"Grand-piano sign" as a femoral rotational indicator in both varus and valgus knees: a simulation study of anterior resection surface in total knee arthroplasty

Component rotational mismatch in the standing position is a potential risk factor for unfavourable functional outcomes after total knee arthroplasty

Purpose

This study assessed rotational mismatch between components after total knee arthroplasty (TKA) in the supine and standing positions and aimed to investigate the effect of rotational mismatch in the standing position on postoperative patient-reported outcome measures (PROMs).

Methods

Seventy-one patients (71 knees) who underwent TKA for medial knee osteoarthritis were used to investigate rotational mismatches between components. Rotational mismatches between components were examined on postoperative standing whole-leg and supine knee radiographs using a three-dimensional-to-two-dimensional model image registration technique, and the angles between the reference axes of the components were measured. Component alignment was evaluated using postoperative computed tomography images, and a questionnaire (2011 version of the Knee Society Score: [KSS 2011]) was mailed to investigate postoperative PROMs.

Results

In the entire cohort, rotational mismatches in the supine and standing positions were similar (p = 0.9315). In 15% of patients, the mismatch was large (>5°) in the supine position but small (<5°) in the standing position (overestimated group). However, in 23% of patients, the mismatch was small (<5°) in the supine position and large (>5°) in the standing position (underestimated group). The underestimated group had severe preoperative varus deformity, resulting in external rotation of both femoral and tibial components. Rotational mismatch in the standing position (p = 0.0032) was a significant risk factor for unfavourable PROMs. Patients with a mismatch in the standing position had significantly lower scores than those without a mismatch (p = 0.0215), exceeding the minimal clinically important difference values.

Conclusions

The underestimated group is clinically important because the surgical procedure and intraoperative assessment of component placement are performed in the supine position. In cases of severe preoperative varus deformity, care should be taken not to place the component in malrotation to avoid rotational mismatch in the standing position.

Level of Evidence

Ⅳ, Case series.

Restoration of the anterior compartment after robotic total knee arthroplasty significantly improves functional outcome and range of motion at 1 year

PURPOSE

This study aims to assess the functional outcomes based on restoring the anterior compartment after total knee arthroplasty (TKA).

METHODS

This retrospective study included 96 primary TKAs performed between 2021 and 2022. Functional positioning principles were applied using an image-based robotic-assisted system. The mean age was 69.2 ± 7.9 years. Knee Society Score (KSS), Kujala score, Forgotten Joint Score (FJS) and knee flexion were collected preoperatively and at 1 year. The depth difference between native and prosthetic trochlea was measured to assess anterior compartment restoration at full extension, 30°, 70° and 90° flexion. The global anterior compartment restoration combined the anterior compartment restoration and the patellar thickness restoration.

RESULTS

The trochlear offset was mostly understuffed after TKA compared to the native anatomy, mainly for medial and lateral condyles at 30° and 70° of flexion. The global anterior compartment restoration was understuffed in full extension (-0.7 mm ± 2), at 30° (-4.4 mm ± 2) and 70° of flexion (-3.6 mm ± 2.5). At 90°, the global anterior compartment restoration was overstuffed (2.2 mm ± 1.8). Functional scores were not significantly influenced by the anterior compartment stuffing at 0° and 30° (n.s.). The anterior compartment overstuffing at 70° and 90° was associated with decreased KSS function score (p = 0.009) and flexion (p = 0.04).

CONCLUSION

Moderate anterior understuffing was frequently observed after TKA performed with functional positioning and an image-based robotic-assisted system. This understuffing did not influence the functional outcomes. The overstuffing of the anterior compartment led to a reduction in KSS function score and flexion measurements at 1 year.

LEVEL OF EVIDENCE

Level III, retrospective cohort study.

"Boot Sign" of Anterior Femoral Condylar Resectional Shape during Total Knee Arthroplasty Is More Frequent in Asian Patients

PURPOSE

There is lack of intraoperative consensus on the distal femur anterior resected surface shape that allows reliable rotational alignment assessment during total knee arthroplasty (TKA). We aimed to evaluate the ratio and prevalence of anterior femoral resection surface intraoperatively.

MATERIALS AND METHODS

The study included 234 osteoarthritis patients with varus knees and not valgus knees or deformities. After conventional medial parapatellar approach, measured resection technique based on the mechanical axis of the femur and preoperative TEA-PCA angle on CT with anterior reference was used among all the patients. The anteroposterior (AP) lengths after distal femoral resection were measured as the femoral lateral AP (FLAP) and femoral medial AP (FMAP) lengths. Based on the medial (MD) and lateral condyle (LD) vertical distance ratios of the femur anterior resected surface, the groups were classified into "boot sign", "grand-piano", and "butterfly sign" groups. For comparison of the mean values, the data were assessed for normality with the Shapiro-Wilk test. One-way ANOVA with post hoc analysis using Tukey's honestly significant difference (HSD) test was used to compare the mean values among the groups. The correlations between the MD/LD and variables were analyzed using the Pearson correlation coefficient. Linear regression analyses were used to find the associated factors to the anterior femoral resection surface shape.

RESULTS

Mean intraoperative femoral rotation and distal femoral cutting angles were 4.9° ± 1.2 and valgus 5.0° ± 0.7, respectively. Mean FLAP was 52.9 ± 4.2 mm. Mean MD/LD (0.61 ± 0.13) was lower than that of typical "grand-piano sign". The morphological shape incidence of the "boot sign" was 62.4%. In the "boot sign" group, the FLAP was found to be smaller than that in the other groups (52.4 ± 4.2 vs. 53.7 ± 4.2 vs. 54.9 ± 2.7; p = 0.02), while the intraoperative femoral rotation angle was found to be larger than in the other groups (5.0 ± 1.2 vs. 4.6 ± 1.1 vs. 4.7 ± 1.2; p = 0.039). The MD/LD-associated factors were FLAP, intraoperative femoral rotation, and distal femoral cutting angles (R2 = 0.268).

CONCLUSION

The femur anterior resection surface shape in TKA was found in the "boot sign" rather than the "grand-piano sign" in Korean ethnics owing to an asymmetric morphology of femoral condyles. Ethnic differences, including distal femoral morphology, should be considered for assessment of the femoral rotation angle using the femur anterior resection surface shape.