T. DEREK V. COOKE, MB, BChir, FRCSC,
Adjunct Professor,

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School of Rehabilitation Therapy,
Queen's University,
Kingston, Ontario, Canada;
ELIZABETH A. SLED, MSc, BScPT, Doctoral Candidate,
School of Rehabilitation Therapy,
Queen's University;
R. ALLAN SCUDAMORE, MA, PhD.
School of Rehabilitation Therapy,
Queen's University.
 
Address reprint requests to Dr. T.D.V. Cooke, 797 Princess Street, Suite 508, Kingston, Ontario K7L 1G1. E-mail: derek@cookes.ca

Evaluations of knee alignment are useful in the diagnosis of arthritic conditions affecting the knee joint, serving also as a guide for conservative management and surgical planning^1-5. They are also fundamental to various aspects of musculoskeletal research. Recently, there has been great interest in frontal plane alignment measures related to the pathogenesis of knee osteoarthritis (OA)^6,7. Several approaches have been proposed over the years to describe and measure alignment^1-5,8-10, but the differences between them have made it difficult to compare or correlate the results of independent studies. Toward a standard approach to the measurement and reporting of alignment data that may be equally applicable to clinicians and researchers, we discuss a system of measurements based on geometric analysis of the femur, tibia, and knee joint surfaces. We also discuss a standardized methodology for measurement and computation of these parameters.

PRINCIPLES AND MEASUREMENTS OF ALIGNMENT

From the anatomical and functional perspective, the orientation of the femur and tibia at the knee is best described in terms of the bones' mechanical axes. The orientation of these axes reflects alignment in stance, which may be neutral, varus (bowlegged), or valgus (knock-kneed) (Figure 1).

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Figure 1. Common frontal plane lower limb alignment patterns. A. Varus alignment: knee center is lateral to the LBA (HKA is negative). B. Neutral alignment: knee center is located on the LBA (HKA = 0°); femoral and tibial mechanical axes are colinear. C. Valgus alignment: knee center is medial to the LBA (HKA is positive). LBA: load-bearing axis, HKA: hip-knee-ankle angle, FM: femoral mechanical axis, TM: tibial mechanical axis.

The mechanical axis of the femur (FM) is located as a line from the center of the femoral head running distally to the mid-condylar point between the cruciate ligaments¹¹. In the case of the tibia, the mechanical axis (TM) is a line from the center of the tibial plateau (interspinous intercruciate midpoint) extending distally to the center of the tibial plafond¹² (Figure 2). The angle between these 2 axes is the hip-knee-ankle (HKA) angle^1,13. In the neutrally aligned limb the HKA angle approaches 180°. At this point FM and TM are colinear, pass through the knee center, and are coincident with the load-bearing axis, which is the line of ground reaction force passing from the ankle to the hip^1,13 (Figure 1B). In varus the knee center is lateral to the load-bearing axis (Figure 1A), whereas in valgus the knee center is displaced medially^13,14 (Figure 1C). As a convention the HKA angle may be expressed as its angular deviation from 180° (i.e., HKA = 0° in neutral alignment). Varus deviations are negative and valgus deviations are positive. Our choice of varus as a negative value and valgus as positive was based on the general observations of a more serious problem of loading and damage in the varus knee.

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Figure 2. Frontal plane angles in a limb with varus alignment. LBA: load-bearing axis, CH: condylar-hip angle, the angle of the femoral condylar tangent with respect to the femoral mechanical axis; varus negative, valgus positive. For the HKA measured as deviations from 90°. PA: plateau-ankle angle, the angle between the tibial margin tangent and the tibial mechanical axis; varus negative, valgus positive. For the HKA measured as deviations from 90°. CP: condylar-plateau angle: the angle between the femoral and tibial joint surface tangents; narrowing medially, negative, and laterally positive. HKA. Hip-knee-ankle angle: the angle between the femoral and tibial mechanical axes; varus negative, valgus positive. Measured as 180° equaling zero. FM: femoral mechanical axis, TM: tibial mechanical axis, FM-FS: angle between the femoral mechanical axis and the femoral shaft axis.

Limb alignment (HKA) depends both on long bone geometry and on the geometry of the articulating surfaces of the femur and tibia¹. In the course of knee arthritis, changes of alignment are usually ascribed to changes in the articulating geometry. Typically, focal erosion in the medial compartment leads to narrowing that, under load, displaces the knee center laterally (varus deformity; Figure 1A). Similarly, narrowing of the lateral compartment imparts medial knee displacement (valgus deformity; Figure 1C). But, on occasion, deformity of the femur and/or tibia also influences alignment. To understand the basis for alignment (and change thereof in the course of disease) it is crucial to be able to measure and document articular surface relationships and define limb bone morphology. Based on simple geometric analysis the following elements usefully define the geometry of the tibial and femoral surfaces and the angle between them when loaded in stance^1,13 (Figure 2).

