The effect of posture change on the position of the skin marks for the transverse horizontal axis K. H. O’Boyle, B Dent SC,~ F. D. Gallagher, B Dent SC,~ M. O’Sullivan, B Dent SC, MSC,~ and W. E. McDevitt, BDS, PhDd University of Texas Health Science Center, San Antonio, Tex., Eastman Dental Hospital, London, United Kingdom, and School of Dental Science, Trinity College, Dublin, Ireland Marks are placed on facial skin in clinical dentistry to indicate the position of more deeply placed landmarks or reference points. In this study the movement of the transverse horizontal axis skin points overlying the craniomandihular articulation were observed and quantified. The time taken for the skin displacement to occur when the posture changed between upright and supine was also studied. The extent of displacement of the skin point was approximately 3 mm in the sagittal plane and 2 mm in the frontal plane when the posture was changed from upright to supine and vice versa. The displacement was complete after 30 seconds in 95% of subjects. The direction of the displacement was primarily cephalad but with a dorsal component of more than 10 degrees in 87% of subjects. The extent of the movement in the sagittal and frontal planes was correlated. There was no gender difference for the skin displacement. Awareness by clinicians of the extent and direction of such facial skin movements can help to prevent errors. (J PROSTHET DENT 1996;75:545-51.1 I n dentistry and the related surgical sciences mark- ings are placed on the facial skin to indicate the positions of more deeply placed landmarks or reference points for various clinical procedures. The skin position over the un- derlying structures may change with a change in posture. There are few references in the dental literature to this phenomen0n.l Clinical procedures are carried out with the patient in the upright and supine positions. If skin mark- ings are placed with the patient in one posture and a mea- surement or incision is made with the patient in another posture, the skin movement could be a source of error. Dentists most commonly place marks on the skin in the region of the craniomandibular articulation (CMA). The purpose of this study was to investigate movements of the skin in the CMA region. The objectives of this study were to establish whether a change between upright and supine posture results in a movement of the skin overlying the craniomandibular articulation, to determine the direc- tion of the movement, and to quantify the extent of the skin movement if it occurs. MATERIAL AND METHODS Forty subjects were chosen from a sampling frame of 55 subjects who already had the transverse horizontal axis (THA) skin point marks tattooed in position. All subjects (15 women and 25 men) were dental students chosen aGraduate Student, Prosthodontics Division, University of Texas Health Science Center. bGraduate Student, Endodontics, Eastman Dental Hospital. CSenior Registrar in Restorative Dentistry, School of Dental Sci- ence, Trinity College. dHead of Department of Restorative Dentistry, School of Dental Science, Trinity College. MAY 1996 sequentially according to their class subdivisions for small- group teaching. Their ages ranged from 20 to 30 years. They were incorporated into the sample on the basis of their voluntary agreement and their availability at the times set for carrying out the field work. The facial tissues functioned normally and were free from disease. Posture position The Systematica 1060T dental chair (Kavo Dental Ltd., Amersham, United Kingdom) was used for all subjects. The upright and the reclined positions were prepro- grammed and standardized to minimize variation in the subject position. The unsupported upright body position would have been the ideal position to use when measure- ments were made with the subject upright. It was found that without support the head tended to move slightly during the measuring process, which caused inaccurate measurement. Hence the most upright position of the chair and its headrest was defined as the upright position. This position stationed the subject comfortably straight upright with the Frankfort plane approximately parallel to the floor. The reclining position of the chair was set so that the line from the dorsum of the skull to the dorsum of the hips was parallel to the floor. The subject was comfortable in the supine position with the Frankfort plane approximately perpendicular to the floor. Instrumentation A tattoo mark that indicated the THA on the right side of the face was the point observed for movement (Fig. 1). A modified Almore axis locator (Almore International Cor- poration, Portland, Ore.) was used to identify and measure the skin movements in the cephalad-caudal and dorsal- ventral dimensions of the sagittal plane (Fig. 2). THE JOURNAL OF PROSTHETIC DENTISTRY 545