Assessment of anterior loop of mandibular canal and its implication in implant therapy.

Context: Precise identification of anatomic structures is imperative for presurgical planning for implant installation to ensure effectiveness of procedures, especially in the region of mental foramen, and to prevent iatrogenic complications. Aims: This study aimed to assess the presence of anterior loop of mandibular canal and its approximation to the alveolar crest using cone-beam computed tomography (CBCT). Materials and
Methods: CBCT scans of the patients scheduled for implant therapy in the region of mental foramen were retrospectively analyzed for the presence of anterior loop of mandibular canal. Findings were compared based upon gender and side of the mandible. Distance between the most prominent part of anterior loop of mandibular canal and the alveolar crest was measured.
Results: Anterior loop of the mandibular canal was identified 66.8% of hemimandibles analyzed with the highest prevalence in the fourth decade of life. Males and females exhibited predilection toward the right and left side, respectively. The presence of anterior loop of mandibular canal is associated with reduced distance between the neurovascular bundle and alveolar crest jeopardizing the placement of standard length dental implant.
Conclusion: The current study emphasizes an increased need for precise identification and preservation of anterior loop of mandibular canal in the third and fourth decades of life, especially in females. Copyright:

INTRODUCTION

A fascinating travelogue through time unravels the evolution of art and science of dental Implantology. With the shift of focus toward comprehensive oral care, mere replacement of missing teeth is taken over by dental and masticatory rehabilitation which renders fulfillment of functional demands as well as esthetic concern. The first evidence of dental implants, around 600 AD, is attributed to the Mayan population who utilized pieces of shells as implants for the replacement of mandibular teeth.[1] With a cumulative survival rate ranging between 89.5% and 92.7% after 10–27 years of function, dental implants are proven to be reliable for offering adequate support to a fixed prosthetic restoration.[2]

Although implant placement is not a highly scrupulous and complex procedure, intraoperative and postoperative complications are frequently reported to be associated with surgical placement of implants.[3456] More or less frequently, the cause of such complications is the injury to the neurovascular bundle. Implant surgical procedures in the interforaminal region of mandible may cause inadvertent injury to the peripheral nerves in the vicinity and may pose significant morbidity to the patient due to neurosensory alterations[3] whereas injury to the local vascular structures may be trivial or uncontrollable resulting in life-threatening hemorrhage causing edema of the floor of mouth and airway obstruction.[78]

A branch of inferior alveolar nerve may form an intraosseous loop, referred to as the anterior loop of mandibular canal by extending beyond the mental foramen.[910] Precise identification of this benign anatomic variant is critical for presurgical planning for implant installation to ensure therapeutic success in the region of mental foramen by prevention of iatrogenic complications.[9101112] Reports regarding the prevalence of anterior loop of the inferior alveolar nerve are highly variable, ranging between 22% and 28%, also up to 88%.[1314]

The ability and reliability of conventional two-dimensional radiographic methods to reveal the presence of anterior loop of mandibular canal is of limited value and questionable significance. Moreover, cone-beam computed tomography (CBCT) is considered as the standard method for assessing bone tissue and is able to precisely demarcate the anterior loop.[15161718] The present study makes the use of CBCT scans to identify the presence of anterior loop of mandibular canal in accordance with the gender, age, and dentate condition and to identify its predilection for side of the mandible. In addition, the proximity of anterior extension of mandibular canal to the alveolar crest is noted in patients scheduled for implant therapy.

MATERIALS AND METHODS

Approval of the protocol was sought by the Institutional Ethical Committee prior to commencement of this retrospective cross-sectional study and informed consent was obtained from the participants. The determination of sample size was calculated considering the prevalence of entity under study and the desired error of margin.[1920]

A comprehensive search of the institutional database belonging to SDKS Dental College and Hospital, Nagpur, Maharashtra, to assess CBCT scans from inception to January 2020 was undertaken. From a pool of 3870 scans, 1463 CBCT scans were undertaken for implant therapy; with 631 scans belonging to the mandible. One hundred and eighty-seven CBCT scans were undertaken for implant placement in the region of mental foramen and were assessed thoroughly. Scans of the adequate diagnostic quality of completely edentulous or partially edentulous male and/or female patients belonging to age group 20–60 years, scheduled for implant placement in the region of mental foramen, were to be included in the study. CBCT scans acquired in a standardized manner maintaining the cephalostat settings, keeping the patient's head positioned such that the occlusal plane was parallel to the floor and median sagittal plane perpendicular to the floor, were to be included.[21] Scans should have been acquired using Orthophos SL 3D Dentsply Sirona (Dentsply Sirona, Germany), time of exposure 14.1 s with 6 mA electric current and 80–85 kV. Scans of low diagnostic quality showing radiolucent and/or radiopaque lesions in the region of mental foramen were to be excluded.[2122] The presence of fracture line, supernumerary or unerupted teeth, and metal artifacts was also excluded owing to compromised acuity.[2122]

