Lower Extremities
Long Bone Measurement
Anatomy & Pathology
Key Anatomical Structures
- Hip joint: Formed by the articulation of the femoral head with the acetabulum of the pelvis.
Serves as the proximal reference point for femoral length and alignment. - Knee joint: Formed by the femoral condyles, tibial plateaus, and patella.
Acts as the central landmark for axis deviation and mechanical alignment. - Ankle joint: Formed by the tibia, fibula, and talus, creating the mortise configuration.
Provides the distal reference point for leg length and alignment evaluation. - Long bone alignment: Includes the continuous visualization of the femoral shaft, tibial shaft, and both joints at each end. Used to assess leg length discrepancy, varus/valgus deformities, and postoperative alignment after orthopedic correction.
- Soft tissues and landmarks: Identify greater trochanters, tibial spines, and malleoli to verify symmetry and rotation.
Common Pathologies
- Leg length discrepancy (anisomelia): Difference in total limb length due to congenital, developmental, or traumatic factors.
- Angular deformities (varus / valgus): Lateral or medial deviation of the femur or tibia relative to mechanical axis.
- Postoperative assessment: Used to verify alignment and hardware placement following arthroplasty or osteotomy.
- Fracture malunion / nonunion: Persistent angulation or shortening after fracture healing.
- Growth disturbances (pediatric): Premature closure of growth plates or leg-length changes after trauma or infection.
Procedure Overview
Long bone measurement—also called a teleoroentgenogram or scanogram—is a specialized imaging procedure used to evaluate leg length, alignment, and mechanical axis from hip to ankle.
Modern digital radiography systems acquire multiple exposures (hip, knee, ankle) that are automatically stitched into a single long-length image using software algorithms.
The study is typically performed upright and weight-bearing, ensuring that physiologic load and alignment are accurately represented. For reproducibility between studies, toes must point forward, pelvis must not rotate, and medial malleoli spacing must be standardized (usually 20 cm apart).
Routine Projection (ARRT Required)
AP Weight-Bearing Hips–Knees–Ankles
CR Location & Positioning
- SID: 40–72 inches (102–183 cm), depending on DR protocol. Longer SID reduces beam divergence and geometric distortion across the lower limb.
- Patient position: Upright, back against vertical DR system. Equal weight-bearing on both feet. Standing on a 2-inch riser to ensure inclusion of ankle joints.
- Adjustments: Medial malleoli spaced approximately 20 cm apart (or measured and documented if not possible).
Toes pointing directly forward in anatomic position. Legs straight; pelvis not rotated. Apply side marker and, if required for surgical planning, place a magnification marker at the level of the limb. - CR: Perpendicular to IR. Automated long-length DR system captures sequential exposures at:
- Hips (proximal)
- Knees (midpoint)
- Ankles (distal)
- Software merges images into one continuous stitched image.
- Patient instructions: Suspend respiration and remain motionless during the exposure.
- Exposure: Department protocol (balanced technique to encompass hip–ankle density range). DR stitching software automatically equalizes brightness across regions.
Evaluation Criteria
- Coverage: Entire femur and tibia visible, including both hip, knee, and ankle joints. Both legs imaged in full anatomic alignment.
- Rotation checks: Patellae centered over femora. Tibial plateaus symmetric side-to-side.
- Motion checks: Sharp cortical bone and trabecular markings from hip through ankle.
- Technique checks: Consistent brightness and contrast from hip to ankle (no density step-offs). Magnification marker visible when required. No visible stitching artifacts.
- Clinical aim: To measure leg length discrepancy, evaluate varus/valgus alignment, and provide baseline or postoperative comparison for orthopedic surgical planning.
Long Bone Measurement – Common Errors & Fixes
| Error | Appearance | Cause | Fix |
|---|---|---|---|
| Ankles missing | Ankles cut off bottom of image | Patient not on riser or IR field placed too high | Always use riser and confirm centering |
| Malrotation | Patellae off-center, asymmetric condyles | Toes not forward | Align toes straight, check patellar centering |
| Inconsistent spacing | Measurements vary on follow-up | Malpositioned legs | Standardize malleolar spacing (20 cm) or document |
| Stitching artifact | Visible seams or density mismatch | Patient moved, software misaligned | Repeat with patient still, reprocess stitch |
| Calibration error | Digital length incorrect | No magnification marker | Place calibration marker per protocol |
Hows & Whys of Long Bone Measurement Radiography
Anatomy
- Why must hips, knees, and ankles all be included?
Because accurate alignment and length can only be measured if both joints at each end of the femur and tibia are visualized. - Why is a magnification marker sometimes required?
To calibrate digital measurements so orthopedic surgeons can plan with true scale.
Positioning
- Why must the patient be upright and weight-bearing?
It shows true lower limb alignment under physiologic load, which is critical for diagnosis and pre-op planning. - Why is a 2-inch riser used?
To ensure both ankle joints are captured on the same stitched image. - Why must toes point straight forward?
Rotation of the legs alters the apparent length and angulation, producing inaccurate results. - Why are the medial malleoli spaced 20 cm apart (or measured)?
Standard spacing ensures reproducibility between follow-up studies and allows reliable comparison.
Technique & Image Evaluation
- How do you know if the patient was rotated?
Patellae are not centered over the femora, and tibial plateaus appear asymmetric. - What error is indicated if the ankles are cut off?
The patient was not on a riser or the IR coverage was positioned too high. - How do you know the DR stitching worked correctly?
No visible seam lines or abrupt density changes between hip, knee, and ankle exposures. - How do you verify reproducibility for follow-up?
Check consistent ankle spacing, upright stance, and proper use of markers across studies.
Clinical Applications
- Why is long bone measurement performed?
To quantify leg length discrepancy, assess angular deformities, and monitor orthopedic correction outcomes. - Why is digital stitching preferred over traditional film methods?
It reduces distortion, provides consistent density across exposures, and allows precise computer-based measurement. - Why should these exams be repeated only when clinically necessary?
Because they require large exposure fields and should be limited to essential follow-up intervals for orthopedic assessment.
