Abstract
Background Although not well understood, foot kinematics are changed with hallux valgus.
Objective The purpose of this study was to examine tarsal kinematics in women with hallux valgus deformity.
Design A prospective, cross-sectional design was used.
Methods Twenty women with (n=10) and without (n=10) deformity participated. Data were acquired with the use of a magnetic resonance scanner. Participants were posed standing to simulate gait, with images reconstructed into virtual bone datasets. Measures taken described foot posture (hallux angle, intermetatarsal angle, arch angle). With the use of additional computer processes, the image sequence was then registered across gait conditions to compute relative tarsal position angles, first-ray angles, and helical axis parameters decomposed into X, Y, and Z components. An analysis of variance model compared kinematics between groups and across conditions. Multiple regression analysis assessed the relationship of arch angle, navicular position, and inclination of the first-ray axis.
Results Both the hallux and intermetatarsal angles were larger with deformity; arch angle was not different between groups. The calcaneus was everted by ≥6.6 degrees, and the first ray adducted (F=44.17) by ≥9.3 degrees across conditions with deformity. There was an interaction (F=5.06) for the first-ray axis. Follow-up comparisons detected increased inclination of the first-ray axis over middle stance compared with late stance in the group with deformity.
Limitations Gait was simulated, kinetics were not measured, and sample size was small.
Conclusions There were group differences. Eversion of the calcaneus and adduction of the first ray were increased, and the first-ray axis was inclined 24 degrees over middle stance in women with deformity compared with 6 degrees in control participants. Results may identify risk factors of hallux valgus and inform nonoperative treatment (orthoses, exercise) strategies.
A better understanding of foot kinematics is needed to improve the treatment of hallux valgus.1,2 The condition represents a deformity in which the hallux abducts (eFig. 1A) and the first metatarsal adducts (eFig. 1B). Hallux angle (eFig. 1A, angle a) refers to the offset in first metatarsophalangeal joint position. The related separation between the first and second metatarsals is called the intermetatarsal (IM) angle (eFig. 1B, angle b). Both angles increase as deformity progresses. Treatment of hallux valgus includes the use of orthoses, exercise, and splinting. Of these, randomized controlled trials3–5 support only orthoses as beneficial in the treatment of hallux valgus. Unfortunately, the mechanism by which orthoses aid in the management of hallux valgus is not known because of a lack of data on the kinematic alterations that accompany the deformity.
The extent to which kinematics are or are not changed in people with hallux valgus deformity has been the focus of research. Separate studies6,7 collected optical motion data during walking trials from surface mounted markers and modeled the foot as 3 rigid segments (hallux, forefoot, and hindfoot). Deschamps purported the change in foot kinematics associated with hallux valgus as isolated to the hallux and that “other segments of the foot are not affected in a major way.”7(p105) Because modeling is unable to capture tarsal positioning, as the result of tracking errors during gait,8 the conclusion may be incorrect. Angular position of the first metatarsal (Fig. 1), navicular, and calcaneus are the variables selected for measurement in this imaging study of kinematics.
Kinematic measurements involving the first metatarsal (ray). (A) Hallux angle, (B) intermetatarsal angle, and (C) arch angle are measurements of foot posture. First-ray–to–navicular transverse, sagittal, and frontal (not shown) position angles were computed by means of computer processes that embedded orthogonal coordinate frames into the respective bones. The first-ray axis, shown as a black “hinge-like” rod, was modeled with the use of a finite helical axis approach.
Tracking bone-to-bone kinematics from imaging eliminates skin motion artifacts and aggregate modeling errors.9,10 Imaging quantifies bone morphology, and, when a series of static images are taken to represent a sequence of body movements, the changes in bone position do approximate joint motion.9 Kinematic imaging also has disadvantages. Conventional scans, for example, are taken with the patient placed supine with arbitrary loads imposed.10,11 Such nonphysiological loading has caused researchers11–13 to question the validity of the measurement. Wolf et al13 compared the change in intertarsal angles computed from images acquired supine with the same angles measured from fixed bone pins while participants (N=3) stood. Angles did not correspond. Mean differences were as high as 10 degrees, a value twice as large as the error attributed to image processing (<3°) or bone pin measurements (2°–6°). To address the issue of postural dependency, advancement in magnetic resonance (MR) scanner technology now permits the patient to stand, giving improved opportunity to simulate gait.13
Currently, orthoses are prescribed to decrease subtalar joint displacements of the calcaneus and navicular.3–5 Nester and colleagues8,14 attached motion sensors to pins screwed into the foot of 6 adults and recorded tarsal motion over repeated walking trials. Calcaneus-with-respect-to-tibial summation of component rotations averaged 17.0 degrees in the sagittal plane, 7.3 degrees in the transverse plane, and 11.3 degrees in the frontal plane.14 The series of Cardan angles represents the change in spatial orientation of the calcaneus relative to the leg. Navicular-with-respect-to-calcaneal component rotations averaged 6.1 degrees in the sagittal plane, 11.3 degrees in the transverse plane, and 9.5 degrees in the frontal plane.8 The magnitude of these relative tarsal motions signifies the importance of the calcaneus and navicular in the transference of load during gait.
