Our study characterized the incidence of ankle OA and its correlations with clinical data, which was assessed by the K&L scale and AOFAS scale. [12, 19]. It was also noteworthy that ankle pain, stiffness, and physical function disability increased significantly with K&L grades and AOFAS ankle-HF Scale scores. After an average of 2.1 years in receiving the reconstruction operation of the tibial defect with an external circular fixator of the Ilizarov technique, forty-nine (20.8 %) of 236 patients developed ankle OA. This differs from the results described by Morasiewicz et al.  for external fixation in the treatment of ankle OA since their subjects were patients who did not receive bone transport for critical bone defects, and the sample size was insufficient. However, the patients in our cohort developed ankle symptoms after bone transport procedure, then the incidence of ankle OA and the healing rate after fusion surgery were not consistent with most similar studies [20,21,22,23]. Compared with other methods, the advantages of the external circular fixator also included rigid immobilization, resistance to shear and torsional stresses, axial loading with the ability to restore early weight-bearing status, and manageability of large soft tissue with bone defects .
Unfortunately, other complications occurred in patients as well, including pin tract infection(18 cases),delayed union on docking site(7 cases), axial deviation(12 cases) and poor regenerate consolidation(1 case), which are consistent with the complications related to the Ilizarov technique described by Liu et al. . These complications may be one of the reasons for such a high incidence of ankle OA in our cohort of patients. According to the published study, the incidence of ankle OA was increased in patients who obtained ankle instability and premature functional exercise [7, 12, 13, 16, 26, 27]. Complications of bone defect reconstruction using the Ilizarov technique had been proven to increase the chance of ankle instability and the compression of the joint surface . For instance, poorly controlled pin tract infection may lead to pin tract loosening, which increases the additional treatment time, the rate of osteoporosis, or delay the connection of the docking site further, and promote the incidence of ankle OA lastly. In this study, the same phenomenon was noticed that the patients who experienced more complications, which postpone the period of whole treatment, gained the ankle OA easier. Thus, it’s of great importance for the prevention of ankle OA to keep the pin tract clean and dry, underwent X-rays radiograph to observe the position of bone transport regularly, and make timely management for the other complications.
A multivariate analysis was conducted among the baseline data of ankle OA patients and the variables with statistical significance were analyzed by logistic regression analysis. Subsequently, age > 45years, male, BMI > 25, double level bone transport, diabetes, hypertension, osteoporosis, glucocorticoid intake, duration of disease > 24 months, EFI > 50days/cm, AOFAS ankle-HF scale scores < 50 were associated with a greatly increased risk of ankle OA as defined by the K&L scale. The incidence of ankle OA was 50–70 % in the existence of three or more risk factors. With the help of meticulous multiple logistic regression statistical analysis, independent risk factors for the development of ankle OA were associated with the BMI > 25, male, diabetes, osteoporosis, AOFAS ankle-HF scale scores < 50.
Despite the changes and clinical symptoms of osteoarthritis assessed by radiography were controversial , a substantial association was found between pain, functional scores, and the appearance of radiographic ankle OA. In hip and knee, the strong relationship between clinical and radiography in a people-based cohort had been disclosed , yet uncommon in the ankle. In our observation, the potentially modifiable risk factors were BMI [13, 26, 30], and chronic diseases such as diabetes, hypertension, and osteoporosis, could be managed to reduce the probability of ankle OA effectively. Thus, patients should be messaged about these additional risks and instructed towards weight reduction and management of chronic diseases. The other reason for risk factors identification was to provide patients with recommendations regarding the future risk of ankle OA (individualized prediction) and match available treatment options to postpone poor prognosis.
Obesity (BMI > 25) had been certified as an independent risk factor for OA in previous studies . In our cohort, the probability of ankle OA with different K&L levels in obese patients (OR1.34, CI0.32-0.56) was as high as 57.1 % after the tibia bone defect reconstruction surgery. The main reason for our consideration was that obesity increased the axial load and inflammatory response of the lower limb mediated by ankle alignment, which accelerated the process of cartilage degeneration [26, 31], and led to ankle OA further. Then joint damages were almost caused by the change of mechanics through altered surface alignment, forced distribution, and varying degrees of ongoing joint instability . Besides, ankle OA was more likely to occur in male patients (OR0.54, CI0.18-0.63) over the age of 45(OR2.29, CI0.63-0.87) when treated with external circular fixators for bone defects in our study. According to our observation, this was probably the more opportunities they had to develop a significant smoking history, diabetes, alcohol abuse, and chronic diseases, as a result of decreased metabolism and excessive physical work than females. This was in agreement with Holzer et al. findings of a higher incidence of OA in the presence of an associated older male.
