In-Service Exam
Orthognathic, TMJ, Chin - 2001





In a 30-year-old woman who is undergoing evaluation prior to orthognathic surgery, cephalometric analysis shows a decreased SNB angle and a normal SNA angle. These findings are most consistent with

(A) mandibular protrusion
(B) mandibular pseudoprognathism
(C) mandibular retrusion
(D) maxillary protrusion
(E) maxillary protrusion and mandibular retrusion

PHOTO

Reproduced with permission of McCarthy JG, ed. Plastic Surgery. Philadelphia, Pa: WB Saunders Co; 1990;2:1198.

The correct response is Option C.

This patient's findings are most consistent with mandibular retrusion, which is defined as a normal SNA (sella-nasion-point A) angle combined with a decreased SNB (sella-nasion-point B) angle on cephalometric analysis. The SNA angle measures the position of point A (anterior maxilla) relative to the anterior cranial base (SN); a normal SNA angle is defined as 82 degrees x 3 degrees. Patients with maxillary protrusion have an increased SNA angle, while patients with maxillary retrusion have a decreased SNA angle. In contrast, the SNB angle measures the position of point B (anterior mandible) relative to the anterior cranial base (SN); a normal SNB angle is defined as 80 degrees x 3 degrees. It is increased in patients with mandibular protrusion and decreased in patients with mandibular retrusion.

The Landes angle, which is formed by the Frankfort horizontal line and the nasion to point A (N-A) plane, is sometimes used instead of the SNA angle because of its greater reliability. A normal Landes angle is measured at 88 degrees x 3 degrees.

The ANB angle measures the position of point A to point B relative to the anterior cranial base; the angle is positive when point A lies anterior to point B. Patients with maxillary protrusion, mandibular protrusion, or a combination of both will have a markedly increased ANB angle. A decreased ANB angle can be seen in patients who have maxillary retrusion, mandibular protrusion, or a combination of both. In patients with mandibular pseudoprognathism, the ANB angle is normal.

References
1. Ferraro JW. Cephalometry and cephalometric analysis. In: Ferraro JW, ed. Fundamentals of Maxillofacial Surgery. New York, NY: Springer-Verlag; 1997:233-245.
2. Grayson BH. Cephalometric analysis for the surgeon. Clin Plast Surg. 1989;16:633-644.

A 19-year-old man has midface hypoplasia, maxillary retrusion, and the appearance of mandibular prognathism. On examination, he has Angle class III malocclusion and a negative overjet of 15 mm. Which of the following is the most appropriate management?

(A) Anterior mandibular subapical osteotomy
(B) Genioplasty
(C) Le Fort I osteotomy
(D) Sagittal split osteotomy
(E) Combined Le Fort I osteotomy and sagittal split osteotomy

The correct response is Option E.

In this patient who has maxillary hypoplasia and mandibular hyperplasia, resulting in midface retrusion and the appearance of mandibular prognathism, the most appropriate management is combined Le Fort I and sagittal split osteotomies. There are several indications for combined surgery in this patient, including correction of the 15 mm of negative overjet and rotation of the maxillary midline. Maxillomandibular correction can be beneficial in patients who have more than 10 mm of either positive or negative overjet.

Anterior mandibular subapical osteotomy can be used to level the occlusal plane, correct asymmetry, or change the anterior positioning and/or axial angulation of the anterior mandibular teeth. Genioplasty alone will not correct this patient's Angle class III malocclusion. If performed as a single procedure, neither Le Fort I osteotomy nor sagittal split osteotomy will correct both of this patient's deformities.

References
1. Ferraro JW. Cephalometry and cephalometric analysis. In: Fundamentals of Maxillofacial Surgery. New York, NY: Springer-Verlag; 1997:233-245.
2. Grayson BH. Cephalometric analysis for the surgeon. Clin Plast Surg. 1989;16:633-644.
3. Guyuron B. Combined maxillary and mandibular osteotomies. Clin Plast Surg. 1989;16:795-802.
4. Wolford LM, Fields RT. Surgical planning. In: Booth PW, Schendel SA, Hausamen JE, eds. Maxillofacial Surgery. London, England: Churchill Livingstone, Inc; 1999;2:1205-1257.

A 30-year-old woman has painful clicking of the jaw six months after sustaining blunt trauma to the face in a motor vehicle collision. Radiographs taken at the time of injury showed no evidence of fracture. Which of the following is the most appropriate diagnostic study in the evaluation of this patient?

