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Calcaneal Fractures Cause a Lateral Load Shift In Chopart Joint Contact Stress and Plantar Pressure Pattern In Vitro

Dieter Rosenbaum,* Gerhard Bauer,† Peter Augat* and Lutz Claes*
* Abteilung Vnfallchirurgische Forschung und Biomechanik; and † Abteilung fiir Unfall-, Hand-, Plastische und
Wiederherstellungschirurgie, Klinikum der Vniversitiit VIm, VIm, Germany

Abstract-In order to evaluate the effects of the anatomical changes after calcaneal fractures on joint loading characteristics we investigated the effects of simulated calcaneal fractures on intra-articular loading in the Chop art joint and on plantar pressure patterns in vitro, Five fresh-frozen lower leg specimens were axially loaded with 500 N in three positions: neutral, 10° plantarflexion and 10° dorsiflexion, The loading characteristics were determined before and after creating a tongue-type fracture by osteotomy, Plantar loading patterns were measured with a capacitive pressure distribution platform. Intra-articular contact areas and pressures were recorded on pressure sensitive film introduced into the talonavicular and calcaneocuboid joint. Increased loading in the calcaneocuboid joint ( + 25%, p = 0.(05) and decreased loading in the talonavicular joint ( - 16%, p = 0.01) corresponded to increased loading of the lateral aspects of the plantar surface. These findings were independent of the foot position. The results suggest that the effects of increased lateral foot loading which have been observed in calcaneal fracture patients (Rosenbaum et al., Clin. Biomech. 10, 345-351, 1995) are caused by changes in joint kinematics and do not appear to result from a protective gait pattern established after the injury. Copyright © 1996 Elsevier Science Ltd.

Keywords: Calcaneal fractures; Chopart joint; Joint contact stress; Plantar pressure distribution; Fuji Prescale film.

The importance of the anatomic reconstruction of the hind foot following intra-articular calcaneal fractures has been increasingly emphasized in recent years. In order to attain satisfactory clinical results the restoration of the anatomy to a pre-traumatic state has been suggested by many authors (Colburn et al., 1989; Hansen, 1991; Mutschler et al., 1993; Myerson and Quill, 1993; Paley and Hall, 1993; Zwipp et al., 1993). Intra-articular fractures of the calcaneus usually cause a depression of the subtalar joint and therefore lead to a reduction of Boehler's angle in combination with a shortening and widening of the calcaneus, The altered joint geometry is clinically manifested in a reduced range of subtalar motion, as seen in eversion and inversion measurements (Mittlmeier et al., 1992), Furthermore, it has been suggested that altered joint loading can contribute to the development of arthrosis (Ross and Sowerby, 1985). Pathologically high local pressures have been reported as a secondary deficit in the ankle joint following calcaneal fractures (Carr et al., 1988) and may cause degenerative changes in the joint cartilage (Paar et al., 1991; Stockenhuber et al., 1991). Arthrosis in the calcaneocuboid joint is often observed since this joint is involved in up to 50% of all calcaneal fractures (Nagar, 1995; Zwipp et al., 1989).

Our investigations on plantar pressure measurements after calcaneal fractures revealed changes in the gait patterns of patients even after average healing periods of four years (Rosenbaum et al., 1995). The patients produced significantly increased pressures under the mid-foot and the fifth metatarsal head and a decrease under the first metatarsal head indicating an overall load shift towards lateral aspects of the plantar surface. Similarly, a lateralization of the foot loading pattern has been described by Mittlmeier and coworkers (Mittlmeier et al., 1992). The authors reported a deflection of the center-of-pressure line to the lateral side of the foot and attributed this finding to an impairment of the dynamic eversion ability after calcaneal injury.

However, it has yet to be determined whether the observed changes in foot loading patterns after calcaneal fractures are adaptational changes or are caused by damage to the hind foot joints. In the first case, it is conceivable that the patients have altered their gait pattern more or less consciously in order to avoid painful joint loading in the early healing period and have retained it even after complete fracture healing, In the second case the transfer of the force from the talus through the calcaneus to the hind foot and through the Chopart joint line to the forefoot may have been affected by post-traumatic changes in the shape of the calcaneus or by cartilage damage at the time of injury.

