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INTRODUCTION
Variability in the ratio of the anterior-posterior (AP) and transepicondylar
(ML) dimensions of the distal femur among males and females may have implications
for implant design and functional outcome after total knee arthroplasty
(TKR) . Current AP and ML size dimensions of standard femoral components
may not optimally account for the variability between male and female
osteoarthritic knees. Only a few studies have analyzed the dimensions
of the distal femur and made comparisons according to gender. 4-7
These studies have been limited because of small sample sizes and the
fact that some were done on non-arthritic cadaveric femoras 5,
7 that were stripped of all soft tissue. In addition, some of these
studies have relied on indirect measurements using plain radiographs 4,
5, 7 and have not accounted for magnification.
MATERIALS
AND METHODS
We prospectively
determined the intra-operative measure- ments of the anterior-posterior
and transepicondylar dimensions in a large series of consecutive osteoarthritic
knees (100 male and 100 female Caucasian patients) undergoing unilateral
primary TKR by two separate surgeons. All patients were skeletally mature
and had no evidence of Paget's or other metabolic bone diseases. All measurements
were done at the time of surgery using a sterile ruler after removing
all osteophytes and after making the distal femoral cut. The distal femoral
cut was 7 millimeters of bone from the more prominent distal condyle (usually
medial). The transepicondylar axis (medio-lateral dimension) was measured
as a transverse line drawn between the medial and lateral epicondylar
prominences at the level of the distal resection. The anterior-posterior
axis was measured as the distance between the anterior femoral cortex
just proximal to the trochlea (usually maximal height) and the transposterior
condylar axis. Measurements were obtained from the larger of the two posterior
condyles. Values for the AP, ML, and AP/ML ratio were evaluated by the
Kolmogorov-Smirnov test 8
and found to deviate significantly from a normal distribution. Therefore,
median values and the range are reported. AP and ML values were compared
using the Wilcoxon signed-ranks test. Differences between males and females
were evaluated by the Mann-Whitney U test 8
. For all comparisons, a two-tailed value of P < 0. 05 was considered
statistically significant.
RESULTS
We found that
the ML dimension was greater than the AP dimension for all osteoarthritic
distal femurs. Female arthritic knees were narrower on the ML dimension
than their male counterparts In addition, females had a significantly
greater AP/ML ratio than males.
DISCUSSION
Determining
the linear dimensions of the distal femur across genders is important
for several reasons. Linear measurements may identify reproducible dimensional
relationships within the knee among genders. Prosthetic manufacturers
can use this data to streamline their production to generate sizes that
are more likely to fit most males and females. Finally, surgeons can be
more aware of these size differences and make adjustments during pre-operative
planning and surgery to avoid medio-lateral impingement 3
and malpositioning of the prosthesis. 1,
2 One rule of thumb in our practice is to cover as much bone as possible
and lateralize the femoral prosthesis without metal overhang. The AP and
ML dimensions showed that all distal femurs are wider than they are tall
and that females have narrower knees than males. Thus, males are more
likely to have bone overhang the outer borders of the prosthesis than
females. In contrast, females are more likely to have the outer borders
of the prosthesis hang over the lateral and medial edges of the femoral
condyles. Both scenarios are susceptible to mediolateral impingement and
decreased range of motion. This is particularly true with more conforming
implant designs. Females are likely to impinge on the soft-tissues, such
as the capsule or retained menisci, while males with uncovered bone, are
likely to impinge on the polyethylene liner. 3
The surgeon should anticipate these possibilities before cementing the
prostheses and should clear the soft tissues and excess bone from the
edges of the femoral prosthesis. Centering the femoral implant on the
distal femoral surface can help avoid excessive bony or metal overhang.
Further studies are also encouraged in this area to assess how much overhang
is critical to cause medio-lateral impingement in poorly fitting femoral
prostheses.
CONCLUSION
Our experience
suggests that males and females have comparable AP/ML ratios although
most female arthritic distal femurs tend to be narrower than males. Femoral
implant designs and surgical techniques should account for these differences
to avoid medio-lateral overhang and possible capsular impingement. Further
studies are encouraged to determine the amount of prosthetic or bony overhang
that is critical to cause clinically significant impingement.
ACKNOWLEDGMENT
We thank David Zurakowski, Ph. D. for his invaluable assistance in performing
the statistical analysis.
Kingsley R. Chin,
MD is a Clinical Fellow in Orthopaedic Surgery at Harvard Medical
School.
David F. Dalury,
MD is an Assistant Professor in Orthopaedic Surgery, John's Hopkins
University, Baltimore, MD
Richard D. Scott,
MD is a Professor in Orthopaedic Surgery at Harvard Medical School.
Address correspondence
to:
Richard D. Scott,
MD
Department of Orthopaedic Surgery
New England Baptist Hospital
125 Parker Hill Avenue
Boston, MA 02120
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