"INTERLOCKED DOUBLE SACRUM SCREW SYSTEM"

This invention relates to "Interlocked Double Sacrum Screw System", which is applied to the sacrum vertebra in posterior spinal stabilization operations.

This invention consists of a system which includes a 1 ^st screw to be applied to the sacrum vertebra following the necessary surgical exposure, and a 2 ^nd screw which passes inside the 1 ^st screw's positioning channel, directs to lateral (to the wing side of sacrum), and is interlocked to the 1 ^st screw from inside.

Pedicle screw is the strongest spinal implant in terms of bone anchoring. When pedicle screws are combined with rods, they make possible to apply large forces to the spine, correct deformities, and achieve a strong fixation. Today, transpedicular stabilization is the most commonly used and standard fixation method in the lumbosacral region.

Transpedicular stabilization is used very frequently in spine fractures, as well as degenerative instability cases.

Pull out and breakage of the pedicle screws results in failure of the stabilization system. In that case, as the stabilization is lost, original complaints of the patient (pain and numbness of the low back and leg, inability to walk, foot weakness) returns. In addition, the patient may feel pain at the implantation area due to loose implant. Loosening or breakage of the pedicle screws is not infrequent, and most commonly seen in the first sacrum (Sl) vertebra. There are various reasons why pedicle screws suffer a high rate of failure at the Sl level. Primarily, the Sl screw bears more load than all the other screws. Because Sl vertebra is the point where the lumbar spine and the sacrum intersect. When a lumbosacral stabilization is performed, all pull-out force from upper screws is transmitted to the sacrum, which moves as a single-piece unit, at the Sl level. Therefore, the Sl screw should be very strong. On the other hand, Sl pedicle screw is weaker than lumbar pedicle screws because of the anatomy of Sl vertebra. The first anatomic reason is, the anterior-posterior diameter of the Sl body is shorter than lumbar vertebrae. Therefore, comparing to the lumbar pedicle screws, a shorter screw can be placed. Secondly, in order to place a longer Sl screw, it is required to direct the screw medially (to the midline). However, that is difficult because of the regional anatomy. The reason for that medialization difficulty is the existence of the iliac bone. Iliac bone prevents screw head from being tilted laterally and causes the screw to be placed straight. To be able to place the screw at the Sl level in a medialized fashion, not straight; a longer skin incision, a larger soft tissue retraction laterally, and even resection of the some bone from ilium may be necessary. AU these factors explain why it is technically difficult and there is increased surgical trauma for the patient when classical pedicle screws are applied at the Sl level, In order to increase the strength of Sl screws, it is recommended to penetrate the cortical bone at the anterior surface of sacrum and advance the screw a few threads (=bicortical application). However, this method is risky. Particularly, in the cases where screws are not sufficiently medialized, iliac arteries and veins located here may be injured when screw penetrates anterior (=front) surface of the sacrum. Such a situation may result in the death of the patient by sudden and massive bleeding, or chronic erosion of the blood vessel wall and subsequent late bleeding or development of an aneurysm.

For all those reasons mentioned, Sl screw is the weakest point of lumbosacral stabilization, and this has led spine surgeons to seek various solutions. These are:

1. Using an additional stabilization point in order to support Sl screw (another screw for Sl pedicle, S2 screw, hook for Sl or S2 foramina, iliac bone screw).

2. Evacuating the L5-S1 disc interspace and placing here a supporting implant such as cage (^interbody fusion).

3. Abandoning the Sl fixation and posterior transpedicular stabilization completely and performing the L5-S1 fixation through other methods. An example for that is AXIALIF technique, which is performed by entering the pelvic cavity posteriorly, and entering the disc interspace by penetrating the anterior surface of the sacrum.

All such techniques have their specific difficulties and disadvantages. To support Sl screw with another screw, it is necessary to expand the surgical field. Preparation which is required for a new screw (preparation of entry point, fluoroscopy) is performed, the screw is placed and the rod is extended here and then fastened to the new screw. The risk of placing such an additional screw is taken, operation time prolongs, and the technical problems are experienced. Because many spine surgeons have never seen or applied such techniques, they do not have enough experience about them. There is no ideal place to insert such a second screw supporting Sl screw: It is not always possible to find another place for a second screw entering Sl pedicle; S2 screw is quite weak screw; iliac screw is strong, however technically it is difficult, and as stabilization system bridges sacroiliac joint, it creates problems related to this joint in the long term. In summary, supporting the S 1 screw through a second screw placed to another region is not an ideal solution. In the literature, the most similar solution to the logic of our invention is the combining two separate screws, one of which extending to the body of sacrum and the other one to the wing of sacrum, using a small plate (blocker). Thus, even if strength of two screws are obtained, such a solution has not been commonly used, because it is difficult to find the entry points for two separate screws at the S 1 level.

