Clinical Evaluation of Implant Suprastructures Depending on the Biomechanical Characteristics of the Materials Used

Abstract

The choice of an appropriate implant suprastructure, which should be made according to the individual characteristics of each patient, is a leading factor in achieving good aesthetic results. The aim of this study was to assess the clinical behavior of implant suprastructures according to functional, biological, and aesthetic criteria. Methods: The study enrolled 23 patients with a total of 27 implants in different areas of the oral cavity. The following groups were studied for a period of 1 year: Group A, the control group (monolithic implant crowns made of zirconium dioxide on a titanium base); Group B (monolithic implant crowns made of lithium disilicate with individualized (custom) titanium alloy abutment); and Group C (implant crowns made of zirconium dioxide with individualized titanium alloy abutment). The functional criteria included suprastructure fracture, crown fracture or chipping, screw fracture or loosening, faceting or wearing of the occlusal surface of the crown, articulation relations, and suprastructure fracture. The biological indicators included the probing pocket depth (PPD) and Mombelli et al.’s plaque index and bleeding on probing. The bone loss level was measured radiographically. Digital measurements of the bone loss level were performed when definitive prosthetics were placed and 1 year after. The platform of the implant was taken as a reference point. Two parallel lines ran distally and medially to the implant and descended to the level of the first thread, which was in contact with the bone. The aesthetic evaluation was based on Jempt’s papilla index and considered differences in the color of the peri-implant mucosa. The suprastructures were examined at 2 weeks and 1 year after the initiation of prosthetic treatment. Results: Regarding the functional criteria, no deviations from the reference range were registered in any of the study groups for both measurements. Radiography showed no bone loss in any of the study groups. Group B showed the best behavior regarding biological complications, followed by Group C. There was no plaque accumulation in Group B, but statistically significant amounts were found in Groups A (p = 0.08) and C (p = 0.01). Group B had the lowest bleeding index, but the differences between the groups were found to be insignificant during the observations at the one-year mark. On the vestibular side of the papillae, p = 0.39, while on the oral side, p = 0.35. The PPD measurements showed that there were statistically significant differences between the three groups when they were compared after the second week and after one year (p = 0.00). Jempt’s papilla height index showed the highest values in Group B, which increased by two times throughout the study period. The increase was statistically significant in Groups B and C, while the growth in Group A remained statistically insignificant (p = 0.10). The aesthetic indicator “Gingiva color around the restoration” showed mild margo gingivalis graying in Groups B and C. Conclusions: Individualized implant abutments made of titanium alloy and monolithic restorations made of lithium disilicate or zirconium dioxide have stable biomechanical behavior and may be the optimal choice for the prosthetic treatment of partial edentulousness. Because this study took place within a limited period of time, clinical trials with a longer follow-up period need to be carried out.

1. Introduction

The complete aesthetic and functional rehabilitation of partial edentulousness is challenging, especially when it is dependent on single implant treatment. Restoring a missing tooth with an implant-supported prosthesis could be the optimal treatment choice; however, it requires good clinical planning and the sophisticated management of hard and soft tissues. Contemporary implant dentistry is constantly searching for solutions to improve biological and surgical outcomes [1].

The choice of an appropriate implant suprastructure, which should be made according to the individual characteristics of each patient, is a leading factor in achieving good aesthetic results [2]. Despite the availability of standardized prosthetic components, treatments often cannot meet the individual needs and personal requirements of each patient and cannot provide the most favorable position for future restoration and gingival architectonics [3,4]. However, individualized (custom) abutments can prevent many of the complications observed in prosthetic treatments using factory-manufactured abutments in cases in which there are anatomical limitations, such as lack of sufficient distance from the antagonist, large interproximal space, unparallel implant position, etc. [5]. New materials and production technologies allow for fast and accurate manufacturing [6,7,8]. Custom-made CAD/CAM abutments have a better survival rate compared to conventional suprastructures, but there are still only a few clinical trials that have been described in the literature.

