Femoroacetabular impingement (FAI) describes abnormal contact between the acetabular rim and the femoral head-neck junction secondary to a spectrum of morphologies about the femoral head-neck junction (cam lesions) and acetabular rim (pincer lesions).1,2 FAI has been increasingly recognized as a source of prearthritic hip pain over the past 20 years.1,2 Acetabular-based FAI, or pincer-type FAI, is characterized by abnormal contact between the acetabular rim and labrum and the femoral head-neck junction stemming from acetabular over-coverage of the femoral head (Fig. 1). The reported incidence of pincer-type FAI varies across the literature. Pincer morphology has been found to be present in up to 67% of asymptomatic patients, and approximately 8% of patients with symptomatic FAI, or FAI syndrome (FAIS).3, 4, 5 It is also found in combination with cam morphology in approximately 45% of cases.3 Patients with symptomatic pincer morphology tend to be young, athletic, and female.2

Pincer morphology can be further classified as true acetabular over-coverage secondary to either focal over-growth of the anterior rim or global over-coverage from the depth of the hip socket (coxa profunda or protrusio acetabuli), or relative over-coverage secondary to acetabular retroversion.6 Relative acetabular over-coverage from acetabular retroversion is associated with both posterior femoral head under-coverage in addition to anterior over-coverage (Fig. 2).7,8 Subspine impingement is an extra-articular form of acetabular impingement that occurs secondary to dysmorphism of the anterior inferior iliac spine (AIIS).6,8, 9, 10 Given the variety of morphologies associated with pincer-type FAIS, it is important to understand their associated pathomechanics and sequelae in order to appropriately manage these patients.