To the best of our knowledge, this study describes the largest series of children with CPMBT. To date, about 200 cases of CPMBT have been reported in the available literature [1–5,7,8,14–18]. For years, CPMBT has been considered a benign self-solving condition, due to the virtually absent risk of fracture or pseudarthrosis and the natural tendency to the spontaneous resolution, with minimal residual deformity. Nonetheless, we noticed that the spontaneous correction of the bowing is sometimes incomplete, the LLD is frequently wide and a residual ankle valgus may persist at the end of growth. We found some interesting correlation between the AP-IPA and the L-IPA, and between the AP-IPA and the LLD%. In other words, the greater is the angular deformity at birth, the wider will be the LLD at skeletal maturity. This finding is confirmed by previous reports [2,3,8,18].
The treatment of CPMBT is still controversial. Given the tendency to the spontaneous correction of the deformity, some authors suggest conservative treatment consisting of manipulation, serial casting, orthoses and shoe lifts; then, a limb equalization is proposed during late childhood, if needed [4,7,14]. Currently, there is no evidence that the use of braces and orthoses may improve the angular correction in CPMBT, since it occurs spontaneously and independently by the use of orthoses. The main goal of braces and orthoses is to aid walking and balance, while the patient is too young for the surgical treatment.
There are three reasons for surgical intervention in CPMBT: equalize the LLD, correct the ankle valgus and correct the residual bowing of the tibia [2]. Nonetheless, there are no clearly defined guidelines for the surgical treatment of CPMBT.
Regarding the LLD, we found that the final discrepancy at skeletal maturity averaged 4.3 cm, corresponding to 13% of the length of the unaffected leg; this percentage, remains rather constant during growth, as confirmed by the majority of the previous reports [2–5,18]. In our series, all the children who reached skeletal maturity underwent limb lengthening by circular external fixator while only one child underwent contralateral epiphysiodesis. Whether contralateral epiphysiodesis has been recommended in CPMBT, due to the lower risk of complications [3], compared to limb lengthening, aesthetical issues can raise due to the loss of body height. Moreover, recent concern has mounted regarding the potential risk of compromising the morphology of the proximal tibia, when a large, congenital LLD must be addressed [19,20]. Therefore, we suggest to reserve this treatment only for children in which the LLD% at 10–12 years is less than 10%, corresponding about 2 to 3 cm. Regarding the tibial bowing, the majority of cases improved spontaneously by the end of growth. Our behaviour consisted in a “waiting strategy”, using braces and orthoses until skeletal maturity, then, correcting in a single stage the length and the potential residual bowing. In our opinion, this strategy should reduce the risks for the patient and the costs for the health service. Nonetheless, a more pronounced reduction of the angular deformity was noticed during the first six years of life; thereafter, the rate of spontaneous correction showed a marked reduction. Other authors reported that, in CPMBT, the greatest rate of correction is observed during the first year of life, and rapidly decreases until the age of four [2–4,15,18]. This aspect may have practical implications, because an extreme bowing of tibia in a school-age child might hamper even the possibility of using braces to aid walking. In this scenario, some authors suggested early corrective osteotomy at the apex of the deformity, by the age of 3 to 6 years [2–4,15,16]. It has been argued that the intense periosteal activity at this age allows for early bone healing of the diaphyseal osteotomy; furthermore, the overgrowth of the tibia due to the physeal stimulation and the tibial straightening could potentially contribute to the leg length equalization [15]. We treated only one case by early corrective osteotomy of the bowed tibia: although we achieved a rapid healing of the osteotomy and a perfect alignment of the tibia, we observed a progressive partial relapse of the bowing, an important ankle valgus, while the LLD remained unchanged. These finding are consistent with those reported by Johari et al. [4], suggesting that the early tibial osteotomy should be proposed only in case of severe, disabling bowing, as an intermediate treatment, to avoid complex bracing and allow walking with simple foot orthosis or shoe lift. Another possible reason to recommend early tibial osteotomy, is given by the possibility to perform intramedullary lengthening by telescopic nails at skeletal maturity [21]. This technique has been reported as safe, effective and more tolerated by the patients, in comparison with external fixation. Nevertheless, the procedure is more simple, safe and effective in a straight, rather than a bowed tibia. Yet, simultaneous correction of the bowing and lengthening by circular external fixation is not simple, requires high compliance by the child and the parents, high proficiency with the technique and it has high risk of complications. In our series, we experienced a relevant rate of moderate and severe complications, in line with other reports: Kaufman et al.[5] reported 17 mild to severe complications in 11 CPMBT treated by external fixation; Johari et al.[4] described complications in all the 6 cases treated by external fixation; Wright et al.[18] reported 16 complications in 17 children treated by external fixation. Moreover, we suggest to perform the lengthening procedures closer to the skeletal maturity, since we did not experience any recurrence of the limb length inequality, in contrast with other authors, which reported recurrence of the limb length inequality, if the lengthening procedure was performed during growth [2–5,18].
Finally, in our series about one third of children with CPMBT presented a valgus ankle by the end of growth (LDTA < 85°). This issue has been reported previously [2,4]. Although the normal range of the LDTA has been established [11,12], an exact cut-off to define a pathologic deformity has not been clearly defined, with proposed values varying from 5° to 10° of valgus [20–23]. In our series, only one patient underwent distal tibial hemiepiphysiodesis to treat ankle valgus. Nonetheless, it is our opinion that, if at age of 10–11 years, the valgus inclination of the distal tibial articular surface persists, a medial distal tibial hemiepiphysiodesis may be performed, when sufficient growth potential is still present [18–19]. This simple procedure can effectively realign the ankle, minimizing the complications [22–27] and avoiding demanding realignment procedures (double or triple corticotomies, complex external fixation constructs) at the end of growth.
Although our study describes the largest series of CPMBT in the available literature, some limitations must be underlined. We conducted a retrospective analysis of cases collected across more than 40 years, sometimes with incomplete information and missing data; moreover, 12 cases out of 44 had no sufficient radiographic follow-up available for the evaluation of the progression of the deformity. In addition, the cases were not collected uniformly at birth and 16 children had the first radiographic evaluation when they were older than 5 years. Finally, almost a half of cases did not reach the definitive treatment. These issues, were reported in most of the previously published case series describing CPMBT [2–5,18], emphasizing the difficulty to achieve complete information about ultra-rare developmental pathologies, in which at most one case per year can be collected, even in the highly specialized referral institutions. In order to address this issues and minimize biases, we used mixed effect models, a complex statistical approach that allows to maximize the prediction power from small and heterogeneous groups of subjects with missing data; nonetheless, we are aware of potentially biased results as well as a loss of statistical power and precision.