Geach Dental

  • How Your Health Begins in the Mouth” Podcast with Dr. Adam Geach

Correcting The Black Border At The Gumline Of Crowns – Before And After

undefined

Dental crowns, commonly known as dental caps, are invaluable  restorations for rebuilding damaged or decayed teeth. These restorations offer both strength and aesthetic benefits, making them a popular choice among patients seeking to regain their confident smiles. However, in some cases, individuals may be left with a “black border” at the gumline of their crowns.

This disheartening occurrence manifests as a noticeable dark line where the crown meets the gum tissue. In our comprehensive guide, we will explore the reasons behind this unsettling phenomenon and shed light on the various professional prosthodontic solutions to rectify this problem. Akin to flipping a before-and-after switch, the transformation achieved by addressing black borders can be remarkable.

A black border at the gumline does not merely affect the visual appeal of your smile; it can also signify underlying dental concerns. Neglecting this issue may lead to gum health problems and jeopardize the longevity of your dental restoration. It is essential to understand the root causes, the diagnostic process, and the treatment options available to correct black borders.

Join us as we explore the intricacies of black borders on dental crowns and explore real-life, transformative before-and-after scenarios guided by the expertise of our prosthodontist. Read on to learn more!

Causes of Black Borders

Poor Marginal Fit

undefined
  • Patient-specific Fitting: Each patient’s dental anatomy is unique. If a crown is not custom-fitted accurately to the patient’s tooth, it can create gaps where bacteria can infiltrate. Over time, this poor marginal fit can lead to the development of a black border. See the black border in the picture above. 
undefined
  • Inadequate Restoration: Insufficiently sealed margins between the crown and the tooth can allow oral fluids and debris to penetrate, causing discoloration and decay. Ensuring a precise, snug fit is essential to prevent black borders. 

Gum Recession

undefined
  • Natural Aging Process: As patients age, gum recession is a common occurrence. Receding gums expose the underlying tooth structure and the crown’s margin, which can create the appearance of a black border. See picture above. 
  • Periodontal Conditions: Certain gum diseases and conditions can accelerate gum recession. Patients with periodontal issues are more susceptible to black borders around their crowns.

Aging Crowns

  • Wear and Tear: Older dental  crowns can deteriorate over time due to wear and  chipping. As the crown ages, the materials used may degrade, leading to the emergence of black borders.
  • Outdated Materials: Older crown materials may be more prone to developing black borders. This is due to the metal that shows at the border of the crown at the gum line. We no longer use dark metals in crown restorations, which greatly improves the dental aesthetics of the crown. 

Understanding these causes is the first step towards effectively addressing and resolving black border issues in dental crown patients.

Diagnosis and Evaluation

Clinical Examination

  • Visual Inspection: A skilled prosthodontist conducts a thorough visual examination of the patient’s oral cavity. This examination includes scrutinizing the crown’s margin, gumline, and adjacent teeth to identify any black border or related issues.
  • Checking Fit: The prosthodontist may use an explorer to assess the crown’s fit and check for any irregularities along the gumline. The periodontal health around the tooth will also be determined and periodontal probe will be used to check for any deep pockets. 

X-rays and Imaging

  • X-rays: Dental X-rays are essential for a comprehensive assessment. They provide insight into the crown’s fit. X-rays can show gaps or open margins where the crown meets the tooth. X-rays also show the health of the root and any periodontal bone loss.
  • Intraoral Scans: 3D intraoral scans are now used instead of uncomfortable impression materials. These scans create precise 3D images, allowing prosthodontists to visualize the shape of the crown and to evaluate the occlusion or biting surfaces.

Treatment Planning

  • Customized Solutions: Based on the clinical examination and imaging results, the prosthodontist develops a personalized treatment plan. This plan outlines the most suitable method to address the black border, taking into account the patient’s specific case.
  • Discussion with the Patient: The prosthodontist communicates the diagnosis and treatment options to the patient, ensuring they understand the recommended course of action and actively participate in decision-making.

A meticulous diagnosis and evaluation process is vital for successfully identifying the causes of black borders and determining the most effective treatment strategy for each patient.

