Pre-prosthetic surgery: Mandible.

Pre-prosthetic surgery is that part of oral and maxillofacial surgery which restores oral function and facial form. This is concerned with surgical modification of the alveolar process and its surrounding structures to enable the fabrication of a well-fitting, comfortable, and esthetic dental prosthesis. The ultimate goal of pre-prosthetic surgery is to prepare a mouth to receive a dental prosthesis by redesigning and smoothening bony edges.

Pre-prosthetic surgery is done to provide a better anatomic environment and to create proper supporting structures for denture construction.[1] Ultimate goal should be rehabilitation of the patient with restoration of the best possible masticatory function, combined with restoration or improvement of dental and facial esthetics. To achieve this goal, maximum preservation of hard and soft tissues of the denture base is of utmost importance. Wearing dentures for prolonged period manifests adverse changes in the denture-bearing areas due to change in the size of the jaw bones resulting in ill-fitting and painful dentures.[23]

Since the end of World War II, through the development of better materials, improved accuracy of processing techniques, and better understanding of oral physiology, dental prosthesis has made great strides in increasing the successful use of prosthetic appliances in edentulous patients. The pattern of bone loss differs in maxilla from mandible. In maxilla, the usual resorption is on the buccal and inferior portion of the alveolar ridge. The pattern of edentulous bone loss results in upward and inward loss of structures. In the anterior maxilla, there is less horizontal bone loss and posterior drift of anterior rest is seen more than in edentulous mandible. In the posterior maxilla, there is invented drift of posterior rest. The width of maxilla is reduced.

Pattern of Resorption - Mandible

The mandible resorbs downward and outward, causing rapid flattening of ridge with greatest loss occurring within 12-18 months after extraction.

Gross anatomic studies[45] of dried jaw bones have shown a wide variety of shapes and sizes of residual ridges. In order to provide a simplified method for categorizing, the most common residual ridge form has been described[5] : Order I, pre-extraction; Order II, post-extraction; Order III, high, well-rounded; Order IV, knife edge; Order V, low, well-rounded; and Order VI, depressed. This self-descriptive system is useful clinically as well as for research purposes and helps one to differentiate the various stages of residual ridge resorption in the individual patient.

Pre-prosthetic surgery is an integral part of oral and maxillofacial surgery and prosthodontics. It comprises both basic procedures and sophisticated techniques of reconstructions and rehabilitation of oral and maxillofacial region. The treatment planning, therefore, should involve coordination between the prosthodontist and oral maxillofacial surgeon. As the goal of the prosthetic appliance construction is to improve functions and esthetics, the requirements to achieve these goals should be discussed by the team members. Good shell had recommended the following criteria for a healthy and edentulous ridge:[6]

The bony ridge should have adequate width and height and should be U-shaped for a denture to be retentive and efficient.

The oral mucosa should have adequate uniform thickness.

The ridge should not have any undercut (or) sharp ridges.

No bony (or) soft tissue protuberance should be present.

Should have adequate buccal and lingual sulci depth.

Objective of Pre-prosthodontic Procedure

Correcting conditions that preclude optimal prosthetic function

Hyperplastic replacement of resorbed ridges

Unfavorably located frenular attachments

Bony prominences, undercuts

Enlargement of denture bearing areas

Vestibuloplasty

Ridge augmentation

Placement of tooth root analogues by means of implants

Treatment Plan

The prognathic patient frequently places considerable stress and unfavorable leverages on maxillary basal seat. This may cause extensive reduction of maxillary residual ridge. A mandibular osteotomy of these cases can create a more favorable arch alignment and improve cosmetics as well.

Alveoloplasty

The bony prominences are removed by means of alveolectomy and alveloplasty. Alveoloplasty is the term used to describe the trimming and removal of the labiobuccal alveolar bone along with some interdental and interradicular bone and is carried out at the time of extraction of teeth and after extraction of teeth.

