I AM BACK

After all this month I am officialy back!!! Sorry for this silence but I had to study really A LOT! For all this time I have been studying anatomy and this post is going to be just about it :P I believe that this subject is extremely important because without human anatomy no medicine studies can be started including dentistry.
I really like anatomy and I believe that a good specialist (even if he will only work in the mouth) must have a big variety of different skills as well as knowledge, so I decided to do an additional work, so called preparation. With a student from another group we prepared the left leg. Firstly, we peeled (don't know if it's the right word to use though :D)  the skin and took off all the fats from the muscles and other areas. Here you can see the result:

This is the back of the left leg and the musles (starting from the bottom and going up): Biceps Femoris muscle, Semitendinosus muscle, Semimembranosus muscle;

The biceps femoris muscle is lateral in the posterior compartment of thigh and has two heads:

-The long head originates with the semitendinosus muscle from the inferomedial part of the upper area of the ischial tuberosity. 

-The short head arises from the lateral lip of the linea aspera on the shalf of the femur

The muscle belly of the long head crosses the posterior thigh obliquely from medial to lateral and is joined by the short head distally. Together, fibers from the two heads form a tendon, which is palpable on the lateral side of the distal thigh. The main part of the tendon inserts into the lateral surface of the head of the fibula. Extensions from the tendon blend with the fibular collateral ligament and with ligaments associated with the lateral side of the knee joint. The biceps femoris flexes the leg at the knee joint. The long head also extends and laterally rotates the hip. When the knee is partly flexed, the biceps femoris can laterally rotate the leg at the knee joint. The long head is innervated by the tibial division of the sciatic nerve and the short head is innervated ny the common fibular division of the sciatic nerve.

The semitendinosus muscle is medial to the biceps femoris muscle in the posterior compartment of thigh. It originates with the long head of the biceps femoris muscle from the inferomedial part of the upper area of the ischial tuberosity. The spindle-shaped muscle belly ends in the lower half of the thigh and forms a long cord-like tendon, which lies on the semimembranosus muscle and descends to the knee. The tendon curves around the medial condyle of the tibia and inserts into the medial surface of the tibia just posterior to the tendons of the gracilis and sartorius muscles. The semitendinosus flexes the leg at the knee joint and extends the thigh at the hip joint. Working with the semimembranosus, it also medially rotates the thigh at the hip joint and medially rotates the leg at the knee joint. The semitendinosus muscle is innervated by the tibial division of the sciatic nerve.

The semimembranosus muscle lies deep to the semitendinosus muscle in the posterior compartment of thigh. It also attached above to the superolateral impression on the ischial tuberosity and below mainly to the groove and adjacent bone on the medial and posterior surfaces of the medial tibial condyle. Expansions from the tendon also insert into and contribute to the formation of ligaments and fascia around the knee joint. The semimembranosus flexes the leg at the knee joint and extends the thigh at the hip joint. Working with the semitendinosus muscle, it medially rotates the thigh at the hip joint and the leg at the knee joint. The semimembranosus muscle is innervated by the tibial division of the sciatic nerve. 

Nerves of the leg: the posterior cutaneous nerve and the sciatic nerve which divides into to branches 1)The common fibular nerve 2) The tibial nerve

The sciatic nerve is the largest nerve of the body and carries contributions from L4 to S3. It leaves the pelvis through the greater sciatic foramen inferior to the piriformis muscle, enters and passes through the gluteal region and then enters the posterior compartment of the thigh where it divides into two major branches: 

1. The common fibular nerve

2. The tibial nerve

Posterior divisions of L4 to S2 are carried in the common fibular part of the nerve and the anterior divisions of L4 to S3 are carried in the tibial part. The sciatic nerve innervates:

- All muscles in the posterior compartment of the thigh

-The part of adductor magnus originating from the ischium

-All muscles in the leg and foot

-Skin on the lateral side of the leg and the lateral side and sole of the foot

The posterior cutaneous nerve of thigh is formed by contributions from S1 to S3 and leaves the pelvic cavity through the greater sciatic foramen inferior to the piriformis muscle. It passes vertically through the gluteal region deep to the gluteus maximus and enters the posterior thigh and innervates:

-A longitudinal band of the skin over the posterior aspect of the thigh that continues into the upper leg

-Skin over the gluteal fold, over the upper medial part of the thigh and in the adjacent regions of the perineum

The popliteal fossa is an important area of transition between the thigh and leg and is the major route by which structures pass from one region to the other.  The popliteal fossa is a diamond-shaped space behind the knee joint formed between muscles in the posterior compartments of thigh and leg. 

