Angle of Inclination

The angle of inclination refers to the frontal plane angle created between the femoral neck and the shaft of the femur (Figure 9-7, A). This angle, which normally measures about 125 degrees, directs the shaft of the femur toward the midline, thereby positioning the knee joint directly under the weight of the body. An angle of inclination of about 125 degrees is typically associated with the most optimal alignment of the hip joint, as indicated in Figure 9-7, A, by the alignment of the red dots.

Deviations in the angle of inclination of the hip can occur from abnormal development in early childhood or from trauma. Coxa valga describes an angle of inclination that is significantly greater than 125 degrees (Figure 9-7, C). Coxa vara, in contrast, describes an angle of inclination that is significantly less than 125 degrees (Figure 9-7, B); note the offset alignment of the red dots in Figure 9-7, B and C. In severe cases of either abnormal deformity, the hip joint can become increasingly unstable or can be subjected to high stress. In these cases, a surgical correction may become necessary to realign the proximal femur or the acetabulum. Without the correction, abnormally high stress on the joint may cause degeneration of the joint, pain, and abnormal gait.

Torsion Angle

Although it is difficult to visualize, the femur is naturally twisted along its long axis. This twist is described as torsion between the shaft and neck of the femur. The angle of torsion can be appreciated by placing a femur flat on a tabletop. Relative to the femoral condyles (which are parallel with the tabletop), the femoral neck normally projects upward about 15 degrees. This 15-degree torsion angle is often referred to as normal anteversion, shown from above in Figure 9-8, A. Similar to the normal angle of inclination, normal anteversion is typically associated with the most optimal alignment between the femoral head and the acetabulum, as indicated by the close alignment of the red dots in Figure 9-8, A. Figure 9-8, B, shows a hip with excessive anteversion.

Figure 9-8, C, shows an example of a hip with retroversion, with a femoral neck-to-shaft angle of significantly less than 15 degrees. In an effort to optimally align the hip joint, an individual may internally rotate the hip (in cases of excessive anteversion) or externally rotate the hip (in cases of retroversion) during standing and walking. Figure 9-8, D, shows the compensatory internal rotation of the hip, or “in-toeing,” that may occur to help align a hip with excessive anteversion. Clinicians should be aware that an individual displaying either “in-toeing” or “out-toeing” may in fact be properly positioning the hip to best disperse the compressive forces of weight bearing.

Muscular Efficiency of Standing at Ease

Most persons can stand for long periods of time using only minimal amounts of muscular energy about the hip. This near “free lunch” is attained primarily because of the relationship between the ligaments of the hip and the line of gravity. While standing in full upright posture, the line of gravity normally travels just posterior to the medial-lateral axis of rotation of the hips (Figure 9-11). Gravity, therefore, provides a passive extension torque at the hips, which, if not opposed, would cause backward bending of the pelvis over the femurs. However, because all three ligaments of the hip are stretched in extension, they generate a passive rubber band–like flexion torque at the hips that offsets the extension torque caused by gravity. This balancing act, created between the tension in the stretched ligaments and the action of gravity, allows us to stand with surprisingly little muscular activation across the hips. In fact, this mechanism is so effective that it can enable standing by persons with paralysis of the lower extremities. For example, Figure 9-12 illustrates an individual with paralysis of the lower extremities after a lumbar spinal cord injury, standing with the aid of crutches and braces at the knees and ankles. By leaning the trunk and pelvis posteriorly relative to the hip joints, the large ligaments of the hip become stretched, stabilizing the trunk, hips, and pelvis without any muscular activation at the hip.

Hip Flexion Contracture

By definition, a hip flexion contracture is a limitation of passive hip extension caused by lack of extensibility of the muscles or ligaments of the hip. Whether mild or severe, hip flexion contractures are relatively common impairments in persons with compromised mobility.

Persons most susceptible to a hip flexion contracture are those who spend a great deal of time in a sitting or otherwise hip-flexed position. Consider the flexed position of the hips in a person who is confined to a wheelchair or who chronically assumes a fetal position in bed. Over time, a slackened ligament or muscle will adapt to its shortened position and remain shortened. Once shortened or contracted, the flexor muscles and ligaments of the hip are difficult to re-lengthen, even with aggressive stretching. The long-term effects of a hip flexion contracture can destabilize the hip and can have a negative impact on many other regions and structures of the body.

