Five-Year Survival Outcomes of Angulated Dental Implants Placed Tangentially to the Inferior Alveolar Nerve in the Posterior Mandible

Introduction

Dental implant therapy has revolutionized the treatment of partial and complete edentulism by providing a predictable and effective solution for dental rehabilitation [1]. Successful implant placement depends primarily on sufficient alveolar bone height and thickness. However, volumetric and morphological deficiencies of the jawbones remain major limiting factors for implant therapy [2].

To achieve optimal biomechanical implant positioning and adequate implant numbers, bone augmentation procedures – such as sinus floor elevation, ridge splitting, horizontal and vertical bone grafting, and the use of titanium or polytetrafluoroethylene membranes – are frequently performed [3–6]. These techniques restore bone volume and improve alveolar morphology. However, they require prolonged healing periods and can be associated with complications, including infection, graft resorption, and membrane exposure [7]. In addition, patient-related factors such as systemic diseases, socioeconomic conditions, deleterious habits, and advanced age can limit the indication and success of bone augmentation procedures [8].

Ideally, implants should be placed vertically to ensure optimal axial force distribution [9]. However, anatomical challenges in the posterior mandible, particularly reduced bone height and proximity to the inferior alveolar nerve (IAN), often complicate implant placement. In severely atrophic posterior mandibles, rehabilitation can require inferior alveolar nerve lateralization or transposition [10]. Although these procedures provide clinical benefits, they are associated with potential complications, most notably neurosensory disturbances [11].

Tilted dental implants that bypass the IAN have therefore emerged as a less invasive alternative. This approach enables the use of available bone without extensive augmentation procedures [12]. Implants are typically placed at angulations between 15° and 45°, allowing the insertion of longer implants [13]. Previous studies have reported high survival rates, minimal marginal bone loss, and reduced surgical morbidity with tilted implants [14–16].

In recent years, tilted implant placement has gained increasing acceptance for prosthetic rehabilitation of the edentulous mandible. Distal angulation allows fixation in multiple cortical regions and reduces cantilever length, thereby improving load distribution. Furthermore, this technique eliminates the need for bone grafting procedures and reduces overall treatment morbidity [12].

The aim of this study was to evaluate linguobuccal-angled implant placement in posterior mandibular regions with severe bone deficiency. The objectives were to avoid IAN injury, enable the use of standard-length implants, and provide functional prosthetic rehabilitation without bone augmentation.

Material and Methods

PATIENT SELECTION:

This study included 37 patients (9 men, 28 women), with a mean age of 62.9 years (range, 49–71 years), who presented to the Department of Oral and Maxillofacial Surgery, Faculty of Dentistry, Çukurova University, between 2016 and 2018 (Table 1) Written informed consent was obtained from all participants. The study protocol was approved by the Clinical Research Ethics Committee of Çukurova University Faculty of Medicine (approval No: 2018/83).

INCLUSION CRITERIA:

The inclusion criteria were as follows: (1) at least 1 implant placed in the posterior mandible; (2) absence of systemic disease; (3) no active periodontal disease; (4) posterior mandibular atrophy due to long-term edentulism; (5) age 18 years or older, with localized posterior tooth loss and inability to use removable partial dentures; (6) residual bone height between 3 and 5 mm and crestal width of 6 mm or greater, rendering short implants unsuitable; (7) refusal of vertical bone augmentation procedures; and (8) lingual positioning of the inferior alveolar nerve.

EXCLUSION CRITERIA:

The exclusion criteria were as follows: (1) smoking more than 10 cigarettes per day; (2) history of head and neck radiotherapy; (3) mucosal diseases in the surgical area, such as oral lichen planus; (4) rapid alveolar bone loss due to stage III/IV, grade C periodontitis; and (5) poor oral hygiene (full-mouth plaque index >25%) [17].

PREOPERATIVE AND POSTOPERATIVE PROTOCOL:

Patients who received at least 1 angled dental implant in the posterior mandible were included. Preoperative cone beam computed tomography (CBCT) scans (Planmeca ProMax 3D, Helsinki, Finland) were obtained for all patients. An acrylic stent with radiographic markers was fabricated prior to imaging. Panoramic radiographs and CBCT images were taken with the stent in place (Figure 1). Follow-up began immediately after prosthetic loading and continued at 6-month intervals for 5 years. Two-dimensional radiographs were obtained at each visit. Marginal bone loss was assessed using CBCT at the 5-year follow-up. Marginal bone loss was defined as the vertical distance from the implant shoulder to the most coronal bone-to-implant contact on mesial and distal surfaces. All CBCT scans were obtained using a 20×15 cm field of view with a voxel size of 0.2 to 0.4 mm. Given the spatial resolution, very small bone level changes approaching voxel size should be interpreted cautiously. Such measurements may fall within the inherent accuracy limits of CBCT-based linear analysis. Metal artefact reduction was applied during imaging. The device’s built-in metal artefact reduction algorithm minimizes beam hardening and streak artefacts caused by high-density materials. It improves grey value consistency and enhances visualization of the alveolar crest and IAN region.

