Host response - Orthopaedic Biomaterials in Research and Practice - page 351

is far more probable in remote organs such as the liver, pancreas, and
lungs, where tissues are capable of concentrating corrosion products,
than in the relatively insensitive primary tissue for most orthopaedic
procedures, including bone. These remote site concerns will be dis-
cussed below.

Systemic and remote host response

acute effects Introduction of implants may produce marked transitory systemic

effects. Perhaps the best known of these is central hypotension after
PMMA insertion. The mechanism is poorly understood but animal stud-
ies suggest that it is potentiated by hypovolemia. It is well controlled in
modern clinical practice by normovolemic (replacement) management
of blood volume. Some researchers have suggested that it is an embolic
phenomenon, since insertion of cement into a closed medullary space,
as in the femur, may produce transient pressures up to more than five
times venous blood pressure. Although there was a brief vogue of vent-
ing the femoral canal by a distal drill hole or by a later withdrawn cath-
eter before PMMA insertion, these practices have fallen out of favor. It
is hoped that the current interest in pressurizing PMMA after insertion
into both the acetabulum and femur will not produce a return of this
phenomenon.
There are also short-term changes in chemical composition and blood
coagulation parameters, unrelated to deep venous thrombosis, second-
ary perhaps to operative stress, to medications used intraoperatively, or
perhaps to the rapidly resolving high levels of metal release seen imme-
diately postoperatively.

Chronic effects Corrosion and wear, produce longer-term changes in blood composi-

tion, primarily in its metal content. Table 14.3 summarizes currently
accepted normal values for implant-contained metals in blood, serum
and urine.
As discussed in Chapter 12, it is extremely difficult to estimate the
true rates of metal release in vivo because of the combination of the
processes possible. Traditional methods of radiologically assessing
wear are not practical for MOM bearings. While serving an important
role in product evaluations, simulator studies are only somewhat use-
ful, as there is no free exchange with blood and synovial fluid. Further,
it is difficult to isolate and characterize Co-Cr particles from lubri-
cant fluid. The alkaline digestion method (used to isolate UHMWPE
particles) may alter the size and can reduce the amount of chromium.
Isolation with enzymatic digestion of periprosthetic tissues has been
more successful, although to understand the potential systemic impli-
cations of metal release, detection of metal ions circulating periph-
erally is required. Of the available methods, measuring serum is the
simplest and most common, though circulating levels may not accu-
rately represent total body ion level. Further, chromium concentrates in
erythrocytes, so measuring serum may not be sufficient in some cases.