Serious injuries like tendon injuries often require treatment by an experienced hand surgeon. Surgeons know how complex and delicate the anatomic structures of the hand are. They understand the need for very careful surgical technique when repairing, reconstructing, transferring, or grafting a tendon.
There are many potential complexities of surgery for tendon transfers. Tendon transfer refers to taking one tendon and moving it to function in place of another. This is done most often when a tendon has been injured, the nerve to the tendon has been damaged, or as in your case, there is a defect of the tendon from rheumatoid arthritis. But tendon transfers are not as simple as they sound.
For example, sometimes a tendon originates (or begins) in a slightly different place than expected. Most tendons start from an attachment directly to the bone. But in some cases, the insertion point could be a ligament instead of the bone. Or the origination could be from the soft tissue over the bone.
If even a single tendon slip is taken from the wrong tendon, it can affect the movement and strength of the finger and/or hand. Likewise, if the surgeon chooses a weaker tendon for a transfer to a stronger tendon that has been injured, the result can be a significant loss of hand function.
The extensor indicis proprius (EIP) of the index finger has many connections to the extensor digitorum communis (EDC). EDC is the main tendon that extends all the fingers. The connection between these two tendons must be carefully cut to avoid losing the benefit of the EIP as a tendon transfer.
There are also thin bands of tissue that connect the extensor digitorum communis (EDC) tendons. These are called the juncturae tendinum. The full function of the juncturae tendinum is not completely understood but it is clear that the anatomy can be quite different from one person to another. Even rare anomalies (differences or variations) of the juncturae tendinum anatomy are important for the hand surgeon to be aware of.
Knowing how many and what kind of anatomical differences that can occur from patient to patient is essential when planning hand surgery. Knowing the anatomic and functional variations of the extensor tendons affects decisions surgeons make before and during tendon transfers. You will most likely want to find someone to do the surgery who has knowlege and understanding of the unique and very complex hand anatomy.