Clinical Efficacy and Long-term Observation of Fiber Post-and-Core Crown versus Onlay Restoration for Posterior Teeth with Post-endodontic Defects

Summary

This retrospective clinical study compares fiber post-and-core crowns and all-ceramic onlays for restoring posterior teeth after root canal treatment and demonstrates that all-ceramic onlays provide improved periodontal health, masticatory function, and oral health-related quality of life.

Abstract

This retrospective study aimed to compare the clinical efficacy and long-term outcomes of fiber post-and-core crown restoration and all-ceramic onlay restoration for posterior teeth with defects following root canal treatment. A total of 100 patients treated between January 2020 and December 2022 were allocated to two groups (n = 50 each): a fiber post-and-core crown group and an all-ceramic onlay group. Restoration integrity, tooth integrity, marginal adaptation, interproximal contacts, periodontal parameters (periodontal pocket depth, gingival index, and plaque index), masticatory function (bite force and chewing efficiency), and oral health-related quality of life (OHIP-14 scores) were evaluated before treatment and at 3 months and 2 years after restoration. No significant differences were observed between the groups in restoration integrity, tooth integrity, marginal adaptation, or interproximal contacts at either follow-up. However, the all-ceramic onlay group demonstrated significantly lower periodontal pocket depth, gingival index, and plaque index at both follow-up time points. Bite force and chewing efficiency were significantly greater in the onlay group, and OHIP-14 scores indicated better oral health-related quality of life. At the 2-year follow-up, two restoration failures occurred in the fiber post-and-core crown group, whereas no failures were observed in the all-ceramic onlay group. These findings suggest that all-ceramic onlay restorations provide superior clinical outcomes for posterior teeth after root canal treatment, particularly with respect to periodontal health, masticatory function, and patient-reported quality of life, and may represent a promising alternative to conventional fiber post-and-core crown restorations.

Introduction

Root canal treatment is a common procedure for managing pulpitis and apical periodontitis. Its primary objectives are to eliminate inflammation, prevent the spread of infection, and preserve the function and appearance of the affected tooth by removing infected pulp tissue, disinfecting the root canal system, and sealing it with an appropriate filling material¹. Advances in endodontic techniques have improved treatment outcomes, enabling teeth with pulpitis, apical periodontitis, and extensive structural defects to be preserved for extended periods following successful root canal treatment². Posterior teeth, including premolars and molars, play a critical role in mastication by grinding food into smaller particles, thereby facilitating digestion and absorption³. However, after root canal treatment, these teeth exhibit reduced structural strength due to extensive removal of carious and infected dentin, as well as the loss of pulpal vitality, thereby increasing their susceptibility to fracture⁴. Consequently, restoration after root canal treatment should be planned with consideration of the remaining tooth structure, the magnitude and direction of occlusal forces, and the most appropriate restorative approach to optimize clinical outcomes.

Successful restoration of tooth defects depends on adherence to both biological and mechanical principles; failure to do so may compromise restoration longevity and damage the supporting tooth structure and surrounding tissues⁵. Selection of an appropriate restorative method requires comprehensive evaluation of multiple factors, including restoration of masticatory function, long-term durability, periodontal and gingival health, esthetic outcome, mechanical stability, and individual patient needs⁶. Because the remaining tooth structure is often limited after root canal treatment, post-and-core restorations have traditionally been used to improve retention and resistance, thereby providing support for subsequent full-coverage crowns⁷. Fiber post-and-core crowns have gained widespread clinical acceptance because of their favorable biocompatibility and mechanical properties, including an elastic modulus similar to that of dentin, which enhances biomechanical compatibility with the restored tooth⁸. Nevertheless, although post-and-core restorations improve crown retention, they require additional removal of radicular dentin during post-space preparation. This secondary invasive procedure increases the risk of root perforation and fracture and conflicts with the principle that preservation of tooth structure contributes to greater fracture resistance⁹.

To minimize tooth preparation and maximize preservation of the remaining tooth structure, minimally invasive restorative concepts and stepwise treatment strategies have received increasing attention in recent years. Clinicians are increasingly selecting restorative approaches based on the condition of the remaining tooth structure while evaluating the clinical performance of different techniques. Advances in dental materials and adhesive technologies have expanded the use of indirect adhesive restorations consistent with minimally invasive principles. An inlay is an indirect restoration fabricated outside the oral cavity and adhesively bonded into a prepared tooth to restore its morphology and function¹⁰. Similar to a full crown, the restoration is fabricated after tooth preparation and subsequently cemented using adhesive techniques¹¹. When sufficient remaining tooth structure provides adequate fracture resistance, compressive strength, and retention form, inlay restoration may be considered. However, conventional inlays restore only the defect itself and provide limited protection for the remaining tooth structure¹². An onlay is a modified form of indirect restoration in which part of the restoration is bonded within the tooth while the remaining portion covers one or more cusps. This design is particularly suitable for posterior teeth with cusp involvement because it redistributes oblique occlusal forces toward more favorable vertical loading, thereby reducing mechanical stress on the restoration¹³. Compared with conventional full-coverage restorations, onlays require less tooth preparation, preserve cervical tooth structure, reduce stress concentration in the cervical region, and lower the risk of cervical fracture. They are therefore appropriate for restoring large posterior defects, fractured cusps, cracked teeth, and teeth requiring occlusal reconstruction while simultaneously restoring anatomical form, occlusion, and proximal contact relationships¹⁴.

