Does unilateral total hip arthroplasty improve pain and function in the non-operative hip joint?

Background: Some reports suggested that the status of the opposite-side hip affects clinical outcomes of unilateral total hip arthroplasty (THA) for hip osteoarthritis (HOA). This study aimed to determine whether unilateral THA could improve pain and movement of the non-operative hip.
Methods: The analysis included 195 patients divided into three groups according to contralateral hip status based on radiographic change assessed by Kellgren-Lawrence (KL) grade: normal hip (Group N: n = 124), HOA with KL grade > 2 (Group O: n = 39), and THA patients who already underwent THA in their opposite hip joint (Group T: n = 32). All patients were interviewed and examined preoperatively and at 1-year intervals after surgery. Hip function was evaluated with Harris Hip Score (HHS) and range of motion (ROM) preoperatively and at 1-year follow-up. We used the Japanese Orthopedic Association Hip-Disease Evaluation Questionnaire (JHEQ) and the Visual Analogue Scale (VAS) for separately evaluated right and left hip joints.
Results: The mJHEQ movement, which indicated the activity, and flexion of ROM in the non-operative hip improved in group N and group T. The VAS and JHEQ pain values did not differ between preoperative and 1-year follow-up in all groups. In the operative side, HHS function, JHEQ movement values in group O were significantly lower than those of groups N and T at 1-year follow-up.
Conclusion: Unilateral THA may improve non-operative hip movement and active daily life except when the non-operated hip shows osteoarthritis. It did not improve non-operative hip pain, regardless of the condition on the other side. Clinical outcomes of unilateral THA may be affected by opposite hip status each other.