Knee X-Ray in Pregnant Patient with Instability
Yes, you can and should perform a knee X-ray on this pregnant patient—the radiation exposure to the fetus from knee radiography is negligible (predicted fetal absorbed dose <1 mGy), and the benefit of diagnosing a potentially serious injury causing falls far outweighs any theoretical risk. 1
Radiation Safety in Pregnancy
The American College of Radiology explicitly states that women can be reassured that benefit far outweighs risk with regard to diagnostic imaging of extremities during pregnancy, because all radiographs not involving the abdomen or pelvis have a predicted fetal absorbed dose of less than 1 mGy 1. This is well below the threshold of concern for fetal harm.
- Knee radiographs should be the initial imaging evaluation because findings may be conclusive, eliminating the need for further imaging 1
- The specific risk to the fetus from radiation appears to be childhood malignancy, with theoretical projections suggesting that for each 10-mGy exposure there is a maximum risk of 1 additional cancer death per 1,700 exposures 1
- There are no diagnostic radiographic procedures considered a risk factor for genetic damage, malformation, or neurodevelopmental effects based on current knowledge 1
Clinical Justification
This patient's presentation of knee pain with instability causing falls represents a significant safety concern that requires immediate diagnostic evaluation:
- Falls during pregnancy pose substantial risk to both mother and fetus, making timely diagnosis critical 2
- Radiographs are the appropriate initial imaging study for acute knee trauma with focal tenderness, effusion, or inability to bear weight 1
- The instability suggests potential ligamentous injury, loose body, or other structural pathology that could lead to recurrent falls if left undiagnosed 3
Imaging Protocol
Obtain standard knee radiographs (minimum anteroposterior and lateral views) as the first-line imaging study 1:
- The lateral view should be obtained with the knee at 25-30 degrees of flexion to evaluate for joint effusion 1
- Additional views (patellofemoral, oblique) can be added based on clinical suspicion without meaningful increase in fetal radiation exposure 1
- If radiographs are negative but clinical concern persists, MRI without contrast is the next appropriate study as it uses no ionizing radiation 1
Common Pitfalls to Avoid
- Do not delay imaging due to pregnancy concerns when there is a legitimate clinical indication—the risk of missing a serious injury outweighs minimal radiation exposure 1
- Do not proceed directly to MRI without obtaining radiographs first, as radiographs may be diagnostic and are more readily available 1
- Ensure proper shielding of the abdomen and pelvis during the X-ray, though this is more for patient reassurance than actual necessity given the distance from the knee 2