DIP extension is NOT what you want if you suspect a central slip rupture!
A test to evaluate integrity of central slip / to r.i. complete rupture of central slip
Have patient attempt to extend PIP jt while finger is comfortably flexed
If central slip injury is left untreated it may lead to Boutonnière's deformity over the course of days and weeks
Instructions:
1. Have patient flex injured finger over edge of a table with MCPs at 0 degrees and PIP at 90 degrees.
2. Apply resistance to middle phalanx.
3. Ask the patient to gently extend the PIP.
4. Rigidity at DIP during PIP extension indicates complete rupture (test cannot detect partial ruptures). A finger with an intact central slip will have a "floppy" DIP.
The Biomechanics:
in case of central slip rupture, the central slip retracts and has no ability to extend the middle phalanx
the distal phalanx instead will be rigid and extend because of the pull of the lateral bands (fibers from intrinsics and EDC) which are no longer restricted (central slip has "checkrein" effect on lateral bands in uninjured finger)
Variation to better compare injured against non-injured fingers as described by Schreuders:
Have patient push middle phalanx of injured finger against non-injured finger with each PIP at 90 degrees.
Have patient push tips of thumbs together to provide additional stability for fingers. Have patient try to extend the PIP.
Asymmetric movement easily noticeable in comparison between two digits. Ruptured central slip leads to lateral bands pulling on the terminal tendon extending the DIP.
References:
1. Elson, RA. Rupture of the central slip of the extensor hood of the finger. A test for early diagnosis. J. Bone Joint Surgery Br 1986;68(2):229-31
2. Schreuders, TA, Soeters JN, Hovius SE, and Stam H. A modification of Elson's Test for the Diagnosis of an Acute Extensor Central Slip Injury. The British Journal of Hand Surgery 2006; 11(4):111-112
