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FOOT& ANKLEINTERNATIONAL
Copyright 0 1995 by the American Orthopaedic Foot and Ankle Society, Inc.
In Vitro Kinematics of the Axially Loaded Ankle Complex in Response to
Dorsiflexion and Plantarflexion
Beat Hintermann, M.D., and Benno M. Nigg, Dr. Sc. Nat.
Calgary, Canada
vitro techniques, Siegler et aL2' observed 21.8% of
dorsiflexion and 22.0% of plantarflexion at the subtalar joint. Using in vivo roentgen stereophotogrammetry, L ~ n d b e r g 'found
~
2.3% to 3.8% of dorsiflexion
and 10.0% to 40.7% of plantarflexion occurring in the
midfoot. Consequently, the subtalar and the interrelated joints of the midfoot must be also involved in foot
dorsiflexion-plantarflexion.
The tibia was shown to rotate externally with respect to the talus, when moving the foot from a dorsiflexed to a plantarflexed position, and internally,
when moving the foot from a plantarflexed into a dorsiflexed p o s i t i ~ n . ' ~Tibia1
" ~ ~ ~rotation,
~~
on the other
hand, has been shown to be linked with foot eversioninversi~n.~
The
, ~effect
~ ~ ~of, ~dorsiflexion-plantarflex~
ion on foot and tibia1 rotation, however, has not yet
been determined.
The purpose of this study was to determine what
motions of the tibia and the calcaneus occurred with
dorsiflexion-plantarflexion of the foot when the ankle
and subtalar joints were constrained only by the normal bony and soft tissue anatomy. The effect of varying degrees of axial loading of the ankle complex was
also studied.
ABSTRACT
The rotational movementsof the tibia and calcaneus that
occur with dorsiflexion-plantarflexion and axial loading
were studied in cadaver foot-leg specimens using an
unconstrained testing apparatus. Independent of the
foot flexion position, significant internal rotation of the
tibia and eversion of the calcaneus were noted after the
ankle complex was axially loaded. Independent of loading, 10" of dorsiflexion resulted in 0.1" of eversion and
2.1" of internal rotation of the tibia. Conversely, 10" of
plantarflexion resulted in 1.6" of inversion and 1.3" of
external rotation of the tibia. The induced rotational
movements of the tibia and the calcaneus differed significantly between the specimens. These results suggest
that the foot "axes" did not change by axially loading the
ankle complex and they support previous reports that
the ankle complex uses different axes for dorsiflexion
and plantarflexion.
INTRODUCTION
The ankle joint was originally modeled as a hinged
joint with an oblique axis9r23to account for some of
the rotatory movements between the foot and the
tibia. More recent studies have shown that dorsiflexion-plantarflexion of the ankle is a complex motion
that involves some movements out of the primary
plane of
By using in vivo roentgen
stereophotogrammetry, Lundberg et al? found
changing axes for dorsiflexion and plantarflexion that
differed by 20" to 30". They also noted that the axes
crossed at, or near, one central point in the talus.
Dorsiflexion and plantarflexion of the foot are generally considered to take place in the ankle (talocrural)
joint. Recent studies, however, showed that dorsiflexion-plantarflexionof the foot does not correspond to a
simple rotation around the ankle joint axis. Using in
METHODS
Foot "eversion" and "inversion" in this paper refer to
rotation of the calcaneus in the frontal plane.
The experimental setup used for this in vitro investigation has been described in detail previously.798
The
six degrees of freedom device (Fig. 1) consisted basically of three parts: the frame, the rod, and the foot
plate assembly. The frame held the rod in a vertical
position by bearings which allowed for free rotational
movement around and translational movement about
its longitudinal axis. The foot plate assembly was fixed
on a floating platform which could translate in the
medial-lateral and anterior-posterior directions. The
foot plate could rotate in both the frontal and sagittal
plane, that is eversion-inversion and dorsiflexion-plan-
From the Human Performance Laboratory, The University of Calgary, Canada. Address requests for reprints to Dr. Hintermann at
Orthopaedic Surgery Department, Kantonsspital, CH-4031 Basel,
Switzerland.
