WO2007069250A2

WO2007069250A2 - Faceted long bone head prosthesis

Info

Publication number: WO2007069250A2
Application number: PCT/IL2006/001434
Authority: WO
Inventors: Yaron Bar-Ziv; Yona Kosashvili
Original assignee: Presrv Ltd.
Priority date: 2005-12-14
Filing date: 2006-12-13
Publication date: 2007-06-21
Also published as: IL172588A0; WO2007069250A3

Abstract

The present invention provides a long-bone endoprosthesis having an essentially spherical head (10) characterized by the presence of a plurality of crater-like depressions (12) distributed over its outer surface, and further characterized in that substantially the entire outer surface has an average micro -roughness in the range of 1 to 1000 micrometers.

Description

FACETED LONG BONE HEAD PROSTHESIS

Field of the Invention

The present invention relates to a new type of prosthetic device for use in a cartilage-preserving joint replacement technique. More specifically, the present invention is directed to a long bone prosthetic head having a novel, external surface design with improved mechanical properties.

Background of the Invention

Fractures of the neck of the femur and humerus have always presented great challenges to orthopedic surgeons and remain in many ways today the "unsolved fracture" as far as methods of treatment and the results obtained thereby are concerned.

until recently, few satisfactory treatment options for the management of long bone neck fractures have been available. However, a radically new method of treatment, in which the long bone head articulating surface is retained and attached to a roughened or textured long bone prosthetic head was developed by the present inventor and disclosed in the co- owned, co-pending international patent application no. PCT/IL04/0000980 (published as WO 05/039439) .

It is a purpose of the present invention to provide an improved long bone head prosthesis that may be used in the treatment of long bone neck fractures in the method and system described in WO 05/039439.

In particular, it is a purpose of the present invention to provide a prosthetic head possessing significant mechanical advantages over the prosthetic head design specifically disclosed in WO 05/039439. Summary of the Invention

It has now been unexpectedly found that a micro-roughened long bone prosthetic head having a series of facets or craters on its outer surface possesses superior mechanical properties when used in the cartilage-preserving long-bone prosthetic replacement method disclosed in WO 05/039439. In particular, the micro-roughened and cratered prosthetic head has been found to have significantly better resistance to shear stress (at the prosthetic head-cement interface) when compared with the micro-smooth, grooved prosthetic head specifically exemplified in the aforementioned published international patent application.

The present invention is thus primarily directed to a long- bone endoprosthesis comprising an essentially spherical head comprising a central internal cavity that perforates one side of the prosthetic head, such that said cavity is suitable for articulation with a long-bone prosthetic stem section, and wherein said head region is characterized in having crater- like depressions distributed over its outer surface, and wherein substantially the entire outer surface has an average micro-roughness (R_α) in the range of 1 - 1000 micrometers. Preferably, the average micro-roughness value should be between 30 and 70 micrometers.

In one preferred embodiment of the invention, the crater-like depressions are distributed across essentially the entire outer surface of the endoprosthetic head, each depression having a maximum depth in the range of 0.2 to 3 mm and a diameter in the range of 3 to 20 mm. The term "micro-roughness" is used throughout the specification to indicate roughness that is due to small irregularities of the surface. This type of roughness is clearly different from the discontinuities in the smooth, essentially spherical envelope shape of the prosthetic head that are due to features such as the craters found on the outer surface of the presently-disclosed device.

For the purposes of the present invention, the parameter R₀, is defined in accordance with the International Standards Organization document ISO 468 ("Surface Roughness Parameters their values and general rules for specifying requirements") .

The aforementioned endoprosthesis is suitable for treating fractures of the neck region of the femur and humerus by means of the surgical methods disclosed in WO 05/039439, the entire contents of which are incorporated herein by reference. In these surgical methods, the head region of the endoprosthesis is inserted into a shell-like cavity comprising the outer layers of the patient's femoral or humeral head, following removal of most of the cancellous bone. In this way, the cartilaginous articular surface of the long bone head, together with the underlying subchondral bone and a small portion of the cancellous bone is preserved, thus retaining the natural articulating surfaces of the proximal long bone ball-and-socket joint. Brief Description of the Drawings

Fig. 1 photographically depicts an endoprosthetic head of the present invention.

