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Correlation between Sagittal Plane Deformities of the Hip and Spinopelvic Alignment in Advanced Hip arthritis patients undergoing Total Hip Arthroplasty: A Prospective Observational Study

This prospective study demonstrates that correcting fixed flexion deformity in advanced hip arthritis patients via total hip arthroplasty significantly improves spinopelvic mobility, with preoperative hip range of motion and dynamic ante-inclination identified as key predictors of early postoperative sacral slope changes.

Original authors: Souvik Paul, Aritra Chattopadhyay, Ankit Gaurav, Siddhartha Singh, Abhishek Meena, Udit Chauhan, Quamar Azam, Roop Bhushan Kalia

Published 2026-07-16

📖 6 min read🧠 Deep dive

CC BY 4.0

Original authors: Souvik Paul, Aritra Chattopadhyay, Ankit Gaurav, Siddhartha Singh, Abhishek Meena, Udit Chauhan, Quamar Azam, Roop Bhushan Kalia

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Body's Tug-of-War: Why Your Hips and Back Are Best Friends

Imagine your body as a high-tech suspension system, like the one on a rugged off-road vehicle. The spine is the main frame, the pelvis is the heavy-duty axle, and the hips are the massive wheels that do the actual moving. For this system to work smoothly, everything has to stay perfectly balanced. If one wheel gets stuck or bends the wrong way, the whole frame has to twist and contort just to keep the car driving straight. In the world of medicine, this "tug-of-war" between the spine and the hips is called spinopelvic alignment.

Doctors have long known that when the hips get stiff or arthritic, the back tries to compensate. If a hip can't bend forward properly, the pelvis tilts backward to make up the difference, kind of like how you might lean your whole body forward if you can't bend at the waist. This study dives into a specific problem: what happens when a hip is stuck in a bent position (called a Fixed Flexion Deformity or FFD) and then gets replaced with a new, smooth joint? The big question is: does fixing the hip also "unstick" the back, or does the back stay confused? Understanding this is crucial because if doctors only look at the back's position before the surgery to plan the new hip joint, they might get the angle wrong, leading to a wobbly or unstable replacement.

The Study: Untangling the Hip-Back Knot

This research paper, titled "Correlation between Sagittal Plane Deformities of the Hip and Spinopelvic Alignment in Advanced Hip Arthritis Patients Undergoing Total Hip Arthroplasty," is a prospective observational study led by Souvik Paul and a team from AMRI Hospitals and AIIMS Rishikesh. They wanted to see if fixing a stiff, bent hip actually helps the spine and pelvis return to their natural, happy positions.

The Setup
The team followed 45 patients who had severe arthritis in one hip and a noticeable "stuck" bend in that hip (FFD). Before surgery, and again three months after, they put these patients through a rigorous check-up. They measured:

How bent the hip was: Using a simple test called the Thomas test.
How much the hip could move: Measuring the Range of Motion (ROM).
The "Back-Bend" Score: They took X-rays of the patients standing up and sitting down to measure the Sacral Slope (SS). Think of the Sacral Slope as the angle of the "floor" inside your pelvis. When you sit, a healthy pelvis tilts, changing this angle. The difference between the standing angle and the sitting angle is called $\Delta$ SS (Delta SS), which tells doctors how "mobile" or flexible the spine-pelvis connection is.
The "Hip Angle": They also measured something called Ante-inclination (AI), which is a fancy way of describing how the hip socket faces forward and up.

The Big Findings
The results were like watching a tangled knot suddenly untie itself.

The Hip Got Fixed: Before surgery, the average patient had a hip stuck at a 16.4° bend. After the Total Hip Arthroplasty (THA), that bend was completely gone (0°). The ability to bend the hip forward jumped from 72.5° to 114.1°.
The Back Got Unstuck: Before surgery, about 26% of the patients had a "hypermobile" spine-pelvis system. This means their backs were doing too much work to compensate for the stiff hip. After the hip was fixed, this group shrank to just 13%, while the "normal" group grew from 63% to 76%. The data suggests that the "hyper-mobility" wasn't a permanent spinal problem; it was just the back panicking because the hip was broken. Once the hip was fixed, the back calmed down and returned to normal.
The Connection: The study found a clear link: the worse the hip was stuck before surgery, the more the spine had to overcompensate. Specifically, the preoperative hip bend correlated positively with the change in sacral slope (r = 0.45, p = 0.002).

What This Means for the Surgeon
The most exciting part of the paper is a warning for surgeons. For a long time, the rule of thumb has been: "Look at the patient's spine and pelvis before surgery, and set the new hip joint based on those numbers."

This paper suggests that rule might be wrong for patients with a stiff, bent hip. Because the spine and pelvis are so busy compensating for the bad hip, the pre-surgery X-rays show a "fake" picture of how the body works. Once the hip is fixed, the spine relaxes, and the angles change.

The authors ran a statistical model to see if they could predict how the spine would behave after surgery. They found that two things were the best clues:

How much the hip could move before surgery (Preoperative ROM).
How much the hip socket angle changed between standing and sitting (Dynamic Ante-inclination).

The model suggested that if a patient had a very stiff hip and a lot of "dynamic" movement in the hip socket before surgery, their spine would likely settle into a new, different position after the operation. The model explained about 30% of the changes, which is a good start, though the authors admit they need more patients to be 100% sure.

The Takeaway
In plain English: If you have a hip that is stuck in a bent position, your back is probably doing a lot of extra gymnastics to help you stand up straight. When you get a new hip, your back doesn't just stay in that "gymnast" pose; it relaxes and goes back to normal.

