MOD onlay
resistance Path of least
Dr Paul A Tipton considers the advantages and disadvantages of MOD onlay preparations
T Fig 1
Reduce the occlusal surface by approxi- mately 0.5-1mm – use depth cuts. Slightly less as you approach the buccal cusp tip – maintaining the correct cuspal inclines (unless in guidance)
he MOD onlay is a variation of the class 2 onlay in that retention is gained from the
inlay part of the preparation, but due to the cuspal coverage, the wedging effect of the inlay is nullified. The wedge-like inlay preparation increases the risk of cuspal fracture without protecting undermined cusps. Further advantages of the
onlay over the inlay include better resistance and reten- tion form, making it a suitable abutment preparation in a fixed movable bridge or fixed/ fixed if the abutment teeth are mobile. The margins are also
protected by taking them from the occlusal surface (inlays) and putting the margins onto non-contact surfaces. This reduces the chances of wear, secondary caries, cement breakdown and leakage. The maxillary MOD
onlay preparation is a viable alternative to the maxillary three-quarter crown when the MOD amalgam (if present) can be removed and prepared onto an onlay preparation. The resistance and reten-
tion forms are similar to the three-quarter crowns, as is the occlusal preparation. The palatal preparation, however, differs from the three-quarter
crown in that rather than being a ‘just supragingival’ margin, the palatal finish line is usually 1mm below the functional cusp contact with the opposing tooth, thus reducing tooth preparation.
Preparation Occlusal reduction is started by placing depth grooves similar to the three-quarter crown described in the previous article. One millimetre is removed from the occlusal surface, with an extra 0.5mm off the functional cusp (palatal cusp) to provide the functional cusp bevel. The preparation reduction
is then tailored to whether the tooth will be in lateral guid- ance or not by reducing the buccal cusp by 1mm or less if not in guidance – to reduce the amount of gold showing on the buccal cusp tips. The occlusal shoulder
follows the termination line of the functional cusp bevel on the palatal axial surface. The shoulder is 0.5 to 1mm wide and is cut with a tungsten carbide bur. It is positioned 1mm wide and is cut with a tungsten carbide bur. It is positioned 1mm apical to the contact point with the opposing tooth. This
Continued »
Fig 2
Reduce the functional cusp bevel by an extra 0.5mm at 45° to the occlusal plane
Fig 3
Use carbide bur with flat cutting edge to place a 0.5mm shoulder
Fig 6 Fig 4
Cut out a MOD cavity. Remove the MOD amalgam and prepare a MOD inlay prepara- tion with a 12 degree angle of convergence
Fig 5 Make sure the contact areas are broken
Complete the preparation by continuing the flare (fine bevel) onto the buccal cusp tip and bevel the palatal shoulder with fine diamond or carbide bur using speed increasing handpiece. Make sure you have a “line of draw” from the buccal
Ireland’s Dental magazine 19
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