In current clinical practice, post-and-core crowns and onlays are the two most frequently used options for restoring posterior teeth with extensive structural defects. However, clear clinical criteria for selecting between these restorative approaches remain lacking, and treatment decisions often rely on clinician experience, introducing substantial subjectivity. To date, few clinical studies have directly compared all-ceramic onlays with fiber post-and-core crowns for restoring posterior teeth after root canal treatment, and most of the available evidence derives from laboratory investigations. In recent years, the biomimetic concept has received increasing attention because it emphasizes maximal preservation of tooth structure and avoidance of invasive procedures such as post-space preparation. Consequently, postless restorative strategies based on adhesive bonding and the ferrule effect have emerged as promising alternatives to traditional intraradicular retention methods. However, comparative clinical evidence evaluating these approaches remains limited. Therefore, it was hypothesized that all-ceramic onlay restoration would provide superior clinical outcomes compared with fiber post-and-core crown restoration by improving periodontal health, enhancing masticatory function, and achieving better oral health-related quality of life at 3 months and 2 years after restoration. To test this hypothesis, this study compared the clinical efficacy of all-ceramic onlays and fiber post-and-core crowns for restoring posterior teeth with defects after root canal treatment by evaluating restoration outcomes, recovery of masticatory function, periodontal health, and oral health-related quality of life. The findings are intended to provide clinical evidence to support restorative decision-making for posterior teeth following root canal treatment.

Protocol

This retrospective study was conducted in accordance with the Declaration of Helsinki. The study protocol was approved by the Ethics Committee of WuYi County Stomatological Hospital (Approval No. 2025-1) on February 25, 2025. Patients treated between January 2020 and December 2022 were enrolled. Written informed consent was obtained from all participants for study participation and publication of de-identified clinical images. The research tools used in this experiment are listed in the Table of Materials. The study workflow is illustrated in Figure 1.

Figure 1: Study workflow. Flowchart illustrating patient enrollment, group allocation, restorative procedures, and follow-up evaluations performed at 3 months and 2 years after restoration. Please click here to view a larger version of this figure.

1. Study design

This retrospective study enrolled patients treated between January 2020 and December 2022 who met the predefined eligibility criteria.

1. Inclusion criteria
   Patients were eligible for inclusion if they met all of the following criteria: posterior tooth defects confirmed by dental radiographs, with root canal treatment completed more than 1 week previously; radiographs demonstrating adequate root canal filling, no significant periapical radiolucency, no alveolar bone resorption, and healthy periodontal tissues; absence of percussion pain or tooth mobility; sufficient remaining tooth structure to support either all-ceramic onlay or fiber post-and-core crown restoration; normal posterior occlusion without parafunctional habits such as bruxism or clenching; no acute or severe systemic disease that would contraindicate treatment; and normal cognitive function with the ability to cooperate throughout treatment.
2. Exclusion criteria
   Patients were excluded if they presented with uncontrolled residual pulpitis, apical periodontitis, temporomandibular joint disorder syndrome, poor oral hygiene, active periodontitis, absence of an opposing tooth, presence of a removable prosthesis as the opposing dentition, abnormal occlusal habits, pregnancy or lactation, anatomical abnormalities such as malformed teeth, or temporomandibular joint disease.

2. Fiber post-and-core crown restoration treatment

The fiber post-and-core crown restoration procedure was performed according to a standardized protocol. Tooth shade was determined within 5 s under natural light using a shade guide to minimize visual fatigue. Based on the working length established during root canal treatment and the root diameter observed on dental radiographs, the post space was sequentially prepared with P-drills and then shaped with a fiber post-shaping drill. The post diameter was maintained at approximately one-quarter to one-third of the root diameter. The post length was planned to be at least equal to the clinical crown length, with the intrabony portion extending beyond one-half of the root length within the alveolar bone while preserving a minimum 5 mm apical seal. The root canal walls were confirmed to be smooth before post placement.

A glass fiber post of appropriate length and diameter was selected and tried in to confirm an accurate fit. The post was immersed in 75% alcohol, shaken, and air-dried. The root canal was irrigated with saline, ultrasonically cleaned, disinfected with 75% alcohol, and dried. A self-etch adhesive was applied to both the root canal walls and the post surface, then light-cured for 15 s. Dual-cure resin cement was injected from the base of the post space, the post was inserted, and the assembly was light-cured for 40 s. A resin core was subsequently built up using core resin and light-cured.

Tooth preparation was performed according to the requirements for an all-ceramic crown, including a uniform 1.0 mm occlusal reduction, elimination of undercuts on all surfaces, and a chamfer finish line at the cervical margin. Impressions were obtained using a two-step addition silicone technique. Following gingival displacement with retraction cords, a heavy-body impression was made, followed by a light-body wash impression using the double-cord technique. A temporary crown was fabricated and cemented to protect the prepared tooth and maintain the prepared space until definitive restoration.

The tooth shade was reconfirmed, and a super-hard plaster model was prepared. The occlusal relationship was recorded using bite registration silicone and submitted to the dental laboratory for fabrication of a zirconia full crown. During the clinical try-in, proximal contacts and marginal adaptation were evaluated using articulating paper (40 µm and 200 µm) and adjusted as necessary.

For final bonding, the intaglio surface of the crown and the prepared tooth were cleaned and dried. Dual-cure resin cement was mixed and applied to the internal surface of the restoration. The crown was fully seated, and excess cement was tack-cured for 1–2 s before removal. Final light curing was then performed from multiple directions for 20 s per surface. Iodine glycerin was applied to the gingival sulcus, and patients were instructed to avoid chewing hard foods on the treated side for 24 h (Figure 2).

