Björk-Jarabak Analysis

The Björk-Jarabak analysis combines Björk's cranial base and mandibular morphology research with the polygon method popularized by Joseph Jarabak in 1972, who incorporated additional refinements from Steiner, Tweed, and Ricketts. Ronald Roth, who trained under Jarabak, later produced the widely used Roth-Jarabak variant.

Rather than focusing primarily on where the jaws sit relative to each other, this analysis is built around predicting facial growth direction — whether a patient's face is likely to continue growing in a way that opens or closes the bite — using a simple four-point polygon traced from the cranial base to the chin.

What It Evaluates

The Björk-Jarabak analysis evaluates the skeletal growth pattern of the face, particularly its vertical direction (horizontal/hypodivergent versus vertical/hyperdivergent growth), by examining the geometry of the cranial base, the ramus, and the relationship between posterior and anterior facial height.

Learn more: Jarabak Cephalometric Analysis — CephX

Key Measurements

  • Saddle angle (N-S-Ar) — describes the cranial base's shape; a smaller angle tends to place the glenoid fossa and condyle more forward and downward.
  • Articular angle (S-Ar-Go) — sometimes called the cranial bending angle; describes the inclination of the ramus relative to the cranial base.
  • Gonial angle (Ar-Go-Me) — describes the divergence of the ramus and mandibular body, and is further split into a superior gonial angle (Ar-Go-N) and inferior gonial angle (N-Go-Me).
  • Sum of Björk's polygon angles (saddle + articular + gonial) — normal average is approximately 396° ± 6°; deviations point toward hyper- or hypodivergent growth tendencies.
  • Posterior facial height (S-Go) and anterior facial height (N-Me) — linear measurements whose ratio (S-Go/N-Me, expressed as a percentage) is the core vertical growth indicator of the analysis.

Learn more: Evaluation of skeletal variations and establishment of Cephalometric Norms in Saudi Sub Population using Bjork Jarabak's analysis — PMC

Clinical Interpretation

The posterior-to-anterior facial height ratio classifies the patient's growth pattern. Using the Roth-Jarabak thresholds: a ratio of 54%–58% suggests a clockwise (hyperdivergent) growth tendency, 59%–63% suggests a neutral growth pattern, and 64%–80% suggests a counterclockwise (hypodivergent) growth tendency.

A larger gonial angle combined with a lower facial height ratio typically indicates a downward-and-backward mandibular growth rotation — a pattern associated with a longer lower face, a tendency toward anterior open bite, and less favorable anchorage during treatment. Conversely, a smaller gonial angle and a higher ratio indicate a more forward, upward mandibular rotation, associated with a deeper bite tendency and typically better anchorage control. Because the polygon is built almost entirely from cranial base and ramus geometry, it is particularly useful for evaluating growth potential in growing patients, alongside sagittal analyses such as ANB or Wits.

Learn more: Gonial Angle by Lateral Cephalogram in Orthodontic Patients of a Tertiary Care Hospital: A Descriptive Cross-sectional Study — PMC

References