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This two-part series provides a comprehensive guide to fabricating definitive metal partial dentures, addressing the challenges dental practitioners face in mastering prosthesis design. Part 1 explores diagnostic procedures, indications for metal partial dentures, design principles, and surveying techniques. It emphasizes the understanding of biomechanical forces and discusses design principles such as support, retention and stability, along with the components that provide these functions in a metal partial denture. Decision algorithms for selecting various components are presented to guide clinical practitioners in efficient designing. The series aims to equip dental professionals with a thorough understanding of the theoretical foundations and practical methods for effective metal partial denture fabrication.
CPD/Clinical Relevance:
This series provides essential knowledge and practical techniques for designing and fabricating effective metal partial dentures.
Article
Conventional removable partial dentures continue to be a viable treatment option, even in the current dental scenario dominated by dental implants. This choice is influenced by factors such as patient-related systemic considerations, intra-oral factors, or cost constraints that may deter patients from choosing implants or other fixed alternatives. Removable partial dentures are available in diverse types,1,2 encompassing interim solutions as well as definitive options, which include both metal-based and metal-free alternatives.3 Among these alternatives, mastering the definitive prosthetic design and understanding the underlying principles has consistently posed a challenge for dental practitioners.
Historically, partial dentures were cast using the lost wax technique, leading to the term ‘cast partial dentures.’ However, recent advancements in fabrication, using computer-aided design and manufacturing (CADCAM), have introduced non-cast options.4 This shift makes the use of the term ‘metal partial dentures’ more appropriate than ‘cast partial dentures’. Despite the growing popularity of metal-free partial dentures with newer polymer materials,3 this series focuses specifically on discussing claspretained definitive (metal) partial dentures owing to their longstanding success. It is essential to note that the foundational principles of design and biomechanics for removable prostheses in partially dentate mouths remain consistent, whether the partial denture is metal or metal-free.
This two-part series serves as a guide to fabricating definitive/metal partial dentures. Part 1 explores key elements, including indications, success factors, design principles, surveying techniques, and decision algorithms. Part 2 covers procedures for impressions, essential laboratory techniques, and practical case examples. This structured approach aims to offer dental professionals a thorough understanding of theoretical foundations and practical methodologies for effective metal partial denture fabrication.
To aid in understanding and recall, we propose the use of the following mnemonic – the 4 Ds: diagnosis, decision-making, designing, and delivery and maintenance (Figure 1).
Figure 1. The 4 Ds mnemonic.
Diagnosis
In the diagnostic stage preceding the selection of a definitive/metal partial denture, a clinician must gather information including, general health, systemic conditions, medications, allergies as well as patient compliance, concerns and expectations. Among all the different factors, intra-oral examination focuses on four key areas:
Evaluation of teeth
Conducting a meticulous tooth-by-tooth prognosis assessment involves evaluating the prosthodontic, endodontic, and periodontal conditions. This detailed examination forms the foundation for informed decision-making regarding the suitability of the patient for a metal partial denture.5
Evaluation of edentulous spaces
Systematically analysing edentulous spaces, both in terms of mesiodistal and inciso-cervical dimensions, ensures a comprehensive space analysis for accurately accommodating the partial denture.
Evaluation of soft tissue
A thorough assessment of the existing soft tissues should evaluate the health and compressibility of those tissues, and include recording features such as high frenal attachments, reduced sulcus depth, presence of tori and prominent undercuts. For partially dentate cases, the use of Siebert's system is suggested, which categorizes edentulous ridges into:
Understanding the inter-arch relationships becomes crucial, as variations in occlusion can significantly influence the design of the metal partial denture.7
Moreover, various other factors require careful evaluation before deciding on a metal partial denture (Figure 2).
Figure 2. Factors influencing the success of a metal partial denture.
Decision making: when to choose a cast/metal/definitive partial denture
Following initial primary disease stabilization, the decision-making process for prosthesis selection should take into account the number of missing teeth (span length and span configuration) and overall periodontal prognosis.7 After this preliminary decision-making process, other factors such as cost, treatment duration, patient preferences etc. will influence the final choice.
The flow chart (Figure 3) excludes the modality of implants and provides guidance on determining whether a fixed or removable prosthesis is an appropriate treatment option. Metal partial dentures are suitable for a wide range of partially dentate cases. When a fixed prosthesis cannot be provided owing to concerns about cost or the preservation of tooth tissue, a metal partial denture becomes a viable option.7,8 In cases where the periodontal condition is poor, an interim acrylic denture may be initially provided until a stable periodontal status develops.7,8 Subsequently, a metal partial denture can be planned for long-term prosthetic prognosis. However, if the poor periodontal condition persists, interim partial dentures may be a more appropriate option.
