Brockhurst P J
Department of Dentistry, University of Queensland.
Aust Dent J. 1996 Oct;41(5):317-23. doi: 10.1111/j.1834-7819.1996.tb03140.x.
Partial dentures made from cast cobalt chromium alloys suffer from inadequacies of the retentive spring members. With similar devices made from Type 4 gold alloy, the retentive members are considered satisfactory. Analyses of the combined modulus and strength properties of the alloys show that the lower modulus of elasticity of the gold doubles its overall effectiveness in spring members such as retentive clasp arms. To match this, the strength of cobalt chromium alloys must be doubled. Since this appears unlikely, improvement to the design of the spring members is the only way to rectify this problem. Retentive clasp arms are required to have adequate elasticity to deflect out of the retentive undercut, adequate stiffness to produce retention and adequate strength to resist accidental damage. Current designs have no published rational basis to meet these requirements, and appear far from ideal. An analysis of the forces applying to clasp arms and design for uniform stress along the arm produced a shape which differs substantially from the currently used conical taper. The cross-section dimensions are related to the angular distance from the tip of the clasp. A theoretical comparison of the difference between the theoretical and current designs cannot be made as the current design for tapered clasps is not specific. It is proposed that improved performance over present clasp designs can be obtained by using clasps of the derived shape. However, the manufacture of clasps of the required complex shape and variable size is impractical, and the next best alternative of clasps of constant cross section along their length was examined. Deflection calculations for this form showed that the loss of deflection in using a clasp of constant cross section instead of the ideal was only 23 per cent. It is proposed that clasps of constant cross section along their length, with a thickness chosen according to the size of the tooth used for the undercut, will present a significant improvement in clasp performance. Further, it will provide a systematic method of selecting clasp thickness which is not presently available.
由铸造钴铬合金制成的局部义齿，其固位弹簧部件存在不足之处。而对于用4型金合金制成的类似装置，固位部件被认为是令人满意的。对这些合金的组合模量和强度特性进行分析表明，金合金较低的弹性模量使其在诸如固位卡环臂等弹簧部件中的整体效能提高了一倍。为了与之匹配，钴铬合金的强度必须提高一倍。由于这似乎不太可能，改进弹簧部件的设计是解决这个问题的唯一途径。固位卡环臂需要有足够的弹性以便从固位倒凹中脱出，有足够的刚度以产生固位力，并有足够的强度来抵抗意外损坏。目前的设计没有公开的合理依据来满足这些要求，而且看起来远非理想。对作用于卡环臂的力进行分析，并设计出沿臂均匀受力的形状，结果得到的形状与目前使用的锥形 taper 有很大不同。横截面尺寸与卡环尖端的角距离有关。由于目前锥形卡环的设计不具体，无法对理论设计和当前设计之间的差异进行理论比较。有人提出，使用推导形状的卡环可以获得比目前卡环设计更好的性能。然而，制造所需复杂形状和可变尺寸的卡环是不切实际的，因此研究了沿其长度具有恒定横截面的卡环的次优替代方案。对这种形式的卡环进行挠度计算表明，使用恒定横截面的卡环而不是理想卡环时，挠度损失仅为23%。有人提出，沿其长度具有恒定横截面、厚度根据用于倒凹的牙齿大小选择的卡环，将在卡环性能上有显著改进。此外，它将提供一种目前尚无的选择卡环厚度的系统方法。
