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Subluxation of the Talocalcaneal Joint in Adults Who Have Symptomatic Flatfoot*

DHEERA ANANTHAKRISNAN, M.D., RANDAL CHING, PH.D., ALLAN TENCER, PH.D., SIGVARD T. HANSEN, JR., M.D. and BRUCE J. SANGEORZAN, M.D.SEATTLE, WASHINGTON

Investigation performed at the Orthopaedic Biomechanics Laboratory, University of Washington School of Medicine, Seattle

	Abstract

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Background: When flatfoot is acquired during adulthood, the shape of the foot changes. In addition to a decreased arch, there may be valgus angulation of the hindfoot or abduction of the forefoot, or both. However, there is little objective information to provide a better understanding of the anatomical or morphological changes that occur in acquired adult flatfoot. We wondered if such an understanding of the three-dimensional anatomy might shed light on the pathway by which these changes occur. We designed this study to measure the three-dimensional position of the talocalcaneal joint in patients who have painful flatfoot. Methods: Computed tomography scans of the feet of eight patients who had symptomatic flatfoot were used to construct a model of the talocalcaneal articulation. The scans were performed on a custom loading frame developed to simulate weight-bearing with the foot in a neutral position while a seventy-five-newton axial compressive load was applied. The digital data from the scans were used to make three-dimensional computer models of the articular surfaces of the talus and calcaneus of each foot. These models then were used to calculate the percentage of the articular surface that was in contact and, conversely, the percentage that was subluxated. Two surfaces were modeled for each bone; the posterior facet formed one surface, and the anterior and middle facets were combined to form the second surface. The data were compared, with use of Mann-Whitney nonparametric U analysis, with those derived from scans of the feet of four patients without a deformity of the hindfoot who served as controls. Results: A mean (and standard deviation) of 68 ± 9 percent of the posterior facet of the calcaneus was in contact with the talus in the patients who had flatfoot compared with 92 ± 2 percent in the controls, and a mean of 51 ± 23 percent of the anterior and middle facets of the calcaneus was in contact with the talus in the patients who had flatfoot compared with 95 ± 6 percent in the controls. These differences were significant (p = 0.0066 for both). Conclusions: Marked subluxation of the talocalcaneal joint occurs in some patients who have symptomatic planoabductovalgus deformity.

	Introduction

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Although the prevalence of flatfoot varies with age, at least one study of approximately 200 adults suggested that as many as 20 percent of adults have a foot that could be described as flat on the basis of the footprint (actual numbers were not given)²⁹. However, only a small number of these flatfeet have functional limitations or cause clinical problems^23,29,31. While it is not known what distinguishes a painful flatfoot from one that is symptom-free, it is known that the shape of the bone is altered in some patients who have a flatfoot². A soft-tissue injury or abnormality also may contribute to an acquired deformity^{4,11,17,21,24}. In addition, many patients (thirty-six of forty-three in the study by Dyal et al.⁸) who have a symptomatic flatfoot also have a decreased arch on the contralateral side. This suggests that there may be an anatomical configuration that predisposes a foot to clinical deformity. However, there is no unifying objective method of understanding different types of flatfoot.

Because of the suggestion on plain radiographs that the talocalcaneal and talonavicular joints are subluxated in some patients who have flatfoot, the term peritalar subluxation was coined by one of us (S. T. H., Jr.) to describe this category of symptomatic flatfoot with severe deformity. However, actual subluxation has never been documented, to our knowledge.

This study was designed to assess quantitatively the relationship between the articular surfaces of the talus and calcaneus while patients who had symptomatic flatfoot were in a simulated weight-bearing position.

