BONE FIXATION SCREWS

Abstract: A bone fixation screw having a shaft 10 with a head 11 at one end and a screw thread 12 at the other end is also provided with a screw thread 15 on the shaft under and adjacent to the head. In use the screw thread 15 will engage with the hard cortex bone to provide a firmer fixation. The head 11 may have in known manner a screw-threaded projection 13 for receiving a nut 14 which can tighten up a fixation plate against the head.

Description: TITLE: BONE FIXATION SCREWS DESCR I Pr ION This invention is concerned with bone fixation screws, and particularly but not exclusively with screws for fixing appliances to the bones of children. Children's bones are usually much softer than those of adults and surgeons, when attempting to tighten a screw into cancellous bone, may be faced with the difficulty that the thread of the screw fails to grip the soft cancellous bone so that the screw will not tighten and an unsatisfactory fixation results. It is an object of the present invention to provide means for overcoming or at least reducing this difficulty. According to the invention, a bone fixation screw having a shaft with a head at one end and a screw thread at the other end is also provided with a screw thread on the shaft under and adjacent to the head, Thereby, the second-said screw thread will in use engage with the hard cortex bone, enabling the surgeon to tighten the second-said screw thread into the hard bone and obtain a firmer fixation. Preferably the second-said screw thread is immediately under the head. The screw threaded parts of the shaft are preferably separated by an unthreaded part, but may abut each other. Preferably, the diameter of the screw thread under the head is greater than that of the first-said screw thread. The screw is preferably cannulated, i.e. it has an axial through hole. Apart from the screw thread under the head, the screw may be of known kind having a head shaped like a nut for engagement and turning by a spanner and a projection from the head on the opposite side from the shaft, which projection is screw-threaded for carrying a nut. The nut is for use in tightening up against the head a fixation plate carried by the projection. The invention further provides a screw of the aforementioned kind having a screw thread under and adjacent to the head in combination with a perforated fixation plate adapted to be received on the projection. Alternatively, apart from the screw thread under the head, the screw may be of another known kind having a head without an aforesaid projection. The head in such screws in usually slotted so that the screw can be turned by a screw-driver or like tool. Such screws may be used for fixing together the parts of a fractured bone and the screw thread at the opposite end of the shaft from the head is usually adjoined by a substantially longer unthreaded part. Preferably, when the present invention is applied in such a screw, the screw thread under the head extends over only a minor part of the length of the shaft and a substantial part of the length of the shaft is unthreaded. While, as aforementioned, the invention is particularly useful for screws for children S surgery because of the softness of the cancellous bone in children, it is of course applicable to screws for general bone surgery. The following is a description, by way of example, of bone fixation screws in accordance with the invention, with reference to the accompanying drawings, in which Fig. 1 is a side elevation of a conventional bone fixation screw, Fig. 2 is a side elevation of a bone fixation screw in accordance with the invention, and Fig. 3 is a sect--iÏnal view through part of a bone showing the screw of Fig. 2 in use. The screws shown in the drawings are in the form of lag screws and each comprises a shaft 10 with a nut like head 11 at one end and a screw thread 12 at the other end. On the opposite side to the shaft, the head has a screw-threaded projection 13 carrying a nut 14. The screw has an axial through hole for passage over a guide wire when being inserted. The conventional screw shown in Fig 1 has an unthreaded part extending from under the head 11 to the screw thread 12. In the screw according to this invention shown in Fig. 2, a screw thread 15 is provided on the shaft 10 under and immediately adjacent to the head 11. The screw thread 15 is of greater diameter than the screw thread 12. As can be seen from Fig. 3, in use the screw thread 12 engages in soft cancellous bone 16 while the screw thread 15 engages in hard cortex bone. After the screw has been inserted into the bone, a fixation plate 18 (of which only part is shown in Fig. 3) is passed over the projection 13 with the nut 14 removed, the projection passing through a hole in the plate. The nut 14 is then screwed on to projection 13 so as to clamp the plate 18 against the head 11. The other part of the plate is of course perforated and secured by screws in known manner to another part of the bone. In another embodiment in accordance with the invention, the projection 13 (with nut 14) is omitted and the head is round and slotted so that it can be turned by a screw-driver or like tool. The major part of the length of the shaft 10 between the screw threads 12 and 15 is unthreaded. In both embodiments, the screw thread 15 extends over only a short length of the shaft.

Claims: CLAIMS: 1. A bone fixation screw having a shaft provided with a head at one end and a screw thread at the other end, which screw is also provided with a screw thread on the shaft under and adjacent to the head. 2. A bone fixation screw according to claim 1 wherein the second-said screw thread is immediately under the head. 3. A bone fixation screw according to claim 1 or claim 2 wherein the shaft has an unthreaded part separating said screw threads. 4. A bone fixation screw according to any preceding claim wherein the diameter of the screw thread under the head is greater than that of the first-said screw thread. 5. A bone fixation screw according to any preceding claim which is cannulated. 6. A bone fixation screw according to any preceding claim having a head shaped like at nut and a projection from the bead on the opposite side from the shaft, which projection is screw-threaded for carrying a nut. 7. A bone fixation screw according to claim 6 in combination with a perforated fixation plate adapted to be received on the projection. 8. A bone fixation screw according to any of claims 1 to 5 wherein the head provides one extremity of the screw. 9. A bone fixation screw according to chim 8 wherein the head is slotted. 10. A bone fixation screw according to claim 8 or claim 9 wherein the screw thread at the opposite end of the shaft from the head is adjoined by a substantially longer unthreaded part of the shaft. 11. A bone fixation screw according to any of claims 8 to 10 wherein the screw thread under the head extends over only a minor part of the length of the shaft and a substantial part of the length of the shaft is unthreaded.

Inventors: HOWSE, DEREK LESLIE
Applicants: D. HOWSE & COMPANY LIMITED
Application year: 1982
Main classification: A61B17
IPC codes: A61B17, F16B5