Because of this, there is a lack of studies considering individualized CAD/CAM-milled abutments. Most of the existing studies did not include control groups or make comparisons with conventional implant abutments. The most commonly reported clinical outcomes that were measured are soft tissue volume and health, marginal bone level, aesthetic score, and success rate. These studies also demonstrated substantial heterogeneity regarding the manufacturing systems and materials of the abutments examined, the specific clinical outcomes observed, the design of the studies, and the measurement techniques employed. Studies represent a follow-up period of 1–3 years. For studies comparing CAD/CAM abutments with conventional abutments, no significant differences have been observed in most of the clinical outcomes at the 1-year follow-up [9].

There are no available studies that consider the pre-milled abutment titanium blanks included in the present research. These blanks are prefabricated with genuine connections to ensure accurate fit and superior performance with the implant connection.

Definite guidance regarding the use of different types of abutments over implant superstructures that considers their specific morphological and biomechanical characteristics, especially in the critical transmucosal area, is needed. These statements indicate the need to formulate recommendations for the optimal application of different implant superstructures according to their biomechanical and clinical behavior.

The working hypothesis of the present research was that the pre-milled abutment titanium blanks with genuine connections in combination with the ceramic material of the crown would have an advantage over the prefabricated Ti-base abutments and zirconium crowns due to the possibility of individual varieties of transmucosal profiles based on functional, biological, and aesthetic indicators, supporting their long-term clinical use.

2. Materials and Methods

The study enrolled 23 patients with a total of 27 TSV implants (Zimmer Biomet, Warsaw, IN, USA) located in different areas of the oral cavity (

Table 1. Distribution of patients by sex and age.

Patients’ Age		20–30		30–40		40–50		50–60		60–75		Total
Sex		M	F	M	F	M	F	M	F	M	F
Number		1	2	2	5	2	4	2	3	0	2
%		4%	9%	9%	21%	9%	17%	9%	13%	0%	9%
Total	Number	3		7		6		5		2		23
	%	13%		30%		26%		21%		9%		100.0%

and

Table 2. Zones of implantation.

Features	Zones of Implantation
	Incisors		Premolars		Molars		Total
	Number	%	Number	%	Number	%	Number	%
Maxilla	3	11%	5	18%	10	37%	18	67%
Mandible	0	0%	2	7%	7	25%	9	33%
Total	3	11%	7	25%	17	62%	27	100.0%

). Patients were selected based on several inclusion criteria, including good general health status, no systemic or chronic diseases, no parafunctions or inflammatory processes in the oral cavity, and non-smokers. All cases were selected at the stage of complete osteointegration and after the formation of the peri-implant mucosa by the provisional restorations. After a thorough extra- and intra-oral examination, segmental radiography, and a clinical evaluation of the transmucosal area, the definitive implant prosthetic restorations were made. The study was conducted in accordance with the Declaration of Helsinki and was approved by the Ethics Committee of Scientific Research with trial registry approval 12/14.05.2020.

The following groups were studied (

Table 3. The three study groups according to the criteria.

Group Name	Samples *	Materials	Sample Number
A (control group)	Monolithic implant crowns made of zirconium dioxide on a titanium base.	- GenTek™ TiBase (Zfx, Zimmer Biomet, USA - Katana (Kuraray Noritake, Tokyo, Japan)	9 suprastructures
B	Monolithic implant crowns made of lithium disilicate with individualized titanium alloy abutment.	- GenTek™ Pre-milled Abutment Blank (Zimmer Biomet, USA) - IPS e.max CAD (IvoclarVivadent, Schaan, Schaan, Liechtenstein)	9 suprastructures
C	Implant crowns made of zirconium dioxide with individualized titanium alloy abutment.	- GenTek™ Pre-milled Abutment Blank (Zimmer Biomet, USA) - Katana (Kuraray Noritake, Japan)	9 suprastructures

* All suprastructures consist of two pieces—a metal abutment and monolithic crown.

):

For each case, prosthetic treatment was performed according to an entirely digital workflow and using CAD/CAM technology (Figure 1, Figure 2 and Figure 3). The prosthetic field was scanned with an iTero Element intraoral scanner (Align Technology, San Jose, CA, USA), and the implant position was registered by a TSV Scan Body (Zimmer Biomet, Warsaw, IN, USA) that matched the diameter of the implant platform. The STL files were imported into ExoCad modeling software (GmbH, Darmstadt, Germany) when the custom abutments for Groups B and C and the monolithic crowns were used for all of the study groups were designed. The custom abutments and the zirconia crowns for Groups A and C were milled in an inLab MC X5 CAD/CAM machine (Dentsply, Sirona, Charlotte, NC, USA). The lithium disilicate crowns for Group B were milled in Lab MC XL (Dentsply, Sirona, Charlotte, NC, USA).

