EXPOSED MENTAL NERVE

ABSTRACT

Mental nerve is the terminal branch of Inferior alveolar nerve. In many of the surgical as well as clinical procedures, mental foramen is a key factor. Anatomic variations of this nerve may occur with respect to size, number and position which can significantly alter the clinical implications of various intraoral treatments. Thus knowing detailed information of the mental nerve is important. Appropriate position helps the clinician to deliver local anesthesia effectively, during implant placement and replacing missing teeth with dentures is an important part in prosthetic point of view. Following tooth extraction, cortical bone suffers large amount of atrophy on the vestibular area and patients usually have low and narrow alveolar crest. In moderate to severe mandibular atrophy, the bone height between alveolar crest and mental foramen is too narrow and patients may feel pain under pressure. Thus CBCT imaging may be useful in the diagnosis, management and treatment of these conditions.
Keywords: Anatomy, Mental foramen, Mental nerve.

INTRODUCTION

Largest and bulky branch of mandibular nerve is the inferior alveolar nerve. An arm of Inferior alveolar nerve departs the mental foramen near the apex of 1st and 2nd premolar in the form of mental nerve.¹ Mental nerve is single, but different branching patterns of this nerve have been reported.² Thus forming the inferior dental plexus and blood vessels it enters the mandibular canal.³ Thus the mental foramen descends slightly in toothless individuals. The mental foramen is one of the two foramina is situated towards the surface of the mandible.
The depressor anguli oris is a facial muscle associated with frowning. It origin is from the mandible and insertion into the angle of the mouth. The anterior region of the mentum and lower lip and labial gingiva of the mandibular anterior teeth and the premolars of the other two branches have afferent sensory nerve which provides sensation.⁴
In perpendicular plane, mental foramen is situated usually at the mid-point which is equidistant from the lower border of the mandible and the alveolar margin. In the horizontal plane, it is exactly at 1/4th distance from the mental symphysis to the posterior border of the ramus of the mandible.⁵ Therefore, the site of mental foramen is variable and shifts with increasing age of an individual. It is found that, in children the opening is near the lower border of the mandible and shifts up to halfway till the upper and lower borders of the mandible in an adults. In edentulous jaw it is near the alveolar ridge.⁶
For any treatment modalities to be carried out, a precise site of the mental nerve is important to prevent iatrogenic injuries and also mental foramen should be differentiated from other radiolucent pathologic lesions.^7,8 Hence, being aware of these canals is of paramount importance in all diagnosis, treatment plan and prognosis aspects.
A 75 year old female patient walks in with a chief complain of pain in the lower left back region. Most the teeth were extracted around 13 years ago due to poor periodontal condition and was edentulous for a longer period of time. Later removable partial dentures were given by the dentist which were around 9 years old as the patient did not agree for any other treatment options available. Patient also had a history of angioplasty done one and half year back with three stents placed and also had high blood pressure and is on medication for the same.
Thus this is case of a 75 year elderly edentulous woman with hyperesthesia and pain of the left mental nerve. Atrophy of the mandibular ridge was examined. (Fig. 1).

With the loss of teeth in lower left region, the alveolar ridge resorption will lead to the bone loss thus changing the relative position of mental foramen from mid-level towards upper border of the mandible towards the alveolar ridge. Most commonly seen in geriatric patients.
On physical examination, significant tenderness to light touch was noted around the area supplied by the mental nerve on left side. Facial muscle weakness was not seen. Treatment given to the patient was a removable partial denture for upper and lower jaw. The denture flanges were extended in to the vestibule. Thus, these flanges impinge on the mental nerve emerging out of the foramen near to the alveolar ridge. Patient was explained about the severity of the dentures that would hamper the alveolar ridge. Due to patient being medically compromised the patient was not recommended nor did they want any further treatment. Patient was also referred to the neurologist, were the patient was prescribed with Carbamazepine (Mezatol). Its primary use is for Trigeminal neuralgia, Epilepsy/ Seizures. Later, after 2 weeks again patient was examined and minimal relief from pain and hyperesthesia.
For removable prosthesis, if on wearing the denture it impinges near the mental foramen region i.e. 1st, 2nd premolar, the denture-bearing area can be relived so that it does not hurt on mastication. Inflammation of mucous membrane and resorption of bone may occur due to transmission of excessive functional forces or occlusal errors.
It is recommended, that in severe cases of alveolar resorption the bone loss is so much that the mental and inferior alveolar nerves are on the ridge and lying just below the mucosa which may cause pain on denture wearing. Thus, in these cases the lateralization of nerve or mental nerve repositioning procedures are to be carried out. This is then done by extending the foramen towards the lower border with the lateral cortex osteotomy and enlargement of foramen inferiorly.⁹
In the modern era because of the availability of CBCT one can easily trace out the branching pattern of the nerve and damage to the nerve can be checked. Also with the emergence of navigation technique and CBCT we can plan our surgery accordingly.¹⁰
The superiorly positioned mental foramen can be appreciated well. (Fig. 2) (Fig. 3)

IDEAL TREATMENT WOULD BE

1. 1. Transposition and reposition of mental nerve is a pre-prosthetic intervention which is effective for patients with hyperaesthesia and which involves an abnormal increase in sensitivity to stimuli of the sense caused due to dental prosthesis on the alveolar ridge.^11-13 Under local anesthesia crestal incision is made and mucoperiosteal flap is removed. Mental nerve is then bluntly dissected from the buccal flap carefully. For left mental nerve a caudal transposition is done with the use piezo-surgery under local anaesthesia. Appropriate connective tissue dissection from the nerve trunk is done so that the opening can be identified. After removing approximately 6 mm posterior-inferior cortical bone, mental nerve is transposed inferiorly to the mental foramen and posterior bone gap is packed with bone wax immediately, once done than two implants are placed in the inter-foraminal area. The flap is than sutured with 3-0 vicryl. Neurosensory function is assessed observed objectively with warm and cold sensory tests that were applied to the lower lip, and measured with a thermal sensory analyser. Hypoesthesia or hyperesthesia of left mental nerve is observed on 1st postoperative day. Neurosensory test and healing is to be done to check normal functioning of the mental nerve. After 3 months of implant placement, the patient is than given implant overdenture prosthesis.

