Effect of extramedullary fixation under different biomechanical modes on osseointegration of short-stem intramedullary implants: an experimental study

The reconstruction of extreme long bone defects after malignant tumor resection remains challenging. Short-stem intramedullary implants offer a solution but are often limited by mechanical instability from suboptimal extramedullary fixation. This study assessed how extramedullary low elastic modulus versus high elastic modulus fixation affects osseointegration of these implants by using animal experiments, providing evidence to refine reconstruction techniques. Twenty four rabbits were randomized into LF (low elastic modulus fixation) and HF (high elastic modulus fixation) groups. All received 3D-printed custom short-stem intramedullary implants, differing only in the elastic modulus of the extramedullary fixation component. Weekly x-rays monitored implant stability and complications. Animals were euthanized at 6 and 12 weeks for CT and histological analysis to quantitatively assess callus formation and bone ingrowth into implant threads. X-rays confirmed stable fixation in all animals throughout the observation period. Subsequent CT quantification at 12 weeks demonstrated that the LF group exhibited a significantly greater percentage of callus length relative to total prosthesis length (LF: 55.7 ± 11.4% vs HF: 32.9 ± 8.8%,<i>p</i>< 0.05) and percentage of prosthesis surface wrapped by external callus (LF: 60.8 ± 5.4% vs HF: 31.0 ± 4.7%,<i>p</i>< 0.001). Histological analysis at 12 weeks further confirmed superior osseointegration in the LF group, as evidenced by significantly higher percentage of new bone area in callus relative to predefined thread gap area (LF: 51.8 ± 9.6% vs HF: 7.3 ± 4.1%;<i>p</i>< 0.001), greater percentage of callus-thread edge contact length relative to total thread edge length (LF: 54.1 ± 17.1% vs HF: 7.4 ± 5.7%;<i>p</i>< 0.001), and increased callus penetration depth (LF: 1.637 ± 0.169 mm vs HF: 0.635 ± 0.447 mm;<i>p</i>< 0.05). The Intra-Extramedullary Combined Reconstruction Technique is a reliable method for major bone defect reconstruction. This study provides direct comparative evidence that low elastic modulus extramedullary fixation, unlike its high modulus counterpart, significantly enhances peri-implant callus formation and osseointegration. This represents a novel and important refinement in reconstruction strategy, contributing directly to the potential for long-term implant stability.