The metallic components of the STAR total ankle arthroplasty are made of
cobalt-chromium alloy. All of the components in the original series had a
100-µm-thick hydroxyapatite layer applied on a smooth metal surface. In the
current version, the metallic components have been coated with an
approximately 300-µm-thick layer of titanium (ISO 5832), applied with
vacuum plasma-spray technology, with a pore size of 75 to 200 µm and a
porosity of 25% to 35%. In addition, an approximately 25-µm-thick
hydroxyapatite layer (porosity, 60%) has been applied with an electrochemical
process (Figs. 1 and
2).
The tibial components are manufactured in five sizes: 30, 30, 35, 40, and
45 mm long and 30, 32, 32.5, 33, and 33.5 mm wide at the anterior aspect,
respectively. The same component is used for the right and left ankles. The
"extra-small" component (30 × 30 mm) has never been used in
our department. The "extra-large" component (45 × 33.5 mm)
has recently been introduced. The talar components are of four different
widths—extra-small, small, medium, and large—and have different
configurations in order to fit the right and left ankles.
The meniscal components used in the original series were made of ultra-high
molecular-weight polyethylene and had been sterilized with gamma irradiation
in air. These components, which range in height from 6 to 10 mm, are today
sterilized with gamma irradiation in nitrogen-vacuum. Because of the risk of a
future fracture of the meniscus, we try to avoid the use of the 6 and
7-mm-thick menisci. The desired position of the components after implantation
is shown in Figure 3.
INDICATIONS:
Rheumatoid arthritis.Inflammatory arthritis of another origin (e.g., psoriatic arthritis or
gout).Hemochromatosis.Primary osteoarthritis.Posttraumatic osteoarthritis.
Rheumatoid arthritis.
Inflammatory arthritis of another origin (e.g., psoriatic arthritis or
gout).
Hemochromatosis.
Primary osteoarthritis.
Posttraumatic osteoarthritis.
CONTRAINDICATIONS:
Neuropathic arthropathy (Charcot joint).Proven or suspected septic arthritis.Osteonecrosis affecting a major part of the talar body.Severe malalignment that cannot be corrected before or at the time of ankle
replacement.Signs of impaired circulation to the distal part of the leg.Compromised soft tissues about the ankle.
Neuropathic arthropathy (Charcot joint).
Proven or suspected septic arthritis.
Osteonecrosis affecting a major part of the talar body.
Severe malalignment that cannot be corrected before or at the time of ankle
replacement.
Signs of impaired circulation to the distal part of the leg.
Compromised soft tissues about the ankle.
PITFALLS:
The lateral malleolus may be cut accidentally when the distal part of the
tibia is resected. This risk is greater in patients with osteoarthritis. Thus,
care must be taken to know when to stop advancing the saw.
When very long saw blades are used together with powerful saws, the tibial
nerve and the tendon of the flexor hallucis longus are in danger. Thus, such
instruments should be avoided.
When the definitive tibial component is introduced, it may get stuck on the
fibula if the drill holes have not been directed somewhat medially. Also, if
the tibial component does not seat in place smoothly, its progress may be
hampered by bone spikes. This carries the risk that the tibial component will
lift out of its tracks at the back, leaving a gap between the component and
the distal part of the tibia.
If the drill holes for the tibial component have been made too deep,
especially if they pass through the posterior cortex of the tibia, the
definitive tibial component may slide too far back and become unstable.
If the holes for the rails of the tibial component are made before the
definitive talar component is seated, the anterior aspect of the distal part
of the tibia may be damaged when the talar component is put into place.
Patients with rheumatoid arthritis are particularly at risk for this problem,
which is why we now implant the definitive talar component earlier in the
procedure.
AUTHOR UPDATE:
Since the original article was published, the surgical approach has
remained unchanged. However, since 1999, the tibial and talar components have
completely new anchoring surfaces, as described above. Since 1998, the
meniscus has been sterilized with gamma irradiation in nitrogen-vacuum.
Recently, new guides for cutting the distal part of the tibia and new trial
templates for checking the talar cuts have been introduced. The postoperative
care and rehabilitation have remained unchanged.
All operations are performed in a clean-air enclosure with vertical airflow
and almost always with the use of spinal anesthesia. An antibiotic is
administered intravenously about thirty minutes before the application of the
tourniquet, again immediately after the tourniquet has been released, and then
after six and after twelve hours.
After skin preparation with the patient in the supine position, the foot is
placed on a 25-cm-high cushion with the ankle flush with the end of the
operating table (Fig. 4). The
tourniquet is applied in a sterile fashion after draping in order to reduce
the time without blood flow to the leg.