The present study aimed to answer the following two questions: (1) Can malunions of calcaneal fractures alone cause the abnormal plantar pressure patterns seen in post-traumatic patients? (2) Can malunions elevate contact stress in the talonavicular and calcaneocuboid joint? Answers to these questions should help to decide whether differences between pre- and post-traumatic plantar pressure patterns under in vitro conditions are comparable to the pressure patterns seen under in vivo conditions of calcaneal fracture patients in the previous study. The additional measurements of the intra-articular pressure patterns in the Chopart joint line ought to indicate whether the load transfer from the tibiotalar joint through the fractured hind foot to the forefoot is related to potential differences in the plantar pressure pattern.

Materials and Methods
Five lower leg specimens without evidence of previous injuries or osteoarthritis were obtained fresh from adult cadavers, deep frozen until ready for use, and thawed in a water bath the night before the experiment. The specimens were dissected below the knee joint to a uniform length of 25 cm measured between the lateral malleolus and the proximal end. The tibia and fibula were freed of soft tissue down to a level of five centimeters above the ankle joint. The proximal ends were embedded with PMMA (Technovit 3040, Heraeus Kulzer GmbH, Wehrheim, Germany) to provide a cylinder for fixation in the testing jig.

The joint capsule was opened medially to provide access to the talonavicular joint. A lateral incision from the lateral malleolus to the base of the fifth metatarsus allowed access to the calcaneus and calcaneocuboid joint. Care was taken to minimize the damage to the joint capsule and to leave the ligaments intact. A suture in the Achilles tendon with a wire loop allowed application of tensile forces for a plantarflexion moment to achieve a more realistic load distribution.

After pre-traumatic testing of the intact specimens a tongue-type calcaneal fracture was simulated by a medio-lateral and an antero-posterior transsection using an oscillating saw (Fig. 1). These osteotomies simulate common primary and secondary fracture lines (Essex-Lopresti, 1951). A wedge was resected to simulate the joint depression typical for this fracture which led to a reduction in Boehler's angle (i.e. the angle formed by the two superior surfaces of the calcaneus). The osteotomy was fixed with Kirschner wires. Lateral X-rays revealed that the post-traumatic Boehler's angle was reduced to 1.6 ± 2.3° (range 0-5°).

Fig. 1. Schematic description of the imposed calcaneal fracture showing (A) the primary and secondary fracture line and the resected wedge (shaded area); (B) the resulting shape of the calcaneus.
Fig. 1. Schematic description of the imposed calcaneal fracture showing
(A) the primary and secondary fracture line and the resected wedge (shaded area);
(B) the resulting shape of the calcaneus.

For the determination of intra-articular and plantar pressures, a foot loading jig was developed (Fig. 2) which allowed fixation of the specimen, axial loading with a pneumatic cylinder, control and measurement of the applied force, and recording of the resulting plantar pressure pattern. The jig consisted of a rectangular frame holding a capacitive pressure distribution platform with a total of 2016 sensors in an area of 225 by 440 mm thus providing a spatial resolution of 2 sensors cm-2 (EMED-SF2, Novel GmbH Munich, Germany). The platform could be rotated in two directions to provide a plantar-/dorsiflexion and eversion/inversion range of motion of up to 20°. A vertical rod extending above the main frame held a pneumatic cylinder (type SA-4918-2, Festo, Stuttgart, Germany) and a strain-gauge force sensor (type 8426, Burster, Gernsbach, Germany). Roller bearings provided longitudinal guidance and minimized frictional losses under loading conditions. The epoxy cylinder at the proximal end of the specimen was engaged in a cylindrical adapter on the lower end of the rod. External weights acting across pulleys were used for specimen weight compensation.

Fig. 2. Foot loading apparatus with EMED SF-platform.
Fig. 2. Foot loading apparatus with EMED SF-platform.

With the tibia and fibula in a vertical position, the specimens were axially loaded for approximately 10 s via a pneumatic cylinder with 600 N. A 100 N tensile force was applied to the Achilles tendon by attaching weights to the wire loop. This plantarflexion force simulated a more realistic standing situation by causing a forward shift of the center of plantar pressure from the heel to a more anterior location. Before and after osteotomy the specimens were loaded in each of three positions in a randomized order: neutral, 10 plantarflexion, and 10° dorsiflexion. A single frame of the plantar pressure pattern was recorded after 5 s of load application.

