Ch. 9 Structure and Function of the Hip

Objectives

• Describe the supporting structures of the hip joint.
• Justify the actions of the hip muscles through knowledge of the muscle’s proximal and distal attachments
• Describe the force-couple involved in producing an anterior pelvic tilt and a posterior pelvic tilt
• Explain the biomechanical consequences of a hip flexion contracture
• Explain how the position of the hip and knee affects the length and ultimate function of the multiarticular muscles of the hip
• Explain the function of the hip abductor muscles during the single-limb support phase of walking
• Describe why a cane is most effective when used in the hand opposite the weakened or painful hip

Osteology

• Pelvis:
  • Also called the innominate bone, meaning “nameless.”
  • Each right and left innominate bone is formed by the union of three bones: the ilium, the ischium, and the pubis.
  • The wedge-shaped sacrum completes the posterior side of the pelvis.
  • The junction of the two innominate bones with the sacrum forms the sacroiliac joints.

Ilium

• Wing-shaped superior portion of the innominate.
• Iliac crest: long, palpable ridge of bone that marks the superior border of the ilium.
  • Clinicians often compare the heights of the right and left iliac crests to determine pelvic symmetry.
• Anterior-superior iliac spine (ASIS).
• Anterior-inferior iliac spine (AIIS).

Ilium Cont’d

• Posterior-superior iliac spine (PSIS).
  • A small dimple located just superficial to each PSIS helps identify the general location of the SI joints.
• Posterior-inferior iliac spine.
  • Marks the superior tip of the greater sciatic notch.
• Greater sciatic notch is a semicircular space between the PIIS and the ischial spine that provides space for the large sciatic nerve to exit the pelvis.

Ilium Cont’d

• Iliac fossa is the smooth, concave, anterior surface of the ilium, which provides the proximal attachment of the iliacus muscle.
• The articular surface of each ilium articulates with the sacrum, forming the sacroiliac joints.

Ischium

• Located on the posterior-inferior aspect of the innominate.
• Ischial tuberosity is a bumpy projection from the posterior-inferior aspect of the ischium that serves as the proximal attachment of three of the four hamstring muscles.
• People sit on their ischial tuberosities and are often the site of pressure sores, which may occur in persons who lack normal sensation and the ability to frequently alter their seated position.

Pubis

• Composed primarily of two arms or rami, which form the pubic crest.
  • Superior pubic ramus
  • Inferior pubic ramus
• The junction between the pubic crests of each innominate is called the pubic symphysis. Completes the anterior “ring” of the pelvis.
• Obturator foramen: formed by the pubic rami and the ischium.
  • Covered by the obturator membrane which serves as a proximal attachment for the obturator internus and obturator externus.

Consider this… Implications of pregnancy on joint laxity

• During pregnancy, a woman’s body receives an influx of a hormone called relaxin, which increases the flexibility of the pelvic ligaments in preparation for childbirth.
• This will cause the pelvis to shift, resulting in malalignment of the pelvic bones, particularly the sacroiliac joints and the pubic symphysis.

Acetabulum

• Deep, cup-shaped structure that encloses the head of the femur at the hip joint. It is formed by the ilium, pubis, and ischium
• Lunate surface: the horseshoe-shaped articular superior surface of the acetabulum, lined with articular cartilage, and the only part of the acetabulum that normally contacts the femoral head.
• Acetabular fossa: depression deep within the floor of the acetabulum; normally does not contact the femoral head and is not lined with articular cartilage.

Femur

• Longest bone in body, helps contribute to the extensive stride length of human gait
• Proximal portion of each femur is composed of a head, neck, and shaft
• Head of the femur contains a small, cup-like depression called the fovea, which accepts the ligamentum teres
• Intertrochanteric line courses anteriorly between the greater and lesser trochanters.
• Linea aspera is a line of slightly raised bone that courses along much of the posterior side of the femur.
  • Serves as the distal attachment for many adductor muscles and as the proximal attachment of 2 quadriceps muscles

Angle of Inclination

• Frontal plane angle created between the femoral neck and the shaft of the femur that helps to transmit and absorb forces.
• 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.
• Coxa valga: angle of inclination that is significantly greater than 125 degrees
• Coxa vara: describes an angle of inclination that is significantly less than 125 degrees
• The hip joint can become increasingly unstable or can be subjected to high stress, and cause pain and abnormal gait.

