The accuracy of guided surgery via mucosa-supported stereolithographic surgical templates in the
hands of surgeons with little experience
In the last few years, the use of stereolithographic surgical templates to guide implant placement has
been heavily promoted. These guides are produced by computer-aided design/computer-assisted
manufacture (CAD/CAM) technology and intend to transfer the ideal implant position from the
computerized planning to the surgical setting (Lal et al. 2006). This planning can ideally be performed
using 3D imaging (MSCT/CBCT; BouSerhal et al. 2002). By loading these 3D images in a software
program, together with images of the planned prosthetic device, it becomes possible to virtually plan
the optimal implant position, taking both anatomical and restorative information into account. As
such, a prosthetically driven implant placement becomes feasible, which benefits the cooperation
between surgeon and prosthodontist (Lal et al. 2006; Vercruyssen et al. 2008; D'haese et al. 2012a,b;
Hultin et al. 2012). Theoretically, this concept may offer a lot of advantages compared to freehanded implant placement (Hultin et al. 2012). A pre-surgical planning minimizes the need for
decision at time of implant surgery, which may shorten the duration of the procedure. Moreover, it
facilitates prosthetic rehabilitation and might eliminate esthetic and biomechanical risks involved in
standard implant surgery (Vercruyssen et al. 2008; Dandekeri et al. 2013; Cassetta et al. 2014). It
offers also the opportunity to apply a flapless approach, which on its own may give extra advantages.
The main advantage of flapless implant placement is reduction of postoperative discomfort (pain,
hematoma, swelling, etc.; Fortin et al. 2006; Hultin et al. 2012). Furthermore, it offers a shortened
surgical time, minimizes changes in crestal bone levels, reduces the risk of inflammation, and limits
bleeding (Becker et al. 2005). These features could especially be advantages in medically
compromised (anticoagulantia, bisphosphonates, etc.) or anxious patients (D'haese et al. 2012a,b).
However, unrealistic clinical expectations concerning the efficacy and ease of computer-aided
implantology may arise (Hämmerle et al. 2009). In a recent systematic review (Hultin et al. 2012)
examining the advantages of computer-guided implant placement, the authors identified a number
of complications that can occur. Some of these complications are specific for the applied technique.
They are often related to the limited access and visibility, due to the flapless approach or due to a
misfit of the guide.
The literature is not consistent on whether a learning curve is present for guided implant placement.
Vasak et al. (2011) reported a clear learning effect over time regarding the accuracy of implants
placed with NobelGuide™ templates. They concluded that guided implant surgery only offers a
simple aid, without any gain in procedural reliability and security, during the first implantations
performed by an inexperienced surgeon. This conclusion was supported by Cassetta et al. (2014) who
concluded that computer-aided implant placement is a useful complement to conventional surgery,
but it only offers a guide and does not replace surgical experience, skill, and knowledge of anatomy.
Three other studies (Valente et al. 2009; Cassetta et al. 2013; Vercruyssen et al. 2014) did not
observe a clear learning curve. However, all surgeons who did the implant surgery in those studies
were experienced in conventional implant placement and only inexperienced in the use of
stereolithographic guides.
The aim of the current study was to investigate the accuracy of implant placement with mucosasupported stereolithographic guides, executed by inexperienced surgeons (inexperienced in both
implant placement and guided surgery) and to compare these data with data of an experienced
implant surgeon (experienced in guided surgery), who used the same treatment and accuracy
analysis protocol (Vercruyssen et al. 2014) and with the data reported by systematic reviews.
Material and methods
Patient
Sixteen patients (mean age: 59.5 years, 35–78; 10 males, 6 females, and 3 smokers), who were in
search of oral implant therapy of at least four implants in a fully edentulous maxilla or mandible,
were recruited. To be included, the patients had to fulfill the criteria given in Table 1 and had to sign
an informed consent form. All patients were treated between November 2011 and January 2014 by
students who followed a 3-year postgraduate training in periodontology at the Catholic University
Leuven, in Belgium. The study was approved by the ethical committee of the University Hospital of
the Catholic University of Leuven (s55275).