The increasing prevalence of OA with age may be the result of cumulative exposure to various risk factors and biological changes [7, 27, 33]. Briefly, mechanical protective mechanisms, due to decreased muscle hypertrophic capacity in the elderly, this is likely to result in loss of joint protection with age during normal gait cycles. And a remarkable advance from the late 90 s was the description of cellular metabolic pathways affected by aging . With the aging of the body, the metabolism and enzymatic reaction of various types of cells in the body, which nourishment of articular cartilage required, are inevitably reduced. Thus, age is a quite important risk factor for weight-bearing joint OA, like the ankle joint.
The published researches had revealed that the double level bone transport shortened the EFT and EFI greatly in the comparison of the single level bone transport [1, 2, 4, 34]. Similarly, the EFT and EFI of patients with double level bone transport in this study were lower than those of patients with single level. However, the phenomenon that the number of ankle OA in patients with double level bone transport (59.1 %) was more than that in patients with single level (40.8 %) was noticed. Afterward, there was a stronger correlation was confirmed by the univariate logistic regression analysis between double level bone transport and ankle OA (OR 3.79, CI1.51-9.53). Moreover, the DS is also the basic factor for choosing double-level bone transport, especially in the DS > 5 cm. Thus, our cognition for this was that double-level bone transport reduced EFI and the period of Ilizarov technique treatment, but it required stronger forces on the tibia and ankle to ensure the stability of the entire external circular fixator frame, which made the movement of the ankle more limited and aggravated the degeneration of articular cartilage. Given the above limitations of this technique and the lack of regenerative capacity of articular cartilage, it was not surprised that the double level bone transport was a responsible risk factor for the development of ankle OA [35, 36]. This had also been verified by studying complications of the Ilizarov technique which point to high rates of osteoarthritis development, even after surgical anatomic joint reduction [25, 37].
However, although mechanical overload was assuredly a risk factor in the development of ankle OA, it was insufficient on its own to explain why degradation of cartilage also occurred in non-overload affected areas of cartilage. For instance, osteoporosis (OR1.58, CI0.21-1.5) and chronic disease subjects, such as diabetes (OR0.31, CI0.13-0.75) and hypertension (OR0.42, CI0.17-0.42), seemed to sustain more severe types of ankle OA in our study. The majority of patients with osteoporosis in our cohort could be explained by the mechanism of bone defects caused by long-term post-traumatic osteomyelitis (> 24 months), which had been treated for sufficient time before bone transport was initiated. This may make a mess of the microenvironment for bone regeneration and soft tissue coverage, which then delayed docking site union and bone mineralization. In Hannah et al. research, researchers found that natural changes were observed in chronic diseases patients’ ankle included structural change of type II collagen and a decreased capacity of chondrocytes to repair the damaged extracellular matrix, accompanied by the progression of diabetes and hypertension [27, 30]. The ankle OA was caused probably by this condition of local high glucose consistency and vascular resistance reduction of chondrocyte differentiation, then the potential cartilage regeneration was undermined. Clinical outcomes from this study may stand by that the chondrocytes in chronic disease patients’ cartilage may be less effective in repairing articular surface damage.
AOFAS ankle-HF Scale scores had been reported to have a greater capacity for diagnosing the degeneration of ankle function [19, 38]. Likewise, patients with high scores (OR1.26, CI1.18-1.34) had a greater risk of obtaining ankle OA in our observation. Therefore, despite the biological mechanism on ankle cartilage degeneration is complex, clinicians should pay more attention to the patients who acquired high scores and make good use of this accessorial tool.
Further, the relationship was also observed between the number of risk factors and risk categories according to logistic regression analysis based on the scale scores. In the presence of 3 or more risk factors, the risk of developing ankle OA appears to increase notably and continued to rise with the number of factors (Fig. 1). More than half of the patients (153 of 236) fell into two or fewer risk factors category with the occurrence rate of ankle OA of 22–47 %, the risk factors of 3 or more (35/236) were disclosed at a higher rate of 53–66 %.
Yet, certain limitations existed in our study. Firstly, given its retrospective nature, small sample size, and exclude the patients older than 60 years, caution should be taken regarding the interpretation of our analytical results. The actual rate of ankle OA could be higher (some ankle insidious pain may not be brought to the attention of patients). Secondly, insufficient postoperative management instruction could give rise to higher numbers of ankle OA due to the complex nature of the postoperative rehabilitation of the Ilizarov technique, and more confounding factors coming into play. Thirdly, to get control of the limitations of our study, further studies with larger sample size, longer follow-up period, and multicenter design should be designed.