Fixed Prosth Secrets Revealed: Prepare to Have Your Dental Mind Blown!

Introduction

Welcome, colleagues! This post synthesizes expert perspectives on fixed prosthodontics, drawing heavily from insightful lectures hosted on RipeGlobal by renowned clinicians: Dr. Lane Ochi, Dr. Michael Melkers, Dr. Zain Rizvi, Dr. Lincoln Harris, and dental photography expert Dr. Karyn Halpern. We'll delve into the critical techniques and foundational principles that underpin successful restorations, moving beyond material choices alone. Presented with professional insight and practical flair, this guide aims to integrate actionable wisdom for achieving predictable, long-lasting clinical outcomes in your fixed prosthodontic cases.

Preparation Ponderings: The Foundation is Everything

Solid preparation is non-negotiable. While materials evolve, the fundamental principles of tooth preparation remain paramount for long-term success.

• Retention and Resistance: The Inseparable Duo: As Dr. Lane Ochi emphasizes, retention (resisting vertical dislodgement) and resistance (resisting dislodgement along other axes) are intrinsically linked. Simply adding a well-designed resistance feature inherently limits the path of insertion and significantly boosts retention. When restorations fail, critically analyze whether it's a material issue or a fundamental design flaw in the preparation. Forget flat-top preps – their susceptibility to failure due to high C-factor stresses during bonding has rendered them obsolete.

  
  

• Taper - Precision Matters: Total Occlusal Convergence (TOC) is critical. Dr. Ochi highlights studies (like Campani's) showing that reducing TOC, particularly aiming for around 8 degrees in the cervical 1.5 mm of axial walls (down from a more forgiving 20 degrees), dramatically increases resistance form and, consequently, retention. If you encounter overly tapered walls (like a tipped buccal wall), take the time to upright them. Consider adding proximal boxes – these features create walls that place cement under compression during lateral loading, further enhancing resistance.

  
  

• Occlusal Veneers are Onlays: Forget the flat-top "occlusal veneer." As Dr. Ochi and Ivoclar Vivadent point out, this design invites shear failure. The modern approach is an "occlusal onlay" preparation that extends over the buccal and lingual cusps. This design puts the restoration and cement under compression, a much more favorable loading scenario. Even incorporating simple internal features like an isthmus or a "pothole" can act as a second wall, putting cement under compression and mechanically hindering displacement.

  
  

• Axial Wall Height – Millimeters Count: Every bit of wall height contributes to resistance. Dr. Ochi references Wake's study demonstrating how significantly restoration failure (under off-axis loads) increases as axial wall height decreases from 4mm down to 0mm. Even losing half a millimeter of zirconia can substantially reduce its strength. Interestingly, well-supported feldspathic porcelain (around 1mm) on a PFM can exhibit surprising strength, sometimes rivaling 1mm thick monolithic zirconia in specific tests – underscoring the importance of adequate, well-supported material thickness and sufficient preparation height.

  
  

• Finish Line Finesse – Chamfer is Champion: If you choose only one margin style, make it a chamfer. It's generally the easiest to read visually, scan accurately, and achieve a good marginal seal. A well-captured, well-sealed margin is crucial; even the most retentive prep will ultimately fail due to leakage if the margin isn't perfect.

  
  

Impression Insights: Capturing Reality Accurately

A flawless impression or scan is the blueprint for success. Achieving this requires meticulous technique and understanding tissue management.

• Gingival Displacement (Not Retraction): Terminology matters. Aim for displacement to expose the margin clearly. Dr. Ochi stresses understanding biologic width principles (Maynard & Wilson, Kois) to avoid damaging the attachment – remembering it takes only ~25 grams of pressure to sever healthy attachment (even less in inflamed tissue). The two-cord technique remains a gold standard: a small first cord (hollow cords work well) for vertical displacement, followed by a larger second cord for horizontal displacement. Dr. Rizvi adds practical advice: place the small cord (e.g., 000) subgingivally, then the larger (e.g., 00 or 0) at the margin. Leave the larger cord for ~5 minutes (consider hemostatic agents), then remove only the larger cord before impressing or scanning, leaving the small cord for contrast. Patience and hemorrhage control are key. (Dr. Ochi also notes rotary gingival curettage as an alternative tissue management technique described by Ingram and Hansing).

