a 1-year randomised controlled trial comparing zirconia versus metal-ceramic implant supported...

� 347

Eur J Oral Implantol 2011;4(4):347–361

RANDOMISED CONTROLLED CLINICAL TRIAL

Mandana Hosseini, DDSPhD student, Department of Oral Rehabilitation, School of Dentistry, University of Copenhagen, Copenhagen, Denmark

Nils Worsaae, DDSSpecialist in Oral and Maxillofacial Surgery, Department of Dental, Oral and Maxillofacial Surgery, Rigshospitalet, Copenhagen, Denmark

Morten Schiødt, DDS, Dr OdontSpecialist in Oral and Maxillofacial Surgery, Department of Dental, Oral and Maxillofacial Surgery, Rigshospitalet, Copenhagen, Denmark

Klaus Gotfredsen, DDS, Odont DrProfessor, Department of Oral Rehabilitation, School of Dentistry, University of Copenhagen, Copenhagen, Denmark

Correspondence to:Mandana HosseiniDepartment of Oral Rehabilitation, School of Dentistry, University of Copenhagen, 20 Noerre Allé, DK-2200, Copenhagen N, DenmarkTel: +45 353 26988Fax: +45 353 26602Email: [email protected]

Mandana Hosseini, Nils Worsaae, Morten Schiødt, Klaus Gotfredsen

A 1-year randomised controlled trial comparing zirconia versus metal-ceramic implant-supported single-tooth restorations

Key words all-ceramic crowns, dental implants, metal-ceramic restorations, randomised controlled trial, zirconia abutments

Purpose: To compare the biological, technical and aesthetic outcomes of single implant-supported all-ceramic versus metal-ceramic crowns.Materials and methods: Thirty-six patients with premolar agenesis were randomly treated with 38 all-ceramic (AC) and 37 metal-ceramic (MC) implant-supported single-tooth restorations. A quasi-randomisation of consecutively included restorations in patients with one or more implants was used, i.e. a combination of parallel group (for 13 patients with one restoration) and split-mouth (for 23 patients with ≥2 restorations). All patients were recalled for baseline and 1-year follow-up examinations. Biological and technical outcomes, including complications, were clinically and radiographically registered. The Copenhagen Index Score and visual analogue scale (VAS) score were used to assess professional and patient-reported aesthetic outcomes, respectively, by blinded assessors.Results: One-year after loading, no patient dropped out and no implant failed, though one MC restoration had to be remade. The marginal bone loss was not significantly different between AC and MC restorations (AC: mean 0.08 mm, SD 0.25; MC: mean 0.10 mm, SD 0.17). Seven out of 10 inflammatory reactions were registered at AC restorations. Two technical complications, one loss of retention and one chipping of veneering porcelain were recorded at two metal-ceramic crowns. The marginal adaptation of the all-ceramic crowns was significantly less optimal than the metal-ceramic crowns (P = 0.014). The professional-reported colour match of all-ceramic crowns was significantly better than metal-ceramic crowns (P = 0.031), but other aesthetic parameters as well as the VAS scores demonstrated no statistically significant difference between AC and MC restorations. Conclusions: Marginal bone loss and the aesthetic outcomes were not significantly different between AC and MC restorations in this short-term follow-up study, though inflammatory reactions in the peri-implant mucosa as well as less optimal marginal adaptation were more frequently registered for all-ceramic compared to the metal-ceramic crowns.

Conflict-of-interest statement: The authors declare no conflicts of interest.

