Patent Application
20120143346
This invention relates generally to a system of modular femoral components for a hip joint prosthesis and, more particularly, is directed to the shape of the intramedullary stem which is inserted into the surgically prepared medullary canal of the proximal femur for fixation or anchorage of the prosthesis into the surrounding supportive cortical bone and also relates to the location for insertion of a neck element into the stem.
As is well-known in the art, the intramedullary stem or blade-like portion of the femoral component of a hip joint prosthesis is inserted into the prepared medullary canal of the proximal femur for fixation within the surrounding bone structure. This fixation occurs by either bone cementing which employs methyl methacrylate, or by biological fixation employing a mechanism of bone ingrowth within minute voids or porosities of a specially applied porous-like metal alloy surface structure. This porous surface structure is metallurgically integrated onto the surface of the prosthesis stem at specially selected locations.
With this in mind, a congruent fit between the femoral prosthesis stem and prepared proximal medullary canal of the femur is an important technical and clinical consideration. One factor that affects the ability to achieve a congruent fit is the natural shape of the proximal medullary canal prior to the canal being prepared to receive the prosthesis stem. It is generally recognized that there are two shape extremes of the natural proximal femoral medullary canals. The first shape extreme is often referred to as a “stove pipe” shape, and the second type of shape is often referred to as a “champagne flute” (conical) or more narrow distally) shape. The third basic shape is the most usual and falls between “champagne” and “stove pipe”. It would be advantageous for a surgeon to have available femoral prosthesis stems that are suited for a particular type of proximal femoral medullary canal. Advantages lie in ease of preparation of the medullary canal and the potential for a more congruent fit between the stem and canal.
Additionally, besides attempting to achieve a stable mechanical press-fit within the proximal femoral canal for porous ingrowth fixation (and for cementation, as well), the surgeon must attempt to position the center of the bearing head of the femoral prosthesis coincident with the articular center of the hip joint for duplication of the natural functional biomechanical characteristics of the hip. The two principal spacial components of femoral head position include proximal-distal offset distance (proximal-distal elevation from the collar or neck resection level to the femoral head center) for leg length considerations and medial-lateral (M-L) offset (medial-lateral dimension between the medullary stem center-line and the head center) for attainment of proper hip joint function and power characteristics.
One way the femoral head position can be changed by the surgeon is by selecting a neck from a range of different length modular neck components. Also head components which allow discrete incremental increases in both the proximal-distal (P-D) and medial-lateral offsets in relation to a neck can be provided. The neck angles of the modular necks can also be varied to change P-D and M-L offsets. Variation in just a medial-lateral (M-L) component of the femoral head position is achieved by selecting an alternative femoral component design, which incorporates a larger (M-L) offset. Such a femoral component may not be readily available for a given distal stem size within the same prosthesis system, and accordingly, may involve consideration of a long lead-time custom device, or more probably, consideration of another prosthesis system may have to be explored.
Another possible way the desired proximal-distal femoral head offset can be achieved is by preparing the proximal femoral canal to accept the prosthesis at a higher position or by selecting an alternative femoral component design of proper distal diameter, but with a broader proximal-transverse or a proximal M-L intramedullary stem width. This allows the prosthesis to seat at a higher position within the femur. The former technique for seating the femoral component at a higher position can result in larger stem/bone interface gaps proximal-medially, a condition reflective of a less desirable, and possibly less stable, stem fit. Conversely, to lower the vertical head offset position, a femoral prosthesis stem of narrower proximal-transverse width can be employed, which allows the prosthesis to seat at a lower position within the femur. Lower prosthesis seating may also result in an inexact fit between the stem and bone, especially if the femur was surgically prepared for a larger proximal stem design of different proximal-medial curvature. Again, these adjustments in femoral head position must be surgically achieved while coincidentally attaining a tight, stable intramedullary stem fit within the proximal femur with minimal interface gaps between the stem and bone.
Another way to provide a surgeon with an ability to alter the position of the center of the prosthetic femoral head relative to the prosthetic femoral stem axis is to provide monolithic stems that, for a given size, offer either different neck angles or pure P-D and/or M-L offsets. In order to determine which type of angle offset is most appropriate, some prosthetic implant systems offer the ability to attach trial necks to the tools used to shape the proximal medullary canal of the femur. This assembly permits a trial reduction of the joint that will reveal if the choice of implants is appropriate for the patient. However, a disadvantage to these systems is that they do not allow a surgeon to trial off of the actual stem implant. Variations in position between the proximal medullary femoral canal shaping tools during a trial reduction and the actual position of the stem implant can result in the femoral head implant center being positioned in a slightly different place than intended relative to the femoral prosthesis stem axis and neck resection level.