Effects of Coxa Valga

• Reduces weight-bearing surface, resulting in an increase in stress applied across other joint surfaces not specialized to sustain such loads
• Shortens the moment arm of hip abductors, placing them in a mechanical disadvantage.
• Genu varum

Effects of Coxa Vara

• Increases shear forces of the hip articulation
• Lengthens the movement arm of the hip abductors, placing them in a mechanical disadvantage
• Genu valgum

Clinical Insight: Relationship between pressure sores and bony anatomy

• Pressure sores (bed sores or decubitus ulcers) often occur when the skin and superficial tissues of the body are compressed over an extended period and are subsequently deprived of adequate blood flow.
• Bony areas around the hip, such as the greater trochanter, sacrum, and ischial tuberosities are at risk for developing pressure sores
• These areas are subject to high localized pressure when the person is sitting or supine.

Torsion Angle

• The femur is twisted along its long axis. This twist is described as torsion between the shaft and neck of the femur.
• Can be appreciated by placing a femur flat on a tabletop. Relative to the femoral condyles, the femoral neck projects upward around 15 degrees.
• Normal anteversion: 8–15-degree torsion angle
• Excessive anteversion: over 15-degree torsion angle
• Retroversion: less than 8-degree torsion angle
• To optimally align the hip joint, an individual may internally rotate the hip or externally rotate during standing and walking. “In- toeing or out-toeing”

Craig’s Test

• Position: prone lying with the hip in neutral and knee flexion to 90 degrees
• Procedure: Palpates the posterior aspect of the greater trochanter while passively internally and externally rotating the hip until the most prominent portion of the greater trochanter is parallel to the surface. The examiner measures the angle between the shaft of the tibia and a line perpendicular to the table.
• Response: 8-15 degrees is normal anteversion; (+) if the angle is >15 degrees indicating femoral anteversion OR <8 degrees indicating femoral retroversion

Arthrology

• Designed to withstand the large and potentially dislocating forces that can routinely occur during walking and other, more vigorous activities
• A thick layer of articular cartilage, muscle, intact labrum, and cancellous bone in the proximal femur helps dampen the large forces that routinely cross the hip
• Failure of any of these protective mechanisms caused by disease, injury, or even advanced age may lead to deterioration and weakening of the joint structure.

Supporting Structures Within the Hip Joint

• Transverse acetabular ligament completes the “cup” of the acetabulum
• Ligamentum teres: connective tissue that runs from the transverse acetabular ligament to the fovea of the femoral head. A branch of the obturator artery travels through to provide the femoral head with a limited amount of its blood supply
• Acetabular labrum: a sharp ring of fibrocartilage that surrounds the outer rim of the acetabulum. Labrum provides increased stability by deepening the socket, and providing a “seal”

Supporting Structures Located Outside the Hip Joint

• Iliofemoral ligament or “Y” ligament is a thick, strong ligament resembling an inverted Y. One of the thickest ligaments in the body, it attaches distally to the intertrochanteric line of the femur. Limits excessive extension of the hip
• Ischiofemoral ligament spirals around the femoral neck and attaches near the apex of the greater trochanter. It limits extension and internal rotation of the hip.
• Pubofemoral ligament attaches distally to the lower half of the intertrochanteric line of the femur. It limits abduction and extension of the hip.

