Solving Orthopedic Device Manufacturing Challenges with Pulsed Electrochemical Machining (PECM)
Learn how Voxel's PECM technology can be utilized to alleviate a variety of design/manufacturability challenges for orthopedic devices such as spinal implants and compression staples.
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Several factors have contributed to increased demand for orthopedic devices, such as the growing number of geriatric patients globally, and a decrease in bone mass density across age groups over time. A significant rise in osteoporosis diseases has been observed as a result. In response, many orthopedic device manufacturers are prioritizing R&D on devices servicing parts of the body undergoing constant stress, notably devices implanted in the hips, spine, and feet.
As demand drives higher part volumes, orthopedic device manufacturers are facing challenges aside from significant regulatory hurdles. Primarily, complex part geometries and tough-to-machine materials common in high-stress orthopedic devices are hindering the design-to-production process for medical device manufacturers. The medical device market also often requires large part volumes to be produced before launch, adding additional strain for manufacturers to optimize efficiency.
Orthopedic device manufacturers should consider Voxel's pulsed electrochemical machining (PECM) technology, as its capabilities may alleviate some of these difficulties. In this article, we’ll review how PECM’s capabilities--such as high repeatability, superfinished surfacing and high level of precision--may help manufacturers enable new designs and cut processing times for a variety of implantable medical devices, particularly high-demand, high-stress devices in the spine, feet, and hips.
The aforementioned pressure on orthopedic device manufacturers generally is further exacerbated for spinal device manufacturers specifically by the well-documented general decrease in posture quality. This problem is especially acute for younger age demographics, leading experts to speculate these issues worsening over the coming decades, driving higher demand for spinal implants in the near future. Let's review how PECM can manufacture devices related to the two primary types of spinal surgery:
Compared to fusion, disc replacement surgery is considered the less invasive and traumatic option, as its purpose is to fully restore patient mobility. As such, manufacturers design artificial disks with the intent to replicate the range of motion of the specific disc they are replacing.
In one example of a lumbar disk replacement (pictured), two cobalt-chrome endplates are separated by a convex-shaped polyethylene part, with a concave geometry on the top endplate, effectively creating a ball-and-joint motion to mimic the original range of motion. High-quality finishing of a concave geometry in a material such as cobalt chromium comes with considerable machinability challenges. These geometries are likely milled, which may produce inadequate surface quality results and insufficient production times, relative to high demand of spinal devices. Cobalt chrome’s abrasive properties wear down tools quickly, inhibiting cost-efficient high-volume production.
PECM, however, could be a viable alternative for machining these geometries—as a non-contact, non-thermal process, there is little to no tool wear, and PECM can potentially produce complex cobalt-chrome parts down to >10µm repeatability.
For the treatment of chronic spinal conditions, or correcting substantial deformities, orthopedic surgeons may perform spinal fusion surgery on two or more vertebral bodies via screws, plates, or cages, effectively grafting them into a single structure. A variety of spinal fusion devices exist depending on the severity of the deformation/trauma, the spinal section, and patient-specific conditions. The constant compression on vertebrae makes mechanical strength and fatigue resistance both top priorities for manufacturing spinal fusion devices--as these directly influence both the part’s relationship with surrounding bone and the product’s lifetime itself. Therefore, many tough-to-machine, fatigue-resistant alloys, notably titanium alloys or cobalt-chromium, are common.
Aside from high material costs, there are notable incentives for product development engineers to miniaturize the design of spinal fusion devices. Smaller devices reduce potential complications during surgery, such as nerve damage or wound infection. A smaller fusion device may also improve patient mobility or comfort by reducing the inhibition already present in fusing vertebrae. Additive manufacturing (AM) is growing in popularity with spinal fusion cages in particular, maximizing the customizability inherent in AM to suit patient-specific conditions and minimize expensive material waste. However, at present AM has notable drawbacks; producing higher part volumes can involve significant costs, and often requires a secondary finishing operation before the product's launch, as an additively-produced part’s surface quality sometimes does not meet the rigorous finishing standards in the medical device market.
PECM is capable of fulfilling the role of a postprocessing operation for certain metal additive parts, for both secondary machining and finishing. PECM is not a good fit for machining porous, osseointegrative surfaces common in spinal fusion devices. Rather, it is optimal for selectively finishing the smooth surfaces often on the sides and inner parts of spinal cages and other spinal fusion devices. More importantly, PECM’s high-precision capabilities can enable manufacturers to continue the miniaturization of spinal fusion parts, advancing surgical capabilities and ultimately improving patient outcomes from minimally invasive surgery and increased mobility.
