Ideal Implant Positioning

FIG 6.1 (A–D) Various examples of malpositioned implants leading to increased morbidity. 

Mesial-Distal (“X” Axis): Implant–Natural Tooth

Insufficient Implant–Root Apex Distance

Etiology

Implants placed too close to an adjacent tooth root are usually the result of poor treatment planning (inadequate space), poor surgical technique (improper angulation), or placement of too wide of an implant body. This may occur when there are root dilacerations of an adjacent tooth or if a tooth has been orthodontically repositioned to where the tooth root has encroached on the intraroot space (Fig. 6.2A).

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FIG 6.2 Root-implant approximation. (A) Implant ideally needs to be >1.5 mm from root apex. (B) Implant impingement causing apical pathology encompassing the root and implant. (C) Root apex damaged by implant surgical drill. (D) Resultant irreversible root damage. (E) Example of a time-related complication involving an implant placed too close to a tooth root (i.e. immediate post-operative radiograph), (F) Pathology present four years later resulting in the loss of the tooth and implant. 

Complications

Damage to Adjacent Periodontal Ligament.

Implants positioned too close to a tooth risk damage to the periodontal ligament and surrounding structures. This may cause displacement of bone into the periodontal ligament (PDL) space and result in altered blood supply to the adjacent tooth, loss of tooth vitality, apical periodontitis, and internal or external resorption.4

Loss of Implant.

Implants that are placed too close to an adjacent tooth may fail due to infection or bone resorption. If more than 1.5 mm of space exists between the implant and the adjacent tooth, any bony defect around an implant will remain a vertical defect and will ususally not cause bone loss on the adjacent natural tooth. If bone loss around an implant is less than 1.5 mm then the bone on the adjacent tooth will maintain the interdental papilla height.

Loss of Tooth.

If adequate space is not maintained between a tooth and an implant, the adjacent tooth may be irreversibly traumatized and may be lost to a fracture or to internal or external resorption (Fig. 6.2B–D).

Prevention

Ideal Position.

The ideal position is to maintain at least 1.5 mm from the adjacent tooth root or tooth structure. Ideal angulation should be strictly adhered to by evaluating initial osteotomy position via a radiograph of a direction indicator after the first pilot drill (Fig. 6.3A).

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FIG 6.3 Ideal Implant Positioning (A) 3-D image depicting implant >1.5 mm for tooth roots. (A) Ideal positioning. (B–C) Errors in positioning are often a result of using study casts or two-dimensional radiographs (which do not show the true root position) for implant placement planning. Without evaluating the third dimension, it is impossible to determine the exact root position. 
Proper Treatment Planning.

The exact measurement of intraroot distance should be determined to prevent implant approximation to the root. Additionally, evaluation for anatomic variants such as dilacerations of adjacent teeth should always be radiographically diagnosed. The use of CBCT images should be utilized to confirm exact space available between the root and intended implant position. The most accurate CBCT image is the axial slice, which can be easily used to verify measurements.

Use of Study Casts.

Study casts should not be used as the sole determinate of implant position. In most cases, root position and angulation cannot be determined from study casts. A common problem is when positional (fully limiting) surgical guides are made from study casts without three-dimensional evaluation (Fig. 6.3B–C).

Two-Dimensional Radiographs.

In evaluation of intraroot distances, caution must be used in using two-dimensional radiographs as the sole determinants of implant positioning. Both periapical and panoramic radiographs have inherent disadvantages in providing accurate measurements (Fig. 6.4A). Ideally, to verify adequate intraroot distance, a three-dimensional (computed tomography [CT] or cone beam computed tomography [CBCT]) scan should be performed. Accurate measurements may be made in the axial images at three locations: the cementoenamel junction (CEJ), midroot, and root apex (Fig. 6.4B–E).

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FIG 6.4 An often problematic positioning area involves the replacement of the maxillary lateral incisor. This situation often results after orthodontic treatment (A) when there is close proximity of roots (converging). Ideal computed tomography evaluation should include evaluation of the axial images at the (B) crestal, (C) midroot, and (D) root apex. (E) Illustration depicting ideal apical spacing with nonideal crestal positioning. (F) Caution should be exercised in using two-dimensional radiographs because of their inherent positioning complications. 
Caution With Orthodontic Implants.