The images in Figure 1 depict the following:

(a)

The emergence profile formed by the provisional prosthetic structure;

(b)

The standard titanium base fixed to the implant at Tooth 14;

(c)

The implant suprastructure of Tooth 14 cemented extraorally;

(d)

The implant suprastructure fixed on the implant.

The images in Figure 2 depict the following:

(a)

Individualized titanium suprastructure fixed to the implant at Tooth 36;

(b)

The implant suprastructure cemented extraorally at Tooth 36;

(c)

The implant suprastructure fixed on the implant—occlusal and vestibular view.

The images in Figure 3 depict the following:

(a)

The emergence profile formed by the provisional prosthetic structure;

(b)

The individualized titanium suprastructure fixed to the implant at Tooth 26;

(c)

The implant suprastructure cemented extraorally to Tooth 36;

(d)

The implant suprastructure fixed on the implant in the mouth—vestibular view.

All of the patients were invited to a follow-up examination at 2 weeks and 1 year after the definitive prosthesis Restoration was assessed according to the functional, biological [10,11,12,13], and aesthetic indicators [12] described in

Table 4. Assessment according to functional, biological, and aesthetic indicators.

Criteria
Functional	Biological	Radiological	Aesthetic
Suprastructure fracture	Mombelli et al.’s plaque index: 0—No plaque detected 1—Plaque only recognized by running a probe across the smooth marginal surface of the implant 2—Plaque can be seen with the naked eye 3—Abundance of soft matter	Bone loss level	Jempt Papilla Index: 0—No papillae present 1—Less than half of the papillae are present 2—Half or more of the papillae height but do not extend all the way up to the contact point 3—The papillae fill up the entire proximal space 4—Hyperplastic papillae cover much of the single implant restoration
Crown fracture or chipping
Connecting screw fracture or loosening	Mombelli et al.’s bleeding on probing index: 0—No bleeding when a periodontal probe is passed along the mucosal margin adjacent to the implant 1—Isolated bleeding spots visible 2—Blood forms a confluent red line on mucosal margin 3—Heavy profuse bleeding		Gingival color around the restoration: 1—Pale pink 2—Pink 3—Coral Pink
Faceting or wearing of the occlusal surface of the crown
Articulation relations	Probing pocket depth
Suprastructure fracture

. The follow-up for all of the cases was carried out by an experienced clinician (Table 2). The statistical analysis was conducted using SPSS 22.0 software (SPSS, IBM, Armonk, NY, USA).

The indicators detailed below were evaluated following the applied methods:

The examination was performed using magnifiers with a magnification of x3.6 (Zeiss, Jena, Germany) and with a straight probe.

The articulation relations were verified using articulation paper with a micro thickness of 40 µ (Bausch, Hainspitz, Germany).

Using the parallel technique (Updegrave 1951), segmental radiography was performed by an experienced X-ray technician for each patient. Digital measurements of the bone loss level were performed in the event of definitive prosthetics and at 1 year. The platform of the implant was taken as a reference point. Two lines parallel to the implant descended medially and distally to the level of the first thread, which was in contact with the bone (Figure 4).

3. Results

Regarding the functional criteria, no deviations from the reference range were registered in any of the study groups for both measurements.

The measurement results for the biological criteria were as follows:

With regard to the plaque retentive index, it can be concluded that in the supra-implant restorations, a combination of an individualized titanium base and monolithic lithium disilicate crown (Group B) exhibits the best behavior (the absence of plaque around the restorations). This conclusion was valid for both observation periods.

The highest amount of plaque was observed in Group C.

In Groups A and C, the plaque increased during the observation period. This increase was more noticeable in Group C. The value of the index reached 0.56 in one year, and the increase was statistically significant (p = 0.01). In Group A, the observed increase from 0.17 to 0.22 was characterized by static insignificance (p = 0.08) (

Table 5. Analysis of the plaque retention index: temporal changes and differences between the groups.

