undefined muscle tear in hip joint

[Van de Voorde Jean]

my patient has knee-arthrosis and will have a prosthesis soon, but she also has pain and difficulty to flex the hipjoint, when she does : a 'bubble' is seen and felt lateral to the inguinal fossa, when standing it dissappears. I presume it is the contracted part of a muscle - due to a muscle tear, but I'm not sure which muscle, since most of them tear at the knee or at the spina iliaca anterior superior, for which in this case there is no evidence. when contracted it seems to me that the vastus lateralis is 'missing' for the tractus iliotibialis is readily visible compared to the other leg.
testing is not obvious. Can anyone suggest anything?
thanks alot.