A durable biological fixation between implant and bone depends largely on the micro-motions [Pilliar et al., 1986]. Finite element analysis (FEA) is a numerical tool to calculate micro-motions during physiological loading. However, micromotions can be simulated and calculated in various ways. Generally, only a single peak force of an activity is applied, but it is also possible to apply discretized loads occurring during a continuous activity, offering the opportunity to analyze incremental micro-motions as well. Moreover, micro-motions are affected by the initial press-fit. We therefore aimed to evaluate the effect of different loading conditions and calculation methods on the micro-motions of an uncemented femoral knee component, while varying the interference-fit.

We created an FE model of a distal femur based on calibrated CT-scans. A Sigma® Cruciate-Retaining Porocoat® (DePuy Synthes, Leeds, UK) was placed following the surgical instructions. A range of interference-fits (0–100 µm) was applied, while other contact parameters were kept unchanged. Micro-motions were calculated by tracking the projection of implant nodes onto the bone surface. We defined three different micro-motions measures: micro-motions between consecutive increments of a full loading cycle (incremental), micro-motions for each increment relative to the initial position (reference), and the largest distance between projected displacements, occurring during a discretized full cycle (resulting) (Fig. 1A). Four consecutive cycles of normal gait and squat movements were applied, in different configurations. In the first configuration, incremental tibiofemoral and patellofemoral contact forces were applied, which were derived from Orthoload database using inverse dynamics [Fitzpatrick et al., 2012]. Secondly, we applied the same loads without the patellofemoral force, which is often used in experimental set-ups. Finally, only the peak tibiofemoral force was applied, as a single loading instance. We calculated the average of micro-motions of all nodes per increment to compare different calculation techniques. The percentage of area with resulting micro-motions less than 5 µm was also calculated.

The percentage of surface area was increased non-linearly when the interference fit changed from 0 to 100 µm particularly for squat movement. Tracking nodes over multiple cycles showed implant migration with interference-fits lower than 30µm (Fig. 1A). Loading configurations without the patellofemoral force, and with only the peak tibiofemoral force slightly overestimated and underestimated the resulting micro-motions of squat movement, respectively; although, the effect was less obvious for the gait simulation when no patella force was applied. Both incremental and reference micro-motions underestimated the resulting micro-motions (Fig. 1B). Interestingly, the reference micro-motions followed the pattern of the tibiofemoral contact force (Fig. 1B).

The calculation technique has a substantial effect on the micro-motions, which means there is a room for interpretation of micro-motions analyses. This furthermore stresses the importance of validation of the predicted micro-motions against experimental set-ups. In addition, the minor effect of loading configurations indicates that a simplified loading condition using only the peak tibiofemoral force is suitable for experimental studies. From a clinical perspective, the migration pattern of femoral components implanted with a low interference fit stresses the role of an adequate surgical technique, to obtain a good initial stability.

Femoral knee implants have promising outcomes, although some high-flex designs have shown rather high loosening rates (Han et al., 2007). In uncemented implants, it is vital to limit micromotions at the implant-bone interface, to facilitate secondary fixation through bone ingrowth (kienapfel et al., 1999). Hence, it is essential to investigate how micromotions of different uncemented implants are affected by various loading conditions when a range of bone qualities as a patient-related factor is applied.

Using finite element (FE) analysis, we simulated implant-bone interface micromotions during four consecutive cycles of normal gait and squat movements. An FE model of a distal femur was generated based on calibrated CT-scans, after which Sigma® and LCS® Cruciate-Retaining Porocoat® components (DePuy Synthes, Leeds, UK) were implanted. Using a frictional contact algorithm (µ=0.95), an initial press-fit fixation was simulated, which was previously validated against experimental data. The micromotions were calculated by tracking the projection of implant nodes on the bone surface excluding overhang area. The applied loading patterns were based on discretized simulations, providing incremental loads for each activity based on implant-specific kinematics, which was derived from Orthoload database using inverse dynamics (Fitzpatrick et al., 2012). This provided the opportunity to calculate incremental micromotions, but also the resulting micromotions for each single cycle, for both activities. In addition, the percentage of implant surface area with resulting micromotions less than a defined threshold was calculated.

