# Optimising Tibial Stem Selection: A Finite Element Study

François Zot, Arnaud Germaneau, Marc Gardegaront, Tanguy Vendeuvre, Mathieu Severyns

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François Zot, Arnaud Germaneau, Marc Gardegaront, Tanguy Vendeuvre, Mathieu Severyns. Optimising Tibial Stem Selection: A Finite Element Study. Multidisciplinary Biomechanics Journal, 2025, 50th congress of the.., 10.46298/mbj.16162. hal-05191714v2

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Submitted on 23 Oct 2025

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Distributed under a Creative Commons Attribution 4.0 International License
# Optimising Tibial Stem Selection:
A Finite Element Study

François Zota*, Arnaud Germaneaua, Marc Gardegarontb, Tanguy Vendeuvrea, c, Mathieu Severynsa, d

a Institut Pprime UPR 3346, CNRS – Université de Poitiers – ISAE-ENSMA, Poitiers, France

b Univ Lyon, Univ Eiffel, Univ Claude Bernard Lyon 1, LBMC UMR T9406, Lyon, France

c CHU de Poitiers, Département de Chirurgie Orthopédique et Traumatologie, Poitiers, France

d Clinique Porte Océane, Département de Chirurgie Orthopédique et Traumatologie, Les Sables d'Olonne, France

* Corresponding author: francois.zot@univ-poitiers.fr

Received date: 03/04/2025

Accepted date: 27/06/2025

Publication date: 27/10/2025

**Keywords:** total knee arthroplasty, extension stem, finite element analysis

© 2025 The Authors

Licence CC-BY 4.0

Published by Société de Biomécanique

## 1. Introduction

Total knee arthroplasty (TKA), or total knee replacement, is a common surgery to treat osteoarthritis. While TKA has a 90% survival rate after 15 years, mechanical complications like aseptic loosening can cause implant migration and early failure. This complication often requires complex surgical revision and is linked to bone fragility and uneven loading at the bone-implant interface. Tibial extension stems can improve (Bae et al., 2012) stability by increasing intramedullary anchoring and creating a robust contact zone in the tibia. However, they risk bone resorption due to stress shielding. Digital models using finite element (FE) methods can aid preoperative planning by analysing the mechanical response of peri-prosthetic bone based on stem choice. These digital twins provide a biomechanical basis for selecting stem length and diameter, currently left to the surgeon's discretion.

This study aims to evaluate the biomechanical effect of tibial extension stems on TKA stability, using patient-specific FE models to determine stress distribution and implant stability based on stem geometry.

## 2. Methods

To develop the digital model, CT-scan images of an 80-year-old male patient were used. The images (voxels of 0.499mm iso) were processed using 3D Slicer to extract the 3D tibia geometry, which was then imported into Ansys SpaceClaim for model preparation. A virtual TKA was performed by making a 2mm orthogonal cut

to the tibial epiphysis at a 3° joint line obliquity. The tibial insert dimensions were chosen to maximise coverage without protrusion. A boolean operation created space for the implant, and multiple models were developed to account for various extension stem dimensions. The geometric models were imported into Ansys Mechanical for finite element analysis, using quadratic tetrahedral elements (TET10) for discretisation. A convergence study determined optimal mesh parameters, setting element size to 1mm for the tibia and implant, and 0.75mm in the contact zone between the tibia and the implant. The tibial baseplate and extension stem were assumed to be made of cobalt-chrome-molybdenum. The tibia's heterogeneous bone density was considered to develop patient-specific models using a custom Python script (QCTMA) that integrated CT-scan values into the mesh. Six models were developed, each with different stem dimensions. In the Model 1, we used the implant without a stem. In the models 2 to 4, the stems had a diameter of 9mm and a length ranging from 20 to 70mm. For the models 5 and 6, the stem had a length of 40mm and a diameter of 12.5 and 14mm. Boundary conditions included fixing the distal tibia and applying compressive loads to the implant plateau, simulating activities like walking (3 times the body weight: 3BW), going up or down stairs (5BW), and running (12BW). A bonded contact condition was imposed between the tibial bone and the implant to represent the case of secondary stability (i.e., after osteointegration).
Strain and stress distribution in the periprosthetic tibial bone were observed, and the risk of fracture (RF) (Piovan et al., 2024) was computed to assess potential damage to implant stability.

### 3. Results and discussion

Figure 1 shows similar strain distribution in peri-prosthetic bone for models with stems. At the 12BW loading step, Model 1 has higher strain levels, Model 5 shows the highest strain value.

