Anatomical Consideration in Oral Implant Surgery

Anatomical consideration in oral implant surgery

Sound knowledge of individual maxillomandibular anatomy, the degree of atrophy, and the quantity and quality of the available host bone is crucial for establishing an accurate diagnosis, achieving implant success, and choosing the appropriate implant design and size. It is of paramount importance to carefully examine the individual anatomic conditions to prevent damage to the inferior alveolar nerve and to avoid possible medicolegal consequences. We will make an overview of the specific anatomic aspects of the masticatory apparatus that are of importance for endosseous implantations. After describing general maxillomandibular anatomic and biomechanical aspects relevant to the implant surgeon, we will discuss specific resorptive patterns during atrophy and changes in the shape of the maxilla and mandible in more detail, examining relevant aspects of the individual parts of the maxilla and mandible. As for the values given in the literature, it should be noted that an osseointegrated implant length of 7.0 mm is considered to be suitable for sustaining functional loads (van Steenberghe et al, 1990). The layer of bone surrounding the implant should be at least 1.0 to 2.0 mm in all directions (Wilson, 1989; Watzek et al, 1991; Haider et al, 1991).

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Anatomy of the maxilla-relevant structures in implant surgery

1. The soft parts of the face are attached to the body of the maxilla, the nasal cavity and orbits extending medially and cranially, respectively. 2.The body of the maxilla is the largest part of the maxilla. It is roughly pyramidal and houses the maxillary sinus. 3.The oral cavity and the maxillary teeth are located caudal to it and the pterygopalatine fossa lies dorsal to it. 4.The anterior wall of the maxilla contains the concave canine fossa, whereas its lateral wall dorsally meets the convex infra-temporal surface that protrudes as the maxillary tuberosity.

5. The entire alveolar process of the maxilla deviates slightly from the vertical plane to the outside and forms an average angle of 60 to 80 degrees with the horizontal plane.

6. The incisal region of the alveolar process always being inclined more strongly labially than is the posterior region. (12) 7. the labial and lingual inclinations of the incisal region and the mandibular alveolar process, respectively, thus compensate for the imbalance in the maxillomandibular relationship that results because the radius of the mandibular basal arch is larger than that of the maxillary basal arch. Without this compensation, the imbalance would result in a crossbite. Also the longitudinal axes of the maxillary teeth form an outwardly open angle. 8. the alveolar process attaches to the lowest part of the maxilla, which is composed of cancellous bone. Because the maxilla offers only limited space for tooth anchorage the anterior surface has long, vertical elevations overlying the roots of incisors and canine teeth, known as alveolar juga. 9. the edge of the alveolar process that becomes visible after tooth loss is called the alveolar margin. Its orientation corresponds to that of the body of the mandible. 10. the ridge of the zygomatic bone, also referred to as zygomaticoalveolar process, separates the canine fossa from the infratemporal fossa and originates just above the first molar. 11. above the frontal portion of the alveolar process is a sharp bony prominence known as the anterior nasal spine. 12. on the palatal side, the incisive foramen is located mediosagittally at the base of the frontal alveolar process, serving as an exit for the incisive nerve. 13. dorsal to the socket of the wisdom tooth, the alveolar process of the maxilla is arched; the convexity, which is filled with fine, spongy bone is known as the post molar or retromolar tuberosity. 14. palatal to the second molar tooth is the greater palatine foramen, which is located 1 to 8 mm from the neighboring alveolus. 15. the maxillary sinus is the largest of the paranasal sinuses and has the shape of a lying pyramid whose base forms the lateral wall of the nasal

cavity. In dentate adults, the floor of the sinus and the nasal cavity are about equal in height. (13) 16. anteriorly, the sinus generally extends to the alveolus of the first premolar and sometimes even to that of the canine. However, in dentate maxillae, sinuses extending almost to the midline have been observed as well. 17. septa spring from the floor of the sinus, dividing it into a variable number of recesses, known as alveolar recesses.

18. in most cases, the lowest recess is located in the region of the molar and premolar teeth, where the sinus reaches the alveolar walls, which often form its floor in this area. 19. occasionally, the bony floor of the sinus dehisces and the apices of the tooth roots directly border the mucous membrane of the sinus. Anteriorly the floor of the sinus ascends and is generally thicker than in the molar region. 20. the degree of sinus pneumatization and extension increases with age, in dentate adults, the size and the shape of the sinuses may differ strongly on each side.

