During flexion of the joint from the fully extended position the collateral ligaments soon become tight as they pass over the apex of angulation of the side margin of the head and remain tight as they move over more vertical plane of the side of the head.
From measurements on the dissected fingers, from measurements on radiographs and from tracings of photographs of the proximal phalanx it was not possible to draw any definite conclusions about the sagittal cam effect comparable to those relating to the metacarpo-phalangeal joint. A few specimens exhibited some degree of this cam but most did not. This may be related to functional variations of individual fingers and requires more detailed study.
If the joint is immobilised for some time in flexion with the collateral ligaments well below the apex of angulation, the slack volar part of the collateral aspects of the capsule with fibres running to the lateral and palmar tubercies of the middle phalanx and the dorsal fibres of the flexor sheath may become contracted. Shortening of the fibres of the lower part of the collateral portions of the capsule (A) and of the most dorsal fibres of the flexor sheath (B) was a constant finding. Curtis (1964) advocated excision of a portion of flexor sheath over the joint in dealing with flexion contracture.
Dissection of two fingers affected by extension contracture suggests that it takes a long time to produce shortening of the soft and pliable more dorsal part of the capsule.
It appears therefore that if immobilisation of the proximal interphalangeal joint cannot be avoided, it should be for as short a period as possible, with the collateral ligaments just riding over the apex of the side margin on the head when fibres A and B are only slightly slack. The results of immobilising the injured finger with the proximal interphalangeal joint flexed not more than 15 degrees might be compared with those after immobilisation with the joint more flexed. Splintage of the joint in extension was advised by Stewart (1962) on functional and clinical grounds. He noted the importance of flexion of the metacarpo-phalangeal joint and used the position of extension in all cases except those of: 1) gross damage leading inevitably to stiffness; and 2) division of flexor tendons or infection in the tendon sheath.
It is suggested that in correction of contracture of the proximal interphalangeal joint caused by Dupuytren's affection it may be advisable to excise fibres A and B.