Gender-sensitive knee design: not size

Properly sized conventional prostheses will overhang the anterior femoral bone in women.

Issue: March 2007

ByRobert E. Booth Jr, MD

John Insall, MD, who recognized 15 years ago that women’s femurs had a more trapezoidal shape and greater AP dimension than their counterparts in men, generated the original impetus for a gender-sensitive total knee. A custom component, 5 mm larger in the sagittal diameter, was created to simultaneously avoid overhang and yet not over-resect the posterior femoral condyles — the same compromise that confronts surgeons today in using generic (and especially non-handed) implants.

Robert E. Booth Jr., MD
Robert E. Booth Jr.

For more than a century, forensic pathologists have been able to distinguish men’s bones from women’s bones. Many articles have described the “aspect ratio” used to design femoral components and its gender bias. Scott et al for example, identified the bimodal gender-based spectrum of femoral size and shape. Mahfouz et al have created a CT-scan based bone atlas to document the many differences (eg, aspect ratio, sulcus angle, femoral shape, intercondylar width, anterior femoral thickness, etc.) between male and female knees. There is no question that these differences are real.

Clinical significance

The clinical significance of these parameters becomes apparent when we look at the outcomes of our knee arthroplasties. Bourne et al showed women to have worse results with greater disability than men after total knee arthroplasty (TKA). Ritter et al have admitted, “Female gender has a tendency to have a little more pain.” Parvisi et al and Fisher et al noted that stiffness and pain are greater in women in large series sorted for gender. Most compelling of all: the results of a 42,217 patient registry of 421 surgeons in 26 countries — looking at neutrally (3°-9°) aligned TKA’s — showed women to have a statistically significant higher incidence of lateral retinacular releases and lateral collateral releases, as well as worse flexion, lower walk tolerance, and lower Knee Society function scores.

Clearly, we are doing a better job for men than women, even though the latter are more numerous and the driving force in knee arthroplasty today. The presence of gender-based distinctions and demands is ubiquitous in modern medicine, and Laura Tosi, MD and colleagues in a 2005 Orthopaedic Forum in the Journal of Bone and Joint Surgery, brilliantly described the concept of “sexual dimorphism.”

The most obvious issue is that shape — not size — determines the appropriate “fit” of a femoral component on the distal femur. If a prosthesis is correctly sized in a woman’s knee, current components will overhang the anterior femoral bone in at least 70% of patients.

The surgeon can either accept the overhang or “downsize” to a smaller implant. The former choice is believed by most surgeons to create anterior knee pain and to require extra soft-tissue releases to optimize patellar positioning. The latter choice requires either anterior bone resection (at the risk of notching) or posterior over-resection (which weakens the cam effect by reducing the correct femoral offset and thus reducing flexion).

Neither of these choices is desirable. Both represent compromises in the optimal reconstruction of an arthritic knee. There is no question that we used generic, non-handed components for decades, but knee surgery has moved to a new level of demand and sophistication. The old non-handed components have served their purpose and have had their day — we can now do better. The new, narrowed anterior profile of the gender-sensitive TKA allows full restoration of the appropriate AP dimension without overhang.

Greater Q angle

A second issue is the angulation of the trochlear sulcus. Hsu et al wrote there is negligible documented difference between the anatomic axis and the mechanical axis by gender, but there is a distinct increase in the quadriceps angle, the “Q-angle,” in women because of their proportionately greater pelvic width. This angle, measured in extension but functional in flexion, is 3° greater in the gender-sensitive TKA, reducing the need for soft tissue releases to enhance patellar tracking.

“Neither of these choices is desirable. Both represent compromises in the optimal reconstruction of an arthritic knee.”
— Robert E. Booth Jr., MD

A third advantage in the gender-sensitive femur is the thinner anterior flange. It is well known and well documented by Mahfouz et al that most women’s knees have less anterior femoral condylar bone than their counterparts in men.

To apply a generic prosthesis — without reducing the thickness of the patellar composite by over-resecting the patella — is to automatically and definitionally “overstuff” the anterior compartment of the knee.

This also results in more anterior knee pain, more soft tissue releases, greater stiffness, and less flexion, which are all characteristics of female outcomes from current prostheses. No true advocate of “measured resection” would consider putting in more metal and plastic than resected bone, yet this is exactly what contemporary designs compel us to do!

Mass customization

The early results of gender-sensitive knees have shown increased motion in women, although this finding is somewhat complicated by the incorporation of high-flexion posterior femoral geometrics in the implants. Most surgeons have found that their spectrum of implant use has increased one size, suggesting that we are no longer downsizing to avoid overhang.

The pain and comfort benefits are yet to be proven, partially because of the patient “buy-in” to the gender concept based on cognitive dissonance. Is marketing to blame for this? Of course, but how different are gender direct-to-consumer promotions than the ceramic hip or mobile knee ads that preceded them?

The best part about gender knees is that they have opened a new era of “mass customization” in arthroplasty, taking us beyond our current asymptote of design.

Gender is now “beyond the bones,” as we accommodate the different kinematics of women’s knees, the issues of ethnicity and developmental variations and the ever-increasing demands of a whole new generation of total knee recipients.

For more information
Booth R, Bertin K. Zimmer NexGen Knee Registry, 2006.
Bourne RB, et al. Patient factors vs. outcomes at 5-10 years status post TKA. Presented at the 2006 Interim Meeting of The Knee Society. Alexandria, Va. 2006.
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):213-217.
Fisher D, et al. Looks good but feels bad. Presented at the Meeting of the American Association of Hip & Knee Surgeons. Dallas. 2006.
Hsu RWW, Himeno S, Coventry MB, Chao EYS. Normal axial alignment of the lower extremity and load-bearing distribution at the knee. Clin Orthop Related Res. 1990; 255:215.
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. Antibes, France.
Ritter MA, et al. Benefits of high flexion after TKA. Presented at the 2006 Interim Meeting of The Knee Society. Alexandria, Va.
Parvisi J, et al. Stiffness after TKA. Presented at The Eastern Orthopaedic Association Meeting, 2006. Boca Raton, Fla.
Tosi L, et al. Does sex matter in musculoskeletal health? J Bone Joint Surg Am. 2005 July; 87(7):1631-1647.
Robert E. Booth Jr., MD, chief of orthopedic surgery, Pennsylvania Hospital, 800 Spruce Street, Philadelphia, PA 19107; 800-789-7366. He has received royalties from Zimmer.