Defining Flatfoot. 5 Associate Professor, Department of Orthopaedic Surgery, Johns Hopkins Hospital. 456

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1 /99/ FOOT & ANKLE INTERNATIONAL Copyright O 1999 by the American Orthopaedic Foot and Ankle Soclety, Inc. Defining Flatfoot Jennifer M. Tareco, M.D.,* Nancy H. Miller, M.D.,t Bruce A. MacWilliams, Ph.D.,* and James D. Michelson, M.D.3 Newark, New Jersey, Baltimore, Mayland, and Salt Lake City, Utah ABSTRACT Pes planus is a term frequently used in describing flatfoot; however, no study has objectively defined flatfoot. We evaluated the single leg stance footprint of 40 feet in 21 people with no history of foot problems, using pressure-sensitive film and a Harris mat. The medial and lateral aspects of the forefoot, midfoot, and hindfoot were assessed. The midfoot was further analyzed by dividing the medial midfoot force by the total midfoot force. The mean medial midfoot forceltotal midfoot force was 11.I % (SD = 6.5%). Pes planus was defined as the medial midfoot forceltotal midfoot force > 24.0% (mean + 2 SD). A population associated with pes planus (124 feet in 63 patients with Marfan syndrome) was then evaluated in the same fashion. Although the mean medial midfoot forceltotal midfoot force was not statistically different (16.0%), a distinct group of patients (25%) had forces that were outside the range of normal midfoot forces. INTRODUCTION Pes planus has been used as a descriptive term for both flexible flatfoot and for pathological conditions such as painful longitudinal arches or symptomatic callosities. There is no universally accepted definition of normal arch height within the general population. A clear understanding of the distribution within the normal population would help us to better evaluate the cause, functional limitations, and prognosis of different forms of pathologic pes planus. It would also aid in Study performed at Department of Orthopaedic Surgery, Johns Hopkins Hospital, Baltimore, Maryland. *Attending Surgeon, Clinical Instructor, Children's Hospital of New Jersey at Newark Beth Israel Medical Center, 201 Lyons Ave., Suite H-1, Newark, NJ, Fax: (973) ; jtareco@sbhcs.com To whom requests for reprints should be addressed. t Assistant Professor, Department of Orthopaedic Surgery, Johns Hopkins Hospital, Baltimore, Maryland. $ Research Associate Professor, Department of Orthopaedic Surgery, University of Utah, Salt Lake City, Utah. 5 Associate Professor, Department of Orthopaedic Surgery, Johns Hopkins Hospital. 456 communication among clinicians regarding both symptomatic and asymptomatic pes planus. This study evaluated the force distribution of feet in a normal population, with attention directed specifically at the midfoot to define pes planus or flatfoot, objectively. This definition was then applied to a group of individuals with Marfan syndrome, a condition characterized by gross ligamentous laxity. MATERIALS AND METHODS The control population consisted of 40 feet in 21 people. No subjects in the control group had active foot problems or a history of foot problems. The single leg stance footprint was recorded using Fuji ultra-low pressure-sensitive film and a Harris-type malg The film was placed in contact with the developing sheet and was placed on a smooth floor. The Harris mat was placed on top of this film such that the dimpled surface of the mat directly contacted the film. A single leg stance footprint was recorded by having the patient step on the mat and maintain his or her balance for 5 sec (with fingertip assistance from the examiner). The pressures under the dimpled surface of the mat caused a grid of circular impressions to be exposed on the Fuji film. The size of each impression directly correlated with the amount of force (patient's load) on that dimple (Fig. 1). The film was then scanned with a flatbed scanner at 150 dpi to obtain a digital image. Custom software was developed to read the resulting image and compute the area and centroidal location of each dimple impression. The area of the impression was then used to compute the local pressure based on the results of a calibration. A manual outline of each subject's foot was also made on plain paper. Calibration of the method was performed using an MTS servohydraulic testing apparatus (MTS; Eden Prairie, MN) to apply constant known loads to a small section (4 dimples x 4 dimples) of the Harris mat-fuji film composite with known area. The area of the exposed film was computed by scanning the image and using the custom software. These areas were then related to the known pressures. This resulted in a near

