Radiographic appearance of the odontoid lateral mass interspace in the occipitoatlantoaxial complex

Study Design. This Is an anatomic study in which the odontoid lateral mass interspace is evaluated ifldio- graphically in various positions of uppor cervical spire rotation.Objectives. The objectives are to Determine whether odontoid lateral mass interspace asymmetry Exists In the ligamenlojslv intact cervical spire and to define odontoid lateral mass Inlerspact? behavior during ationtoaxia: Rotation.Summary of Background Data. Controversy exists regarding the behevior of the odontoid lateral mass interspace during atlantoaxial rotation. The meaning of interspace asymmetry varies depending on the author and the method of study. Atlas fractures account for 7Va of cervical sp ne fractures and arc frequently associated with other cervical fractures, so anunderstanding of the radioeraphic anatomy is crucial lo tne evaluation of these patients.Methods. Ten human cadaveric ccrvlcal spines were dissected of all but ligamentous soft tissue and were mounted and prepared for radiographic study in neu¬tral position and n varying degress of right and left rotation. Radiographs were evaluated for odontoid lateral mass interspace asymmetry in neutral and in rotated positions, and data were analyzed.Results. Measurable asymmalry could be shown when comparing neutral positions and when comparing rotated with neutral positions. A statistically significant difference could not be proven in this asymmetry, although clinical significance is suggested by the ana ysls of the data.Conclusion. Measurable asymmetry chh be present in the neutrally positioned, ligamentously Intact atlanto¬axial complex and is not ncccssarily Indicative o; instability. This agrees with the concept of the "neutral zone" aid described by White Hnd Panjabi In Clinical Biomezhanics of the Spine-1 There is a trend for increasing odontoid lateral mass interspace on the side to which the head is rotated, and this odontoid lateral mass interspace is measurably different from the contralateral ?donto id lateral mass Inlcrspaw, indicating that odontoid 'alaral mass interspace asymmetry Is not a good indicator for cervical instability in the otherwise asymptomatic individual.