Background: Posterior dislocation continues
to be a relatively common complication following total hip arthroplasty.
In addition to technical and patient-associated factors, prosthetic
features have also been shown to influence stability of the artificial hip
joint. In this study, a dynamic model of the artificial hip joint
was used to examine the influence of the size of the head of the
femoral component on the range of motion prior to impingement and posterior
dislocation following total hip replacement.
Methods: Six fresh cadaveric specimens were
dissected, and an uncemented total hip prosthesis was implanted in
each. Each specimen was mounted in a mechanical testing machine
and loaded with use of a system of seven cables attached to the
femur and pelvis that simulated the action of the major muscle groups
crossing the hip joint. The hip was taken through a range of motion
similar to that experienced when rising from a seated position. The
three-dimensional position of the femur at the points of impingement
and dislocation was recorded electronically. The range of joint
motion was tested with prosthetic femoral heads of four different
diameters (twenty-two, twenty-six, twenty-eight, and thirty-two
Results: Significant associations were noted
between the femoral head size and the degree of flexion at dislocation
in ten (p = 0.001), twenty (p < 0.001), and thirty (p = 0.003)
degrees of adduction. Increasing the femoral head size from twenty-two
to twenty-eight millimeters increased the range of flexion by an
average of 5.6 degrees prior to impingement and by an average of
7.6 degrees prior to posterior dislocation; however, increasing
the head size from twenty-eight to thirty-two millimeters did not
lead to more significant improvement in the range of joint motion.
The site of impingement prior to dislocation varied with the size
of the femoral head. With a twenty-two-millimeter head, impingement
occurred between the neck of the femoral prosthesis and the acetabular
liner, whereas with a thirty-two-millimeter head, impingement most
frequently occurred between the osseous femur and the pelvis.
Conclusions: With the particular prosthesis
that was tested, increasing the diameter of the femoral head component
increased the range of motion prior to impingement and dislocation,
decreased the prevalence of prosthetic impingement, and increased the
prevalence of osseous impingement.
Clinical Relevance: These results suggest that
femoral heads with a twenty-eight-millimeter diameter increase the range
of motion after total hip replacement. This may be beneficial when
additional factors compromising joint stability are encountered.