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Basic and Clinical Research

Freehand Versus Guided Surgery

Factors Influencing Accuracy of Dental Implant Placement

Choi, William DMD*; Nguyen, Bao-Chau DDS; Doan, Andrew DMD; Girod, Sabine MD, DDS, PhD§; Gaudilliere, Brice MD, PhD; Gaudilliere, Dyani DMD, MPH

Author Information
doi: 10.1097/ID.0000000000000620
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Abstract

Proper angulation and positioning of dental implants is essential to achieve acceptable prosthetic outcomes. Poor angulation and positioning of the implant is associated with increased risk of complications, such as perforation of the lingual plate or inferior alveolar canal. The prosthesis can also be compromised causing unfavorable occlusal forces to be transmitted to the implants or poor aesthetics of the prosthesis.1–3 In addition, anatomical concerns, such as the impact of the interimplant distance on crestal bone height and papilla contour rely on accurate planning and placement.4 It is critical, therefore, to strategically place the implant fixtures. The challenge is that all surgical sites are different.

Improving implant accuracy has been the subject of substantial interest. Accumulating evidence suggests the use of a surgical guide is the primary determinant associated with implant accuracy.5–7 Several methods for surgical guidance have been proven effective in increasing accuracy.8–22 However, guided surgery is not always the chosen option due to lack of resources or the urgency of the case.5,23 Given appropriate presurgical planning, including 3-dimensional radiographic imaging24–28 and proper case selection, freehand surgery may be an acceptable alternative. Therefore, it is critical to identify the factors that affect the accurate positioning of the implant fixture.

Previous research regarding factors affecting implant positioning in freehand surgery predominantly focused on surgical drilling methods and the provider's years of experience. These studies provided important information with regards to the effect of drilling speed, utilization of all bur diameters in succession, and interpractitioner differences.5,29 However, research relating to case selection based on the patient's anatomical and demographic characteristics as well as research comparing one practitioner's accuracy from year to year is lacking. As these other factors affecting implant accuracy in freehand cases have not yet been systematically explored, providers are left to make decisions about the choice of guided or freehand approaches largely based on their own preference or experience.

The aim of this retrospective, cross-sectional study was to systematically interrogate the influence of major demographic, surgical, and anatomical factors that contribute to the accuracy of implant fixture positioning and angulation (accuracy factors). Identification of such factors is the necessary first step for an evidence-based protocol, providing practitioners with a rigorous method for optimal case selection and surgical planning.

Materials and Methods

Subjects

The Institutional Review Board of Stanford University School of Medicine (Stanford, CA) approved this study, protocol #37438. Inclusion criteria included endosteal dental implants placed using a freehand method by one practitioner between 2011 and 2015 with a preoperative cone beam computed tomography (CBCT) and postoperative periapical radiograph, consistent with the International Congress of Oral Implantologists and American Academy of Oral and Maxillofacial Radiology recommendations for perioperative images in dental implantology.26,27 Postoperative CBCT scans were also evaluated when taken for clinical reasons, but they were not taken routinely to avoid unnecessary radiation. Exclusion criteria included implants placed in fully edentulous arches and implants where no reproducible radiographic landmark could be identified. All implants were from the Hiossen Implant System and were placed according to the same protocol using the CBCT software to visualize the surgical site in 3 dimensions with anatomical measurements by the EZ3D software preoperatively.

Study Approach

Paired preoperative CBCT images and postoperative periapical radiographs were chosen with reproducible landmarks. Information on whether the extracted tooth outline could be seen within the bone, indicating varying bone density or incomplete bony remodeling was recorded. The same practitioner who placed the final implants reviewed the cross-sectional images and electronically drew a line over the preoperative CBCT image retrospectively to represent the ideal mesiodistal position and angulation for the implant. This procedure was performed in duplicate, and the practitioner was blinded to the patient information and postoperative radiograph.

