Hip Biomechanics Flashcards
(24 cards)
Angle of Inclination
Adult= 125˚, Child= 150˚
- Increase in the angle of inclination = coxa valgum
- Decrease in the angle of inclination = coxa varum
Anteversion
Measured in transverse plane)
Measures the angle btwn the axis of the head and neck VS the transcondylar axis
Normal angle of anteversion = 8-15° -Increased angle (15-30*)
Decreased angle (5-8*) =Retroversion
Excess Anteversion
Consequences of excessive anteversion
Apparent excess medial rotation and decrease external rotation
Unnatural (compensatory) toe-out stance or gait
Natural (noncompensatory) toe-in stance or gait
The greater the degree of anteversion, the more prone to degeneration or injury the hip becomes
Excess Anteversion
Left
Compensatory gait for anteverted hip
we see a compensatory gait, trying to keep the foot forward.
Excess Anteversion
Right
Noncompensatory toe-in gait for anteverted hip
we see a natural, non-compensatory toe-in gait.
Though awkward, it’s better for the joint.
Intrinsic Stability
Provided by 3 characteristics:
- The Acetabulum and Labrum
- Coaptation Force
- Strong Ligaments
Intrinsic Stability
1. The Acetabulum and Labrum
The hip has 3 degrees of freedom like the shoulder. However, it is much more stable due to the fact that the acetabulum forms a much deeper socket than the glenoid cavity.
The acetabulum and labrum engulf more than half of the femoral head and passively maintain joint integrity.
It would be very difficult to pull the head from the acetabulum unless tearing of the labrum were to occur.
Intrinsic Stability
2. Coaptation Force
It is strong bc of the large surface areas in relative contact to one another.
This creates a vacuum force of 25 kg of atmospheric pressure.
The coaptation force is strong enough to passively hold the joint together even if all the muscles were to be removed.
Intrinsic Stability
3. Strong Ligaments
Enhance stability b/c they twist on themselves in extension, like a screw home mechanism, and approximate the joint surfaces closer.
Weight Bearing
On two legs
Each femoral head receives force equivalent to half the weight of the upper body
Weight Bearing
On one leg
Each femoral head receives 3 times the weight of the body
This is because the centre of mass is midline (S2), whereas the femoral heads are lateral to the midline
This creates a rotary force around the femoral head
Strong abductor muscles are needed to counteract this rotary force
With abductor weakness, the stance leg dips into adduction, which appears as the non-stance leg dropping
The abductors primarily responsible for single-leg stance strength
are
Glute med, glute min, TFL
Weight Bearing - Gait
Two common gait patterns occur with weak abductors
Pelvis Drops (aka Trendelenburg gait)
Torso lurches
Weight Bearing - Gait
Pelvis Drops (aka Trendelenburg gait)
the pelvis will drop noticeably to the opposite side of weakness
presents as short swing phase on the unaffected side (short stance phase affected side)
Weight Bearing - Gait
Torso lurches
the person may ‘lurch’ their upper body toward the side of weakness attempting to shift the centre of gravity lateral to the fulcrum (S2), to prevent dropping of the opposite side of the pelvis
Both can occur due to weakness or as antalgic responses to pain
Weight Bearing - Gait
Using a cane
Using a cane can prevent the torso lurching towards the side of weakness if the cane is held in the hand of the unaffected side.
Force now acts through the cane up to the lateral trunk muscles, shoulder depressors, elbow extensors and wrist flexors.
Articular Surfaces & Arthriti
High prevalence of DJD (OA) at hip
The mechanical axes of the acetabulum and femoral neck are situated as such so that they’re almost in alignment with one another
In anatomic position, both are directed anteriorly. Therefore, in normal standing position, the articular surface of the head of femur is exposed anteriorly and a small area of the posterosuperior aspect effectively weight-bears.
Articular Surfaces & Arthritis
Two positions that align the axes:
a.) 90˚ hip flexion, slight ER and AB
b.) Extension, abduction and internal rotation
Articular Surfaces & Arthritis
This is important b/c there is a correlation between the relative contact of the articular surfaces and the high occurrence of OA
During normal walking only a small portion of the articular surfaces are in contact (sup/post). The larger area of ‘exposed’ cartilage, therefore, does not undergo the intermittent compression needed for adequate nutrition –> high occurrence of Degenerative Joint Disease at the hip.
The greater the angle of torsion/ the more a hip is anteverted –> axes are less aligned –> less effective contact area between joint surfaces. –> predisposition towards DJD
Observation
Gait
Look for a lurch (suggesting hip pain or abductor weakness or both) or a dropping pelvis (abductor weakness)
Observation
Movement
Guarded movements with removing shoes and socks, crossing legs, or bending over to touch the toes
Observation
Posture
Is weight evenly distributed between legs?
Hyper/Hypolordosis; Ant/post pelvic tilt; Upslip/downslip; Med/lat facing patella
Observation
If the trochanters are level but ASIS’s, PSIS’s and the iliac crests are asymmetrical, when compared bilaterally, (that is one side is lower than the other) one of several issues can be suggested:
a. The high side: coxa valgum, longer femoral neck (uncommon)
b. The low side: short femoral neck, *coxa varum, cartilaginous narrowing from hip joint degeneration – “” is most common.
Observation
A way to unilaterally check for the extent of coxa varum or cartilaginous narrowing from hip joint degeneration is
to draw a line from the ASIS to the corresponding ischial tuberosity (Nélaton’s line)
