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Metatarsophalangeal Hyperextension Movement Pattern Related to Diabetic Forefoot Deformity

Victor A. Cheuy, Mary K. Hastings, Michael J. Mueller
DOI: 10.2522/ptj.20150361 Published 1 August 2016

Abstract

Background Metatarsophalangeal joint (MTPJ) hyperextension deformity is common in people with diabetic neuropathy and a known risk factor for ulceration and amputation. An MTPJ hyperextension movement pattern may contribute to the development of this acquired deformity.

Objective The purpose of this study was to determine, in people with diabetes mellitus and peripheral neuropathy (DM+PN), the ankle and MTPJ ranges of motion that characterize an MTPJ hyperextension movement pattern and its relationship to MTPJ deformity severity. It was hypothesized that severity of MTPJ deformity would be related to limitations in maximum ankle dorsiflexion and increased MTPJ extension during active ankle dorsiflexion movement tasks.

Design A cross-sectional study design was used that included 34 people with DM+PN (mean age=59 years, SD=9).

Methods Computed tomography and 3-dimensional motion capture analysis were used to measure resting MTPJ angle and intersegmental foot motion during the tasks of ankle dorsiflexion and plantar flexion with the knee extended and flexed to 90 degrees, walking, and sit-to/from-stand.

Results The MTPJ extension movement pattern during all tasks was directly correlated with severity of MTPJ deformity: maximum ankle dorsiflexion with knee extended (r=.35; 95% confidence interval [CI]=.02, .62), with knee flexed (r=.35; 95% CI=0.01, 0.61), during the swing phase of gait (r=.47; 95% CI=0.16, 0.70), during standing up (r=.48; 95% CI=0.17, 0.71), and during sitting down (r=.38; 95% CI=0.05, 0.64). All correlations were statistically significant.

Limitations This study was cross-sectional, and causal relationships cannot be made.

Conclusions A hyperextension MTPJ movement pattern associated with limited ankle dorsiflexion has been characterized in people with diabetic neuropathy. Increased MTPJ extension during movement and functional tasks was correlated with severity of resting MTPJ alignment. Repetition of this movement pattern could be an important factor in the etiology of MTPJ deformity and future risk of ulceration.

One in 4 patients with diabetes mellitus (DM) will develop a foot ulcer, and each treatment of a wound infection is estimated to cost approximately $28,000 for the 2 years following diagnosis.1–3 The Task Force of the Foot Care Interest Group of the American Diabetes Association indicated that the “most common triad of causes that interact and ultimately result in ulceration has been identified as neuropathy, deformity, and trauma.”4(p1679),5 The most common of these deformities is metatarsophalangeal joint (MTPJ) deformity, often called “hammertoe” or “claw toe,” and characterized by hyperextension of the proximal phalanx of one or multiple lesser toes, most frequently occurring at the second MTPJ.6–11 The resulting metatarsal head prominences are then exposed to repetitive, excessive pressures, which—when coupled with an insensate foot—lead to skin breakdown under the metatarsal heads.12 Understanding the causes of MTPJ deformity would have profound implications for intervention strategies for treatment or prevention, with the ultimate goal of reducing the incidence of ulceration and amputation.

The etiology is unclear, but MTPJ deformity may arise from the interplay of intrinsic foot muscle deterioration and limited ankle joint mobility. Intrinsic foot muscle deterioration precedes extrinsic extensor digitorum longus muscle deterioration as a result of distal-to-proximal peripheral neuropathy (PN) and has been shown to be related to MTPJ alignment.13–16 The accumulation of intermuscular adipose tissue and loss of lean muscle tissue weaken the intrinsic foot muscles, resulting in a muscular imbalance between the flexors and extensors at the MTPJ.13 This muscle imbalance destabilizes the joint as the force couple hyperextends the proximal phalanx.