Hosseini et al Zirconia versus metal-ceramic restorations348 �


� Introduction

The long-term survival rate of single-tooth implants has been documented by clinical studies1-8. As a consequence of these promising results, there has been an increasing focus on the aesthetic outcomes of the implant-supported treatments9-11. To achieve an optimal treatment outcome in the aesthetic zone, not only the surgical, but also the prosthetic pro-cedures are particularly important. According to a consensus report on aesthetics in implant dentistry, optimal standards for fixed implant restorations include healthy peri-implant tissue and restorations in harmony with the surrounding dentition12. The implant-supported restorations traditionally include titanium abutments and metal-ceramic crowns. However, metal abutments have been suggested to induce a greyish discolouration of peri-implant mucosa13-15. The high-strength oxide ceramics, especially zirconia, were introduced as an alterna-tive material to metal abutments and crown cop-ings to improve the colour harmony of peri-implant soft tissue and restor ations with the natural denti-tion16,17. Additionally, biological properties such as soft tissue integration and the bacterial adhesion of the oxide ceramics are comparable to the titanium abutments18-22. However, the number of clinical studies that include zirconia implant-supported sin-gle crowns (ISSC) is limited15,23. One randomised controlled clinical trial (RCT) compared the aesthetic outcome of the peri-implant soft tissue between zir-conia and titanium abutments24, and another clin-ical study analysed the influence of the abutment materials on the colour of the peri-implant soft tissue a short time after insertion of ISSCs25, but clinical studies comparing the colour and morphology of all-ceramic and metal-ceramic crowns are sparse. Although a recently published RCT also included a number of aesthetic parameters of all-ceramic and metal-ceramic crowns, the implant-supported single-tooth restorations in that study were screw-retained, and the core material of all-ceramic crowns was made of glass-infiltrated alumina26. Thus, RCTs comparing both crown and peri-implant soft tissue aesthetic parameters of the cement-retained zirco-nia-based and metal-ceramic ISSCs are missing.

The purpose of this clinical study was to compare the biological, technical and aesthetic outcome of

zirconia abutments and zirconia all-ceramic crowns (AC restorations) with metal abutments and metal-ceramic crowns (MC restorations) supported by sin-gle-tooth implants in patients with tooth agenesis in the premolar region.

The present article is reported according to the CONSORT statement for improving the quality of reports of randomised trials27.

� Materials and methods

� Study design

In this RCT, the inclusion criteria were patients with tooth agenesis in the premolar region, having no contraindications for oral implant treatment (e.g. uncontrolled diabetes, metabolic bone disorders, history of radiotherapy in the head and neck, cur-rent chemotherapy, or other diseases with an influ-ence on bone healing). Furthermore, patients had to be treated with Astra Tech® (Mölndal, Sweden) implant systems to be included. The study protocol was accepted by the Danish Regional Committee on Biomedical Research Ethics (H-1-2009-119), and the patients were informed about the purpose and design of the study by the treating dentists. The participants were consecutively included from Janu-ary 2008 to December 2009. Initially, 40 patients accepted the informed consent; however, 4 patients were subsequently excluded from the study, as 2 patients were treated with implants other than Astra Tech, 1 patient had missing teeth because of trauma, and the other patient was treated with implants in the molar region.

The 36 patients included in the study (18 men and 18 women) had a mean age of 28.1 years (SD 9.2, range 19–57 years), and they had a total of 95 missing premolars (27 first premolars and 68 second premolars). They were restored with 75 ISSCs; thus, each patient was restored with 1 to 5 (median 2) premolar ISSCs (Table 1).

The prosthetic treatments were randomised at site level between AC (test group n = 38; maxilla n = 19, mandible n = 19) and MC (control group n = 37; maxilla n = 21, mandible n = 16) restorations (Fig 1). To ensure an equal number of each restor-ation type at the end of the study and to insert both

Hosseini et al Zirconia versus metal-ceramic restorations � 349


types of restorations in the patients with two or more premolar implants, a quasi-randomised design was used. As the randomisation included restorations both in patients with one (n = 13 patients) and in patients with two or more premolar implants (n = 23 patients), the current study was a combination of a parallel group and a split-mouth study design. The first restoration material was chosen by drawing lots between two envelopes containing the randomisa-tion codes (AC or MC). Thereafter, the subsequent restorations were alternately made of AC and MC materials. To conceal the random allocation as long as possible, a person independent of the treatments informed the treating dentist about the type of res-toration (AC or MC) after the impression was taken.

For patients with several premolar restorations, the allocation of restoration material started with the most distal restoration in the first quadrant and con-tinued to the most distal restoration in the fourth quadrant.

The number of patients enrolled in the study was primarily decided through duration of the study period, i.e. how many referrals of patients with tooth agenesis in the premolar region the depart-ment received. A power calculation was based on peri-implant marginal bone loss of a comparable patient group provided from results of an unpub-lished 3-year prospective study:

n1 = n2 = 2(�/�)2 f(�, �) = 2(0.3/0.2)2

f(0.05, 0.2) = 35

Table 1 The number of premolar implant-supported single-crowns (ISSC) per patient, and the distribution of all-ceramic (AC) and metal-ceramic (MC) restorations in patients with different numbers of premolar ISSCs.

