Summary

A percutaneous method for impaction bone grafting using structural chain-milled, partially-demineralized allograft.

Abstract

Introduction

Open reduction and internal fixation for fractures of the subchondral bone (including articular impaction fractures, insufficiency fractures, and early-collapse in avascular necrosis) is difficult or impossible in difficult-to-reach anatomic regions. Indirect reduction methods utilizing calcium phosphate cementation has been tried with a near 50% failure rate and 22% conversion to arthroplasty. Subchondral polymethyl methacrylate cementation has similar failure possibly due to iatrogenic stiffness or thermal injury. Open impaction bone grafting has been employed with success but has associated surgical approach morbidity. Here we present the first cadaveric series demonstrating fracture reduction by percutaneous impaction grafting using chain-milled partially-demineralized allograft bone.

Methods

Thirteen intact cadaveric proximal femur specimens were used for this study. A 2 cm diameter by 2 cm deep osteochondral core impaction defect was created, with a 1cm deep portion of the bone core morselized to simulate failure of deep subchondral bone with an intact osteochondral cap. Linear force-control loading (control) was performed before allograft impaction grafting using 800N approximating body mass (MTS 244.21, MTS Systems Corp, Eden Prairie, MN) with the resultant stiffness and displacement recorded. A metaphyseal-entry retrograde fluoroscopic approach using a 5mm cannula was used (VBA set, LENOSS Medical, Bristol, RI) to perform percutaneous allograft impaction grafting to effect fracture reduction. 2cc of chain-milled partially-demineralized allograft bone was used per cadaveric specimen (OsteoPearl by LENOSS, MTF Biologics, Edison, NJ). Linear loading was repeated using the same force-control protocol (800N) to test post-grafting stiffness and displacement. Surface volumetric displacement was evaluated by 3D surface scanning (Creaform, AMETEK, Berwyn, PA) on the femoral head surface before and after grafting. Baseline and post-grafting computed tomography (CT) was performed to evaluate 3D defect filling.

Results

All specimens successfully underwent percutaneous impaction grafting. Qualitatively, upon impaction of the chain-milled allograft, which softens with 1 minute of hydration, graft material uniformly fills the bony defect apparently following paths of least resistance. Near-anatomic and powerful reduction of the osteochondral cap was experienced in all specimens utilizing 2cc of chain-milled allograft. No leakage of the graft material outside of the femoral head was encountered. Subsequent to impaction grafting, the osteochondral cap was biomechanically tested demonstrating a 2.4-fold increase in stiffness as compared to control and importantly, immediate elastic re-expansion of the repaired elements to reproducibly achieve less than 2 mm of fracture stepoff by 3D surface and CT analysis.

Conclusion

This is a cadaveric study demonstrating biomechanical advantages of subchondral impaction grafting using chain-milled, partially-demineralized allograft bone through a percutaneous approach. Unlike subchondral cementation, impaction grafting has the apparent benefits of: 1. Ability to generate mechanical power to effect fracture reduction; 2. Graft tends to follow paths of least resistance to reinforce a fracture plane; 3. No-leakage; and 4. Subchondral mechanical reinforcement of the osteochondral fragment. Although not shown in this study, the likelihood for superior graft incorporation, and the possibility of utilizing the hydrating properties of chain-milled allograft permitting marrow aspirate or drug delivery, enables partially-demineralized chain-milled allograft impaction grafting as a prime method for the treatment of peri-articular and subchondral fractures.