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Abstract:

The introduction of implants to dentistry has helped many edentulous patients to have a more reliable functional and esthetic alternative to fixed and removable prosthetic appliances. The optimal 3-dimensional positioning of the implants secures the best function and esthetic of the final restoration and at the same time avoids the biomechanical complications and failure that might take place subsequent to the wrong positioning of the implants. The aim of this project: is to highlight the importance of placing the implants in the optimal 3-D position. Computer guided template-based implant placement will be discussed as a precise and predictable tool in the planning and in the execution of the implant placement. The objective: is to enable the placement of the implant in a way that secures the highest predictable success with the least complications. Materials and Methods: Out of 350 downloaded relevant articles only 161 articles were chosen and referenced. The excluded articles were either written in languages other than English, descriptive of new fabrication methods of surgical guides, single case reports or experimental done on small sample size{less than 5}. Results: Most of the revised papers are case series or experimental studies done on animals or cadavers. Even the systematic reviews were based on those articles. Computerized tomography (CT) and interactive software programs are proven to be important for accurate treatment planning. The accuracy of transferring the treatment plan precisely by the aid of CAD/CAM fabricated surgical guide to the patient mouth are reported to be more predictable than free hand implant placement especially in the compromised cases. Conclusion: The published literature highlighted the higher predictability, accuracy and precision of computer guided template based implant placement over conventional free hand placement; but supporting strong evidence is lacking. Good controlled clinical studies with long term follow up is needed in this regard. For the time being and with the reported high success of conventional free hand implant placement, it seems that free hand implant placement seems to be predictable at least in the hands of experts or with the uncompromised cases.

Introduction:

In the 19th century many aspects of life were affected by the industrial revolution, especially in sciences and manufacturing. Dentistry was not an exception of this. However, it was in the Victorian era when the basis of modern dental care was first set. A large number of the developments in dentistry were just modifications of industrial inventions. Many of such developments were impossible before the introduction and use of electricity, which led to the invention of more complex surgery equipments {Gelbier S, 2005}. Another very important development took place as a consequence of the invention and the use of computers in the various fields of dentistry {Schleyer Titus K.L., 1999}.Computer guided implant placement is one of the aspects that shows how dentistry has got use to the computer and its science { Azari A. and Nikzad S.; 2008}.

In fact, it is true that the advancement in dentistry and the increased public awareness public of the importance of keeping their teeth healthy have led to the decreased frequency of edentulism. Such declination varies not only among different countries but also among the different geographic regions within the same country and among the different groups of people of different cultural and social backgrounds. However, it is predicted that in the next few decades, there will be very low proportions of edentulism in elderly persons (over 65 years). Teeth loss increases with age, this means that in the future edentulism will occur at later stages in life {Lang NP & Muller F. 2007; Lang N.P.

& De Bruyn H., 2009}. In such situations and in addition to the "natural delays" in the healing of elder individuals {Goodson 3rd WH& Hunt TK 1979}; the patients at that age will be most properly affected by co-morbidities and unfavorable ageing conditions like osteoporosis, uncontrolled diabetes mellitus, hypothyroidism and chronic renal disease which are among the diseases that negatively affect the bone quality and consequently implant success. Some older age related diseases like Parkinsonism, Alzheimer can affect the ability to perform adequate oral hygiene and this may lead to inflammation and bone loss around implants {Roberts WE. et al.1992; Elsubeihi ES. & Zarb GA. 2002; Marder MZ. 2004}.For this, dental professionals need to develop their skills and to use new techniques that offer the patients with the safest, accurate results and the least morbidity and to be able to manage such elder patients and their unfavorable conditions which sometimes may preclude the placement of the implants {Lang NP & Muller F, 2007; Lang N.P. & De Bruyn H., 2009}.

Back ground:

Traditionally, lost teeth were replaced by removable partial dentures {RPD}, fixed partial dentures {FPD} and complete dentures in cases of full edentulism { Bragger U et al 2005}. Since the introduction of implants to dentistry by Branemark{1977}, many of the edentulous patients have been able to have more reliable functional and esthetic alternative to fixed and removable prosthetic appliances{Lang N.P. & De Bruyn H., 2009}. The increasing Demand for implant retained restorations in the last few decades resulted in the need for new improved techniques to ensure the most proper implant position to restore the patient properly {Ganz S.D,2001} .

Historically the implants were placed where there is bone { Kopp et al 2003}. They were considered successful when integration is achieved {Branemark et al.1977}. However, as the functional and esthetic demands of the patients have increased significantly; the success of implant-supported restorations is no more only related to the level of implant integration in the bone but also to the proper positioning of the implants and subsequent prosthetic outcome{ Lal et al.2006}. For this proper treatment planning has become mandatory. The main goal of the treatment plan is to place the implants in the optimal position as dictated by function and esthetics of the definitive restoration. This in turn avoids the biomechanical complications and failure that might take place subsequent to the wrong positioning of the implants {Garber DA& Belser UC. 1995; Kopp et al. 2003 & Park et al. 2009}.

The aim of this project: is to highlight the importance of placing the implants in the optimal 3-D position. Computer guided template-based implant placement will be discussed as a precise and predictable tool in the planning and in the execution of the implant placement.

The objective

is to enable the placement of the implant in a way that secures the highest predictable success with the least complications.

Materials and Methods:

Multiple searches have been made through Science Direct and Google Scholar. The following key words were used with different combinations: systematic review, computer-guided, computer-assisted, image-guided, robots, dental implants, complications, treatment planning, radiography, computerized tomography, accuracy, flapless surgery, zygomatic implant, bone density, augmentation volume, immediate loading, free hand surgery, Steriolithographic templates. As the topic of computer guided implant placement is a modern topic; the selected articles were limited to the articles published from the year 2000 up to 2010. However some older but relevant articles were hand searched, selected and referenced. Almost 350 articles were found to be relevant to the different points to be covered in the project were downloaded; after reading their abstracts it was found that many of them were either clinical case series and reports or experimental studies made on animals or cadavers, many of the systematic reviews were found but again nearly all of them were based on the mentioned articles. The articles of single case reports and the experimental studies with very small sample {less than five} size were excluded. Also, the articles that focus on the fabrication of new surgical guides were excluded. Articles in languages other than English are excluded too. Only 161 were selected and referenced in this project.

