Comparison of bilateral tibial lengthening with circular external fixator and simultaneous deformity correction with hexapod external fixator in Achondroplasia

Achondroplasia is a growth plate dysplasia caused by FGFR-3 gene mutation. In addition to dysmorphic short stature, genu varum is very common. Therefore, a certain part of the achondroplasic population is interested in extensive cosmetic limb lengthening and deformity correction to achieve a functional height so that they can attain total autonomy in everyday life. All patients who underwent bone lengthening treatment from March 2014 to December 2022, were retrospectively analyzed in terms of inclusion and exclusion criteria. To determine any angular deformity in the coronal plane, the Mechanical Axis Deviation was calculated. Patients were divided into two groups: group A included patients who had no angular deformity and therefore performed exclusively cosmetic bilateral tibial lengthening, group B included patients undergoing cosmetic lengthening and simultaneous bilateral multiplanar correction of tibial deformities with hexapod external fixator. The achieved distraction, the external fixator index and the consolidation index were calculated and compared for the two groups. Any adverse event was recorded. According to inclusion and exclusion criteria, 14 patients (28 tibias) diagnosed with Achondroplasia were included (20 tibias group A and 8 tibias group B). We found a statistically significant difference between the two groups regarding the mean age at the time of surgery (p 0.01). The mean MAD value within group B showed a substantial improvement from 56.50 ± 19.1 mm to 14.6 ± 5 mm. Comparing the two groups, we observed a statistically significant difference between both preoperative (p 0.0001) and postoperative (p 0.006) MAD values. The difference found between the two groups was significant also in terms of mean external fixator index (35.9 days/cm versus 52.5 days/cm) and consolidation index (24.2 days/cm versus 37.9 days/cm). Comparing the adverse events found in the two groups no significant differences emerged (p 0.7). In the context of cosmetic lower limb lengthening program for achondroplasic patients, most studies have focused on linear lengthening often underestimating the importance of any associated deformities correction. In our experience, the genu varum correction with the hexapod system showed relevant results in term of MAD improvement. Our results show a significant difference between the two groups in consolidation (p 0.002) and thus also in external fixation (p 0.001) time to the disadvantage of the hexapod group probably due to: higher age (p 0.01) with lower healing potential, greater difficulty in weightbearing and in dynamizing the fixator.