Figure 2: Representative clinical images of posterior tooth restoration with a fiber post-and-core crown. (A) Clinical appearance of the tooth 1 week after root canal treatment, before restoration. (B) Placement of the fiber post within the prepared root canal. (C) Bonding and core build-up following fiber post placement. (D) Immediate postoperative appearance after placement of the definitive crown. Please click here to view a larger version of this figure.

3. All-ceramic onlay restoration

The all-ceramic onlay restoration procedure was performed according to a standardized protocol. Tooth shade was determined within 5 s under natural light using the same shade guide described for the fiber post-and-core crown group. Discolored, softened, and structurally weakened cusp tissue was removed. A P-drill was used to remove 1–2 mm of gutta-percha from the root canal orifice, after which the canal was irrigated with saline, dried, disinfected with 75% alcohol, and treated with a self-etch adhesive. The canal orifice was sealed with flowable resin and light-cured for 40 s. The cavity was prepared with a flat pulpal floor, straight axial walls, and no undercuts. Occlusal reduction consisted of 1.5 mm on functional cusps and 1.0 mm on non-functional cusps while maintaining a remaining tooth thickness greater than 3 mm. The cavity walls were flared at approximately 6°, with rounded internal line angles, a convergence angle of 2°–6°, and a 90° shoulder finish line.

Impressions were obtained using the same two-step addition silicone technique described for the fiber post-and-core crown group. A temporary onlay was fabricated and placed to protect the prepared tooth. The occlusal relationship was recorded using bite registration silicone, and the working model was submitted to the dental laboratory for fabrication of a lithium disilicate ceramic onlay. During the clinical try-in, marginal adaptation was evaluated using a periodontal probe, interproximal contacts were assessed using dental floss, and the shape and shade of the restoration were confirmed with the patient.

For final bonding, the intaglio surface of the onlay was etched with 9.5% hydrofluoric acid for 40 s, rinsed thoroughly, neutralized with sodium bicarbonate, ultrasonically cleaned in 95% alcohol for 5 min, and dried. A silane coupling agent followed by an adhesive was applied to the restoration surface. The prepared tooth was isolated, etched with 35% phosphoric acid for 30 s, rinsed, and dried. Adhesive was applied and light-cured for 15 s. The onlay was bonded using dual-cure resin cement, excess cement was removed, and the restoration was light-cured for 40 s. Occlusion was adjusted as required, and the restoration was polished (Figure 3).

Figure 3: Representative clinical images of posterior tooth restoration with an all-ceramic onlay. (A) Clinical appearance of the tooth 1 week after root canal treatment, before restoration. (B) Tooth preparation for the all-ceramic onlay. (C) Lithium disilicate ceramic onlay before placement. (D) Immediate postoperative appearance after onlay try-in and definitive bonding. Please click here to view a larger version of this figure.

4. Operator standardization and calibration

To minimize operator-related variability, all restorative procedures were performed by three prosthodontists who followed standardized clinical protocols. Before patient enrollment, all operators completed a 2-week calibration program using typodont models. A detailed operative manual was provided, and weekly review sessions were conducted to reinforce adherence to the standardized procedures. Intraoperative checklists were used to ensure consistency during critical procedural steps. All completed restorations were subsequently reviewed by a senior prosthodontist for quality assurance. In addition, quarterly inter-operator calibration sessions were conducted to review representative cases and standardize clinical decision-making. These measures were implemented to ensure procedural consistency throughout the study.

5. Clinical evaluation

Clinical outcomes were evaluated at 3 months and 2 years after restoration by two dentists with more than 15 years of clinical experience who were not involved in the treatment procedures. Both examiners were blinded to group allocation. Before the formal evaluations, the examiners completed three calibration sessions using 20 representative cases to standardize the interpretation of all evaluation indices. Inter-examiner agreement was excellent, with Cohen's κ values ranging from 0.82 to 0.91 and intraclass correlation coefficients (ICC) ranging from 0.88 to 0.94.