Limited ankle joint mobility is also a common impairment in individuals with DM+PN, likely caused by an accumulation of advanced glycation end products and thickening of the Achilles tendon.17–20 Goniometric measurements from people with MTPJ deformity have shown limited ankle joint dorsiflexion range of motion (ROM).9 We hypothesize that limited ankle dorsiflexion ROM contributes to MTPJ deformity in 2 ways. First, we hypothesize that a strategy of toe extension during the swing phase of gait is used to physically shorten the length of the foot and assist in clearing the toes. Second, we hypothesize that in order to actively dorsiflex the stiff and limited ankle, the extrinsic toe extensor muscle (extensor digitorum longus) is recruited to assist in dorsiflexion while extending the toes. We are calling this pattern of a substantial increase in MTPJ extension with active dorsiflexion of the ankle joint the MTPJ hyperextension movement pattern. We hypothesize that MTPJ deformity occurs when the MTPJ hyperextension movement pattern related to limited ankle dorsiflexion motion is coupled with the inability of the intrinsic foot muscles to stabilize and correct MTPJ hyperextension. Increased reliance on the extensor digitorum longus muscle, coupled with the poor MTPJ stability from intrinsic foot muscle deterioration, may cause the extensor digitorum longus muscle to shorten and the joint capsule to become lax, resulting in hyperextension of the MTPJ at rest.21

The purpose of this study was to determine, in people with DM+PN, the ankle and MTPJ ROMs that characterize a MTPJ hyperextension movement pattern and the relationship of this movement pattern to MTPJ deformity severity. We hypothesized that limitations in maximum ankle dorsiflexion and increased MTPJ extension during movement tasks would be related to severity of MTPJ deformity. The tasks performed were full ankle dorsiflexion and plantar-flexion ROM with the knee extended and flexed, walking, and sit-to/from-stand. These tasks were chosen to elicit the movement pattern because they require active ankle dorsiflexion. Repetition of this movement pattern could be an important factor in the etiology of MTPJ deformity and future risk of ulceration.

Method

Participants

Thirty-four individuals (16 male, 18 female; mean age 59 years, SD=9) participated in this study. Informed consent was obtained from all participants. The complete demographic summary is shown in Table 1. An a priori power analysis was used to predict the sample size necessary to identify significant correlations between the independent variable and MTPJ deformity using G*power 3.1 (University of California–Los Angeles, Los Angeles, California). Estimated effect sizes were taken from the literature, as follows: 1.0 in maximum ankle dorsiflexion ROM and 0.88 in skin intrinsic fluorescence (not included in this portion of the study), with a correlation of −.51 between intrinsic foot muscle deterioration and MTPJ angle (not included in this portion of the study).9,16,22 A sample of 34 participants was estimated for a minimal statistical power of 0.80 and α=.05. The inclusion criteria for all participants were the presence of type 2 DM and diabetic PN. Peripheral neuropathy was assessed using the lower extremity examination of the Michigan Neuropathy Screening Instrument (MNSI), where PN was defined as an MNSI score >2.23,24 The bilateral examination included a foot inspection, a test of ankle reflexes, and sensation tests of the dorsum of the great toe using a 128-Hz tuning fork (vibration) and a 5.07g Semmes-Weinstein monofilament (pressure). Further sensory testing for descriptive purposes was performed using Semmes-Weinstein monofilaments and a biothesiometer (PN ≥25 V) to test pressure and vibration perception thresholds of the plantar surfaces of both feet (Tab. 1).25 Exclusion criteria were age ≤40 and ≥75 years, nondiabetic PN (ie, alcoholic, chemotoxic, lumbar radiculopathy), dialysis, severe arterial disease (Ankle-Brachial Index <0.9 or >1.3), unable to physically complete testing for the study, lower extremity amputations, and presence of a neuropathic ulcer.4

View this table:
Table 1.

Participant Demographics and Characteristicsa

Procedure

Spiral CT scans were used to measure second MTPJ alignment to assess severity of deformity at rest. Although CT scans are not part of routine clinical assessments, CT is considered the “gold standard” for measuring bony MTPJ deformity and needed to be measured as accurately as possible to investigate the relationships described in this study. Participants were positioned supine on the CT scanner table using previously published reliable and precise methods.26–29 The following CT parameters on a multi-slice Siemens Sensation 64 CT scanner (Siemens Medical Systems Inc, Iselin, New Jersey) were used to acquire the scans: 0.5-second rotation time, 64- × 0.6-mm collimation, 220 mA, 120 kVp, a pitch of 1, and a 512 × 512 matrix. A positioning device standardized ankle position to 30 degrees of plantar flexion to approximate mid-position of physiological ROM. The MTPJ angle was defined as the complement of the angle between the longitudinal axes of the second metatarsal and second proximal phalanx. A larger angle corresponds to more severe MTPJ hyperextension (Fig. 1). Participants were specifically recruited with a broad spectrum of MTPJ angles to allow a full examination of the relationship between the movement pattern and severity of MTPJ deformity (Tab. 1).

