Vita TU, Occupational Therapist, Sunshine Social Welfare Foundation
When dealing with hand burns, exercises are one of the intervention strategies that therapists use to prevent or minimize complications like contractures that can lead to severe deformity. As part of their “arsenal”, therapists can use various types of exercises: passive, active, stretching, muscle strengthening, etc. But the type, frequency and intensity of exercise must be carefully considered based on the location and the extent of injury to avoid causing secondary damage. This is especially true in the case of the distal interphalangeal (DIP) joints.
At the level of the proximal interphalangeal (PIP) joint, the common extensor tendon (EDC) splits into two lateral bands, which finally merge as the terminal tendon at the level of the distal interphalangeal joint. The terminal tendon of the common extensor tendon is primarily responsible for DIP extension . The flattened terminal tendon attaches to the base of the distal phalanx and blends with the joint capsule. Because its excursion is only about 4mm, even a very small gap can lead to a lack of extension, therefore causing an extension lag .
In a study done by Satoshi et al., computed tomography images were used to analyze the grasping movements of the thumb and index finger, and the flexion of MP, PIP and DIP joints when picking up objects of different sizes. The study found that the MP and PIP joints significantly adjusted their flexion angle and posture according to the size of objects, while the DIP flexion angle and posture didn’t change much and tended to be fixed . For the overall performance of hand function, the range of motion requirements for DIP joints is not as high as for PIP and MP joints. This means that even with some DIP limitation, hand function will not be significantly affected and the patient can still perform grasping.
Why passive flexion exercises are not recommended for DIP joints?
The extensor tendon at the DIP joint has less elasticity than other tendons. Also, this area is less vascularized, which makes the tendon more vulnerable to injury . Because of the terminal tendon’s lack of elasticity, passive flexion stretching in itself may cause the terminal tendon to become too loose or it may even rupture, which will affect active movements.
Burns can also cause damage, which can then be made more serious when passive flexion is done. Burns can affect the terminal slip of the extensor tendon in different ways: (1) direct exposure to heat causing injury; (2) hypoxia caused by being pinched between an eschar or when DIP is in flexion; (3) reduced tendon gliding caused by adhesion of the extensor mechanism3. All of this will result in limitation of DIP active movement.
Because there is a higher risk of damaging the tendon, and also because the DIP flexion requirement for performing grasp movement is minimal to begin with, it’s better to avoid passive flexion stretching exercises for the DIP joint, and instead opt for active exercises which carry less risk of causing injuries and creating further complications.
 Rebecca Bassett, Extensor tendon injury of the distal interphalangeal joint (mallet finger). https://www.uptodate.com/contents/extensor-tendon-injury-of-the-distal-interphalangeal-joint-mallet-finger
 Kai Megerle & Günter Germann, Extensor tendon injuries., Plastic Surgery Key-Fastest Plastic Surgery & Dermatology Insight Engine. https://plasticsurgerykey.com/extensor-tendon-injuries/
 Cathy N. H., Reginald L. R., and Marlys J. S.: Soft tissue Management and Exercise. In Reginald L. R. and Marlys J. S.(ed): Burn Care and Rehabilitation-Principles and Practice. F. A. Davis Company, 1993, P349.
 Satoshi S., Yoshiaki N., Masataka N. and Hideharu S., Flexion Angles of Finger Joints in Two-Finger Tip Pinching Using 3D Bone Models Constructed from X-Ray Computed Tomography (CT) Images. Appl Bionics Biomech. 2020 Sep 10. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7501542/