2 Foot & Ankle International/Vol. 20, No. 7/July 1999 DEFINING FLATFOOT 457 AP - axis: Line Connecting Bisector of Widest Metatarsal and Heel Segments \ Medial Border Widest Segment Under Metatarsal Heads Widest Segment Under Heel Fig. 1. Example of footprint created with pressure-sensitive film and mat. linear relationship across the range of physiologic values. A least squares fit was used to obtain the linear calibration equation, which enabled local pressures to be determined from measured areas. The foot was divided into six segments: medial and lateral aspects of the forefoot, midfoot, and hindfoot. Line segments identifying the widest points on the heel and forefoot were overlaid on the footprint. The long axis of the foot was defined as a line connecting the bisectors of these two line segments. The foot was then divided into equal thirds, based on length measured from a trace of the foot (Fig. 2). The actual load per segment was normalized using each patient's total load to give a percentage of load per segment. The midfoot was further analyzed by dividing the percentage of force in the medial midfoot by the force in the entire midfoot. The evaluation of 124 feet in 63 patients with Marfan syndrome was performed in the same manner as the control patients. Additionally, a general medical history (specifically seeking presence or absence of scoliosis, aortic root dilation, and lens dislocation) and detailed foot history was taken from each Marfan subject. The examiner also recorded a subjective assessment of the arch as high, normal, or flat in the stance phase for the group of Marfan patients. Examination of feet and an evaluation for generalized ligamentous laxity (knee and elbow hyperlaxity, thumb to forearm Fig. 2. Division of the foot into six segments: medial and lateral aspects of the forefoot, midfoot, and hindfoot. test) was performed by the examiner. Laxity was considered present when two of the three tests were indicative. The data was evaluated using Student's t-test for parametric data and chi-square analysis for nonparametric categorical data. RESULTS Evaluation of the medial midfoot force divided by the total midfoot force in the control patients revealed a mean percentage of 11.1 % (SD = 6.5%). Pes planus was defined as a percentage of force in the medial midfoot divided by the force in the total midfoot of >24.0% (mean + 2 SD). Using this criteria, 97.5% of the population were within the normal range (Fig. 3). A total of 2.5% of the control population had pes planus by our definition. In the Marfan population, the mean force in the medial midfoot divided by the total midfoot was 16.0% (SD = 17.4%). The mean was not statistically different from the mean in the control population (P = 0.093). The distribution, however, revealed two distinct types of feet, one inside and one outside the distribution curve for normal feet (Fig. 4). Thirty-one Marfan patients (25%) were outside the range of normal midfoot force. In the Marfan group, those feet which were defined

3 458 TARECO ET AL. Foot & Ankle International/Vol. 20, No. 71July Percentage of Medial Midfoot Loading In Feet of the Normal Controls TABLE 1 Subjective vs Objective Flatfootedness in Marfan Subjects (N = 103) - - Defined Observed Normal Flat Normal Flat Marfan syndrome; that is, aortic dilatation (P = 0.96), scoliosis (P = 0.20), and ectopia lentis (P = 0.29). DISCUSSION Medial MidfootAotal Midfoot Loading Fig. 3. Distribution of the medial midfoot force divided by the total midfoot force in the control population. to have pes planus correlated significantly (P < 0.001) with those who were subjectively observed to have flatfoot by the examiner (Table 1). Marfan patients with objective ligamentous laxity did not have a higher percentage of pes planus (P = 0.09) than the normal control population. There was no statistically significant correlation between type of weightbearing pattern and the major manifestations of Many studies have been conducted to evaluate pressure distribution in feet. The purpose was often to diagnose areas of abnormal pres~ure,~." to evaluate patients with foot disorders,'8235 to evaluate results of treatment.3,8z'2 Only one previous study has attempted to define flatfoot objectively. Footprints in 84 adults were evaluated, and the findings showed a mean instep width of 34 mm (SD = 11 mm) and defined flatfoot as an instep width > 75 mm." The methodology of obtaining the definition of flatfoot was not documented. Additionally, the results are reported as an absolute measurement of size, with no relationship to foot length or body habitus. A study compared the pressure distribution from pedobarograph analysis of 15 infants (range, Percentage of Medial Midfoot Loading in Marfan's Patients Mean + 2SD (NL) Fig. 4. Distribution of the medial midfoot force divided by the total midfoot force in the Marfan populat~on. Medial MidfooVTotal Midfoot Loading

