The posterior-stabilized knee prosthesis is designed specifically to provide the posterior stability to a knee arthroplasty when PCL is deficient or has to be sacrificed. Posterior dislocation of such prosthesis is rare but dreaded complication. There are several causes of postoperative dislocation such as malposition of the prosthesis, preoperative valgus deformity, a defect of the extensor mechanism and overwidening of the flexion gap. Posterior-stabilized rotating-platform mobile-bearing knee implants have been widely used to further improve the postoperative range of motion by incorporation of the post and cam mechanism to improve the posterior roll back during flexion and to overcome the wear and osteolysis problems due to significant undersurface micromotion of posterior-stabilized fixed-bearing knees. But, spin-out or rotatory dislocation of the polyethylene insert can occurs as result of excessive rotation of the rotating platform accompanied by translation of the femur on the tibia after mobile-bearing total knee arthroplasty, but that is very rare. Here, authors describe an unusual case of acute 180° rotatory dislocation of the rotating platform after posterior dislocation of a posterior-stabilized mobile-bearing total knee arthroplasty. A 71-year-old male with knee osteoarthritis underwent a TKRA using posterior-stabilized mobile-bearing prosthesis. The posterior dislocation of the total knee arthroplasty occurred 5 weeks postoperatively(Fig. 1). We underwent closed reduction of posterior dislocated total knee arthroplasty resulting in a complete 180° rotatory dislocation of the rotating platform (Fig. 2). He was treated with open exploration and polyethylene exchange with a larger component. This case illustrates that dislocation of a posterior-stabilized mobile-bearing total knee arthroplasty can occur with valgus laxity, cause 90° spin-out of the polyethylene insert and closed reduction attempts may contribute to complete 180° rotatory dislocation of the rotating platform. Special attention needs to be paid to both AP and lateral view to ensure that the platform is truly reduced and not just rotated 180° as was in this case

Introduction. Post cam structure, which is the main structure of posterior-stabilized design (PS), is useful to realize the intrinsic stability of a knee prosthesis replaced for a case with the severe degeneration. A large size post might, however, shorten the range of knee motion. On the other hand, retrieval studies sometimes reveal the ultrahigh molecular weight polyethylene (UHMWPE) deformation or severe failure of the tibial post of PS knee. Strength of a tibial post of available design is obviously insufficient to prevent the severe deformation. Therefore, minimally required size of the post should be clarified for polyethylene inserts. In the present study, we performed finite element (FE) analysis assumed the mechanical conditions of a tibial post in a PS knee and aimed to design criterion of a post of polyethylene insert of a knee prosthesis. Method. The shape of one commercially available knee prosthesis was referred as a posterior-stabilized knee prosthesis. The contour of the metallic femoral component was traced and digitized by hand. The contour of the UHMWPE insert was digitized by a micro computed tomography apparatus. Three dimensional finite elements were generated by a modeling software (Simpleware, Ltd. UK) as total 83000 four-noded tetrahedral elements. The bottom of the tibial insert was fully constrained. Load on femoral component was assumed to realize the tibial post impingement under several kinds of knee motions. Posterior load 100 N or 500N at the 10 degree hyperextension, anterior load 500N or 1000N during 120 degree flexion were applied (Fig. 1). The software of FE analysis was LS-DYNA ver.971 (Livemore Software Technology Corp. USA). The hardware was Endeaver Pro-4500 (EPSON Corp. Japan). The distributed values of von Mises stress and plastic strain of the tibial post were shown as the results of the analysis. Results. At the 10 degree hyperextension, the maximum values of von Mises stress and plastic strain of anterior aspect of tibial post were 26.0 MPa, 0.054 at posterior load 100 N., 35.3 MPa, 0.383 at posterior load 500N, respectively (Fig. 2). At 120 degree flexion, these values of posterior aspect of tibial post were 27.6 MPa, 0.086 at anterior load 500 N, 32.1 MPa, 0.208 at anterior load 1000N, respectively (Fig. 3). Plastic deformation has occurred on the contact area, as shown in Fig. 2, 3. Discussion. Our results showed that large plastic deformation may occur in the anterior or the posterior aspect of a tibial post by impingement during common exercises like running, climbing up, or squatting. In the femoro-tibial articulation, the true-stress decreases with increase in load because the compressive deformation can widen the contact area on the UHMWPE. The true-stress in the tibial post, however, increases with increase in load because bending and tensile deformation reduces the section area. Therefore, the design criterion of tibial post of PS knee prosthesis including the size of the post must be revised the safety coefficient that realize the sufficiently lower stress generated in the tibial post than the yield stress of UHMWPE