Treatment Options

Crown Replacement

undefined
  • Customized Crowns: In cases where the existing crown is the root cause of the black border, prosthodontists may recommend a new, precisely fitted crown. This ensures a snug fit and eliminates the risk of future black borders. See case 1 above. 
  • Material Advancements: Modern crown materials offer superior aesthetics and longevity.  All ceramic crown materials such as zirconia are used, for natural-looking and durable results. Having an in-office laboratory technician, where our crowns are fabricated in our office allows us to achieve excellent aesthetic results that match your surrounding teeth. 

Gingival Reshaping

  • Surgical Procedure: Gingival reshaping, or gum contouring, involves a minor surgical procedure to adjust the gum line. By reshaping the gums to the appropriate level, we can create better harmony and improve the proper aesthetic tooth proportions. New crowns are often needed in conjunction with this surgical procedure to hide the black borders.  

Porcelain Veneers

  • Thin, Aesthetic Facings: Porcelain veneers are thin ceramic facings that are custom-made to cover the front surface of teeth. They can effectively mask black borders, providing a natural and aesthetically pleasing outcome.
  • Minimally Invasive: Veneers require less tooth reduction, helping to preserve the underlying tooth structure.

Composite Bonding

  • Resin Application: Composite bonding involves the application of tooth-colored resin to cover the black border area. This resin is skillfully shaped and polished to match the surrounding tooth structure.
  • Cost-Effective: Bonding is a cost-effective solution for concealing black borders and enhancing the crown’s appearance.

Prosthodontists employ these treatment options with precision and expertise, tailoring the approach to each patient’s unique circumstances and preferences. Treatment choice depends on factors such as the cause of the black border, the patient’s oral health, and their aesthetic goals.

Recovery and Aftercare

Post-Treatment Care

  • Follow Professional Guidance: After any dental procedure, it’s vital to adhere to the specific instructions provided by your prosthodontist. This may include dietary restrictions, oral hygiene practices, and medication as needed.
  • Monitor Symptoms: Pay attention to any symptoms such as lingering tooth sensitivity and report any concerns or complications to your prosthodontist promptly.

Long-Term Maintenance

  • Regular Dental Check-ups: Scheduling routine dental appointments is crucial for monitoring the health of your crown and gums. Your prosthodontist can detect any issues early and address them promptly.
  • Oral Hygiene: Maintain excellent oral hygiene by gentle brushing, flossing, and using mouthwash as your prosthodontist recommends. A clean and healthy mouth can reduce the risk of future black borders.
  • Avoiding Risk Factors: Be mindful of habits that can contribute to black borders, such as smoking or excessive force on your teeth. Minimizing these risk factors can help preserve your dental restoration.

By following a comprehensive post-treatment care plan and committing to long-term maintenance, you can ensure the longevity and aesthetics of your dental crown while preventing the recurrence of black borders. Your prosthodontist will guide you through the steps to keep your smile healthy and beautiful.

Summary

The presence of a black border around your dental crown can significantly mar your smile’s appearance and erode your confidence. Geach Dental, with two convenient locations in Downtown Los Angeles and Laguna Niguel, Orange County, specializes in resolving this issue. Our prosthodontic team will use advanced knowledge, specialized expertise, and cutting-edge technology to ensure you regain the attractive and confident look you deserve.

We are dedicated to providing every patient with tailored treatment in a comfortable environment. Our commitment includes comprehensive consultations to ensure you fully understand the procedure, its potential challenges, and the rewards it offers. Your satisfaction and comfort are our top priorities.

To take the first step toward eliminating the black border and achieving a healthier, more attractive smile, contact us today at (213) 622-3339  or click here to schedule a consultation. Geach Dental is your trusted partner on the journey to a more radiant and confident smile.

ABOUT THE AUTHOR: Meet Dr. Adam Geach, distinguished prosthodontic specialist and owner of Geach Dental. His dental education includes Harvard School of Dental Medicine and the University of Connecticut, where he earned a D.M.D. and an M.D.Sc. in Prosthodontics, respectively. Dr. Geach holds Diplomate status with the American Board of Prosthodontics, underscoring his exceptional expertise in dental care. 

At his clinics in Los Angeles and Laguna Niguel, Dr. Geach offers a wide range of services, from cosmetic treatments to full-mouth reconstructions, all delivered with a focus on personalized patient care.Take the first step toward your ideal smile and improved oral health by clicking here.

Search for Blog Articles

Categories

Pinteresting Board
Share

Meet Dr. Adam Geach, distinguished prosthodontics specialist and owner of  Geach Dental. 