Clinical Data

The primary clinical outcomes were (1) mesiodistal position of the center of the implant at the crest of bone and (2) the angulation of the implant in the mesiodistal plane. The influence of 12 demographic, surgical, and anatomical factors (accuracy factors) was considered. Accuracy factors included age and sex (demographic factors); practitioner experience, timing of implant placement relative to tooth extraction (immediate vs delayed), the number of adjacent implants placed (from one to 3 adjacent implants), the implant length and width (surgical factors); the presence of adjacent teeth (tooth-borne status), the mesiodistal width of the edentulous space (for tooth-borne cases), the side of the mouth, the arch, and the location in the arch (anatomical factors).

Quantification of Angulation Accuracy

Mesiodistal angulation of the adjacent tooth and the retrospectively determined ideal implant were recorded. The difference between the 2 angulations was calculated using the method shown in Figure 1. This procedure was repeated with the postoperative periapical radiographs to calculate the difference between the mesiodistal angulation of the achieved implant and the same adjacent tooth for each implant individually. The discrepancy in angulation between the ideal and achieved implants was calculated in degrees (Fig. 1).

Fig. 1
Fig. 1:
Calculation of the discrepancy in angulation between the ideal and achieved implant.A, Axial and coronal views, (B) sagittal view of the preoperative CBCT from a representative patient, and (C) matched postoperative periapical radiograph are shown for a posterior, single implant case. D, Axial and coronal views, (E) sagittal view of the preoperative CBCT, and (F) matched postoperative maximum intensity projection from a posterior multiple-implant case are shown. (B, C, E, and F) the angulation discrepancy between the ideal and achieved implant was calculated as follows for each implant individually in the single- or multiple-implant cases: |(ai° − bi°) − (aa° − ba°)| = angulation discrepancy where, ai°, angulation of the ideal implant; bi°, angulation of the adjacent reproducible landmark; aa°, angulation of the achieved implant; ba°, angulation of the same landmark.

Quantification of Positioning Accuracy

Measurements from EZ3D CBCT software were taken, either of the distance between the 2 adjacent teeth at the crest of bone of the tooth-borne edentulous site or another reproducible distance on a partially edentulous site, such as the width of the adjacent tooth. The same distance was then measured by hand on the image to calculate the relationship between the image size and the actual anatomy. This distance was then measured by hand on the postoperative radiograph to calculate the distance between the center of the implant and the adjacent tooth at the crest of bone in millimeters for both the ideal and the achieved implant. This method was performed for each implant individually in the single- or multiple-implant cases (Fig. 2).

Fig. 2
Fig. 2:
Calculation of discrepancy in position between ideal and achieved implants at crest of bone.A, Axial and coronal views and (B) sagittal view of the preoperative CBCT from a representative patient, and (C) matched postoperative periapical radiograph are shown for the posterior, single-implant case. D, Axial and sagittal views, (E) coronal view of the preoperative CBCT, and (F) tmatched postoperative periapical radiograph are shown for the anterior multiple-implant case. (B, C, E, and F) The CBCT software has a ruler function that estimates the true width of the edentulous space (or another reproducible width such as that of the adjacent tooth) in yellow (labeled 1a in panels B and E). The positional discrepancy between the ideal and achieved position at the crest of bone was calculated as follows for each implant separately in the single or multiple-implant cases:
Fig. 1
, where 1a, true anatomical width of landmark from the CBCT software; 1b, width of landmark measured on image; 1c, measured image width from adjacent landmark to the center of ideal implant at crest of bone; xi, calculated true distance from adjacent landmark to the center of ideal implant; 2b, width of the same landmark measured on postoperative image; 2c, measured width from adjacent landmark to the center of the achieved implant; xa, calculated true distance from adjacent landmark to the center of achieved implant.

One investigator performed all measurements and then measured the duplicate ideal implants for validity. Duplicate measurements of the ideal placement for a subset of 50 implants were highly correlated for position (P = 8.69e-08) and angulation (P = 8.40e-10).

Statistical Analysis

The dependent variables of interest were discrepancy in mesiodistal position and angulation between the ideal and achieved implant placement assessed radiographically. Pearson correlations were conducted to find the relationship between the 2 outcome variables and continuous accuracy factors (age, practitioner experience, width of edentulous space, implant width, and height). Continuous accuracy factors were then categorized into binary variables and independent samples t tests were conducted to find the differences in mean for the outcome variables grouped by each of the 12 accuracy factors. Linear regressions were used to determine the confounding effect of potentially related independent variables. Related variables included the number of adjacent implants placed with tooth-borne status; age with tooth-borne status; practitioner experience with number of adjacent implants, tooth-borne status, and timing of placement; as well as location on the arch with tooth-borne status. Finally, outlier analysis was performed by z-scoring the angulation and position discrepancies with outliers defined as having a z score of 2.5 or greater.

