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Prosthodontics

INTRODUCTION:

The fracture of acrylic resin dentures is an unresolved problern in removable Prosthodontics. .Acrylic resin denture bases display warpage during processing and after being placed in service. Skinner16’ states, “The results of our research are definitely in agreement with those of Worner that the dimensional accuracy of the modern acrylic resins is not as good as that exhibited by the vulcanite of past years.

Peytona states, “In general, it is established that all resins receive some internal

strain during processing, which may be released, as warpage, when placed in service.”

CAUSES OF ACRYLIC RESIN DENTURE FRACTURE :

Fractures in dentures result from two different types of forces, namely,

  1. flexural fatigue
  2. and impact fatigue

. Flexural fatigue occurs after repeated flexing of a material and is a mode of fracture whereby a structure eventually fails after being repeatedly subjected to loads that are so small that one application apparently does nothing detrimental to the component. Catastrophic failure results from a final loading cycle that exceeds the mechanical capacity of the remaining sound portion of the material.3The midline fracture in a denture is often a result of flexural fatigue.
Impact failures usually occur out of the mouth as a result of a sudden blow to the denture or accidental dropping whilst cleaning, coughing or sneezing. Additional factors which form areas of stress concentration such as a large frenal notch;7 dentures with thin or under-extended flanges; poorly fitting dentures or a lack of adequate relief; dentures with a wedged or locked occlusion; poor clinical design and dentures which have been previously repaired.8 Despite the high frequency of denture fracture, there is surprisingly little discussion of the subject in the literature.
Such dentures are easily damaged because the structures of partial dentures are quite complex. Hargreaves9 and Smith10 have both indicated that midline fractures in dentures are most likely to occur after 2 to 3 years of use. The present study confirmed that most upper complete dentures (29.4%) were damaged after 3 years of use. Impact failure (80.4%) was the most common cause of damage of the dentures in the study done in 2006 by Ali M. El-Sheikh and Saied B. Al-Zahrani1,. This agrees with that of Lambrecht and Kydd6, and Hargreaves.9 This could be explained by the lack of attention being paid by the patients towards the care of their dentures.

MIDLINE FRACTURE OF DENTURES

While denture bases deform under loading, this deformation may be exacerbated by other factors such as changes in the denture base, tooth wear, and Sharp changes in contour, pin holes, inclusions, deep scratches, and residual processing stresses may
all cause stress intensification. A survey of denture fractures,’ however, has indicated that most failures occurred when there was deep notching at the midline labial frenum. This supports the contention that the incisal notch is the most important causative
factor in midline fracture and that cracks initiate at the tip of the notch where there is high local stress concentration.‘. ”

PREVENTION OF MIDLINE FRACTURE

Various approaches can help reduce the incidence of midline fracture of denture bases. A good processing technique .which reduces or eliminates residual stress within the denture and avoids surface defects and inclusions is essential. Using higher strength
polymers, notably impact-resistant materials, will reduce the tendency to fracture. (Laboratories using this type of material all reported a low incidence of fractures in the survey.) Constructing dentures with metal palates for patients with heavy occlusions has the dual advantage of providing greater strength and better thermal stimulation of the underlying mucosa.
Setting maxillary central incisors too far palatally should be avoided since, with c:orrect labial positioning, the amount of acrylic resin in the gingival portion of the midline and in areas lingual to it will be increased and thus strengthen the base at its weakest point.

In these situations metal alloys have been used to strengthen the bases and prevent fracture.17,18,19 Metals and metal alloys used in denture bases display excellent strength-to-volume ratios 20,21,22 and may be cast in thin sheets maintaining rigidity and fracture resistance.

Metal based dentures
(1) are more retentive,28,29,30
(2) have less occlusal discrepancy,
(3) cause fewer sore spots,
(4) have a reduced incidence of fracture,31,32,33
(5) feel better to the patient,29
(6) are better thermal conductors,28,29 act as a stable record base,34
(8) have a thinner palate that aids speech,
(9) preserve the residual alveolar ridge,
(10) are less porous,
(11) deform less during lateral mandibular function,and
(12) are more accurate in tissue detail.

