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Wear testing cannot replicate the variations in wear rates and wear mechanisms seen in vivo, which may be related to differences between in vivo and in vitro conditions. A considerable difference exists between the in vivo synovial fluid volume (few milliliter) and the in vitro substituted bovine serum volume (several hundred milliliter). The aim of this study was to analyze the effects of a reduced fluid volume on the wear behavior in a knee wear simulator study. Four wear tests with decreasing fluid volumes (250, 150, 75, and 45 ml) were carried out. Using a large fluid volume of 250 ml for wear testing resulted in a wear rate of 9.7±1.2 mm3/10(6)  cycles. Decreasing the fluid volume consecutively reduced the wear rate to down to 8.8±1.4 mm3/10(6) for 150 ml (p=1.00), 5.6±1.2 mm3/10(6) for 75 ml (p=0.01), and 1.0±0.2 mm3/10(6) cycles for 45 ml fluid volume (p≤0.01). Additionally, higher serum degradation and larger wear particles were observed with smaller fluid volumes used for testing. This study demonstrates the high relevance of the protein-based lubricant on the wear behavior and the technical limitation to replicate the synovial fluid in simulator tests. Wear testing should be carried out using larger fluid volumes (e.g., 250 ml) to generate physiological relevant wear masses., (© 2014 Orthopaedic Research Society. Published by Wiley Periodicals, Inc.)

Wear mechanisms in polyethylene components for total knee replacements are inherently mechanical; the local stresses or strains exceed some material limit. Retrieval analysis and knee simulators have provided the means to quantify the damage observed in vivo or in vitro. These results have been circumstantially linked to the material stresses obtained from computational simulations using finite element analysis, knee simulator tests, and computational simulations of two condylar knee designs. We hypothesize that if an equivalent loading environment is produced in the computational simulation, we can correlate the distribution of computed stresses with observed damage of simulator specimens and further relate design differences to in vivo performance from retrieval analyses. The finite element model agreed with the knee simulator kinematics and kinetics within 2-13%, and composite FEA contact areas matched 66-90% of the damage areas due to burnishing on the simulator specimens. Burnishing was the primary mode of damage for both the simulator and retrieval specimens corresponding with the relatively low magnitudes of contact stress observed. Both the computational and experimental techniques underpredicted the amount determined from retrieval analysis, but the differences between the two designs were consistent for all three methods. Combining these techniques strengthens the applicability of the computational simulation while highlighting the complementary approach of these methods for preclinical testing and assessing the link between material state and damage., (Copyright (c) 2006 Orthopaedic Research Society.)

This study evaluated the influence of the geometric configuration of the tibial joint area on the kinematics of the knee. Twenty-two patients with noninflammatory arthritis and minor preoperative deformity were studied. They each received an AMK total knee replacement with retention of the posterior cruciate ligament. Eleven patients without any knee abnormalities were used as controls. The patients were stratified to either the flat (terminology of the manufacturer: standard) or concave (terminology of the manufacturer: constrained) polyethylene insert (n = 11 in each group). Knee kinematics were assessed 1 year after the operation by having the patient ascend a platform corresponding to an extension of the knee from 50 to 70 degrees of flexion. During this motion, two film-exchangers simultaneously exposed six to 13 pairs of serial stereoradiographs. The concave geometric configuration of the tibial insert resulted paradoxically in increased anterior-posterior translations compared with the flat insert but no significant change of rotations and translations in the other directions. Compared with normal knees, the most obvious abnormality was increased anterior-posterior translations (p < 0.004). At 50 degrees of flexion, the implants with the flat tibial polyethylene insert had displaced 2 times and the concave ones had displaced 2.5 times more posteriorly than the normal knees (p < or = 0.001). Less internal tibial rotation was also recorded in the flexed positions for both types of inserts compared with the normal knees (p < 0.02). Four knees in four patients, who reported symptoms of instability and abnormal knee function, showed significantly increased proximal displacement of the center of the tibial plateau in the flexed position. The findings suggest that current prosthetic designs and surgical technique do not restore normal knee kinematics and indicate that design improvements should rely on in vivo kinematic studies.

This study was undertaken to determine how accurately total graft force and load-sharing between graft segments could be set and maintained during augmented anterior cruciate ligament (ACL) reconstruction in the goat knee. Special procedures were developed to reduce the effect of tissue creep and to overcome difficulties in the setting of graft force. Five knees from goat cadavers were reconstructed using a bone-tendon-bone graft (PT) and a synthetic augmentation device (LAD). Prescribed levels of total graft force and load-sharing between the autograft and LAD were set under a standardized external joint load. Immediately after fixation, the set force declined an average of 9 and 3% in the LAD and PT, respectively. After three subsequent exercise sequences, the set forces fell from their initial level by an average of 25% for the LAD and 28% for the PT. An analysis of variance did not show the loss of force with exercise to be statistically significant. We conclude from this in vitro study that our method can be used to set forces in an ACL reconstruction with reasonable maintenance of load-sharing but that losses of approximately 30% of total graft force after exercise of the reconstructed joint are to be expected.

The fixation of cemented and noncemented tibial components in 19 total knee arthroplasties was examined 3 to 10 weeks postoperatively with roentgen stereophotogrammetric analysis. Physiologic external forces were applied either in outward-inward rotation or as eccentric posterior loading generated by squatting. In one case there was no motion, while in the remaining 18 cases relative interface motion of up to 1.3 mm was found. In some cases, the largest motion was found during inward-outward rotatory stress, while in others, the largest motion was found during squatting. Posterior tilt was weakly correlated with posterior displacement of the femoral component on the tibia during squatting (r2 = 0.323, p < 0.05). For the noncemented cases, the initial fixation to bone was probably insufficient, and ingrowth of bone would not have been achieved. For the cemented cases, motion of the implant was reduced. Given the short postoperative time and the probable absence of any substantial fibrous tissue membrane, we suggest that the observed motion represented elastic deformation of the bone.

This study measured the vertical displacement of three kinds of cementless tibial components [Porous Coated Anatomical (PCA), Tricon, and Whiteside], under eccentric loading up to 2,225 N. Displacement between the tibial tray and the proximal tibia was measured with linear variable differential transformers at the anterior and posterior side when anteriorly or posteriorly loaded, and at the medial and lateral side when medially or laterally loaded. The general pattern of motion was sinking at the loaded side and lift-off at the opposite side. Lift-off opposite the loaded side was fairly small for all components at all measurement sites. Among the three components, the Whiteside showed the smallest displacements. The Tricon (when anteriorly or posteriorly loaded), and the PCA (when medially or laterally loaded) showed sinking at the loaded side. Anterior screw fixation of the PCA was not effective in preventing anterior lift-off. The tilting motion of the tibial components observed in this study implies instability of the initial fixation, which could possibly compromise bony ingrowth. Furthermore, this tilting could cause uneven distribution of load, and potentially result in fracture of the underlying bone.