Complications and Survival of Teeth Restored With Crowns Following Crown Lengthening: A Retrospective Cohort Study

Abstract

Purpose

The aim of this retrospective cohort study was to evaluate the complications and survival of teeth after single-crown restoration following surgical crown lengthening.

Materials and Methods

Patients who underwent surgical crown lengthening prior to receiving single-crown restorations by postgraduate residents at Dubai Dental Hospital, between January 2011 and January 2021 were contacted to participate in the study. The restored teeth were clinically and radiographically examined. Demographic data, patient- and tooth- related details, prevalence of prosthodontic, periodontal, and endodontic complications, as well as failure rates with underlying reasons, were documented and analysed.

Results

A total of 179 teeth from 81 patients were included in this study. Statistical analysis revealed a survival rate of 96.8% at 5 years and 95% at 10 years. Among surviving teeth, clinically significant complications of prosthodontic, endodontic and periodontal origin occurred in 8.8%, 11.2%, and 15.9% of cases, respectively. Regular dental attendance emerged as a protective factor against complications in multivariable analysis (OR = 0.38; p = .013).

Conclusion

Within the limitations of this study, compromised teeth treated with crown restorations following crown lengthening showed a high long-term survival rate.

Clinical Relevance

The high survival rate of teeth preserved through crown lengthening prior to crown restoration provides valuable evidence to support clinical decision-making regarding the rehabilitation of structurally compromised teeth with unclear prognosis.

Introduction

Dealing with structurally compromised teeth has always been challenging in routine dental practice. A compromised state refers to any reduction in the functional longevity or performance of a tooth. The restorability of a tooth should be assessed and confirmed prior to initiating any treatment, particularly in cases with a short or insufficient clinical crown. Tooth position, strategic value, periodontal condition, crown-to-root ratio, inter-arch space, occlusion, feasibility of endodontic treatment, and aesthetics are crucial factors to take into account when assessing tooth restorability.¹

A commonly practiced treatment for seemingly restorable teeth with massive structure loss is placing full coverage crowns to prevent further breakdown.² Although these restorations are intended to protect the remaining tooth structure, their success depends on the amount of tooth structure supporting them. In other words, parallel dentin walls coronal to the planned crown margin, when encompassed by a crown, provide protection by reducing internal pressure within the tooth and preventing root fracture. This phenomenon is known as the “ferrule effect”.³ Surgical crown lengthening procedures (CLP) with osseous recontouring could be considered to increase exposed tooth structure to better support future restorations and improve retention and resistance.⁴ Otherwise, the clinician may opt for tooth extraction followed by prosthetic replacement of the missing tooth to restore occlusion, or choose not to, if it is the last tooth in the arch, accepting a shortened dental arch.⁵

Considering that existing evidence demonstrates high survival rates for implants and implant-supported restorations^6,7 and remains inconclusive regarding long-term outcome of CLP, the decision between performing endodontic treatment and crown lengthening followed by crown restoration versus tooth extraction and implant placement remains a clinically challenging dilemma. In general, there is no agreement on the most effective treatment approach for compromised teeth. When endodontic therapy is combined with CLP and full coverage restorations, these procedures place additional financial and temporal obligations on the patient.⁸ It is essential for the dentist to explore various treatment options and provide the patient with the necessary information, including potential complications and associated costs, to support informed and suitable clinical decision-making.

There is limited evidence available for assessing the outcomes of crowned teeth after crown-lengthening. The survival rates for compromised teeth restored with CLP were reported to be 88.3% at 5 years, 78.4% in 10 years, and 68.1% in 15 years.⁹ Also, Patil et al.¹⁰ reported that the survival rates of teeth following combined endodontic and periodontal therapy were 82.2% and 51%, respectively, after 5- and 10-year periods. In other words, endodontically treated teeth that underwent CLP were 2.3 times more likely to be extracted compared to those that did not undergo CLP. The survival rate of teeth with an inadequate crown-to-root ratio (1:1) was as low as 40%, whereas teeth with an adequate crown-to-root ratio (<1:1) had high survival rates (93%), which may indicate that while osseous CLP itself may not affect the tooth’s overall survival, an inadequate crown-to-root ratio after CLP may potentially jeopardize the tooth’s long-term survival. In a recent study, Oh et al.¹¹ conducted a retrospective study analysing the long-term survival of teeth following CLPs and subsequent crown insertion over a period of up to 14 years. Among 268 treated teeth, the overall survival probability was 87% at 5 years and 70% at 10 years. The most common reasons for tooth loss were coronal tooth fracture and endodontic failure. The study found no significant impact on survival from crown material, provider experience, or post placement. These findings highlight the importance of occlusal dynamics and pulpal status in predicting long-term outcomes of teeth restored after CLPs.

The aim of this retrospective cohort study was to evaluate the survival rates and complications of structurally compromised teeth restored with full-coverage crowns following CLP, in a postgraduate dental setting.

Materials and methods

Study design: This study was a single-arm retrospective cohort study with no control group. It was conducted in accordance with the principles of the Declaration of Helsinki (1975), as revised in 2024¹², for biomedical research involving human subjects. Ethical approval was obtained from Mohammed Bin Rashid University of Medicine and Health Sciences IRB committee as follows: MBRU-IRB-2022-97. The study was registered in the ClinicalTrials.gov database (Identifier: NCT07259187). This study followed Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) guidelines.¹³

In this retrospective cohort study, the records of patients who received surgical CLP prior to receiving full coverage crowns at Dubai Dental Hospital between January 2011 and January 2021 were obtained and reviewed by A.AH and F.AR for possible inclusion in the study. The study was conducted within a postgraduate training environment where both periodontics and prosthodontics residents were supervised by the same set of calibrated clinical faculty to ensure standardization and adherence to institutional protocols. The surgical and prosthetic phases were coordinated under this unified supervision. This was a single-centre study involving multiple residents trained within the same structured framework.

