Lower Extremities
14 Long Bone Measurement
Anatomy
- Hip joint – femoral head and acetabulum
- Knee joint – femoral condyles, tibial plateaus, patella
- Ankle joint – tibia, fibula, talus, mortise joint
- Long bone alignment – femur and tibia shafts visualized continuously to assess length and angulation
Projection
AP Weight-Bearing Hips–Knees–Ankles
Also called Teleoroentgenogram or Scanogram
SID:
- 40–72″ (102–183 cm), depending on DR protocol
- Extended SID reduces divergence and distortion for long-length exposures
Patient Position:
- Upright, back against vertical DR system
- Equal weight-bearing on both feet
- Standing on 2-inch riser (to include ankles on the image)
Adjustments:
- Medial malleoli positioned 20 cm apart (or actual spacing measured and documented if not possible)
- Toes pointing forward in anatomic position
- Legs straight, pelvis not rotated
- Side marker placed, and a magnification marker attached to the IR if required for surgical planning
- Gonadal shielding when possible
CR:
- Perpendicular to IR
- Long-length DR system captures a stitched image from:
- Hips (proximal)
- Knees (midpoint)
- Ankles (distal)
- Computer software automatically merges the exposures into one full-length image
Patient Instructions:
-
Suspend respiration and remain motionless
Exposure:
- kVp: department protocol (balanced for hip–ankle range)
- DR stitching software corrects density differences between hip, knee, and ankle regions
Evaluation Criteria
Coverage
- Entire femur and tibia visible, including hips, knees, and ankles
- Both legs imaged in full anatomic alignment
Rotation Checks
- Patellae centered over femora
- Tibial plateaus symmetric side-to-side
Motion Checks
-
Cortical bone and trabecular markings sharp from hip through ankle
Technique Checks
- Consistent brightness/contrast from hip to ankle (software-stitched evenly)
- Magnification marker visible if protocol requires
- No visible stitching artifacts
Clinical Aim
- Detect leg length discrepancy
- Evaluate varus/valgus deformities (bowleg/knock-knee)
- Provide reproducible baseline and follow-up images 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, or 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.