21. because no important muscles are attached to the maxillary sinus, the most important functional load acting on this region is masticatory pressure. (14) 22. after loss of the maxillary teeth and reduction of the forces acting on the sinus, the walls of the sinus gradually get thinner; however, they are not at risk of being fractured Unlike the mandible, the maxilla is not supplied with any nerves and vessels relevant to implant surgery.

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Anatomy of the mandible-relevant structures in implant surgery Basically, the mandible is composed of tow parts: A. the ascending mandibular ramus consisting of an articular process (condylar process) and a muscular process (coronoid process). B. the body of the mandible, which is formed by the mandibular base and the alveolar part of the body of the mandible, which attaches cranially. In dentate adults, the alveolar part and the base of the mandible have approximately the same height. An imaginary border could be drawn through the mental foramen.

1. in the chin region of the mandibular base, the triangular mental protuberance juts out far beyond the arch of the alveolar process. Lateral to it, in the canine region, the mental tubercle, whose size varies from one individual to another, protrudes from the mandibular base.

2. a slight concavity known as the incisive fossa is located above the mental protuberance.

(16) 3. on the outside of the mandibular base is an oblique smooth ridge known as oblique line it forms the extension of the sharp, anterior edge of the coronoid process and runs anteriorly, terminating in the region of the second molar. 4. a ridge reinforcing the mandibular base on the lingual side is the mylohyoid line, to which the mylohyoid muscle attaches. Ascending obliquely along the body of the mandible, it runs as far posteriorly as the third molar and separates tow slight concavities, known as the submandibular and sublingual fossae.

5. especially posteriorly, buccolingual bone concavities can often be found caudal to the mylohyoid line. This region also houses the mylohyoid groove, which contains the mylohyoid branch of the inferior alveolar nerve. 6. while the mylohyoid line becomes less distinct as it runs anteriorly to posteriorly, the oblique line becomes more pronounced posteriorly.

7. anteriorly and posteriorly, the body of the mandible thus displays a thicker and thinner lingual cortex respectively.

(17) 8. just lingual to the midline on each side are the digastric fossae to which the digastric muscles are attached. 9. the mental spine protrudes from the midline in the oral direction, the genioglossus and geniohyoid muscles originating from it. 10. vestibularly, the alveolar part of the mandible has long, vertical bone prominences, referred to, as alveolar juga that are generally not present in the posterior region. 11. the portion of the alveolar part that is located cranially to the mylohyoid line protrudes markedly, especially on the posterior side, and forms the bed for the molar teeth, whose posterior border is the retromolar triangle. 12. the free edge of the alveoli, also known as the alveolar margin, is located more inwardly than the basal arch. Poseriorly In the molar region, the alveolar arch travels lingually. 13. coming from the mandibular foramen, the mandibular canal courses through the ramus and the body of the mandible in a double-S curve. 14. it conveys the inferior alveolar nerve, artery, and vein, the nerve bundle being located on the lingual and buccal sides in the molar and premolar regions, respectively. 15. in the premolar region, the canal branches off into the dental canal (incisive canal) anteriorly and ascends from the mandibular base toward the mental foramen as the mental canal.

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16. because nerve tissue reacts sensitively to mechanical strain, most of the mandibular canal is located within the statically neutral portion of the mandible; ie, elastic bone deformation only causes minimum stretching of the canal. 17. while running through the cancellous portion of the mandible, the mandibular canal is encased by a lamella of compact bone that seems to result from stimulation of esteotoblastic activity by the pulsation of the inferior alveolar artery. 18. according to Carter and Keen (1971), the mandibular canal is located near the lower cortex (type 2) in most subjects or more cranially near the tooth roots (type 1) in rare cases. Some times the canal displays a bipartite posterior portion. 19. the mental canal, ascending from the mandibular canal, is often described as a semicircular arch, first swerving anteriorly, buccally, and cranially, and then posrteriorly and buccally. 20. it is thus often clearly located anterior to the mental foramen.

21. the mental foramen has an oval shape with a sharp anterior end and a mean diameter of 5 mm. (19) 22. in 92.6% of the population, it is located in the premolar region. In only 7.4% is situated more posteriorly or anteriorly. 23. the distance between the tow mental foramina varies extensively from one individual to another and ranges between 40 and 56 mm.

