Christoph Stotter1,3, Erich Reiter1, Werner Schretter2, Philippe Reuter3, Stefan Nehrer1, Thomas Klestil1,3

1Department For Health Sciences, Medicine and Research, Danube University Krems, Krems, Austria
2Ws, Design, Tulfes, Austria
3Department of Orthopedics and Traumatology, Landesklinikum Baden, Mödling, Austria

Keywords: Computed-aided design, entry point, intramedullary instrumentation, total knee arthroplasty.


Objectives: The aim of this study was to simulate different entry points and investigate potential angulation errors of the intramedullary device and resulting changes in the distal femoral cut using a computer-aided design (CAD) approach.

Materials and methods: We used a CAD approach to simulate various distal femoral entry points for intramedullary instrumentation. Simulations were performed on (i) a commercially available three-dimensional (3D) CAD model of a human femur (DigitalFemur) and (ii) a digital 3D model of an analogue large femur model produced using a coordinate measuring machine (FaroFemur). Divergent insertion points medial, lateral, anterior and posterior to the ideal position were simulated. Angulation deviations of the resulting positions of the intramedullary rod were measured and changes in the anatomical-mechanical axis angle were calculated. Differences between the two simulation models were quantified.

Results: The ideal entry point in the FaroFemur was 9.9 mm anterior and 4.3 mm medial to the apex of the intercondylar notch, and 9.2 mm anterior and 3.6 mm medial in the DigitalFemur. A medial entry point increased the angle between the anatomical femoral axis and the alignment rod in the FaroFemur and DigitalFemur (with 5 mm displacement 2.510° and 2.363°, respectively; with 10 mm displacement 3.239° and 3.283°, respectively). In contrast, a lateral entry point decreased the angle between the anatomical femoral axis and the alignment rod (with 5 mm displacement 2.267° and 2.262°, respectively; with 10 mm displacement 3.158° and 3.731°, respectively). An anterior entry point changed the angle between the anatomical femoral axis and the alignment rod towards extension (1.802° in the FaroFemur; 2.142° in the DigitalFemur), while a posterior entry point generated a deviation toward flexion (2.045° in the FaroFemur; 2.055° in the DigitalFemur). The mean difference between the two models was 0.108±0.121° with the highest difference for anterior displacement.

Conclusion: Minor deviations of the entry point for intramedullary instrumentation during total knee arthroplasty can result in malalignment of several degrees.

Citation: Stotter C, Reiter E, Schretter W, Reuter P, Nehrer S, Klestil T. Influence of the femoral entry point for intramedullary alignment in total knee arthroplasty: A computer-aided design approach. Jt Dis Relat Surg 2022;33(2):294-302.

Author Contributions

Conceptualized the study developed the study design: C.S., E.R. and W.S.; Performed the simulations: P.R., S.N. and T.K.; Discussed the results: C.S.; Wrote the manuscript: P.R., S.N. and T.K.; Provided valuable comments on the manuscript. All authors agreed to the final version of the manuscript for submission and have given approval for this version to be published. All authors contributed extensively to the work presented in this manuscript.

Conflict of Interest

The authors declared no conflicts of interest with respect to the authorship and/or publication of this article.

Financial Disclosure

The authors received no financial support for the research and/or authorship of this article.