Subchondral bone remodeling: comparing nanofracture with microfracture. An ovine in vivo study

Abstract

Purpose: microfracture, providing direct stimulation of chondrogenic mesenchymal stem cells (MSCs) in the subchondral bone, remains the most frequently used primary cartilage repair technique. However, the newly formed type I collagen-rich fibrocartilaginous tissue has poor biomechanical properties and a tendency to degenerate. To overcome these limitations the nanofracture technique was introduced. Our purpose was to compare subchondral bone remodeling 6 months after microfracture versus nanofracture (subchondral needling) treatment in an ovine model.

Methods: full-thickness chondral lesions were created in the load-bearing area of the medial femoral condyles in four adult sheep. Each animal was then treated on one side with microfracture and on the contralateral side with nanofracture. Subchondral bone remodeling was assessed by micro-CT using a Bruker(®) SKYSCAN and CTVOX 2.7 software (Bruker Corp., Billerica, MA, USA) for image reconstruction; trabecular bone density measurements were performed through a color-representation structure thickness analysis.

Results: at the six-month endpoint, the microfracture-treated samples showed limited perforation depth and cone-shaped channels with large diameters at the joint surface. The channel walls displayed a high degree of regularity with significant trabecular bone compaction leading to a sealing effect with limited communication with the surrounding trabecular canals. Condyles treated with nanofracture showed channels characterized by greater depth and smaller diameters and natural irregularities of the channel walls, absence of trabecular compaction around the perforation, remarkable communication with trabecular canals, and neo-trabecular remodeling inside the channels.

Conclusions: nanofracture is an effective and innovative repair technique allowing deeper perforation into subchondral bone with less trabecular fragmentation and compaction when compared to microfracture; it results in better restoration of the normal subchondral bone architecture at six months.

Clinical relevance: our data support the use of smaller-diameter and deeper subchondral bone perforation for MSC stimulation; this technique may prove to be an attractive alternative to standard microfracture procedures.

Keywords: mesenchymal stem cell stimulation; micro-CT; microfracture; nanofracture; ovine; subchondral bone.

Fig. 1

A full-thickness chondral lesion in the load-bearing area of each medial femoral condyle was created using an arthroscopic burr.

Fig. 2

A: Treatment of the defect with microfracture. B: Treatment of the defect with nanofracture.

Fig. 3

A: The cone-shaped channels had large diameters at the joint surface with significant trabecular bone compaction. B: Limited communication of the channel with the surrounding trabecular canal-safter microfracture.

Fig. 4

Subchondral cysts ranging from 7 mm to 12 mm were found near the perforation after microfracture.

Fig. 5

A: Micro-CT scan of the condyles treated with nanofracture showing perforations with a greater depth and a smaller diameter compared to the perforations in the microfracture-treated ones, natural irregularities of the channel walls, and absence of trabecular compaction around the perforations. B: Remarkable communication between pre-existing trabecular canals and the perforation after nanofracture – Sagittal view. C: Remarkable communication between pre-existing trabecular canals and the perforation after nanofracture – Axial view.

Fig. 6

A: Color-representation structure analysis in the nanofracture-treated condyles. B: Less trabecular fragmentation around the channels as well as intra-channel bone remodeling with a trabecular structure remarkably similar to that of native subchondral bone.

Fig. 7

Immediate on-site formation of a super-clot after nanofracture.