Читать книгу Fundamentals of Fixed Prosthodontics - James C. Kessler - Страница 118

Rigid connectors (eg, solder joints) between pontics and retainers are the preferred way of fabricating most fixed partial dentures. A fixed partial denture with the pontic rigidly fixed to the retainers provides desirable strength and stability to the prosthesis while minimizing the stresses associated with the restoration.

However, a completely rigid restoration is not indicated for all situations requiring a fixed prosthesis. An edentulous space can occur on both sides of a tooth, creating a lone, freestanding pier abutment (Fig 7-21). Physiologic tooth movement, arch position of the abutments, and a disparity in the retentive capacity of the retainers can make a rigid fiveunit fixed partial denture a less-than-ideal treatment plan.

Studies in periodontometry have shown that the faciolingual movement ranges from 56 to 108 μm,¹⁶ and intrusion is 28 μm.¹⁷ Teeth in different segments of the arch move in different directions.¹⁸ Because of the curvature of the arch, the faciolingual movement of an anterior tooth occurs at a considerable angle to the faciolingual movement of a molar (Fig 7-22).

These movements of measurable magnitude and in divergent directions can create stresses in a long-span prosthesis that will be transferred to the abutments. Because of the distance through which movement occurs, the independent direction and magnitude of movements of the abutment teeth, and the tendency of the prosthesis to flex, stress can be concentrated around the abutment teeth as well as between retainers and abutment preparations.

It has been theorized that forces are transmitted to the terminal retainers as a result of the middle abutment acting as a fulcrum, causing failure of the weaker retainer.¹⁹ However, photoelastic stress analysis and displacement measurement indicate that the prosthesis bends rather than rocks. Standlee and Caputo²⁰ suggest that tension between the terminal retainers and their respective abutments, rather than a pier fulcrum, is the mechanism of failure. Intrusion of the abutments under the loading could lead to failure between any retainer and its respective abutment.

The loosened casting will leak around the margin, and caries is likely to become extensive before discovery. The retention on an anterior tooth is usually less than that of a posterior tooth because of its generally smaller dimensions. Because there are limits to increasing a retainer’s capacity to withstand displacing forces, some means must be used to neutralize the effects of those forces. The use of a nonrigid connector has been recommended to reduce this hazard.¹⁹

In spite of an apparently close fit, the movement in a nonrigid connector is enough to prevent the transfer of stress from the segment being loaded to the rest of the fixed partial denture (Fig 7-23). The nonrigid connector is a broken-stress mechanical union of retainer and pontic instead of the usual rigid connector. The most commonly used nonrigid design consists of a T-shaped key that is attached to the pontic and a dovetail keyway placed within a retainer.

Fig 7-23 A nonrigid connector on the middle abutment isolates force to the segment of the fixed partial denture to which it is applied. (Reprinted from Shillingburg and Fisher¹⁹ with permission.)

Fig 7-24 If a nonrigid connector is placed on the distal side of the retainer on a middle abutment, movement in a mesial direction will seat the key into the keyway. (Reprinted from Shillingburg and Fisher¹⁹ with permission.)

Fig 7-25 If a nonrigid connector is placed on the mesial side of the middle abutment, mesially directed movement will unseat the key. (Reprinted from Shillingburg and Fisher¹⁹ with permission.)

Use of the nonrigid connector is restricted to a short-span fixed partial denture replacing one tooth.²¹ The magnification of force created by a long span is too destructive to the abutment tooth under the soldered retainer. Prostheses with nonrigid connectors should not be used if prospective abutment teeth exhibit significant mobility. There must be equal distribution of occlusal forces on all parts of the fixed partial denture.

A nonrigid fixed partial denture transfers shear stress to supporting bone rather than concentrating it in the connectors. It appears to minimize mesiodistal torquing of the abutments while permitting them to move independently.²² A rigid fixed partial denture distributes the load more evenly than a nonrigid design, making it preferable for teeth with decreased periodontal attachment.²³ If the posterior abutment and pontic are either opposed by a removable partial denture or unopposed, and if the three anterior units are opposed by natural teeth, the key and the posterior units that are subjected to little or no occlusal forces may supererupt.

The location of the stress-breaking device in the five-unit pier-abutment restoration is important. It is usually placed on the middle abutment because placement on either of the terminal abutments could result in the pontic acting as a lever arm.

The keyway of the connector should be placed within the normal distal contours of the pier abutment, and the key should be placed on the mesial side of the distal pontic. The long axes of the posterior teeth usually lean slightly in a mesial direction, and vertically applied occlusal forces produce further movement in this direction. Nearly 98% of posterior teeth tilt mesially when subjected to occlusal forces.²⁴ If the keyway of the connector is placed on the distal side of the pier abutment, mesial movement seats the key into the keyway more solidly¹⁹ (Fig 7-24). Placement of the keyway on the mesial side, however, causes the key to be unseated during mesial movements²⁰ (Fig 7-25). In time, this could produce a pathologic mobility in the canine or failure of the canine retainer.

Fig 7-26 When a mandibular molar tilts mesially, there is a discrepancy between its long axis and that of the premolar.

Fig 7-27 This fixed partial denture will not seat because the tooth distal to the fixed partial denture intrudes on the path of insertion (arrow).

Fig 7-28 Orthodontic appliance for uprighting a tilted molar: (a) occlusal view; (b) facial view.