Rotational mobility and early clinical outcomes of a posterior-stabilised insert with reduced posterior lip: a prospective cohort study.

BACKGROUND: A newly designed polyethylene insert with a reduced posterior lip has been introduced to enhance rotational mobility in deep flexion after total knee arthroplasty (TKA). This design may facilitate postures requiring deep flexion and rotation, such as cross-legged sitting. This study evaluated whether the new insert improves cross-legged sitting and clinical outcomes at two years postoperatively. METHODS: Fifty TKAs implanted with the new insert (2021-2023) were compared with 62 historical controls implanted with the conventional insert during an earlier period (2013-2020). Knee range of motion, KSS-2011, FJS-12, subjective instability, radiographic alignment, complications, and survivorship were assessed. Intra-operative rotational kinematics was measured by navigation system in the new insert group. Predictors of cross-legged sitting were evaluated using multivariate logistic regression. RESULTS: Cross-legged sitting was possible in 20% (10/50) of new insert group versus 5% (3/62) of conventional insert knees (p = 0.017). Postoperative knee flexion angle was greater with the new insert group (125.6 ± 6.0° vs 122.2 ± 9.7°, p = 0.033). No differences were observed in KSS-2011, FJS-12, subjective instability, alignment, or complications. Greater external rotational capacity in deep flexion independently predicted cross-legged sitting (odds ratio 1.21, 95%CI 1.01 to 1.44, p = 0.041). Two-year survivorships were 96.0% (95%CI 84.9 to 99%) and 96.6% (95%CI 87.6 to 99.2%) in the new and conventional groups, respectively (p = 0.819). CONCLUSION: The newly designed polyethylene insert was associated with a higher feasibility of cross-legged sitting after TKA, potentially related to enhance external rotational capacity in deep flexion. LEVEL OF EVIDENCE: II (prospective cohort study).