The Lisfranc, or tarsometatarsal, joints form the transverse
arch of the human foot and contribute to the shape of the medial
longitudinal arch1. These joints
are flat, are surrounded by tough ligaments, and have little motion1-10. The tarsometatarsal joints are
believed to transfer the forces generated during gait from the hindfoot to
the forefoot as the foot moves from heel-strike through toe-off11. However, there is no quantitative
documentation of the pathway and magnitude of the forces involved
in this process7,9,12-16. The
straightforward structural appearance, and seemingly simple function,
may explain why their role in quantitative analyses of tarsal joint
anatomy and kinesiology has been disregarded4,16.
The lack of information regarding tarsometatarsal joint biomechanics
may be partially responsible for suboptimal clinical results. Limited
treatment efficacy is manifested as inadequate patient satisfaction;
limited mobility; and unrestored function, which often leads to
some degree of long-term disability17-21.
Furthermore, the prevalence of severe, symptomatic degenerative
arthritis secondary to injury of the tarsometatarsal joints has
been reported to be as high as 50%22.
The objective of the present study was to quantify the contact
mechanics (areas, pressures, and forces) in the tarsometatarsal
joints of normal cadaveric feet. The motivation for the study was
the need for objective data, from normal feet, to enable future quantitative
assessments of the therapeutic efficacy of tarsometarsal joint treatments.
Specimen Preparation
Six fresh cadaveric lower-leg specimens were transected approximately
20 cm proximal to the ankle, sealed in double plastic bags, and
stored in a freezer maintained at —20°C. Lateral and anteroposterior
radiographs of the thawed specimens were made to check for skeletal
abnormalities. All soft tissue was removed from the proximal parts
of the tibia and fibula of the six specimens, but the interosseous
membrane was left intact. The dissected proximal aspects of the
tibia and fibula were then potted in automobile-body filler (Bondo; Dynatron/Bondo,
Atlanta, Georgia) while the foot was maintained in a neutral stance
position.
When the potting compound had hardened, a transverse incision
was made in the dorsum of the midfoot over the tarsometatarsal joint
spaces. The dorsal tendons and supporting dorsal ligaments were
transected. The articulating surfaces of the tarsometatarsal joints
were exposed by making four incisions into the tarsometatarsal joint
spaces. Since the fourth and fifth tarsometatarsal joints are contained
within one joint space, they were evaluated as one joint and are
referred to as the fourth/fifth tarsometatarsal joint in
this study. The articulating surfaces were visually examined for
abnormalities. The remaining soft tissue surrounding the joint spaces,
plantar pad, plantar fascia, and strong supporting plantar ligaments
were left intact.
Transducer Preparation
Intra-articular pressures and contact areas of the tarsometatarsal
joints were quantified with single-use pressure-sensitive film (Fuji
Prescale Super Low and Low; C. Itoh, New York, NY). Two different
types of pressure-sensitive film (Super Low, with pressures ranging
from 0.49 to 2.45 Mpa, and Low, with pressures ranging from 2.45
to 9.80 MPa) were needed to measure the range of pressures. The
pressure-sensitive film was cut to shape with use of four custom
steel-rule dies (American Steel Rule Die, Elkhart, Indiana). These
dies accurately and reproducibly cut the film into shapes approximating
the contours of the four distal articulating surfaces of the tarsometatarsal
joints. These pieces of film were then formed into transducers by sealing
them on both sides with clear packaging tape (3M, Minneapolis, Minnesota),
thus preventing the intrusion of artifact-causing moisture. The excess
sealant tape was removed with use of a second steel-rule die shaped
12% larger than the die used to cut the film. The thickness
of the Super-Low-film and the Low-film sealed transducers was 0.278
and 0.283 mm, respectively.
Transducer Calibration
Calibration of the two types of transducers was performed with
two sets of transducers cut into long calibration filmstrips (one
strip per transducer type) on each day of testing. The calibration
filmstrips were prepared with use of the same method as that used
for the tarsometatarsal joint transducers. The Super-Low-film transducer
used for calibration consisted of one long piece of film with maximum pressures
of 0.49, 0.82, 1.14, 1.47, 1.80, 2.12, and 2.45 MPa applied to seven
different locations on this filmstrip. Similarly, maximum pressures
of 2.45, 3.68, 4.90, 6.13, 7.35, 8.58, and 9.80 MPa were applied
to seven different locations on the Low-film transducer calibration
strip. The applied pressures were calculated from the known load applied
over a constant, known surface area. These maximum pressures were
applied to the filmstrips with use of a 3.00-cm-diameter precision
flat-ground stainless-steel cylinder attached to the crosshead of
a servohydraulic materials-testing machine (Instron 8521; Instron,
Canton, Massachusetts). The film was placed on a precision flat-ground
square block during calibration loading23.
