In the difficult patient population presented in this study, after fixator removal and under full load in a carbon orthosis, there was an increase in the bony consolidation of the arthrodesis zone in 58.33% (n = 14) of all 24 cases. This took place after the fixator had already been worn for an average of five months, and further risky surgeries were thus avoided. In most cases, there was consolidation between 41–60% after fixator removal, but an increase could also be recorded in patients with 21–40%, 5–20% and 0% consolidation. Mobilization under full load was possible for all patients.

In complex patient populations such as that described in this study, with multiple previous surgeries, already failed arthrodesis attempts, an infection history, poor soft tissues, a long medical history and/or relevant previous illnesses, the decision to repeat surgical treatment in cases of nonunion or partial union after wearing the Ilizarov fixator for several months is often difficult. In our opinion, the risk of a repeated infection following introduction of internal osteosynthesis material would have been too high for many of these patients, and/or the soft tissue situation and previous illnesses would have meant an increased risk of a defect/wound healing disorder. An extension of the wearing time of the fixator or even another attempt at arthrodesis using the Ilizarov fixator is often not tolerated by the patient due to its bulky structure and the possibility of recurrent pin infections, further complications, injuries to the opposite leg and destruction of clothing and bed linen. The alternative is amputation. To prevent this, a carbon orthosis was first applied in this patient population with the provision of full weight bearing. To the best of our knowledge, the possibility of further treatment using a carbon orthosis in the case of partial consolidation or nonunion has not yet been described in cases of arthrodesis via an Ilizarov fixator.

In recent studies, primarily good results have been achieved with the Ilizarov fixator [1,2,3,4,5,6,7,8,9,10,11,12,13, 18]. However, most of these studies only described a union or a failure in cases of nonunion or infection. Arthrodesis is considered permanent if it is clinically stable and if X-ray or CT shows bridges, although this is described differently depending on the author. Khanfour et al. described arthrodesis as the detection of bridging trabeculation in at least 2 planes at the arthrodesis site on the radiogram [8]. Katensis et al. required evidence of bridging trabeculae without a change in the position of the ankle under weight bearing, and Salem et al. confirmed successful arthrodesis by painless weight bearing as well as radiologically using plain radiographs or CT scans [7, 18]. Since X-ray was not always sufficiently meaningful in the complex courses of the patients in the current study, CT was also performed for almost all patients after the fixator was removed. A much more precise statement about the consolidation of the arthrodesis could thus be made [14,15,16,17]. The abovementioned patients all showed only partial consolidation or nonunion. To avoid breaking the existing bone bridges in this complicated patient group and to achieve a further increase in consolidation even without a fixator or additional internal osteosynthesis material, an orthosis was applied. This allowed all patients to walk, and repeated surgical treatment with all risks and complications was avoided.

The grouping of the consolidation rate as sets of percentages was based on Jones et al. [14]. He described a system for calculating the extent of consolidation and divided it into the following groups: 0–33%, nonunion, 34–66%, partial consolidation, and 67–100%, consolidation. However, this was via the application of an internal arthrodesis procedure. Further studies by Dorsey et al. and Glazebrook et al. followed this system for internal arthrodesis as well and stated that arthrodesis is stable from over 33% or over 25- 49% [15, 17]. For internal arthrodesis, there is therefore no consensus regarding the consolidation rate on CT when partial consolidation is sufficiently stable. After external arthrodesis using an Ilizarov fixator, very few authors describe partial consolidations or tight pseudarthroses that were subsequently treated with an orthosis. However, the further course of the patients remains unclear. Kugan et al. described a patient with nonunion who could mobilize with an orthosis with pain. El Alfy et al. also described a patient with fibrous nonunion who was able to mobilize with an orthosis, and Zarutzy et al. described five patients who needed a supporting orthosis (four stable pseudarthroses and one malunion) [4, 9, 10]. In a study on tibiocalcanear arthrodesis by Reinke et al., a patient with a partial consolidation of 40–50% and four patients with stable pseudarthrosis were described, all of whom were also treated with an orthosis [1].

