Background: With the advent of more wear-resistant bearings, there
is renewed interest in resurfacing total hip arthroplasty. However, there is a
paucity of information on the biomechanical results of this type of
arthroplasty compared with those of contemporary total hip arthroplasty.
Methods: Using standardized radiographs, we measured and compared
the biomechanical parameters that affect the hip joint reactive forces in
fifty hips that had a metal-metal surface replacement with those parameters in
forty hips that had a contemporary cementless total hip replacement performed
during the same time-period by the same surgeon.
Results: On the average, the arthritic hips that were treated with
metal-metal surface replacement had had a more valgus preoperative neck-shaft
angle and less horizontal femoral offset than the normal, contralateral hips
(p = 0.0003). After both the metal-metal surface replacements and the
cementless total hip replacements, the hip center of rotation was medialized
by approximately 6 mm. Both procedures were associated with an average
increase in limb length of approximately 3 or 4 mm. After the metal-metal
surface replacements, the horizontal femoral offset was essentially equal to
the preoperative value, but both values averaged about 8 mm less than those on
the normal, contralateral side (p < 0.00001). In the hips with a
conventional total hip replacement, the horizontal femoral offset increased an
average of 9.5 mm compared with the preoperative value and was an average of 5
mm more than that for the normal, contralateral hip (p = 0.001).
Conclusions: The biomechanical results of total hip resurfacing
depend on the preoperative anatomy of the proximal part of the femur. Limb
lengthening of 1 cm can be achieved, but horizontal femoral offset is
essentially unchanged by hip resurfacing. Horizontal femoral offset can be
increased reliably with a contemporary total hip replacement. Arthritic hips
of limbs that are more than 1 cm shorter than the contralateral limb or that
have a comparatively low horizontal femoral offset may be better served by a
contemporary total hip replacement. These biomechanical limitations should be
considered in the selection of hips for resurfacing.
Level of Evidence: Therapeutic study, Level III-1
(case-control study). See Instructions to Authors for a complete description
of levels of evidence.