Number of premolar ISSCs per patient (number of patients)

1 (13)

2 (13)

3 (5)

4 (4)

5 (1)

Total number of restorations

Number of restorations

AC 7 13 7 8 3 38

MC 6 13 8 8 2 37

Total number of restorations 13 26 15 16 5 75

AC

MC

Fig 1 A zirconia abut-ment and all-ceramic crown (AC: all-ceramic restoration, test group), and a titanium abut-ment and metal-ceramic crown (MC: metal-ceramic restoration, control group).

AC

MC



The clinically relevant difference of mean marginal bone loss (�) between two patient groups was set to 0.2 mm, standard deviation of marginal bone loss (�) was set to 0.3 mm, and type 1 (�) and type 2 (�) errors were set to 5% and 20%, respectively. Thirty-five implant-supported single-tooth restor-ations should be used in the test as well as in the control group.

� Treatment procedure

All 75 implants (Astra Tech) were inserted by four experienced oral surgeons at the Department of Oral and Maxillofacial Surgery, Glostrup University Hospital (Copenhagen, Denmark) according to the standard surgical guidelines from the manufacturer. In 35 implant regions with atrophy in the bucco-lingual dimension, a local bone grafting procedure with a combination of autologous bone and Bio-Oss® (Geistlich, Wolhusen, Switzerland) was per-formed. Twenty implants were inserted in the eden-tulous premolar regions with two missing teeth as the alveo lar bone widths were insufficient for two implants (Fig 1). The remaining 55 implants replaced one missing permanent premolar (Fig 2). Thirty out of these 55 implants replaced primary second molars, which were retained and were extracted close to or at the time of implant insertion.

After a healing period of 4 to 6 months, the patients were referred to the School of Dentistry in Copenhagen for ISSCs, where one of four experi-enced prosthodontists performed the restorations. Impressions at implant level were taken with a poly-ether material (Impregum™; 3M ESPE, Seefeld, Germany). All zirconia abutments were ZirDesign™ (Astra Tech), titanium abutments were TiDesign™ (Astra Tech), and two gold alloy abutments were Cast-to (Astra Tech). The restoration materials are listed in Table 2.

Figure 2 illustrates the abutment preparations. The zirconia and metal abutments were all prepared with chamfer preparations using a torpedo-formed cylindrical diamond. According to the clinical guide-lines, the finish line design of the crown margins were located 1 to 1.5 mm sub-mucosally in visible regions and <1 mm sub-mucosally in non-visible regions. At AC and at MC crowns, an all-porcelain butt joint was designed buccally. The all-ceramic copings for

the AC restorations were manufactured out of pre-sintered zirconia blanks by computer-aided design/computer-aided manufacturing (CAD/CAM). The metal-ceramic copings for the MC restorations were fabricated using the conventional lost-wax casting technique. Although the dental technician used dif-ferent crown materials (Table 2), the fabrication pro-cedures were similar.

All abutments were screw-retained with a screw torque of 25 Ncm in accordance with the manufac-turer’s recommendations. All crowns were cement-retained: 71 ISSCs with phosphate cement (DeTrey® Zinc, Dentsply, Konstanz, Germany) and 4 (3 AC crowns and 1 MC crown) with resin cement (Pan-avia®, Kuraray, Okayama, Japan). The treatments were completed with careful instruction of all patients in oral hygiene at the time of crown cementation and at each follow-up examination. Supportive therapy was initiated if indicated.

� Follow-up examinations

All patients were recalled for baseline (median 2.4 months after crown insertion, range 0.5–12 months) and 1-year (median 13.5 months after crown inser-tion, range 11–20 months) follow-up examinations. The clinical and radiographic assessments were blinded and were performed by an observer who was not involved in the treatment of the patients; unfortunately the differences in radiopacity of zirco-nia and titanium abutments reduced the reliability of blinded radiological assessments. The clinical photo-graphs of the restorations, including the neighbour-ing teeth and marginal peri-implant mucosa, were taken at the time of crown cementation, at baseline and at the 1-year examination. The digital intraoral radiographs (Digora® Optime digital films, Soredex, Tuusula, Finland) were taken by using the long-cone paralleling technique with Eggens film holders at both follow-up examinations and were assessed with the Digora Optime system. Biological, technical and aesthetic variables were registered at all exami-nations.

� Biological outcome variables

The implant survival and clinical absence of mobility were recorded28. The peri-implant health and oral



Fig 2 All-ceramic (AC) and metal-ceramic (MC) restorations before and after crown cementa-tion.

