Following tooth extraction, a well-described sequence of wound healing takes place in the remaining socket. A series of physiological events occurs, resulting in wound healing and bone generation within the socket. Bone remodeling results in external changes to the alveolar ridge. The soft tissue architecture follows the remodeling of bone. Importantly, healing of the extraction socket will be compromised unless a minimally traumatic tooth extraction approach is applied.

Healing events underpin the biological rationale for the ITI placement protocols following tooth loss, which are as follows: Type 1: immediate or same day placement, Type 2: placement following early soft tissue healing (at 4-8 weeks) or partial bone healing (at 12-16 weeks), and Type 3: late placement (after more than 6 months). The outcome of this remodeling is most often a significant modification of the pre-extraction outline of the alveolar ridge, which may present a problem for later prosthetic reconstruction. For this reason, a knowledge of the healing events following tooth extraction is essential for appropriate prosthodontically driven implant treatment planning. This module will present anatomic characteristics of the alveolar bone and extraction socket and will describe the histologic and dimensional changes following tooth extraction.

After completing this ITI Academy Module, you should be able to: describe the anatomic features of the alveolar process, tooth socket, and alveolar ridge; describe the biological events that occur following tooth extraction; describe soft tissue healing; define ridge alterations following tooth extraction; and relate understanding of healing processes to the biological rationale for the ITI placement protocols following tooth extraction.

The bone that supports teeth in the maxilla and mandible may be divided into two parts. As can be seen in this diagram of the mandible, the part that contains the roots of the teeth and developing tooth buds of unerupted teeth is known as the alveolar process, or alveolar bone. The bone of the alveolar process is continuous with the second part, the basal bone of the jaws, with no distinct boundary dividing them. The alveolar process is also made of two parts, the alveolar bone proper and the supporting bone. The bone lining the socket of a tooth is referred to as the alveolar bone proper. The rest of the bone that supports the teeth is the supporting bone. After a tooth is extracted, the alveolar process heals with formation of bone within the socket and external resorption or shrinkage of the bone, forming the alveolar ridge.

The alveolar process contains the roots of the teeth and developing tooth buds of unerupted teeth. It is a tooth-dependent part of the jaws, since it forms in response to the development and eruption of the teeth. Its final volume and shape are determined by the form of the teeth, their axis of eruption, and their eventual inclination.

The alveolar process, as previously mentioned, is composed of two parts, the alveolar bone proper and the supporting bone. The alveolar bone proper is the bone that lines the socket of a tooth. The function of the alveolar bone proper, together with the root cementum and the periodontal ligament, is to form the attachment apparatus of the tooth.

There are several different terms used to describe the alveolar bone proper. The gross anatomic term is alveolar bone proper, which is synonymous with the term cribriform plate, so-called because it is perforated by many minute openings for vascular and nervous components. The histologic term is bundle bone. In histologic sections, the alveolar bone proper appears as a thin lamella of cortical bone lined with bundle bone. Extrinsic collagen fiber bundles of the periodontal ligament known as Sharpey's fibers are embedded into the bundle bone. Their purpose is to connect the bundle bone with the root cementum. The radiographic term for alveolar bone proper is lamina dura. This structure appears more dense than adjacent bone on radiographs. Widening or disruption of the lamina dura may indicate periodontal pathology.

As shown in this histologic section, bundle bone, periodontal ligament, and cementum form an anatomic unit. When a tooth is extracted, the cementum and much of the periodontal ligament are removed. The bundle bone resorbs after tooth extraction.

In addition to the alveolar bone proper, the rest of the bone that supports the teeth is the supporting bone. This portion of the alveolar process consists of two parts: the outer cortical plates, buccal and lingual to the tooth, and the trabecular bone between the cortical plates and the tooth socket.

The cortical plates of the supporting bone are continuous with the alveolar bone proper lining the tooth sockets. The cortical bone is comprised of functional units called osteons that consist of concentric lamellae and canaliculi surrounding a central Haversian canal. The cortical plates are thicker in the mandible than in the maxilla, and they are thickest in the molar and premolar regions of the mandible.

The trabecular bone is located between the alveolar bone proper and the cortical bone plates. Trabecular bone, also known as cancellous bone, consists of bony trabeculae and marrow spaces. In adult patients the marrow spaces are rich in adipocytes and mesenchymal cells. Mesenchymal cells have bone-forming potential and may be induced to form bone, but they also support the differentiation of hemopoietic cells into osteoclasts that will initiate bone resorption.

