Definition of Centric Relation

The definition of CR remains somewhat controversial, and it is frequently misstated.46–49 In 1987, the definition of CR underwent a change in terminology from being considered a mandibular position in which the condyles are seated in the most “posterior superior” position to one in which the condyles are placed in the most “superior anterior” position in the glenoid fossa.^45,46 For analytic model planning in preparation for orthognathic surgery, defining the exact position of each condylar head during mandibular protrusion and lateral excursions is not critical. However, it is essential to locate a mandibular position that is reproducible with the teeth in occlusion. This must be obtained without the assistance of the patient (i.e., with fully relaxed masticatory muscles), and it needs to be reliably accomplished both preoperatively and intraoperatively. This position must also be transferable to the articulator in the laboratory for model planning to be accomplished accurately. An erroneous and inconsistent recording of CR will lead to the malpositioning of the maxilla or mandible during surgery.

A survey study conducted by Truitt and colleagues confirmed that 98% of oral and maxillofacial surgeons define CR as the most posterior superior seating of the condyle in the fossa.⁵¹ Interestingly, a majority of the orthodontists surveyed agree with the surgeons’ definition of CR, but there was less consistency.^22,41 According to the survey, surgeons and orthodontists do agree that, in preparation for orthognathic surgery, analytic model planning requires that casts of the teeth be mounted in CR on a semi-adjustable articulator.

Capturing Centric Relation

Hellsing and McWilliam used a subtraction technique to assess the repeatability of CR that is recorded with a one-handed push-back technique.²¹ They found that, with the use of this technique, the mandibular condyles can be seated in a reproducible position and thus be well suited for the registration of CR. Campos and colleagues compared the swallowing technique with the chin-point guidance technique when the patient was in both upright and supine body positions.^3,5,9,15,21 Both techniques were found to establish physiologic CR in a reproducible manner. Interestingly, when the chin-point guidance method was used, the work of these authors suggested that reliability was greater with the patient in the upright, seated position as compared with the supine position.

Simon and Nicholls compared the chin-point guidance, chin-point guidance with ramus support, and bimanual manipulation techniques.⁴³ They found that there were no significant differences in the range of mandibular positions that were observed. Interestingly, their study patients did not include individuals with jaw deformities. It is known that patients with specific patterns of jaw deformity (e.g., Class II retrognathism) are more likely to involuntarily posture the mandible forward during the swallowing technique.

When a reliable CR can be obtained, we find the chin-point guidance technique—when used both preoperatively and intraoperatively by the same clinician—to be both reliable and convenient for the orthognathic surgery patient. This is the method that has been adopted by this author, and it has been used successfully for orthognathic patients that have a reliable CR. By using one hand to apply pressure to the subject’s chin and the other hand to apply opposing pressure to the occiput, the clinician is able to apply posterior and superior “vectored” force to the anterior mandible. This will seat the condyles in a superior-anterior location within each glenoid fossa (i.e., CR).

Unstable (Unreliable) Centric Relation

Alternative methods to the standard analytic model planning approach to bimaxillary orthognathic surgery have been described in an attempt to improve the accuracy and predictability of outcomes.8,11,34,37,50,52 Recently, surgeons have also begun looking to computer-aided design and computer-aided modeling software to assist with the planning and implementation of orthognathic procedures (see Chapter 13).^{3,4,7,14,16,18,19,20,29,30,31,38,42,44,54,55,56}

The subset of patients in which obtaining an accurate CR is not feasible are well suited for a “mandible-first” approach. Common clinical examples of these patients include the following: 1) those with specific congenital anomalies (e.g., hemifacial microsomia with Kaban Type IIB or III malformations) (Fig. 14-2 and 14-3) 2) those with specific types of trauma (e.g., after condylar fracture with resorption or after ankylosis release with condylar resection) 3) those with specific pathology (e.g., after condylar resection for tumor) and 4) those with the inability to cooperate in the office setting (e.g., those with neuropsychiatric disorders that involve uncontrolled masticatory muscle spasticity).

A mandible-first approach to analytic model planning is also indicated in unique clinical circumstances in which significant posterior maxillary lengthening is planned to the extent that the clockwise rotation of the mandible will require the translational movement of the condyles in the glenoid fossa during the use of the intermediate splint (Fig. 14-4). Fortunately, these cases are not common; very few patients require the actual lengthening of the posterior maxillary height. Furthermore, those who do generally either require the closure of baseline posterior open bites (i.e., cases that involve the failure of the posterior dentition to erupt) or are undergoing simultanous advancement. For example, the advantage of the counterclockwise rotation of the maxillary and mandibular planes (i.e., pitch orientation) is frequently seen in the clinical setting of a long face growth pattern. This counter clockwise rotation is generally accomplished via the differential intrusion of the maxilla (i.e., anterior more so than posterior). This does not require the lengthening of the posterior maxillary height or the overall posterior facial height (see Chapter 21).