Better joint motion and muscle activity are achieved using kinematic alignment than neutral mechanical alignment in total knee replacement

Introduction: In total knee replacement (TKR), neutral mechanical alignment (NMA) is generally targeted during prosthetic component implantation. An original implantation approach has been recently proposed, referred to as kinematic alignment (KA). This is based on the alignment of the pre-arthritic lower limb undergoing TKR, which is reconstructed using suitable image-based techniques during the surgical planning phase. Particularly, KA is thought to allow better soft-tissue balance [1] and restoration of knee physiological function than NMA. Patient-specific instrumentation (PSI), recently introduced in TKR to execute more accurate and personalized prosthesis component implantation, can be used in the achievement of KA. In detail, KA approach via PSI has the potential to result in more physiological knee motion, including relevant muscle activity, but this has not been demonstrated yet. The aim of this study was to report knee kinematics and electromyography (EMG) for a number lower limb muscles from two TKR patient groups, i.e. operated according to NMA via conventional instrumentation, or according to KA via PSI. Methods: A four-centre randomized study of 144 patients was designed; in each centre, 36 patients affected by primary gonarthrosis were recruited for TKR and implanted with a cruciate-retaining fixed-bearing prosthesis with patella resurfacing (Triathlon® by Stryker®, Kalamazoo, MI-USA). In our centre 20 patients were implanted so far. 17 of these patients, i.e. 11 operated targeting NMA (group A) via convention instrumentation and 6 targetingKA(group B) via PSI (ShapeMatch® by Stryker®, Kalamazoo, MI-USA), were assessed clinically using the International Knee Society Scoring (IKSS) System and biomechanically at 6-month follow-up. Knee kinematics during stair-climbing, chair-rising and extension-against-gravity was evaluated by 3D video-fluoroscopy (CAT® Medical System, Monterotondo, Italy) synchronized with 4-channel EMG analysis (EMG Mate, Cometa®, Milan, Italy) of the main knee ad/abductor and flexor/extensor muscles. Knee motion data were reconstructed to calculate flex/extension (FE), ad/abduction (AA), and internal/external rotation (IE), together with the rotation of tibial base-plate contact-line (CLR), this being the line connecting the medial (MCP) and lateral (LCP) condyle contact points. MCP and LCP antero-posterior translations were also calculated and reported in % of tibial base-plate length. Results: Postoperative knee and functional IKSS scores in group A were 78±20 and 80±23, worst than in group B, respectively 91±12 and 90±15. Knee motion patterns were much more consistent over patients in group B than A. In both groups, normal ranges were found for FE, IE and AA, the latter being generally smaller than 3◦. Average IE ranges in the three motor tasks were respectively 8.2±3.2◦, 10.1±3.9◦ and 7.9±4.0◦ in group A, and 6.6±4.0◦, 10.5±2.5◦ and 11.0±3.9◦ in group B. Corresponding MCP translations were 13.8±5.6% anterior, 17.0±6.6% posterior and 15.4±6.9% posterior in group A, and 13.0±3.4%, 16.6±5.3% and 16.6±5.6% all posterior in group B; LCP values were all posterior, i.e. 9.5±3.6%, 11.1±4.3% and 8.7±2.6% in group A, and 10.2±2.1%, 13.7±8.6% and 14.6±9.8% in group B. Relevant CLRs were 8.2±3.2◦, 10.2±3.7◦ and 8.8±5.3◦ in group A, and 7.3±3.5◦, 12.6±2.6◦ and 12.5±4.2◦ in group B. EMG analysis revealed prolonged activation of the medial/lateral vasti muscles in group A. Such muscle co-contraction was not generally observed in all patients in group B, this being proving more stability in the knee joint after TKA according KA. Discussion: These results reveal that better function occurs usingKAthanNMAin TKR. Though small differences were observed between TKR groups in terms of motion data, the higher data consistency and the less prolonged muscle activations identified using KA support the claim of a more natural soft tissue balance in corresponding knees. More patients are needed to establish the superiority of KA. Reference [1] Eckhoff DG, et al. J Bone Joint Surg Am 2005;87(Suppl. 2):71–80.