A 10.0-N preload was applied to the calibration strip for five seconds.
It was followed by a single ramp load, which was applied for five
seconds to the maximum pressure, was held at the maximum pressure
for five seconds, and then was released.
To quantify the accuracy of the transducers, pressures and areas
were measured from the calibration filmstrips in three separate
trials, and the forces were calculated. These values were compared
with the known applied pressures, areas, and forces. On the basis
of the differences between the measured and known (fiduciary) values,
the accuracy of the force, area, and pressure measurements was determined.
Specimen Loading
A foot-specimen positioning jig (Fig. 1) was designed to reproducibly hold
the specimen, in five different anatomical positions: 10° of dorsiflexion,
30° of plantar flexion, neutral, 10° of inversion, and 10° of eversion24. This jig was securely mounted on
the load-cell of the test system. Dorsiflexion and plantar flexion
of the ankle were performed with neutral inversion and eversion.
Inversion and eversion of the hindfoot were performed with the foot
in neutral position—that is, neither dorsiflexed nor plantar
flexed. For each position, the specimen was loaded in compression
to four different maximum axial loads (350, 700, 1050, and 1400
N), which correspond to 0.5, 1.0, 1.5, and 2.0 times the loading
used previously in a similar model24.
If it is assumed that the average weight of the subjects (donors
of the cadaveric feet) was 700 N, then these loads would correspond
to 0.5, 1.0, 1.5, and 2.0 times body weight, respectively.
A specimen was mounted on the loading platform of the test apparatus
and kept moist with gauze soaked in Ringer lactate solution. The
loading platform had a high-friction surface to prevent anterior-posterior
or medial-lateral sliding of the foot. This high-friction surface
also gave rise to the generation of shear forces in the system,
but the load-cell of the test apparatus did not measure these forces.
Displacement was applied to the tibia coincident with the tibial
axis, and the compressive component of the foot load parallel to
this axis was measured by the load-cell. Load distribution on the
plantar surface of the foot was not determined, and the foot-ground
shear forces were not measured. Brackets were attached to the loading
platform to assist with reproducible alignment of the foot on the
platform.
Experimental Protocol
Four shape-specific tarsometatarsal joint pressure transducers
were inserted through the dorsal incisions (previously described)
into their respective tarsometatarsal joint spaces. A 10.0-N compressive preload
was axially applied and single-ramp-loaded in five seconds to the
desired maximum load—that is, 350, 700, 1050, or 1400 N.
The maximum load was maintained for five seconds and then released25. Each specimen, at each foot position,
was initially loaded at the maximum 1400-N level; subsequent tests
were conducted with use of loads sequentially reduced in 350-N increments
to the minimum load (350 N). Testing for each position was performed from
the highest to the lowest loads to allow the specimen to find a
steady-state location and position on the loading apparatus that
would be consistent throughout the remainder of the loading sequence.
The loaded pressure transducers were removed and checked for moisture
artifact. This complete sequence was repeated at least once (two trials)
with both the Super-Low-film and Low-film transducers for each foot,
joint, load amplitude, and foot position until two consistent film-pressure
patterns were obtained. The reproducibility of the films with respect
to the location and the density of the pressure print was evaluated
visually. The Low-film transducer was used at all loads except 350
N because the pressures were below the manufacturer’s recommended
threshold level. The Super-Low film was used for all loads.
Transducer Analyses
The loaded transducers were scanned with use of a flatbed scanner
(Microtek International, Hsinchu, Taiwan), at 300 dots/in
with a 255-level gray scale, within twenty-four hours of testing.
The scanned images were analyzed with custom image-analysis software
(Quantim; Zedec Technologies, Burlington, North Carolina), which
enabled the contact area, average pressure, combined average pressure (Super
Low film and Low film), and total force to be determined.
The calibration strips were scanned, and the images were used
as the basis for converting transducer film-image densities to known
pressures. The average pressures recorded individually by the Super-Low-film
and Low-film transducers, for both trials, were combined with use
of an algorithm (Appendix). The calibration images were analyzed
to create a transformation relationship that enabled film-color
(red) intensity to be converted to a joint-pressure value.
