Achieving Precise, Complex Features for Orthopedic and Dental Applications
Compared with a lot of other medical devices, bone screws have a seemingly simple structure: a threaded cylinder with a head on one end and a tip on the other. Yet, there can be a surprising amount of complexity in the design of bone screws (also called anchors).
That is why precision CNC Swiss-style machining plays an important role in realizing the designs that give bone screws the proper features and functionality for their end uses.
Basic Structure of Bone Screws
Like the common wood screw most people are familiar with, the structure of a bone screw has certain basic characteristics:
- The head of the screw is the top, flattened surface that aids in the insertion of the screw.
- The tip is the other end of the screw, opposite from the head.
- The length of the shaft (or body) of a screw is measured from the head to the tip.
- The pitch of a screw is the distance traveled by the screw with each 360º turn.
- The major diameter of a screw is the total thickness of the screw from one peak of the thread to the other peak on the opposite side.
- The minor diameter is the thickness of a screw not including the threads. This is important to know because the pilot hole into which a screw will be inserted should be the same size as the minor diameter.
Why Bone Screws Are Used
The main function of a bone screw is to help repair bone fractures. Typically, the screw converts the force of bone movement into compression that holds the bone in place so it can heal more quickly.
The use of bone screws dates back to the early twentieth century. That was when surgeon William O’Neill Sherman pioneered the fixation of bone fractures using plates and conventional screws that he modified to attach to bone.
Today, bone screws are used in a wide range of orthopedic and orthodontic applications. Orthopedic screws are used mostly for fixation of bone or help attach soft tissue (such as tendons) to bone. In dentistry, bone screws generally serve as anchors over which something else — such as a replacement tooth — is fastened.
As expected, modern bone screws are more precise and specialized than Doc Sherman’s version. Unsurprisingly, CNC Swiss machining is the method often used to produce precise features that enhance the screw’s fixation method or ability to adhere to bone or other tissue.
Different Bones, Different Screws
In a study comparing the strength of four commercially available bone screws having different pitches, the holding power of the screws correlated with the density of the bone, the design of the threads, and number of threads engaging the bone. Since different types of bones have different densities, it makes sense, then, that bone type has an impact on screw design.
Cortical (or compact) bone makes up the hard, dense outer layer of bones that provide the human body with a support structure and help to protect the internal organs. These smooth, white, solid-looking bones make up about 80% of an adult’s body mass.
Cancellous bone (also known as spongy bone) forms the internal tissue network of human bones and is generally found at the tip of other bones, near joints, and inside vertebrae. Cancellous bone is less dense, somewhat softer, and more flexible than cortical bone.
Accordingly, CNC Swiss machining can be used to make screws that vary in threading, pitch, and other features based on the type of bone in which they will be used:
- Cortical screws — also called cortex screws — are designed to hold tightly and provide maximum stability when inserted into cortical bone. These strong screws have a small pitch, with closely spaced, shallow threads along the entire length, and usually have a blunt end. Cortical screws typically come in lengths from 0.059” (1.5 mm) to 0.177” (4.5 mm).
- Cancellous screws, which are designed for fixation of cancellous bone, are longer and have a larger pitch than cortical screws. Cancellous screw threads are more deeply cut and widely spaced, and the shaft can be fully threaded or only partially threaded. The screws generally come in lengths from 0.138” (3.5 mm) to 0.256” (6.5 mm).
Other Variables in the Design of Bone Screws
Besides variations in threading and pitch, bone screws can have holes, steps, slots, and other features produced thanks to the precision capabilities of CNC Swiss machining. For instance, different types of bone screw heads can be created to meet specific needs:
- Screws for anchoring teeth may have a tapered head onto which a replacement tooth can be pressed.
- A threaded screw head can be used to lock the bone screw and its attachment (such as a plate) into place, for greater stability.
- A hexagonal screw head is commonly utilized to allow the use of a screwdriver for insertion.
Swiss machined bone screws can also take advantage of different tip designs that support different screw insertion techniques:
- Non-tapping screws have a smooth, round tip that is inserted by making a pilot hole and tapping threads inside the hole.
- Self-tapping screws have cutting flutes so that the screw taps its own threads once it is inserted in a pilot hole. For example, since cancellous bone is much less dense than cortical bone, a cancellous screw is self-tapping — that is, it cuts its path in the bone as the screw is inserted.
