Patent Description:
The DRUJ is a synovial pivot-type joint between the distal ends of the two bones in the forearm -- the radius and the ulna. It is one of two joints between the radius and ulna, the other being the proximal radio ulnar joint ("PRUJ"). The DRUJ is the articulation between the crescent-shaped convex distal head of ulna and the concave "sigmoid notch" on the distal radius.

The function of the DRUJ and PRUJ is to rotate the forearm and distribute loads from the wrist across the radius and ulna. The joints constrain the forearm to motion in one degree of freedom, pronation-supination. In simple words, this is a rotatory movement by which the forearm and hand rotate around the long axis of the forearm. In pronation, the palm of the hand faces downwards, while in supination, it faces upwards. In supination, the forearm can rotate from <NUM> degrees neutral to approximately <NUM>-<NUM> degrees. In pronation of the forearm can rotate from <NUM> degrees neutral to approximately <NUM>-<NUM> degrees.

In some cases of DRUJ dysfunction, replacement of the joint with a prosthesis is necessary to address bone pathology, degenerative changes, or trauma. Total DRUJ replacement prostheses can be constrained or unconstrained. In a constrained prosthesis, both radial and ulnar components are included, and the components are positively linked. In an unconstrained prosthesis, although ulnar and radial components may be included, they are not linked and function in much the same way as a healthy anatomic joint. Unconstrained prostheses often include a total replacement for the head of the ulna bone. Optionally, if the sigmoid notch in the radius (also sometimes referenced as the "ulnar notch" or "sigmoid cavity" of the radius) is damaged or otherwise dysfunctional, an unconstrained prostheses may include a component that is implanted on the radius to replace the sigmoid notch and provide an articulating surface for articulation with the replacement ulnar head. Prostheses systems for replacing part of a joint are known in the art. For example <CIT> discloses a joint prosthesis system indicated for a different intended use, namely for use in a proximal radio ulnar joint, comprising some of the features of the present invention, but not an ulnar head component comprising an oblong body having a rear surface and two opposite side surfaces.

The instant invention provides a novel unconstrained DRUJ prosthesis including a <NUM>-piece replacement head system and, optionally, a cooperating sigmoid notch component. The three components of the replacement head system are a stem component adapted for implanting onto a prepared ulna, a head component adapted to pivotally engage the stem component, and a set screw to positively secure the head component to the stem component once the two components are in the desired pivotal alignment.

The replacement DRUJ prosthesis system can comprise several different sized stems components and head components providing options to a surgeon depending on the patient's anatomy and the desired interaction between the components.

It is an object of the present invention to provide a prosthesis for replacement of the distal radio ulnar joint to restore the pronation-supination motion of the forearm as well as stability between the ulna and radius.

It is another object of the present invention to provide a DRUJ prosthesis adapted for rotational alignment of the replacement joint along the axis of forearm rotation.

It is another object of the present invention to provide a DRUJ prosthesis which allows for the use of several different sized heads having differing geometries.

It is another object of the present invention to provide a DRUJ prosthesis adapted for variable pivoting alignment between the replacement distal ulnar head and the ulna.

It is another object of the present invention to provide a DRUJ prosthesis adapted for adjustment in the alignment between the articulating surface of the ulnar head and the sigmoid notch.

Disclosed is a DRUJ prosthesis system comprising an ulnar stem component, an ulnar head component, and a set screw.

The stem component comprises a substantially cylindrical elongated body having proximal and distal ends. The proximal end of the stem component comprises a medullary stem adapted for insertion into a prepared ulnar canal upon resection of the anatomical ulnar head. The distal end of the stem component comprises a spherical ball end attached to the stem component through a neck, the neck having a diameter substantially smaller than the diameter of the ball end. Interposed between the neck and the medullary stem is a flange which restraints the maximum penetration of the medullary stem into the ulnar canal.

The ulnar head component comprises an oblong body having a top surface, a bottom surface, an articulating surface, a rear surface, and two opposite side surfaces. The ulnar head component further comprises a ball cavity adapted to receive the stem component's ball end. The ball cavity is accessible through an entrance on either the rear or side surfaces having a diameter sufficiently wide to permit entry of the stem component's ball end. The ball cavity is also accessible through a bottom opening having a diameter sufficiently wide to accommodate the neck of the stem component, but narrower than the diameter of the stem component's ball end. The entrance and bottom opening are connected permitting insertion of the ball end of the stem component through the entrance and engagement of the stem component's neck with the bottom opening once the ball end is sufficiently inserted into the cavity.

