Cobb method or Harrison posterior tangent method

Study Design. Thirty lateral cervical radiographs were digitized twice by t hree examiners to compare reliability of the Cobb and posterior tangent met hods. Objectives. To determine the reliability of the Cobb and Harrison posterior tangent methods and to compare and contrast these two methods. Summary of Background Data. Cobb's method is commonly used on both anteropo sterior and lateral radiographs, whereas the posterior tangent method is no t widely used. Methods. A blind, repeated-measures design was used. Thirty lateral cervica l radiographs were digitized twice by each of three examiners. To evaluate reliability of determining global and segmental alignment, vertebral bodies of C1-T1 were digitized. Angles created were two global two-line Cobb angl es (C1-C7 and C2-C7), segmental Cobb angles from C2 to C7, and posterior ta ngents at drawn at each posterior vertebral body margin. Cobb's method and the posterior tangent method are compared and contrasted with these data. Results. Of 34 intraclass and interclass correlation coefficients, 28 were in the high range (>0.7), and 6 were in the good range (0.6-0.7). The Cobb method at C1-C7 overestimated the cervical curvature (- 54 degrees) and, at C2-C7 it underestimated the cervical curve (-17 degrees), whereas the post erior tangents were the slopes along the curve (-26 degrees from C2 to C7). The inferior vertebral endplates and posterior body margins did not meet a t 90 degrees (C2: 105 degrees +/- 5.2 degrees, C3:99.7 degrees +/- 5.2 degr ees. C4: 99.9 degrees +/- 5.8 degrees, C5: 96.1 degrees +/- 4.5 degrees, C6 : 97.0 degrees +/- 3.8 degrees, C7: 95.4 degrees +/- 4.1 degrees), which ca used the segmental Cobb angles to underestimate lordosis at C2-C3, C4-C5, a nd C6-C7. Conclusions. Although both methods are reliable with the majority of correl ation coefficients in the high range (ICC > 0.7), from the literature, the posterior tangent method has a smaller standard error of measurement than f our-line Cobb methods, Global Cobb angles compare only the ends of the cerv ical curve and cannot delineate what happens to the curve internally. Poste rior tangents are the slopes along the curve and can provide an analysis of any buckled areas of the cervical curve. The posterior tangent method is p art of an engineering analysis (first derivative) and more accurately depic ts cervical curvature than the Cobb method.