Differences Between the Sexes in the Anatomy of the Anterior Condyle of the Knee

Fehring, Thomas K. MD; Odum, Susan M. MEd, CCRC; Hughes, Josh BS; Springer, Bryan D. MD; Beaver, Walter B. Jr. MD

Journal of Bone & Joint Surgery - American Volume:
doi: 10.2106/JBJS.H.00834
Scientific Articles
Abstract

Background: Claims that there are dramatic differences in anterior condylar anatomy between the sexes have led to the design of total knee implants with thinner anterior condyles specifically for use in women. We had observed, in our patients, differences in anterior condylar anatomy that appeared to be highly variable and dependent on the size, height, and ethnicity of the patient as well as his or her sex. Because of this observed variability, we sought to determine if differences in anterior condylar anatomy between the sexes actually exist.

Methods: Two hundred and twelve randomly selected magnetic resonance images (112 of men and 100 of women) were evaluated. The anterior condyle was defined as the area of bone anterior to the anterior femoral cortex, 10 mm above the joint line. The medial and lateral heights of the anterior condyles were measured in millimeters directly from magnetic resonance imaging data obtained in two planes. The so-called aspect ratio was calculated to determine whether patient size had an effect on the size of the anterior condyles.

Results: On the basis of the numbers available, there was no significant difference (p = 0.16) between the sexes with regard to lateral condylar height. The average difference was only 0.5 mm. There was a significant difference (p = 0.001) between men and women with regard to medial condylar height. However, the average difference was only 1.1 mm. While the difference between the sexes with regard to anterior condylar height was nominal, the measurements were highly variable regardless of sex. On the basis of the numbers available, there were no significant differences between men and women with regard to the condylar aspect ratios.

Conclusions: The difference in anterior condylar anatomy is mentioned as one of three reasons for the need for a so-called gender-specific knee implant. The aspect ratio reported here, which is a surrogate for patient size, seems to negate any difference in anterior condylar anatomy based on sex. We have shown that anterior condylar anatomy is highly variable regardless of sex. We believe that implants as well as surgical techniques should be designed with the variability of anterior condylar anatomy taken into account and with an attempt to reproduce such anatomy regardless of sex.

Author Information

1OrthoCarolina Hip and Knee Center, 1915 Randolph Road, Charlotte, NC 28207. E-mail address for T.K. Fehring: Thomas.Fehring@orthocarolina.com

2OrthoCarolina Research Institute, 4601 Park Road, Suite 250, Charlotte, NC 28209

Article Outline

Claims that there are dramatic differences in anterior condylar anatomy between the sexes has led to the design of total knee implants with thinner anterior condyles specifically for use in women. Proponents of this modification maintain that using a conventional implant in women may overstuff the patellofemoral joint, leading to a feeling of tightness and/or limitation of the range of motion1. While overstuffing of the patellofemoral joint has been described as a clinical problem by certain authors2-4, others have claimed that it is not clinically relevant5,6.

One can theorize that a mismatch between the anteroposterior dimension of the host bone and the anteroposterior dimension of the prosthetic implant may negatively affect knee kinematics6. A patient in whom a small anterior condyle has been replaced with an implant with a thicker anterior flange is at risk for anterior knee pain and a diminished range of motion. A patient in whom a large anterior condyle has been replaced with an implant with a thinner anterior flange may have a decreased quadriceps moment arm and a functional disadvantage.

We had observed, in our patients, that anterior condylar anatomy appeared to be highly variable and dependent on the size, height, and ethnicity of the patient as well as his or her sex. Because of the observed variability in anterior condylar anatomy, we sought to determine if differences in anterior condylar anatomy between the sexes actually exist. The null hypothesis was that there is no difference between men and women in terms of anterior condylar dimensions.

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Materials and Methods

After this study was approved by our institutional review board, 212 magnetic resonance images (112 of men and 100 of women) that had been randomly selected from our orthopaedic hospital's imaging database were evaluated. The anterior condyle was defined as the area of bone anterior to the patellofemoral sulcus, 10 mm above the joint line (Fig. 1). In order to perform the anterior condylar measurements, the anterior cortex of the femur was identified on the sagittal midline image (Fig. 2-A). At a point 10 mm above the joint line, an axial cut was made (Fig. 2-B). This point was selected to simulate a distal femoral cut in a routine total knee replacement. No attempt was made to account for the variability in the length of the trochlear groove. All measurements were made by a single observer (J.H.). Measurements were made directly from the digital magnetic resonance images with use of radiographic measuring software (Stentor; Philips Radiology Informatics, San Francisco, California). The medial and lateral heights, in millimeters, of the anterior condyle were then measured directly from the axial magnetic resonance imaging data. Bone above the anterior cortical line was considered to represent the anterior condyle (Fig. 2-B). Additionally, the medial-lateral dimension (width) between the epicondyles was measured at this level (Fig. 2-B). In order to determine whether patient size had an effect on the size of the anterior condyles, a so-called aspect ratio between these two measurements was calculated by dividing the anterior condylar height by the medial-lateral dimension.