Figure 1.
Figure 1.

Metatarsophalangeal joint (MTPJ) angle is defined as the complement of the angle between the longitudinal axes of the second metatarsal and proximal phalanx; a larger angle corresponds to more severe MTPJ hyperextension.

Temporal and kinematic data for the shank and foot were captured with an 8-camera, 200-Hz Vicon 3-dimensional (3D) motion analysis system (Vicon MX, Los Angeles, California). Twenty-four 10-mm reflective markers were attached to the skin with tape or to thermoplastic plates to define 5 segments on the target lower extremity, determined as the foot with the most severe deformity. The 5 segments were the shank, hindfoot, forefoot, second metatarsal, and second proximal phalanx. Marker placement followed methods previously described.30,31 Hindfoot and forefoot marker placement followed the modified Oxford multisegmental foot model, and 2 customized trimarker jigs were constructed for the second metatarsal and proximal phalanx segments. The multisegmental foot model was built in Visual3D software (C-Motion Inc, Germantown, Maryland). Modified Oxford foot models similar to that used in the present study have been used previously in participants with diabetes and with and without foot deformity, as well as in other populations with foot deformity.31–34 Also, the Oxford foot model's method of referencing a neutral stance has been shown to improve reliability between trials and to reduce angle error compared with not referencing stance.35 Details of foot marker placement, segment and coordinate system definitions, and illustrations of the movement tasks are provided in the eAppendix.

Three-dimensional kinematic data were collected during the following movement tasks. These tasks, representing a variety of common daily activities, were selected through pilot testing. All tasks required active dorsiflexion, the action hypothesized to elicit the MTPJ hyperextension movement pattern that, if repeated countless times throughout daily life, could contribute to the development of deformity.

Ankle dorsiflexion/plantar-flexion task.

Participants were instructed to complete 3 trials of full ankle dorsiflexion and plantar flexion with the knee in complete extension (participant long sitting with foot off edge of table) and in 90 degrees of flexion (participant short sitting with leg and foot off edge of table). Each trial contained 3 repetitions through the full ankle ROM. The repetition within each trial that had the largest magnitude of ankle dorsiflexion was selected for analysis to ensure that every participant's true maximum was attained. The variables of interest were maximum ankle dorsiflexion and maximum MTPJ extension. Ankle dorsiflexion was calculated from the hindfoot on shank intersegmental motion, and MTPJ extension was calculated from second proximal phalanx on second metatarsal intersegmental motion.

Walking task.

Participants were instructed to complete 5 barefoot walking trials at a normal comfortable speed. Start and finish lines were marked on the floor several meters before and after the forceplate. Heel-strike and toe-off events were determined from the forceplate data. Walking speed was measured using a digital stopwatch. The fastest and slowest trials were eliminated, and the remaining trials were selected for analysis of a single stride (heel-strike to heel-strike of target foot). The variable of interest was MTPJ extension at mid-swing, measured as described above. This variable was chosen because active ankle dorsiflexion is necessary for foot clearance during swing and may elicit MTPJ extension for assistance.

Sit-to/from-stand task.

Participants were seated in a chair with an adjusted height so that the feet rested comfortably on the floor when the knees were in approximately 90 degrees of flexion. Participants could then adjust how far forward in the chair they sat to stand up comfortably and were not allowed to use their hands to assist during the task. Participants could bring their feet back symmetrically only as far as to be able to ensure the entire plantar surface of the foot including the heel were still on the ground. No further attempts to control their posture were made. The commands given were to take 3 seconds to stand up, remain standing for 3 seconds, and take 3 seconds to return to a sitting position, as maintained with a metronome. Five trials were completed, and the 3 trials with the greatest magnitude of MTPJ extension were selected for analysis. The variables of interest were peak MTPJ extension excursion during standing up and sitting down. Extension excursion was calculated as the change in MTPJ extension angle compared with the standing MTPJ angle. The standing angle is an average position measure, calculated as the average MTPJ angle of the central 20% of the entire sit-to/from-stand trial, corresponding to upright standing.

