Early dynamization by reduced fixation stiffness does not improve fracture healing in a rat femoral osteotomy model

Abstract

Dynamization of fracture fixation is used clinically to improve the bone healing process. However, the effect of early dynamization remains controversial. This study evaluated the effect of early dynamization, by reduced stiffness of fixation on callus stiffness and size after 5 weeks of healing in a rat diaphyseal femoral osteotomy. An external unilateral fixator allowed either a rigid (R-group; n ¼ 8) or a flexible (F-group; n ¼ 8) fixation. The dynamized group (D-group: n ¼ 8) had a rigid fixation for 1 week, and then a flexible fixation for the remaining 4 weeks. The pre-and postoperative activity of the rats was measured. After 5 weeks, the rats were sacrificed, and healing was evaluated by biomechanical and densitometric methods. The R-group had a higher activity more closely approaching preoperative levels, compared to the D-group throughout all time points measured. This difference was significant after 14 days and 21 days. The flexural rigidity of the R-group was 82% (tested in the anterior-posterior direction; p ¼ 0.01) and 93% (tested in the medial-lateral direction; p ¼ 0.002) greater than the flexural rigidity of the D-group. The rigid fixation led to a stiffer callus with a smaller callus volume, but better mineralized tissue in the whole callus and at the level of the osteotomy gap than the flexible or the dynamized fixation. Early dynamization did not improve healing compared to rigid or flexible fixation in a rat femoral osteotomy model. ß

Temporal Variation in Fixation Stiffness Affects Healing by Differential Cartilage Formation in a Rat Osteotomy Model

“…We divided 53 male Wistar rats (weight at operation, 414 ± 22 g) into five groups: stiff fixation (S group: n = 10), flexible fixation (F group: n = 11), dynamized at 1 week (D1 group: n = 10) [7], dynamized at 3 weeks (D3 group: n = 11), and dynamized at 4 weeks (D4 group: n = 11). The S and F groups had a stiff or a flexible fixation, respectively, for the entire 5-week experimental period (Fig.…”

“…These calculations assumed the Type I error rate would be controlled at 5% and the desired power of 80%. The resulting sample size calculations suggested 12 rats would likely provide sufficient power for the end point of bending rigidity data, which was the initial outcome performed on the samples before histologic analysis [7].…”

“…We previously found early dynamization leads to lower flexural rigidity of the healing bone [7] while late dynamization leads to flexural rigidity similar to continuously stiff fixation. However, these observations did not document how the tissue differentiation in the healing callus is affected by the time of dynamization.…”

“…Clinically, the mechanical environment can be altered through increasing the amount of interfragmentary movement (IFM) by reducing the fixation stiffness of interlocked nails [2,4,[38][39][40][41][42] or external fixators [12,17,28]. The IFM is also influenced by the load on the operated leg due to gravity and muscle forces [7,14,20,36,43]. For a similar loading condition, the fixation stiffness is the dominating factor for the IFM [14].…”

“…The recent development of well-controlled external fixation systems for the rat allows exploration of how mechanical stimulation in terms of the IFM can temporally influence the healing process [7,24,29,30,32,37].…”