Keywords
angulated screw channel - screw-retained prosthesis - computer aided design- computer
aided manufacturing - implant
Introduction
The implant-supported prosthesis is a successful strategy to restore dental esthetics
and function.[1] The definitive restoration can be attached to the implant by cement-retention or
screw-retention mechanisms.[2] While cementing implant restorations appears a simple procedure, clinical evidence
suggests an association between biological complications and excess cement.[3]
[4]
[5]
[6] Moreover, the difficulties in their maintenance and their retrievability have made
screw-retention mechanism a more preferred choice.[3]
[5]
[7] However, implants are not always placed in favorable positions for screw-retained
restorations. This can be frequently encountered in the anterior maxilla due to centripetal
resorption pattern and concave bone configuration. Further, angulated implant placement
in the posterior regions can be indicated to avoid critical structures like maxillary
sinus or mandibular canal. The treatment options to correct implant angulation may
involve the use of an angled abutment with a cement-retained restoration, or surgical
bone augmentation to allow for more ideal implant placement.[8] Alternatively, angle correction via intermediate abutments or restorations retained
by a lateral screw could be considered. However, these systems have been shown to
increase the treatment complexity, maintenance burden, and incur additional costs.[8]
[9]
Almost two decades ago, angled screw channel (ASC) abutment design (Dynamic Abutment;
Talladium International Implantology) was introduced to allow restoring angulated
implants with simple direct to fixture screw-retained restoration. The ASC design
uses a screw with a hexalobular head shape that can be engaged with a hexagonal faceted
sphere screwdriver at various angles between 0 and 28 degrees with 360-degree freedom
of rotation.[10] This allowed tightening the abutment screw at an orientation different from the
center axis of the implant. Earlier, ASC abutments were casted on a hemisphere base
via a burnout sleeve that could be rotated freely to direct the screw access channel
away from the area of concern.[10] Additionally, this concept was limited to certain implant systems. Recently, advancements
in implant software programs and manufacturing systems have made it possible to design
and fabricate ASC restorations digitally. Moreover, prefabricated titanium bases incorporating
the ASC are available from some implant manufacturers. The versatility of ASC has
been confirmed by a cone bean computed tomography (CBCT) analysis study that showed
screw-retained restorations are achievable with the use of ASC abutments in 76% of
cases in the anterior maxilla.[11] Despite the increased popularity, the efficiency and survival of the ASC systems
remain unclear. The purpose of the present systematic review was to investigate laboratory
and clinical outcomes of ASC restorations through the available literature to determine
their survival and the influencing factors.
Methods
This systematic review adapted the Preferred Reporting Items for Systematic Reviews
and Meta-Analysis (PRISMA) statement. The aim was to answer the following focused
question “what are the outcomes of implants restored with angled screw channel prostheses?”
The main search strategy was developed for PubMed ([Table 1]) and supplemented with additional search in Science Direct, Scopus, and Cochrane
Library. The search was conducted in March 2021 and updated in May 2021. The search
aimed to identify all the available clinical and laboratory studies on ASC. The inclusion
criteria were peer-review publication, prospective or retrospective clinical study
with at least 1-year observation period after implant restoration, clinical study
that evaluated biological and/or mechanical outcomes, clinical study that clearly
listed outcome related to ASC restorations, and laboratory study that evaluated ASC
performance variables with clinical relevance. The studies were excluded if they were
not in English language, the ASC-related data could not be determined, or the restorative
stage details were not clearly stated. Duplicated articles from different searches
were discarded by a reference manager software program (Endnote X9, Clarivate Analytics,
Philadelphia, Pennsylvania, United States). Following this, the titles and abstracts
were screened. The articles of interest were selected for full-text analysis and matching
against the inclusion criteria. Further, the reference lists of the included studies
were manually searched. Quality of the selected clinical studies was scored with Newcastle–Ottawa
scale for nonrandomized studies ([Table 2]) which were designed to assess the quality of cohort studies based on selections
of exposed and nonexposed cohorts, comparability influenced by the controls of risk
factors, and completeness of outcomes.[12] The risk of bias of the included laboratory studies were assessed by using an adaptation
of the methods applied in two previous systematic reviews.[13]
[14] Descriptions of the following parameters were used to assess each article's risk
of bias ([Table 3]): sample size calculation, presence of a control group, type of component used (genuine/nongenuine),
statistical analysis performed, reliable analytical methods or statistical indicators,
blinding of the evaluation assessors, and utilizing clinically relevant restoration
material. A “yes” was assigned where the parameter was reported in the text and a
“no” if the information was absent. The risk of bias was classified according to the
sum of “yes” received as follows: 1 to 3 = high, 4 to 5 = medium, and 6 to 7 = low
risk of bias.
