The PIP joint falls in flexor zone 2 and extensor zone 3 and is a notoriously tricky area of the body to rehab mostly because of the complex anatomy and the numerous structures present around the PIP on both the dorsum of the hand as well as the palmar side. A colleague of mine was once overheard saying "you just look at the PIP joint funny and it gets aggravated!" So if a patient presents with edema, pain over the dorsal aspect of the joint and/or P2, has an PIP extensor lag and your Elson's test if negative - you can rule out central slip rupture. But a little more scrutiny is warranted: if, with the PIP in slight flexion there is also an extensor lag at the DIP joint, place the PIP in even more flexion - if the DIP STILL has an extensor lag (NOTE: the OPPOSITE of what you'd see with Elson's test, i.e. rigidity of the DIP with the cause being central slip rupture), the differential diagnosis is WEAKNESS of the central slip. Reverse blocking exercises are indicated as, are place and hold extension exercises for the central slip / PIP joint and if symptoms worsen, a referral to a hand surgeon is indicated.