Varus/valgus stability in imageless robotic-assisted total knee arthroplasty applying three-dimensional assessment of varus/valgus stress X-rays.

BACKGROUND: The postoperative varus/valgus stability assessment in stress X-rays has been established as an evaluation index. However, it is performed by the two-dimensional (2D) method rather than the three-dimensional (3D) method. OBJECTIVE: This study aimed to identify the precision and reproducibility of measuring varus/valgus stress X-rays three-dimensionally and to examine varus/valgus stability under anesthesia in imageless robotic assisted total knee arthroplasty (rTKA). METHODS: This prospective study analyzed 52 consecutive rTKAs (five males, 67 ± 5.3 years; 47 females, 74 ± 5.9 years). Postoperative varus/valgus stress X-rays in knee extension under anesthesia at manual maximum stress were three-dimensionally assessed by 2D-3D image matching technique using the 3D bone and component models. Varus/valgus angle between components (VV angle) in no stress, valgus stress, varus stress, medial joint opening (MJO), and lateral joint opening (LJO) were evaluated, clarifying this method's precision and reproducibility and valgus/varus stability. RESULTS: All parameters' precision and reproducibility had <1° mean differences and high intra- and inter-class correlation coefficients. Bland-Altman plots showed no fixed and proportional bias. Non-stress VV angle, valgus VV angle, varus VV angle, MJO, and LJO were 3.6 ± 1.2°, 1.0 ± 1.4°, 7.1 ± 1.9°, 1.5 ± 1.0 mm, and 2.8 ± 2.7 mm, respectively. CONCLUSION: This prospective study demonstrated that (1) the three-dimensional measurement method provided sufficient precision and reproducibility, and (2) the rTKAs could achieve good postoperative varus/valgus stability with a small standard deviation.

The morphology of the femoral posterior condyle affects the external rotation of the femur.

PURPOSE: The purpose of this study was to identify factors related to the external rotation of the femur during knee flexion. METHODS: Three-dimensional (3D) digital models of the femur and tibia were reconstructed from computed tomography images of 41 healthy Japanese subjects. Thirteen parameters related to femoral and tibial morphology and alignment of the lower extremities were evaluated, including the inclination angle of the posterior lateral and medial femoral condyles, the ratio of the medial and lateral posterior condyle radii approximated as spheres, the spherical condylar angle, the posterior condylar angle, the medial and lateral posterior tibial slope, the difference of medial and lateral posterior tibial slope, the tibiofemoral rotation angle, the 3D femorotibial angle, the 3D hip-knee-ankle angle, and the passing point of the weight-bearing line (medial-lateral and anterior-posterior). The rotation angle of the femur relative to the tibia during squatting was investigated using a 3D to 2D image matching technique and the relationships with the13 parameters were determined. RESULTS: The femur externally rotated substantially up to 20° of knee flexion (9.2° ± 3.7°) and gently rotated after 20° of knee flexion (12.8° ± 6.2°). The external rotation angle at 20°-120° of knee flexion correlated with the spherical condylar angle, the tibiofemoral rotation angle and the inclination angle of the posterior medial condyles (correlation coefficient; 0.506, 0.364, 0.337, respectively). CONCLUSION: The parameter that was most related to the external rotation of the femur during knee flexion was the spherical condylar angle. LEVEL OF EVIDENCE: IV.

Comparison of post-operative three-dimensional and two-dimensional evaluation of component position for total knee arthroplasty.

