Flexion deformity is detected by -

Even a gross limitation of extension, causing a fixed flexion deformity, can be completely masked simply by arching the back into excessive lordosis. Founately it can be just as easily unmasked by performing THOMAS TEST : both hips are f lexed simultaneously to their limit, thus completely obliterating the lumbar lordosis; holding the 'sound' hip firmly in position (and thus keeping the pelvis still), the other limb is lowered gently; with any flexion deformity the knee will not rest on the couch. Meanwhile the full range of flexion will also have been noted; the normal range is about 130 degrees. REF:Apley&;s system of ohopaedics- 9th edn- pg no 495.