Effects of foot orthoses on center-of-pressure patterns in women.

Effects of Foot Orthoses on Center-of-Pressure Patterns in Women Osseous deformities of the forefoot resulting from abnormal development of the midtarsal joint can create changes in the typical ground-reaction force pattern. [1] Katoh et al suggested that the use of foot orthoses may troduce a more typical ground-reaction force pattern by improving the alignment of the foot. [2]

The two major developmental deformities of the midtarsal joint are a forefoot varus or forefoot valgus. The cause of forefoot deformities is believed to be a delay in the normal ontogeny of the midtarsal joint. [3] Forefoot varus is a structural variation in which the plane of the metatarsal heads is inverted in relation to the bisector of the posterior surface of the calcaneus when the subtalar joint is in a neutral position and the midtarsal joint is fully locked. [4] Forefoot valgus is a structural variation in which the plane of the metatarsal heads is everted in relation to the bisector of the posterior surface of the calcaneus with the subtalar joint in a neutral position and the midtarsal joint fully locked. [4] The incidence of forefoot deformities in a sample of healthy women has been reported to be 44.8% and 8.6% for forefoot valgus and forefoot varus, respectively. [5] Forefoot deformities can cause abnormal foot motion during walking. [4,6] This abnormal foot motion can be a cause of pain in the forefoot. [6-8]

Several investigators have used cinematography and electrogoniometry to determine whether foot orthoses effectively modify abnormal movement patterns of the foot. [9-13] Only two studies used components of the ground-reaction force pattern to study the effectiveness of foot othoses. Schoenhaus et al recorded the torque curve caused by transverse rotation of thelower extremity during the stance period for three patients with various foot abnormalities. [14] The torque curves were interpreted by the authors as representing an indirect measure of subtalar joint motion. They concluded, after assessment of changes in the torque curve before and after the fitting of rigid foot orthoses, that the wearing of foot orthoses caused a more normal force pattern. Scranton et al investigated the effects of a foot orthosis and two foot-strapping techniques on ground-reaction forces in five subjects. [15] Changes in the pattern of forces occurred for all three treatments. From their findings, however, the authors did not conclude that any of the treatments effectively controlled abnormal force patterns.

Neither of the previous studies considered the effect of forefoot deformities on the ground-reaction force pattern. This is an important consideration, because the design and materials used in the construction of foot orthoses will vary depending on the type of forefoot deformity. [3,16] No investigator has used the center-of-pressure pattern as a method of determining the effectiveness of foot orthoses. Katoh et al stated that the center-of-pressure pattern can be an effective method for quantitatively evaluating foot orthoses. [2] The center of pressure calculated from force-platform data refers to the centroid of the pressure distribution as the ground-reaction components are applied over the plantar surface of the foot. One disadvantage of center-of-pressure patterns is that they cannot be quantified easily. To permit quantitative comparison, Nigg plotted the mediolateral (Ax) and the anteroposterior (Ay) components of the center of pressure as a function of time. [17] The area under the Ay and Ax distance-time curves, termed the integral, can then be calculated and used for analyses.

The purpose of this investigation was to determine the effects of three types of foot orthoses on the center-of-pressure pattern for women with forefoot deformities during walking. The hypotheses were 1) the integral values of the Ax and Ay distance-time curves walking barefoot would be greater than values obtained walking with shoes only for subjects with forefoot deformities, and 2) the integral values of the Ax and Ay distance-time curves walking with shoes only would be greater than values obtained walking with each of the three types of foot suports for subjects with forefoot deformities.

The two independent variables for our study were Factor A, forefoot deformity, and Factor B, foot treatments. The two levels of Factor A were 1) varus and 2) valgus. The five levels of Factor B were 1) barefoot, 2) shoes only, 3) Thermocork (*1) orthoses (CORK) with shoes, 4) Nickelplast [TM] (*1) orthoses (NICKEL) with shoes, and 5) Plastizote [R] II (*1) orthoses (PZII) with shoes.

Method

Subjects

Eighteen women between the ages of 21 and 46 years (X = 24.0, s = 5.6) participated as subjects. The subjects had no history of injury to the foot, ankle, or lower leg 12 months before data collection. We performed a foot-type assessment on each subject using the procedure described by McPoil and Brocato. [18] Nine subjects had a forefoot varus, and nine subjects had a forefoot valgus. Subjects selected demonstrated 1) a dynamic walking angle of 15 degrees, 2) a dynamic walking base of 5.1 to 10.2 cm, and 3) at least 10 degrees of talocrural joint dorsiflexion. No subject had a subtalar valgus or a subtalar varus of greater than 4 degrees. A subtalar varus in excess of 4 degrees has been described as a cause of abnormal foot motion. [3] The study procedure was approved by an institutional review board, and all subjects signed an informed consent form before participation.

