(Translated from German)

Choosing the right lateral view at the hip is crucial to avoid complications and ensure the best possible healing for the patient.

One of the most important decisions in the diagnostic process is the choice of the right diagnostic procedure – and this especially holds true for x-ray exams of the hip. The lateral view of the hip according to either the lateral frog leg view or the horizontal beam (also called cross-table, or horizontal ray) should be familiar to the reader. In practice, it can be observed that often only the frog leg view is taken. Why is this so and in what situation is this projection contraindicated? And what should be considered in the respective setting? In this article, the two different possibilities are examined.

(If you are unfamiliar with these methods, get an overview on “Adult Hip Radiographs” by orthobullets.com)

Why do many clinics or practices only perform frog leg lateral view?

In many clinics or practices, the frog leg lateral view is preferred and used as a standard for all cases, as it is an easy and fast method to carry out. It also has the advantage that it causes a lower radiation dose compared to the horizontal ray. And yes, it's true, performing the horizontal ray is more time-consuming, more complex to set up, and the dose-area product (DAP) can be higher than the frog leg view. Nevertheless, there are clear indications/contraindications for the respective variants.

When frog leg and when horizontal beam?

For a simple overview of the variants to be chosen, here is a short presentation:

Especially if a femoral neck fracture is suspected, the horizontal view should be used as a lateral view. In this case, the frog leg is not only unsuitable, but can significantly aggravate the patient's condition (dislocation of the fracture) and is usually performed only with additional pain. The horizontal ray is, if performed safely, a less painful method to visualize the hip joint laterally, without the risk of further dislocating or worsening of a fracture.

Here is an example of how an acetabular fracture was aggravated by the frog leg view.

In patients with limited mobility, the leg often cannot be tilted outwards by 45°, and lifting the opposite side is sometimes not expedient. Then the frog leg view reaches its limits, as can be seen below.

Examples of diagnostically inadequate Frog leg images

Since, as described above, horizontal beam is far less well known, the procedure for the most common indications is discussed here.

A) Horizotnal Beam: the correct procedure for suspected hip fractures 

Hip overview ap projection with measuring ball

In a patient with a leg length difference, there is a high probability of a femoral neck fracture or a femoral fracture. The first diagnostic step in case of suspected hip fracture is the preparation of a hip overview in anteroposterior (AP) projection using a measuring ball.

At least half of the measuring ball must be depicted in the image and positioned in the middle at the level of the femur bone.

This AP image serves as a comprehensive overview and allows for a basic assessment of the hip joints and femur. The use of a measuring ball is essential here, as it allows precise measurements for the surgical planning of the femoral head. This is particularly important to precisely determine the required implant sizes and to ensure optimal preparation for the surgical procedure.

Normally, the legs are turned inwards from the hips (15° internal rotation) to compensate for femoral antetorsion. However, if a femoral neck fracture is suspected, caution should be exercised. Especially in the presence of pain or a leg length difference, internal rotation should be avoided to minimise the risk of aggravation of the injury.

Pictures: Agata Epler Adjustment method hip overview with measuring ball and X-ray image with surgery of the drawing for planning

If a fracture is detected in the hip overview, the frog leg view or the angled lateral hip view (also “rolled lateral view”) should not be performed as a lateral view under any circumstances.

The abduction and rotation of the affected leg required for frog leg imaging can significantly aggravate the patient's condition. This movement can lead to further dislocation of the fracture and is also extremely painful for the patient.

Example: Frog leg shot collimations with more or less of the femur in the picture. Including the femur could be necessary because of a long endoprosthesis. Required positioning aid – 45° wedge       

Angled lateral hip view: The unaffected side of the patient is lifted by 45° and the leg is rotated outwards. This positioning is not suitable for a hip fracture because it is painful and can lead to further dislocation of the fracture.

Pictures: Agata Epler, Example: Angled lateral hip view

Horizontal beam as lateral view

The horizontal offers the necessary alternative for a lateral view. It allows the projection of the hip joint without the need to move the affected leg. The patient lies in a supine position, with the unaffected leg bent and lifted. The affected leg remains extended. The detector / wall stand and the X-ray tube are directed at a 45° angle to the hip. This adjustment technique significantly minimises pain and the risk of exacerbation of the fracture.