When a person with a hip flexion contracture attempts to stand upright, the line of gravity shifts anterior to the medial-lateral axis of rotation (Figure 9-13). This disables the passive standing mechanism discussed earlier, resulting in the need for continuous activation of the hip and back extensor muscles to maintain an upright position. Often a person becomes fatigued and opts to sit, which places the hip flexor muscles and ligaments in a shortened position, thereby perpetuating the vicious cycle.

In order to “right” the trunk in the presence of a moderate to severe hip flexion contracture, the lower spine must be overly extended, resulting in increased lordosis of the lumbar spine (see Figure 9-13). If severe, this eventually can create tightness of the low back extensor muscles and can increase wear and tear on the lumbar facet joints.

This hip-flexed posture as depicted in Figure 9-13 interferes with the body’s ability to optimally dissipate the compression forces that naturally cross the hips. Normal standing, with the hips in extension, directs compressive forces through the thicker regions of articular cartilage, more evenly dispersing the compressive loads. Standing with a hip flexion contracture directs the compressive forces from weight bearing through regions of the hip that are not anatomically designed to dissipate these large forces (indicated by the offset alignment of the red dots in Figure 9-13). Over time, this may generate abnormal wear and tear on the joint, increasing the chance that the individual may develop osteoarthritis. Standing with a severe hip flexion contracture usually involves the need to stand with the knees flexed. Such a posture requires continuous activation of the quadriceps muscles and increases the likelihood of developing knee flexion contractures.

Hip Flexion

Hip flexion involves a movement between the femur and the pelvis within the sagittal plane about a medial-lateral axis of rotation. Each kinematic strategy involving flexion reduces the distance between the anterior aspect of the pelvis and the anterior surfaces of the femurs, as follows:

• Femoral-on-Pelvic Motion

    Femoral-on-pelvic flexion of the hip occurs as the femur moves anteriorly about a fixed pelvis: This motion can be observed as the knee (or thigh) is brought toward the chest (Figure 9-15, A). The normal range of motion for hip flexion is about 0 to 120 degrees, with 0 degrees being the neutral, “straight” position of the hip.

• Long-Arc Pelvic-on-Femoral Motion

    Long-arc pelvic-on-femoral flexion of the hip is often performed in bending over and touching the toes or picking an object up off the ground (Figure 9-15, B). The pelvis rotates anteriorly about the stationary femoral heads, while the lumbar spine flexes to exaggerate the forward bend of the entire trunk.

• Short-Arc Pelvic-on-Femoral Motion

    An anterior pelvic tilt occurs as a short-arc pelvic-on-femoral hip motion, with the trunk remaining upright (Figure 9-15, C): The pelvis rotates, or tilts, anteriorly around the medial-lateral axis of rotation of both hips. As introduced in Chapter 8, the term tilt implies a movement through a relatively short arc of motion. This action is easily understood by the following activity: While sitting or standing, tilt just the top of your pelvis anteriorly, keeping your trunk and chest upright. If performed correctly, the low back will have arched into greater extension (increased lordosis). Because the pelvis is rotating forward, the lumbar spine must extend—just as far—to keep the chest and trunk upright. In this way, the lumbar spine allows the pelvis to rotate over the femoral heads, while allowing the trunk to remain upright. The range of motion of an anterior pelvic tilt is about 30 degrees, determined mostly by the available extension of the lumbar spine. This important relationship between the lumbar spine and the hip joints is discussed further as the chapter progresses.

Hip Extension

Hip extension involves a movement between the femur and the pelvis within the sagittal plane about a medial-lateral axis of rotation. Each kinematic strategy for extension reduces the distance between the posterior aspect of the pelvis and the posterior surface of the femurs, as follows:

• Femoral-on-Pelvic Motion

    Femoral-on-pelvic extension of the hip occurs as the femur rotates posteriorly about a fixed pelvis (Figure 9-16, A). This motion can be observed as an individual extends the leg to walk backward. The hip normally can be extended 20 degrees beyond the neutral position. As depicted in the illustration, this motion is typically limited by tension in the ligaments and muscles located on the anterior side of the joint.