Table 1
Search strategy
|
Search strategy
|
Query
|
|
Population: screw-retained implant supported restorations
|
(((Screw-retained[All Fields] AND implant[All Fields])) OR (“prostheses and implants”[MeSH
Terms] OR (“prostheses”[All Fields] AND “implants”[All Fields]) OR “prostheses and
implants”[All Fields])) OR (screw-retained[All Fields] AND restoration[All Fields])
|
|
Intervention: ASC restorations
|
((((abutment screw) AND (off[All Fields] AND “axis”[All Fields]))) OR ((“single”[All
Fields] AND implant[All Fields] AND restoration[All Fields]))) AND ((((((angled[All
Fields] AND “screws”[All Fields] AND channel[All Fields])) OR (angulated[All Fields]
AND screws”[All Fields])) OR (non-axial[All Fields] AND “screws”[All Fields] AND channel[All
Fields])) OR (Abutment[All Fields] AND “screws”[All Fields] AND channel[All Fields]))
OR (two[All Fields] AND piece[All Fields] AND abutment[All Fields]))
|
|
Outcome:
|
(((((reverse[All Fields] AND (“torque”[MeSH Terms] OR “torque”[All Fields]))) OR (technical[All
Fields] AND (“complications”[Subheading] OR “complications”[All Fields]))) OR (mechanical[All
Fields] AND complication[All Fields])) OR (fractures[All Fields] AND resistance[All
Fields])) OR “survival rate”[MeSH Terms]
|
Table 2
Quality assessment of selected clinical studies using Newcastle–Ottawa scale for cohort
studies
|
Quality assessment criteria
|
Acceptable
|
Greer et al[23]
|
Anitua et al[25]
|
Tallarico et al[27]
|
Friberg and Ahmadzai[24]
|
Pol et al[22]
|
Anitua et al[26]
|
Shi et al[28]
|
Nastri et al[29]
|
|
Representativeness of exposed cohort?
|
Representative of average adult in community
|
1
|
|
|
1
|
|
|
|
|
|
Selection of nonexposed cohort
|
Drawn from same community as exposed cohort
|
|
|
|
|
|
|
|
|
|
Ascertainment of exposure
|
Secured records
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
|
Demonstration that outcome of interest not present at start of the study
|
Yes
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
|
Study controls for the degree of screw access angulation
|
Yes
|
|
1
|
|
|
|
1
|
1
|
|
|
Study controls for additional risk factor?
|
restorative material
|
1
|
1
|
1
|
|
1
|
1
|
1
|
|
|
Assessment of outcome
|
Secure records
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
|
Follow-up long enough
|
Follow-up (>1 year)
|
|
|
1
|
|
|
1
|
|
1
|
|
Adequacy of follow-up
|
Small number of subject loss
|
1
|
1
|
1
|
|
1
|
1
|
1
|
1
|
|
Overall quality score (maximum = 9) >7: good/5–7: fair/< 5 poor
|
6
|
6
|
6
|
4
|
5
|
7
|
6
|
5
|
Table 3
Quality assessment and risk of bias of laboratory studies considering aspects (reported
in “Materials and Methods” section)
|
Author (year)
|
Sample size calculation
|
Control group
|
Genuine component used
|
statistical analysis performed
|
reliable analytical methods or statistical indicators
|
blinding of the evaluation assessors
|
Utilization of clinically relevant restoration material
|
Risk of bias
|
|
Goldberg et al (2019)[21]
|
No
|
Yes
|
No
|
Yes
|
Yes
|
No
|
No
|
High
|
|
Hu et al (2019)[20]
|
Yes
|
Yes
|
Yes
|
Yes
|
Yes
|
No
|
NA•
|
Medium
|
|
Opler et al (2019)[18]
|
No
|
Yes
|
No
|
Yes
|
Yes
|
No
|
NA
|
High
|
|
Swamidass et al (2021)[17]
|
No
|
Yes
|
Yes
|
Yes
|
Yes
|
No
|
Yes
|
Medium
|
|
Drew et al (2020)[16]
|
No
|
Yes
|
Yes
|
Yes
|
Yes
|
No
|
Yes
|
Medium
|
|
Garcia-Hammaker et al (2021)[15]
|
Yes
|
Yes
|
Yes
|
Yes
|
Yes
|
No
|
Yes
|
Low
|
|
Mulla et al (2021)[19]
|
No
|
Yes
|
Yes
|
Yes
|
Yes
|
No
|
Yes
|
Medium
|
Abbreviation: NA, not applicable.
