Contraindications for Non-metal clasp dentures (NMCD)

There are multiple patient situations where NMCDs are successfully used for replacing missing teeth, however, when inappropriately treatment planned and/or designed, they can lead to increased stresses on the abutment teeth and accelerated resorption of the alveolar ridge.¹¹ It is critical to understand the clinical situations in which the NMCDs (non-rigid) should not be indicated, they include the following:¹¹

1. Lack of occlusal stops: When there is a lack of vertical occlusal stops (Figure 2), rotation and sinking of the thermoplastic denture base towards the tissue may occur; This may lead to accelerated ridge resorption and concentration of excessive forces in the resin clasp resulting in its deformation/fracture. A deformed clasp may place increased compressive forces on the marginal gingival and cause tissue injury. In such situations, NMCDs (non-rigid) should not be indicated; They should be designed with metal rests and a framework (rigid) to control the displacement of the dentures while in use.

   Figure 2 - Lack of posterior occlusal stops

   

   Figure 2 - Lack of posterior occlusal stops

2. Lack of vertical restorative space: NMCDs are contraindicated when there is insufficient vertical restorative space (Figure 3). Most of the thermoplastic denture base resins do not chemically bond to the prosthetic teeth, hence, diatorics are created to mechanically retain the prosthetic teeth. In patients with limited or inadequate restorative space, the artificial teeth may have to be modified/shortened to compensate for the lack of restorative space leading to the formation of cracks and ultimately their fracture.

   Figure 3 - Lack of vertical restorative space

   

   Figure 3 - Lack of vertical restorative space

3. Short clinical crowns: NMCDs may also be contraindicated when there are short clinical crowns and inadequate vestibular depth. Abutment teeth with short clinical crowns and inadequate vestibular depth/associated deep alveolar undercuts may pose a challenge in designing resin clasps of appropriate shape and width. Inappropriately shaped and/or a narrow resin clasp may lack sufficient retention and strength ultimately leading to its fracture.

4. Lack of oral hygiene: The thermoplastic resin clasp of NMCDs covers the cervical third of the abutment teeth, the marginal gingiva, and a part of the labial/buccal mucosa; Poor oral hygiene may lead to the development/exacerbation of caries and periodontal disease. Hence, NMCDs should not be indicated in patients with a lack of motivation and/or inability to maintain optimum oral hygiene.