To expose the ankle joint, we use a straight anterior incision
(Fig. 5). After the superficial
peroneal nerve has been identified, the ankle is approached lateral to the
tibialis anterior tendon, which is retracted medially. The neurovascular
bundle and the remaining structures are retracted laterally. The ankle joint,
including the space between the malleoli and the talus, is cleaned, and any
anterior osteophytes are removed with an osteotome
(Fig. 6). Currently, when we
are treating a patient with rheumatoid arthritis, we introduce two Kirschner
wires percutaneously through the medial malleolus at this point in the
procedure as prophylaxis against intraoperative fracture. The guide is then
aligned so that its rod is parallel to the anterior crest of the tibia and,
when seen from the side, parallel to the fibula
(Fig. 7). The lower end of the
guide is equipped with an adjustable device to facilitate horizontal and
parallel cuts of the distal part of the tibia and proximal part of the talus.
This part of the guide has multiple holes through which it is firmly fixed to
the tibial metaphysis by steel pins. The accompanying cutting blocks can be
adjusted in proximal and distal directions as well as medially and laterally
without removing the steel pins. In order to protect the medial malleolus
while the distal tibial resection is being performed, a u-shaped device is
fitted into the two holes that will be used to prepare for the rails of the
tibial component later in the procedure
(Fig. 8).
Any valgus or varus malposition is corrected as much as possible by holding
the heel in one hand. Next, the distal part of the tibia is cut with use of a
thin oscillating saw blade a few millimeters proximal to the diseased or
eroded area; it is then removed.
The next step is to cut and remove a 4 to 5-mm-thick slice from the dome of
the talus (Fig. 9). With use of
separate guide blocks in three widths, applied and fixed to the top of the
talus, the medial and lateral sides of the talar body are removed
(Fig. 10, left). Then the
posterior and anterior aspects of the talar body are trimmed in a sloping
fashion with use of another guide block centered under the tibia. The
posterior slope is created with use of a long oscillating saw blade that is
kept flush with the posterior slope of the guide block
(Fig. 10, right). The
manufacturer provides an angulated saw blade with which to create the anterior
slope, but we find this difficult to use. We prefer to chisel, nibble, and
mill away bone anteriorly.
The joint is now cleaned of any bone debris, and as much of the posterior
capsule as possible is removed. A groove, in which the stem of the talar
component is to be introduced, is now created from the top of the talus with a
drill and punch (Fig. 11).
This groove will accommodate a trial talar template that is used to confirm
that the anterior and posterior talar cuts, which are the most difficult to
create, have been performed correctly. If they have not been performed
properly, careful adjustments must be made with use of the oscillating saw
blade (Fig. 12). At this
point, a lateral fluoroscopic view should be obtained with this trial template
in place. On this image, it can be seen if the angle of the cut distal tibial
surface is perpendicular to the long axis of the tibia, and the appropriate
length of the definitive tibial component can be measured
(Fig. 13).
We prefer to introduce the definitive talar component at this stage, in
contrast to what is recommended by the manufacturer. If this component is
introduced after the holes for the rails of the tibial component have been
drilled, there is a risk that the anterior aspect of the distal part of the
tibia will be crushed. Patients with rheumatoid arthritis are particularly at
risk for this complication, which can be avoided by implanting the definitive
talar component at this point.
At this stage, it is suitable to check whether it is possible to introduce
an 8-mm-thick and approximately 10-cm-long trial meniscus without too much
resistance. If it is not possible, the distal part of the tibia should be
resected more proximally to allow the insertion of a meniscus of this
thickness.
The cutting block is then adjusted so that its distal part is flush with
the distal tibial cut. Two parallel 6-mm holes are drilled through holes in
the guide block while the talar component is protected by the trial meniscus.
These drill holes should not pass through the posterior cortex
(Figs. 14-A and 14-B). We then
check to see if there is a need to remove more bone by using a trial tibial
component before introducing the definitive tibial component
(Fig. 15). These trial
components are not part of the standard equipment; they have to be ordered
separately. The drilled holes are then opened into the joint space with a
special punch. The definitive tibial component is carefully driven into place,
while the talar component is protected with the trial meniscus
(Fig. 16).
Trial menisci are used to determine the optimal height of the definitive
polyethylene meniscus, which is then inserted. Finally, cancellous bone is
packed into the holes left in front of the rails of the tibial component.
A final check of the position of the components is made fluoroscopically.
One 10-mm suction drain is inserted, and the wound is closed in layers. A
posterior plaster slab is applied, and the tourniquet is released.
After two days, the drain is removed, the wound is inspected, and a short
leg cast made of fiberglass is applied. Radiographs are made to confirm that
the components have been implanted correctly
(Figs. 17-A and 17-B). Full
weight-bearing with two crutches is then allowed. After two weeks, the wound
is inspected, the sutures or staples are removed, and a new fiberglass short
leg walking cast is applied. The cast is worn for two weeks by patients with
osteoarthritis and for one or two weeks longer by patients with rheumatoid
arthritis. After the cast has been removed, walking exercises and training in
ankle motion and balance by a physiotherapist are prescribed.