The acetabular “full moon sign” resulting from aseptic loosening of a total hip arthroplasty

A 64-year-old woman presented with right hip and buttock pain irradiating into her knee for the past 4 months. The patient had a total right hip arthroplasty 5 months prior. She describes a feeling of “movement” in her hip. There is no recent history of trauma.

X-ray

Round hyperdense image, resembling a full moon and representing the acetabular component of the total hip arthroplasty, which has migrated within the acetabulum. No peri-prosthetic fracture or lucency was visible (Figure 1).

CT-scan

Loosened acetabular component of the total hip arthroplasty, which has rotated through 90 degrees in the sagittal and has positioned itself in the posterior part of the acetabulum and is thus seen face-on, appearing as a “full moon” as opposed to the “half moon” that is habitually seen when it is correctly positioned (Figures 2, 3 and 4).

Background

Total hip replacements are common procedures, with around 100,000 performed per year, a number that continues to grow [1]. Aseptic loosening of a hip prosthesis, defined as “inadequate initial fixation, mechanical loss of fixation over time, or biologic loss of fixation caused by particle-induced osteolysis around the implant” may require a revision [1]. However, this is a relatively rare occurrence, with an incidence of 2% in the first 18 months and 1% per year subsequently [2,3]. Complications such as loosening are difficult to diagnose using x-rays, and other modalities, such as CT, are often called upon to help problem-solve [1].

Imaging Perspective

The patient was referred to radiology by the emergency department for a diagnostic ultrasound-guided hip-arthrocentesis for a suspected hip effusion. The arthrocentesis was attempted, yielding no fluid. However, there was a high suspicion of a cotylar fragment, which prompted a follow-up CT scan.

In this case, the imaging pearl is the round, “full moon” appearance of the acetabular component, being seen head-on on the AP hip x-ray. This appearance is due to the acetabular component, which is a concave semi-sphere, being seen in its entirety and thus fully facing towards or away from the x-ray machine. If the concave section of the semi-sphere were not facing towards or away from the x-ray machine, then it would result in a non-spherical, grossly ellipsoid image. Normally, one expects to see a “half moon” appearance of the acetabular component when it is well positioned due to it being rotated within the acetabulum to properly accommodate the head of the femoral prosthesis. In cases where doubts remain, a CT-scan can be used to evaluate the position of the acetabular component [1]. X-rays are the initial screening modality, which should first be performed post-operatively in a standing position, once the patient is able to do so, and not immediately post-operatively, as in that case, they are ineffective for complication screening [1]. The standard incidences are a weight-bearing AP radiograph and a lateral radiograph [4]. The radiographs of the patient performed one year before the loosening of the prosthesis are normal (Figures 6 and 7). Although x-rays are able to detect most forms of prosthesis failure, other modalities can be useful if x-rays are inconclusive [1]. A CT-scan can be used to detect subtle lucencies which can go unnoticed on x-rays and provide better evaluations of the layout of the prosthetic components, as demonstrated in this case, where we are definitively able to show the mispositioning of the acetabular component [1]. Furthermore, PET-CT may be of use in order to differentiate between septic and aseptic loosening [1].

Outcome

Imaging is important in cases of failing hip replacement to determine the type of failure which is occurring and thus dictate the management. Once the diagnosis of a displaced acetabular component has been made, the patient should undergo revision surgery to replace the hip prosthesis, as was the case for our patient [5].

Take Home Message / Teaching Point

The “full moon" sign should raise the suspicion of mispositioning of the acetabular component of a hip prosthesis and prompt further imaging to confirm the diagnosis.

All patient data have been completely anonymised throughout the entire manuscript and related files.