Hip Joint Anatomy

• The hip joint is formed by the union of the acetabulum of the pelvis and the head of the femur.
• The hip joint is a diarthrodial ball-and-socket joint with three degrees of freedom.

Structure of the Hip Joint

• The cuplike concave socket of the hip joint is called the acetabulum.
• It is located on the lateral aspect of the pelvic bone.
• The acetabular labrum is a ring of wedge-shaped fibrocartilage.
• It deepens the socket.
• Increases the concavity of the acetabulum.
• Grasps the head of the femur to maintain contact with the acetabulum.
• The head of the femur is the convex component of the ball and socket configuration of the hip joint.
• It forms two-thirds of a sphere.
• The femoral neck joins head and shaft.
• It attaches to the shaft of the femur between the greater and lesser trochanters.

Angulations of the Femur

• There are two angulations made by the head and neck of the femur on the shaft.
• The angle of inclination occurs in the frontal plane.
• It is between the axis of the femoral neck and the axis of the femoral shaft.
• The angle of torsion occurs in the transverse plane.
• It is between the axis of the femoral neck and the axis of the femoral condyles.

Angle of Inclination

• The angle of inclination of the femur in early infancy is about 150°.
• The inclination decreases to an average of 125° in the normal adult.
• In women, the angle is somewhat smaller compared to men.
• This is due to the greater width of the female pelvis.

Angle of Torsion

• The angle of torsion of the femur is viewed from the top down the length of the femur.
• An axis through the femoral neck makes an angle with an axis through the femoral condyles.
• It reflects the medial rotation of the femoral condyles
• It decreases with age.
• In the newborn, the angle of torsion ranges from 30° to 35°, decreasing substantially in the first 2 years.
• In the adult, the angle of torsion is normally around 12°, but may vary from 12° to 20°, and like the angle of inclination, it varies between sexes.

Center Edge Angle (CE)

• The acetabulum is oriented on the pelvis facing obliquely forward, outward, and somewhat downward.
• A line conncects the lateral rim of the acetabulum and the center of the femoral head.
• It forms an angle with the vertical, known as the center edge (CE) angle.
• It indicates the amount of inferior tilt of the acetabulum.
• It measures the stability of the hip joint.
• Average center edge CE angles in adults:
• 38° in men.
• 35° in women.
• With ranges in both sexes of about 22 to 42°.
• A smaller CE angle results in diminished coverage of the head of the femur.
• The angle increases with age
• This means children have less coverage over the head of the femur
• Joint stability decreases compared to adults.

Bony Architecture of the Hip Region

• The upper end of the femur has a special structural pattern.
• It accommodates the mechanical stresses and strains created by different loading modes on the femur.
• The trabeculi of bone lines up along lines of stresses
• This forms a system to meet stress requirements.
• There are major trabeculi:
• Arcuate bundle.
• Vertical or supporting bundle.
• Lateral (trochanteric).

The Articular Surface of the Hip (Stability)

• Articular congruence of the hip joint.
• Atmospheric pressure.
• Ligaments and capsule.
• Periarticular muscles.

Hip Joint Fibrous Capsule

• The articular capsule of the hip is strong and dense.
• It provides joint stability.
• The capsule is attached to the entire periphery of the acetabulum.
• It blends with the acetabular labrum.
• It extends beyond the acetabulum superiorly.
• The capsule covers the femoral neck and attaches to the base of the neck.

Ligaments

• The anterior portion of the capsule is reinforced by two strong capsular ligaments.
• The ligaments are named by their attachments.

Iliofemoral Ligament

• The iliofemoral ligament covers the hip joint anteriorly.
• It arises from the anterior iliac spine.
• It inserts into the intertrochanteric line.
• This ligament:
• Is a fan-shaped that resembles an inverted letter y.
• Is the strongest ligament at the hip.
• Is taut during flexion to check hip hyperextension.
• The superior fibers may become tensed during adduction

Pubofemoral Ligament

• The pubofemoral ligament covers the hip joint anteriorly.
• It arises from the pubic bone and margin of the obturator foramen.
• It inserts into the femoral neck deep to the iliofemoral ligament.
• The Pubofemoral ligament is taut in hip abduction and in extension.
• The bands of the iliofemoral and the pubofemoral ligaments form a Z on the anterior capsule.
• It is similar to that of the glenohumeral ligaments.

Ischiofemoral Ligament

• The ischiofemoral ligament covers the hip joint posteriorly.
• It arises from the ischium.
• Inserts into the greater trochanter of the femur.
• The spiral fibers tighten during' extension.
• All of the capsular ligaments of the hip are coiled or twisted.
• They pass from the pelvis to the femur in a neutral position.
• The ligaments are taut during extension and slack during flexion.

Flexor Muscles of the Hip

• These muscles lie anterior to the frontal plane.
• They pass through the center of the joint.
• They all run anterior to the axis of flexion and extension lying in this frontal plane.

Extensor Muscles of the Hip

• These muscles lie posterior to the frontal plane.
• They pass through the center of the joint.
• They are divided into two groups:
• The first group inserts into the femur.
• The second group inserts in the vicinity of the knee joint.
• Hip extensors play an essential role in stabilizing pelvis.
• They assist in the antero-posterior direction.