The margins of the upper part of the diamond are formed medially by the distal ends of the semitendinosus and semimembranosus muscles and laterally by the distal end of the biceps femoris muscle;

The margins of the smaller lower part of the space are formed medially by the medial head of the gastrocnemius muscle and laterally by the the plantaris muscle and the lateral head of the gastrocnemius muscle;

The floor of the fossa is formed by the capsule of the knee joint and adjacent surfaces of the femur and tibia, and, more inferiorly, by the popliteus muscle;

The roof is formed by the deep fascia, which is continuous above with tha fascia lata of the thigh and below with deep fascia of the leg. 

The major contents of the popliteal fossa are the popliteal artery, the popliteal vein, and the tibial and common fibular nerves

The gastrocnemius muscle

The soleus muscle

The calcaneal tendon (Achilles tendon)

The gastrocnemius muscle is the most superficial of the muscles in the posterior compartment and is one of the largest muscles in the leg. It originates from two heads, one lateral and one medial:

-The medial head is attached to an elongate roughening on the posterior aspect of the distal femur just behind the adductor tubercle and above the articular surface of the medial condyle;

-The lateral head originates from a distinct facet on the upper lateral surface of the lateral femoral condyle where it joins the lateral supracondylar line.

At the knee, the facing margins of the two heads of the gastrocnemius form the lateral and medial borders of the lower end of the popliteal fossa. In the upper leg, the heads of the gastrocnemius combine to form a single elongate muscle belly, which forms much of the soft tissue bulge identified as the calf. In the lower leg, the muscle fibers of the gastrocnemius converge with those of the deeper soleus muscle to form the calcaneal tendon, which attaches to the calcaneus (heel) of the foot. The gastrocnemius plantarflexes the foot at the ankle joint and can also flex the leg at the knee joint. It is innervated by the tibial nerve.

The soleus is a large flat muscle under the gastrocnemius muscle. It is attached to the proximal ends of the fibula and tibia, and to a tendinous ligament, which spans the distance between the two heads of attachment to the fibula and tibia:

-On the proximal end of the fibula, the soleus originates from the posterior aspect of the head and adjacent surface of the neck and upper shaft of the fibula;

-On the tibia, the soleus originates from the soleal line and adjacent medial border;

-The ligament, which spans the distance between the attachments to the tibia and fibula, arches over the popliteal vessels and tibial nerve as they pass from the popliteal fossa into the deep region the posterior compartment of leg

In the lower leg, the soleus muscle narrows to join the calcaneal tendon that attaches to the calcaneus. The soleus muscle, together with the gastrocnemius and plantaris, plantarflexes the foot at the ankle joint. It is innervated by the tibial nerve. 

How did you like the anatomy? Have you ever done something like this? Let me now!! ;)

Prosthodontics: The Metal - Ceramic Crown Preparation PART 2

Hello!!

My studies have started and I am having anatomy this month, so you probably know how much to learn I have right now because as far as I saw on Instagram most of you are dentistry students or doctors... Anyway, I think quality beats quantity haha 

This is the second part of "The Metal-Ceramix Crown Preparation". I am also planning to do the third one with short summary of part 1 and part 2 + videos. 

Axial reduction of the proximal and lingual surface

Sufficient tooth structure must be removed to provide a distinct, smooth chamfer of about 0.5 mm width.

M and N: Proximal reduction. O:Placing a 0.5-mm lingual chamfer. 

A football-shaped diamond is recommended for lingual reduction of anterior teeth.

6. Reduce the proximoaxial and linguoaxial surfaces with the diamond held parallel to the intended path of placment of the restoration. These walls should converge slightly from cervical to incisal or occlusal. A taper of approximately 6 degrees, as measured as the angle between opposing axial walls, is recommended. On anterior teeth, a lingual concavity is prepared for adequate clearance for the restorative materials. Typically, 1 mm is required if the centric contacts in the completed restoration are to be located on metal. When contact is on porcelain, additional reduction is necessary. For anterior teeth, usually only one groove is placed, in the center of the lingual surface. For molars, three grooves can be placed in a manner similar to that described for the all-metal complete cast crown.

