JBJS | Three-Dimensional Morphology of the Distal Part of the Femur Viewed in Virtual Reality

The Journal of Bone & Joint Surgery, Volume 83, Issue 2 suppl 1

Scientific Article | October 01, 2001

Three-Dimensional Morphology of the Distal Part of the Femur Viewed in Virtual Reality

Donald G. Eckhoff, MD; Thomas F. Dwyer, MD; Joel M. Bach, PhD; Victor M. Spitzer, PhD; Karl D. Reinig, PhD

Donald G. Eckhoff, MD
Thomas F. Dwyer, MD
Anschutz Outpatient Pavilion, Box 6510, Mail Stop F-722, Aurora, CO 80045-0510

Joel M. Bach, PhD
Victor M. Spitzer, PhD
Karl D. Reinig, PhD
University of Colorado Health Sciences Center at Fitzsimons, Box 6508, Mail Stop F-432 (J.M.B.) or F-435 (V.M.S. and K.D.R.), Aurora, CO 80045-0508

In support of their research or preparation of this manuscript, one or more of the authors received grants or outside funding from National Library of Medicine Contract N01-LM-0-3507. None of the authors received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.

The Journal of Bone & Joint Surgery. 2001; 83:S43-50

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Introduction

Introduction | Methods | Results | Discussion and Conclusion | References

The morphologic shape of the distal part of the femur dictates the shape, orientation, and kinematics of prosthetic total knee replacement. Traditional prosthetic designs incorporate symmetric femoral condyles with a centered trochlear groove. Traditional surgical techniques center the femoral component to the distal part of the femur and position it relative to various bone landmarks. However, failure patterns documented in retrieval studies^1,2, case series³, and kinematic studies demonstrate how traditional designs and surgical techniques reflect a poor understanding of distal femoral morphology and knee kinematics.

It has been shown that the flexion/extension axis of the knee is fixed within the femur and that the articular surfaces of the condyles are circular in profile^4,5. Ligament length patterns are significantly altered by abnormal axial alignment when a hinged knee brace is used⁶. It is expected that a malaligned femoral component would have the same effect in total knee arthroplasty.

The purpose of this report is to demonstrate, with conventional images and with interactive animations in virtual reality, the three-dimensional shape of the naturally asymmetric distal part of the femur, with illustration of the sulcus axis of the trochlear groove and the flexion/extension axis of the condyles relative to conventional axes (mechanical, anatomic, epicondylar, and posterior condylar). Correlations between the morphologically determined rotation axes and experimentally determined kinematic axes are illustrated.

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Fig. 1:: The stereotactic micrometer, originally designed to localize intracranial lesions in neurosurgery, was modified to hold cadaveric femora for topographical mapping of the condyles and the trochlear groove.

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Fig. 2:: The stylus of the micrometer moves horizontally and vertically in millimeter increments to allow measurements of depth, in millimeters, of the articular surface of the condyles and the trochlea in the horizontal plane.

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Fig. 3:: A side view of the knee simulator, with the femoral unit on the left and the tibial unit on the right. The knee is oriented with the patella pointed down and the tibia parallel to the horizontal. The femoral carriage swings through a vertical plane to allow flexion/extension.

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Fig. 4:: An end view of the knee simulator, with the viewer looking at the tibial unit from distal to proximal. Movement of the tibial carriage to the left or right allows abduction/adduction. The three large wheels in this image have been replaced by air-bearings to further reduce friction. A downward motion represents anterior tibial displacement, whereas an upward motion would be posterior displacement.

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Fig. 5:: Schematic representation of the knee simulator. The upper image is a top view oriented from posterior to anterior. The lower image is a side view, with the tibial unit on the left and the femoral unit on the right. A/P = anterior/posterior, I/E = internal/external rotation, M/L = medial/lateral, F/E = flexion/extension, C/D = compression/distraction, and A/A = abduction/adduction.

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Fig. 6:: A side view of the knee simulator focusing on the details of the flexion/extension actuation. The hamstring muscle actuator can be seen in the upper-right portion of the image, and the axial rotation actuator can be seen in the foreground of the lower-left portion of the image.

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Fig. 7:: A table showing the design load capabilities of the knee simulator. The design external loads are capable of disrupting the structures of the healthy knee joint. The muscle loads approach physiologic levels. F/E = flexion/extension, I/E = internal/external rotation, A/A = abduction/adduction, M/L = medial/lateral, A/P = anterior/posterior, C/D = compression/distraction, QUA = quadriceps, HAM = hamstrings, and GAS = gastrocnemius.

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Fig. 8:: The femoral unit of the knee simulator with the femoral alignment fixture installed. This alignment fixture allows six-degrees-of-freedom adjustment of the femur with respect to the femoral unit. There is a similar alignment fixture for the tibia (not shown).