(A) Arthroscopy
(B) CT scan
(C) Digital subtraction angiography
(D) MRI
(E) Tomography

The correct response is Option D.

MRI is the most appropriate diagnostic study in the evaluation of this patient who has painless clicking of the jaw after sustaining blunt trauma to the face. In fact, MRI is preferred for evaluation of all conditions involving the articular disk and temporomandibular joint (TMJ), including internal derangement of the TMJ. This painless noninvasive modality allows examination in multiple planes and produces an accurate visualization of the soft tissues without exposing the patient to ionizing radiation. Fast-scanning techniques can be used to assess TMJ pathology.

Arthroscopy involves insertion of a scope into the joint space to allow for direct visualization of internal structures. Although this more invasive procedure allows surgeons to visualize the TMJ, it puts the patient at increased risk for the development of adhesions.

CT scans are more effective than conventional radiographs for the evaluation of fractures and tumors within the jaws; they can also be used to evaluate the bony structures of the TMJ. However, this modality exposes the patient to ionizing radiation and provides a less accurate representation of the soft tissues.

Digital subtraction angiography is used for visualization of the vascularity of the head and neck and not the soft tissues of the TMJ.

Tomography has been replaced by the more accurate CT scan and MRI, and is no longer used.
References
1. Mendes D, Jacobs JS. Traumatic deformities and reconstruction of the temporomandibular joint. In: Cohen M, ed. Mastery of Plastic and Reconstructive Surgery. Boston, Mass: Little, Brown & Co; 1994;2:1220-1229.
2. Westesson P. Magnetic resonance imaging of the temporomandibular joint. Oral Maxillofac Surg Clin North Am. 1992;4:183-206.
3. Zide BM. The temporomandibular joint. In: McCarthy JG, ed. Plastic Surgery. Philadelphia, Pa: WB Saunders Co; 1990;2:1475-1513.

A 24-year-old woman with maxillary hypoplasia is scheduled to undergo Le Fort I osteotomy. In order to protect the maxillary dentition, the osteotomy must be carried out above the dental apices. Which of the following maxillary teeth have the longest roots?

(A) Central incisors
(B) Lateral incisors
(C) Cuspids
(D) First bicuspids
(E) First molars

The correct response is Option C.

The cuspids, or canine teeth, have the longest roots in both the maxilla and mandible. The average length of a cuspid tooth from the tip of the root to the tip of the crown is 27 mm.

Knowledge of the length and position of the dental roots will help to prevent injury during Le Fort I osteotomy and placement of internal fixation during fracture reduction. The dentition can also be injured during stabilization of maxillary or mandibular fractures.
The average length of a molar tooth from the tip of the root to the tip of the crown is 24 mm. The length of the upper first molar is 21.3 mm.

The cuspid tooth is named for its single cusp; bicuspids have two cusps (buccal and lingual), and molars have three cusps (mesiobuccal, distobuccal, and mesiolingual). The average adult has two maxillary cuspids, four maxillary bicuspids, and six maxillary molars.

References
1. Bannister LH. Alimentary system. In: Bannister LH, et al, eds. Gray's Anatomy. New York, NY: Churchill Livingstone, Inc; 1995:1683-1812.
2. Ferraro JW. Oral anatomy. In: Ferraro JW, ed. Fundamentals of Maxillofacial Surgery. New York, NY: Springer-Verlag; 1997:127-157.

The Frankfort horizontal line passes through which of the following points?

(A) Gonion-pogonion
(B) Porion (tragion)-nasion
(C) Porion (tragion)-orbitale
(D) Sella-nasion
(E) SNA-SNB

PHOTO

Reproduced with permission of McCarthy JG, ed. Plastic Surgery. Philadelphia, Pa: WB Saunders Co; 1990;2:1190.
The correct response is Option C.

The Frankfort horizontal line passes through the porion (tragion) and orbitale. Anatomists in Germany in the last century determined this point to be a horizontal reference line for skull orientation. In addition, the sella-nasion line is used as a reference line; it is oriented at 6 to 8 degrees from the Frankfort horizontal. This reference line is used to define the length of the cranial base.

Maxillary relations can be evaluated using the anterior and posterior nasal spines, which can be used for maxillary orientation, as well as the SNA angle. This measures the position of point A (anterior maxilla) relative to the anterior cranial base (SN). A normal SNA angle is identified as 82 degrees  4 degrees. Decreased width indicates maxillary retrusion, while increased width indicates maxillary protrusion.