Results:

Most of the revised articles concerning the accuracy of CAD/CAM template based implant placement are case series or experimental studies done on animals or cadavers. Even the systematic reviews were based on those articles. Based on the revised articles; computerized tomography (CT) and interactive software programs are proven to be important for accurate treatment planning. The accuracy of transferring the treatment plan precisely by the aid of CAD/CAM fabricated surgical guide to the patient mouth are reported to be more predictable than free hand implant placement especially in the compromised cases.

Discussion:

Complications associated with improper implant positioning:

Marginal bone loss and consequent mechanical and esthetic problems:

The correlation of marginal bone loss and improper positioning of the implant is reported in the literature. The wrong angulation of the implant is usually compensated by the use of angled abutments, in this situation the load carried by the implant is mostly off-axis, such unfavorable load leads to the bone destruction around the implant and other mechanical complications like screw loosening, fracture and/or implant fracture {Chun-Li Lin et al., 2005 & Saab X. E. et al 2007}.The improper placement labially, will lead to thin labial bone and subsequent bone loss and gum recession{Buser D. et al. 2004}, lingually, in addition to the thinning of lingula bone it results in emergence problems as seen with ridge-lap restorations. Such restorations are difficult to maintain and consequent inflammation and bone loss is unavoidable {Tarnow DP. 1995; Belser UC. et al. 1998}. The placement of the implant too close to the adjacent tooth can cause resorption of the inter-proximal alveolar crest {Esposito M. et al. 1993; Thilander B. et al. 1999}. If the implant is placed too far apically using extensive countersinking, the polished implant collar will come into contact with the bone and this in turn induces bone resorption because polished surface does not integrate {Buser D. et al. 1991a; Hämmerle et al. 1996}, also the micro gap will come closer to bone and unnecessary bone loss will take place. Disuse atrophy due to subnormal mechanical stimulation is another explanation for bone loss around polished implant neck or crest modulous {Al-Sayyed, A. et al. 1994; Vidyasagar L. & Apse P. 2004}.

Nerve injury:

Altered sensation of the lower lip as a result of inferior alveolar nerve injury is one of the serious complications of mandibular implant osteotomies {Bartling et a. 1999; Vazquez L. et al. 2007}.This is especially if the magnification factor on the panoramic radiograph is misinterpreted {Vazquez L. et al. 2007}.

The injury of the mental nerve can also lead to the same symptoms; the mental foramen is an important landmark during surgical procedures in the lower premolar area. it is usually located at the apex of the second mandibular premolar or between apices of thetwo premolars. However, in some cases its location can vary from the mandibular canine to the first molar. Unfortunately, The foramen may not appear on conventional two dimensional radiographs, in this condition a computerized tomography (CT) scans are important as they are more accurate for the detection of the mental foramen than conventional radiographs { Bartling et a. 1999 , Greenstein& Tarnow, 2006}.

Lingual bundle:

Though it is rare complication, the wrong implant placement in the mandible may lead to the perforation of lingual cortex with a great possibility of lingual bundle injury with a subsequent fatal bleeding and hematoma formation. Under the effect of profuse bleeding and as a result of the progressive expansion of the lingual, sublingual, sub- mandibular, and sub-mental hematomas ; the tongue and the floor of the mouth can be displaced leading to the rapid and complete obstruction of the airway {Mordenfeld A et al. 1997 and Kalpidis & Setayesh , 2004}.

Devitalization of the adjacent teeth:

In addition to the risk of losing the inter-proximal bone {Esposito M. et al. 1993; Thilander B. et al. 1999}; the placement of the implants in close proximity to the adjacent teeth may lead to the loss of the teeth vitality especially if the roots are injured during drilling for implant placement {Jemt T.& Pettersson P. 1993; Rubenstein J.E.& Taylor T.D. 1997; Goodacre CJ. et al. 1999 & Schwartz-Arad D. et al. 2004}.

Other possible rare but serious complications:

Some other rare complications can take place due to poor treatment plan and wrong implant placement like perforation of nasal and sinus floor {Nahlieli O. et al 2008},injury of the submandibular and/or sublingual salivary glands {Nahlieli O. et al 2008},mandibular fracture especially in osteoporotic and atrophied mandible{ Raghoebar G.M. et al.2000 & Meijer H.J.A. et al. 2003}.

Criteria of optimal implant position:

The ideally placed implant should be surrounded by uniform bone volume and density; this provides the implant with a good bony support against the multi directional long term loading. A minimum of more than 1 mm bone thickness is recommended to keep around the body of the implants {Nancy L.C., 1993}. In addition, the implant ideally should be placed in the geometric centre of the crown; this reduces the off axis loading and prevents many of the biomechanical complications and its subsequent poor esthetic outcomes {Galanis C.C et al. 2005& 2007}.

Implant tooth distance:

A minimum of 1- 1.5 mm distance between the implant surface and the adjacent teeth needs to be respected {Buser D. et al 2004}. However, a 3 mm distance between the implant and an adjacent natural tooth is recommended to minimize the potential for damage to the supporting structures of the natural teeth {Adell et al, 1986; Hobo et al, 1989}.

Implant to implant distance:

To avoid bone resorption in-between the adjacent implants; a minimum of 3 mm inter-implant distance is recommended {Hobo et al, 1989& Traini et al. 2007}.

Implant to vital structure distance:

To avoid nerve injury during implant surgery in the mandible, some guidelines should be considered with respect to verifying the position of the mandibular and/ or the mental foramen and to validate the presence of the anterior loop of the mental nerve. These guidelines included leaving a 2 mm as safety zone between the implant and the nerve. Once the safety zone is identified, implants can be placed safely; and before the placement of any implant anterior to the mental foramen that is deeper than the safety zone, the mental foramen must be explored to verify the possibility that an anterior loop is there {Buser &Von Arx 2000, Greenstein& Tarnow, 2006}. In consideration of the risk of surgical complications during implant placement, bone grafting or other surgical procedures where risk is anticipated, a CT examination should routinely be performed before any surgical approach { Ganz S.D,2001; Scaravilli MS, et al 2009& Naitoh et al. 2010}.