When surgery is planned on the edentulous ridge, incision should be made on the crest of alveolar ridge; usually the envelope flap would suffice, but releasing incision can be made on the labial side to provide broad base to the flap. Bony contouring is accomplished with bone files, rongeurs, or burs. Digital palpation can be used to determine the uniformity of the ridge. If bone resorption in the mandible has been extreme, the mental foramen may open directly at the crest of the residual bony process. This causes the margins of the mental foramen to extend and have very sharp edges. Pressure from the denture against the mental nerve will cause pain. If, however, constant irritation develops as a result of soft tissue being pinched between the denture and the bone, the spicules and the knife-edged ridge most be reduced.

Removal of Sharp Ridge (or) Knife Edge Ridge

Bony prominences, undercuts, and spiny ridges[56] are usually removed to avoid undercuts and to make possible a border seal beyond them against the floor of the mouth.

A sharp knife edge like edentulous ridge causes great denture irritation. It is usually found in the anterior part of mandible.

Place an incision on the crest of the ridge and elevate the mucoperiosteum as minimally as possible in order to maintain the vestiblular depth. Irregular and sharp bony edges are trimmed to a depth of 1-2 mm with the help of rongeurs, bone files, or burs, and the wound is closed with silk sutures.

Reduction of Genial Tubercle

The genial tubercles are extremely prominent as a result of advanced ridge reduction in the anterior part of the body of mandible. If the activity of the genioglossus muscle has a tendency to displace the lower denture, the genial tubercle is removed and the genioglossus muscle detached.

Genial tubercles are the bony projections located on the lingual aspect of the mandible, two on either side of the midline, which gives attachment to the genial muscles. The two genial tubercles located superiorly are more prominent than the inferior ones due to the gross resorption of the mandibular ridge. This may elevate the ridge lingually, giving a shelf-like appearance and making the anterior lingual seal impossible. Genial tubercles are exposed by blunt dissection. Using bur, chisel, or rongeurs, the tubercle is removed, and the rough bony margins are smoothened using file.

Ridge extension procedure

Vestibuloplasty[7-9] is a surgical procedure wherein oral vestibule is deepened by changing the soft tissue attachments. Vestibuloplasty can be done either on the labial or on the lingual side.

Kazanjian′s technique

An incision is made in the mucosa of the lip and a large flap of labial and vestibular mucosa is retracted. The mentalis muscle is detached from the periosteum to the required depth and the vestibule is deepened by supraperiosteal dissection. A flap of the mucosa is turned downward from the attachment of the alveolar ridge and is placed directly against the periosteum to which it is sutured. A rubber catheter stent can be placed in the deepened sulcus and secured with percutaneous sutures. This catheter helps to hold the flap in its new position and maintains the depth of the vestibule. It is removed after 7 days. The labial donor site is coated with tincture of benzoin compound, and the surface heals by granulation and secondary epithelialization. Contracture of the wound margins take place.

Ridge Augmentation

Superior border augmentation

It was described by Davis[10-12] in the year 1970. This procedure is indicated when mental foramen is situated in the superior border. In this procedure, autogenous bone graft is used. The rib graft can be fixed to the superior border of the mandible. Two segments of the rib, about 15 cm long, are obtained from the 5thand 9th ribs. The rib is contoured by vertical scoring in the inner surface. The second rib is cut into small pieces to laterpack against the solid rib. Fixation is done by means of transosseous wiring or circumferential wiring.

Disadvantage

Morbidity of the donor site

Secondary surgical site

Necessity of the patient to withdraw denture till the surgical wound heals for period of 6-8 months

Inferior border augmentation - Visor osteotomy

This technique was first described by Sanders and Cox in the year 1986 for reconstruction of a resected mandible. This procedure is indicated to prevent and manage fractures of an atrophic mandible.[13-14]

Visor osteotomy was described by Harle to overcome the resorption of free onlay bone graft. This technique is followed where the muscle insertion to the mandible and nutrient supply is maintained. In this procedure, mandible is divided buccolingually by a vertical osteotomy from external oblique ridge of one side of the mandible to the other side. The osteotomized lingual segment is pushed superiorly and fixed with the buccal segment using stainless steel wire in the lower border of the lingual segment.