Therefore, if a surgeon plans the new hip joint based only on the pre-surgery X-rays (when the back is still doing gymnastics), they might place the new joint at the wrong angle. The paper suggests that for patients with this specific "stuck hip" problem, surgeons should expect the body to change its alignment after the surgery and plan accordingly. They also note that for patients with actual stiff spines (like those with fused vertebrae), the back won't change, and those patients might need a different type of hip bearing (dual mobility) to stay stable. But for the "stuck hip" crowd, fixing the hip fixes the whole system.

Technical Summary: Correlation between Sagittal Plane Deformities of the Hip and Spinopelvic Alignment in Advanced Hip Arthritis Patients Undergoing Total Hip Arthroplasty

Problem Statement
Total Hip Arthroplasty (THA) is a standard intervention for advanced hip arthritis, yet optimal outcomes depend on accurate component positioning within a dynamically functioning spine–pelvis–hip complex. Abnormal spinopelvic alignment is a known risk factor for postoperative instability, impingement, and dislocation. While the concept of "hip–spine syndrome" highlights the interdependence of these regions, prospective data specifically addressing patients with clinical Fixed Flexion Deformity (FFD) remain limited. In patients with FFD, compensatory pelvic adaptations often develop preoperatively. Consequently, relying solely on preoperative static spinopelvic measurements for acetabular component planning may fail to reflect the true postoperative functional state, as pelvic alignment frequently readjusts following the correction of hip deformity.

Methodology
This prospective observational study was conducted at a tertiary-care university hospital involving 45 patients with unilateral hip arthritis and clinical FFD undergoing primary THA.

Participants: Patients included those with primary/secondary osteoarthritis, healed tubercular/infective sequelae, or Crowe type I–II dysplasia. Exclusion criteria included prior spinal fusion, previous hip arthroplasty, and severe dysplasia (Crowe III–IV).
Intervention: All surgeries utilized a standard lateral approach with acetabular components positioned within the Lewinnek safe zone. No iliopsoas releases were performed.
Assessments: Patients were evaluated preoperatively and at three months postoperatively.
- Clinical: Thomas test for FFD, hip range of motion (ROM), Harris Hip Score (HHS), and Visual Analogue Scale (VAS).
- Radiographic: Standing and sitting lateral radiographs were used to measure Pelvic Incidence (PI), Sacral Slope (SS), Pelvic Tilt (PT), Ante-inclination (AI), and Pelvic–Femoral Angle (PFA).
Definitions: Spinopelvic mobility was defined by the change in Sacral Slope ( $\Delta$ SS) between standing and sitting positions, categorized as stiff ( $<10^\circ$ ), normal ( $10–30^\circ$ ), or hypermobile ( $>30^\circ$ ).
Analysis: Statistical analysis included correlation tests (Pearson/Kendall-tau) and multiple linear regression to identify predictors of postoperative $\Delta$ SS.

Key Results

Clinical Outcomes: Mean FFD was corrected from $16.4^\circ$ preoperatively to $0^\circ$ postoperatively. Hip flexion improved significantly from $72.5^\circ$ to $114.1^\circ$ . Pain scores (VAS) decreased from 7.96 to 1.24, and functional scores (HHS) increased from 48.1 to 90.1.
Spinopelvic Changes:
- Preoperatively, the cohort distribution was 11% stiff, 63% normal, and 26% hypermobile.
- Postoperatively, the distribution shifted toward normalization: 11% stiff, 76% normal, and 13% hypermobile.
- Mean $\Delta$ SS decreased slightly from $23.9^\circ$ to $20.9^\circ$ , while standing SS decreased and sitting SS increased slightly.
Correlations:
- Preoperative FFD correlated positively with preoperative $\Delta$ SS ( $r=0.45, p=0.002$ ).
- Preoperative FFD correlated negatively with standing PFA ( $r=-0.61, p<0.001$ ).
- Reduced preoperative ROM was associated with larger preoperative $\Delta$ SS ( $r=-0.33, p=0.02$ ).
Predictive Modeling: Multiple linear regression identified preoperative hip ROM ( $\beta \approx -0.35$ ) and dynamic Ante-inclination change ( $\Delta$ AI, $\beta \approx 0.23$ ) as independent contributors to postoperative $\Delta$ SS. The model explained approximately 30% of the variance ( $R^2 \approx 0.30$ ). However, the authors note that neither predictor reached statistical significance ( $p > 0.05$ ), likely due to the limited sample size.

Significance and Claims
The study demonstrates that THA in patients with FFD leads to significant clinical improvement and a normalization of spinopelvic mobility. The authors claim that apparent preoperative spinopelvic hypermobility in this population is largely a compensatory, hip-driven mechanism that becomes reversible following the correction of the flexion deformity.

The primary significance of the findings is the caution they raise regarding preoperative planning. The authors assert that planning acetabular orientation based solely on preoperative spinopelvic parameters may be misleading in patients with substantial FFD, as the pelvic alignment readjusts post-deformity correction. Instead, the study suggests that preoperative hip ROM and dynamic AI should be considered when anticipating postoperative pelvic realignment. The authors conclude that while dual-mobility bearings are often considered for spinopelvic issues, they should be reserved for patients with fixed spinal pathology, whereas patients with hip-driven hypermobility may achieve normalization through standard THA.

Limitations
The authors acknowledge the study's modest sample size (45 patients) and short-term follow-up (three months) as limitations. They explicitly state that the predictive model requires validation with larger cohorts to confirm the statistical significance of the identified predictors.

The Body's Tug-of-War: Why Your Hips and Back Are Best Friends

The Study: Untangling the Hip-Back Knot

Technical Summary: Correlation between Sagittal Plane Deformities of the Hip and Spinopelvic Alignment in Advanced Hip Arthritis Patients Undergoing Total Hip Arthroplasty

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