Minimum sample size of N = 352 for independent samples t tests was calculated for a modest effect size (Cohen's d = 0.3) with the goal of at least 0.80 power and a significance level of 0.05 before Bonferroni correction. Tests of the 12 a priori hypotheses for the 12 factors affecting angulation and position were conducted using Bonferroni-adjusted α levels of 0.00417 per test (0.05/12). Statistical analysis was conducted in SPSS version 20 and R version 3.3.0.

Results

Subjects

A total of 545 implants in 288 patients met inclusion criteria; 95 implants were then excluded from the analysis because of lack of reproducible radiographic landmarks in the existing imaging. The final data set comprised data from 450 implants in 251 patients. Average discrepancy in position and angulation between the ideal and achieved implant was calculated to quantify the accuracy of freehand implant placement and to determine the overall clinical acceptability of the method of measurement. The average mesiodistal angulation discrepancy between ideal and achieved surgical placement was 5.43 ± 4.57 degrees SD. Average mesiodistal position discrepancy between ideal and achieved surgical placement was 1.13 ± 1.48 mm SD. These discrepancies are within the range of those found by previous studies for freehand and guided surgery.5,12,20,22,30

Systematic Evaluation of the Accuracy Factors' Influence on Implant Position and Angulation

The contribution of the 12 accuracy factors to implant placement accuracy was systematically examined (Fig. 3). Four factors (tooth-born status, the number of adjacent implants, timing relative to extraction, and practitioner experience) significantly influenced implant position (Fig. 3, A), whereas 3 factors (the number of adjacent implants, tooth-borne status, and the width of the edentulous space for the subset of tooth-borne, single-implant cases) significantly influenced implant angulation (Fig. 3, B). Three accuracy factors (number of adjacent implants, tooth-borne status, and timing relative to extraction) remained significantly associated with either implant position or angulation after correction for multiple hypothesis testing (Fig. 3, A–B, red bars).

Fig. 3
Fig. 3:
Relative contribution of accuracy factors to positional and angulation discrepancy.A and B, Bar graphs on the left panels represent the P values of each of the 12 factors studied on a −log(10) scale, listed from the most significant to the least significant factor. The factors shown in red are significant to the Bonferroni-adjusted α level of 0.00417, whereas the factors shown in black have P values below the 0.05 level. The bar graphs on the right panels represent the influence of significant accuracy factors on implant placement accuracy. A, Tooth-borne status (P < 0.000001), the number of adjacent implants placed (P = 0.000003), timing of implant placement relative to tooth extraction (P = 0.003), and practitioner experience (P = 0.014) were associated with position accuracy. Age, width of the edentulous space (for the subset of tooth-borne single-implant cases), location in the arch, side of the mouth, implant width, arch, patient sex, and implant length were not associated with significant differences in position discrepancy (P > 0.05, t test). B, The number of adjacent implants placed (P = 0.002), tooth-borne status comparing fully tooth-borne to not fully tooth-borne (P = 0.006), and the width of the edentulous space for the subset of tooth-borne single-implant cases (P = 0.043) were the factors associated with angulation accuracy. Practitioner experience, patient sex, arch, timing of implant placement relative to tooth extraction, implant width and length, patient age, location in the arch, and the side of the mouth were not associated with significant differences in angulation discrepancy (P > 0.05, t test).