Inclusion and exclusion criteria

Participants were selected based on predefined eligibility criteria to ensure consistency in treatment modality and outcome assessment.

Inclusion Criteria: Patients were included in the study if they met all the following conditions:

• 1.Setting and Providers: The tooth received crown lengthening surgery (CLP) performed by postgraduate residents in Periodontology and subsequent definitive crown placement by postgraduate residents in Prosthodontics at Dubai Dental Hospital.
• 2.
  Patient Characteristics:

  • a.
    Age between 18 and 70 years at the time of the CLP.
  • b.
    Able and willing to participate, with written informed consent obtained.
• 3.
  Follow-up and Documentation Requirements:

  • a.
    A minimum of 12 months had elapsed since placement of the definitive crown at the time of data collection.
  • b.
    Baseline intraoral radiographs available prior to CLP to document periodontal and structural condition.
• 4.
  Tooth-Level Clinical Criteria:

  • a.
    Tooth mobility not exceeding Miller Class I at baseline.
  • b.
    A planned or anticipated crown-to-root ratio ≥1:1 following CLP based on clinical and radiographic evaluation.

Exclusion Criteria: Patients were excluded if any of the following conditions applied:

• 1.
  Procedure Purpose or Type:

  • a.
    CLP performed exclusively for aesthetic indications.
  • b.
    Cases involving intentional endodontic treatment and CLP performed solely to correct supra-eruption for prosthetic clearance (i.e., without biological indication).
• 2.
  Baseline Dental and Periodontal Condition:

  • a.
    Teeth exhibiting furcation involvement at baseline.
  • b.
    Mobility greater than Miller Class I preoperatively.
  • c.
    Teeth functioning as abutments for fixed dental prostheses, due to altered biomechanical loading.
• 3.
  Treatment Completion and Record Accuracy:

  • a.
    Absence or incompleteness of clinical or radiographic documentation prior to CLP.
  • b.
    Incomplete endodontic, periodontal, or definitive restorative treatment at the time of review.
  • c.
    The final restoration was not a full-coverage definitive crown.
• 4.
  Follow-up and Participant Availability:

  • a.
    Follow-up duration less than 12 months after crown placement.
  • b.
    Patients who declined participation or could not be contacted for consent.

Crown lengthening procedure and crown provision details: The surgical crown lengthening procedure entailed placement of submarginal incisions 0.5–1.0 mm apical to the gingival margins of the involved teeth, ensuring a minimum of 2–3 mm of attached keratinized gingiva, followed by full-thickness mucoperiosteal flap reflection to expose the tooth structure from the predetermined crown margins to the alveolar crest. Osseous recontouring of the supporting bone was performed as indicated to re-establish a biologic width of approximately 3 mm.^{14, 15} Provisional restorations were fabricated using acrylic resin and maintained throughout the healing period to support soft tissue stability. A minimum healing period of 8-12 weeks was allowed before initiating prosthetic restoration.^{16, 17}

Data collection: Retrospective data were collected manually from digital records of registered patients (Dental4windows, Centaur Software Development Co Pty Ltd., Australia). All personal information was de-identified and securely stored with a study code number. The data were then organized into categories based on a pre-determined form for single crowns with CLP (Appendix 1).

Clinical and radiographic examinations were completed for the treated teeth. However, for participants who had radiographs within the past one year of examination no further radiographs were obtained. Demographic parameters, systemic and patient-related factors such as smoking, diabetes mellitus, parafunctional habits (self-reported or diagnosed bruxism and clenching), location of teeth (maxilla or mandible, anterior or posterior), history of treated periodontitis, history of endodontic treatment, and history of regular dental attendance were documented. Furthermore, periodontal status was evaluated by assessing mobility, probing pocket depth, bleeding index, plaque index, restorative margin position in relation to the gingival margin, and crown-to-root ratio based on periapical radiographs taken using long-cone parallel technique. In addition, prosthetic factors including crown material and opposing dentition, namely tooth, crown, or removable partial denture were recorded. Subsequently, prosthodontic complications such as crown chipping, open margin, secondary caries, repeated dislodgement of crowns, and tooth fracture, endodontic complications including presence of apical radiolucency or sinus tract, and periodontal complications of furcation involvement or radiographic mesial/ distal bone loss were recorded. In case of extraction, reasons for failure were categorized as due to restorative, endodontic, and/or periodontal reasons for failure.

Case definition: Failure was defined as lost tooth at the time of examination, whereas survival was defined as tooth remaining in situ with or without complications. Complications were further classified as 'clinically significant complications' when they met predefined severity thresholds. Surviving teeth with clinically significant complications were classified according to the origin of complications: (1) prosthodontic origin: secondary caries or repeated dislodgement of the crown (defined as ≥2 episodes of crown dislodgement requiring re-cementation); (2) endodontic origin: defined as the presence of sinus tract, development of a new apical radiolucency in periapical radiographs, or an increase in the size of pre-existing apical radiolucency; and (3) periodontal origin: including grade II or III tooth mobility (Miller classification), grade II-IV furcation involvement (Hamp classification), or severe bone loss. These categories were not mutually exclusive; teeth could be classified under multiple origins if they met criteria for more than one category.

All teeth presenting with complications were referred to the relevant speciality as a part of the treatment protocol to address the issues identified. Radiographic timelines illustrating examples of surviving teeth with and without complications are shown in Fig. 1, Fig. 2.

Fig. 1.

Radiographic timeline of surviving teeth without complications: Case 1: (A) Preoperative radiograph demonstrating presence of recurrent caries for lower right second premolar and first molar; (B) baseline radiograph depicting definitive crown placement post endodontic retreatment and CLP; (C) Four-year follow-up radiograph showing intact crown margins with stable endodontic/periodontal status; indicative of surviving teeth without complications; Case 2: (D) Preoperative radiograph demonstrating presence of limited tooth structure and minimal supracrestal tissue attachment for upper left second premolar; (E) baseline radiograph depicting definitive crown placement post endodontic retreatment and CLP; (F) Ten-year follow-up radiograph showing intact crown margins with stable endodontic/periodontal status; indicative of surviving teeth without complications

Fig. 2.