Superlow and ultralow Pressensor film (Fuji Film, Tokyo, Japan) with a measurement range of 0.5-2.5 MPa and 0.2-0.6 MPa, respectively, were used for recording the intra-articular pressure (Atheshian et al., 1994; Brown et al., 1991). The two sheets of Pressensor film were cut according to the shape of the respective joint surfaces of each specimen and protected against moisture with surgical foil (OpSite Flexgrid, Braun-Smith & Nephew GmbH, Spangenberg, Germany).

The Pressensor samples were filmed with a digital camera and analyzed on an image analysis system (ProgRes 3000 and IMCO 1000, Kontron GmbH, München, Germany). The images were displayed on a high-resolution monitor and the stained area was digitized with a cursor by a single observer. From each Pressensor film sample, the contact area and the average and maximum gray level were determined.

For calibration purposes, standardized pressure samples were obtained in a commercial testing machine (Model 1445, Zwick Co., Ulm, Germany) using two different flat-surfaced metal rods (6 and 10 mm diameter) in a pressure range between 0 and 2.5 MPa for the superlow film and between 0 and 0.7 MPa for the ultralow film. The comparison of the mean gray level and the applied pressure indicated a good linearity in the range from 0.5 to 2.0 MPa (superlow) and 0.2 to 0.6 MPa (ultralow) (Fig. 3). Using the calibration curve, the above-mentioned parameters were converted to provide the following information. The joint contact area was determined as the stained area on the ultralow film (i.e. pressures exceeding 0.2 MPa). A high pressure area was determined as the stained area on the superlow film (i.e. pressures exceeding 0.5 MPa). Average pressures were calculated from the mean gray level of the superlow film, maximum pressures from highest gray level of the superlow film. The total force transmitted through the respective joint was obtained by the product of average pressure times total joint contact area. For each of the six conditions (3 positions, 2 film sensitivities) two repeated trials were performed with immediate visual inspection of the stains in order to control for crinkling artifacts.

Fig. 3. Calibration curves for the superlow and ultralow Pressensor film.
Fig. 3. Calibration curves for the superlow and ultralow Pressensor film.

Plantar loading was measured with the previously mentioned capacitive pressure distribution platform. Due to the static loading characteristics (i.e. no considerable change in the load distribution over time) only one frame was recorded after about five seconds of load application. A second recording from each experimental condition was taken during the loading of the second Pressensor film. In accordance with the in vivo measurements (Rosenbaum et al., 1995) the maximum pressure values were determined in the following anatomical regions: central (CH), medial (MH) and lateral heel (LH), mid-foot (MF), first (M1), second (M2) and fifth metatarsal head (M5). Due to the lack of a tensile force on the flexor hallucis longus and brevis tendons, no load was measured under the hallux. Additionally, the size of the loaded plantar surface and the ground reaction force (as determined by the integration of the pressure values on all loaded sensors) were recorded. All measurements were repeated twice and the results averaged.

The effect of the injury condition (pre- vs posttraumatic) and the loading position (normal, plantar-, dorsiflexion) was statistically evaluated using a twofactor repeated measures ANOV A (Stat-View 4.02, Abacus Concepts, Berkeley, CA).

A comparison of the two repeated measurements indicated a high degree of reliability. For all parameters obtained with the plantar pressure measurement system the Spearman Rank Correlation Coefficients were above r = 0.9 (p < 0.000 1) with the exception of the total force (r = 0.8, p = 0.001). For the intra-articular measurements the coefficients were between r = 0.8 (p = 0.001) and r = 0.9 (p < 0.0001).

The vertical ground reaction force recordings from the pressure distribution platform revealed that the measurements compared well before and after the osteotomy (Table 2). Virtually no differences ( < 1 %) were seen. Therefore, the external loading conditions proved to be well controlled.

