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Original Articles: Knee

Femoral Condyle Geometry in the Normal and Varus Knee

Matsuda, Shuichi MD*; Matsuda, Hideo MD, PhD**; Miyagi, Tomoyuki MD*; Sasaki, Kunio MD*; Iwamoto, Yukihide MD, PhD; Miura, Hiromasa MD, PhD

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Clinical Orthopaedics and Related Research: April 1998 - Volume 349 - Issue - p 183-188
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Abstract

Correct rotational alignment of the femoral component is one of the most important factors in successful total knee arthroplasty. The posterior condyles have been used as a femoral rotational alignment landmark. Slight external rotation of the femoral component relative to a line connecting the posterior aspects of the femoral condyles places the patellar groove in a favorable position in a normal knee.1,8 However, the posterior aspect of the femoral condyle does not serve as a reliable rotational landmark for a valgus knee because the lateral femoral condyle is abnormally small.2 The transepicondylar axis and an anteroposterior axis recently have been suggested as more reliable rotational landmarks because they are not affected by distortion of the femoral condyles.2,3,5,7,9

In varus knees, degenerative changes or hypoplasia of the posterior part of the medial condyle possibly may make the posterior condyle an unreliable rotational landmark. However, detailed geometric measurements of varus knees are not available. Thus, a question remains regarding whether or not the posterior condyles can serve as a proper rotational landmark for varus knees.

This study was designed to evaluate femoral condylar geometry in normal and varus knees using magnetic resonance (MR) imaging. The reliability of the posterior condylar axis as a rotational landmark for varus knees also was evaluated compared with the transepicondylar axis and the anteroposterior axis.

MATERIALS AND METHODS

Thirty knees in 27 healthy volunteers (normal knee group) and 30 knees in 30 patients with symptomatic osteoarthritis and varus deformity (varus knee group) were evaluated by MR imaging (Table 1). The normal knee group had no knee symptoms, and plain radiographs showed no osteoarthritic changes. They had normal results on physical examination and normal knee range of motion. Patients in the varus knee group had signs and symptoms of osteoarthritis, and standing radiographs showed complete loss of the medial joint space. Clinical data for the varus knees are shown inTable 2. No evidence of trauma was identified in either group.

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TABLE 1:
Demographic Data
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TABLE 2:
Clinical Data for Varus Knee Group

T1 weighted MR imaging was performed according to the following protocol on a 0.5 Tesla Shimadzu SMT-50A MR imaging system (Shimadzu Co Ltd, Kyoto, Japan), and all subjects were restrained during the scanning process to prevent movement. Sagittal and transverse images of the knee joint were obtained with a repetition time (TR) of 500 ms and an echo time (TE) of 20 ms.

Sagittal plane sections through the most prominent part of both femoral condyles were used for measurement. The articular surface of each femoral condyle was divided into distal and posterior parts (Fig 1). A line was drawn from the most anterior point on the surface articulating with the tibia (Point A) to the most superior point of the femoral articular surface (Point B) (Line A-B). Another line was drawn perpendicular to Line A-B from the midpoint of Line A-B to the femoral articular surface. The point where this line met the femoral articular surface was defined as Point C. The arc A-C and the arc C-B were defined as the distal part and the posterior part of the femoral condyle, respectively. Each part was assumed to fit a circular arc,4 and the radius of the arc was calculated using three points on the articular surface of each part. The radius of the circular arc was calculated using the following equation when the distances between two of the three points were a, b, and c. Equation 1

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Fig 1:
Magnetic resonance image of the sagittal view of the lateral femoral condyle. Point A: the most anterior point on the femoral articular surface that articulates with the tibia; Point B: the most anterior point on the femoral articular surface that articulates with the tibia; Point B: the most superior point of the femoral articular surface; Point C: the point where a line bisecting and perpendicular to the Line A-B crosses the femoral articular surface; Point D: the most distal point on the femoral articular surface; and Point E: the most posterior point on the femoral articular surface.

Points A and C and the most distal point on the articular surface (Point D) were used for the distal part of the condyle, whereas Points B and C and the most posterior point (Point E) were used for the posterior part(Fig 1).

The medial to lateral radius ratio was defined as the radius of the medial condyle divided by that of the lateral condyle. The medial to lateral radius ratio was calculated for each distal and posterior condyle.

In the transverse view, sections through the most prominent part of both femoral condyles were used for measurement. The transepicondylar axis was defined as a line between the most medial and most lateral prominences of the epicondyles, whereas the posterior condylar axis was a line connecting the posterior aspects of the femoral condyles. The anteroposterior (AP) axis was a line connecting the deepest part of the patellar groove anteriorly and the center of the intercondylar notch posteriorly (Fig 2).2 The angle between the transepicondylar axis and the posterior condylar axis and the angle between the line perpendicular to the AP axis and the posterior condylar axis were measured.