	Temporal Changes		Differences between Groups (p-Value)
	2 Weeks	1 Year	2 Weeks	1 Year
Group A	0.17 (0.28)	0.22 (0.32)	0.13	0.00
Group B	0.00 (0.00)	0.00 (0.00)
Group C	0.22 (0.29)	0.56 (0.44)

in brackets—standard deviation.

).

Group A showed the worst performance in terms of bleeding on the oral side at the 1-year follow-up. The best general mean values of the bleeding indicator were observed in Group B, where initially, there was no bleeding in the second week, while at 1 year, the value remained relatively low at 0.22 on both the vestibular and oral sides. On both sides—vestibular and oral—there were statistically significant differences in the second week due to the increased values of the bleeding indicator in Group A. The differences became insignificant between the groups after one year of observations. On the vestibular side, p = 0.39, while on the oral side, p = 0.35 (

Table 6. Temporal analysis of bleeding index in patients with different types of implant suprastructures.

	Vestibular		Oral
	2 Weeks	1 Year	2 Weeks	1 Year
Group A	0.56 (0.73)	0.56 (0.73)	0.67 (0.71)	0.56 (0.53)
Group B	0.00 (0.00)	0.22 (0.44)	0.00 (0.00)	0.22 (0.44)
Group C	0.00 (0.00)	0.67 (0.87)	0.11 (0.33)	0.22 (0.67)
p-value	0.01	0.39	0.01	0.35

in brackets—standard deviation.

).

Group B had the lowest periodontal pocket depth (PPD) values throughout the study period, followed by Group C. The highest values were reported in the prosthetic restorations used in Group A, increasing by almost two times over a period of 1 year.

In Group C, there was a statistically insignificant decrease in the pocket depth value (p = 0.46). In Groups A and B, there were increases in the observed period, as shown in the table. In both cases, these were statistically significant, with p = 0.03 and p = 0.00 (

Table 7. Pocket depth analysis while probing: temporal changes and differences between groups.

	Temporal Changes		Differences between Groups (p-Value)
	2 Weeks	1 Year	2 Weeks	1 Year
Group A	1.54 (0.31)	2.41 (0.53)	0.00	0.00
Group B	0.93 (0.20)	1.12 (0.30)
Group C	1.42 (0.25)	1.39 (0.75)

in brackets—standard deviation.

).

Despite the observed differences in the dynamic development of the three groups, the patients in Group B maintained the lowest pocket depth values during probing compared to the other groups. It is also important to note that statistically significant differences were observed when the three groups were compared in the second week and after one year (p = 0.00). In both measurements, the indicator remained statistically significantly lower in Group B compared to the other two groups.

The radiography results showed no bone loss in any of the study groups.

A series of T-tests were performed in order to assess the significance of the variations in the mean values of bone loss for each group on the basis of the two time periods (

Table 8. Bone loss measurements—statistical findings (mm).

		Medial		Distal
		T	T-1	T	T-1
Group A	Mean (std. div.)	2.422 (0.429)	2.422 (0.429)	2.433 (0.406)	2.443 (0.394)
	p-value	No change		0.347
Group B	Mean (std. div.)	1.8 (0.235)	1.789 (0.252)	1.8 (0.292)	1.8 (0.292)
	p-value	0.347		No change
Group C	Mean (std. div.)	2.344 (0.288)	2.344 (0.288)	2.322 (0.379)	2.333 (0.361)
	p-value	No change		0.347

).

Jempt’s papillae height index had the highest values in Group B, increasing by two times over the course of the study period.

The results showed an increase in this index in all three patient groups, as measured by the means. The increase was statistically significant in Groups B and C, while the growth in Group A remained statistically insignificant (p = 0.10).

In the distal papillae, Group B once again showed the highest mean for Jempt’s papillary height index compared to the other two groups, reaching an average of 2.56. However, the observed increases were only statistically significant only Group C, where the largest increase in the indicator was observed. In Group B, the increase was statistically insignificant over the study period (p = 0.17). Similarly, Group A also recorded a statistically insignificant increase (p = 0.30) (

Table 9. Jempt’s papillae height analysis for each group.