514
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Foot & Ankle InternationalIVol. 16, No. 81August 1995
KINEMATICS OF AXIALLY LOADED ANKLE COMPLEX
Fig. 1. This illustration shows the experimental setup. F = frame;
R = rod; B = bearings for rod; J
ball-and-socket joint; U =
U-plate for calcaneal fixation; S = gliding screw through the second
metatarsal head; T = translation mechanism for foot plate; H =
holding lever; G = ground platform (medial/lateral, and anteriorposterior translation); P = pillars; L = load applied to rod and tibia;
El = eversion-inversion axis with goniometer; DP = dorsiflexionplantarflexion axis with goniometer; TR = potentiometer for tibial
rotation.
tarflexion. Vertical loading was applied by adding
weights on top of the rod. Electrical potentiometers
were used to measure axial tibial rotation, and manual
goniometers were used to measure foot plantarflexion-dorsiflexion and calcaneal eversion-inversion.
Fourteen fresh-frozen foot-leg specimens were
used from donors who had an average age of 69 years
(range, 48-81 years) at the time of death. All specimens appeared normal both visually and by sharp
dissection at the conclusion of the experiment. The
soft tissue was removed distally to 3 cm above the
ankle joint. The medial and lateral aspects of the calcaneus were also exposed. After removing the tibial
plateau, a stem was inserted and cemented into the
bone canal, which allowed for tight fixation between
the tibia and rod of the device. The axis of the tibia was
adjusted to the axis of the rod by means of a ball-andsocket joint, which included a translation mechanism
at its base. The foot was mounted on the foot plate
and fixed by means of screws and cement on the
calcaneus and additionally by a screw through the
second metatarsal head, avoiding abduction-adduc-
515
tion movement of the foot with respect to the tibia.
Because this screw could glide anterior-posteriorly,
intrinsic motion of the foot was not limited by the
fixation on the foot plate. The foot plate represented
the position of the calcaneus, and the possible intrinsic motion of the foot was not compromised.
The ankle joint axis, determined visually by an average axis through the malleoli, was adjusted to the
dorsiflexion-plantarflexion axis of the device. The mediolateral position of the calcaneus was chosen so
that the longitudinal axis of the tibia intersected the
eversion-inversion axis of the foot plate.
The neutral position was defined as the zero eversion-inversion and tibial rotation position of the footleg specimen, when the foot was in 0" dorsiflexionplantarflexion. Then, the foot plate was rotated about
the dorsiflexion-plantarflexion axis in steps of 10" from
neutral to 20" dorsiflexion (defined as negative plantarflexion), and then from neutral to 30" plantarflexion.
The resulting tibial rotation and calcaneal eversioninversion were recorded for each of these plantarflexion positions: -20", -lo", 0", lo", 20", and 30". The
measurements were performed with 0 N, 200 N, 400
N, and 600 N vertical loading conditions.
Test experiments (10 trials, with refixation of the
specimen in the jig) with a single specimen were used
for evaluation of the setup error. For the measured
output value, one standard deviation corresponded to
1.89% of the mean tibial rotation, and to 2.21 % of the
mean eversion-in~ersion.~
Statistical analyses were performed by using the
3-way analysis of variance test. The level of significance was set to P < 0.05.
RESULTS
Tibia1 Rotation
A dorsiflexion movement rotated the tibia internally.
On average, every 10" of dorsiflexion corresponded to
about 2" of internal tibial rotation. On the other hand,
plantarflexion rotated the tibia externally by about 1.5"
for each 10" of plantarflexion (Fig. 2). Although there
was a significant range between the different specimens, each specimen exhibited the same trend in the
relationship of dorsiflexion/plantarflexion of the foot
and axial rotation of the tibia. While the amount of
movement transferred from dorsiflexion into tibial rotation did not change by axial loading of the ankle
complex, the movement transferred from plantarflexion into tibial rotation did consistently decrease with
increasing axial load (Table 1). Foot dorsiflexion-plantarflexion of 50" resulted, on average, in 7.7" of tibial
rotation (Fig. 2). When loaded, the amount of resulting
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516
HINTERMANN AND NlGG
Foot & Ankle International/Vol. 16, No. 8/August 1995
Tibial Rotation
10
Tibial Rotation
T
*FI
*FI
-5 --"---y-
7
e
400
600
10
20
-10
-20
-10
20
10
0
-
Dorsiflexion
-10 1
0
30
Plantarflexion
Fig. 2. Relationship of dorsiflexion-plantarflexionand tibial rotation of the unloaded foot. The results indicate the average values
and the range for all specimens; although there was a significant
range between the different specimens, each specimen exhibited
the same trend in the results, as shown.
tibial rotation was 6.4" (200 N), 5.8" (400 N),and 6.0"
(600 N),respectively. Loading the ankle complex up to
400 N resulted in a continuous increase of internal
tibial rotation for all flexion conditions, and slightly
more for extreme plantarflexion (Fig. 3). Further loading up to 600 N did not change the resulting tibial
rotation any more.
