Evolution of Posterior Stabilized Knee Implants

Timothy Wright, PhD, from Hospital for Special Surgery, New York, NY, assisted in the design of 1G posterior stabilized implant, which is technically an evolutionary modification of IB-II. Unlike IB-II, the 1G system has cruciate retaining (CR) in addition to posterior stabilized (PS) implants. The CR implant was designed and developed by StelKast.
The feedback from 1G users and the results from explanted components after revision retrieval paved the way for a design change evaluated by a validated computational finite element model analysis. Also, 1G PS implant had a symmetric patella flange whereas the CR implant had an anatomic patella flange, and the options of femoral sizes were limited to a few in 1G knee system.
The new design has created more tibio- and patello-femoral contact areas for the PS and less for the CR implants. These changes have resulted in a more consistent range of contact areas in the 2G system between CR and PS implants, leading to a more similar wear pattern between the two.
The following changes have been made to the 1G system femoral component: debulking, implementing an asymmetric patellar flange and trochlear groove, reducing the medial-lateral profile of the anterior flange, reducing the posterior condyle length, and modifying to a trapezoidal anterior-posterior profile. For the tibial inserts, the changes included adding patellar tendon relief, implementing a round rather than a pointed post for the PS insert, and reducing the posterior lip (Figs. 1 and 2). The posterior slope of the CR insert was increased to 6°. Two different inserts, high flexion (HF) and ultra-congruent (UC), are available for the new CR implants. The UC insert is designed to offer greater conformance to the femur in the sagittal plane, an increased posterior lip height, resistance to subluxation, and an elevated anterior wall height relative to HF insert (Fig. 3). All inserts used with CR 2G in this study were UC, and were HF for 1G. The design changes were intended to improve the function of 2G over that of 1G.
Fig. 1

1G and 2G tibial and femoral components (PS and CR) and the corresponding inserts

Fig. 2

1G and 2G tibial inserts used in this study

Fig. 3

Comparison between UC and HF inserts. Conformity percentage has been measured at various degrees of knee flexion. The highest conformance is between full extension and 15° of knee joint flexion HF = High Flexion; UC = Ultra Congruent

All statistical analyses were performed by SPSS statistical software (version 20, IBM, Armonk, NY, USA). Independent two-sample t-tests were used to compare the means between two groups with continuous data. A Chi-squared test was used to compare two frequencies between the two groups. A p-value ≤ 0.05 was considered to be significant. For the cases lost to follow up, the latest available follow-up data were used to determine the outcome at that point. Based on a power analysis to determine the sample size, it was found that If the true difference between the means of the two groups was 2.1(the minimum difference found between average PRO scores), we would need to study 65 subjects in each group to be able to reject the null hypothesis that there was no difference between the groups with a probability (power) of 0.8. The Type I error probability associated with testing the null hypothesis was 0.05.