RESEARCH FRONTIERS IN TOTAL JOINT REPLACEMENT

There is absolutely no question that total joint replacement is one of the miracles of modern medicine. Nevertheless, it is not yet perfect. Many areas still exist where improvements can be made, and it is in these areas where research is being conducted.

One major area of research is improving the fixation of artificial joints to bone. As has been stated, this initially was done without bone cement (polymethylmethacrylate) but was later markedly improved with the use of cement. During the past ten years, scientists have been trying to further improve fixation by either improving the cementing techniques or by returning to a biologic method of fixation without cement. Both appear to be successful and both have current indications. Cemented fixation works well for older patients and seems most appropriately used in the femur during total hip replacement and also frequently in knee, shoulder and elbow replacements. Biologic fixation works well in most situations for young patients, especially in the hip socket, the knee and occasionally in the shoulder.

Even with these improvements in fixation, better techniques are being explored. Newer cements which are less brittle and more biologically inert are being studied. Better cementing techniques, including eliminating stress concentration points in the cement and using composite structure fixation, are being applied. Biologic fixation is being improved by adding chemicals to porous surfaces to increase bone growth and adhesion to the prosthesis.

Different materials are being investigated to improve the prosthesis. This includes many different metals, plastics, ceramics, and composites. The search is for durable, inert materials that are compatible with long term use. Some of these have similar mechanical characteristics to living tissue, and others are very different to meet different needs.

Design changes have been frequent over the years. At the knee, these have been progressively evolving into a prosthesis that is looking more and more like the normal knee. As this has occurred, functional results have improved. At the hip, most investigators are proposing designs that are also quite similar. In spite of these similarities, small design changes are being made to try and improve patient satisfaction and surgical success. Normal function and long-term success are the goals of designers. Other advances in medicine have spin-off benefits in orthopaedics. Work in anesthesia, prevention of infection, improved operating room conditions, and better X-ray techniques are making joint replacement surgery more successful. There is absolutely no question that total joint replacement is one of the miracles of modern medicine. Nevertheless, it is not yet perfect. Many areas still exist where improvements can be made and it is in these areas where research is being conducted. One major area of research is improving the fixation of artificial joints to bone. As has been stated, this initially was done without bone cement (polymethylmethacrylate) but was later markedly improved with the use of cement. During the past ten years, scientists have been trying to further improve fixation by either improving the cementing techniques or by returning to a biologic method of fixation without cement. Both appear to be successful and both have current indications. Cemented fixation works well for older patients and seems most appropriately used in the femur during total hip replacement and also frequently in knee, shoulder and elbow replacements. Biologic fixation works well in most situations for young patients, especially in the hip socket, the knee and occasionally in the shoulder. Even with these improvements in fixation, better techniques are being explored. Newer cements which are less brittle and more biologically inert are being studied. Better cementing techniques, including eliminating stress concentration points in the cement and using composite structure fixation, are being applied. Biologic fixation is being improved by adding chemicals to porous surfaces to increase bone growth and adhesion to the prosthesis. Different materials are being investigated to improve the prosthesis. This includes many different metals, plastics, ceramics, and composites. The search is for durable, inert materials that are compatible with long term use. Some of these have similar mechanical characteristics to living tissue, and others are very different to meet different needs.

Current Research to Improve Total Joint Replacement:

1. Hip Research—There are three areas of investigation currently being pursued by Dr. Bertin:

 
1. Minimizing wear to maximize long-term successful hip replacement It has been recognized that the single factor that may limit lasting success after hip replacement is progressive wearing of the plastic acetabular or socket portion of the prosthesis. This acetabular wear occurs by the metal ball rubbing against the plastic socket and abrading off small particles of plastic. This has two detrimental effects. First, the plastic gets thinner and eventually could wear completely through. Secondly, the small particles that are worn away and float around in the area of the hip can cause the body to generate a type of reaction to these particles. The body tries to isolate and dissolve the particles. Since the particles cannot be dissolved, they continue to generate this response from the body. The body’s response to remove the plastic is not specific and instead of dissolving the plastic, some bone in the area of the hip may be dissolved. This insidious loss of bone can progress to the point that the artificial hip itself becomes loose or the bone fractures.  These reasons point to the fact that one of the most important things that could be achieved today in hip surgery is to decrease the wear process.  There are many things that can be done to accomplish this routinely.  One includes packaging the plastic in an oxygen free environment.  This isolation from oxygen keeps the plastic from oxidizing and then subsequently deteriorating.  Designing an acetabular prosthesis to minimize movement between the metal shell of the prosthesis and the polyethylene liner also minimizes wear.  Design is further important to decrease the chance for impingement between the femoral neck and the polyethylene liner.  Prosthesis design is again important because it is desirable to isolate the fixation interface from the wear debris that is formed.  Therefore the metal shell should be implantable in a stable position without the need for screws or screw holes through the socket. This will require precise instrumentation and a reproducible technique. Thus the present directions for research include:

 
1. Wear reduction by using a new and improved ultra high molecular weight polyethylene. Dr. Bertin is one of six investigators nationwide participating in a multicenter study to evaluate wear using “Longevity” polyethylene. This new material was developed by researchers in Boston (William Harris, M.D. and M.I.T.) and has been tested extensively in the laboratory. The plastic is treated to increase the crosslinking between the polyethylene molecules and this makes it much more wear resistant. It appears to have almost no wear in the laboratory under the stresses and environment that is similar to ones seen in hip replacements. Indeed this material has been tested in hip simulators and performed superbly. The F.D.A. has recently released this product and it can be used in patients undergoing total hip replacement. The study is a five-year prospective comparison between regular polyethylene and Longevity. Radiographs of patients in the study will be used to calculate the amount of wear of the two types of plastic.
 
2. Improving prosthesis design. Dr. Bertin has been extensively involved with hip prosthesis design. He has helped design hip replacements for primary (first time operations) and revision (redo or repeat) operations. These include the Trilogy Acetabulum which he designed with Drs. William Harris and Murali Jasty of Harvard University in Boston. He also helped develop the VerSys System for hip replacement. This is a family of femoral prostheses (the ball side of a total hip) to meet the needs of various problems, disorders and philosophies of surgically treating hip disease. Other designers of this system include Bobyn (Montreal), Burke (Boston), Cheribino (Italy), Collis (Eugene), Dunn (Utah), Galante (Chicago), Glassman (Arlington), Goldberg (Cleveland), Harris (Boston), Jasty (Boston), Johnston (Des Moines), Kyle (Minneapolis), Maloney (St. Louis), Poss (Boston), Rosenberg (Chicago), Rubash (Boston), Salvati (New York), Vaughn (North Carolina), and White (Albuquerque). Dr. Bertin also collaborated with Dr. Rubash (Boston) to design a prosthesis used commonly to treat patients with hip fractures, which allows them to rapidly resume walking after surgery. The goals of new prosthesis design are to:

 
1. Create a prosthesis that reestablishes patient anatomy to allow function as normal as possible.
2. Provide the surgeon with an implant that can be predictably used to obtain excellent clinical results treating a variety of disorders including a wide variety of patient sizes and shapes.
3. Develop a technique that is compatible with the new prosthesis allowing the doctor to readily apply the prosthesis to appropriate patients.
4. Incorporate new materials and technology, which will advance the current high standards and success rates.

Current ongoing research involves:

1. Reviewing the clinical results of patients who have received one of these new prostheses. Pain relief, return of function, and radiographic (xray) results are compiled and compared to other techniques and implants.
2. Designing a new femoral prosthesis to be used in revision hip surgery. This new modular prosthesis will provide the operating surgeon great flexibility in treating a wide variety of problems that are encountered when a hip replacement fails. New designs and technology will be available to make this system unique and helpful. This work is in conjunction with Drs. Cheribino (Italy), Dunn (Utah), Goldberg (Cleveland), Gross (Toronto), Jones (Dallas), Maloney (St. Louis), Rosenberg (Chicago), Rubash (Boston).
 
3. Establishing prosthesis fixation. One of the most important features of a successful total hip replacement is to have adequate attachment of the hip to the skeleton. This is called fixation and can be accomplished with bone cement or with bone ingrowth into the metal of the hip prosthesis.  It is recognized that cement fixation is advantageous for some situations and cementless fixation is better for others. For example, acetabular replacement is essentially always done without cement.  Initially (1960-1980) cement was used and there was a high rate of late fixation failure.  Since changing to cementless acetabular replacement, the long-term results have markedly improved.  The femoral component can be successfully attached with cement and with bone ingrowth.  There are definitely better results in the elderly patient, particularly with osteoporosis, if cement is used.  In the younger patient, cementless fixation may provide longer successful implantation.