To reduce the probability of such serious complication, some preventive measures should be taken before, during, and after implant placement in the anterior part of the mandible, among such measures are the awareness of the regional arterial anatomy, proper treatment planning through radiographic and clinical evaluation of the osseous morphology, and the right angulation and length of the selected implant and finally the skill of the surgeon {Kalpidis & Setayesh , 2004}.

Dental CT is a valuable tool for the assessment of jaw bone anatomy and can easily demonstrate the occurrence, position, diameter and course of the lingual vascular canals of the mandible; for this, a CT examination should routinely be performed before any surgical procedure to the anterior region of the mandible to verify the presence of the mandibular lingual vascular canal {MLVC} and to evaluate the lingual cortical bone thickness and density to avoid perforations and the life threatening bleeding {Scaravilli MS, et al 2009& Naitoh et al. 2010}.

Distribution of the implants in edentulous jaws:

Ideal implant distribution and placement is critical in order to secure the optimal mechanical and esthetic outcome of the definitive restorations as well as enabling the patients to maintain proper hygiene. The placement of the implants in the inter-proximal positions may cause problems from an aesthetic, mechanical and hygiene perspective {Jivarj S., 2006}. Also, the antero-posterior distribution of the implants should allow equal distribution of load over a wide area with minimal cantilever length {Adell R et al. 1990; Palmqvist S et al 1994 & Jivarj S., 2006}. When multiple implants are placed to retain a prosthetic appliance; parallelism between the implants should be secured otherwise the unfavorable off-axis loading will not be avoidable {Arfai N.K. & Kiat-amnuay S. 2007}. In the case of implant and tissue supported overdentures both implant placement and distribution become critical; where the Implants have to be placed so that when a bar is constructed it has a straight line connection between the implants and does not encroach on the palatal/lingual denture bearing area. The distribution of implants should also be in the way so that adequate space is available for the clip {Jivarj S., 2006}. When more force - from the opposing occlusion- on the implants are anticipated more implants should be placed to share the load {Jivarj S., 2006}.

Treatment planning:

Until recently the main concern was directed to the surgical aspect of implant placement {Ganz S.D, 2001}. The implants were planned to be placed where the bone is found {Kopp et al 2003}. The esthetic and functional outcome of the final prosthesis was not much considered {Ganz S.D, 2001}. The new concept of prosthetically driven treatment planning and implant placement requires careful evaluation of the surgical site. In addition, it must relate the 3-D location of the future prosthetic restoration to the optimal 3-D implant position. This position must be discussed and agreed on between the restorative dentist and the dental surgeon {Garber DA& Belser UC, 1995, Kopp et al 2003 & Park et al. 2009}.

In the early days dentists who were believing in this concept {prosthetic driven} were mostly dependant on conventional radiography, wax-up prostheses and/or surgical templates made on the hard stony surfaces of the study casts, and to overcome the problem of transferring the plan to the operative site, customized radiographic and surgical templates have become an integral part of treatment (Becker CM & Kaiser DA. 2000, Almog DM et al.2001}. Very soon later, it was found that the hard surface of casts is not equal to the soft tissue surface of the oral cavity, and this method may not be as accurate as necessary for treatment purposes. Additionally, it was established that templates fabricated on the study cast without knowledge of the exact anatomy below the surface cannot be considered reliable {Lal K. et al. 2006 and Widmann G& Bale JR 2006}.

The traditional tools for the treatment planning of dental implants include detailed clinical examination, panoramic, cephalometric and peri-apical x-ray films, diagnostic wax-up and articulated study models. Other diagnostic aids may include photography and ridge mapping technique for the assessment of the implant bone sites. Advanced diagnostic tools such as tomography, digital radiography, and CT scan film allow for a more accurate pre-surgical evaluation sites { Traxler M. 1992, Tyndall D. A. et al. 2000, Flanagan D. 2001, Ganz S.D,2001, Perez A.M. et al 2005, Guerrero M. E. 2006, Chen Lung-Cheng 2008, Loubele M. et al 2008}.

Study models:

Accurately mounted casts are critical in assessing prosthetic and inter-occlusal space limitations. Spatial constraints must be considered as a matter of practicality {Jivraj S et al 2006}. Study casts are also valuable tool to evaluate occlusion {Hayasaki et al. 2005 } and to help in the treatment planning through diagnostic wax- up {Katsoulis J. et al. 2008}. Moreover, radiographic and surgical templates can be constructed out of such study models {Lal K.et al. 2006; Katsoulis J. et al. 2008 & Rubio-Serrano M. et al 2008}.

Bone sounding

No doubt that the direct measurement {DM} of the ridge size is the most accurate diagnostic tool. However, treatment planning especially in the big cases calls for collecting information before surgery. This saves time and money, avoids the unexpected complicated surgeries like harvesting bone for ridge augmentation and increases the predictability of the treatment. When ridge mapping {RM} is compared to direct measurement {DM} of the ridge size and to Linear tomography {LT} & cone beam computerized tomography {CBCT}; ridge mapping {RM} seems to be the most reliable pre- operative clinical method to determine the ridge size {Perez L.A. 2005 & Chen L.C. 2008}., linear tomography is reported to underestimate the ridge size {Perez L.A. 2005} while cone beam computerized tomography is reported to overestimate the ridge measurements {Chen L.C. 2008}.

However, ridge mapping is not only an invasive procedure but also a difficult to use in the cases of shallow labial and/or lingual vestibules. Additionally, in ridge mapping the actual position of the inferior alveolar nerve cannot be verified {Perez L.A. 2005 & Chen L.C. 2008}.