Procedure
All postgraduate students had a limited surgical experience in implant therapy (30–80 implants
placed). The patients in this study were treated by nine different postgraduate students (see
distribution in Table 2). During planning and surgery, the same protocol was used as described in
(Vercruyssen et al. 2014), with the exception of the pre-surgical scan. In this study, a pre-surgical
CBCT (at 85 kV and 6 mA, voxel size 250 μm, Scanora® 3D; Soredex, Tuusula, Finland) was taken
instead of the MSCT scan (at 120 kV and 90 mA, 0.6 mm slice thickness, voxel size 330 μm, Somatom
Definition Flash®; Siemens, Erlangen Germany). All the postgraduate students had to follow “strictly”
a series of steps:
Preparation of scan prosthesis
The scan prosthesis had to be well fitting, stable and had to fulfill the esthetic demands of the
patient. If the existing denture fulfilled these criteria and contained all information necessary for the
future prosthetic restoration, this denture was transformed into a scan prosthesis by inserting at
least eight small glass markers. These glass markers served as radio-opaque markers during the
scanning procedure. If the existing denture did not fulfill the criteria, a relining procedure was
performed to adapt the prosthesis in the best way to the soft tissues, or a whole new prosthesis was
made at the prosthetic department of the University Hospital of Leuven. To secure an optimal fit of
the scan prosthesis during the scanning process, a bite index was prepared in putty material
(SheraEx-act® 85; Shera GmbH & Co., Lemförde, Germany).
Scanning procedure
A first CBCT scan (at 85 kV and 6 mA, voxel size 250 μm, Scanora® 3D; Soredex) of the patient with
the scan prosthesis and the bite index in place was taken. With this scan, the anatomical structures
and glass markers were visualized. After this, the scan prosthesis alone was scanned under adapted
settings, in a way that not only the glass markers, but also the entire denture became visible.
Pre-surgical planning
The DICOM images of both CBCT scans (patient with prosthesis and scan prosthesis alone) were
imported in a software program (Simplant®; DENTSPLY Implants, Mölndal, Sweden) to perform the
pre-surgical planning. Via the radio-opaque glass markers, both sets of DICOM images could be
matched. The implants were virtually placed in the most optimal position according to bone anatomy
and prosthetic design. This pre-surgical planning was checked by both an experienced
periodontologist and an experienced prosthodontist. If needed, the planning was adapted. Once the
planning was approved, it was sent to the manufacturer for the fabrication of a mucosa-supported
stereolithographic drill guide (SIMPLANT SAFE Guide by DENTSPLY Implants).
Surgery
Surgery was performed following the implant manufacturers instructions (Facilitate™; DENTSPLY
Implants) at the department of Periodontology of the University Hospital of Leuven, by an
inexperienced postgraduate student, who also made the planning. After local anesthesia, the
mucosa-supported stereolithographic guide was positioned using a bite index to secure proper
seating. The guide was fixed to the jaw by three anchor pins, equally distributed in the jaw. All
implants were placed flapless. There was a physical stop during drilling. Seventy-six AstraTech
OsseoSpeed Tx™ implants (DENTSPLY Implants) were inserted (distribution see Table 2). All surgeries
were supervised by an experienced surgeon.
Accuracy analysis
A postoperative CBCT (at 85 kV and 6 mA, voxel size 250 μm, Scanora® 3D; Soredex) was taken to
check the final position of the implants. These images were superimposed on the virtual planning via
the Mimics® software (Materialise, Leuven, Belgium). An accuracy measurement protocol (DENTSPLY
Implants) was used to determine the deviations between planned and placed implants. This process
is based on surface registration, which consists of a minimization of distances between the pre-op
and post-op jaw model. In this way, an iterative closest point algorithm could be used to match both
jaws. Using the same software, coordinates of the planned and placed implants could be calculated,
and several inaccuracy parameters could be defined (Fig. 1a). The global deviation is defined as the
3D distance between the coronal/ apical centers of the planned and placed implants. The angular
deviation is calculated as the 3D angle between the longitudinal axes of both. Depth deviation is the
distance between coronal/apical center of the longitudinal axis of the planned implant and a plane
parallel through the coronal/apical center of the placed implant. Moreover, a reference plane was
set in bucco-lingual direction by which both the mesio-distal and bucco-lingual deviation could be
calculated (Fig. 1b). Also, the inter-implant deviation was calculated, as the difference between the
planned and real inter-implant distance.
