5 Functional In Vivo Kinematic Analysis of the Normal Knee
A. Williams, C. Phillips
Summary
The concept of tibiofemoral “roll-back”driven by tension in the cruciate ligaments (the “four-bar linkage” theory) as a model of tibiofemoral motion during knee flexion has dominated thinking for the past 30 years. Some obvi- ous flaws have been overlooked, however. An interven- tional MRI scanner has been used to allow study, for the first time, of the weight-bearing living knee during a squat,in three dimensions.Results show that during knee flexion the lateral femoral condyle does move posterior- ly, whereas in the active range of flexion the medial femoral condyle does not move significantly. This differ- ential motion equates to femoral external rotation (or tibial internal rotation). It is proposed that this axial rotation is driven by the shapes of the articular surfaces, and not the ligaments. The findings have far-reaching implications for arthroplasty and the understanding of ligament function.
Introduction
The biomechanics of the normal knee has been a subject of on-going speculation since 1836. Different theories as to how the tibia, femur, and patella articulate have devel- oped as a result of research involving cadaveric and liv- ing subjects. One of the biggest challenges still encoun- tered is how to study functional kinematics of the knee, taking into consideration how muscle contraction,move- ment, and loading affect joint position.
Methods of Investigating Knee Motion
The majority of methods incorporate either invasive or irradiating techniques or sometimes both, therefore re- ducing acceptability to the volunteers being studied. In addition there can be problems in analysis such as the phenomenon of “cross-talk” in Röntgen Stereophoto- grammetric Analysis (RSA) [1]. Magnetic Resonance Imaging (MRI) is an attractive tool, being a noninvasive technique that does not involve ionizing radiation and
which produces high-resolution images in any plane, thereby allowing accurate three-dimensional analysis of the knee joint. However, due to the space constraint in conventional MRI scanners, studies have been non- weight bearing and involve a small range of knee motion.
“Interventional” Magnetic Resonance Imaging
Although many different types of “open”scanner are reg- ularly used in the clinical setting, few vertical-access
“interventional” scanners exist worldwide. One is based at St.Mary”s Hospital,London,UK.This model design in- corporates a 0.5-T magnet housed in two vertical coils spaced 56 cm apart (⊡Fig. 5-1).
Despite the magnet”s field strength being a third of that encountered in conventional scanners, the images produced are of satisfactory resolution, enabling dynam- ic analysis of bony and soft-tissue structures within the knee.As a result of the space,subjects can be scanned dur- ing active movement from full extension through to full flexion in both non-weight-bearing (seated) and physio- logical weight-bearing positions.
⊡Fig. 5-1.0.5 Tesla interventional MR scanner
This scanner design incorporates two methods of image registration, known as “Flashpoint Tracking” and “MR Tracking”,which allow images to be continually obtained from one chosen plane in the knee joint, irrespective of significant movement between consecutive scans. Either of these “tracking”devices and a receiver coil are attached to the subject”s knee (⊡Figs. 5-2 and 5-3).
This facility makes it easy to accurately assess relative movement of femur on tibia, during a full range of mo- tion, while analyzing medial and lateral compartments simultaneously but individually. To achieve this, the posi- tion of the posterior femoral condyles relative to the tib- ia are measured in the sagittal plane at mid-medial and mid-lateral positions of the knee, according to the method of Iwaki et al. [2]. On individual scan images of the medial and lateral compartments in increasing incre- ments of flexion, the centres of the posterior circular sur- faces of the femoral condyles were identified and used as
fixed femoral reference points [2,3].The distance between these and a vertical line drawn from the ipsilateral poste- rior tibial cortex was measured for each position with a Vernier caliper and corrected for magnification (⊡Fig.
5-4). Changes in this distance, with progressive incre- ments of knee motion, thus represent relative motion of the femur on the tibia occurring with knee flexion.
Recent cadaveric studies have established the sagittal contours of the medial and lateral joint surfaces [1].
Through several dynamic MR studies,the consequence of this articular geometry on knee kinematics has become apparent [3–5].
Weight-bearing Tibiofemoral Motion Using Open-access MRI
The use of this technique has produced some dramatic findings. Through collaborative work, our findings have been compatible with results of other studies employing conventional MRI of cadaveric specimens [2, 6], horizon- tal access open MRI of the non-weight-bearing living knee [3, 7], and RSA [8]. Primary results from the St. Mary”s Interventional MRI Unit, analyzing weight- bearing knees in living subjects,have now been reproduced in a number of studies [3–5,9].Knees have been scanned at 10° increments from hyperextension to 140°. The results of the most detailed study of normal tibiofemoral motion are
⊡Fig. 5-2.Scanning in non-weight-bearing position
⊡Fig. 5-3.Diagram of scanning in full weight-bearing position
FFC d
⊡Fig. 5-4.Measurement of the position of the posterior femoral condyles relative to the tibia FFC, Flexion Facet Center; d, distance measured to ipsilateral posterior tibial cortex. (after [2])
summarized in the graphs of mid-medial and mid-lateral compartments [4] (⊡Fig. 5-5a, b).
