Anatomical Features of the Alveolar Process
The Alveolar Process
The bone that supports teeth in the maxilla and mandible may be divided into two parts: the alveolar process and the basal bone, which are continuous with no distinct border dividing them. The alveolar process of the jaw develops along with the eruption of the teeth and is composed of the alveolar bone proper and the supporting bone. The volume and the shape of the alveolar process are determined by the form of the teeth, their axis of eruption, and eventual inclination (Schroeder 1985).
The Alveolar Bone Proper
Compact bone that composes the alveolus (tooth socket). Also known as cribriform plate or lamina dura, the fibers of periodontal ligament insert into it (American Academy of Periodontology, 2001). The function of the alveolar bone proper is to form the attachment apparatus of the tooth in conjunction with the root cementum and the periodontal ligament.
Alveolus and Extraction Socket
Alveolus is the socket in the bone into which a tooth is attached by means of the periodontal ligament. (American Academy of Periodontology, 2001). 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 with the buccal bone plate being usually thinner (<1 mm in the anterior region) than the lingual or palatal wall.
The Alveolar Ridge
When the alveolar process heals following loss of teeth, the remaining bone is referred to as the alveolar ridge and is the result of bone formation within the socket and bone resorption externally.
Classification of Remaining Bone
Lekholm and Zarb (1985) classified the “quality” of the bone in the healed edentulous ridges into four categories according to which classes 1 and 2 are characterized by thick cortical plates and small volume of bone marrow. Sites that belong to classes 3 and class 4 present with thin walls of cortical bone surrounding large amount of cancellous bone (spongiosa), including trabeculae of lamellar bone and marrow.
Alterations of the Alveolar Process Following Tooth Extraction
Biological Events and Histologic Ridge Alterations
In the study by Amler (1969), the socket healing was monitored in volunteers, after tooth extraction and soft tissue biopsies were harvested from the extraction sites after varying intervals; from 48 hours to 32 days. From his observations, Amler concluded that a blood clot formed within the socket soon after the removal of a tooth. Within two to three days the clot was gradually replaced with granulation tissue and subsequently with young connective tissue and osteoid with signs of mineralization after three weeks. After six weeks of healing, there was pronounced bone formation in the socket and trabeculae of newly formed bone.
The limitations of this study are the following: (i) The tissue sampled was not demineralized prior to sectioning as a result only events of that preceded hard tissue formation could have been analyzed. (ii) The study was of short duration and did not include the important later phase of socket healing that involves the process of modeling and remodeling. (iii) The biopsy was restricted to the superficial region of the wound and the marginal portions of the socket. Thus, the tissue composition of the fully healed extraction site was not documented in the study.
Cardaropoli et al. (2003) published the results of a long‐term experimental study in dogs, which described in more detail the various phases of socket healing including processes of both modeling and remodeling.
Overview of the Histologic Sequence of Healing
The healing of an extraction socket is characterized by a sequence of histologic events taking place in four clear stages:
- Blood clotting: The blood clot acts as a physical matrix that directs cellular movements, contains growth factors, and enhances the activity of inflammatory cells.
- Wound cleansing: When neutrophils and macrophages that migrate into the wound clean the site from bacteria and damaged tissue before formation of new tissue can start.
- Tissue formation: A new tissue, i.e. granulation tissue gradually, replaces the blood clot. The granulation tissue contains macrophages, a large number of fibroblast‐like cells, and newly formed blood vessels. A provisional connective tissue is established through a combination of fibroplasia (intense synthesis of matrix components exhibited by the mesenchymal cells) and angiogenesis (formation of new vessels).
- Tissue modeling and remodeling: Within few weeks the entire socket is filled with woven bone which is replaced gradually with lamellar bone and marrow.
Dimensional Ridge Alterations
Knowledge of healing events following tooth extraction is essential for prosthetically driven treatment planning.
Studies measuring the mean apico‐coronal and bucco‐lingual change in ridge dimensions following tooth extraction:
In the study by Pietrokovski and Massler (1967), the authors had access to 149 dental cast models (72 maxillary and 77 mandibular) in which one tooth was missing (and not replaced) on one side of the jaw. The outer contours of the buccal and lingual/palatal portions of the ridge at a tooth site and at the contralateral edentulous site were determined by the use of a profile stylus and an imaging technique. A pilot study was also carried out to test the accuracy of the technique and the degree of symmetry of normal and edentulous ridges. The authors reported the following findings:
- From the pilot study the human dental arches found to be symmetrical but not identical with a small and not significant difference.
- The buccal plate of the maxillary and mandibular arch was resorbed to a greater extend shifting the position of the edentulous ridge toward the palatal/lingual site and shortening the total arch length.
- The amount of tissue resorption was significantly greater in the edentulous molar region than in the incisor and premolar region of both jaws.
