HISTOMORPHOMETRY OF DISTRACTION OSTEOGENESIS IN A CAPRINE TIBIAL LENGTHENING MODEL

Standardized histomorphometry of bone formation and remodelling during distraction osteogenesis (DO) has not been well characterized, Increa sing the rhythm or number of incremental lengthenings performed per da y is reported to enhance bone formation during limb lengthening, In 17 skeletally immature goats, unilateral tibial lengthenings to 20 or 30 % of original length were performed at a rate of 0.75 mm/day and rhyth ms of 1, 4, or 720 times per day using standard Ilizarov external fixa tion and an autodistractor system, Two additional animals underwent fr ame application and osteotomy without lengthening and served as osteot omy healing controls, Histomorphometric indices were measured at prede termined regions from undecalcified tibial specimens, Within the distr action region, bone formation and remodelling activity were location d ependent, Intramembranous bone formed linearly oriented columns of int erconnecting trabecular plates of woven and lamellar type bone. Total new bone volume and bone formation indices were significantly increase d within the distraction and osteotomy callus regions (Tb.BV/TV, 226% [p < 0.05]; BFR/BS, 235-650% [p < 0.01]) respectively, compared with c ontrol metaphyseal bone, Bone formation indices were greatest adjacent to the mineralization zones at the center of the distraction gap; min eral apposition rate 96% (p < 0.01); mineralized bone surfaces 277% [p < 0.001]); osteoblast surfaces 359% [p < 0.001]); and bone formation rate (650% [p < 0.01]), There was no significant difference (p < 0.14; R = 0.4) in the bone formation rate of the distracted callus compared with the osteotomy control callus. Within the original cortices of th e lengthened tibiae, bone remodelling indices were significantly incre ased compared with osteotomy controls; activation frequency (200% [p < 0.05]); osteoclast surfaces (295% [p < 0.01]); erosion period (75%); porosity (240% [p < 0.001]). Neither the rhythm of distraction nor the percent lengthening appeared to significantly influence any morphomet ric parameter evaluated. Distraction osteogenesis shares many features of normal fracture gap healing, The enhanced bone formation and remod elling appeared to result more hom increased recruitment and activatio n of bone forming and resorbing cells rather than from an increased le vel of individual cellular activity.