His dental education includes Harvard School of Dental Medicine and the University of Connecticut, where he earned a D.M.D. and an M.D.Sc. in Prosthodontics, respectively. Dr. Geach holds Diplomate status with the American Board of Prosthodontics, underscoring his exceptional expertise in dental care. At his clinics in Los Angeles and Laguna Niguel, Dr. Geach offers a wide range of services, from cosmetic treatments to full-mouth reconstructions, all delivered with a focus on personalized patient care.Take the first step toward your ideal smile and improved oral health by clicking here.

Related blog posts

Zirconia Implants vs. Titanium: Health Benefits and Differences Explained

Dental implants have been a major advancement in replacing missing teeth. When placed correctly, titanium implants have incredibly high success rates and are the standard for tooth replacement. Dental implants can be used to replace a single tooth or can be strategically placed to support multiple missing teeth or a whole arch of teeth. The great benefit of dental implants is that they eliminate a removable denture and provide the function and appearance of natural teeth.  Over the past couple years, patients have become concerned about the idea of having metal titanium implants placed in their body. Some patients are not comfortable with the idea of having a “foreign metal object” placed into their jawbone. I can understand their concern especially if they are unsure of the science, materials used and potential risks. So much information is available on the internet and it is difficult to find answers to these questions. Because of this, some patients are more comfortable with the idea of non-metal zirconia implants as an alternative. So, are zirconia dental implants healthier than titanium dental implants?  Composition of titanium dental implants Titanium (Ti) is a metallic element with the atomic number 22 and has a silvery color. It is the ninth most abundant element in the earth’s crust. The alloy Ti-6Al-4V (Titanium, 6-aluminum, 4-vanadium), which was originally used for aerospace applications, is currently the most frequently used material for medical and dental implantation. Titanium alloys are widely used for orthopedic implantation (joint replacements) because of their lightweight, high biocompatibility and favorable strength/density ratios. Titanium alloys are superior to stainless steel and cobalt-chrome (Co-Cr) alloys with regard to biocompatibility and corrosion resistance.  Titanium plays a pivotal role for dental implants because the roughened surface of these implants attract bone cells to deposit bone and fill the space between the jawbone and the implant surface. This process is called osseointegration and is critical for implant stability. Titanium dental implants quickly form a layer of titanium dioxide on the surface and this layer forms a boundary between the metal surface and the body, preventing corrosion and deterioration of the material. This is the reason for the high biocompatibility.  Can titanium dental implants cause an allergic reaction?  Exposure to titanium from implants is common, but reports on clinical allergy are rare. Titanium dioxide on the surface of dental implants is more stable than pure titanium. It is possible that titanium ions could be released from the surface and sometimes into distant tissue. However, evidence for the possible toxic effects and risk of allergic reactions is weak (1). One study showed that titanium allergy can be detected in dental implant patients, even though its estimated prevalence is very low (0.6%) (2). It should be noted that no material, including any dental implant, can be considered universally biocompatible. As mentioned above, the Ti-6Al-4V alloy is the preferred choice for dental implants. Small amounts of other elements associated with titanium alloys can act as “impurities.” Titanium alloys can contain very small amounts of beryllium, cobalt, chromium, copper, iron, nickel and palladium. These impurities may be causative factors for different allergic reactions in patients with dental implants, but the likelihood is low.  It is therefore advisable to test for titanium allergy in patients with a history of multiple and/or severe allergies. These include allergy to jewelry, nickel and cobalt. Furthermore, patients with previous unexplained implant failure could be tested. In the rare situation that a patient is allergic to titanium, it is preferable to use alternative treatment options or to use an alternative dental implant material such as zirconia (3).  What are zirconia implants?  The idea of an alternative to titanium dental implants has been growing for years due to the questions regarding sensitivity to titanium that was discussed above. Another drawback of titanium is its grey color. When a titanium implant is placed in an area with very thin gingival tissue, there is a risk that a dark grey shadow may be visible.  Zirconium dioxide (zirconia) has been introduced as an alternative to titanium dioxide. Zirconia was first used for the fabrication of dental crowns and implant abutments. Currently, tetragonal zirconia poly-crystal is the ceramic of choice for dental implants. The white, opaque color of zirconia along with the good biocompatibility, favorable mechanical properties and low affinity for bacterial plaque, make it a good potential material for dental implants.  Are zirconia implants as successful as titanium implants?  Osseointegration is a major factor in the success of modern dental implants. Titanium remains the material of choice for obtaining and maintaining osseointegration. Early studies show that zirconia implants have similar 1 and 2 year survival rates when compared to titanium implants. More clinical long-term studies providing detailed information about implant failures, biological and technical complications, and prosthetic and esthetic outcomes are urgently needed (3). Potential implant abutment and crown complications Once a dental implant has integrated and healed with the jawbone, an implant abutment and crown are then made to complete the replacement of the tooth. The abutment and crown is held into the implant by a screw. These 3 components are shown in the picture below.   When titanium is used for the implant and abutment, this connection is incredibly strong and stable with very few mechanical complications from chewing. On the other hand, zirconia is a more brittle material compared to titanium. When zirconia is used for the implant and abutment, more mechanical complications (breakage of the abutment) have been reported, see picture below. This has been reported in studies and I have also experienced this in my practice. The challenge with zirconia is the strength and stability of the connection between the abutment and implant. So far, this connection is not as reliable as titanium. When implant abutments break, this expensive abutment and crown needs to be replaced and has the possibility of breaking again.   Conclusions References At Geach Dental in Downtown Los Angeles and Laguna Niguel, Orange County our team has extensive experience in dealing with dental implant issues. If you are