The first metatarsal plays a role in the development of hallux valgus deformity.15–17 The first (metatarsal-cuneiform) ray moves as a single arch segment during gait.8,18 Rotation occurs about an axis that runs horizontal from the navicular to the base of the third metatarsal (Fig. 1).19 In theory,20 should the arch flatten under imposed load caused by excess subtalar joint pronation, the navicular would descend in a plantar direction, thus lowering the most medial point of the first-ray axis. Accordingly, the position of the navicular influences the biomechanical behaviors of the first ray. Glasoe et al21 quantified orientation of the first-ray axis in 9 cadavers while simulating gait. Inclination (tilt toward vertical) of the axis was inversely related to height of the navicular (r=−.73), and, when the axis inclined, the first ray adducted. Although the specimens tested did not have hallux valgus, extrapolation of this result suggests that inclination of the first-ray axis may precipitate adduction of the first-ray arch segment.
The overall aim of this study was to examine the change in weight-bearing alignment of the calcaneus, navicular, and first ray in women with hallux valgus. Because our methods posed participants standing to simulate gait mid-stance (MS), heel-off (HO), and terminal stance (TS), published bone-pin measurements of tarsal motion provide a baseline for interpreting the data recorded in the present report. For the face validity comparison, Nester and colleagues8,14 reported the navicular everts 5 degrees relative to the calcaneus until HO, which corresponds with subtalar and midtarsal joint pronation, and it then inverts an equivalent amount over remainder of stance. Data collected in clinical gait studies22–24 also record lowering of the arch accompanied by eversion of the calcaneus continuing until HO, when the foot transitions from pronation to supination.
This study tested 3 hypotheses. Hypothesis 1 examined hallux angle, intermetatarsal (IM) angle, and arch angle (as a parameterization of arch height) measurements of foot posture, with the expectation that each would be enlarged with deformity.
Hypothesis 2 examined calcaneus and navicular positions across simulated gait events (MS, HO, TS) to compare the effects of group and condition:
2.i. The calcaneus would evert more in relation to the fibula in women with deformity at each test condition.
2.ii. The navicular would evert more in relation to the calcaneus at HO with deformity.
2.iii. Regardless of group, the navicular would evert more in relation to the calcaneus at HO compared with MS and invert more at TS compared with HO.
A difference found may distinguish hindfoot position as a modifiable risk factor of hallux valgus deformity.
Hypothesis 3 examined first-ray position, again comparing the effects of group and condition:
3.i. The first ray would adduct more in relation to the navicular in women with deformity, and, accordingly, the first-ray axis would be more inclined (tilt toward vertical) over the middle and the late stance gait conditions. Inclination of the axis toward vertical would allow greater adduction in the transverse plane and restrict dorsiflexion in the sagittal plane and inversion in the frontal plane.
3.ii. The first ray would evert more relative to the navicular at HO with deformity.
3.iii. Arch angle considered in combination with the change in navicular eversion angle measured over middle and late stance as variables in a regression model would predict inclination of the first-ray axis.
No previous study has investigated the position of the navicular as a potential contributing mechanism of first-ray adduction in people having hallux valgus.
Method
Participants
Twenty women were studied because of their prevalence as 90% of the patient cohort.25 Ten had hallux valgus, and 10 served as control participants. A power calculation completed from pilot data26 indicated that a 5-degree group difference in relative angle between the calcaneus and leg (fibula) had 80% power to reach significance if sampling 9 per group. A 5-degree group difference in the measurement of hind foot eversion is considered an indicator of hindfoot deformity.27
A flyer announcing a call for study participants (ages 18–75 years) with “bunion deformity” was posted at the University of Minnesota medical clinics. As many as 30 people responded to the advertisement, but only the first 10 people having what the principal investigator (W.M.G.) determined as being hallux valgus deformity were invited to participate provided that they met all other aspects of the study inclusion and exclusion criteria. Control participants were group-matched according to age and body mass index as closely as possible (Tab. 1). All participants were independent in daily activities. Excluded were those having a contraindication to MR imaging and those with deformity other than hallux valgus or insensate to monofilament testing, a precursor of Charcot deformity. Also excluded were individuals with rheumatoid arthritis, those with self-reported episodic gout, history or visible evidence of ulceration or forefoot surgery, or stiffness that limited dorsiflexion of the hallux to 50 degrees or less or ankle to 10 degrees or less, to allow standard test conditions. Consent was obtained in accordance with University of Minnesota Institutional Review Board (No. 0709M16823) guidelines.
