Firstly the Achilles tendon is the main tendon that attaches to the back of the heel bone (calcaneus) which bends the foot/ankle down to the ground, it is obviously of key importance in jumping and athletic activity. The plantaris muscle is a small muscle of unknown importance, it attaches at the back of the knee joint at the top end, and down on the heel bone at the bottom end.
The statements released by various parties explaining Vermaelen's surgery/injury last season and this season do not all add up in a consistent fashion. Firstly he was out for a prolonged period last year after having surgery to address the 'inflamed' plantaris tendon, from this it has to be assumed that he had a degree of Achilles tendinosis, meaning 'degeneration' of the tendon, a condition which is particularly tricky to treat and which obviously took Vermaelen some time to settle down.
This season, the other side has flared up after an incident in the Udinese game and obviously the same surgery has been performed. The following statement was released by Arsenal FC:
"Vermaelen has undergone a minor procedure to remove an inflamed plantaris tendon from his left ankle. This is the same treatment that he had on his right ankle in January this year and has nothing to do with his Achilles tendon."
This statement is slightly economical with the truth in my opinion, it has everything to do with his Achilles tendon just as the same surgery did last season. In the scientific literature there is only a small amount of evidence relating to removing the plantaris tendon and it all links in with Achilles tendinosis. There is one large case series of re-operations involving removing the plantaris tendon in Achilles tendinosis and one far smaller series of cases involving removing the plantaris tendon in chronic Achilles tendinosis. The conclusions of these low quality studies state:
"The medial pain might be based on the involvement of the plantaris tendon in the process." and "The role of the plantaris tendon in midportion Achilles tendinosis needs to be further evaluated and should be kept in mind when treating this condition."
In conclusion Vermaelen's surgery is based on very little scientific evidence. Many experts would say that the surgery is of very dubious benefit, ie it is almost 'sham' surgery, a placebo treatment that only works because the patient is forced to rest for a decent period. Certainly no one could argue that there is any strong evidence to say that this surgery will definitely settle down his Achilles problems. It is worth remembering last season it took him many months to get back to full training, he experienced several setbacks after his plantaris surgery.
Achilles tendinosis is frequently a chronically painful condition, much like patella tendinosis (Owen Hargreaves has this), and there are few reliable treatments for them. Personally I am not quite sure why Arsenal are being slightly economical with the truth in this instance, it appears likely that they do not wish to scare supporters by revealing that Thomas Vermaelen's surgery may not be the long term definitive cure for his pain that they pretend it is. In reality we simply cannot reliably predict whether or not Thomas Vermaelen will recover fully, he may do, he may not. This is the truth about Thomas Vermalen's injury, it is not reassuring but it is the truth.