Field of the Invention
This invention relates generally to a dental post used to anchor a crown and, more particularly, to a dental post including a quadrangle split shank for increased compliance and root stress reduction, a thread-free shank tip option to eliminate the possibility of root fracture, a radiused transition from a primary thread to a secondary thread for stress reduction, and an axially grooved post head for increased crown retention.
Description of the Related Art
A dental crown is a tooth-shaped cap that is placed over a tooth to cover the tooth to restore its shape and size, strength, and improve its appearance. The crown, when cemented into place on a post, fully encases the entire visible portion of a tooth that lies at and above the gum line. The crown is mounted to a post which extends deep down into the root canal of the tooth in order to provide strength and stability.
A crown post must provide a strong mounting to the tooth and root—including the ability to withstand lateral forces and bending moments, and resistance to pull-out. Post designs are known in the art which thread down into the root canal to provide the grip and mounting strength required for a solid crown mount. However, these threaded designs may put excessive stress on the tooth and root, and these stresses can cause a root to fracture—resulting in more extensive dental repair work. Other prior art posts do not thread into the root, but rather are simply cemented in place. These thread-free designs minimize the risk of root fracture, but suffer from very limited pull-out strength due to the absence of any mechanical grip.
A crown is needed which provides high pull-out resistance while minimizing the risk of root fracture, and incorporates other features for increased post strength and increased post-to-crown retention.
In accordance with the teachings of the present invention, a post for a dental crown is disclosed having improved structural features—including increased post strength and reduced likelihood of root fracture. In one embodiment, the post includes a quadrangle (4-way) split shank for increased compliance and root stress reduction while providing very high mechanical grip strength and pull-out resistance. In another embodiment, a shortened threaded section and a thread-free shank tip minimize the possibility of root fracture while still providing high grip strength and pull-out resistance. Embodiments also include a radiused transition from a primary thread to a secondary thread portion of the shank for stress reduction, and an axially grooved post head for increased crown retention strength.
Additional features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.
The following discussion of the embodiments of the invention directed to a dental crown post with improved structural features is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses.
A dental crown is a tooth-shaped cap that is placed over a tooth to cover the tooth to restore its shape and size, strength, and improve its appearance. The crown is mounted to a post which extends deep down into the root canal of the tooth in order to provide strength and stability. The crown post must be securely mounted to the tooth and root—providing the ability to withstand lateral forces and bending moments, and resistance to pull-out.
The post 10 includes a primary thread 18 which runs along the length of the shank 14, and a secondary thread 20—typically only a single turn—located near the gum line 16. The primary thread 18 cuts into the tooth and root as the post 10 is threaded into position, while the secondary thread 20 threads into the tooth. The primary thread 18 and the secondary thread 20 provide a secure attachment of the post 10 to the tooth and root due to the significant mechanical grip of the threads. However, the threaded design of the post 10 may put excessive stress on the tooth and root, and these stresses can cause a root to fracture—resulting in more extensive dental repair work. The shank 14 of the post 10 includes a slot 22—running axially through the shank 14—intended to reduce stress in the tooth and root by allowing compliance of the shank 14. However, in actual usage, the shank 14 of the post 10 offers only limited compliance and causes root fracture in a significant number of cases. This will be discussed further below.
The post 10 includes a transition 24 from the shank to a middle section containing the secondary thread 20. The transition 24 is typically a step function change of diameter which acts as a stress concentration and has been known to cause post breakage. The post 10 also includes a vent groove 26 which is intended to allow cement to extrude up out of the root hole during installation of the post 10. In practice however, the vent groove 26 also causes a stress concentration in the middle section of the post 10, where torque is highest during installation, resulting in even more post failures.
Finally, the post 10 includes rings 28 along the length of the head 12, and drive slots 30 in the top of the head 12. The rings 28 provide mechanical retention of the crown to the post 10, while the drive slots 30 are designed to receive a screwdriver-like tool which is used to install the post 10 in the patient. The post 10, having threads which engage the tooth and root, is sometimes known as an active post design.
Other prior art posts do not thread into the root, but rather have an unthreaded shank portion which is simply cemented in place. These thread-free designs—sometimes known as passive post designs—minimize the risk of root fracture, but suffer from very limited pull-out strength due to the absence of any mechanical grip. A new type of crown post is needed which overcomes the limitations of the post 10 and other known post designs.
The post 40 also includes a secondary thread 52 at the top of the shank 42 and just below the head 44, and a radiused transition 54. The radiused transition 54 eliminates the step-function change in diameter of the post 40, thereby reducing the stress concentration and resulting in a stronger post. The primary thread 46 and the secondary thread 52 are preferably orthopedic threads designed specifically for efficiently cutting into organic structures such as bone, tooth or root.