AC

MCAC

MC

AC

MCAC

MC



hygiene were assessed by recording probing pocket depth (PPD) ≥ 5 mm, modified Plaque (mPlI) and Sulcus Bleeding Index (mBI)29 at four aspects of each implant at the baseline and 1-year examina-tions. The median values of four scores of mPlI and mBI at each implant were used for statistical analyses. The marginal bone levels at the implants were assessed as the most coronal bone-implant contact mesially and distally at both examinations (Fig 3), and peri-implant radiolucency and peri-radicular radiolucency of neighbouring teeth were radiographically recorded. The peri-implant mar-ginal bone loss was the mean value of change in the mesial and the distal marginal bone level from the baseline to the 1-year examination.

Additionally, vitality loss in adjacent teeth, neuro-sensory disturbances and inflammatory reactions such as fistula, exudation/suppuration and pain adja-cent to the implant reconstructions were recorded at the baseline and the 1-year examinations.

� Technical outcome variables

The clinical examinations included assessments of crown survival, loosening or fracture of the abut-ment screw, loss of retention, and fracture including chipping of the veneering ceramics.

Radiographs were examined to record any excess of cement materials and to evaluate mar-

ginal fit of the crowns using a modified marginal adaptation score30: score 1 was excellent fit, score 2 was distinguishable misfit (mesially and/or dis-tally), score 3 was distinct misfit (mesially and/or distally), and 4 was unacceptable misfit. The marginal adaptation score of each ISSC restor-ation corresponded to the highest score detected on radiographs from the baseline and the 1-year examination.

� Professional-reported aesthetic outcome variables

The professional-reported aesthetic outcome of the restorations was evaluated at the baseline and the 1-year follow-up examinations by using the Copen-hagen Index Score (CIS)30,31. According to this index, each score ranged from 1 for the best to 4 for the poorest aesthetic outcome. The following five variables were used in this study:• crown morphology score • crown colour match score • mucosal discolouration score• papilla index score, mesially• papilla index score, distally.

The overall professional-reported aesthetic outcome was expressed by CIS, which was the sum of these five aesthetic scores. In addition, the cast models fabricated before crown cementation were used to measure the mesiodistal distance in the edentu-lous space (DES) as the minimum coronal distance between the proximal surfaces facing the implant site (Fig 4).

� Patient-reported outcome variables

The patient-reported aesthetic outcome was assessed by a visual analogue scale (VAS)—a 100 mm line with the end phrases ‘Very bad aesthetic’ on the left (0 mm) and ‘Very good aesthetic’ on the right (100 mm)—1 year after crown cementa-tions.

� Statistics

The statistical analyses of outcome variables were performed with the SAS 9.1 package. Descriptive

Table 2 Material composition of all-ceramic and metal-ceramic restorations.

Type of restor ation

Abutment* Crown coping Veneering ceramic

All-ceramic restorations (n = 38)

Zirconia (ZirDesign™) (n = 38)

KaVo Zirconia†

(n = 27)HeraCeram Zirkonia‡

(n = 27)

Procera Zirconia§

(n = 11)HeraCeram Zirkonia‡ (n = 7)

IPS e.max CeramII

(n = 4)

Metal-ceramic restorations (n = 37)

Metal (TiDesign™: n = 35, Cast-to*: n = 2)

HeraNordic 75‡

(n = 34)HeraCeram‡

(n = 34)

ORION WX�

(n = 3)IPS d.SIGNII

(n = 3)

* Astra Tech, Mölndal, Sweden† Kavo, Biberach, Germany ‡ Heraeus Kulzer, South Bend, IN, USA§ Nobel Biocare, Gothenburg, Sweden II Ivoclar Vivadent, Schaan, Liechtenstein� Elephant Dental BV, Hoorn, Netherlands



analyses of data were performed. To account for the correlation between several restorations applied to the same patient, models had to incorporate a random subject level. For the quantitative data (dif-ferences in bone level, bone loss, DES, VAS and CIS), evaluation was performed by using a traditional mixed model of ANOVA. For ordinal categorical data (differences in mPlI, mBI, marginal adaptation score and professional-reported aesthetic scores in the test and control groups), a nonlinear mixed model was applied using PROC NLMIXED32. The statistical sig-nificance level was set at P < 0.05.

� Results

Thirty-six patients attended the 1-year follow-up examination, and an additional 4 patients were excluded from the study. One of the 36 patients did not show up to the baseline examination, where the radiographs and photos from the day of the crown cementation visit were used to replace the missing data.

