Orthognathic Surgery
1.13.1 Discuss common cephalometric measurements and what they tell you.
The purpose of cephalometric measurements is to determine quantitative deviations from the normal dental/facial morphology. There are many different systems, i.e. Down’s, Steiner’s, Tweed’s, McNamara’s, etc. No need to remember for HNS perspective. Just remember 2 main approaches. 1 = Metric approach with use of selected linear and angular measures with reference to stable anatomic landmarks (Maxilla, mandible, etc.) 2 = Graphic approach where you overlay an individual’s x-ray over a template and see what’s off. Nowadays, number 2 is used the most. Basically take x-ray and see what’s off compared to normal.
Metric method: Everything is relative to 2 main axes: Frankfort horizontal, and 90 degrees to that (true vertical).
There are certain angles and lengths for proportions, no need to remember.
Graphic Overlay Method
1.13.2 Tell us what Frankfort’s horizontal and the aesthetic line of Ricketts are and name all the points on an included diagram. If your SNA is increased, what does that mean? If your SNB is increased, does this mean that you are immensely intelligent?
Franfort horizontal line= line drawn from top of EAC (porion) to infraorbital rim (orbitale).
Aesthetic line of Ricketts= line drawn from nasal tip (pronasale) to chin (pogonion)
No included diagram, but:
S= sella
N= nasion
A= point of deepest concavity on maxilla (when viewed from lateral)
B= point of deepest concavity on mandible (when viewed from lateral)
Standard values for white caucasians (UK study):
SNA = 81 degrees (±3)
SNB = 79 degrees(±3)
SNA indicates whether or not the maxilla is normal, prognathic, or retrognathic.
SNB indicates whether or not the mandible is normal, prognathic, or retrognathic. Increased SNB
ANB determines class of occlusion
1.13.3 Make a table: symptoms, signs, occlusions, treatment, and cephalometric analysis of Long Face syndrome, maxillary horizontal deficiency, mandibular horizontal excess, mandibular horizontal deficiency.
1.13.4 Tell us how to surgically do the following operation-what complications might you expect? LeFort I and bi-sagittal split osteotomies.
Incredible article that would do severe injustice to try and summarize. So I’ll list the article for you and discuss
Patel PK, Novia MV. The surgical tools: the LeFort I, bilateral sagittal split osteotomy of the mandible, and the osseous genioplasty. Clinics in plastic surgery. 34(3):447-75. 2007. [pubmed]
1.13.5 What soft tissue response do you expect from skeletal changes?
No good measurement system other than 3D reconstruction. Generally less than 1:1 ratio.
1.13.6 When would you use distraction osteogenesis? Does it work? What are the expected complications? Comment on the following articles.
Distraction osteogenesis is indicated in the following cases:
Severe retrognathia associated with a syndrome (eg, Pierre Robin syndrome, Treacher Collins syndrome, Goldenhar syndrome), especially in infants and children who are not candidates for traditional osteotomies
Patients who have unilateral hypoplasia of the mandible (eg, hemifacial microsomia)
Nonsyndromic mandibular hypoplasia associated with a dental malocclusion (especially if the advancement exceeds the capabilities of a traditional osteotomy or if the patient is hesitant to undergo a bone graft harvest with the associated morbidity)
Mandibular transverse deficiency associated with a dental malocclusion and dental crowding
Patients with severe OSA (respiratory disturbance index [RDI] >60) and patients who are obese (body mass index [BMI] >28).
Mandibular hypoplasia due to trauma and/or ankylosis of the temporomandibular joint
Mandibular continuity defects resulting from excision of tumors and/or aggressive developmental cysts
The optimal rate of distraction is 1 mm per day. A rate of distraction of 0.5 mm or less per day may cause premature consolidation of the bone. Distraction of more than 1.5 mm per day may cause delayed ossification or pseudoarthrosis due to local ischemia in the interzone.
Complications include the following: fibrous nonunion or premature union of bone, infection that may hinder osteogenesis, noncompliant patient with treatment failure, scarring of the skin with external devices, hardware failure, and malocclusion because of improper vectors.
McCarthy JG, Katzen JT, Hopper R, Grayson BH. The first decade of mandibular distraction: lessons we have learned. Plast Reconstr Surg. 2002 Dec;110(7):1704-13. PMID:12447053
Imola MJ, Hamlar DD, Thatcher G, Chowdhury K. The versatility of distraction osteogenesis in craniofacial surgery. Arch Facial Plast Surg. 2002 Jan-Mar;4(1):8-1. PMID:11843671