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What is the mechanism behind Thomas test?

Asked by Terry 

Fixed flexion deformity of hip is compensated by pelvic extension resulting from increased lumbar lordosis. The combined effect allows patient to walk with feet touching the ground in conjunction with knee flexion. When we flex the normal hip, the deformity in pelvis gets corrected first resulting in simultaneous correction of the compensatory lumbar lordosis.

As the pelvis is drawn upwards - in order to keep the alternate leg flat on the examination table, the hip flexors and rectus femoris must be supple and stretch enough to allow the leg to lie flat. If these are shortened, the hip cannot remain extended.

Method of eliciting Thomas test: Thomas Test and Staheli Test | Epomedicine