Instrumentation

We used a Kistler Type 9281B11 force platform (*2) placed in the center of a 10.4-m walkway to measure the ground-reaction forces. The amplified signal from the force platform was input to a Hewlett-Packard Model 9217 computer. (*3)

Shoes and Fabrication of Foot

Orthoses

Three different pairs of foot orthoses were fabricated for each subject using neutral-position foot molds. The foot molds were made from neutral cast impressions of each subject's feet using the prone-position procedure. [18] We chose the prone position based on the advantages described by Valmassey. [16] The design and fabrication method used to construct the foot orthoses was identical to the procedure described by McPoil and Brocato [18] except for the final layer of material (Fig. 1). The final layer of each of the three pairs of orthoses was varied by using CORK, NICKEL, or PZII, respectively.

All subjects wore the Nike [R] Vortex [TM] athletic shoe. (4) The Vortex [TM] model provided rear-foot stability using a rear-foot stabilizer and rigid counter without restricting medial or lateral motion of the forefoot.

Procedure

Initial ground-reaction force data were collected for each subject walking barefoot before issuing shoes and foot orthoses. Each subject was asked to walk over the walkway to ensure proper contact with the force platform and to establish a consistent, self-selected walking cadence. With then asked the subject to walk barefoot over the force platform while data for three barefoot trials were collected. Each subject was then instructed to wear the shoes twice for two hours the first day, increasing to three hours twice a day for the remaining six days (40 hours total). At the end of the 40 hours of shoe-wearing time, we instructed the subjects to place one pair of orthoses in the shoes and to repeat the initial break-in procedure until 40 hours of wearing time was completed. All subjects were instructed to repeat the same procedure for each pair of foot orthoses and to use the orthoses only in the shoewear provided. We used a table of random numbers to determine the break-in order for the foot orthoses and requested that each subject record wearing time on a calendar provided. Final testing was scheduled when the subject had recorded a total wearing time of 160 hours for the shoes, which included 40 hours for each of the three pairs of orthoses.

Before final data collection, a metronome was set at the self-selected cadence used by the subject during the barefoot trials, and the subject practiced walking for 10 minutes. Just before data collection, the metronome was turned off, and the subject continued to practice walking. During this time period we recorded ground-reaction force data to determine the duration of the stance phase. If the stance-phase duration was within 0.02 second of previously calculated barefoot durations, final testing was conducted. If the duration was not within those limits, the subject continued practicing until this criterion was met.

When we determined walking cadence to be satisfactory, each subject was assigned to one of the four remaining treatment conditions (shoes only, shoes with CORK, shoes with NICKEL, or shoes with PZII). We used an incomplete counterbalancing design to determine the order of testing to prevent possible practice effects. Ground-reaction force data were collected as the subject walked over the force platform three times for each treatment. In addition to the three barefoot trials, each subject performed three trials for each treatment for a total of 15 trials. Force data were sampled during each trial for 2 seconds at 1,000 Hz.

Data Analysis

The duration of the stance phase was calculated using a "start-stop" computer subroutine. To calculate the integral of the Ax distance-time curve (AxI), the Ax center-of-pressure time pattern was plotted by the computer. A baseline was then drawn to connect the first and last data points of the pattern. The area between the baseline and the Ax center-of-pressure time pattern was then measured to determine the AxI. An example of the AxI is showin in Fig. 2. The same procedure was used to calculate the integral of the Ay distance-time curve (AyI) from the Ay center-of-pressure time pattern. An example of an AyI is shown in Fig. 3.

To determine the reliability of the stance-phase durations, type three intraclass correlation coefficients (ICCs) were performed on the 15 trials for the 18 subjects. [19] A two-factor, mixed analysis of variance (ANOVA) for repeated measures was used to determine whether the overall F-ratio tests for the main effects of Factor A (deformity), Factor B (foot treatments), or the A X B interaction were significant. A Tukey post hoc comparison was used to determine differences among the treatment means.

Results

Realibility of Stance-Phase

Durations

The mean stance-phase duration for all 18 subjects was 0.64 second. The standard deviation was 0.05 second. The ICC was .84.

Mediolateral Distance-Time

Curve Integral

Factor B and the A X B interaction were significant for the variable AxI. The means and standard deviations are listed in Table 1, and the results of the ANOVA are listed in Table 2. A plot of the simple main effects for AxI is shown in Fig. 4 to illustrate the interaction.