PURPOSE: The purpose of this study was to evaluate the post-operative three-dimensional (3D) femoral and tibial component positions in total knee arthroplasty (TKA) by the same co-ordinates' system as for pre-operative planning and to compare it with a two-dimensional (2D) evaluation. MATERIALS AND METHODS: Sixty-five primary TKAs due to osteoarthritis were included. A computed tomography (CT) scan of the femur and tibia was obtained and pre-operative 3D planning was performed. Then, 3D and 2D post-operative evaluations of the component positions were performed. KneeCAS (LEXI, Inc., Tokyo, Japan), a lower-extremity alignment assessment system, was used for the 3D post-operative evaluation. Standard short-knee radiographs were used for the 2D post-operative evaluation. Differences between the pre-operative planning and post-operative coronal and sagittal alignment of components were investigated and compared with the results of the 3D and 2D evaluations. RESULTS: According to the 3D evaluation, the difference between the pre-operative planning and actual post-operative sagittal alignment of the femoral component and the coronal and sagittal alignments of the tibial component were 2.6° ± 1.8°, 2.2° ± 1.8° and 3.2° ± 2.4°, respectively. Using the 2D evaluation, they were 1.9° ± 1.5°, 1.3° ± 1.2° and 1.8° ± 1.4°, making the difference in 3D evaluation significantly higher (p = 0.013, = 0.003 and < 0.001). For the sagittal alignment of the femoral component and the coronal and sagittal alignment of the tibial component, the outlier (> ± 3°) ratio for the 3D evaluation was also significantly higher than that of the 2D evaluation (p < 0.001, = 0.009 and < 0.001). CONCLUSIONS: The difference between the pre-operative planning and post-operative component alignment in the 3D evaluation is significantly higher than that of the 2D, even if the same cases have been evaluated. Two-dimensional evaluation may mask or underestimate the post-operative implant malposition. Three-dimensional evaluation using the same co-ordinates' system as for pre-operative planning is necessary to accurately evaluate the post-operative component position.

No differences in objective dynamic instability during acceleration of the knee with or without subjective instability post-total knee arthroplasty.

INTRODUCTION: Instability after total knee arthroplasty is a critical problem. The purpose of this study was to clarify the stability of implanted knees during walking by comparing differences in dynamic instability during knee acceleration between individuals with or without previously experienced subjective instability, as measured by self-reported questionnaire. MATERIALS AND METHODS: We examined 92 knees with medial pivot implants. Mean patient age and follow-up duration were 78.4 years and 32.8 months, respectively. An accelerometer was used to investigate the accelerations along three axes; that is, vertical (VT), mediolateral (ML), and anteroposterior (AP) directions in 3-dimensional (3D) space. The analysis in the stance phase and gait cycle was performed by: (1) root mean square (RMS) values of acceleration and (2) frequency domain analysis using fast Fourier transformation (FFT). A self-reported knee instability score was used for the subjective feeling of instability. RESULTS: A total of 76 knees did not feel unstable (group 0), but 16 knees felt unstable (group 1) in patients during activities of daily living. Regarding the RMS, there were no differences in each direction between the groups. For FFT, the cumulative amplitude in the frequency < 30 Hz also showed no significant differences in all directions between the groups during the stance phase (VT, p = 0.335; ML, p = 0.219; AP, p = 0.523) or gait cycle (VT, p = 0.077; ML, p = 0.082; AP, p = 0.499). DISCUSSION: Gait analysis based on the acceleration data showed that there were no between-group differences in objective dynamic instability during acceleration of the knee, with or without reports of previously experienced subjective instability, as assessed by the self-reported questionnaire.

Association between anteroposterior laxity in mid-range flexion and subjective healing of instability after total knee arthroplasty.

PURPOSE: Flexion instability following total knee arthroplasty (TKA) is a common indication of early revision. The association between the objective anteroposterior (AP) laxity direction in mid-range flexion and the subjective healing of instability remains unclear; thus, this study aimed to clarify this association. METHODS: In this study, 110 knees (74 females, 92 knees; 16 males, 18 knees) with medial pivot implants were examined with a median age of 79 (range 60-92) years for a median follow-up duration of 22 (range 6-125) months. AP laxity was measured using a KT-1000 arthrometer. Self-reported knee instability score was used for the subjective healing of instability. RESULTS: Eighty-seven knees did not feel unstable (Group 0), whereas 23 knees felt unstable (Group 1). There was a significant difference in AP displacement [Group 0: median 6 mm; range 2-15 mm and Group 1: median 8 mm; range 4-14; p < 0.0001]. The threshold value of 7 mm was determined using the area under receiver operating characteristic curve of 0.79 [95% confidence interval (CI) 0.69-0.88, p < 0.0001]. In multivariate analysis, AP displacement of ≥7 mm was an independent risk factor for feelings of instability (odds ratio 7.695; 95% CI 2.306-25.674; p = 0.001). CONCLUSIONS: AP laxity of ≥7 mm represents a known cause of feelings of instability. By controlling AP laxity in TKAs, without stiffness in the knee, it is possible to prevent feelings of instability. The clinical relevance is that AP laxity of <7 mm is one of the target areas in TKA. LEVEL OF EVIDENCE: IV.