RU194022U1 - Osteosynthesis screw - Google Patents

Abstract

The utility model relates to medicine. The screw for osteosynthesis is a cylindrical rod, in the lower part of which a thread is made with a lead-in turn and a head with a turn-key indentation in the form of a profile of a rounded triangle. The invention provides reliable fixation and simplification of the operation of osteosynthesis. 1 tablet, 3 ill.

Description

The utility model relates to medical equipment, traumatology and orthopedics, in particular to devices for fastening bone fragments in the treatment of bone fractures.

Known screw for osteosynthesis of the ulnar process, containing a body with a threaded part and a head, which is made in the form of a segment of the ball with a flat supporting surface and provided with a circular groove of variable depth, forming an eccentric (RU Patent No. 18630, IPC АВВ 17/58, publ. 10.07. 2001, Bulletin No. 19. S.B. Korolev, S.V. Turushkin, Screw for osteosynthesis of the ulnar process).

As a prototype, a screw for osteosynthesis made of a titanium alloy containing a cylindrical rod with a thread with a lead-in thread and a head with an internal hexagon (patent No. 26396 RU, IPC АВВ 17/00, publ. 10.12.2002, bull. No. 34. I.K. Maslov, Yu.P. Zhizhin, A.I. Tereshchenko, A.S. Ovchinnikov, A.D. Shavelkin, O. B. Saigushev, Screw for osteosynthesis).

However, the disadvantage of these screws is the possibility of screw breakage due to high stresses arising in its head, which can lead to damage to soft tissues during surgery.

The objective of the proposed utility model is to reduce the stresses arising in the screw head for osteosynthesis and, as a result, to increase reliability due to the use of the developed special profile of a turn-key recess in the screw head.

The problem is solved due to the fact that in a screw containing a body with a threaded part and a head, a turn-key recess in the screw head is made in the form of a profile of a rounded triangle, which is a geometric figure of constant width, which means that in all sections its width will be the same .

In FIG. 1 shows a general view of a screw for osteosynthesis, and FIG. 2 is a view of a special turnkey hole profile. And in FIG. 3 - diagrams of the operation of the key-hole pair for profiles of the type “hexagon” (a), “square” (b), “triangle” (c) and “rounded triangle” (d).

The screw for osteosynthesis made of a titanium alloy contains a cylindrical rod 1, in the lower part of which there is a thread 2 with a lead-in turn 3 and a head 4 with a turn-key recess in the form of a special profile 5. The profile has the following calculated parameters:

D is the diameter of the inscribed circle of the profile (main parameter), mm;

R is the large radius of the profile, mm; R = 3D;

r is the small radius of the profile, mm; r = 0.355D;

H is the distance of the center of the large radius of the profile from the main center, mm; H = 2.5 D;

h is the distance of the center of the small radius of the profile from the main center, mm; h = 0.27D.

The screw for osteosynthesis is set as follows: access to the fragments and compare them. Previously, in the connected fragments, a hole is made for screws with a diameter equal to or close to the diameter of the cylindrical rod 1. For small screw diameters for osteosynthesis, it is advisable to cut a preliminary thread with a tap. After that, a screw for osteosynthesis is screwed into the hole obtained with its threaded part 2. If the thread is not pre-cut, then, to facilitate screw entry into the hole and high-quality connection of the screw with bone tissue, a lead-in turn 3 of smaller diameter is performed on the threaded part 2 of the screw. For less tightening of the screw in the head 4 of the screw, a turn-key recess is made in the form of a rounded triangle of profile 5, where a key is installed, during rotation of which the screw is installed in the bone tissue.

There is a technological gap between the key and the key-hole recess of the screw head, which depends on the nominal size of the screw and is determined by the size tolerance, on average it is in the range of 0.005-0.03 mm. Because of this, the key has a free path and for the profile forms of a turnkey recess with sharp edges, the contact area between the key and the recess at the moment of applying the force necessary to screw in the screw is very small (Fig. 3a, 3b, 3c). Therefore, at the moment of application of force, the corners of the key are crushed, as a result of which the key becomes inoperative, or these sharp corners on the key crush the profile of the recess in the screw head and the screw becomes unsuitable for further use.

The profiles of the recesses that do not have sharp angles practically do not collapse due to the fact that they have a larger contact area (Fig. 3d).

The stress-strain state of the screw head was simulated using the SolidWorks software, the stress criterion in this program is estimated according to Mises. Since the most commonly used screws for osteosynthesis with a nominal thread diameter of 3.5 mm, it was under these parameters that the heads were modeled in accordance with GOST R 50582-93 (ISO 5835-91). The choice of material was determined taking into account the requirements for metallic materials intended for use in the production of surgical implants according to GOST R ISO 5832-2-2014. Therefore, chemically pure titanium Ti was used as material for modeling the stress-strain state of screw heads for osteosynthesis.

The table shows the maximum values of stresses obtained as a result of three-dimensional modeling of the process of screwing screws with different profiles of turnkey indentations.

Table - Maximum stress values using various standard profiles

Profile form Maximum stress, σ MPa Triangular 42732 Square 35039 Hexagon 27418 Rounded triangle 21351

From the results obtained, it can be seen that with equal force of screw tightening, the smallest stresses occur in the proposed profile in the form of a rounded triangle. This means that under the same operating conditions, a screw with a profile of a turn-key hole in the form of a rounded triangle will provide a more reliable fixation than screws with a triangular, square and hexagonal profiles of key-hole recesses.

The practical implementation of the hexagonal and triangular profiles shows unstable results when installing screws for osteosynthesis, that is, during their screwing and operation, there is a risk of breaking the screw head for osteosynthesis as a result of crushing. At the same time, in the manufacture of turnkey hexagonal and triangular profiles of the recess, certain difficulties arise due to the fact that the existing screw manufacturing process involves obtaining a turnkey profile using stamping operations, due to which the profile angles are rounded. At the same time, the corresponding keys do not have such rounding, as a result of which this difference in the shape of the key and the profile of the turn-key recess leads to an uneven distribution of stresses in the screw head.

The use of turn-key deepening profiles without sharp corners makes it easier to manufacture screws and improve their quality. The larger contact area between the key and the profile, in comparison with the hexagonal and triangular profiles, provides an even distribution of stresses inside the screw head for osteosynthesis, thereby reducing the number of breakdowns.

Given the obtained voltage values, the contact area between the key and the profile of the turn-key recess, as well as the manufacturability of using the profile in the form of a rounded triangle, will increase the reliability and accuracy of the manufacture of screws for osteosynthesis.

Claims (1)

A screw for osteosynthesis made of a titanium alloy and containing a head, a cylindrical rod, in the lower part of which there is a thread with a lead-in turn, characterized in that the profile of the keyed recess in the screw head is a rounded triangle, which is a figure of constant width.

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