In order to reduce the load on Sl screw, emptying disc space and placing a cage type supporting implant with the purpose of removing L5-S1 disc space movement, namely, interbody fusion can be applied. This process considerably decreases pull-out risk of S 1 screw. However, some risks of interbody fusion do exist. First of all, whether it is made from the region opened (posterior) or front (from abdominal region, transperitoneal or retroperitoneal), it is a separate operation and lengthen the total operation time considerably. Performing the operation from the back or front has their specific problems. Posterior (back) approach requires removal of the facet joints, retraction of the nerves and dural sac, and trying to place the implant (called cage) from here, and this may create the risk of nerve damage and dural tear. Anterior (front) approach is a completely a different operation, and holds the risk of damaging retroperitoneal tissues, vessels and nerves. In men, interbody fusion attempts via anterior approach may cause sexual dysfunction called retrograde ejaculation. In summary, when the L5-S1 interbody fusion is performed due to weakness of Sl screw, the patient takes the risks of a completely different second operation. Similar situation is true for AXIALIF technique, in which L5-S1 interbody fusion is made by entering pelvic cavity posteriorly. In this technique, patient undergoes another operation from another region, specific risks of the operation are taken, it is an technology-driven and extremely expensive operation, and is not commonly applied.

In spine surgery, there is a need for an implant to be used for lumbosacral stabilization which would overcome the difficulty of S 1 screw medialization, increase pull-out resistance of this screw, achieve those tasks without requiring additional exposure or another operation, and be easily applicable. Interlocked Double Sacrum Screw System has been designed to cope with those needs. This invention consists of a system in which there is a 1st screw which is applied to the sacrum vertebra after standard exposure for lumbosacral stabilization, and a 2nd screw which passes through positioning channel situated on the neck of 1st screw, extends to the lateral, namely, to the wing side of sacrum, and is locked to the 1 st screw. Because this screw system is compatible with standard rods, they can be used as a part of standard pedicular screw fixation. The advantages of those interlocked two screws over standard pedicle screws are as follows: a) Compared to a single screw, two screws provide considerably higher resistance against pull- out.

b) An additional entry point for the bone is not needed; entry point of single screw is used.

c) Placing the 1 st screw is easier as it does not require medialization; it is not necessary to extend skin incision, increase muscle retraction or remove from the iliac bone. d) As the positioning housing within the 1st screw functions as a guide for the 2nd screw, 2nd screw can be placed securely and without any effort.

The invention has been presented in the pictures attached,

Figure 1 shows the screw system applied within the sacral bone,

Figure 2 shows the final view of the two screws which are interlocked to each other,

Figure 3 shows detailed view of the 1st screw is illustrated. The part numbers used and their equivalents are as follows;

1) 1 ^st screw

2) 1 ^st screw's head

3) Rod channel

4) Intra-channel threads

5) Rod

6) Nut

7) Screw neck

8) 2 ^nd screw positioning channel

9) Recess for the 2 ^nd screw's head

10) Lock hole

11) Screw lock

12) Screw body

13) Screw thread

14) Screw tip

15) 2 ^nd screw

16) 2 ^nd screw's head

17) 2 ^nd screw's neck

18) 2 ^nd screw's body

19) 2 ^nd screw's tip

20) Body of the sacrum

21) Wing of the scrum

22) Iliac bone

The application of Interlocked Double Sacrum Screw is quite simple. After standard exposure, the 1st screw (1) is placed to the first sacral vertebra (20). Body (12) of the 1 ^st screw (1), its threads (13), and its tip (14) are not different from standard monoaxial screws. However, the placement method is different. During application, contrary to the standard methods, it is not necessary to direct the 1st screw (1) to the medial as much as possible. A little medialization is enough, and even 1st screw can be placed straight. Thus, friction of the 1st screw head (2) to the iliac bone (22) or the necessity of bone removal is not necessary, unlike standard application. The only point to be considered during application of the 1st screw (1) is that the recess (9) for the 2 ^nd screw head should be at the medial, not to the lateral. After the 1st screw (1) is applied, second screw (15) is applied as follows: 2 ^nd screw (15) passes through 2 ^nd screw positioning channel (8) on the screw neck (7) of the first screw (1) which is a thicker neck than that of standard pedicle screws, and the head of this 2 ^nd screw (16) is seated in the recess (9) for the 2 ^nd screw head at the beginning part of 2 ⁿ screw positioning channel (8). There is no thread in the 2 ^nd screw positioning channel (8) and 2 ^nd screw's neck region (17), and these regions fit each other fully. Threaded body part of the 2 ^nd screw (18) and the tip (19) are not different from standard bone screws. However, both 2 ^nd screw positioning channel (8) on the 1 ^st screw (1) and the 2 ^nd screw

(15) can have variable geometries. The angle of 2 ^nd screw positioning channel (8) on the 1 ^st screw neck (7) directs the 2 ^nd screw (15) from the medial to the lateral, and sends it exactly to the wing of the sacrum (21). Therefore, for the 2 ^nd screw (15), the surgeon need not to make a special preparation or take a fluoroscopy. After the 2 ^nd screw (15) is completely advanced and its head

(16) is seated in the recess (9) for the 2 ^nd screw head on the 1 ^st screw (1); the screw lock (11) within the threaded lock hole (10) in the 1 ^st screw head (2) is tightened. Thus, 2 ^nd screw (15) is secured to the 1 ^st screw (1). The following stages of the operation are the same as standard pedicle screw fixation. The rod (5) is positioned into the rod channel (3) on the 1 ^st screw head (2), and the system is completed by tightening the nut (6) which fits into the intra-channel threads (4) within the rod channel (3).