Analysis of the geometry of finishing lines for full crown restorations

Contemporary concepts on periodontal complications from prosthetic and restorative therapies

The clinical outcome of every prosthetic and restorative procedure depends on the maintenance of a healthy periodontium. It is, therefore, important that the prosthodontist and restorative dentist cause no harm or permanent damage to the underlying hard and soft tissues when performing clinical procedures necessary to carry out the planned treatment. Several factors involved in these procedures have been described to have an impact on gingival health. For the present article, a selection of four of these factors are presented with the goal of evaluating the current trends and their influence on periodontal structures: (1) tooth preparation configuration and apical extension, (2) gingival tissue sulcular expansion/retraction, (3) prosthetic contours, and (4) prosthesis marginal adaptation and the consequences of excess cement remnants. Based on the available scientific evidence and clinical experience, recommendations for the practitioner are given.

Marginal Misfit of 3D-Printed (Selective Laser Sintered), CAD-CAM and Lost Wax Technique Cobalt Chromium Copings with Shoulder and Chamfer Finish Lines: An In-Vitro Study

Background and Objectives: The aim was to compare the Misfit of 3D-Printed, Selective laser melting (SLM), milled (Computer aided design-Computer aided manufacture CAD-CAM) and Lost wax technique (LWT) fabricated Cobalt chromium (CoCr) alloy copings on shoulder (SH), radial shoulder (R-SH) and chamfer (CH) finish line configuration. Materials and Methods: Ninety resin, second maxillary premolar teeth were prepared for metal-ceramic crowns, equally divided into (n = 30) SH, R-SH and CH margin preparations. For each preparation design (SH, R-SH and CH), CoCr copings were prepared using SLM, CAD-CAM and LWT. This resulted in nine study groups with 10 CoCr copings each. The marginal misfit of specimens was assessed with a high-resolution digital microscope. Misfit was evaluated in vertical and horizontal dimensions in μm. Data were analyzed using ANOVA and a post hoc multiple comparisons test. Results: For vertical misfit, the highest was observed in SLM samples with chamfer margin (167.96 ± 24.1), and the least was shown by CAD-CAM samples with radial shoulder (58.8 ± 12.53). CAD-CAM and shoulder margins showed the least vertical misfit. For horizontal misfit, the maximum was observed in SLM samples with shoulder margin (137.94 ± 37.85) and the least by LWT samples with chamfer (89.38 ± 14.81). Chamfer margins and LWT samples showed the least horizontal misfit among the group samples. Fabrication technique and finish line design play a critical role in reducing the marginal misfit of CoCr copings. Conclusions: For vertical misfit, SLM copings showed poor outcomes compared to CAD-CAM specimens, however comparable outcomes to Cast specimens. SLM copings showed comparable horizontal misfit outcomes to CAD-CAM specimens and low misfit compared to Cast copings, respectively. Vertical misfit was low with shoulder margins, and horizontal misfit was better with chamfer marginal configuration.

Impact of a Core Ferrule Design on Fracture Resistance of Teeth Restored with Cast Post and Core

Objectives. To investigate the influence of a contra bevel on the fracture resistance of teeth restored with cast post and core. Materials and Methods. Sixty plastic analogues of an upper incisor were endodontically treated and prepared with 6° internal taper and 2 mm of ferrule in order to receive a cast post and core. The prepared samples were divided into two groups (n = 30); the first group serves as control while the second group was prepared with an external 30° bevel on the buccal and lingual walls. All samples crowned were exposed to a compressive load at 130° to their long axis until fractures occurred. Fracture resistance loads were recorded and failure modes were also observed. Mann-Whitney test was carried out to compare the two groups. Results. Mean failure loads for the groups were, respectively, 1038.69 N (SD ±243.52 N) and 1078.89 N (SD ±352.21 N). Statistically, there was no significant difference between the two groups (P = 0.7675 > 0.05). Conclusion. In the presence of a ferrule and a crown in the anterior teeth, adding a secondary ferrule to the cast post and core will not increase the resistance to fracture.

