Introduction
William B. Downs introduced a method of recording the skeletal and denture (dental) pattern to measure facial form on a cephalogram in 1948. Although several analyses have been introduced since then, Downs’ analysis is still in use. It gives reasonable information on the subject’s lateral skeletal and denture profile. His analysis was not presented as the basis for an orthodontic treatment goal. Instead, it was a method for examining and quantifying the relationships of the skeletal components of the face, that is, the maxilla and mandible and its dentition, essentially the incisors. ,
Downs said, ‘The ten figures used in the appraisal do describe skeletal and denture relationships, but single readings are not so important; what counts is the manner in which they all fit together and their correlation with type, function, and esthetics’.
Original sample and the primary reference plane
Downs’ norms were based on 20 Caucasian subjects of age range 12–17 years of both sexes. All individuals possessed clinically excellent occlusion. The Frankfort horizontal plane was used as a reference plane because of its clinical visibility and familiarity with clinicians.
Basis of downs’ analysis
Downs considered the sagittal position of the ‘chin’ of greater importance in determining the four basic facial types.
Downs facial types are:
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Orthognathic
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Retrognathic
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Prognathic
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Prognathism
He felt that the subjects with malocclusion, whose skeletal pattern variation is within the range of his norms, could be treated to these standards. However, subjects whose skeletal and dental patterns were severely beyond the range could not be treated to a harmoniously balanced face within Downs’ deviation range. Although facial patterns varied from orthognathic to a mild state prognathic, the face was still considered harmonious and balanced.
Downs’ analysis provides information by which we can determine whether the individual’s pattern shows comparatively harmonious relations and whether dysplasia in a person is in the facial skeleton, the dentition or both.
Reference planes
Downs used the following reference planes ( Figs 22.1–22.3 ):
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1.
Facial plane: A line is drawn from nasion (N) through pogonion (Pg).
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2.
Mandibular plane: A tangent from the gonion and the lowest point of the symphysis, menton (Me).
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3.
Occlusal plane: It is drawn by bisecting the overlapping cusps of the first molars and the incisal overbite. In the cases in which the incisors are grossly malposed, Downs recommended drawing the occlusal plane through overlapping cusps of the premolars and the molars.
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4.
Y-axis: It is formed by drawing a line from sella turcica (S) to gnathion (Gn).
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5.
FH plane: It is drawn using the superior border of machine porion (Po) and orbitale (Or).
Skeletal reference planes and variables according to Downs.
Skeletal reference planes: (1) F-H plane Po-Or; (2) facial plane N-Pg; (3) Y-axis plane S-Gn; (4) mandibular plane Go-Me; (5) A-B plane. It also shows (i) facial angle; (ii) mandibular plane angle; (iii) Y-axis angle.
Skeletal reference planes and variables according to Downs.
Skeletal angular variables: (i) Angle of convexity; (ii) A-B plane angle.
Skeletal pattern ( Figs 22.1 and 22.2 ; Table 22.1 )
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1.
Facial angle : It is measured as an inferior posterior angle at the intersection of the facial plane with the Frankfurt horizontal plane; it essentially indicates the degree of recession or protrusion of the mandible in relation to the upper face at the point where Frankfurt horizontal plane (FHP) is related to the facial plane. An increase in facial angle is suggestive of chin protrusion. Mean 87.8 degrees, range 82–95 degrees ( Fig. 22.1 ).
TABLE 22.1
Downs’ cephalometric norms
Caucasians North Indians , South Indians (Keralites) Koreans Downs (1948, 1956) Kharbanda (1989) Roy (1987) Park et al. (1989) 14.5 ± 2.5 yrs 18–25 yrs 17–25 yrs 18 yrs 10 M, 10 F 25 M, 23 F 50 F 35 M 45 F S. no. Parameters degrees Norm Range Mean Range Mean Range Mean ± SD Skeletal pattern 1 Facial angle 87.8 82–95 84.09 77–90 85 76–93 89.1 ± 2.4 89.3 ± 3.0 2 Angle of convexity 0 −8.5 to 10 4.3 −14 to 12 7.5 −4.5 to 18 3.7 ± 4.4 3.6 ± 4.4 3 A-B plane to N-Pg angle −4.8 0 to −9 −6 −11 to 6 −6.7 −13.5 to 5 −4.9 ± 2.6 −4.5 ± 2.7 4 Mandibular plane angle 21.9 17–28 24.22 13–35 26.7 16.5–36.5 23 ± 4.6 23.4 ± 3.9 5 Y-axis angle 59.4 53–66 62.78 55–70.5 62 53–70 60.6 ± 2.2 60.8 ± 2.9 Dental pattern 6 Occlusal plane angle 9.3 1.5–14 10.67 3–28 11 0.5–17 7.1 ± 3.0 7.7 ± 3.3 7 Inter-incisal angle 135.4 130–150.5 124 112–144 119 113–148.5 124.9 ± 7.8 128.2 ± 7.3 8 L1-MP 1.4 −8.5 to 7 11 4–24 13.8 0–27 6.8 ± 6.2 4.3 ± 5.7 9 L1-OP 14.5 3.5–20 14.5 11–36.5 28.8 9.5–42 22.9 ± 4.9 20.8 ± 5.4 10 UI to A-Pg line (mm) 2.7 −1 to 5 6.41 3.5–10 8.3 0–14.5 7.6 ± 2.2 7.0 ± 1.7 
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