The results of the analysis of the simple main effects on Factor B were significant for both the Varus Group (F = 9.81; df = 4,32; p [is less than] .001) and the Valgus Group (F = 5.50; df = 4.32; p [is less than] .01). Tukey's pair-wise comparisons on Factor B for the Varus Group resulted in significant differences between the following treatment means: 1) barefoot and shoes only, 2) barefoot and shoes with CORK, 3) barefoot and shoes with NICKEL, and 4) barefoot and shoes with PZII. Tukey's pair-wise comparisons on Factor B for the Valgus Group resulted in significant differences between the following treatment means: 1) barefoot and shoes with CORK, 2) barefoot and shoes with NICKEL, 3) and barefoot and shoes with PZII.

Anteroposterior Distance-Time

Curve Integral

Factor A, Factor B, and the A x B interaction were not significant for the variable AyI. Means and standard deviations for AyI are listed in Table 3, and the results of the ANOVA are listed in Table 4.

Discussion

Our first concern in analyzing the results of this study was the reliability of the stance-phase durations for each subject. Changes in walking cadence between testing trials affecting the duration of the stance phase can result in force-platform data variations. [20] Based on the ICC reported, we believe that the protocol used in this investigation to reproduce the subjects' self-selected cadence was effective.

The hypotheses for our investigation were based on the premise that displacement on the center-of-pressure pattern would be greater walking barefoot in comparison with walking with shoes only and with shoes with orthoses. The results of the analysis on the AxI data were significant between the barefoot and shoes-only treatments for the Varus Group but not for the Valgus Group. Our first hypothesis, therefore, was supported for the Varus Group only.

No significant differences were found in the AxI data between the shoes-only treatment and each of the three types of foot orthoses with shoes for either the Varus or Valgus Groups. Based on these results, we rejected the second hypothesis for both the Varus and Valgus Groups.

On further examination of the AxI data, we noted a significant difference between the barefoot condition and each of the three types of foot orthoses for the Valgus Group. We believe these findings support the use of foot orthoses, in addition to shoewear providin rear-foot stability, for women with a forefoot valgus deformity. Women with a forefoot varus deformity would not appear to benefit, however, from wearing foot orthoses while walking if they wore shoewear that provided rear-foot-stability.

The results we obtained were not significant for the variable AyI, which measured the Ay displacement of the center-of-pressure pattern. Root et al stated that abnormal motion of the foot caused by forefoot deformities occurs in the frontal, or Ax, plane. [1] Because AxI is a measure of Ax displacement of the center-of-pressure pattern, we expected AxI to be a better measure of foot orthosis effectiveness than AyI.

No significant differences were found among any of the three types of foot support after 40 hours of wearing time. This result indicates that the type of material used in the construction of the foot orthoses, within the 40-hour break-in period used in this study, did not affect the center-of-pressure pattern. Although the intent of our investigation was not to evaluate the durability of the materials used for fabrication of the orthoses' final layer, we recognize that longer periods of wearing time could alter the material.

Summary and Clinical

Implications

The purpose of our investigation was to evaluate the effect of walking with shoes only and walking with shoes with foot orthoses on the center-of-pressure patterns of women withe forefoot deformities using the barefoot condition as the baseline measure. The addition of shoewear resulted in a reduction of the AxI for both the Varus and Valgus Groups. Only the Varus Group, however, demonstrated a significant difference between the barefoot and shoes-only treatments. A significant difference between barefoot and shoes-only treatment conditions was not found in the Valgus Group, but shoes plus any of the three types of foot orthoses used in this study significantly reduced the AxI in comparison with the barefoot treatment.

Based on our results, the first step in the treatment of foot problems in women caused by forefoot deformities would be the recommendation of shoewear. Foot orthoses appear to be an important addition to the treatment regimen for women with a forefoot valgus deformity. The necessity of prescribing foot orthoses for individuals with only forefoot varus deformities would be suspect, based on our findings. Continued research in this area is needed.

Afinal observation from our investigation is that active, healthy women may have foot deformities without foot pain or discomfort. In certain individuals, therefore, such deviations in forefoot alignment may still be within a normal range.

Acknowledgments

We gratefully acknowledge Tom Brunick and Thomas Clarke, PhD, for their assistance in obtaining the shoewear used in our investigation.

(*1) AliMed, Inc, 297 High St, Dedham MA 02026-2839.

(*2) Kistler Instrument Corp, 75 John Glenn Dr, Amherst, NY 14120.

(*3) Hewlett-Packard Co, PO Box 3640, Sunnyvale, CA 94088-3640.

(*4) Nike [R] Inc, 9000 SW Nimbus Dr, Beaverton, OR 97005.

