Patellar fixation technique plays a greater role on affecting the biomechanics of patellofemoral joint after TKR. If surgeon chooses HXLPE patellar component, inset technique is recommended for avoiding the possible complications of component loosening or peg fracture.
Owing to excellent wear resistance of highly cross-linked polyethylene (HXLPE) over conventional polyethylene (UHMWPE), the application of HXLPE in total knee replacements (TKR) is getting popular. However, the reduced mechanical strength of the HXLPE has been concerned if used for TKR, particularly when patellar maltracking occurs, a great shear force at the bone-implant interface could cause premature component loosening or peg failure. To enhance the fixation strength between bone and polyethylene patellar component, contemporary patellar fixation technique can be further classified into onlay and inset configurations. However, little is known about the important characteristics of shear force that occurs at the bone-implant interface in the onlay and inset fixation techniques between choices of novel materials. Current study therefore aimed to compare the biomechanical influence among different patellar implantation methods and biomaterial choices.
Two commercial polyethylene including UHMWPE (GUR 1020) and 75 kGy HXLPE (Orthoplastics Ltd, Lancashire, United Kingdom) were machined to three-pegs all-polyethylene patellar component. Each patellar component was bound into onlay and inset configurations with a self-made aluminum block using cement. Each group has 6 specimens. A 0.5 mm implanted depth was pre-drill on the block to simulate the inset configuration. A constant displacement rate of 50.8 mm/min of lateral shearing force was applied through MTS machine (MTS Corporation, Minneapolis, MN). The test completes until component disassociation or peg fracture. Peak shear force was recorded and the fractographic analysis was observed via microscope (Leica, Germany) in order to identify the failure mode and relevant surface features. Finite element (FE) analysis (ABAQUS 2017, France) was performed according to the current experimental study. von Mises stress and equivalent plastic strain were compared among different implantation procedures and biomaterial choices.
Current result found that the ability of avoiding shear loosening was significantly improved in inset group. The peak shear force of inset configuration increased 54% in UHMWPE group (from 1705 N to 2631 N) and 53% in 75 kGy HXLPE group (from 1650 N to 2530N). No obvious distinguish was found between materials. From the fractographic observation, the fracture mode of single peg side had a smooth surface that was parallel to the direction of shear force, indicating breakage was caused by shear force. From FE analysis, single peg side showed greater difference between inlay and onlay configurations. The inset fixation technique has advantages in reducing stress and decreasing plastic deformation on single peg side.
Current biomechanical study demonstrated that the fracture strength was significantly improved with the inset patellar implantation. The onlay fixation configuration offer lower strength to resist shear stress at the implant-bone interface comparing to the inset one. The pegs at the backside surface of patellar component were shear failure at the single peg side first and then remaining pegs pulled out consequently. Patellar fixation technique has a greater effect on affecting the peak shear force comparing to the factor of material selection. If surgeon chooses HXLPE patellar component, inset patellar implantation technique is recommended for lowing the risk of component loosening or peg fracture.