Radiographic timeline of surviving teeth with clinically significant complications after receiving endodontic therapy, crown lengthening, and definitive crown placement; Case 1: (A) Preoperative radiograph demonstrating presence of limited tooth structure and recurrent caries for lower incisors; (B) baseline radiograph depicting definitive crown placement post endodontic therapy and CLP; (C) Five-year follow-up radiograph showing increasing periapical radiolucencies and horizontal bone loss associated with increasing probing pocket depths and grade II mobility; indicative of surviving teeth with clinically significant complications due to endodontic/ periodontal reasons. Case 2: (D) Preoperative radiograph demonstrating presence of limited tooth structure and recurrent caries for upper right second premolar; (E) baseline radiograph depicting definitive crown placement post endodontic therapy, CLP and cast post and core; (F) Eleven-year follow-up radiograph showing periapical radiolucency extending distally up to the middle third of the root; and grade II mobility; indicative of surviving teeth with clinically significant complications due to endodontic reasons. Case 3: (G) Preoperative periapical radiograph; (H) preoperative bite-wing radiograph demonstrating presence of limited crown structure and minimal supracrestal tissue attachment for lower left first molar; (I) baseline bite-wing radiograph depicting definitive crown placement post CLP; (J) Ten-year follow-up radiograph showing radiolucency around the mesial and distal roots with grade III furcation involvement and grade II mobility; indicative of surviving teeth with clinically significant complications due to endodontic/periodontal reasons.

Data reliability: an experienced prosthodontist (F.AR) conducted a training session on data collection and entry of examination data to the established data collection form. The principal investigator (A.AH) and F.AR collected and compared data for five recruited patients. The inter-examiner reliability was determined, by calculating Cohen kappa coefficient.

Statistical analysis: the collected data were transferred to Statistical Package for Social Sciences (SPSS) for Windows (IBM- SPSS) version 28.0 (SPSS Inc.).

Categorical variables were described using proportions while continuous variables (such as age) were described by measures of tendency and dispersion. Cross-tabulation was performed to evaluate the independence of categorical variables. These variables were evaluated by using the chi-2-square test or Fisher’s exact test. For all categorical variables with unequal distribution across categories (e.g., jaw location [112 maxillary vs. 67 mandibular teeth], tooth position [51 anterior vs. 128 posterior teeth], and other patient- and tooth-related factors), statistical comparisons were based on failure proportions (number of failures divided by total teeth at risk within each category), ensuring valid statistical inference despite unequal sample sizes. To identify independent predictors of clinically significant complications, a multivariable logistic regression model was constructed, with occurrence of clinically significant complications as the dependent variable and variables showing univariate associations at p ≤ .2 entered as independent variables. Due to the low number of failure events multivariable analysis for tooth failure was not feasible, as it would violate the events-per-variable principle and risk model overfitting. A p-value of .05 was considered statistically significant level for all the tests.

Results

Based on the inclusion and exclusion criteria, a total of 148 eligible individuals were initially identified and screened. Eventually, 179 teeth from 81 patients were included. The inter-examiner kappa value was 0.83, indicating excellent data collection agreement.

Demographic data and patient-related characteristics

Patient age ranged from 23 and 70 years, with a mean age of 51 ± 12.8 years. Women comprised 59% of the sample population. Twenty-one patients had diabetes (25.9%) and 14 (17.3%) participants were reported as current smokers in this study. Furthermore, 10 patients (12.3%) had parafunctional habits, whereas the majority of the participants, 52 out of 81 individuals (74.2%), were not regular attendees (Table 1).

Table 1.

Patient-related and systemic characteristics.

Characteristic	Participants (total = 81)	Teeth (total = 179)
	n (%)	n (%)
Demographics
Age, years (mean ± SD)	51 ± 12.8	—
Gender
Male	33 (40.7)	73 (40.8)
Female	48 (59.3)	106 (59.2)
Systemic factors
Diabetes mellitus
Yes	21 (25.9)	49 (27.4)
No	60 (74.1)	60 (74.1)
Behavioural factors
Smoking status
Never smoker	20 (24.7)	22 (12.3)
Former smoker	47 (58)	141 (78.8)
Current smoker	14 (17.3)	16 (8.9)
Parafunctional habits
Yes	10 (12.3)	16 (8.9)
No	71 (87.7)	71 (87.7)
Regular dental attendance
Yes	29 (35.8)	56 (31.3)
No	52 (64.2)	123 (68.7)

Note: Data represent patient-level and tooth-level baseline characteristics. Total sample comprised 81 patients (33 male, 48 female) contributing 179 teeth. Tooth-level percentages reflect the distribution of teeth across patient characteristics and are not statistically independent observations. Percentages may not sum to exactly 100% due to rounding.

Abbreviations: n, number; SD, standard deviation.

Status of the teeth and/or crowns

A total of 67 mandibular (37.4%) and 112 maxillary teeth (62.6%) were included in this study. Posterior teeth accounted for approximately twice the number of anterior teeth, constituting 128 (71.5%) and 51 (28.5%), respectively. Among the included teeth, 149 teeth were endodontically treated, of which 59 teeth (33%) had a post and core restoration as part of the definitive restoration (Table 2).

Table 2.

Tooth-related characteristics of the study sample.

