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Only physiotherapy will improve your ROM up to some extent Its genetically decided problem mm No specific reason

In long-standing ankylosing spondylitis, hip movement can remain limited even after bilateral total hip replacement, especially when surgery is done for protrusio deformity with bone grafting. Over many years, AS causes severe soft-tissue contractures of the capsule, muscles, and tendons, along with pelvic and spinal rigidity (bamboo spine), which mechanically restrict hip flexion despite a well-positioned implant. The posterolateral approach and the need to protect grafted bone can also lead to postoperative stiffness and scar formation. at 10 months post-surgery, improvement in ROM is usually slow and often incomplete, and full flexion is not always achievable in advanced AS cases. Achieving 70–80° of hip flexion depends on whether the limitation is mainly from soft-tissue tightness rather than implant positioning. Gradual, pain-guided stretching, prolonged hold positions, and consistency over months are key, but gains are typically modest and incremental.