This image shows a cross section of the maxillary alveolar process at the mid-root level of the teeth. The bone between the roots of a tooth is called interradicular bone or interradicular septum. The bone between the roots of adjacent teeth is called interdental bone or interdental septum. Trabecular bone occupies most of the interdental bone. The cortical plates are the outer walls of the alveolar process. The alveolar bone proper can be seen on the inside of the tooth socket. It is characterized by numerous perforations present on the bone surface facing the roots that are known as Volkmann's canals.

Traumatic extraction or tooth loss with bone damage leads to unpredictable extraction socket healing and bone remodeling. The extraction socket is a term used to describe the tissues remaining after tooth removal. The outer walls of the extraction socket consist mainly of cortical bone. The buccal bone plate is usually thin, less than 1 mm, especially in the anterior dentition, and it consists mainly of bundle bone. On the contrary, the lingual or palatal wall of the socket is usually thicker than its buccal counterpart, and some trabecular bone may be present.

The term alveolar ridge refers to the alveolar bone following loss of teeth. After a tooth is extracted, the alveolar process heals with formation of bone within the socket and bone remodeling, leading to external changes. The outer walls of the alveolar ridge consist of cortical bone. The buccal bone plate is comparatively thinner than the lingual or palatal plate. The cortical plates enclose trabecular bone that consists of bony trabeculae and marrow. As a rule, the ridge of an edentulous site in the maxilla contains comparatively more trabecular bone than a site in the mandible.

Anatomical Features of the Alveolar Process, Key Learning Points: The alveolar process or alveolar bone contains the roots of the teeth and developing tooth buds of unerupted teeth. The alveolar process has two parts, the alveolar bone proper and the supporting bone. Alveolar bone proper lines the tooth sockets; histologically, it is known as bundle bone, and radiographically, as the lamina dura. Alveolar bone proper, periodontal ligament, and cementum constitute the attachment apparatus of the tooth. Supporting bone is comprised of outer cortical plates and a central portion of trabecular bone. The alveolar ridge is the alveolar bone that remains after tooth loss.

The healing of an extraction socket is characterized by a sequence of histologic events taking place in five distinct stages: hemostasis, inflammation, proliferation, mineralization, and remodeling. Following tooth extraction, the socket fills with blood, leading to formation of a blood clot. The remaining cascade of events in the extraction socket is similar to intramembranous bone formation in other types of osseous defects, beginning with the inflammation stage. During this stage, the blood coagulum in the socket matures into granulation tissue. In the third stage, the granulation tissue is replaced by a provisional connective tissue matrix, which, following a mineralization process, becomes woven bone. The newly formed woven bone, following a remodeling process, becomes trabecular bone in the last stage of this healing process.

Hard and soft tissue composition in the extraction socket during longer healing periods was documented in a study by Cardaropoli and coworkers, in which the entire healing cascade after tooth extraction was evaluated in a dog model. The following images are histologic specimens representing the stages of healing observed in their study. On the day of extraction, the outline of the socket can be seen as a region of pink-stained bone. The inner lining of the bone is bundle bone, which was previously attached to the extracted tooth by the periodontal ligament. Within the socket, the coagulum appears as a non-homogenous mixture of fibrin, red blood cells, and inflammatory cells.

On day 3, the socket is filled with a blood coagulum comprised of red blood cells and platelets trapped in a fibrin network together with isolated inflammatory cells such as neutrophils. Close to the bundle bone, mesenchymal cells, severed periodontal fibers, and dilated vascular units can be observed. At the end of this initial healing period, small segments of the coagulum are replaced by a highly vascularized granulation tissue and an inflammatory cell infiltrate that is stained dark red/blue.

After 1 week of healing, the wound in the extraction site has significantly changed. In the central and apical part of the socket, large areas of the coagulum have been replaced with a provisional connective tissue matrix, which is lightly stained in the histologic section. Regions of darker-staining granulation tissue can still be seen. This provisional matrix is made of newly formed connective tissue fibers, blood vessels, mesenchymal cells, and various types of leukocytes. At the margins of the socket, the bone has begun to lose its continuity due to the action of osteoclasts, which have started to resorb the bundle bone. The gaps in the bone that connect the inner part of the socket to the surrounding trabecular bone are also known as Volkmann's canals. These gaps allow new blood vessels to grow into the socket from the surrounding bone.

After 14 days of healing woven bone, which appears as a loose, unstructured network of bone, has started to fill the socket except in the central region, where significant amounts of the provisional connective tissue matrix still remain. This is because the woven bone forms first at the periphery of the socket and gradually extends from the walls towards the center of the socket. It is characterized by a poorly organized collagen matrix that deposits along the newly formed blood vessels that originated from the surrounding trabecular bone. At this stage most of the bundle bone has been resorbed, and the bone marrow spaces in the interdental septa communicate directly with the newly formed bone.