The selected trials from each kinematic activity were processed in Visual3D, and the variables of interest (Tab. 2) were calculated in Microsoft Excel (Microsoft Corp, Redmond, Washington). We used IBM SPSS version 21 software (IBM Corp, Armonk, New York) to determine correlations between the variables of interest from kinematic analysis and resting MTPJ angle from CT for the entire cohort (N=34), adjusted for the covariate of ankle dorsiflexion (Tab. 2). Pearson correlation coefficients were calculated, and a significance level of P<.05 was used for all analyses. To better visualize kinematic differences and appreciate correlations between variables, the movement patterns of 2 participant subsets during the performed tasks were illustrated. The subsets were 5 participants with the least resting MTPJ extension (≤40°) (LE subset) and 5 participants with the highest resting MTPJ extension (≥74°) (HE subset) as measured by CT. Demographic data of these participants are shown in Table 1.

View this table:
Table 2.

Kinematic Outcome Variables During Active Dorsiflexion Tasks for Total Group and 2 Participant Subsetsa

Role of the Funding Source

This study was funded by grants from the National Institute of Diabetes and Digestive and Kidney Diseases (F31 DK101286), the National Institute of Child Health and Human Development (T32 HD007434 and K12 HD055931), and the National Center for Advancing Translational Sciences (KL2 TR000450), US Department of Health and Human Services, National Institutes of Health, during the conduct of the study.

Results

Participants were recruited to have a broad variance in the severity of MTPJ deformity (X̅=59°, SD=16°, range=32°–100°). Correlations between the variables of interest and resting MTPJ angle measured by CT are shown in Table 3 for all participants.

View this table:
Table 3.

Correlations Between Kinematic Outcome Variables and Resting MTPJ Anglea

Ankle Dorsiflexion/Plantar-Flexion Task

In the knee extended condition, resting MTPJ angle was directly correlated with maximum MTPJ extension (r=.35; 95% CI=.02, .62) while adjusting for the covariate of ankle dorsiflexion. The zero-order correlation was moderately stronger (r=.47; 95% CI=.16, .70), indicating that ankle dorsiflexion ROM does have an influence in controlling for the relationship between severity of deformity and MTPJ extension. The HE subset had approximately 6 degrees less dorsiflexion than the LE subset but had similar plantar flexion (Fig. 2A). Although both subsets exhibited the same general pattern of more MTPJ extension during dorsiflexion and less during plantar flexion, the MTPJ was consistently more extended throughout the motion for the HE subset (Fig. 2C). The MTPJ angle from peak ankle plantar flexion to peak dorsiflexion was plotted for both subsets (Fig. 2E). There was a steep increase in MTPJ extension as the HE subset started to dorsiflex (solid black line). The MTPJ extension of approximately 10 degrees (vertical dotted line) corresponded to only a 6-degree change at the ankle toward dorsiflexion (horizontal dotted line). In contrast, the LE subset did not extend the MTPJ 10 degrees until the ankle dorsiflexed twice as much, approximately 12 degrees. The HE subset maximally extended the MTPJ nearly 30 degrees more than the LE subset during the task.

Figure 2.
Figure 2.

Ankle (A, B) and metatarsophalangeal joint (MTPJ) (C, D) kinematic movement patterns in the knee-extended and knee-flexed conditions. Metatarsophalangeal joint versus ankle motion from peak plantar flexion to peak dorsiflexion (E, F). Slope of initial increase in MTPJ extension at the start of dorsiflexion indicated by solid black line. Ten-degree change in MTPJ extension indicated by vertical dotted line, and corresponding change in dorsiflexion indicated by horizontal dotted line. Least hyperextended MTPJ subset indicated by solid line/diamonds. Most hyperextended MTPJ subset indicated by shaded region/squares.

In the knee-flexed condition, resting MTPJ angle was directly correlated with maximum MTPJ extension (r=.35; 95% CI=.01, .61) while adjusting for the covariate of ankle dorsiflexion. The zero-order correlation was only slightly stronger (r=.38; 95% CI=.05, .64), indicating that ankle dorsiflexion ROM had minimal influence in controlling for the relationship between severity of deformity and MTPJ extension in this task. The HE subset had ankle dorsiflexion motion similar to that of the LE subset (Fig. 2B), and both subsets had the same general pattern of MTPJ motion over the task, as in the knee-extended condition (Fig. 2D). The HE subset again was consistently more extended and showed one more rapid increase in MTPJ extension as the ankle initially dorsiflexed, comparable in magnitude and in difference to the LE subset in the knee-extended condition.