Fig. 2 schematically depicts the manner in which the endoprosthetic head of the present invention is assembled together with cement, a thin cancellous bone layer and the outer cortical bone / cartilage shell. Fig. 2A shows the order of assembly of the aforementioned layers and components, while fig. 2B depicts the completed assembly.

Fig. 3 is a cut-away illustration of a femoral endoprosthetic head of the present invention in situ, i.e. in articulation with the acetabular cavity.

Fig. 4 schematically illustrates the apparatus used to compare the faceted endoprosthetic head of the present invention with a grooved endoprosthetic head, with respect to resistance to shear stress.

Fig. 5 is a bar chart comparing the faceted endoprosthetic head of the present invention (B) with a grooved endoprosthetic head (A) , with respect to resistance to ' first noticeable failure'.

Detailed Description of Preferred Embodiments

As disclosed hereinabove, the long-bone endoprosthetic head of the present invention is characterized by the presence of a plurality of facets or craters distributed over its outer surface. In addition, essentially the entire outer surface of the prosthetic head is micro-roughened. It was unexpectedly found by the present inventor that the combination of these two surface features (i.e. the cratering and the micro-roughness) significantly improves the mechanical stability of the attachment between the prosthetic head and the cartilage/bone shell that is cemented thereonto, thereby rendering the presently disclosed and claimed endoprosthesis eminently suitable for use in the cartilage- sparing surgical methods for treating, inter alia, femoral neck fractures that are disclosed in WO 05/039439.

Fig. 1 depicts a typical prosthetic head of the present invention, generally indicated as 10. As may be seen, the generally spherical shape of the head is interrupted on its inferior side by a flattened base region 14, said base region being perforated by an opening 16 leading into the internal cavity of the prosthesis.

The external dimensions of the endoprosthetic head 10 are essentially the same as those of prior art femoral and humeral prostheses. Thus, in the case of a prosthesis of the present invention having a head region that is essentially spherical in shape, the diameter of the spherical head is generally in the range of 20 to 42 mm. A preferred head diameter for the femoral head prosthesis is in the range of 28 to 32 mm. In practice, however, the head diameter may also be outside of these preferred ranges, in certain circumstances being as small as 12 mm or as large as 60 mm.

The dimensions of the internal cavity of prosthetic head 10 are such that a standard long-bone prosthetic stem may be fitted therein. In practice, this generally requires that the diameter of opening 16 be in the range of 18 to 38 mm, preferably 10 to 15 mm. The endoprostheses of the present invention may be constructed from cobalt-based alloys (e.g. cobalt-chrome), titanium, titanium-based alloys, stainless-steel, combinations of the above-mentioned metals, and other metals and metal combinations used in orthopedic implants. The prostheses are manufactured from the aforementioned metals by means of one or more of: electro-erosion, casting, sintering, milling and turning.

In addition, the endoprostheses may also comprise components that are constructed from non-metallic materials such as biocompatible plastics and polymers, such as polyurethane and polyethylene, as well as other synthetic biocompatible materials that are softer than the aforementioned metals, and hard materials such as ceramics. The aforementioned lists are intended to exemplify some of the more common materials, and are not to be considered as limiting. Various combinations of the different materials mentioned hereinabove also form part of the scope of the present invention. Thus, for example, prosthetic heads may be constructed with a metal trunion in contact with a plastic body. Another example of the use of a combination of materials would be the case in which the central bulk of the prosthetic head is constructed of a metal or metal alloy, whereas the outer portion (having an external surface that is cemented to the femoral head shell and an inner surface that is bonded to said metal or alloy) may be constructed of a non-metallic, polymeric material .

In a preferred embodiment of the invention, the prosthetic head is manufactured from a Cobalt-Chrome alloy conforming to ASTM F1537, such as can be procured from Allvac, Monroe, NC, USA, item number TJA-1537.

As shown in Fig. 1, the generally spherical contour of the prosthetic head has been interrupted or modified by the presence of plurality of craters or facets 12 that are distributed across the outer head surface. These craters have a diameter in the range of 3 to 20 mm, preferably 12.5 mm, and a maximum depth in the range of 0.2 to 3 mm. A typical prosthetic head of the present invention having an external diameter of 28 mm will have approximately 11 facets on its outer surface. These craters are created by one or more of the following means: electro-erosion, milling, casting or sintering.