7. Make a lingual alignment groove by positioning the diamond parallel to the cervical plane of the facial reduction. When the round-tipped diamond of appropriate size and shape is aligned properly, it is submerged almost halfway into the tooth structure. Verify the alignment of the groove, and carry the axial reduction from the groove along the lingual surface into the proximal; maintain the originally selected alignment of the diamond at all times.

8. As the lingual chamfer is developed, extend it buccally into the proximal to blend with the interproximal shoulder placed earlier.

A lingual chamfer is prepared to allow adequate space for metal. A smooth transition from interproximal shoulder to chamfer is essential.

Alternatively, a facial approach may be used. Although this is slightly more difficult initially, after some practice it should be easy to eliminate the lingual guiding groove and to perform the proximal and lingual axial reduction in one step; however, this requires that the diamond be held freehand parallel to the path of placement. The proximal flange that resulted from the shoulder preparation can be used as a reference for judging alignment of the rotary instrument. 

A: Proximal reduction of the flange with a facial approach. B: One sufficient tooth structure has been removed, the cervical chamfer is prepared simultaneously with the lingual axial surface. After distolingual preparation had been completed, the mesial chamfer is blended into a smooth transition with the shoulder.

The interproximal margin should not be inadvertently placed too far gingivally and thereby infringe on the attachment apparatus. It must follow the soft tissue contour. On posterior teeth, the lingual wall reduction blends into the functional cusp bevel placed during the occlusal reduction. Anterior teeth require an additional step: After preparation of the cingulum wall, one or more depth grooves are placed in the lingual surface. These are approximately 1 mm deep.

9. Use a football-shaped diamond to reduce the lingual surface of anterior teeth. It is helpful to stop when half this reduction has been completed, to evaluate clearance in maximum intercuspation and all excursions. The remaining intact tooth structure can serve as a reference.

Finishing

The margin must provide distinct resistance to vertical displacement of an explorer tip, and it must be

smooth and continuous circumferentially. (A properly finished margin should feel like smooth glass slab.) All other line angles should be rounded, and the completed preparation should have a satin finish free from obvious diamond scratch marks. Tissue displacement is particularly helpful when subgingival margins are being finished. Sometimes this step is postponed until just before impression making after tissue displacement.

Controlled tissue displacement can be helpful when finishing the margin with a fine-grit diamond or another rotary instrument.

10. Finish the margins with diamonds, hand instruments, such as the off-angle hatchets or carbides. 

All internal line angles should be rounded to facilitate the impression-making and die –pouring steps. 

The finishing steps for the facial margin depend on the design of margin chosen 

A: completed preparation. Note that the transition from incisal to axial walls is rounded, and a distinct 90-degree or slightly sloping shoulder had been established. B: An even chamfer width and a smooth transition between lingual and axial surfaces. The chamfer is distinct and blends smoothly into the facial shoulder.

 A porcelain labial margin requires proper support for the porcelain. A shoulder with a 90-degree cavosurface angle is recommended. This type of shoulder can also be used for a crown with a conventional metal collar and offers the advantage of allowing the collar to be kept narrow. However, there is then the risk of leaving unsupported enamel. For this reason, the margin is often beveled or sloped to create a more obtuse cavosurface angle. A flat-ended diamond in a low-speed handpiece creates the 90-degree shoulder. Any unsupported enamel must be removed subsequently by careful planing with a sharp chisel. Care must also be taken to orient the rotary instrument as it moves around the tooth if inadvertent undercuts are to be avoided. When a metal-collar design of ceramic restoration is planned, the need for a 90-degree shoulder is less crucial. A sloping shoulder has been advocated to ensure the elimination of unsupported enamel and to minimize marginal gap width. Such a shoulder (cavosurface angle of about 120 degrees) can be accomplished with a flat-ended

diamond by changing its alignment, with particular attention to the configuration of the tooth structure cervical to the margin. Alternatively, a hatchet can be used to plane the margin to the correct angulation. Again, be careful to avoid undercutting the axial wall of the preparation where it meets the shoulder during finishing. A shoulder-bevel margin is most effectively achieved with a flame-shaped carbide bur or hand instrument, depending on the length of bevel required. 

The shoulder bevel.

In general, a short bevel with a cavosurface angle of 135 degrees is advocated, although longer bevels have been recommended for improved marginal fit. Special care must be exerted where the bevel meets the interproximal chamfer. The chamfer and bevel should be continuous with each other. Care must be taken not to damage the epithelial attachment during beveling; tissue displace ment before preparation of subgingival bevels is recommended.

