Host response - Orthopaedic Biomaterials in Research and Practice - page 361

1 7

2 6

3 5

4

FIGUre 14.6 (See color insert.) Gruen zones for sectionalizing bone
resorption in the proximal femur.

metaphysis, proximal to the lesser trochanter, and seems to affect the
cortex more severely than the cancellous bone. Bone loss assessed with
dual-energy x-ray absorptiometry has been used to estimate the bone
loss around porous coated, uncemented stems in the proximal femur to
be 7% to 50% primarily in Gruen zones 1 and 7, though this is depen-
dent on cemented versus cementless fixation (Bauer and Schils 1999).
(See Figure 14.6 for a graphical description of Gruen zones.) In a study
using DEXA comparing cemented and cementless stems, it was shown
that bone loss in cemented stems was isolated to the calcar region while
loss in the uncemented stem was much more significant (Pandit et al.
2006). Coating in general is often associated with less bone resorption—
radiographic investigation has shown that hydroxyapatite (HA)-coated
stems have better bone remodeling and implant fixation when compared
to porous coated implants. Prevalence of radiographic osteolysis in
HA-coated stems has been demonstrated to be 27% lower.
Periprosthetic bone loss seems to stabilize after about 2 years. Some
studies on cementless stems have shown an initial postoperative bone
loss, followed by a recovery of density somewhere near 2 to 3 years post-
operatively, possibly owing to osseointegration of the implant and better
resulting load transfer. Reduced physical activity may also play a role
in the initial bone loss. Despite it being a significant risk factor, stress
shielding is rarely the sole cause of aseptic loosening.
A goal of implant design is to ensure that the quality and integrity of
the host bone are sufficient for providing long-term structural support of
any implant. Methods undertaken have been to reduce the stiffness of the
stem material (e.g., by using Ti instead of CoCr or steel) or to modify the
fit and fill of the stem to better conform to bone morphology. Below is a
brief discussion on clinical outcomes for four alterations to traditional hip
stems: tapered stems, hip resurfacing, isoelastic implants, and short stems.
Understanding differences in bone resorption response to implant type has
implications for decisions for patients known to have poor bone quality.