Participant Demographics and Foot Posture
Scan and Image Preparation Procedures
Participants were imaged with the use of a FONAR (Fonar Inc, Melville, New York) Upright 0.6-T magnet. Images were obtained in the sagittal plane, with the use of a T1-weighted, three-dimensional (3D) gradient-recall-echo sequence with fat suppression (flip angle=60°, number of excitations=1) scanning protocol. The scan field, composed of 128 slices, was centered and rotated across the image sequence to capture the foot/ankle in a 256 × 256 matrix. Image resolution was 1.0 × 1.0 mm, giving a precise location of each voxel in the scanner field. Each scan took 6 minutes to complete; the entire scanning protocol was completed in 1 hour.
A 3-setting image sequence simulated gait MS, HO, and TS. The static conditions represent the incremental instances of continuous foot motion during the gait cycle, with HO staged to occur halfway between the beginning of MS and the end of TS.21 For each scan, the participant stood on a level scanner platform with weight distributed on both feet while partially kneeling into a bolster (Fig. 2A). Conditions were standardized by placing the foot on identically sized wedges and then flexing the participant's knee to pose the ankle at a predetermined angle (Fig. 2B) targeting the average of values reported in the literature.8,28,29 The ankle was dorsiflexed 5 degrees at MS and 10 degrees at HO and was plantar flexed 10 degrees at TS. The hindfoot wedges supported the load-bearing surface of the calcaneus only. Composed of hard rubber, the wedges did not slip or compress under weight. No wedging was used at MS (Fig. 2B). One wedge, having a 15-degree angle, was placed beneath the calcanei bilaterally at HO. Two of the 15-degree angle wedges were stacked and placed beneath the calcanei bilaterally at TS. A smaller wedge having a 10-degree angle was placed beneath the toes at TS (Fig. 2B) to tighten the plantar fascia.
(A) Participant stood partially kneeling into a bolster (fixed in the magnetic resonance scanner) with the foot posed on wedges to simulate gait conditions. (B) Sagittal view of the 3-setting sequence of imaged bone datasets. The bones shown are the proximal phalanx of the hallux, first metatarsal, navicular, calcaneus, and fibula.
From the scanned images, the proximal phalanx of the hallux, the first and second metatarsals, the navicular, calcaneus, and distal end of the fibula were segmented (reconstructed) with the use of MIMICS (Materialise, Leuven, Belgium) software. These tarsals were selected for analysis in anticipation that deformity would affect their relative anatomical alignment.20,21 The fibula was segmented because its truncated length best represents the spatial orientation of the leg.10
Once segmented, the virtual bone datasets displayed in the global-vertical MR scanner coordinate system (eFig. 2). The global reference is henceforth called the “MR reference.” The bone models segmented at MS were imported into the MIMICS projects at HO and TS to “register” the image sequence together. Registration gives valid representation of a bone to within 1 degree.12 This final image-processing step also removed any variability associated with reconstructing one bone model over multiple scans.26
Kinematic Measurements and Calculations
A single examiner (W.M.G.) measured the foot posture hallux angle, IM angle, and arch angle from a planar view of the MS bone dataset reconstructed for each participant. The hallux angle is positive when the proximal phalanx of the hallux deviates laterally (abduction) with respect to the first metatarsal. The IM angle defined the transverse-plane alignment of the first and second metatarsals.25 Arch angle (Fig. 1C) defining the alignment of the first metatarsal and calcaneus was measured in the sagittal plane.30 The reliability of the measurements was excellent (intraclass correlation coefficient [ICC (3,1)]≥.97; standard error of the mean ≤2°) in all cases.26
Principal component inertial-axes orthogonal systems (frames) were used to define tarsal kinematics across gait conditions.9,10 Through the use of automated computer processes, each bone was embedded with an inertial coordinate frame. Definitions of the coordinate frame axes correspond to Z-axis dorsiflexion and plantar flexion, Y-axis adduction and abduction, and X-axis inversion and eversion (eFig. 2). All local coordinate frames, except for the frame embedded into the navicular, aligned nearly coincident with the MR reference (Fig. 1). The navicular frame aligned out-of-plane. To allow us to describe the first metatarsal (ray) position angle in the clinically meaningful anatomical planes, the local coordinate frame embedded to define the navicular was mathematically corrected to zero in the MR reference at the MS gait condition. The correction factor then applied to the participant's navicular at HO and TS allowed for the identification of any change in its position between gait conditions and groups.