6. Outcome measures

1. Restoration integrity
   Restoration integrity was classified as follows: Grade A, the restoration was intact without defects; Grade B, the restoration exhibited minor damage that did not affect function; and Grade C, the restoration was fractured or detached.
2. Tooth integrity
   Tooth integrity was classified as follows: Grade A, the tooth hard tissue was intact without defects; Grade B, minor defects or cracks were present in the tooth hard tissue; and Grade C, significant hard-tissue defects were present that could potentially affect tooth function.
3. Marginal adaptation
   Marginal adaptation was classified as follows: Grade A, no detectable gap was present between the tooth and the restoration margin, and the periodontal probe did not catch; Grade B, a slight marginal gap was present, with slight probe catch; and Grade C, a significant marginal gap allowed insertion of the periodontal probe.
4. Interproximal contacts
   Interproximal contacts were classified as follows: Grade A, dental floss passed through the contact area with resistance; Grade B, dental floss passed with slight resistance and mild food impaction was present; and Grade C, dental floss passed without resistance, accompanied by severe food impaction.
5. Periodontal pocket depth
   Periodontal pocket depth was measured using a periodontal probe that was cleaned and disinfected before each examination. The probe was held using a modified pen grasp, and a standardized probing force of 20–25 g was applied. The probe was inserted to the base of the periodontal pocket, and the probing depth was recorded in millimeters. Greater probing depths indicated poorer periodontal outcomes.
6. Gingival index
   The Gingival Index was used to evaluate the severity of gingival inflammation according to gingival color, consistency, and bleeding tendency. Examinations were performed under standardized conditions using a blunt-ended periodontal probe at six sites per tooth (mesiobuccal, midbuccal, distobuccal, mesiolingual, midlingual, and distolingual). The highest score recorded for each tooth was used for analysis. Scores ranged from 0 to 3, where 0 indicated normal gingiva with no bleeding on probing; 1 indicated mild erythema and edema without bleeding on probing; 2 indicated moderate redness, edema, a shiny appearance, and bleeding on probing; and 3 indicated severe redness, swelling, spontaneous bleeding, or ulceration. Higher scores indicated poorer periodontal health.
7. Plaque index
   The Plaque Index was used to evaluate the thickness and amount of plaque accumulation at the gingival margin of the index teeth. Examinations were performed by visual inspection combined with gentle probing of the tooth surface. Each tooth was scored from 0 to 3 according to the following criteria: 0, no plaque at the gingival margin; 1, a thin plaque film adherent to the gingival margin that was not visible to the naked eye but detectable by probing; 2, moderate plaque accumulation visible at the gingival margin or on adjacent tooth surfaces; and 3, abundant soft deposits filling the gingival sulcus and covering the gingival margin and adjacent tooth surfaces. Higher scores indicated poorer oral hygiene and greater plaque accumulation.
8. Chewing function
   Chewing function was assessed by measuring bite force and masticatory efficiency. Bite force was measured using a bite force tester. Masticatory efficiency was determined using the sieving and weighing method and expressed as the percentage of food comminuted within a standardized chewing period. Participants were instructed to chew 5 g of peanuts for 20 s before expectorating all chewed material into a collection container. The oral cavity was then rinsed to recover any remaining food particles. The collected material was passed through a 2.0 mm sieve, and the residue retained on the sieve was dried and weighed. Masticatory efficiency was calculated as:
   (total weight - residual weight)/total weight × 100%.
9. Oral health-related quality of life
   Oral health-related quality of life was evaluated using the Chinese version of the Oral Health Impact Profile-14 (OHIP-14), a validated instrument widely used to assess the psychosocial impact of oral conditions on daily life. The questionnaire comprises 14 items across seven domains: functional limitation, physical pain, psychological discomfort, physical disability, psychological disability, social disability, and handicap. Each item is scored on a 5-point Likert scale ranging from 0 ("never") to 4 ("very often"), yielding a total score of 0–56, with higher scores indicating poorer oral health-related quality of life. The Chinese version of the OHIP-14 has demonstrated good reliability and validity (Cronbach's α = 0.93)¹⁵, supporting its use for evaluating oral health outcomes in Chinese populations.

7. Statistical analysis

Statistical analyses were performed using SPSS software. Normally distributed continuous variables were expressed as mean ± standard deviation and compared using the independent-samples t-test. Non-normally distributed continuous variables were expressed as median (Q1, Q3) [M (Q1, Q3)] and compared using the Mann-Whitney U test. Categorical variables were expressed as numbers (percentages) and compared using the χ² test, continuity-corrected χ² test, or Fisher's exact test, as appropriate. Ordinal variables were compared using the Mann-Whitney U test. All statistical tests were two-sided, and P < 0.05 was considered statistically significant.

Results

A total of 100 eligible posterior teeth were included, with 50 teeth in the fiber post-and-core crown group and 50 in the onlay restoration group. All patients completed clinical follow-ups at 3 months and 2 years postoperatively.

Baseline characteristics
No statistically significant differences were observed in baseline demographic or clinical characteristics between the two groups (all P > 0.05; gender, P = 0.841; age, P = 0.462; education level, P = 0.687; body mass index, P = 0.398; disease duration, P = 0.455; tooth type, P = 0.545; jaw position, P = 0.836), indicating that the groups were comparable before treatment (Table 1).

Characteristic	The fiber post-and-core crown group (n=50)	The all-ceramic onlay group (n=50)	χ²/t value	P value
Gender(n, %)
male	23 (46.00)	24 (48.00)	0.040	0.841
female	27 (54.00)	26 (52.00)
age(years)	34.42±7.24	35.56±8.17	-0.738	0.462
Education degree (n, %)
Below junior college level	27 (54.00)	29 (58.00)	0.162	0.687
College degree or above	23 (46.00)	21 (42.00)
Body mass index(kg/m2)	23.60±2.40	24.00±2.31	-0.849	0.398
Disease course (months)	5.58±2.26	5.26±1.98	0.750	0.455
Affected tooth site(n, %)
premolars	23 (46.00)	20 (40.00)	0.367	0.545
molars	27 (54.00)	30 (60.00)
Jaw position(n, %)
maxilla	19 (38.00)	18 (36.00)	0.043	0.836
mandible	31 (62.00)	32 (64.00)

Table 1: Baseline demographic and clinical characteristics. Data are presented as n (%) for categorical variables and mean ± standard deviation (SD) for continuous variables. P values were calculated using the χ² test for categorical variables and the independent-samples t-test for continuous variables. No statistically significant differences were observed between the two groups at baseline.

Restoration integrity, tooth integrity, marginal adaptation, and interproximal contacts
Restoration integrity, tooth integrity, marginal adaptation, and interproximal contacts were compared between groups using the Mann-Whitney U test for ordinal variables. At 3 months after restoration, all restorations in both groups were classified as Grade A for restoration integrity and tooth integrity (50/50 cases), with no significant between-group differences (P = 1.000 for both outcomes). At the 2-year follow-up, the all-ceramic onlay group maintained Grade A ratings for all restorations and teeth (50/50), whereas the fiber post-and-core crown group had 48 Grade A restorations and two Grade C failures. These differences were not statistically significant (restoration integrity, P = 0.155; tooth integrity, P = 0.155).

