368 Future Directions
Joint and/or implant instability
Instability after TKR may be due to deficiencies
in implant design, but is most commonly the
result of surgeon error. At surgery, tibial and
femoral ostectomies are performed to balance
collateral ligament tension in flexion and exten-
sion while at the same time preserving axial
alignment. Excessive laxity will lead to joint
instability and an increased risk of prosthesis
subluxation resulting in accelerated implant
wear. Excessive tension in the collateral liga-
ments will restrict range of motion and may
lead to an increased risk of collateral ligament
failure.
Another cause of instability is ligament
injury, either intraoperatively or postopera-
tively. The medial collateral ligament (MCL) is
susceptible to iatrogenic injury during the tib-
ial ostectomy. Care must be taken to ensure that
the ligament is located and protected during the
ostectomy. If a collateral ligament is inadver-
tently lacerated, a primary repair should be per-
formed immediately. The repair should be pro-
tected with a Robert Jones bandage, a brace, or a
splint. It is important to note that the rehabilita-
tion of a collateral ligament injury in a dog with
a TKR is more challenging compared to that in
a dog with an intact cranial cruciate ligament
(CrCL). This is because of the interrelationship
between the CrCL and MCL. Deficiency of the
CrCL increases strain on the MCL (Lujanet al.
2007). Preliminary studies using a computa-
tional model of the canine stifle have shown that
MCL strain patterns are sensitive to the posi-
tion of the TKR prosthesis (G. Bertocci & N.
Brown, personal communication). In dogs with
primary MCL repair failure, it may be prefer-
able to convert to a custom-constrained implant
instead of struggling with repeated revision of
the MCL repair.
Neurovascular injury
The popliteal artery is a major structure caudal
and medial to the joint. It is at risk of injury
during the tibial ostectomy as the saw blade
exits the caudal tibial cortex. The risk of vascu-
lar injury is reduced by joint flexion during the
cut. This reduces the tension on the artery and
allows it, along with the adjacent neurovascular
bundles, to move caudally away from the exit
point of the blade. Placement of a blunt-tipped
Hohmann retractor immediately caudal to
the tibial plateau protects the neurovascular
bundle.Wear
Wear-generated UHMWPE particulate debris is
an important cause of long-term implant fail-
ure in humans (Reayet al. 2009). Any articu-
lating implant material will undergo a variable
amount of mechanical wear.In vivo, material
wear rates are influenced by both iatrogenic
(e.g., implant malalignment) and environmen-
tal (e.g., activity level, body weight) factors. In
preclinical research studies with canine TKR
implants, wear was found to be relatively mild
12 months after implantation (Allenet al. 2009)
(Figure 44.6). However, the long-term perfor-
mance of TKR implants has not been assessed
and will likely only be determined through
biomechanical testing and a systematic analysis
of implants that are retrieved either at the time
of revision surgery or through retrieval studies.Aseptic loosening
A presumptive diagnosis of aseptic loosening is
made on the basis of radiographic evidence of
periprosthetic osteolysis in the face of negativeFigure 44.6 Wear of the ultra-high-molecular-weight
polyethylene (UHMWPE) component of a canine total
knee replacement (TKR). A large area of the UHMWPE
(arrowheads) has delaminated from the lateral margin of
this implant. The biological response to this fragmented
UHMWPE can lead to osteolysis, fixation failure, and
aseptic loosening. The wear seen with this research
implant was, at least in part, the result of sub-optimal
implant design. Wear rates with the new clinical
TKR implant system have not been determined to date.