In this study, 14 (58.33%) patients under load in the orthosis showed an increase in consolidation six months after the start of arthrodesis. In nine (64.29%) patients, there was an initial consolidation of 41–60%, and in some cases, an orthosis might not have been necessary. However, given the long course of the disease, we did not want to take any risks, and the orthosis initially provides the patient with security so that a full load is possible. In four other patients, the consolidation was 0% (1), 5–20% (1) and 21–40% (2). The risk of a break in areas with few bridges would have been too high, and a safe increase could be achieved with the orthosis. Among the seven (29.17%) patients with no change, the majority only had 0% (2) or 5–20% consolidation (3), which could indicate that an increase occurs sooner when at least 21–40% ossification has already occurred. However, the follow-up for these patients was shorter at 11 months (average) than for the patients with an increase (17.4 months), and with such a small number, no definitive statement can be made.

The 58.33% of the patients who experienced an increase in consolidation does not seem very high at first, but this must be considered against the background of a difficult patient population. Eleven (45.83%) patients were smokers, the average BMI of 31.0 was in the range of obesity grade 1, four (16.67%) patients had diabetes mellitus, three (12.5%) patients presented with wound healing disorders during as well as after fixator removal, and other difficult diseases such as chronic kidney failure with renal osteopenia, osteoporosis, rheumatoid arthritis, PVD, depression, and alcohol abuse were recorded (Tables 2, 3, 4 and 5). Twelve (50%) patients experienced a second attempt after a previous frustrating arthrodesis, and one patient experienced a third attempt. In a study on 88 patients with internal subtalar arthrodesis, Chahal et al. showed that smokers and patients with DM had a 3.8- and 18.7-fold higher probability of malunion and that the worst functional outcome was observed in patients with DM [22].

The results of the FAAM score were significantly worse for the ADL subscale with 49% (SD ± 18) and for the sport subscale with 24% (SD ± 17) than for other studies. Kerkhoff et al. described an average value of 70% (SD ± 22.3) for the ADL subscale and 29% (SD ± 27.8) for the sport subscale in 122 patients. However, these were primary internal arthrodeses in the case of degenerative changes, and the BMI was lower at 27.5 (SD ± 4,9); no statement was made regarding DM [23]. Strasser et al. described a score of 81.5 (SD ± 18.3) in patients over 70 after internal arthrodesis [24]. The BMI or other previous illnesses were not reported. A division into subscales was not made here either. Morasiewicz et al. achieved values of 79% (56–88) after Ilizarov arthrodesis and 70% (49–91) after internal arthrodesis [25]. Although this included patients with infection, vulnerable soft tissues and deformities, there are no reports of complicated courses with frustrating arthrodesis in advance. The average age of 44 years was also significantly younger than that of 59 years in this study. However, it shows that the overall outcome with partial consolidation and orthosis for complicated patients (multiple interventions, frustrating previous arthrodesis attempts, long disease courses and/or multiple previous illnesses) is worse overall, so orthosis should be seen as a salvage procedure and only be provided an alternative for individual cases.

This study has several limitations. The study sample of 24 patients was small, and the study mainly had a retrospective design. However, these are complex individual patients after following Ilizarov arthrodesis, which is otherwise a well-investigated therapeutic procedure; therefore, it is not expected that a large number of patients should be encountered. Even if CT had been performed for almost all patients after fixator removal, only 10 (41.67%) patients underwent follow-up CT. In the remaining patients, CT and X-rays were compared with only X-rays. However, further CT diagnostics would in some cases not have been ethically justifiable if the change was already clearly visible on comparable X-ray images or because there would have been no further consequences for the patient. Thus, the radiological percentages could not be determined exactly.

For patients without a change, the follow-up was significantly shorter than for patients who did demonstrate change, so no statement can be made here as to whether a change might still occur in the course of the process, which would potentially increase the rate of ossification even further. The outcome scores could not be recorded for all patients because some had moved to an unknown location or had already died from other causes. A longer follow-up would be necessary here to make a reliable statement regarding the long-term outcome. In addition, there was no comparable control group without an orthotic device, so no statement can be made regarding the course in this case.