Read Up

The Ultimate Guide to Teeth Whitening

Explore the ultimate guide to successful teeth whitening, covering professional vs. at-home treatments, personalized plans, in-office procedures, take-home kits, and essential aftercare tips. Read on to discover the path to a brighter, healthier smile with our expert insights.

Read Up

Zirconia Implants vs. Titanium: Health Benefits and Differences Explained

Dental implants have been a major advancement in replacing missing teeth. When placed correctly, titanium implants have incredibly high success rates and are the standard for tooth replacement. Dental implants can be used to replace a single tooth or can be strategically placed to support multiple missing teeth or a whole arch of teeth. The great benefit of dental implants is that they eliminate a removable denture and provide the function and appearance of natural teeth.  Over the past couple years, patients have become concerned about the idea of having metal titanium implants placed in their body. Some patients are not comfortable with the idea of having a “foreign metal object” placed into their jawbone. I can understand their concern especially if they are unsure of the science, materials used and potential risks. So much information is available on the internet and it is difficult to find answers to these questions. Because of this, some patients are more comfortable with the idea of non-metal zirconia implants as an alternative. So, are zirconia dental implants healthier than titanium dental implants?  Composition of titanium dental implants Titanium (Ti) is a metallic element with the atomic number 22 and has a silvery color. It is the ninth most abundant element in the earth’s crust. The alloy Ti-6Al-4V (Titanium, 6-aluminum, 4-vanadium), which was originally used for aerospace applications, is currently the most frequently used material for medical and dental implantation. Titanium alloys are widely used for orthopedic implantation (joint replacements) because of their lightweight, high biocompatibility and favorable strength/density ratios. Titanium alloys are superior to stainless steel and cobalt-chrome (Co-Cr) alloys with regard to biocompatibility and corrosion resistance.  Titanium plays a pivotal role for dental implants because the roughened surface of these implants attract bone cells to deposit bone and fill the space between the jawbone and the implant surface. This process is called osseointegration and is critical for implant stability. Titanium dental implants quickly form a layer of titanium dioxide on the surface and this layer forms a boundary between the metal surface and the body, preventing corrosion and deterioration of the material. This is the reason for the high biocompatibility.  Can titanium dental implants cause an allergic reaction?  Exposure to titanium from implants is common, but reports on clinical allergy are rare. Titanium dioxide on the surface of dental implants is more stable than pure titanium. It is possible that titanium ions could be released from the surface and sometimes into distant tissue. However, evidence for the possible toxic effects and risk of allergic reactions is weak (1). One study showed that titanium allergy can be detected in dental implant patients, even though its estimated prevalence is very low (0.6%) (2). It should be noted that no material, including any dental implant, can be considered universally biocompatible. As mentioned above, the Ti-6Al-4V alloy is the preferred choice for dental implants. Small amounts of other elements associated with titanium alloys can act as “impurities.” Titanium alloys can contain very small amounts of beryllium, cobalt, chromium, copper, iron, nickel and palladium. These impurities may be causative factors for different allergic reactions in patients with dental implants, but the likelihood is low.  It is therefore advisable to test for titanium allergy in patients with a history of multiple and/or severe allergies. These include allergy to jewelry, nickel and cobalt. Furthermore, patients with previous unexplained implant failure could be tested. In the rare situation that a patient is allergic to titanium, it is preferable to use alternative treatment options or to use an alternative dental implant material such as zirconia (3).  What are zirconia implants?  The idea of an alternative to titanium dental implants has been growing for years due to the questions regarding sensitivity to titanium that was discussed above. Another drawback of titanium is its grey color. When a titanium implant is placed in an area with very thin gingival tissue, there is a risk that a dark grey shadow may be visible.  Zirconium dioxide (zirconia) has been introduced as an alternative to titanium dioxide. Zirconia was first used for the fabrication of dental crowns and implant abutments. Currently, tetragonal zirconia poly-crystal is the ceramic of choice for dental implants. The white, opaque color of zirconia along with the good biocompatibility, favorable mechanical properties and low affinity for bacterial plaque, make it a good potential material for dental implants.  Are zirconia implants as successful as titanium implants?  Osseointegration is a major factor in the success of modern dental implants. Titanium remains the material of choice for obtaining and maintaining osseointegration. Early studies show that zirconia implants have similar 1 and 2 year survival rates when compared to titanium implants. More clinical long-term studies providing detailed information about implant failures, biological and technical complications, and prosthetic and esthetic outcomes are urgently needed (3). Potential implant abutment and crown complications Once a dental implant has integrated and healed with the jawbone, an implant abutment and crown are then made to complete the replacement of the tooth. The abutment and crown is held into the implant by a screw. These 3 components are shown in the picture below.   When titanium is used for the implant and abutment, this connection is incredibly strong and stable with very few mechanical complications from chewing. On the other hand, zirconia is a more brittle material compared to titanium. When zirconia is used for the implant and abutment, more mechanical complications (breakage of the abutment) have been reported, see picture below. This has been reported in studies and I have also experienced this in my practice. The challenge with zirconia is the strength and stability of the connection between the abutment and implant. So far, this connection is not as reliable as titanium. When implant abutments break, this expensive abutment and crown needs to be replaced and has the possibility of breaking again.   Conclusions References At Geach Dental in Downtown Los Angeles and Laguna Niguel, Orange County our team has extensive experience in dealing with dental implant issues. If you are

Read Up

The Ultimate Guide to Teeth Whitening

Explore the ultimate guide to successful teeth whitening, covering professional vs. at-home treatments, personalized plans, in-office procedures, take-home kits, and essential aftercare tips. Read on to discover the path to a brighter, healthier smile with our expert insights.

