STJ axis transverse plane

Why is the (STJ) axis transverse plane projection(?) a line? why not a zig-zag as each additional joint is ‘actively engaged’ or a set of curves as additional joints are included and the tension from the restraining structures (proximally and distally) changes slightly as the assessor’s thumb moves distally?

Why is the (STJ) axis transverse plane projection(?) a line? why not a zig-zag as each additional joint is ‘actively engaged’ or a set of curves as additional joints are included and the tension from the restraining structures (proximally and distally) changes slightly as the assessor’s thumb moves distally?

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STJ axis transverse plane