Results
The initial electronic search yielded 358 publications. After elimination of duplicates,
326 remained for title and abstract review. Twenty-seven articles were selected for
full-text analysis, 13 of which were laboratory and clinical studies that fulfilled
the inclusion criteria. Fourteen studies were excluded as listed in [Table 4] along with the reasons of exclusion. The supplementary manual search through the
bibliography of the included studies yielded two additional articles. Therefore, 15
publications were eligible for the current review ([Fig. 1]). The included studies were published between 2017 and 2021, with the majority being
published in the past 2 years. Out of seven laboratory investigations, two studies
evaluated the fracture strength of the zirconia crowns with ASC ([Table 5]),[15]
[16] four studies evaluated the reverse torque value of nonaxially tightened screws ([Table 6]),[17]
[18]
[19]
[20] and one examined both variables.[21] Only four studies reported the exact direction of force in their investigations
that was 30 degrees for all of them.[15]
[17]
[19]
[21] A total of eight nonrandomized cohort studies (four prospective and four retrospective)
reported the outcomes of 281 implants restored with ASC restorations in 254 patients
([Table 7]).[22]
[23]
[24]
[25]
[26]
[27]
[28] Of these, two investigations focused on technical complications[23]
[25] and the rest reported the technical and biological outcomes of ASC restorations.[22]
[24]
[26]
[27]
[28] Six studies included only single-crown restorations[22]
[23]
[24]
[27]
[28] and two studies included partial or full arch restorations.[25]
[26] All but two studies reported cumulative survival rates after at least 1 year of
loading.
Table 4
Reasons for exclusion of discarded studies after full-text analysis
|
Reasons for exclusion
|
Did not evaluated ASC restorations
|
Not indicative of variables with clinical relevance
|
No data about prosthetic stage of treatment
|
|
Excluded studies
|
Anitua et al[40]
Chen and Pan[41]
Guljé et al[42]
Hotinski and Dudley[43]
Lin et al[30]
Menéndez-Collar et al[44]
Mokhtarpour et al[45]
Paolantoni et al[46]
Vélez et al[47]
|
Edmondson et al[11]
González-Martín and Veltri [48]
Farré-Berga et al[49]
Farronato et al[50]
|
Wang et al[51]
|
Abbreviation: ASC, angled screw channel.
Table 5
Detailed data of included laboratory studies (fracture/fatigue studies)
|
Author (year)
|
Sample size
|
Implant system
|
Abutment system
|
Restoration material and design
|
Evaluated screw channel angulation (degree)
|
Tightening torque value
|
Aging
(cyclic loads and thermocycles)
|
Fracture test
|
Results
|
Mode of failure
|
|
Goldberg et al (2019)[21]
|
n = 7
|
External hexagon implants, Osseotite; Zimmer Biomet (Fixture)
|
1. Dynamic Abutment (DA); (Dynamic Abutment Solutions.)
2. UNISG Abutment with gold square screw (GS); (Zimmer Biomet.)
|
Full contour casted Ni–Cr crown
Maxillary central incisor
|
GS: 0
DA: 0
DA: 20
DA: 28
|
GS: 0, 35 Ncm
DA: 0,20, 28, 25 Ncm
|
Cyclic load: in dual axis mastication simulator under axial load of 40 N for 1200,000
cycles. (all specimens were retightened at 9,205 cycles)
|
Fracture strength was tested by a universal testing machine under compressive load
at 30-degree angle until failure
|
Fracture strength (N):
GS-0 degrees: 989.01
DA-0 degrees: 869.59
DA-20 degrees: 715.88
DA-28 degrees: 789.84
|
Screw fracture:
GS-0 degrees: 2, DA-0 degrees:2, DA-20 degrees: 1, DA-28 degrees:0.
All crowns remained intact but implant platform was severely deformed or fractured
|
|
Drew et al (2020)[16]
|
n = 5
|
4.3 mm × 10 mm, NobelActive, Nobel Biocare (Fixure)
|
1. Nobel Procera, (Nobel Biocare) with a titanium adapter.
|
CAD/CAM monolithic zirconia crown
Maxillary central incisor
|
0
25
|
35 Ncm
|
|
Off-axis compressive sinusoidal fatigue load cycles of 10 to 200 N at 15 Hz with a
maximum number of 334,800 (250,000 cycles is equivalent to 1 year of clinical service)
|
Mean number of cyclic loads for incisal-cervical fractures/catastrophic failure:
ASC abutments: 16,650/83,385
SSC abutments: 135,270/212,940
|
All ceramic fracture occurred in the cingulum area in an incisal-cervical direction.
The crack occurred from the apical part of the screw access opening to the level of
titanium adapter
|
|
Garcia-Hammaker et al (2020)[15]
|
n = 10
|
Conical connection regular platform, Nobel Biocare (Analog)
|
Nobel Procera, (Nobel Biocare) with a titanium adapter
|
CAD/CAM monolithic zirconia crown
Maxillary central incisor
|
0
25
|
35 Ncm
|
Not conducted
|
Perpendicular compressive static force on samples mounted with 30-degree angulation
(2-mm below the incisal edge) until failure, using a universal testing machine
|
Mean value of the load to failure:
0 degrees: 534.05
25 degrees: 215.49
Maximum load:
0 degrees: 762.70
25 degrees: 420.51
|
25-degree zirconia abutments: fracture at the most apical portion of the zirconia
piece with some minor damage to the screw head. Eight specimen showed loss of screw
channel continuity
|
Table 6
Detailed data of included laboratory studies (reverse torque studies)
|
Author (year)
|
Studied samples and sample size
|
Implant system
|
Abutment system
|
Restoration material and design
|
Evaluated screw channel angulation (degree)
|
Tightening torque value
|
Aging
(cyclic loads and thermocycles)
|
Results
|
|
Goldberg et al (2019)[21]
|
n = 7
|
External hexagon implants, Osseotite; Zimmer Biomet (Fixure)
|
1.Dynamic Abutment (DA); (Dynamic Abutment Solutions)