Without wishing to be bound by theory, it is believed that craters 12 are able to improve the stability of the prosthesis-cement attachment by virtue of transforming shear stresses in the enveloping cement into compressive stresses. During the process of manufacturing the craters, all surface contours are gently rounded, thereby minimizing the development of localized concentrations of stress forces.

Following creation of the craters or facets, as described hereinabove, the outer surface of the prosthetic head is micro-roughened. This micro-roughening may be produced in several different ways, by the use of any standard procedure including mold-casting techniques, machine-cutting, electro- erosion, sintering and grit blasting techniques.

Fig. 2 depicts the various layers of the cartilage-preserving prosthesis of the present invention. As shown in the exploded view in Fig 2A, the prosthetic head 20 is covered by a layer of biocompatible cement 22. Generally, this cement layer has a thickness of 2-5 mm. Examples of suitable cements include polymethylmethacrylate (PMMA) cement, Palacos cement, Simplex, CMW and Cementech. In many cases, the preferred cement is PMMA cement. The aforementioned cement layer is used to stably grasp the cartilage preserving prosthesis inside the bone-cartilage long-bone head shell. Conceptually, the head shell can be considered as consisting of two layers: a thin (approximately lmm deep) layer of residual cancellous bone 24 infiltrated by the cement layer, and an approximately 5mm thick layer of cortical bone overlaid with articular cartilage 26. It is to be recognized, however, that in practice, the two outer most layers - the cancellous bone layer 24 and the cortical bone/cartilage layer 26 - are present as a single head shell, following removal of most of the cancellous bone. The process of removing most of the cancellous bone from the long-bone head is achieved by reaming the cancellous bone with a high or low speed burr or any other conventional acetabular reamer.

The complete prosthetic head-cement-bone/cartilage assembly is shown (in exterior view) in Fig. 2B.

Fig. 3 is an in situ cut-away view of the prosthetic head of the present invention 30 after it has been cemented to a femoral head shell. As shown in the figure, the prosthetic head, with the plurality of craters 35 on its surface, is attached to the femoral head shell (consisting of a residual amount of cancellous bone 37 and cortical bone with its overlay of articular cartilage 38) with a layer of cement 36. The natural interface between the two anatomical articular surfaces (the femoral head cartilage 38 and the acetabular cavity cartilage 39) is thus preserved. The external entrance to the prosthetic head's interior cavity 34 (located in the centre of flattened inferior surface 32) will then receive a stem trunion (not shown) , which in turn will be inserted and bonded into a pre-prepared femoral canal.

In one preferred embodiment of the present invention, the prosthetic head having the micro-roughened and cratered outer surface may be part of a bipolar prosthesis. As is well known in the art, bipolar prostheses for use in the management of long bone neck fractures comprise the following components :

1. An outer head for articulation with the joint socket

(e.g. acetabular) surface. In the case of the present invention, the outer head has a micro-roughened and cratered outer surface, in order to permit optimal attachment thereof to the prepared femoral head, as described herein.

2. An intermediate layer or portion fixed within the inner cavity of the outer head. Typically, the intermediate portion is constructed of a material that is softer than metal (such as polyethylene) . Alternatively, this portion may be constructed of the same material as the outer head, thereby forming a single unit therewith.

3. An inner head, the outer surface of which movably articulates with the inner surface of the aforementioned intermediate portion, and the inner cavity of which is immovably fixed to a femoral stem trunion. 4. A locking ring, whose function is to retain the inner head in movable contact with the intermediate portion within the inner cavity of the outer head.

Further details regarding bipolar devices are to be found in the co-owned, co-pending WO 05/039439, incorporated herein by reference.

In another preferred embodiment of the present invention, the prosthetic head having the micro-roughened and cratered outer surface may be part of a bipolar monoblock prosthesis, wherein the inner head and femoral stem of said prosthesis are provided as a single integral unit. As in the case of the bipolar prosthetic head described hereinabove, the inner head of the monoblock unit is retained in movable contact with the intermediate layer within the cavity of the outer head and locked in place with the above-described locking ring.

In the context of the present invention, the term "long-bone" is used primarily to refer to the femur and humerus.