A popular relatively new implant modality is the use of orthodontic implants temporary anchorage devices (TADs) for anchorage. Orthodontic implants utilize the use of smaller diameter implants, which are inserted perpendicular to the long axis of the tooth in the interradicular spaces of the maxilla and mandible. TADs are utilized for tooth movement (e.g., labial segment retraction or mesial movement of teeth) or for intraoral anchorage, in which tooth movement in all three planes may be accomplished. Interradicular orthodontic implant complications include loss of tooth vitality, tooth loss, osteosclerosis, and dentoalveolar ankylosis.5,6 These implants should be cautiously placed because they often are placed in areas of minimal intraroot distance and above the mucogingival line in attached tissue, which often leads to detrimental effects on adjacent tooth structure (Fig. 6.5A–B).7

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FIG 6.5 Implant-root impingement. (A) Orthodontic implants are often problematic because of their intended position between tooth roots within attached tissue. (B) Temporary anchorage device (TAD) placement in close approximation to an adjacent tooth. (C) A maxillary canine root is often slanted 11 degrees distally and has a distal curvature 32% of the time. When the implant is placed parallel to the first premolar, it may inadvertently encroach upon the canine root. (D) A maxillary first premolar implant may need to be parallel to the canine. (C–D, From Misch CE: Contemporary implant dentistry, ed 3, St Louis, 2008, Mosby.)
Maxillary Lateral Incisor Region.

In regards to implant position, one of the most common problematic areas is the replacement of congenitally missing lateral incisors. Often after orthodontic treatment, there exists a normal mesiodistal distance of the clinical crowns; however, compromised intraroot distance will result. This is most likely to occur in the apical area. Lack of space may contraindicate implant placement or require orthodontic treatment for repositioning of the roots (see Fig. 6.4).

Maxillary First Premolar Site.

Another common area for root approximation problems is in the maxillary first premolar edentulous site. Careful consideration for the angulation of a natural canine must be evaluated. The 11-degree average distal inclination and distal curvature of the canine root frequently place the apex of the root into the first premolar implant area. The implant should be angled to follow the root of the canine and prevent contact with or perforation of the natural root. A shorter implant often is indicated, especially when a second premolar is also present (Fig. 6.5C–D).

FP-2, FP-3, RP-4, and RP-5.

More latitude exists with the mesiodistal positioning of implants for FP-2, FP-3, RP-4, and RP-5 prostheses; however, the anteroposterior (A-P) spread should always be maximized. Because the soft tissue is replaced in these types of prostheses (pink acrylic or porcelain), implants need not be placed in specific tooth positions. Implant positioning is usually dictated by 3-mm spacing between implants and maximizing the A-P spread whenever possible.

Treatment

Initial Placement.

If there is insufficient space between an implant and a natural tooth, the implant should be removed and repositioned, especially if the adjacent tooth is symptomatic. If space is compromised (<6.0 mm), the roots should be repositioned via orthodontics or treatment plan changed to a different type of prosthesis.

Past Placement.

If an implant has been restored and root approximation (<1.5 mm) exists, the tooth/implant should be monitored on a regular basis and the patient informed of the possible morbidity. If symptomatic or radiographic pathology is present, the implant should be removed and repositioned along with vitality testing of the tooth.

Lack of Implant–Coronal Distance

Etiology

Lack of space between the implant platform and the coronal aspect of the adjacent tooth occurs most likely from poor initial osteotomy positioning, poor treatment planning, or the use of too large of an implant body, leading to a situation where the implant encroaches upon the adjacent tooth. Implant clinicians must be aware that most implant crestal platforms are larger than the implant body, which will result in decreased space between the adjacent tooth (e.g., a 3.8-mm implant may have a 4.1-mm platform) (Fig. 6.6 and Table 6.1).

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FIG 6.6 Mesiodistal distance. Implant body diameter vs. implant crest module. The crest module of an implant is often wider than the implant body dimension. (Pictured: External hex implants. Courtesy BioHorizons Implant Systems, Inc.)

TABLE 6.1

Average Mesiodistal Width of Permanent Teeth

Tooth Mandibular (mm) Maxilla (mm)
Central incisor 5.3 8.6
Lateral incisor 5.7 6.6
Cuspid 6.8 7.6
First bicuspid 7.0 7.1
Second bicuspid 7.1 6.6
First molar 11.4 10.4
Second molar 10.8 9.8

From Hebel MKS, Gajjar R: Anatomic basis for implant selection and positioning. In Babbush C, editor: Dental implants: the art and science, ed 2, Philadelphia, 2001, Saunders.

