Patent Application
20080221697
1. Field of the Invention
The present invention relates generally to prosthetic medical implant devices, especially those associated with surgical joint replacement. The present invention relates more specifically a partial joint replacement implant for the bones of the human foot, especially the first metatarsophalangeal joints.
2. Description of the Related Art
Prosthetic implant devices have been used for some time to fully or partially replace existing skeletal joints in humans. Among the many joints associated with the bones of the human foot, one that is known to cause frequent problems is the metatarsophalangeal joint between the first metatarsal and the first phalanx in what is commonly known as the hallux or great toe. A number of efforts have been made in the past to partially or fully replace this joint. Some efforts have focused on the partial replacement of the joint using silicone based materials to construct a prosthetic device that is attached to the phalanx (where, for example, the phalanx cartilage has degenerated) and which operates against the metatarsal head which may still remain intact. Silicone material, however, has generally been found to be too soft for the purpose of maintaining an appropriate joint surface and will eventually break down into particles that can have damaging effects on the human body.
Other efforts have been made in the past to fully replace the joint with a unitary flexible silicone implant in such a manner that does not result in an abrasive motion of the remaining bone against the silicone material (as in the case of the silicone hemi-implant). In these cases, the implant actually forms a hinge between the metatarsal and the phalanx and is attached to the resected or planed faces of the metatarsal and the phalanx in cases where both joint surfaces have degraded. Despite the absence of an abrasive motion of the silicone material against a bone surface in the joint, degradation of the silicone material formed into this hinge type joint replacement continues to cause concern for its potential damaging effects on the body. Such full joint replacement using silicone material, therefore, is also not a preferred solution even where both surfaces of the joint require replacement.
A number of metal implant devices, usually made of titanium or its alloys, have been used in place of the above described silicone devices. Full or partial (hemi) replacement of the joint has been provided for in the many efforts to design metal implants for this purpose. The preferred approach where the metatarsal head generally remains intact is to provide for a metal implant device fixed to the phalanx head of the joint. In general, it is preferred not to replace both sides of the joint with metal implants, as this will frequently result in joint discomfort and/or progressive dislocation of the joint often resulting in joint stiffness.
For the above reasons, a hemi-joint replacement is the preferred surgical procedure when the proximal phalanx in the metatarsophalangeal joint has deteriorated while the metatarsal head remains generally sound. Such a replacement is often indicated where the individual has painful arthritis in the joint, painful hallux valgus, hallux limitus, or hallux rigidus.
A number of efforts have been made in the past to provide different types of toe implants for the purpose of partially replacing a metatarsophalangeal joint. An example of a hemi-implant used for replacing the proximal phalanx in the hallux is disclosed in U.S. Pat. No. 5,326,366 issued on Jul. 5, 1994 to Pascarella et al., entitled Biomechanical Great Toe Implant. The device described incorporates a concave surface that imitates the shape of the head of the metatarsal to provide a round or elliptical perimeter to the bearing surface for the joint.
Complete joint replacement has been addressed in a number of other existing patents describing a range of implant designs including: U.S. Pat. No. 5,458,648 issued on Oct. 17, 1995 to Berman et al., entitled Great Toe Joint Implant and Method of Implantation; U.S. Pat. No. 5,314,486 issued on May 24, 1994 to Zang et al., entitled Non-Constrained Total Joint System; U.S. Pat. No. 5,037,440 issued on Aug. 6, 1991 to Koenig, entitled Orthopedic Toe Implant; U.S. Pat. No. 4,903,031 issued on Mar. 13, 1990 to Frisch, entitled Toe Implant; U.S. Pat. No. 6,699,292 issued on Mar. 2, 2004 to Ogilvie et al., entitled Interphalangeal Joint Replacement; U.S. Pat. No. 5,776,203 issued on Jul. 7, 1998 to Spalding et al., entitled Metatarsal Phalangeal Sesamoid Prosthetic Joint; U.S. Pat. No. 4,642,122 issued on Feb. 10, 1987 to Steffee, entitled Toe Implant; U.S. Pat. No. 4,156,296 issued on May 29, 1979 to Johnson et al., entitled Great (Large) Toe Prosthesis and Method of Implanting; and U.S. Pat. No. 6,319,284 B1 issued on Nov. 20, 2001 to Rushdy et al., entitled Toe Implant.
