Don’t Let Your CAD Choices Screw You Over

For some engineers, specifying a screw thread is no more than determining diameter and length and then searching through a parts catalog. For others, a custom thread is the difference between a successful surgical outcome with enhanced quality of life or a disappointing result that may require a revision surgery. A bone screw, whether used in a reconstructive, spinal, or other application, can be such an example.

A bone screw typically begins with a conical tip. The major diameter of the screw quickly increases to provide purchase in order to draw the screw into the bone. However, the minor diameter increases more gradually to reduce insertion force until the final portion is engaged into the bone. At this point the increasingly large tapered minor diameter provides a press fit into the outer (cortical) layer of the bone. This thread geometry is illustrated in Figure 1. The overall thread geometry is critical to providing stability to the affected area to allow bone growth and ultimately healing.

Figure 1: Typical Bone Screw Thread Geometry

Figure 1: Typical Bone Screw Thread Geometry

EAC’s Engineering and Design Services team recently worked on a project that clearly showed the advantage of using Creo versus SolidWorks to create this critical thread geometry. Several years ago the Engineering and Design Services team worked with an orthopedic company to create a series of bone screws varying in diameter and length for reconstructive surgeries. They used PTC Creo to create CAD models and drawings. The company was recently acquired by a global orthopedic corporation that used Solidworks 2016. The larger company insisted that the CAD models and drawings be recreated in SolidWorks 2016 for compatibility with internal corporate standards.

While working on the task, the Engineering and Design Services team found difficulty in reproducing the identical thread geometry using SolidWorks. Creo uses a single thread feature containing two individual sketches; one for thread profile and another for thread trajectory. This feature easily creates the varying thread tapers for the screw. Figure 2 shows the completed part and associated features in the model tree on the left hand side of the picture.

Figure 2: Bone screw modeled in CREO 3.0 by Parametric Technologies, Inc.

Figure 2: Bone screw modeled in CREO 3.0 by Parametric Technologies, Inc.

Recreating the same geometry in Solidworks requires 11 separate features and 8 sketches as shown in the model tree in Figure 3. Separate threaded features were required for the straight thread and the tapered thread at the tip of the screw. In addition, two helical curves were needed for each of the threaded features. One curve was required to provide the thread trajectory while the second helical curve was needed to fix the thread profile normal to the longitudinal axis of the screw.

Figure 3: Bone screw modeled in SolidWorks 2016 by Dassault Systemes

Figure 3: Bone screw modeled in SolidWorks 2016 by Dassault Systemes

The additional time to generate the needed features in Solidworks was significant for each part when compared to Creo and combined to substantial time savings for the entire project. This example illustrates why Creo is a superior application to SolidWorks.

The pressure to develop more and better products in less time is increasing. Your 3D CAD solution should enable you to provide your best work to produce your innovative ideas quickly and add advanced capabilities when you need them. The best tool for this is PTC Creo. PTC delivers the most scalable range or 3D CAD product development packages on the market today. Read more about why you should design using PTC Creo here.