CASE REPORT WITH PROCESS OF FABRICATION OF METAL DENTURE BASE:

A patient aged 65 year reported to the Dept of Prosthodontics GDC Rohtak for complete denture insertion, Chief complaint of the patient was frequent midline fracture of maxillary complete denture. (fig.1) On examination, the cause of frequent fracture of complete denture was found to be high frenal attachment , for which patient refused to remove it surgically. So, it was decided to use a metal denture base instead of conventional acrylic resin denture base.

Fig-1


1. The preliminary phases of metal-base denture construction do not differ significantly from conventional resin-base techniques. Impressions and casts were generated using accepted prosthodontic procedures.

Fig-2

Fig-3 Fig-4

2. Final cast of the patient was duplicated using refractory material and placing spacer wax below the metal flange where acrylic will engage the metal( Fig-2,3,4)

 

Fig-5


3. Wax-up done over refractory cast for metal framework.(fig-5) The resin-metal junction was properly placed as placing the resin-metal junction too far laterally will result in an under contoured alveolar ridge and inefficient contact with lateral border of the tongue and placing it too far medially will produce “crowding” of the tongue.

4. Sprue attached ,investing and casting done using metal alloy (fig-6,7)

 

Fig-6

Fig-7
  1. even and adequate bulk of denture base material cured to achieve optimum polymerization and
  2. free of porosity;
  3. relief of incompressible tissue in the center of the hard palate;
  4. addition of labial flange to increase rigidity of denture base as well as
  5. even and balanced occlusion to reduce wedging effect and locking of occlusion.
  6. Improvements in denture base resin and
  7. the reduction of stress concentrators such as notches and diastema to minimum would also help prevent these fractures.

7. Now th finished and polished denture inserted in patient’s mouth and proper post-insertion instruction was given. (Fig-10,11)


 

Pre-operative view:


Post-operative view:


DISCUSSION:

Matthews and Wain15 have shown that under load the maximum tensile stresses are on the palatal aspect of the denture. Factors that contribute to stress concentrations will enable the initiation and propagation of cracks thereby influencing the rate of failure. Both the presence of notches and diastema act as stress concentrators thereby influencing the risk of failure. A majority of the midline fractures can be avoided by the application of established prosthodontic principles during denture construction. The principles include


The problem of acrylic resin fracture can be reduced by the use of the improved high impact resins. There is also need for a new and more suitable method of reinforcing the denture base during preparation. This could be achieved by using continuous electrical-glass (E-glass) partial fiber reinforcement..11 Continuous, unidirectional E-glass partial fiber reinforcement has been shown to considerably improve the mechanical properties of removable complete and partial dentures in vitro.12. It has been reported that the insertion of metal wire or metal mesh as ‘strengtheners’ into acrylic resin dentures is not very satisfactory. It is probable that the acrylic resin shrinks away from the ‘strengthening’ material leaving a material with a network of voids which weakens the structure by creating new points of stress concentration

DISADVANTAGES OF THE TECHNIQUE:

  • 1. The cost of the denture is greater than that of one with a plastic base.
  • 2. Refitting of the denture is difficult.
  • 3. The technique is more time-consuming than those for making plastic base
    dentures. However, due to the uniformity and excellence of results, the advantages
    seem to outweigh the disadvantages.
CONCLUSION:

Metal bases for complete dentures have been used successfully and provide many advantages over the more commonly used acrylic resin. The few disadvantages are far outweighed by the many advantages. The possibility of allergy to the metal, although a valid concern, varies with the composition and electrochemical properties of the alloy and the susceptibility of the patient. With metal bases for dentures, the patient benefits by having a more comfortable, better fitting, and stronger prosthesis. The dentist benefits by reducing postinsertion visits and providing a restoration that will better satisfy the patient.

