What makes the safe position safe? Part 1
Vita TU, Occupational Therapist and Project Manager
Sunshine Social Welfare Foundation
We all know safe position is the basis of good positioning in burns…
It’s common knowledge that when immobilizing the burned hand, we must position it in the safe position. The wrist in 10 to 15 degree extension, while the fingers are positioned with MP joints in 70 to 90 degree flexion, and PIP and DIP joints are in extension. The thumb’s CMC joint is in radial and palmar abduction, while the thumb’s MP joint is in extension . But have you ever wondered why this position is actually safe? In this first post, we’ll tell you why, starting with the MP and IP joints.
It’s the anatomy, dummy!
There’s an anatomical explanation to why we position the MP joint in flexion and the IP joints in extension. First, let’s look at the MP joint.
There are two structures that ensure stability of the MP joint: the collateral ligaments and the volar plate.
Stability of the MP joint is ensured by surrounding soft tissues: the collateral ligaments and the volar plate.
Why is it safer to put MP joints in flexion instead of extension? The shape of the articular surface of the metacarpal head is eccentric, cam-shaped so it extends farther volarly than dorsally, and it is wider volarly than dorsally. When the metacarpophalangeal joint is in flexion, we can see the difference. The distance in extension is shorter than the distance in flexion. This in turn will affect the soft tissues that surround the MP joint and ensure its stability: the collateral ligaments and the volar plate. When the MP joint is at 90 degrees, the collateral ligament will be at its tightest.
The distance in flexion is bigger, which will put the collateral ligament at its tightest.
The volar plate of the metacarpophalangeal joint is made of crisscrossing bands of fibers that expand during extension and collapse like an accordion during flexion. 
The volar plate of the MP joint expands during extension and collapses like an accordion during flexion.
The safe position for the MP joint is flexion, first because it provides better stability, second because the collateral ligament will be in a stretched state, which will prevent adaptive shortening, which would otherwise lead to hand deformity.
IP joints also have collateral ligaments and volar plate, so why should IP joints be in extension instead of flexion? First, the volar plate of the IP joint is different from the one of the MP joint. The volar plate is located on the palmar surface of each interphalangeal joint and converges laterally with the collateral ligaments to form more robust attachments. The volar plate’s function is to avoid finger hyperextension, but its structure in the IP joints is different. If we look at the proximal joint from the volar side, the proximal end of the volar plate is attached to the proximal phalanx by two thick check rein ligaments, which tighten as the middle phalanx is extended and thus prevent excessive hyperextension.
The proximal end of the IP volar plate is attached to the proximal phalanx by two thick check rein ligaments.
But contrary to the MP joint, the volar plate of the IP joint is made of more rigid cartilaginous structure, it does not fold and expand like the MP joint volar plate. Instead, it glides proximally and distally with movement of the joint. It will be taut in extension and more relaxed in flexion. 
During extension, the volar plate is taut, and during flexion, the volar plate is looser.
The shape of the IP joint is more evenly round, which means that the tightness of the collateral ligaments on either side of the IP joint will not vary greatly in flexion and extension. This means that the collateral ligament is not really a decisive factor in the development of contracture. However, the volar plate is. During extension, the volar plate is taut, and during flexion, the volar plate is looser. Prolonged positioning in flexion will result in adaptive shortening of the volar plate and its check-rein ligaments, which will make extension difficult, and can also lead to flexion contracture. This is why the safe position of the interphalangeal joint is in extension.
Safe position of MP and IP joints
In our next post, we’ll explain the rationale for the safe position of the wrist and thumb.
If you’re curious to know more about “how” and “why” anatomy influences many principles behind the rehabilitation of the burned hand, don’t miss our course Rehabilitation of the Burned Hand – Introduction and Basic Concepts.
For more tips, how to's and videos about burn rehabilitation, you can also visit our Sunshine Training Youtube channel.
 Serghiou MA, Niszczak J, Parry I, Richard R. Clinical practice recommendations for positioning of the burn patient. Burns. 2016 Mar;42(2):267-75.
 Serghiou MA, Ott S, Whitehead C, Cowan A, McEntire S, Suman O. Comprehensive rehabilitation of the burn patient. In: Herndon DN, editor. Total burn care. 4th ed. Edinburgh: Saunders Elsevier; 2012. p. 517–49.
 Neal E. Pratt. Anatomy and Kinesioloy. In: Terri M. Skirven, A. Lee Osterman, Jane M. Fedorczyk, Peter C. Amadio, editors. Rehabilitation of the Hand and Upper Extremity. 6th ed. Elsevier Mosby; 2011. P3-51.