Complications

If implants are positioned too close to the coronal portion of the tooth, many complications can result.

Interproximal Bone Loss.

When there is a lack of space between the tooth and implant, bone resorption will occur due to lack of a sufficient blood supply. Esposito has shown a correlation between increased bone loss and decreased distance of the implant from the adjacent tooth.8

Compromised Emergence Profile.

Due to a lack of space between the adjacent clinical crown and implant, it may be difficult, if not impossible, to form an ideal emergence profile in the new final prosthesis. Lack of proper emergence profile leads to esthetic, hygienic, and soft tissue complications, which increases implant morbidity (Fig. 6.7A–C).

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FIG 6.7 Coronal positioning. (A) Ideal positioning 1.5 to 2.0 mm from tooth. (B) Implant/abutment too close to adjacent crown exhibiting bone loss. (C) Abutment in close approximation to adjacent tooth. (D) Lack of papilla height resulting from lack of space from implant to adjacent tooth. 
Hygiene Difficulties.

Because of the unnatural contours of the prosthesis and the lack of space for cleansibility, proper hygiene techniques will be difficult. This will result in plaque buildup and related periodontal complications.

Reduced Papilla Height.

Because of interproximal bone loss due to the proximity of the implant to the coronal portion of the tooth, a lack of or reduction in the size of papilla will be present. This will result in periodontal conditions and esthetic issues (Fig. 6.7D).

Prevention

Treatment Planning.

The use of an accurate radiographic modality (CBCT) is paramount in determining if sufficient space exists for ideal clinical crown size (measurement on axial slices). Additionally, a study cast and diagnostic wax-up may be utilized. Ideally, 1.5 to 2.0 mm should be present from the implant neck to the adjacent tooth (Fig. 6.8).

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FIG 6.8 Prevention. (A) The use of CBCT with interactive treatment planning allows for the accurate placement and positioning of the implant for ideal prosthetic replacement. (B) Study cast may be used in conjunction with CBCT to evaluate coronal space available. 
Preoperative Modification.

Upon preoperative evaluation, if inadequate space exists for implant treatment, the following options may be completed to increase mesiodistal distance:

1. Enameloplasty (modification of the interproximal contact areas) may be completed on the proximal contours of the adjacent teeth to increase mesiodistal dimensions. However, aggressive modification may lead to hypersensitivity and possible endodontic intervention (Fig. 6.9A).

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FIG 6.9 Possible treatment options for inadequate spacing. (A) Enameloplasty of adjacent tooth allows for additional space for prosthesis emergence. (B) Orthodontic repositioning allowing more space additional spacing (when 12 to 14 mm of space is available, the mesial and distal contour of the adjacent teeth is modified to gain addition space). (C) A periapical radiograph of two 3.7-mm implants to replace one molar, in which the proximal contours of adjacent teeth were reduced. (D) An intraoral view of two osteotomy sites for the replacement of a mandibular first molar. (E) When the mesiodistal space is 12 to 14 mm, the implants may be offset to increase space between the implants. (F) In the mandible, the distal implant is positioned more buccal and the mesial implant more lingual. In the maxilla, the mesial implant is more buccal and the distal implant is more lingual. (G) Intraoral view of mandibular molar with offset implants, the distal implant being more lingual to facilitate easier flossing (hygiene). (C–G, From Misch CE: Contemporary implant dentistry, ed 3, St. Louis, 2008, Mosby.)

2. Orthodontic intervention may be utilized to upright a tilted adjacent tooth to increase the intratooth space. For larger spaces (multiple spaces), one implant may be placed and an orthodontic spring incorporated in the transitional crown. The spring pushes the distal tooth more distal and, after orthodontic movement, the second implant may be inserted with less risk and improved hygiene between each implant. Another option is to reduce the space orthodontically and place only one implant and crown (Fig. 6.9B).