Functional Importance of the Extendable Hip

• While standing in the upright position, the line of gravity travels posterior to the medial-lateral axis of rotation of the hips.
• Since all three ligaments of the pelvis are stretched in extension, they generate a passive rubber band-like flexion torque at the hips that offsets the extension torque caused by gravity. (passive-length tension relationship)
• This balancing act allows us to stand upright with minimal muscular effort. Even individuals with paralyzed BLE can stand with the aid of crutches, and leaning the trunk and pelvis posteriorly relative to the hips.

Consider This…Gaining Strength by “Winding Up” Passive Structures

• Athletes such as field goal kickers enhance their kicking power by storing energy in the capsular ligaments of the hip. By “winding up,” quickly extending the hip before the kick, the large ligaments are stretched and store energy to produce a flexion torque.

Why a painful hip can lead to hip flexion contracture…

• When the hip is injured or suffering from joint degeneration such as arthritis, swelling can build up within the intracapsular joint space. This increases the intracapsular pressure and can cause pain.
• Maintaining a flexed position for a prolonged period can lead to adaptive shortening of the capsular ligaments and hip flexor muscles, resulting in hip flexion contracture.

Hip Flexion Contracture

• People most susceptible to a hip flexion contracture are those who spend a long time in a sitting or hip-flexed position.
• Over time, a muscle will adapt to its shortened position and become difficult to re-lengthen
• Can destabilize the hip and negatively affect other regions of the body, such as developing a knee flexion contracture.
• 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. This disables the balancing mechanism of our hip ligaments, resulting in a continuous activation of the hip and back extensor muscles to maintain an upright position.
• Usually, people attempt to fix the trunk, by increasing the lumbar lordosis, which can increase the tightness of the low back extensor muscles and can increase wear and tear on the lumbar facet joints. This can result in osteoarthritis.

How to Prevent a Hip Flexion Contracture

• Strengthening of the hip extensors: the hips are actively moved out of flexion and the muscular strength bias of the hips is shifted toward extension
• Patient education: moving out of the flexed position on a regular basis greatly reduces the likelihood of developing a contracture.
• Encourage standing if tolerable versus sitting and lying flat
• Encourage prone lying
• Avoid sleeping with pillows under the knee
• Stretching hip flexors: Encourage stretching for HEP

Kinematics

• Femoral on pelvic motion: the femur rotates about a relatively fixed pelvis (open-chain motion)
• Long-arc pelvic-on-femoral motion: the pelvis can rotate through a long arch relative to fixed femurs (closed-chain motion).
  • Often performed to maximize the range of motion of the trunk. To maximize the displacement of the trunk, the lumbar spine moves in the same direction as the pelvis.
  • Ex. Bending forward or to the side to pick up an object off the floor
• Short-arc pelvic-on-femoral motion: pelvis can rotate relative to a fixed femur (closed-chain motion).
  • Like the movement strategy for the long-arc motion, the trunk remains relatively still. The lumbar spine must rotate in the opposite direction. Ex. Anterior or posterior pelvic tilt.

Hip Flexion

• Sagittal plane about a medial-lateral axis of rotation
• Femoral-on-pelvic motion (open-chain): Ex. bring knee to chest
• Long-arc pelvic-on-femoral motion (closed-chain): Ex. Bending over and touching toes or pick an object from the floor
• Short-arc pelvic-on-femoral motion (closed-chain): Ex. Anterior pelvic tilt with trunk remaining upright. This tilt allows your lower back to excessive extend or have a greater arch

Hip Extension

• Sagittal plane about a medial-lateral axis of rotation
• Femoral-on-pelvic motion (Open-chain): An individual extends the leg to walk backward
• Long-arc pelvic-on- femoral (closed-chain): Lumbar spine and pelvis rotated in the same posterior direction
• Short-arc pelvic-on- femoral (closed-chain): posterior pelvic tilt

Hip Abduction

• Frontal plane about an anterior-posterior axis of rotation
• Femoral-on-pelvic motion (open-chain): femur moves away from midline
• Long-arc pelvic-on-femoral motion (closed-chain): laterally flexing the trunk to pick up an item off the floor
• Short-arc pelvic-on-femoral motion (closed-chain): Hip hiking
• Hip hiking: a rise in the contralateral side of the pelvis

