Year : 2006 | Volume
: 6 | Issue : 4 | Page : 179--184
Removable partial dentures designing: Forces as primary concern
SG Singla1, Jagmohan Lal2,
1 Department of Prosthodontics, Dr. H. S. J. Institute of Dental Sciences, P.U, Chandigarh, India
2 Bhojia Dental College and Hospital, Budh, Nalagarh (H.P.), India
S G Singla
H. No 1901, Sector 39-B, Chandigarh
All structural analysis and designing of an removable partial dentures (RPD) require a knowledge of the forces that will be applied and the ability of the structure to withstand these forces. An RPD is an appliance that allows «SQ»controlled«SQ» movement in function under load to avoid impingement of tissues and injury to abutments. The load transfer characteristics of various RPD designs are important for best prognosis and longevity. Thus judicious incorporation of various components in an RPD involves counteraction of vertical, horizontal, and rotational forces to which an appliance is subjected in the oral cavity. The purpose of this article is to design an appliance based on isolation of various forces to which it is subjected during function.
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Singla S G, Lal J. Removable partial dentures designing: Forces as primary concern.J Indian Prosthodont Soc 2006;6:179-184
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Singla S G, Lal J. Removable partial dentures designing: Forces as primary concern. J Indian Prosthodont Soc [serial online] 2006 [cited 2019 Nov 20 ];6:179-184
Available from: http://www.j-ips.org/text.asp?2006/6/4/179/30692
Removable partial dentures (RPD) are objects that move or are allowed to move when placed in function. Since, nature demands equilibrium for every object, living or non-living, RPD has to be in a state of equilibrium, i.e., a state in which opposing forces or influences are balanced. Keeping in mind Devan's statement 'to preserve that remains,' forces should be given major consideration while designing a partial denture so as to ensure the dynamics of these appliances without deleterious effects to the supporting structures.
This article aims at designing equilibrate appliances taking into consideration various forces to which it is subjected.
[Table 1] and [Figure 1] give a quick glance at various parts of RPD. According to Davis Henderson, stability is that characteristic of removable restoration which resists forces that tend to alter the relationship between the denture base and its supporting bone.
The forces acting on the masticatory apparatus and there by on appliance can be vast in magnitude and direction. They can be broadly categorized as in [Figure 2].
First consider the vertical forces, which can bodily move the saddle towards or away from the tissues.
Movement of the saddle toward the tissues [Figure 3]
When vertical forces are applied, any compression that occurs is uniform over the entire seat. Difference in transmission of load is due to difference in resiliency of periodontal ligament and denture bearing mucosa. This problem can be tackled with
Broad stress distributionPhysiologic basingStess equalization.
Periodontal ligament is better able to resist masticatory forces as compared to residual ridge. Therefore, latter is likely to get the larger brunt of load than tooth. Gradually with passage of time, resorption occurs, especially at the distal end. There is sinking of the saddle at the distal end, which may ultimately contribute towards rotational action along transverse axis [Figure 4].
Movement of saddle away from the tissues
This means total lifting of the saddle away from the base rather than lift at one end [Figure 5]. This may be the case in any type of saddle. Major forces of retention, i.e., physical forces, as in complete denture must be employed with.
Intimate contactWider area coveragePosterior sealPolished surfaces
Then we have the powerful Direct retainer. Use as many as you like depending on the:
Number of saddlesSize of saddlesLocation of saddles
'Don't be too miser or too liberal, be judicious and have a simple design' Always remember the fundamental principle of placing the retaining element nearer to the saddle.
Masticatory forces and forces exerted by orofacial musculature including tongue [Figure 6] can cause these movements, i.e., lateral bodily movement and anteroposterior movement.
Lateral bodily movement
The morphology of supporting tissues and the clasp design provide sufficient bracing. In addition, resistance will be provided by rigid parts of a denture i.e., saddle and major connector resting on the slopes in the stippled areas [Figure 7] against the forces whose directions are shown by arrows. Lateral bodily movement is more problematic in resorbed ridges and weak abutments wherein alternatives are:
Use of more number of rigid bracing elements so as to distribute the forces over wider areas. Reduction in cusp height to reduce lateral forces [Figure 8].
Horizontal forces can also cause anteroposterior movement, which can be forward or backward.
In case of Bounded saddles, contact of saddle itself with abutment teeth resists A-P movement.
In case of free end saddle; (distal extension cases).
Forward movement is prevented by:
Extension of major connector on anterior part of palate [Figure 9]Using a Linguoplate major connector in case of mandibular arch.