In contrast, the gonion-pogonion, which represents the mandibular plane, and the SNB angle can be used to evaluate mandibular relations. The SNB angle measures the position of point B (anterior mandible) relative to the anterior cranial base (SN). A normal angle is defined as 79 degrees  3 degrees. A wide angle denotes mandibular protrusion, while a narrow angle denotes inadequate mandibular development.

References
1. McCarthy JG. Introduction to plastic surgery. In: McCarthy JG, ed. Plastic Surgery. Philadelphia, Pa: WB Saunders Co; 1990;1:28.
2. McCarthy JG, Kawamoto H, Grayson BH. Surgery of the jaws. In: McCarthy JG, ed. Plastic Surgery. Philadelphia, Pa: WB Saunders Co; 1990;2:1190.
3. Zide B, Grayson B, McCarthy JG. Cephalometric analysis: part I. Plast Reconstr Surg. 1981;68:816.

A 27-year-old man has increased lower incisal show on physical examination. Three months ago he underwent horizontal sliding genioplasty with anterior displacement (8 mm) of the chin followed by fixation with three 24-gauge steel wires.

Which of the following is the most likely cause of his current findings?

(A) Excessive traction on the lower lip
(B) Inadequate fixation of the osteotomy segment
(C) Inadequate repair of the mentalis muscle
(D) Injury to the anterior belly of the digastric muscle
(E) Injury to the mental nerve

The correct response is Option C.

The most likely cause of the increased lower incisor show in this patient who underwent horizontal sliding genioplasty, as well as in patients who undergo placement of a chin implant, is improper or inadequate repair of the mentalis muscle. Whenever a lower buccal sulcus incision is made, the mentalis muscle is the first structure seen in the midline after the mucosa is divided. It must be divided properly in order to obtain exposure of the symphyseal mandible for either osteotomy or implant placement. It is essential to precisely close the muscle as a separate layer in order to prevent downward displacement of the lower lip as healing occurs.

Excessive traction on the lower lip usually has no long-term sequelae. Inadequate fixation of the osteotomy segment is an unlikely cause of this patient's increased incisal show because the three 24-gauge steel wires would have provided adequate fixation for horizontal genioplasty (plate and screw fixation could also have been used). Injury to the anterior belly of the digastric muscle would be more likely to result in necrosis of the advanced chin segment because this muscle supplies blood to the chin segment after horizontal osteotomy. Injury to the mental nerve would cause numbness of the lower lip on the side of the injury.

References
1. Guyuron B, Kadi JS. Problems following genioplasty: diagnosis and treatment. Clin Plast Surg. 1997;24:507-514.
2. McCarthy JG, Kawamoto H, Grayson BH, et al. Surgery of the jaws. In: McCarthy JG, ed. Plastic Surgery. Philadelphia, Pa: WB Saunders Co; 1990;2:1188-1474.

A 34-year-old woman desires an improved aesthetic appearance of the chin. On physical examination, she has a bony chin deformity characterized by sagittal deficiency and vertical mandibular excess. She has class I occlusion. Which of the following genioplasty procedures should be performed in this patient?

(A) Asymmetric genioplasty
(B) Interposition genioplasty
(C) Jumping genioplasty
(D) Sliding genioplasty
(E) Silastic implantation

The correct response is Option C.

This 34-year-old woman who has a bony chin deformity should undergo jumping genioplasty. In this procedure, the transverse osteotomy is performed initially, decreasing the vertical dimension of the chin; following this, the osteotomized segment is transposed anteriorly with its attached suprahyoid musculature to augment the sagittal deficiency.

Asymmetric genioplasty involves adjusting the lines of osteotomy in multiple vectors in order to correct a misshapen chin. Interposition genioplasty can be performed to increase the vertical and sagittal dimensions of the chin; autogenous bone or hydroxyapatite can be grafted to the osteotomy site. Although sliding genioplasty can be used for correction of a sagittal deficiency or an excessively large chin, the change in vertical chin dimension seen following this procedure is only minimal. Silastic implantation will correct the sagittal deficiency but not the vertical excess.

References
1. McCarthy JG, Kawamoto H, Grayson BH, et al. Surgery of the jaws. In: McCarthy JG, ed. Plastic Surgery. Philadelphia, Pa: WB Saunders Co; 1990;2:1188-1474.
2. McCarthy JG, Ruff GL, Zide BM. A surgical system for the correction of bony chin deformity. Clin Plast Surg. 1991;18:139-151.