Wear and fracture of patellar components has been frequently reported as a failure mode for cemented and press-fit patellar components. Malalignment of the patellar components may cause higher contact stresses, which may lead to excessive wear, delamination, and/or component fracture. A 6 station MTS (Eden Prairie, MN) knee joint wear simulator and Alpha Calf Fraction serum (Hyclone Labs, Logan, UT) diluted to 50% with a pH-balanced 20-mMole solution of deionized water and EDTA was used (protein level = 20 g/l) for testing. Asymmetric, all-polyethylene, patellar components with an overall construct thickness of 11 mm (Duracon®, Stryker Orthopaedics, Mahwah, NJ) were used. Appropriately sized cobalt-chrome femoral components articulated against the patellae. The patellae were cemented (Simplex, Stryker Orthopaedics, Mahwah, NJ) to delrin fixtures, which placed the patella in 10° of lateral tilt (Figure 1). This angle was chosen based off the work of Huang et al, which was one of the larger average tilt angles reported The loading and kinematic profiles used for testing were published previously (maximum axial load: 2450N and maximum patellofemoral angle: 54°. Variations of the loading profile were studied by evaluating the effects of heavier patients, which increased the maximum axial load to 3100N(250lb patient) and 3750N(300lb patient) (Figure 2). Lateral offset was tested to evaluate the effect of malalignment. Increments of 1mm were analyzed starting from the neutral position, eventually reaching a maximum lateral offset of 5mm. A 6-dof load cell was placed beneath the patella fixturing to capture dynamic loads (ATI, Apex, NC). The axial and medial/lateral shear loads where used to calculate the resultant medial/lateral shear force being applied to the patellar pegs.INTRODUCTION
MATERIALS AND METHODS
Many studies have looked at the effects of titanium tibial baseplates compared to cobalt chrome baseplates on backside wear. However, the surface finish of the materials is usually different (polished/unpolished) [1,2]. Backside wear may be a function not only of tray material but also of the locking mechanism. The purpose of this study was to evaluate the wear performance of conventional polyethylene inserts when mated with titanium tibial trays or cobalt chrome tibial trays that both have non-polished topside surfaces. Three titanium (Ti) trays were used along with three cobalt chrome (CoCr) trays. The Ti trays underwent Type II anodization prior to testing. All trays were Triathlon® design (Stryker Orthopaedics, Mahwah, NJ). Tibial inserts were manufactured from GUR 1020 conventional polyethylene then vacuum/flush packaged and sterilized in nitrogen (30 kGy). Appropriate sized CoCr femoral components articulated against the tibial inserts (Triathlon®, Stryker Orthopaedics, Mahwah, NJ). Surface roughness of the tibial trays was taken prior to testing using white light interferometry (Zygo Corp, Middlefield, CT). A 6-station knee simulator (MTS, Eden Prairie, MN) was used for testing. Two phases were conducted. The first phase used a normal walking profile, as dictated by ISO 14243-3 [3]. The second phase used waveforms created specifically for stair climbing kinematics. Testing was conducted at a frequency of 1 Hz for 2 million cycles for each test with a lubricant of Alpha Calf Fraction serum (Hyclone Labs, Logan, UT) diluted to 50% with a pH-balanced 20-mMole solution of deionized water and EDTA (protein level = 20 g/l) [4]. The serum solution was replaced and inserts were weighed for gravimetric wear at least every 0.5 million cycles. Standard test protocols were used for cleaning, weighing and assessing the wear loss of the tibial inserts [5]. Soak control specimens were used to correct for fluid absorption with weight loss data converted to volumetric data (by material density). Statistical analysis was performed using the Student's t-test (p<0.05).INTRODUCTION
MATERIALS AND METHODS
Rotational mal-alignment of the patella-femoral interface will result in increased wear. Highly cross-linked polyethylene will decrease wear even if mal-aligned. A biomechanical model based on high load and flexion was used to measure wear of rotationally aligned and mal-aligned all-polyethylene patellae. The components were articulated against “aligned” and “mal-aligned” (60 internally rotated) femoral components. The patella were subjected to a constant 2224 N force and the femoral components rotated from 600 to 1200 at 1.33 Hz. Patellae of identical geometry made of conventional UHMWPE and highly crosslinked UHMWPE were tested to 1 000 000 cycles. Wear was determined by gravimetric measurement relative to cemented soak controls. Conventional UHMWPE: All samples demonstrated damage (burnishing and scratching) of the articulating surfaces. There was a significant increase in wear (p<
.05) in the mal-aligned patella. Highly cross-linked patellae: All components fractured in the mal-aligned construct (gamma irradiated remelted n=6, gamma irradiated and annealed n=2). Failure first occurred at the cement interface then at the posts. Correct femoral rotation is important during TKA. The intertrochlear line, tibial cut, epicondylar axis and posterior condyles are helpful landmarks, but there is still eyeball control of rotation. It is clear from this study that rotational mal-alignment will result in increased polyethylene wear. Highly cross-linked polyethylene has decreased wear in THA. Unfortunately, the decrease in ductility and toughness may make the use of these materials unsuitable for TKA. Based on this study model, patellar components would need to be redesigned if highly cross-linked polyethylene were to be applied. The wear rate of conventional UHMWPE patellae is increased by rotational mal-alignment. Highly cross-linked components were a poor solution to problem. Use of highly cross-linked polyethylene resulted in component fracture.