Read Up
Related blog posts

Zirconia Implants vs. Titanium: Health Benefits and Differences Explained

Dental implants have been a major advancement in replacing missing teeth. When placed correctly, titanium implants have incredibly high success rates and are the standard for tooth replacement. Dental implants can be used to replace a single tooth or can be strategically placed to support multiple missing teeth or a whole arch of teeth. The great benefit of dental implants is that they eliminate a removable denture and provide the function and appearance of natural teeth.  Over the past couple years, patients have become concerned about the idea of having metal titanium implants placed in their body. Some patients are not comfortable with the idea of having a “foreign metal object” placed into their jawbone. I can understand their concern especially if they are unsure of the science, materials used and potential risks. So much information is available on the internet and it is difficult to find answers to these questions. Because of this, some patients are more comfortable with the idea of non-metal zirconia implants as an alternative. So, are zirconia dental implants healthier than titanium dental implants?  Composition of titanium dental implants Titanium (Ti) is a metallic element with the atomic number 22 and has a silvery color. It is the ninth most abundant element in the earth’s crust. The alloy Ti-6Al-4V (Titanium, 6-aluminum, 4-vanadium), which was originally used for aerospace applications, is currently the most frequently used material for medical and dental implantation. Titanium alloys are widely used for orthopedic implantation (joint replacements) because of their lightweight, high biocompatibility and favorable strength/density ratios. Titanium alloys are superior to stainless steel and cobalt-chrome (Co-Cr) alloys with regard to biocompatibility and corrosion resistance.  Titanium plays a pivotal role for dental implants because the roughened surface of these implants attract bone cells to deposit bone and fill the space between the jawbone and the implant surface. This process is called osseointegration and is critical for implant stability. Titanium dental implants quickly form a layer of titanium dioxide on the surface and this layer forms a boundary between the metal surface and the body, preventing corrosion and deterioration of the material. This is the reason for the high biocompatibility.  Can titanium dental implants cause an allergic reaction?  Exposure to titanium from implants is common, but reports on clinical allergy are rare. Titanium dioxide on the surface of dental implants is more stable than pure titanium. It is possible that titanium ions could be released from the surface and sometimes into distant tissue. However, evidence for the possible toxic effects and risk of allergic reactions is weak (1). One study showed that titanium allergy can be detected in dental implant patients, even though its estimated prevalence is very low (0.6%) (2). It should be noted that no material, including any dental implant, can be considered universally biocompatible. As mentioned above, the Ti-6Al-4V alloy is the preferred choice for dental implants. Small amounts of other elements associated with titanium alloys can act as “impurities.” Titanium alloys can contain very small amounts of beryllium, cobalt, chromium, copper, iron, nickel and palladium. These impurities may be causative factors for different allergic reactions in patients with dental implants, but the likelihood is low.  