1. Condylar-hip (CH): the angle of the femoral condylar tangent with respect to the FM axis;

2. Plateau-ankle (PA): the angle between the tibial margin tangent and the TM axis;

3. Condylar-plateau (CP): the angle between the femoral and tibial joint surface tangents.

Since HKA is expressed as degrees of deviation from 180°, CH and PA angles are expressed as degrees of deviation from 90°, negative for varus and positive for valgus. A joint space angle (CP) that narrows medially is designated varus (–) and laterally valgus (+). When these conventions are observed, the following relationship applies^1,13:

HKA = CH + PA + CP

Surveys^8,9,13,15,16 have been made in attempts to establish "normal" alignment values in populations without arthritis. The results shown in Table 1 are based on measurements of mechanical axial alignment, depicted as the HKA angle. As might be expected, the data indicate that the average for nonarthritic alignment is close to neutral (–1.0° to –1.3°), although the large standard deviations denote that significant numbers support a pronounced varus or valgus alignment, possibly a predisposing factor for arthritis in later years.

Table 1. Frontal plane knee alignment of asymptomatic adults. Hip-knee-ankle (HKA) is the angle between the mechanical axes of the femur and tibia (deviation from 180°; negative angle denotes varus alignment).

METHODOLOGY

Limb alignment and joint angular data are ideally obtained using long radiographs for measurement. Accuracy will be further enhanced by use of standardized systems, especially for positioning of the subjects. Commonly, however, femorotibial alignment is measured from short views of the knee, which may only define limited aspects of the bones' anatomical axes^17-19. The femoral anatomical (shaft) (FS) axis may be a reasonable approximation of the FM axis, since these axes are offset from each other by only 5° or so, with small variance^11,17. However, anatomic axes located on short views poorly predict mechanical axes, especially with proximal or distal bone deformity^16,20. Thus, in the presence of proximal coxa vara or distal tibia vara (developmental or acquired) a varus malalignment may be missed. It is self-evident that the shorter the view of the knee the greater the risk of missed deformity. Currently, the prevalence of such conditions is not well documented, and requires surveys using full-length views^1,20.

To combat error it is critical to consider a standardized approach to image acquisition and data extraction. One approach is to standardize the positioning of subjects by use of a frame, avoiding inaccuracy due to random variation of limb rotation and knee flexion^21,22. Angles comprising the HKA (i.e., those based on mechanical axes) were measured with less error than similar angles based on anatomic axes^21-23. We have reported a system in which the ankles are located over fixed marks on a rotatable platform and the limbs are then rotated to align the plane of flexion within the sagittal plane; the hips are supported to restrict further movement^1,20-22. This set-up allows for rotation of the subject (maintaining the position) so as to obtain orthogonal lateral views using a cassette between the knees^1,20.

Errors may also be associated with particular types of deformity. For this reason we have avoided the "patella-ahead" alignment of the limbs during set-up, which may actually produce mal-rotation, since patellar malalignment is common²⁴. Similarly, positioning so that the posterior profiles of the condyles are coincident is prone to error because of frequent condylar asymmetry^11,13. Another feature relevant to alignment is knee subluxation (frontal plane), since it may influence the measured HKA angle. This association is not well documented, but in any case compensation may be made by measuring the displacement (lateral or medial) of the mid-spinous point of the tibia with respect to the mid-condylar point of the femur.

Digital imaging systems provide a fast and convenient approach to data acquisition and processing, with excellent reliability and precision. These systems use software with the digital equivalents of ruler, circle, goniometer, and midline tools to define bony landmarks and make measurements. In general, digital systems provide results that are in good agreement with manual methods^{18,19,23,25-28}.

ALTERNATIVE APPROACHES

There are several other methods for measuring frontal plane alignment. Some are based on a single knee center-point defined apart from the bones' mechanical axes, e.g., the midpoint between the tibial spines and apex of the femoral intercondylar notch⁹. This knee center-point is then used as an origin in constructing the femoral and tibial anatomic axes^8,9,17,29,30. The various center points identified in the literature are illustrated in Figure 3⁹. Our recommendation for use of separate center points for the femoral and tibial knee surfaces is based on anatomic study^11,12 and on the very positive feature of being able to define the extent of femoral and tibial contributions to overall alignment for a comprehensive analysis (a feature not available when using a single knee center-point). Further, by identifying each bone's moment arm, the presence of subluxation is indexed, not hidden. Although several studies have shown the utility of anatomic measurements in defining incident knee OA and progression with alignment^29,30, the findings of such studies should be equally accessible through analysis of mechanical instead of anatomic axes.

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Figure 3. Centrally located points used in measures of alignment (as modified9). The centers of the femoral intercondylar notch and tibial spines, respectively, denote the locations of the femoral axis distally and the tibial axis proximally in our recommended approach.