Background. Both minimally invasive surgery(MIS) and computer-assisted surgery(CAS) in total knee arthroplasty have been scientifically linked with surgical benefits. However, the long-term results of these techniques are still controversial. Most surgeons assessed the surgical outcomes with regard to knee alignment and range of motion, but these factors may not reflect subjective variables, namely patient satisfaction. Purpose. To compare satisfaction and functional outcomes between two technical procedures in MIS total knee arthroplasty, namely computer-assisted MIS and conventional MIS procedure, operated on a sample group of patients after 10 years. Methods. Seventy cases of posterior-stabilized total knee prostheses were implanted using a computer-assisted system and were compared to seventy-four cases of matched total knee prostheses of the same implant using conventional technique. Both groups underwent arthrotomy by 2 centimeter limited quadriceps exposure minimally invasive surgery (2 cm Quad MIS). At an average of ten years after surgery, self-administered patient satisfaction and WOMAC scales were administered and analyzed. Results. Demographic data of both groups including sex, age, preoperative WOMAC and post-operative duration were not statistically different. Post-operative WOMAC for the computer-assisted group was 38.94±5.68, while the conventional one stood at 37.89±6.22. The median of self-administered patient satisfaction scales of the computer-assisted group was 100 (min37.5-max100), while the conventional one was 100 (min25-max100). P-value was 0.889. There was 1 re-operative case in the conventional MIS group due to peri-prosthetic infection which was treated with debridement, polyethylene exchanged and intravenous antibiotics. Conclusion. The long-term outcomes of computer-assisted MIS total knee arthroplasty are not superior to that of the conventional MIS technique. Computer assisted MIS total knee arthroplasty is one of the treatment options for osteoarthritis of the knee that has comparable levels of satisfaction to the conventional MIS technique

Introduction. Post cam is useful to realize the intrinsic stability of a posterior-stabilized (PS) knee prosthesis replaced for a case with the severe degeneration. Some retrieval studies reveal the ultrahigh molecular weight polyethylene (UHMWPE) deformation or severe failure of the tibial post of PS knee. Strength of the tibial post of available design is obviously insufficient to prevent the severe deformation. The large size post might, however, shorten the range of knee motion. Therefore, minimally required size of the post should be clarified for polyethylene inserts. In the present study, we performed finite element (FE) analysis assumed the mechanical conditions of a tibial post in a PS knee and aimed to design criterion of a post of polyethylene insert of a knee prosthesis. Method. The shape of three commercially available knee prostheses, product A, B, and C was referred as PS knee prosthesis. The contour of the metallic femoral component and the UHMWPE insert were digitized by a computed tomography apparatus. Three dimensional finite elements were generated by modeling software (Simpleware, Ltd. UK) as four-node tetrahedral elements. In FE analysis, we used LS-DYNA ver.971 (Livemore Software Technology Corp. USA) as the software and Endeaver Pro-4500 (EPSON Corp. Japan) as the hardware. These bottoms of the tibial insert were fully constrained. The value of 30MPa was defined as yield stress of UHMWPE. 500N posterior load was applied to each femoral component at 10 degree hyperextension. Then, 1000N anterior load at 120 degree flexion, after tibial insert was located 10 degree internal rotation (Fig. 1). These loads were assumed to realize the two types of tibial post impingement under several kinds of knee motions. The distributed values of von Mises stress and plastic strain on the tibial post were shown as the results of the analysis. Results. At the 10 degree hyperextension, these maximum values of von Mises stress were 24.5, 3.23, 27.09MPa on anterior aspect of tibial post of the product A, B, and C, respectively (Fig. 2). These plastic strains were 0.045, 0.001, 0.064. At the 120 degree flexion, these maximum values of von Mises stress were 33.67, 4.53, 27.03MPa on posterior aspect of the product A, B, and C, respectively (Fig. 3). These plastic strains were 0.28, 0.004, 0.061. The stress of product A was higher than yield stress of UHMWPE. The strain was obviously higher than that of product B and C. Discussion. Our results showed that plastic deformation may occur in the posterior aspect of a tibial post by impingement during common exercises like climbing up, or squatting. In the femoro-tibial articulation, the true-stress decreases with increase in load because the compressive deformation can widen the contact area on the UHMWPE. The true-stress in the tibial post, however, increases with increase in load because bending and tensile deformation reduces the section area. Therefore, the design criterion including the post size must be revised the safety coefficient which realizes that the generated stress in the tibial post is sufficiently lower than the yield stress of UHMWPE