• Long-Arc Pelvic-on-Femoral Motion

    Pelvic-on-femoral extension of the hips can occur with the lumbar spine and pelvis rotated in the same posterior direction. This action maximizes trunk displacement, as when arching the trunk backward (as in Figure 9-16, B), or as one returns upright after touching the toes.

• Short-Arc Pelvic-on-Femoral Motion

    A posterior pelvic tilt is a short-arc, pelvic-on-femoral extension of the hip with the trunk remaining upright (Figure 9-16, C). This motion is the opposite of that described for the anterior pelvic tilt. During a posterior pelvic tilt, the lumbar spine must flex slightly (decreasing its lordosis) to maintain an upright position of the trunk.

Hip Abduction

Hip abduction involves a movement between the femur and the pelvis within the frontal plane about an anterior-posterior axis of rotation. Note that, regardless of which kinematic strategy is used, abduction reduces the distance between the iliac crest and the lateral aspect of the thigh, as follows:

• Femoral-on-Pelvic Motion

    Femoral-on-pelvic abduction of the hip occurs as the femur moves away from the midline relative to a fixed pelvis (Figure 9-17, A). The normal range of motion for this open-chain motion is about 0 to 40 degrees.

• Long-Arc Pelvic-on-Femoral Motion

    A long-arc pelvic-on-femoral abduction of the hip often occurs as the lumbar spine and the pelvis move in the same frontal plane direction, relative to a stationary femur. This motion maximizes trunk displacement, as when one laterally flexes to the side to pick up a suitcase off the floor.

• Short-Arc Pelvic-on-Femoral Motion

    Hip hiking is a short-arc, pelvic-on-femoral abduction of the hip motion with the trunk remaining upright (Figure 9-17, B). Similar to anterior and posterior tilting of the pelvis, the lumbar spine rotates slightly in the opposite direction of the pelvis, so that the trunk remains fixed and upright. As shown in Figure 9-17, B, hip hiking causes a rise in the contralateral side of the pelvis. This closed-chain movement is a type of compensation commonly used to clear the “swing leg” from the ground during ambulation—a gait pattern often used by patients requiring extra clearance to advance a leg, such as those with marked weakness of the ankle or foot.

Hip Adduction

Hip adduction involves a movement between the femur and the pelvis within the frontal plane about an anterior-posterior axis of rotation. Regardless of which kinematic strategy is used, adduction results in a reduced distance between the midline of the pelvis and the medial aspect of the femur, as follows:

• Femoral-on-Pelvic Motion

    Femoral-on-pelvic hip adduction occurs as the femur moves toward, or across, the midline relative to a fixed pelvis (Figure 9-18, A). The normal range of motion for this open-chain action is 0 to 25 degrees.

• Long-Arc Pelvic-on-Femoral Motion

    Pelvic-on-femoral adduction of the hip can occur as the lumbar spine and the pelvis both rotate in the frontal plane, away from the fixed femur. This motion is relatively rare during normal activities but may be visualized by standing on the right leg and laterally flexing the body far to the left.

• Short-Arc Pelvic-on-Femoral Motion

    A hip drop is a short-arc pelvic-on-femoral motion that results in a slight drop of the contralateral side of the pelvis (Figure 9-18, B). For example, when one is standing on the right leg, the left side of the pelvis drops; to keep the trunk upright, the lumbar spine must laterally flex (just as far) to the right. This motion may be observed in individuals with weak hip abductor muscles, who are unable to hold their pelvis level during the stance phase of gait.

Internal and External Rotation of the Hip

Internal and external rotation of the hip occurs in the horizontal plane about a vertical or longitudinal axis of rotation. Because of their similarities, the kinematics of internal and external rotation are described together. As with other movements, three kinematic strategies are possible, as follows:

Femoral-on-pelvic internal and external rotation of the hip can be observed as an individual rotates an extended leg so that the foot and the knee point inward (during internal rotation) and outward (during external rotation). Palpation of the greater trochanter can verify that these motions are the result of rotation of the femur. Normal range of motion is about 0 to 35 degrees for internal rotation, and 0 to 45 degrees for external rotation. Figure 9-19 shows external rotation only.

Related posts:

Fundamentals of Human Gait Structure and Function of Joints Basic Principles of Kinesiology Structure and Function of the Knee Structure and Function of the Shoulder Complex Structure and Function of the Vertebral Column

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