2. UNISG Abutment with gold square screw (GS); (Zimmer Biomet.)
|
Full contour casted Ni–Cr crown
Maxillary central incisor
|
GS-0
DA-0
DA-20
DA-28
|
GS-0: 35 Ncm
DA-0,20,28 degrees: 25 Ncm
|
Cyclic load: in dual axis mastication simulator under axial load of 40 N for 1,200,000
cycles (all specimens were retightened at 9,205 cycles)
|
Mean difference between baseline and removal torque (ΔRT) values:
GS-0 degrees: −1.04
DA-0 degrees: 1.09
DA-20 degrees: −0.51
DA-28 degrees: −2.57
|
|
Hu et al (2019)[20]
|
n = 16
|
regular platform; iMilling, Chantilly, VA (Analog)
|
4.3 mm regular platform S-Link abutment; iMilling, Chantilly, VA
|
No restoration fabricated
|
0
10
20
|
35 Ncm
|
Not conducted
|
Mean reverse torque values:
0 degrees: 31.16
10 degrees: 32.07
20 degrees: 30.09
|
|
Opler et al (2019)[18]
|
n = 10
|
NobelActive, Regular platform; Noble Biocare
|
Dynamic Abutment, Talladium International Implantology
|
No restoration fabricated
|
0
10
15
25
28
|
25 Ncm
|
Not conducted
|
Mean input torque/ mean reverse torque value (Ncm)
0 degrees: 25/22.18 28 degrees: 25/17.44
Mean input torque/mean output torque (the ultimate delivered torque) value range for
all study groups (Ncm): 24.97 to 25.01/24.36 to 25.81
|
|
Swamidass et al (2020)[17]
|
n = 10
|
Nobel Biocare Replace Conical Connectio 4.3 mm ×11.5 mm; Nobel Biocare (Fixure)
|
1.Nobel Biocare Gold-Adapt (GA)
2. Nobel Biocare SSC (NB)
3. Nobel Biocare ASC (NB)
4.Dynamic Abutment Solution (DA)
5.Core3dcentre angle correction (C3D)
|
CAD/CAM monolithic zirconia crown
Maxillary central incisor
|
NB-0
GA-0
NB-20
DA-20
C3D-20
|
NB-20 degrees, NB-0 degrees, GA-0 degrees: 35 Ncm
DA-20 degrees: 25 Ncm
C3D-20 degrees: 20 Ncm
|
Thermocycle: 5°C to 55°C for 5,000 cycles.
Cyclic load: 0 N to 100 N, 30 degrees to the implant long axis at 10 Hz for 1 million
cycles (equivalent to 1 year of masticatory cycle)
|
Median of torque removal values: precyclic loading/postcyclic loading, median percentage
torque loss (Ncm)
NB-0 degrees: −22.9/ – 20.6, 10.9%
GA-0 degrees: −17.7/− 11.0, 35.9%
NB-20 degrees: −23.9/− 19.4, 20.8%
DA-20 degrees: −14.1/− 8.95, 31.9%
C3D-20 degrees: −12.8/− 7.75, 34.5%
|
|
Mulla et al (2021)[19]
|
n = 7
|
Nobel Replace Conical Connection 4.3 mm ×10 mm; Nobel Biocare (Analog)
|
1.Dynamic Abutment Solutions (DA)
2. AngleBase; Dess Dental Smart Solutions (DE)
3. Angled Screw Channel Solutions; Nobel Biocare (ASC)
4. Universal Base; Nobel Biocare (UB)
|
CAD/CAM monolithic zirconia crown
Maxillary central incisor
|
UB-0
DA-25
DE-25
ASC-25
|
DA: 25 Ncm
DE, ASC, UB: 35 Ncm
|
Cyclic load: in masticatory simulation machine at 30° under a 200 N load at 2 Hz for
5 million cycles, (equivalent to 5 years of functional loading)
The mastication simulator was stopped every 500,000 cycles to retighten any loose
crowns, with each stop simulating a 6-month recall appointment (no specimen found
loose. Thus no retightening was performed)
|
Mean of input torque deviation value (ITV) from the target value (ITV1/ITV2; Ncm):
UB: 0/− 0.2
DA: −0.7/− 0.2
DE: −3.9/− 2
ASC: −4.1/− 2.1
Mean of RTV after 24 hours/after cyclic loading (Ncm):
UB: 27/16
DA: 17/11
DE: 27/15
ASC: 26/18
Preload efficiency:
UB: 48.5%
DA: 43.8%
DE: 46.8%
ASC: 54.2%
|
Table 7
Detailed data of included clinical studies
|
Author (year)
|
Study type
|
Observation period
|
Number of patients
|
Number of restorations (unit)
|
Restoration material
|
Location in the arch
|
Implant system
|
Time and technique of implant placement
|
Angulation of screw access channel
|
Results
|
|
Greer et al (2017)[23]
|
Retrospective
|
Mean: 216.3 days (range: 14–784 days)
|
60
|
84
Single crown
|
Nobel Biocare Procera ASC 90%
Procera full contour zirconia 10%
|
Maxilla 90%/mandible 10%
Incisor (>70%)/premolar (>20%)/canine (>5%)/molar (<5%)
|
Nobel active internal connection (64%)
Nobel replace conical connection (36%)
|
Not reported
|
Not reported
|
96% with no complications.