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Fig. 9:: Forty knees (forty patients) with osteoarthritis of the medial compartment presenting for total knee arthroplasty were scanned with computed tomography, and the measurements were compared with those for forty normal knees in age-matched controls.

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Fig. 10:: Thirty-four knees (thirty-four patients) with anterior pain were scanned with computed tomography, and the measurements were compared with those for thirty-four normal knees in age-matched controls.

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Fig. 11:: Computed tomographic scans were performed with the knee in extension by transversely "cutting" the femur proximal to the intercondylar notch and the tibia proximal to the tubercle.

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Fig. 12:: The distal femoral and proximal tibial computed tomography cuts were superimposed to measure relative translation (see Fig. 19) and rotation (see Fig. 20). The proximal and distal femoral cuts were superimposed to measure femoral version (see Fig. 21).

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Fig. 13:: The transverse image shown in Fig. 13-A is one of the 1877 1-mm-spaced transverse images of the Visible Human Male. The collection of 1877 images forms a volume of photographic data. This volume can be "resliced" at any angle to form other cross-sectional images, such as the coronal one shown in Fig. 13-B and the sagittal one shown in Fig. 13-C.

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Fig. 14:: The transverse image shown in Fig. 14-A is one of the 5189 0.33-mm-spaced transverse images of the Visible Human Female. The resolution of this transverse image is comparable with that of the image of the male counterpart shown in Fig. 13-A. Like the volume of the male, this volume can also be "resliced" at any angle to form other cross-sectional images, such as the coronal one shown in Fig. 14-B and the sagittal one shown in Fig. 14-C. The resolution of these "resliced" images is more than three times higher than the resolution of the images of the male counterpart because of the thinner slice thickness for the female.

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Fig. 15:: The transverse image shown in Fig. 15-A is one of the 2500 0.10-mm-spaced transverse images of a right-knee specimen. Like the whole-body volumes, this knee volume can also be "resliced" at any angle to form other cross-sectional images, such as the coronal one shown in Fig. 15-B and the sagittal one shown in Fig. 15-C. The resolution of these "resliced" images is more than ninety times higher than the resolution of the images of the male counterpart and nearly thirty times higher than that of the images of the female counterpart because of thinner slice thickness and higher "in-plane" resolution.

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Fig. 16:: The femur of the Visible Human Male has been extracted from the volumetric computed tomography data represented in Figs. 13-A, 13-B, and 13-C. A cylinder was then fit to the osseous surface of the distal part of the femur. Note that the radius of the cylinder fit to the medial femoral condyle is slightly larger than the radius of the cylinder fit to the lateral femoral condyle, but the center of each cylinder lies on a single axis.

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Fig. 17:: The sulcus (lowest point) is a nearly linear depression in the trochlear groove that lies lateral to the midplane, defined as the plane perpendicular to the posterior condylar axis.

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Fig. 18:: The sulcus is oriented between the traditional mechanical axis (the line joining the center of the femoral head and the center of knee) and the anatomic axis (the center of the femoral shaft).

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Fig. 19:: The cross-sectional centers of the distal part of the femur and the proximal part of the tibia are not superimposed but are translated 4±6 mm anteroposteriorly and 5±4 mm mediolaterally in both normal knees and those with osteoarthritis or anterior pain.

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Fig. 20:: In an extended knee with pathological findings, there is rotation of the tibia in relation to the femur fixed in soft tissue—a "rotation" contracture. The tibia is externally rotated in relation to the femur in both the osteoarthritic knees (4°±1°) and the knees with anterior pain (7°±1°). There is no rotation of the tibia in relation to the femur, or rotation contracture, in the normal knees.

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Fig. 21:: The distal part of the femur is malrotated relative to the proximal part of the femur in the osteoarthritic knees and the knees with anterior pain when compared with the normal knees.

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Fig. 22:: The distal part of the femur of a knee with anterior pain, viewed here with computed tomography, is rotated when the proximal part of the femur is normally oriented in relation to the acetabulum.

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Fig. 23:: Experimental data for eight cadaveric knees. These data show the coupled axial rotation of the tibia during flexion/extension of the femur following functional alignment. Note the extremely small variability attributable to the alignment procedures.

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Fig. 24:: One view of the Interactive Anatomic Animation of the distal part of the femur illustrates the three-dimensional relationship between the epicondylar axis (green line) and the "cylindrical axis" (red line), defined by the center of the cylinders that most closely reproduce the geometry of the condyles.

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Fig. 25:: A femoral component designed with asymmetric features addresses the natural asymmetry of the human knee.

Methods

Introduction | Methods | Results | Discussion and Conclusion | References

Eighty-five mummified cadaveric knees were measured with a stereotactic micrometer (Fig. 1)⁷. The location and orientation of the sulcus were obtained by repeated horizontal passes of the stereotactic stylus over the distal part of the femur, beginning at the top of the articular surface and progressing down to the intercondylar notch (Fig. 2). With each horizontal pass, the lowest depression of the trochlea (sulcus) was identified by the stereotactic stylus and the coordinates were recorded. After each horizontal pass, the stereotactic device was lowered by 2 mm to provide sequential horizontal tracings.