11. After a satisfactory facial margin has been obtained, round all sharp line angles within the preparation.

S: Finishing the preparation with a fine-grit diamond. T: The completed preparation.

 This facilitates surface wetting and expedites subsequent procedures (impression making, pouring of casts, waxing, and investing). A fine-grit diamond operating at low speed is particularly useful. However, where access allows, a slightly larger tapered diamond may be preferred because the greater diameter of its tip prevents "ditching" of the chamfer. Blend all surfaces together, and remove any sharp transitions.

Facial (A) and lingual (B) views of metal-ceramic preparations.

Evaluation

Areas often missed during finishing are the incisal edges of anterior preparations and the transition from occlusal to axial wall of posterior preparations. The completed chamfer should provide 0.5 mm of space for the restoration at the margin. The chamfer must be smooth and continuous, and when it is evaluated, a distinct resistance to vertical displacement of the tip of an explorer or periodontal probe should be felt. The chamfer should be continuous with the interproximal shoulder or beveled shoulder. The cavosurface angle of the chamfer should be slightly obtuse or 90 degrees. Under no circumstances should any unsupported tooth structure remain, especially at the facial margin. Care is also needed to avoid creating an undercut between the facial and lingual walls. This aspect of the preparation should be thoroughly evaluated. Excessive convergence should also be avoided, because this may lead to pulpal exposure. All residual debris is removed with thorough irrigation. 

Metal-ceramic crowns used to restore maxillary incisor teeth.

A: Metal-ceramic preparations on the maxillary premolars in conjunction with more conservative preparations on the molars. B: Buccal view of the preparations. Note that, by comparison, considerable tooth reduction was needed on the premolars to accomodate metal-ceramic restorations. C: Except for the molars, all remaining teeth in this patient have been prepared for metal-ceramic restorations. Note that subtle variations and modifications of the same underlying theme: wing - type preparations on the anterior teeth, wingless on the premolars. D: Mandibular arch of the same patient. Many of the smaller mandibular teeth were prepared with wingless restorations. Because of previously existing restorations, excessively heavy shoulder - like chamfers resulted on some of the posterior teeth.

Prosthodontics: The Metal-Ceramic Crown Preparation PART 1

In many dental practices, the metal-ceramic crown is one of the most widely used fixed restorations. This has resulted in part from technologic improvements in the fabrication of this restoration by dental laboratories and in part from the growing amount of cosmetic demands that challenge dentists today. 

The restoration consists of a complete-coverage cast metal crown (or substructure) that is veneered with a layer of fused porcelain to mimic the appearance of a natural tooth. 

To be successful, a metal-ceramic crown preparation requires considerable tooth reduction wherever the metal substructure is to be veneered with dental porcelain. Only with sufficient thickness can the darker color of the metal substructure be masked and the veneer duplicate the appearance of a natural tooth. The porcelain veneer must have a certain minimum thickness for esthetics. Consequently, much tooth reduction is necessary, and the metal-ceramic preparation is one of the least conservative of tooth structures. 

1. Recommended minimum dimensions for a metal-ceramic restoration on an anterior tooth (A) and a posterior tooth (B). Note that the significant reduction needed compared to that for a complete cast or partial veneer crown.

Historically, attempts to veneer metal restorations with porcelain had several problems. A major challenge was the development of an alloy and a ceramic material with compatible physical properties that would provide adequate bond strength. In addition, it was initially difficult to obtain a natural appearance.

The technical aspects of the fabrication of this restoration will be discussed in later posts. For now, only a brief description is provided. The metal substructure is waxed and then cast in a special metal-ceramic alloy that has a higher fusing range and a lower thermal expansion than do conventional gold alloys. After preparatory finishing procedures, this substructure, or framework, is veneered with dental porcelain. The porcelain is fused onto the framework in much the same manner as household articles are enameled. Modern dental porcelains fuse at a temperature of about 960° C (1760° F). Because conventional gold alloys would melt at this temperature, the special alloys are necessary.