Marginal discrepancy as affected by selective placement of die-spacer: an in vitro study

An increase in the marginal discrepancy is seen after cementation with a luting agent and provision of cement space with a die-spacer is the most preferred method to avoid it. Recommended thickness of die-spacer is 25-40 μm. Smaller die-spacer thickness was consistently found at the axio-occlusal line angles as compared to the other surfaces which has been postulated to that the spacer paint tends to flow away from the sharp line angles and cusp tips as a result of increased surface tension. The absence of adequate relief spaces in these areas impedes the flow of cement beyond the occlusal portion of the casting, which would result in incomplete seating because of hydraulic pressure. Fifty stone dies were duplicated from a steel die and were divided into five groups of sample size 10, where the die-spacer was selectively placed. Measurements were taken at four points, 90° apart from each other with the help of optical microscope. Later all the castings were cemented using Glass Inomer cement as a luting agent, under a 10 kg static load and measurements were recorded. Statistical analysis showed samples with no spacer had the maximum pre and post cementation gap while the least discrepancy was seen in group with additional layer of die-spacer painted over the axio-occlusal line angle. The results were highly significant which clearly indicated the superiority of this group over others. Within limitations of the study, it can be said that application of additional layer of die-spacer at the axio-occlusal line angle will help in decreasing the post cementation marginal discrepancy in full cast metal crowns.

Comparative analysis of the clinical techniques used in evaluation of marginal accuracy of cast restoration using stereomicroscopy as gold standard

PURPOSE

This study assessed the effect of preparation design on marginal adaptation and also compared the sensitivity and specificity of clinical evaluation techniques for marginal accuracy of cast restorations to stereomicroscopy.

MATERIALS AND METHODS

Three Ivorine molar teeth of different designs were prepared. (A)-A complete crown preparation with buccal shoulder and beveled finish line. (B)-A complete crown preparation with chamfer finish line. (C)-A three-quarter crown preparation with proximal boxes and beveled finish line. Twenty four castings were prepared with eight castings for each design respectively. Each casting underwent examination with an explorer, disclosing media, and a stereomicroscope. Stereomicroscopy at a value less than or equal to 30 microns was used as a gold standard to evaluate the significance of different designs on marginal adaptation. Chi-square tests of independence and Kruskal-Wallis were used to evaluate the effect of preparation design and compare the agreement between examination methods for detection of marginal gap size of greater than or equal to 30 microns (α=.05). Sensitivity and specificity for explorer and disclosing media as compared to stereomicroscope was calculated using statistical formula given by Park.

RESULTS

The preparation design did not significantly affect overall marginal adaptation. Examination by explorer and disclosing media at 30 µm revealed 39% and 10.06% sensitivity and 91% and 82% specificity respectively.

CONCLUSION

Preparation designs examined in this study did not significantly affect the marginal adaptation of the castings. Commonly used clinical evaluation techniques using explorer and disclosing media appeared to be inadequate for assessment of marginal accuracy.

Consistency of labial finish line preparation for metal ceramic crowns: an investigation of a new bur

PURPOSE

Previous studies have reported on the difficulties inherent in preparing the labial aspect of teeth for metal ceramic crowns with consistency and also the implications for the definitive restoration of underprepared and overangled finish lines. In this study, a bur designed to prepare a 1.2-mm deep chamfer was tested and compared with two other bur kits.

MATERIALS AND METHODS

Seventy-two teeth were prepared to receive metal ceramic crowns in vitro by students using one of the two bur kits or the new bur. Using a coordinate measuring machine (CMM), replicas of the prepared teeth were scanned in the mid-labial plane, and shoulder width and cavosurface angles were measured.

RESULTS

The new bur produced preparations with a mean shoulder width and cavosurface angle of 1.146 +/- 0.241 mm and 108 +/- 11 degrees, respectively, compared with 0.626 +/- 0.234 mm and 128 +/- 14 degrees produced by a commercially available standardized crown cutting bur system and 0.626 +/- 0.218 mm and 124 +/- 15 degrees produced by the bur kit in use at our center at the time of the study. These differences were statistically significant at the 5% level, using Tukey's comparison of means.

CONCLUSIONS

Teeth prepared by the new bur had wider shoulders and lower cavosurface angles than teeth prepared either with a standardized crown cutting system or locally selected bur kit. The quality of labial preparations produced by the new bur in terms of closeness to the postulated "ideal" width and angulation warrants further investigation.

Influence of margin design on the fit of high-precious alloy restorations in patients

OBJECTIVE

It was the objective of this study, to analyze the influence of the marginal design on the marginal accuracy of a casting in a clinical setup in patients.