Figure 3. Preliminary decision-making chart.
Design
The process of design requires the clinician to have a thorough understanding of the biomechanical forces that will be applied to both the prosthesis and the supporting tissues.
Biomechanical forces in partial dentures: impact and correlation with classification
Removable partial dentures, owing to their lack of rigid attachment to teeth, necessitate control of potential movement of the denture during function to provide optimal stability, retention, and patient comfort. The dynamic nature of prosthesis movement under functional load exerts stresses on the teeth and soft-tissue denture-bearing areas.9 It is imperative to manage these stresses, ensuring that they remain within the bounds of physiological tolerance without disruption or traumatic consequences.9
A crucial determinant in achieving optimal design lies in the distinction between partial dentures receiving support solely from teeth, and those relying on both teeth and soft tissues. Kennedy's classification categorizes partially edentulous arches into four primary classes,
Class IV (Figure 4d) with modifications (Figure 4c) as described by Applegate.10
Figure 4.
(a) Kennedy class I: bilateral edentulous areas positioned posterior to the natural teeth. (b) Kennedy class II: a unilateral edentulous area situated posterior to the remaining natural teeth. (c) Kennedy class III with modification: a unilateral bounded saddle with natural teeth either side of the edentulous space and a modification space. (d) Kennedy class IV: a single bounded saddle that crosses the midline, anterior to the remaining natural teeth.
The movement of the denture during function in Class III and even short span Kennedy's class IV cases are generally minimal as they are primarily tooth supported. In contrast, the rotational movement of Class I and II removable partial dentures occurs in three cranial planes (sagittal, frontal, horizontal) due to variations in the support characteristics of the abutment teeth and the soft tissue enveloping the residual ridge. Despite the potential minimal actual movement of the denture, this may introduce a lever force acting on the abutment teeth (Figure 5).9
Figure 5. Lever forces acting in different partial denture situations.
Understanding these variations is essential for designing an effective partial denture, given that the nature of the edentulous span and support used influences the denture's movements.10
Key principles of designing and component insights
A partial denture prosthesis consists of multiple components that provide support, retention and stability, all of which are joined together by connectors to ensure function, and aesthetics, ultimately leading to better patient comfort and acceptance.
Support
Support refers to the foundational area upon which a dental prosthesis rests.11 Specifically, concerning dental prostheses, it denotes the resistance to forces directed towards the basal tissue or underlying structures. In a metal partial denture, the primary support is provided by the rests being placed on the hard tissues, with additional secondary support provided by a well-adapted denture base contacting the soft tissues.12
Retention
Retention is the quality inherent in the dental prosthesis acting to resist the forces of dislodgement along the path of displacement.11 Components in the metal partial denture providing retention are direct and indirect retainers. Direct retainers consist of a clasp assembly or precision attachments.13 These retainers can be categorized into two main types: occlusally or gingivally approaching which describe where the components originate from the framework. Tables 1–4 include commonly used clasp designs. However, the material choices and specifics of the different designs is beyond the scope of this article.
Name
Indication
Advantages
Disadvantages
Synonyms: Supra bulge/occlusally approaching/circumferential/Akers clasp designs
Simple design Provides sSupport, reciprocation and passivity The mesial rest on the distal abutment acts as an indirect retainer in long-span Kennedy class IV situations
Mesio-lingually tipped mandibular molar with mesiolingual undercut (can be used in other situations)
Maximal support, bracing, retention, encirclement
Distortion and fracture of clasp assembly
Demineralization of the tooth
Additional rest/bracing strut are required to reduce the flexibility of the clasp arm
Combination clasp
In Kennedy class I and II situations when the undercut is away from the edentulous area
Reduces stresses transmitted to abutment next to distal extension
Most cases of Class I and II when a distal undercut is present
Aesthetic
Cannot be used in the presence of soft tissue undercuts
Food lodgement
RPI clasp
Distal extension RPD
Reduces stresses transmitted to an abutment next to distal extension base Mesial rest helps to resist rotational movements of the denture base during mastication
Cannot be used in the presence of soft tissue undercuts
Food lodgement
Kennedy class III situationsShort-span Kennedy class I and II with adequate lingual sulcus depth (8 mm or more)
Minimum interferes with function
Not as rigid as other designsDifficult to add additional prosthetic teeth
Sublingual bar
All Kennedy classes with inadequate lingual sulcus depth (6 mm or less)
Good rigidityGood hygiene