Characteristic	n (%)
Anatomical location
Jaw
Maxilla	112 (62.6)
Mandible	67 (37.4)
Tooth position
Anterior	51 (28.5)
Posterior	128 (71.5)
Endodontic and Periodontal history
History of treated periodontitis
No	105 (58.7)
Yes	74 (41.3)
History of endodontic treatment
No endodontic treatment	30 (16.8)
RCT only	90 (50.3)
RCT with post	59 (33)
Periodontal status
Tooth mobility
No	97 (57)
Grade 1	59 (34.3)
Grade 2	13 (7.6)
Grade 3	1 (0.6)
Probing pocket depth
1–3 mm (normal)	88 (51.8)
4–5 mm	56 (32.6)
>5 mm	26 (15.1)
Bleeding on probing
No	24 (14.1)
Yes	146 (85.9)
Plaque index†
0 (no plaque)	33 (19.4)
1 (minimal)	84 (49.4)
2 (moderate)	52 (30.6)
3 (abundant)	1 (0.6)
Prosthodontic status
Crown-to-root ratio
Unfavourable (≥1:1)	10 (5.9)
Favourable (<1:1)	160 (93.6)
Crown material
Zirconia	130 (76.5)
Porcelain-fused-to-metal	40 (23.5)
Crown margin position
Supragingival or equigingival	101 (59.4)
Subgingival margin	69 (40.6)
Opposing occlusion
Missing	1 (0.6)
Natural tooth	86 (48)
Crown or pontic	92 (51.4)

Note: Anatomical location and endodontic/periodontal treatment history were documented for all teeth included at baseline (n = 179). Clinical periodontal status and prosthodontic characteristics were assessed at final follow-up examination among surviving teeth (n = 170). Nine teeth (5.0%) were lost to failure during the follow-up period and are excluded from follow-up assessments.

^†Plaque index scored according to Silness and Löe classification.

Abbreviations: RCT: root canal treatment.

With respect to the periodontal status, 59 (34.3%), 13 (7.6%), and 1 tooth (0.6%) had mobility of grades I, II and III, respectively. Six-point periodontal probing revealed normal periodontal probing depths of 1-3mm for nearly half of the teeth, while the remaining teeth presented pocket depths of 4-5 mm in 56 teeth (32.6%), and greater than 5 mm, in 26 teeth (15.1%). One hundred forty-six teeth (85.9%) demonstrated bleeding upon probing, 84 teeth (49.4%) had plaque index of score 1 and the majority teeth (93.6%) showed an acceptable crown-to-root ratio (Table 2).

Of the prosthetic restorations, 130 teeth (76.5%) were restored with monolithic zirconia crowns, while 40 (23.5%) had porcelain-fused-to-metal (PFM) crowns. Regarding the status of opposing dentition, 92 teeth (51.4%) opposed crowns, while 86 teeth (48%) opposed natural dentition. Only one tooth (0.6%) had no occlusion due to extraction of the opposing tooth. One hundred and one crowns (59.4%) had equi-gingival or supra-gingival margins, whereas the remaining 69 crowns (40.6%) had sub-gingival margins (Table 2).

Prosthodontic, endodontic, and periodontal complications

Complications were categorized into prosthodontic, endodontic, and periodontal types. The highest percentage of the complications was prosthodontic-related, with crown chipping observed in 21 crowns (12.4%), followed by repeated crown dislodgement in 13 crowns (7.4%), secondary caries in 10 teeth (5.8%), and open margin in 9 crowns (5.3%). Periodontal complications included developing Grades I and II furcation involvement in 7 (4.1%) and 8 teeth (4.7%), respectively. Grades III and IV furcation involvement, each, were observed in 2 cases (1.2%). Mild bone loss (<15% or 2 mm attachment loss from CEJ) was observed in 87 teeth (50.9%), moderate bone loss (15% ≤; involves coronal third of root) in 79 teeth (46.2%), and severe bone loss (involves middle third of root) in only one case (0.6%). Endodontics complications consisted of persistent apical radiolucency for 19 teeth (11.2%) and presence of a sinus tract at 6 sites (3.5%) (Table 3).

Table 3.

Prevalence of prosthodontic, periodontal, and endodontic complications among surviving teeth.

Type of complication	n (%)
Prosthodontic complications
Crown chipping
No	149 (87.6)
Yes	21 (12.4)
Repeated crown dislodgement
No	157 (92.6)
Yes	13 (7.4)
Open margin
No	161 (94.7)
Yes	9 (5.3)
Secondary caries
No	160 (94.1)
Yes	10 (5.8)
Periodontal complications
Furcation involvement
No	151 (88.8)
Grade I	7 (4.1)
Grade II	8 (4.7)
Grade III	2 (1.2)
Grade IV	2 (1.2)
Presence of bone loss (according to British Society of Periodontology)34
No bone loss	4 (2.3)
Mild (< 15% or ≤ 2 mm from CEJ)	87 (50.9)
Moderate (>15%; involves coronal third of root)	79 (46.2)
Severe (involves middle third of root)	1 (0.6)
Very severe bone loss (involves apical third of root)	0
Endodontic complications
Apical radiolucency
No	151 (88.8)
Yes	19 (11.2)
Sinus tract
No	165 (96.5)
Yes	6 (3.5)

Note: Data represent the number and percentage of teeth presenting with each complication type among surviving teeth at final follow-up. Some teeth presented with multiple complications.

Abbreviation: CEJ, cementoenamel junction.

