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PurposeTo achieve a higher level of satisfaction in patients having undergone Total Knee Arthroplasty (TKA), a more personalized approach has been discussed recently. It can be assumed that a more profound knowledge of bony morphology and ligamentous situation would be beneicial. While CT/MRI can give 3D information on bone morphology, the understanding of the ligamentous situation in diferent lexion angles is still incomplete. In this study, the dynamic gap widths of a large number of varus knees were assessed in various lexion angles, to ind out whether all varus knees behave similar or have more individual soft tissue patterns. Additionally, it was investigated whether the amount of varus deformity or other patient factors have an efect on joint gap widths. Methods A series of 1000 consecutive TKA patients, including their CAS data and patient records were analyzed. Joint gap widths in multiple lexion angles (0°, 30°, 60°, 90°) were measured in mm and diferences between the joint gaps were compared. A "standard" varus knee was deined as follows: (1) Lateral extension gap greater than medial, (2) lateral lexion gap greater than medial, and (3) lexion gap greater than extension gap. The percentage of fulillment was tested for each and all criteria. To measure the inluence of varus deformity on gap width diference, three subgroups were formed based on the deformity. Data were analyzed at 0°, 30°, 60° and 90° lexion. The efect of patient factors (gender, BMI, age) on gap sizes was tested by performing subgroup analyses. Results Only 444 of 680 (65%) patients met all three varus knee criteria. The lateral extension gap (4.1 mm) was signiicantly larger than the medial extension gap (0.6 mm) in 657 (97%) patients and the gap diference highly correlated with the amount of varus deformity (r 2 = 0.62). In all lexion positions, however, no correlation between gap diferences and varus deformity existed. Women had signiicantly larger extension and lexion gaps. Age and BMI showed no signiicant efect on gap widths. Conclusion Varus knees show a large inter-individual variability regarding gap widths and gap diferences. The amount of varus deformity correlates highly with the medio-lateral gap diference in extension, but not in any lexion angle. As varus knees are not all alike, a uniform surgical technique will not treat all varus knees adequately and the individual gap sizes need to be analyzed and addressed accordingly with an individualized balancing technique. Which inal balancing goal should be achieved needs to be analyzed in future studies. Level of evidence Level III.

Purpose Navigated, gap-balanced adjusted mechanical alignment (AMA) including a 0° varus tibial cut and modiication of angles and resections of the femoral cuts to obtain optimal balance accepting minor axis deviations. Objectives of this study were (1) to analyse to what extent AMA achieves the goals for leg alignment and gap balance, and ( 2) in what percentage non-anatomical cuts are needed to achieve these goals. Methods Out of 1000 total knee arthroplasties (TKA) all varus knees (hip-knee-ankle (HKA) angle < 178°; n = 680) were included. All surgeries were performed as computer assisted surgery (CAS) in AMA technique. CAS data at the end of surgery were analysed with respect to HKA and gap-sizes. All bone cuts were quantiied. Depending on the amount of deformity, a subgroup analysis was performed. It was analysed whether the amount of deformity inluences the non-anatomical cuts by correlation analysis. Results AMA reached the goals for postoperative HKA (3° corridor) in 636 cases (93.5%). While extension and lexion gap balance were achieved in more than 653 cases (96%), lexion and extension gap size were equalled in 615 knees (90.4%). The resections of the lateral tibia plateau and distal and posterior medial femoral condyle were anatomical (Tibia: 7.0 ± 1.7 mm; medial condyle distal: 7.8 ± 1.4 mm; medial posterior: 8.2 ± 1.8 mm). The number of non-anatomical resections for those cuts were low; 67 (9.9%); 24 (3.5%); 32 (4.7%). For the medial tibia plateau and the lateral posterior condyle, the cuts were non-anatomical in a high percentage of cases; Tibia: 606 (89.1%), lateral posterior condyle: 398 (58.5%). Moderate but signiicant correlations were found between resection diferences and amount of deformity (medio-lateral: tibia: 0.399; distal femur: 0.310; posterior femur: 0.167). No correlations were found between resection diferences and gap values. Conclusion AMA reaches the intended target for HKA and gap balance in over 612 (90%) of cases and maintains the medial femoral condyle anatomically. Non-anatomical tibial resection causes increased external rotation of the femoral component and by that non-anatomical cut of the posterior lateral condyle. Nonanatomical resections of AMA might be one reason for the persisting high rate of unsatisied patients after TKA. Anatomical and individual alignment philosophies might help to reduce this rate of dissatisfaction.

This new CAS software in combination with the presented dynamic gap measurement provides accurate gap values and therefore facilitates balancing TKA. This technique works reproducibly for different surgeons and has proven robustness also for repeated measurements of any surgeon in this study.

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