Bone fixation plates are the most common implantable devices used for the treatment of fractures. Fixation plates must be designed with regard to surface quality and adequate tensile strength. As such, materials that are both biocompatible and sturdy, such as titanium alloys and stainless steel, are the most common. For manufacturers, a crucial variable to consider is the specific bone(s) being fixated. Consider a fixation device in a patient’s foot, an area under constant stress with more joints than anywhere else in the human body. Larger fixation devices would render these joints unmoving and drastically reduce both patient mobility and comfort. Hence, orthopedic surgeons will generally prefer to implant smaller, more flexible devices in a patient’s foot (nitinol is often used). This is in direct opposition to, for example, a femur fracture, where orthopedic surgeons would prefer a larger, stiffer implant design with a higher yield strength.
Manufacturing fixation devices for a patient’s foot is particularly challenging. Nitinol, an increasingly common material for these devices, requires large amounts of cutting force, resulting in increased rates of tool wear and decreased machining efficiency, especially when machining higher part volumes. These issues are exacerbated by smaller geometries (plates can be less than an inch long, with a 2mm thickness) and the high surface quality standards necessary to meet FDA guidance. Voxel Innovations has proven PECM as a viable alternative process to machine both larger, sturdier fixation devices and smaller, more tensile devices--with optimal surface quality (capable of surfaces down to .005-.4µm Ra). For example, Voxel has machined .5mm diameter x 10mm deep holes in nitinol, and .38mm diameter holes in stainless steel, with with a higher level of repeatability than conventional processes, such as CNC machining.
Specific to podiatry, compression staples are another nitinol-based implant. These devices are utilized in arthrodesis (bone joint fusion) surgery, but are expanding to a variety of other applications, such as treating trauma or orthopedic deformities in the spine and even wrists/hands. As a shape memory alloy, nitinol is the ideal material for compression staples, due to its ability to retain its original geometry despite elastic deformations from constant joint movement within the ankles and wrists.
Compression staples (pictured) have serrated features along the inner part of the forceps, and this unusual geometry may present additional machinability challenges. Voxel is capable of machining multiple parts in a single process, including geometries similar to compression staples, which may reduce machining times and improve efficacy of the parts for manufacturers.
Humeral and Femoral Implants
Most arthroplasty devices (the procedure of restoring a joint’s functionality with an implant) have similar design-to-manufacturability challenges as other implants. Many are comprised of a medical-grade polymer and/or bioinert metal, usually a titanium or cobalt chrome alloy. The manufacturing process of humeral and femoral joint replacement devices may have additional benefits from utilizing PECM technology.
Humeral implants function as a replacement for a shoulder joint, the three primary components being a convex ‘head’, a ‘stem’ inserted in the humerus, and the concave-shaped glenoid, made of a medical-grade polymer, acting as the ‘socket’. The heads are generally comprised of a cobalt-chrome alloy or a titanium alloy and are usually between 37-55mm in diameter.
Aside from the challenge of superfinishing the head and glenoid geometries in high volumes, manufacturers also face challenges with machining the other stem components, present in both humeral and femoral implants. The lower part of the stem, inserted into the humerus or femur, is usually a smoother surface, designed to minimize corrosion. In contrast, other areas of the implant closer to the head have a porous, osseointegrative surface quality designed to promote fusion of the device with the surrounding bone. Due to the variation of surface features on a single implant, a number of steps are required to process these parts, lengthening cycle times.
Pulsed electrochemical machining may reduce the number of steps necessary for processing both heads and stems on humeral and femoral replacement devices. PECM has capabilities to machine both convex and concave geometries on tough-to-machine materials to meet, or potentially exceed, standards related to both biocompatibility and minimize friction on a joint replacement part, assisting both manufacturers and patients alike.
While circumstances vary depending on material and geometries, PECM could potentially machine multiple parts of humeral/femoral replacement devices in tandem using a single cathode, as opposed to the conventional method of always machining the stem, head, and glenoids separately. PECM can create a selective surface finishing capability, particularly important for humeral and femoral stem parts, which can benefit from a selective surface finishing on some aspects, while leaving others open for osseointegrative, porous surfaces. Other processes such as AM are adept at creating osseointegrative surfaces, while PECM’s selective finishing capabilities could improve surface quality where necessary without disturbing the porous features.
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