INDICATIONS

The metal-ceramic crown is indicated on teeth that require complete coverage and for which significant esthetic demands are placed on the dentist (e.g., the anterior teeth). It should be recognized, however, that, if esthetic considerations are paramount, an all-ceramic crown has distinct cosmetic advantages over the metal-ceramic restoration; nevertheless, the metal-ceramic crown is more durable than the all-ceramic crown and generally has superior marginal fit. Furthermore, it can serve as a retainer for a fixed dental prosthesis because its metal substructure can accommodate cast or soldered connectors. Whereas the all-ceramic restoration cannot accommodate a rest for a removable prosthesis, the metal-ceramic crown may be successfully modified to incorporate occlusal and cingulum rests as well as milled proximal and reciprocal guide planes in its metal substructure.

Typical indications are similar to those for all-metal complete crowns: extensive tooth destruction as a result of caries, trauma, or existing previous restorations that precludes the use of a more conservative restoration; the need for superior retention and strength; an endodontically treated tooth in conjunction with a suitable supporting structure (a post and core); and the need to recontour axial surfaces or correct minor malinclinations. Within certain limits, this restoration can also be used to correct the occlusal plane.

CONTRAINDICATIONS

Contraindications for the metal-ceramic crown, as for all fixed restorations, include patients with active caries or untreated periodontal disease. In young patients with large pulp chambers, the metal-ceramic crown is also contraindicated because of the high risk of pulp exposure. If at all possible, a more conservative restorative option such as a composite resin or porcelain laminate veneer or an all-ceramic crown with less reduction is preferred.

A metal-ceramic restoration should not be considered whenever a more conservative retainer is feasible, unless maximum retention is needed, as for a long-span fixed dental prosthesis. If the facial wall is intact, the practitioner should decide whether it is truly necessary to involve all axial surfaces of the tooth in the proposed restoration. Although perhaps technically more demanding and time consuming, a more conservative solution that satisfies the patient’s needs and may provide superior long-term service can usually be found.

ADVANTAGES

The metal-ceramic restoration combines, to a large degree, the strength of cast metal with the esthetics of an all-ceramic crown. The underlying principle is to reinforce a brittle, more cosmetically pleasing material through support derived from the stronger metal substructure. Natural appearance can be closely matched by good technique and, if desired, through characterization of the restoration with internally or externally applied stains. Retentive qualities are excellent because all axial walls are included in the preparation, and it is usually quite easy to ensure adequate resistance form during tooth preparation. The complete-coverage aspect of the restoration permits easy correction of axial form. In addition, the required preparation is often much less demanding than for partial-coverage retainers. In general, the degree of difficulty of a metal-ceramic preparation is comparable to that of preparing a posterior tooth for a complete cast crown.

DISADVANTAGES

The preparation for a metal-ceramic crown requires significant tooth reduction to provide sufficient space for the restorative materials. To achieve better esthetics, the facial margin of an anterior restoration is often placed subgingivally, which increases the potential for periodontal disease. However, a supragingival margin can be used if significant cosmetic concerns do not preclude it or if the restoration incorporates a porcelain labial margin.

In comparison with an all-ceramic restoration, the metal-ceramic crown may have slightly inferior esthetics, but it can be used in higher stress situations or on teeth that would not provide adequate support for an all-ceramic restoration.

Because of the glasslike nature of the veneering material, a metal-ceramic crown is subject to brittle fracture (although such failure can usually be attributed to poor design or fabrication of the restoration). A frequent problem is the difficulty of accurate shade selection and of communicating it to the dental ceramist. This is often underestimated by the novice. Because many procedural steps are required for both metal casting and porcelain application, laboratory costs generally render the metal-ceramic restoration among the more expensive of dental procedures.

PREPARATION

The recommended sequence of preparation is illustrated for a maxillary right central incisor;

2. Preparation of a maxillary incisor for a metal-ceramic crown. A, Heavily restored maxillary central incisor. B and C, Rotary instrument aligned with the cervical one third and incisal two thirds to gauge correct planes of reduction. D and E, Guiding grooves placed in the two planes. The cervical groove is made parallel to the path of placement, which usually coincides with the long axis of the tooth. The secondary facial depth groove is prepared parallel to the facial contour of the tooth. F and G, Incisal guiding grooves are placed. H, Incisal edge reduction. I to K, Facial reduction accomplished in two planes. L, Breaking proximal contact, maintaining a lip of enamel to protect the adjacent tooth from inadvertent damage. M and N, Proximal reduction. O, Placing a 0.5-mm lingual chamfer.

 however, the same step-by-step approach can be applied to other teeth. 

