridge) will be hardly deformed at all, but the fatty soft bits (eg the pear shaped pad) will be squeezed flatter and flatter until they offer the same resistance as the stiff bits When the denture is made, the mucosa is permanently deformed by the denture -not much over the ridge, but more on the soft bits which can take it without suffering There are practical difficulties with the altered cast technique: The technical procedures, although easily understood, are difficult to do well, and can easily lead to too much or too little pressure The technique takes no account of the different pressures applied to the mucosa as a result of saddle rotation: it assumes even force is applied along the length of the saddle The denture framework must seat very positively for any hope of success -a minimum of three widely spaced rests are needed The amount of pressure exerted by the mucocompressive impression is not under measurable control to suit each patient's individual need A clinical judgement (ie guess) needs to be made on how viscous the impression material should be It is very difficult to tell whether pain is being caused by the fitting surface or the occlusion If the pain is localised, it is usually the fitting surface, but it could be due to rotational movement of the saddle (ie occlusion)

5 5 Stress Breakers If one prefers more force to be applied to the saddle mucosa than the abutments, then stress breakers give the saddles a degree of I independent movement This is useful if the abutments are poorly angled and would De torqued unacceptably by normal denture movement, or if their periodontal support is weak Off the ridge Retention If the metal substructure of the saddle rests on the ridge, it is very difficult to ease, and impossible to reline The casting should sit lor 2mrn above the ridge, to allow acrylic to flow under it when flasked To aid trying-in, ask the technician to leave a tag of metal ( called a foot ) dropped down from the mesh onto gum level Balance Of Forces The mesial rest is classical A mesial plate provides a guide plane, but is unsightly A lingual clasp arm goes into the distal undercut Relining Laboratory relines of free end saddles usually disrupt the occlusion by being too thick It is best to reline with cold-cure acrylic at the chairside This relies on accurate positioning of the metal framework To aid future relining, design the framework to have a positive three-point contact with the teeth If there are only two good contacts (egocclusal rests) the framework will rock while it is being positioned While relining, do not let the cold-cure acrylic set in the undercuts! RPI The mesial REST is classical The distal PLATE provides a guide plane to resist displacement by sticky foods The I-bar clasp is gingivally approaching It disengages on functional load as the saddle sinks The RPI system is the logical result of considering how a free end saddle denture moves during function