Mesenchymal stem cell-enhanced ACL grafts performed better

At four weeks, treated grafts had much higher ultimate load-to-failure.

Issue: August 2003

Hamstring grafts for anterior cruciate ligament reconstruction treated with mesenchymal stem cells were 46% stiffer than control grafts in an animal model.

While studies have been done to investigate the use of bone morphogenetic proteins (BMPs) in enhancing the healing of tendon grafts to bone tunnels, this is the first to examine the use of mesenchymal stem cells (MSCs), researchers said.

“We have demonstrated that mesenchymal stem cell-enhanced ACL (anterior cruciate ligament) grafts heal with a graft-tunnel junction that is more similar to the ACL chondral enthesis,” said Jit-Kheng Lim, MD, of the Tissue Engineering Lab at the National University of Singapore.

“We show that mesenchymal stem cell-enhanced ACL grafts were overall 46% stiffer compared to controls, and this improved at four and eight weeks to almost two times that of controls. This compares very favorably with the two papers that used bone morphogenetic proteins, both of which demonstrated improved biomechanical properties,” he said.

Faster return to activity?

“This may allow a faster return to activity,” Lim told Orthopedics Today, “which is in keeping with the trend toward earlier and more aggressive accelerated rehabilitation programs after ACL surgery.”

Bilateral ACL reconstructions were performed in 42 adult rabbits with hamstring tendon autografts. In one limb the graft was coated with MSCs in a fibrin glue carrier; the other limb served as a control and the graft was coated with fibrin glue only. The test limbs were randomized and the surgeon was blinded to the MSC-treated limb; the reconstructions were assessed at two, four and eight weeks.

Samples were gathered from 12 rabbits and histological analysis was done using hematoxylin/eosin and safranin-O staining. They were also analyzed for collagen types I, II and III by immunohistochemistry.

A blinded observer performed biomechanical testing of the ACL grafts for stress, strain and elastic modulus in the other 30 rabbits. Another six rabbits were used for histological and biomechanical testing of the normal intact ACL.

Lim said the results in the control group were not unexpected; at four and eight weeks mature scar tissue formed at the interface between the bone and the tendon graft. In the MSC-treated group, at four weeks immature cartilage cells were present and there was an intermediate zone of cells blending into the tendon graft substance.

Distinct fibrocartilage

“By eight weeks we could see a distinct zone of fibrocartilage from bone through fibrocartilage and into tendon substance,” Lim said. This was a zone of type II collagen between the type I and type III collagen of the tendon substance and bone on either side of the graft, according to immunohistochemical analysis. The findings were similar to those in the specimens of animals with intact ACLs.

The MSC-enhanced grafts were similar to controls at two weeks in biomechanical testing, but by four and eight weeks, the MSC grafts had much higher ultimate load-to-failure compared to controls (P<.05), Lim said.

For more information:

Lim J, Hui J, Li L, et al. Enhancement of tendon graft osteointegration using mesenchymal stem cells in anterior cruciate ligament reconstruction. #SS-41. Presented at the Arthroscopy Association of North America 22nd Annual Meeting. April 24-27, 2003. Phoenix.