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Maxillomandibular biomechanics

The implant surgeon must become accustomed to a new model in which the two jaws are not statically neutral anymore, because loading causes marked bone deformation that also affects bone morphology in the long run. To guarantee an even distribution of functional loads and sufficient supply with appropriately oriented tissue, bone is able to perform adaptive bone remodeling. This property of bone is due to the fact that bone is active metabolically.

Biomechanics of the maxilla The maxilla is a lightweight structure consisting of thin plates (body of the maxilla) that display a broad cancellous margin caudal to the site of pressure absorption. The edentulous alveolar arch generally has a thinner layer of compact bone buccally than palatally. The cancellous portion of the arch is always less dense than in the mandible, ie, it is composed of a relatively coarse mesh that gets tighter only in the incisal region. The contact areas, where the masticatory force is transmitted to the adjacent bones, display several reinforced processes (frontal process, zygomatic process, and pterygoid process). Load transfer from the alveolar process to the other processes takes place via three supporting pillars of the maxillary skeleton:

1. the anterior fronto-maxillary pillar, or fronto-nasal pillar, transmits the masticatory force from the incisal and canine regions to the neurocranium.

(21) 2. the lateral zygomatico-maxillary pillar transfers the masticatory force from the premolar and molar regions via the zygomatico-alveolar process to the zygomatic process and from there, via the lateral orbital margin and zygomatic arch, to the region of the temporal line and to the base of the mastoid process, respectively. 3. the ptergo-maxillary pillar transmits the masticatory force to the body of the sphenoid bone.

Biomechanics of the mandible

-The body of the mandible is a U-shaped arched long bone whose compact portion is supported by the trajectory systems of the cancellous bone and bears the compressive strength imposed by chewing. To perform this function, the mandible has large dimensions. -In more than 68% of the population, the mandible is asymmetric, its right side generally being longer because 93% of all humans chew more on the right side. (22) - The forces acting on the mandible are mostly bending stresses that cause a marked deformation of the mandible. - Asymmetric loading of the mandible during forceful mastication leads to the development of basal convexity on the lower edge of the loaded side and a basal concavity on the unloaded side. - And the horizontal mandibular rami are also bent in the vertical direction. The chin region is affected by bending, shearing, and torsional stresses and moves dorso-ventrally. - Mouth opening and protrusion cause approximation of the molar regions by up to .7 mm and 1.o respectively, a fact that should be considered especially when a super-construction is planned. - The compact bone substance is generally developed more strongly at the caudal margin of the mandiblar body than cranially, buccally, or lingaully, while the cancellous substance is denser in the mandibular base than in the alveolar part of the mandible. - Between the two tension areas of the base and the alveolar part of the mandible is a statically neutral region that houses the mandibular canal. Thus the vessels conveyed by the canal are affected only minimally by the bending and compressive stresses that act on the mandible. - The trajectories are defined as line systems developed by cancellous trabeculae, indicating the direction of the main normal forces (compressive and tensile forces) while being affected only by tension and pressure and not by bending forces. The mandible consists of relatively consistent three-dimensional tension trajectories.

- The two main tension systems are located in the ramus and in the mandibular base. - Other local tension systems can be found at the sites of muscular attachment and in the alveolar process. - Several tension trajectories oriented parallel to the occlusal plane pass through the alveolar part, which absorbs the strain through the teeth hanging suspended from Sharpey’s fibers. (23) - The trabeculae are oriented parallel to the direction of tensile and compressive deformation. The greater the number of the trabeculae, the less deformation can be caused by the compressive forces. - Normally, the trajectories follow the same paths in both dentate and edentulous mandibles - In the inter-foraminal region, the ratio of compact to cancellous bone is approximately 1:1; in the molar region, the amount of compact bone slightly exceeds that of cancellous bone.

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Resorptive processes affecting the maxilla and mandible

Resorption and atrophy of the maxilla and mandible are caused or influenced by the following factors: 1.Mechanical causes: a. Functional factors: - Frequency, direction, and strength of the forces acting in the bone (atrophy due to pressure) - Bruxism b. Prosthodontic factors: - Type and fit of the prosthesis - Duration of prosthodontic treatment - Hours of prosthesis wearing per day - Malocclusion - Lack of prosthodontic treatment (atrophy due to inactivity) c. Surgical factors: - Extraction or other local surgical procedures 2. Inflammatory causes: a. Periodontal inflammatory processes that can cause a loss of alveolar bone prior to tooth loss b. Local inflammatory processes such as osteomyelitis (or

complications related to subperiosteal implants) 3. Systemic and metabolic causes: a. Age b. Sex: - On average, women are affected by atrophy earlier than men - Periodontopathy of pregnancy - Postmenopausal osteoporosis c. Hormonal balance and imbalance - Cushing’s syndrome - Acromegaly (25) - Hyperparathyroidism - Hyperthyroidism d. Further cofactors: - Diabetes mellitus - Type of nutrition - Mineral deficiency - Artheriosclerosis - Generalized osteoporosis - Malabsorption - Anemia - Hypertension -Vitamin C deficiency