Table 2. Plantar force, area and pressure parameters for neutral, dorsi- and plantarflexion; pre- and post-traumatic mean values of five specimens (standard deviations are indicated in paranthesis)
Table 2. Plantar force, area and pressure parameters for neutral,
dorsi- and plantarflexion; pre- and post-traumatic mean values of five
specimens (standard deviations are indicated in paranthesis)

The intra-articular pressure patterns revealed a significant load shift from the talonavicular joint to the calcaneocuboidjoint (Table 1). In both joints the simulated fracture caused significant changes in the total force (talonavicular joint p = 0.01, calcaneocuboid joint p = 0.005), joint contact area (talonavicular joint p = 0.007, calcaneocuboid joint p = 0.002) and high pressure area (talonavicular joint p = 0.001, calcaneocuboid joint p = 0.017). In both joints neither the average nor the maximum pressure showed a significant change. In the neutral position the total force transmitted through the calcaneocuboid joint increased after the osteotomy by 25% and decreased in the talonavicular joint by 16%. The maximum pressure decreased in the calcaneocuboid joint by 2% and increased in the talonavicular joint by 11 %. In the calcaneocuboid joint the total joint contact area increased by 62% and the high pressure area increased by 19%. In the talonavicular joint the total joint contact area decreased by 17% and the high pressure area decreased by 26%.

Table 1. Intraarticular force, pressure and area parameters for neutral, dorsi- and plantarflexion measured in the talonavicular and calcaneocuboid joint; preand post-traumatic means of five specimens (standard deviations are indicated in parenthesis)
Table 1. Intraarticular force, pressure and area parameters for neutral,
dorsi- and plantarflexion measured in the talonavicular and calcaneocuboid
joint; preand post-traumatic means of five specimens (standard deviations
are indicated in parenthesis)

The intra-articular load changes induced by the osteotomy were not affected by foot position. Similar changes as observed in the neutral position were seen in the dorsiflexed and plantarflexed positions.

The plantar surface area that contacted the platform increased significantly after inducing the fracture (p = 0.006) by 14% in the neutral position and by 11 % in dorsi- and plantarflexion (Table 2). This coincided with a markedly (p = 0.017) increased midfoot width (pre-traumatic 0.4 cm, post-traumatic 2.0 cm). Sample pressure patterns from one specimen demonstrate good repeatability within one condition (Fig. 4).

Fig. 4. Example of plantar pressure measurements (Foot # 3) with the pre-traumatic (left) and posttraumatic calcaneus (right) demonstrating the load shift to the mid-foot and lateral forefoot region. No marked differences are seen between the neutral position, 10 dorsiflexion, 10 plantarflexion. Two repeated trials in each condition are shown to demonstrate the repeatability.
Fig. 4. Example of plantar pressure measurements (Foot # 3) with the pre-traumatic (left)
and posttraumatic calcaneus (right) demonstrating the load shift to the mid-foot and lateral
forefoot region. No marked differences are seen between the neutral position, 10°
dorsiflexion, 10° plantarflexion. Two repeated trials in each condition are shown
to demonstrate the repeatability.

The plantar pressures were highest in the heel regions and lowest in the mid-foot region. The comparison before and after osteotomy demonstrated a transfer of the load towards lateral aspects of the foot with significant differences in the central heel (p = 0.005), mid-foot (p = 0.007), 1st (p = 0.014) and 2nd metatarsal head (p = 0.0001). In the neutral position the local peak pressure values decreased in the central heel region by 34%, the 1st metatarsal head by 19% and 2nd metatarsal head by 50% while 77% higher pressures were seen in the mid-foot region.

The present study demonstrates that anatomical changes in the hind foot caused by simulated calcaneal fractures significantly affect the adjoining Chopart joint line even though the talonavicular and calcaneocuboid joints are not directly involved in the fracture mechanism. In the talonavicular joint we found a decreased joint contact area and intra-articular force whereas in the calcaneocuboid joint the contact area and force were increased.

During the stance phase of the gait cycle the talus serves as a weight bearing link between the leg and the foot (Perry, 1992). The body weight is delivered to the talus and from there distributed to the hind foot via the subtalar joint and to the forefoot via the talonavicular and anterior talocalcaneal joint (Kapandji, 1985). The talona vicular joint transfers the forces to the three medial metatarsals while the calcaneocuboid joint serves the two lateral metatarsals. Therefore, it is not surprising that the internal load transfer coincided with a lateral load shift on the plantar surface that was seen in decreased pressures under the first and second metatarsal head and increased pressures under the mid-foot and fifth metatarsal head. With respect to the posed questions we found that the malunions associated with calcaneal fractures may play an important role in causing an abnormal foot loading pattern similar to that observed in post-fracture patients. The abnormal pattern is caused by a change in the load transfer through the Chopart joint line which is not necessarily associated with an increase in joint contact stress but is characterized by a load redistribution from the talonavicular to the calcaneocuboid joint.

