If the fibres are near the
insertion, (you have to know that a tendon connects a muscle to a bone) then sometimes it is difficult to heal the natural way, where you need surgical intervention.
1) The tendon should be positioned perfectly
2) The nerve supply should be intact
3) There should be no damage at origin, tendon and insertion place.
These are the anatomical things which the surgeon has to correct and ensure that your exensor tenson works perfectly after this.