At a Glance

• Hip and spine pathology often influence one another through shared biomechanics.
• Compensation for spinal deformity frequently involves the pelvis, hips, knees, and ankles.
• Better communication between hip and spine surgeons may improve patient outcomes.

Estimated read time: 5 minutes

At the The Way I See It session “Evolving Considerations in Hip-Spine Surgical Patients” during the AAOS 2026 Annual Meeting, speakers repeatedly returned to a central theme: hip and spine surgeons can no longer afford to evaluate patients through separate biomechanical frameworks.

Many of the complications seen after hip or spine surgery may not originate in the operated region alone; instead, they reflect the complex relationship among the spine, pelvis, hips, and lower extremities. A patient with a painful hip may actually have a spine-driven problem, and a patient with back symptoms may be compensating for pathology in the hip.

The session brought together hip and spine specialists to build a common understanding of the biomechanics that connect their disciplines.

The pelvis links the hip and spine
Virginie Lafage, PhD, director of research at Lenox Hill Hospital in New York, provided the biomechanical foundation for much of the discussion. “We cannot talk about spine shape without talking about the pelvis,” she said.

Dr. Lafage explained that pelvic morphology varies substantially among patients. Pelvic incidence, a fundamental measure of pelvic anatomy, influences the amount of lumbar lordosis required to maintain an efficient standing posture. Because pelvic anatomy differs from person to person, spinal alignment goals must also differ.

A central concept in her presentation was the “economy of posture,” first described by Jean Dubousset, MD. This principle suggests the body naturally adopts positions that minimize muscular effort while keeping the head balanced over the pelvis and feet. When alignment deteriorates, however, compensatory mechanisms begin to emerge.

Compensation involves the entire body
One of the most important themes of the session was that patients compensate for spinal deformity using much more than the spine alone.

As the trunk tilts forward, muscle activity increases, and posture becomes more difficult to maintain. Patients initially compensate through the spine, but when that capacity is exhausted, the lower extremities become involved.

Dr. Lafage described a sequence of compensatory strategies: The hips hyperextend, the knees flex, and the ankles dorsiflex to help bring the body’s center of gravity back over the feet.

These adjustments allow patients to remain upright despite significant deformity, but they come at a cost. Increased muscular effort, fatigue, and altered joint mechanics can develop over time. Importantly for both hip and spine surgeons, these compensatory changes also alter pelvic orientation and acetabular positioning, meaning pathology in one region may influence the biomechanics of another.

As Dr. Lafage emphasized, this is why appearance alone can be misleading. Many patients who appear balanced are actually relying on a complex series of compensatory mechanisms. “Even when you have a deformity, patients are able to stand,” she said. A patient standing upright, therefore, does not necessarily indicate normal alignment.

Hip motion depends on spinal mobility
Nathanael D. Heckmann, MD, FAAOS, associate professor of orthopaedic surgery at the University of Southern California Keck School of Medicine, focused on the implications of spinal motion for hip arthroplasty. He challenged a common assumption regarding sitting mechanics.

“Only about two-thirds of that motion actually comes from the hip joint,” he said. The remaining motion results from lumbar lordosis flattening and posterior pelvic rotation.

This interaction becomes particularly important when the spine loses mobility. As lumbar motion decreases, the hip must compensate with increased sagittal motion. Complicating matters further, hip arthritis and lumbar degeneration can influence pelvic orientation in opposing directions, creating a constantly evolving biomechanical environment.

To quantify this phenomenon, Dr. Heckmann discussed the hip user index (HUI), which measures the proportion of sagittal-plane motion coming from the hip. Patients with stiff spines rely disproportionately on hip motion, increasing the risk of impingement and instability after total hip arthroplasty.

In patients with dislocating hips, he reported that the hip accounted for approximately 80% of sagittal-plane motion, far exceeding the proportion seen in individuals without substantial spinal disease.

Isolated decision making can create new problems
Alan H. Daniels, MD, FAAOS, chief of spine surgery at The Miriam Hospital and Rhode Island Hospital and professor at Brown University, illustrated the practical consequences of hip-spine interactions through multiple patient cases.

In one example, a patient developed rapidly progressive hip degeneration following extension of a spinal fusion. “Her hips were okay until we extended her fusion,” he said.

In another case, correction of spinal alignment ultimately led to instability of a previously functional total hip arthroplasty. “I did something that led a nicely placed total hip to dislocate,” he said.

These examples demonstrated how changes in spinal alignment can alter pelvic orientation and hip mechanics, sometimes producing unintended consequences despite technically successful surgery.

The lesson, Dr. Daniels emphasized, is that treating the spine without considering the hip, or vice versa, can result in avoidable complications.

Surgical sequencing remains challenging
The panel also explored one of the most common questions in hip-spine care: Which problem should be addressed first?

Dr. Daniels summarized the current philosophy succinctly: “You generally fix the most symptomatic issue first.”

However, he noted an important exception. When substantial changes in pelvic orientation are expected following spinal correction, spine surgery may need to occur first because pelvic tilt changes can influence acetabular component position and long-term hip stability.

The challenge, panelists acknowledged, is that surgeons cannot always predict exactly how pelvic orientation will change after corrective procedures.

Static safe zones are being reconsidered
Returning to the hip side of the discussion, Russell J. Bodner, MD, FAAOS, challenged traditional concepts of acetabular component positioning.

“The static Lewinnek safe zone that hip surgeons have used since 1978 is obsolete,” he said. His argument was not that component positioning no longer matters, but rather that pelvic orientation changes throughout daily activities and continues to evolve with age and degenerative disease. For example, the immobile lumbar spine due to either degeneration or fusion increases the risk of dislocation in total hip arthroplasty (THA). However, even though surgeons know lumbar pathology impacts THA stability, the orthopaedic literature has yet to define or revise the Lewinnek safe zone in THA patients.

Pelvises do not remain in a single static position, he explained. “The pelvis moves.” Or the pelvis moves less in the presence of lumbar immobility, which leads to THA instability issues.

Because pelvic motion changes acetabular orientation, implant positioning must account not only for anatomy but also for function. The speakers repeatedly emphasized that future planning may rely more heavily on dynamic assessments and individualized functional targets than on traditional static measurements.

Looking forward
The panel ultimately returned to its central message: successful treatment of complex hip-spine patients requires communication across specialties.

“We are at the beginning,” Dr. Lafage said while discussing efforts to integrate biomechanical understanding across disciplines.

Dr. Bodner ended the session with a similar call to action. “We can continue these conversations and learn from each other, because our patients will benefit.”

Theresa Witham is managing editor of AAOS Now.