3. For larger spaces (multiple implants) the implants may be offset with one implant placed buccal and the other implant on a diagonal toward the lingual.9 The diagonal dimension increases the mesiodistal space by 0.5 to 1 mm. In the mandible, the most anterior implant is placed to the lingual aspect of the midcrest and the more distal implant is placed to the facial aspect to facilitate access of a floss threader from the vestibule into the intraimplant space. The occlusal contacts also are slightly modified on the buccal aspect of the mesial implant to occlude over the central fossa. In the maxilla, the anterior implant is placed facially and the distal implant palatally to improve esthetics. The distal occlusal contact is placed over the lingual cusp, and the mesial occlusal contact is located in the central fossa position. The cervical esthetics of the maxillary molar are compromised on the distal half of the tooth to achieve greater intratooth distance and easier access for home care. This maxillary implant placement requires the intraimplant furcation to be approached from the palate, rather than the buccal approach, as for the mandible (Fig. 6.9C–D).

Surgical Adjuvants.

A large number of implant clinicians are utilizing positioning devices that allow for ideal osteotomy preparation and adherence to an implant placement at least 1.5 to 2.0 mm from the adjacent tooth. A surgical spacer may be used, which enables the initial osteotomy site to be placed at the correct position, allowing for adequate space between the tooth and final implant position (Fig. 6.10A–B). Surgical guidance systems (Salvin) may also be used to ensure ideal implant placement (buccal-lingual and mesial-distal spacing) and may be used with any surgical drill system (Fig. 6.10C–D). The most accurate positioning adjunct is the use of CBCT-generated surgical templates (tooth supported) (Fig. 6.10E).

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FIG 6.10 Ideal placement. (A–B) Positioning device placed on the distal contact of adjacent tooth allowing for ideal osteotomy site in the edentulous space. (C–D) Surgical guidance systems may be used for various situations and spacing between teeth. (E) Tooth-supported surgical template allowing for accurate implant positioning. (C, Courtesy Salvin Dental Specialties, Inc., Charlotte, NC.)

Treatment

Initial Placement.

If the position of the implant is less than 1.0 mm from the adjacent clinical crown, removal and reposition of the implant should be completed. If the implant is positioned 1.1 to 1.5 mm from adjacent tooth, removal or modification (enameloplasty) of the adjacent tooth may be completed, as long as irreversible damage to the tooth is not done.

Past Placement.

If implant has been restored and root approximation (<1.5 mm) exists, the tooth/implant should be monitored. If symptomatic, the implant should be removed and repositioned along with vitality testing of the tooth.

Too Great a Distance Between Implant and Tooth

Etiology

Too much space between an implant and adjacent tooth is the direct result of poor treatment planning and/or surgical technique (Fig. 6.11). When implants are placed more than 2 mm from an adjacent tooth, a cantilever effect will result on the marginal ridge of the implant crown. In some cases, this may lead to biomechanical overload or esthetic issues with resulting bone loss and increased morbidity.

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FIG 6.11 Illustration depicting implant placement too far from a tooth (mesial) leading to a cantilever effect and biomechnical issues. 

Complications

Overcontoured Crowns.

Because of the excessive space between the implant and tooth, overcontouring of the final prosthesis is required to achieve a contact area with the adjacent tooth. This results in biomechanical issues as well as esthetic complications.

Atypical Prosthesis.

Because of the need to obtain interproximal contact, the final prosthesis will be atypical, which may lead to increased difficulty in prosthetic impression, laboratory, and insertion procedures (Fig. 6.12).

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FIG 6.12 Implant positioning too far from tooth. (A) Implant placement too far from adjacent crown resulting in an excessively large, cantilevered crown. (B) Resultant prosthesis gives rise to overcontouring/cantilever effect. (C–D) Atypical prosthesis because of nonideal implant placement and need to obtain contact area, which results in biomechanical complications and food impaction. 
Cantilever Effect (Biomechanics).

The resultant cantilever from a malpositioned implant results in a biomechanical disadvantage with damaging moment forces, which may result in bone loss. Cantilevers present on implant prostheses are more problematic than on natural teeth for several reasons. Forces are magnified to the entire implant system, which may result in implant screw loosening, cement retention failure, or even possibly the mobility and failure of the implant itself. Secondly, because the implant is void of a periodontal ligament, there is no stress release system in place to protect the implant. Weinberg et al have shown a 10-degree increase in cusp inclination leads to a 30% increase in the force applied to the restoration. Additionally, studies have shown that a 10-degree increase in implant inclination can lead to a 5% increase in force-related function.10 A 1-mm increase in the horizontal offset of an implant restoration may produce a 15% increase in torque during function, and a 1-mm increase in the vertical offset introduces a 5% increase (Fig. 6.13).11 The overcontoured crown leads to resultant shear forces, which may lead to component failure (i.e., screw loosening, screw fracture, implant fracture).

