Use the Right Looking Glass When You Do Caliper-verified Kinematically Aligned TKA!

Abstract

“Through the Looking-Glass” refers to the Lewis Carroll novel (ie, the sequel to Alice in Wonderland) where Alice crosses over into a “bizarre universe” when she enters the flipped world on the other side of a mirror. The phrase implies unpredictable and strange happenings, a poignant metaphor when the mechanical alignment (MA) total knee arthroplasty (TKA) surgeon confronts kinematic alignment (KA) for the first time! Interest in caliper-verified KA TKA is justifiably growing. 11 of 13 randomized trials, case-matched studies, and case series of bilateral TKA with a KA TKA and MA TKA from centers worldwide, including a 2022 publication from the Hospital for Special Surgery [[1]Elbuluk A.M. Jerabek S. Suhardi J. et al.Head-to-Head comparison of kinematic alignment versus mechanical alignment for total knee arthroplasty.J Arthroplasty. 2022; Abstract Full Text Full Text PDF PubMed Scopus (1) Google Scholar], show better clinical outcome scores, knee range of motion, alignment, fewer ligament releases, or better knee kinematics than MA TKA. Equally important are the 2021 Australian and New Zealand registry results, and a case series study showed implant survival is comparable or better than MA at 7 to 10 years, respectively [[2]Howell S.M. Shelton T.J. Hull M.L. Implant survival and function ten years after kinematically aligned total knee arthroplasty.J Arthroplasty. 2018; 33: 3678-3684Abstract Full Text Full Text PDF PubMed Scopus (101) Google Scholar,[3]Klasan A. de Steiger R. Holland S. et al.Similar risk of revision after kinematically aligned, patient-specific instrumented total knee arthroplasty, and all other total knee arthroplasty: combined results from the Australian and New Zealand joint replacement registries.J Arthroplasty. 2020; 35: 2872-2877Abstract Full Text Full Text PDF PubMed Scopus (17) Google Scholar]. The concept of caliper-verified KA is simple. The KA surgeon adopts the dentist's 3-D approach of fitting a crown on the worn tooth by focusing on surface relationships inside the knee and not on distant landmarks (ie, the femoral head and ankle centers) or a preoperative radiographic image. For the femur, the surgeon sets the femoral component coincident to the patient's distal and posterior prearthritic joint lines without releasing ligaments, including the PCL. First, the surgeon computes the femoral resection target to restore the joint lines by subtracting 1 mm for the saw blade's kerf and 2 mm for complete cartilage loss from the thickness of the condyle of the femoral component. Next, intraoperative caliper measurements determine whether the thickness of these femoral bone resections is within the ±0.5 mm of the KA target. Performing the caliper measurements identifies unintended target deviations, enabling a correction before femoral component implantation. A 2022 report analyzing over 1000 femoral resections performed using manual instruments by less-experienced and experienced surgeons showed comparable and greater accuracy than the femoral resections performed by robotics [[4]Howell S. Nedopil A.J. Hull M. Negligible effect of surgeon experience on the accuracy and time to perform unrestricted caliper verified kinematically aligned TKA with manual instruments.Knee Surg Sports Traumatol Arthrosc. 2022; Crossref Scopus (2) Google Scholar]. For the tibia, the surgeon uses the gap-balance technique to fine-tune the tibial resection until a check with a spacer block and trial components verifies a rectangular extension space with negligible varus-valgus laxity. Simultaneously, the surgeon cuts the slope to match the patient's medial prearthritic posterior slope when retaining the PCL. These steps, by default, restore medial and lateral tibial compartment forces and laxities close to those of the native knee. The postoperative radiographic expectation is that angular component alignment closely matches the contralateral knee joint. Surgeons transitioning from MA to caliper-verified KA need to appreciate t