Walking Task

The MTPJ extension at mid-swing was directly correlated with resting MTPJ angle (r=.47; 95% CI=.16, .70) while adjusting for the covariate of ankle dorsiflexion. The zero-order correlation was not changed (r=.47; 95% CI=.16, .70), indicating that ankle dorsiflexion ROM did not influence in controlling for the relationship between severity of deformity and MTPJ extension in this task. Those participants with more severe MTPJ deformity kept the MTPJ extended even after toe-off. Walking speed was not correlated with MTPJ extension (r=.05). The MTPJ angle over the gait cycle is illustrated for both subsets in Figure 3A. The general pattern of the subsets was similar during the stance phase, although the HE subset was consistently more extended. The HE subset had greater, sustained MTPJ extension following toe-off through the entire swing phase, approximately 26 degrees more at mid-swing. The decrease in MTPJ extension between the maximum at toe-off and mid-swing was only one-third the magnitude of the LE subset (7° versus 21°, respectively).

Figure 3.
Figure 3.

(A) Metatarsophalangeal joint (MTPJ) angle during stance and swing phases of walking. (B) Metatarsophalangeal joint extension excursion during sit-to/from-stand task. Excursion is defined as the change in MTPJ extension angle compared with the standing MTPJ angle (average MTPJ angle spanning the 40th–60th percentiles). Least hyperextended MTPJ subset indicated by solid line. Most hyperextended MTPJ subset indicated by shaded region.

Sit-to/From-Stand Task

The MTPJ extension excursion was directly correlated with resting MTPJ angle during both sit-to-stand (r=.48; 95% CI=.17, .71) and sit-from-stand (r=.38; 95% CI=.05, .64) while adjusting for the covariate of ankle dorsiflexion. The zero-order correlations were r=.47 (95% CI=.16, .70) and r=.38 (95% CI=.05, .64), respectively, indicating that ankle dorsiflexion ROM had minimal influence in controlling for the relationship between severity of deformity and MTPJ extension. The MTPJ extension excursion, defined as the change in MTPJ from standing baseline, is plotted for the 2 subsets in Figure 3B. Both subsets showed peaks in MTPJ excursion when the ankle was dorsiflexed during standing up and sitting down. During both of these instances, the HE subset extended the MTPJ from standing baseline approximately twice as much as the LE subset (16° versus 7°, respectively, for standing up; 15° versus 7°, respectively, for sitting down).

Discussion

The results of this study show that there is a pattern of motion characterized by an increase in MTPJ extension during active ankle dorsiflexion associated with MTPJ hyperextension deformity severity in people with DM+PN. The amount of MTPJ extension excursion during daily tasks is related to the severity of resting MTPJ extension alignment.

During the non–weight-bearing ankle dorsiflexion/plantar-flexion task, maximum MTPJ extension and maximum ankle dorsiflexion were related to resting MTPJ angle. The relationship between limited ankle dorsiflexion and MTPJ extension suggests an increase in extensor digitorum longus muscle recruitment during the ankle dorsiflexion task and an inability of the intrinsic muscles and MTPJ joint capsules to stabilize the MTPJ.

A larger difference in maximum ankle dorsiflexion between the HE and LE subsets was observed in the knee-extended condition versus the knee-flexed condition (Fig. 2). When the knee is extended, the gastrocnemius muscle is stretched across the knee and ankle and limits ankle dorsiflexion ROM more than when the knee is flexed.36 When the knee was flexed, removing the gastrocnemius muscle as a source limiting ankle dorsiflexion, there was no relationship between maximum dorsiflexion and resting MTPJ angle. Resting MTPJ angle and maximum MTPJ extension were still correlated in the flexed condition, possibly a result of the overutilization of the movement pattern, where MTPJ hyperextension was utilized even when ankle dorsiflexion was not as limited.

The timing of MTPJ extension by the HE subset also supports the hypothesized movement pattern. Plotting MTPJ and ankle motion against each other, the HE subset extended the MTPJ much more than the LE subset as the foot moved initially from maximum plantar flexion to dorsiflexion (Fig. 2E). The steep increase in MTPJ extension as the ankle moved out of maximum plantar flexion suggests that, compared with the LE subset, the HE subset had increased reliance on the extensor digitorum longus muscle at the very beginning of dorsiflexion, possibly aiding in initiating the movement.

