One of the most important things that the medical examiner must determine in any death investigation is the time of death. This can spotlight or exonerate a suspect and support or refute a witness statement. An alibi can be upheld or shattered.
But it’s not that easy. The medical examiner uses things such as body temperature, degree of rigor mortis, lividity, stomach contents, degree of decomposition, insect activity, and a few other parameters to make a best guess as to time of death. Another determination that often proves helpful is the potassium level within the vitreous humor, the liquid inside the eyeball. The potassium level tends to increase in a linear fashion and at a known rate after death and this can help the medical examiner with his estimation.
Now it seems that assessing the level of potassium in the victim’s synovial fluid might be just as accurate. Synovial fluid is the liquid that lubricates our joints. This could prove useful in cases where the victim has been decapitated or the head is so damaged that the eyes have been destroyed.