Functional restoration and risk of non-union of the first metatarsocuneiform arthrodesis for hallux valgus: A finite element approach

Abstract

First metatarsocuneiform arthrodesis is one of the surgical interventions to correct hallux valgus, especially those with hypermobile first ray. There is lacking of biomechanical investigations to assess this operation. The objective of this study was to explore the functional restoration and the risk of non-union after the surgery via finite element analysis.

A three-dimensional foot model was constructed from a female aged 28 via magnetic resonance imaging. Thirty bones and encapsulated bulk tissue were modeled. Walking stance was simulated by the gait analysis data of the same participant. Parts of the first metatarsal and cuneiform were resected and the bone graft was assigned with the same stiffness as adjacent bones to resemble the surgery of first metatarsocuneiform arthrodesis.

The third principal stress of the first metatarsal at midstance (25% stance) and push off (60% stance) was increased by 76% and 139% respectively after the operation, while that of the second metatarsal was decreased by 14% and 66%. The operation reduced the medial deviation of the first metatarsal head by about 3.5 mm during initial push off (60% stance). Besides, the bone graft could experience tensile stress inferiorly (26.51 MPa).

In conclusion, the increase of stress on the first metatarsal and the reduced medial excursion of the first metatarsal head after the simulated operation reflected that metatarsocuneiform arthrodesis could restore the load-bearing function of the first ray. However, inter-fragmentary compression could not be guaranteed. The appropriate course of hardware and non-weight-bearing protocol should be noted and further investigated.

Introduction

Hallux valgus (HV) is one of the most common foot problems with a prevalence of 23% in adult and 35.7% in elderly population (Nix et al., 2010). While surgical interventions are indicated for moderate to severe deformity, arthrodesis of the first metatarsocuneiform (MC) is particularly used for patients with hallux valgus and hypermobile first ray (Robinson and Limbers, 2005). The association between first ray hypermobility and hallux valgus (HV) has long been studied (Robinson and Limbers, 2005). Arthrodesis can reduce recurrent HV via retaining first ray stability (Johnson and Kile, 1993). Lapidus procedure, one common type of MC joint arthrodesis, improves radiological and clinical outcome (Faber et al., 2004, King et al., 2014). Though the recurrence rate is low, it is argued whether the rate of Lapidus procedure is comparable to other operations regardless the presence of hypermobility (Faber et al., 2013).

The relationship between recurrent HV and hypermobile first ray is conceived (Scranton and McDermott, 1995). Clinically, hypermobility of the first ray is assessed by manual excursion which is inaccurate and unverifiable (Smith and Coughlin, 2008). It is difficult to assess the effect of an intervention aiming to reduce hypermobility. However, MC arthrodesis is recognized as salvage after failed surgical treatment and recurrent hallux valgus (Coetzee et al., 2004), despite that there is no strong evidence on its effectiveness in reducing hypermobility (Easley and Trnka, 2007).

MC arthrodesis has a relative high failure rate (Coughlin and Shurnas, 2003). The non-union rate can be up to 75% (Coughlin and Mann, 2012). The operation is difficult to obtain satisfactory fusion and technically challenging (Coughlin and Mann, 2012). To determine the correction, the surgeons need to continuously assess the intermetatarsal alignment and the shortening of the 1st metatarsal. It is also crucial to achieve rigid inter-fragmentary compression to hold the arthrodesis site (Coughlin and Mann, 2012). However, the compression is difficult to ascertain as the joints born complicated loading in gait and other activities. Post-operatively, the compression of the fusion site is supported by hardware, such as compression screw and interlocking plate (Basile et al., 2010). Many surgeons placed the patients on a non-weight-bearing protocol up to 6 weeks for the healing of the fusion site (Basile et al., 2010). However, the appropriate courses of hardware and post-operative protocol are under debate. Some studies advocated subsequent hardware removal due to the high rate of hardware pain and re-operation possibly due to foreign body reaction, while clinically the hardware often remains inpatients indefinitely unless the patients complain or a revision surgery is performed (Mallette et al., 2014).

Previous studies controlled the bending loads to the fusion site to determine the strength and stability of different hardware configurations in cadaveric studies (Cottom and Rigby, 2013). However, this clinical problem has yet to be addressed by a validated finite element approach that provides information about the soft tissue and bones within and beyond the fusion site. Moreover, a platform to study and compare the effect of different surgical corrections and protocols on hypermobility could better predict and improve the outcome of surgical interventions.

In this study, we constructed a three-dimensional finite element model of the foot and ankle from a normal subject. To resemble the simulation on the hallux valgus deformity and MC arthrodesis procedure, we had made two assumptions. The first assumption was that hallux valgus deformity and first ray hypermobility presented similar biomechanical characteristics (Scranton and McDermott, 1995). Therefore, the hallux valgus deformity was presented as a normal foot with hypermobility. The second assumption was that the biomechanics and alignment of a normal foot with MC arthrodesis would be similar to that of the corrected hallux valgus foot. Therefore the simulated MC arthrodesis procedure was conducted on a normal foot.

The objectives of this study were (1) to evaluate the functional restoration of the first ray after MC arthrodesis; (2) to assess the risk of non-union of the bone graft after MC arthrodesis. Computational simulation of the MC arthrodesis was carried out to study its biomechanics during stance phase. Sufficient hallux and first ray loading was imperative to functional restoration upon hypermobility (Koller et al., 2014) and could be reflected by the third principal (compressive) stress of the metatarsals in this study. The effectiveness of the surgery should also be represented by the reduction of medial deviation of the first metatarsal upon load bearing. Stress analysis of the bone graft was performed to investigate its tensile and compressive behavior and thus the risk of non-union in arthrodesis. This study reveals the advantages and problems of MC arthrodesis that will enhance the understanding and future planning of the intervention.