Backward movement is prevented by [Figure 10]:
Coverage of pear shaped retromales padMinor connector contacting the mesiolingual surface of abutment toothBy encircling more than 180o of the abutment with the clasps.
Rotational forces cause rotational movement, which can be anteroposterior rotation or lateral rotation.
Anteroposterior rotation in free end saddle [Figure 11]
Rotation around transverse axis can be away from the tissues or towards the tissues.
A-P rotation away from the tissues (Lift at the heel) [Figure 12]
This movement should be distinguished from bodily lift of saddle away from the basal seat, i.e., complete loss of retention. Loss of retention in this case is only at the distal end with the direct retainers still firmly in place. This movement cannot be prevented, because we can not put a stop to the dislodging forces acting on the saddle. However, definitely, certain measures can be taken to minimize the deleterious effects this movement will have on the remaining supporting structures.
Consider a bar with a single support than behaves like a sea-saw rotating around the single fixed axis [Figure 13]. This rotational movement can be prevented by tying the bar from above or providing additional solid support.
If this situation is simulated to that of an RPD, of course it cannot be tied to the maxilla from above so as to prevent the A-P rotation. The only alternative in this case is to provide a solid support on the other hand as far as possible from the fulcrum line in the form of a rest (indirect retainer). This limits the sinking at the anterior end and consequently, lift at the heel is prevented [Figure 14].
In the process, this additional support at the anterior end is subjected to load, which might be deleterious to the said tooth. We must aim at minimizing this load or distributing it widely.
A Class I lever situation is shown in [Figure 15]. Wherein, greater the mechanical advantage, lesser is the force required to lift the bar. In an RPD, by shifting the resistance anteriorly, mechanical advantage can be reduced and consequently forces transmitted at the anterior end are reduced. We can also widely distribute this load by involving more number of teeth for providing indirect retention.
Rotation towards the tissues (Sinking at the distal end) [Figure 16]
With passage of time, ridge resorption, especially at the distal end leads to sinking of the saddle and rotational movement toward the basal seat. In such a situation an RPD can behave like an extraction forcep, with consequent damage to the supporting structures [Figure 17].
One solution to this problem is 'repeated rebasing' which is not practically feasible for the dentist as well as for the patient.
A viable solution is use of mesial rests instead of distal rests, which permits more even distribution of load and less stress on abutment teeth.
In [Figure 18], when a disto-occlusal rest is used, fulcrum 'F', lies near the distal marginal ridge (DMR) and as the vertical loading occurs, denture sinks at the distal end. The clasp terminal moves up to engage the undercut, hence constituting the resistance arm in accordance with Class I lever and thereby exerts tipping forces on the abutment.
In case of mesio-occlusal (MO) rest, when effort 'E' is applied, fulcrum F shifts to mesial marginal ridge (MMR) and clasp terminal rotates downward and mesially. At the same time, remaining part of clasp, i.e., shoulder portion (that lies above the survey line) provides resistance R to downward sinking. Thus, a Class II lever situation is constituted which is beneficial.
Since this resistance R is situated closer to the rotational center F than the clasp terminal, these forces are well resisted as in case of a pole embedded in sand [Figure 19]. We can also extend the occlusal rest to the adjacent tooth for wider distribution of load [Figure 20].
With the use of a MO rest there is an increase in length of lever arm, which makes rotational action more vertical in gingival area of abutment tooth [Figure 21].
RPI system [Figure 22] is one such system designed to incorporate MO rest and allows vertical rotation of saddle towards mucosa without damaging the supporting structures of abutment tooth.
An additional retainer anteriorly if permissible (esthetically) can prevent upward rotation at the anterior end of RPD, hence minimizing the sinking at the distal end of the saddle [Figure 23].
Lateral rotation in free end saddle [Figure 24]
This occurs along sagittal axis. This can be checked by Cross arch bracing, i.e., rigid major connector is extended onto the opposite side of the arch as far posteriorly as possible and a retainer unit is provided at its distal most end [Figure 25].
When planning treatment for partially edentulous patients, the dentist is confronted with myriad combinations of edentulous spaces and remaining teeth. It is up to the dentist to understand the functions of parts and to select the ones that will counter various forces generated around fulcrum lines by levers or inclined planes. When a patient comes, view the diagnostic models, outline the saddle and try to imagine the forces to which it can be subjected and movements it can make. After this make judicious use of various components without complicating the design.
Just remember, You are to prescribe and lab is to execute and not the opposite. RPD is a Tertiary prevention aid. Without mechanical and biological consideration, an RPD can be and often is unknowingly designed as a destructive machine.
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