Medicine by Alexandros G. Sfakianakis,Anapafseos 5 Agios Nikolaos 72100 Crete Greece,00302841026182,00306932607174,[email protected]
Osteoporotic lumbar spine - Principles of pedicle screw fixation and interbody fusion
» Screw Designs
Various screw designs have evolved over a period of time to address various issues and drawbacks that emerge while operating upon an osteoporotic vertebra, like the more frequent instances of screw pullout
The standard screw has a cylindrical core and a cylindrical thread [Figure 1]. Here, the core provides inner surface and the inner diameter, while the outer border of thread provides the outer diameter of the crew. The distance in between the two threads is called the 'pitch' [Figure 2]. In cortical screws, the pitch is smaller, and in cancellous screws, the pitch is wider. This area between the threads is vital in providing bone purchase, and hence, is an important deciding factor in determining the 'pull-out strength' of the screw. The pull-out strength is assessed by a constant axial force applied to dislodge the screw from a vertebra where it was secured earlier in a laboratory condition. The force utilized to dislodge the screw is called the 'pull-out force'. A good pedicle screw should have a good pull-out strength particularly in an osteoporotic bone. Deeper threads, a larger pitch, and longer screws with a bicortical purchase can improve the pull-out strength of a screw. However, these types of screws are not without limitations when being placed in an osteoporotic vertebral body where a breach of bone may occur during the insertion of the screw. In the dorsal and lumbar regions of the spine, a bicortical purchase of the vertebra is fraught with danger as there is the risk of injuring the ventrally placed large vessels. The length of the screw that is inserted within the body is not very important since the cancellous part of the vertebral body contributes less than 20% in improving the pull-out strength. The pedicle contributes around 60% of the pull-out strength, and in combination with the cortical bone that comprises of the proximal part of the vertebral body near the pedicle, contributes much more to the pull-out strength. Hence, the only way one can improve the pull-out strength, and in turn, achieve a strong fixation is to achieve a bicortical purchase; however, the limitation in achieving a bicortical purchase has already been discussed.
Figure 1: The standard screw has a cylindrical core and a cylindrical thread
Figure 2: The core provides inner surface and the inner diameter, while the outer border of thread provides the outer diameter of the crew. The distance in between the two threads is called the 'pitch'
Conical screws have a conical core with cylindrical threads, which means that the inner diameter of the screw is progressively larger at the proximal end [Figure 3]. Since the threads are cylindrical all along the screw, the area of bone purchase is larger and there will be constant compression of cancellous bone around the screw distally that improves the pull-out strength. Hence, this design is popular and relied upon. Moreover, the screw insertion in the case of a conical screw is easier compared to the standard one. Attempting further improvement in these screws by adding self-tapping ridges to avoid tapping as a separate step of the pedicle screw insertion procedure did not practically improve the pull-out strength of these screws [Figure 4]. Recently, double threaded conical screws have been introduced that aim to provide appropriate threads at the pedicle (containing the cortical bone) and the body (containing the cancellous bone) [Figure 5]. Inspite of these modifications in the vertebral screw technology, there are still less-than-satisfactory outcomes in the presence of osteoporosis due to the destruction of the bony architecture of the vertebral body. To obviate this and to enhance the screw purchase strength, different varieties of bone cement have been introduced at the distal end of screw through a cannulated screw design. These cannulated conical screws have pores at the distal end between threads through which a fast setting cement can be injected into the osteoporotic cancellous bone which can grip the screw and hold it in place, and hence, can increase the pull-out strength of the screws [Figure 6]. This system is gaining popularity in the recent times and is used both in the open and percutaneous forms of spinal fixation. PMMA (polyme