What usually causes anterior unilateral hip pain (with anteromedial thigh pain) when sitting in a car and especially when getting in and out of a car?

Could be: Osteoarthritis of the hip or if the pain comes across the thigh, consider meralgia parathesthica. Hope that helps.

Hip OA: Biomechanically, when getting up from sitting, there is an enormous amount of joint reactive force exerted on the hip. That is to say, muscle contractions drive the femur into the acetabulum, which is the fulcrum for movement. Therefore, any derangement to the joint (oa, avn, impingement, plica, etc) would be exacerbated. Typically, hip pain is felt into the groin, thigh, and to the knee.

Hip Labral Tear: This is one of the most common complaints I see inpatients with a labral tear of the Hip. Occasionally, you can see it in arthritis as well. The torn labral fragment gets pinched int he joint and causes pain with hyper flexion and rotation....ie getting in and out of a car.