The inner surface of the entrance of the ulnar head component is peripherally threaded. In some embodiments, the cavity is frusto-conical in shape, narrowing gradually between the entrance and the opposing wall of the cavity. In other embodiments, the cavity is cylindrical but lined with a plurality of splines that gradually increase in depth between the entrance and the opposing wall in the cavity. The frusto-conical cavity or splines are adapted for centering the stem component's ball end as it is inserted into the cavity. For maximum stability, the splines are sharp and adapted to partially penetrate the ball end of the stem component when sufficient pressure is applied to the ball end upon insertion into the cavity.

The articulating surface of the ulnar head component is cylindrically or spherically convex and, when projected onto a plane parallel to the top surface of the ulnar head, defines an arc having a center point. In some embodiments, the center point of the arc defined by the articulating surface is offset relative to a point projected from the longitudinal axis of the stem component which also traverses the center of the ball end of the stem component to the same parallel plane.

The set screw comprises a generally cylindrically shaped body having leading and trailing ends and a peripheral external thread between the two ends adapted to engage the thread on the inner surface of the entrance of the ulnar head component. The trailing end of the set screw includes a driving tool socket to permit the application of torque using a driving tool. The leading of the set screw optionally comprises a spike adapted to impinge on the ball end of the stem component when the set screw is torqued into the ulnar head/stem component assembly.

The DRUJ prosthesis system may optionally include a sigmoid notch component having an articulating surface and a bone-facing surface. The articulating surface is adapted to cooperate with the articulating surface of the ulnar head. The bone-facing surface is adapted to engage and be fastened to the sigmoid notch on the patient's radius. Engagement to the sigmoid notch is achieved by means of any method suitable, including, without limitation, screws, pegs, adhesives, and combinations thereof.

The disclosed DRUJ prosthesis system comprises a stem component, an ulnar head, and a set screw. In instances where the anatomical sigmoid notch has deteriorated or is otherwise ineffective, an optional sigmoid notch component can be included.

Referring to <FIG> and <FIG> shown is an embodiment of the present DRUJ prosthesis system adapted to completely replace the anatomical distal ulnar head and the sigmoid notch. <FIG> illustrates the system implanted with representations of the ulna and radius shown for reference. <FIG> shows the same embodiment with the bones completely removed for better visualization.

As is shown in <FIG> and <FIG> the DRUJ prosthesis system <NUM> comprises a stem component <NUM>, an ulnar head component <NUM>, a set screw <NUM>, and, optionally, a sigmoid notch component <NUM>. The stem component <NUM> is implanted on the ulna <NUM> after resection of the damaged or dysfunctional anatomical distal ulnar head (not shown). The ulnar head component <NUM> is pivotally attached to the stem component <NUM> and secured into position by the set screw <NUM> once proper alignment is achieved. The optional sigmoid notch component <NUM> is attached to the radius <NUM> in alignment with the ulnar head component <NUM>.

Shown in <FIG> is stem component <NUM> which comprises a substantially cylindrical elongated body <NUM> having a proximal end <NUM> and a distal end <NUM>. The elongated body <NUM> comprises a medullary stem adapted for insertion into a prepared ulnar canal upon resection of the anatomical ulnar head. The proximal end <NUM> may optionally include one or more straight or spiral (not shown) flutes <NUM> to facilitate penetration of the prepared ulnar canal. Elongated body <NUM> and proximal end <NUM> may be smooth or may be partially or completely textured to promote faster bone growth. Proximal end <NUM> may be rounded, flat or pointed.

The distal end <NUM> of the stem component <NUM> comprises a spherical ball end <NUM> attached to the stem component through a neck <NUM>. The neck <NUM> has a diameter that is substantially smaller than the maximum diameter of ball end <NUM>. Disposed between the neck <NUM> and the medullary stem is a flange <NUM> which acts as a stop to exceed the maximum penetration depth of the medullary stem into the ulnar canal.

Shown in <FIG>, <FIG>, <FIG>, <FIG> are various views of the ulnar head component <NUM> of the present invention.

The ulnar head component <NUM> comprises an oblong body having a top surface <NUM>, a bottom surface <NUM>, an articulating surface <NUM>, a rear side <NUM>, and two opposed side surfaces <NUM>. The ulnar head component <NUM> further comprises a ball cavity <NUM> sized and adapted to receive the stem component's ball end <NUM>. The ball cavity <NUM> is accessible through an entrance <NUM> on either the rear side <NUM> or either side surface <NUM> (the illustrated embodiment has a cavity entrance on rear side <NUM>) having a diameter sufficiently wide to permit entry of the stem component's ball end <NUM>.

