Patent Application
20140257510
This invention relates to a monolithic (one-piece) femoral hip prosthesis and more particularly to a femoral hip stem component having an anatomic shape which produces a line-to-line contact with a prepared femoral medullary canal.
Hip arthroplasty procedures involve the implantation of a prosthetic stem component within a femoral intramedullary canal. A ball-shaped head on the proximal end of the stem cooperates with a socket of an acetabulum to provide for articulation between the femur and the acetabulum. In order to maintain pain-free articulation of the hip joint following implantation of the stem, it is important that the stem be securely fastened in the intramedullary canal. When a press-fit or line-to-line contact is desired with the intramedullary canal, the stem contour should closely match the contour of the prepared intramedullary canal.
The anatomy of the femur varies considerably from one individual to another. Version of the proximal femur refers to the relationship of the axes of the femoral neck to the transcondylar access of the distal femur. Femoral anteversion refers to the condition where the femoral axis is rotated anteriorly with respect to the transcondylar femoral axis, with the femoral head directed anteriorly to the coronal plane of the femur. The anteversion angle is defined as the angle by which the femoral neck deviates forwards from the axis of the femoral condyles, projected on to the horizontal plane, and it measures the anterior rotation of the neck of the femur around the shaft. Methods for in vivo measurement of the anteversion angle include the trochanteric prominence angle test and the Hermann biplanar radiological technique, although today the method of choice is considered to be CT. Femoral anteversion along with acetabular anteversion provides inherent stability to the hip joint. Such a femoral component implant is shown in U.S. Pat. Nos. 5,139,522; 5,358,534; and 5,762,204, the disclosures of which are incorporated herein by reference.
Previously when the plurality of neck version angles were desired, femoral components with modular necks were provided with the necks at various angles to the coronal plane bisecting the hip stem. However, a modular design has several drawbacks, for example, a large number of different parts must be provided in kit form and because of the modular design, stresses within the neck and the connection location within the proximal femur produce a hip stem less resistant to the forces applied during use. Therefore, it is advantageous to provide a one-piece hip stem with the neck made part of the proximal and distal stem portions during manufacture such as by casting or forging.
By analyzing databases of femur images for a large number (greater than 500) femur images and analyzing sales of modular necks for anatomic femoral implant stems. it was determined that the femoral anteversion showed a bi-modal data distribution (two neck anteversion). The bi-modal distribution was not the distribution that one of ordinary skill in the art would expect. One of ordinary skill would expect that three or more anteversion angles and related head offsets would be required for each stem size in a given patient population As a result of this bi-modal distribution, the patient population can be well served by a one-piece, i.e., monolithic or nonmodular implant neck stem system that offers only two anteverted head offsets for any given size or stem. It was determined that these two angles were 0° (neutral version, i.e., the center of the neck and therefore the head lies in the coronal plane) or an anteversion angle which produces a predetermined anteversion. The amount of anteversion offset from neutral associated with each size of stem is the same and is between 4 and 5 mm. The amount determined to be most appropriate is approximately 4.57 mm.
Thus, the distances between the center of the trunnion for the neutral stem and the center of the trunnion for the anteverted stem is the same regardless of the size of the rest of the stem. This situation is the result of the prosthetic hip implant stem being anatomic. Anatomic implant stems are designed to more closely fill the anterior portion of the femoral canal as compared to tapered wedge stems (such as Accolade® from Stryker and Taperloc® from Biomet) and as a result require a right and left stem. One design philosophy of an anatomic stem is that the centroid of the stem is at the same location as the centroid of the femoral canal. As a result, any size-specific anteversion changes are accounted for in the design of the proximal portion of the stem. For example, larger sizes of bones would be expected to, in general, have more anteversion. Thus, the anteversion increase that is a function of size in many cases is partially addressed in the design of the proximal stem portion leaving the additional offset for the head center relatively constant from size to size.
Thus, for each size stem there would be four monolithic anatomical hip stems, two of which would be shaped for the left femur and two of which would be anatomically shaped for the right femur with two different anteversion angles (0° and approximately 4.5 to 11 degrees selected to produce a 4 to 5 mm head anteversion). Typically there are four to eight stem sizes for a particular hip implant system.
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.
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Thus, for each stem size, an anatomic femoral component stem would be provided for the right (not shown) and left femur, each having a distal portion 14 extending along the anterior bow of the femoral shaft with two femoral components provided for the left femur (as shown), one neutral and one anteverted. Typically the anteversion is between 4.5 and 11°, and it has been found from a study of the femur referred to above that an anteversion angle that produced a 4.57 mm offset would fit a large portion of the population. It should also be noted that as stem sizes increase or decrease from the medium size shown the anteversion offsets remain relatively constant within groups of patients. The femoral database includes the data that may vary the 4 to 5 mm anteversion offset based on a population of individuals requiring a large or small femoral components. In other words, an analysis of a group of individuals requiring larger femoral components may produce a slightly different medium anteversion (males). This would be likewise for the group of individuals requiring a smaller femoral component (female). Obviously, any number of population groups can be addressed by different size femoral components, however, the population is usually grouped into three to five different groupings (male, female, Asian, etc.).
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Thus, the hip system of the present invention provides four stems, two neutral inversion, and two having an anteversion angle, which shifts the centroid of the trunnion approximately 4 to 5 mm from that of the neutral stem so that only four stems per size are required. It has unexpectedly been found that the four stems provided, two for the left and two for the right femur, with one stem being neutral inversion and the other being anteverted as described above, provide adequate choices for covering a large portion of a patient population. Thus, a surgeon performing a total hip replacement on, for example, a right femur has to select an appropriate size monolithic implant and then merely select a neutral or anteverted stem. As indicated above, an anatomic stem design, which produces a line-to-line contact with a prepared femoral canal in combination with the four stems per size described herein provide the desired anteversion for the majority of patients in a given population group.
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.