Five different 3D distal-relative-to-proximal bone position angle calculations were made: calcaneus-to-fibula, navicular-to-calcaneus, navicular-to-MR reference; first ray–to–navicular, and the proximal phalanx of the hallux–to–first ray (Tab. 2). Tracking the angular position of the calcaneus, the navicular, and the first ray was of primary interest. The hallux–to–first-ray angles are presented as mean results but not analyzed further. Any change recorded in tarsal position between gait conditions gives a measure of motion,9 but the motion cannot be attributed to any single joint, the exception being the hallux–to–first-ray angle, which represents first metatarsophalangeal (1-MTP) joint kinematics.
Position Angle Group Comparisons Recorded Across Gait Conditionsa
Transformation matrices were calculated from one custom-written Matlab (Mathworks Inc, Natick, Massachusetts) code. Currently, no standards have been recommended for calculating the order of rotations for the tarsals individually. The hindfoot calcaneus and navicular angles were calculated by means of a Y-Z′-X′′ sequence, with expectation that the sagittal Z-axis component rotation would be largest.31 The hallux and first-ray angles were calculated by means of a Y-Z′-X′′ sequence because, with deformity, calculating the first rotation about the Y-axis may best represent rotations outside the sagittal plane. Left-sided data were converted to right-sided equivalency. The overall image data processing error was ≤3 degrees.26
A finite helical axis32 calculation expressed orientation of the first-ray axis relative to the navicular between MS and HO (middle stance) and between HO and TS (late stance) in the MR reference.21 A tag-tracking33 adjustment made within the helical axis computation kept the MR reference vertical and consistent across gait events (eFig. 2). The orientation components where the direction cosines provide the cosine of the angle in each component direction (X, Y, Z) to the helical axis vector and total rotation (Phi) were computed.32 Important to this research, the Y component (Yc) defined vertical orientation of the first-ray axis within the MR reference and was not specific to the participant's anatomy.
Data Analysis
All data were screened for the assumptions of normality. Independent t tests assessed for group differences in demographics and foot posture. A 2-way mixed-model analysis of variance assessed for the significance of group × condition interaction for the relative calcaneus and navicular position angles and main effects for group and condition. In the presence of significant interaction or when the a priori hypothesis required, simple effects of groups at each level of condition were completed. Gait condition (MS, HO, TS) was used as a within-subject factor. Additional runs of the analysis of variance model assessed the significance of main effects (group and condition) and interaction for Yc. In this model, the finite periods between successive gait conditions (middle stance; late stance) were treated as independent variables affecting Yc. If significant interaction was identified, post hoc simple effects of condition for each group were tested. In final analyses, multiple regression analysis assessed the relationship of arch angle in combination with the navicular eversion angle in predicting Yc. Significance was set at P<.05. All data runs were conducted with the use of NCSS (Kaysville, Utah) version 7.0 software.
Role of the Funding Source
This work was funded by the Arthritis Foundation North Central Chapter and in part by the Foundation for Physical Therapy.
Results
In screening for non-normality, each variable fell within the cutoffs of −2 to 2 for skewness and −4 to 4 for kurtosis.
Demographic and Foot Posture Comparisons
There was no group difference in age, height, or body mass index (Tab. 1). Foot posture was changed with deformity (hypothesis 1). Both hallux and IM angles were statistically larger (P<.01), although arch angle was not different (P=.46) between groups.
Calcaneus and Navicular Comparisons
There was no group × condition interaction (F≤1.77; P≥.18) in any of the 3-D calcaneus-to-fibula, navicular-to-calcaneus, or navicular-to-MR reference position angles. Several simple effects were found (Tab. 2) and are reported in response to the a priori hypotheses tested.
Hypothesis 2.i tested pair-wise group comparisons in the frontal calcaneus-to-fibula angles at each condition. The calcaneus everted more (P<.05) with deformity by an average of 6.6 degrees at MS, 7.4 degrees at HO, and 7.9 degrees at TS.
