Associations of Blood Biomarkers of Bone Turnover with Static Histomorphometry Parameters at the Hip in Patients with Chronic Kidney Disease Undergoing Surgery for Hip Fracture

This cross-sectional study of hip fracture patients found that while most exhibited low bone turnover, circulating biomarkers BAP, TRAP5b, and sclerostin were significantly associated with static histomorphometry parameters of bone formation and resorption, suggesting their potential utility as adjuncts to PTH for assessing bone turnover and guiding therapy in patients with and without chronic kidney disease.

The Gist

The Big Picture: A Broken Hip and a Broken Filter

Imagine your body is a busy construction site. Your bones are the buildings, and they are constantly being renovated. Old bricks are taken down (resorption), and new bricks are laid (formation). This cycle is called bone turnover.

Now, imagine you have a kidney problem (Chronic Kidney Disease, or CKD). Your kidneys are like the site's water filtration system. When the filter gets clogged, the water gets dirty, and the whole construction site gets confused. The workers (bone cells) might stop working, work too fast, or lay down bricks without the proper cement.

This study looked at people who had just broken their hips. The researchers wanted to answer a tricky question: Can we tell what's going on inside the bone just by looking at the blood, or do we need to take a piece of the bone out to see?

The Problem: The "Speedometer" is Broken

Usually, doctors use a test called PTH (Parathyroid Hormone) to check how fast the bone construction is moving. Think of PTH as the speedometer on a car.

• The Issue: In people with healthy kidneys, the speedometer works great. If the needle is high, the car is speeding (high bone turnover). If it's low, the car is crawling (low turnover).
• The Glitch: In people with kidney disease, the speedometer is broken. The needle might point to "fast," but the car is actually sitting still. Or it might point to "slow," but the car is racing. Because of this, doctors often guess the wrong treatment. If they give medicine to slow down a car that's already stopped, the bones get even weaker.

The Experiment: The "Bone Biopsy" vs. The "Blood Test"

To find the truth, the researchers did two things on 97 patients who were having surgery for a broken hip:

1. The "Ground Truth" (The Bone Sample): Since these patients were having their hip bones removed or drilled into for surgery, the doctors took tiny samples of the actual bone. They looked at them under a microscope. This is like sending a drone down to the construction site to count exactly how many workers are laying bricks and how many are taking them down. This is the most accurate way to know what's happening.
2. The "Blood Test": They also took blood samples from the same patients to measure various chemical markers (like BAP, TRAP5b, Sclerostin, and PTH). This is like looking at the exhaust fumes coming out of the car to guess how fast it's going.

The Findings: Who Got the Message Right?

The researchers compared the "drone footage" (bone samples) with the "exhaust fumes" (blood tests). Here is what they found:

• The Old Speedometer (PTH) Failed: The PTH blood test was not a good predictor of what was happening in the bone. It was like a broken speedometer; it couldn't tell the difference between a car that was stopped and one that was moving.
• The New Dashboard Lights Worked: They found three other blood markers that acted like new, accurate dashboard lights:
  • BAP (Bone Alkaline Phosphatase): This is a marker for the bricklayers. When BAP was high in the blood, the drone footage showed lots of bricklayers working.
  • TRAP5b: This is a marker for the demolition crew. When TRAP5b was high, the drone footage showed lots of workers taking bricks down.
  • Sclerostin: This is a "brake pedal." When Sclerostin was high, the construction slowed down.

The "Aha!" Moment:
In patients with kidney disease, the bone was often in a state of "low turnover." This means the construction site was almost empty. The bones were thin, and the new bricks weren't getting cemented (mineralized) properly. The BAP and TRAP5b blood tests correctly identified this "empty construction site," whereas the PTH test was confused.

Why This Matters: Choosing the Right Medicine

This is a huge deal for treatment.

• Scenario A: If a patient has a "fast" construction site (high turnover), doctors usually give medicine to slow it down (anti-resorptives).
• Scenario B: If a patient has a "stopped" construction site (low turnover), slowing it down makes the bones brittle and prone to breaking again. These patients need medicine that wakes up the workers (anabolic drugs like Teriparatide).

The Conclusion:
Because the standard PTH test is unreliable for kidney patients, doctors might be giving the wrong medicine. This study suggests that by adding BAP, TRAP5b, and Sclerostin to the blood test panel, doctors can finally see the "drone footage" without actually drilling into the bone.

The Takeaway in One Sentence

For patients with kidney disease who break a hip, the standard blood test (PTH) is a broken speedometer, but new blood tests (BAP and TRAP5b) act like a reliable GPS, telling doctors exactly how to fix the bone so it heals strong instead of getting weaker.

Technical Summary

1. Problem Statement

Patients with Chronic Kidney Disease (CKD) face a significantly elevated risk of hip fractures and post-fracture mortality compared to the general population. A critical clinical challenge in managing these patients is distinguishing between age-related osteoporosis and CKD-related Metabolic Bone Disease (MBD).

• The Diagnostic Gap: Bone Mineral Density (BMD) is the standard for fracture risk assessment but fails to characterize bone turnover status (low, high, or mixed). In CKD, low turnover (adynamic bone) and high turnover (hyperparathyroidism) can exist at similar BMD levels.
• Therapeutic Dilemma: Treatments are diametrically opposed; anti-resorptive agents (e.g., bisphosphonates) may worsen low-turnover disease, while anabolic agents (e.g., teriparatide) are preferred for low turnover.
• Current Limitations: The gold standard for assessing turnover is iliac crest bone biopsy with tetracycline double-labeling, which is invasive and rarely performed. Parathyroid Hormone (PTH) is the most common blood biomarker used, but its reliability as a standalone indicator of turnover in CKD is debated.
• Research Question: Can easily obtainable blood biomarkers accurately reflect local bone turnover status at the hip (a common fracture site) in patients with and without CKD, specifically when compared against static histomorphometry?