The loading changes in the Chopart joint line and the plantar pressure pattern may be explained by the disturbed hind foot mechanics. The fracture often causes an in congruency of the talocalcaneal joint surface, especially in the posterior talar facet of the calcaneus. During eversion and inversion the talus glides on the posterior facet of the calcaneus. A destruction of the facet thus leads to an impaired eversion/inversion range of motion (Bauer et al., 1993; Brunner et al., 1991; Pozo et al., 1984; Stockenhuber et al., 1991). During the stance phase of walking, the eversion movement is necessary for transferring the body weight from the lateral edge of the foot at heel strike to the medial structures of the forefoot at push-off. Therefore, the foot is forced to roll over the more lateral surface if eversion is disturbed. Another possible mechanism responsible for the demonstrated changes could be an altered tension in the plantar aponeurosis as a result of the osteotomy. However, since the wedge was resected along the secondary fracture line and the fragments were fixed with Kirschner wires it did not affect the length of the calcaneus in our model. Therefore, it is unlikely that the surgical procedure affected the anteroposterior position of the calcaneus and the tension in the plantar aponeurosis.

Some limitations of the present study have to be considered. The Pressensor film works only in a certain pressure range and disregards pressures below or above the specified range. Therefore, we chose to repeat the measurements with films of two sensitivities in order to get reliable contact area measurements from the ultralow film and reliable peak pressure values from the superlow film. Furthermore, Pressensor film is very sensitive to artifacts due to crimping, sliding and shear stress staining (Atheshian et al., 1994) that may occur during insertion into a narrow space or when applying it on extremely curved surfaces. We were able to minimize artifacts by opening the joint space enough to insert the film without bending. The articular surfaces of the talonavicular and calcaneocuboid joint revealed only slight curvatures that did not cause a problem. If, regardless of our precautions, artifacts became apparent, we visually inspected and manually corrected the stained regions as suggested by Brown (Brown et al., 1991). In spite of these limitations Fuji Pressensor film appears equally suited or superior to other contact measurements in congruent joints like stereophotogrammetry, dye staining and silicone rubber casting (Atheshian et al., 1994). Fuji film gave consistent results in all investigated joints and required less data processing than stereophotogrammetry. Furthermore, Pressensor film has been used extensively for intraarticular pressure measurements in various joints of the foot such as the talocrural joint (Calhoun et al., 1994; Christensen et al., 1994; Kimizuka et al., 1980; Moody et al., 1992; Paar and Ungerechts, 1988; Vrahas and Veenis, 1994), the subtalar joint (Wagner et al., 1992; Wang et al., 1995), the metatarsal-cuneiform joint (Firoozbakhsh and Orcutt, 1994) and the talonavicular joint (Beaudoin et al., 1991). Therefore, the applied method can be considered well established and adequate for the purpose of the present study.

Information about the loading characteristics of the Chopart joint line is scarce. Only two studies report contact characteristics of the talonavicular joint (Beaudoin et al., 1991; Kitaoka et al., 1994) and one considered the calcaneocuboid joint (Porras et al., 1980). Therefore, a comparison of the present data to reported results is impaired. We can state, however, that the values for the talonavicular joint contact area (92.5 mm² in the neutral position) are in the same range as those reported before under similar loading conditions: 111.2 mm² under a load of 445 N (Beaudoin et al., 1991) and 131.7 mm² under a load of 668 N (Kitaokaet al., 1994).

The present findings may be clinically important since pathological joint loading has been recognized as a secondary deficit in the ankle joint following calcaneal fractures (Carr et al., 1988) and may lead to arthrosis (Ross and Sowerby, 1985; Stockenhuber et al., 1991). A possible detrimental mechanism is not only an increase in peak pressure but a load redistribution in the joint surface since each persisting change in the joint contact area is likely to promote the development of arthrosis (Paar et al., 1991). The latter effect leads to cartilage loading in formerly less affected regions that are not accustomed to it and have no time to adapt to the new stress situation. Arthrotic conditions have been observed in the ankle, talonavicular and calcaneocuboid joints after conservative (Pozo et al., 1984) as well as surgical treatment of calcaneal fractures (Paley and Hall, 1993).

