The hip X-ray is used primarily to demonstrate/exclude a fracture. Hip X-rays are also frequently opted for as initial test in chronic hip symptoms, e.g. osteoarthritis.
The hip joint can be imaged under various angles. A standard hip X-ray examination generally includes an anteroposterior (PA) image and a lateral image. Ideally, the AP image shows both hip joints (which strictly speaking makes it a pelvis X-ray) to allow comparison with the other hip. The lateral direction may be opted for in axiolateral images or a frog leg lateral image. The various directions are explained in more detail below.
The patient is placed on his/her back and the X-rays will pass through the hip joint from anterior to posterior (fig. 1). The leg is internally rotated 15˚ – 20˚ to achieve femoral anteversion. This will extend the femoral neck (collum), improving its evaluability. When the leg is rotated externally, the greater trochanter will project over the neck and improve imaging of the lesser trochanter (fig. 2).
Figure 1. Technique for AP image of the hip.
Figure 2. Hip image with internal rotation and external rotation. Note that internal rotation improves the evaluability of the femoral neck because the greater trochanter does not project over the neck.
There are various techniques for lateral imaging. The most commonly used images are the axiolateral image, the frog-leg lateral image and the Lauenstein image.
The patient is placed on his/her back. The unaffected hip is abducted and lifted (using a cushion/block as support), allowing a medial view of the affected hip (fig. 3). The X-ray machine is positioned so as to direct the X-ray beams exactly through the femoral neck (horizontal beam), achieving optimal unobstructed images and minimizing overprojection of the lesser/greater trochanter. The primary benefit is that the patient can leave the painful leg flat. After trauma or in immobile/postoperative patients therefore, the axiolateral image is first choice for lateral imaging.
Figure 3. Technique for axiolateral hip X-ray.
Frog leg lateral image
The hip is abducted (about 45˚) with the knee in flexion (about 30˚- 45˚). The foot can rest on the inside of the contralateral knee (fig. 4). The X-rays pass through the hip joint from medial to lateral. This technique can also be used to image both hip joints; the so-called Lauenstein image (= frog-leg image). The feet are positioned together (fig. 5). This image is particularly useful to evaluate the shape of the femoral heads and head/neck transitions; e.g. to confirm epiphysiolysis and Perthes disease (= avascular necrosis).
Figure 4. Technique for frog-leg lateral image.
Figure 5. Technique for Lauenstein image.
Note: it is important not to make a frog-leg lateral image/Lauenstein image if a fracture or hip luxation is suspected. This is very painful and may complicate the fracture/luxation.