Single-Implant Cases Are More Accurate than Those With 2 or 3 Implants Being Placed Adjacently

The number of adjacent implants was the only accuracy factor whose influence on both implant position and angulation remained significant after correction for multiple hypothesis testing (Fig. 3, red bars). Of the 450 implants placed, 238 were single-implant cases, 179 were cases with 2 adjacent implants, and 33 were cases with 3 adjacent implants. Single-implant cases were significantly more accurate in position with smaller discrepancy (M = 0.79 mm, SD = 0.78 mm) than double- or triple-implant cases (M = 1.59 mm, SD = 2.00 mm); t(195) = −4.82, P = 0.000003 (Fig. 3, A, right panel). Single-implant cases also had a smaller discrepancy in angulation (M = 4.79 degrees, SD = 3.56 degrees) than multiple-implant cases (M = 6.15 degrees, SD = 5.41 degrees); t(358) = −3.12, P = 0.002 (Fig. 3, B, right panel). Cases with 3 adjacent implants were the least accurate in angulation (M = 6.55 degrees, SD = 5.55 degrees) and position (M = 1.83 mm, SD = 2.21 mm).

Fully Tooth-Borne Implant Cases Were More Accurate in Angulation and Position than Either Partially Tooth-Borne or Edentulous Cases

Of all the accuracy factors examined, the presence of adjacent teeth on either side of the implant site (tooth-borne cases) affected the accuracy of position the most (Fig. 3, A). Tooth-borne status also influenced implant angulation accuracy but to a lesser extent than the number of adjacent implants. Tooth-borne cases were more accurate in terms of position discrepancy (M = 0.68 mm, SD = 0.64 mm) than non-tooth-borne cases (M = 1.49 mm, SD = 1.83 mm); t(269) = 5.98, P < 0.000001 (Fig. 3, A, right panel). The same was true for the angulation discrepancy of tooth-borne cases (M = 4.75 degrees, SD = 3.55 degrees) compared with non-tooth-borne cases (M = 5.88 degrees, SD = 5.09 degrees); t(446) = 2.75, P = 0.006 (Fig. 3, B, right panel).

To determine whether multiple implants being placed adjacently was a confounder for the effect of tooth-borne status on position and angulation accuracy, a linear regression was conducted with the number of adjacent implants placed and tooth-borne status as covariates. Results indicated that the effect of tooth-borne status remained significant as an independent predictor of position accuracy (β = −0.51, P = 0.014) but not angulation accuracy. Similarly, the number of adjacent implants placed also remained a significant predictor of position (β = 0.35, P = 0.036) but not angulation accuracy. When compared to tooth-borne status, the number of adjacent implants placed explained more of the variance in angulation accuracy.

Immediate Implant Cases Are More Accurate in Position, but Not Angulation

Information on the timing of implant placement relative to tooth extraction was available for 445 implants. Of these, 54 implants were placed the same day as the tooth extraction (immediate placement cases) and 391 were placed on a later date (delayed placement cases). The delayed placement cases were significantly less accurate (P = 0.003) with higher mean position discrepancy between ideal and achieved implant placement (M = 1.19 mm, SD = 1.55 mm) than the immediate cases (M = 0.74 mm, SD = 0.83 mm) (Fig. 3, A, right panel). There were no statistical differences in mesiodistal angulation between immediate and delayed cases (Fig. 3, B, left panel). Together, the data suggest that immediate implants are more accurate in mesiodistal positioning but not angulation.

Bone remodeling is an important factor associated with the timing of the implant and was separately quantified for each implant case to differentiate between early-delayed and late-delayed cases. All cases had radiographic information on whether the bone had completely remodeled after tooth extraction versus almost complete remodeling or only partial remodeling by classifying the clarity of the extracted tooth outline on the preoperative radiograph. Cases where there was an obvious radiographic outline of the extracted tooth suggesting large bone density discrepancies in the site were less accurate in mesiodistal angulation (P = 0.07) and significantly more accurate in mesiodistal positioning (P = 0.031).

The most severe angulation discrepancies in the subset of lower-molar implants were seen in cases with obvious radiographic outlines of the previously extracted tooth (Fig. 4). Cases where no radiographic trace could be seen of the extracted tooth's bony outline, or only a mild trace, were significantly less accurate in mesiodistal positioning (P = 0.002) but not in angulation compared with cases where a marked radiographic outline could be seen or in immediate cases. Overall, incomplete/partial radiographic bone remodeling after extraction predicted higher positioning accuracy and a trend toward lower angulation accuracy.

Fig. 4
Fig. 4:
Radiograph illustrating influence of the extracted tooth's root form on angulation inaccuracy. The outline of the extracted molar is clearly visible, representing incomplete bony remodeling. The lower density of the newly formed bone seems to have driven the implant off course during placement.