Hypothesis 2.ii tested the group effect in the frontal navicular-to-calcaneus angles. The planned pairwise comparison detected a group difference (P<.05) at HO. Inversion (not eversion as hypothesized) of the navicular increased by an average of 5.8 degrees with deformity.
Hypothesis 2.iii tested the condition effect in the frontal navicular-to-calcaneus angle. An omnibus condition effect (F=20.00; df=2,36; P<.001) was found. The navicular increasingly inverted across the gait progression (Tab. 2). The pair-wise comparisons identified the navicular as more (P<.05) inverted at TS compared with MS in both groups.
Two other calcaneus and navicular position angle comparisons merit highlight (Tab. 2). First, the sagittal calcaneus-to-fibula angle was not different between groups (F=1.10; df=1,18; P=.31), a result expected because sagittal alignment of the participant's ankle was controlled as the independent test variable during scanning. Second, the sagittal navicular-to-MR reference angle increasingly plantar flexed by an average of 15 degrees between conditions. The 15-degree increment is identical to the angle of wedge used to lift the heel and simulate gait.
First-Ray Comparisons
Several significant interactions and main effects were found in the first-ray angles and helical axis mean comparisons. These results are also reported in response to the hypotheses tested.
Hypothesis 3.i tested the group effect in the transverse first-ray–to–navicular angle and inclination of the first-ray helical axis. Adduction of the first ray was increased (F=44.17, df=1,18; P<.001) by 10.3 degrees at MS, 11.9 degrees at HO, and 9.3 degrees at TS with deformity (Tab. 2). First-ray helical axis total rotation (Phi) averaged 5.6 degrees (components: Yc=0.40, Zc=0.08, Xc=−0.20) over middle stance in women with deformity (Tab. 3). The largest component, positive Yc (0.40), indicates that adduction was dominant; the smaller negative Xc (−0.20) indicates eversion to a lesser degree; the smallest positive Zc (0.08) indicates that dorsiflexion contributed minimally. Total rotation (Phi) averaged 11.3 degrees over late stance (components: Yc=−0.05, Zc=−0.33, Xc=−0.62) with deformity. The largest component, negative Xc (−0.62), indicates that eversion was dominant; the smaller negative Zc (−0.33) indicates plantar flexion to a lesser degree; the smallest negative Yc (−0.05) indicates that abduction contributed minimally. Table 3 also records the helical axis parameters for control participants. There was an interaction (F=5.06; df=1,18; P=.037) for Yc. Vertical Yc was larger over middle stance (0.40) compared with late stance (−0.05) in women with deformity.
Helical Axis Parameters Reported as Total Rotations (Phi) and Y, Z, and X Components (Direction Cosines: the Cosine of the Angle of the Helical Axis Vector With Respect to the Component Axis)a
Hypothesis 3.ii tested the group effect in frontal first-ray–to–navicular angles at HO (Tab. 2). There was an interaction (F=3.56; df=2,36; P<.04). The simple effects comparisons revealed no group difference at HO.
Hypothesis 3.iii tested the relationship of arch angle, navicular position, and the first-ray axis. Multiple regression analysis revealed no significance (eTable); arch angle considered with the change in the frontal navicular-to-MR reference angle was not related (F≤0.39; R2≤.04; P≥.68) to Yc.
Discussion
Hypothesis 1: Foot Posture Angles
Contrary to what the name would imply, the deformity is not isolated to the hallux. Adduction of the first metatarsal enlarges both the hallux and IM angles. The hallux angle (Tab. 1) averaged 36 degrees in women with deformity compared with 8 degrees in control participants. A hallux angle measuring 36 degrees indicates a moderate deformity, on the basis of the clinical grading system in which normal is 15 degrees or less, mild is 15 to 25 degrees, moderate is 25 to 40 degrees, and severe is 40 degrees or greater.25 The IM angle measured 17 degrees with deformity compared with 11 degrees in control participants.34
Arch angle was hypothesized to be larger in women with deformity. Premised on Inman's published perspective,35 pronation of the subtalar joint lowers the arch (increasing the arch angle), which, in turn, predisposes hallux valgus. Inman did not report data. The arch angle was not different between groups. All measurements (Tab. 1) fell within the narrow range from 126 degrees to 143 degrees. This range is normal in adults, compared with the extremes of 104 degrees and 166 degrees recorded in people having a cavus or planus foot, respectively.30,36 While accepting that no single planar measure is adequate to represent the complex shape of the arch,10,37 not finding a group difference both counters and supports the observations of Inman.35 To clarify, he specified that pronation (eversion) of the calcaneus, not flatness of the arch, precipitates deformity. The difference here is subtle because eversion of the calcaneus often manifests as a flat arch.10
Hypothesis 2: Calcaneus and Navicular Angles
Local coordinate frames defined the relative spatial position angle between bones, from which the relationship of the tarsals was assessed between groups and across conditions. For example, the frontal calcaneus-to-fibula angle computed at MS averaged −27.8 degrees with deformity compared with −21.2 degrees in control participants (Tab. 2). A negative value indicates eversion. Therefore, in relative terms, the calcaneus everted 6.6 degrees more (P<.05) at the MS gait condition with deformity. The relative angle records the relationship between the local coordinate frames embedded to spatially define the segmented bone model, and nothing more.
