By Arturo Corces, MD
Total hip arthroplasty (THA) remains one of the most successful procedures in orthopedic surgery. Yet even as implant design, surgical technique and component positioning have advanced, instability and dislocation remain important causes of failure. Increasingly, evidence suggests that some of these failures cannot be understood by looking at the hip alone.
The spine, pelvis and hip function as an interconnected biomechanical system. Changes in spinal alignment and mobility alter pelvic position, which in turn changes the functional orientation of the acetabulum during everyday activities. For orthopedic surgeons treating an aging population with increasingly prevalent degenerative disease in both the hip and lumbar spine, this relationship has important implications for THA. Historically the position of the acetabular cup and the orientation of the femoral component was considered to play a major role in the stability of the implant.
Lewinnek’s classic paper delineating the safe zone for acetabular component placement has more recently been challenged by data showing that 58% of dislocations occurred despite the acetabular component being positioned within the Lewinnek safe zone.
The growing hip-spine challenge
When a healthy individual transitions from standing to sitting, coordinated motion through the lumbar spine and pelvis changes the functional position of the acetabulum to accommodate hip flexion. Degenerative lumbar disease, spinal deformity and particularly lumbar spinal fusion can restrict or alter that movement.
The result is an important distinction between the anatomic position in which an acetabular component is implanted and its functional orientation as the patient moves.
This helps explain why a technically well-positioned conventional THA can nevertheless become unstable in certain patients.
A systematic review of the spinopelvic relationship in THA concluded that altered spinopelvic motion can affect acetabular orientation and contribute to complications including dislocation, impingement, aseptic loosening and component wear.
The association becomes particularly apparent among patients who have undergone lumbar fusion. A large database study reported a 7.4% prevalence of hip dislocation among patients who underwent lumbar fusion before THA, compared with 4.8% among patients without prior fusion (https://link.springer.com/article/10.1186/s12891-024-07823-1).1 The fusion group experienced an 80% higher risk of dislocation at six months, 71% higher risk at one year and 60% higher risk at two years. The investigators also reported increased risk of failure leading to revision.
More recent evidence reinforces the concern. A 2024 systematic review and meta-analysis of patients with hip-spine syndrome found significantly greater THA dislocation risk when lumbar spinal fusion preceded THA (https://link.springer.com/article/10.1186/s12891-024-07823-1).2 The authors emphasized the importance of spinopelvic biomechanics when planning THA in patients with instrumented spines. Even patients with stable implants can experience dislocation after undergoing spinal fusion.
Why conventional THA geometry can be challenged
Traditional THA places the ball on the femoral side and the polyethylene-lined cup on the acetabular side. Successful articulation therefore depends, in part, upon the relationship between component orientation and the changing position of the pelvis.
Surgeons have responded to instability risk through improved preoperative assessment, larger femoral heads, dual-mobility constructs, navigation and increasingly sophisticated evaluation of spinopelvic mobility. These approaches remain important.
But the growing prevalence of hip-spine pathology raises a more fundamental question: Can implant geometry itself be made more tolerant of changes in functional pelvic orientation?
That question is especially relevant because the population requiring THA increasingly overlaps with the population experiencing lumbar degeneration, spinal deformity and previous spinal surgery.
Investigating a different mechanical approach
One investigational approach reverses the traditional geometry of the hip articulation.
The Hip Innovation Technology Reverse Hip Replacement System (Reverse HRS) places the ball on the acetabular side and the polyethylene-lined cup on the femoral side. The design is intended to provide enhanced mechanical stability at extended ranges of motion and reduce dependence on precise component orientation.
Importantly, that proposed benefit must be demonstrated clinically.
I am currently serving as principal investigator at the Miami Institute for Joint Reconstruction for the U.S. pivotal IDE study evaluating the Reverse HRS. The randomized, controlled, multicenter trial is designed to enroll 288 patients undergoing primary THA and compare the investigational system with commercially available conventional uncemented THA systems. The study is evaluating safety and effectiveness, including patient-reported outcomes, device survivorship, radiographic findings and other measures over long-term follow-up.
The study therefore provides an opportunity to determine whether changing the mechanics of the articulation can translate into meaningful clinical performance.
What do we know so far?
Earlier clinical research conducted in Canada provides an initial evidence base.
A prospective study published in Bone & Joint Open in 2023 evaluated 22 patients implanted with the Reverse HRS using radiostereometric analysis (RSA), a highly sensitive method for measuring implant migration and assessing fixation.
At 24 months, mean acetabular subsidence was 0.087 mm and mean femoral subsidence was -0.002 mm, both below the published thresholds used by the investigators. Patient-reported outcome measures also improved significantly, with good-to-excellent clinical results. The investigators concluded that RSA demonstrated excellent fixation with a predicted low risk of revision for loosening at ten years.
Longer-term follow-up presented subsequently has continued to evaluate implant migration, polyethylene wear and clinical outcomes through five years. These findings are encouraging, but they do not substitute for the larger randomized pivotal trial now underway in the United States.
Thinking beyond the hip
Perhaps the most important lesson from the growing body of hip-spine research is that THA can no longer always be considered an isolated reconstruction of the hip joint.
For patients with spinal degeneration, deformity or fusion, the functional environment surrounding a hip implant may be substantially different from the environment assumed when the components are positioned on the operating table.
Understanding that relationship should influence patient evaluation, surgical planning and implant development.
Importantly, patients differ in their baseline spinopelvic mechanics. Some rely more on spinal and pelvic motion during the transition between standing and sitting, while others rely more on hip motion. This underlying “spine-mover” or “hip-mover” pattern may influence how strongly spinal degeneration, stiffness or fusion alters functional acetabular orientation and ultimately THA stability.
The next generation of THA innovation may therefore depend not simply on improving individual components, but on designing reconstruction strategies around how the hip, pelvis and spine actually function together.
For surgeons treating patients with increasingly complex spinopelvic pathology, that may prove to be one of the more consequential changes in how we think about total hip replacement.
- Malkani AL, et al. Total Hip Arthroplasty in Patients With Previous Lumbar Fusion Surgery: Are There More Dislocations and Revisions? The Journal of Arthroplasty. 2018.
- Huppert A, Ambrosio L, Nwosu K, et al. Previous lumbar spine fusion increases the risk of dislocation following total hip arthroplasty in patients with hip-spine syndrome: a systematic review and meta-analysis. BMC Musculoskeletal Disorders. 2024;25:732.
Dr. Arturo Corces has spent nearly forty years in Orthopaedic surgery, most of it in South Florida. After Medical School at New York University he trained at SUNY for his residency and chief residency, then went on to a fellowship in adult reconstruction at the University of Pennsylvania in 1988.He has held faculty appointments at the University of Miami, Barry University, and the University of South Alabama and currently teaches as an Affiliated Clinical Assistant Professor at NSU’s Dr. Kiran C. Patel College of Medicine. He has been the Program Director of the Orthopedic Surgery Residency at Larkin Community Hospital since 2019, and he directs the Implant Service at the Miami Institute for Joint Reconstruction, where he’s practiced for over two decades. He sits on the AAOS Adult Reconstruction Hip Program Committee and is a reviewer and editor for several peer reviewed journals.Over the course of his career, Dr. Corces has published extensively on hip and knee arthroplasty, revision surgery, and periprosthetic infection, and has lectured at conferences throughout the Americas, Asia and Europe. What he enjoys talking about most are the residents and fellows he’s trained along the way. A lot of his energy goes into mentoring the next generation of orthopedic surgeons, through Larkin’s residency program and the Miami Orthopedic Research Foundation a not for profit organization .