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Statistical Methods

Standard descriptive statistics, including the mean and standard deviation, were calculated. A Kolmogorov-Smirnov test for normality was performed, and the data were found to be normally distributed. Differences in condylar dimensions were determined with use of an independent t test. To determine whether differences between the sexes in anterior condyle height were affected by the medial-lateral epicondylar width, an analysis of covariance was performed, with two models tested. With the first model, the independent variable was sex, the dependent variable was medial condylar height, and medial-lateral width was the covariate. With the second model, lateral condylar height was used as the dependent variable, and the other variables remained the same. The significance level for all statistical tests was established a priori to be 0.05. A post hoc power analysis was performed with use of the anterior and medial condyle ratio variables.

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Source of Funding

DePuy Orthopaedics (Warsaw, Indiana) provided partial financial support to OrthoCarolina Research Institute for research staff.

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Results

Condylar Height

On the basis of the numbers available, there was no significant difference between the sexes with regard to lateral condylar height. The average difference was only 0.5 mm. The average lateral condylar height (and standard deviation) was 7.3 ± 3.0 mm (95% confidence interval = 0.58) for men compared with 6.8 ± 3.1 mm (95% confidence interval = 0.59) for women (p = 0.16, 1 − β = 0.63).

There was a significant difference between the sexes with regard to medial condylar height. However, the average difference was only 1.1 mm. The average medial condylar height was 5.7 ± 2.4 mm (95% confidence interval = 0.48) for men compared with 4.6 mm ± 2.6 (95% confidence interval = 0.49) for women (p = 0.001, 1 − β = 0.92).

While the difference in the anterior condylar height between the sexes was nominal, the measurements were highly variable regardless of sex. The range for both medial condylar height and lateral condylar height was from 0 to 18 mm (Figs. 3-A and 3-B).

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Epicondylar Dimension
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There was a significant difference in the medial-lateral epicondylar dimension between the sexes. The variability of this measurement was greater for women. The average medial-lateral dimension between the epicondyles was 86.6 ± 4.9 mm (95% confidence interval = 0.92) for men compared with 76.6 ± 6.5 mm (95% confidence interval = 1.29) for women (p < 0.0001, 1 − β = 0.99).

In the analysis of covariance models, there were no significant differences, on the basis of the numbers, between the sexes with regard to medial condylar height (F[1,209] = 0.14, p = 0.71]) or lateral condylar height (F[1,209] = 3.45, p = 0.0.06]) after we adjusted for the medial-lateral epicondylar width.

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Aspect Ratio (Anterior Condylar Height/Medial-Lateral Width)

On the basis of the numbers, there was no significant difference between men and women with regard to the condylar aspect ratio. The average medial condylar aspect ratio was 0.07 ± 0.03 mm (95% confidence interval = 0.005) for men compared with 0.06 ± 0.03 mm (95% confidence interval = 0.006) for women (p = 0.13, 1 − β = 0.67). The average lateral condylar aspect ratio was 0.08 ± 0.03 mm (95% confidence interval = 0.006) for men compared with 0.09 ± 0.04 mm (95% confidence interval = 0.007) for women (p = 0.50, 1 − β = 0.44). A graphical illustration of the aspect ratios resulted in two nearly identical bell-shaped curves for each sex (Figs. 4-A and 4-B).

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Discussion

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There appear to be differences between men and women with regard to musculoskeletal disease. Women are at higher risk for the development of osteoarthritis and have higher rates of disability attributable to osteoarthritis7. A number of authors have noted that the results of total knee arthroplasty in women are equal or superior to those in men8-11. Despite these favorable results, the necessity for a so-called gender-specific total knee replacement has been marketed to patients and surgeons alike. The difference in anterior condylar anatomy is cited as one of three reasons for a gender-specific knee implant1. However, differences in anterior condylar anatomy between the sexes were negligible both in the series reported here and in previously published reviews of the subject. Like us, Poilvache et al.12 found that the anterior extent of the condyles was highly variable. They reported an average difference between men and women of 1.48 mm anterolaterally and 1.67 mm anteromedially. Conley et al.1 noted that the difference between the sexes with regard to condylar anatomy averaged only 1.3 mm anteromedially and 0.8 mm anterolaterally.