� Biological outcome variables

Table 3 describes the results of the biological out-come variables. All implants survived and no mobility

Table 3 Biological outcome variables at the all-ceramic (AC) or metal-ceramic (MC) restorations recorded at the baseline and 1-year examinations.

Biological variables Baseline 1-year follow-up

AC MC AC MC

Implant survival 100% 100% 100% 100%

Implant mobility 0% 0% 0% 0%

Modified Plaque Index (mPlI) median 1 (range 0–2)

median 1 (range 0–2)



Modified Sulcus Bleeding Index (mBI) median 0 (range 0–2)




Marginal bone level mean 0.58 mm, SD 0.62

mean 0.33 mm, SD 0.33

mean 0.66 mm, SD 0.68

mean 0.43 mm, SD 0.36

Marginal bone loss - - mean 0.08 mm, SD 0.25

mean 0.10 mm, SD 0.17

Marginal bone loss ≥ 1.6 mm - - 0% 0%

Fig 3 The marginal bone level was determined by measur-ing the distance between a reference point at the top of the implant (R) and the most coronal bone-implant contact (B).

Fig 4 Mesiodistal distance in the edentulous space (DES).

DES

BB

R R



was recorded after 1 year of function. At the MC and the AC restorations, no significant differences in mPlI and mBI were found at both examinations. The marginal bone level at the baseline examin ation was significantly (P = 0.034) more apically pos-itioned at the AC than at the MC restorations. At the 1-year examination, the mean marginal bone

loss at all restorations was 0.09 mm (SD 0.21, range 0–1.4 mm), and was not significantly different (P = 0.69) between the AC and the MC restorations. One patient with an AC and MC ISSC had marginal bone defects ≥2 mm at both sites at the baseline examination, but did not demonstrate marginal bone losses after 1 year (Fig 5).

Fig 5 Radiographs of patient with mar-ginal bone defects ≥2 mm at implants in maxillary premolar regions restored with metal-ceramic (MC) and all-ceramic (AC) restor ations. Although the marginal adapta-tion score of the AC restor ation was 3, no marginal bone loss was registered at either implant.

Table 4 Biological complications at the all-ceramic (AC) or metal-ceramic (MC) restorations recorded at the baseline and 1-year examinations.

Follow-up examinations

Number of patients

Complications Number of ISSCs

Number and type of restoration (marginal adaptation score)

Baseline 1 Mandibular paraesthesia 1 1 AC

1 year 1

3

3

2

1

Buccal marginal fistula, swelling, pain and suppuration

Suppurations on probing

Suppurations and PPD* ≥ 5 mm

PPD ≥ 5 mm

Continuous, weak pain

1

3

3

2

1

1 AC (1)

3 AC, (2, 2, 1)

3 MC (1, 1, 1)

2 AC (2, 2)

1 AC (2)

* PPD: probing pocket depth

BaselineMC

BaselineAC

One-yearMC

One-yearAC



The biological complications are presented in Table 4. One patient with paraesthesia at the baseline examination had regained normal sensibility after 1 year. The 10 other complications registered in 10 other patients at the 1-year examination were inflammatory reactions detected at 7 AC and 3 MC restorations. Five of the 7 all-ceramic crowns with complications had a marginal adaptation score of 2, while the 2 other all-ceramic crowns and the 3 metal-ceramic crowns had a marginal adaptation score of 1. Figure 6 illustrates an AC and MC restoration with marginal adaptation scores of 2 and 1, respectively, in a patient with good oral hygiene (mPlI = 0). At the 1-year examination, suppuration was registered at the AC restoration. Generally, at implants with biological complications, the mean marginal bone loss was less than 0.5 mm.

� Technical outcome variables

The survival rates of the AC and MC restorations after 1 year were 100% and 97.4%, respectively.

No complications involved the abutments or the abutment screws. At the baseline examination, no technical complications were recorded. At the 1-year examination, one chipping fracture of the veneer-ing ceramic (Fig 7) and one loss of retention were registered, both at MC restorations. The fractured crown was repaired by grinding and polishing, while the insufficient retention at the other crown resulted in fabrication of a new MC restoration.

The radiographic examinations revealed minor cement excess at one MC restoration with a marginal adaptation score of 2 and a marginal bone loss of 0.53 mm at the 1-year examination.