Statistical Analyses
Polynomial regression was used to determine the relationship
between color intensity and the pressure applied to the calibration
filmstrips. Contact forces were calculated from the ratio of pressure
to area. All data obtained from the tarsometatarsal joints were
normalized by the values for pressure and force that were determined
in the second tarsometatarsal joint of that specimen under the 700-N load
with the foot in neutral position. Three-way analyses of variance
were used to analyze the contact pressure, contact area, and joint-force
data to determine if there were significant differences in each
of these parameters as a function of joint, load, or foot position.
The post hoc Scheffé correction was used
to identify specific significant differences. A value of less than
0.05 was considered significant. Statistical analysis was performed
with StatView 5.0 statistical software (SAS Institute, Cary, North
Carolina).
All procedures associated with the Fuji film technique, including
force application, film-scanning, image-outlining, and image-combining
and analysis, but not insertion of the film into the individual tarsometatarsal
joint spaces, resulted in an error of 0.6% for contact-area
measurements, 7.5% for contact-force measurements, and
8.0% for the calculated contact pressure. A total of 210
Fuji film transducers were used for calibration in this study.
A total of 1680 loaded Fuji film transducers were used to measure
the normal contact forces and areas in each of the four joints of
the six cadaveric feet as a function of foot position, applied load,
and specific tarsometatarsal joint. As expected, the intra-articular
contact forces (Fig. 2-A), areas (Fig. 2-B), and, to
a slightly lesser extent, pressures (Fig. 2-C) in each of the tarsometatarsal
joints (with the foot in neutral position) increased linearly (r2 = 0.923
to 0.998) with increasing applied load. The second and third tarsometatarsal
joints bore approximately 50% to 200% more of
the load applied to the foot in neutral position than did the first
and fourth/fifth tarsometatarsal joints. The second tarsometatarsal
joint, and, to a lesser extent, the third tarsometatarsal joint,
demonstrated the largest increases (3.5 and 3.3 times, respectively)
in contact force as a function of the applied load (Fig. 2-A). The forces
and areas across the second and third tarsometatarsal joints with
the foot in neutral position were typically two to three times the
forces and areas across the first and fourth/fifth tarsometatarsal
joints. The proportion of forces, areas, and pressures among the
individual joints as well as between groups consisting of the second
and third tarsometatarsal joints and the first and fourth/fifth tarsometatarsal
joints appeared to remain relatively constant throughout the range
of loads applied with the ankle and foot in neutral position.
The second and third tarsometatarsal joints bore the majority
of the force as the foot position changed in the sagittal (Fig. 3-A) and frontal
(Fig. 4-A)
planes, just as they did as a function of applied load with the
foot in neutral position (Fig. 2-A). However, the first and fourth/fifth
tarsometatarsal joints had a more active role in these foot positions than
they did in neutral position—that is, the contact area
(Figs. 3-B and 4-B) of the first and
fourth/fifth tarsometatarsal joints varied approximately
1.4 to 2.8 times more with the foot in varying positions than it
did with varying loads with the foot in neutral position (Fig. 2-B). It is of
note that during changes in foot position in the sagittal or frontal
plane, the forces borne by the first and fourth/fifth tarsometatarsal
joints were comparable with each other, as were the forces borne
by the second and third tarsometatarsal joints. There was a similar
relationship with respect to the contact areas of the first and
fourth/fifth tarsometatarsal joints and the second and
third tarsometatarsal joints (Figs. 3-A and 4-B).
The range of values observed for tarsometatarsal joint-contact
force (2 to 541 N) and contact area stand in sharp contrast to the
uniformity in contact pressure (0.5 to 5.7 MPa) measured among all
of the tarsometatarsal joints during changes in foot position in
the sagittal or frontal plane (Figs. 3-C and 4-C). These variations (or lack thereof)
reflect the contact mechanics of the tarsometatarsal joints and
not the variability in Fuji film measurement technique.
The results of the present study suggest that the tarsometatarsal
joints have a complex role in regulating joint pressures in the
midfoot. This role belies the simplicity of these joints and refutes
their previously stated lack of biomechanical importance4. These data clearly show that the
tarsometatarsal joints respond with varying joint-contact areas
in response to increasing loads and changing foot positions. This
occurs not only through the individual action of each tarsometatarsal
joint but also through the collective action of all of the tarsometatarsal
joints acting as a unit. These joints thus maintain relatively constant
bearing-surface pressure by altering the contact area within the
individual joints or by redistributing force to other tarsometatarsal joints.