SURGICAL TECHNIQUE:

On preoperative CBCT images, a reference line perpendicular to the IAN canal was drawn. The planned buccal implant position relative to the nerve was determined, and the angle between the implant axis and the reference line was measured (Figure 2A). Implants were placed using a custom surgical guide designed to indicate the linguobuccal angulation (Figures 2B, 3). Linguobuccal-positioned implants refer to implants whose apical portion is inclined from the buccal side toward the lingual aspect. After local anesthesia (4% articaine with 1: 100 000 epinephrine), a full-thickness mucoperiosteal flap was elevated (Figure 4A). Osteotomy was performed using the custom surgical guide with predefined reference sections. The horizontal arm indicated the ideal 7-mm inter-implant distance and was aligned parallel to the crestal plane. The vertical arm formed a 90° angle and included guiding lines at 15° and 25° relative to a plane perpendicular to the crest plane. These markings ensured accurate implant angulation according to CBCT planning (Figure 4B). The implant entry point and angulation were marked with a pilot drill. Osteotomy preparation was completed according to the manufacturer’s protocol (Figure 4C). A minimum safety distance of 1 mm from the IAN was maintained as recommended [18] (Figures 2B, 5). The tapered design of the hybrid-type implant (MGM dental implant, Baesweiler, Germany) further supported the safety margin. Implants were placed with an insertion torque of 35 and 40 Ncm to ensure adequate primary mechanical stability while minimizing excessive cortical bone compression. Postoperatively, patients received amoxicillin-clavulanate 1 g twice daily or clindamycin 150 mg twice daily, naproxen 250 mg twice daily, and a 0.12% chlorhexidine mouth rinse 3 times daily.

IMPRESSION AND PROSTHETIC WORKFLOW:

Three months postoperatively, multi-unit abutments were placed, followed by a 15-day healing period (Figure 6). A preliminary closed-tray impression was taken to assess implant angulation. Based on the resulting cast, a customized open-tray impression was fabricated.

The final impression was obtained using the open-tray technique. Impression copings were secured, and addition silicone material was used due to its dimensional stability. After polymerization, transfer screws were loosened, and the impression was removed parallel to the implant angulation. Impressions without distortion or rotation were forwarded for laboratory procedures.

Custom abutments were fabricated to compensate for implant non-parallelism and achieve restorative axis alignment. Abutments were torqued according to manufacturer recommendations. A metal framework try-in was performed to evaluate passive fit and parallelism. Radiographs confirmed proper seating. The restoration was then completed (Figure 7).

SURVIVAL CRITERIA:

Implant success required the absence of mobility, no peri-implant radiolucency, annual marginal bone loss of 0.2 mm or less after the first year, and no persistent symptoms of pain, infection, or neuropathy. Implant survival referred to clinically stable implants not meeting all success criteria. Failure was defined as implant removal for any reason [19].

STATISTICAL ANALYSIS:

Marginal bone loss was defined as the dependent variable. Age and sex were included as covariates. Implant angulation, implant brand, and opposing dentition were analyzed as independent variables. Group comparisons were performed using Quade nonparametric analysis of covariance. Multiple linear regression was conducted to evaluate independent associations. A P value <0.05 was considered statistically significant.

Intraexaminer reliability was excellent, with intraclass correlation coefficients (ICCs) of 0.98 and 0.97, and interexaminer reliability was high (ICC=0.94). Minor differences (≤0.10 mm) were observed only in the few implants with measurable bone loss (maximum difference, 0.30 mm).

Results

A total of 63 implants were placed in 37 patients. Of these, 24 implants were inserted at 15° and 39 at 25° (Table 1). All implants had a diameter of 4.3 mm and a length of 11 mm. All restorations were fixed implant-supported bridges.

At the 5-year follow-up, all implants remained functional. No implant failures were recorded. Marginal bone loss of approximately 1 mm was observed in only 1 implant placed at 15°. No measurable marginal bone loss was detected in the remaining implants. The overall mean marginal bone loss was 0.016±0.13 mm.

Multiple linear regression analysis was performed to assess the association between marginal bone loss and the studied variables. Age and sex showed statistically significant associations with marginal bone loss. Increasing age was positively associated with marginal bone loss (β=0.001, P=0.016). Male patients exhibited higher marginal bone loss than female patients (β=0.020, P=0.015).