Another way to provide a surgeon with an ability to alter the position of the center of the prosthetic femoral head relative to the prosthetic femoral stem axis is to provide a femoral prosthesis stem that is able to accept any one of a number of modular femoral neck prosthesis options. These femoral neck prosthesis options can be configured to allow any practical number of neck angles and lengths or independent P-D and M-L offsets. The disadvantages to these modular stem-neck implant systems is that due to the junction between the two mating parts the resulting implant may not be as strong as a monolithic implant of the same geometry. However, the advantage is that the femoral head center may be able to be more accurately placed relative to the femoral prosthesis stem axis and neck resection level.
In the past, stems with longer monolithic and or modular necks had to be used to achieve the extra offset that patients on the high end of the spectrum for neck length and head offset needed. Alternately one can change the neck angle such as 127° or 132° for a given size stem. This results in two different head center locations
Neck designs in both monolithic and modular stem designs are subjected to a very high amount of stress when loaded in the hip. The longer the neck is for a given patient's body weight, the more stress is applied to both the neck and the mating coupling feature on stem. Current modular neck designs have been developed with relatively expensive materials that are much stronger in order to deal with this high stress. In some cases, modular neck length is also limited for a given set of stem sizes due to the maximum stress limits of the materials that are currently used. This results in a lack of appropriate head offset values for some patient's anatomy. Again the neck angle can be changed with different modular necks. Thus the neck angle can be changed independently.
The key difference for the system of the present invention is that a single stem size can have variations in the position of the mating feature for the modular neck and this leads to the ability to give the patient more head medial-lateral offset without changing the neck length, neck angle and/or proximal-distal head position. An alternate embodiment shifts only the P-D head location without changing the M-L location.
Each stem size has two or more variations in medial curvature for a given size. The anterior/posterior widths of the stem could also be varied along with the medial calcar offset values and head offset values relative to the proximal axis of the femur. The first variation is designed to fit into patients with lower than average sets of medial calcar head offsets. The second stem variation will be designed to fit into patients who have a higher than average set of medial calcar offset values and corresponding medial-lateral head offset values relative to the proximal axis of the femur. A third variation may fit into patients who have average medial calcar offset values and head offset values.
In the modular neck stem design, the location of the neck coupling element which is typically a mating female pocket that would receive the modular neck in the first variation of the modular stem design will be shifted towards M-L the low end of the distribution of data for medial-lateral head offset. The location of the female pocket or tapered bore in the second variation of the modular stem design will be shifted towards the high end of the population distribution of data for M-L head offset. The location of the female pocket or tapered bore in the third variation of the modular stem design would be positioned near the average values for both medial calcar offset and M-L head offset. In addition a second M-L offset of the pocket in which the neck is inserted will give an identical M-L head offset. Thus a shorter neck can be used to obtain the desired M-L offset.
Statistical analysis data from a virtual bone database has been utilized to determine the appropriate medial curvatures and corresponding head offsets for each variation. Just like in the conventional modular neck designs, the medial-lateral head offset can be controlled both by the modular neck angle to the longitudinal stem axis, neck length and the position of the receiving pocket or tapered bore that is located in the proximal stem. The key difference for this system is that a single stem size can have variations in medial-lateral head offset without changing the neck length, neck angle, and/or head proximal-distal position. The standard deviation in data for M-L head offset in a large group of patients on average is approximately +/−2.5 mm and approximately 2-3 mm over the entire range of bone sizes analyzed. This range could be increased or decreased if additional optimization is required. The subset of patients shown in
The modular stem/neck design with two or more mating pocket positions for each stem size allows a surgeon to reconstruct leg length and medial-lateral head offset without adding extra neck length in patients who have a higher than average head position and corresponding medial canal offset values along the length of the calcar. This results in a stronger system with a shorter moment arm.
Each stem size has multiple medial curvature options. One version of this stem design has a modular neck pocket and would utilize modular necks in a new way that will allow surgeons to attain the appropriate medial-lateral head offset range, leg length range and anteversion angle range with reduced neck lengths for patients that require high head offsets and longer then normal neck lengths.