One hundred and twenty-five CBCT scans met the eligibility criteria of the present study and were investigated for determination of predilection for gender, age, and side and dentate condition with respect to the presence of anterior loop of mandibular canal. The distance between the most prominent part of anterior loop of mandibular canal and alveolar crest was measured. With the help of Sidexis 4 imaging software (Build Version 4.2.0.60310, Sirona Dental Systems GmbH, Germany), 3D reconstruction was obtained [Figure 1]. Multiplanar reconstructions including sagittal, coronal, and axial images were obtained and evaluated concurrently by two trained observers. Tangential section reveals virtual implant placement and an intervening anterior loop [Figure 2].

Figure 1

Three-dimensional reconstruction showing the course of mandibular nerve and formation of anterior loop of mandibular canal in the region of mental foramen (purple shadow)

Figure 2

Virtual implant placement and demarcation of intervening anterior loop of mandibular canal in the region of mental foramen (purple shadow)

Commencing the assessment with the axial reconstruction, the volume was rotated toward the side being analyzed. The long axis of the mandibular canal was positioned parallel to the sagittal plane and the coronal reconstruction was adjusted perpendicular to the region of interest[22] [Figure 3]. Using the coronal reconstruction, the most mesial part of mental foramen was located to ascertain the presence of anterior loop.[22] The CBCT scans with the presence of an anterior loop of mandibular canal were selected and were keenly observed for the presence of a single and/or two hypodense areas of variable shape and diameter.

Figure 3

Axial reconstruction within the region of mental foramen such that mandibular canal is positioned parallel to the sagittal plane

According to Apostolakis and Brown,[11] the anterior loop can be differentiated from the incisive canal based on its diameter. Often, the incisive canal has a diameter of <3 mm. Subsequently, observation of a single round hypodense area of <3 mm diameter in the present study was interpreted as the incisive canal, whereas a hypodense area with a diameter exceeding 3 mm was considered to be an anterior loop of mandibular canal. Moreover, less frequently, two round hypodense areas were observed. In that instance, one is considered to be corresponding to the lumen of the mandibular canal that traverses the mental foramen anteroinferiorly, whereas the other is interpreted to reflect the doubling back (loop) of the mandibular canal leading to nerve externalization[1122] [Figure 4].

Figure 4

Coronal reconstruction showing the course of anterior loop of mandibular canal. Image 9–13: Confluence of two round hypodense areas suggestive of anterior loop of mandibular canal. Image 17–20: Single round hypodense area ≥3 mm suggestive of anterior loop of mandibular canal

Subsequently, the most prominent part of the anterior loop of mandibular canal and the highest point over the alveolar ridge were marked. Unplanned extractions at different visits result in uneven remodeling leading to irregularities of the alveolar ridge. For such alveolar ridges, the distance was recorded at two different points over the ridge. One corresponding to the most prominent and the other corresponding to the least prominent point over coronal reconstruction and the mean value was considered. The distance between the alveolar crest and the most prominent point of the anterior loop of mandibular canal was noted [Figure 5]. Prevalence of anatomic extension was noted as per gender, age, and dentate condition and further segregated according to the side of the mandible. The recorded data were summarized as absolute and relative frequencies over MS Office Excel sheet. The Pearson Chi-square test was applied to evaluate the presence of an anterior loop of mandibular canal with respect to gender, age, and side of the mandible. P < 0.05 was considered to depict statistical significance.

Figure 5

Measurement of distance between the alveolar crest and most the prominent part of anterior loop of mandibular canal

RESULTS

The anterior loop was visualized in 66.8% of the hemimandibles analyzed with age ranging between 20 and 60 years. Percentage analysis of the presence of anterior loop according to gender, age of the patients, side of the mandible, and dentate condition was carried out [Table 1].

