Gender Specific Implants
If one thinks about having an artificial knee prosthesis that is size and shape specific for gender, it seems to be a natural conclusion. Indeed, why would someone conclude otherwise! Knee replacements were first implanted in patients in the late 1960s. Since then there have been many changes to improve the results. The most recent significant change has been the development and use of total knee implants that are gender specific.
The total knee prosthesis which has been used for over thirty years had an average shape and various sizes. It turns out that the average more closely approximates male anatomy and is very different from a woman’s knee shape. Surgeons noticed this difference and now implants of specific size and shape for a woman’s knee are available.
Why is this so important for women?
About two thirds of the 400,000 knee replacement operations done in the United States are performed in women. Because the implants that were previously available did not fit female anatomy accurately, surgeons had to make adjustments in the surgical technique to get the implants to fit. This is very similar to forcing everyone’s feet into the same size or shape of shoe. Because of these forced adjustments in surgery, the results of knee replacements in women could be compromised.
How was the new Gender Solution prosthesis developed?
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When the differences between men and women knees were recognized, they had to be quantitated. Therefore over 800 detailed CT scans of knees were analyzed to determine the exact differences between male and female knees. The scans showed that the overall shape was very different. This illustration shows the first difference. The female knee is narrower or thereby taller for any given width. This can be seen easier if one of the knees is laid on top of the other:
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It became very clear that for any “height” of knee, there was a unique width for men and for women. This is the first distinctive for this new knee.
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The second difference that was incorporated into this new prosthesis is the thickness of the “anterior flange” of the prosthesis. This is the front third of the implant and in women it is much thinner than in men as show by the following drawings that summarize the CT scan data. This is important because it makes the prosthesis less bulky and helps with the way the kneecap tracks or moves across the front of the knee.
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Associated with this thinner anterior flange is a narrower anterior flange. This also helps with patellar tracking and reduces the mass effect of the prosthesis in the front of the knee.
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The third major difference between men and women’s knees is the angle that the knee bends in relation to the hip and the kneecap. This is called the “Q angle”. In women the Q angle is larger than in men. In all other previous implants, there has been no consideration for this important difference. In the Gender Solutions Femoral prosthesis, this correct Q angle is incorporated in the design.
All of these unique features are part of the new prosthesis. This is in addition to the fact that this implant, the NexGen, has the recognized highest success rates (as reflected by the lowest revision rates) in the Swedish Knee Arthroplasty Registry. In addition the NexGen Flex prosthesis is designed to accommodate high flexion, up to 155 degrees. This makes the prosthesis uniquely appropriate for those women who desire higher degrees of flexion. Surgical techniques also exist for it to be implanted with a less invasive surgical procedure. All of these significant changes have made knee replacements much better for women. We have recognized significantly improved results in using this prosthesis.
Dr. Bertin’s Contribution to Gender Specific Implants
After performing many knee replacement operations, Dr. Bertin recognized the distinct differences between a man’s and a woman’s knee anatomy. He then approached Zimmer, an orthopedic manufacturing company that he had worked with for many years, with the idea to have a gender specific knee prosthesis. Other orthopedic surgeons were recognizing this same issue of gender differences. Zimmer assembled a design and development team of orthopedic surgeons and engineers to work on this project. This team included Dr. Bertin and other experienced individuals from around the world. Years of work and research culminated in the manufacture of the Zimmer NexGen.Gender Solutions Knee Prosthesis and the approval by the FDA for use in the United States in May 2006. Dr. Bertin in Salt Lake City and Dr. Robert Booth in Philadelphia implanted the first knees in patients on May 9, 2006.
Resources for more information about gender specific implants:
www.genderknee.com –This website is specifically about the differences between male and female knees and explains the Zimmer NexGen Knee prosthesis in more detail.
www.zimmer.com –The company that has been innovative in supporting this prosthesis is the worlds largest pure orthopedic company.
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi –Many of the references listed below can be found on this research site for all published, reviewed medical articles.
www.pacewithlife –The Pace With Life web site explains the gender solution
knee from the perspective of the manufacturer.
Technical References:
1. Hitt K, Shurman IIJ, Greene K, et al.
Anthropometric measurements of the human knee: correlation to the sizing of
current knee arthroplasty systems.
J Bone Joint Surg. 2003;85:155-122.
2. Poilvache PL, Insall JN, Scuderi GR,
Font-Rodriguez DE. Rotational landmarks and sizing of the distal femur in total
knee arthroplasty, Clin Orthop.
1996;331: 35-46.
3. Vaidya SV, Ranawat CS, Aroojis A, Laud NS. Anthropometric measurements to design total knee prostheses for the Indian population. J Arthroplasty. 2000;15(1):79-85.
4. Urabe K, Miura H, Kuwano T, et al. Comparison between the shape of resected femoral sections and femoral prostheses used in total knee arthroplasty in Japanese patients. J Knee Surg. 2003;16(1):27-33.
5. Chin KR, Dalury DF, Zurakowski D, Scott RD. Intraoperative measurements of male and female distal femurs during primary total knee arthroplasty. J Knee Surg. 2002;15(4):213-214.
6. U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Center for Health Statistics. 2003 National Hospital Discharge Survey, Advance Data No. 359. July 8, 2005; Table 8:14.
7. U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Center for Health Statistics. 2003 National Hospital Discharge Survey, Advance Data No. 359. July 8, 2005; Table 10:16.
8. Hawker G, Wright J, Coyte P, et al. Differences between men and women in the rate of use of hip and knee arthroplasty, The New England Journal of Medicine, 342:1016-1022, 2000.
9. Mahfouz M, Booth R Jr, Argenson, J, Merkl, BC, Abdel Fatah EE, Kuhn MJ. Analysis of variation of adult femora using sex specific statistical atlases. Presented at: Computer Methods in Biomechanics and Biomedical Engineering Conference; 2006.
10. Data on file at Zimmer.
11. Scott NW. Pearls on avoidance and treatment of intraoperative and postoperative complications – exposure of the stiff knee. Presented at: American Association of Hip and Knee Surgeons, Knee Society Specialty Day; March 25, 2006.
12. Csintalan RP, Schulz MM, Woo J, McMahon PJ, Lee TQ, Gender Differences in Patellofemoral Joint Biomechanics, Clin Orthop. September 2002; 402:260-269.
13. Aglietti P, Insall JN, Cerulli G. Patellar pain and incongruence. I: Measurements of incongruence. Clin Orthop. 1983;176:217-224.
14. Hsu RWW, Himeno S, Coventry MB, Chao EYS. Normal axial alignment of the lower extremity and load bearing distribution at the knee, Clin Orthop. 1990;255:215-227.
15. Woodland LH, Francis RS. Parameters and comparisons of the quadriceps angle of college-aged men and women in the supine and standing positions. American Journal of Sports Medicine. 1992;20:208-211.
16. Measurements taken from x-ray templates.
