Hallux valgus (HV): A multi-approach investigation analysis

Abstract

Objective

this study aims to develop a 3D FE model of the foot suffering from valgus hallux in order to investigate the plantar pressure distributions between bony structures.

Methods

in a first phase a baropodometric analysis was performed, successively a FE analysis was performed comparing results and obtaining information on the stress shielding.

Results

the valgus hallux deforms the correct spreading of the stress inside the bony structures causing an overloading of pressure located on the hallux and downloading the other toes.

Conclusion

This comparative study can furnish important indications about the distribution of the stress patterns on the foot.

1. Introduction

Metatarsalgia in hallux valgus (HV) deformity is seen as a result of increased pressure and load transfer to the lateral metatarsal region. A load and pressure transfer from the big toe to the central metatarsal region is described, indicating the functional impairment of the big toe and the simultaneous worsening of the loading conditions at the metatarsals.1, 2, 3, 4, 5 Others describe a pressure transfer from the first metatarsal to the lateral metatarsals. However, the critical pressure that is likely to result in symptoms of metatarsalgia is unknown. The purpose of this study was to determine specific differences in the loading patterns in patient s with HV deformity with and without metatarsalgia and to find predictive pressure variabilities that are likely to result in metatarsalgia. The wearing of constricting and high heel shoes are extrinsic factors, which are important in the development of hallux valgus.^1,2 The role of pes planus is complex. It is unlikely that it is an important initiating factor in hallux valgus but in the presence of pes planus the progression of hallux valgus is more rapid.³ Heredity is likely to be a major predisposing factor in some patients, with up to 68% of patients showing a familial tendency. This is particularly so in those patients with a compromised medial joint capsule as in rheumatoid arthritis, collagen deficiency or a neuromuscular disorder.⁴ The presence of pes planus does not reduce the rate of success of operations for hallux valgus.^5,6 Radiological assessment Weight-bearing anteroposterior (AP) and lateral radiographs of the foot are taken to help assess the deformity and assist in pre-operative planning. The hallux valgus angle (HVA) (normal < 15°) and intermetatarsal angle (IMA) (normal < 9°) are measured. The distal metatarsal articular angle (DMAA) (normal < 10°) is the angle between the articular surface of the head and shaft of the first metatarsal. In most cases, the DMAA is normal and the first metatarsophalangeal joint is subluxed. This is commonly termed an incongruent hallux valgus. In a small percentage of patients who are usually young, the joint is congruent and not subluxed. In these cases the DMAA is increased, the metatarsal articulation points more laterally than normal, and there is no subluxation of the metatarsophalangeal joint. A congruent hallux valgus is less prone to progression than one, which is incongruent. The DMAA is difficult to measure, with high inter- and intra-observer variability. Hallux valgus interphalangeal deformity is present if there is significant angulation between the proximal and distal phalanges. The relationship of the first metatarsal head to the sesamoids, the size of the medial eminence and the presence of degeneration should also be recorded. It has become traditional to classify the severity of the deformity using radiological criteria in order to help formulate an algorithm for surgical treatment: mild (HVA up to 19°, IMA up to 13°); moderate (HVA 20° to 40°, IMA 14° to 20°); severe (HVA > 40°, IMA > 20°), see Fig. 1. Many problems can be evidenced by taking into account different loading conditions and for this reason, other researchers used the entire bony chain of leg7,8,9, and 10 to evaluate the stress occurring in healthy feet, examining various applications. In this paper, the hallux valgus syndrome has been investigated by using a baropodometric approach in order to estimate the plantar pressure aging in this case and using the obtained data to calibrate a FE model of the entire foot able to predict the stress shearing on the bony parts.

Fig. 1.

The Valgus Hallux syndrome,pathological causes.

