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The oft-forgotten sagittal projection is now extremely

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useful to evaluate the angles and displacements of the

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linear structures of the wrist and hand, including the

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metacarpal, the capitate, the lunate, and the radius.

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Let's imagine for a moment that we're going to draw

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in our lunate, which is not seen in this image.

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The lunate normally would sit like this, and

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straddling atop of it would be the capitate.

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And on top of the capitate would be a metacarpal.

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So they would be linear.

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But what happens if our lunate

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decides to face palmar, or forward?

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This.

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Which is known as volar

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intercalary segmental instability.

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If we were to draw a line through

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its center, perpendicular to this

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axis, the line might look like this.

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On the other hand, if our lunate was dorsal

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facing, let's draw it again in yellow, dorsal

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facing, facing the dorsum of the wrist, then

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our line would look something like this.

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So now let's take our scaphoid line, which

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bisects the scaphoid parallel to its long axis,

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and compare it to the line that is perpendicular

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to the lunate, the so-called lunate line,

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and they are virtually parallel to one another.

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So the scapholunate angle, in other words, this

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line and this line, look something like this.

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Whoops, gotta get my pen to work.

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Look something like this.

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In other words, the angle is closed.

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What's the normal angle?

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What's the normal angle between the

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scaphoid and a normal perpendicular lunate?

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It's about 30 to 60 degrees, closer to 60.

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But when there's VISI, volar intercalary

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segmental instability, this angle now closes.

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The orange line and the yellow line

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are parallel to one another virtually.

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What about in DISI?

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In DISI, now, our lunate line goes this way.

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And the angle between our lunate line and our scaphoid

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line, which I'll draw in blue, is markedly increased.

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So let's summarize.

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In the normal setting, the scapholunate

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angle is 30 to 60 degrees, closer to 60.

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In patients with a volar facing lunate,

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volar intercalary segmental instability,

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this angle closes or gets small.

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In patients with dorsal intercalary segmental

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instability, dorsal facing lunate, this angle gets big.

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60, 70, 80, 90, 100, or more degrees.

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This patient, let's scroll it,

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demonstrates VISI, a volar facing lunate.

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Here's our lunate right here.

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There's our lunate.

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It's facing volarly.

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The lunate line, to compare with the

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scaphoid line, would be drawn thus.

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That.

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That is how you begin to use the sagittal projection.

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We will also focus a little bit later on the

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degree of displacement of the lunate relative

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to the linear position of the radius, the

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capitate, and the metacarpal, and that will be a

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discussion for perilunate and lunate dislocations.