Implant Accuracy Increases With Practitioner Experience

The year in which the implant was placed was used as a proxy for the experience of the practitioner as the implants were placed over 5 years, early in the practitioner's career. There was a correlation between the year the implant was placed and the positional (r = −0.14, n = 379, P = 0.007) and angulation (r = −0.11, n = 450, P = 0.017) accuracy of the implants such that greater experience correlated with higher accuracy.

The practitioner's experience remained an independent predictor of angulation (P = 0.035) and positional (P = 0.024) accuracy when controlling for the number of adjacent implants placed simultaneously, the tooth-borne status, and the timing of placement relative to tooth extraction by linear regression. As the implants were placed in the first 6 years of the practitioner's postresidency career, 3 years were used as a cutoff to determine whether this was an appropriate length of time to see a difference in accuracy. Implants placed after 3 years of experience were significantly more accurate in position (P = 0.014, Fig. 3, A, left and right panels), whereas the difference in angulation accuracy using the 3-year cutoff was not statistically significant (Fig. 3, B, left panel).

The Width of the Edentulous Space is a Key Anatomical Indicator of Implant Accuracy

To evaluate the influence of the size of the edentulous space on implant accuracy, the mesiodistal width of the space was measured in the CBCT software. In tooth-borne, single-implant cases, a smaller width of the edentulous space measured at the crest of bone was correlated with higher angulation accuracy (r = 0.18, n = 164, P = 0.025) and higher positional accuracy (r = 0.23, n = 155, P = 0.004). The average mesiodistal width of the edentulous space for single-implant cases was 11.09 mm, so 11 mm was used as a cutoff for categorizing these cases into a narrow or wide edentulous space to make a clinical recommendation more readily. Independent samples t tests showed that cases with narrow mesiodistal width of the edentulous space were more accurate in angulation (P = 0.043, Fig. 3, B) but not position (P = 0.097). Therefore, smaller mesiodistal width of the edentulous space may be an important indicator for angulation accuracy in tooth-borne, single-implant cases.

Conversely, implant angulation accuracy was decreased in tooth-borne, triple-implant cases when the width of the edentulous space was smaller (r = −0.65, n = 12, P = 0.022), but there was no association for positional accuracy. There was no significant correlation between accuracy and width of the edentulous space in tooth-borne, double-implant cases.

Patient Age, but Not Sex, Influences Implant Accuracy

Age and sex were considered in the analysis because of the anatomical differences they imply. The age of the patient was significantly correlated with the mesiodistal position accuracy such that greater age was associated with a higher discrepancy between ideal and achieved implant placement (r = 0.15, n = 367, P = 0.003). However, age did not remain a significant predictor of position accuracy when controlling for tooth-borne status by linear regression (P = 0.143).

The data also revealed a trend toward higher angulation inaccuracy on women (N = 215, M = 5.81 degrees, SD = 4.90 degrees) as compared to men (N = 235, M = 5.08, SD = 4.23), but this difference was not statistically significant (P = 0.09). There was no significant sex difference in position accuracy. These results suggest neither age nor sex is an independent risk factor for implant inaccuracy. Rather, age is associated with edentulism, which critically affects accuracy.

Arch, Location on Arch, and Implant Dimensions Are Not Associated With Implant Accuracy

The tooth type being replaced and the location within the arch were not significant predictors of implant accuracy. Second molar implants, of which 67 were included in our sample, were the least accurate in terms of position discrepancy (M = 1.58 mm, SD = 1.65 mm) compared with all other teeth (M = 1.03 mm, SD = 1.43 mm). However, this effect did not remain significant when controlling for tooth-borne status, as second molar implants were typically not fully tooth-borne.

Although upper implants were more accurate than lower, left-sided implants were more accurate than right-sided implants (the practitioner is right-handed), and anterior implants were more accurate than posterior in terms of mean position and angulation discrepancy, these differences were not statistically significant. Implant width and implant length were also not correlated with accuracy of position or angulation.

