The PIP joint is prone to deformity with even slight alterations in the delicate balance of forces around the joint. Around the outside of the PIP joint are the lateral bands, which are made up of fibers from the EDC (more lateral) and fibers from the lumbricals and interossei (medially). The lateral bands are weak extensors of the PIP: it can keep the PIP extended if it is positioned in extension passively and it contributes slightly to full extension but the central slip does the brunt of the work.
The lateral bands are kept in place by retinacular fibers around the PIP joint: the dorsal fibers of the triangular ligament keeps it from moving volarly and the transverse retinacular ligament keeps it from shifting dorsally. The oblique retinacular ligament (also known as the ligament of Landsmeer) originates on the volar plate of the PIP joint and travels obliquely along the lateral aspect of the middle phalanx to insert on the DIP joint with the terminal tendon - linking PIP and DIP joint motion: with PIP flexion it relaxes and allows for ease of DIP flexion and with PIP extension it assists with extending the DIP along with the lateral bands and the terminal tendon.
Interruption of the central tendon and the triangular ligament causes proximal migration of the extensor complex and palmar displacement of the bands during flexion of the PIP joint. The extensor's lateral bands now become flexors in their new position below the axis of normal motion and adds to the flexion force of the flexors. Lateral bands subluxed in this manner can become caught under the condyles of the base of the middle phalanx in extreme cases.
At the acute stage for most people, the flexion deformity is still passively correctable and responds well to treatment with custom orthotics which positions the joint in as much tolerable extension as the tissues will allow. However, if left untreated the lateral bands will scar down & become fixed the the underlying collateral ligaments and capsule and the retinacular ligaments (especially the ORL) and the palmar plate will tighten. The DIP joint will lose active flexion, develop hyperextension and over even more time, will progressively lose even passive flexion and the finger will posture in what is referred to as a Boutonniere deformity.
To assess for tightness in the ORL/Landsmeer ligament and rule out joint capsule tightness:
Position the PIP joint in extension and attempt to passively flex the DIP joint. Assess resistance relative to contralateral side.
Assess resistance with PIP joint in some flexion. If there is no difference in DIP joint motion regardless of PIP joint position, the joint deformity is likely due to capsular tightness, however if there is more flexion available at the DIP with the PIP in flexion, it is due to tightness of the ORL / Landsmeer's ligament.
A Boutonniere deformity responds well to treatment of orthotic wear for 8 weeks with slow weaning off of orthotic thereafter to assess joint response. More long-standing deformities (>4 weeks) will require more complicated treatment and the prognosis is altered.