The above values were strikingly similar to those noted in the current series, in which there was a 0.5 mm difference in the lateral height of the anterior condyle and a 1.1 mm difference in the medial height. While there are no firm data to substantiate whether this small a difference would be clinically relevant, we doubt that it is. Additionally, neither of the two previously published reports1,12 included an aspect ratio to adjust for patient size, as we did in this study. We believe that correcting for the medial-lateral dimension is a surrogate for correcting for patient size regardless of sex and is an important factor to take into account when analyzing this type of data. The aspect ratios reported in our study (0.07 for men compared with 0.06 for women) seem to negate any difference based on sex. This finding corroborates the findings of Grelsamer et al.13, who contradicted another claim—that there is a significant difference in the Q angle between the sexes1.

It has been suggested that overstuffing of the patellofemoral joint is a major clinical problem. Star et al.14 noted increased patellofemoral compressive forces with increased patellar bone and implant thickness. In a review article, Bong and DiCesare2 stated that stiffness and tightness of the extensor mechanism were a result of inadequate resection of the patella or anterior placement of the femoral component. Shoji et al.4 found that an increase in patellar thickness of 20% led to diminished flexion.

In contrast, Daluga et al.3 failed to find a correlation between a change in the anteroposterior measurement and the postoperative knee range of motion. They found that at least a 12% increase in the anteroposterior dimension was necessary before the need for manipulation was increased. Mihalko et al.6 found that, when a cut was made flush with the anterior femoral cortex, the lateral condylar height of a conventional implant increased the lateral height of the anterior condyles by only 1.1 ± 2.6 mm and the medial anterior condylar height was increased by only 0.5 ± 2.2 mm. This negligible increase with use of conventional implants of three different manufacturers is noteworthy. Pierson et al.5 stated that overstuffing of the patellofemoral joint had no effect on clinical symptoms. They found that increasing the height of the patellofemoral joint did not change the range of motion and concluded that overstuffing does not diminish the clinical outcome.

Shortcomings of our data analysis are that it is based solely on magnetic resonance images of patients for whom we had no clinical history. Because these magnetic resonance images were randomly selected, we do not know the stature or ethnicity of these patients. Regardless, our data were nearly identical to those in previous studies1,12. Additionally, we did not measure the absolute length of the trochlear groove in each case, which may have affected our results. We chose to measure at the level of a routine distal femoral resection, which more closely replicates the clinical situation. On the basis of our data, we cannot comment on the other aspects of a so-called gender-specific knee implant—that is, whether there is a need for a narrower femoral component or an increased Q angle1. However, the necessity of a thinner anterior condyle for women is not substantiated by the data presented in our study or our review of the literature.

We conclude that anterior condylar anatomy is highly variable with a bell-shaped distribution regardless of sex. While narrowing the prosthetic anterior condyle may diminish anterior knee pain or improve the range of motion, the clinical advantage of doing so has not been studied, to our knowledge. Placing a thinner anterior condyle in someone who used to have a large anterior condyle may have a negative effect on quadriceps function as the quadriceps lever arm would be diminished. We believe that implants as well as surgical techniques should be designed with the variability of anterior condylar anatomy taken into account and an attempt to reproduce such anatomy regardless of the sex of the patient.

Disclosure: In support of their research for or preparation of this work, one or more of the authors received, in any one year, outside funding or grants of less than $10,000 from DePuy Orthopaedics. In addition, one or more of the authors or a member of his or her immediate family received, in any one year, payments or other benefits in excess of $10,000 or a commitment or agreement to provide such benefits from a commercial entity (DePuy Orthopaedics). Also, a commercial entity (DePuy Orthopaedics) paid or directed in any one year, or agreed to pay or direct, benefits of less than $10,000 to a research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which one or more of the authors, or a member of his or her immediate family, is affiliated or associated.

A commentary is available with the electronic versions of this article, on our web site (www.jbjs.org) and on our quarterly CD-ROM/DVD (call our subscription department, at 781-449-9780, to order the CD-ROM or DVD).

Investigation performed at the OrthoCarolina Hip and Knee Center, Charlotte, North Carolina

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