2. Methodology

Study Design:

• Type: Cross-sectional study.
• Population: 97 adults (mean age 80 ± 11 years; 67% female) undergoing surgical repair for hip fracture between December 2020 and March 2025.
• Cohorts: 68% had CKD (defined as eGFR < 60 mL/min/1.73m²); 32% had normal/mild kidney function (eGFR ≥ 60).
• Data Collection:
  • Blood: Fasting plasma collected pre-anesthesia.
  • Bone: Samples collected intra-operatively from the femoral head (hemiarthroplasty) or greater trochanter (intertrochanteric fracture).
  • Histomorphometry: Undecalcified bone sections were stained with toluidine blue. Static parameters were analyzed using the OsteoMetrics system.
  • Biomarkers: Seven markers measured: $\alpha$-Klotho, Bone Alkaline Phosphatase (BAP), Dickkopf-1 (DKK-1), Fibroblast Growth Factor 23 (FGF23), Tartrate-Resistant Acid Phosphatase 5b (TRAP5b), Parathyroid Hormone (PTH), and Sclerostin.

Statistical Analysis:

• Turnover Definition: Since tetracycline labeling was impossible in acute fracture settings, the study utilized static histomorphometry cutoffs validated in prior work. Low turnover was defined as Osteoblast Surface/Bone Surface (Ob.S/BS) < 9.74% and Eroded Surface/Bone Surface (ES/BS) < 9.68%.
• Tertile Analysis: Tertiles for Ob.S/BS and ES/BS were established in the non-CKD group and applied to the entire cohort to categorize turnover levels.
• Models: Multinomial and multivariable linear regression models were used to assess associations between blood biomarkers and histomorphometry parameters, adjusting for age, gender, eGFR, and osteoporosis status.

3. Key Contributions

• Site-Specific Validation: This is the first study to correlate blood biomarkers with hip-specific bone histomorphometry in a CKD population, addressing the limitation that iliac crest data may not reflect hip physiology.
• Static vs. Dynamic: Demonstrated the utility of static histomorphometry parameters (Ob.S/BS and ES/BS) as surrogates for turnover status in acute trauma settings where dynamic labeling is unfeasible.
• Biomarker Hierarchy: Provided evidence that BAP and TRAP5b are superior to PTH for assessing turnover status in this specific high-risk population.

4. Key Results

Demographics and Bone Quality:

• Turnover Status: 96% of the cohort (including 68% with CKD) exhibited low bone turnover based on histomorphometry cutoffs.
• CKD vs. Non-CKD: Patients with CKD had significantly thinner trabeculae (lower Trabecular Thickness) and higher unmineralized bone (higher Osteoid Thickness, Volume, and Surface), suggesting a defect in mineralization despite low turnover.
• PTH: PTH levels were significantly higher in CKD patients but were not associated with histomorphometric measures of bone turnover (Ob.S/BS or ES/BS).

Biomarker Associations:

• Positive Associations (Direct):
  • BAP (Bone Alkaline Phosphatase): Per doubling of BAP, there was a 222% increase in Ob.S/BS and an 87.5% increase in ES/BS. BAP was strongly associated with higher turnover tertiles.
  • TRAP5b: Per doubling of TRAP5b, there was a 319% increase in Ob.S/BS and a 142% increase in ES/BS.
  • These associations persisted after adjusting for eGFR, suggesting they are robust markers of turnover independent of kidney function.
• Negative Associations (Inverse):
  • Sclerostin: Per doubling of sclerostin, there was a 28.9% decrease in ES/BS. Higher sclerostin correlated with lower turnover.
• Non-Associations:
  • PTH: No significant correlation with Ob.S/BS or ES/BS.
  • FGF23: Showed an inverse association with ES/BS in some models but was less consistent than BAP/TRAP5b.
  • $\alpha$-Klotho and DKK-1: No significant associations with turnover parameters.

5. Significance and Clinical Implications

• Redefining Turnover Assessment: The study suggests that BAP and TRAP5b are more reliable indicators of bone turnover status at the hip than PTH in patients with CKD undergoing hip fracture surgery.
• Therapeutic Guidance: Given that 96% of these patients had low turnover, the findings support the potential use of anabolic therapies (e.g., teriparatide, abaloparatide) rather than anti-resorptives for this specific population, provided turnover status is confirmed.
• Clinical Utility: Blood biomarkers (specifically BAP and TRAP5b) could serve as a non-invasive "liquid biopsy" to guide treatment decisions, potentially reducing the need for invasive bone biopsies.
• Sclerostin Nuance: While sclerostin showed an inverse relationship with turnover, its levels are influenced by eGFR; thus, it requires further study regarding normalization for kidney function before routine clinical adoption.
• Limitations: The study is cross-sectional, and the high prevalence of low turnover (96%) limits the ability to study high-turnover dynamics. Additionally, the inability to use tetracycline labeling necessitated reliance on static parameters, though these were validated in prior work.

Conclusion:
In patients with CKD presenting with hip fractures, blood biomarkers BAP and TRAP5b are strongly associated with local bone turnover at the hip, whereas PTH is not. These biomarkers offer a promising, non-invasive tool to stratify bone turnover status and guide appropriate pharmacologic therapy in a population where treatment errors can be catastrophic.