	Materials and Methods

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Images made with a computed tomography scanner (9800 Hilite Advantage; General Electric Medical Systems, Waukesha, Wisconsin), with the patient in the supine position with simulated weight-bearing, were used to construct three-dimensional models of the articular surfaces of the talus and calcaneus. The digital data from the images (slice thickness, 1.5 millimeters) were transferred in digital format from the computed tomography workstation to a personal computer (Power Macintosh 9500; Apple Computer, Cupertino, California) (Fig. 1). The articular surfaces of each image were digitized with use of image processing and analysis software (NIH Image; National Institutes of Health, http:// rsb.info.nih.gov/nih-image/). The two-dimensional images were combined to construct a three-dimensional model of each joint (Fig. 2), from which the measurements were made. These measurements then were compared, with use of the Mann-Whitney nonparametric U test, with those obtained for the surfaces of the control feet.

View larger version (94K): [in this window] [in a new window]   The digital from the computed tomography scans were transferred to a personal computer (Power Macintosh 9500; Apple Computer, Cupertino, California) in digital format. Imaging processing and analysis software (NIH Image; National Institutes of Health, http://rsb.info.nih.gov/nih-image/) was used to digitize the articular surfaces (outer black lines). In addition, a line representing the region of overlap was drawn on each slice of the scan that involved the talocalcaneal joint (middle black line).

View larger version (62K): [in this window] [in a new window]   Three-dimensional polygons were built from the digitized surface lines shown in Fig. 1. The lines representing each joint surface (for example, the posterior facet of the talus and the anterior facet of the calcaneus) were exported to a file for that surface and were assembled as a series of stacked objects to create a three-dimensional image. In this example, the light lines represent the calcaneus and the dark lines represent the talar surface, both viewed from above.

Selection of Images
Since 1994, computed tomography scans performed at our institution as part of the clinical evaluation for problems related to the hindfoot have been done with the foot in a simulated weight-bearing position. Computed tomography, which was used only when an operation was being considered (although not all patients went on to have operative treatment), was employed as a tool to determine the location of the deformity and the degree of degenerative changes in the intertarsal joints. Between June 1994 and October 1995, when this investigation was begun, thirty-five feet had been scanned with the patient bearing weight, and twenty-four of them were flatfeet. The digital data were stored on optical discs. However, most of the scans had been discarded.

The scans of eight patients (eight feet) who had a presumed diagnosis of rupture of the posterior tibial tendon were available for study. The ages of the patients ranged from forty to seventy years. All patients had symptoms that were severe enough for them to consider operative reconstruction. Seven patients had valgus angulation of the hindfoot and abduction of the forefoot in addition to the decrease in the arch. One patient who had had a recent acute rupture of the posterior tibial tendon had only a slight decrease in the arch and no valgus angulation of the hindfoot or abduction of the forefoot.

Computed tomography scans of four normal feet (four patients, ranging in age from thirty-one to fifty-five years) were used as controls. These scans had been made because of unilateral problems other than a flat or cavus foot. Two of these patients had a posttraumatic deformity (of the ankle in one and of the subtalar joint in the other), one had a symptomatic stress fracture of the navicular joint, and one had a painful ankle and an osteochondritis dissecans lesion.

Loading Frame
Weight-bearing was simulated with the patient in the supine position and the legs placed in a custom loading frame that holds the feet in a neutral position while a seventy-five-newton axial compressive load is applied. The scans were performed in the semicoronal plane at 1.5-millimeter intervals. The seventy-five-newton load was chosen for pragmatic reasons. When the loading frame was being built, we tried several loads. The load that was selected both recreated the standing habitus of the foot and could be tolerated comfortably for the time that it took to perform the scan. This load represents approximately 10 percent of the load on the foot during quiet standing; however, it may have led to an underestimation of the actual degree of deformity.

Joint Model
After the digital data from the computed tomography scans had been transferred from the computed tomography workstation to the personal computer in digital format and the articular surfaces were both viewed and digitized with use of the image processing and analysis software, lines along the articular surfaces and a line representing the region of overlap were drawn on each slice of the scan that involved the talocalcaneal joint (Fig. 1). These two-dimensional surface lines were exported to a file and stacked to construct a three-dimensional representation of each joint surface (for example, the anterior facet of the calcaneus and the posterior facet of the talus) (Fig. 2). The stacked line polygons, representing the articular facets, were rendered into a single smooth surface with use of computer-aided-design (CAD) software (FORM Z; Autodessys, Columbus, Ohio). Rendering is the process by which the software program extrapolates the surface in the space defined by the polygons. In this case, rendering allowed the completion of a surface that otherwise would have appeared to be a series of interconnecting polygons (Figs. 3, 4, and 5). The surface areas were measured from the individual smoothed (rendered) surfaces.