At 30 days of healing, a significant part of the extraction socket is filled with newly formed bone. This bone contains a large number of primary osteons and is continuous with the original bone of the socket walls. In some areas, the process of modeling and remodeling of the newly formed bone has begun. Osteoclasts are present on the surface of the original cortical bone lateral to the crestal region of the extraction socket.

At 60 to 90 days of healing, most of the woven bone in the socket has been replaced with trabecular bone and bone marrow. Bone marrow spaces include large blood vessels, inflammatory cells, and adipocytes. A bridge of new bone, mainly woven bone, has formed over the entrance of the socket.

At 120 days of healing, the entrance of the socket, denoted with an arrow, has become reinforced by layers of cortical bone that are deposited over the previously formed woven bone.

After 180 days of healing, beneath the marginal cortical bone at the entrance of the socket as denoted by the arrow, most of the socket is filled with trabecular bone that includes large marrow spaces. The bone is characterized by a limited number of trabeculae of lamellar bone. The bone marrow contains large numbers of adipocytes but only a few inflammatory cells. It should be noted that the time frame for healing in this dog study would be quicker than that in human beings, and therefore caution should be exercised in interpreting these results.

Trombelli and coworkers monitored the healing of human extraction sockets for a 6-month period and presented a semi-quantitative analysis of tissues and cell populations involved in various stages of socket healing. They showed that granulation tissue was present in comparatively large amounts in the early phases of socket healing. At 6 to 8 weeks, the granulation tissue was replaced with a provisional connective tissue matrix and woven bone.

However, hard tissue formation within extraction sockets is highly variable in humans. Although formation of a provisional connective tissue is consistent during the first weeks of healing, the interval during which mineralized bone is laid down is not as predictable.

Biological Events After Tooth Extraction, Key Learning Points: Extraction socket healing is characterized by a sequence of events: hemostasis, inflammation, proliferation, mineralization, and remodeling. The healing results in woven bone, which is remodeled to trabecular bone. Variability exists in humans with respect to the timing and the amount of hard tissue formation within extraction sockets.

The soft tissue healing at the entrance of the socket also follows a specific pattern: At day 1 after extraction, the marginal portion of the coagulum is covered with a layer of inflammatory cells, mainly neutrophils. After 3 days, small segments of the coagulum at the margins of the socket are replaced by a highly vascularized granulation tissue with an inflammatory cell infiltrate.

After 4 to 5 days, the epithelium from the margins of the soft tissue starts to proliferate to cover the granulation tissue in the socket. At 14 days, the connective tissue at the marginal portion of the extraction socket is partially lined by epithelial cells. After 21 to 30 days, the marginal soft tissue compartment of the socket is characterized by a well-organized fibrous connective tissue lined with a keratinized epithelium.

After 60 to 90 days, newly formed woven bone forms a bridge across the entrance of the socket. The epithelium covering the bone is keratinized. Soft tissue healing at this stage has been completed. At 90 to 180 days after tooth extraction, the woven bone is gradually remodeled into cortical bone. A periosteum is established with collagen fibers from the lining mucosa inserting into the new cortical bone.

Tooth Socket Soft Tissue Healing, Key Learning Points: Epithelial migration from the socket margins begins within days after extraction. Several weeks are required for keratinized epithelium to cover the extraction socket. Cortical bone and periosteum will be formed after several months of healing.

Following multiple- or single-tooth extraction and the subsequent loss of masticatory function, the alveolar ridge will present a series of adaptive alterations known as alveolar atrophy. The alveolar atrophy is characterized by a reduction in the dimensions of the alveolar ridge that is a combination of hard and soft tissue changes. This reduction occurs in both the horizontal and the vertical dimension, and as a result the arch is shortened. The amount of tissue atrophy following the loss of a single tooth can be substantial and is variable between different teeth and areas of the alveolar process. The amount of tissue atrophy can also be influenced by factors such as pre-existing pathological processes and excessive pressure from a removable prosthesis.

Studies utilizing clinical or cast model measurements have shown that the reduction in ridge dimensions is three-dimensional, but it is greater along the buccal surface than along the lingual or palatal surfaces. Changes in bone height are usually moderate. For example, Schropp and colleagues observed that, after 12 months of healing, the height of the buccal bone plate was 1.2 mm apical to its lingual or palatal counterpart. On the contrary, the width of the alveolar ridge in single-rooted teeth will be decreased approximately by 50%, and two-thirds of this reduction will occur within the first 3 months after tooth extraction.