The amount of MTPJ extension at mid-swing and excursion during standing and sitting were directly related to resting MTPJ angle. The functional tasks of walking and sit-to/from-stand both showed the MTPJ hyperextension movement pattern during ankle dorsiflexion. During walking, the MTPJ joint extended at toe-off, and the HE subset sustained the extension throughout the swing phase until heel-strike (Fig. 3). At mid-swing, MTPJ extension decreased only 7 degrees from maximum for the HE subset compared with the 21-degree decrease in the LE subset. Ankle dorsiflexion at mid-swing (data not shown) for both subsets was identical (7°), despite the nearly 30-degree difference in MTPJ extension at the same time point (72° versus 46°, respectively). The difference in MTPJ extension for the same amount of dorsiflexion suggests that the extensor digitorum longus muscle is being recruited to assist with ankle dorsiflexion for proper foot clearance during swing. Similarly, MTPJ extension during the sit-to/from-stand task coincided with active ankle dorsiflexion as body weight shifted forward and the shank rolled over the foot during standing up and sitting down (Fig. 3). Similar to the knee-flexed condition mentioned earlier, resting MTPJ angle and MTPJ extension were correlated in these tasks, whereas ankle dorsiflexion had a minimal influence, possibly a result of the overutilization of the movement pattern, where MTPJ hyperextension was utilized even when ankle dorsiflexion was not as limited.

Although we cannot determine cause and effect from this cross-sectional study, we hypothesize that countless repetition of the activity tasks in this study may result in overutilization of this movement pattern and contribute to MTPJ deformity onset and progression. The overuse of repeated MTPJ hyperextension movement pattern may lead to ligament and capsule lengthening on the plantar surface of the MTPJ and shortening of the extensor digitorum longus muscle, ligaments, and capsule on the dorsal surface of the MTPJ, resulting in MTPJ hyperextension deformity at rest. These MTPJ joint changes alter the pressure profile of the plantar surface due to the now prominent metatarsal head.7–9,12,37 The HE subset was consistently more extended at the MTPJ throughout stance phase (Fig. 3), which renders the toes less capable of weight bearing, decreases the contact area of the forefoot, and exposes the metatarsal heads to more focal and increased plantar pressures.12,37 The HE subset also showed higher indicators of disease, specifically longer duration of diagnosed DM, higher glycosylated hemoglobin (HbA1C), and higher MNSI examination scores compared with the LE subset (Tab. 1). The combination of DM severity, insensitivity due to PN, excessive peak plantar pressures under bony prominences, and repetitive stress during walking increases the risk of skin breakdown and predisposes the limb to amputation.5,38,39

Intrinsic foot muscle deterioration likely plays a role in the development of the MTPJ hyperextension movement pattern and resultant MTPJ deformity in people with DM+PN. Intrinsic foot muscle deterioration can destabilize the MTPJ because there is no longer a flexion force to act as an antagonist to the extensor digitorum longus muscle. This muscle imbalance contributes to MTPJ hyperextension. Previous studies have shown a relationship between intrinsic foot muscle deterioration and MTPJ angle. Using magnetic resonance imaging-based methods, muscle deterioration was measured as the ratio of adipose to lean muscle volume.40 In a cohort of 23 participants with DM+PN, varying MTPJ angles, and some with medial column deformity (n=11/23), midfoot intrinsic foot muscle deterioration was directly correlated with MTPJ angle in standing (r=.51, P=.01).16 Using the same methods to quantify muscle deterioration on the cohort of the present study, remaining lean muscle volume of the intrinsic muscles of the forefoot was inversely correlated with resting MTPJ angle (r=−.52, P<.01), where less lean muscle volume was associated with more hyperextension.41 Taken together, these studies suggest that limited ankle dorsiflexion and deterioration of intrinsic muscles contribute to the development of this MTPJ hyperextension movement pattern.