Regardless of the type of loading, in all simulations, the predicted micromotions were highest in the first cycle, suggesting settling of the implant during initial cycle. The Sigma®implant displayed a 30% larger area with micromotions below the threshold of 5 microns, for both loading conditions (Fig. 1A). The highest micromotions occurred at the anterior flange, regardless of type of activity or design. Squatting had a more detrimental effect on the primary stability, with smaller areas of low micromotions as compared to the gait load (Fig. 1B). Bone stiffness had a minor effect, which was more apparent for squatting (Fig. 1B).

We found acceptable low ranges of micromotions in both implant designs, although demanding activities such as squatting generated higher motions. In addition, LCS® experienced higher micromotions, probably caused by the smaller contact area at bone-implant interface compared with Sigma®. Nevertheless, the predicted micromotions were all below the clinically relevant threshold for bone ingrowth (<40 microns) (kienapfel et al., 1999). Furthermore, our simulated settling behavior stresses the necessity for simulating multiple loading cycles, rather than just a single cycle. The effect of bone stiffness was evident, but only to a limited extent. The main current limitation of our study is the utilization of an elastic material model for the bone which is probably the reason to predict a low range of micromotions. We are planning to make the material model more realistic, by including plasticity and viscoelastic bone behavior.

Local infiltration analgesia (LIA) is promoted as an effective treatment modality for pain control after total knee arthroplasty (TKA) (1). A mixture of drugs is used to provide a multimodal analgesic effect. Previous studies reported that the use of these drugs is safe. After we carefully implemented a LIA study protocol in our practice, concerns raised about patient safety with probably higher infection rates. This forced us to perform an interim analysis after the first 58 cases.

58 patients underwent a unilateral TKA with a standardised LIA protocol (2), which consisted of a mixture of ropivacaine, epinephrine, and triamcinolone acetonide. Complications, knee function and patient satisfaction scores were prospectively recorded during regular outpatient control.

Four patients (6.9%) presented with signs of periprosthetic joint infection (PJI) within two months after surgery. Baseline characteristics were similar between the infected and non infected group. All infections were treated with debridement and retention, and antimicrobial treatment was started. One patient who suffered an infection died during followup. At two years followup all implants could be retained. Knee function and KSS score were acceptable for the patients who suffered PJI.

There is no consensus on the combination of drugs used for LIA. The application of corticosteroids in LIA is reported to be safe (3), but arguable results about the injection of local corticosteroids around knee arthroplasty surgery in the past have raised suspicion in literature (4). Combined with our unacceptable high rate of PJI, we believe that the current body of evidence, with small heterogeneous series, does not support the safe use of corticosteroids in LIA.

Pegs are often used in cementless total knee replacement (TKR) to improve fixation strength. Studies have demonstrated that interference fit, surface properties, bone mineral density (BMD) and viscoelasticity affect the performance of press-fit designs. These parameters also affect the insertion force and the bone damage occurring during insertion. We aimed to quantify the effect of the aforementioned parameters on the short-term fixation strength of cementless pegs.

6 mm holes were drilled in twenty-four human femora. BMD was measured using calibrated CT-scans, and randomly assigned to samples. Pegs were produced to investigate the effect of interference fit (diameters 6.5 and 7.6 mm), surface treatment (smooth and rough- porous-coating [friction coefficient: 1.4]) and bone relaxation (relaxation time 0 and 30 min) and interactions were studied using a DOE method. Two additional rough surfaced peg designs (diameters 6.2 and 7.3 mm) were included to scrutinize interference. Further, a peg based on the LCS Porocoat® (DePuy Synthes Joint Reconstruction, Leeds, UK) was added as a clinical baseline. In total seven designs were used (n = 10 for all groups). Pegs were inserted and extracted using an MTS machine (Figure 1), while recording force and displacement. Bone damage was defined as the difference between the cross-sectional hole area prior to and after the test.

BMD and interference fit were significant factors for insertion force. BMD had a significant positive correlation with pull-out force and subsequent analyses were therefore normalised for BMD.

Pull-out force increased significantly with interference for both surface coatings at time 0 (p < 0.05). However, after 30 minutes the effect remained significant for rough pegs only (p < 0.05-Figure 2A).

Additionally, the pull-out force increased with diameter in a non-linear manner for the rough pegs (Figure 2B). The two surface treatments were not significantly different to the clinical comparator. Interference fit was the only significant factor for bone damage.