4% with mechanical complications reported (a loose screw, a ceramic fracture, an implant
failure)
|
|
Anitua et al (2018)[25]
|
Retrospective split mouth
|
1 year
|
52
|
0 degrees: 55
Median number of units: 3 (2–12)
ASC: 55
Median number of units: 3 (2–13)
|
CAD/CAM Cr–Co framework veneered with porcelain
|
0 degrees: maxilla: 21/mandible: 34
ASC: maxilla: 26/mandible:29
Premolar and molar regions
|
Dental implants, BTI Biotechnology Institute
|
Not reported
|
0 degrees (n = 55)
15 degrees (n = 46)
30 degrees (n = 9)
|
Seven complication in experiment group (five porcelain fracture, one screw loosening,
and one screw fracture)
Four complications in control group (three porcelain fracture and one screw fracture)
|
|
Tallarico et al (2018)[27]
|
Prospective case series
|
Up to 3 year (mean duration: 38.2 months)
|
10
|
23
Single crown
|
CAD/CAM zirconia framework veneered with feldspathic porcelain
|
Posterior maxilla
|
NobelReplace Conical Connection; Nobel Biocare AG
|
Delayed placement. flapless procedure and immediately loaded
|
Not reported
|
Prosthesis and implant cumulative survival rate: 100%
Mean marginal bone level:
Placement and loading: 0.29 ± 0.34 mm/1-year follow-up: 0.37 ± 0.32 mm/2-year follow-up:
0.38 ± 0.33 mm/3-year follow-up: 0.50 ± 0.42 mm
|
|
Friberg and Ahmadzai (2019)[24]
|
Prospective
|
Up to 1 year
|
47
|
51 (ASC: 42)/22 implants were followed
Single crown
|
Not reported
|
Maxillary incisors and canine
|
NobelParallel CC, Nobel Biocare
Narrow platform: 27
Regular platform:24
|
4 immediate placement and 47 late placement, all with two-stage procedure. 14 implant
sites were bone augmented
|
Not reported
|
Implant cumulative survival rate: 98.0%
Mean marginal bone loss: 0.41 ± 0.36 mm
No complication reported in relation to the ASC restorations.
|
|
Pol et al (2020)[22]
|
Prospective case series
|
1 year
|
30
|
30
Single crown
|
CAD/CAM full contour zirconia crowns with metal adapter
|
Maxilla: 12 Mandible: 18
Molars
|
NobelParallel CC, Nobel Biocare
Regular platform
|
Delayed placement. Two-stage procedure
|
Not reported
|
Prosthesis success rate: 100%
Implant survival rate: 100%
Mean marginal bone loss: 0.16 ± 0.26
|
|
Anitua et al (2020)[26]
|
Retrospective controlled split mouth
|
Mean duration: 45.5 ± 15.02 months
|
22
|
0 degrees: 34
Partial: 88.2% Complete: 11.8%
ASC: 34
Partial: 94.1% Complete: 5.9%
|
CAD/CAM Cr–Co framework layered with porcelain
|
0 degrees: maxilla: 55.9%/mandible: 44.1%
ASC: maxilla: 47.1%/mandible: 52.9%
Anterior and posterior regions of the arch
|
Dental implants, BTI Biotechnology Institute
|
Delayed placement. Surgical technique: not reported
|
0 up to 30 degrees
|
Survival rate: 100%
Mean marginal bone loss: −0.29 ± 0.51 mm
|
|
Shi et al (2020)[28]
|
Prospective
|
1 year
|
44
|
CR: 20
ASC: 24
Single crown
|
CR: zirconia base restoration veneered with ceramic on a custom-made zirconia abutment.