The degree of resorption is also influenced by basic anatomic features of the alveolar ridge. It has also been stated that, compared with other bones in the human body, the alveolar bone shows a higher turnover rate that accounts for its higher predisposition to osteoporosis and resorption. In any case, atrophy is considered to be a multifactoral process.

Vertical resorption Examining a large group of patients, Atwood and Coy (1971) found that the total loss of vertical bone volume averages 0.5 mm per year in complete-denture wearers and that the vertical dimensions of the mandible and the maxilla are reduced by 0.4 mm and 0.1 mm, respectively. The typical senile face is caused by an approximation of the chin region to the tip of the nose.

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Horizontal resorption 1. the pressure exerted by the lips, cheeks, and tongue increases horizontal resorption of the bony alveolar process and accelerates resorption of the buccal, lingual, and palatine walls of the alveoli. 2. in both arches, resorption initially occurs on the buccal surface and progresses to lingual surface and may result in sharp bony edges. 3. the anterior and the posterior margins of the alveolus remain unaffected. 4. because horizontal resorption generally starts at the thinner alveolar wall, the new alveolar margin is located more lingually during the first months after tooth loss than was the former row of teeth (in the maxilla and the mandible, the alveolar wall is generally thinner facially than lingually). 5. in advanced stages of resorption, the direction of horizontal alveolar ridge resorption varies markedly between mandible and maxilla: while the margin of the mandibular alveolar ridge moves

buccally particularly in the posterior region (centrifugal, eccentric), the maxillary alveolar ridge moves palatally (centripetal, concentric). 6. compared with the mandibular base, the alveolar part of the mandible shows a slightly interior inclination; progressive resorption causes the ridge to move further buccally and approximately to the mandibular base, however, in the anterior region of the mandible, the alveolar ridge generally shows a slightly labial inclination. 7. the alveolar process of the maxilla, on the other hand, is clearly inclined toward the buccal aspect, gradually approximating the palatally located base in progressive resorption. This leads to absolute expansion of the mandibular arch compared to the maxillary arch

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Resorptive patterns of the maxillary alveolar process

Atrophy of the alveolar process in the maxilla progresses at a markedly slower rate in different patterns than in the mandible. This difference seems to be mainly due to the fact that the alveolar process of the maxilla offers a larger denture-bearing area than does the mandible. Atrophy is most pronounced in the first year after tooth loss and subsequently becomes less intense. According to the findings of Atwood and Coy (1971) the rate of vertical resorption of the maxilla averages 0.1 mm per year; other authors have also reported values up to 0.5 mm per year (Jackson and Ralph, 1980). Fallschuessel (1986) established a classification of the resorptive processes affecting the alveolar process of the maxilla: RRO 0 refers to the fully preserved, dentate alveolus. RRO 1 describes a moderately wide and high, well-rounded alveolar ridge that has not been affected by pronounced resorption yet. RRO 2 residual ridges are small and high.

RRO 3 residual ridges are sharp and high. RRO 4 includes wide ridges that are markedly reduced in height. RRO 5 describes a severely atrophic, flat alveolar process. The majority of maxillae (68%) can be assigned to RRO 1 and RRO 2. In the anterior region, the amount of bone resorption may be up to 65% and the degree of vertical resorption is significantly higher than the posterior region. The posterior region loses considerably less bone during atrophy.

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However, because it is affected by the progressive maxillary

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- however, because it is affected by the progressive maxillary sinus pneumatization, the absolute amount of bone loss up to 80% is considerably greater than in the anterior region. For this reason, the amount of vertical bone available in the posterior region of the alveolar ridge is often less than 10 mm. - other than in the anterior region of the maxilla (the horizontal resorption rate is nearly twice that of vertical resorption), horizontal alveolar ridge atrophy and vertical atrophy are about equally intense in the posterior region. Small knife-edged ridges are rarely found in the posterior region of the maxilla. - in RRO 1, the ridge offers adequate horizontal and vertical bone for implantation. Only in rare cases is sinus pneumatization so pronounced that the vertical ridge dimension becomes inadequate for insertion of an implant. The width of the alveolar ridge generally exceeds 4.5 mm, offering sufficient bone for primary implant stability.