Statistical analysis
The outcome variables were the global deviation at the entry and apical point of the implant, the
angular deviation, the depth deviation at the entry and apical point, and the mesio-distal and buccolingual deviations at the coronal point. A linear mixed model with side (left/right) as fixed factor and
patient as random factor was fit for the comparison between sides (left/right). The latter was
corrected for simultaneous hypothesis testing according to Sidak (1967). An alternative comparison
was made between the data of the inexperienced group and data previously presented by
Vercruyssen et al. (2014) and Vercruyssen et al. (submitted, depth and lateral deviations). She can be
considered an experienced surgeon in both guided and non-guided implant placement and used a
similar protocol, both for the surgery as well as for the inaccuracy evaluation. Again a linear mixed
model was applied, with the experience as the only fixed factor.
Results
In the inexperienced group, 17 edentulous jaws in 16 patients were included. No patients were lost
before the second scan was taken. One implant (upper jaw) in one patient showed an early failure,
without obvious reasons. This implant was excluded for accuracy analysis. All other implants could be
loaded after 3 months healing time.
Seventy-five implants were analyzed for accuracy. The deviations between the planned and placed
implants are summarized in Table 3. The mean global deviation coronally was 0.87 mm (SD 0.50) and
at the apex 1.10 mm (SD 0.53). In depth, the deviation was around 0.5 mm (0.0–2.4 mm). The
deviation in angulation was 2.8° ranging from 0.2 to 7.0°. At the implant shoulder, the bucco-lingual
inaccuracy was 0.39 mm (ranging from 0.0 to 1.5 mm) and in the mesio-distal direction 0.42 mm
(ranging from 0.0 to 1.7 mm). These observations were also compared with the data for the
experienced clinician (with 12 jaws and 52 implants). Only for deviation in angulation, the
inexperienced group scored inferior to the experienced clinician (Table 4).
These data are further visualized in detail in a boxplot in Figs 2 and 3 showing the accuracy data from
the inexperienced group together with the data of a recent systematic review about accuracy of
guided surgery (Van Assche et al. 2012) and with the data of the experienced group. The deviations
in depth, mesio-distal and bucco-lingual direction of the inexperienced group, together with the
median deviation of the experienced surgeon are shown in Fig. 4. In Fig. 5 a boxplot is showing the
inter-implant distance.
The implants placed by right-handed surgeons (inexperienced group) were placed more accurate at
the right side of the patient (quadrant 1–4) than at the left side (quadrant 2–3). The opposite finding
could be made for the left-handed surgeon, whose implants placed at the left side of the patient
were more accurate. Statistically significant differences between left and right could be found for
following parameters: coronal depth (P = 0.02), apical depth (P = 0.01), and mesio-distal (P = 0.002).
The coronal inter-implant deviation (0.32 mm, SD 0.52) was clearly smaller than the global coronal
deviation, pointing to the fact that part of the inaccuracy was primarily due to incorrect positioning
of the guide.
Discussion
To our knowledge, this is the first study to assess the accuracy of dental implants placed in vivo by
inexperienced surgeons with a stereolithographic guide. The impact of experience on the chance of
osseointegration for oral implants has, however, been explored extensively (Preiskel & Tsolka 1995;
Lambert et al. 1997; Kohavi et al. 2004; Melo et al. 2006; Zoghbi et al. 2011). Some of these studies
comparing implants placed by inexperienced and experienced surgeons and concluded that the rate
of osseointegration was significantly higher for experienced surgeons (Preiskel & Tsolka 1995;
Lambert et al. 1997; Zoghbi et al. 2011). A possible explanation for the poor outcome of implants
placed by inexperienced surgeons is that with less experience the frequency of problems such as
excessive heat during drilling, non-stabilization of the implant, lack of adequate planning may
increase. This finding could not be confirmed by others (Kohavi et al. 2004; Melo et al. 2006). In the
study of Melo et al. (2006), the inexperienced surgeons were supervised by experienced surgeons,
which can explain the better outcomes. In the present study, all the different steps necessary in
planning and implant placement were strictly supervised by an experienced surgeon. This may be a
reason for the comparable outcomes in accuracy between both groups of experience.
