Medical Design Projects
Baxano Surgical, Inc. received 510k clearance from the FDA in mid-April of 2014 to market a line of pedicle screws. I developed the initial design and made subsequent changes based on feedback from consulting surgeons, in-house engineers, marketing analysts, and vendors. Pedicle screws are placed in the vertebrae above and below the damaged disc. The screws ranged in length from 25mm to 85mm and in diameter from 4.5mm to 8.5mm. The screws have a custom thread form designed to provide maximum resistance to pullout. They are very similar to other screws within the industry. However, they consist of a two lead thread at the start of the screw to approximately 20mm from the head of the screw. Then it transitions to two additional thread leads for a total of 4 thread leads over the last 20mm in length. This is believed to provide the best resistance to pull out.
Pedicle screws are part of a system used to promote fusion between two vertebrae when the disc between them has been removed due to damage or disease. They usually consist of four components: a bone screw with a spherical head; a "tulip" with a hemispherical recess and an opening through which the screw passes; a seat/saddle, which usually contains a hemispherical recess and a "U" shaped seat to accepts a titanium or cobalt rod; and finally a locking cap or set screw, which locks the entire construct down and prevents it from moving. The "pedicle screw" is so named for the portion of the vertebral anatomy through which the screw passes. The hemispherical recesses in the seat/saddle and the "tulip" provide bearing surfaces which create a friction lock on the spherical head of the screw when locked down. A titanium or cobalt rod passes through the "U" shaped seat of the seat/saddle and the tulip to connect two pedicle screws. Once the screws and rods are in their desired position, the locking cap or set screw is used to "lock down" the rod and prevent any axial movement between the "tulip" and the head of the screw. This usually happens after the damaged or diseased disc has been removed and a biologically inert object, such as a PEEK cage, is placed between the two vertebrae along with a quantity of bone graft material to preserve spacing. The entire construct is intended to eliminate motion between the two vertebrae and to promote bone growth between the two vertebrae over the course of 6 - 12 months.
When it becomes necessary to remove damaged or diseased disc in the spine, often the spacing between the vertebrae has collapsed, causing the vertebral body to compress nerves passing between vertebrae into the spinal column. Distraction is the process of restoring the spacing between vertebrae and thereby relieving pressure on the nerves. The initial AxiaLIF design achieved this through differential pitch. The thread pitch on the upper portion of the screw differed slightly from the thread pitch on the lower portion of the screw such that the act of advancing the screw through the two adjacent vertebrae creates additional space between the vertebrae. The amount of distraction was pre-determined by the pitch and length of the screw, so the surgeon had to choose from a limited set of screws to produce the desired result. Among my responsibilities were creating different screw designs based on customer feedback for different screw lengths and desired distraction for the one piece screws. This occurred when the surgeon felt the limited production set was not sufficient for a particular case.
Later design changes to the screw introduced a three piece construct which allowed more flexibility in determining the amount of distraction once the screws were in place, or in situ, in the vertebral bodies. My role was to work with existing models to incorporate the internal features of the design without compromising the thread form and reducing impact to instrumentation to minimize the number of additional instruments required for the new design.
A PEEK (polyether ether ketone) cage is one of the components mentioned above that can be used to stabilize the spine. Diseased or damaged disc tissue is replaced by the biologically inert PEEK cage and bone graft material to promote bone growth to fuse the adjacent vertebrae. My role on this project was to use the base model provided by the project lead engineer to produce models and drawings for the various sizes being offered. This was done by populating a design table to supply the appropriate variables to the formulas in the underlying sketch which controlled the features of the cage. The number of serrations varied according to the radius of the cage profile, which varied according to the length of the cage. There was a second basic design that added a 7° included angle across the width of the cage to help restore spinal curvature within the affected vertebrae.
The axial approach to spinal fusion used and pioneered by TranS1, Inc., presents a unique challenge in how you prepare and abrade the vertebral endplates during damaged/diseased disc removal in preparation for spinal fusion. The working surfaces are roughly 90 degrees from the path used to access the space. TranS1 developed discectomy cutters which were formed from loops of Nitinol (NiTi). For those who are unfamiliar with it, Nitinol is known for its shape memory. The blades are formed at an angle and subjected to heating until they reach a narrow window of superelasticity just above their transformation temperature. Once allowed to cool, the blades retain this shape but are flexible enough to retract into a tube. This allows the surgeon to pass the blades retracted into the tube through the working channel and into the disc space. Once there, the blades can be extended from the tube where they return to their original shape and can be rotated to cut away the damaged/diseased disc. I worked with my supervisor to develop a way to refine and control the angle of the deployed blade by using the axial motion to constrain the angle of the deployed blade and provide more control of how much surface contact was achieved when abrading the bony surface of the endplates to induce bleeding, which helps to facilitate bone growth and subsequent fusion. A patent was issued to Baxano Surgical, Inc., the name TranS1, Inc. later adopted, for this device on April 15, 2014 with my supervisor and myself listed as the co-inventors.
The following is taken directly from the Baxano Surgical marketing text:
“The VEO Lateral System employs similar patient positioning, discectomy and implant placement as other lateral systems on the market, and is compatible with most intraoperative neuromonitoring (IOM) systems.”
“However, VEO is uniquely designed for direct visualization of the psoas prior to dissection. This is accomplished with the Reveal two-stage retraction system: a tubular retractor for access to – but not through – the psoas, followed by a pair of independent blades for controlled, manual psoas retraction in the anterior/posterior plane. The VEO Lateral System’s goal is to provide optimal control and minimal tissue disruption.”
The following marketing image displays many of the components unique to the system. While I worked on many of the components of this system, of particular note are the two retractor blades, which provided significant modeling challenges due to their form factors. The basic system design was provided by the project lead engineer and further refinements of the models and additional enhancements and modifications to the system were made by myself and other designers and engineers over the course of the project.