ASC: NobelProcera zirconia framework veneered with ceramic
|
Anterior maxilla
|
NobelActive internal connection and NobelReplace conical connection, Nobel Biocare
|
Immediate flapless placement and delayed loading
|
Mean angulation of screw channel: 13.7 degrees (0–25 degrees)
|
Implant survival rate: 100%
Mean marginal bone loss:
ASC: 0.31 ± 0.30 mm
CR: 0.41 ± 0.38 mm
Mechanical complication rate:
ASC: 13.0% (2 screw loosening, 1 ceramic fracture (fracture between zirconia coping
and veneer material)
CR: 5.0% (1 loss of retention)
|
|
Nastri et al (2021)[29]
|
Retrospective
|
Minimum 2 years
ASC: 36.4 ± 10.3 months
CR: 52.3 ± 5.7 months
|
20
|
CR with custom abutment: 10
ASC: 10
Single crown
|
Not reported
|
Central incisor: 4
Maxillary lateral incisor: 8
Maxillary canine: 4
Maxillary first premolar: 1
Mandibular lateral incisor: 3
|
Nobel Biocare
|
Not reported
|
Not reported
|
No mechanical complication reported
Total white esthetic score/pink esthetic score: baseline: ASC: 16.6 CR: 17.3; follow-up:
ASC: 16.2; CR: 17.1
Mean MBL: ASC: −0.22 ± 0.19 mm/CR: −0.29 ± 0.11 mm
Mean bleeding on probing: Baseline: ASC: 0/10 CR:2/10 Follow-up: ASC: 2/10 CR: 2/10
Mean probing depth: baseline: ASC: 3.7 mm CR: 3.3; follow-up: ASC: 4 CR: 3.7
|
Abbreviation: ASC, angled screw channel; CC, conical connection; CR, cement retained.
Fig. 1 Flowchart for search process according to PRISMA guideline. PRISMA, preferred reporting
items for systematic reviews and meta-analyses.
Drew et al[16] and Garcia-Hammaker et al[15] examined the fracture strength of the two piece CAD/CAM monolithic zirconia crown
with 25-degree ACS by using cyclic and static loads, respectively. Drew et al[16] showed no statistically significant difference in the mean number of cyclic loads
to failure between ASC and straight screw channel (SSC) crowns. However, the SSC crowns
survived a greater number of cycles prior to failure. Garcia-Hammaker et al[15] reported a significantly higher mean fracture load by 2.4 times and maximum load
before failure by 1.8 times for the SSC crowns. Despite the superior fatigue loading
of SSC, the specimens in both groups resisted the physiologic loads and failure occurred
at loads that resembled a parafunctional situation. Goldberg et al[21] used external connection hexagon implants for the comparison of the fracture strength
of SSC gold screw abutments with 0, 20, and 28 degrees of Dynamic abutments casted
to nickel–chromium (Ni–Cr) crowns, after aging in a mastication simulator. The study
reported screw fracture before implants' mechanical failure in 17.8% of specimens
with only 0.03% of fractures attributed to the ASC restorations. Furthermore, no significant
difference was reported among the fracture strength values of the studied groups.
In the same study, Goldberg et al[21] compared the reverse torque value (RTV) of the nonaxially tightened dynamic abutment
screws (DAS) with 0-degree DAS and SSC gold screw. Although not statistically significant,
the 0-degree DAS demonstrated the highest RTVs, while the 28-degree DAS showed the
lowest values. The study suggested that increased off-axis loading resulted in higher
tensile forces on abutment screws.
Hu et al[20] showed that the screwdriver insertion angulation has a significant impact on the
RTVs of the abutment screws tightened in 0-, 10-, and 20-degree angles. The lowest
mean RTV was in the 20-degree group and was described as the loss of applied torque
due to increased screwdriver angle from the action of force. The highest mean RTV
was shown in 10-degree specimens. This was explained by the possibility of a more
intimate engagement of the screwdriver tip and the abutment screw at 10-degree angulation
for the chosen system. Another study investigated the effect of screwdriver insertion
angulation on the RTVs, input torque values delivered at various angulations (0, 10,
15, 25, and 28 degrees), and the transmitted output torque values in five different
angulations. While the study revealed no significant difference in input torque values
among the groups, the mean RTV between the 0- and 28-degree groups was significantly
different with lower values for the 28-degree group. Regarding the mean output torque
values, which was measured by the strain induced in the screw body, the results revealed
no significant difference between 0- and 15-degree angulations. However, significantly
lower output torque transmission to abutment screws was found when the angulation
increased to 25 and 28 degrees. Furthermore, photography of the specimens revealed
evidence of wear at the screw head predominantly with the 28-degree group, suggesting
slipping of the driver.[18]
Swamidass et al[17] investigated the differences in RTVs of ASC abutments from different manufacturers
before and after simulated aging by temperature and cyclic loading. The RTV was measured
10 minutes after tightening as the initial value and subsequent to aging as the ultimate
value. The percentage of the differences of RTVs was calculated and analyzed. In general,
while the systems with higher initial torque value showed lower percentage torque
loss, the differences between the 0- and 20-degree groups were not significant. On
the other hand, when the abutment screws and implant were from the same manufacturer,
no significant difference between the SSC and ASC groups was identified. In contrast,
differences between the SSC group with genuine abutment screws and the ASC group with
abutment screws from alternate brands were significant. An additional finding was
the wear of the screw head against the zirconia crown in groups with friction fitted
two-piece zirconia abutments (Nobel Biocare). Congruent with this study, Mulla et
al[19] evaluated a higher magnitude of cyclic forces for a longer duration of time on 25-degree
access channel restorations from three different manufacturers. RTVs were measured
24 hours after initial tightening and after simulated 5 years of functional load.