In a further aspect, the present invention is also directed to a method for treating fractures of the neck region of a long-bone in a patient in need of such treatment, wherein said method comprises the steps of: a) removing most or all of the cancellous bone from the head of said long-bone, thereby forming a long-bone head shell; b) preparing the long bone canal to accept a prosthetic stem; c) inserting, and optionally cementing, said prosthetic stem into said long-bone canal; d) inserting and cementing a long-bone prosthetic head into said long-bone head shell; and e) reduction of the stem region into the recessed region of said prosthetic head.

Further examples of methods for treating long-bone neck fractures that are suitable for use with the presently disclosed and claimed invention are described in WO 05/039439.

The retention of the natural articular surfaces of the ball- and-socket joints in the abovementioned surgical methods results in several clinical advantages, including prevention of acetabular erosion, obviation of the problem of ball/socket mismatch that is seen with prior art approaches, and pain reduction. Many of the above advantages arise from the fact that the retention of the anatomical articulating surfaces preserves the natural clearance between the long bone head and the socket within the joint. It should be noted that the viability and integrity of the long bone head articular cartilage is maintained by virtue of the fact that the nutritional requirements of this tissue are met by synovial fluid that bathes said tissue. In order to provide the aforementioned clinical advantages, an essential prerequisite is that the attachment of the prosthetic head region to the cartilaginous head shell is sufficiently strong to withstand the mechanical forces generated during normal, routine use of the prosthesis after implantation.

The presently disclosed and claimed endoprosthetic device has been found to be superior to the device characterized by the micro-smooth, grooved head region which was specifically disclosed in WO 05/039439, with respect to certain mechanical properties which are of relevance for the intended use of the device. In particular, the prosthetic head of the present invention has been found to possess significantly greater retention to the cartilage/cortical bone head shell when compared with the grooved prosthetic head of the aforementioned publication. While not wishing to be bound by theory, it is believed that the mechanical advantages of the device of the present invention are related to the transformation of shear stress in the cement to compressive stress. This is important since the cement can withstand compressive stress much better than shear stress. In addition, localized concentration of stress in the cement is minimized by the present invention, thereby significantly reducing possible fatigue damage.

The aforementioned mechanical advantage will be described and demonstrated in more detail in the following Example, which presents comparative data illustrating the superior resistance to shear stress at the cement-prosthesis interface shown by the present invention in relation to the preferred embodiment of WO 05/039439.

(■ Example

Failure force: comparison of a faceted prosthetic head of the present invention with the grooved prosthetic head disclosed in WO 05/039439

The aim of this study was to compare the presently-claimed prosthetic head with the micro-smooth grooved prosthesis specifically disclosed in WO 05/039439, with respect to resistance of the cement-prosthesis interface to shear stress .

Methods and materials :

Prostheses: Each of the two prostheses tested was constructed from a spherical or semi-spherical ball made of Cobalt- Chrome-Molybdenum alloy (meeting the standards of ASTM F1537, for use as a femoral head replacement) . In the case of the prosthesis of the present invention, a plurality of facets was formed in the outer surface of the head by means of milling. The outer surface was then sandblasted using a commercial sandblasting machine employing standard grit of the type that is normally used for the removal of scale from metal surfaces prior to painting.

Following sandblasting, the outer surface of the prosthetic head had a micro-roughness (Ra value) of 30-70. The head had 11 craters distributed across its outer surface, each crater having an average maximum depth of 0.6 mm, and an average diameter of 12.5 mm. The external appearance of the cratered prosthesis thus formed was similar to that depicted in Fig. 1. The micro-smooth (polished) grooved prosthesis was prepared according to the details provided in the Example of WO 05/039439.

A conical cavity on the rear surface of each prosthesis tested enabled the mounting of said prosthesis onto a conical stem. For the purposes of this test all prostheses had an equatorial diameter of 28mm.

Cement: Johnson & Johnson polymethylmethacrylate (PMMA) Bone Cement IG AB 40 gm. CMWl with antibiotics, P/N 3015040

Prosthesis Container: An aluminum container with a cylindrical cup-like cavity was used to house each of the prosthetic devices during the test. The cavity of the container was filled with cement and the prosthesis was then embedded in the cement, prior to connecting the container to a beam (metal strut) . The purpose of the beam was to magnify the torque applied by the Force / Displacement measuring machine described in the next section.

