Segmenting femurs and analyzing femoral length and femoral axis

Fig 1.

Total hip replacement is widely considered a very successful surgery to relieve pain and restore mobility to patients. Since the shape of every patient's femur is different, the femoral offset, anteversion angle and length can vary widely between patients. Therefore, to achieve good functional results, an accurate restoration of hip biomechanics with appropriate implant and sizing options is essential. Professor Scheerlinck and his coworkers at the University Hospital of Brussels wanted to know whether two different cemented femoral implant systems would be able to restore hip biomechanics (both anatomic and 5 mm medialized rotation centers) in 30 patients. Using the Scripting module in the Materialise Mimics Innovation Suite allowed them to answer this research question efficiently.

Scripting to virtually implant femoral stems

Fig. 2a

Scripting to virtually implant femoral stems

Fig. 2b

Performing the Virtual Implantations

Thirty femurs were segmented and their hip rotation center, proximal femoral axis and femoral length were determined (Figure 1). The implants, listed in Table 1, were landmarked and the implants were automatically sized and aligned along the proximal femoral axis to restore an anatomical and 5 mm medialized hip rotation center. In total, the script took about 10 seconds to generate each stem/femur combination (Figure 2a and 2b), which would have taken about 30 minutes per combination if performed manually

Type Exeter type
(CPT - Zimmer Biomet)
Kerboul type (Centris - Mathys)
Sizing options 18 12
Offset options 5 4
Femurs 30 30
Centers of rotation to restore 2 2

The proposed stem size and position could be manually fine-tuned by the surgeon through a custom user interface (Figure 3). Python allows the user to create user-friendly interfaces and they can be made to interact with Mimics and 3-matic.

User-friendly interface through Python scripting in the Mimics Innovation Suite

Fig. 3

Analyzing the Results

Stems 1 and 2 allowed an accurate restoration of the anatomic hip rotation center (mean distance stem rotation center to anatomic rotation center, Stem 1: 0.97±0.88 mm, Stem 2: 1.66±1.59 mm). Stem 1 allowed the restoration of the rotation center to within 5 mm of the anatomical rotation center in all 30 cases (max. 4.31 mm), whereas stem 2 achieved this in only 28/30 hips (max. 6.72 mm).Aiming for a 5 mm medialized rotation center was more difficult to achieve with both stems (mean distance stem rotation center to anatomic rotation center, Stem 1: 1.38±1.63 mm, Stem 2: 3.61±2.73 mm). Stem 1 allowed the restoration of the rotation center within 5 mm of the medialized rotation center in 29/30 cases (max. 8.09 mm), whereas Stem 2 achieved this in only 25/30 cases (max. 11.15 mm). Professor Scheerlinck and his coworkers concluded that although both stem systems allowed the restoration of hip biomechanics accurately in most cases, Stem 1 was superior to Stem 2 for achieving both anatomic and medialized rotation centers. This could be explained by the wider range of implant sizes available in Stem 1 vs. Stem 2 (18 vs. 12) and that Stem 1 was a so-called 'undersized stem', offering more freedom to correct version. They also found that both stems had difficulties achieving a medialized center of rotation. In some cases, differences between aimed and planned rotation centers were close to 1 cm, which might negatively impact the clinical outcome. Therefore, they recommend that to avoid suboptimal reconstructions with the available implants, templating should be considered mandatory – especially when aiming for a medialized reconstruction.

The Scripting Module's Main Benefits

The study presented here would not have been possible without the Scripting module in the Mimics Innovation Suite. These are its main advantages:

  • Scale up your work from one or two patients to dozens or even hundreds of patients
  • Avoid repetitive work and focus on more important tasks
  • Get more consistency between Mimics users
  • Create more user-friendliness and consistency through Python's support for custom user interfaces that interact with Mimics and 3-matic
  • The Python programming language is relatively easy to learn, and there is a wealth of knowledge available on the Internet
  • Scripting tutorials are included in the Scripting module, allowing you to get a head start in scripting your workflows

 

For more information:

If you want to discover the possibilities of either or both of these modules for yourself, check out our free MIS 20 webinar series or contact your local Account Manager or Application Engineer.