Jean-Noël A Argenson MD
The Aix-Marseille University
Department of Orthopedic Surgery
Total knee arthroplasty (TKA) has become a successful method of treating advanced osteoarthritis of the knee that provides pain relief and restoration of knee function, including high flexion activities. It can now be performed using less invasive procedures, which results in limited disruption of the soft tissues and muscles surrounding the joint and quicker patient recovery time. The sex of patients undergoing TKA is predominately female (between 65% and 70%) – principally due to the fact that osteoarthritis affects more women than men – especially in the knee. Anther reason is the longer life expectancy in women, meaning that at the time of TKA (usually around 65 years of age) there are more women than men. Finally, it appears that body mass index (BMI) is proportionally higher in women and there is a direct relation with a high BMI and the incidence of osteoarthritis.
Statistical bone atlases
Previous studies evaluating the anatomy of the distal femur have provided rudimentary information regarding geodesic and angular measurements. These measurements are, however, subject to reproducibility errors as a result of human interaction.(1,2) A novel 3D matching method (which matches a 3D surface with a high degree of accuracy) has since been developed and employed, the results of which were used to automatically locate landmarks useful for prosthesis design and accounted for sex anatomical differences.
The specific 3D reconstruction method developed by Dr Mohamed Mahfouz at the University of Tennessee was used to create a male and female statistical bone atlas, based on CT scan data of 144 adult femora. A comparison of global shape differences observed between male and female femora was then made by comparing same-sized male and female femora as defined by the sex-specific atlases.(3) The main purpose of this was to discover how differences in shape between males and females change with relative size. Using these atlases, a wide array of comparisons and measurements can be made, including general morphologic measurements (eg, mechanical axis, anatomical axis, mediolateral [ML] width), and other surgically relevant measurements.
Size differences in prosthesis design
Quantifying differences in femoral size and shape between males and females has important connotations for prosthesis design. Although it seems intuitive that males would generally have larger femora, they do not scale in the same manner as females. As a result, large differences (in the order of 1–2cm) exist in the area of the distal femur, which is instrumental for sizing femoral implants. The best anteroposterior (AP) size versus the best ML size represents one of the main trade-offs that surgeons must decide on when performing TKA. For example, if a patient has a ML overhang then the surgeon must size that patient with a smaller implant to achieve conformity with the bone and avoid impinging on the soft tissue. However, this may create problems that result in excessive bone loss – especially at the anterior aspect of the femur. For a given AP height (the only one measured during surgery with current instrumentation) there is a range of measured ML width values. A general trend towards smaller ML widths for a given AP height in females was observed, while the aspect ratio was more constant in male.
Significant anatomical differences were not limited to the AP versus ML ratio. There were also differences in the lower medial and lateral condyles of females resulting in less prominent anterior condyles. This difference is clinically relevant because of the possible contact and subsequent risk of impingement between the anterior condyles and the soft tissues around the knee such as muscles or articular capsule. Finally, when aligning male and female knees along their transepicondylar axes, the anatomical axis changes angle – as demonstrated by internal rotation of the female knee. This difference in rotation, added to the fact that the sulcus is significantly flatter in females, may increase the risk of patellar instability after TKA.
Gender solution knee implants
All these data imply that male and female morphology is essentially different and, therefore, a basis for a gender-specific implant exists. Such comparisons led to the design of the NexGen series (Zimmer) for TKA (see Figure 1).
Accounting for the morphological differences described above, the Gender Solutions knee implants include an aspect ratio in which the ML width has been reduced to minimise the potential of overhang. Additionally, the anterior condyle has been reduced to account for the lower anterior condyle seen in females and prevent possible patello-femoral overstuffing issues. Finally, the sulcus has been recessed in order to improve the trochlear angle accounting for the differences found between male and female for patellar tracking.
All these improvements in design will potentially improve the tolerance of the prosthesis in the joint by limiting the possible impingement between implant and soft tissue, which can be a source of discomfort for the patient even with a well-functioning TKA. Clinicians might therefore see benefit for their patients after TKA, but only large-scale clinical feedback will be able to demonstrate that benefit.
- Yoshioka Y, Siu D, Cooke TD. The anatomy and functional axes of the femur. J Bone Joint Surg Am 1987;69:873-80.
- Hitt K, Shurman JR 2nd, Greene K, et al. Anthropometric measurements of the human knee:correlation to the sizing of current knee arthroplasty systems. J Bone Joint Surg Am 2003;85:115-22.
- Mahfouz M, Booth R, Argenson JN, et al. Analysis of variation of adult femora using sex-specific statistical atlases. To be presented at the International Conference Biomedical Engineering; 2006 Dec 11-14; Kuala Lumpar, Malaysia.