The ball cavity is also accessible through a bottom opening <NUM> having a diameter sufficiently wide to accommodate neck <NUM> of the stem component <NUM>, but narrower than the diameter of the ball end <NUM> of stem component <NUM>. The entrance <NUM> and bottom opening <NUM> are connected permitting insertion of the ball end of the stem component <NUM> through the entrance <NUM> and engagement of the stem component's neck <NUM> with the bottom opening <NUM> once the ball end is sufficiently inserted into the cavity <NUM>. It should be noted that due to the narrow diameter of the bottom opening <NUM>, once the stem component <NUM> and ulnar head component <NUM> are assembled, the only way to disengage them is to slide ball end <NUM> back out through cavity entrance <NUM>. The inner surface of the entrance <NUM> of the ulnar head component includes a peripheral internal thread <NUM>.

The bottom opening <NUM> may optionally include a collar <NUM> to support stem neck <NUM> and to serve as an attachment point for an alignment tool (not shown) that can be used to properly align the ulnar head <NUM> and stem component <NUM>.

In some embodiments (not shown), cavity <NUM> has smooth walls and is cylindrical in shape, having a diameter that closely matches that of ball end <NUM>. In some embodiments (also not shown), cavity <NUM> has smooth walls and is frusto-conical in shape, narrowing gradually between the entrance <NUM> and the opposing wall <NUM> of the cavity <NUM>. In other embodiments (shown here in <FIG> and <FIG> but best observed in the cross sectional view shown in <FIG>), the cavity is cylindrical but lined with a plurality of splines <NUM> that gradually increase in depth between the entrance <NUM> and the opposing wall <NUM> of the cavity <NUM>. The frusto-conical cavity (not shown) or splines <NUM> are adapted to center the stem component's ball end <NUM> as it is inserted into the cavity <NUM>. For maximum stability, the splines <NUM> may have sharp edges and be adapted to penetrate the ball end <NUM> of the stem component <NUM> when sufficient pressure is applied to the ball end <NUM> upon insertion into the cavity <NUM>.

As shown in <FIG>, the articulating surface <NUM> of the ulnar head component <NUM> is cylindrically or spherically convex and, when projected onto an imaginary plane parallel to the top surface <NUM> of the ulnar head, defines an arc having a center point <NUM>. In some embodiments, the center point <NUM> of the arc defined by the articulating surface <NUM> is offset relative to a point projected onto the same imaginary plane by the longitudinal axis <NUM> of stem component <NUM> which also traverses the center of the ball end <NUM> of the stem component <NUM>. The length of this offset <NUM> can vary depending on the geometry of the ulnar head <NUM> selected. Offset <NUM> can range from zero, to the range normally found in the anatomy of a healthy ulnar head (<NUM> - <NUM>), or greater. A greater-than-anatomical offset could be used in situations where a greater tensioning force of the ligaments between the radius and ulna is desirable.

Referring next to <FIG>, set screw <NUM> comprises a generally cylindrically shaped body having a leading end <NUM> and a trailing end <NUM> and a peripheral external thread <NUM> between the two ends. Thread <NUM> is adapted to engage the thread <NUM> on the inner surface of the entrance <NUM> of the ulnar head component <NUM>. The trailing end <NUM> of set screw <NUM> includes a driving tool socket <NUM> to permit the application of torque using a matching driving tool (not shown). The leading end <NUM> of set screw <NUM> optionally comprises a spike <NUM> adapted to impinge on, and partially penetrate, the ball end <NUM> of the stem component <NUM> when set screw <NUM> is torqued into the entrance <NUM> and thread <NUM> of the ulnar head <NUM> once assembled with the stem component <NUM>.

The stem component <NUM>, ulnar head component <NUM>, and set screw <NUM> are preferably each of unitary construction. The material used for each of the components is any suitable bio-compatible metallic material having the required strength and durability to withstand the loads normally imposed on the DRUJ. As discussed above, in order to achieve maximum stability once the various components are implanted and properly aligned, the spike <NUM> of the set screw <NUM>, as well as the splines <NUM> of the ulnar head <NUM> are designed to impinge on, and partially penetrate the ball end <NUM> of the stem component <NUM>. To achieve this result, it is desirable to use a material for the stem component <NUM> that is slightly softer than that used in the ulnar head component <NUM> and the set screw <NUM>. An exemplary suitable combination of materials is cobalt-chromium for the ulnar head <NUM> and set screw <NUM> and titanium for the stem component <NUM>.