Current ongoing research involves:

1. Evaluation of the clinical results of various types of fixation in different patient populations and comparing these results to each other and to the results of other centers.
2. Developing new techniques to improve fixation with and without the use of cement.
 
2. Knee Research—The following research is being performed to improve total knee replacement:
    
   1. Minimizing wear to maximize long-term successful knee replacement It has been recognized that the single factor that may limit lasting success after knee replacement is progressive wearing of the plastic tibial portion of the prosthesis.  This wear occurs by the metal femoral prosthesis rubbing against the plastic tibial prosthesis and abrading off small particles of plastic.  This has two detrimental effects.  First, the plastic gets thinner and eventually could wear completely through.  Secondly, the small particles that are worn away and float around in the area of the knee can cause the body to generate a type of reaction to these particles.  The body tries to isolate and dissolve the particles.  Since the particles cannot be dissolved, they continue to generate this response from the body.  The body’s response to remove the plastic is not specific and instead of dissolving the plastic, some bone in the area of the knee may be dissolved.  This insidious loss of bone can progress to the point that the artificial knee itself becomes loose or the bone fractures.  These reasons point to the fact that one of the most important things that could be achieved today in knee surgery is to decrease the wear process.  There are many things that can be done to accomplish this routinely.  One includes packaging the plastic in an oxygen free environment.  This isolation from oxygen keeps the plastic from oxidizing and then subsequently deteriorating.  The manufacturer can also use a process of manufacture called ‘net shape molding’ to create the plastic part.  This has been shown to make a significant decrease in the amount of wear particles generated.   Wear is also minimized in prosthesis design by designing a knee prosthesis to maximize contact between the metal and plastic and thereby decrease the stress per unit area and thus decrease the wear that occurs in each cycle.   The challenge then becomes to maintain normal function and movement with a highly conforming articulation. Thus the present directions for research include:
   
    
   1. Decreasing polyethylene wear through application of new prosthesis designs. Dr. Bertin is one of five centers in the United States using a new Mobile Bearing Knee (called the MBK).  This knee is designed to have two rather than one moving or mobile articulations.  By having a second moving site, the two surfaces can be completely conforming.  This should allow the knee to have the normal movement imposed by otherwise normal ligaments and muscles and at the same time decrease the stress in the plastic.   This lower stress on the polyethylene should prolong the life of the replaced knee.  This research will be performed by implanting a limited number of these knees in appropriate individuals and then following the results by clinical parameters and radiographic evaluations.  The study has been approved by the F.D.A. and the hospital Institutional Review Board.  If this proves successful, it would mean a new design direction for knee replacements in the future.
    

        

   2. Evaluation of current designs and techniques.  Dr. Bertin was one of the designing surgeons of the NexGen Total Knee System.  Other designers include:  Insall (New York), Walker (London), Andriacchi (Palo Alto), Vince (Los Angeles), Dunn (Salt Lake City), Galante (Chicago),  Booth (Philadelphia),  Goldberg (Cleveland).  This system has become the most popular knee system in the world.  Dr. Bertin continues to evaluate the results he has had with this system and present the results in different countries.  By better understanding this system including any disadvantages as well as advantages, Dr. Bertin and others will be able to use it most appropriately today for patients.  It will also serve as the basis for the improvements in the next generation of knee replacements.  Current research involves:
   
    
   1. Prospective evaluation of the clinical and radiographic results of knee replacement patients. These are recorded in a computer database that allows for comparison of multiple variables and analysis of various groups of patients.  As results become available, some techniques and suggestions for future practice are formulated.
    
   3. Improvement of results of revision total knee replacement (reoperation of failed knees).   With the age of patients needing knee replacements decreasing and the average age of the population increasing, the need to eventually revise failed knees in increasing.  Dr. Bertin has had a special interest in developing techniques and implants to be used to solve this complex problem.  He helped develop the revision portion of the NexGen system, which now allows patients to have a second chance at a successful knee replacement.  These implants are used to solve loosening, infection or prosthesis fracture.
    

Dr. Bertin has been involved with many innovative and influential developments in the design and development of total hips and total knees. His fifteen patents in the United States and others internationally evidence this. He continues to collaborate with other physicians to improve the care of each patient and create better techniques for other doctors.