Chapter 14 Disorders of the Pelvic Limb: Diagnosis and Treatment 365normal hip function, range of motion, and
pelvic and crural muscle bulk. It is unusual
that osteoarthritic degeneration will preclude
THR. However, there is an increased degree of
surgical difficulty in hips with advanced OA.
Juvenile dogs with luxation of dysplastic hips
(luxoid conformation) can develop rapid
remodeling of the femur, notably lateral drift of
the proximal medial femoral cortex, which will
preclude insertion of a diaphyseal stem. In such
cases the window of opportunity for routine
THR may be short.
Selection criteria for THR are stringent, and
common contraindications to THR include
obesity, bacterial pyoderma, other clinically
significant orthopedic disease (CCL rupture
almost invariably should be addressed prior to
consideration for THR), neurological impair
ment, infective arthritis, immunosuppression,
and polyarthropathy. Resolution of some of
these conditions is practical and may permit
THR at a later date if necessary.
Anchorage of the prosthetic cup and stem is
implant‐specific and may involve cemented or
cementless fixation. In cemented THR, a mantle
of acrylic cement is used to bond the implant
to bone. Cementless anchorage of the cup can
involve press‐fit (BioMedtrix and Kyon) or
screw home (Innoplant) devices. Cementless
fixation of the stem may be simply press‐fit, or
augmented with locking bolts through the
lateral cortex (Kyon and BioMedtrix EBM lat
eral bolt) or screw home (Innoplant). The initial
press‐fit should be stable enough to allow
osteointegration, which will contribute to the
permanent fixation of the implant (Figure 14.14).
In selected patients, when using the modular
BioMedtrix system, a hybrid fixation—most
commonly a cementless cup with a cemented
stem—can offer the most durable fixation
(Figure 14.15).
The primary advantage of the cemented stem
over press‐fit cementless systems, is that it
offers maximum strength shortly after implan
tation. It is ideal for patients with more brittle
cortical bone, thinner cortices, a cylindrical
(rather than conical) proximal femoral diaphy
sis, and poor‐quality trabecular bone (implying
increased risk of cementless stem subsidence)
as seen typically in the older shepherd breeds.
The new BioMedtrix “micro” and “nano” THR
systems are fully cemented and specifically
address hip disorders in small dogs and cats
(Liska, 2010).
Complications of THR are uncommon but
include sciatic neuropraxia, prosthetic luxation,
femoral fissure/fracture, stem subsidence,
periprosthetic infection, aseptic loosening, and
implant failure. Aseptic loosening is a biological
process initiated by polyethylene wear debris
that stimulates macrophage‐mediated osteo
clastic bone resorption at the implant–bone
interface. Debonding of the cement–implant
interface, a mechanical deterioration, can also
occur (Finkelstein et al., 1991; Edwards et al.,
1997).
The early experience with press‐fit cementless
stems was associated with relatively high rates
of subsidence and fissure. Technical refinement
in stem implantation and the development of
porous ingrowth textures with higher coeffi
cient of friction (Biomedtrix EBM) and aug
mented fixation (e.g., medial collar, lateral bolt)
have all contributed to more consistently suc
cessful cementless THR (Zhang et al., 1999;
Biemond et al., 2011).Figure 14.13 Progression of osteoarthritis remains
possible with both the JPS and TPO (as shown here)
procedures, particularly in patients that already have
damage to joint ligament or cartilage.