  
  

• Material Handling: Flood the sulcus and prep generously with low-viscosity light body material, extending it onto the tray material over the prep to minimize voids, especially in undercuts. Ensure the heavy body material actively pushes the light body ahead of it as the tray seats or the patient bites, ensuring complete capture without voids. Dr. Rizvi reminds us to allow full setting time before removal to prevent distortion. For gaggers, consider using a lower tray for the upper arch impression to minimize posterior material excess.

  
  

• Digital vs. Analog Fit & Challenges: While labs often prefer milling for speed and cost, Dr. Melkers cautions that pressed restorations (like EMax) often exhibit superior internal adaptation compared to milled ones. Studies (like Johnny's, noted by Ochi) suggest CAD/CAM restorations can have larger internal gaps ("slop") and thicker cement layers. While CAD margins can be excellent, the internal fit might compromise retention, especially on short preps. For maximum retention/resistance on challenging preps, Dr. Ochi suggests considering a cast restoration over a milled one – you might "out-cast" a poor prep where milling can't compensate. Furthermore, labs report challenges with digital scans, including incorrectly marked margins or poor quality scans, particularly with deep subgingival margins where optical capture is difficult without adequate tissue management/displacement. Surveys noted by Dr. Ochi indicate digital scanning hasn't universally reduced remakes for labs.

  
  

• Common Impression Errors: Studies highlighted by Dr. Ochi found a staggering 86% of impressions sent to labs had detectable errors, with the majority being critical errors involving the finish line. The #1 critical error? Lack of complete gingival displacement. Always evaluate the entire impression, including the opposing side, for potential distortion, not just the prepared tooth margin.

  
  

• Triple Tray Limitations: Triple trays excel at capturing Maximum Intercuspation (MIP). However, Dr. Melkers warns they are inherently limited in accurately capturing lateral excursions. Because the tray itself becomes the hinge axis reference, its proximity to the arch results in a different arc of rotation compared to the patient's actual condylar movement. This can lead to built-in lateral interferences. Historically, labs sometimes tried to compensate (often poorly understood instructions), but a better approach, according to Melkers, is designing preparations with shallower fossae and "landing pads" to minimize potential interferences inherent in the technique.

  
  

Material Musings: Choosing Your Armament

Material science offers incredible options, but understanding their properties and historical context is key.

• Gold Standard: Cast gold restorations cemented with traditional water-based luting agents set the benchmark for longevity. As Dr. Melkers notes, even gold can fail, but its relative "weakness" allows it to adapt to forces, which can be a strength. Dr. Ochi points out that historical reviews show lower 5-year retention failure rates for PFMs compared to early zirconia studies (e.g., Pacia & Posia RCT showing 90.2% PFM vs 74.8% zirconia survival regarding retention).

  
  

• Ceramic Considerations: All-ceramics offer excellent aesthetics and strength but are inherently more brittle than metals. Even robust monolithic zirconia (e.g., 1.5mm thick) can fracture under sufficient force.

  
  

• PFM Principles: The success of PFMs relies on adequate metal support for the porcelain. The fact that 1mm of well-supported feldspathic porcelain performed respectably against 1mm zirconia in one test highlights the importance of the underlying structure and proper material thickness.

  
  

Cementation Sensations: The Crucial Bond

Modern resin cements offer predictable bonding but aren't a substitute for good preparation.

• Dr. Ochi reminds us that bonding to flat surfaces creates an "infinite C-factor," leading to high shrinkage stress and potential cohesive/adhesive cracks – another reason flat-top preps failed. The ideal approach combines sound retentive preparation design with the benefits of resin cementation.

  
  

• After try-in, thoroughly clean the intaglio surface of saliva contaminants. Dr. Rizvi recommends Ivoclar IvoClean.

  
  

• For conditioning ceramics like lithium disilicate (e.max), Dr. Rizvi suggests Monobond Etch & Prime as an efficient and safer alternative to separate hydrofluoric acid etching and silanating steps. It combines both actions with comparable bond strength. Follow manufacturer instructions carefully (e.g., rub 60s, wait 60s, rinse). Then proceed with your chosen dual-cure or light-cure resin cement.

  
  

Occlusion Observations: Where Function Meets Longevity

Occlusion is arguably the most complex and critical aspect of restorative dentistry, influencing the success or failure of our work.

• Centric Relation (CR) – The Great Debate: Dr. Melkers highlights the historical and ongoing confusion surrounding CR definitions (JROS survey, 7 definitions in GPT 2005, Spear's count of 29 changes!). Early definitions were often clinically impractical. The current GPT definition (2017) still references clinically invisible parts. Importantly, a 2016 study found no coincidence between CR and MIP in participants. Regardless of definition, Dr. Melkers emphasizes Pete Dawson's principle: CR should be achieved via gentle manipulation and result in a comfortable, stable, and repeatable position.

