What Is the Björk-Jarabak Analysis?
The Björk-Jarabak analysis synthesises the independent work of Arne Björk (a Danish-Swedish orthodontist who studied mandibular implant markers to observe actual growth rotation) and Jaroslav Jarabak (who formalised the angular and linear measurements into a clinically applicable cephalometric system). The analysis focuses primarily on the vertical dimension and growth pattern — the features that most influence the stability of orthodontic treatment and the long-term prognosis for malocclusion correction.
Its central contribution is the concept of mandibular growth rotation. Björk's implant studies demonstrated that the mandible does not simply grow downward and forward as a unit — it rotates about a condylar centre of rotation, and the direction of that rotation varies dramatically between individuals. A patient growing with strong anterior rotation will tend to close their bite over time; a patient growing with posterior rotation will tend to open it. These tendencies have profound implications for deep bite management, open bite prognosis, and the stability of any achieved occlusal result.
For context on how growth rotation fits into the broader diagnostic picture alongside angular and linear analyses, see the cephalometric analysis guide. For the complementary linear measurement of lower facial height, see McNamara analysis.
Björk-Jarabak Measurements — Norms and Interpretation
| Measurement | Mean Norm | SD | Clinical Range | High Value | Low Value |
|---|---|---|---|---|---|
| Saddle Angle (N-S-Ar) | 123° | ± 5° | 118° – 128° | Post. condylar position | Ant. condylar position |
| Articular Angle (S-Ar-Go) | 143° | ± 6° | 137° – 149° | Posterior ramus inclination | Anterior ramus inclination |
| Gonial Angle (Ar-Go-Me) | 130° | ± 7° | 123° – 137° | Open / hyperdivergent | Closed / hypodivergent |
| Upper Gonial Angle (Ar-Go-N) | 52° – 55° | — | 47° – 60° | Ramus flared forward | Ramus inclined posteriorly |
| Lower Gonial Angle (N-Go-Me) | 70° – 75° | — | 65° – 80° | Steep mandibular body | Flat mandibular body |
| Sum of Posterior Angles | 396° | ± 6° | 390° – 402° | >402° = Posterior rotation | <390° = Anterior rotation |
| Post. Facial Height (S-Go) | ~80 mm | ± 5 mm | Sex/age dependent | Long posterior face | Short posterior face |
| Ant. Facial Height (N-Me) | ~122 mm | ± 6 mm | Sex/age dependent | Long anterior face | Short anterior face |
| PFH/AFH Ratio (S-Go/N-Me) | 62% – 65% | — | 59% – 68% | >65% = Anterior rotation | <62% = Posterior rotation |
Norms from Jarabak & Fizzell (1972) and Björk's original growth studies. PFH/AFH ratio norms reflect white Western European and North American samples.
The sum of posterior angles is the single most reliable predictor of growth rotation type. A sum below 390° indicates anterior rotation (deep bite tendency, favourable for Class II correction in growing patients). A sum above 402° indicates posterior rotation (open bite tendency, less favourable prognosis for Class II or open bite correction). A sum between 390° and 402° is neutral.
Landmarks Required for Björk-Jarabak Analysis
Articulare vs Condylion: The Björk-Jarabak analysis uses Articulare (Ar) — a constructed landmark at the intersection of the ramus posterior border and cranial base — rather than the anatomical Condylion (Co) used in McNamara analysis. Articulare is more consistently identifiable on standard lateral cephalograms where the condylar outline may be obscured by superimposition.
Clinical Interpretation of Björk-Jarabak Analysis
Anterior vs Posterior Growth Rotation
An anterior rotator (sum < 390°, PFH/AFH > 65%) grows with the mandibular symphysis rotating forward and upward relative to the anterior cranial base. This rotation tends to close the bite, increase the overbite, and may convert a Class II malocclusion into a more favourable skeletal relationship over time. Anterior rotators are generally good candidates for functional appliances and have better long-term stability for deep bite correction.
A posterior rotator (sum > 402°, PFH/AFH < 62%) grows with the symphysis rotating downward and backward. The bite tends to open over time, the mandibular plane angle steepens, and Class II malocclusions become more severe with growth. Posterior rotators are significantly more challenging to treat, particularly for open bite correction, and relapse rates are higher. Treatment decisions in posterior rotators must account for the ongoing negative growth vector.
The Gonial Angle Subdivision
The full gonial angle (Ar-Go-Me, norm 130°) can be subdivided by a line from Go to Nasion. The upper gonial angle (Ar-Go-N, norm 52–55°) reflects the relationship between the ramus and the anterior cranial base. The lower gonial angle (N-Go-Me, norm 70–75°) reflects the angle of the mandibular body. In anterior rotators, the upper gonial angle tends to be larger (the ramus is relatively more anteriorly inclined); in posterior rotators, the lower gonial angle tends to be larger (the mandibular body is more steeply inclined).
Using Björk-Jarabak Alongside Other Analyses
The Björk-Jarabak analysis complements the vertical measurements in McNamara analysis (which provides the lower facial height in absolute millimetres), the vertical component of Downs analysis (mandibular plane angle and Y-axis), and the vertical growth indicators in Ricketts analysis (facial axis and mandibular plane angle). When multiple analyses consistently classify a patient as hyperdivergent, the growth rotation classification from Björk-Jarabak adds the prognostic dimension: not just how much vertical excess exists now, but which direction it is heading.
Run Björk-Jarabak Analysis Free in BCeph
BCeph includes a complete Björk-Jarabak module computing all three posterior angles, their sum, the upper and lower gonial angle subdivisions, S-Go, N-Me, and the PFH/AFH ratio. Results are displayed with norm ranges and growth rotation classification (anterior, neutral, or posterior).
The Articulare landmark used in Björk-Jarabak overlaps with the Ricketts analysis in BCeph, and the Nasion, Sella, Gonion, and Menton landmarks are shared across most analysis modules. Running a full diagnostic session across all BCeph modules requires a single unified landmark placement workflow.