The DRUJ prosthesis system <NUM> may optionally include a sigmoid notch component <NUM> such as that illustrated in <FIG>. Sigmoid notch component <NUM> comprises articulating surface <NUM> and a bone-facing surface <NUM>. The articulating surface <NUM> is adapted to cooperate with the articulating surface <NUM> of the ulnar head <NUM>. The bone-facing surface <NUM> is adapted to engage and be fastened to the sigmoid notch on the patient's radius. Engagement to the sigmoid notch is achieved by means of any method suitable, including, without limitation, screws, pegs, adhesives, stems, 3D-printed meshes, and combinations thereof. To this end, the bone-facing surface may include one or more pegs or spikes <NUM> and one or more threaded holes <NUM> to accept a bone screw. Suitable materials for sigmoid notch component <NUM> include, without limitation, ultra-high-molecular-weight polyethylene (UHMWPE) including highly crosslinked varieties, polyetheretherketone (PEEK), and polyphenylsulfone (PPSU). In addition, bone-facing surface <NUM> may be manufactured from titanium or other osteointegrating metal.

In operation, the surgeon first gains clear access to the site of the ulnar head to be replaced and resects the ulnar head by performing a cut across the ulna below the ulnar head. Once resected, the ulnar head is measured to determine the appropriate size of ulnar head component <NUM> to be used as a prosthesis. Next, the ulnar canal is identified on the ulna and the opening to the ulnar canal is marked using an awl or similar tool. The ulnar canal is next reamed and expanded to a diameter and depth suitable for insertion of the medullary stem of stem component <NUM>. The cut surface of the ulna can then be prepared with a planer to ensure it is substantially flat and perpendicular to the prepared medullary canal.

The stem component <NUM> is next inserted into the prepared medullary canal until set, with flange <NUM> in flush contact with the cut surface of the ulna. With the stem component <NUM> in place, the ulnar head component <NUM> is next mated to it by introducing the ball end <NUM> of stem component <NUM> through entrance <NUM> of the ulnar head component <NUM>. The ball end <NUM> is inserted until neck <NUM> is seated in bottom opening <NUM>. Set screw <NUM> is next threaded into entrance <NUM> to prevent ball end <NUM> from exiting the ulnar head <NUM> but set screw <NUM> is not completely fastened. At this point, with set screw <NUM> not fastened, ulnar head <NUM> is free to pivot and swivel with respect to stem component <NUM>.

Next, an alignment tool (not shown) may be placed on collar <NUM> of the ulnar head <NUM> and manipulated as necessary to align ulnar head <NUM> with other anatomical features of the patient. For example, an alignment tool may be used to align the axis of rotation of the replacement ulnar head with the axis of rotation of the forearm. Once the proper alignment is reached, set screw <NUM> is fully torqued causing spike <NUM> and splines <NUM> to impinge and partially penetrate ball end <NUM>.

If it is determined that the anatomical sigmoid notch is damaged or ineffective, the sigmoid notch component <NUM> may be implanted by aligning the articulating surface <NUM> of the sigmoid notch component <NUM> with the articulating surface <NUM> of the ulnar head component <NUM> and affixing the sigmoid notch component <NUM> to the radius bone.

Claim 1:
A distal radioulnar joint prosthesis system comprising:
an ulnar stem component (<NUM>) comprising an elongated cylindrical body (<NUM>) having a proximal end (<NUM>) and a distal end (<NUM>), the proximal end (<NUM>) adapted for insertion into a ulnar medullary canal, the distal end (<NUM>) comprising a spherical ball end (<NUM>) attached to the ulnar stem body by means of a neck (<NUM>) having a diameter narrower than the spherical ball end (<NUM>), and a flange (<NUM>) disposed between the neck(<NUM>) and the ulnar stem body;
an ulnar head component (<NUM>) comprising an oblong body having a top surface (<NUM>), a bottom surface (<NUM>), an articulating surface (<NUM>), a rear surface (<NUM>), two opposite side surfaces (<NUM>),
an interior cavity (<NUM>) adapted to receive the ulnar stem component's ball end (<NUM>),
an entrance (<NUM>) to the cavity (<NUM>) having a diameter sufficient to accommodate the ulnar stem component's ball end (<NUM>),
internal threads disposed on the entrance, and
a bottom opening (<NUM>) to the cavity (<NUM>) having a diameter sufficiently wide to accommodate the neck (<NUM>) of the ulna stem component (<NUM>) but being narrower than the diameter of the ball end (<NUM>) of the ulna stem component (<NUM>), wherein the entrance to the cavity (<NUM>) is connected to the bottom opening (<NUM>); and
a substantially cylindrical set screw (<NUM>) having a trailing end (<NUM>) including a driving tool socket (<NUM>), a leading end (<NUM>) with a spike (<NUM>) adapted to partially penetrate, and fix the position, of the ball end (<NUM>) of the ulnar stem component (<NUM>), and a peripheral thread adapted to engage the internal threads disposed on the entrance (<NUM>) of the ulnar head component.