For marginal adaptation, both groups demonstrated 49 Grade A and one Grade B restoration at 3 months (P = 1.000). At 2 years, the all-ceramic onlay group had 48 Grade A and 2 Grade B restorations, whereas the fiber post-and-core crown group had 47 Grade A, 2 Grade B, and 1 Grade C restorations (P = 0.635). For interproximal contacts, the all-ceramic onlay group demonstrated 50 Grade A contacts, compared with 48 in the fiber post-and-core crown group at 3 months (P = 0.155). At 2 years, 49 Grade A contacts were observed in the all-ceramic onlay group and 46 Grade A contacts in the fiber post-and-core crown group (P = 0.165). No statistically significant differences were identified between the treatment groups for any of these outcomes at either follow-up time point (Tables 2–4).

Grade (Restoration integrity)	3 months after restoration		2 years after restoration
	The fiber post-and-core crown group (n=50)	The all-ceramic onlay group (n=50)	The fiber post-and-core crown group (n=50)	The all-ceramic onlay group (n=50)
A	50 (100.00)	50 (100.00)	48 (96.00)	50 (100.00)
B	0 (0.00)	0 (0.00)	0 (0.00)	0 (0.00)
C	0 (0.00)	0 (0.00)	2 (4.00)	0 (0.00)
Z value	0.000		-1.421
P value	1.000		0.155
Grade (Tooth integrity)	3 months after restoration		2 years after restoration
	The fiber post-and-core crown group (n=50)	The all-ceramic onlay group (n=50)	The fiber post-and-core crown group (n=50)	The all-ceramic onlay group (n=50)
A	50 (100.00)	50 (100.00)	48 (96.00)	50 (100.00)
B	0 (0.00)	0 (0.00)	0 (0.00)	0 (0.00)
C	0 (0.00)	0 (0.00)	2 (4.00)	0 (0.00)
Z value	0.000		-1.421
P value	1.000		0.155

Table 2: Restoration and tooth integrity. Restoration integrity was graded as follows: Grade A, intact restoration; Grade B, minor damage without functional impairment; Grade C, fractured or detached restoration. Tooth integrity was graded as follows: Grade A, intact hard tissue; Grade B, minor cracks or defects; Grade C, significant defects that could potentially affect tooth function. Data are presented as the number of teeth. P values were calculated using the Mann-Whitney U test. The identical numerical results reflect that the same two cases in the fiber post-and-core crown group at the 2-year follow-up were classified as both restoration failure and tooth fracture.

Grade	3 months after restoration		2 years after restoration
	The fiber post-and-core crown group (n=50)	The all-ceramic onlay group (n=50)	The fiber post-and-core crown group (n=50)	The all-ceramic onlay group (n=50)
A	49 (98.00)	49 (98.00)	47 (94.00)	48 (96.00)
B	1 (2.00)	1 (2.00)	2 (4.00)	2 (4.00)
C	0 (0.00)	0 (0.00)	1 (2.00)	0 (0.00)
Z value	0		-0.475
P value	1.000		0.635

Table 3: Marginal adaptation. Marginal adaptation was graded as follows: Grade A, no detectable marginal gap; Grade B, a small marginal gap with slight probe catch; and Grade C, a significant marginal gap allowing probe insertion. Data are presented as the number of teeth. P values were calculated using the Mann-Whitney U test.

Grade	3 months after restoration		2 years after restoration
	The fiber post-and-core crown group (n=50)	The all-ceramic onlay group (n=50)	The fiber post-and-core crown group (n=50)	The all-ceramic onlay group (n=50)
A	48 (96.00)	50 (100.00)	46 (92.00)	49 (98.00)
B	1 (2.00)	0 (0.00)	2 (4.00)	1 (2.00)
C	1 (2.00)	0 (0.00)	2 (4.00)	0
Z value	-1.421		-1.387
P value	0.155		0.165

Table 4: Interproximal contacts. Interproximal contacts were graded as follows: Grade A, dental floss passed with resistance; Grade B, dental floss passed with slight resistance and mild food impaction; and Grade C, dental floss passed freely with severe food impaction. Data are presented as the number of teeth. P values were calculated using the Mann-Whitney U test.

Periodontal indicators
Periodontal pocket depth, gingival index, and plaque index were not normally distributed and are presented as median (Q1, Q3). Before restoration, no significant differences were observed between groups for periodontal pocket depth (P = 0.820), gingival index (P = 0.784), or plaque index (P = 0.777).

At 3 months after restoration, the all-ceramic onlay group demonstrated significantly lower periodontal pocket depth (median [Q1, Q3]: 2.00 [3.00, 4.00] versus 3.00 [4.00, 4.00], P < 0.001), gingival index (0.00 [0.00, 1.00] versus 0.00 [1.00, 1.00], P = 0.031), and plaque index (0.00 [0.00, 1.00] versus 0.00 [0.00, 1.00], P = 0.043) than the fiber post-and-core crown group. Similar findings were observed at the 2-year follow-up, with significantly lower periodontal pocket depth (P < 0.001), gingival index (P = 0.008), and plaque index (P = 0.006) in the all-ceramic onlay group (Tables 5–7).

Point of time	The fiber post-and-core crown group (n=50)	The all-ceramic onlay group (n=50)	Z value	P value
Before restoration	4.00 (4.00, 6.00)	4.00 (5.00, 5.25)	-0.227	0.820
3 months after restoration	3.00 (4.00, 4.00)	2.00 (3.00, 4.00)	-3.583	<0.001
2 years after restoration	3.00 (3.00, 4.00)	2.00 (2.00, 3.00)	-4.104	<0.001

Table 5: Periodontal pocket depth [median (Q1, Q3), mm]. Periodontal pocket depth was measured using a periodontal probe with a standardized probing force of 20–25 g. Data are presented as median (Q1, Q3). P values were calculated using the Mann-Whitney U test.