A recent systematic review evaluated the amount of change in height and width of the residual ridge after tooth extraction. The weighted means showed that the clinical loss in width is greater than the loss in height. The mean reduction in width of the alveolar ridge was calculated to be approximately 4 mm, and the mean reduction in height was approximately 1.7 mm. The mean crestal height change as assessed on the radiographs was 1.53 mm.

Araujo and Lindhe described the edentulous ridge profile alterations following tooth extraction in an experimental study in a dog model. During the first week of post-extraction healing, the socket area is occupied by coagulum and granulation tissue. A large number of osteoclasts are seen on the outer as well as on the inner surfaces of the buccal and lingual bone walls. The presence of osteoclasts on the inner surface of the socket walls indicates that the bundle bone resorption has been initiated.

At 2 weeks after tooth extraction, the apical and lateral parts of the socket are filled with woven bone, while the central and marginal portions of the socket are occupied by provisional connective tissue. On the inner and outer surfaces of the socket walls, numerous osteoclasts can be seen. In several areas of the socket wall, the bundle bone has been resorbed and replaced with woven bone.

At 4 weeks after tooth extraction, the socket is filled with woven bone. Osteoclasts are present on the outer surfaces at the margin of the buccal and lingual walls, signaling resorption of cortical plates. The resorption of the bundle bone is almost complete. Osteoclasts also line the trabeculae of woven bone present in the central and lateral aspects of the socket, contributing to the remodeling of newly formed woven bone into a more mature type of trabecular bone.

At 8 weeks after tooth extraction, the entrance to the extraction site is bridged with cortical bone. The woven bone in the socket is replaced with bone marrow and some trabeculae of lamellar bone. At the crests of the buccal and lingual cortical plates, there are signs of ongoing bone resorption.

The margin of the buccal wall is shifted apically by approximately 2 mm over the 8 weeks of healing, as indicated by the yellow arrow. Bone loss is greater in the buccal wall than in the lingual wall during socket healing for several reasons. First, the crestal portion of the buccal bone wall, especially in the anterior region, is occupied by bundle bone. As mentioned earlier, bundle bone is a tooth-dependent tissue that completely resorbs after tooth extraction. On the contrary, the lingual or palatal socket wall is typically composed of less bundle bone. Also, the lingual bone wall of the socket is wider than the buccal wall.

Many factors have been suggested as having an influence on post-extraction ridge atrophy. Among these, the most significant are: pre-existing pathological processes that have damaged the bone prior to extraction; excessive pressure from a removable prosthesis; the presence of a thin bone phenotype; and the number of missing teeth, that is, the more teeth that are missing, the greater the atrophy.

Ridge Alterations Following Tooth Extraction, Key Learning Points: The loss of one or more teeth results in significant alveolar ridge alteration, called alveolar atrophy. Tooth extraction will result in resorption of the bundle bone. The amount of ridge alteration after bundle bone resorption is dependent on the thickness of the alveolar bony walls. Since in most extraction sites the buccal bone wall is thinner than the palatal or lingual wall, dimensional changes are more pronounced on the buccal aspect. Factors influencing ridge resorption include pre-existing pathological processes, pressure from removable prostheses, a thin bone phenotype, and the number of missing teeth.

Healing of the Extraction Socket, Module Summary: The alveolar process or alveolar bone contains the roots of the teeth and developing tooth buds of unerupted teeth. The alveolar process has two parts, the alveolar bone proper and the supporting bone. Alveolar bone proper lines the tooth sockets; histologically, it is known as bundle bone, and radiographically, as the lamina dura. Alveolar bone proper, periodontal ligament, and cementum constitute the attachment apparatus of the tooth. Supporting bone is comprised of outer cortical plates and a central portion of trabecular bone. The alveolar ridge is the alveolar bone that remains after tooth loss.

Extraction socket healing is characterized by a sequence of events: hemostasis, inflammation, proliferation, mineralization, and remodeling. The healing results in woven bone, which is remodeled to trabecular bone. Variability exists in humans with respect to the timing and the amount of hard tissue formation within extraction sockets. Epithelial migration from the socket margins begins within days after extraction. Several weeks are required for keratinized epithelium to cover the extraction socket. Cortical bone and periosteum will be formed after several months of healing.

The loss of one or more teeth results in significant alveolar ridge remodeling, leading to alveolar atrophy. Tooth extraction will result in resorption of the bundle bone. The amount of ridge alteration after bundle bone resorption is dependent on the thickness of the alveolar bony walls. Since in most extraction sites the buccal bone wall is thinner than the palatal or lingual wall, dimensional changes are more pronounced on the buccal aspect. Factors influencing ridge remodeling include pre-existing pathological processes, pressure from removable prostheses, a thin bone phenotype, and the number of missing teeth.