It is important to note that lifting the healthy leg for this procedure can also be challenging for some patients. The patient must be able to hold up the leg that is not to be examined by holding it with both hands at the back of the knee or by using an aid such as a knee roll. Ideally, a dedicated positioning aid is used that offers stability, is radiolucent and can be adjusted at the appropriate height. The leg rest in the picture is manufactured by Pearl Technology and suits this position nicely.

Picture: Agata Epler – Example of aids and positioning agents for the axial hip according to the horizontal beam view

Wobbly or unstable positioning aids increase the risk of further dislocating the fracture due to abrupt slipping out of position during horizontal beam examination. 

B) Horizontal Beam: the right approach to postoperative control 

Postoperative Control 

Even during the first postoperative check-up, the frog leg or angled lateral view should be avoided. If the patient is not yet mobile, the horizontal beam should be used as a lateral view. The first view is the anteroposterior (AP) hip overview, which is great for marking the length of the implant and the upper boundary. This makes the adjustment technique of the lateral view much easier. The use of a measuring ball in hip overview imaging can also be helpful in postoperative control but does not necessarily have to be done. However, it usually doesn't hurt.

 Picture: Agata Epler, Post-op control hip overview without measuring ball and lateral hip according to the horizontal beam.

The horizontal beam allows lateral projection of the hip joint without moving the affected leg, which is crucial for painless and safe postoperative control. By choosing the horizontal beam, you can create suitable projections more reliably and avoid insufficient results as shown below.

First postoperative day: Control with Redon drain.

When trying to perform a frog leg view, the leg could not be turned outwards sufficiently due to severe pain, which makes the projection unsuitable as a lateral view. 

Horizontal Beam: what do you need to consider?

As mentioned earlier, it is true that performing the horizontal beam is more time-consuming, more complex to adjust, and the dose-area product (DAP) can be higher than in frog leg view. Often, however, inadequate adjustment technique or exposure are the cause of inadequate horizontal beam scans. To achieve diagnostically usable results, it is crucial to correctly adjust, position and expose the horizontal beam.

Examples of diagnostically inadequate horizontal beam images

When using automatic exposure, the exposure time is often not sufficient to adequately display the femoral neck and acetabulum. This is because the ionisation chamber is positioned on the femur and soft tissues. Therefore, free exposure is preferable. As a tip for the horizontal beam: To compensate for the differences in thickness, a bean bag (NOT a sandbag!) can be positioned on the thigh and exposed freely. For a mobile detector with a virtual grid, 75-85 kV and 16-24 mA are recommended, while 75-85 kV and 20-50 mA should be used for the bucky stand.

1. Example of a potential source of error 

The suspicion of a fracture is confirmed by the AP image. It must be ensured that a measuring ball has been used correctly and that a hip overview image has been registered and carried out. In the present example, however, a pelvic overview was incorrectly registered, although a hip overview image would have been required for surgical planning.

2. Example of a potential source of error 

The AP view lacks the measuring ball that would be required for surgical planning.

Again, the AP projection is crucial to determine which lateral view should be set. In the case of a femoral neck fracture, the lateral hip should therefore be adjusted as horizontal beam, as in this example. The prerequisite is that the healthy leg is stably elevated at a 90° angle. Either the patient holds the leg in the hollow of the knee with both hands, or appropriate positioning aids are used to hold this position safely.

Conclusion 

The choice of the right lateral view is a basic prerequisite for careful and painless diagnostics and the first step towards successful treatment and healing.

To avoid complications in the event of a femoral neck fracture or acetabular fracture and to ensure optimal healing, the horizontal beam is the suitable variant for the lateral view. It is a misconception that the horizontal beam is more painful and makes the fracture worse. In fact, it is much less painful for the patient. Frog leg lateral view is more painful for the patient, can further dislocate the fracture and is therefore contraindicated.

Horizontal beam should also be preferred postoperatively until the patient is mobilized.

To perform it successfully, the patient should be able to hold up the leg that is not to be examined safely or a suitable positioning aid should be used to support the leg. When choosing suitable aids, it is important to ensure that they offer stability, can be adjusted to the appropriate height and are radiolucent.

Agata Epler, Expert Radiology Technology – Lecturer – “Alles Einstellungssache"

For a patient positioning course (in German): optimal positioning and exposure of the horizontal beam-image: alleseinstellungssache.com