View larger version (60K): [in this window] [in a new window]   The surface objects were modeled with use of computer-aided-design (CAD) software (FORM Z; Autodessys, Columbus, Ohio), which was used to render the surface of each facet. The process of rendering involves extrapolation of the surface between the lines by the software to create a smooth, continuous surface. This model of a right foot in the control group is viewed from posteriorly in order to display the curve of the joint surface. The dark gray surface (top) is the surface of the talus, and the light gray surface (bottom) is that of the calcaneus. The middle surface (white) is an artificial surface that represents the area of overlap on the computed tomography scan. Note that the saddle-shaped surface of the posterior facet and the more planar surface of the middle facet are nearly coincident.

View larger version (78K): [in this window] [in a new window]   Rendered surface of a right foot in the control group, viewed as if looking down at the talocalcaneal joint. The surface of the talus is represented by dark gray and that of the calcaneus, by light gray. Note that the surfaces are aligned.

View larger version (93K): [in this window] [in a new window]   Rendered surface of a right flatfoot, viewed as if looking down at the talocalcaneal joint. The surface of the calcaneus is represented by light gray and that of the talus, by gray. Note that the calcaneal surfaces are posterior and lateral to the talar surfaces. The middle and anterior facets are nearly dislocated.

The area of each of these six surfaces was measured with use of computer-aided-design software analysis. For each joint that was scanned, we measured the total surface of each facet of the talus and calcaneus and the area of each that was in contact, and we compared the overlap of each facet with its total area. The contact area could be expressed as a ratio of the area that was in contact with the total area or as a percentage of the facet surface. Thus, a ratio of 0.68 means that 68 percent of the entire surface of a facet is in contact with the other bone. The surfaces could be rotated and viewed from any perspective (Fig. 3).

We calculated the ratio of the overlap (contact) area to the total area of each facet of both the talus and the calcaneus because the talus and calcaneus are not perfectly juxtaposed and the joint surfaces are not the same size. (The anterior and middle facets of the talus tended to have a slightly greater surface area than the facets of the calcaneus.) For the purposes of this study, the areas that were not in contact were presumed to represent subluxation. The percentage of subluxation (the area that was not in contact) was calculated as a percentage of the calcaneal surface and of the talar surface. The data were compared with use of the Mann-Whitney U test (Statview software; Abacus Concepts, Berkeley, California).

	Results

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Posterior Facet (Table I)
The ratio of the part of the posterior facet of the calcaneus that was in contact with the talus to the entire surface area of the posterior facet was a mean (and standard deviation) of 0.68 ± 0.092 in the patients who had flatfoot compared with 0.92 ± 0.022 in the controls; this difference was significant (p = 0.0066). Alternatively, it could be stated that 32 percent of the surface of the posterior facet of the calcaneus was subluxated in the patients who had flatfoot compared with 8 percent in the controls. The ratio of the part of the posterior facet of the talus that was in contact with the calcaneus to the entire surface area of the posterior facet was a mean of 0.70 ± 0.116 in the patients who had flatfoot compared with 0.89 ± 0.046 in the controls; this difference was significant (p = 0.0415). Conversely, it could be said that 30 percent of the posterior facet of the talus was subluxated in the patients who had flatfoot compared with 11 percent in the controls.