In a recent in vitro study, the impact of operator experience on the accuracy of implant placement
with stereolithographic bone-supported guides was examined (Cushen & Turkyilmaz 2013). Implant
placement was performed with the Facilitate™ system (DENTSPLY Implants). Two operators had
some experience (over 100 implants placed), and 2 had little experience (less than 10 implants
placed). Statistical analysis showed significant difference between the experienced and
inexperienced group for angular and horizontal error at implant apex and at entry point, with more
accuracy for the experienced clinicians. The same conclusion could be drawn out of an in vitro study
on the accuracy of micro-implant placement performed by experienced and inexperienced dentists
(Cho et al. 2010).
In the current study, an initial power analysis was not performed as we included all the patients that
visited us in the time span of the study, and this may be a shortcoming of the study.
As can be seen in the boxplots (Figs 2 and 3), the accuracy of the implants placed by inexperienced
surgeons is comparable with the experienced surgeon (Vercruyssen) and with the results of a recent
systematic review (Van Assche et al. 2012). Of this systematic review, only the data of in vivo studies,
using a mucosa-supported drill guide, were taken into account. The data of that systematic review
are based on seven studies that included 462 implants.
The 'Material and methods' of the present study and the study of Vercruyssen et al. (2014) were
exactly the same, except the pre-operative radiography. In this study, we decided to take a CBCT
instead of the CT image, because of the lower radiation dose. Vercruyssen and co-workers already
mentioned in the discussion that this is a good option, because the accuracy between both imaging
techniques is similar (Arisan et al. 2013; Poeschl et al. 2013).
In the present study, one of the inexperienced surgeons was left-handed. For the statistical analysis
of the accuracy comparison of the implants placed at the patients left (quadrant 2–3) and right
(quadrant 1–4) side, we decided to mirror the data for the left-handed person. Implants in quadrant
1–4 placed by this surgeon were considered to be placed at the left side (quadrant 2–3) and vice
versa. The implants placed at the right side of the patient by right-handed surgeons are significantly
more accurate than the accuracy at the left side. A possible explanation can be the tolerance within
the guide. The tolerance between the drill – drill guide and drill guide – sleeve can be responsible for
deviation in several directions (Van Assche & Quirynen 2010; Koop et al. 2013). The authors advise
the clinician to move the drill several times in and out to feel the smoothest drill position. So, it is
obvious that the protocol of computer-aided implantology involves a sequence of many steps
including production of a radiographic template, scanning procedure, planning, and surgery, and
mistakes can arise at all different stages. The final (in)accuracy of the implant placement is a
summation of all steps. Therefore, it is crucial to understand the importance of each step and to
realize the magnitude of the cumulated inaccuracy.
In this study, the mean inter-implant distance between the planned and placed implants is 0.32 mm,
and thus much lower than the global coronal inaccuracy. Based on these findings, one can conclude
that part of the observed inaccuracy is caused by the malpositioning of the guide during the surgery
(D'haese et al. 2012a,b). The chance for malpositioning is more likely to increase when the mucosa is
thicker (Vasak et al. 2011; D'haese & De Bruyn 2013; Ochi et al. 2013). In the present study, we tried
to minimize this effect using a bite index and fixation pins (Horwitz et al. 2009; Van Assche et al.
2012; Cassetta et al. 2013).
The EAO consensus conference (2012) launched the warning that the belief that less training is
needed for computer-guided implant therapy is far from accurate. “Surgical skills and experience go
above and beyond those necessary for providing regular implant surgery” (Sicilia et al. 2012).
The good result for the inexperienced surgeons is surprising, especially when comparing the results
of the in vitro studies, like already mentioned before (Cho et al. 2010; Cushen & Turkyilmaz 2013) It
might be explained by the supervision by trained colleagues during each step in the procedure of
planning and surgery or by the guiding system itself. Probably both factors played a significant role.
Conclusion
Based on these findings, one could assume that surgical experience has no major influence on the
accuracy of implant placement with mucosa-supported stereolithographic drill guides in fully
edentulous jaws, when all steps needed for the procedure are supervised by experienced dentists.
The major factor of inaccuracy can be found in malpositioning of the guide. Implants placed at the
right side of the patient by a right-handed surgeon are more accurate than those placed at the left
side.