We speculate that limited ankle joint mobility and intrinsic foot muscle deterioration are the primary factors contributing to the MTPJ hyperextension movement pattern. These contributing factors suggest targets for specific intervention to reduce the incidence of deformity, ulceration, and amputation. A progressive exercise and movement retraining program may help to improve intrinsic foot muscle strength, ankle dorsiflexion ROM, and extensor digitorum longus muscle length and reduce MTPJ extension with active dorsiflexion to minimize MTPJ deformity. A recent randomized controlled trial showed that a progressive walking program and exercises given to participants with DM+PN to improve balance and ankle strength resulted in increased ankle dorsiflexion ROM and Foot and Ankle Ability Measure (self-report of foot function) scores.42 A surgical intervention of Achilles tendon lengthening also can increase ankle dorsiflexion ROM and reduce ulcer recurrence in people with DM+PN and limited ankle dorsiflexion.43 Targeted exercises for the intrinsic foot flexor muscles have resulted in improved strength and function in healthy adults, which would reduce the muscular imbalance between intrinsic toe flexion and extrinsic toe extension and stabilize the MTPJ.44 The presence of PN may limit restorative goals in neuropathic muscle, but there is a lack of research in this area. Future research should investigate whether a foot-specific intervention can affect ankle dorsiflexion ROM, intrinsic foot muscle quality, and MTPJ extension during movement tasks to improve MTPJ alignment.

This study has limitations to consider. The cross-sectional design only allows for correlations, and causal relationships cannot be determined. The confidence intervals for the correlation coefficients were quite wide, indicating the need for further information. Confidence intervals are highly dependent on sample size, and the low number of participants in this study may have contributed to the wide interval. Replicating this study with a larger sample size would give more precise estimates of effects and narrower confidence intervals. Skin marker kinematic profiles to represent underlying bones are an inherent limitation of kinematic models. We hypothesize that any improvements in accuracy would only strengthen the correlations and narrow the confidence interval widths presented in this study. Although CT is not always a feasible option, this study serves as an important first step preceding future studies that could investigate the accuracy of other, more clinic-friendly methods of MTPJ angle measurement. Electromyography of the duration of extensor digitorum longus muscle activity during these functional tasks also could provide information about the muscle recruitment of MTPJ extension during ankle dorsiflexion. Each of the kinematic variables explained 12% to 23% of the variance of the MTPJ deformity (Tab. 3). Other potential risk factors that account for the remaining unexplained variance in MTPJ angle include traumatic injury, arthritis, changes in bone shape and capsule laxity that affect joint position and mobility, ill-fitting footwear, and a genetic predisposition for abnormal foot structure or biomechanics.9,45 Finally, future studies could incorporate pressure data with the described gait analysis to better understand the plantar pressure changes associated with MTPJ deformity.

A hyperextension MTPJ movement pattern associated with limited ankle dorsiflexion has been identified in people with diabetic neuropathy. Increased MTPJ extension during various movement and functional tasks that require active dorsiflexion was correlated with severity of resting MTPJ alignment. Repetition of this movement pattern could be a primary factor in the etiology of MTPJ deformity and future risk of ulceration.

Footnotes

  • All authors provided concept/idea/research design, writing, data collection and analysis, project management, fund procurement, participants, and consultation (including review of manuscript before submission). Dr Hastings and Dr Mueller provided facilities/equipment and institutional liaisons. The authors thank Kay Bohnert and Darrah Snozek for their assistance with data collection.

  • This study was approved by the Washington University Institutional Review Board.

  • This study was funded by grants from the National Institute of Diabetes and Digestive and Kidney Diseases (F31 DK101286), the National Institute of Child Health and Human Development (T32 HD007434 and K12 HD055931), and the National Center for Advancing Translational Sciences (KL2 TR000450), US Department of Health and Human Services, National Institutes of Health, during the conduct of the study.

  • Received June 26, 2015.
  • Accepted February 20, 2016.

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Vol 96 Issue 8 Table of Contents
Physical Therapy: 96 (8)

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Metatarsophalangeal Hyperextension Movement Pattern Related to Diabetic Forefoot Deformity
Victor A. Cheuy, Mary K. Hastings, Michael J. Mueller
Physical Therapy Aug 2016, 96 (8) 1143-1151; DOI: 10.2522/ptj.20150361

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Metatarsophalangeal Hyperextension Movement Pattern Related to Diabetic Forefoot Deformity
Victor A. Cheuy, Mary K. Hastings, Michael J. Mueller
Physical Therapy Aug 2016, 96 (8) 1143-1151; DOI: 10.2522/ptj.20150361
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