2. Lateralization

When an osteotomy is carried at the mental foramen, a drill is used around the orifice of the external cortical bone. At the anterior curvature there is a possibility of damaging the nerve, so to avoid that a 5mm window is created which is extended ahead of the foramen. To avoid the risk of nerve damage a round drill is used to create a window which is than replaced with a diamond drill extending ahead of the foramen. The incisor branch which is located at about 5 mm from the foramen must be sectioned in order to secure complete mobilization of the alveolar nerve. The dental implants are placed under direct visualization once the nerve is completely lateralized. The vestibular cortical layer is replaced once the implants are placed in position. When osteotomy is carried out, or the nerve is properly positioned in either of the cases, arrival of the nerve comes out to be more distal.¹⁴

Advantages

1. 1. 1. Implants with longer length can be placed.
      2. Bicortical mandibular fixation is possible because of greater primary implant stability.^15-17
      3. Clinical and radiological study is required.
      4. Increased protection of the dental neurovascular bundle.^14,18,19
      5. No bone graft needed.
      6. Donor site morbidity is avoided.

All on 4 concept

Alveolar atrophy in the posterior regions of edentulous patients often hinders with the placement of dental implants. Thus without being augmented to increase the height and width of available alveolus it is complicated. This technique-sensitive augmentation surgery for the atrophied unfavourable posterior areas is basically for tilted implants to allow a better anterior-posterior spread of dental implants. This in turn favours a better load of distribution. This concept is known as “All-on-Four,” which was described by Paulo Malo and co-workers.²⁰ Two implants are placed vertically in the anterior region and the other two are placed up to an angle of 45° in the posterior region. When used in the mandible, tilting of posterior implants makes it possible to achieve good bone anchorage without interfering with mental foramina. Thus these implants are loaded immediately with a provisional fixed dental prosthesis. The use of All-on-4 surgical guide assists in ensuring the placement of implants with correct positioning, angulation and emergence.²¹

Advantages of All on 4 concept²²

1. 1. • Anatomical structural injury is avoided with angled implants in posterior region.
      • Longer implants have better anchorage in the bone.
      • In edentulous maxilla and mandible bone grafting is avoided.
      • High success rates and good biomechanics.
      • Aesthetics and function can be given immediately.
      • Final restoration can be fixed or removable.

Limitations²²

1. 1. • Oral health to be maintained well.
      • Minimum of 10mm bone in length is required for four implants.
      • Sufficient stability is attained from implants for immediate function.

Disadvantage of All on 4 concept²²

1. 1. • Free hand arbitrary surgical placement of implant is not always possible.
      • Cantilever of the prosthesis cannot be extended beyond limit.
      • Pre-surgical preparation such as CAD/CAM, surgical splint is required.
      • Technique sensitive.

An implant impinging on the nerve causes edema, hematomas and various sensory disturbances on mental nerve because of complete dentures or a partial denture.^23,24 Rupture of the mandibular canal or mental foramen during an osteotomy may result in injury of the inferior alveolar nerve, mental nerve.

According to Seddon 1943²⁵

1. 1. 1. Neurapraxia: Injury to a nerve that interrupts conduction causing temporary paralysis but not degeneration and is followed by complete and rapid recovery.
      2. Axonotmesis: Nerve is damaged but not severed and sensations will return within 2 to 6 months.
      3. Neurotmesis: Complete physiologic disruption of entire nerve trunk. Patients with such nerve injuries experience unexpected and unpleasant sensations. Difficulty in performing common activities with the face and mouth which is unacceptable to patients and negatively impact their physiology and psychology.

DISCUSSION

Damage to the nerve will lead to sensory disturbances and cause severe discomfort. The vital structures are salvaged when there is proper knowledge of anatomical landmarks. Preoperative diagnosis will help in better treatment planning. In 1977 inferior alveolar repositioning was done by Alling²⁶, in patients with major resorption and nerve arises close to the alveolar crest. In 1987 Jensen and Nock¹⁵ were the first ones to place osseointegrated implant in case nerve repositioning. However, Sakkas et al²⁷ did only a caudal transposition of the right mental nerve trunk using piezosurgery under local anesthesia. After 3 months, two dental implants were placed to the interforaminal area of the mandible and 3 months later mandibular overdenture prosthesis was inserted.²⁷ Kale and colleagues²⁸ mobilized the nerve 8 mm distally and 7 mm inferiorly keeping in mind a scope for further vestibuloplasties also the incisive branch was sacrificed as the patient was edentulous and the nerve supplies only the lower anterior teeth.²⁸ In order to reduce the risk of neurosensory alterations careful nerve manipulation and mobilization is required. Despite the various complications; the technique is well bearable by the patients.^28,29 Thus, there have been several reports about inferior alveolar nerve transposition^16,30,31 there are only few reports presenting the mental nerve transpositioning.^28,29 Metzger et al.³² proved that the epineurium became roughened without the deeper structures being affected after piezosurgery for transposition of the inferior alveolar nerve. Advantages of piezosurgery are precise cutting and minimal bleeding at the operating area. From the tip of the instrument the soft tissues could be damaged by extra mechanical force, thus operating techniques is critical.³³ The disadvantage is that it requires a longer time for the procedure.³⁴

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