  
  

• MIP vs. CR – Clinical Relevance: MIP is simply teeth together, regardless of joint position. CR is a specific joint position, regardless of tooth contact. Dr. Melkers clarifies his current philosophy: treat primarily to MIP unless there's a specific indication for CR. CR is a position of restorative convenience, not necessarily inherent health or pathology. Restoring complex cases in CR (as advocated by Pankey/Dawson) allows forces to be distributed axially when teeth occlude without deflection and facilitates immediate anterior guidance. Conversely, restoring in MIP when the condyles aren't fully seated can lead to premature posterior contacts if the patient later seats into CR, potentially damaging restorations or teeth.

  
  

• When to Utilize CR: Dr. Melkers suggests using CR as a diagnostic and treatment tool in specific situations:

  
  

  • Screening broken terminal teeth: To assess potential loss of restorative room when seating into CR (using the approximate 1mm anterior leaf gauge opening = 1/3mm posterior change ratio).

    
    

  • Screening pain patients: Especially those with lateral pterygoid symptoms or risk of anterior open bite.

    
    

  • Severe wear cases: To evaluate if a seated condylar position provides needed restorative space.

    
    

• Tools for Finding CR: The Leaf Gauge and Lucia Jig/Panky Programmer are valuable tools to disclude posterior teeth, allowing muscles to relax and condyles to seat more fully towards CR. Melkers describes using the leaf gauge to find the first CR contact (marking with black film after screening with red), then adding leaves (~2.5mm total opening) for bite registration. The Lucia jig achieves similar deprogramming. Bimanual manipulation is another key technique. Verification relies on achieving a repeatable record.

  
  

• Occlusal Adjustment – Precision Required: Avoid indiscriminately deepening fossae during adjustments, as this eliminates desirable axial contacts and promotes incline interactions. Dr. Melkers advocates creating "landing pads" – flat areas for stable, axially directed contacts. Trust thin (e.g., 19-micron) articulating paper for accuracy; thicker papers can create false positives.

  
  

• Force Management: The core goals are to reduce destructive forces, distribute loads favorably, and achieve axially loaded contacts in compression. Real-world occlusion (Class II, III, rotations, wear) requires adapting principles, not applying cookie-cutter solutions. Think "how to think," not just "what to think."

  
  

• Anterior Guidance – Protecting the Posterior: Dr. Melkers' finger-biting exercise vividly demonstrates the rationale: anterior teeth have greater proprioception and elicit a protective decrease in muscle force. Anterior guidance (ideally canine-initiated disclusion, transitioning to incisors in protrusion) serves to disclude posterior teeth during excursions, reducing stress on them. EMG studies show significantly lower muscle activity in protrusive guidance compared to posterior clenching or even canine guidance alone.

  
  

• Red Flags & Specific Challenges:

  
  

  • Second Molar Issues: Severe wear, exclusive second molar wear, super-eruption, and dished-out anatomy are red flags requiring careful management (equilibration, reshaping, restoring, repositioning via ortho/TADs) to avoid problems.

    
    

  • Severe Wear: Often involves an acid/erosion component, not just grinding. Assess etiology carefully before jumping to splints. CR assessment can be vital.

    
    

  • Dished-Out Anatomy: Indicates potential lack of tooth structure. Use calibrated photos (Melkers) for digital planning to ensure adequate reduction without over-prepping. Design wax-ups with landing pads.

    
    

  • Quadrant Dentistry: Accurate bite registration is key (Melkers suggests a "button bite" around the prep). Understand how distance from the condyle affects escape pathways and compensate for triple tray limitations by shallowing fossae/using landing pads.

    
    

• Philosophy: Embrace Melkers' adaptation of the Serenity Prayer: accept unavoidable factors (like severe parafunction) and focus on managing the treatable aspects through careful design and material choice. Understand the "Why" behind your treatment (Purpose, Process, Presentation - PPP sheet).

  
  

Lab Liaison: The Art of Communication

Clear, detailed communication with your technician is essential for predictable outcomes.

• Shade Communication: As Dr. Rizvi advises, take shade tabs before prepping (teeth dehydrate). Provide the lab with your preferred shade, plus one shade lighter and one darker, held parallel and in the same plane as the tooth. Take stump shades after preparation if needed.