As hypothesized (2.i), the frontal calcaneus-to-fibula angle was significantly (P<.05) everted by an average of ≥6.6 degrees at each gait condition with deformity (Tab. 2). There was no interaction; thus, deformity affects position more than it does motion. eFigure 3 gives added spatial meaning to this result. Because the coordinate frame defining the fibula aligned close to coincident with the MR reference, eversion of the calcaneus, for practical purposes, can be described in relation to the horizontal (ground). Tanaka et al38 found a similar weight-bearing result on the basis of measurements acquired from radiographs in 58 patients seeking surgical consultation for deformity. Results suggest that eversion of the calcaneus may be part of the larger sequelae of hallux valgus.
Hypothesis 2.ii predicted that the navicular would evert more in relation to the calcaneus at HO with deformity. Group difference was identified; however, inversion (not eversion) was increased by an average of 5.8 degrees (Tab. 2). This relative angle result probably reflects the group difference detected in the weight-bearing position of the calcaneus instead of there being any real difference in navicular positioning. Support for this statement is seen in the navicular-to-MR reference data (Tab. 2), because the frontal angles differ by <2 degrees between groups at HO.
Hypothesis 2.iii evaluated the change in the navicular-to-calcaneus angles across conditions for the purpose of assessing the face validity of the data recorded (Tab. 2). Total frontal-plane rotation of the navicular averaged 6.3 degrees with deformity and 6.7 degrees in control participants. The amount is proportional to the 9.5-degree frontal navicular-to-calcaneus rotations measured over the entire stance phase of gait by Nester et al8 with bone-mounted sensors and agrees in general description to other gait studies that acquired kinematics data with surface-based marker systems.18,22,24,27 We hypothesized that the navicular would evert from MS to HO and invert from HO to TS. The change in angle (F=20.0, P<.001) was consistent between groups (no interaction), with the navicular becoming increasingly inverted across conditions (Tab. 2) by an average of 2 degrees from MS to HO and 5 degrees from HO to TS. The part of hypothesis 2.iii that predicted that the navicular would evert from MS to HO was rejected, but the part that predicted the navicular would invert from HO to TS was accepted. Our method of simulating gait produced navicular motion comparable to data published in the gait literature.8,14,18,22,24,27
Hypothesis 3: First-Ray Angles
As hypothesized (3.i), adduction of the first ray was increased (P<.01) with deformity. The second part of hypothesis 3.i tested for group difference in the orientation of the first-ray axis, extracted as 3-dimensional helical axis direction cosine components (Tab. 3). A direction cosine in this case is simply the cosine of the angle between the calculated oblique joint axis and one of the respective X, Y, or Z scanner (MR) reference frame axes (Fig. 3). Hypothetically, if Yc equals 1.00, the other 2 components (Z and X) of the array must compute to zero. This particular vector (axis) would point up in the vertical (coincident with the MR reference) and describe single-plane adduction. Because rotation of most tarsals including the first ray is triplanar, a more realistic example would output an array of direction cosine components that are equivalent in magnitude. A positive array (Yc=0.33, Zc=0.33, Xc=0.33) would describe a vector that projects equally in an oblique fashion up (Y), lateral (Z), and forward (X), with the rotation expressed in shared proportions of adduction, dorsiflexion, and inversion. Figure 3 illustrates a result close to this. The vectors drawn represent the first-ray axis for 3 individuals studied here, with their data collected over middle stance (MS to HO). Two participants had deformity. Figure 3 does not highlight group characteristics but instead gives spatial meaning to the helical axis component result.
Estimates of the first-ray helical vector (axis) for 3 representative participants. Vectors are hand-drawn in the magnetic resonance reference (shown superimposed on the foot) from the indicated direction cosine components. HV indicates hallux valgus group; C indicates control group.