METHODS

Ninety volunteer patients' teeth-which were intended for extraction due to medical reasons-were prepared prior to extraction. Three different types of finishing line-chamfer, 135 degrees shoulder and 90 degrees shoulder-were employed. Two each c-silicone and pvs impressions were taken of each tooth using either a two-stage or a one-stage putty-wash technique. After preparation and impression taking the teeth were extracted. Gypsum casts were poured from the impressions and high precious alloy castings fabricated on the dies and marginal discrepancies were determined on the extracted teeth.

RESULTS

The median value of marginal discrepancies was lower than 150 microm for all groups. The difference between the three different preparation types was significant (H-test, p<0.05). The lowest median values were obtained for the chamfer preparations, while the 90 degrees shoulders always produced the highest median values. Preparations at gingva level exhibited more accurate marginal fit than subgingival preparations. No significant differences could be observed between the pvs and c-silicone materials or the one-step and two-step putty-wash techniques.

CONCLUSIONS

The marginal designs of the preparations had much less influence on the marginal fit of high precious alloy castings than expected. There is considerable reason to assume that technical but clinical parameters influence the quality of fixed prosthodontics much more than has been believed in the past.

Cervical margin design with contemporary esthetic restorations

The contemporary dentist has a wide variety of options to use in the restoration of extensively damaged or previously restored teeth. Metal-ceramic and all-ceramic crowns are frequently used to restore esthetics and function. One of the essentials for success with either option is proper tooth preparation, which includes proper selection and preparation of the cervical margin of the preparation.

Influence of cold working and thermal treatment on the fit of implant-supported metal-ceramic fixed partial dentures

STATEMENT OF PROBLEM

The application of porcelain may alter the fit of metal-ceramic frameworks. In fixed partial denture frameworks, this altered fit may cause marginal opening and require sectioning and soldering of the prosthesis.

PURPOSE

The purpose of this study was to compare the fit of implant-supported metal-ceramic fixed partial dentures when different thermal treatments were performed.

MATERIAL AND METHODS

Fifteen implant-supported metal-ceramic fixed partial denture frameworks were fabricated and divided into 3 treatment groups: Group 1 = cold working and thermal oxidation; Group 2 = unrestrained thermal treatment, cold working, and oxidation; and Group 3 = investment-restrained thermal treatment, cold working, and oxidation. After the appropriate treatment procedures, porcelain was applied. Measurements were made with a digital caliper, with the help of fiduciary marks, after each step in 3 dimensions and at 2 locations. These measurements were compared with those of the as-cast framework (baseline) to determine framework distortion. Distortion data among groups were compared with repeated-measures analysis of variance and Tukey's Studentized range test (P<.05).

RESULTS

Absolute values of distortion (regardless of direction) after thermal treatment and after porcelain application were not significantly different among groups. However, total raw values of distortion, which describe the specific direction of distortion, were significantly different for 4 of the 6 recorded measurements between Groups 1 and 3 (P<.05). Although the absolute distortion values did not differ among groups, heat treatment and finishing of the alloy affected the timing and direction of distortion.

CONCLUSION

Within the limitations of this study, heat treatment of implant-supported metal-ceramic fixed partial denture frameworks before cold working did not decrease the total absolute distortion. Definite patterns of distortion were, however, found in Groups 1 and 3, suggesting that specific manipulations and heat treatments of the alloy affected the direction of distortion.

Evaluation of high-temperature distortion of high-palladium metal-ceramic crowns

STATEMENT OF PROBLEM

Crown fit is a prerequisite for long-term clinical success; however, crown distortion may occur during porcelain firing. The dimensional stability of some high-palladium alloys at high temperatures has been questioned.

PURPOSE

The purpose of this study was to use a new method to measure the distortion of copings for metal-ceramic single units of selected high-palladium alloys with compositions representative of commercial alloys.

MATERIAL AND METHODS

Four high-palladium alloys containing copper and 3 containing no copper were tested. A palladium-silver alloy was included for comparison, and a gold-palladium alloy served as the control. By using reference points scribed on the margin, the mesiodistal and buccolingual margin diameters of identical copings were measured with a traveling microscope at 4 stages: as-cast, oxidized, after 2 simulated opaque porcelain firings, and after 2 simulated dentin porcelain firings. The margin distortions for the various specimen groups representing combinations of alloys, stages, and measurement diameters were compared with the use of 1-way analysis of variance and a multiple range test.