3. Preparation of a maxillary premolar for a metal-ceramic crown. A, Depth holes. B, Occlusal depth cuts. C, Completed occlusal reduction. D and E, Lingual chamfer and facial shoulder are prepared on half the tooth (lingual view [D] and facial view [E]. F, Completed preparation.

As with all tooth preparations, a systematic and organized approach to tooth reduction saves time.

Armamentarium

The instruments needed to prepare teeth for a metal-ceramic crown include: 

• Round-tipped rotary diamonds (regular grit for bulk reduction, fine grit for finishing) or carbides 
• Football- or wheel-shaped diamond (for lingual reduction of anterior teeth)
• Flat-ended, tapered diamond (for shoulder preparation)
• Finishing stones
• Explorer and periodontal probe
• Off-angle hatchets (B to D)

4. Armamentarium for the metal-ceramic crown preparation. A, Diamond rotary instrument. B to D, Off-angle hatchets. These are useful for smoothing the shoulder margins of metal-ceramic crown preparations.

The actual sequence of steps can be varied slightly, depending on operator preference.

Step-By-Step Procedure

The preparation is divided into five major steps: guiding grooves, incisal or occlusal reduction, labial or buccal reduction in the area to be veneered with porcelain, axial reduction of the proximal and lingual surfaces, and final finishing of all prepared surfaces.

Guiding grooves

1. Place three depth grooves, one in the center of the facial surface and one each in the approximate locations of the mesiofacial and distofacial line angles ( Fig. 2 A to E).

5. Depth grooves in the facial wall are placed in two directions: incisally, parallel to the tooth contour, and cervically, parallel to the path of placment. The grooves should be 1.3 mm deep.

These are in two planes: the cervical portion to parallel the long axis of the tooth and the incisal (occlusal) portion to follow the normal facial contour (Fig. 2 D and E).

2. Perform the facial reduction in the cervical and incisal planes. The cervical plane determines the path of placement of the completed restoration. The incisal or occlusal plane provides the space needed for the porcelain veneer; it should be approximately 1.3 mm deep to allow for additional reduction during finishing. The secondary facial grooves usually extend halfway down the facial surface, although (depending on the shape of the tooth) they may extend to include the incisal two thirds. Cervical grooves are generally made parallel to the long axis of the tooth. However, they can be adjusted slightly to create a more desirable path of placement; in particular, some labial inclination improves retention on a tooth with little cingulum height. On small teeth, it may be advisable to keep the cervical grooves somewhat shallower near the margin.

3. Place three depth grooves (about 1.8 mm deep) in the incisal edge of an anterior tooth. These provide the needed reduction of 2 mm and allow finishing (Fig. 2 F and G). Verify the depth of these grooves with a periodontal probe. On posterior teeth on which the occlusion is to be established in porcelain, 2 mm of clearance must exist. If the occlusion is to be established in the metal, the same minimum clearances are needed as for a complete cast crown. Posterior occlusal reduction incorporates a functional cusp bevel on the lingual cusp, similar to that for a complete cast crown. When the diamond is initially positioned for anterior teeth, it may be helpful to observe the long axis of the opposing tooth in maximum intercuspation and to orient the instrument perpendicular to that. 

6. A: depth grooves 1.8 mm deep placed in the incisal edges to ensure adequate and even reducton. B:  incisal reduction completed on the left central and lateral incisors. Note that angulation of the diamond, perpendicular to the direction of loading by the mandibular anterior teeth.

The grooves must not be too deep; otherwise, an overreduced and undulating surface results. 

Incisal (occlusal) reduction

The completed reduction of the incisal edge on an anterior tooth should allow 2 mm for adequate material thickness to permit translucency in the completed restoration. Posterior teeth generally require less reduction (1.5 mm) because esthetics is not as critical. Caution must be used, however, because excessive occlusal reduction shortens the axial walls and thus is a common cause of inadequate retention and resistance form in the completed preparation. This can be particularly problematic on the anterior teeth (on which, as a consequence of tooth form, most of the retention is derived from proximal walls). 

4. Remove the islands of remaining tooth structure. On anterior teeth, access is usually unrestricted, and the thickest portion of the cutting instrument can be used to maximize cutting efficiency (Fig. 2 H). On the posterior teeth, the same protocol is followed as in preparing depth grooves for a complete cast functional. This includes the use of a functional cusp bevel, although additional occlusal reduction is needed where the porcelain is to be applied (Fig. 3 A to C).