Additionally, their study analyzed the deviation of the input torque value from the
target value recommended by the manufacturer. Two hexalobular systems (Nobel Biocare
and Dess Dental Smart Solutions) delivered significantly lower input torque values
at 25 degrees compared with the SSC group. Nonetheless, their measured RTVs were not
significantly different from the RTVs of SSC group. The 25-degree dynamic Ti-base
system revealed insignificant input value torque deviation compared with 0-degree
group. Unlike the other three study groups, this hexalobular system exhibited a high
amount of torque loss at both times of RTV measurement. The statistical analysis of
this study revealed significant differences in mean RTVs for both 24-hour RTV and
after cyclic loading among all groups. In addition, the RTV means before and after
cyclic loading for each group were significantly different. Furthermore, out of five
catastrophic failures reported with the 25-degree groups, three were related to zirconia
fracture initiated from the area surrounding Ti-base. Other two failures were Ti-base
and screw head fracture in systems with cemented two-piece zirconia abutment. Overall,
no significant difference was found in the survival rate among the groups.
Overall outcomes from the seven clinical studies showed a high survival rate for both
the ASC restorations (88–100%) and the dental implants (98–100%) with low mean marginal
bone loss (MBL; 0.16–0.41 mm). However, the majority of the studies were conducted
for a short duration of time (1 year). Ceramic fracture was the most frequent complication
that was reported in three studies (seven events),[23]
[25]
[28] mostly related to 15-degree metal–ceramic multiunit restorations with equal incidents
in both arches (two in mandible and two in maxilla). Poor occlusal management was
the attributed causes of ceramic fracture in the Greer et al study.[23] Screw loosening was reported in three studies (four events),[23]
[25]
[28] and screw fracture was reported in one study (one event).[25]
Two studies included data for ASC and SSC restorations.[25]
[26] These studies evaluated multiunit metal ceramic fixed prosthesis in the posterior
region (premolar and molar) of both arches, whereas the abutment screws were tightened
either in axial or nonaxial direction. During follow-up, a total of 11 technical events
were reported, 7 in the ASC group and 4 in SSC group ([Table 6]). However, analysis of the results indicated that neither the frequency of technical
complications nor the implants' survival and MBL were affected by the angulation of
the screw channel. Tallarico et al[27] used ASC zirconia abutments for immediate restoration of tilted implants placed
in the posterior region of the atrophic maxilla. Although the studied sample size
was limited, no biological or technical complications were reported in the 3-year
follow-up. The authors suggested that the combination of tilted implants and ASC abutments
might be a safe alternative to maxillary sinus floor augmentation procedures when
patients refuse additional surgical procedures. Friberg and Ahmadzai[24] and Pol et al[22] evaluated the ASC restorations with conical connection implants for replacement
of single missing teeth in incisor/canine and molar region, respectively. None of
the studies indicated any complication after 1 year of function. Shi et al[28] compared the technical and biological outcomes of ASC and cement-retained single
restoration in anterior maxilla after 1 year of loading. Their report indicated that
while the difference in MBL was insignificant among the two study groups, bleeding
on probing percentage was significantly higher in the cement-retained group. In addition,
four events of technical complications were reported. Screw loosening and ceramic
chipping were the complications associated with the ASC cohorts. Contrary to these
findings, Nastri et al[29] reported no mechanical complication and no significant difference in bleeding on
probing among their study cohorts (cement-retained versus ASC single crowns) through
the 2-year follow-up. Additionally, the differences in probing depth, mean MBL, and
white esthetic score between the two study groups were insignificant. However, the
cement-retained restorations had significantly higher pink esthetic scores both in
baseline and follow-up recordings.
Discussion
This systematic review critically appraised the existing evidence from the laboratory
and the clinical studies on the implant-supported ASC restorations. Concerning laboratory
studies, fracture strength and screw resistance to loosening were investigated. According
to fracture strength studies, the ASC restorations appear to fail at less cycles of
loads and lower forces than SSC. However, since they sustained the expected physiological
forces, the ASC restorations were considered mechanically comparable with SSC counterparts.[15]
[16]
[21] Fractographic examinations of anterior two-piece zirconia abutments revealed that
critical cracks were initiated in the cingulum from the most apical part of the screw
access channel in the friction-fitted system.[15]
[16]
[19] Consistent with this finding, some clinical studies have reported early catastrophic
failures with the same pattern in two-piece friction fitted zirconia abutments in
anterior and premolar SSC restorations.[30]
[31] As the screw head seats on the internal surface of the zirconia restoration, hoop
stresses and/or incompatible hardness between the zirconia and titanium components
may be responsible for such pattern of failure.[16]
[17]
[19] This could suggest that the abutment-titanium base interface design might play more
considerable role than the ASC in the long-term performance of the hexalobular systems.