Calcaneal Eversion-Inversion
No change in calcaneal position was observed
when dorsiflexing the foot, whereas plantarflexion induced substantial calcaneal inversion. The resulting
calcaneal inversion was higher with increasing plantarflexion (Fig. 4). On average, every 10" of plantarflexion corresponded to about 1.5" of calcaneal inversion.
Again, although there was a significant range between
the different specimens, each specimen exhibited the
same trend in the relationship of dorsiflexion/plantarflexion of the foot and eversion-inversion of the calcaneus. The amount of movement transferred from
dorsiflexion-plantarflexion into calcaneal eversion-inversion did not change for all loading conditions of the
ankle complex (Table 2). Loading of the ankle complex
resulted in a continuous increase of calcaneal eversion
for all flexion positions, and slightly more for extreme
200
"1
Fig. 3. Mean values of the axial rotation position of the tibia for the
various dorsiflexion-plantarflexion positions of the foot when loading the ankle/foot complex up to 600 N.
plantarflexion. The effect of axially loading the ankle
complex on calcaneal eversion lessened with increased axial loads (Fig. 5).
DISCUSSION
Studies of movement in the ankle other than dorsiflexion and plantarflexion have generally dealt with
instability rather than physiological movement.' 1,'2,21
Close4 mentioned rotation about a vertical axis in the
ankle joint during walking, and McCullough and
Burge," in one of the few investigations of such
movement, analyzed the range of movement about
the vertical axis in anatomical preparations. Van
Langelaan22using cadaver specimens, and Lundberg
et a1.,14 in vivo, made roentgen stereophotogrammetric analysis of joint movement and discrete joint helical
axes. Siegler et aI.*' were the first to analyze the
coupled movement to dorsiflexion and plantarflexion.
They used a testing apparatus that placed no constraints on the ankle motion, but their testing procedure did not provide for axial loading.
Although previous
have shown that
stability of the ankle (tibiotalar) joint is increased by
axial loading, attempts to analyze the effect of axial
loading on the mechanics of the ankle joint complex
have been few. Michelson et
using a minimally
TABLE 1
Dorsiflexion-Plantarflexion:Tibial Rotationa Dependent on Axial Loading
Load
ON
200 N
400 N
600 N
Dorsiflexion
Plantatflexion
0.19 (0.14-0.27)
0.12 (0.06-0.19)
0.20 (0.13428)
0.09 (0.04-0.1 6)
0.22 (0.13-0.28)
0.07 (0.04-0.1 5)
0.19 (0.12-0.26)
0.06 (0.01-0.12)
a Tibial rotation indicates amount of coupled motion (average movement transferred from dorsiflexion into internal tibial rotation and from
plantarflexion into external tibial rotation, respectively). Mean values and range are indicated.
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Foot &Ankle InternationalfVol. 16, No. 8fAugust 1995
KINEMATICS OF AXIALLY LOADED ANKLE COMPLEX
-
-
Eversion Inversion
Eversion Inversion
-
10
T
C
.-0
2
a,
517
5
C
+FI
-10
*FI
10
20
t
C
.-0UJ
&
I
+FI
-5
5
w
I
-10
0
-10
-20
Dorsiflexion
20
10
-
30
0
Plantarflexion
Fig. 4. Relationship of dorsiflexion-plantarflexion and calcaneal
eversion-inversion of the unloaded foot. The results indicate the
average values and the range for all specimens; although there was
a significant range between the different specimens, each specimen
exhibited the same trend in the results. as shown.
constrained testing apparatus, have determined the
motion occurring at the ankle joint with axial loading.
They found, by loading the ankle, a significant shift
between the tibia and talus, and a significantly increased valgus motion of the ankle. However, Michelson et al.’s investigation did not involve the application of moment forces. The present study attempts to
address a part of this issue by using an axially loaded
ankle complex taken through a range of dorsiflexion/
plantarflexion motion without external constraints.
