JP2535197B2 - Dynamic self-locking stem for hip prostheses - Google Patents

Description

The present invention relates to a stem for a joint prosthesis, in particular for a hip prosthesis, which stem comprises a proximal (near the center of the body) neck, lateral position. It has a tension part, a middle compression part, and a central part located between the side part and the middle part.

Loosening of the prosthesis remains a major and difficult factor in joint replacement, proving that the problem of matching load-bearing implants to living bone remains unsolved. The prerequisites for a reliable long-term anchorage of the joint prosthesis are: proper load transfer between the prosthesis and the bone, and a non-moving interface between the prosthesis and the bone.

Bone as a living tissue imposes these conditions that must be considered when designing a prosthetic device.

The stem anchoring of the femoral component to the hip joint replacement (prosthesis) may be due to the fact that the replacement has to date generally outperformed any other prosthesis component. The absence of bone cement for stem anchoring makes it even more uncertain and difficult to achieve the above requirements even in this rather simple rod-in-tube arrangement.

SUMMARY OF THE INVENTION The claimed invention solves the problem of a method of designing a dynamic self-locking stem for a prosthesis that allows the stiffness of the joint prosthesis to be matched to the stiffness of the surrounding bone tissue. is there.

The stem of the present invention includes: -the stem has a structure that facilitates load transfer from the prosthesis to the bone; -the stem has a mechanism that secures a non-moving interface with the bone; It is unique in that it is adaptable to the new mechanical environment and adapts the unavoidable bone response to that environment.

The advantages provided by the present invention are the result of design changes within the stem, as opposed to most changes in state of the prosthesis traditionally found on the stem. Each of the medial and lateral portions can be appropriately shaped and sized to accommodate its function, thereby allowing gradual load transfer to the bone, i.e., the medial and lateral portions. The lateral compliance can be well matched to that of the bone cortex. This provides a basis for structural design, while the segment between the lateral and medial portions is used to control interfacial stress. The joint between the mid / side and the segment consists of a very thin flexible bridge that acts as a hinge. The inclination of the segment with respect to the stem axis determines the amount of stem spread that results from the loading. With physiological loading, the lateral portion tends to move proximally (closer to the center of the body) as the middle portion moves distally (towards the distal end). Yes, and the segment deforms to increase the medial / lateral separation. Optimal design criteria require the stem to widen at all levels producing sufficient vertical interface stress to prevent any movement due to shear stress. This guarantees absolute dynamic stability at the bone-prosthesis interface.

The flexural rigidity of the stem depends on the amount of shear coupling provided by the bone to the medial and lateral sides of the stem. Assuming a stable bone-prosthesis interface, stem stiffness increases with increasing stiffness of the surrounding bone. Regulation of stem stiffness by bone stiffness plays an important role in reaching equilibrium for bone after stem insertion.

Any stem "stresses" some bone within the load transfer zone. In general, a higher stem stiffness / bone stiffness ratio provides greater stress protection.
This results in a decrease in bone stiffness and increases the stress protection effect. The result of this positive feedback process is overall stress protection of the affected bone. Regulation of stem stiffness by the bone avoids the above effects because the reduction in bone stiffness provided by stress protection also renders the stem flexible.

In addition to the unique features discussed above, the stems of the present invention provide the important practical advantage of immediate stability. At the time of insertion, the stem is inserted by striking the insertion force in the lateral-distal direction with the anvil. As a result, the stem reduces in width and elastically preloads the interface with bone.
This preload is eventually eliminated by bone remodeling and does not provide permanent stability.
This preload allows for bone ingrowth to the stem and thus improves conformability. A good fit between the stem and bone is necessary for the locking mechanism to work as described above.

The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this specification.

For a better understanding of the present invention, the operational advantages of the present invention and the specific objects achieved by utilizing the present invention, reference should be made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated and described.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a front and rear view of a stem according to the present invention, and FIG. 2 is an enlarged sectional view of a central segment of the stem according to FIG.

DESCRIPTION OF PREFERRED EMBODIMENTS FIG. 1 represents a stem according to the present invention. The lateral portion 2 of the stem is separated from the intermediate portion 3 by a number of full thickness cuts 5 that span the central portion 4 of the stem from the proximal portion 1 to the distal portion 6. Cut 5 is preferably
Made by EDM wire cutting.

FIG. 2 represents an enlarged central segment of the stem according to FIG. The tension transition side truss (bracket) 2 and the compression transition mid truss 3 are separated from the center 4 by a section of each said cut 5 running along each of the lines 8 and 9. A thin flexible hinge 7 connects the segment 10 to the trusses 2 and 3. The line connecting the hinges of one segment is inclined by an angle α with respect to the horizontal axis, said angle being between 20 ° and 40 °, preferably between 25 ° and 35 °. A cutting wire is inserted through the hole 11 and guided along the Z-shaped trajectory 5 for cutting.

Claims (9)

(57) [Claims] 1. A proximal neck portion (1), a lateral tension portion (2), a middle compression portion (3), and a portion between the lateral portion (2) and the middle portion (3). In a stem for a joint prosthesis having a central part (4), in particular a hip prosthesis, said central part (4) having a serpentine front and rear cut (5)
With a regular pattern of cuts each having two distinct ends, the pattern of cuts extending from the proximal portion (1) of the stem to the distal end (6), To accommodate the stiffness of the receiving bone cavity so that a series of flexible hinges (7) are formed along the boundaries (8), (9) of the lateral (2) and medial (3) sections. The stem (5) is located in the stem, and at the same time the central portion (4) of the stem allows movement of the medial segment (10) formed by the cut (5). 2. Stem according to claim 1, characterized in that the cut (5) is Z-shaped. 3. Hinges (7) of one segment (10)
The angle formed by the line connecting the stem and the transverse axis of the stem is varied to produce sufficient stem-bone compression along the stem to prevent movement at the stem-bone interface. The stem according to claim 2, characterized in that 4. The stem according to claim 3, wherein the angle is 20 ° to 40 °. 5. Stem according to claim 4, characterized in that the distance between the hinges (7) in the near-distal direction is between 6 and 10 mm. 6. The stem according to claim 4, wherein the angle is 25 ° to 35 °. 7. The stem according to claim 5, wherein the distance is 7 to 9 mm. 8. Hinges (7) of one segment (10)
The angle formed by the line connecting the stem and the transverse axis of the stem is varied to produce sufficient stem-bone compression along the stem to prevent movement at the stem-bone interface. The stem according to claim 1, characterized in that 9. A proximal neck portion (1), a lateral tension portion (2), a middle compression portion (3), and between the lateral portion (2) and the middle portion (3). In a stem for a joint prosthesis having a central portion (4), the central portion (4) is usually provided with a series of serpentine and narrow repetitive anterior and posterior cuts (5), each of which has two distinct ends. Having a series of cuts extending from the proximal portion (1) of the stem to the distal end (6), the cuts having a plurality of cuts between the proximal portion (1) and the distal end (6). The lateral segment (10) is located, and the cut is
Positioning a series of flexible hinges (7) connecting the opposite sides of the medial segment along the medial to medial boundaries (8), (9), To allow the stem stiffness to adapt to the stiffness of the receiving bone cavity, and further to allow the flexible hinge (7) to provide limited movement of the medial segment (10) formed by the cut (5). A stem for the joint prosthesis, characterized in that the cut is arranged.