Table 1 summarises simulation results, showing the lowest mechanical field values for Model 1. Models 2 to 6 have similar equivalent stress values. Model 4 shows values of and 29.82MPa at 12BW, compared to Model 3 with slightly higher values. Equivalent elastic strain results are similar, with peak strain values of 0.0037 at 12BW.

<table><caption>**Table 1.** Summary of the results for stress, strain and risk of fracture for the 12BW loading state.</caption><thead><tr><th></th><th>M1</th><th>M2</th><th>M3</th><th>M4</th><th>M5</th><th>M6</th></tr></thead><tbody><tr><td>Stress (MPa)</td><td>26.40</td><td>30.64</td><td>30.73</td><td>29.82</td><td>30.58</td><td>30.22</td></tr><tr><td>Strain</td><td>0.0033</td><td>0.0037</td><td>0.0037</td><td>0.0034</td><td>0.0036</td><td>0.0037</td></tr><tr><td>RF</td><td>13.44</td><td>15.41</td><td>15.94</td><td>15.25</td><td>17.78</td><td>16.91</td></tr></tbody></table>

The study developed FE models of knees with implants to evaluate the biomechanical effects of adding an extension stem on total knee replacement stability. Stem diameter significantly impacts fracture risk, with larger diameters showing larger zones of high fracture risk and strain. This aligns with previous findings that larger diameters improve cortical bone contact, potentially increasing contact loads during compression.

The study confirms that stem geometry affects stress shielding and concentrations, with short stems inducing fewer issues than long stems. Optimizing force transmission through the tibial plate can reduce stress shielding, preserving physiological bone loading and minimizing bone resorption and periprosthetic fracture risk (Completo et al., 2009; Eidel B et al., 2021; Scott & Biant, 2012).

Future work could explore primary stability, relative displacements between the implant and bone, and consider a broader patient base to account for variability in bone geometries and properties. Varying the direction

<figure><img src="image_2.png" /><figcaption>**Figure 1.** Strain distribution in the bone.</figcaption></figure>

of applied forces in simulations could better represent daily activities.

### 4. Conclusions

This study shows that tibial extension stems enhance the stability of total knee arthroplasty but increase stress, strain, and fracture risk in the periprosthetic bone, especially with larger stem diameters. Patient-specific finite element models help optimize implant selection, potentially improving the longevity and outcomes of knee replacements.

### Acknowledgements

We would like to thank United Orthopedic for providing the 3D models of the implants.

### Conflict of Interest Statement

None.

### Contributor Roles

FZ: Conceptualisation, Methodology, Validation, Formal Analysis, Writing; AG: Formal Analysis, Investigation, Writing; MG: Methodology, writing; TV: Methodology, Project Administration, Writing; MS: Conceptualisation, Investigation, Supervision, Writing.

### References

Bae, D. K., Song, S. J., Park, M. J., Eoh, J. H., Song, J. H., & Park, C. H. (2012). Twenty-year survival analysis in total knee arthroplasty by a single surgeon. *The Journal of Arthroplasty*, 27(7), 1297–1304, e1. https://doi.org/10.1016/j.arth.2011.10.027
Completo, A., Talaia, P., Fonseca, F., & Simões, J. A. (2009). Relationship of design features of stemmed tibial knee prosthesis with stress shielding and end-of-stem pain. *Materials & Design, 30*(4), 1391–1397. https://doi.org/10.1016/j.matdes.2008.06.071

Eidel, B., Gote, A., Fritzen, C. P., Ohrndorf, A., & Christ, H. J. (2021). Tibial implant fixation in TKA worth a revision? How to avoid stress-shielding even for stiff metallic implants. *Computer methods in biomechanics and biomedical engineering, 24*(3), 320–332. https://doi.org/10.1080/10255842.2020.1830274

Piovan, G., Bori, E., Padalino, M., Pianigiani, S., & Innocenti, B. (2024). Biomechanical analysis of patient specific cone vs conventional stem in revision total knee arthroplasty. *Journal of Orthopaedic Surgery and Research, 19*, 439. https://doi.org/10.1186/s13018-024-04936-0

Scott, C. E. H., & Biant, L. C. (2012). The role of the design of tibial components and stems in knee replacement. *The Journal of Bone and Joint Surgery. British Volume, 94-B*(8), 1009–1015. https://doi.org/10.1302/0301-620X.94B8.28289
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