A kit of modular prosthetic hip components is provided which includes a first neck element having a coupling element thereon. The kit has first and second hip stem elements each having a coupling element at a proximal end thereof for connecting to the coupling element of the first neck element. The first and second stem elements each have a progressively larger medial-lateral dimension in a proximal portion thereof. The coupling element or pocket on each of the first and second stem elements is located at a different medial-lateral location thereon. The coupling element or pocket is located more medially and proximally on the second stem element than the first stem element. The first neck element forms an identical first neck angle with a proximal-distal axis of the respective first and second stem elements and locates a femoral head mounted on the neck element in the same proximal-distal location but a different medial-lateral location with respect to the patient's acetabulum. By not increasing the neck length the moment arm between the loading point of the femoral head and the coupling element or pocket on the stem is not increased.
The dimension in the medial-lateral direction at a proximal most surface of each stem element increases in increments from the first stem element to the second stem element such as in increments of 2.5 mm. The distance between the proximal-distal axis of each stem and a lateral side of the first and second stem elements are equal. The first neck element may have a second coupling element, such as a male taper for attaching a prosthetic femoral head. The kit may further comprise a second neck element forming a second angle with the proximal-distal axis of a respective first, second and third stem elements. The second neck angle is different than the first neck angle such as 135° and 145°. The first and second stem elements each have an identical distal stem portion including a distal tip of each stem element. The first neck element has a longitudinal axis wherein a second end of the first neck element along the longitudinal axis when the neck and stem coupling elements are assembled is offset in the medial direction a distance which increases in increments from the first stem element to the second stem element. The coupling element on each stem element may be a tapered bore which has a central axis and the central axis moves medially in the increments (2.5 mm) from the first stem element. Each stem element has a superiorly facing proximal surface including the tapered bore, the superiorly facing proximal surface moving superiorly in incremental distances from the first stem element to the second stem element. The first stem element has a coupling element such as the tapered bore located more laterally and distally than the second stem element. A medial side of the first and second stem elements may be curved with the curve of the second stem element having a smaller radius than the first stem element. Generally the curve starts distally approximately 40 mm below a proximal most medial edge of the proximal portion of the stem element.
As used herein when referring to bones or other parts of the body, the term “proximal” means close to the heart and the term “distal” means more distant from the heart. The term “inferior” means toward the feet and the term “superior” means toward the head. The term “anterior” means toward the front part or the face and the term “posterior” means toward the back of the body. The term “medial” means toward the midline of the body and the term “lateral” means away from the midline of the body.
The kit or system may also have groups of different size stem elements (A-P dimension, length and M-L dimensions), each group having at least two stems with identical distal portions which distal portions differ in dimension from group to group.
a shows a prior art femoral component, including a modular spherical head with the neck extending at an angle of 127° with respect to the longitudinal axis of the stem;
b is a prior art femoral component similar to that of
a is a prior art head for a femoral component having a pocket for receiving the proximal end of a neck component located in a first location with the part spherical head;
b is a part spherical head for a femoral component having the same diameter as that of
a is a prior art femoral component, including a modular neck having a first length connecting a part spherical head to the stem component;
b is a second prior art femoral component somewhere to that of
Referring to
A second way of changing the center location is shown in
Yet another way of the prior art changes the location of the center of the spherical head with respect to the acetabulum is shown in
Referring to
The size of the femoral component in the medial dimension, are selected to provide the largest coverage of individuals shown in
Referring to
As discussed above, stem 100 defers from stem 200 in that the calcar area is shifted medially and proximally on moving from stem 200 to stem 100. The medial shift is approximately the same as the shift in the locations of cavity 206 to cavity 106. Thus, when moving from point 202 to point 102, a shift may typically be 5 mm which is similar to the M-L head offset difference between point 204 and point 104. The proximal face 32 and 34 of stems 100 and 200 are rotated superiorly from a point 36 on the lateral side of the femoral component 100, 200. The distance of rotation from location 34 on stem 200 to location 32 on stem 100 is sufficient to move cavity 106 superiorly with respect to cavity 206 a sufficient distance to place head centers 104, 204 in the same proximal distal location with respect to a specific patient's acetabulum. Thus all stem dimensions lateral of center line 24 stay the same.
Referring to
As could be understood by one skilled in the art, the pocket could also be shifted in the anterior-posterior direction and the proximal-distal direction.
Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.
