To achieve a long-lasting fixation of uncemented femoral knee implants, an adequate primary stability is required. Several factors, including the applied load, bone quality, surgical preparation, and implant characteristics affect the primary fixation. Recently, novel Attune® cementless femoral component has been proposed by DePuy Synthes (Warsaw, IN, USA). We aimed to compare the primary stability of this novel high-flex design against the conventional LCS® under different loading conditions (gait, deep knee bend (DKB), and high-flex loading), while accounting for the effect of bone quality and cut accuracy.

Six pairs of femora were prepared following the normal surgical procedure. Calibrated CT-scans and 3D-optical scans of the bones were obtained to measure bone mineral density (BMD) and bone cut accuracy, respectively. After implantation of the appropriate size implants (Left legs: Attune; right: LCS), a black-and-white speckle pattern was applied to each specimen (Fig.1B). The micromotion measurement was repeated three times in nine regions of interest (ROIs): the medial and lateral condyles from the posterior view; anterior, distal, and posterior regions from the medial and lateral views; the proximal tip of the anterior flange. The reconstructions were subjected to a gait load and a portion (around 50%) of the peak force of a DKB to prevent fracture of the proximal femur (Fig. 1A and Table. 1). The loads were derived from the Orthoload database using implant-specific inverse dynamics [1]. In addition, a sequence of DIC-images synchronized with the applied load was captured to find the relationship between micromotion and load. Afterwards, implants were pushed-off simulating 150° of flexion, while force-displacement graph was recorded.

BMD and bone cut accuracy were not significantly different between the groups. Under both loading conditions, Attune had a significantly lower micromotion (Table. 1). Cut accuracy was not a significant factor, and BMD was only significant for the comparison under the gait loading (not under DKB conditions). High-flex push-off force was not significantly different. However, Attune required a significantly higher load to reach a micromotion of 50 or 150 µm during the push-off test. Different relations between micromotion and applied load, depending on the loading configuration and implant design, were found (Fig. 2).

Our study has shown a clearly lower range of micromotion for the novel implant. Potential factors to explain the higher micromotion of LCS are parallel anterior and posterior bone cuts in the LCS versus the tapered bone cuts of the Attune. In addition, LCS has a less surface area in contact with bone due to the presence of a rim at the borders of the implant, which may have resulted in lower pre-stresses at the bone-implant interface.

Taking to account, the promising clinical outcome of LCS and also the lower range of micromotion of Attune, we suggest that the Attune has a potential to be at least as successful as the LCS system from a bone fixation point of view. However, further clinical evaluation of the Attune is necessary to assess its performance on the longer term.

Complications of the patellofemoral (PF) joint remain a common cause for revision of total knee replacements. PF complications, such as patellar maltracking, subluxation, dislocation and implant failure, have been linked to femoral and patellar component alignment. Computational analyses represent an efficient method for investigating the effects of patellar and femoral component alignment and loading on output measures related to long term clinical success (i.e. kinematics, contact mechanics) and can be utilized to make direct comparisons between common patellar component design types. Prior PF alignment studies have generally involved perturbing a single alignment parameter independently, without accounting for interaction effects between multiple parameters. The objective of the current study was to determine critical alignment parameters, and combinations of parameters, in three patellar component designs, and assess whether the critical parameters were design specific.

A dynamic finite element (FE) model of an implanted PF joint was applied in conjunction with a 100-trial Monte Carlo probabilistic simulation to establish relationships between alignment and loading parameters and PF kinematics, contact mechanics and internal stresses (Figure 1). Seven parameters, including femoral internal-external (I-E) alignment, patellar I-E, flexion-extension (F∗∗∗∗∗E) and adduction-abduction (A-A) rotational alignment, and patellar medial-lateral (M-L) and superior-inferior (S-I) translational alignment, as well as percentage of the quadriceps load on the vastus medialis obliquus (VMO) tendon, were perturbed in the probabilistic analysis. Ten output parameters, including 6-DOF PF kinematics, peak PF contact pressure, contact area, peak von Mises stress and M-L force due to contact, were evaluated at 80 intervals during a simulated deep knee bend. Three types of patellar component designs were assessed; a dome-compatible patellar component (dome), a medialized dome-compatible patellar component (modified dome), and an anatomic component (anatomic). Model-predicted bounds at 5 and 95% confidence levels were determined for each output parameter throughout the range of femoral flexion (Figure 2). Traditional sensitivity analysis, in addition to a previously described coupled probabilistic and principal component analysis (probabilistic-PCA) approach, were applied to determine the relative importance of alignment and loading parameters to knee mechanics in each of the three designs.

The dome component demonstrated the least amount of variation in contact mechanics and internal stresses, particularly in the 30–100° flexion range, with respect to alignment and loading variability. The modified dome had substantially reduced M-L contact force when compared with the dome. The anatomic design, while wide bounds of variability were predicted, had consistently greater contact area and lowered contact pressure than the dome and modified dome designs. The anatomic design also reproduced more natural sagittal plane patellar tilt than the other components. All three designs were most sensitivity to femoral I-E alignment. Thereafter, sensitivity to component alignment was design specific; for the anatomic component, the main alignment parameter was F-E, while for the domed components it was a combination of F-E and translation (M-L and S-I) (Figure 3). Understanding the relationships and design-specific dependencies between alignment parameters can add value to surgical pre-operative planning, and may help focus instrumentation design on those alignment parameters of primary concern.