Point of time	The fiber post-and-core crown group (n=50)	The all-ceramic onlay group (n=50)	Z value	P value
Before restoration	1.00 (2.00, 3.00)	1.00 (2.00, 3.00)	-0.275	0.784
3 months after restoration	0.00 (1.00, 1.00)	0.00 (0.00, 1.00)	-2.161	0.031
2 years after restoration	0.00 (0.00, 1.00)	0.00 (0.00, 0.25)	-2.66	0.008

Table 6: Gingival index [median (Q1, Q3)]. The Gingival Index was scored from 0 to 3 based on gingival appearance and bleeding on probing (0 = healthy gingiva; 1 = mild inflammation without bleeding on probing; 2 = moderate inflammation with bleeding on probing; 3 = severe inflammation with spontaneous bleeding or ulceration). Data are presented as median (Q1, Q3). P values were calculated using the Mann-Whitney U test.

Point of time	The fiber post-and-core crown group (n=50)	The all-ceramic onlay group (n=50)	Z value	P value
Before restoration	1.00 (2.00, 2.00)	1.00 (2.00, 3.00)	-0.283	0.777
3 months after restoration	0.00 (0.00, 1.00)	0.00 (0.00, 1.00)	-2.019	0.043
2 years after restoration	0.00 (1.00, 1.00)	0.00 (0.00, 1.00)	-2.767	0.006

Table 7: Plaque index [median (Q1, Q3)]. The Plaque Index was scored from 0 to 3 according to plaque accumulation at the gingival margin (0 = no plaque; 1 = a thin plaque film detectable only by probing; 2 = moderate visible plaque accumulation; and 3 = abundant soft deposits). Data are presented as median (Q1, Q3). P values were calculated using the Mann-Whitney U test.

Masticatory function
Bite force and masticatory efficiency were normally distributed and are presented as mean ± standard deviation. No significant differences were observed between groups before restoration (bite force, P = 0.548; masticatory efficiency, P = 0.599).

At 3 months after restoration, the all-ceramic onlay group demonstrated significantly greater bite force (44.17 ± 2.47 kg versus 42.32 ± 3.69 kg, P = 0.004) and masticatory efficiency (88.60 ± 5.88% versus 83.58 ± 5.75%, P < 0.001) than the fiber post-and-core crown group. These differences remained significant at the 2-year follow-up, with higher bite force (45.17 ± 3.03 kg versus 42.75 ± 2.86 kg, P < 0.001) and masticatory efficiency (90.27 ± 4.22% versus 84.22 ± 4.59%, P < 0.001) in the all-ceramic onlay group (Tables 8 and 9).

Point of time	The fiber post-and-core crown group (n=50)	The all-ceramic onlay group (n=50)	t value	P value
Before restoration	30.26±3.34	29.88±2.93	0.062	0.548
3 months after restoration	42.32±3.69	44.17±2.47	-2.931	0.004
2 years after restoration	42.75±2.86	45.17±3.03	-4.018	<0.001

Table 8: Bite force (mean ± SD, kg). Bite force was measured using a bite force tester. Data are presented as mean ± standard deviation (SD). P values were calculated using the independent-samples t-test.

Point of time	The fiber post-and-core crown group (n=50)	The all-ceramic onlay group (n=50)	t value	P value
Before restoration	65.46±7.09	64.64±8.41	0.527	0.599
3 months after restoration	83.58±5.75	88.60±5.88	-4.313	<0.001
2 years after restoration	84.22±4.59	90.27±4.22	-6.859	<0.001

Table 9: Masticatory efficiency (mean ± SD, %). Masticatory efficiency was assessed using the sieving and weighing method. Participants chewed 5 g of peanuts for 20 s, and the chewed material was passed through a 2.0 mm sieve. Data are presented as mean ± standard deviation (SD). P values were calculated using the independent-samples t-test.

Oral health-related quality of life
OHIP-14 scores were normally distributed and are presented as mean ± standard deviation, with lower scores indicating better oral health-related quality of life. Before restoration, no significant difference was observed between groups (P = 0.543). At 3 months after restoration, the all-ceramic onlay group had significantly lower OHIP-14 scores than the fiber post-and-core crown group (11.24 ± 2.24 versus 14.64 ± 3.92, P < 0.001). This difference remained significant at the 2-year follow-up (11.42 ± 2.45 versus 13.78 ± 2.26, P < 0.001), indicating better oral health-related quality of life in the all-ceramic onlay group (Table 10).

Point of time	The fiber post-and-core crown group (n=50)	The all-ceramic onlay group (n=50)	t value	P value
Before restoration	29.34±3.51	28.94±3.03	0.610	0.543
3 months after restoration	14.64±3.92	11.24±2.24	5.325	<0.001
2 years after restoration	13.78±2.26	11.42±2.45	5.005	<0.001

Table 10: Oral Health Impact Profile-14 (OHIP-14) scores (mean ± SD). OHIP-14 scores range from 0 to 56, with higher scores indicating poorer oral health-related quality of life. Data are presented as mean ± standard deviation (SD). P values were calculated using the independent-samples t-test.

Both restorative approaches demonstrated favorable clinical performance during the 2-year follow-up period, with no significant differences in restoration integrity, tooth integrity, marginal adaptation, or interproximal contacts. However, compared with fiber post-and-core crown restorations, all-ceramic onlay restorations were associated with significantly better periodontal outcomes, greater bite force and masticatory efficiency, and lower OHIP-14 scores at both follow-up time points. These findings support the study's hypothesis that all-ceramic onlay restorations provide superior clinical outcomes for posterior teeth after root canal treatment.