Figure 8 illustrates the frequency of marginal adaptation scores that were significantly (P = 0.014) lower at the MC (score 1: 76%, score 2: 21%, score 3: 3%, score 4: 0%) compared to the AC restor-ations (score 1: 37%, score 2: 60%, score 3: 3%, score 4: 0%). There was no statistically significant relationship between marginal adaptation scores and the marginal bone loss.

Fig 6 Marginal adaptation score 2 at all-ceramic (AC) and score 1 at metal-ceramic (MC) restoration (base-line radiograph), and clinical observation of suppuration at the AC restoration at the 1-year examination.

Fig 7 Photographs of a metal-ceramic crown (region 25) at the baseline examination, and chip-off fracture of the veneering ceramic recorded at the 1-year examination.

Baseline One-year

Baseline One-year



cantly different between the AC and the MC restor-ations at the baseline and the 1-year examinations (Table 6). At the 1-year examination, the frequency of the mucosal discolouration score was almost unchanged for both types of restorations, while the frequency of the papillae with a score of 1 (mesially and distally) increased at the AC and the MC restor-ations (AC: mesial 16.8%, distal 15.7%; MC: mesial 13.5%, distal 10.5%). The overall professional-reported aesthetic outcome, measured by using CIS, was not significantly different between the AC and MC restorations after 1 year (AC: mean 9.3, SD 1.9; MC: mean 9.1, SD 1.4; P = 0.705).

� Patient-reported outcome variables

The patient-reported overall aesthetic evaluations demonstrated no significant difference in the VAS scores between the AC and the MC restorations (AC: mean 84.9, SD 18.4; MC: mean 83.1, SD 18.8; P = 0.92).

Table 7 describes the relationship between the patient-reported (VAS) and professional-reported aesthetic outcomes. The mean of the VAS scores increased significantly with lower scores of the crown colour match, crown morphology and papilla index (mesially). However, the VAS scores had no significant relationship with the overall professional aesthetic outcome (CIS), the mucosal discolouration scores and the papilla index scores (distally).

� Discussion

After 1 year of function, no implant failed and no implant lost more than 1.5 mm of marginal bone. Only one metal-ceramic restoration with poor reten-tion was remade.

In the present study as well as in another clinical study of customised zirconia and titanium abutments with a 1-year follow-up period33, all the zirconia abutments in premolar regions survived. This sug-gests that high-strength zirconia abutments as well as metal abutments can be used in posterior regions at least according to short-term follow-ups33.

In the current study, the difference in marginal bone level was generally small, and no difference

Fig 8 The frequency of marginal adaptation scores at restorations with all-ceramic (AC) and metal-ceramic (MC) crowns.

Table 5 The mean (mm) mesiodistal distance in the edentulous space (DES) for the different crown morphology scores at AC and MC restorations.

Crown morphology score*

Number of

premolar restorations

DES

Mean (SD)

range

AC MC AC MC

Score 1 12 12 7.09 (0.64)

5.90–7.95

7.15 (0.82)

5.80–8.25

Score 2 19 21 8.19 (1.26)

6.15–10.25

7.82 (1.26)

5.40–10.30

Score 3 7 4 9.56 (2.12)

6.60–12.90

9.59 (1.79)

8.40–12.25

Score 4 0 0 - -

* Crown morphology scores: 1: excellent, 2: almost optimal, 3: moderate, 4: poor crown morphology

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� Professional-reported aesthetic outcome variables

The professional-reported aesthetic evaluation dem-onstrated that the crown morphology scores did not vary significantly (P = 0.233) between the all-ceramic and the metal-ceramic crowns. However, higher scores for crown morphology were signifi-cantly correlated to increasing DES values at the AC (P < 0.001) and at the MC (P = 0.002) restorations (Table 5). The frequency of the crown colour match scores at the AC and MC crowns are illustrated in Figure 9. The all-ceramic crowns had significantly (P = 0.031) lower crown colour match scores than the metal-ceramic crowns.