Ten cadaveric knees were studied with use of a six-degrees-of-freedom motion-analysis apparatus⁸ (Figs. 3, 4, 5, and 6) based on a floating coordinate system⁹. Initial alignment of the flexion/extension axes of the specimens with the corresponding axis of the apparatus was performed with use of the transepicondylar axis. The specimens were then passively flexed between -5 (hyperextension) and 120 under physiologic loads (Fig. 7) while six-degrees-of-freedom kinematics were measured. The position of the femur was interactively adjusted in the alignment fixture while the coupled abduction/adduction and compression/distraction motion was observed during flexion/extension (Fig. 8). Following this functional alignment^8,10, passive flexion of the specimens between —5 (hyperextension) and 120 was repeated while six-degrees-of-freedom kinematics were measured. The kinematics of the two alignment conditions were compared.

Seventy-four knees of seventy-four patients with osteoarthritis of the medial compartment or anterior knee pain (Figs. 9 and 10) and seventy-four knees of thirty-seven volunteers (controls) were evaluated with computed tomography^11-15. A computed tomography scan of the femoral condyles, immediately proximal to the notch, and of the tibial plateau, immediately proximal to the tubercle, was performed for each knee in full extension (Figs. 11 and 12). The knee was extended because this is the only position in which the tibia assumes a reproducible position in rotation relative to the femur. Additional computed tomography cuts were obtained through the femoral head, through the femoral shaft at the lesser trochanter, and across the malleoli to determine limb rotation.

The morphologic and biomechanical characteristics of the knee defined in these studies were measured in three additional cadaveric specimens in cyberspace and illustrated with a computer visualization program as an Interactive Anatomic Animation. These three cadaveric knees were sectioned with computed tomography into 0.1 to 1.0-mm slices, digitized, and reconstructed in virtual space (Figs. 13-A, 13-B, 13-C, 14-A, 14-B, 14-C, 15-A, 15-B, and 15-C). Computer-generated cylinders were "grown" within the confines of the articular surface of the distal part of the femur to confirm and illustrate the cylindrical geometry of the condyles as well as to demonstrate the position of the cylindrical axis relative to conventional axes (mechanical, anatomic, epicondylar, and posterior condylar) (Fig. 16).

Results

Introduction | Methods | Results | Discussion and Conclusion | References

The sulcus of the trochlear groove lies lateral to the midplane and is oriented between the mechanical and anatomic axes of the femur (Figs. 17 and 18). The cross-sectional center of the femur lies medial and anterior to the cross-sectional center of the tibia (Fig. 19). The tibia is not rotated relative to the femur in normal knees, but it is externally rotated in osteoarthritic knees and in knees with anterior pain (Fig. 20). Femoral version is decreased in osteoarthritic knees and increased in knees with anterior pain compared with femoral version in normal knees (Figs. 21 and 22).

Kinematic analysis demonstrated that, when the coupled abduction/adduction motion was eliminated, the functional flexion/extension axis of the femur was parallel to the flexion/extension axis of the apparatus. Subsequently, when the coupled compression/distraction motion was eliminated, the functional flexion/extension axis of the femur was coincident with the flexion/extension axis of the apparatus. Variability was extremely small (Fig. 23).

The medial and lateral femoral condyles geometrically approximate cylinders. Although the medial condyle has a larger radius than the lateral condyle, both cylinders are oriented around a common axis (Fig. 16). This common "cylindrical" axis represents a single, fixed, flexion/extension axis in the human knee, which is distinct from other conventional axes (mechanical, anatomic, epicondylar, and posterior condylar) (Fig. 24).

Discussion and Conclusion

Introduction | Methods | Results | Discussion and Conclusion | References

This study documents the asymmetric morphologic features of the distal part of the femur and correlates these features with the kinematics of the knee. The femoral sulcus is located lateral to the midplane between the femoral condyles and is oriented between the anatomic and mechanical axes of the femur. The center of the femur in cross section is offset, medially and anteriorly, from the center of the tibia. These asymmetric, offset morphologic features of the distal part of the femur should be incorporated into the design (Fig. 25) and positioning of prosthetic replacements in the knee.

This study also documents the asymmetric cylindrical shape of the condyles and establishes the cylindrical axis of rotation of the condyles about which the tibia rotates. The measured kinematic data support the morphologic finding of a fixed, cylindrical flexion/extension axis of rotation. This study provides kinematic and morphologic validation for a single cylindrical flexion/extension axis of the knee.

This study was the first to illustrate these observations in three-dimensional stereo with use of interactive animations and virtual reality.

References

Introduction | Methods | Results | Discussion and Conclusion | References

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