On the other hand, as the angulation of the screw channel is increased, the bulk of
the palatal walls of anterior zirconia abutments is reduced.[15]
[16] This could lead to a weak point in zirconia restorations where the thickness my
reach to less than 0.7 mm.[32] Thus, precaution should be taken when using ASC until more robust evidence is available
on the interaction between the angulation amount and abutment thickness. Since ASC
restorations were mostly required in the anterior maxillary region,[11] the investigated restoration materials by laboratory studies (monolithic zirconia
or full-contour Ni–Cr crowns) may not be fully representative of the clinical application
of ASC in highly demanding esthetic clinical situation. Therefore, further studies
are required to determine the reliability of other restorative materials with ASC.
When resistance to screw loosening is considered, studies showed that the RTV was
influenced by initial torque value,[17]
[19] configuration of the screwdriver,[20] screw design,[17]
[19] abutment system,[17]
[19] and angulation of the screw channel.[17]
[18]
[19]
[20]
[21] As the torque is applied to an abutment screw, it elongates and the threaded surface
elastically deforms. Adequate screw elastic elongation, referred to as preload, could
secure the implant-abutment joint by appropriate clamping forces.[33] A reduction in the input torque could compromise the screw joint by reducing the
applied preload.[34] A consistent outcome of the two studies investigated various ASC systems indicated
that the higher the initial torque value, the less susceptible the screw to loosening.[17]
[19] Although the measurement methods were different, two studies showed that the actual
torque delivered to the screw is reduced when the insertion angle of the screwdriver
exceeds 15 degrees (25 and 28 degrees).[18]
[19] However, RTVs of screws from different systems showed variable behaviors. For example,
while 25-degree Nobel Biocare and 25-degree Dess abutment screws performed similarly,[19] the 28-degree DAS exhibited a significant 23% reduction in RTV when compared with
the 0-degree control group.[18]
Intimate engagement of the screwdriver and screw head showed to play a role in the
amount of the delivered torque. This is further influenced by the screwdriver's sphere
and facet design, as well as, the surface treatment of the screw head.[17]
[18]
[19]
[20] Likewise, difficulty in the engagement of the screwdriver, which was reported with
certain systems, led to stripping and wear of the screw head that might impact the
crown retrievability and the usability of the screws after multiple tightening in
long duration of time.[17]
[18]
The use of nongenuine component can influence the RTV and may jeopardize the joint
stability of the implant-abutment complex.[17] A proper match and integration of the components within an implant system is important,
especially when using implants with conical/internal connection as shown by the included
studies.[35] Further investigations is required to evaluate the RTV with other types of implant
connection.
The majority of the included clinical studies reported no major complications after
1 year of treatment of posterior regions with conical connection Nobel Biocare implants
and ASC restorations.[22]
[25]
[26]
[27] Consistent with these, the anterior ASC restorations revealed favorable clinical
performance as well.[23]
[24]
[29] Technical complications were reported in three studies[23]
[25]
[28] with no significant differences between ASC and SSC or cement-retained restorations.[25]
[26]
[28] A previous clinical study with up to 9 years of follow-up has reported that two-piece
zirconia SSC abutments with bonded titanium insert can be a suitable option for anterior
and premolar region. However, in the molar area, the use of the same abutment without
a complete metal-to-metal connection platform (friction-fitted titanium insert) to
support the restorations have led to a high incidence of fracture.[30]
The data on MBL around the ASC restorations revealed favorable outcomes. Previous
systematic reviews have indicated that changes in the crestal bone level is not significantly
affected by angulated implant placement as compared with axial placement.[36]
[37] However, other factors related to the abutment design may affect the amount of MBL,
such as the abutment height, and contour,[38] and repeated disconnection and reconnection of the abutment.[39] In the present systematic review, this complementary information was mostly missing.
In addition, although the MBL in ASC and SSC were comparable, the number of studies
comparing them was scares.
While ASC restorations show promising short-term results, some clinical questions
are yet to be answered. For example, there is a need to determine their long-term
clinical performance, cost-effectiveness, and the management of their complications.
Thus, the availability of ASC should not be a justification for injudicious angular
implant placement.
Limitations
The present systematic review is limited with the fact that few clinical studies provided
clear comparison between ASC and SSC. In addition, the fair quality of the majority
of clinical studies and high-to-medium risk of bias of the laboratory mandate the
need for stronger future evidences. Another limitation is that the clinical data were
incomplete regarding the amount of angle correction. Moreover, most of the clinical
studies had only 1-year duration of observation which does not reflect the long-term
performance of ASC. Thus, the current evidence for use of ASC is limited and a more
robust clinical guidance on the application of ASC abutments is needed.
Conclusion
Within the limitation of this systematic review, the following inferences can be drawn:
-
According to laboratory studies, the fracture resistance of SSC and ASC restorations
were comparable. The incidence of screw loosening might be lower in ASC systems with
higher insertion torque and genuine components.
-
According to clinical studies, although the ASC restorations demonstrated favorable
performance in anterior and posterior regions of the mouth in short-term, evidence
is insufficient to determine their long-term survival.
