A Novel Calibration Method for Long Bone Lengthening with Intramedullary Magnetic Nails

Background:
The PRECICE® intramedullary lengthening nail enables gradual distraction of the bone, and has been used for thousands of cases since 2011. This study aimed to improve the accuracy of obtaining the actual distraction gap on standard X-rays with the PRECICE® magnetic nail as a calibration tool.

Methods:
A retrospective review of all femoral lengthening procedures with the PRECICE® antegrade intramedullary lengthening nail was carried out between 2013 to 2018. A novel parameter, the calibration ratio (CR), was introduced in order to calculate the calibrated length of the distraction gap (CDG), defined by the measured distraction gap (MDG) multiplied by the CR (CDG = MDGxCR). The MDG and CDG were compared to the planned distraction gap (PDG). The actual nail length (ANL) was measured on all available extracted nails and compared to the PDG, MDG and CDG. All measurements were performed with Trauma Cad® software applied by a consultant orthopedic surgeon.

Results:
Thirty-one femurs were lengthened in 30 patients (mean age at surgery 17.1years, 16 males). The average amount of lengthening was 4.4 cm (range, 2.4 to 7.9 cm). There was a significant difference between the PDG (45.5 mm) and the MDG (47.8 mm), t(29) = 1.99, p = 0.06. There was no significant difference between the PDG (45.5 mm) and CDG (44 mm), t (30) = -2.72, p = 0.01. The ANL and the MDG and PDG differed significantly, t (14) = -4.71, p = 0.000 and t (14) = -2.53, p = 0.024, respectively, while the ANL and the CDG did not, t (14) = -2.11, p > 0.05.

Conclusions:
The CDG adjusted by the CR more accurately correlated with the PDG and the number of days of lengthening than the non-calibrated MDG. More over , comparing the ANL on extracted nailsThe CDG was more accurate than the PDG and MDG. This calibration technique is simple and accurate, and does not require special tools.