Table 1

Distribution of cases with and/or without an anterior loop of mandibular canal in the region of mental foramen amongst patients scheduled for implant therapy with respect to gender, age, side of the mandible and dentate condition

Group (number)	Total number of hemimandibles	Absence of anterior loop, n (%)	Presence of anterior loop, n (%)
Gender
Males (74)	148	52 (35.1)	96 (64.9)
Females (113)	226	72 (31.9)	154 (68.1)
Total	374	124 (33.15)	250 (66.84)
Age group (years)
20-30 (34)	68	35 (51.5)	33 (48.5)
31-40 (51)	102	27 (26.5)	75 (73.5)
41-50 (54)	108	23 (21.3)	85 (78.7)
51-60 (48)	96	39 (40.6)	57 (59.4)
Total	374	124 (33.15)	250 (66.84)
Side of mandible
Males
Right	78	6 (15.4)	33 (84.6)
Left	34	10 (58.8)	7 (41.2)
Bilateral	36	10 (55.6)	8 (44.4)
Total	148	26	48
Females
Right	48	11 (45.8)	13 (54.2)
Left	116	14 (24.1)	44 (75.9)
Bilateral	62	11 (35.5)	20 (64.5)
Total	226	36	77
Dentate condition	Number of patients	Absence of anterior loop, n (%)	Presence of anterior loop, n (%)
Completely edentulous	89	40 (44.9)	49 (55.05)
Partially edentulous	98	22 (22.4)	76 (77.5)
Total	187	62	125

n – number

With respect to gender-wise comparison, the anterior loop was present among 64.86% of males and 68.14% of females; however, this difference did not reach statistical significance (P = 0.51). The highest prevalence is found in the fourth decade of life (P = 0.000078). Statistically significant differences were noted regarding the prevalence of anterior loop toward the right side of the mandible among male population (P = 0.05). Females exhibited increased prevalence toward the left side of the mandible; however, this difference did not reach statistical significance (P = 0.13) A significantly higher prevalence of anterior loop is noted among completely edentulous patients (P = 0.0011).

Among 374 hemimandibles analyzed, anterior extension is found to be present in 250 scans and absent in 124 scans. The baseline characteristics of the study population are illustrated in Table 2. Among the male population, the distance between the most prominent part of anterior loop of mandibular canal and the alveolar crest is found to be in the range of 7–9 mm for 16 scans, in range of 9–11 mm for 14 scans, between 5–7 mm for 11 scans, between 3–5 mm for 9 cases, and within the range of 11–13 mm for 6 cases [The findings are presented in Figure 6], whereas among females, the distance between the most prominent part of anterior loop of mandibular canal and the alveolar crest is found to be in the range of 7–9 mm for 40 scans, in range of 5–7 mm for 32 scans, in range of 9–11 mm for 12 scans, between 3–5 mm for 8 cases, and within the range of 11–13 mm for 5 cases [The findings are presented in Figure 7].

Table 2

Demographics of the study population

Demographic characteristics	Number of cases
Gender
Males	48
Females	77
Age group (years)
20-30	27
31-40	34
41-50	37
51-60	27
Side of mandible
Right	125
Left	125
Dentate condition
Completely edentulous	49
Partially edentulous	76

Figure 6

X-axis showing the distance between the most prominent part of anterior loop of mandibular canal and alveolar crest in males Y-axis showing the number of hemimandibles with the presence of anterior loop of mandibular canal amongst males

Figure 7

X-axis showing the distance between the most prominent part of anterior loop of mandibular canal and alveolar crest in females Y-axis showing the number of hemimandibles with the presence of anterior loop of mandibular canal in females

DISCUSSION

The study of anatomical relationship of neurovascular bundle in the mandible is a blooming area of research with emphasis on its anatomical variations typically noted in the region of mental foramen.[22] An exponential rise is noted in the number of studies conducted over the assessment of anterior loop of mandibular canal in the last two decades probably attributed to the proportional rise in a number of surgical procedures undertaken for implant placement in the interforaminal region.[1112131718232425] Incidence of permanent neurosensory disturbance within the distribution of mental nerve subsequent to the placement of dental implants has been reported in 7%–10% cases.[26] This may be the result of underestimation of nerve proximity prior to implant placement attributable to the use of evaluative methods with less acuity or utter negligence of the operator.