Most of the efforts in the past that have included an implant component designed to be integrated into the phalanx, and more specifically the proximal surface of the phalanx, incorporate a concave bearing surface that is generally in the shape of the distal face of the metatarsal. This provides a generally circular or elliptical perimeter to the bearing surface which helps to maintain the joint in alignment with the implant.
Various efforts have been made to design implant stems that reduce the likelihood of inadequate or improper placement of the implant into the phalanx bone. Many such efforts have created circular or conical stems that, while presenting increased surface area for contact, frequently suffer from rotation after placement. Other efforts at creating suitable stem configurations for the implant have focused on a variety of non-rotating configurations, often at the cost of decreased surface contact. Some of the more effective designs present rectangular shafts that offer serrations and sharp edges to facilitate placement and insertion of the implant. Unfortunately, the adequate use of such rectangular wedge shaped stem designs depends greatly on the condition of the phalanx bone and the ability of the physician to provide a receptor opening in the bone that is neither too large nor too small. Excavating or broaching too much of the bone, of course, provides a loose placement of the implant, while failing to excavate or broach enough of the bone can result in fractures during placement.
Most efforts in the past to design optimally shaped and sized stems for joint replacement implants have, for a number of reasons, centered the stem on the back face of the concave joint replacement structure. Such centered placement, however, fails to recognize that the largest, most stable portion of the phalanx bone available for use as a means for retaining the implant does not lie immediately distal to the center axis of the joint. Rather, as can be seen in the side views associated with the typical human foot skeletal structure, the major portion of the bone available to receive and support the implant is offset above a center line of the joint. Placing the stem of the implant device along the center line of the joint significantly limits the quantity (and often quality) of bone surrounding the stem and further limits how wide the stem can be and how long the stem can extend into the phalanx bone.
Addressing the above concerns, and in fulfillment of the above stated objects, the present invention provides an improved hemi-implant for use primarily in conjunction with the first metatarsophalangeal joint. The hemi-implant is constructed with an elliptical, concave, joint surface component that is positioned on a stem component, and is designed to be placed into a prepared proximal face of the first phalanx bone. The implant is sized according to the requirements of the patient and may be provided in a number of incremental standardized implant sizes. The improvements of the present invention relate primarily to the structure, size, and position of the stem that is designed to extend into the phalanx bone to support the joint surface component of the implant. The stem component of the present invention is positioned off-set from a center line axis of the elliptical joint surface component of the implant. The off-center placement of the stem allows for an extended length to the stem, even for the smallest embodiments of the design. A simpler but more effective retention structure is provided on the acute angle edges of the stem. An implant size selection tool is provided for use by the physician as a means to select and place an appropriately sized implant. The size selection tool further provides a template for positioning and placing the stem through the use of a broach tool directed into the planed proximal surface of the phalanx bone. A method for sizing and selecting the appropriate implant; as well as positioning, placing, and fixing the implant as a partial joint replacement, especially for the first metatarsophalangeal joint, is also described.
Structures, geometries, and methods of use associated with the present invention allow the physician to more securely place the implant with less risk of bone damage and further to provide greater comfort to the patient as a result of more accurate placement and more appropriate sizing of the implant. Further benefits of the structural and functional design of the hemi-implant of the present invention will become apparent to those skilled in the art upon an understanding of the detailed description of the preferred embodiments which refers specifically to the drawing figures attached, a brief description of which follows immediately below.