These findings have led us to hypothesize that pressure regulation
in the tarsometatarsal joints functions through two specific mechanisms.
The first mechanism involves the adjustment of the contact area
within the individual joints so that the pressure distribution among
the joints remains relatively constant. The second mechanism involves
the transfer of force from the second and third tarsometatarsal
joints, which initially have large loads, to the first and fourth/fifth
tarsometatarsal joints. This mechanism was most evident during plantar
flexion. Specifically, these data indicate that the first tarsometatarsal
joint contributes to force transfer during foot movements in the
frontal plane (inversion and eversion) and the fourth/fifth
tarsometatarsal joint contributes to force transfer during foot motion
in the sagittal plane (dorsiflexion and plantar flexion). We believe
that this transfer of force to the first and fourth/fifth
tarsometatarsal joints, the outer tarsometatarsal joints, is an
important, but previously unrecognized, means of limiting pressure
in the second and third tarsometatarsal joints, the inner tarsometatarsal
joints. The second mechanism would also come into operation with
the foot in neutral position to limit pressure in the second and
third tarsometatarsal joints if larger loads (more than two times
body weight) were applied. If this occurred, the ratio of pressures
that were shown during neutral position would be altered, and the forces
borne by the first and fourth/fifth tarsometatarsal joints
would substantially increase to protect the second and third tarsometatarsal
joints from pressure overloads.
The second tarsometatarsal joint of the Lisfranc joint complex
has been referred to as the "keystone" and the
main force-transferring joint of the complex11,12.
In contrast, the findings in the present study clearly show that
the third tarsometatarsal joint bore the most force at virtually
all loads and foot positions. The results of our study support the
concept that the second and third tarsometatarsal joints are stiffer
and not as mobile and, because of this, are able to withstand large
force amplitudes and maintain the anatomic congruency of the tarsometatarsal
joint complex. This role, in turn, contributes to the ability of
the foot to keep its shape and to maintain healthy joint-cartilage
function regardless of the magnitude or direction of the activity-relevant
loads that are repetitively applied during normal function26,27. In contrast, we believe that
the mobility of the first and fourth/fifth tarsometatarsal
joints has a key role (complementary to the role of the second and
third tarsometatarsal joints) in handling force overloads emanating
from radical changes in foot position or high loads.
The present study has several limitations, most of which arise
from the in vitro cadaveric model that was used.
First, all measurements of tarsometatarsal joint contact were performed
statically, and therefore we were not able to observe the contact
mechanics of joints loaded dynamically. Second, our measurements
of cadaveric specimens did not account for the additional forces
generated by muscle contraction. Finally, even though the use of
Fuji film for the evaluation of intra-articular contact
mechanics is well documented and accepted23,24,28,
this film could measure only compressive forces, and the joint shear
forces, which are likely to be present, were not measured. For this
reason, the contact areas, forces, and pressures are probably undervalued.
The average pressure for each specimen, joint, applied load,
and foot position was determined with use of weighted averages from
the two types of pressure-sensitive film. The final average pressure for
a tarsometatarsal joint at a specific load, position, and specimen
was calculated with use of the equation:
PT = (PSL) · (ASL/AT) + (PLOW) · (ALOW/AT).
PT is the contact pressure (in megapascals) across a given tarsometatarsal
joint. This value was calculated by adding the average pressure
contributions (calculated from the average of the two trials) from both
the Super-Low film and the Low-film transducers. PSL is the average
pressure (in megapascals) as measured from the unsaturated region
of the Super-Low film. This value was calculated from the average
of the two trials. PLOW is the average pressure (in megapascals)
that was measured by the Low-film type over the same area (as determined on
a pixel-by-pixel basis) in which the Super-Low film was saturated.
This value was calculated from the average of the two trials. ASL
is the average unsaturated contact area (measured by counting pixels
but expressed in square millimeters) of the Super-Low film from
the two trials. ALOW is the average saturated contact area (measured
by counting pixels but expressed in square millimeters) of the Super-Low
film from the two trials. AT is the total average contact area (measured
by counting pixels but expressed in units of square millimeters) from
the two trials with use of the Super-Low film. Note that AT = ASL+ ALOW.
The contact force was equal to the product of PT times AT (in newtons).