4 Foot & Ankfe InternationalfVoi. 20, No. 7/Ju/y 1999 DEFINING FLATFOOT 459 months old) and 11 1 adults. The results showed significantly higher midfoot pressures in infants7 This was consistent with a developing arch and decreasing midfoot pressures as the child aged and developed a more fixed arch, similar to adult pressure patterns. The results of the footplate analysis in the 11 1 adults were not separated into medial and lateral midfoot, to assess the arch more accurately. No definition of flatfoot was derived from the data. Several studies were evaluated that used various parameters of a footprint tracing; e.g., arch width or arch angle.' These studies were compared with each other as well as with actual measurement of arch height. The findings showed statistically significant variability in the assessment of flat and high arched feet. Correlation and regression analysis showed that footprint-tracing techniques, with resulting parameters, were invalid as a basis for the prediction of arch height. The current study used objective biomechanical methodology for the measurement of foot-contact pressure in single leg stance. Normal arch pressure is defined based on the standard distribution of foot pressures within a normal population, without existing foot pathologic condition. The definition of flatfoot as >24% of the midfoot pressure borne in the medial aspect is generated from the SD within this control population. The definition of pes planus as being present in 2.5% of controls is certainly arbitrary, as are all such classification systems. We realize that foot morphology consists of a continuous spectrum, the extremes of which are easily recognized as pes cavus and pes planus. The object of making a definition is to use it in another population to see if it is a reasonable predictor of pes planus (when compared to a subjective assessment of pes planus). There are no objective standards of pes planus to use in the comparison. The results in the second group of subjects (Marfans), indicated that there was good agreement between our objective measure and the subjective judgement of pes planus. This can be shown by at least three different methods. First, one can use predictive values (PV) of the objective test in comparing it to the subjective assessment, These results are: PV (pes planus by objective criteria) = 89%. PV (non-pes planus by objective criteria) = 70%. Overall accuracy = 75%. A second way to look at it is to perform a logistic regression, using the subjective assessment as the parameter to be predicted by the entire array of objective measures (including objective ligamentous laxity, presence of scoliosis, Fuji film measure, etc.). Using forward stepwise selection, the significant predictors of observed pes planus were the Fuji-film measure (P = 0.003; r = 0.34) and objective ligamentous laxity (P = 0.04; r = 0.14). Using this regression equation, the sensitivity for the prediction of pes planus was 57.6%, with a specificity of 97.4%. The PV (pes planus) was 95%. The PV (non-pes planus) was 72.5%. A third way is that one can simply look at the Spearman rank correlation between the objective and subjective measures of pes planus. In our study, r = 0.52 (P < ). The Marfan population has been reportedly known for remarkable pes planus, which is believed, intuitively, to result from the increased ligamentous laxity secondary to the underlying connective tissue pathologic condition. However, this study has shown that within this group, the mean arch pressure distribution was not significantly different from an age-matched control population. However, a well defined subset of the Marfan population existed above the limit of normal medial midfoot force. A more detailed look at the Marfan subjects is being presented in a separate report. The interobserver variability was studied in assessment of the arch as compared to four-plane weightbearing photos4 The probability of interobserver consistency in assessing a foot as flat ranged from 32% to 75%. Our study, however, showed good correlation between those patients believed to have flatfoot by the examiner and those who were objectively diagnosed with flatfoot (P < 0.001), with a sensitivity of 88.9% and specificity of 69.7%. We believe that this method allows for objective evaluation of any type of foot or foot disorder by providing a control population with which to compare the abnormal foot. REFERENCES 1. Bordelon, R.L. Hypermobile Flatfoot in Children. Ciin. Orthop., 181:7-14, Bordelon, R.L.: Management of disorders of the forefoot and toenails associated with running. 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