Contemporary
Canine Cement Technique
Historically, the canine
total hip replacement has been a successful procedure. However in recent years,
the premature breakdown of the cement mantle has become the Achilles heel as the
primary reason for canine THR failure1. A review of the human orthopaedic
literature over the last 30 years, has revealed an evolutionary development in
cement technique in order to improve the survivorship of cemented THR in people.
Since 1983, surgeons have been using, what is known as, the third generation cement
technique and the clinical results have been excellent2.
Listed below are the variables associated with a contemporary cement technique.
Optimizing these variables will help to ensure a higher quality cement mantle
and a more durable cement-bone interface.
-
- Modern Cement Technique
- A canal preparation the
preserves cancellous bone,
- Cement pressurization
using a cement plug,
- Minimizing porosity or
voids in bone cement,
- Proper mixing of antibiotics
in bone cement,
- Pulsatile lavage and adrenaline
soaked sponges,
- A smooth surface femoral
component.
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Preservation of valuable cancellous
bone:
With most contemporary canine THR systems,
the femoral preparation involves an aggressive reaming and broaching technique
that removes precious cancellous bone within the femoral canal in order to make
the femoral component fit. This approach creates a smooth, tube-like structure
where the cement has nothing to adhere to. When rotational loads are applied to
the hip, this cement-bone interface has a higher tendency to breakdown prematurely.
Instead, the hip system of
choice ought to be one whose surgical technique best preserves cancellous bone.
Cancellous bone provides a three-dimensional surface for the cement to be pressurized
into. Cement can better resist rotation and micromotion if it is intimately secured
to bone, rather than a smooth tube.
Cement Pressurization:
“Using a medullary cement plug, cement gun, and a cement pressurizer, the
distribution of cement into periprosthetic bone is substantially improved3”.
MacDonald, et. al., demonstrated that “the shear strength at the interface
between polymethyl methacrylate cement and the cancellous bone is linearly dependent
on the depth of penetration of cement into bone. The penetration of cement can
be increased and the shear strength of the interface increased by medullary plugging
lavage and pressurization of cement4”. Simply put, pressurizing
cement into cancellous bone improves the durability of the cement-bone interface.
Proper mixing of antibiotics
in bone cement:
As always, following the cement manufacturers mixing instructions allows the monomer
and polymer to polymerize properly. In addition, if antibiotics are to be added
to the cement to prevent the onset on infection, the antibiotics must be added
after the monomer and polymer are mixed together5. Mixing the antibiotics with
the polymer powder first then adding the monomer, may not polymerize the cement
properly5.
Pulsatile lavage and
adrenaline sponges:
“Femoral canal preparation has been improved by the use of pulsatile lavage
and adrenaline soaked sponges3”.
Use of smooth surface
femoral stems:
A grit blasted surface or a plasma sprayed surface on the proximal aspect of contemporary
canine femoral stems was added in order to enhance the cement-prosthesis interface
to prevent loosening. However, studies, in the human literature, have shown that
this strong bond seems to impart significant stresses to the cement-bone interface
leading to premature failure of this interface. Just imagine, if we cement a stem
into a smooth tube, should we be surprised that the cement-bone interface breaks
down?
Mann et al, “found that the addition of a proximal plasma-sprayed surface
to a femoral component resulted in an increase in the normal stresses across the
cement-bone interface. These authors speculate that there may be an increased
risk of cement-bone interface failure with the addition of a plasma-sprayed surface3”.
Gardiner et al, “perhaps improvement of the bond at the cement-prosthesis
interface transfers increased stress to the cement-bone interface and thus promotes
early failure at this interface3”.
Conclusion:
Veterinary surgeons and pet owners demand high quality products and services for
their pets. The quality of the cement technique should not be underestimated.
It is as integral a component of a successful THR procedure as the THR device
itself. Adopting the modern cement technique described in this document will enable
veterinary surgeons to provide the canine patient with the best chance for a successful,
long-term surgical outcome.
Reference:
1 Edward, M.R., DVM; Egger, E.L., DVM; Schwarz, P.D., DVM; “Aseptic Loosening
of the Femoral Implant After Cemented Total Hip Arthroplasty in Dogs: 11 Cases
in 10 Dogs (1991-1995)”, JAVMA, Vol. 211, No. 5, Spetember, 1997.
2 Goetz, D.D., Harris, W.H., “Why Have We Left Charnley Low Friction Arthroplasty?”,
Vol. 13 The Iowa Orthopaedic Journal.
3 Gardiner, R.C., Hozak, W.J., “Failure of the Cement-Bone Interface, A
Consequence of Strengthening the Cement-Prosthesis Interface?”, JBJS, Vol.
76-B, No. 1, January 1994.
4 MacDonald, W., Swarts, E., Beaver, R., “Penetration and Shear Strength
of Cement-Bone Interfaces.”, Clin Orthop, 286:283, 1993.
5 Personal communication, Arlen Hanssen, MD, Rochester, MN.
Caveats for Cementless
Canine Total Hip Replacement
As a result of some recent unfavorable cemented results, there is a trend towards
cementless fixation for canine total hip replacement. As outlined on the opposite
page, excellent cemented results can be achieved using an improved cement technique.
Nevertheless, the cementless canine THR is a more challenging and technically
demanding procedure when compared to its cemented counterpart. For the cementless
procedure to be successful, there are several technical aspects that must be considered.
The ability to achieve exceptional initial stability cannot be underestimated.
Without it, the chance for a successful long-term clinical result is greatly diminished.
Initial stability is derived from both the implant geometry and the surgeon’s
ability to achieve an intimate fit with the bone. To do so, the prosthesis must
be wedged, or press-fit into the bone. In an attempt to achieve a tight press-fit,
there is an increased potential for femoral fracture that may need to be stabilized
using cerclage wires, which adds time and complexity to the procedure.
If initial stability isn’t achieved at the time of surgery, or is unable
to be maintained, motion at the bone-implant interface will occur. As the dog
walks or gets up off the floor, it applies a torsional load to the hip. If this
load is unable to be resisted by the implant design or the press-fit, micromotion
occurs. This motion results in a fibrous membrane being formed around the implant
and not the desired bone ingrowth, as is the objective of a cementless procedure.
Micromotion can also result in thigh pain. If the implant does not circumferentially
fill the distal canal the implant can move within the distal canal. Pain occurs
when the tip of the implant impinges up against the endosetal cortex.
Another concern with micromotion is bone loss. If the stem is permitted to move
within the canal, over time, the stiff metal stem will wear out the more pliant
bone, thereby further complicating a revision procedure.
Finally, the convalescence period for a cementless device is longer than for a
cemented one. Immediate weight bearing may begin sooner with a cemented device,
as the arthroplasty is as strong as it will ever be. For a cementless procedure,
bone ingrowth begins to occur between 4-8 weeks, providing excellent stability
is achieved.
Using a contemporary cement technique similar to the human procedure can only
build upon the successful cemented canine THR results without the potential problems
and increased costs associated with a cementless procedure.
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