2. Materials and methods

In this paper, a comparative study has been performed in order to characterize the stress shielding of a foot suffering from hallux valgus. In a fist phase, a Rx was performed on a patient suffering by HV, data revealed pathologic values, in particular the distal metatarsal articular angle (DMAA) was found to be about 13°, the hallux valgus angle was 36°, while the inter metatarsal angle was found to be 17°. In a second phase, the patient was subjected to a baropodometric investigation. Postural evaluation and biomechanical analyses were carried out by a Maxi Platforms FreeMed® on a normal adult patient (72 [kg] BM). Successively two numerical models of the foot ware obtained by matching nuclear magnetic resonance (MRI) for soft tissues, and a computerized tomography (CT) for bones, carried on the two feet of the patients. Obtained data were imported into the commercial Hypermesh code by Altair®, where the final finite element model of the foot was reconstructed. The healthy foot was reconstructed by using 43892 nodes and 178.579 tethraedric elements, while for the pathological one were used 43920 nodes and 178.591 elements. Ligaments, other connective tissues, and the plantar fascia were defined, by 98 truss mono dimensional elements, connecting the corresponding attachment points on the bones. All the bony and ligamentous structures were embedded in a volume of soft tissues. To simulate the frictionless contact between the joint surfaces, ABAQUS automated surface-to-surface contact option was used. According to the model developed by Gefen et al.¹¹ adopting the linear elastic material law, the Young's modulus and Poisson's ratio for the bony structures were assigned as 7300 MPa and 0.3, respectively. The mechanical properties of ligaments¹² were selected from the literature, see Table 1. A rigid wall of steel was used as ground support using surface-to-surface contact elements in combination with the penalty algorithm with a normal contact stiffness of 600 N/mm and a friction coefficient of 0.4. A vertical force of approximately 360 N and the corresponding reaction produced by the Achilles tendon measuring of 175 N were applied on the top of the talus and on the calcaneus, corresponding to a BW of 72 kg, as depicted in Fig. 2.

Table 1.

Mechanical and geometrical properties of the FE model.

Component	Element type	Young modulus	Poisson's ratio	Cij and Di material parameters
Bony parts	3d Tetrahedrons	7.300 [MPa]	0.3	C10 = 0.08556	C02 = 0.00851
Soft tissue	3d Tetrahedrons	hyperelastic	/	C01 = -0.05841	D1 = 3.65273
ligaments	1d Truss	350 [MPa]	/	C20 = 0.03900	D2 = 0.00000
Rigid wall	3d Tetrahedrons	210.000 [MPa]	0.3	C11 = -0.02319

Fig. 2.

Baropodometric measurement system and FE numerical model.

3. Results

In Fig. 3 is reported the contour maps of pressures obtained by the baropodometric analysis. As it is possible to notice There is an evident tendency towards valgus (editor's note: outward deviation of the big toe toward the second toe) of the right big toe which determines the “hammer-like” characteristic of the second toe with there not being any pressure applied by the distal phalange (the bone at the outer extremity of the toe). This is not the case with the left foot. The first phalange of the big toe applies only modest pressure aging from 0,03 to 0,06 MPa if compared to that of the opposite foot from 0,06 to 0,09 MPa. In Table 2 are reported the principal results obtained by the analysis.

Fig. 3.

Countour map of plantar pressures obtained by baropodometric analysis.

Table 2.

Numerical data obtained by the baropodometric analysis.

>	Left foot	Right foot
Forefoot
Area [cm2]	90	81
Load [%]	27	22
Ratio R/A %	51	47
Backfoot
Area [cm2]	78	76
Load [%]	26	25
Ratio R/A %	49	53
Total
Area [cm2]	168	157
Load [%]	53	47
Weight [kg]	38	34

In Fig. 4 is reported the Eq. Von Mises contour map obtained by the stress analysis. In Fig. 4a is reported the stress shielding measured on the skin and its maximum values is about 0,34 MPa. As it is possible to see, the numerical map is quite similar to the baropodometric one. Infact, Hallux evidences a pathological overloading while the second toe is almost discharged. In Fig. 4b is reported the stress shielding on the bony parts. The maximum value reached is about 11 MPa and the trend followed remarks what described in the previous picture. In Fig. 5 are reported results obtained for displacements and the equivalent elastic strain. The maximum values of displacements are about 6,83 mm while the equivalent elastic strain has values ranging around 1,34 E−002 μmm/mm.

Fig. 4.

Eq. Von Mises stresses measured on the skin a)and on the bony parts b).

Fig. 5.

Displacements a)and Eq. Elastic strain b) localized on the bony part.