In the lateral compartment the femur moves posteri- orly – fairly rapidly at first, then steadily until 120° (pro- ducing about 20 mm of displacement), and thereafter rather abruptly (a further 10 mm) into a deep squat.
Medially the situation is very different. In the range of flexion to 120° there is little anteroposterior movement of the femur on the tibia, but from this point to full flexion there is a modest sharp posterior displacement akin to the lateral side (9 mm). The limit of active knee flexion is 120°, and the kinematics from here to a deep squat are a passive phenomenon and distinct from that occurring in earlier flexion.
The differential medial and lateral motion equates to longitudinal axial rotation with knee flexion; internal tibial rotation/external femoral rotation occurs around a
medial axis.For an average-sized male knee this produces 20° of rotation.
Recent fluoroscopic studies have also confirmed this finding of longitudinal rotation with flexion [10]. It is this axial rotation which, when viewed as a lateral projection of the knee fluoroscopically,gives the “illusion”of femoral
“roll-back”, since the lateral femoral excursion, but not the medial, is appreciated at first glance.
Knee flexion can be divided into three arcs: the screw- home arc, the functional active arc, and the passive deep- flexion arc.
Screw-Home Arc
The screw-home arc is the movement of the knee between approximately 20° of flexion to terminal extension. Little
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-5 0 5 1 0 1 5 2 0 2 5 3 0 3 5
-10 0 1 0 2 0 3 0 4 0 5 0 6 0 70 80 90 10 0 11 0 120 130 140 150
Flexion Angle (degrees)
Distance (mm)
FFC ADJUSTED
⊡Fig. 5-5a, b.Mean AP translation of lateral (a) and medial (b) femoral condyles from extension to deep flexion
-5 0 5 10 15 20 25 30 35
-10 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150
Flexion Angle (degrees)
Distance (mm)
FFC ADJUSTED
a
b
is known about this arc and its functional significance. In contrast to the functional active arc there is profound asymmetry between the shapes of the medial and femoral condyles articulating with the tibia [1] (see below). The medial femoral condyle articulates with the upward slop- ing anterior tibial surface. This contributes to the poste- rior part of the medial femoral condyle rising 1–2 mm with progressive terminal extension. As the lateral femoral condyle rotates internally when it moves forward in extension, it rolls down over the anterior edge of the lateral tibial plateau to compress the anterior horn of the lateral meniscus; hence, presumably, the presence of a re- cess in the lateral tibial plateau and the sulcus terminalis of the lateral femoral condyle. It is not yet known if the terminal rotation observed with screw-home is obligato- ry and it is the subject of on-going study.
Functional Active Arc
The functional action arc from approximately 20° to 120°
of flexion is influenced by neuromuscular control.During this phase longitudinal rotation with flexion is not oblig- atory and can, to a large extent, be reversed by voluntari- ly externally rotating the tibia during flexion, allowing the knee to function almost as a uniaxial hinge [3]. Knee motion can vary within an “envelope” of kinematic boundaries [11]. The mechanisms responsible for axial rotation with flexion are not defined and do not appear to be simply under the control of the cruciate ligaments as was previously thought. As well as voluntary control, the different shapes of the articulations are very impor- tant in this regard (see below).
Passive Deep-Flexion Arc
In the arc of 120º–140º of deep flexion,tibiofemoral motion is passive,as a result of external force (usually body weight) allowing extra flexion. Medially the femoral condyle rises about 2 mm as it moves posteriorly, riding up on the posterior horn of the medial meniscus. This may explain why degenerate posterior horn tears of the medial menis- cus often occur in deep flexion. On the lateral side of the knee there is extreme movement of the lateral femoral condyle,which drops approximately 2 mm as it nearly sub- luxes off the tibia. Therefore, in a deep squat both medial and lateral condyles now move backwards close to sublux- ation, largely balanced, presumably, by extensor mecha- nism tension and posterior anatomical impingement.
Articular Contact Points
It is natural to assume at first that relative motions of the medial and lateral tibiofemoral articular surface contact points will “mirror” the motion of the bones in terms of direction and in magnitude [12]. If the sagittal profiles of the femoral condyles were single radius curves (i.e., a circle) or “J”-shaped (closing helix) curves and the tibial surfaces flat, this would have to be true. The situation for a circle would be analogous to the wheel of a car moving on the road: Whether sliding or rolling, the contact point would lie on a line perpendicular to the road passing through the center of the wheel. Hence, as the wheel moved, so, correspondingly, would the contact point. In the knee, however, the situation is different and the actu- al anatomy present “disassociates” the movements of the articular contact points and of the bones. Through de- tailed study of cadaveric specimens the sagittal shapes of the medial and lateral joint surfaces have been established [1]. The medial tibia is flat for its posterior half, leading anteriorly to an “up-slope”.With the well-fixed and there- fore relatively immobile posterior horn of the medial meniscus [13], the distal articular surface is significantly concave, thereby stabilizing the femur. Laterally the tibia presents a broadly convex surface to the femur. In this much less stable arrangement the lateral meniscus is highly mobile [13] to provide important load sharing with the articular surfaces.The medial femoral condyle surface describes arcs of two circles. The more anterior is short- er and has a larger radius than the posterior. Laterally the anterior arc is very small or even absent, so that the articular surface is effectively described by the arc of a single circle (⊡Fig. 5-6a, b).