Conventional two - dimensional radiography:

Actually, conventional 2-D radiography {panoramic, cephalometric and intraoral views}, which is widely used for the treatment planning, has important diagnostic limitations, such as magnification and distortion, setting errors and position artifacts {Tyndall D.A. & Brooks S.L.; 2000& White SC et al, 2001}. Moreover, these 2-D radiographs do not show lingual anatomy or provide complete three dimensional (3-D) information about the dental arch {Nikzad and Azari. 2008 }. These limitations make the 2-d radiography is less than optimal tool for the diagnosis and treatment planning of dental implants; where according to recommendations provided by The American Academy of Oral and Maxillofacial Radiology (AAMOR), The aim of the preoperative dental implant treatment planning is to place the optimum number and size of implants to secure the best prosthetic outcome. This can be achieved only if a thorough knowledge of the patient's bony anatomy in 3- dimensions is provided in the radiographic examination .

Proper treatment planning requires that the clinician evaluate the suitability of the remaining bone for placement of implants. The clinician must determine if there is enough height, density, width of bone, and an appropriate axis of orientation for a successful prosthetic outcome { Tyndall D.A.& Brooks S.L 2000& White SC et al, 2001}.

CT scanning

Although Computerized Tomography {CT} scans have been used in the medical field since 1973; it was not before 1987 when this new technology became available for dental purpose {Ganz S.D, 2001}. In implant dentistry; Computerized Tomography {CT} scan is one of the most important diagnostic tools which significantly improved the clinician's ability to diagnose and to put accurate treatment plan because it helps in viewing the anatomy and dental related anomalies of the jaws {Dula K. et al. 1994; Abrahams JJ& Berger SB,1998 & AbrahamsJ.J.& Hayt M.W.,1999 & Lal K. et al. 2006}and in the proper choice of implant size and angulation and this in turn helps to avoid injury of critical structures such as the mandibular canal or maxillary sinus {Ganz S.D,2001; Scaravilli MS, et al 2009& Naitoh et al. 2010}.

Moreover the CT scan allows the visualization of the scanned jaw bone in a series of cross sectional, axial and panoramic views .This makes the planning of implant placement more precise in relation to the bone and future prosthesis especially when the a radiographic template {scano-guide} is used during scanning{Lal K.et al. 2006& Rubio-Serrano M. et al 2008 }.However CT scan by itself is nothing but series of axial and coronal 2-D images and the clinician needs to integrate such images in his mind to gain the desired information in 3-D {Gillespie J.E. & Isherwood I.1986}.

Interactive software programs:

The present development of clinical computer applications allows the clinicians to obtain 3-D models to plan virtually real situations {Rubio-Serrano M. et al 2008}.

Interactive computer software is now increasingly used as a tool for implant diagnosis, planning and treatment execution. Firstly, it is used in connection with imaging techniques, such as computerized tomography (CT) or magnetic resonance imaging (MRI) { Hassfeld S, Mühling J 2001}. Secondly, it is used for the construction of surgical templates carrying the information necessary to transfer that planning to the mouth of the patient. In most of the cases, this procedure is based on stereolithographic models { Ewers R . et al 2005 & Ganz S.D. 2005 , Schneider D. et al 2009 }. There are different commercialized soft ware programs are available, such as: Implametric, SimPlant { Ganz S.D. 2005& Parel SM & Triplett RG 2004}, Nobel Guide {Rocci A et al. 2003}, med3D { Engelke W& Capobianco M. 2005}, etc. Most of the programs display an axial cut and a panoramic cut with multiple bucco-lingual cuts {parasagittal} and reformatted 3-D image {Parel SM & Triplett RG 2004}. In the 3D image, bony structures are visualized with the possibility of incorporating other anatomical structures or even soft tissues {Schneider D. et al 2009}.

Bone density

One of the good tools in the software programs is the ability to evaluate the bone density {quality} during the analysis of CT data. The importance of bone quality {density} for the success of dental implants is agreed on in the literature { Jemt T, Lekholm U 1995; Esposito M et al 1998; Shahlaie et al 2003 & Park et al 2008}.As suggested by Lindh et al.1996 Site-specific measurements are important, not only for a general prediction of treatment prognosis but also in the evaluation of how long of an interval between first- and second-stage surgical procedure and loading is needed { Friberg B et al 1991,1995a &1995b}. Moreover, to have accurate preoperative measurements of the bone density helps in avoiding the placement of the implants in the areas of poor quality {Norton M.R. and Gamble C. 2001& Shahlaie et al 2003}.

The strong correlation between the average CT number and the concentration of hydroxyapatite in bone is reported {Maki et al 1997}, and the quantitative CT in Hounsfield units {HU} are accepted as a valuable supplement to the subjective bone density classification defined by Lekholm and Zarb {1985}. The Hounsfield index is a standardized scale for reporting the reconstructed CT values. It is a measure of the attenuation coefficient which varies among different tissues, it is based on the density of air (-1000),water (0) and dense bone(1000){ Shapurian T. et al. 2006}. Misch CE {1993} stated that the bone density measurements using CT scan is more accurate than radiographic assessment. And he classified bones into 5 categories according to density: D1 bone had density above "1250 HU"; D2 = "850-1250' HU"; D3 = "350-850 HU"; D4 = "150-350 HU"; and D5, below"150 HU". For this, the use of CT scanning and interactive software programs is considered as a viable and accurate method to measure bone density {Norton M.R. and Gamble C. 2001& Shahlaie et al 2003}.

Recently, efforts in the oral imaging field have focused on developing tools that accurately and automatically measure bone density by measurements of x-ray absorption{ de Oliveira R. C. G. et al. 2008}. Thanks to such efforts, now the CT images in DICOM {Digital Imaging and Communications in Medicine} format contain the required data of bone density which enables the different software programs can measure it {Norton M.R. and Gamble C. 2001& Park et al 2008}.