(31) - in RRO 2, the implant must be narrow enough to prevent excessive weakening or even perforation of the bone lamella of the lateral alveolar

process. On average, the required minimum width can be found 3.0 mm above the alveolar ridge. Only in the tuberosity region is sufficient horizontal bone available directly subgingivally. - despite offering adequate vertical bone volume, the ridges in RRO 3 are generally not wide enough for implant insertion. A buccolingual bone width of 4.5 mm can only be found near the base approximately 7.0 mm below the alveolar ridge. As in RRO 2, the tuberosity is an exception and appears to offer adequate bone for insertion. -In RRO 4 and RRO 5, the anterior-canine region offers a small, but often adequate amount of bone for implant accommodation. The posterior region in RROs 4 and 5 never offers sufficient bone for implant insertion because the vertical ridge dimension is too low. This can be regarded as contraindication for endosseous implantation

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Resorptive patterns of the mandibular alveolar process

- following tooth loss, the alveolar ridge of the mandible undergoes extensive remodeling and resorptive process that are characterized by irreversible bone reduction. - in extreme cases of atrophy, the mandible may lose up to 70% of its original bone volume in the region of the mandibular body. - because prostheses will always have a poor fit in the edentulous, severely resorbed mandible than in the atrophic maxilla, implant surgical treatment will be indicated more often and more urgently in the atrophic mandible than in the atrophic alveolar process of the maxilla. - the average rate of vertical resorption is 1.2 mm in the first year after tooth loss and progresses to up to 0.4 mm per year. - for functional adaptation to the reduced cross-section of the mandiblular body, bone apposition to the inner cortex is increased, and the initially coarse-meshed cancellous substance either narrows or is compacted. Atwood (1963, 1971) was the first to draw attention to the fact that these resorptive processes follow a characteristic pattern that may vary individually. He established a classification of six residual ridge orders (RRO) that are characterized by the size and shape of the alveolar process: RRO 1 corresponds to the dentate alveolus. RRO 2 describes the empty alveolus immediately after extraction or natural tooth loss; ie, the ridges grouped in these orders have not been affected by resorption yet. After tooth loss, external resorption leads to a reduction of bone height and a rounding of the sharp alveolar process. The other four residual ridge orders correspond to the different stages of alveolar ridge reduction in the mandible. (33)

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RRO 3 is characterized by high, well-rounded alveolar ridges. RRO 4 includes small, knife-edged ridges. RRO 5 describes low, well-rounded, or even flat alveolar parts. RRO 6 the surface of the mandibular body is depressed and the entire alveolar part has been resorbed. Through use of this classification of residual ridges, the vertical bone volume at each stage of alveolar ridge resorption can be assessed, because there is a correlation between the height of the alveolar process and the residual ridge orders.

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- in RRO 1 to 5, the base of the mandible is hardly affected by resorption. - in RRO 6, on the other hand, reduction of the mandibular base can occasionally be so sever that it even affects the bone substance on both sides of the mandibular canal, leaving a prominent mandibular canal. - statistical analyses have shown that, on average, the premolar and molar teeth of the mandible are lost much earlier than the anterior and canine teeth. - For this reason, the posterior region of the edentulous mandible usually shows a markedly higher degree of resorption and is more frequently affected at the mandibular base (RRO 6) than the anterior-canine region, in which a high, knife-edged ridge can be found in many cases (RRO 4). - according to the findings of Tallgren (1972), bone reduction in the first year after tooth loss is approximately 10 times higher than in the following years. - in most cases, the implant-surgically favorable, high and well-rounded alveolar process of RRO 3 is only available during the first 2 years after tooth loss. After that, resorption results in a knife-edged ridge form (RRO 4) that is maintained for several decades. - in RRO 5, only the inter-foraminal region can be used as a host site, because the risk of damaging the inferior alveolar nerve is too high in the posterior region. - in RRO 6, the entire alveolar ridge has been resorbed so that there may even be complications when an implant is inserted into the inter-foraminal region.