Abductor Muscles of the Hip

• These muscles are divided into two groups:
• The first group includes tensor fasciae latae, anterior fibers of gluteus medius, and the bulk of gluteus minimus.
• It produces abduction-flexion-medial rotation.
• The second group includes posterior fibers of glutei minimus and medius, and the abductor fiber of gluteus maximus.
• It produces abduction-extension-lateral rotation.
• These muscles act as balanced synergists-antagonists to produce pure abduction.

Deltoid of the Hip

• This consists of a wide muscular fan covering the external aspect of the hip joint.
• Its apex pointing inferiorly, having anatomical and functional resemblance to the deltoid muscle of the shoulder joint.

Action of Gluteus Medius

• On the lever arm of the femoral neck varies with the degree of abduction and can be resolved into two vector components:
• Acting towards the center of the joint and promoting coaptation.
• Acting at right angles (tangential) representing the effective force of the muscle.

Hip Muscles

• Adductor muscle of the hip.
• Lateral rotator muscles of the hip (the pelvi-trochanteric muscles): with lateral rotation as the primary function.
• Medial rotator muscles of the hip.

Hip Kinematics

• Hip joint motions are easiest to visualize as movement of the convex femoral head within the concavity of the acetabulum.
• The femoral head will glide within the acetabulum in a direction opposite to the motion of the distal end of the femur.
• Flexion and extension occur from a neutral position as an almost pure spin of the femur head around a coronal axis through the head and neck of the femur.
• The head spins posteriorly in flexion and anteriorly in extension.
• Flexion and extension from other positions must include both spinning and gliding of the articular surfaces.
• This is dependent on the combination of motions.
• The motions of abduction, adduction, and medial-lateral rotation must include both spinning and gliding of one surface on another.
• This would occur opposite to the motion of the distal end of the femur when the femur is the moving segment.
• Whenever the hip joint is weight-bearing
• The femur is relatively fixed.
• The hip joint is produced by movement of the pelvis on the femur.

Close and Maximum Loose Packed Position of the Hip

• Close-packed position:
• Maximal extension and maximal medial rotation of the hip.
• Maximum loose-packed position of the hip:
• 30 degrees of flexion, abduction, and slight lateral rotation of the hip.

Movement of Hip Joint - Abduction

• Abduction is normally limited by:
• Tension of adductors.
• Pubofemoral ligament.
• Medial band of iliofemoral ligament (ROM: pure abduction is 30° degrees, but is increased by tilting of the pelvis).

Movement of Hip Joint - Adduction

• With extended hip:
• Contact with opposite leg (ROM: 30° beyond midline).
• With flexed hip:
• Tension of abductors.
• Lateral band of the iliofemoral ligament.

Hip Rotation

• Medial rotation is limited by:
• Lateral rotators.
• Ischiofemoral ligament.
• Posterior part of the fibrous capsule (ROM: Medial rotation is 30°).
• Lateral rotation is limited by:
• Medial rotators.
• Lateral band of iliofemoral ligament (ROM: Lateral rotation is 60°).

Hip Flexion

• Flexion is limited by:
• Tension of the hamstring.
• Contact of the thigh and abdominal soft tissues.
• (ROM: 120° with flexed knee).
• (ROM: 90° with extended knee).
• Extension is limited by:
• Tension of the vertical band of the iliofemoral ligament.
• (ROM: 20° with extended knee).
• (ROM: 10° with flexed knee).

The Range of Hip Joint Motion in Different Activities

• During tying shoe while foot is on floor:
• Sagittal 124, Frontal 19, Transverse 15.
• During tying shoe with foot across opposite thigh:
• Sagittal 110, Frontal 23, Transverse 33.
• During sitting down on chair and rising from sitting:
• Sagittal 104, Frontal 20, Transverse 17.
• During stooping to obtain an object from floor:
• Sagittal 117, Frontal 21, Transverse 18.
• During squatting:
• Sagittal 122, Frontal 28, Transverse 26.
• During ascending stairs:
• Sagittal 67, Frontal 16, Transverse 18.
• During descending stairs:
• Sagittal 36

Stability of the Hip Region

• During double stance, the stability in the transverse plane is secured by the simultaneous contraction of the ipsilateral and contralateral adductors and abductors.
• When these antagonistic actions are properly balanced, the pelvis is stabilized in the position of symmetry.
• During single limb stance, the stability of the pelvis is provided solely by the action of abductor muscles, explained by the abductor mechanism.
• During standing on one limb:
• Gluteus medius and minimus pull the rim of the pelvis toward the greater trochanter of the femur.
• This action prevents the pelvis from dropping on the contralateral side.
• The potential drop is caused by the weight of the trunk and non-stance leg.
• The body weight creates a situation of unequilibrium.
• Abductors work to create equal counterclockwise moment instability to restore equilibrium.
• The moment leads to trunk leaning towards the non-stance leg.
• This mechanism is compensated by using a cane held in the opposite hand.
• It works through a large moment arm.
• Force is applied to the cane to create a counter clockwise moment
• This decreases demand on the abductors.
• Reducing body weight or reducing gravitational force moment arm by leaning towards the stance leg decreases demand on abductor muscles.