In helical axis results, Yc was 0.40 over middle stance, with adduction being the dominant component rotation (Tab. 3) in women with deformity. Yc was 0.10 over middle stance, with adduction being the smallest component rotation in control participants (there was an interaction: F=5.06, P=.037). The follow-up comparisons revealed that Yc was larger over middle stance (0.40) compared with late stance (−0.05) in the group with deformity. An arccosine trigonomic function converts cosine values to angles. Arccosine 0.40=66 degrees; arccosine −0.05=93 degrees. Because the baseline computation was made in the MR reference (eFig. 2), and, staying consistent with language used throughout, the complementary Yc (66°) indicates that the axis inclined 24 degrees from horizontal over middle stance, and it declined 3 degrees over late stance in women with deformity. Conversion of Yc for control participants equates to their axis being inclined 6 degrees over middle stance and 3 degrees over late stance. As theorized,20,21 the first-ray axis inclined most (24°) over middle stance in the group having deformity. Inclination of the axis, by effect, transfers the dorsiflexion component of rotation toward adduction.21
We hypothesized (3.ii) that women with deformity would demonstrate increased eversion of the first-ray–to–navicular angle at HO because maximal pronation occurs at this delayed instant of the gait cycle in people with hallux valgus.6,7 No group difference was found (Tab. 2). Canseco et al6 reported a different result. With the use of surface markers and foot-modeling techniques, they measured the forefoot everted with respect to the hindfoot 10 degrees more in patients with hallux valgus. Because modeling does not isolate the measurement of kinematics,8 Canseco and colleagues may have actually detected a relative change in the hindfoot position instead of the forefoot. Additional research is needed.
Hypothesis 3.iii investigated the usefulness of arch angle (height) and navicular position in predicting inclination of the first-ray axis (Yc). The multiple regression models (eTable) output R2 values to estimate the variance explained by arch angle and navicular eversion in predicting Yc at middle (R2=.04) and late stance (R2=.00). The variables explained a nonsignificant (P≥.68) amount of the variance of Yc. Subsequent to this, a post hoc regression analysis was performed to examine the relationship between the eversion of calcaneus (measured with respect to the fibula at mid-stance) to Yc computed over middle stance. The linear model revealed F=2.62, R2=.12, and P=.12. The coefficient of determination (R2) explained only 12% of the overall variance in the prediction equation. Variability in talus morphology or individual differences in talocalcaneal and talonavicular joint alignment or stiffness may account for such unexplained variance. The low P value (P=.12) gives merit for research to continue to test for a link between the frontal position of the calcaneus and first-ray kinematics.
Clinical Applications
Watchful waiting, that is what Torkki et al5 called the practice of not treating hallux valgus until, eventually, the severity of impairments justify surgery. We question the rationale for withholding care.
Nearly 40 years ago, Inman35 advocated for use of a “heel cup, shoe insert, or arch support” to control excess pronation in the treatment of hallux valgus. We concur and point to evidence from randomized controlled trials to make the case.3–5 Budiman-Mak et al3 studied the outcomes on 102 patients treated for rheumatoid arthritis and related foot pain. Those fit with orthoses were 73% less likely to have development of hallux valgus compared with a control group. More recently, Torkki and colleagues4,5 reported outcomes on 209 consecutive patients seeking care for symptomatic hallux valgus. Patients treated with orthoses described themselves as improved on a global assessment scale at 1-year follow-up and at 2-year follow-up. These sample patients were as satisfied as were the patients treated with surgery, and more satisfied than were patients who acted as the control group. Generally, the evidence1,2 shows that orthoses aid in the management of symptoms but do not correct deformity. So, what else can be done?
Muscles of the leg contribute to support the alignment of the hindfoot, midfoot, and first ray. The tibialis posterior tendon travels beneath the medial malleolus and inserts into the navicular.39,40 Continuation of the tendon then branches into the midfoot tarsals, onto the flexor hallucis brevis muscle,39 and, sometimes, onto the fibularis (peroneus) longus tendon at the base of the first metatarsal.40 The line-of-pull of the tibialis posterior is ideal to limit eversion of the subtalar and transverse tarsal joints, stabilize the navicular, and stiffen the midfoot and first ray to the demands of weight bearing.41 The fibularis longus also should be strengthened.42 It inserts onto the first metatarsal and cuneiform, and pull of this tendon abducts and plantar flexes the first ray relative to the navicular,43 which, in theory,20,35 could prevent vertical inclination of the first-ray axis.
