Hip biomechanics are quite complex due to pelvic motion associated with it and range of movements it produces. Biomechanics is the science that studies forces acting on a living body.
Hip is a mobile and stable joint that because of its anatomy and strong attachments of muscles and ligaments.
The neck of femur and its orientation plays an important role in the mobility of the hip. The neck is angulated in relation to the femur in the sagittal and coronal plane.
Neck-shaft angle is about 140 degrees at birth and reduces to 120-135 degrees in adults.
The anteversion is about 40 degrees at birth but reduces to 13-16 degrees in adults.
The orientation of acetabulum is also important for hip stability and transfer of the forces.
It is directed forwards 15-20 degrees and 45 degrees downwards.
Mechanical axis of the lower limb passes between the center of the hip joint and center of the ankle joint.
Anatomical axis line is between the tip of the greater trochanter to the center of the knee joint.
There is about 7 degrees of angle between the two axes.
Joint reaction force
Force generated within a joint in response to forces acting on the joint
in the hip. The joint reaction force is the result of the need to balance the arms of the body weight and abductor tension that maintains the pelvis at level.
This refers to how well the two joints surfaces conform to each other. A higher congruence increases the joint contact area.
Instant center of rotation
It is the point about which a joint rotates. It may remain same or may change depending on translation of the joint surfaces [as in knee]
Center of gravity
The center of gravity is the average location of the weight of an object. In humans, it is just anterior to S2.
Forces Acting on the Hip
The hip joint is the first class lever. In a first class lever, the fulcrum is placed between the effort and load.
The fulcrum here is hip and the load is body weight. Abductor tension is the effort.
To maintain a stable hip, the torques that are produced by the body weight are opposed by the pull of the abductor muscles. [see the diagram below]
Abductor force x lever arm A = weight x lever arm
Thus the forces acting across hip joint are
- Body weight
- Abductor muscles force
- Joint reaction force
As we noted before, the joint reaction force is the force that is generated within a joint in response to forces that act on the joint.
In the hip, it is generated to balance the moment arms of the body weight and abductor tension to maintain a level pelvis.
The joint reaction force is measured in terms of body weight.
In two leg stance, the weight of the upper body is equally divided on both hips and there is little need for muscular force.
Each hip supports about one-third of body weight in two leg stance.
During single leg stance, this force is 3 times body weight. The limb on which weight is supported would have to bear the upper body weight and contralateral, non supporting, limb.
In single leg stance, the effective centre of gravity shifts to the non-supportive leg and a downward force attempts to tilt pelvis on unsupported side.
the abductors on the hip which supports the body, must exert a downward counter balancing force with right hip joint acting as a fulcrum.
The use of the cane may relieve upto 60% of the load on hip in stance phase because cane transmits weight to the ground and results inlesser force required.
During walking it is 5 times in and during running it is 10 times.
Roughly, the joint reaction force would be equal to the sum of body weight and abductor force.
Weight on each femoral head is half of the body weight above hips.
joint reaction forces during supine straight leg raising is more than 3 times body weight.
Similarly, getting on & off bed pan puts a load equal to four times body weight.
The factors that affect the joint reaction force are
- Body weight and its moment arm
- Abductor force and its moment arm
The body weight can be taken as the load applied to a lever arm extending from the body’s center of gravity to the center of the femoral head.
The abductor mechanism acts on the lever arm extending from the lateral aspect of the greater trochanter to the center of the femoral head.
This force by abductors must be equal to the load applied by weight to hold the pelvis level when in a one-legged stance and a greater moment to tilt the pelvis to the same side when walking.
The ratio of the length of the lever arm of the body weight to that of the abductor musculature is about 2.5: 1. Therefore the force needed by abductor muscles must approximate 2.5 times the body weight to become equal to the force applied by body weight.
The estimated load on the femoral head in the stance phase of gait is equal to the sum of the forces created by the abductors and the body weight and is at least three times the body weight.
Changes During Gait
During normal gait, on heel-strike, the hip moves into 30 degrees of flexion and at toe-off [when the foot is finally off the ground] about 10° of extension. The range of abduction to adduction is about 11°, and for internal-external rotation, the range is about 8°.
During different phases of the gait cycle, different forces act on the femoral head. Approximately two-thirds of the hip force is produced by the abductors.
The directions of the resultant force on the joint are important to the function of total hips.
It is useful to consider the forces relative to axes based on the long axis of the femur.
In the coronal plane, the forces acting make an angle of 15° to 27° to the long axis of the femur during the stance phase of gait which results in axial compression, varus, and mediolateral forces. In the sagittal plane, anteroposterior forces on the femoral head, resulting in torsion.
This is done to check abductor function.
The patient stands on a single limb. As described before, the body weight center of gravity shifts to the lifted limb and lowers the pelvis.
The abductor muscles on the side the patient is standing on exert the force to lift the pelvis up on the contralateral side.
A failure to do so results in dropping of the pelvis on single leg stance and indicates abductor malfunction.
It is called a positive Trendelenburg sign.
Factors Affecting Hip Biomechanics
Shortening of abductor lever arm
Shortening of Abductor lever arm would result in increase in abductor workout.
The abductor lever arm may be shortened in
- Supratrochanteric shortening
- Femoral anteversion
- External rotational deformities
- Developmental dysplasia of the hip.
If the muscles are not able to generate the rquisite force to counterbalance body weight, it would result in a lurch during the gait or pelvic tilt.
We have seen that forces acting on the hip are measured as multiple of body weight. Therefore, the increase in weight leads to increase in forces acting on the joint.
Biomechnics in Hip Prosthesis Design
Hip biomechanics are used in designing of the hip prostheses. The idea of the design is to decrease the the joint force by
- Centralization of femoral head by deepening of Acetabulum – decreases body wt lever arm
- Increase in neck length and Lateral reattachment of trochanter – lengthens abductor lever arm
This aims at decreasing abductor force, joint reaction force, & so the wear of the implants.
Femoral offset is the distance between the center of the head to the center of axis of the stem
An inadequate medial offset shortens the moment arm leading to increased abductor work. It could lead to limp and increase bony impingement
If the offset is increased, there is increased stress on the stem and leads to implant loosening as an increase in offset would reduce the force but causes an increase in the bending moment on the stem.
In the saggital plane, the forces act to bend the stem posteriorly and become more pronounced when the hip is flexed. It has been found that the joint reaction force was lower when the hip center was placed in the anatomical location compared with a superior and lateral or posterior position.