Page 37 - ISAKOS 2020 Newsletter Volume 2

02 The table summarizes the year, author/country, journal of publication, and principle results of eight randomized and case-controlled trials, published between 2012 and 2019, that compared kinematic and mechanical alignment. Six studies showed that kinematic alignment restored better motion, better alignment, better clinical outcomes, and comparable tibial component migration when compared with mechanical alignment. The studies by Waterson and Young, which showed comparable outcomes, included only patients with small deformities and set limits on the degree of postoperative correction, which adulterated the testing of kinematic alignment.
Calipered Kinematic Alignment is FDA and CE Approved
In 2018, one implant company received FDA and CE approval for the “calipered” kinematic alignment technique with use of a medial stabilized ball-and-socket design and a flat lateral compartment design as described by Freeman and Pinskerova. The medial stabilized design can function with either a PCL-retaining (CR) or PCL-substituting (CS) insert (GMK Sphere; Medacta) (Fig. 3).
03 The left images show the tibiofemoral relationships in the medial
and lateral compartments of the native knee in full extension and full extension, with the medial femoral condyle hardly moving (orange square) and the lateral femoral condyle rolling posterior in full flexion (orange rectangle). The right images show the design of the CR insert, which features a medial ball-and-socket and a lateral flat surface without a posterior rim. The use of this medial-stabilized implant design is a promising strategy for promoting anteroposterior stability and reducing the risk of late tibial component failure resulting from posterior rim wear of the insert.
04 The composite of images shows the verification worksheet (left) and the millimeter recordings of the thicknesses of the distal and posterior femoral bone resections compared with those of the condyles of the femoral component (right). The femoral component is kinematically aligned when the femoral resections match the thicknesses of
the condyles of the femoral component within ± 0.5 mm after compensating for ~1 mm from the loss of bone from the kerf of the saw blade and 2 mm for cartilage loss.
The regulatory agencies mandated the use of caliper measurements of the bone resections and tibiofemoral relationships and completion of a series of intraoperative verification checks to confirm precise restoration of the native joint lines of the femur to within ±0.5 mm (Figure 4).
The caliper measurements and verifications function as quality-control methods that more accurately set the femoral and tibial components coincident to the native joint lines than the use of patient-specific, navigational, and robotic instrumentation that sets the components perpendicular to the femoral and tibial mechanical axes (Figure 4)1.
Calipered Surgical Technique with Intraoperative Verification Checks
Watching a narrated video is one efficient method for learning the surgical steps of calipered kinematic alignment and the intraoperative decisions that restore the native tibial compartments without ligament release. Briefly, the distal femoral cuts are made, resecting precisely the thickness of the components and allowing for cartilage loss and the kerf of the saw blade. The same process is carried out for the posterior parts of the femoral condyles. When a CR insert is used, the tibial resection should restore the native varus-valgus angulation and posterior slope of the plane of the native proximal tibial joint line. All cuts are measured and checked and are adjusted when necessary. The anteroposterior axis of a properly designed and anatomically shaped tibial baseplate is oriented parallel to the flexion- extension plane of the native knee when best-fit to the cortical boundary of the tibial resection.
The flexion-extension plane is perpendicular to the transverse axis of the native femur and the distal and posterior femoral joint lines.
CURRENT CONCEPTS
ISAKOS NEWSLETTER 2020: VOLUME II 35