Reference is made first to
Joint surface component 12 is generally constructed with a concave profile to provide the contact surface for the first metatarsophalangeal joint. This surface construction may generally be described as comprising joint surface concavity 16 and joint surface rim 18. In general, the shape and size of both joint surface concavity 16 and joint surface rim 18 are determined by the overall size of implant device 10 and may vary according to the individual patient's requirements.
The back face of joint surface component 12 is generally planar in configuration and is intended to contact the plane of resection on the phalanx bone. As indicated above, stem component 14 extends from this back face of joint surface component 12 and may in the preferred embodiment be machined from the same metal solid as the joint surface component. Alternately, stem component 14 may be attached to the joint surface component 12 with any number of rigid metal-to-metal attachment methods known in the art. Stem component 14 generally comprises a wedge having a diamond-shaped cross section that terminates in stem wedge tip 22.
On each of the acute angle edges of stem component 14, a retention indentation 20 is provided to facilitate the adherence of the stem within the bone, especially after post-operative bone growth. This single deep indentation 20 on each edge of stem 10 is preferred over the multiple shallow serrations found in the prior art. The single indentation facilitates the easy insertion of the stem into the bone after a broaching tool (described below) has initially formed a receptive cavity. The deep indentation provides greater adherence within the bone, especially over time as bone growth extends into the indentation and firmly retains the implant 10 in place. The indentations, as shown in
The view shown in
A primary benefit of the design of the implant device 10 of the present invention is the ability to utilize a stem component 14 of extended length L greater than that typically implemented in the prior art. For reasons discussed in greater detail below, length L of stem component 14 may generally extend 5 mm-10 mm longer than similar implant stems in the prior art. The length L of stem component 14 in the preferred embodiment of the present invention will generally be in the range of 10 mm-40 mm, with a preferred length of approximately 25 mm. Of course the actual length of the stem depends in part on the overall size of the implant with larger implants requiring (and allowing for) larger stem lengths. In this regard it may be practical to view a preferred length dimension L for the stem to be on the order of 5 mm-10 mm longer than the measure of the major axis diameter D1. Once again, the increased length of dimension L is a result of the off-center placement of stem component 14 as described in more detail below. Width dimension W of stem component 14 is generally related to the overall size of implant device 10 and varies according to the same.
The view shown in
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Of critical note in
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Reference is finally made to
If the selected template size is not the proper fit as determined at Step 108 the physician selects a new size of template at Step 110 and again places the sizer tool over the prepared phalanx face at Step 106. If at Step 108 a proper template fit is identified, then the physician proceeds at Step 112 to orient and align the template and thereafter mark the cavity location for the stem of the implant. This location is determined by appropriate marking of the articular surface of the phalanx (as resected) through the template window 44 (best seen in
At Step 116 a cavity is established for the implant stem utilizing a broach tool directed in to the marked location on the proximal surface of the phalanx. Once the cavity is established the physician then orients and places the implant device stem within the established cavity at Step 118. At Step 120 an impact tool is implemented to set the implant in place within the phalanx as described and shown above. The physician then proceeds to re-set the joint and suture any tendons that have been displaced in the process of placing the implant at Step 122. The surgical site is closed at Step 124 and the surgery is finalized at Step 126 in a manner will known in the art.
Although the present invention has been described in terms of the foregoing preferred embodiments, this description has been provided by way of explanation only, and is not intended to be construed as a limitation of the invention. Those skilled in the art will recognize modifications of the present invention that might accommodate specific patients and bone or joint structures. As is known in the art, it is necessary to provide various sizes of a similarly structured implant device in order to accommodate patients of different ages and different bone structures. Such modifications as to components, size, and even configuration where such modifications are merely coincidental to the size of the patient, do no necessarily depart from the spirit and scope of the invention. It is further anticipated that some variation may occur, for example, in the configuration of the various sections of the implant device to allow variations in the force required when placing the device. Again, all of these various modifications and variations do not necessarily depart from the spirit and scope of the invention.