RESULTS

Most of the high-palladium alloys had high-temperature distortions that were not significantly different from those of the control alloy. The distortions occurred principally during the oxidation cycle. The effect of mesiodistal groove reinforcement on preventing distortion was not the same for all alloys.

CONCLUSION

The results suggest that small observed distortions of these alloys will not produce clinical problems. Several laboratory techniques are available to counteract the distortions.

Method for marginal measurements of restorations: accessory device for toolmakers microscope

Standardization of measurements for marginal fit of castings is critical. This study describes the fabrication of a device that allowed fixation of specimens on a Toolmakers microscope with identical conditions according to tri-dimensional positioning of specimens, measuring location, and seating force. The device also allows mapping of the marginal discrepancies on the entire marginal perimeter of the tooth preparation.

The effects of die-spacing on post-cementation crown elevation and retention

This paper reviews the relevant dental literature concerning the effect of die-spacing on crown elevation and pre- and post-cementation crown retention. Techniques of providing die-spacing and measurement of the thickness of die-spacer are discussed. A review of the role of the provision of a cement space in reducing post-cementation crown elevation is presented. Factors which may affect crown retention prior to and following cementation are also reviewed. The influence of variables in techniques and experimental design on the results of the studies reviewed is discussed.

Restoration of pulpless teeth: application of traditional principles in present and future contexts

Posts were recommended more than 100 years ago to retain artificial crowns. Recent studies suggest that posts can weaken teeth; therefore restorative procedures that help preserve pulpal vitality and eliminate the need for posts are desirable. If endodontic therapy is unavoidable, conservation of remaining tooth structure is most important. When a post is required to retain a core for an artificial crown, a custom cast post is the most effective means of conserving tooth structure. The length of the post should not be compromised, although 4 to 5 mm of apical gutta-percha must be maintained. The restorative prognosis is improved if the width of the post does not exceed one half the width of the root, and the cemented artificial crown should extend apical to the core to provide a 1.5 to 2 mm ferrule. Complex procedures have allowed the dentist to restore extensively damaged teeth. However, extraction and replacement with implant-supported prosthodontics may be more prudent with severely compromised teeth.

Effects of firing cycle and surface finishing on distortion of metal ceramic castings

The "as-cast" fit of metal ceramic restorations has been reported to deteriorate during the high-temperature firing cycles used for application of porcelain veneer. In this study, thermocycling and surface finishing or cold working were examined for their effects on marginal adaptation of metal ceramic castings. Methods for minimizing the loss of marginal adaptation were evaluated, and casting variables were eliminated by construction of acrylic resin measuring dies directly in the restorations. Thermocycling of metal ceramic restorations resulted in increased marginal openings, and all of the loss of marginal fit occurred during the first thermocycling of the alloy. The restorations that were cold worked and then oxidized by conventional manipulation had substantially more marginal opening than any other group. A fourfold, statistically significant improvement (p < 0.001) in the marginal adaptation of a metal ceramic restoration was observed when the initial thermal cycle was completed before the specimens were finished.

Postcementation marginal fit of a new ceramic foil crown system

The in vitro marginal fit of a new foil crown was investigated. Ten standardized polycarbonate master dies filled with 30 wt% carbon having a 90-degree butt joint margin were used to evaluate the marginal fit of Sunrise porcelain foil crowns. The crown specimens were made on stone dies and treated with three coats of die spacer. After fabrication, they were adjusted, cemented with zinc phosphate cement to their respective master dies with 5 kg loading for 10 minutes and stored in water. A Gaertner traveling microscope was used to measure the marginal fit of each specimen. Readings were recorded at five locations randomly marked but equally dividing the margin on each crown, and the overall mean for the crowns was then calculated. The results from the 50 areas revealed a mean marginal discrepancy of 25.3 microns, and this compared favorably with the discrepancies reported for other ceramic crown systems.