Failure and survival analysis

In total, 9 crowned teeth (4 maxillary and 5 mandibular) were missing at the time of examination. Statistical analysis demonstrated an overall estimated survival rate of 96.8% at five years and 95% at ten years. In bivariate analyses of failure proportions, there were no significant associations with gender (male 8.2% [6/73] vs female 2.8% [3/106]; p = .102), smoking status (current 7.1% [1/14], former 5.6% [8/143], never 0%; p = .499), parafunctional habits (yes 0% [0/16] vs no 5.5% [9/163]; p = .422), history of treated periodontitis (4.1% [3/74] vs 5.7% [6/105]; p = .477), history of regular dental attendance (1.8% [1/56] vs 6.5% [8/123]; p = .167), jaw (maxilla 3.6% [4/112] vs mandible 7.5% [5/67]; p = .210), tooth position (anterior 3.9% [2/51] vs posterior 5.5% [7/128]; p = .501), or crown material (zirconia 3.0% [4/134] vs porcelain-fused-to-metal 11.1% [5/45]; p = .774). The only significant association was with history of endodontic treatment (p < .001), whereby teeth without prior endodontic treatment exhibited a higher failure proportion (20.0%, 6/30) compared with those with root canal treatment (1.1%, 1/60) or post placement (3.4%, 2/59). All comparisons were conducted on failure proportions to account for unequal category sizes (Table 4).

Table 4.

Bivariate analysis of tooth failure by patient- and tooth-related variables.

Systemic and patient-related factors:	Total teeth at risk, n	Failures, n (%)†	p-value
Gender
Male	73	6 (8.2)	0.102
Female	106	3 (2.8)
Diabetes
Yes	49	5 (10.2)	0.065
No	130	4 (3.1)
Smoking status
Current smoker	14	1 (7.1)	0.499
Former smoker	143	8 (5.6)
Never smoker	22	0
Parafunctional habits
Yes	16	0	0.422
No	163	9 (5.5)
Regular dental attendance
Yes	56	1 (1.8)	0.167
No	123	8 (6.5)
Tooth-related factors:
Jaw
Maxilla	112	4 (3.6)	0.210
Mandible	67	5 (7.5)
Tooth position
Anterior	51	2 (3.9)	0.501
Posterior	128	7 (5.5)
History of treated periodontitis
Yes	74	3 (4.1)	0.477
No	105	6 (5.7)
History of endodontic treatment
No endodontic treatment	30	6 (20)	<0.001*
RCT only	60	1 (1.1)
RCT with post	59	2 (3.4)
Crown material
Zirconia	134	4 (3)	0.774
PFM	45	5 (11.1)

Note: Data are presented as number of failures and percentages calculated as proportions within each category to account for unequal distribution across groups. Total sample: 179 teeth with 9 failures (5.0%). Statistical comparisons were performed using chi-square test for variables with adequate cell counts and Fisher's exact test for variables with expected cell counts <5. All statistical tests were based on proportions, not absolute counts.

Abbreviations: RCT: root canal treatment; PFM: porcelain-fused-to-metal.

^†Percentages calculated as (number of failures / total teeth at risk within category) × 100.

^⁎p < 0.05, statistically significant.

Survival with clinically significant complications

The number and percentage of the surviving teeth with clinically significant complications are presented in Table 5. The following categories were used to group the surviving teeth with a clinically significant complication: (a) prosthodontic reasons (secondary caries or repeated dislodgement of crowns) accounted for 15 (8.8%), (b) endodontic reasons (presence of apical radiolucency and sinus tract), accounted for 19 (11.2%), and (c) periodontal origin (severe bone loss, severe furcation involvement [grades II, III, IV], or severe mobility [grades II and III]) accounted for 27 surviving teeth (15.9%). In bivariate analyses, diabetes (p = .001) and a history of regular attendance (p < .001) were significantly associated with prosthodontic clinically significant complications, while opposing occlusion was significantly associated with endodontic clinically significant complications (p = .002). In contrast, gender, smoking status, parafunctional habits, jaw, tooth position, history of treated periodontitis and history of endodontic treatment showed no significant associations with the prosthodontic, endodontic, and periodontic clinically significant complications (all p > .05)(Table 6). However, in the multivariable logistic regression, history of regular dental attendance remained the only independent predictor significantly associated with lower odds of clinically significant complications (OR = 0.38, 95% CI 0.18–0.82, p = .013). By contrast, diabetes (OR = 1.54, 95% CI 0.69–3.42, p = .294), opposing occlusion (OR = 0.97, 95% CI 0.48–1.96, p = .940), smoking (OR = 1.24, 95% CI 0.74–2.06, p = .417), and jaw (OR = 1.19, 95% CI 0.59–2.39, p = .626) were not significantly associated with the outcome, while tooth position showed a non‑significant trend toward higher odds of clinically significant complications (OR = 1.95, 95% CI 0.92–4.13, p = .083). Overall model discrimination was modest (Cox–Snell R² = 0.059; Nagelkerke R² = 0.081), indicating that additional, unmeasured factors likely contributed to these outcomes (Table 7).

Table 5.

The underlying reasons for clinically significant complications among surviving teeth

Category of clinically significant complications	Teeth, n (%)
Teeth with clinically significant complications	47 (27.6)
Prosthodontic origin†	15 (8.8)
Endodontic originꜿ	19 (11.2)
Periodontal origin⁎	27 (15.9)

Note: Data represent reasons for clinically significant complications among surviving teeth at final follow-up (n = 170). Complication categories are not mutually exclusive; individual teeth may present with complications of multiple origins simultaneously (see Figure 3 for overlap patterns). Percentages are calculated as proportions of all surviving teeth.

^†Prosthodontic origin: secondary caries or repeated crown dislodgement.

^ꜿEndodontic origin: presence of sinus tract, development of new apical radiolucency, or increase in size of pre-existing apical radiolucency.

^⁎Periodontal origin: grade II or III tooth mobility (Miller classification), grade II–IV furcation involvement (Hamp classification) or sever bone loss.

Table 6.

Bivariate association between patient- and tooth-related variables and prosthodontic, endodontic, and periodontal clinically significant complications among surviving teeth.