The results of this study indicate that there is a
significant internal rotation of the tibia and eversion of
the calcaneus when the ankle is loaded. While dorsiflexion of the foot induces a significant internal rotation
of the tibia, no eversion of the calcaneus occurs. Plantarflexion of the foot results in a significant external
rotation of the tibia and inversion of the calcaneus as
well. These findings support the previous observations
that the “ankle joint axis” is not the same for dorsiflexion and plantarflexion of the foot.’s6s13
They also suggest that the ability of the joints distal to the talocrural
joint to participate in dorsiflexion-plantarflexion of the
foot may be, as previously recognized,10y19
of considerable importance. Lundberg et aI.l4 have reported
large individual variation in the distribution of motion
200
400
600
“1
Fig. 5. Mean values of the eversion-inversion position of the calcaneus for the various dorsiflexion-plantarflexion positions of the
foot when loading the ankle/foot complex up to 600 N.
between the individual joints. They found that substantial amounts of rotation occurred in the joints
proximal and distal to the navicular when the foot was
plantarflexed. In contrast, dorsiflexion in the joints of
the arch was limited.
With increasing plantarflexion, there was less foot
rotation transferred into tibial rotation. This suggests
an “axis” of the ankle/foot complex that becomes
closer to the horizontal in the frontal plane, as more
and more motion occurs in the small joints of the
midfoot while the relative ankle motion decreases.
This, again, concurs with in vitro5 and in vivo10*14
investigations that the small joints of the midfoot contribute substantially to foot plantarflexion.
No calcaneal eversion resulted as the foot dorsiflexed. The foot rotation, thus, occurred about a horizontal axis in the frontal plane.I3 Plantarflexion, however, was accompanied by a substantial calcaneal
inversion. There may be at least two reasons: first, the
“axis” of the ankle joint may be moved away from the
horizontal in the frontal plane; second, a relevant rotation about the axis of the subtalar joint may also
contribute to overall foot plantarflexion.
In the present study, the resulting tibial rotation and
calcaneal eversion-inversion for a given dorsiflexionplantarflexion did not change as a function of axial
loading of the tibia. Thus, the structures that resist
TABLE 2
Dorsiflexion-Plantarflexion: Eversion-Inversion” Dependent on Axial Loading
Load
ON
200 N
400 N
600 N
Dorsiflexion
Plantarflexion
0.0 1 (0.00-0.03)
0.15 (0.06-0.21)
0.00 (0.00-0.02)
0.13 (0.04-0.22)
0.0 1 (0.00-0.04)
0.1 1 (0.04-0.20)
0.0 1 (0.00-0.04)
0.12 (0.05-0.19)
a Eversion-inversionindicates amount of coupled motion (average movement transferred from dorsiflexion into calcaneal eversion and from
plantarflexion into calcaneal inversion, respectively). Mean values and range are indicated.
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518
HINTERMANN AND NlGG
Foot & Ankle International/Vol. 16, No. 8/August 1995
deformation under loading conditions, such as bone
and joint structures, must be primarily responsible for
this transfer of movement.
Pilot investigations without the metatarsal screw
showed that the foot tended to abduct when the tibia
was rotated externally. This did not correspond to
L ~ n d b e r g ’ sin
’ ~vivo observation that, when externally
rotating the tibia, the forefoot slightly rotates in adduction with respect to the calcaneus. In addition, testretest investigations with and without the screw did
not reveal any significant differences in the output
modes of foot and tibial motion. Reasons to use the
screw were primarily that it allowed the foot to be fixed
and held exactly in its defined longitudinal axis on the
center of the foot plate.
In conclusion, this study has shown that axial loading of the ankle complex resulted in a significant internal rotation of the tibia and eversion of the calcaneus. This study has also demonstrated that
dorsiflexion-plantarflexion of the foot is connected to
calcaneal and tibial rotation. The amount of resulting
calcaneal and tibial rotation, however, was not constant thorough the whole range of dorsiflexion-plantarflexion. Axial loading of the ankle complex did not
change this relationship. These results suggest that
the foot “axes” did not change by axially loading the
ankle complex and they support previous reports that
the ankle complex uses different axes for dorsiflexion
and plantarflexion.
ACKNOWLEDGMENT
The authors thank the Swiss Orthopedic Society for
financial support of this study.
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