Can hip osteoarthritis (OA) be unilateral?

Can Hip Osteoarthritis Be Unilateral?

Yes, hip osteoarthritis (OA) can definitely be unilateral, affecting only one hip joint, though patients with unilateral hip OA are at increased risk of developing OA in the contralateral hip over time. 1, 2

Epidemiology and Natural History of Unilateral Hip OA

• Unilateral hip OA is a common clinical presentation, with studies showing that many patients initially present with symptoms and radiographic changes in only one hip 1
• Research demonstrates that approximately 81% of patients with unilateral hip OA remain free from requiring arthroplasty on the contralateral hip 10 years after their initial hip replacement 1
• About 59% of patients with unilateral hip OA remain completely symptom-free in their contralateral hip at 10-year follow-up 1

Risk Factors for Progression to Bilateral Hip OA

Certain radiographic features in the "normal" contralateral hip can predict future development of OA:

• Reduced minimum joint space width (hazard ratio 0.299) 1
• Low center-edge angle (hazard ratio 0.941) 1
• Low head-to-neck ratio (hazard ratio 1.555) 1
• Presence of osteophytes (hazard ratio 1.453) 1
• Weight-bearing surface (sourcil) inclination has a strong influence on the development of contralateral OA 2

Biomechanical Considerations in Unilateral Hip OA

• Patients with unilateral hip OA adopt characteristic gait patterns to reduce pain in the affected hip 3
• These compensatory mechanisms lead to abnormal loading patterns in both the affected and non-affected limbs:
  • The non-affected limb shows greater external hip adduction moments (+15%) compared to healthy controls 3
  • The affected limb demonstrates reduced external knee adduction moments (23-30% lower) compared to the non-affected limb and healthy controls 3
• These biomechanical alterations may accelerate OA progression in the non-affected hip due to increased loading 3

Pattern of OA Progression in Lower Extremities

• When OA progresses beyond a single joint, it follows a non-random pattern 4
• After one joint replacement, the contralateral cognate joint (e.g., opposite hip) is the most likely next joint to require replacement 4
• For non-cognate progression (hip to knee or knee to hip), the contralateral limb is >2-fold more likely to develop end-stage OA requiring replacement than the ipsilateral limb 4
• This pattern differs from rheumatoid arthritis, where progression follows a random pattern without laterality preference 4

Clinical Implications

• Patients with unilateral hip OA should be monitored for early signs of contralateral hip involvement 1, 2
• Treatment of unilateral hip OA (such as total hip arthroplasty) may improve movement in the non-operative hip if that hip is normal, but does not improve pain in the non-operative hip 5
• Clinical outcomes of unilateral total hip arthroplasty may be affected by the status of the opposite hip 5
• Patients with idiopathic OA in one hip have a significantly higher risk of developing OA in the contralateral hip compared to those with secondary OA due to congenital hip diseases 2

Conclusion

Hip OA can present as a unilateral condition, though there is a significant risk of progression to the contralateral hip over time. Understanding the biomechanical changes and risk factors for bilateral progression can help guide monitoring and management strategies for patients with unilateral hip OA.