Clawed Hallux Deformity

Introduction

       A clawed hallux is a deformity of the great toe (Figures 1, 2 and 3) potentiallyresulting from a functional overpull of three extrinsic muscles: the peroneus longus (PL), extensor hallucis longus (EHL), and flexor hallucis longus (FHL). The deformity leads to etension of the first metatarsophalangeal joint (MTPJ) and flexion of the first interphalangeal joing (IPJ). While the peroneus longus, EHL, and FHL have been implicated in the development of the clawed hallux, the actual causes (specifically, the overpull combinations) have not been presented in previous research.This deformity is believed to increase plantar pressures under the great toe, which in turn increases the incidence of foot ulcers.

_{Figure 1 A medial/lateral radiograph of a patient with a clawed hallux}

_{Figure 2 A medial/lateral photograph of a patient with a clawed hallux}

_{Figure 3 A plantar ulcer under the first metatarsal head of the same clawed hallux patient}

    These increased pressures are of a great concern to diabetic patients that have a loss of sensation in their limbs. The purpose of our research was to quantitatively address the contribution of each muscle alone, as well as combined, in the creation of the clawed hallux deformity. By quantifying the role of peroneus longus, FHL, and EHL in the creation of the clawed hallux and the associated plantar pressure elevation, we hope to develop a more thorough understanding of the true etiology of this deformity with the potential to improve treatment strategies for diabetic patients with this disorder.

Methods

      Eight cadaver feet were tested in a specially designed acrylic frame (Figure 4) that statically simulated movement through a gait cycle. Forces (tensile and compressive) were applied via air cylinders that were calibrated to within 5% error. Customized software was written to control the cylinders.

_{Figure 4 A photograph of loaded foot}

     Each foot was supported to simulate midstance in gait. Plantar pressures were measured using the Pedar® pressure measurement system. Three-dimensional orientation of the bones of interest were measured using the Fastrak® electromagnetic motion analysis system (Polhemus, Inc.; Colchester, VT). Normal muscle forces were calculated using physiological cross sectional areas (PCSA), electromyographic (EMG) measurements, and a muscle fiber scaling factor (Table 1). [MuscleForce = (PCSA x %EMG) x scaling factor ]

    The total compressive force loaded on the tibia was calculated as a combination of the BW contribution and summation all extrinsic muscle forces simulating their compressive forces seen at their respective sites of origin on the lower extremity (Table 1). The degree of overpull corresponds to a four fold increase compared to the forces calculated at midstance. The degree of overpull in protocol 2 corresponds to a three-fold increase compared to the calculated forces at midstance.

Table 1: Loading Protocol 1 (All values are Newtons) 

peroneus longus (PL), tibialis anterior (TA), tibialis posterior (TP), flexor hallicus longus (FHL), extensor hallicus longus (EHL), flexor digitorum longus (FDL)
"+" muscle or muscles that are overpulled
^a 3X overpull at 1/4 BW forces
^b 3X overpull at 1/2 BW forces
^c Compression = (1/4 of 82% BW @700N = 143N) + force of opposing extrinsics

Results

     Overpulling peroneus longus resulted in the greatest plantar pressure increase under the head of the first metatarsal producing a 2.4 N/cm2 (42%) increase from 5.7 N/cm2 at midstance (Figure 5).

_{Figure 5 Change in Plantar pressure beneath the first metatarsal heel for the various imbalances}

     Overpulling FHL resulted in the greatest plantar pressure increase under the distal hallux producing a 5.0 N/cm2 (167%) increase from 3.0 N/cm2 at midstance and a 4.8 N/cm2 (109%) increases from 4.4 N/cm2 at midstance for protocols 1 and 2 respectively (P < 0.0001 for both protocols, Figure 6). We also found that EHL was statistically significant in lowering the plantar pressure under the distal hallux (protocol 1: P = 0.0029, protocol 2: P = 0.0005).

_{Figure 6 Change in plantar pressure beneath the distal hallux for the various imbalances}

     Overpulling EHL produced the greatest angular change in the MTP joint measured at 5.63˚ for protocol 1 and 3.96˚ for protocol 2 (protocol 1: P = 0.0148, protocol 2: P < 0.0001, Figure 7).

_{Figure 7 First Metatarso phalangeal angular change for the various imbalances}

     Overpulling FHL produced the greatest angular change in the IP joint measured at -8.69° (Figure 8). FHL and EHL are each significantly different from PL and midstance but they are not significantly different from each other.

_{Figure 8 First Inter Phlangeal joint angular change for the various inbalances}

Discussion

     With our cadaver simulations, we found that peroneus longus, FHL, and EHL all contribute to the etiology of the clawed hallux deformity in a distinct fashion. Relevant output parameters included bony rotations and the associated elevation of plantar pressure seen in clinical patients.

     The limitations of this study include our estimation of force values generated by the extrinsic foot muscles for both normal subjects and patients with this disorder. Our protocol, using small magnitudes of overpulled forces, showed even slight muscular imbalances around the first ray are capable of producing this clinical deformity. Limitations also included the relatively small angular changes we measured when compared with patients seen clinically and it was impossible for our frame to match the degree of angular changes seen in many severe clinical presentations of clawed hallux. Both of these findings may be explained in part by the time course of the disease. Our protocol occurred over a period of minutes, where clinical disease of this type occurs over years. Other limitations involved with our study include the use of static cadaveric feet to model a dynamic disorder in living patients.

     Our results implicate the peroneus longus as the primary muscle involved in increasing plantar pressure under the first metatarsal. Although EHL played a similar but lesser role, peroneus longus is likely unbalanced in patients presenting with a clawed hallux and complaints of elevated plantar pressure or ulcerations under their first metatarsal head. This elevated pressure may result from an unopposed pull of peroneus longus seen with paralysis of tibialis anterior.

     Surgical treatment for the clawed hallux is, traditionally, the modified Robert Jones procedure, which addresses the EHL alone for correction of the deformity. Our research has shown that there may be more than one etiology for this deformity. For clinicians, the physical problem should be closely examined before deciding on a treatment. For example, if the clinical problem is increased plantar pressure, transfer of the EHL might not be the most effective treatment. If IP flexion is the greatest problem, perhaps transfer of the FHL should be considered. Though this research does not measure the effect of treatment, it does suggest that there might be more than one way to address this deformity.

Acknowledgement

This research was supported in part by VA Grant A0806C, RR&D Center for Limb Loss Prevention and Prosthetic Engineering.

This project is fully described in:

Olson SL, Ledoux WR, Ching RP, Sangeorzan BJ., Muscular imbalances resulting in a clawed hallux., Foot Ankle Int. 2003 Jun;24(6):477-85.

Research Team

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