Anterior and Middle Facets (Table I)
The ratio of the contact area of the anterior and middle facets of the calcaneus to the total surface area of those facets was a mean of 0.51 ± 0.229 in the patients who had flatfoot compared with 0.95 ± 0.057 in the controls; this difference was significant (p = 0.0066). Alternatively, it could be stated that 49 percent of the posterior facet of the calcaneus was subluxated in the patients who had flatfoot compared with only 5 percent in the controls. The contact ratio of the anterior and middle facets of the talus in the patients who had flatfoot was 0.44 ± 0.211 compared with 0.87 ± 0.076 in the controls; this difference was significant (p = 0.0066). Alternatively, it could be stated that 56 percent of the middle facet of the talus was subluxated in the patients who had flatfoot compared with only 13 percent in the controls.

Ratio of Talar Surfaces to Calcaneal Surfaces (Table II)
As an internal test of the surface measurements, ratios of talar surfaces to calcaneal surfaces were compared. If the surfaces of one or the other of the bones were increased, it would give a false impression of subluxation. The surface-area ratio of the anterior and middle facets (the ratio of the total surface of the anterior and middle facets of the calcaneus to the total surface of the anterior and middle facets of the talus) was a mean of 0.86 ± 0.137 in the patients who had flatfoot compared with 0.92 ± 0.036 in the controls; with the numbers available for study, we could not detect a significant difference (p = 0.3082). The surface-area ratio of the posterior facet (the ratio of the total surface of the posterior facet of the calcaneus to the total surface of the posterior facet of the talus) was 1.03 ± 0.111 in the patients who had flatfoot compared with 0.97 ± 0.059 in the controls; with the numbers available for study, we could not detect a significant difference (p = 0.1742). These data show that, although the areas that were in contact were significantly different between the normal feet and the flatfeet, no significant difference was detected with regard to the ratios of the talar surfaces to the calcaneal surfaces. In other words, a difference in the sizes of the opposing joint surfaces did not create the appearance of subluxation where none existed (Fig. 6).

View larger version (45K): [in this window] [in a new window]   Bar graphs showing the ratio of the surface area of the calcaneus to that of the talus (Calc:Tal), the ratio of the overlap area (the area in contact) to the total surface area of the calcaneus (OL:Calc), and the ratio of the overlap area to the total surface area of the talus (OL:Tal). Significant differences are marked with an asterisk. All data are expressed as ratios to normalize for the size of the foot. A ratio of 1.0 indicates that one articular surface is perfectly coincident with the other. The subluxated area is the converse of the overlap area. There were no detectable differences in size between the surfaces of the flatfeet and those of the control (normal) feet or between the surface area of the talus and that of the calcaneus. There were significant differences between the flatfeet and the control feet with regard to subluxation of both the anterior and middle and the posterior facets. Note the very small I-bars (indicating the standard deviation) for the control feet compared with the larger I-bars for the flatfeet.

Contact Patterns
The contact values varied substantially among the patients who had flatfoot but very little among the controls, as shown by the very small standard deviations. One patient who had flatfoot had a contact pattern similar to that of the controls. This was the patient who had a mild deformity following a recent rupture of the posterior tibial tendon. None of the control surfaces demonstrated substantial subluxation.

	Discussion

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The stimulus for this investigation was our belief that the treatment of symptomatic flatfoot in adults is not always based on clear, objective data. Many treatments have been proposed, each of which may effect change at a different anatomical location. A possible starting point to understanding the rationale for the various methods of treatment is a better definition of the pathological condition. We attempted to define the relationship between two of the major tarsal bones, the talus and the calcaneus.

While there are many unknowns, some observations regarding flatfoot are supported in the literature. First, flatfoot is more often a normal finding than it is a pathological state. Second, flatfoot has many forms. It may consist of only a diminished arch or of abduction of the forefoot or valgus angulation of the hindfoot, or both. Third, flatfoot is described in clinical and radiographic terms in the literature; however, clinical parameters such as the observed arch height or footprint ratios do not seem to be consistently associated with the radiographic arch height in adults. Radiographic parameters are more consistent than clinical indices but may show substantial variation^{3,5,12-14,18,21,23,25-27,29,31}. This suggests that more than one change is responsible for the variations in the shape of the foot.