Testing Apparatus :

A Hounsfield T. H. E. Force / Displacement machine equipped with a THE-500 load cell, rated for a maximum load of 500 (± 0.01) Newton was used. Fig. 4 pictorially depicts the various parts of the testing apparatus. Thus, the prosthesis (not visible) was cemented into the prosthesis container 44 with bone cement. The prosthesis was rigidly connected via a conical stem 46 to an immobile base 40. The prosthesis container was then connected to the end of the long metal unsupported beam 42 (described in the previous section) . The force / displacement machine applied a vertical force 48 to the opposite end of the beam at a distance of 500 (±1) mm from the prosthesis cup's axis. The vertical force was translated via the beam into a torque force on the prosthesis cup, thereby creating shear forces in the cement surrounding the prosthesis and in the cement-prosthesis interface.

The vertical force on the beam end was increased until either the cement-prosthesis interface failed significantly (evidenced by a discontinuity in the graph of the recorded load profile), or the machine's maximum displacement was reached.

The force vs. displacement data was collected on a standard PC, using the instrument's software. The compressive force was applied at a stroke rate of 3 mm/minute.

Testing procedure :

Α total of three sessions were performed for each prosthesis. In each testing session, the following steps were performed:

1. Check that prosthesis is clean and that all remains from previous test have been removed. Note and document surface geometry and roughness.

2. Prepare cement according to cement manufacturer instructions .

3. Cement prosthesis in prosthesis container, taking care to position prosthesis in middle of container until full curing of cement (about 20 minutes) .

4. Connect prosthesis container to beam.

5. Place prosthesis on stem keeping beam parallel to base.

6. Position base + prosthesis container + beam under force / displacement machine. Check that distance between prosthesis container vertical axis and pressing point on beam is 500 (± 1) mm.

7. With the force / displacement machine apply a downwards stroke at a rate of 3 mm/minute, and record force vs. displacement behavior.

8. Halt downward stroke when one of the following occurs:

• Appearance of significant discontinuity in the force vs. displacement profile

• Maximum stroke has been reached

9. Dismantle prosthesis container from beam

10. Remove cement + embedded prosthesis from prosthesis container (by pushing it out) .

11. Remove any remaining cement from prosthesis container.

12. Carefully break cement open and remove prosthesis

13. Clean prosthesis and remove any remaining cement

14. Verify that file recording all experiment data has been saved.

The results of the testing procedure were expressed as the force (in Newtons) required for induction of the first noticeable failure. This parameter represents the minimal force at which a discontinuity, even slight, appears in the force-displacement curve. It is assumed that this discontinuity (accompanied by a cracking noise from the specimen) indicates a slight rotation of the prosthesis inside the cement mantle (as indicated by the arrow in Fig. 4) . The results of three separate measurements on each of the two different prostheses are shown in the following table: Force required for inducing the first noticeable failure FNewtoni

The averaged data shown in the above table are also displayed graphically in Fig. 5, from which it is evident that the cratered, micro-rough prosthesis of the present invention (B) is able to withstand a much higher force than the grooved, polished prosthesis of WO 05/039439 before showing the first noticeable signs of failure.

In view of the foregoing results, it is concluded that the prosthesis-cement interface of the cratered, micro-roughened head prosthesis of the present invention is significantly better at resisting shear forces than the prosthesis specifically exemplified in WO 05/039439. The implication of this conclusion is that of the two devices tested, the prosthetic device of the present invention will display significantly greater resistance to mechanical failure in clinical use.

Λfhile specific embodiments of the invention have been described for the purpose of illustration, it will be understood that the invention may be carried out in practice by skilled persons with many modifications, variations and adaptations, without departing from its spirit or exceeding the scope of the claims.

Claims

1. A long-bone endoprosthesis having an essentially spherical head characterized by the presence of a plurality of crater- like depressions distributed over its outer surface, and further characterized in that substantially the entire outer surface has an average micro-roughness (R_α) in the range of 1 to 1000 micrometers.

2. The long-bone endoprosthesis according to claim 1, wherein the micro-roughness value is in the range of 30 to 70 micrometers .

3. The long-bone endoprosthesis according to claim 1, wherein the crater-like depressions are distributed across essentially the entire outer surface of the endoprosthetic head, and wherein the maximum depth of each depression is in the range of 0.2 to 3 mm and the diameter of each depression is in the range of 3 to 20 mm.