In the lateral joint compartment, the femoral surface moves posteriorly by a combination of rolling and sliding and, akin to a wheel, takes the articular contact point back with it. Medially the joint surface motion is almost exclu- sively by sliding (i.e., “spinning on the spot”), initially in the early part of flexion, about the center of the more an- terior “extension facet center” and then from about 30°–40° about the center of the more posterior arc (the
“flexion facet center”). The shift in position of the “active”
center of rotation is quite abrupt.This shift is accompanied by a corresponding posterior change in position of joint surface contact (similar to the change in position of the lat- eral joint surface contact point), but not a posterior bodi- ly transition of the femur [14]. This phenomenon is possi- ble only due to the shapes of the articulating surfaces.
Implications and Future Developments
While caution is necessary in extrapolating these results of knee motion,observed in a controlled squat,to normal daily activities such as walking and running, the authors
believe the findings of differential compartment motion in the knee to be very important. Primarily, the results challenge the popular concept of femoral “roll-back”. It is reasonable to argue that “roll-back”exists laterally.Due to lack of anteroposterior translation medially, in the active range of flexion (up to 120°) this term is not appropriate for the bone itself. However, what of the contact area?
First, “rolling” cannot be sensibly applied to change in position of an area. Second, there is no steady transfer of contact through knee flexion provided by “rolling”;
rather, as the knee flexes, the medial femur spins only abruptly changing the center about which it rotates and so allowing a change in articular contact position. This is certainly not the description of “roll-back” that has hith- erto been popularized.
Furthermore, the kinematics presented here produce the perceived benefits of the “roll-back” model. The pos- terior shift of joint contact and femoral external rotation with knee flexion increase the extensor mechanism lever arm. Femoral external rotation allows avoidance of pos- terior bone impingement, thereby maximizing flexion and providing the further benefit of reducing the “Q angle”, so aiding patellar kinematics.
Dynamic MRI allows analysis of not only bony struc- tures, but also of the ligaments. Previous mathematical
models suggested that, when taut, the cruciate ligaments act as a rigid four-bar link, guiding TF motion. Imaging of knees with both intact and deficient anterior and pos- terior cruciate ligaments during the full range of flexion, in loaded and unloaded positions [9,15], makes it evident that the ligaments do not tend to play a great role in guid- ing motion in normal physiological movement of the knee when taut,but rather during excessive application of force, such as that encountered during sporting activity.
The ACL assists in controlling the static weight-bearing tibiofemoral position in the lateral compartment and the PCL acts similarly in the medial compartment. Never- theless, neither ligament influences the extent of active motion during weight-bearing flexion of the knee [4].
It would seem likely that the articular surface geometry is a more potent factor driving knee kinematics. The dramatic differences in sagittal shapes of the medial and lateral compartments account for the similarly clear differences in medial and lateral kinematics.
Much of the interest in knee kinematics has been directed towards optimizing prosthetic design. The his- tory of knee replacement shows that improvements in implant performance were associated with the designs becoming closer in shape to the natural knee.Current de- signs have produced very successful functional outcomes
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⊡Fig. 5-6a, b.Sagittal MRI images of the lateral (a) and medial (b) tibiofemoral joints showing the posterior (FFC, Flexion Facet Center) and anterior (EFC, Extension Facet Center) circular arcs of the femoral condyles
a b
in the 0°–90° range of flexion. Most are designed to pro- duce femoral “roll-back” either by preserving the PCL (PCR) or substituting it for the cam-post mechanisms common to the posterior stabilized (PS) designs. Both types perform well,despite the argument that is raging for and against the two groups. Only the PS designs produce femoral roll-back; in reality, the PCR designs have rather erratic motion, including paradoxical anterior sliding of the femur during flexion [16].Since no prosthesis,total or unicompartmental, reproduces normal joint geometry, none can rightly claim to restore normal joint kinemat- ics. This is not to say that they do not perform well; many do, but not by restoration of normal kinematics. Rather, their functional success lies in the fact that the changes they impose are well tolerated.
Application of our observed tibiofemoral kinematics might be useful, particularly in restoring physiological knee function, including flexion. However, one must pro- ceed with caution.A simplistic view would be that a pros- thesis allowing external femoral rotation about a medial axis during knee flexion, so as to provide more normal kinematics, might produce better results. However, al- though we do not believe in the four-bar linkage model, there will be some price for sacrificing the cruciate liga- ments, and at best the prosthetic articular surfaces in current designs remain far from normal. This means that these designs probably will not confer any advantage over current standard total condylar designs.
Perhaps the next generation of total knee replace- ments will require articular surfaces shaped in the anatomical manner, to guide more physiological knee motion and achievement of higher levels of function.
Acknowledgements. We thank the English Football Association/Professional Footballers Association for generously funding Miss Carol Phillips”post, and Profes- sor W. Gedroyc, MRCP, FRCR, Director of The Interven- tional MRI Unit and Consultant Radiologist, St. Mary”s Hospital, London.
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