Bone graft volume

Accurate evaluation of the 3-D bone volume before surgery is another advantage of the use of soft ware programs in the treatment planning in implant dentistry. The lack of enough bone volume frequently precludes the conventional implant placement. In this condition the bone volume needs to be improved by different augmentation techniques and /or materials {Esposito M. et al, 2008}.The detailed information about the needed bone volume before surgery is of much help in determining the best donor site { Krennmair G. et al 2006 & Verdugo F. et al 2009} and can help in estimating the amount and costs of the xenographic bone substitute required for the augmentation surgery{Clavero J.& Lundgren S. 2003}. Moreover, knowing the needed bone volume in advance helps in minimizing the duration of the surgery and this in turn minimizes the chances of complications and reduces the expenses for the patients {Cricchio G. & Lundgren S.2003}. The computerized tomography {CT} can produce series of accurate cross sectional images and by the aid of soft ware programs like Simplant {Materialise, Leuven, Belgium} the 3-D volume of area to be augmented can be calculated {Hatano N. et al. 2004& Krennmair G. et al. 2006}.

Flapless implant surgery:

Predictable flapless implant surgery is one of the fruits of the application of modern technology like CT scan, interactive software programs and CAD/CAM fabricated surgical guides {Sclar A.G. 2007}. Minimal invasive surgery techniques are applied to a wide variety of interventions. The main aim is to reduce the costs of the treatment and patient healing time {Rubio-Serrano M et al 2008& Valente F. et al 2009}.

The traditional implant protocol set by Branemark requires a duration of a few months for osseointegration of the endosseous implants before the connection of definitive dental prostheses {Adell R. et al 1981; Branemark PI 1983 & Lindquist LW et al 1996}. When compared with the surgical phases, implant prosthesis fabrication is relatively time consuming {Rodrigues AH et al, 2003}.

When implants are placed without flap elevation, both the amount of osseointegration and bone height around the implants are significantly greater than in implants placed with flap elevation. This enhancement is most probably due to the preservation of bone vascularization {Pennel B.M. et al 1967; Wilderman M.N et al. 1970& Jeong S-M et al 2007}. Moreover, the small sized punched mucosa lead to small, clean, closed wounds are known to heal quickly with little scar formation, whereas large open wounds heal slowly and with significant scarring{ Mathes S.J.,2006 & Lee D-H et al. 2009}.18 D.C. Sabiston and H.K. Lyerly, Textbook of previous termsurgery,next term Saunders, The biological basis of modern surgical practice. Philadelphia (1997) p. 207-20.

Recently, the use of flapless surgery for implant placement has become popular. This can be attributed to its numerous advantages that include improved patient comfort and healing, decreased surgical time, and the ability to resume normal hygiene procedures immediately following surgery. However, the flapless approach is only indicated when the surgeon is confident that the underlying osseous anatomy is ideal relative to the planned implant size and its 3-D position in the alveolus. If this is not the case; many problems may arise like: injury of the unseen vital structures, thermal damage secondary to inadequate irrigation during osteotomy preparation, malposed angle or depth of implant placement, and inability to appropriately contour osseous topography to facilitate restorative procedures {Sclar A.G. 2007& Van De Velde T. et al. 2007}. For this, the use of the conventional flapless implant placement should be limited to clinicians with advanced clinical experience and good surgical assessment {Sclar A.G. 2007}.

By no doubt, the use of soft ware programs for the treatment planning is of great benefit in implant dentistry {Sclar A.G. 2007}. This benefit is double folded when the computer guided implant placement protocol using steriolithographic surgical guides are used, where in addition to the mentioned advantages of flapless surgery; the implants are precisely positioned in a safe way { Tardieu P. & Vrielinck L. 2003; Ganz 2005;Tardieu P. et al. 2007; Valente F. et al 2009 & Verielinck L. & Tardieu P. 2009}.

Immediate restoration {immediate smile}:

Immediate loading is another outcome of the use of high tech in the implant dentistry {Malo P et al. 2007}. Where the provided adequate precision of implant placement makes the prefabrication of the final prosthetic restorations and immediate loading a predictable possibility (van Steenberghe et al. 2005; Sanna et al. 2007; Komiyama et al. 2008 & Yong L.T 2008).

Recently, a treatment protocol "Immediate Smile" or "Teeth in- an-Hour" was introduced which allows the simultaneous placement of endosseous implants and a computer- assisted design/computer-assisted manufacturing (CAD/CAM)-guided, immediately loaded, definitive prosthesis {van Steenberghe D et al. 2002; 2005; Tardieu P. 2003 & Sanna AM et 2007}. High success rate is reported with this protocol {van Steenberghe D et al. 2004; 2005}. The viability of the procedure is supported by its accuracy, allowing the transfer of the virtual planning 3-D model to the surgical template, placement of the implants, and attachment of the prosthesis immediately after abutment connection {Malo P et al. 2007}.

Zygomatic implants:

The use of CT data and interactive software program for the treatment planning makes the management of compromised cases much easier and more predictable. CT scan is extremely useful tool in evaluating the trajectory of the bone in the posterior maxilla. When a patient loses his/her teeth for a significant period of time, pneumatisation of the sinuses takes place making the placement of implants very difficult. In such conditions maxillary sinus lift procedures are frequently performed to create adequate bone volume for predictable implant placement {Raja S.V. 2009}. The donor bony site varies depending on the volume of bone required. Bone from the iliac crest is mostly harvested when both sinuses require augmentation. However the disadvantages of sinus lift are the morbidity of the donor site, lengthy operation , high cost and patients mostly request alternative less invasive options {Jivarj S. 2006& Fortin T et al. 2009}.

With information from the CT scan implants can be inclined to avoid the maxillary sinuses { Krekmanov L et al. 2000 &Fortin T et al. 2009} or alternative procedures that use existing anatomical sites that offer reduced morbidity and minimal invasion of the existing structures can be used{Jivarj S et al 2006}. Zygomatic implants can be placed to engage the zygomatic bone infero-lateral to the orbital rim and provide anchorage for a fixed prosthesis in conjunction with anterior implants { Henry P J.2002& Vrielinck et al. 2002}.

