Despite consequent advancement in Total Knee Arthroplasty (TKA) up to 20% of patients are not satisfied after having been operated. Beside correct implantation, the design of the TKA-system is supposed to be a key factor of a successful TKA. Consequently it has been tried to restore natural kinematics by the design of the prosthesis. A medially stabilized design therefore is supposed to allow a lateral translation with a medial pivot. Our study compared posterior stabilized (PS) with medially stabilized (MS) TKA-design in terms of kinematics, femorotibial and patellofemoral contact patterns in vitro.Introduction
Objectives
The complex cellular mechanisms of the aseptic loosening of total joint arthroplasties still remain not completely understood in detail. Especially the role of adherent endotoxins in this process remains unclear, as lipopolysaccharides (LPS) are known to be very potent modulators of the cell response on wear particle debris. Contributing factors on the LPS affinity of used orthopedic biomaterials as their surface roughness have to be investigated. The aim of this study was to evaluate the affinity of LPS on the surface roughness of different biomaterials Cubes with a side length from ultra-high-molecular-weight-polyethylene (UHMWPE), crosslinked polytethylene (XPE), carbon fibre reinforced poly-ether-ether-ketone (CFR-PEEK), titanium, titanium alloy, Polymethyl methacrylate (PMMA), implant steel (CoCr) and instrument steel (BC) were produced (figure 1). Cubes of each material have been produced with a rough and a smooth surface. Before the testings, all cubes and used materials were treated with E-Toxa-Clean(®) to eliminate pre-existing LPS on the used surfaces. The cubes were then fixed on the cap of a glass that was filled with a LPS solution with a concentration of 5 IE/ml. After 30 minutes the cube was removed and the LPS concentration in the supernatant was measured. The endotoxin content of each sample was evaluated by a Limulus Amoebocyte Lysate (LAL) - Test (Lonza, Verviers, Belgium). The detection level of endotoxin was set at < 0.005 EU/ml diluted 1/10.Introduction
Materials and methods
A pain free motion of the patella after total knee arthroplasty (TKA) is still a challenge for surgeons and TKA-designers today. After TKA, the restricted guidance of the patella and kinematic alterations of the femorotibial joint results in increased retropatellar pressure and unphysiological patellar tracking. The alignment of the prosthetic components can influence patellofemoral stresses and tracking of the patella. The aim of this study was to demonstrate the consequences of different alignments of the tibial baseplate on patellar stress and knee kinematics. Different alignments of the tibial baseplate were simulated with five different UHMWPE-Inlets. Inserts with medial and lateral translation (±3mm; Figure 1A) as well as internal and external rotation (±3°; Figure 1B) were manufactured. Original inlays were used to define the neutral position. Eight human knee specimens without TKA were tested in a custom made knee rig. This rig mimics a loaded squat from approximately 20°−120° of flexion under six degrees of freedom in the knee joint. Retropatellar pressure (IScan, Tekscan, USA) as well as knee kinematics (CMS 20, Zebris, Germany) were recorded during squatting. Afterwards, TKA components were implanted in a neutral position via subvastus approach in tibia first technique. Each of the 5 tibial inlets was tested consecutively with the knee rig under the same conditions. Results were compared using mixed effects models with a random intercept per specimen. Component alignment as well as moving direction (flexion/extension) and flexion degree were defined as fixed effects in our model (SPSS, IBM, USA). After TKA in neutral position, retropatellar peak pressure increased by 0.71MPa (p<0.01), femorotibial rollback was reduced (−2.24mm; p<0.01) and the patella kinematics, in particular patella flexion (−2.02°; p<0.01) and rotation (−0.97°; p<0.01), were changed during squatting. Compared to the neutral position, internal rotation of the tibial baseplate increased retropatellar pressure by 0.20 MPa, while an external rotation provided a reduction of −0.24 MPa (p<0.01). In contrast a medialization or lateralization showed no effect on retropatellar pressure (p=0.09). Both, rotation and translation of the tibial baseplate influenced tibiofemoral kinematics significantly. A reduction of the femorotibial rollback was measured in external alignment (rotation and lateral translation; both p<0.01). An internal rotation showed more femoral rollback (0.93mm p<0.01). Patellar kinematics was changed primarily by component translation rather than rotation. A lateralisation of the tibial baseplate resulted in a medial shift of the patella by −0.43mm and vice versa (p<0.01). Rotation of the tibial baseplate had no influence on the patella shift (p=0.8) The findings in this study suggest that the alignment of the tibial baseplate influences patellar biomechanics significantly in vitro. An external rotation of the tibial baseplate decreased retropatellar pressure and patella kinematics tend more to the in situ situation of a natural knee. An internal alignment of the tibial baseplate seems to reconstruct natural tibiofemoral rollback in parts. However, studies (i.e. Nicoll et al.) show higher anterior knee pain by an internal alignment and a higher rollback after TKA might lead to higher wear.