Hip Adduction

• Frontal plane about a medial-lateral axis of rotation
• Femoral-on-pelvic motion (open-chain): Femur moves toward or across, the midline of the body
• Long-arc pelvic-on-femoral motion (closed-chain): lumbar spine and the pelvis both rotate in the frontal plane, it is a rare motion, but may be visualized by standing on the right left and laterally flexing the body far to the left side
• Short-arc pelvic-on-femoral motion (closed-chain): hip drop

Internal and External Rotation

• Femoral-on-pelvic motion (open-chain): rotate an extended leg so that the foot and knee point inward and outward.
• Long-arc pelvic-on-femoral motion (closed-chain): pelvis, lumbar spine, and trunk all rotate in the same direction in the horizontal plane
• Short-arc pelvic-on-femoral motion (closed-chain): used during walking or running, allowing the shoulders to remain square to the direction of progression. Rotation of the pelvis over the supporting limb as the lumbar spine rotates slightly in the opposite direction. Difficult to observe.

Arthrokinematics

• Convex femoral head moving within the concave acetabulum of the pelvis.
• Femoral-on-pelvic perspective: abduction, adduction, internal, and external rotation involve a roll and slide in opposite directions.
• Flexion and extension, the femoral head spins in place about a medial-lateral axis of rotation.

Thomas Test

• Checks for tightness of hip flexors.
• The patient is in supine and holds the non-tested hip and knee fully flexed while the other leg is lowered toward the treatment table. If the leg does not lower to the level of the table, the hip flexors are considered tight.
• If the non-tested leg is not flexed, it will allow the patient to anteriorly tilt the pelvis and result in an invalid test.

Primary Hip Flexors

• Iliopsoas: most powerful hip flexor in the body
• Rectus femoris: only muscle in the quadriceps group that crosses the hip and knee. It is a hip flexor and knee extensor.
• Sartorius: longest muscle in the body
• Tensor fascia latae: tightens the IT band to provide increased stability across the lateral aspect of the hip and knee
• Line of pull is anterior to the medial-lateral axis of rotation

Primary Hip Extensors

• Gluteus maximus
• Semitendinosus
• Semimembranosus
• Biceps femoris (long head)
• Adductor magnus (extensor head)

Using the Gluteus Maximus to Extend the Knee

• Can perform the last 20 or 30 degrees of knee extension, provided the foot is in firm contact with the ground.
• It is a valuable substitution technique for those with knee extensor paralysis or a prosthetic leg, who lack true knee extensors
• Strong contraction of the gluteus maximus pulls the femur posteriorly, which draws the attached tibia posteriorly; thereby extending the knee.

Primary Hip Abductors

• Gluteus medius
• Gluteus minimis
• Tensor fasciae latae
• A demanding activity imposed on these muscles occurs during closed-chain activities such as single leg stance or single limb support.
• The most frequent demands are during walking. The hip abductors must supply an adequate contraction force to keep the pelvis from “falling into the space”

Trendelenburg Sign

• Weakness in the hip abductor muscles can be demonstrated as a Trendelenburg sign. The test is performed by having a patient assume a position of single-limb support over the affected leg.
• For example, if the right hip is suspected to be weak, the pt. will lift the left leg and stand only on the right leg, if the pelvis drops toward the unsupported leg, a positive Trendelenburg sign is diagnosed.
• Patients can compensate by leaning their trunk towards the side of the weakened stance leg. This type of compensation is known as a gluteus medius limp or compensated Trendelenburg.