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FIG 6.13 (A) A posterior implant with a cantilevered crown to the mesial. (B) The implant fractured within a few years. It is often more predictable to join an implant to a natural tooth than to cantilever from one implant. (From Misch CE: Dental implant prosthetics, ed 2, St. Louis, 2015, Mosby.)
Food Impaction.

Food impaction is a common complaint from patients with an increased implant-tooth distance because periodontal maintenance is difficult as a result of related soft tissue complications.

Periodontal Complications.

Because of difficulty in hygiene, chronic tissue problems often result, which may cause perimu­cositis or peri-implantitis, leading to implant morbidity (see Figs. 6.116.12).

Prevention

Positioning Devices.

Prevention for malpositioning may include the use of special positioning devices that allow for ideal osteotomy placement and adherence to the ideal placement of 1.5 to 2.0 mm from the adjacent tooth. These predetermined distance spacers will minimize the possibility of placing the implant too close or too far from the adjacent tooth (Fig. 6.14).

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FIG 6.14 Ideal calculation for implant spacing. 
Surgical Templates.

A CBCT-generated template may be used to accurately place the implant. When an implant is to be placed adjacent to a tooth, a tooth-supported guide is the most accurate in comparison to bone- or tissue-borne guides (see Fig. 6.10A–B, E).

Treatment

Initial Placement.

If nonideal placement is determined during surgery, the implant should be repositioned in the ideal position (i.e., 2 mm from adjacent tooth). The osteotomy should be initiated with the following formula:

12 diameter of the implant+2.0 mm from tooth

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In other words, a 4.0-mm implant pilot osteotomy would be 2.0 mm + 2.0 mm = 4.0 mm from the adjacent tooth. If the initial osteotomy is not ideal, a Lindemann drill (side cutting) is used to reposition the osteotomy into the correct position.

Past Placement.

If the implant has already been placed and is ready to be restored, the amount of occlusal force should be assessed to determine the ideal treatment:

Minimal Occlusal Forces: If favorable force factors exist, then a cantilever (overcontoured crown) may be fabricated with (Fig. 6.15A):

Narrow occlusal table

Minimal cusp height: It has been reported that every 10-degree increase in cusp inclination leads to a 30% increase in the torque applied to the restoration during function10

No lateral contacts

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FIG 6.15 Treatment of excessive distance. (A) Prosthesis with narrow occlusal table, minimal cusp height and no lateral contacts. (B) To decrease implant-tooth distance, the natural tooth may be elongated or overcontoured by the use of a crown or bonding (arrow). 

High Occlusal Forces: If unfavorable forces are present, a cantilever is contraindicated and the mesiodistal distance is reduced by either:

Overcontouring adjacent crown (e.g., crown, composite) (Fig. 6.15B)

Remove implant and reposition

Mesial-Distal (“X” Axis): Implant–Implant

Lack of Implant-Implant Distance

Etiology

When implants are placed too close together, it is usually the result of poor treatment planning or surgical technique (Fig. 6.16). Treatment planning issues can be prevented by using ideal spacing rules. The guidelines include approximately 3.0 mm between implants, and >1.5 to 2.0 mm from adjacent teeth.

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FIG 6.16 Illustration depicting inadequate space between two implants. 

Complications

Bone Loss.

Because of the lack of interproximal bone a decreased blood supply will result in bone loss. Tarnow et al have shown that implants placed less than 3.0 mm apart may have adequate stability and function; however, this placement will likely result in crestal bone loss. In this study, implants with greater than 3 mm distance between implants resulted in a 0.45 mm bone loss, while implants positioned less than 3 mm had over twice the amount of bone loss, or approximately 1.04 mm.12

Lack of Interimplant Papilla.

When lack of space exists between the implants, the resultant bone loss will be responsible for the loss of the papilla. As the bone resorbs, the distance between the contact point of the crowns and the bone level increases. As this distance increases (i.e., above 5 mm), the papilla will become smaller in size and contour.

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Jan 12, 2018 | Posted by in Implantology | Comments Off on Ideal Implant Positioning
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