The mucosal discolouration scores and the papilla index scores, mesially and distally, were not signifi-



in marginal bone loss was registered when implants with AC and MC restorations were compared. After 1 year, all biological complications in the present study were inflammatory reactions, which were mainly observed at all-ceramic crowns. One reason for this observation may be the less optimal marginal fit of the all-ceramic compared to the metal-ceramic crowns. An insufficient marginal fit of the crowns has been suggested to cause chronic inflammation and subsequent breakdown of the surrounding tissue34. In addition, it was suggested that the all-ceramic crowns are more frequently remade because of unacceptable margins35. One of the explanations of less optimal marginal fit of the all-ceramic crowns in the current study may be the fabrication procedures of pre-sintered zirconia copings. The enlarged par-tially sintered zirconia copings are easy to mill, but compared to fully sintered blanks, they are sintered after the milling process to obtain the final strength, which results in shrinkage of the material. It has been

Table 6 The frequency and P values of the mucosal discolouration scores and the papilla index scores (mesially and distally) at the all-ceramic (AC) and the metal-ceramic (MC) restorations evaluated at the baseline and 1-year examinations.

Aesthetic parameters of peri-implant mucosa Baseline 1 year

AC restorations MC restorations AC restorations MC restorations

Mucosal discolouration*

Score 1 71.1% 70.3% 76.3% 70.3%

Score 2 26.3% 29.7% 18.4% 27.0%

Score 3 2.6% 0.0% 5.3% 2.7%

Score 4 0.0% 0.0% 0.0% 0.0%

P = 0.916 P = 0.614

Papilla index, mesially†

Score 1 28.9% 24.3% 44.7% 37.8%

Score 2 57.9% 54.1% 42.1% 40.6%

Score 3 7.9% 13.5% 7.9% 16.2%

Score 4 5.3% 8.1% 5.3% 5.4%

P = 0.620 P = 0.339

Papilla index, distally†

Score 1 13.2% 13.5% 28.9% 24.3%

Score 2 57.9% 35.2% 50.0% 43.3%

Score 3 26.3% 37.8% 21.1% 24.3%

Score 4 2.6% 13.5% 0.0% 8.1%

P = 0.114 P = 0.505

* Mucosal discolouration scores: 1: no discolouration, 2: light greyish discolouration, 3: distinct greyish discolouration, 4: metal (zirconia) visible†Papilla index scores: 1: papilla filling the entire proximal space, 2: papilla filling at least half the entire proximal space, 3: papilla filling less than half the proximal space, 4: no papilla

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Fig 9 The frequency of the crown colour match scores at restorations with all-ceramic (AC) and metal-ceramic (MC) crowns.

indicated that this process is sensitive and results in a possible deformation of the restoration and marginal adaptation36,37. Additionally, the subsequent porce-lain veneering process has also been shown to influ-ence the marginal fit of the zirconia copings38-40.



A limited number of studies have compared the marginal adaptation of all-ceramic and metal-ceramic restorations. In an in vitro study41 as well as in a clinical study42, the marginal gaps at zirconia-based, all-ceramic restorations were greater than at metal-ceramic restorations, which agrees with the results of the present study. In contrast, another la boratory study43 demonstrated less marginal misfit at zirconia-based than at metal-ceramic restorations.

In the current study, the annual frequency of loss of retention as well as the frequency of chip-off frac-tures was 1.33%, which is almost in the same range as the estimated annual complication rate reported in a systematic review6. In this review, the estimated rate of loss of retention and ceramic chipping of ISSCs was 1.13 and 0.92, respectively. Another systematic review7 found no significant difference in the rate of crown loosening or chipping of the veneering ceramic at ISSCs on ceramic abutments compared to those on metal abutments. However, the number of technical complications was only two in the present study.

The aesthetic factors and a high survival rate have in recent studies been suggested as important fac-tors for treatment success, especially in the aesthetic zone44,45. In the present study, the morphology of the AC and the MC crowns was comparable, which agrees with findings of another study26. The majority of the premolar crowns in the present study were placed in edentulous regions with mesiodistal distances greater than the anatomic width of natural premolars46. It should be noted that only patients with premolar agenesis were included in the current study. However, the replacement of two adjacent missing teeth with two implants was not possible in 27% of the premolar regions, where only one implant was inserted. Further-

Table 7 The relationships (P values) between VAS and CIS scores, and between VAS and the five aesthetic parameters at all restorations (AC and MC), at the all-ceramic (AC) and at the metal-ceramic (MC) restorations after 1 year.

CIS* Crown morphology score

Crown colour match score

Mucosal discolouration score

Papilla index score, mesially

Papilla index score, distally

VAS†

AC and MC 0.446 0.005 0.002 0.623 0.030 0.228

AC 0.810 <0.001 0.955 0.852 <0.001 0.188

MC 0.199 0.106 0.005 0.392 0.666 0.758

* CIS: overall professional-reported aesthetic outcome (sum of the crown morphology score, crown colour match score, mucosal discolouration score, and papilla index score [mesially and distally])† VAS: overall patient-reported aesthetic outcome (visual analogue scale)

more, 40% of the premolar ISSCs replaced retained primary second molars, which have a greater mesio-distal width than permanent premolars47.