The introduction of zygomatic implant is a promising alternative for the management of the severely atrophied maxilla. Due to the anatomic nature of the zygomatic bone and the implant length, the placement of zygomatic implants poses a challenge for prosthodontists {Vrielinck et al. 2002 & Xiaojun C et al. 2009}.

On the basis of computer aided preoperative treatment planning with CT data, the technique of customized drill guides produced through stereolithography has evolved to dictate the location, angle, and the implant insertion depth, so as to provide a link between the planning and the actual surgery by transferring the virtual treatment plan accurately to the patient mouth{ Vrielinck et al. 2002; Vrielinck L. et al 2003; Lal K et al 2006 and Stie´venart M.& Malevez C. 2010 } Figure{1,2,3,4}; However, two main problems may affect the accuracy of Zygomatic implant placement in severe atrophic edentulous maxilla, the first is is the stability of the drill guide, which has to rest on the underlying tissue such as jawbone or mucosa, unstable surgical guide has a major negative influence on the accuracy of the translation of the preoperative treatment planning to the real operative field {Vrielinck L. et al 2003; Galanis C.C et al 2007 & Kreissl M.E et al 2007 [13] C.C. Galanis, M.M. Sfantsikopoulos, P.T. Koidis, N.M. Kafantaris and P.G. Mpikos, previous termComputernext term methods for automating preoperative dental previous termimplantnext term planning: previous termimplantnext term positioning and size assignment, Comput. Methods Programs Biomed. 86 (2007), pp. 30–38. Article | http://www.sciencedirect.com/scidirimg/icon_pdf.gifPDF (665 K) | View Record in Scopus | Cited By in Scopus (6)}. The second is the manual placement of the implant where the guidance right now is only for drilling and not implant placement {Vrielinck L. et al 2003}.

Accuracy of free hand implant placement vs. computer guided implant placement:

No doubt that proper treatment planning is the key for successful implantation. However, having a perfect treatment plan does mean that implants will be placed optimally. Only, the precise transfer of the treatment plant to the patient mouth can secure the implant optimal positioning {Wanschitz F. et al. 2002}.

In fact the comparison between the accuracy of free hand implant placement and computer guided implant placement is unfair and difficult if not impossible. It is unfair because in the free hand implant placement; the whole procedure is approximate where the bone volume verified by clinical examination and 2-D radiographic views yields approximate {less than accurate} figures but not true accurate ones {Tyndall D.A.& Brooks S.L.; 2000& White SC et al, 2001; Perez L.A. 2005 & Chen L.C. 2008} and the precise existence and position of the intra-bony vital structures like nerves and/or vascular canals cannot be verified easily{ Perez L.A. 2005 & Chen L.C. 2008}.

Moreover, the execution of the treatment plan is guided by nothing more than what is called mental navigation {Vrielinck L. & Tardieu P. 2009} and this in turn will lead to an inaccurate positioning of the implants in most of the cases as shown in one of the studies evaluating the implant positions in posterior maxilla where it is found that only 20% of those free hand placed implants can be categorized as ideally placed {Massey B.C. & Alder M.E. 2002}. And even if the conventionally constructed surgical templates are used they do not secure any accuracy at any level where the created holes or slots does not secure the right axis of the implants intra-bony nor it secures the parallelism when multiple implants are placed {Engleman M.J. et al 1988; Mason W.E. & RuganiL F.C. 1999; Lal K. et al. 2006 & Widmann G; Bale JR 2006}; only they secure the entry point of the drills at the crest of the ridge which is translated later as implant head position{Nikzad S.& Azari A.2008}. However, the judgment of the position of the implant through the location of its head at crest of the bone is misleading as shown in figures {5, 6, 7 & 8}. Additionally, in case of multiple implant placement; the appearance of the implants as parallel in the 2-D radiographs does not mean that they are actually parallel as shown in figure {7& 8}. Finally, considering the number of drills {almost five} used in the conventional free hand protocol and the possibility of each drill deviation from the original plan { Wanschitz F. et al.2002& Tardieu P. et al. 2007 } ending by the unguided implant placement{ Tardieu P. et al. 2007 & Verielinck L. and Tardieu P. 2009} in the prepared implant site; accuracy and precision are two terms that are impossible to judge or to ensure in the free hand implant placement.

It is difficult or impossible to compare the accuracy of the two protocols because in the case of computer guided implant placement the accuracy can be judged by comparing the postoperative outcome achieved to the documented and saved preoperative accurate 3-D plan { Nickenig H-J et al 2009}; while in the case of conventional free hand implant placement there is no accurate preoperative plan reference to compare the postoperative outcome against {Valente F. et al.2009}. Even when the CT scans and software programs were used for the creation of preoperative treatment plan, the unguided free hand placement shows significant deviation from the ideal predetermined position {reference} in the preoperative plan {Van de Velde T. et al. 2007; Nickenig H-J et al 2009& Park C.et al. 2009}.

The literature is confusing when it talks about "computer guided implant placement" because in most of the cases the term is misused and applied to drill guidance {implant site preparation} only and not true guided implant placement. Also, though there is an agreement on the value and necessity of accuracy, there is no clear agreement on its definition or the parameters used to evaluate such accuracy {Verielinck L. and Tardieu P. 2009}. Moreover, a universally agreement on valid value in millimeters regarding an ‘acceptable' deviation is not and cannot be defined because in some clinical situations even the smallest deviations might be significant {e.g. nerve injury} while in other situations the improper implant position can be tolerated and/or compensated { Jivarj S., 2006& Schneider D et al 2009}.