Does not provide indirect retention
Lingual plate
All Kennedy classes with less than 8 mm of lingual sulcus depthPeriodontally compromised anterior teethInoperable toriModification: step back design can be used in case of diastemas
Easy to add additional prosthetic teethRigid and provides indirect retention
May cause anterior tipping of teeth if not properly designedIncreased tooth surface coverage
Kennedy bar (double lingual bar)
Class I and II situations where anterior teeth have open cervical embrasures
Less coverage of teeth in the lingual surface
May cause anterior tipping of teeth if not properly designedFood entrapmentInterference to the tongue
Labial or buccal bar
Malpositioned or lingually inclined anterior teethLarge lingual tori or exostoses that cannot be removed surgically
Indicated when other major connectors cannot be used due to lingually inclined teeth
Low patient acceptance Poor aesthetics and can alter lip fullness
Continuous bar or cingulum bar
Large tori or exostoses that cannot be removed surgicallySevere undercut in the lingual alveolus
Can be used when other major connectors cannot be used owing to large lingual tori
Must be bulky to have sufficient rigidity and thus may be objectionable to the patient
Name
Characteristics/salient features
Minor connectors that join clasp assemblies to major connectors (Figure 14, A)
A sufficient bulk is needed to ensure rigidity Must be positioned to avoid irritating the oral tissues Located on proximal surfaces of teeth adjacent to edentulous areas
Minor connectors that join indirect retainers or auxiliary rests to major connectors (Figure 14, B)
Positioned in lingual embrasures to disguise their bulk and promote patient comfort
Minor connectors that join denture bases to major connectors (Figure 15)
Three types:Open/lattice constructionTooth-tissue supported cases, with limited inter-arch spaceRelining the denture base is possibleMesh constructionTooth-tissue supported cases, with ample available inter-arch spaceRelining the denture base is possibleWeaker acrylic attachmentDifficult to pack acrylic as compared to open designBead, wire, nail head or braided postShort-span tooth-supported cases, where relining of denture base may not be required Improved hygiene and enhanced thermal stimulationDifficult to adjust and reline Weak acrylic attachment
They support gingivally approaching claspsOnly minor connectors with some degree of flexibility
Figure 6. Simple circlet (circumferential clasp).Figure 7. Embrasure clasp.Figure 8. Ring clasp.Figure 9. Cingulum rests (two indirect retainers in Kennedy class I).Figure 10. Occlusal rest (one indirect retainer in Kennedy class II).Figure 11.
(a) Decision algorithm for selecting a direct retainer for Kennedy class I and II. (b) Decision algorithm for selecting a direct retainer for Kennedy class III and IV.Figure 12. Horseshoe.Figure 13. Lingual bar.Figure 14. (A) Minor connectors that join clasp assemblies to major connectors. (B) Minor connectors that join indirect retainers or auxiliary rests to major connectors.Figure 15. Minor connectors that join denture bases to major connectors: (a) open; (b) mesh; and (c) braided.Figure 16. For vertical projection/bar-type clasps.
Indirect retainers are components that assist the direct retainer(s) in preventing displacement of the distal-extension denture base. They function through lever action on the opposite side of the fulcrum line when the denture is displaced away from the tissues. Cingulum rests on canines and occlusal rests on premolars are typical examples of indirect retainers in distal extension cases.14 Different types of direct and indirect retainers with their indications and advantages are presented in Tables 1 and 2.
A decision algorithm for selecting the type of direct retainer is presented in Figure 11. These decision algorithms facilitate design, but should be used cautiously and take into account local patient factors.
Stability
The quality of stability for a removable partial denture is the resistance to displacement by functional horizontal or rotational forces.15 The bracing or stabilizing elements of a metal partial dentures are functional when the denture is fully seated. These components include the denture base contacting the slopes of the residual alveolar ridge, proximal plates contacting the guide planes and the reciprocal arm of the clasp assembly when placed to resist the displacing force.15
Connectors
The parts of the metal partial denture that unite all the components are connectors. Major connectors join the components on one side of the arch to those on the opposite side.16 Minor connectors are the connecting link between the major connector and the other units of the prosthesis, such as the clasp assembly, indirect retainers, and denture bases.16 Different types of these connectors with their indications and advantages are presented in Tables 3 and 4.
Decision algorithms for selecting the type of maxillary and mandibular major connectors are presented in Figures 17–19.
Figure 17. Decision algorithm for selecting a maxillary major connector for Kennedy class I and II.Figure 18. Decision algorithm for selecting a maxillary major connector for Kennedy class III and IV.Figure 19. Decision algorithm for selecting a mandibular major connector for Kennedy classes I–IV.