Schropp et al. introduced subtraction radiography as a new method for assessing morphologic changes and remodeling processes of extraction sites during the healing period. This prospective clinical trial demonstrated that major changes of an extraction site take place during the 12 months following atraumatic tooth extraction. The width of the alveolar ridge was reduced by 50% during the observation period and two thirds of this reduction occurred within the first three months after tooth extraction. Regarding the soft tissue height, only slight changes, less than 1 mm, took place in both jaws during the 12 months of healing (Schropp et al. 2003).
Araújo and Lindhe (2005) described alterations in the edentulous ridge profile following tooth extraction in an experimental study in a dog model. Over eight weeks of healing, the margin of the buccal wall shifted apically by approximately 2 mm. According to the authors, bone loss during socket healing is greater along the buccal than the lingual wall for several reasons. First, the crestal portion of the buccal bone wall is primarily compromised of bundle bone, especially in the anterior region. Bundle bone is a tooth‐dependent tissue that completely resorbs after tooth extraction. On the contrary, bundle bone typically comprises a smaller proportion of the lingual or palatal socket wall. Also, the lingual bone wall of the socket is thicker than the buccal wall.
Clinical Implications for Ridge Preservation and Implant Treatment
Adequate dimensions of the alveolar ridge promote implant placement in the proper three‐dimensional position and provide proper mechanical support for the implant and soft tissues. Numerous experimental and clinical studies have shown that resorption of the alveolar process is an inevitable consequence after tooth extraction.
Many factors have been suggested to influence post‐extraction ridge reduction. These include:
- Pre‐existing pathological processes
- Thin bone and soft tissue phenotype
- Number of missing teeth
- Traumatic tooth extraction
Preventing or reducing this resorption is desirable for long‐term success in implant dentistry since ideally the implant must be circumferentially surrounded by healthy bone and a wide band of keratinized mucosa.
Atraumatic Tooth Extraction
Tooth extraction is the first step in treatment and even though it has been considered a simple and straightforward procedure, it is an invasive procedure. It disrupts vascular structures and damages soft tissues and associated periodontal ligament. It should be performed as minimally traumatic as possible, with care to avoid additional bone resorption. Flapless tooth extraction is important and has been shown to reduce the amount of bone loss in the early healing phase four to eight weeks post extraction compared with full‐thickness flap elevation (Fickl et al. 2008).
Indications for Ridge Preservation
Ridge preservation may be defined as a procedure that aims to preserve the ridge volume existing at the time of extraction. General indications:
- Maintenance of the existing soft and hard tissue envelope
- Maintenance of a stable ridge volume for optimizing functional and esthetic outcomes
- Simplification of treatment procedures subsequent to the ridge preservation (Hämmerle et al. 2012).
It is also indicated when an immediate or early implant placement is not feasible due to:
- Adolescent people (too young for implant placement)
- Medical reasons
- Financial reasons
- Extended apical bone defects where primary stability may be compromised
- Extended soft tissue deficiency
- Multi‐rooted sites
Contraindications for Ridge Preservation
- General contraindication against oral surgical interventions
- Infection at the extraction site, which cannot be taken care of during the ridge preservation surgery
- Patients radiated in the area planned for ridge preservation
- Patients taking bisphosphonates
- Implant placement anticipated within six to eight weeks of extraction
- Thick periodontal phenotypes with expected sufficient amount of bone after extraction and healing
Ridge Preservation Procedures
Several procedures have the potential to modulate the degree of the inevitable ridge changes.
Maintenance of the Root and the Socket Shield Technique
Historically, the first therapeutic attempts to prevent alveolar ridge resorption were performed by root retention, with primary goal of maximizing the stability of removable prostheses (Osburn 1974). Later in 1994, Langer introduced the spontaneous in situ gingival augmentation technique to increase the quantity of gingival tissue around a tooth scheduled for extraction.
Salama et al. in 2007 described the advantages of the “Root Submergence technique” for pontic site development in esthetic implant dentistry. Hürzeler et al. published in 2010 the “Socket‐shield” technique, according to which the retaining of the buccal aspect of the root during implant placement did not appear to interfere with osseointegration and could be beneficial in preserving the buccal bone plate.
Forced eruption has been used as a site development for future implant placement.
Over the past two decades, multiple studies have been conducted to evaluate the efficacy of different socket grafting approaches (Avila‐Ortiz et al. 2014). In these studies a large variety of biomaterials have been employed and tested, including autologous bone, bone substitutes (allografts, xenografts, and alloplasts), autologous blood‐derived products, and bioactive agents (Darby et al. 2009).
Socket Grafting Technique: Clinical Recommendations
- Minimally traumatic tooth extraction
- Socket debridement
- Raising of a flap and placement of biomaterials (biomaterial for ridge contouring and/or barrier membrane)
- Primary wound closure when possible (soft tissue punch, connective tissue graft, barrier membrane, soft tissue replacement matrix, pedicle flap)