3-D Cephalometric Soft Tissue Landmarks

Abstract

According to the ancient Greeks, facial beauty derives from harmony and balance of the proportions of the individual components of the face. In daily clinical practice, orthodontists and maxillofacial and plastic surgeons often decide on what is beautiful and bal-anced based on anthroscopy (from the Greek " anthro-pos " , human and " copein " , examine) or visual assess-ment. Because the examiner's judgement is inherently influenced by his or her aesthetic perception and per-sonal experience, anthroscopy remains highly subjec-tive. Anthropometry (" metron " , measure), in contrast, is the biological science of studying and measuring human physical dimensions. Anthropometry of the head and face was pioneered by Aleš Hrdlička (1869–1943) but extensively developed and popular-ized by Leslie G. Farkas, whose clinical and scientific work on direct and indirect (photogrammetry) an-thropometry has made a major contribution on today's clinical assessment of the head and face. Potential sources of error in anthropometry are incorrect land-mark definition, improper use of measuring equip-ment and/or an inadequate measuring method. The major shortcoming of direct anthropometry is that it necessitates a wide range of measurement tools (slid-ing and spreading callipers, soft measuring tape, etc.), demands great experience on the part of the investiga-tor and is very time-consuming. The disadvantages of indirect anthropometry or photogrammetry are diffi-culties in standardization of photographs, inaccuracy in definition of bone-related soft tissue landmarks (e.g. soft tissue orbitale, soft tissue gonion and zygion) and its two-dimensional character. Advances in both computer hardware and software technology led to 3-D anthropometric methods such as laser surface scanning, stereo photogrammetry, CT and MRI (Chap. 10). Recently a new innovative voxel-based method of 3-D cephalometry was developed by our research group. CT based 3-D cephalometry of soft tissues has the advantage that bone-related landmarks can be defined in a more accurate and reliable way be-cause the 3-D virtual scene approach allows visualiza-tion of the underlying hard tissues. Therefore, no sur-face analogues of bony landmarks are necessary. More-over, analysis bias caused by improper use of measur-ing equipment or inadequate measurement technique is decreased because of standardized virtual position-ing of the skull with the set-up of a 3-D cephalometric reference system (Chap. 3) and the automatic genera-tion of measurements (Chap. 7). The important dis-advantages of spiral-CT based 3-D cephalometry are the radiation dose (Chap. 1) and the horizontal posi-tion of the head during scanning, which has an impact on the facial soft tissue mask due to gravity. These problems will be resolved in the near future with the application of cone-beam CT in clinical routine be-cause of its low radiation dose and its ability to scan the patient in the vertical rest position (Chap. 1). Cone-beam-CT-based 3-D cephalometry will allow the gen-eration of 3-D cephalometric reference data including hard and soft tissue data and bone–soft tissue move-ment ratios. As already mentioned in Chap. 4, landmarks situat-ed on the skin are referred to as 3-D cephalometric soft tissue landmarks and their abbreviations (symbols) are marked in lower-case letters as in anthropometry and conventional soft tissue cephalometry. This chap-ter offers " step-by-step " guidelines for precise defini-tion of such landmarks. Sometimes the anthropo-metric definitions had to be modified. A total of 28 landmarks with regard to the facial soft tissue units (forehead, nasal, periorbital, midface and mandible) are described in detail. Additionally, ear-and head-related landmarks are listed at the end of the chapter because they have not yet been validated.An important shortcoming of CT-based 3-D cephalometry of soft tissues remains improper or impossible identification of soft tissue landmarks that are related to the hair (trichion, superciliare, frontotemporale) or eye-lids (palpebrale superius, palpebrale inferius). Registration of the natural texture of the face by means of 3-D pho-tographic techniques could be a solution and is there-fore an interesting topic for future research (Chap. 10).