  
  

• Prescription Power: Be explicit. Specify material, desired internal treatment (etching/silanating preferences), margin marking, contact points, and desired occlusal guidance (e.g., canine guidance vs. group function). Discuss ingot selection and translucency concerns with your technician.

  
  

• Managing Fit Issues: Dr. Rizvi suggests trying interproximal stripping/wedges first for tight contacts. If adjustment is needed, seat the restoration, mark contacts with articulating paper on floss, carefully adjust the restoration (not the tooth), and polish thoroughly. For under-reduction, the best option is repreparation; alternatives include asking the lab for a reduction coping or adjusting an over-erupted opposing tooth (use cautiously).

  
  

Photography Phantoms: Documenting and Diagnosing with Light

Dental photography, expertly executed by clinicians like Dr. Karyn Halpern, is indispensable.

• Why Bother? It's crucial for diagnosis, treatment planning, unparalleled lab communication, patient education, and medico-legal documentation.

  
  

• Fundamentals (Halpern):

  
  

  • Angulation & Composition: Camera must be level and perpendicular to the teeth's facial surfaces to avoid distortion. Frame tightly on the subject. Patient and photographer should be at the same eye level.

    
    

  • Shade Matching: Position the tab parallel and in the same plane as the hydrated (not wet) tooth. Ensure sharp focus.

    
    

• Equipment Essentials (Halpern): DSLR body, macro lens (100mm recommended), and macro flashes (twin flash preferred for anterior/shade, ring flash useful for posterior/occlusal). Essential accessories include retractors, contrasters, and occlusal/buccal mirrors. Keep extra batteries/cards handy! (Sources: photomed.com, smilelineusa.com).

  
  

• Disinfection (Halpern): Use barriers (cling wrap). Wipe camera body (cap on) with bleach-free alcohol wipes, avoiding lens/rubber. Clean lenses/mirrors with microfiber cloths. Autoclave mirrors protected in microfiber. Always check manufacturer guidelines first.

  
  

• Simplified Settings (Halpern): Stick to Manual mode. Set ISO (100-200), Shutter Speed (1/125), White Balance (Flash), Metering (Center-Weighted), Focus (AF-S). Shoot RAW+JPEG. The main variable becomes the f-stop (aperture): higher number = greater depth of field. General ranges: f/18-f/22 (close-up smiles), f/22-f/29 (intraoral), f/22-f/25 (shade with twin flash).

  
  

• Key Shots (Halpern): A standard diagnostic series includes portraits (natural & max smile), profiles, close-up smiles, anterior retracted (+/- contraster), posterior buccal retracted, occlusal views (max/mand), and shade shots. Lab bench photos are useful for documenting characterization.

  
  

Treatment Triumphs (and Tribulations): Planning & Managing Reality

Effective planning and honest patient communication are paramount.

• Planning Principles (Harris): Often, posterior treatment planning starts with assessing aesthetics and patient goals, then integrating occlusion. Choose the easiest path to a good (not necessarily perfect) outcome. Document everything, especially when patients decline recommended options. If extensive decay is found mid-procedure on a tooth planned for a crown, complete work on salvageable teeth, place a provisional on the questionable one, charge appropriately for work done, and re-evaluate with the patient using photos. When doing core build-ups, sandblast existing structure, bond effectively, and prep to an ideal form, not just replicating the old prep. Manage tissue carefully for vertical preps; if bleeding persists, temporize and reschedule.

  
  

• Managing Failure (Harris/Melkers): Be upfront with high-risk patients (e.g., severe bruxers) that restorations can break. Focus on designing restorations to minimize the frequency and severity of potential failures. Sometimes, accepting a degree of managed failure is part of the long-term plan.

  
  

Conclusion: The Continuous Journey

Mastering fixed prosthodontics is an ongoing journey, not a final destination. By integrating the foundational principles revisited by Dr. Ochi, the nuanced occlusion insights from Dr. Melkers, the practical restorative tips from Dr. Rizvi and Dr. Harris, and the documentation power highlighted by Dr. Halpern, we can elevate our clinical practice. Keep questioning "why," embrace continuous learning (resources like RipeGlobal offer invaluable access to these educators), and strive to provide restorations that are not only beautiful but predictably functional and durable. Go forth and restore with confidence and precision!

Dr. Noor N. AyToghlo

(You can watch the full lectures referenced in this post on RipeGlobal.com)