Although corrective alignment of the arch and hindfoot should be targeted first, other characteristics of lower extremity joint positioning also have been considered causal factors.44–46 With the use of methods of discriminant function analysis, Steinberg et al46 identified measurements of hip internal rotation (an indicator of femoral anteversion) and tibio-femoral (Q-angle) alignment to discriminate between women with and without hallux valgus; they also identified the hindfoot angle of eversion as the single best discriminator between bilateral and unilateral hallux valgus deformity. Other authors44,45 reported finding an association between knee disorders (osteoarthritis and patellofemoral joint pain) and the incidence of hallux valgus. Although such relationships are not fully understood, on the basis of current evidence and knowing the long-term debilitations that result from deformity,47 early intervention is justified.
Synthesis
This study measured malalignment of the hallux, first ray, and calcaneus in women with hallux valgus. Joint laxity may be incidental15,48 or it may be the catalyst that activates other precursor traits.46,49–52 Research investigating the mechanisms of deformity should explore the importance of tissue stiffness and joint laxity. Our next study will examine the association between hypermobility of the first-ray arch segment22 and tarsal kinematics in patients with hallux valgus.51 Research moving forward will use the helical axis approach to measure whether treatment can effectively change the weight-bearing behaviors of the first ray and hallux.
This study had limitations. The sample size was small (N=20), and scanning only 3 conditions reduced the generalizability of the results, though all gait events presumed critical were captured. The experimental group had moderate deformity. They represent the population most likely to seek conservative treatment. Participants were not patients, although it is doubtful that kinematics would be different in patients unless gait was impaired. Posing a participant standing to collect static images does not replicate the transfer of loading between the foot and ground as would occur in gait, but standing is much improved over non–weight-bearing or partial weight-bearing imaging methods.10,11 Standing precludes limb accelerations and related muscle contractions. Despite this reality, good consistency was noted in comparison to the magnitude and direction of the navicular rotations collected in gait trials.8,14,18,22,24,27
Although not considered a limitation, there was a large amount of variability in the helical axis direction cosine components calculated across participants (Tab. 3). Therefore, despite being sufficiently powered to detect group differences in the first-ray angles, group difference in Yc was not detected directly. A post hoc power calculation estimates that recruitment of 3 more women having deformity would be required to achieve an 80% chance to detect a group difference of 0.3 (an amount equal to the average difference in Yc measured between groups at middle stance) in the helical axis component.
The measurement techniques used in this study, although novel, were demonstrated to be scientifically sound.26 We offer insight to researchers wishing to adopt our methods to examine tarsal kinematics. In work that followed, the requirement of standing upright in the MR scanner for 1 hour was determined to be too strenuous for women deconditioned from the comorbid disorders of rheumatoid arthritis and hallux valgus deformity. As an accommodation, these participants were placed semireclined from vertical while preserving the foot and ankle postures that defined the gait conditions of MS, HO, and TS. The analysis of the data is ongoing. One final insight, to select the Cardan sequence that best represents expected hallux and first-ray kinematics, in a side experiment, we computed angles with different ordered rotations. The analysis was necessary because currently no standards31 exist for describing 1-MTP joint kinematics.53–55 Results, published as a research abstract,56 lead us to recommend a transverse, sagittal, and frontal (Y-Z′-X′′) sequence as preferred, particularly for research on patients with hallux valgus deformity.
In summary, hallux valgus is a complex foot deformity. Research that implicates the genesis of hallux valgus as occurring proximal to the hallux marks a paradigm shift for how the onset of the deformity is most often described in clinical practice. We present data that both support and challenge theory. Adduction of the first ray, inclination of the first-ray axis, and eversion of the calcaneus were found in women with moderate deformity. Characterized as risk factors, alterations in tarsal alignment may be correctable with nonoperative treatment. This report may lead to a more specific approach for evaluation and may contribute to interventions aimed at preventing hallux valgus deformity.
Footnotes
Dr Glasoe, Dr Nuckley, and Dr Ludewig provided concept/idea/research design. Dr Glasoe and Dr Nuckley provided writing. Dr Glasoe and Dr Phadke provided data collection and analysis. Dr Glasoe and Dr Ludewig provided project management and fund procurement. Dr Ludewig provided facilities/equipment and institutional liaison. Dr Pena and Dr Ludewig provided consultation (including review of manuscript before submission).
This study was approved by the University of Minnesota Institutional Review Board.
This work was funded by the Arthritis Foundation North Central Chapter and in part by the Foundation for Physical Therapy.
- Received January 28, 2013.
- Accepted June 24, 2013.
- © 2013 American Physical Therapy Association
References
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