Pelvic tilt
Sagittal balance of pelvis can influence position of acetabular component (Fig.1,2).
In case of imbalance the compensatory mechanism can cause shift in pelvic tilt to anterior or posterior side and accelerated hip osteoarthritis.
With pelvic tilt posteriorly,
acetabular component anteversion may increase as the patient's position changes from standing to sitting.
Acetabular version angle
Acetabular version angle is the angle between coronal plane and edges of acetabular component.
Acetabular version angle can be:
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Radiological- angle between the acetabular axis and the coronal plane
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Anatomical- the angle between the acetabular axis and the transverse axis of the patient
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Operative- is an angle of the acetabular axis on the sagittal plane and longitudinal axis of the patient
Radiological version is measured by drawing a line tangential to the face of the acetabular cup and angle to sagittal plane is measured(Fig.3).
It should be from 5° to 25° anteversion.
High anteversion of acetabular component can lead to anterior instability and retroversion can lead to iliopsoas tendinopathy.
It can be also influenced by anterior or posterior approach of the surgery.
Acetabular inclination
Acetabular inclination (acetabular abduction) is defined as an angle between articular side of acetabulum and transverse axis (Fig.4).
Line is drawn between ischial tuberosities and angle with lateral aspect of acetabular component is measured.
Normally it should vary between 30-50°.
Smaller inclication provides stability but reduces abduction,
otherwise greater angle is associated with dislocation.
Rotation center
To relate rotation center is comparing opposite sides (Fig.5).
Horizontal center of rotation is defined by perpendicular distance between centre of femoral head and lateral border of teardrop.
Vertical center of rotation is defined by perpendicular distance between femoral head and transischal tuberosity line.
Other method,
in case of hip dyslasia or contraleral total arthoplasthy,
position of femur head rotation center can be predicted using an isosceles triangle which is 20 % of pelvis height- femur head center should be in hypotenuse (Fig.6).
There are two lines drawn at level of iliac crest and ischial tuberosities and a perpendicular line is drawn which should cross A point.
A point is set in 5 mm lateral to intersection of Kohler’s (ilioischial line or line from ilium to the ischial tuberosity) and Shenton’s (superior border of the obturator foramen and along the inferomedial border of the neck of femur) lines.
Point B is at junction of subchondral acetabular roof and vertical line- AB distance should be ⅕ of pelvic high.
Point C is at level of subchondral acetabular roof and BC is equal to AB.
These measurements is compared to healthy side.
When rotation center is moved superiorly by 5 mm or more,
it can lead to increase of wear,
cause limping and loosening.
Height of femoral head center
It is measured by drawing a perpendicular line to femoral anatomical axis and measure the the vertical distance between the center of the femoral head and the perpendicular line to the shaft of femur was measured (Fig.
7).
Femoral component version
Femoral component version is defined as an angle between long axis of femoral neck and posterior condylar line (Fig.8).
It is influenced by femoral neck osteotomy angle: higher osteotomy angle results in higher anteversion.
Increase in femoral anteversion or retroversion can lead to dislocation.
Femoral component valgus- varus tilt
Femoral component valgus- varus tilt is defined as an angle of stem long axis and femur anatomical axis in coronal plane (Fig.9).
It is influenced by femoral neck osteotomy level: higher level of osteotomy results in more varus stem position.
Femoral component should be in relative valgus position- 5°-10°.
Malalignment of femoral stem in valgus or varus position can lead to early loosening.
Femoral tilt
Femoral tilt (or stem tilt) is defined as an angle between femoral anatomical axis and stem long axis (Fig.10) and it represents an alignment in sagittal plane.
Increase in femoral tilt directly correspond to femoral antetorsion.
Leg length discrepancy
To evaluate difference in leg length a method by Murphy (Fig.11) was suggested.
Using this method a line is drawn at the inferior border of teardrop and difference in distance of this line to apex of lesser trochanter is measured.
Acceptable discrepancy is lower than 1 cm.
Offset of femur
Femoral offset is a perpendicular distance from rotation center of femoral head to long axis of femur (Fig.12) and it shows displacement of hip from pelvis.
Hip offset (horizontal or global offset) incorporates femoral offset and it is a perpendicular distance from teardrop,
through femoral rotation center to femoral axis.
In case of higher rotation center leads to reduction of abductor strength and a larger offset is required to compensate.