Specificity of the present study to include scans of prospective implant therapy patients is in contrast to the study conducted by Do Nascimento et al,[22] wherein CBCT images were obtained for various clinical indications. This lack of specificity in inclusion criteria is probably attributed to a lack of specificity in a surgical procedure. The present study shows that the anterior loop of the mandibular canal was present in 21.53% of all the cases scheduled for implant installation in the interforaminal region, although it is comparable to the prevalence reported by previous studies by Kuzmanovic et al.[27] and Ngeow et al.[28] Utilizing panoramic radiographs, precise identification in the interforaminal region was not reported previously. The low prevalence rates reported in those studies might reflect the failure of panoramic radiography to detect the presence of the anterior loop negating its reliability for evaluation of anatomical variant. Nevertheless, the present study investigates the presence of anterior loop of mandibular canal in patients planned for subsequent implant therapy. A relatively higher prevalence is reported by Kaya et al.[29] who made use of spiral computerized tomography in addition to panaromic radiographs indicating reliability of more sophisticated equipment than panaromic radiographs.

Shankland[30] showed that 6.62% of the Asian Indians (from 138 mandibles) possessed accessory mental foramen; moreover, other authors have reported the presence of accessory branches of mental nerve frequently associated with accessory mental foramen.[31] Literature reveals anatomic studies conducted to detect the presence of anterior loop of the mandibular canal and its length. The presence of loop extension lies between the variable range of 24% and 60%, also up to 88%.[143233] This variability more often attributes to the geographic area of the study. The length of anterior loop is reported to be as low as the mean value of 0.15 mm, whereas length of up to 11 mm is also noted.[143233] Although anatomical studies involve accurate demarcation of anatomic structures, the aforesaid studies have been conducted on cadaveric mandibles and are considered to be highly invasive with respect to human subjects. A reliable, noninvasive diagnostic modality fortified with precision to aid in presurgical planning would be a suitable alternative.

In the present study, the presence of anterior extension was determined by the existence of two separate canals beyond the mental foramen and by measuring its diameter in cross-sections. For this purpose, a minimum diameter of 3 mm was established, similar to the values used in other studies.[111824] In contrast, de Oliveira-Santos et al.[13] and Kaya et al.[29] did not measure the diameter of the canal and considered the anterior loop to be present only in cross-sections with two round hypodense images, with a structure that they referred to as “8-like,” This method may have underestimated the real loop length.

The patients in the present study were aged between 20 and 60 years and presence was observed in majority of females. This finding is in contrast with previous studies in the literature which denotes male predilection.[22] Moreover, studies report significant relationship between the length of anterior loop of mandibular canal and male gender.[910111724] A gender-wise predilection of the presence of anterior loop is consistently reported in literature and the present study emphasizes its accurate demarcation, especially in females. Furthermore, gender-wise prevalence of unilateral and bilateral presence of anterior extension was evaluated in the present study. Such co-relation is less frequently reported in the literature. Among females, unilateral (left) presence is noteworthy followed by bilateral and right side prevalence, whereas males exhibit predilection for the anterior loop toward the right side of mandible followed by bilateral occurrence and presence toward the left side of mandible with little significant difference.

The length of a standard implant is >8 mm.[34] A common axiom is to place as long as possible an implant to decrease the stress applied to the system and most often, the length corresponds to the height of the available bone at the edentulous site.[35] Misch[36] stated that iatrogenic nerve injury can be avoided by placing the dental implant leaving a 2 mm safety zone above the nerve. Moreover, if the implant is placed anterior to the mental foramen, the distal surface of the implant should be ≥2 mm mesial to the foramen.[36] With the focus on implication of anterior extension of the mandibular canal in presurgical planning for implant installation, the distance between the most prominent part of anterior extension and the alveolar crest is found to be between 7 and 9 mm for 41.23% of females and 28.57% of the male population. Maximum distance of 11–13 mm is present for a meager 10.71% of males and 5.15% of females.

CONCLUSION

Sensorineural damage is partially attributed to negligence toward the thorough investigation of anatomic structures as well as benign anatomic variants. Results of the present study emphasize the need to broaden the perspective from just mental foramen to also including anatomic variants in presurgical planning. Subsequently, the choice of dental implant and the need for augmentation should be governed by accurate consideration of mental foramen and anatomic variants frequently reported in the vicinity to avoid injury to the neurovascular bundle. With the assessment of distance between alveolar crest and anatomic extension of the mandibular canal, the present study emphasizes the need of its precise identification and urges its inclusion in standard guidelines for implant installation in the region of mental foramen. Nevertheless, choice of radiographic modality plays a pivotal role in assessment. Although computed tomography scan has proved to be a reliable option to detect anterior extension of the mandibular canal; however, CBCT is preferable for dentomaxillofacial structures owing to its comparative low cost, low radiation dose, and comparable image quality as that of a multi-slice CT.[15161718]

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.