REFERENCES
  1. El Sheikh and Al-Zahrani. Causes of denture fracture : A survey .Saudi Dental J.2006:18(3):206-212
  2. Darbar UR, Huggett R, Harrison A. Denture fracture – A survey. Br Dent J 1994; 176: 342-345.
  3. Wiskott HWA, Nicholls JI, Belser UC. Stress fatigue: Basic principles and prosthodontic implications. Int J Prosthodont 1995; 8: 105-116.
  4. Jagger DC, Harrison A. The fractured denture-solving the problem. J Primary Dent Care 1998; 5: 159-162.
  5. Kydd WL. Complete base deformation with varied occlusal tooth form. J Prosthet Dent 1956; 6: 714-718.
  6. Lambrecht JR, Kydd WL. A functional stress analysis of the maxillary complete denture base. J Prosthet Dent 1962; 12: 865-872.
  7. Rees JS, Huggett R, Harrison A. Finite element analysis of the stress concentrating effect of fraenal notches in complete dentures. Int J Prosthodont 1990; 3: 238-240.
  8. Yunus N, Harrison A, Huggett R. Effect of microwave irradiation on the flexural strength and residual monomer levels on an acrylic repair material. J Oral Rehabil 1994; 21: 641-648.
  9. Hargreaves AS. The prevalence of fractured dentures. Br Dent J 1969; 126: 451-455.
  10. Smith DC. The acrylic denture: Mechanical evaluation mid-line fracture. Br Dent J 1961; 110: 257-267.
  11. Uzun G, Hersek N, Tincer T. Effect of five woven fiber reinforcements on the impact and transverse strength of a denture base resin. J Prosthet Dent 1999; 81: 616-620.
  12. Vallittu PK. Comparison of the in vitro fatigue resistance of an acrylic resin removable partial denture reinforced with continuous glass fibers or metal wires. J Prosthodont 1996; 5: 115-121.
  13. Vallittu PK, Lassila VP, Lappalainen R. Transverse strength and fatigue of denture acrylic-glass fiber composite. Dent Mater 1994; 10: 116-121.
  14. Kim S-H, Watts DC. The effect of reinforcement with woven E-glass fibers on the impact strength of complete dentures fabricated with high-impact acrylic resin. J Prosthet Dent 2004; 91: 274-280.
  15. Matthews E, Wain EA. Stresses in denture bases. Br Dent J 1965; 100: 167-171.
  16. Skinner, Eugene W.: Acrylic Denture Base Materials: Their Physical Properties and
    Manipulation, J. PROS. DEN. 1:161-167, 1951.
  17. Grunewald AH. Gold base lower dentures. J Prosthet Dent 1964;14:432-41.
  18. Lundquist DO. An aluminum alloy as a denture-base material. J Prosthet Dent 1963;13:102-10.
  19. Campbell DD. The cast waged aluminum denture base. J Am Dent Assoc 1935;22:2082-9.
  20. Philadelphia: WB Saunders, 1991:177-213.
  21. Craig RG. Restorative dental materials. 9th ed. St Louis: Mosby- Year Book, Inc, 1993:502-50.
  22. Faber BL. Lower cast metal base denture. J Prosthet Dent 1957: 51-4.
  23. Allen LR. Improved phonetics in denture construction. J Prosthet Dent 1958;8:753-63.
  24. Roth GJ. An analysis of articulate sounds and its use and application in the art and sci.enee of dentistry. Am J Orthod 1940;26:1-23.
  25. Rothman R. Phonetic considerations in denture prosthesis. J Prosthet Dent 1961;11:214-23.
  26. Ylppii A. The effect of dentures on speech. Int Dent J 1955;5:225-40.
  27. Mattie P.A, Phoenix R.D., A precise design and fabrication method for metal base maxillary complete dentures.J. Prosthet Dent 1996;76:496-9.
  28. DeFurio A, Gehl DH. Clinical study on the retention of maxillary complete dentures with different base materials. J. PROSTHET DENT 1970;23:374-80.
  29. Moore FD. Organic or metal bases for dentures. J PROSTHET DENT 1967;17:227-31.
  30. Swartz WH. Retention forces with different base materials. J PROSTHET DENT 1966;16:458-63.
  31. Farmer JB. Preventive prosthodontics: maxillary denture fracture. J PROSTHET DENT 1983;50:172-5.
  32. Johnston EP, Nicholls JI, Smith DE. Flexure fatigue of 10 commonly used denture base resins. J PROSTHET DENT 1981;46:478-83.
  33. Nimmo A, Kratochvil FJ. Preventing fractures of maxillary overdentures. J PRCSTHET DENT 1986;55:773-5.
  34. Halperin AR. The cast aluminum denture base. Part I: Rationale. J PROSTHET DENT 1980;43:605-10.

6. Now, try-in was done and after dewaxing, the metal framework was placed on the cast and curing was done using conventional methods and th denture was finished and polished.(Fig-9)


Fig-9


5. The framework was finished and polished (Fig-8)
Fig-8