Systemic and patient-related factors:	Prosthodontic clinically significant complications	p-Value	Endodontic clinically significant complications	p-Value	Periodontal clinically significant complications	p-Value
Gender
Male	9 (12.5)	0.435	7 (10.4)	0.517	11 (15.1)	0.586
Female	11 (10.6)		12 (11.5)		16 (15.1)
Diabetes
No	8 (6.3)	0.001*	16 (12.6)	0.225	20 (15.4)	0.530
Yes	12 (24.5)		3 (6.8)		7 (14.3)
Smoking status
Never smoker	2 (9.1)	0.838	0 (0)	0.064	1 (4.5)	0.327
Former smoker	17 (12.1)		19 (14)		24 (16.8)
Current smoker	1 (7.7)		0 (0)		2 (14.3)
Parafunctional habits
No	20 (12.5)	0.132	19 (12.3)	0.138	24 (14.7)	0.446
Yes	0 (0)		0 (0)		3 (18.8)
Regular dental attendance
No	20 (16.7)	<0.001*	16 (13.9)	0.074	20 (16.3)	0.341
Yes	0 (0)		3 (5.4)		7 (12.5)
Tooth-related factors:
Jaw
Maxilla	10 (9)	0.149	10 (9.2)	0.206	16 (14.3)	0.428
Mandible	10 (15.4)		9 (14.5)		11 (16.4)
Tooth position
Anterior	3 (6.1)	0.135	7 (14.3)	0.279	8 (15.7)	0.526
Posterior	17 (13.4)		12 (9.8)		19 (14.8)
History of treated periodontitis
No	7 (6.8)	0.22	10 (10)	0.378	15 (14.3)	0.440
Yes	13 (17.8)		9 (12.7)		12 (16.2)
History of endodontic treatment
No endodontic treatment	5(17.2)	0.549	3 (12.5)	0.888	2 (6.7)	0.369
RCT only	9 (10)		9 (10)		15 (16.7)
RCT with post	6 (10.5)		7 (12.3)		10 (16.9)
Opposing occlusion
Missing	0	0.921	1 (100)	0.002*	0 (0)	0.405
Natural tooth	10 (11.9)		4 (5.1)		10 (11.6)
Crown or pontic	10 (11)		14 (15.4)		17 (18.5)

Note: Statistical comparisons were performed using chi-square test for categorical variables with adequate cell counts and Fisher's exact test for variables with expected cell counts <5.

Abbreviation: RCT: root canal treatment.

^⁎P value <0.05, statistically significant

Table 7.

Multivariable logistic regression analysis of factors associated with clinically significant complications.

Variables	B	P-value	OR	95% CI
Diabetes	0.428	0.294	1.535	0.689-3.416
History of regular dental attendance	-0.959	0.013*	0.383	0.180-0.817
Smoking	0.211	0.417	1.235	0.742-2.055
Jaw	0.173	0.626	1.189	0.592-2.388
Tooth position	0.666	0.083	1.946	0.916-4.134
Opposing occlusion	-0.027	0.940	0.973	0.484-1.959
Constant	-0.786	0.393	0.456	-

Note: Results of multivariable logistic regression analysis examining factors associated with clinically significant complications among surviving teeth (n = 170; 47 events).

Model fit: Cox–Snell R² = 0.059; Nagelkerke R² = 0.081.

Abbreviations: B: unstandardised regression coefficient; OR: odds ratio; CI: confidence interval.

^⁎P value <.05, statistically significant

In this study, periodontal reasons for clinically significant complications were the highest with 19 affected teeth (40.4%), followed by endodontic reasons for 10 (21.3%) and prosthodontic reasons for 9 teeth (19.1%). With regards to combination of multiple underlying reasons for clinically significant complications, 5 teeth (10.6%) showed combination of periodontal, endodontic and prosthodontic causes, one case (2.1%) had combined prosthodontic and endodontic complications, 3 cases (6.4%) showed combined endodontic and periodontal complications, and no cases presented combined periodontal and prosthodontic clinically significant complications (Figure 3).

Fig. 3.

Overlap of origins of clinically significant complications among surviving teeth with complications.

Venn diagram illustrating the distribution and overlap of prosthodontic, endodontic, and periodontal clinically significant complications among 47 surviving teeth (27.6% of 170 total).

Discussion

This study evaluated failure, survival, and complication rates of teeth following CLPs followed by single-crown restorations in a postgraduate setting. The primary finding of the present retrospective cohort study was that crowned teeth maintained following CLP showed high survival (96.8% at five years and 95% at ten years).

These outcomes derive from a cohort treated within a postgraduate dental institute, where endodontic, surgical, and prosthodontic procedures were performed by postgraduate residents under close faculty supervision. While all treatments were overseen by experienced clinicians, the potential influence of the residents’ learning curve should be taken into consideration when interpreting the results. While one might assume that survival rates could have been even higher if procedures were performed by experienced clinicians, previous research has shown no significant differences in survival outcomes based on provider experience level (i.e. dental student, residents, or faculty).¹¹ Nevertheless, case selection in the present study emphasized favourable root length and crown-to-root ratios, together with a conservative osseous approach during CLP, which likely minimized furcation involvement and unfavourable crown-to-root ratios and may partly explain the high survival observed. This is consistent with the findings of Patil et al,¹⁰ who also reported a 93% survival rate in teeth with adequate crown-to-root ratio, compared to only 40% in teeth with a 1:1 ratio. Further, in sites with less than adequate crown-to-root ratio detected in advanced periodontitis conditions, a 11- year longitudinal retrospective study reported a nonlinear association between crown-to-root ratio and risk of tooth loss, supporting the importance of preserving a favourable residual root support when assigning long-term prognosis.¹⁸ Together, these findings suggest that favourable survival observed in the current cohort may reflect not only the CLP-restorative approach itself, but also strict case selection with respect to periodontal support and crown-to-root ratio.

Similarly, Dibart et al.¹⁹ reported a 100% survival rate for 26 teeth after 5 years. Nevertheless, 38.5% of the molars that underwent CLP in Dibart’s study exhibited furcation involvement. Considering that no radiographs were taken immediately following CLP, one may question that the osseous contouring performed during CLP, rather than pathological bone loss, may have contributed to radiographic detection of furcation involvement 5 years post-treatment.

