This invention concerns surgical procedures, but more specifically, systematic procedures using computer generated tomographic data to implant a hip prosthesis in a patient. Systematic procedures assure accuracy and consistency in orienting and sizing components of the hip prothesis to attain anatomically correct functioning results.
Surgical procedures involving total hip replacements (THR) began in 1959 and have since dramatically evolved. Initially, extraordinarily excellent results of cemented THR in the short term caused surgeons to believe that deterioration over time would not occur. Recent long term follow-up studies, however, have caused alarm by reporting aseptic loosening rates of 11-29% on the acetabular side and 29-40% on the femoral side after ten years. New cementing techniques in the 1980's including pressurization and plugging of the femoral canal, as well as improved prosthetic design, have increased the life of the hip implant. Customized sizing of femoral components also showed an improvement over the cemented THR. More recently, an entirely different technique known as bone ingrowth fixation has shown promising results, but long-term studies are yet to be made.
Nowadays, about 40% of hip replacements are being done without the use of bone cement at all. However, the success of these new implants requires that two criteria be met during implantation. They include establishing intimate contact of a porous implant surface with viable bone, e.g., that bone which can "grow" into the porous implant surface; and an initial rigid fixation of the prosthesis within the bone. Upon meeting these criteria, bone ingrowth normally occurs and early results are good.
Meeting these criteria is more difficult than using cement to provide fixation. Success entails careful preoperative planning and precise preparation of the bone at surgery. An infinite number of anatomical variations of the femoral canal exists whereas only a limited number of prosthesis sizes are available. In most instances, bone must be machined to accept a 18 standard size and shape of prosthesis. The task requires an experienced surgeon to obtain predictably good results, but as a 20 matter of fact, the majority of hip replacements are performed by orthopaedic surgeons who do not specialize in joint replacement surgery and thus may have limited expertise in this area.
Preoperative planning of bone ingrowth (and also cemented) THR is key to successful implantation. Nearly all available hip systems emphasize planning in their surgical technique manuals. These manuals give considerable attention to details of using acetate x-ray templates to plan the procedures. However, standard preoperative planning using plain x-rays and overlay templates is subject to many errors due to the magnification distortion of x-ray films and the difficulty in obtaining right-angle radiographic views of the hip joint. Most planar x-ray templates are enlarged about 20% relative to the actual patient's anatomy which represents an average magnification factor assuming a 40" distance between the x-ray beam source and the x-ray film table. This variable alone can interject 5-10% or 1-2 mm. error in the correct size of the femoral component and a 2-3 mm. error in the size of the acetabular component.
Most surgical THR protocols call for determination of femoral canal size during reaming or broaching of the femoral canal. This is a good method of determining a tight fit, if the reamer or rasps are always sharpened to the same degree. However, if the surgeon decides to use the next larger stem size because the reamer just used does not appear to be large enough, he must then determine whether the next larger stem fits into the femur prior to proceeding. It is critical to determine the fit preoperatively to avoid potential femoral fractures during surgery or potential leg length discrepancies by a failure to insert the prosthesis to the desired depth because it is too large. This problem is compounded by the fact that most orthopaedic surgeons do not do an adequate job of preoperative planning with templates. Furthermore, templating and is somewhat time-consuming.
It is also difficult to attain acetabular component (sometimes called a "cup") orientation and equalization of leg lengths during a THR operation. Proper cup placement depends on a knowledge of the exact position of the patient's pelvis on the operating room table and a system of visually estimating the position of the component orientation guide instrument. Both of these factors are subject to considerable error making this part of the procedure one of the most difficult and requiring considerable judgment and experience. At present, there are no known methods for systematically determining cup orientation relative to the patient's pelvis.
Leg length equalization both depends on the preoperative planning and on intraoperative measurement. Preoperative planning by the overlay templates only provides a rough estimate of leg length equalization. Intraoperative leg length measurement is usually done by subjective and inaccurate tests such as "shuck" and assessment of abductor muscle tension. At least one prior study substantiated leg-length differentials as much as 1.5 cm in approximately 20% of patients following unilateral THR. This is a distressing fact, since leg length inequality is quite obviously apparent to the patient immediately after surgery.
Prior solutions addressing some of the aforementioned problems include the work of Dr. David G. Mendes published in an article titled "The Role of Computerized Tomography Scan in Preoperative Evaluation of the Adult Dislocated Hip" which appeared in the November-December 1981 issue of Clinical Orthopeadics and Related Research, Vol. No. 161. At page 201 thereof, he discusses the use of three-dimensional CT data to help determine the choice of placement of the socket and to help determine component sizes. A German publication by Gerngro.beta., et al. titled "Moglichkeiten geometrischer Rontgenuntersuchung des Beckens fur den halbseitigen Beckenersatz" appearing at page 331-336 of Zeitschrift fur Orthopadie, Vol. 118, describes the use of CT data to reconstruct a hemipelvis with which a femur is to function. Drs. Richard P. Giliberty, et al., in a publication titled "A Prototype Femoral Stem Utilizing CAT and CAD/CAM" appearing in the periodical Orthopaedic Review, Vol. XII, No. 8, describe the geometical aspects of designing a femoral stem in connection with a THR process using CT data, and specifically discusses factors to be considered for achieving a desired angle of femoral neck inclinations in the frontal and transverse (anteversion) planes, as well as the design of the femoral stem in a fashion to minimize stress points in the intramedullary canal. Still relating to femoral stem design, Messrs. Aldinger, Fisher and Kurtz also describe a CT design process in their publication titled "Computer-aided Manufacture of Individual Endoprostheses" appearing in the Archives of Orthopeadic and Tramatic Surgery, Vol. 102, No. 1.
In view of the foregoing, it is an objective of the present invention to remove some of the "guess work" and poor judgment often occurring in a THR surgical procedure.
It is a further objective of the present invention to attain greater accuracy in sizing and orientation of prosthetic components utilizing computer tomographic data, such as CT scans, NMR scans and PET (positron emission transmissions) data.
It is a more specific objective of the present invention to establish systematic procedures for defining a reference plane about the patient's pelvis (or acetabulum) by which an acetabular socket can be correctly oriented in order to implant an anatomically correct hip system.
It is a further objective of the present invention to provide a method for properly selecting the optimum size and dimensions of femoral and acetabular components of a hip prosthesis.
It is yet another objective of the present invention to provide mathematical means for leg-length equalization based upon dimensions of the hip components and certain fixation points defined within the patient's acetabulum.
It is a further objective of the present invention to utilize computer tomographic data, in conjunction with certain defined surgical procedures, to accurately implant a hip prosthesis.