The simultaneous recording of the intra-articular and plantar pressure distribution has not been reported before. We chose this combination of methods to provide the necessary link to relate the in vitro results to the previously obtained in vivo data. Plantar pressure measurements have proven useful in clinical research to assess foot function, e.g. after surgical procedures (Alexander et al., 1990; Hughes, 1993; Mittlmeier and Morlock, 1991). Pressure distribution measurements under the foot during static and dynamic conditions (i.e. in stance and gait) do not reveal directly the joint contact pressures but have the potential to suggest inferences about joint loading in patients. We were able to show that deviations from a "normal" plantar pressure pattern are associated with changes in the load transfer through the joints of the foot. We compared the results of the in vivo plantar pressure measurements obtained from 14 post-fracture patients during bipedal standing (Rosenbaum et al., 1995) with the in vitro results of the present study. A similar load shift towards lateral foot structures in the forefoot became evident in both conditions (Fig. 5.). The patients exhibited significantly increased pressures under the midfoot and fifth metatarsal head and reduced pressures under the heel center, the second metatarsal head and the hallux of the fractured foot compared with the non-affected foot. These findings correspond well with the changes observed in the in vitro experiments. However, a basic difference should be noted: In the in vivo conditions the hind foot and forefoot are more equally loaded whereas in the in vitro study high loads are transferred to the heel regions, reduced pressures occur under the metatarsal heads, and no pressure is produced by the hallux. These differences are due to the chosen experimental procedure that transferred the load passively through the tibia and fibula. We simulated only one muscle group, the triceps surae, by means of a tensile force acting on the Achilles tendon in order to shift the center of pressure forward. The fact that the loading situation in the in vitro conditions still overestimated the heel loading that can be attributed to the lack of intrinsic foot muscle action that is usually present during midstance (Perry, 1992). This would have been necessary to induce a further load transfer to the forefoot but deemed too difficult to simulate. A combined pulling force on the toe flexors, peroneals and tibialis posterior in addition to an axial load and Achilles tendon pull had been shown to increase the load in the calcaneocuboid joint (Porras et al., 1980) but probably would not have changed the effects of the calcaneal fracture.

In spite of these general differences in the loading patterns between the in vivo and in vitro conditions the chosen model should be valid since all systematic errors or limitations affect the pre- and post-traumatic conditions similarly. Even though the obtained values might not reflect the actual intra-articular loading conditions the changes can be attributed to the induced fracture since all other conditions were kept constant. The changes observed after the artificial fracture compared well with the differences between the healthy and fractured pressure distribution patterns of the standing patients. Therefore, it appears that the changes in hind foot geometry have an important influence on the foot loading patterns that are characteristic for calcaneal fracture patients. The changes in the plantar pressure patterns cannot only be attributed to a protective mechanism to avoid the loading of certain foot structures or joint configurations after trauma and are likewise induced by the anatomical changes in the hind foot geometry. The results stress the importance of restoring the hind foot geometry to avoid pathological loading of the adjoining joints and reduce the risk of developing arthritis.

We were able to demonstrate that malunions caused by calcaneal fractures lead to a disturbance in the loading of the Chopart joint line which is associated with the load transfer to the forefoot and in turn leads a load shift onto the lateral rays. The observed effects in the present in vitro investigation are similar to those seen in gait analyses performed in calcaneal fracture patients. These observations may help to understand clinical long-term results after calcaneal fractures which reveal a high incidence of arthritis in the calcaneocuboid joint even though this joint is not directly involved in the fracture mechanism. We have to realize that the calcaneal fracture is a complex event that leads to changes in the threedimensional shape of the calcaneus and affects the restoration of foot function. In order to improve long-term success of the fracture treatment, an anatomical reconstruction of the calcaneus as close as possible to a pretraumatic shape is highly desirable.

We wish to thank Herbert Schmitt for the construction of the foot loading jig and Patrizia Horny for the illustrations.


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