S. Berahmani

An experimental study to investigate biomechanical aspects of the initial stability of press-fit implants

Initial fixation of press-fit implants depends on interference fit, surface morphology, and bone material properties. To understand the biomechanical effect of each factor and their interactions, the pull-out strength of seven types of CoCrMo tapered implants, with four different interference fits, three different surface morphologies (low, medium and high roughness), and at two time points (0 and 30 min) were tested in trabecular bone with varying density. The effect of interference fit on pull-out strength depended on the surface morphology and time. In contrast with our expectations, samples with a higher roughness had a lower pull-out strength. We found a similar magnitude of bone damage for the different surface morphologies, but the type of damage was different, with bone compaction versus bone abrasion for low and high frictional surfaces, respectively. This explains a reduced sensitivity of fixation strength to bone mineral density in the latter group. In addition, a reduction in fixation strength after a waiting period only occurred for the low frictional specimens. Our study demonstrates that it is essential to evaluate the interplay between different factors and emphasizes the importance of testing in natural bone in order to optimize the initial stability of press-fit implants.

FE analysis of the effects of simplifications in experimental testing on micromotions of uncemented femoral knee implants

Experimental testing of orthopaedic implants requires simplifications concerning load application and activities being analyzed. This computational study investigated how these simplifications affect micromotions at the bone‐implant interface of an uncemented femoral knee implant. As a basis, validated in vivo loads of the stance phase of gait and a deep knee bend were adopted. Eventually, three configurations were considered: (i) simulation of the complete loading cycle; (ii) inclusion of only tibiofemoral loads (ignoring patellofemoral loads); and (iii) applying only a single peak tibiofemoral force. For all loading conditions the largest micromotions found at the proximal anterior flange. Without the patellofemoral force, peak micromotions increased 6% and 22% for gait and deep knee bend, respectively. By applying a single peak tibiofemoral force micromotions were overestimated. However, the peak micromotions corresponded to the maximum tibiofemoral force, and strong micromotion correlations were found between a complete loading cycle and a single peak load (R2 = 0.73 and R2 = 0.89 for gait and deep knee bend, respectively). Deep knee bend resulted in larger micromotions than gait. Our study suggests that a simplified peak force can be used to assess the stability of cementless femoral components. For more robust testing, implants should be subjected to different loading modes.

The effect of surface morphology on the primary fixation strength of uncemented femoral knee prosthesis: a cadaveric study.

We investigated the effect of surface morphology on the mechanical performance of uncemented femoral knee prosthesis. Eighteen implants were implanted on nine paired femurs and then pushed-off (left legs: a novel surface morphology; right legs: Porocoat as baseline). Bone mineral density (BMD) and anteroposterior dimension were not significantly different between groups. The insertion force was not significantly different, however, the loosening force was significantly higher in the novel group (P=0.007). BMD had a direct relationship with the insertion and loosening force (P<0.001). The effect of surface morphology on implant alignment was very small. We conclude that the surface properties create a higher frictional resistance, thereby providing a better inherent stability of implants featuring the novel surface morphology.

Experimental pre-clinical assessment of the primary stability of two cementless femoral knee components

To achieve long-lasting fixation of cementless implants, an adequate primary stability is required. We aimed to compare primary stability of a new cementless femoral knee component (Attune®) against a conventional implant (LCS®) under different loading conditions. 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 cut accuracy, respectively. Micromotions were measured in nine regions of interest at the bone-implant interface using digital image correlation. The reconstructions were subjected to the implant-specifics peak tibiofemoral load of gait and a deep knee bend loading profiles. Afterwards, the implants were pushed-off at a flexion angle of 150°. Micromotions of Attune were significantly lower than LCS under both loading conditions (P ≤ 0.001). Cut accuracy did not affect micromotions, and BMD was only a significant factor affecting the micromotions under simplified gait loading. No significant difference was found in high-flex push-off force, but Attune required a significantly higher load to generate excessive micromotions during push-off. Parallel anterior and posterior bone cuts in the LCS versus the tapered bone cuts of the Attune may explain the difference between the two designs. Additionally, the rims at the borders of the LCS likely reduced the area of contact with the bone for the LCS, which may have affected the initial fixation.

Experimental and computational analysis of micromotions of an uncemented femoral knee implant using elastic and plastic bone material models

It is essential to calculate micromotions at the bone-implant interface of an uncemented femoral total knee replacement (TKR) using a reliable computational model. In the current study, experimental measurements of micromotions were compared with predicted micromotions by Finite Element Analysis (FEA) using two bone material models: linear elastic and post-yield material behavior, while an actual range of interference fit was simulated. The primary aim was to investigate whether a plasticity model is essential in order to calculate realistic micromotions. Additionally, experimental bone damage at the interface was compared with the FEA simulated range. TKR surgical cuts were applied to five cadaveric femora and micro- and clinical CT- scans of these un-implanted specimens were made to extract geometrical and material properties, respectively. Micromotions at the interface were measured using digital image correlation. Cadaver-specific FEA models were created based on the experimental set-up. The average experimental micromotion of all specimens was 53.1±42.3µm (mean±standard deviation (SD)), which was significantly higher than the micromotions predicted by both models, using either the plastic or elastic material model (26.5±23.9µm and 10.1±10.1µm, respectively; p-value<0.001 for both material models). The difference between the two material models was also significant (p-value<0.001). The predicted damage had a magnitude and distribution which was comparable to the experimental bone damage. We conclude that, although the plastic model could not fully predict the micro motions, it is more suitable for pre-clinical assessment of a press-fit TKR implant than using an elastic bone model.

Evaluation of interference fit and bone damage of an uncemented femoral knee implant

Background: During implantation of an uncemented femoral knee implant, press‐fit interference fit provides the primary stability. It is assumed that during implantation a combination of elastic and plastic deformation and abrasion of the bone will occur, but little is known about what happens at the bone‐implant interface and how much press‐fit interference fit is eventually achieved. Methods: Five cadaveric femora were prepared and implantation was performed by an experienced surgeon. Micro‐CT‐ and conventional CT‐scans were obtained pre‐ and post‐implantation for geometrical measurements and to measure bone mineral density. Additionally, the position of the implant with respect to the bone was determined by optical scanning of the reconstructions. By measuring the differences in surface geometry, assessments were made of the cutting error, the actual interference fit, the amount of bone damage, and the effective interference fit. Findings: Our analysis showed an average cutting error of 0.67 mm (SD 0.17 mm), which pointed mostly towards bone under‐resections. We found an average actual AP interference fit of 1.48 mm (SD 0.27 mm), which was close to the nominal value of 1.5 mm. Interpretation: We observed combinations of bone damage and elastic deformation in all bone specimens, which showed a trend to be related with bone density. Higher bone density tended to lead to lower bone damage and higher elastic deformation. The results of the current study indicate different factors that interact while implanting an uncemented femoral knee component. This knowledge can be used to fine‐tune design criteria of femoral components to achieve adequate primary stability for all patients. HighlightsInterference fit and bone damage was assessed using micro‐CT scan of distal femurs.Average actual interference fit was close to the nominal interference fit.It was revealed that damage goes beyond the bone‐implant interface.Bone density was the most effective factor to achieve high interference fit.Bone particles can penetrate into the inter‐trabecular spaces.