Delivery
Workflow
Once a diagnosis and the decision to fabricate a definitive partial denture has been made, it becomes crucial to comprehend the sequential steps that must be undertaken both chairside and in the laboratory phases.
Primary impressions;
Survey of the diagnostic casts and component design;
Tooth preparation according to the specific design;
Secondary impressions;
Survey of the master casts;
Anterior tooth try-in supported by wax (in cases where metal backings will be incorporated into the framework design);
Fabrication of framework;
Framework try-in;
Denture tooth try-in on metal framework supported by wax (anterior and posterior teeth try-in);
Final processing to obtain final partial denture prosthesis;
Delivery and maintenance.
These steps can be either performed by conventional method, digital method or a combination of these methods.
Diagnostic impressions
The diagnostic impressions can be made either conventionally by using irreversible hydrocolloids or digitally by using intra-oral scanners or by scanning the diagnostic cast as determined by the clinician and appropriate to the patient case.17
With the introduction of intra-oral scanners (Ex. Cadent iTero, Align Technology, CEREC Omnicam, Dentsply Sirona, TRIOS, 3 Shape) an alternative technique for recording impressions has been added. The digital process entails conducting multiple scans for both arches. The software subsequently combines these scans to generate a comprehensive fullmouth image. The Standard Tessellation Language (.STL) file generated by the scanner is imported into the designing software to plan and complete designing.17
Surveying
The survey is a crucial step in designing removable partial dentures, involving analysis of the potential abutment teeth and associated structures on diagnostic casts. Using a dental surveyor, this process identifies the path of insertion (POI) and path of withdrawal (POW), determines guide planes, locates areas that may provide suitable retention, assesses interferences, and plans for aesthetics.18 The POI is the direction the denture takes when initially contacting abutment teeth, aiming for minimal interference and enhanced stability. The POW is the opposite direction. The path of displacement (POD) is the direction in which the denture is dislodged during function and is assumed to be at a right angle to the occlusal plane.18,19 Metal partial denture design incorporates components to limit POI, POW to a single path, and resist POD. Guiding planes, parallel axial surfaces of abutment teeth, define POI and POW.18,19 The survey line delineates the height of contour, distinguishing between desirable undercuts (retentive areas) and undesirable undercuts (interferences to be eliminated or blocked out) (Figures 20 and 21).
Figure 20. Surveying.Figure 21. Location of desirable undercut.
Undercuts should be analysed using undercut gauges for both depth and location. Aesthetics are considered, ensuring suitable positioning of components and artificial teeth for a pleasing appearance. The initial cast is horizontally oriented (zero-degree tilt) for marking survey lines and checking interferences. Tilting the cast helps to reduce spaces between abutments and the framework avoiding the creation of unaesthetic spaces in the anterior zone or difficult to clean spaces in the posterior zone. Lateral tilting can be incorporated to limit the undesirable undercuts. Substantial tooth preparation should be avoided when preparing guide planes, an antero-posterior tilt will often aid in this.
The final tilt minimizes tooth modifications, but if it exceeds 10 degrees, then crowns with denture features may be considered. The final orientation of the cast is recorded in relation to the path of placement using reference points and tripoding (Figure 22).19 Marking the external surface of the cast with parallel lines is another method to record the tilt.
Figure 22. Tripoding points.
CAD-based surveying
The process of surveying has been digitized using specialized software. Stereolithographic (STL) files can be acquired either directly by using an intra-oral scanner or by scanning conventional diagnostic casts. The STL file is then processed using specialized CAD software (3Shape CAD points, 3Shape, Partial Framework CAD, and Geomatic Touch X, 3D Systems).17 Using a digital survey tool the program assesses undercut depth on the potential abutments and parallelism between teeth. Using this information, the three-dimensional rotation of the cast is optimised to determine the path of insertion (Figure 23).17
Figure 23. Digital surveying.
Virtual survey lines are generated based on these calculations. The software generates survey lines facilitating virtual blocking out of undesired undercuts (Figure 24).17
Figure 24. Digital block-out.
Conclusion
In this first part, we focused on case selection and design principles, including surveying which is a crucial prerequisite to the design process. The upcoming second part will focus on tooth preparation, laboratory procedures, and prosthetic delivery, highlighting both conventional and digital techniques.
To achieve an optimal patient outcome, it is imperative that the clinician is familiar with the complexity of denture design, fully understanding the function and use of each component.
The absence of a single best design emphasizes the decision-making complexity and multiple acceptable designs for a partially edentulous arch may be reasonable. Understanding the indications and contraindications for metal partial dentures is crucial, as overlooking these turns the design process into a speculative endeavour rather than a methodical and controlled procedure.
