The objective of this study was to compare differences in alignment following robotic arm-assisted bi-unicompartmental knee arthroplasty (Bi-UKA) and conventional total knee arthroplasty (TKA). This was a prospective, randomised controlled trial of 70 patients. 39 TKAs were implanted manually, as per standard protocol at our institution, and 31 Bi-UKA patients simultaneously received fixed-bearing medial and lateral UKAs, implanted using robotic arm-assistance. Preoperative and 3-month postoperative CT scans were analysed to determine hip knee ankle angle (HKAA), medial distal femoral angle (MDFA), and medial proximal tibial angle (MPTA). Analysis was repeated for 10 patients by a second rater to validate measurement reliability by calculating the intra-class correlation coefficient (ICC). Mean change in HKAA towards neutral was 2.7° in TKA patients and 2.3° in Bi-UKA patients (P=0.6). Mean change in MDFA was 2.5° for TKA and 1.0° for Bi-UKA (P<0.01). Mean change in MPTA was 3.7° for TKA and 0.8° for Bi-UKA (P<0.01). Mean postoperative MDFA and MPTA for TKAs were 89.8° and 89.6° respectively, indicating orientation of femoral and tibial components perpendicular to the mechanical axis. Mean postoperative MDFA and MPTA for Bi-UKAs were 91.0° and 86.9° respectively, indicating a more oblique joint line orientation. Inter-rater agreement was excellent (ICC>0.99). Early functional activities, according to the new Knee Society Scoring System, favoured Bi-UKAs (P<0.05). Robotic arm-assisted, cruciate-sparing Bi-UKA better maintains the natural anatomy of the knee in the coronal plane and may therefore preserve normal joint kinematics, compared to a mechanically aligned TKA. This has been achieved without significantly altering overall HKAA

Introduction. In valgus knees, ligament balance remain difficult when implanting a total knee arthroplasty (TKA), this leads some authors to systematically propose the use of constrained devices. Others prefer reserving higher constraints to cases where it is not possible to obtain final satisfactory balance: less than 5 of residual frontal laxity in extension in each compartment, and a tibiofemoral gap difference not in excess to 3mm between flexion and extension. The goal of the study was to assess if is possible to establish preoperative criteria that can predict a constrained design prosthetic implantation at surgery. Materials and Methods. A consecutive series of 93 total knee prostheses, implanted to treat a valgus deformity of more than 5 was retrospectively analysed. Preoperatively, full weight bearing long axis AP views A-P were performed: hip knee angle (HKA) averaged 195 (186 to 226), 36 knees had more than 15 of valgus, and 19 others more than 20 of valgus. Laxity was measured by stress radiographies with a TelosTM system at 100 N. Fifty-two knees had preoperative laxity in the coronal plane of more than 10. Fourteen knees had more than 5 laxity on the convex (medial) side, 21 knees had more than 10 laxity on the concave (lateral) side. Statistical assessment, using univariate analysis, identified the factors that led, at surgery, to an elevated constraint selection level; these factors of independence were tested by multivariate analysis. Logistical regression permitted the classification of the said factors by their odds ratios (OR). Results. High-constraints prostheses (CCK type) were used in 26 out of 93 TKA, the other TKA were regular posterostabilized (PS) prostheses. Statistically, the preoperative factors that led to the choice of a constrained prosthesis were: (1) valgus severity as measured by HKA (PS = (PS = 193, CCK = 198), (2) increased posterior tibial slope (PS = 4.8, CCK = 6.5), (3) low patellar height (using Blackburne and Peel index PS = 0.89, CCK = 0.77), (4) severity of laxity in valgus (PS = 2.3, CCK = 4.3). Among all these factors, the only independent one was laxity in valgus (convex side laxity) (p = 0.0008). OR analysis showed a two-fold increased probability of implanting an elevated constraints prosthesis for each one degree increment of laxity in valgus. Discussion. This study demonstrated that it was not the valgus angle severity but rather the convex medial side laxity that increased the frequency of constrained prostheses implantation. Other factors, as a low patellar height or an elevated posterior tibial slope, when associated, potentiate this possible prosthetic switch (to higher constraints) and should make surgeons aware, in these situations, of encountering difficulties when establishing ligament balance