The study conducted by Moghaddam et al.⁸ reported on 245 cases that required interventions across endodontics, periodontics, and prosthodontics, with survival rates of 96% at 5 years and 83.1% at ten years. In comparison, the present study included 179 teeth, of which 149 (83.2%) received care involving all 3 disciplines. Notably, while the 5-year survival rate reported by Moghaddam et al. was comparable, the ten-year survival rate observed in the current study was higher. It is noteworthy to highlight that the study by Moghaddam et al. classified failures as either tooth loss or presence of severe complications, which may account for differences in reported outcomes between the 2 studies. In the current study, periodontal, endodontic, and prosthodontic factors contributed to clinically significant complications in 15%, 14.7%, and 13.5% of surviving teeth, respectively, demonstrating minimal difference among these categories. The reasons for tooth loss in the present study could not be definitively identified due to incomplete documentation. The study population also included patients with extractions performed at external centres for which the aetiology could not be ascertained from available records. In contrast, Moghaddam et al.⁸ found that prosthodontic issues, particularly secondary caries, were the most frequent reason for failure, accounting for 27.8% (5 out of 18) of cases. Vertical root fractures were the second most common reason, responsible for 22.2% of failures.

In a recent retrospective study by Oh et al.,¹¹ the survival probability of teeth following CLP and crown insertion was approximately 87% at 5 years and 70% at ten years. Compared to Oh et al.,¹¹ the current study demonstrated a significantly higher survival rate of 96.8% at 5 years and 95% at 10 years. Interestingly, while Oh et al.¹¹ identified endodontic treatment as a risk factor for failure, the current study found a different pattern. In this cohort, endodontic treatment was associated with clinically significant complications but not with outright failure. In fact, the failure rate was highest among teeth without any history of endodontic treatment, compared to those that had undergone endodontic treatment or post placement. This may suggest that compromised teeth without endodontic treatment developed emerging pathology or structural complications (e.g., cracked teeth progressing to root fracture) that manifested after CLP and crown placement, which ultimately led to their loss or influenced patients’ decisions to proceed with extraction. These findings are consistent with those of Al-Qudah et al.,²⁰ who reported a 12.5% prevalence of apical periodontitis in non-endodontically treated teeth restored with crowns. This prevalence may be attributed to technical factors during crown preparation, such as excessive reduction and compromised marginal integrity, particularly when the margin is placed on the crown-lengthened portion of the tooth, which can lead to pulpal irritation and subsequent periapical pathology. Al-Qudah et al.²⁰ further reported that the quality of the coronal restoration, rather than the restorative material itself, was significantly associated with apical periodontitis, with inadequately restored teeth being more likely to show periapical disease. This supports the importance of meticulous restorative execution and long-term maintenance when interpreting endodontic complications in crowned teeth following CLP. Interestingly, Oh et al.¹¹ also found tooth loss was most prevalent among endodontically treated teeth that had CLPs and crowns when opposing implant-retained prostheses. In contrast, the present study highlighted additional associations, including diabetes, irregular dental attendance, particularly with prosthodontic and endodontic complications. While restorative failure was the leading cause of tooth loss in Oh et al., periodontal complications were the most prevalent cause of clinically significant complications in the current study. This observation is consistent with long-term periodontal maintenance data showing that, although most periodontally compromised teeth can be retained, prognosis is still strongly influenced by periodontal factors such as furcation involvement, tooth mobility, and clinical attachment loss.^21,22

Bivariate analyses highlighted several variables significantly associated with clinically significant complications that informed multivariable modelling. In this study, a significant association was found between irregular attendance and prosthodontic complications, which may possibly explain the course of secondary caries development due to missed opportunities of early caries detection. The multivariable analysis identified regular dental attendance as the sole independent protective factor against complications, underscoring the importance of preventive care and professional monitoring in long-term tooth survival. At the treatment planning stage, patients with high caries risk or irregular attendance should be considered to have higher risk for clinically significant complications in long term. Motohashi et al.,²³ proposed that a DMFT (decayed, missing, filled, teeth) score of 5 or higher may serve as a predictive indicator for developing recurrent caries. It is therefore recommended to take this “cut-off” score into account when treatment planning and determining tooth prognosis. This emphasis on risk-based treatment planning is consistent with Khalaf et al.,²⁴ who showed that patient caries-risk status significantly influenced restorative management decisions, with higher-risk scenarios receiving more operative intervention. Overall model performance was modest (Cox–Snell R² = 0.059; Nagelkerke R² = 0.081), suggesting unmeasured determinants, such as detailed occlusal parameters, caries risk indices, or adherence to maintenance protocols, may further explain complication risk. Given that 64.2% of the cohort were irregular attenders, the protective effect of regular attendance represents a modifiable, high‑yield target for service design and patient education.

In addition, the results revealed a significant correlation between the presence of an indirect restoration on the opposing tooth and endodontic complications of teeth receiving crowns following CLP. This may imply the possible overlooked impact of occlusal trauma over the service life of the restoration. Although limited, existing case reports have highlighted the relationship between occlusal trauma and endodontic diseases,²⁵ and its impact on periapical healing.²⁶ Consequently, careful adjustment of the occlusion at the time of crown placement is of great importance to minimize the risk of such complications.

