The ‘vacuum sign’ (hip vacuum sign): a new radiographic phenomenon in femoroacetabular impingement

Femoroacetabular impingement syndrome is a risk factor for the development of lesions of the labrum and of the articular cartilage of the acetabulum, and for subsequent osteoarthritis of the hip joint. It has a high incidence in young and physically active patients and typically presents with exercise-related pain in the groin. The radiographic findings encountered in femoroacetabular impingement syndrome are relatively common. They are seen in 14-35% of the asymptomatic population. In patients with hip pain and symptomatic tears of the labrum this incidence reaches 90%. The radiographic findings play an important role in identifying the underlying pathology, and conventional radiographs form the basis of the diagnostic process. In addition to anteroposterior views, at least one lateral view is needed, and in such cases the preferred view is the one taken with the patient in the frog-leg position. Besides the well-known radiographic findings of femoroacetabular impingement syndrome that have been widely described in the literature, a new sign has also been described, the so-called ‘vacuum sign’ (hip vacuum sign). Vacuum phenomena have been reported in various joints, such as those of the spine, the shoulder or the knee. They may be seen in joints under traction or in degenerative disease. In some cases, such as in the spine or the shoulder, these findings are associated with instability of the joint. In the hip joint the ‘vacuum sign’ is very widely recognised during hip arthroscopy. Until recently, however, the ‘vacuum sign’ had never been described in the hip joint without traction. Recent publications have described this sign on conventional preoperative radiographs as well, and of course without the joint being under traction. Specifically, it has been observed on views taken with the patient in the frog-leg position. The sign is described as a radiolucent line appearing within the joint space. It is important to note that the rate at which this sign appeared in symptomatic patients with femoroacetabular impingement syndrome reached 14.6%. The existence of this sign is difficult to explain, but the prevailing interpretation of its aetiology is the vacuum phenomenon, produced by a levering mechanism between the acetabular rim and the femoral neck in cases of femoroacetabular impingement. According to this theory, the finding is caused by a subluxation, or at least a distraction of the joint, as a result of the underlying mechanism of femoroacetabular impingement. It is therefore very likely that this sign is also associated with a minor degree of instability of the hip joint, as this has been described in the recent literature. Source: Jörg H. Schröder, Nils Marschalek, Martin Hufeland, Carsten Perka. The ‘Hip Vacuum Sign’—a new radiographic phenomenon in femoro-acetabular impingement. J Hip Preserv Surg (2016) 3 (4): 346-351