Hip kinematics and kinetics in persons with and without cam femoroacetabular impingement during a deep squat task, by Bagwell, Snibbe, Gerhardt & Powers, in Clinical Biomechanics (2015)
Femoroacetabular impingement (FAI), which has also been called hip impingement syndrome is a condition in which the head of the femur moves in an abnormal way within the acetabulum of the pelvis or contacts structures in such a way that it causes damage. Underlying changes in hip joint structure lead to a reduction in hip clearance that results in impingement during normal daily range of motion. There are several forms of FAI but the most common is dynamic impingement of the hip, which can be caused by one of several factors, including a cam lesion, a pincer lesion, a combined cam and pincer lesion, proximal femoral retroversion, or coxa vara. However, the only two types of dynamic impingement that are commonly discussed in respect of FAI are cam lesions and pincer lesions. A cam lesion, which is short for “camshaft lesion” because of the way that the bone growth resembles a camshaft in this case, occurs where there is an increase in the width of the femoral neck at the femoral head-neck junction. The reduced femoral head-to-neck offset distance pushes the femur into contact with the acetabulum early in the arc of internal rotation of the hip. A pincer lesion, occurs where there is an excess of growth of the upper lip of the acetabulum. A combined cam and pincer lesion displays elements of both of these deformities. For completeness, the other deformities are discussed below.
Proximal femoral retroversion is where the femoral neck contacts the labrum of the acetabulum of the pelvis earlier in the arc of functional motion. Coxa vara, which is a deformity of the hip, in which the angle between the ball and the shaft of the femur is reduced to <120 degrees. Coxa vara frequently results in a limp and is often caused by injury but can also occur when the bone of the femur is softer than it should be and bends under the weight of the body. Since there are many causes of FAI, the underlying anatomical, biomechanical or environmental risk factors are likely to be multifactorial and difficult to ascertain. However, some researchers have identified that there are certain anatomical factors that can contribute to FAI, including poor anterolateral orientation, increased relative depth of the hip acetabulum, and offset abnormalities of the femoral head-neck junction. In addition, some researchers have observed that hip strength is reduced in many individuals who display FAI, although whether this reduction is simply a result of inhibition due to the painful experience, disuse atrophy or previous lack of hip strength is unclear.
OBJECTIVE: To compare the joint angle movements at the hip, including peak hip flexion, peak hip abduction, and peak hip internal rotation, at the pelvis (all measured using an 11-camera motion analysis system) and the associated joint moments (as estimated using inverse dynamics) in a deep squat between individuals with a diagnosis of cam FAI and pain-free, healthy control subjects.
POPULATION: 15 individuals with cam FAI, aged 32.2 ± 7.8 years, and 15 age- and sex-matched individuals, 31.9 ± 7.6 years, without cam FAI.
Joint angle movements
The researchers found that the individuals with cam FAI displayed less peak hip internal rotation than the healthy control subjects (15.2 ± 9.5 vs. 9.4 ± 7.8 degrees). They also displayed less posterior pelvic tilt in the eccentric phase of the squat descent compared to the healthy control subjects, resulting in more anterior pelvic tilt at the point of peak hip flexion (12.5 ± 17.1 vs. 23.0 ± 12.4 degrees).
The researchers found that the individuals with cam FAI displayed lower mean hip extensor moments (0.56 ± 0.12 vs. 0.45 ± 0.15Nm/kg).
What did the researchers conclude?
The researchers concluded that the individuals with cam FAI displayed reduced squat depth, reduced peak hip internal rotation, and more anterior pelvic tilt at the point of peak hip flexion.
The study was limited in that it was cross-sectional and it is therefore unclear whether the features of the individuals with cam FAI were present before their injury or whether the injury led to the features being displayed in compensation for pathology.