Concerning the computer guided implant placement there are three different tools reported in the literature in this regard, Navigation, Robots and steriolithographic surgical guides manufactured by the CAD/CAM technology. Robots are reported as accurate tool to transfer the treatment plan to the patients, however its high cost and availability limits its use {Troccaz J. et al 1998; Hein A.& Lueth T.C 1999& Troulis M.J et al.2005}. The use of a navigation system has been reported as a good tool that results in improved precision of insertion regarding the position, angulation, and depth of implant placement in comparison to conventional free hand placement{Casap N. et al. 2005; Hoffmann J. et al. 2005; Kramer F.J. et al. 2005& Ruppin J. et al. 2008}. When the accuracy of navigation system is compared to the accuracy of the protocol using the steriolithographic surgical guides manufactured by CAD/CAM technology; both showed almost the same level of accuracy {Ruppin J. et al. 2008}. When compared with the free hand implant placement; the accuracy of the axis and implant position has been significantly more precise with the 3-D steriolithographic surgical guide {Nickenig H-J et al 2009 &Park C. et al 2009}. Nickenig H-J et al. (2009) compared the accuracy of placing twenty-three implants in 10 patients using 3-dimensional CAD/CAM fabricated surgical guide to manual implantation performed in anatomical casts of the same patients by a prosthodontist and a maxillofacial surgeon. The postoperative images of casts were superimposed onto the preoperative image of virtually planned ideal position of the implant. The 3-D surgical guide template produced significantly smaller variation between the planned and actual implant positions at the implant shoulder and apex compared with the free hand implantation .The accuracy of axis was significantly more precise with the 3-D surgical guide compared to the free-hand method performed by the maxillofacial surgeon or by the prosthodontist . In their study; Park C et al. (2009) achieved similar results. They compared the accuracy of 45 implants placed on manikin through CAD/CAM fabricated surgical to that of freehand placement. The lateral deviation and angulation of the implant axis was found significantly less in CAD/CAM guided implants than that of free hand placed implants.

This accuracy is improved significantly when the guidance extends to the implant placement itself and not the drilling guidance alone, where the same stabilized surgical guides used for drill guidance is used to place the implants. In this technique not only the horizontal deviation is minimized and angulation of the implants is more accurate but the depth of the implants also is fully controlled by the mean of the special stoppers created on the special drills and implant holder specifically designed for this purpose as in SAFE System { Materialise, Leuven, Belgium} as shown in Figure: 9 {Tardieu P. & Vrielinck L. 2003;Tardieu P. et al. 2004; Park c.et al. 2009;Tardieu P. et al. 2007 & Verielinck L. & Tardieu P. 2009}.

Steriolithographic surgical guides:

Steriolithographic surgical guide is the latest link in the development chain of surgical guides. Historically the implants were placed first without the use of surgical guides but with the increased patients' demands proper positioning of the implants for a better predictable esthetics and function calls for the evolvement of surgical guides {Fortin T et al. 2003} no, doubt that these surgical guides added much to the accuracy of free handed implant placement {Kopp et al 2003}. Traditionally, the fabrication of the surgical guide begins with a diagnostic tooth positioning, either through a diagnostic waxing, denture teeth arrangement, or through the duplication of the pre-existing teeth or restorations {Neidlinger J et al. 1993}. Next to the use of CT scan and interactive software for diagnosis and treatment planning, the steriolithographic surgical guide has been developed for the transfer of the treatment plan to the surgical field precisely. Steriolithographic surgical guides can be defined as precision metallic guides closely matched to the diameter of the drills and/or implants {Park C et al. 2009}. These guides are fabricated with the aid of computer assisted design/computer assisted manufacturing (CAD/CAM) technology and rapid prototyping machine {Marchack CB.2007& Balshi SF et al 2008}. They are made of resin which is laser polymerized layer by layer of 1mm thickness each corresponding to the slice intervals in the CT formatting process. Usually they are made in set of three each is incorporating metal sleeves (5 mm in height) and of different diameters that correspond to the diameter of the predetermined surgical drills to be used in the surgery {Lal K. et al 2006}.they can be fabricated to fit to bone, teeth, mucosa or teeth and mucosa per the request of the surgeon { Tardieu P. & Vrielinck L. 2003; Tardieu P. et al. 2007 & Verielinck L. & Tardieu P. 2009}. The surgical guide can be fabricated to guide the drills as well as the implant placement {Safe System, Materialise, Leuven, Belgium},in this condition two drills only are used, the pilot drill and the final drill. Both drills are stepped so they both can be guided through the same and one guiding tube. Moreover, both drills have depth control stops. As the same drill can be used for drilling osteotomies of different depths, the surgical guide is fabricated so that the height of the different guiding tubes is put at different levels to adapt for the depth difference of osteotomies. The drilling is performed till the drill is blocked by the depth control stops, Figure (10). Using the same surgical guide, the implants are placed, the depth of implant placement is secured by using the specially designed implant holders of different lengthens Figure (11) {Tardieu P. & Vrielinck L. 2003;Tardieu P. et al. 2004; Tardieu P. et al. 2007 and Verielinck L. & Tardieu P. 2009}.

Benefits of Computer guided implant placement:

The main benefit of the computer-aided design (CAD)/computer-aided machining (CAM)- guided dental implant planning and subsequent template-guided implant placement is a thorough preoperative diagnostic and a more predictable implantation procedure { Fortin T.et al 2003 & Widmann G.& Blae RJ 2006}. It reduces the risk of damage to adjacent structures {Fortin et al., 2003; Suzuki and Suzuki, 2008}. Accurate presurgical planning also permits the implementation of restorative goals (Lal K. et al., 2006; Nickenig H-J. & Eitner, 2007; Katsoulis et al., 2008& Nickenig H-J, et al 2009}.

Bone augmentation procedures can eventually be avoided in some patients by an optimal utilization of present bone. In selected cases even flapless minimally invasive surgery, resulting in less postoperative morbidity, shorter surgery duration, procedures can be considered { Campelo LD& Camara JRD 2002;van Steenberghe et al. 2005; Sanna et al. 2007; Komiyama et al. 2008). The fabrication of an accurate surgical guide permits the surgeon to place the implants precisely into planned positions so that a prefabricated final prosthesis may be delivered immediately {Van Steenberghe et al. 2002& Yong L.T 2008}.