It is therefore advisable to test for titanium allergy in patients with a history of multiple and/or severe allergies. These include allergy to jewelry, nickel and cobalt. Furthermore, patients with previous unexplained implant failure could be tested. In the rare situation that a patient is allergic to titanium, it is preferable to use alternative treatment options or to use an alternative dental implant material such as zirconia (3).  What are zirconia implants?  The idea of an alternative to titanium dental implants has been growing for years due to the questions regarding sensitivity to titanium that was discussed above. Another drawback of titanium is its grey color. When a titanium implant is placed in an area with very thin gingival tissue, there is a risk that a dark grey shadow may be visible.  Zirconium dioxide (zirconia) has been introduced as an alternative to titanium dioxide. Zirconia was first used for the fabrication of dental crowns and implant abutments. Currently, tetragonal zirconia poly-crystal is the ceramic of choice for dental implants. The white, opaque color of zirconia along with the good biocompatibility, favorable mechanical properties and low affinity for bacterial plaque, make it a good potential material for dental implants.  Are zirconia implants as successful as titanium implants?  Osseointegration is a major factor in the success of modern dental implants. Titanium remains the material of choice for obtaining and maintaining osseointegration. Early studies show that zirconia implants have similar 1 and 2 year survival rates when compared to titanium implants. More clinical long-term studies providing detailed information about implant failures, biological and technical complications, and prosthetic and esthetic outcomes are urgently needed (3). Potential implant abutment and crown complications Once a dental implant has integrated and healed with the jawbone, an implant abutment and crown are then made to complete the replacement of the tooth. The abutment and crown is held into the implant by a screw. These 3 components are shown in the picture below.   When titanium is used for the implant and abutment, this connection is incredibly strong and stable with very few mechanical complications from chewing. On the other hand, zirconia is a more brittle material compared to titanium. When zirconia is used for the implant and abutment, more mechanical complications (breakage of the abutment) have been reported, see picture below. This has been reported in studies and I have also experienced this in my practice. The challenge with zirconia is the strength and stability of the connection between the abutment and implant. So far, this connection is not as reliable as titanium. When implant abutments break, this expensive abutment and crown needs to be replaced and has the possibility of breaking again.   Conclusions References At Geach Dental in Downtown Los Angeles and Laguna Niguel, Orange County our team has extensive experience in dealing with dental implant issues. If you are

Read Up

The Ultimate Guide to Teeth Whitening

Explore the ultimate guide to successful teeth whitening, covering professional vs. at-home treatments, personalized plans, in-office procedures, take-home kits, and essential aftercare tips. Read on to discover the path to a brighter, healthier smile with our expert insights.

Read Up

Follow Our Newsletter To Stay Tuned

Sign up to receive exclusive guides & tips on health, fashion, travel, beauty and more!

Follow Our Newsletter To Stay Tuned

Sign up to receive exclusive guides & tips on health, fashion, travel, beauty and more!
Request an Appointment
(213) 622-3339
Menu
Request an Appointment