References

[1] Root ML, Orien WP, Weed JH: Clinical Biomechanics: Normal and Abnormal Function of the Foot. Los Angeles, CA, Clinical Biomechanics Corp, 1977, vol 2, pp 175-177

[2] Katoh Y, Chao EYS, Laughman RK, et al: Biomechanical analysis of foot function during gait and clinical applications. Clin Orthop 177: 23-33, 1983

[3] Sgarlato TE: A Compendium of Podiatric Biomechanics. San Francisco, CA, California College of Podiatric Medicine Press, 1971

[4] Burns MT: Biomechanics. In McGlamry ED (ed): Fundamentals of Foot Surgery. Baltimore, MD, Williams & Wilkins, 1987, pp 128-132

[5] McPoil TG, Knecht HK, Schuit D: A survey of foot types in normal females between the ages of 18 and 30 years. Journal of Orthopaedic and Sports Physical Therapy 9:406-409, 1988

[6] Schoenhaus HD, Jay RM: Cavus Deformities. J Am Podiatr Assoc 70:235-238, 1980

[7] Hlavac HF: Compensated forefoot varus. J Am Podiatr Assoc 60:229-233, 1970

[8] McPoil TG, Schuit D: Management of metatarsalgia secondary to biomechanical disorders: A case report. Phys Ther 66:970-972, 1986

[9] Bates BT, Osternig LR, Mason B, et al: Foot orthotic devices to modify selected aspects of lower extremity mechanics. Am J Sports Med 7:338-342, 1979

[10] Rodgers MM, LeVeau BF: Effectiveness of foot orthotic devices used to modify pronation in runners. Journal of Orthopaedic and Sports Physical Therapy 4:86-90-, 1982

[11] Sims DS Jr, Soderberg GL: The effect of a foot orthosis in compensated forefoot varus. Abstract. Phys Ther 64:726, 1984

[12] Smith LS, Clarke TE, Hamill CL, et al: The effects of soft and semi-rigid orthoses upon rearfoot movement in running. J Am Podiatr Med Assoc 76:227-233, 1986

[13] Smart G, Robertson G: Triplanar electrogoniometer analysis of running gait. In Winter DA et al (eds): Biomechanics IX-A. Champaign, IL, Human Kinetics Publishers Inc, 1985, pp 145-148

[14] Schoenhaus HD, Gold M, Hylinski J, et al: Computerized analysis of gait: Clinical examples relating to torque. J Am Podiatr Assoc 69:11-16, 1979

[15] Scranton PE Jr, Pedegana LR, Whitesel JP: Gait analysis: Alterations in support phase forces using supportive devices. Am J Sports Med 10:6-11, 1982

[16] Valmassey RL: Advantages and disadvantages of various casting techniques. J Am Podiatr Assoc 69:707-712, 1979

[17] Nigg BN: Experimental techniques used in running shoe research. In Nigg BN (ed): Biomechanics of Running Shoes. Champaign, IL, Human Kinetics Publishers Inc, 1986, pp 27-61

[18] McPoil TG, Brocato R: The foot and ankle: Biomechanical evaluation and treatment. In Gould JA, Davies GJ (eds): Textbook of Physical Therapy: Orthopaedic and Sports Physical Therapy, St Louis, MO, C V Mosby Co, 1985, pp 313-341

[19] Shrout PE, Fleiss JL: Intraclass correlations: Uses in assessing rater reliability. Psycho Bull 86:420-428, 1979

[20] Gronley JK, Perry J: Gait analysis techniques: Rancho Los Amigos Hospital Gait Laboratory. Phys Ther 64:1831-1838, 1984

T McPoil, PhD, PT, ATC, is Assistant Professor, Department of Physical Therapy, Northern Arizona University, NAU Box 15105, Flagstaff, AZ 86011 (USA). He was Assistant Professor, Department of Physical Therapy, College of Associated Health Professions, University of Illinois at Chicago, Chicago, IL, when this study was conducted.

M Adrian PED, is Director of Biomechanics Research Laboratory and Professor of Physical Education, University of Illinois at Irbana-Champaign, Urbana, IL 61801.

P Pidcoe, BS, is Research Associate, Department of Physical Therapy, College of Associated Health Professions, University of Illinois at Chicago, 1919 W Taylor St, Chicago, IL 61612.

This study was conducted in partial fulfillment of the requirements of Dr McPoil's doctoral degree, the University of Illinois at Urbana-Champaign. This study was funded by a grant from the Chicagoland Orthopaedic Physical Therapy Study Group, Chicago, IL.

This article was submitted January 4, 1988; was with the authors for revision for 21 weeks; and was accepted August 30, 1988.