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Loukas Medical, Systematic Displacement Screw (SDS)

Loukas Medical, Inc. is seeking partnerships to license the SDS for their orthopaedic screw applications.

Approximately 6.8 million fractures that need medical attention occur in the U.S. each year. Many of these require bone screws to stabilize the fractured bone. Add to that the many orthopaedic surgeries that require the use of bone screws, such as spinal, hip and knee, and the number of bone screws that are implanted each year is astronomical.

A typical orthopaedic screw may have a straight or tapered minor diameter and a straight or tapered major diameter. Screws with both a straight minor and major diameter have been around for centuries. Many times, when the screw is inserted into the bone, the ability of the screw to securely fasten to the bone is hampered by osteoporotic or soft cancellous bone. When the screw is not securely fastened to the bone, the screw will toggle, creating complications. To reduce this toggling or lack of purchase, orthopaedic screw manufacturers have created various designs.

Tapered Threads: The screw thread is tapered so that the diameter increases from the tip of the screw to the head, which increasingly compresses the bone as the device is screwed deeper into the bone, resulting in a tight fit. If a tapered bone screw is ever backed out of the bone for adjustment or replacement of the screw, the replaced bone screw becomes loosened and will not be securely held by the bone due to the mating of the tapered bone screw with the tapered bore in the bone. This applies to all types of tapered screws, even if only part of the thread is tapered. Another danger in using a tapered screw is use in pedicles. The tapered portion of the screw thread produces radial forces that could fracture the pedicle, resulting in poor fixation and even nerve damage.

Multiple Pitch Threads: This design adds a second helical thread at the proximal end of the bone screw. This second thread is about halfway up the thread and is placed in between the existing primary thread. The second thread increases screw purchase, but could disrupt the bone at the proximal end of the construct. When the second thread comes in contact with bone, the new thread cuts into the bone between the existing threads that were formed from the distal end of the screw. If the second thread does not successfully form, the bone becomes disrupted and weak. This also becomes an issue if the screw needs to be removed and replaced with another. Another problem that could occur is, if the second thread is not formed correctly, too much radial pressure is applied to the bone, thus causing it to fracture. There is also the chance that, if the second thread does not form properly, the threads in the bone formed by the primary thread become stripped.

Variable Pitch Threads: This is very similar to the above multiple pitch thread, but instead of adding an additional series of threads, the pitch is decreased at the proximal end to half that of the primary thread. The same concerns stated above for the multiple pitch screw applies to this design. In addition, the change in pitch causes the primary thread that was formed in the bone to strip because the two pitches are working against each other.

In contrast, the Systematic Displacement Screw (SDS) has a constant minor and major thread diameters instead of tapered, multiple or variable threads. The pitch of the thread is also constant throughout the entire length of the screw; however, the flattened crests at the tip of the screw are narrower than the crests near the screw head, resulting in a smaller distance between the thread flanks, which displaces and/or compresses more bone matter. Because the major diameter remains constant, the diameter of the hole made by inserting the screw is consistent over the device’s length, allowing it to be adjusted or replaced without loosening.

Rather than increasing the size of the hole, the SDS can be used for the both the original screw and the replacement screw because the pitch is the same, and additional bone is compressed and/or displaced by the smaller distance between the flanks of the new screw.

Testing, performed by Mar-Test, Inc. in Cincinnati, Ohio, showed a 33% to 40% increase of insertion torque in 20 lb. test foam and a 20% to 25% increase of insertion torque in 40 lb. test foam. Additional pull-out and push-out data is available.