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Fig 2:
Magnetic resonance image of the transverse view of the distal femoral condyle. The transepicondylar axis (TE): a line between the medial and lateral most prominent epicondyle; the posterior condylar axis (PC): a line connecting the posterior aspects of the femoral condyles; and the anteroposterior axis (AP): a line through the deepest part of the patellar groove anteriorly and the center of the intercondylar notch posteriorly.

The medial to lateral radius ratio of the distal and posterior part of the condyle, the angle between the transepicondylar axis and the posterior condylar axis, and the angle between the line perpendicular to the AP axis and the posterior condylar axis were compared between normal and varus knees. Statistical significance was evaluated using Student's t test (p < 0.05).

RESULTS

In the normal knees, the radius of the distal part of the condyle was smaller on the medial side than on the lateral side (mean medial to lateral radius ratio = 0.63), but the radius of the posterior part was approximately the same on both sides (mean medial to lateral radius ratio = 0.96)(Table 3). In the varus knees, the radius of the distal part of the medial condyle was close to that of the lateral condyle (mean medial to lateral radius ratio = 0.89), and the medial to lateral radius ratio of the distal condyles in the varus knees was significantly larger than that in the normal knees (p < 0.05). The radius of the posterior part of the condyle was approximately the same on the medial and lateral sides (mean medial to lateral radius ratio =0.99) in the varus knees. There was no significant difference between the normal and the varus knees in the medial to lateral radius ratio of the posterior condyles.Figure 3 shows the MR image of a normal knee with the distal part of the medial condyle being more round than that of the lateral condyle. However, the distal parts of the medial and lateral condyles were flattened in a varus knee (Fig 4). No difference was detected in the posterior part of the medial condyle in the normal and varus knees.

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TABLE 3:
Medial to Lateral Radius Ratio of the Distal and Posterior Condyles in Normal and Varus Knees
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Fig 3A:
C. (A) Standing AP radiograph of a normal knee. Magnetic resonance images of (B) the medial condyle and (C) the lateral condyle in that normal knee. Distal part of the medial condyle was rounder than that of the lateral condyle. No significant difference was seen in the shape of the posterior between the medial and lateral sides.
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Fig 4A:
C. (A) Standing AP radiograph of a varus knee. Magnetic resonance image of (B) the medial condyle and (C) the lateral condyle in that varus knee. Distal part of the medial and the lateral condyles were flattened. There was no significant difference in the shape of the posterior condyle between the medial and lateral sides.

Transverse measurements showed that the transepicondylar axis and the line perpendicular to the AP axis had approximately 6° external rotation relative to the posterior condylar axis in the normal and the varus knees(Table 4).

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TABLE 4:
Angles Between the Posterior Condylar Axis and the Transepicondylar Axis and Between the Posterior Condylar Axis and the Perpendicular to the Anteroposterior Axis

DISCUSSION

The rotational alignment of the femoral component in total knee arthroplasty affects varus and valgus stability and the position of the patellar groove.1,8 The posterior margins of the femoral condyles serve as reliable rotational alignment landmarks for normal knees. However, it is possible that the posterior condylar axis may not be a useful rotational landmark in varus knees because the distal geometry of the femur in these knees is not well understood.

The current study used MR imaging to evaluate the distal geometry of the femur because MR imaging detects the articular cartilage (not the bony outline) and is useful for evaluating geometry in the sagittal plane. The findings of this study showed that the distal part of the condyle in the normal knee was rounder on the medial side than on the lateral side, as reported previously.6 The varus knees had a flattened distal portion of the medial and the lateral condyles. This probably was because of the degenerative changes to the distal medial condyle, but some degree of medial condylar dysplasia in the varus knees cannot be excluded.

Posterior condyle geometry is reported to be similar on the medial and lateral sides in normal knees,6 and the current study showed the same finding. In addition, there was no significant difference in the varus knees between the shape of the posterior condyles on the medial and lateral sides. These results suggest that there is no hypoplasia of the posterior part of the medial condyle in varus knees, although the posterior part of the lateral condyle is distorted severely in valgus knees, as reported previously.2 The data of the current study also suggest that degenerative changes of the posterior part of the medial condyle are minimal, even in advanced varus knees.

The transepicondylar axis or AP axis of the varus knees was approximately 6° externally rotated, as in the normal knees, because the posteromedial condyle was not distorted in the varus knees. The transepicondylar axis or AP axis is reported to be a reliable rotational alignment landmark in varus or valgus knees.2,3,5,7,9 Thus, the posterior condyles can be used as a rotational landmark in varus knees, unlike in valgus knees. However, the large range of the measured angle suggests that the shape of the distal femur varies among individuals, and the posteromedial articular cartilage might be destroyed in more severely deformed knees. All of these rotational indices should be used to achieve correct rotational alignment.

The findings of this study suggest that there is no posterior hypoplasia of the medial condyle, and posteromedial degenerative changes are minimal, even in varus knees. The posterior condylar axis can be a reliable rotational alignment landmark in varus knees.

References

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