	Medial Papilla		Distal Papilla
	2 Weeks	1 Year	2 Weeks	1 Year
Group A	1.44 (0.53)	1.78 (0.67)	1.56 (0.53)	1.67 (0.50)
Group B	1.78 (0.67)	2.33 (0.50)	2.33 (0.50)	2.56 (0.53)
Group C	1.33 (0.50)	1.89 (0.78)	1.22 (0.67)	1.89 (0.78)
p-value	0.25	0.19	0.03	0.01

in brackets—standard deviation.

).

With regard to the medial papilla, there were no statistically significant differences between the groups in the two measurements at both two weeks and one year.

The aesthetic indicator “Gingiva color around the restoration” showed a mild margo gingivalis graying of the otherwise clinically healthy PIM in Groups B and C, with index scores of 1–2 (pale pink to pink) compared to Group A (Figure 5).

4. Discussion

No functional or technical complications were observed during the study period, which is consistent with the results reported by Bosch et al. [14]. This allows us to conclude that the standard and individualized abutments have equally favorable behavior in terms of mechanical stability.

In their study, Bompolaki et al. [15] found a greater number of functional complications in supra-implant restorations made with factory abutments. De Angelis et al. [16] reported isolated cases of chipping in the lithium disilicate crown group; however, the replacement was unnecessary. All of the authors of previously published studies indicated a low rate of functional complications between study groups, which was statistically insignificant.

Gulje et al. [17] followed up on certain zirconium implant restorations with individualized titanium abutments for a period of 1 year and achieved low values of the considered biological indicators, similar to our results. Schepke et al. [18] conducted a comparative assessment of factory-made and individualized abutments and reported that they did not observe any statistically significant differences in the biological factors, which is not consistent with our findings.

However, we found statistically significant values for some of the biological indicators, highlighting that the group with individualized titanium abutments with monolithic restorations made of lithium disilicate had the lowest values in all of the measured criteria.

In their study, Joda и Bragger [19] compared digitally fabricated implant suprastructures according to aesthetic criteria and noted that “graying” was visually observed in the marginal area of prosthetic restorations with individualized abutments. This opinion was confirmed by other authors [19,20,21] and coincided with our observations.

Our results confirmed the opinion of Borges et al. [22] that prosthetic restorations on individualized abutments manufactured with CAD/CAM technology improve aesthetics more than standard abutments due to the better design of the soft tissues of the peri-implant and the restoration of the interproximal papillae.

The presence of a statistically significant difference between the groups and the best values of Group B best shows the effectiveness of the material and design of permanent supra-implant prosthetics in achieving significantly higher Jempt scores for papillae growth than alternative methods. At present, there have been no other studies considering pre-milled abutments with titanium blanks.

The study that was conducted in this work enrolled 23 patients with a total of 27 implants in different areas of the mouth, and a clinical evaluation of the biomechanical parameters of the individualized titanium alloy abutments showed that individualized implant abutments made of titanium alloy and monolithic restorations made of lithium disilicate or zirconium dioxide have stable biomechanical behavior and may be the optimal choice for prosthetic treatment The working hypothesis of this study, which was that the pre-milled abutments with titanium blanks with genuine connections in the combination ceramic material of the crown would be advantageous due to the possibility of individual varieties of transmucosal profiles based on functional, biological, and aesthetic indicators, was proven. Definitive guidance regarding the use of pre-milled abutments with titanium blanks was presented in this study.

Our findings coincide with the opinions of a number of authors that this type of abutment recreates the emergence profile of the natural tooth as accurately as possible, eliminating the possibility of over contouring the entire suprastructure caused by the volumetric cylindrical cross-section of the standardized abutment and the required thickness of the definitive crown [13,23,24,25].

5. Conclusions

Individualized implant abutments made of titanium alloy and monolithic restorations made of lithium disilicate or zirconium dioxide have stable biomechanical behavior and may be the optimal choice for the prosthetic treatment of partial edentulousness. Because this study was carried out over a limited period of time, clinical trials with a longer follow-up period need to be carried out.