Outlier Analysis

Outlier analysis was performed to identify implants with the furthest deviation from the practitioner's ideal for either position or angulation, using a z score of 2.5 or greater to define an outlier. A z score of 2.5 corresponded to an angulation deviation of 15.5 degrees and a mesiodistal positioning deviation of 3.15 mm. The analysis identified 7 outliers for angulation only (Fig. 5, A), 11 outliers for position only (Fig. 5, D), and one outlier for both position and angulation, for a total of 19 outliers. Eighteen outliers (95%) were in partially or fully edentulous sites, 16 (84%) were implants placed as part of a multiimplant case, and 18 (95%) were posterior teeth. Graphs highlighting outliers and radiographic examples of outliers for single-, double-, and triple-implant cases can be seen in Figure 5, A–F.

Fig. 5
Fig. 5:
Implant angulation and position outliers.A, Box plot showing the angulation discrepancy in degrees between ideal and achieved implants for all implants in the study. Outliers, defined as a z-scored value of 2.5 (or actual value of 15.5 degrees) or greater are highlighted in red. B, Postoperative periapical radiograph of a single implant with an angulation discrepancy of 15.6 degrees between ideal and achieved placement. Direction of angulation discrepancy and location of maxillary sinus are highlighted in red. C, Postrestorative periapical radiograph of double-implant case with U-shaped alveolar crest showing tooth #30 placed with neck tipped 15.9 degrees to the distal. D, Box plot showing the position discrepancy at crest of bone in millimeters between ideal and achieved implants for all implants in the study. Outliers, defined as a z-scored value of 2.5 (or actual value of 3.15 mm) or greater are highlighted in red. E, Postrestorative periapical radiograph of a 3-unit fixed partial denture case supported by 2 implants, with the distal implant placed 4.02 mm further distal than planned. F, Postrestorative periapical radiograph of a triple-implant case with the most distal implant placed 3.6 mm further distally than planned.

Discussion

This study identified 5 critical factors that influence the accuracy of implant placement. The number of adjacent implants was a major predictor of both mesiodistal position and angulation accuracy whereas the tooth-borne status and the timing relative to tooth extraction were significantly associated with position accuracy. The practitioner's experience and the width of the edentulous space were also important factors contributing to implant position and angulation, respectively.

The results have several clinical implications, particularly for practitioners who perform freehand implant placement: (1) The effect seen by tooth-borne status of the implant and the number of implants placed adjacently on both position and angulation accuracy suggest that fully tooth-borne, single implants may be the most appropriate cases for freehand implant placement. In contrast, special considerations should be made for multiple-implant cases placed in edentulous or partially edentulous areas of the arch. (2) Narrow edentulous spaces with less than 11 mm mesiodistal dimension between teeth may also be more appropriate for freehand placement in single-implant cases as the variation in position and angulation is lower. However, the possibility of approaching the root of an adjacent tooth must be carefully weighed. (3) With respect to timing of the implant placement, the data suggest that immediate placement improves the accuracy of mesiodistal positioning. This was also the case for recent extractions with newly formed bone suggesting that the presence of fresh anatomical landmarks may facilitate the procedure. However, our results also indicate that in cases with incomplete bone remodeling and splayed roots (such as in lower molars), guided surgery should be used to counteract the tendency of the drill to be driven off course toward the newly formed bone (Fig. 4). (4) The data also suggest that 3 years of postgraduate experience was sufficient to statistically improve positional accuracy, indicating this may be an important milestone for implant providers who place implants regularly. This is in accordance with previous literature which showed that implant accuracy was higher for more experienced practitioners.29

The importance of accuracy becomes particularly clear in the restorative phase. Figure 5, B shows a tooth-borne, single-implant case where the proximity of the sinus caused the practitioner to angulate the implant with the apex closer to the mesial tooth to avoid a sinus lift procedure. The resulting restoration had an enlarged embrasure volume, possibly leading to food impaction. Another angulation outlier (Fig. 5, C) was a double-implant case with implant #30's neck tipped to the distal. This resulting restoration was over-contoured to achieve mesial contact. As in the previous example, the practitioner avoided additional surgeries to correct the U-shaped alveolar crest; he compensated by placing the implant perpendicular to the existing ridge contour. In our position outlier analysis, one of the cases involved an implant placed as a distal abutment for an implant-supported fixed partial denture (Fig. 5, E). This implant was placed without any adjacent teeth to act as guides causing a shift distal to the ideal. In this case, the lack of landmarks led to inaccuracy. However, the resulting restoration is still clinically acceptable as the presence of a pontic allows for some room for adjustment in the restorative phase. The opposite case of having a small margin for error was found in a position outlier where 3 implants were placed adjacently (Fig. 5, F). The position discrepancy here caused a restoration which was under-contoured on the distal and over-contoured on the mesial, as well as a narrow interimplant space between implants #4 and 5 possibly leading to hygiene challenges and crestal bone loss. In each of these outlier cases, the tissue profile surrounding these implants may be compromised because of suboptimal crown contouring.31