The post 40 further includes a flange 56 at the base of the head 44. The flange 56 is substantially wider (greater diameter) than the rest of the post 40, thereby providing better contact with the base of the tooth into which it is implanted, resulting in greater strength and stability of the post 40. The wide flange 56 is a feature not seen on prior art posts such as the post 10. Above the flange 56, continuing toward the top of the post 40, is a series of alternating circumferential rings 58 and grooves 60. The rings 58 and grooves 60 provide the head 44 with a shape which allows for a solid mechanical grip of the crown when cemented onto the post 40. Proceeding from the flange 56 toward the top of the head 44, each of the rings 58 has a slightly smaller diameter, giving the head 44 a tapered “Christmas tree” shape. The tapered shape of the head 44 on the post 40 provides dentists with a post option including a wide base which is suitable for some situations. Other embodiments—discussed below—include a head shape which is not tapered.
Also visible in
In order to avoid root fracture and its undesirable consequences, the post 40 is designed to offer compliance in all radial directions during installation. The 360° radial compliance is due to the slots 48 and 50, which not only offer compliance in the directions perpendicular to the slots 48 and 50 (up/down/left/right directions in
The post 40 discussed above offers a number of structural features which reduce the likelihood of root fracture due to post installation, and also increase the strength of the post itself. Other post features—illustrated and described below—may be used in combination with or replacement of certain features of the post 40, thus providing dentists with the flexibility to best meet the needs of any individual patient.
Studies have shown that the top three threads of a threaded fastener provide almost all of the axial pull-out resistance. The post 70 therefore includes three full turns of the primary thread 78, which along with a secondary thread 80, anchor the post 70 securely. In one embodiment, the shank 72 is approximately equally divided between the unthreaded tip 76 and the section containing the primary thread 78.
The head 74 of the post 70 is of the same design as the head 44 of the post 40, with a flange and a tapered “Christmas tree” shape of rings. The head 74 includes a slot 82—visible in
The post 90 also includes a region 102—just below the head 94 and above the secondary thread—where material properties may be locally improved through shot peening, cold forging or other processes. The region 102 is an area known to be susceptible to failure in prior art post designs. Localized material treatments—particularly in metal materials—can both increase material strength and reduce surface imperfections to minimize post failure in this area.
Although the post 90 of
Looking back at the posts 40/70/90/110 of the preceding figures, it can be seen that there are two basic head styles—straight-sided and tapered, and two basic shank styles—4-way split shank and hybrid shank, in the disclosed embodiments. As shown in the figures, either style of head may be paired with either style of shank, resulting in four different post designs. The particular post design used for an individual patient can be determined based on the patient's circumstances and the expertise of the dentist.
All of the embodiments of the posts 40/70/90/110 described above include the radiused transition from the primary thread to the secondary thread region for stress reduction and improved post strength. All of the embodiments may also include localized shot peening or other treatments, if appropriate, to improve material properties in certain regions of the post. All of the embodiments also include the notches in the rings of the head, for crown rotation prevention. Furthermore, either the slotted drive head or the internal drive socket head may be implemented in any of the posts 40/70/90/110.
The crown posts 40/70/90/110 may be made of any material with sufficient strength properties and which is suitable for implanting in the human body. For example, the crown posts may be made of a stainless steel, such as SAE 316, or of a titanium alloy. The posts may also be made of a carbon fiber composite material. In addition, the posts may be made of polyethyl ethyl ketone (PEEK), which is a semicrystalline thermoplastic with excellent mechanical and chemical resistance properties. Other materials, not listed here, may also be used for the posts, as long as the material possesses suitable physical properties and can be fabricated in the desired shape.
The primary threads 134 on the post 130 are also different in that they have a buttress thread shape.
The post 130 includes a flange 140 similar to those shown on the posts 40 and 70. However, the flange 140 has a different shape than discussed on earlier embodiments. The flange 140 includes a radiused leading edge 142 which serves to reduce radial stress at the location where the flange 140 bears against the tooth/root of the patient. The radiused leading edge 142 along with the flange outer diameter allow the flange 140 to be countersunk in the tooth/root of the patient for further increased stability of the post 130.
Additionally, the post 130 includes a secondary thread 144, which acts as a locking thread to prevent the post 130 from backing out of the root cavity over time. The secondary thread 144 is effective as a locking thread due to its larger diameter than the primary threads 134, and also due to the fact that the secondary thread 144 only penetrates a limited distance into the tooth/root (typically less than two full turns) so that the fit of the secondary thread 144 into the tooth/root remains very tight.
At the right of
The post 130 also includes a head or coronal end 150 which features a different concept for preventing rotation in the crown-to-post attachment.
The post 130 is also shown with a hex drive socket 156, visible in
The dental crown post designs described above include numerous features which are advantageous over prior art designs—including features which minimize the likelihood of root fracture during post installation, features which increase the strength of the post itself, and features which increase the pull-out strength of the post in the root and the strength of the crown-to-post attachment. The structural features of the different shank designs and different head designs can be combined into a variety of posts which give dentists a range of options for successfully and securely installing posts and crowns which meets the needs of their patients.
The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.
This application claims the benefit of the priority date of U.S. Provisional Patent Application Ser. No. 62/257,576, titled, DENTAL POST DESIGNED FOR INCREASED STRENGTH AND REDUCED ROOT DAMAGE, filed Nov. 19, 2015.
Number | Date | Country | |
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62257576 | Nov 2015 | US |