Host response - Orthopaedic Biomaterials in Research and Practice - page 362

Total hip
resurfacing

For hip resurfacing, clinical studies have observed that remodeling is
focused primarily on narrowing of the femoral neck, possibly exacer-
bated or driven by necrosis. In some studies, thinning beyond 10% has
been observed in select cases. The most frequent reason for early revi-
sion in hip resurfacing is neck fracture, possibly the result of the com-
bination of notching owing to suboptimal positioning and a narrowing
of the neck. In some resurfacing implants, there is a pronounced stress
concentration (up to 5× strain in some cases) on the anterior and poste-
rior head-neck region, which is the known region correlating with clini-
cal fractures (Cristofolini et al. 2009). Remodeling is thought to depend
heavily on proper placement of the prosthesis— in vitro studies have
reported lower stress shielding if the implant is appropriately placed.
Computational models predict initial stress shielding/bone resorption
directly beneath the prosthetic head, along with density increases near
the distal stem section (Ong et al. 2008). The bone resorption region
extends distally and peripherally toward the middle of the neck, likely
the driver for clinically observed neck narrowing from a mechanistic
standpoint. Similar to remodeling in other devices, resorption stabilizes
at about 3 years, though these patients are still considered at potentially
higher risk for loosening complications in the long term, along with a
continued risk for femoral neck fracture.

Isoelastic implants Isoelastic implants are intended to match the stiffness of the femur in an

effort to reduce stress shielding and bone resorption around the implant.
Isoelastic implants may include any of the following: a composite of
a metallic core coated with a polymer or porous polymer, a laminated
metal core, or a carbon fiber-reinforced polymer with or without a metal
core. Historically, there have been numerous setbacks in bringing iso-
elastic THA prostheses to the market. Higher debris production and poor
primary fixation are believed to be the main reason for the high failure
rates in early generations.
Studies on bone remodeling in isoelastic stems support the conclusion
that reducing stiffness mismatch can reduce bone resorption around the
implant. Nagi et al. (2006) used DEXA to evaluate periprosthetic bone
remodeling in 92 patients implanted with isoelastic stems and found a
change in BMD averaging 15% for all Gruen zones. They concluded that
the isoleastic prostheses preserved bone better than cemented or unce-
mented metallic implants. In a randomized control trial comparing the
three-part macrocomposite Epoch stem (including a layer of polyaryle-
therketone resin) to an HA tricalcium phosphate-coated Ti stem of a
similar geometry, Karrholm et al. (2002) confirmed the isoelastic Epoch
stem to have lost as little as half the bone mineral in four Gruen regions.
A study of the Bradley stem, which has a proximal HA coating, a tapered
metallic core, and a CFR-PEEK outer layer, has also shown increased
bone mineral density medially (Kurtz et al. 2012).

Tapered stems Tapered stems aim to transfer loads to the metaphysis and prevent proxi-

mal femoral stress shielding. A study of tapered cementless stems by
Pandit et al. (2006) using quantitative computed tomography to map bone