- Self-drilling screws can be inserted as is, with no pilot hole or other preparation required. The screw creates its own pilot hole and is also self-tapping.
Unique Screw Design Advantages
Medical device companies may also come up with their own bone screw design variations to create something unique that sets them apart and provides a competitive advantage.
Besides being a benefit to the anchor manufacturer, a patentable, proprietary design creates opportunities — and challenges — for CNC Swiss-style machining to make what may be a very complex design. For example, more complicated shapes are a perfect example of where the technique of segmentation in Swiss machining allows the more sophisticated machine shop to meet customer’s complex design needs.
Another guiding principle — designing medical devices with one-piece construction — can often make a part more complex from a machining standpoint. Rising to this challenge, Swiss machining can help manufacturers achieve single-component designs that minimize the risk of a device coming apart or having something go wrong due to the failure of one of multiple parts.
In addition, different surface modifications (see below) can be added to bone screws, post-machining, to have a positive impact on bone growth and adhesion, further setting a particular anchor design apart from other manufacturers’ options
Specific Material Considerations for Bone Screws
Bone screws and anchors for orthopedic and dental use have been made from metal alloys for more than a century. That means there has been a lot of time to learn which materials deliver the right mix of high strength, corrosion resistance, and biocompatibility.
Today’s manufacturers generally choose implant-grade materials that meet American Society for Testing and Materials (ASTM) or ISO specifications. Accordingly, making bone screws with the correct features and functionality requires the ability to Swiss machine a wide range of different materials, with each material having its own unique characteristics.
While stainless steel is popular in medical devices and can be used for bone screws, the last few decades have seen the rise in the use of titanium and an array of specialty metals chosen for their unique characteristics and advantages.
For example, titanium — which is available in pure and alloyed versions — is often used when the goal is light weight and high strength or when a device needs to mate with or connect to another titanium component. Titanium screws are frequently used when treating mandibular fractures.
The titanium alloy Ti-6Al-4V ELI (titanium-6aluminum-4vanadium extra low interstitials) is lightweight, corrosion resistant, strong, and biocompatible. Tight controls during the melt process result in a material with added ductility and strength, making titanium ELI a popular and durable choice for bone screws and other medical devices.
Nitinol (NiTi) is sometimes chosen for its ability to withstand a high amount of stress without deforming. That means it can be used for applications where the bone screw or other device needs to stretch and change slightly without taking a permanent set.
Other high strength and biocompatible materials that are often used interchangeably in the Swiss machining of bone screws and anchors include:
- MP35N® and the similar 35N LT® (low titanium), which is a melt of MP35N with the trace amount of titanium removed, giving it better fatigue life
- Elgiloy®, Conichrome®, and other alloys specified by ASTM F1058 and ISO 5832-7
Surface Treatment to Promote Osseointegration
Surface finish is a very important topic here at Metal Cutting. For bones screws and Swiss machining, surface treatment is a unique topic.
While some bone screws are temporary and thus, eventually removed, most are put in place to stay. It turns out, a screw’s surface finish has a lot to do with promoting bone growth so that the screw is firmly embedded and the bone grows around it.
While scientists continue to work on developing special coatings that can be applied to promote osseointegration (read about one example), screw manufacturers have found that rougher finishes and other different types of surfaces can help to promote bone growth.
The invention of direct laser sintering (otherwise known as 3D metal printing) is allowing previously unattainable levels of porosity to be achieved, going beyond just a rough surface finish and promoting osseointegration. That is why 3D manufacturing may very well be the future of bone screws and many other medical device applications.
However, most of the materials used to make orthopedic bone screws and dental anchors are not heat treatable. Instead, they gain their strength from cold working, which is something you can’t do with 3D metal printing.
That means you still have to start with a Swiss machined rod or wire prior to laser sintering!
Swiss Machining vs. 3D Laser Printing for Bone Screws?
While 3D metal printers will say they can do anything, the cold working is among the challenges they have to overcome before all applications for bone screws can be 3D printed. Until then, Swiss-style machining has the advantage of starting with cold work material as bar stock.
The magic of 3D printing is that, as an additive process, that it can produce voids — something that, for the sake of light weight and porosity, is an advantage Swiss machining can’t match. However, for threads, holes, steps, and other features that may be crucial to a screw’s function, additive and subtractive processes both yield the same dimensional end result.
To learn more about the attributes and advantages of CNC Swiss-style turning for your medical devices or other applications, download our free guide Swiss Machine FAQs.