HOW TO PERFORM A KNEE RESURFACING?
at the end of the page the surgical technique...but before learn the basic principles
MULTI RADIUS & INSTANT ROTATIONAL CENTER THEORY
Before the Kinematic Alignment origin, surgeons and engineers, striving to improve the maximum flexion of the knee after total knee replacement, embraced the MULTI RADIUS theory with INSTANT ROTATIONAL CENTER (Fick's studies).
ANATOMICAL SINGLE CURVATURE RADIUS
Three-dimensional studies have subsequently shown that between 0 ° and 90 ° of flexion the radius of curvature is SINGLE and that the radii of the medial and lateral condyles are COMPARABLE.
The medial and lateral radii are identical both in the varus knees and in the valgus knees. For this reason the supposed hypoplasia of the lateral condyle compared to the medial one would seem to be under discussion
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fig.1 a-b from: Eckhoff DG, Bach JM, Spitzer VM, Reinig KD, Bagur MM, Baldini TH, Flannery NM. Three-dimensionalmechanics, kinematics, and morphology of the knee viewed in virtual reality. J Bone Joint Surg Am 2005;87 Suppl 2:71-80.
CARTILAGE WEAR
The cartilage wear of the femoral epiphysis in its DISTAL and POSTERIOR component is predictable.
In the varus arthritic knee, cartilage wear is confined to the medial distal femoral condyle (typically it is up to 2 mm).
In valgus arthritic knee, cartilage wear is confined to the lateral distal femoral condyle (typically it is up to 2 mm) and posterior (typically it is up to 1 mm).
PLEASE NOTE: the posterior condylar wear in the VARUS knee is usually negligible (<1 mm).
BONE WEAR
In both varus and valgus knees bone wear of the distal and posterior femoral condyles are usually negligible (fig.2)
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These assumptions are the background of TWO FOUNDATIONS of the Howell’s Calipered Kinematic Alignment Technique. Actually, the Medacta Individualized Kinematic Alignment (MIKA) instrumentation is the only one FDA approved and CEE instrument specific for this technique.
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FIRST FOUNDATION
The distal femoral cut of the unworn condyle must be of the same thickness as the prosthesis (after compensating for 1-mm kerf from the saw cut)
The distal femoral cut of the worn condyle must be 2 mm less than the thickness of the prosthesis
(after compensating for 1-mm kerf from the saw cut)
SECOND FOUNDATION
In the VARUS knee the posterior cuts of both condyles are of equal thickness of the prosthesis
(after compensating for 1-mm kerf from the saw cut)
In the VALGUS knee, only if lateral cartilage wear is present, the posterior cuts of the medial condyle have the same thickness of the prosthesis and 1-mm less on the lateral condyle
(after compensating for 1-mm kerf from the saw cut)
SUMMARY: THERE IS NO BONE WEAR ON THE FEMUR. THE CARTILAGE WEAR IS THE ONLY ONE PARAMETER WE HAVE TO CONSIDER BECAUSE IT IS PREDICTABLE IN THICKNESS AND LOCALIZATION. CONSIDER THE FEMORAL CUTS, WE MUST COMPENSATE THE WORN CONDYLE WITH A THICKNESS.
fig.2 - from: Lin KM, Howell SM, Hull ML. Is the pattern of cartilage and bone wear predictable in the osteo-arthritic knee with a varus or valgus deformity? Knee Surg Sports Traumatol Arthrosc. 2014
"EQUAL" gap balancing
Prior to the advent of the concept of kinematic alignment, striving to improve the results of the femoral and tibial "independent cuts" (measured resection) technique, the concept of EQUAL BALANCE GAPS in flexion and extension has been introduced.
The EQUAL is considered a rectangular space that must be obtained both in flexion and in extension.
The ASYMMETRICAL gap balancing
The Mechanical Aligners surgeons are questioning the EQUAL gap balancing. It’s easy and simple to note that during arthroscopic surgeries in flexion the lateral compartment is significantly looser compared to the medial one. It means that the flexion GAP is TRAPEZOIDAL with a greater lateral external opening.
Stephen Howell has shown that in EXTENSION the lateral and medial laxities are negligible and the GAP is RECTANGULAR. From his study it appears that in extension the gap is predictable (quadrangular) while in flexion it could be subjectively variable
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According to the kinematic alignment, it is mandatory to reproduce the asymmetric native gap balancing. This concept better fit the idea of femoral roll back in a medial pivot knee, described by Freeman and Pinskerova
SUMMARY: IN A HEALTHY KNEE, THE LIGAMENTS AND THE FEMUR ARE THE MAIN REFERENCES FOR PROXIMAL TIBIAL CUT ON THE CORONAL PLAN (VARUS-VALGUS). THE TIBIAL CUT MUST BE CARRIED OUT, AND SOMETIMES CORRECT, REACHING A QUADRANGULAR SPACE IN EXTENSION WITHOUT TOUCH THE LIGAMENTS. AUTOMATICALLY A TRAPEZOIDAL SPACE MORE OPENED LATERALLY IN FLEXION WILL BE CREATED, SPECIFIC AND DIFFERENT FOR EVERY PATIENT.