Implant-related factors were not significantly associated with marginal bone loss. Implant angulation (15° and 25°) showed no significant effect (β=0.005, P=0.410). Similarly, the type of opposing dentition did not significantly influence marginal bone loss (β=−0.003, P=0.550).

The constant term of the regression model was not statistically significant (β0=0.002, P=0.688), indicating no meaningful baseline marginal bone loss when all independent variables were set to zero (Table 2).

Discussion

Intraexaminer reliability was excellent (ICC=0.98), and interexaminer reliability was also high (ICC=0.94). Minor differences (≤0.10 mm) were observed in the few implants with measurable bone loss. However, given the minimal variability in marginal bone loss values, ICC estimates should be interpreted with caution, as restricted outcome range can artificially increase agreement coefficients.

Although age and sex reached statistical significance in the regression model, the observed effect sizes were extremely small (β=0.001 for age and β=0.020 for sex). These values are not clinically meaningful. For example, the estimated increase of 0.001 mm per year of age does not represent a relevant clinical change. Importantly, only 1 implant demonstrated clinically relevant bone loss during the 5-year follow-up. Therefore, the limited variability of the outcome variable restricts the robustness and interpretability of the regression analysis.

This study demonstrates the feasibility of placing standard-length and standard-diameter implants in atrophic posterior mandibles with a width of 6 mm or greater and limited vertical bone height above the IAN. In these cases, short implants or vertical augmentation were not suitable. A custom surgical guide was used to facilitate angled placement. Chen et al reported successful placement of a 10-mm tilted implant in a case with only 4.5 mm of residual bone height above the IAN using dynamic navigation [20]. Krekmanov et al placed 86 tilted implants in severely resorbed mandibular molar regions, with no implant failure or permanent neurosensory complications observed during 18 months of follow-up [21]. Filipov et al reported a 100% survival rate for linguobuccally or buccolingually tilted implants; however, transient and mild permanent paraesthesia occurred in 2 patients [22]. In the present study, no short-term or long-term nerve-related complications were observed. No implant failures occurred due to osseointegration problems or prosthetic loading.

Implant-supported fixed prosthetic restorations are widely accepted for edentulous rehabilitation. In the traditional Brånemark concept, implants were placed vertically. This approach often resulted in long distal cantilevers, sometimes up to 20 mm [23]. Long cantilevers can increase bending moments and stress concentration, potentially contributing to marginal bone loss [24].

Tilted implants has been increasingly advocated for the rehabilitation of fully edentulous mandibles. Posterior angulation may help avoid the IAN injury and allow the placement of longer implants with improved cortical engagement and primary stability [25]. However, finite element analyses suggest that excessive angulation may increase stress concentration around the implant [26]. Therefore, angulation should be carefully planned.

Vertical bone deficiency in the posterior mandible remains a significant clinical challenge. Conventional approaches such as bone grafting, distraction osteogenesis, and IAN repositioning are associated with potential complications, including neurosensory disturbances and prolonged healing periods [10]. Tilted implants provide a less invasive alternative by bypassing the nerve and eliminating the need for vertical augmentation procedures [14]. Although biomechanical studies have suggested that tilted implants can exhibit less favorable stress distribution than axially placed implants, systematic reviews indicate that they are a viable clinical alternative. Reported implant failure rates are generally low. In several studies, 6 failures were reported in each comparison group, while 4 studies documented no implant failures during follow-up [24,27–29]. However, most of these studies were observational, and some had relatively short follow-up periods. A minimum follow-up duration of 5 years has been recommended for reliable assessment of implant survival [30].

The criteria for implant success have been extensively discussed in the literature. Albrektsson et al proposed guidelines that remain widely accepted [31]. Implant success is generally defined as the presence of a functional implant without mobility, symptoms, or the need for removal [19]. While bone quality and quantity have traditionally been considered primary determinants of implant outcomes, increasing emphasis has been placed on peri-implant tissue health. Marginal bone levels are now regarded as a key indicator of implant stability. According to the 2017 World Workshop on the Classification of Periodontal and Peri-Implant Diseases and Conditions, implant success can be assessed based on radiographic bone levels [32]. When baseline radiographs are available, success is defined by the absence of additional bone loss beyond the initial crestal remodeling. In the absence of baseline radiographs, bone levels should not exceed 2 mm apical to the most coronal intraosseous portion of the implant [19,33].

In the present study, 5-year follow-up data demonstrated favorable clinical outcomes for long implants placed tangentially to the IAN in cases with limited vertical bone height in which augmentation procedures were contraindicated. Measurable marginal bone loss was observed in only 1 implant (approximately 1 mm). This bone loss was considered to be unrelated to prosthetic overload and was attributed to parafunctional activity. Parafunction was managed with botulinum toxin therapy [10,34], and peri-implant inflammation resolved following non-surgical periodontal treatment.