Labial (buccal) reduction

When completed, the reduction of the facial surface should have produced sufficient space to accommmodate the metal substructure and porcelain veneer. A minimum of 1.2 mm is necessary for the ceramist to produce a restoration with satisfactory appearance (1.5 mm is preferable). This requires significant tooth reduction. For comparison, the cervical diameter of a maxillary central incisor averages between 6 and 7 mm. 

In the cervical area of small teeth, obtaining optimal reduction is not always feasible. A compromise is often made with lesser reduction in the area where the cervical shoulder margin is prepared. 

5. Remove the remaining tooth structure between depth grooves (Fig. 2 I to L), creating a shoulder at the cervical margin. 

7. The cervical shoulder is established as the tooth structure between the depth grooves is removed. The rotary instrument is moved parallel to the intended path of placement during this procedure.

8. Fig. left: The facial reduction should be completed in two phases; initially, one half is maintained intact for assessment of the adequacy of reduction. Note the two distinct planes of reduction on the facial. Fig. right: Facial reduction completed. A 6-degree taper has been established between the proximal walls.

If a restoration with a narrow subgingival metal collar is to be fabricated and sufficient sulcular depth is present, place the shoulder approximately 0.5 mm apical to the crest of the free gingiva at this time. Additional finishing then results in a margin that is 0.75 to 1 mm subgingival. Use adequate water spray during the entire phase of preparation, because a significant amount of tooth structure is being removed and copious irrigation (along with intermittent strokes) expedites the preparation process. Such a cautious approach prevents unnecessary trauma to the pulp. The resulting shoulder should be approximately 1 mm wide and should extend well into the proximal embrasures when viewed from the incisal (occlusal) side. 

9. A: the facial shoulder preparation should wrap around into the interproximal embrasure and extend at least 1 mm lingual to the promixal contact. B: the shoulder preparation extends adequately to the lingual side of the proximal contact. Note that on the mesial (visible) side, the preparation extends slightly farther than on the distal (cosmetically less critical) side. 

Where access allows, establishing this shoulder from the proximal gingival crest toward the middle of the facial wall is preferred. This minimizes placement of the initial shoulder preparation too close to the epithelial attachment. If the margin is established from facial to proximal, a tendency exists to "bury"the instrument and encroach on the epithelial attachment. Proper margin position must be maintained in relation to the crest of the free gingiva. The location and specific configuration of the facial margin depend on several factors: the type of metal-ceramic restoration selected, the cosmetic expectations of the patient, and operator preference.

From a periodontal point of view, a supragingival margin is always preferred. Its application is restricted, however, because patients often object to a visible metal collar or discolored root surface. Such objections are common, even when the gingival margin is not visible during normal function, as in patients with a low lip line, and generally limit the use of supragingival margins to posterior teeth and to undiscolored anterior teeth (in which case a porcelain labial margin is preferred).

10. Supragingival margins on the maxillary premolars. They were possible because of a favorable lip line hiding the cervical aspect of these posterior teeth. The subgingival margins on the mandibular premolars were prepared only because of previously existing restorations. 

The optimum location of the margin should be carefully determined with the full cooperation of the patient. Where a subgingival margin is to be placed, careful tissue manipulation is essential; otherwise, there will be damage that leads to permanent gingival recession and subsequent exposure of the metal collar. This is most effectively avoided through meticulous gingival displacement with cord before finishing. 

11. Gingival displacement cord (under tension) is placed in the interproximal sulcus.

The configuration of the margin is also finalized at this time.

12. A: after tissue displacement, the facial margin is extended apically. Caution is needed, because if the diamond inadvertently grabs the cord, it may be ripped out of the sulcus and traumatize the epithelial attachment. B: note the additional apical extension of the shoulder on the distal aspect. C: the entire facial shoulder is placed at a level that will be subgingival after the tissue rebounds. D: the facial margin has been  prepared to the level of the previously placed cord.

References:

• All information is taken from "Contemporary fixed prosthodontics" Fourth Edition by Stephen F. Rosenstiel BDS MSD, Martin F. Land DDS MSD, Junhei Fujimoto DDS MSD DDSc
• http://akshaylaserdentalcare.com/wp-content/uploads/2014/04/metal-ceramic-1024x995.jpg
• http://www.glidewelldental.com/images/dentist/chairside/V7-1/articles/photo-essay-anterior-bruxzir/lightbox/fig_22.jpg