Some other assumptions are widely held but are not broadly referenced in the literature. It is presumed that when a symptomatic flatfoot is acquired in adulthood, there is a loss of the primary dynamic support followed by secondary relaxation of the static supports^4-7,11,17. A tight gastrocnemius-soleus complex^10,13 as well as the bone morphology² may also play a role. In addition, patients who have symptomatic flatfoot are likely to have a decreased arch on the contralateral side⁸. The latter two findings suggest that there may be an anatomical configuration that predisposes the foot to pathological degeneration. Because there are data suggesting that the changes are in the soft tissue or in the shape of the bone, or both, only a three-dimensional model will help to solve the puzzle.

In addition, because the numerous operative procedures include soft-tissue releases and transfers as well as osteotomy and arthrodesis, an understanding of the anatomy is critical to deciding which operation to perform^{1,7,9-11,15,16,19,20, 22,30}. As each of these procedures is successful in some patients, it is possible that anatomically different types of flatfoot may respond to different procedures. It would be helpful to know the pathological anatomy of the flatfoot so that some structurally based treatment plan could be formulated.

This study demonstrates that, in some adults who have severe symptomatic flatfoot and a weak or absent posterior tibial tendon, the talocalcaneal joint subluxates during weight-bearing. The axis of the talocalcaneal joint is an oblique line, 42 degrees from the horizontal axis and 23 degrees from the midline of the foot and passing through the foot near the lateral process of the talus²⁷. This axis is not in the true coronal plane that is traditionally used to describe motion of the joint. If the calcaneus everts along this axis without a soft-tissue checkrein, the facets would be expected to subluxate. As the axis is centered nearer to the posterior facet than to the anterior facet, the latter should subluxate to a greater degree. The pattern of subluxation measured in the present study is consistent with the loss of soft-tissue constraint that allows the calcaneus to evert along its usual axis.

This study had several shortcomings. First, we evaluated a small number of feet and included predominantly patients who had severe flatfoot and were sufficiently symptomatic to consider an operation. All but one of these feet were markedly deformed, with a diminished arch, valgus angulation of the hindfoot, and abduction of the forefoot. Despite these limitations, the differences between the normal feet and the flatfeet were large and significant. Although we cannot say what percentage of patients may have subluxation and we cannot associate the subluxation to a specific degree or type of flatfoot, we can say that subluxation does occur. Whether this is a common end point for all symptomatic flatfeet is not known.

Another shortcoming of our study was that we used an applied load of only seventy-five newtons, which is approximately 10 percent of the load imposed by quiet standing. This may have led to an underestimation of the degree of subluxation, but it certainly does not change the conclusion that subluxation occurs. In addition, the loading frame required that the knee be flexed to 90 degrees. This position eliminates the pull of the gastrocnemius. The small load and the elimination of the pull of the gastrocnemius would tend to diminish the subluxation, not to exaggerate it.

In conclusion, there is greater subluxation of the talocalcaneal joint during simulated weight-bearing on symptomatic flatfeet than there is on feet with a normal arch. Subluxation of the anterior and middle facets is greater than that of the posterior facet.

	Footnotes

 
*No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. Funds were received in total or partial support of the research or clinical study presented in this article. The funding sources were the Department of Veterans Affairs, Rehabilitation Research and Development, and the University of Washington Department of Orthopaedics Research and Training Fund.

Read in part at the Annual Meeting of the American Orthopaedic Association, Colorado Springs, Colorado, June 11, 1996, and at the American Orthopaedic Foot and Ankle Society Specialty Day during the Annual Meeting of the American Academy of Orthopaedic Surgeons, New Orleans, Louisiana, March 22, 1998.

Department of Orthopaedics, Harborview Medical Center, University of Washington, 325 Ninth Avenue, Box 359798, Seattle, Washington 98104. E-mail address: bsangeor@u.washington.edu(Dr. Sangeorzan).

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