The minimal marginal bone loss observed may be associated with the relatively high cortical bone density of the posterior mandible. Proper prosthetic planning and controlled loading may also have contributed to the favorable outcomes.

Accepted success criteria generally allow up to 1 mm of marginal bone loss during the first year of function, followed by an annual loss not exceeding 0.2 mm. Marginal bone changes are influenced by multiple factors, including the implant design and surface characteristics, implant-abutment interface geometry, prosthetic planning, soft tissue thickness, oral hygiene, periodontal history, smoking habits, and other patient-related and treatment-related variables [35].

The implant system used in the present study features a nano-roughened surface, platform switching, and an internal conical connection. These characteristics have been associated with reduced marginal bone loss in previous reports [36–38].

Short implants are often recommended in atrophic posterior mandibles as an alternative to augmentation or IAN lateralization [39–42]. Some biomechanical studies suggest that extra-short implants do not significantly alter von Mises stress, compared with axially placed long implants. It has also been proposed that short implants may offer more favorable biomechanical behavior than do standard-length implants placed at angulations exceeding 17° [43]. However, short implants can present unfavorable crown-to-implant ratios and related biomechanical risks [44]. In the present cohort, residual bone height was insufficient to allow the placement of short implants. Additionally, vertical augmentation and IAN lateralization were contraindicated. Therefore, angled implant placement was selected as an alternative treatment strategy.

Recent meta-analyses indicate no statistically significant differences in implant failure rates between tilted and axially placed implants [45]. Finite element analyses further suggest that splinted, distally tilted implants may reduce cantilever length and improve stress distribution compared with axially placed implants, thereby supporting longer cantilevers [46]. Placement of longer implants may also increase bone-implant contact and enhance primary implant stability [47].

Splinting plays an important role in reducing stress at the bone-implant interface by distributing occlusal forces more evenly [45]. Finite element studies evaluating tilted implants within splinted full-arch prostheses have reported favorable stress patterns and reduced marginal bone loss [48,49]. Additionally, shorter cantilevers have consistently been associated with reduced peri-implant bone loss [50]. Consistent with these findings, meta-analytic evidence has not demonstrated increased marginal bone loss around tilted implants compared with axially placed implants [45].

It is important to note that most previous studies classified implant angulation based on mesiodistal orientation. In contrast, angulation in the present study was predominantly in the linguobuccal direction. This distinction should be considered when comparing outcomes. Some authors describe implant orientation based on apex position rather than anatomical reference, defining implants with lingual apices as buccolingual and those with buccal apices as linguobuccal [51]. Karimov et al evaluated stress and strain patterns in an atrophic mandibular model with implants placed in buccolingual and linguobuccal orientation at 20° and 25° angulations. Although no significant differences were observed between angulation degrees, lower maximum stress values were reported for linguobuccal configurations [51].

Despite the favorable clinical outcomes in the present study, several limitations should be acknowledged. First, the study focused primarily on prosthetic and mechanical factors and did not evaluate biological parameters such as probing depth, bleeding on probing, or soft tissue phenotype. Marginal bone loss is multifactorial and influenced by biological and mechanical factors. Marginal bone loss around dental implants is a multifactorial process in which biological tissue characteristics – including peri-implant tissue depth, bleeding on probing, and soft tissue phenotype (thickness/keratinized mucosa) – are key determinants in addition to mechanical and prosthetic factors [52,53]. Second, the sample size was limited, and no control was included. Third, only 1 implant demonstrated clinically relevant bone loss, which restricts the strength of statistical modeling. Finally, the single-center design and use of a single implant system can limit generalizability.

Conclusions

All implants were placed using a single implant system with identical diameter and length in a single-center setting. Implant dimensions were standardized based on biomechanical considerations for the posterior mandible, where narrower implants may present reduced mechanical resistance. However, the absence of comparisons across different implant systems, diameters, and lengths restricts broader clinical interpretation. All procedures were conducted by an experienced operator using a modified guiding instrument to facilitate angulated placement. While this ensured procedural consistency, operator experience may have contributed to the favorable outcomes. In addition, standardized buccolingual angulation may not fully reflect the variability encountered in routine clinical practice. Within the limitations of this study, angled implant placement bypassing the inferior alveolar nerve may represent a feasible treatment option for the rehabilitation of posterior mandibular atrophy. When appropriate surgical planning and prosthetic loading principles are applied, the technique appears to provide favorable long-term outcomes with high implant survival and limited marginal bone loss. Therefore, multicenter prospective studies with larger sample sizes, incorporating different implant systems, dimensional variations, and operators with varying levels of experience, are warranted to validate and generalize these findings.