In filling compound approximal cavities- disto-morsal- in bicuspids and molars with the various forms of crystal or sponge gold, the matrix will be found of great service, not only in simplifying the cavity but in securing a more perfect adaptation of the gold to the enamel margin, and by facilitating the operation. In fact, many operators utilize these instruments in filling all disto-morsal cavities in the posterior teeth, no matter what form of gold may be used. Their greatest value, however, lies in their use as just indicated, and in the introduction of plastic materials.
In the introduction of gold into disto-morsal cavities in the posterior part of the mouth, it is impossible to use a straight instrument for packing the gold ; it therefore becomes necessary to curve or bend the shaft of the plugger near its point to suitable angles for reaching the various surfaces of the cavity ; consequently the force or impact applied to the shaft of the instrument by the hand or the mallet is not directly expended upon the gold at the point of the plugger, but is more or less dissipated by the elasticity of the steel at the curve or angle, and by the tendency of each impact to drive the gold out of the cavity in a distal direction. By the adjustment of a suitable matrix a compound cavity is converted into a simple one, and the matrix used as a wall against which the gold may be packed. Objections have been raised to the use of these devices on account of the difficulty experienced in giving a proper contour to the approximal surfaces, and of securing perfect adaptation of the gold to the enamel margins which are contiguous to the matrix.
These objections are readily overcome by a proper preparation of the cavity margins, which leaves them strong and straight, the careful adjustment of the matrix, and the same degree of skill exercised in introducing and condensing the gold against the margins that would be used in other cavities.
Several forms of matrices have been invented, all of them possessing more or less'valuable features.
The Jack matrices, are so shaped as to provide for contouring, and are made in pairs, adapted for use upon the right and left sides of the mouth respectively. These are held in place against the tooth to be filled by wooden wedges driven between the matrix and the adjoining tooth, the wedges being first dipped in sandarach varnish to keep them from slipping; or it may be held in position by some quick-setting oxy-phosphate cement. The matrix is applied by an especially designed forceps, which grasp's them firmly, permitting easy adjustment and withdrawal.
When the rubber dam is used, this should be first adjusted, and the matrix applied afterwards.
The matrix, to fulfil the object of its placement, must be immovably fixed against the tooth to be filled. Motion of the matrix results in imperfect adaptation of the filling to the walls and margins by; reason of the difficulty in packing the gold against a shifting body.
To overcome this difficulty, loop and band matrices were devised of various forms and sizes to accord with the differences in the size of the teeth.
The Brophy and the Guilford patterns are the best of their kind, and are easy to adjust. The only difficulty experienced in the use of loop or band matrices is to obtain a close adjustment at the cervix of the tooth, by reason of the smaller size of the tooth at this* location as compared with the morsal surface of the crown.
This difficulty may be overcome by driving a wooden wedge between the band and the adjoining tooth at the cervix, as previously described.
The Woodward matrix is also an ingenious device, and in some features superior to the others just described.
It has the great advantage of being readily adapted to the cervix of the tooth, and is maintained in position by means of two set screws, which rest against the tooth opposite the approximal cavity to be filled and act as separators.
As the teeth move under the pressure of the screws, it becomes necessary to occasionally tighten them. The space thus gained is of great value during the process of finishing the filling.
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