BMD was significant for insertion and pull-out forces, reinforcing the need to account for this factor in biomechanical studies and clinical practice. This study also highlights the importance of time in studying bone interactions, with surface treatment and interference showing different interaction effects with relaxation time. Although smooth pegs initially have a higher pull-out force, this effect reduces over time whereas the pullout force for rough pegs is maintained. Smooth pegs also show time sensitivity in relation to interference and the benefit of increased interference reduces over time, whereas it is maintained in rough pegs. This may be explained by different damage (compressive and abrasive) mechanisms associated with different surface treatments.

In conclusion, BMD and interference fit are significant factors for initial fixation. Bone relaxation plays an important role as it reduces the initial differences between groups. Therefore, these findings should be strongly considered in the design development of cementless TKR.

The effect of an advanced porous surface morphology on the mechanical performance of an uncemented femoral knee prosthesis was investigated. Eighteen implants were inserted and then pushed-off from nine paired femurs (Left legs: advanced surface coating; right legs: Porocoat® surface coating as baseline). Bone mineral density (BMD) and anteroposterior dimension were measured, which both were not significantly different between groups. The insertion force was not significantly different, but push-off force was significantly higher in the advanced surface coating group (P = 0.007). BMD had direct relationship with the insertion force and push-off force (p < 0.001). The effect of surface morphology on implant alignment was very small. We suggest that the surface properties create a higher frictional resistance thereby providing a better inherent stability of implants featuring the advanced surface coating.

To achieve desirable outcomes in cementless total knee replacement (TKR), sufficient primary stability is essential. The primary stability inhibits excessive motions at the bone-implant interface, hence providing the necessary condition for osseointegration [1]. Primary stability for cementless TKR is provided by press-fit forces between the bone and implant. The press-fit forces depend on several factors including interference fit, friction between bone and implant surface, and the bone material properties. It is expected that bone mineral density (BMD) will affect the stability of cementless TKR [2]. However, the effect of BMD on the primary stability of cementless femoral knee component has not been investigated in vitro.

Phantom calibrated CT-scans of 9 distal femora were obtained after the surgical cuts were made by an experienced surgeon. Since the press-fit forces of the femoral component mainly occur in the Anteroposterior (AP) direction, the BMD was measured in the anterior and posterior faces for a depth of 5 mm; this depth was based on stress distributions from a Finite Element Analysis of the same implant design. In addition, four strain gauges were connected to different locations on the implant's outer surface and implant strain measured throughout as an indication of underlying bone strain. A cementless Sigma CR femoral component (DePuy Synthes Joint Reconstruction, Leeds, UK) was then implanted using an MTS machine. In order to simulate a ‘normal’ bone condition, the implanted bone was preconditioned for one hour at a cyclic load of 250–1500 N, and a rate of 1 Hz. Finally, the implants were pushed-off from the bone in a high-flex position. Forces and displacements were recorded both during insertion and push-off tests.

Strong correlations were found for insertion and push-off forces with BMD, R² = 0.88 and R² = 0.88, respectively (p < 0.001), so although implantation may be harder in patients with higher BMD, initial stability is also improved. A correlation was also found between final strain and push-off forces (R² = 0.89, p < 0.01) and BMD also showed a strong reverse correlation with total bone relaxation (R² = 0.76, p = 0.023). These results indicate that higher BMD induces higher bone strain, which can lead to improved fixation strength.

There is no consensus on the best fixation method for the TKR but some surgeons prefer a cementless design for young and active patients. The results of our study showed that the primary stability of a cementless femoral knee component is directly correlated with the bone mineral density. Therefore, patient selection based on bone quality may increase the likelihood of good osseointegration and adequate long-term fixation for cementless femoral knee components.

Introduction

A few follow-up studies of high flexion total knee arthoplasties report disturbingly high incidences of femoral loosening. Finite element analysis showed a high risk for early loosening at the cement-implant interface at the anterior flange. However, femoral implant fixation is depending on two interfaces: cement-implant interface and the cement-bone interface. Due to the geometry of the distal femur, a part of the cement-bone interface consists of cement-cortical bone interface. The strength of the cement-bone interface is lower than the strength of the cement-implant interface.

The research questions addressed in this study were: 1) which interface is more prone to loosening and 2) what is the effect of different surgical preparation techniques on the risk for early loosening.