We investigated the characteristics of patients who achieved Japanese-style deep flexion (seiza-sitting) after total knee replacement (TKR) and measured three-dimensional positioning and the contact positions of the femoral and tibial components. Seiza-sitting was achieved after surgery by 23 patients (29 knees) of a series of 463 TKRs in 341 patients. Pre-operatively most of these patients were capable of seiza-sitting, had a lower body mass index and a favourable attitude towards the Japanese lifestyle (27 of 29 knees). According to two-/three-dimensional image registration analysis in the seiza-sitting position, flexion, varus and internal rotation angles of the tibial component relative to the femoral component had means of 148° (sd 8.0), 1.9° (sd 3.2) and 13.4° (sd 5.9), respectively. Femoral surface contact positions tended to be close to the posterior edge of the tibial polyethylene insert, particularly in the lateral compartment, but only 8.3% (two of 24) of knees showed femoral subluxation over the posterior edge. The mean contact positions of the femoral cam on the tibial post were located 7.8 mm (sd 1.5) proximal to the lowest point of the polyethylene surface and 5.5 mm (sd 0.9) medial to the centre of the post, indicating that the post-cam contact position translated medially during seiza-sitting, but not proximally. Collectively, the seiza-sitting position seems safe against component dislocation, but the risks of posterior edge loading and breakage of the tibial polyethylene post remain.

Cite this article: Bone Joint J 2013;95-B:782–7.

Introduction:. Although commonly used, the clinical performance of highly crosslinked polyethylene in total knee arthroplasty (TKA) remains unknown and concerns exist regarding fatigue resistance and oxidation, particularly in posterior-stabilized (PS) designs. The purpose of this study is to compare highly crosslinked and conventional polyethylene in a PS TKA design at a minimum of 5-years. Methods:. A prospective cohort study of 114 consecutive TKAs in 83 patients was performed as a subset of a multi-center prospective study. All TKAs utilized an identical PS design. Conventional polyethylene inserts were used in 50 knees and second-generation highly cross-linked polythethylene inserts were implanted in 64 TKAs. All patients were followed with clinical outcome measures (Short-Form 36, Knee Society Scores, WOMAC and LEAS) and radiographically for a minimum of 5 years. Results:. The mean age of the highly cross-linked polyethylene group was 4 years less than the conventional group (p = 0.03). There was no difference in BMI (p = 0.3) or preoperative outcome measures between groups with numbers available. Seven patients died or were lost to follow up and one underwent revision for infection at 3 months postoperatively. 103 TKAs obtained minimum 5-year follow up. Mean Knee Society Scores were 12 points higher (p = 0.01) and 14 points higher (p = 0.005) in the physical function subset of the SF-36 in the highly cross-liked polyethylene group. There was no difference in the other outcome measures with the numbers available. There was no radiographic osteolysis or mechanical failures related to the tibial polyethylene in either group. Conclusion:. Mechanical failure or radiographic osteolysis was not observed with either conventional or highly cross-linked polyethylene in this PS TKA design at mid-term follow up. To our knowledge, this is the first minimum 5-year follow-up of highly cross-linked polyethylene in a posterior-stabilized design. While the results support comparative safety, longer-term follow-up is warranted to determine if wear resistance and mechanical properties of highly crosslinked polyethylene are maintained. Significance: Concerns regarding early fatigue failure and mechanical complications related to the PS post-cam articulation of highly-crosslinked polyethylene in TKR were not substantiated at a minimum of 5 years clinical followup in this prospective cohort study. Highly cross-linked polyethylene demonstrated clinical equivalency compared to conventional polyethylene, even when used in a younger and presumably more active patient group