Component impingement due to poor positioning can limit range
of motion after total hip arthroplasty. Contact stresses on ultra-high
molecular weight polyethylene are also dependent on the orientation
of the acetabular component. In this study, a computer kinematic
model was used to determine the effects of component position and
variation of head:neck ratios on prosthetic impingement and hip range
of motion, and a finite element model was employed to calculate
polyethylene stresses at different cup positions.
Range-of-Motion Analysis
Head:neck ratios substantially affected hip range of motion (Table I). The maximum
range of motion before impingement was plotted as a cone. The base
of the cone denoted the overall range of motion available. Larger
head:neck ratios increased the base of the cone (Fig. 4). The addition
of a wide chamfer to the hemispheric liner also affected the range
of motion. Offsetting the inner edge of the chamfer above the head
center increased the range of motion, while offsetting it below
the head center decreased the range of motion (Fig. 5).
Acetabular orientation defined the orientation of the cone in
space and substantially affected the individual components of hip
range of motion. Acetabular abduction angles of <45° reduced
flexion and abduction, while higher angles tended to reduce adduction
and rotation (Fig. 6). Acetabular anteversion increased
flexion and internal rotation but reduced extension and external rotation.
Overall, acetabular abduction angles between 45° and 55° permitted
better range of motion and stability when combined with appropriate
acetabular and femoral anteversion.
Polyethylene Stress Analysis
A consistent decrease in contact area with an increase in the
acetabular abduction angle (from 35° to 55°) and with a corresponding
increase in mean and peak contact stresses was predicted with use
of the finite element model (Fig. 7). At each acetabular abduction
angle, increasing cup anteversion from 0° to 30° resulted in an increase
in contact area and a decrease in von Mises stresses, contact stresses,
and shear stresses. The stresses at the posterosuperior edge of
the liner also increased with increasing abduction: there was a 55% and
69% increase in von Mises stresses from 35° to 45° and
from 35° to 55°, respectively. When the body weight was increased
from 63 kg (139 lb) to 90 kg (198 lb), peak contact stresses increased
by 36%.
A wide chamfer that was offset above the center of the head also
decreased contact area and increased stresses in the liner. The
inner bearing surface area of the liner decreased in a linear fashion
from 1232 mm2 to 1144, 1056, and 968
mm2 for the 1, 2, and 3-mm offsets, respectively.
A corresponding increase in contact stresses was noted for the three
chamfer offsets tested (Figs. 8-A through 8-D).
Larger head:neck ratios increased hip range of motion. However,
this model did not account for bone and soft-tissue impingement.
Data have been reported that suggest that, at larger head:neck ratios (especially
with 32-mm-diameter heads), range of motion may be restricted by
osseous rather than prosthetic impingement2.
The addition of a wide chamfer to the liner design resulted in an
increase in overall range of motion if the inner edge of the chamfer
was offset above the center of the head. The range-of-motion cone
angle increased in a linear fashion with the magnitude of the chamfer
offset.
There is a complex interplay between femoral and acetabular orientation
angles. Acetabular abduction is often constrained by available bone
coverage, while femoral anteversion may be dictated by femoral shaft
geometry. For each combination of acetabular abduction and femoral
anteversion, there is an optimum range of acetabular anteversion
that creates the potential for maximum range of motion without prosthetic
impingement after total hip arthroplasty. Cup abduction angles between
45° and 55°, when combined with appropriate acetabular and femoral
anteversion, resulted in the maximum overall range of motion and
stability with respect to prosthetic impingement.
The finite element analysis suggests that peak contact stresses
range between 4.5 and 5.5 MPa for a hemispheric liner (without a
chamfer), depending on the abduction angle and the anteversion angle. These
stresses are well within the yield strengths reported for ultra-high
molecular weight polyethylene. However, these stresses were generated
with use of a very benign hip load representative of the stance
phase of level walking by a 63-kg (139-lb) individual. Increasing
the body weight to 90 kg (198 lb) substantially increased contact
stresses. Much higher hip loads are generated during stair-climbing,
rising from a chair, and more vigorous activities. Therefore, care
should be taken when approaching the higher limit of cup abduction.
Cup abduction reduces contact area and thus increases contact stresses.
In addition, higher stresses are generated near the posterosuperior
edge of the liner. These results suggest that these stresses can
be alleviated by judicious anteversion of the liner. Overall, the
results suggest that cup abduction angles at or slightly higher
than 45° provide the optimal combination of adequate range of motion
and low contact stresses. If higher abduction angles are necessary, appropriate
anteversion helps to reduce contact stresses.
Chamfers are usually designed to increase range of motion. However,
chamfer offsets above the head center also increase contact stresses
and should be used in moderation. Other design features that can affect
range of motion and contact stresses but were not evaluated in the
current study include elevated-rim, high-wall, anteverted, and lateralized
liners.