4. Discussion

Hypermobility of the first tarsometatarsal joint is thought by some^7,8 and ¹³ to be a causative component in some cases of hallux valgus. In these patients a fusion of the first tarsometatarsal joint (the Lapidus procedure), should be considered for surgical correction as opposed to an osteotomy. There is a correlation between hypermobility of the first ray and hallux valgus^9,10 and a higher incidence of hypermobility at this site causes a hallux valgus deformity which is painful.¹¹ The proximal phalanx drifts into valgus and the metatarsal head into Varus. A groove appears on the medial side of the articular cartilage of the metatarsal head as it atrophies from the lack of normal pressure and this gives rise to the apparent prominence of the medial exostosis. The medial bursa develops in response to the excessive pressure of shoes over this prominence. As the soft tissues on the medial side become further attenuated, the metatarsal head moves medially so that the medial sesamoid lies under the eroded metatarsal ridge and the lateral sesamoid articulates with the lateral side of the metatarsal head in the first intermetatarsal space. The tendons of extensor hallucis longus and flexor hallucis longus are carried laterally with the phalanx, thus becoming adductors and exacerbating the deformity. The adductor hallucis and lateral head of flexor hallucis brevis contribute further to this and with time, they become contracted, as does the lateral joint capsule. The abductor hallucis and medial head of flexor hallucis brevis also lose their abduction moment. The resultant imbalance causes dorsiflexion and pronation of the first toe rendering its pulp non-functional. The resultant reduction in plantar pressure under the first ray leads to insufficiency of the first ray and overload of the lesser rays. As a result, the second toe may claw and eventually the second metatarsophalangeal joint will dislocate. Pain may also be felt in the distribution of the dorsal cutaneous nerve, due to pressure. Deformities of the lesser toes such as corns and calluses are often a source of symptoms and are largely due to insufficiency of the first ray and overcrowding. Synovitis of the second metatarsophalangeal joint with pain and swelling is particularly painful. The physical examination begins with the patient standing as this often increases the hallux valgus and associated deformities. It is important to assess the hindfoot as well as the forefoot. Planovalgus deformities and tightness of the gastrocnemius and soleus can often exacerbate loading and pain under the forefoot. The severity of the hallux valgus deformity and whether it is correctable is documented. Any pronation of the great toe is noted. The first metatarsophalangeal joint is examined to assess the range of movement. The lesser toes should be examined for associated deformities and callosities. The intermetatarsal spaces should be palpated for interdigital neuromas. The plantar surface of the foot should be checked for tender callosities under the lesser metatarsal heads (transfer lesions). In order to assess first tarsometatarsal instability, the examiner immobilises the lesser metatarsals with the thumb and fingers of one hand. The thumb and index finger of the other hand grasp the first metatarsal and move it from a plantarlateral to dorsomedial direction. Movement of more than 9 mm indicates hypermobility.⁸ The patient should also be examined for signs of generalised ligamentous laxity. Different techniques have been developed to study different pathologies, many FE models have been developed. Analytical models allow to determinate physical condirions of solid or fluid components, for example CFD analyses can investigate artery deseases.[15], [16] In order to establish the stress shielding in the bony parts or in the surgical tools describing mechanical behavior of the implanted tibia[17], 18, 19, 20, 21 or implanted femur.22, 23, 24 Other researches^25,26 presented an FE foot model based on MR images investigating the stress map distribution on the different bony part of the foot. Filardi²⁵ found pressure mostly concentered on the calcaneus, reaching values acting around 0.26 MPa, the same as on the lateral plantar fascia, and from the first to the fifth metatarsal head regions. The equivalent von mises contour map evaluated on the bony parts constituting the foot reaches its peak on the calcaneus, and it is about 8.23 MPa. In another paper²⁶ the stress of 5.48 MPa is reached on the talus. The metatarsals bones are differently loaded from the first metatarsal, with a stress of about 2.74 MPa. On the first proximal phalange are reached values of 6.40 MPa, following a decreasing trend on the remaining others proximal phalanges where the stress reaches values of 1.82 MPa. The stress shielding acting on the bony parts reaches its peak on the calcaneus, about 8.23 MPa, spreading on the medial plantar fascia with values of 2.74 MPa.

5. Conclusions

In progressive HV deformity, the decreased weight bearing function of the first and second toes is associated with an increased transfer of weight to the lateral metatarsals. This mediolateral load transfer indicates the functional impairment of the great toe and the simultaneous worsening of the loading conditions at the metatarsals.1, 2, 3, 4, 5^,14 The development of the varus position of the first metatarsal is associated with a dorsal movement of the metatarsal head and a load transfer to the second metatarsal. The results of this study confirm the mediolateral load transfer in a HV deformity. The major load transfer took place from the great toe to the lateral forefoot. The load transfer from the first metatarsal to the lateral forefoot was of minor importance. It is likely that one of the reasons for this mediolateral transfer lies in a compromised windlass mechanism. A progressive HV deformity is associated with a modified orientation of the plantar fascia. This distribution of the loading parameters provides an insight into pedographic characteristics of metatarsalgia in a patient with HV deformity and suggests a threshold of pressure that is likely to cause symptomatology. This could be used for an early pedographic screening of patients with HV deformity, since methods of predictability for metatarsalgia are not available.