Serial bone mineral density ratio measurement for fixator removal in tibia distraction osteogenesis and need of a supportive method using the pixel value ratio

Distraction osteogenesis is one of the common procedures for limb lengthening. However, attempts are being made constantly to establish objective guidelines for early and safe removal of a fixator using a sensitive and quantitative measurement technique. Dual-energy X-ray absorptiometry (DEXA) has been evaluated in the past for understanding callus stiffness, and the present study is a step further in this direction. The purpose of this study was to evaluate the correlation between bone mineral density ratio (BMDR) obtained by a DEXA scan and the pixel value ratio (PVR) on plain digital radiographs at each cortex and various callus pathways and callus shapes as described by Ru-Li's classification. A retrospective analysis of 40 tibial segments in 23 patients operated upon for various indications for limb lengthening was carried out. There were 11 male and 12 female patients with a mean age of 18 years. The Ilizarov method was applied after monofocal osteotomy, and distraction and consolidation were monitored using digital radiographs and DEXA scanning. BMDR was positively correlated with PVR, and the optimal BMDR for removal of the fixator was found to be 0.511. PVR of all cortices, except the anterior cortex, showed significant positive correlation with BMDR of the regenerate. There was good correlation between BMDR and PVR in the homogenous or heterogenous pathway according to callus shape and pathway. Thus, this study shows that BMD measurement can provide an objective and noninvasive method for assessing the rate of new bone formation during tibial distraction osteogenesis. It can thus function as an effective adjunct to measure callus stiffness, along with PVR, using digital radiographs, especially in cases in which callus maturation and stiffness is doubtful. Further studies especially dealing with callus progression through the lucent pathway as well as those dealing with regenerate fractures may be needed to conclusively prove the efficacy of this method for measurement of callus maturation.