Revision TKR-Methods for
Establishing Equal Flexion & Extension Gaps
Kelly G. Vince, M.D.,
F.R.C.S.(C) Assistant
Clinical Professor University
of California - Irvine Irvine,
California Associate
Surgeon, Kerlan-Jobe
Orthopedic Clinic, Los Angeles, California General Rules for equalizing
flexion and extension gaps:
Three Step Plan for Revision TKR Step
1 Reestablish the Tibial Platform
The tibial surface comprises both the flexion and
extension gaps and is the natural starting point for reconstruction. When
residual tibial bone is adequate, standard components may be implanted.
However, when the proximal tibia is osteopenic or has defects that must be
rebuilt, it is frequently necessary to augment fixation with intramedullary
stems. If this option is selected, the medullary canal must be opened. The position and orientation of the intra-medullary rod will
determine the position of the component attached to it. A tibial rod must
enter the proximal tibia at a point over the center of the diaphysis, which
may not correspond to the center of the cut surface of the proximal tibia. If
the rod is not oriented parallel to the mechanical axis of the tibia, then
only a smaller rod will fit, and malalignment will ensue. The diameter of the intramedullary canal should be
determined from preoperative radiographs. With these dimensions in mind, the
size of the canal can be probed with reamers-and active reaming of the tibia
should be minimized. Assemble and insert a trial tibial component with an
intramedullary rod. Ignore tibial defects, but confirm the alignment and posterior slope. The articular platform
should not slope anteriorly. The trial component will help define the defect in
the proximal tibia. If there is contact
between the trial and bone circumferentially, then any defects are
"contained" and can be filled with bone cement or particulate bone
graft. If the angle of the proximal tibial bone does not match the
undersurface of the tibial component, the proximal tibia should be sawn to an
angle that corresponds to an available augment if this can be done with
minimal loss of bone. Attach the augment to the trial and reinsert. Do not reconstruct tibial defects yet, but keep
the trial tibial component in place to protect the bone. Complete a general
plan for balancing gaps and selecting component sizes and positions first,
before attending to details such as reconstructing bone defects. Bone
Defects
Step 2 Stabilize the Knee in
Flexion
Summary
The flexion gap is determined by the position of
the posterior femoral condyles and the tibial articular surface. The tibial
surface has already been established and the posterior articular surface of
the femoral condyles will be determined by the size of the femoral component
and how it is attached to the femur. A. Rotation
of the femoral component should be parallel to the epicondylar axis.
Palpation of residual posterior condylar bone after the removal of the failed
component will help determine if ithad been placed in internal rotation. B. Choose a
femoral component that will be big enough to stabilize the knee in the flexed
position, without concern for fixation or bone defects. This is important. If
intramedullary rods are used they will determine the anterior-posterior
position of the component and consequently the tightness of the flexion gap. Assess the gaps that remain between the interface
side of the posterior femoral component and the remaining bone on the posterior
femur. Gaps should be reconstructed with augments. Do not apply the posterior
femoral component directly to a deficient surface. This leaves the knee
unstable in flexion and displaces the extensor mechanism anteriorly. Failure
to support the flange posteriorly has however been implicated in femoral
component loosening. (King & Scott) C. The
combination of femoral component and tibial polyethylene will establish the
joint line. Choose the pair that stabilize the knee in flexion and that keep
the inferior pole of the patella above the joint line, if possible. Step 3 Balance the Extension
Gap
The final major variable to be solved in the
reconstruction is the position at which the femoral component will be seated
to create an extension gap that equals the flexion gap. A space between the
tibia and femur in extension that is larger than the flexion space can be
diminished by attaching distal femoral augments to the femoral component.
Asymmetric bone defects, or distal femoral cuts that were cut with the wrong
angle originally can be corrected with augments on only one side, or larger
augments on one side than the other. Correct valgus alignment is ensured with
press fit medullary stems if the entry point in the distal femur has been
selected correctly and the stems are large enough to make endosteal contact
in the femur, yet not so large as to be deflected by asymmetry in the bone..
Within limits, a relatively tight extension gap can be enlarged by resecting
additional distal femur. In general terms, there is usually scant bone to
spare, and resection of distal femur creates problems with patella baja,
extensor lag and ultimately with the attachment of the collateral ligaments. Conventional ligament releases may be required at
this point. If so, it will be necessary to return to Step 2 to confirm that
there is not a need for a thicker polyethylene insert as a result of the
releases. Critical Decision Points
Two circumstances may arise that force one of two
choices upon the surgeon. 1. Flexion
extension instability. In some
very difficult reconstructions, the flexion gap sags open and is much more
spacious than the extension gap. It will not be possible to find a femoral
component large enough in the AP dimension to stabilize the knee in flexion-
such a component would overhang tremendously medial to lateral. If an
inordinately thick tibial insert is selected for these knees, it will not be
possible to extend the knee. This knee will require a constrained condylar
prosthesis or a ligament reconstruction, transposing the medial collateral
ligament anteriorly on the femur for example to tighten the knee selectively
in flexion
2. Valgus
instability. When no amount
of release of the lateral structures can place the medial structures under
tension, then progressively thicker polyethylene inserts will only create a
flexion contracture. This calls for a constrained condylar insert or a
proximal advancement of the medial collateral ligament on the femur, with or
without an augmentation. Ligament reconstructions are required only rarely.
Choices
1.
Constraint in the prosthesis
2.
Ligament reconstruction and or
advancement. These are required very rarely, should be regarded as
experimental and are never a substitute for conventional ligament releases.
Treatment for Specific Problems
The stiff knee
Whether lacking flexion, extension or both, the stiff knee may
be one of the most dismaying and difficult knees to revise. The problem will
be unlikely to respond to revision if the knee had poor motion prior to the
original arthroplasty. Arthroscopic release of intra-articular fibrosis,
followed by manipulation and physical therapy has been of modest value in
some patients with stiff knee arthroplasties. Manipulation alone, after
6 weeks, once fibrosis has occurred is unlikely to help, and risks disrupting
the extensor mechanism or fracturing the femur. Mechanical impediments- Potential for
Revision
Poor
flexion
1. Non surgical
2. Surgical
Poor
extension
i. Tight
flexion gap ii.
Tight PCL
iii. Posterior
condyle osteophytes
iv. Proximal
joint line migration
v. Malrotation
of components (especially the femoral)
Other Considerations
i. Tight
extension gap Resection of additional distal femoral bone may be an important
part of revision surgery. ii. Tight posterior structures iii. Quadriceps weakness and failure to comply
with physical therapy iv. Consider spastic disorders References:
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