Thermal cycling distortion of metal ceramics: Part II--Etiology

The three-dimensional geometry of conventional fixed prostheses complicates the study of the thermal cycling distortion in metal ceramic alloys. Any explanation of the etiology of thermal cycling distortion in metal ceramic restorations must account for the observed magnitude, timing, and direction of the deformation. The simplified experimental geometry developed in Part I was applied to elucidate the etiologic factors involved in metal ceramic deformation. Techniques to minimize the thermal cycling distortion were also studied. It was found that all of the significant distortion occurred during the first thermal cycling of the alloy (oxidation) and that no distortion resulted from the application of body porcelain. The specimens that were cold worked and then oxidized had significantly more distortion than any other group. A significant reduction in distortion was observed when the initial thermal cycling was completed before the specimens were cold worked. It was determined that the release of casting- and cold working-induced stresses had a synergistic effect.

Thermal cycling distortion of metal ceramics: Part I--Metal collar width

The complex three-dimensional geometry of conventional full crown restorations severely complicates the study of thermal cycling distortion in metal ceramic castings. A simplified experimental geometry was developed to (1) maximize the measuring sensitivity, (2) eliminate the casting variables, (3) allow the direct measurement of casting distortion, and (4) evaluate the thermal cycling distortion of a wide range of metal collar widths. It was found that all of the one-walled castings distorted during the initial thermal cycling (oxidation) of the alloy. There was no significant distortion associated with porcelain application or glazing. The castings with an 0.8 mm metal collar had significantly less distortion than those with 0.1 and 0.4 mm collars at 2 of the 10 sites measured (inferior or facial margins).

Marginal accuracy and geometry of cast titanium copings

The biocompatibility of titanium with human tissue is well established; its potential as an alternate metal for crown restorations has yet to be fully investigated. This study measured the geometry and accuracy of coping margins cast in Ti, 99.5% chemically pure. Twenty castings with 45-degree facial and 90-degree palatal margins were made by indirect technique with a commercial system using recommended procedures. They were then cemented with ZnPO4 cement. The coping and die assemblies were embedded in resin, sectioned longitudinally, and measured with a microscope at x50 power. Casting shrinkage was characteristic, particularly along the horizontal axis in the plane of the shoulder. Although horizontal linear shrinkage of 45-degree margins was greater than that for the 90-degree form, the surface of marginal discrepancy was greatest with the 90-degree configuration.

Gingival margins for crowns: a review and discussion. Part II: Discrepancies and configurations

While there is variation regarding the maximum acceptable marginal discrepancy, there is little argument that poorly fitting margins are a frequent finding. Large discrepancies are clinically significant, since they facilitate plaque retention. Margins incorporating slip joint geometry have usually been favored as a method of minimizing seating and sealing discrepancies. However, many of these discussions largely ignored the effects of the cementing medium and the clinical applicability of slip joint geometry is based on questionable assumptions with regard to casting accuracy and seating. Greater understanding of the role of restorative margins and gingival health indicates the need for shallow margin placement within the crevice, which requires a reassessment of the use of long bevels. Horizontal margins can be made accurately and, when combined with procedures to maximize crown seating, may provide the best method of minimizing seating discrepancies and maximizing gingival health.

Comparison of conventional paint-on die spacers and those used with the all-ceramic restorations

The study compared the thickness of conventional (Tru-Fit and Belle de St. Claire) and all-ceramic (Dicor and Cerestore) die spacers. Three coats of Tru-Fit, two coats of Belle de St. Claire, and two coats of Cerestore die spacers provided a 25 microns thickness. A single coat of the Dicor spacer resulted in a 37 microns layer. A significantly thinner layer occurred at the convex occlusal line angles for all of the materials tested. This could impede the flow of the luting agent at the time of cementation.

Use of an elastomeric material to improve the occlusal seat and marginal seal of cast restorations

A method to improve the fit of castings by the use of a light-bodied condensation silicone impression material was tested. Eighteen-full coverage castings were made on individual resin dies and divided into two groups. The experimental group was adjusted internally to a uniform precementation space by using the silicone impression material as a disclosing agent for binding of the castings on the axial or occlusal walls. Measurements of marginal fit were made with a light microscope using a filar eyepiece both externally and internally by sectioning after cementation with a zinc phosphate cement. Results demonstrated a significant improvement in marginal seal and occlusal seating in the experimental group compared with the control group. The condensation silicone material proved to be an appropriate research tool for nondestructive, three-dimensional evaluation of the postcementation space and offers a new method of evaluation of cement thickness because seating was found to be not significantly different from that with zinc phosphate cement.