Hip Abductor Force and Joint Reaction

• Hip abductor muscle force must be significant because it is producing torque along a small internal moment arm
• Body weight is producing torque in the opposite direction
• The abduction torque must counterbalance the pelvic-on-femoral adduction torque produced by body weight
• Must generate a force that is twice the body weight
• Most studies indicate a total hip joint reaction force of 3 \times BW when standing on one limb

Principles of Joint Protection

• Use a cane in the hand opposite of the affected side
• Lose excess body weight
• Walk slowly
• Correct faulty postures such as hip flexion contracture
• Maintain relative flexibility of muscles about the hip and lower back
• Avoid carrying heavy loads
• Avoid excessive bending or standing on one limb

Why use a cane in the hand opposite to the affected hip?

• In single leg stance, the hip abductors must produce a large force to maintain the pelvis at a normal level.
• One goal of using a cane is to reduce the demands of the hip abductor muscles, thereby reducing compression forces on the diseased or unstable hip.
• The cane acts upward because the earth returns the same pushing force as is applied downward by the hand, because of this, it acts as a hip abductor muscle.

Hip Adductors

• Pectineus: has the most proximal attachment to the femur of any hip adductor muscle
• Adductor Longus: one of the most superficial adductor muscles
• Gracilis: forms part of the pes anserine, providing medial support to the knee
• Adductor Brevis
• Adductor Magnus – both heads
  • Posterior head: extensor head; innervated by the sciatic nerve and can assist in extending the hip
  • Anterior head: innervated by the obturator nerve and has hip adduction and flexion ability

Hip External Rotators

• Gluteus maximus
• Sartorius
• Piriformis
• Gemellus superior
• Obturator internus
• Gemellus inferior
• Obturator externus
• Quadratus femoris

Stretching Piriformis

• Tightness, spasm, or inflammation of the piriformis muscle can lead to excessive sacroiliac joint stress or compression of the sciatic nerve – piriformis syndrome
• Symptoms such as radiating pain, deep posterior hip pain, and a painful “trigger” point near the piriformis muscle
• The line of pull of the piriformis with the hip flexed past 90 degrees shifts anteriorly to the longitudinal axis of rotation, which turns the piriformis into an internal rotator
• Therefore, the piriformis must be placed with the hip flexed and externally rotated.

Hip Internal Rotators

• Gluteus medius (anterior fibers)
• Gluteus minimus
• Tensor fasciae latae
• These muscles help change the direction of the advancing limb while walking and help balance the force of other flexor muscles that are also external rotators such as the sartorius.
• A typical gait pattern demonstrated by individuals that ambulate with the assistance of a cane or crutches is called crouched gait. Crouched gait refers primarily to the flexed and internally rotated position of the hips while walking.

Key Muscles by Segmental Innervation

• L2: iliopsoas, weak sartorius, weak adductors, weak rectus femoris
• L3: sartorius, rectus femoris, adductors
• L4: gluteus medius, TFL, hamstrings, tibialis anterior
• L5: gluteus maximus, tibialis posterior, toe extensors

Quiz Questions

• Coxa Valga describes:
  • A. The position of the hip when standing with an anterior pelvic tilt
  • B. An angle of inclination significantly greater than 125 degrees
  • C. The position of the hip when standing with a posterior pelvic tilt
  • D. An angle of inclination significantly less than 125 degrees

Quiz Questions

• The iliofemoral, Ischiofemoral, and pubofemoral ligaments all limit:
  • A. Hip extension
  • B. Hip flexion
  • C. Hip abduction
  • D. Hip internal rotation

Quiz questions

• If the abdominals are weak, resisted hip flexion will likely result in:
  • A. Increased lordosis of lumbar spine
  • B. Decreased lordosis of lumbar spine
  • C. Coactivation of the gluteus maximus
  • D. Rupture of the pubofemoral ligament

References

• Mansfield, P. J., & Neumann, D. A. (2024). Essentials of kinesiology: for the physical therapist assistant (4th edi.). Elsevier
• Mansfield, P. J., & Neumann, D. A. (2019). Essentials of kinesiology: for the physical therapist assistant (3rd ed.). Mosby, Inc.
• Dutton, M. (2012). Orthopaedics for the physical therapist assistant. Jones & Barlett Learning, LLC