The current study was conducted to investigate the accuracy of the open and closed implant impression techniques in partially edentulous patients with two adjacent implants. The ethical principles were adhered to, and ethical approval to conduct the study was duly obtained from the Ministry of Health, State Khartoum, Khartoum University Teaching Hospital, number: [WK/OS/AETEA/44/1].
Patients who were scheduled to receive two adjacent implants were invited to participate in the study. The sample that would have sufficient power for analysis was calculated based on data from the previous clinical study by Stimmelmayr in 2013 [9]. The sample size was determined using the following formula:
$$ n=\frac{{\left(\ z\sigma \right)}^2}{(d)^2} $$
where:
n = the required sample size
Z = is the critical value of the normal distribution
σ = the standard deviation taken from the previous study
d = the margin of error (10% × mean).
The sample size was 31 patients; this was increased to 40 participants to accommodate patient dropouts during the study.
The inclusion criteria were patients over 18 years of age and willing to participate. A prerequisite to participation was a treatment plan that would involve two adjacent implants. The patient should also be category ASA I or ASA II medical history (American Society of Anesthesiologists Classification) [10]. Furthermore, evidence of bone loss or implant mobility at the time of impression making, formed part of the exclusion criteria [11, 12].
Informed consent was made, and participants who agreed to participate signed the consent form. For every patient, a surgical positioning guide was fabricated from a diagnostic wax-up that correlated the anatomic conditions. The implant (Osstem Implant System, Seoul, Korea) installation directions were carried out according to the amount and status of the available bone [13]. A Specialist Oral Surgeon placed the implants using the manufacturer’s standardized technique, and similarly, a Specialist Prosthodontist carried out the related restoration steps. For making the impressions, individual trays were initially checked intraorally, and the final impressions made using Virtual Monophase vinyl polysiloxane impression material (Ivoclar Vivadent AG). Before impression making, the horizontal distance between the two impression copings was measured inside the patient’s mouth using a digital caliper (HSL 246-15, Karl Hammacher GmbH, Germany) and recorded (Fig. 1). This recorded intraoral horizontal distances would later be compared against similar horizontal measurements on the master casts, to evaluate discrepancies or horizontal displacements between the positions intraorally and on the master casts. The same impression evaluation criteria used in our previous study were also used here [8], which was described by Lee and Gallucci as follows [14]:
- 1.
There should be accurate imprints of the implant areas.
- 2.
There should be no voids in the occlusal, buccal, lingual, and interproximal surfaces of the neighboring teeth.
- 3.
There should be a proper reproduction of the implant area.
- 4.
There should be no impression material in the analog-impression coping interfaces.
- 5.
The impression material should not be separated from the custom tray.
- 6.
The transfer copings should not be displaced from the impression.
Any impression not meeting these criteria was repeated until the criteria were met. Two impressions were made for every patient by the same clinician, one using the open and a second with the closed tray technique.
For the analysis of accuracy in the vertical direction or marginal discrepancy, verification jigs were constructed to connect the two impression copings [15]. These verification jigs were used to transfer the relationships between the two impression copings and their implants from the patients’ mouths to the master casts. To construct the verification jig, the two impression copings would be attached to their implant, inside the patient mouth, and a string of dental floss is wrapped around to connect the two impression copings (Fig. 2). A light cure acrylic resin (Al dente dental products GmbH, Germany) adapted over the dental floss in increments and cured according to the manufacturer’s instructions. The impression coping’s screws would then be loosened, and the jig removed [16].
The impression copings for both the open and closed tray techniques were re-assembled and fixed into their corresponding implant analogs. Dental Stone Type IV (Elite Rock, Zhermack) was mixed using a vacuum machine for 30 s, then poured using the boxing technique over a vibrator, and the casts separated after 45 min according to the manufacturer’s instruction [17, 18]. The master casts were then sectioned to a base of 20 mm, to allow their allocation under the stereomicroscope (AmScop14370, Myford Road, #150, Irvine, CA 92606 USA), to be examined at a × 50 magnification, and to evaluate the presence or absence of marginal discrepancy [8, 19].
Two examiners were involved in the evaluations, and inter-examiner reliability of 0.932 was obtained.
Statistical analysis
All the data were tabulated and statistically analyzed using IBM SPSS Statistics software version 22. The t test was used to compare intraoral and master cast horizontal distances as well as sub-groups of the open and close impression tray techniques. Where data are not normally distributed, Wilcoxon signed test was used for numerical dependent data and paired data; Mann-Whitney U test was used for independent numerical groups. Chi-square test was used for the association between categorical variables. The p value was set at p ≤ 0.05 and regarded as statistically significant.