Limitations:

Despite the clear benefits reported in the literature in regard to computer guided implant placement some limitations do exist. Of these limitations are: the high cost of the treatment { purchase of the soft ware ,fabrication of radiographic template, CT scan, conversion into 3-D, time consumed for treatment planning , steriolithographic surgical guide fabrication and implant surgery} and the high radiation dose of the CT scan {Frederiksen N.L. 1995 ; Dula K. et al. 1996& Katsoulis J. et al. 2008}. Such high radiation risk can be reduced considerably by lowering the dose output of the scanner {Ekestubbe A. et al 1993} and in some cases; the diagnostic benefits of CT scan may outweigh the lower risks associated with modern improved scanners that produce low doses of radiation. Alternatively , some (Fortin T et al 2002; Fortin T et al, 2003 ; Guerrero ME et al 2006 ; Loubele M. et al., 2008; Nickenig H-J, et al. 2009} do recommend the use of cone-beam tomography that can decreases both cost and radiation dose. Cone-beam CT appears to be the standard of the future, consequently, many changes should be made to improve this imaging tool {Guerrero ME et al 2006}.

The under/overestimation of bone volume during CT driven treatment planning and virtual implant planning in this condition seems to reduce the predictability of implant positioning with sufficient implant stability and the need for bone augmentations, however, the increase in the resolution of CT data in combination with a enough exposure during scanning, might overcome the problem of misinterpretation of the bone volume and consequently contributes to a more predictable and precise implant placement{ Schneider et al.2009}.Inaccuracy of the CT scan { Block M.S. & Chandler C.2009; Park C. et al. 2009 &Schneider et al.2009}, the lack of stability and reproducibility of the radiographic template during CT will lead to wrong scanned data for treatment planning with subsequent wrong clinical implant positioning. Improperly fitting or unstable surgical guide is one of the factors considered to affect the precision of implant placement {Wong N.Y. et al 2007; Block M.S. & Chandler C.2009; Park C. et al. 2009& Schneider et al.2009 } , poor access in the posterior areas due to the thickness of the surgical guide and/ or the height of the guiding cylinders incorporating the metal sleeves and long surgical drills { Yong L.T.& Moy P.K. 2008 and Park C. et al. 2009}, Limited inter-occlusal space in the posterior segments is problematic where it can make the insertion of the drills through the surgical guide impossible and the implantation procedure cannot be carried out as was planned where the free hand protocol will be followed {Park C. et al 2009& Schneider et al. 2009}, poor visibility during drilling can make the verification of the proper depth of drilling and instrumentation difficult especially in the posterior areas{ Yong L.T.& Moy P.K. 2008} ,heat generation due to the enclosement of the drill within the guiding cylinder of the surgical guide with the subsequent inability of the coolant to reach to the tip of the drill and the probable clogging of flutes of the drill with subsequent implant failure { Yong L.T.& Moy P.K.2008}, Deviation between the planned and actual implant position may lead to a misfit of the prosthetic restoration fabricated for the sake of immediate restoration{Oyama K. et al. 2009&Schneider et al. 2009}.sharp bone surrounding the placed implants is reported as possible reason of the misfit of the restoration { Yong L.T.& Moy P.K.2008}.

Conclusion

Unfortunately, an evidence based conclusion cannot be drawn from reviewed literature because most of the published papers are either just clinical case reports or experimental studies conducted on animals or cadavers. Even the few available systematic reviews are also based on same weak papers.

Based on the available data in the literature reviewed it seems that the use of CT scan and interactive soft ware programs for the treatment planning and fabrication of CAD/CAM based steriolithographic surgical template is a viable and promising tool. It may secure more predictable, safer, precise and less invasive implant placement compared with conventional free hand implant placement. The high accuracy reported for computer guided implant placement can help making the clinical management of complicated cases more predictable, safer and easier. Moreover, the accuracy and the high precision of transferring the treatment plan to the patient mouth can make the prefabrication of the final prosthesis a reliable option for those cases seeking immediate restorations.

Though high level of accuracy is reported in the literature, computer guided implant placement is not free of limitations or drawbacks. It is a technique sensitive procedure. Starting by study models and passing through diagnostic waxing up, fabrication of radiographic templates, Ct scanning, interpretation of the CT data, treatment planning and fabrication & stabilization of the surgical guide in the patient mouth during surgery; any error at any of the mentioned steps may significantly affect the degree of accuracy and precision of implant placement. The high radiation doses with CT scan and the high cost of the treatment are two important limitations with computer guided implant placement.

Although computer guided implant placement is a promising technology with the potential for more predictable and less invasive implant placement, its performance has to be critically evaluated. This is because it is already available on the market and in the hands of many clinicians who rely totally on it. The limited data and relatively short observation periods available in literature and the lack of randomized controlled trials (RCT) calls for further research that should involve prospective clinical studies with long term follow up and strive for an improvement of the systems and procedures regarding accuracy, predictability and reproducibility of implant placement as well as surgical and prosthetic outcomes.

For the time being, the conventional free hand implant placement is reported to provide high success similar to that reported with computer guided implant placement and it seems it is enough to use for most of the cases.

Reflection

In fact I am practicing the implant placement in my clinic for the last eight years. In the beginning I used the free hand technique to place implants; I had no problems placing single implants in partially edentulous patients. The problems I had was mostly with multiple implants where parallelism is required to avoid the use of angled abutment and subsequent off axis loading. When the Simplant soft ware program and CAD/CAM fabricated surgical guides were first introduced to me I found it a solution that secure a better treatment plan and ensure higher level of safety and more accurately placed implants. I spent much money and time in training before introducing this technology in my practice. I used it as routine with all patients. No doubt the placement of multiple implants became easier and more precise with the use of CAD/CAM fabricated surgical guide. However similar results are achieved after starting to place implants for the cases required for my Msc using free hand placement. The learning curve and subsequent higher level of self confidence by no doubt are the main reasons for this improvement.

The project in hand came to confirm that the developed knowledge and the skill of the clinicians are the main keys for predictable and successful implant placement. Computer guided implant placement is promising but till now is not evidenced to be superior to the free hand placement. Moreover, it is not free of limitations and hazards. In addition to its high cost; blind dependence on this technology may lead to serious complications as equal as to that of free hand placement.

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