10.2 Femur Selection: Right, Left, and Dual Femur Scanning

10.2 Femur Selection: Right, Left, and Dual Femur Scanning

Quick Answer: Dual femur scanning acquires sequential DXA scans of both proximal femurs during one session. ISCD draws a sharp line between the two uses: when both hips have been scanned, diagnostic classification uses the lowest T-score of the right or left femoral neck or total hip and explicitly not the mean, while mean bilateral total hip BMD should be used for monitoring on repeat tests. Non-standard clinical challenges include severe hip osteoarthritis (hypertrophic osteophytes and sclerosis falsely elevate BMD, requiring exclusion of affected ROIs and reliance on the lumbar spine or 33% radius), total hip arthroplasty (metal components invalidate the scan, requiring scanning of the contralateral hip), and orthopedic pinning. In pediatric patients (under 20), proximal femur scanning is strictly contraindicated by the ISCD due to rapid skeletal growth, maturity variability, and lack of reproducible pediatric normative hip databases; pediatric evaluation is restricted to the PA spine and whole body less head (WBLH).

Dual Femur Scanning Protocols and Clinical Advantages

Dual femur densitometry involves scanning both proximal femurs consecutively during a single appointment, utilizing automated software that moves between hips without repositioning the patient:

Clinical Indications and Rationale

1. Detecting Asymmetry and Discordance: Up to 30%–40% of patients exhibit left-to-right BMD discordance exceeding 0.5–1.0 T-score standard deviations due to limb dominance, trauma, or osteoarthritis. Scanning both femurs identifies the true skeletal nadir, preventing diagnostic under-staging.
2. Improved Diagnostic Sensitivity: Evaluating bilateral femoral necks and total hips captures four validated candidate sites, identifying osteoporosis that would be missed on a unilateral hip.
3. Establishing a Superior Monitoring Baseline: Averaging bilateral total hips produces a combined metric with exceptional statistical stability.

The Mathematical Precision Advantage

In serial monitoring, precision error (% CV) determines the Least Significant Change (LSC) ($\text{LSC} = 2.77 \times \text{CV}$): Averaging bilateral total hips into a Mean Total Hip BMD cancels random positioning errors and quantum noise across independent scans, reducing precision error by a factor of $\sqrt{2}$ (~1.414). A baseline precision error of 1.2% drops to ~0.85%, tightening the LSC and enabling earlier detection of true biological response to therapy.

ISCD Positions on Dual Femur Interpretation

• For diagnosis: when both hips have been scanned, use the lowest T-score among the four candidate regions (left femoral neck, right femoral neck, left total hip, right total hip). ISCD states explicitly that the mean T-score or Z-score must not be used for diagnostic classification.
• For monitoring: when both hips have been scanned on repeat tests, mean bilateral total hip BMD should be used. This is where the precision advantage of averaging is realized.
• The distinction is deliberate. Averaging protects a monitoring comparison from random positioning noise, but it also conceals the lower of two hips — and diagnostic classification is defined by the weakest valid site, not the average one.
• Serial monitoring can track either the Mean Total Hip or the lower individual Total Hip, maintaining the same protocol across follow-up scans.

Non-Standard Hips and Challenging Pathologic Scenarios

Severe Hip Osteoarthritis (Degenerative Joint Disease)

Osteoarthritis produces joint space narrowing, subchondral sclerosis, and hypertrophic marginal osteophytes:

• Radiologic Impact: Sclerotic spurs project over the femoral neck and intertrochanteric regions, artificially elevating measured BMD and T-scores and masking osteoporosis.
• Technologist Protocol: If severe spurring affects one hip, scan the contralateral hip. If both hips exhibit severe degenerative changes, the proximal femur must be excluded from diagnostic consideration, and the technologist must acquire the PA lumbar spine or 33% (one-third) radius of the non-dominant forearm.

Total Hip Arthroplasty (THA) and Metallic Hardware

• Impact: Metallic stems, screws, and cement cause complete beam attenuation, invalidating edge detection.
• Protocol: Scan the contralateral non-operated hip. If bilateral total hip replacements are present, central hip densitometry cannot be performed; scan the PA lumbar spine and 33% radius.

Pediatric Proximal Femur Scanning: ISCD Official Positions

ISCD Pediatric Official Position: Proximal femur DXA scanning is not recommended for diagnosis or monitoring in growing children and adolescents prior to skeletal growth completion and epiphyseal fusion.

Developmental Justifications

1. Dynamic Skeletal Growth: Non-linear growth alters the neck-shaft angle from infancy (~150°) through adolescence (~125°–130°), confounding automated edge-detection.
2. Growth Plate (Physis) Variability: Capital femoral and trochanteric physes display variable mineralization based on biological maturity.
3. Absence of Normative Databases: Universal, robust pediatric normative hip databases comparable to adult datasets do not exist.
4. Poor Precision: ROI placement on developing pediatric femurs produces high precision errors.

Approved Pediatric DXA Sites

In patients under age 20, densitometry must be performed using:

1. PA Lumbar Spine (L1–L4): Highly reproducible, responsive trabecular site with robust pediatric reference curves.
2. Whole Body Less Head (WBLH): Evaluates systemic mineralization while excluding the skull.
3. Pediatric Reporting Rules: Results must be expressed strictly in Z-scores adjusted for age, sex, and height-for-age Z-scores (HAZ), never T-scores.