The Influence of Abnormal Hip Mechanics on Knee Injury: A Biomechanical Perspective
In physically active individuals, the knee is the joint in the lower extremity which sustains the highest percentage of injuries. Research conducted over the last several decades suggests that knee injury may have proximal influence from the hip and trunk. The authors of the commentary, The Influence of Abnormal Hip Mechanics on Knee Injury: A Biomechanical Perspective, provide a literature review on this topic, specifically addressing common knee injuries such as anterior cruciate ligament tear (ACL tear) iliotibial band syndrome (ITBS), and patella-femoral pain (PFP) and the role of the hip and the trunk in these injuries.
Proximal Contributions to Abnormal Tibiofemoral Joint Kinetics
During the stance phase of running and jumping, the femur adducts, internally rotates (IR), and flexes at the hip joint. These motions are primarily controlled by the hip extensors, abductors, and external rotators. Excessive femoral motion into flexion, IR, and adduction places abnormal stresses at the tibiofemoral joint.
In the presence of proximal hip weakness, some compensatory strategies of the pelvis and altered movement patterns of the lower extremity occur. The first compensatory strategy is in the presence of hip abductor weakness, the contralateral pelvis drops (Trendelenburg sign). This compensation causes increased strain to be placed on the iliotibial band and lateral collateral ligament. In compensated Trendelenburg, where the trunk leans over the stance limb, the stress is shifted to the medial knee, particularly the ACL and medial collateral ligament. The second compensatory strategy is in the presence of hip extensor weakness, a posterior trunk lean is common. This places stress through the anterior knee, possibly leading to quadriceps muscle strain, patella tendinopathy, patellofemoral joint compression, and ACL strain.
Tibiofemoral Joint Injury Mechanisms
ACL tears are one of the most common knee injuries sustained by individuals who engage in athletic and recreational activities. The rate of ACL tears is significantly higher in women, possibly due to the predisposed factor of a wider pelvis which causes an increased Q-angle at the knee. The Q-angle is the angle of the patella in relation to the pelvis and the tibia. Several studies have reported that reduced hip strength is related to greater knee valgus angles. Weakness of the hip extensors and abductors has been theorized to play a role in increased risk of females sustaining an ACL injury, however, the finding is not consistent across all studies. Additional factors, such as impaired motor control, may play a role with respect to movement pattern that are thought to be associated with ACL injury.
ITBS is a common cause of lateral knee pain and is the second most common overuse injury in runners, behind PFP. The strongest predictors of athletes who develop ITBS are excessive hip adduction and knee IR with running due to the increased stress on the lateral knee with these motions. In addition, studies show that hip abductor strength of the involved limb in runners with ITBS was significantly reduced when compared to the noninvolved limb and the control groups.
PFP is the most prevalent lower extremity condition seen in orthopedic practice and has been cited as the most common overuse injury in persons who are physically active. Females with PFP tend to have excessive IR of the femur and a greater dynamic Q-angle during running. This increased angle on the knee displaces the patella medially and increases the pressures within the patellofemoral joint.
Those with PFP tend to exhibit impaired strength of the hip extensors, abductors, and external rotators. Despite the fact that altered hip motion and diminished hip strength are common findings in females with PFP, only two studies have evaluated hip strength in conjunction with hip kinematics in this population. One study reported significant reductions in isometric hip external rotator and hip abductor strength in females with PFP compared to a control group, but no differences in hip adduction and internal rotation motion during stair descent were observed. A second study reported that females with PFP exhibited diminished hip muscle strength, but only isotonic hip extension endurance was found to be correlated with excessive hip internal rotation during running.
Pelvis and Trunk Stability
The hip abductor muscles are important to maintaining the pelvis level in the frontal plane. In theory, improving performance of the hip abductors would result in a more optimal alignment of the pelvis during single-limb activities and, in turn, protect the knee joint from excessive frontal plane moments created by compensatory adjustments of the trunk and the resulting movement of the body center of mass.
Excessive anterior tilting of the pelvis resulting from weakness of the posterior rotators of the pelvis (ie, gluteus maximus, hamstrings, and abdominals) and/ or tightness of the hip flexors may result in a posterior shift in the trunk position, placing more stress on the knee joint. Given the fact that impaired trunk proprioception and deficits in trunk control have been shown to be predictors of knee injury, the development of “core” programs should consider dynamic pelvis stability as an important piece.
Dynamic Hip Joint Control
While there is some debate whether abnormal hip kinematics are the result of diminished hip muscle strength or impaired motor control, both aspects of muscle performance should be considered when implementing a rehabilitation or injury prevention program. In particular, the gluteus maximus and gluteus medius should be the targeted muscles due to their important roles in dynamic hip stability. The gluteus maximus is best suited to provide 3-dimensional stability of the hip, as this muscle resists the motions of hip flexion, adduction, and internal rotation and is a strong hip extensor and external rotator. In contrast, the gluteus medius mainly functions to stabilize the femur and pelvis in the frontal plane.
Summary and Physical Therapy First Implications
It can be argued that interventions which address proximal impairments including pelvis and trunk stability and dynamic hip control may be beneficial for patients who present with various knee conditions. The therapists at Physical Therapy First are proficient at evaluating the strength and dynamic control of the trunk and pelvis, and creating individualized treatment plans and home exercise programs to address proximal impairments which could be contributing to knee pain.
Powers, Christopher (2010). The Influence of Abnormal Hip Mechanics on Knee Injury: A Biomechanical Perspective. Journal of Orthopedic and Sports Physical Therapy. Vol 40, Number 2, pages 42-51.