DATA AVAILABILITY:
The de-identified dataset underlying the analyses reported in this study is provided as Supplementary File 1 to facilitate reproducibility of the reported results.

Supplementary File 1: De-identified dataset underlying the analyses reported in this study. Please click here to download this file.

Discussion

Dental caries is the leading cause of tooth defects, particularly in posterior teeth, and affects not only tooth morphology but also masticatory function, dentofacial esthetics, and patients' quality of life¹⁶. With advances in oral healthcare and increasing public awareness, both patients and clinicians have placed greater emphasis on preserving natural teeth whenever possible. Complete root canal treatment is essential for retaining teeth affected by pulpal and periapical disease because it enables thorough debridement, effective apical sealing, and subsequent coronal restoration¹⁷.

Common restorative materials include metal, metal-ceramic, and all-ceramic systems¹⁸. Compared with metal-based restorations, all-ceramic materials provide superior esthetics and favorable mechanical properties, contributing to their widespread clinical use¹⁹. Zirconia is an inorganic ceramic material with excellent mechanical strength, chemical stability, and biocompatibility, and zirconia crowns have demonstrated high long-term clinical success²⁰. In the present study, all full crowns in the fiber post-and-core group were fabricated from zirconia. Lithium disilicate glass-ceramic, particularly the second-generation IPS e.max Press system, has also demonstrated excellent esthetic and mechanical performance and has been widely used for indirect restorations. All onlays in this study were fabricated from lithium disilicate ceramic. Malament et al.²¹ reported a 10-year cumulative survival rate of 95.6% for 556 lithium disilicate onlays placed in 304 patients, supporting the favorable clinical performance observed in the present study. Although numerous laboratory investigations have evaluated onlay restorations after root canal treatment, clinical studies with adequate sample sizes remain limited. The present retrospective study therefore compared the clinical performance of all-ceramic onlays and fiber post-and-core crowns for restoring posterior teeth after root canal treatment by evaluating restoration outcomes, periodontal health, masticatory function, and oral health-related quality of life over a 2-year follow-up period.

Restoration integrity is an important indicator of restoration longevity and clinical success²². At the 3-month evaluation, restoration integrity was 100% in both groups. At the 2-year follow-up, all restorations in the onlay group remained intact, whereas two restoration failures occurred in the fiber post-and-core crown group. Although this difference was not statistically significant, the favorable clinical performance of the onlays may be related to their conservative preparation design, which preserves more sound tooth structure by removing only weakened cusps and restoring undercuts with flowable resin. This minimally invasive approach reduces tooth preparation, shortens chair time, and may improve patient comfort. Furthermore, the 2°–6° preparation divergence simplifies restoration seating, while supragingival margins facilitate adhesive removal and polishing, thereby reducing gingival contamination.

Fiber post-and-core crowns remain a commonly used restorative option for teeth with extensive structural loss because they improve retention and resistance²³. However, post-space preparation requires additional removal of radicular dentin, which may compromise tooth integrity and increase the risk of root fracture²⁴. In contrast, all-ceramic onlays require less invasive tooth preparation, reduce stress on the root canal walls, and may minimize periodontal irritation, making them an attractive option for restoring posterior teeth with extensive defects²⁵. Although no statistically significant difference in tooth integrity was observed between groups, two tooth fractures occurred exclusively in the fiber post-and-core crown group during the 2-year follow-up. Because of the small number of failures, definitive conclusions cannot be drawn. Nevertheless, both fractured teeth exhibited limited remaining coronal structure and inadequate ferrule height, suggesting that insufficient structural support may increase susceptibility to fracture under cyclic occlusal loading²⁶. Reduced residual root dentin following post-space preparation and stress concentration at the post-remaining tooth structure interface may further contribute to this risk, particularly in structurally compromised teeth. These observations suggest that although fiber post-and-core crowns can provide satisfactory clinical performance, minimally invasive onlay restorations may reduce the risk of catastrophic failure when sufficient tooth structure remains.

Although both restorative approaches provide favorable mechanical performance, onlays have longer restoration margins that may increase plaque accumulation and the risk of secondary caries if adhesive procedures are inadequate^27,28. In the present study, however, marginal adaptation did not differ significantly between groups, indicating that both techniques achieved satisfactory marginal fit. The supragingival margin design of onlays facilitates impression making, removal of excess cement, and finishing and polishing, which may improve long-term periodontal stability^29,30. In addition, because onlays preserve more natural tooth structure and avoid extensive retentive preparation, they are consistent with minimally invasive restorative principles while maintaining long-term clinical stability³¹. Although interproximal contacts did not differ significantly between groups, more contact deficiencies were observed in the fiber post-and-core crown group. This finding may reflect the greater amount of tooth preparation required for full-coverage crowns, which can alter natural proximal contacts, whereas subgingival margins may increase gingival irritation and food impaction³². In contrast, preservation of natural marginal ridges and proximal contours during onlay preparation may help maintain physiological contact relationships and reduce food impaction³³. Careful tooth preparation and meticulous finishing remain essential regardless of the restorative technique.

The all-ceramic onlay group also demonstrated significantly better periodontal outcomes. This finding may be attributable to the supragingival margin design, which facilitates plaque control, surface cleaning, and periodontal maintenance³⁴. Furthermore, preservation of the natural buccolingual contour may enhance physiological self-cleansing and contribute to improved periodontal health³⁵. Masticatory function was likewise superior in the onlay group. Onlays more closely reproduce natural tooth morphology, enabling efficient transmission and distribution of occlusal forces while preserving remaining tooth structure³⁶. Lithium disilicate ceramic exhibits wear resistance comparable with that of natural enamel, thereby minimizing wear of opposing teeth while maintaining chewing efficiency and excellent biocompatibility³⁷. The conservative preparation design also reduces stress concentration and preserves healthy tooth tissue, which may facilitate functional recovery and patient adaptation³⁸.

