The hallux MP arthrodesis technique for the treatment of hallux varus was discussed as early as 1989, when Mills and Menelaus [11] reviewed the results obtained after an average follow-up of 12.7 years. Arthrodesis is generally reserved for severe rigid deformity, failure of soft tissue surgery, or the development of osteoarthritis in the MP joint. Evidence in the literature concerning arthrodesis of MP joint in hallux varus is scarce and based on studies with a small sample size and limited analysis of clinical and radiological results and complications.

The aim of this study was to report clinical and radiological results and complication of arthrodesis in iatrogenic hallux varus. All patients included in the present study had a diagnosis of rigid and severe iatrogenic hallux varus. The study population analysed can therefore be considered homogeneous. To our knowledge, the present study presents results of the largest case series reported in the literature.

This study demonstrates that arthrodesis of the first MP joint is an effective treatment option in all cases of rigid and severe IHV, despite the different techniques used in the primary hallux valgus surgery, the severity of the deformity and the extent of residual bone stock.

Fusion of the first MTP joint, in our case series, was obtained in neutral rotation, with a 10–12° hallux valgus angle and approximately 20° of functional dorsiflexion. The literature suggested a position of the first MTP joint to achieve a stable fusion between 5° and 15° of valgus and between 20° and 25° of dorsiflexion with respect to the first metatarsal [12]. In our experience, the extent of dorsiflexion can be determined intraoperatively with a flat board to simulate the loading condition. The choice of the appropriate dorsiflexion angle is a cornerstone of the procedure. A lower dorsiflexion would increase pressure on the interphalangeal joint, leading to osteoarthritis at that level. Conversely, a greater angle would increase the pressure on the first metatarsal head, overloading the sesamoids and causing a claw deformity of the interphalangeal joint with severe difficulties with footwear [13].

Biomechanical studies in the literature indicate the dorsal plate and interfragmentary compression screw as the most stable fixation method to achieve fusion of hallux metatarsophalangeal joint [14]. In our experience, shape memory staples have proven to be effective for synthesis, affordable and less invasive than plates, reducing post-operative pain related to the hardware.

Considering the radiological parameters, the arthrodesis performed in the context of rigid IHV resulted in an increase of the 1st-2nd Intermetatarsal Angle (IMA) of 4.4 ± 2.4°. This implies that, while the Hallux Valgus Angle (HVA) was corrected during the procedure, the deforming forces, including those exerted by the abductor hallucis and flexor digitorum brevis, were repositioned in such a way that the 1–2 IMA was subsequently normalised. The correction obtained three months after surgery was then maintained at medium- to long-term follow-up demonstrating, a high rate of fusion obtained with the procedure.

Furthermore, the analysis of the preoperative clinical and functional data shows that iatrogenic hallux varus significantly results in severe discomfort when wearing shoes and during walking, joint pain and impairment of an individual’s quality of life. The main objective of arthrodesis is to alleviate pain symptoms and allow loading on the first ray. The level VAS at the last follow-up demonstrates the achievement of the first treatment goal. Similarly, AOFAS scores also demonstrate a significant and lasting functional improvement after surgery.

Grimes et al. [15] described that higher AOFAS scores are recorded in primary arthrodesis. Despite the lower AOFAS scores observed after a revision surgery. The fusion of the MP joint of the first ray appears to be a viable treatment option for hallux varus characterised by severe rigid deformity and development of MP osteoarthritis. Furthermore, when analysing the satisfaction rates reported by patients, 100% stating they would undergo the same procedure again. The great AOFAS improvement may be also related to a higher postoperative HVA, which is better adapted to most stocking designs. In addition, in our patient cohort we performed surgery to remove excess skin in the first interphalangeal space, which often causes discomfort. We recommend this procedure whenever a skin fold forms after hallux varus correction.

No significant complications were noted in the case series. In only one case, revision surgery was performed for delayed fusion of the first MP ray fusion. There were no cases of non-union in the last follow-up x-rays.

Pseudarthrosis, whether symptomatic or not, is one of the most relevant complications in arthrodesis of MP joint. In the literature, the fusion rate varies between 71% and 100% of cases depending on the procedure used, with no significant difference currently being demonstrated between the techniques used [16,17,18]. During a hallux arthrodesis, it is essential to adequately prepare the joint, as well as reduce and fix it. Non-union rates are influenced by the stability of the implant used. There is no consensus in the literature as to which fusion method is best, although several biomechanical studies have shown greater stability with the combination of dorsal plate and interfragmentary screw [19].

In our experience, shape memory staples have proven to be effective for synthesis, affordable, and less invasive than plates, reducing the risk of postoperative pain due to plate prominence. In our case series, delayed fusion was observed at the arthrodesis site fixed with crossed screws that required surgical revision to enhance fusion. Our preferred fixation technique (79%) of the fusion site was two orthogonal shape memory staple configurations. At the last available follow-up, no patients reported pain in the arthrodesis site.

The first MP joint fusion approach in IHV is a complex technique even for highly skilled foot surgeons, with a long learning curve. However, this technique can be mastered because the incidence of IHV ranges between 2% and 15.4% in the literature [20].

A limitation of the present analysis is the inclusion of patients undergoing concomitant corrective procedures; however, in foot surgery this element is common. Lastly, the study design is retrospective.