It is interesting to point out the trend toward higher angulation accuracy when implants were placed on men compared with women. Several factors may have contributed to this finding, one of which being the differences in bone density between males and females. In a study by Hiasa et al32 (2011), women were found to have significantly lower jaw densities than men when measured on CT in Hounsfield units. Sex differences have also been previously found in intercanine and intermolar arch width, which could have implications for implant accuracy.33

This study has several limitations. First, the patient population was recruited from a single practitioner's office, and may not be representative of the population at large. However, this was justifiable in the present study to maintain a consistent practitioner approach. Second, 3-dimensional imaging was not available postoperatively, which prevented accuracy assessment in the buccolingual plane. In this regard, preference was given to follow the current guidelines for postoperative radiography26–28 aimed at preventing unnecessary radiation, whereby CBCT scans are routinely taken preoperatively in the surgical planning phase, but not postoperatively unless there is a specific indication, such as neurosensory impairment or when implant retrieval is planned.26,27 In future studies, it will be instrumental to improve the 3-dimensional resolution of implant accuracy assessment, for example by using the method described by Nickenig et al7 (2010), matching preoperative 3-dimensional imaging to postoperative imaging of models with implant analogs. Third, surgical planning based on radiographic images alone does not account for intraoperative adjustments when the practitioner is able to interface directly with the 3-dimensional anatomy and nuances of bone and tissue quality. This may have led to an over-estimation of the discrepancy between ideal and achieved position and angulation. Future prospective studies will address this limitation by allowing the recording and accounting of intraoperative adjustments.

Conclusion

High accuracy of implant placement is essential for ideal outcomes. Although guided surgery is known to significantly improve accuracy, this practice is not used exclusively. Practitioners who place implants freehand do not currently have sufficient data for appropriate case selection. The present study identified important surgical and anatomical factors that improve implant accuracy in freehand placement. These factors provide the foundational basis for a set of clinical guidelines that will improve the chance of success for freehand surgery and increase utilization of guided surgery when the risk for inaccuracy is most present.

Disclosure

The authors claim to have no financial interest, either directly or indirectly, in the products or information listed in the article.

Approval

The Institutional Review Board of Stanford University School of Medicine (Stanford, CA) approved this study, protocol #37438.

Role/Contribution by Each Author

W. Choi, DMD: contributed in the following ways: he placed all implants in the study, contributed to conception and design of the study, integral to the acquisition of the data as he retrospectively determined an ideal implant location for all implants in the study, contributed to manuscript drafting, provided technical support, supervision, and approval of the final version of the manuscript. B.-C. Nguyen, DDS: contributed substantially to acquisition of the data, drafting of the manuscript, and approval of the final version of the manuscript. A. Doan, DMD: contributed to data acquisition, analysis of data, revising the manuscript critically for intellectual content, and approval of the final manuscript. S. Girod, MD, DDS, PhD: contributed to design of the study, interpretation of data, revising the manuscript critically for important intellectual content, and approval of the final version of the manuscript. B. Gaudilliere, MD, PhD: contributed to the conception and design of the study, interpretation of data, drafting the manuscript, figure creation, revising the manuscript critically for important intellectual content, and approval of the final version of the manuscript. D. Gaudilliere, DMD, MPH: contributed to the conception and design of the study, acquisition of data, analysis and interpretation of data, drafting the manuscript, revising the manuscript critically for important intellectual content, technical support, supervision, and approval of the final version of the manuscript.

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Keywords:

angulation; position; tooth-borne

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