Most of spinal cord injuries like traumatic, degenerative diseases and injuries caused by infections and tumors are treatable by internal fixation of pedicle screw. Various equipment is used for spinal fixation, for example Herrington system and all kinds of hooks that are placed in the posterior part or anterior plates and replacement cages which located on the anterior part of the spine.
In the meantime, pedicle screws can be located on both posterior and anterior parts of spine. Being easy to locate, applicable for various lesions and preferred by doctors compared to other equipment, have made this product one of the widely used implants. On the other hand, there have been a variety in designing pedicle screws recent years. Although all of the pedicle screws are almost similar but variety in designing make a huge difference in ease of insertion during surgery and also postoperative effects. Now let’s see what characteristics a suitable pedicle screw should have.
First thing that comes to mind about the ease of implanting a screw during surgery is that it should quickly fit in without any excessive force. To achieve this parameter, pitch of screw threads should be high so it increases speed during insertion. On the other hand, it should be kept in mind that, in high pitch threads screws, the number of threads on the screw are decreased. Screws with less threads have less joint to the bone so reduces fixation effect and it will be problematic after surgery (Figure 1).
1. One-way screw with low pitch and high threads
2. One-way screw with high pitch and low threads
3. Two-way screw with high pitch and high threads
For solving the problem of threads quantity, a screw can be designed in a way which without decreasing pitch, the number of threads are increased. This means that instead of using one-way screw, two-way screw should be used.
The second solution to solve the problem of fixation is that the pitch depth at the top should be more so reaching the tip to the sponge bone would be easier and other portion that is in contact with the corneal layer fixes into the bone and prevents screw to out.
In addition to these parameters, all of which are in accordance with the AO standard, there are other parameters such as screw head and nut design which are less important than what mentioned. However, the design of a suitable pedicle screw based on the technical knowledge of mechanical designers, physicians and technicians’ experiences, and the surgical feedbacks, does not seem complicated. But it should be noted that the idea and design of a piece is only a part of the production process, the implementation and construction of the piece in accordance with ideas is another important part of the process, which in case of success, can lead to production.
As mentioned, there are various designs for pedicle screws which meet minimum standards at least. However, considering that these implants should be approved by the physician for ease of placement and should also be placed in patient’s body for a long time, it should be considered beyond the minimum of standards for design.
It is possible of course, as development of science, these plans and even the materials used for producing them undergo major changes and requires new designs.
– Thesis for designing and manufacturing of spine stabilization equipment, SAIRAN Electro Optic Industries Co.