The use of AC crowns has been suggested to result in a better aesthetic outcome than MC crowns, as all-ceramic crowns have a light transmission similar to nat-ural teeth48. In the present study, a significantly greater optimal colour match of the AC over the MC crowns was registered. In contrast, another study26 found no significant difference in the colour and translucency between AC and MC crowns, and they found a lower translucency of both types of crowns compared to the natural neighbouring teeth. The grey colour of the screw access hole has been indicated as compromising the aesthetic outcome of the crowns, which hardly can be eliminated by composite materials49,50. Thus, the conflicting results may be due to the limited number of crowns included in the other study26.

In order to reduce the greyish discolouration of the marginal peri-implant mucosa, ceramic abutments have been used. Nevertheless, in the present study as well as in another study33, the use of the zirconia abutments compared to the titanium abutments did not result in significantly less mucosal discolouration after 1 year. An animal study13 demonstrated that the thickness of the mucosa has a crucial influence on the mucosal colour, suggesting that in clinical situations with a mucosa thickness of 2 mm or less, the titanium abutments (in contrast to zirconia abutments) may cause a change in colour of the peri-implant mucosa. In a recent clinical study25, it was demonstrated that significantly less discolouration of the peri-implant mucosa was seen at zirconia than at titanium abut-ments, and no effect of mucosa thickness on this aes-thetic parameter was registered.



In the present study as well as in other clinical studies24,51, the height of the papilla was not affected by the use of titanium and zirconia abutments. The papilla height increased at all restorations from crown insertion to 1 year after function, which agrees with the observations reported in other clinical studies52,53.

The aesthetic outcome reported by the patients in the present study indicated that the difference between the AC and the MC crowns was not notice-able by the patients, which is consistent with results reported in another clinical study26. In the present study, a high level of patient-reported aesthetic sat-isfaction with the implant-supported single crowns was recorded. The results of the patient- and the professional-reported aesthetic outcomes were not significantly related in the present study, which agrees with other clinical studies8,45,54-56. Conflict-ing results concerning the most important aesthetic factors evaluated by the patients have been pub-lished54,56-58. However, in the current study, the professional-reported evaluation of crown colour match followed by the crown morphology and the papilla height (mesially) were significantly corre-lated with the patient-reported evaluations of the overall aesthetic outcome of restorations. A pos-sible reason for this observation may be due to the treatment regions in the present study and that the mesial papilla was more visible than the distal papilla.

The main limitation of this study was the com -bination of a split-mouth with a parallel group design. The split-mouth design was preferred to minimise possible inter-individual variability but it was decided to include also patients with single crowns in order to increase the sample size. Although the statistical analysis incorporated the differences at both restor ation and subject level to account for this combined study design, it would be preferable to conduct either a split-mouth study, in which two different crowns per subject were included, or a par-allel group study including patients with one crown.

The present study included only premolars, which decreased the external validity. Ideally, differ-ent tooth types should have been included. Another limitation was the short follow-up time; longer observation periods are essential for analysing the influence of restoration materials on peri-implant tis-sue reactions.

� Conclusions

Within the limits of this clinical study, it was demon-strated that 1 year after oral function:• The changes in marginal bone level at AC as well

as at MC restorations were minimal, and no sig-nificant difference was found between the two groups. However, the frequency of inflammatory reactions was higher at the AC compared to the MC restorations.

• The frequency of technical complications was generally low for both groups, but the marginal adaptation of MC crowns was better than at AC crowns.

• The professional-reported aesthetic outcome demonstrated no significant difference between groups. However, a more favourable colour match at the AC crowns was detected.

• The patient-reported aesthetic outcome was not different at AC and MC restorations.

� Acknowledgments

The authors express special thanks to Astra Tech, Sweden for financial support and delivery of abut-ments and are grateful for the financial support from DSOI (Danish Society for Oral Implantology) and the KOF/Calcin Foundation of The Danish Dental Asso-ciation (Tandlægeforeningen). We also acknowl-edge Associate Professor Lene Theil Skovgaard at the Department of Biostatistics for her help in the statistical analyses.

� References

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