Surgical crown lengthening, orthodontic forced eruption/extrusion, and surgical extrusion are strategies available to obtain an adequate ferrule respecting the supra-crestal tissue attachment for the management of severely compromised teeth, each presenting different advantages and limitations. In a pilot study involving 35 anterior and premolar teeth in 30 participants, forced orthodontic extrusion demonstrated a high survival rate of 94% over a mean observation period of 3.3 years. The most frequently reported complication was orthodontic relapse.²⁷ Surgical extrusion provides a one-step alternative preserving both soft and hard tissues and allowing for ferrule reestablishment in a single session. Graf et al.²⁸ reported achieving success rates of 100% in a pilot study involving endodontically treated teeth in anterior or premolar region in 9 patients with an average of 36.5 months follow-up period. Teeth showed no biological complications. In a prospective clinical study involving 13 single-rooted endodontically treated teeth with insufficient ferrule were treated with surgical extrusion and received crowns, a survival rate of 100% was reported over a mean follow-up of 18.8 months. The success rate, defined by normal function, healthy periodontal tissues, and absence of apical lesions or significant bone loss, was 92.3%. Biological complications were minimal, with no cases of root resorption.²⁹ It is important to note that in the aforementioned studies on forced orthodontic and surgical extrusion, molars were excluded due to the potential exposure of furcation after extrusion.

Surgical crown lengthening can restore biologic width; however, it may negatively impact the aesthetics and attachment levels of adjacent teeth, especially in the anterior region.^{3, 30} Systematic review and meta-analysis of in-vitro studies suggest that partial ferrule designs represent a conservative alternative when a complete ferrule is not present. Configurations of 2 mm buccal, lingual, or combined have demonstrated increased fracture resistance and markedly improved outcomes compared to the absence of complete ferrule.^{31, 32}

Implant-supported single crowns also demonstrate high survival rates, 98.3% for metal-ceramic and 97.6% for zirconia-based crowns over five years but are associated with biological complications such as peri-implantitis (5.1%) and marginal bone loss (up to 4.3%).³³ The availability of implant rehabilitations may have diminished the essential need for conservative preservation of severely compromised teeth to some degree. Nevertheless, preserving natural tissues through the alternative methods may motivate dentists to "restore the unrestorable." It should be noted that in the event of failure, treatment with an implant or fixed partial denture is still feasible.

Study Limitations: Loss of follow-up was the primary limitation of this study, especially among patients who completed treatment between 2011 and 2015. This attrition reduced the analysable sample and may limit the generalizability of the findings. Several factors likely contributed to this attrition, including changes in contact information, relocation to more accessible clinics, and reluctance to attend follow-up visits, especially among individuals with demanding schedules who perceived asymptomatic visits as unnecessary. Furthermore, a proportion of the study population was expatriates who received selective treatment during their period of residence in Dubai and subsequently returned to their home countries, making long-term follow-up unfeasible.

A significant methodological limitation was inconsistency in electronic documentation, which prevented systematic analysis of important clinical factors including the amount of bone removal during surgery, available ferrule length, and occlusal conditions at the time of definitive crown insertion. This incomplete data capture may introduce residual confounding and limits our ability to assess effect modification or perform stratified risk analysis. Prospective studies employing standardized data collection protocols with mandatory documentation fields for these parameters are essential to enable multivariable modelling and more complete inference. Furthermore, limited explanatory power of our regression model and the insufficient number of failure events (n=9) precluded multivariable analysis for tooth survival, highlighting the need for larger, multicentre studies to comprehensively identify independent risk factors and validate these preliminary findings.

Although all included teeth were structurally compromised, the study did not compare whether outcomes differed based on the initial indication for CLP (e.g., caries, subgingival restorations, fractures, or cracked teeth). However, the selection criteria clearly defined the tooth-level clinical conditions required for inclusion. Similarly, the analysis did not stratify results according to pulpal status, such as differentiating teeth requiring primary endodontic treatment from those receiving retreatment of previous endodontic therapy prior to the CLP. In addition, the study did not compare outcomes based on the type of opposing dentition. The absence of these subgroup comparisons limits the ability to determine whether specific etiologic or occlusal factors influence prognosis. The absence of such detailed clinical data limits the generalizability of the findings to broader dental practice settings.

This study included teeth with heterogeneous clinical characteristics, such as varying degrees of periodontal support and restorative history, which may influence prognosis and survival outcomes. Notwithstanding these variations, we were unable to stratify cases due to the limited sample size. Similarly, while we categorized teeth into anterior and posterior groups, we acknowledge that survival rates are also affected by root morphology and furcation involvement, particularly in distinguishing premolars from molars. Future studies with larger samples are needed to allow for more detailed subgroup analyses.

Future Directions: To strengthen the evidence in this area, future research should adopt prospective, long-term clinical trials using standardised treatment protocols and comprehensive data collection. Such designs would allow more robust identification of predictors of failure and provide clearer insight into how clinical, procedural, and patient‑related factors influence long‑term outcomes. Systematic assessment of caries risk, patient compliance, and occlusal conditions should be incorporated, as these factors may materially influence prognosis and treatment planning.

Additionally, future research should include comparative‑effectiveness studies evaluating crown lengthening with full‑coverage restoration against alternatives such as extraction and implant placement. Well‑designed prospective cohorts, matched analyses (help reduce confounding by comparing groups with similar baseline characteristics) could assess long‑term survival, complication profiles, cost‑effectiveness, and patient‑reported outcomes, providing stronger evidence to guide treatment planning for structurally compromised teeth.

Conclusion

Within the limitations of this study, the 5- year and 10-year survival rates of structurally compromised teeth treated with surgical CLP and subsequent restoration in a supervised postgraduate dental setting exceeded 90%. These findings add to the existing literature by demonstrating that such teeth can achieve high long‑term survival when carefully selected and managed. It is important to emphasize, however, that these outcomes reflect treatment provided in a teaching institution, where additional supervision and patient-related confounding factors may influence results. Patients with high caries risk and poor compliance with maintenance appointments are more likely to experience treatment failures and should receive attentive management. This information should be clearly communicated to patients to guide them in making an informed decision about tooth retention versus extraction. Nevertheless, the study’s limitations underscores the need for prospective research with long-term follow-up periods to more precisely identify predictors of failure and to strengthen the evidence base for clinical decision-making.

Conflict of interest

None disclosed.