Margin design for esthetic posterior metal ceramic crowns

Variations in margin designs for ceramic metal restorations have been used to fulfill the many requirements for such restorations. The trend in our population toward greater esthetic awareness places increased emphasis on esthetics. This trend necessitates a high level of esthetics in posterior restorations. Commonly used posterior margin designs have been described and their inherent weaknesses and strengths have been reviewed. An alternative design has been proposed with its advantages in esthetics, tissue compatibility, cleansibility, and versatility. The clinical and laboratory procedures for this design have been described.

The porcelain butt margin with hydrocolloid impression technique

After preliminary scientific data, the dentist commonly evaluates the success of a recommended clinical technique. However, the accuracy of a restoration constructed in the laboratory depends directly upon the abilities of the laboratory technician. The communication between the dentist and the laboratory technician is crucial for success. This study examined the practicality of the porcelain/butt margin. When combined with the appropriate tissue management, sulcular placement, and impressions, this consistent procedure was easily evaluated, financially sound, and time-conserving.

An analysis of margin configurations for metal-ceramic crowns

Major types of margin configurations for ceramometal crowns are reviewed. This is one of the most commonly misunderstood and poorly practiced aspects of fixed prosthodontics. The resultant underprepared teeth produce discolored gingiva and chalky crowns.

The effect of proximal cavity design on the cementation of Class II inlays

This article indicates that the gingival opening of a two-surface cast restoration after cementation is minimized by the use of a preparation with proximal grooves.

A mathematical treatise on the fit of crown castings

A mathematical analysis was given to calculate the marginal cement thickness, Dm, of a cemented crown casting as a function of the minimum cement film thickness, F; the width, W, of the base of the tapered region; the width, W', at the bevel (if present) of the tooth preparation; the taper angle, theta; the marginal angle, beta; and the compensatory expansion, X, of the metal investment system: Dm = [F/sin theta - 0.5(X - 1) (W cot theta - W' cot beta)] sin beta. The elevation of the cemented casting, delta H, above the tooth preparation may also be obtained from this equation by substituting beta = 90 degrees, in which case Dm = delta H. The effect of the preparation of a chamfer margin or bevel on the marginal film thickness may be calculated from this equation. An additional analysis showed that the minimum marginal cement thickness, Dm = F, can be obtained for a shoulder preparation and X values of common metal-investment systems by applying a spacer of appropriate thickness on the entire occlusal and axial walls of the die. This renders the preparation of bevels superfluous. The thickness of the spacer should allow for the cement film thickness, roughness of the tooth and casting surfaces, dimensional inaccuracies of the die, and distortions of the wax pattern. If the spacer is thicker than required, a minimum value of Dm is still obtained for preparations without bevels. If the horizontal part of the die shoulder is covered as well with the spacer, a greater spacer thickness is required to obtain a minimum value of Dm than if the spacer extends to the end of the axial walls. In this case the spacer thickness should be greater on the occlusal die wall than on other regions of the die for values of X smaller than 1. The spacer should also be thicker in the occlusal region if restriction of the investment setting expansion by the wax pattern occurs mainly in this region.

Effect of internal relief, vibration, and venting on the vertical seating of cemented crowns

The presence of a vent hole in the occlusal surface, vibration during seating, and internal relief inside cast crowns were investigated individually and in all combinations for their effect on vertical seating on metal dies, using zinc phosphate cement. It was found that: 1. Vibration, applied horizontally, had no effect by itself or any interaction with venting or internal crown relief. 2. Venting and internal relief alone effectively achieved good seating. 3. Venting and internal relief together were the most effective combinations. 4. When crowns were seated well, they tended to tilt less.

The effect of various finish line preparations on the marginal seal and occlusal seat of full crown preparations

The influences of the marginal design of a full crown on the occlusal seat and marginal seal of a cemented full crown restoration was examined. Under the conditions of the study, the featheredge and parallel bevel preparations demonstrated the best marginal seal, followed in order by the full shoulder, 45-degree shoulder, and finally the 90-degree shoulders with 30-degree and 45-degree bevels. With regard to seating of the restoration, the 90-degree full shoulder demonstrated the best seat, followed in order by the 45-degree shoulder, 90-degree shoulder with 45-degree bevel, featheredge, 90-degree shoulder with 30-degree bevel, chamfer with parallel bevel, and finally 90-degree shoulder with parallel bevel.