Several limitations should be acknowledged. As a retrospective study, selection bias cannot be completely excluded, although baseline characteristics were comparable between groups. The sample size of 100 patients limited statistical power and generalizability, and the 2-year follow-up period was insufficient to evaluate long-term restoration survival. Although all participants received standardized oral hygiene instructions, compliance and professional maintenance were not systematically recorded and therefore may have influenced periodontal outcomes and restoration longevity. In addition, premolars and molars were analyzed together despite differences in occlusal loading, root morphology, and fracture susceptibility. Site-specific subgroup analyses may therefore provide additional insight in future studies. Cost-effectiveness was also not evaluated despite its importance in clinical decision-making. Finally, the present findings are applicable only to posterior teeth with sufficient remaining coronal tooth structure. In cases of severe coronal destruction, inadequate ferrule, or compromised root support, fiber post-and-core crowns remain an important restorative option because they provide intraradicular retention that cannot be achieved with onlay restorations. Future prospective, multicenter, randomized controlled studies with larger sample sizes, longer follow-up periods, and tooth-specific subgroup analyses are warranted to validate these findings.

Conclusion
Within the limitations of this retrospective, single-center study with a 2-year follow-up, all-ceramic onlay restorations demonstrated superior periodontal health, masticatory function, and oral health-related quality of life compared with fiber post-and-core crowns for restoring posterior teeth after root canal treatment, while achieving comparable restoration integrity, tooth integrity, marginal adaptation, and interproximal contacts. These findings support the use of all-ceramic onlays as an effective minimally invasive restorative option for endodontically treated posterior teeth with moderate-to-large defects when sufficient tooth structure remains. However, these conclusions should be interpreted within the limitations of the study design, and further long-term prospective clinical studies are required to confirm the durability and generalizability of these findings.

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
35% phosphoric acid etchant	Bisco, USA	E-59110P	Etching of enamel/dentin before bonding
9.6% hydrofluoric acid	Pulpdent, USA	PEG-3	Etching of ceramic restoration surface
Addition silicone impression material kit	President Dental, Switzerland	PRD.01.PGUMP01	Heavy-body and light-body for final impression
Alginate impression material	Kerr, USA	27477DX-2	Preliminary impression for study model
Bausch occlusion paper	Bausch, Germany	BK-288-3	Occlusal adjustment and contact marking
Bite force tester	West China Medical, China	YHL001	Measurement of maximum bite force
Core resin (for post-and-core build-up)	3M, USA	56864-4	Resin core build-up over fiber post
Dental floss	Johnson & Johnson	1993.0107.205	Checking interproximal contacts
Dual-cure resin cement (part of Multilink N kit)	Ivoclar Vivadent, Liechtenstein	V30339-8	Bonding of crowns and onlays
Fiber post drill (for post space preparation)	3M ESPE (RelyX), USA	56864-4	Part of the core resin kit; used for post space drilling
Flowable resin	Dentsply Sirona (SDR), USA	61C130-5	Cavity sealing and base placement
Full-zirconia crown (prefabricated)	Vita Zahnfabrik, Germany	20182170377	Crown restoration for post-and-core group
Gingival retraction cord	Pascal International (KnitTrax), USA	07-585-7	Gingival displacement for impression
Glass fiber post	3M (RelyX Fiber Post), USA	306831603-8	Post-and-core restoration
Hard plaster / Super-hard plaster	Heraeus Kulzer, Germany	OCTA-SUPERROCK	Pouring of study models and working casts
Hiluster Plus polishing cup	Kerr Rotary, Switzerland	278925	Final polishing of restorations
Iodine glycerol	Xinlong, China	4011244-10	Gingival soothing after retraction and bonding
IPS e.max press onlay (lithium disilicate)	Ivoclar Vivadent, Liechtenstein	U01738	Onlay restoration fabricated in lab
Light-curing lamp	3M, USA	76973	Light curing of adhesive resins (wavelength: 440-480 nm)
Multilink N bonding kit	Ivoclar Vivadent, Liechtenstein	V30339-8	Self-adhesive resin cement for definitive bonding
Occlubrush polishing brush	Kerr, USA	2520	Polishing of restorations and margins
Occlusal registration silicone	President Dental, Switzerland	PRD.01.PGUML01-1	Bite registration and occlusal record transfer
Periodontal probe	Hu-Friedy, USA	PCPUNC15	Measurement of periodontal pocket depth
Saline (0.9% sodium chloride)	Baxter, USA	2F7124	Irrigation and rinsing of root canal and operative field
Self-etch adhesive	3M, USA	41925	Bonding agent for post and crown
Silane coupling agent (Monobond N)	Ivoclar Vivadent, Liechtenstein	Monobond N	Surface pretreatment of ceramic for adhesive bonding
Sodium bicarbonate	Sigma-Aldrich, USA	V900182	Neutralization of hydrofluoric acid after etching
SPSS software	IBM, USA	Version 26.0 ; RRID: SCR_002865	Statistical analysis of clinical data
Ultrasonic device	E.M.S. Electro Medical Systems S.A. (Switzerland)	PIEZON® 150	Cleaning of instruments and ultrasonic activation of root canal irrigants
Vita 3D-master shade guide	Vita, Germany	B360	Shade matching before restoration fabrication