Stiff, slip-resistant fixation maintains ACL graft tension best

Surgeon warns against using sutures alone without immobilizing the knee.

Stiff, slippage resistant fixation is the key to preventing ACL ligament grafts from losing tension and becoming lax, according to a surgeon speaking at ORTHOPEDICS TODAY NY 2005, a comprehensive CME course.

Improper fixation causes tension loss and laxity, said Stephen M. Howell, MD, an assistant professor at the University of California, Davis. He showed placement, tensioning and fixation techniques that have proved effective in preventing tension loss and laxity in ACL ligament grafts. He also demonstrated that cyclical loading, such as moving the knee and walking, causes the greatest loss if graft tension.

Loads, tension and laxity

Howell cited an earlier study by Douglas W. Jackson, MD, ORTHOPEDICS TODAY’s Chief Medical Editor, in which researchers froze a goat ACL in situ and rendered them acellular and avascular. Freezing did not affect tension because the ACL remained anatomically positioned and fixed. Mechanical testing of the animals’ knees showed no graft weakening or laxity at 6 and 12 months, suggesting that ACL graft strength loss may be due to improper tensioning or to tension loss, Howell said.

Cyclical loads such as walking and knee extension cause most tension loss, he noted. “The penalty from losing tension is a bit of an increase in laxity. The cause of it is slippage, so it's a fixation issue, primarily ...” he said.

Howell showed how applying 60 AP load cycles of 170 N sharply decreased interarticular tension. He found that friction inside the tibial tunnel, and inserting the fixation devices, reduced tension slightly. Spike washers, double staples, interference screws and WasherLoc produced different friction levels, tension loss and laxity rates under varied cyclical loads.

Limiting tension loss and anterior laxity depends on fixation choice as well as bone quality, Howell said. “[Slippage] of a soft tissue graft with an interference screw in a 17-year-old soccer player is going to be less than in a 40-year-old female that smokes with softer cancellous bone,” he said.

Stiff fixation sites

Howell stressed the need to use slippage-resistant, stiff fixation during surgery so early motion can be used to prevent stiffness and disuse.

“So, what’s the cause of this increase in anterior laxity and tension loss? ... It’s slippage at the sites of fixation and not creeps. You should not spend a lot of time tensioning your graft out. You should pretension the interface between the fixation device and graft ... .,” he said.

He noted a study in which laxity increased after 225,000 cycles, the equivalent of six weeks’ walking.

“If you use slippage resistant fixation devices that are also stiff devices, you have a better resistance to an increase in laxity,” he said. “I suggest ... that you consider the use of slippage-resistant, high-stiffness femoral and tibial fixation devices to compensate for the obligatory loss in graft tension that's going to occur when you tension the graft. This may help you minimize any increase in anterior laxity.”

Howell advised against tying sutures to a post because sutures are the “least stiff fixation method and are prone to slippage.” He also cautioned against using sutures alone without immobilizing the knee. In dealing with soft bone, he suggested “backing up” a cancellous device or using a single device to grip cortical bone, a stronger bone type.

Howell showed how a WasherLoc and bone dowel were as good as interference screws in restoring anterior laxity. He cited a study in which interference screws provided 340 N/mm stiffness. Tandem washers and screws yielded 414 N/mm stiffness. The WasherLoc and dowel produced 506 N/mm, Howell said.

“So, you don't have to fix the joint line to get good restoration of stability," Howell said. "You need to use high stiffness fixation methods.”

ORTHOPEDICS TODAY NY 2005 took place Nov. 19-20 in New York City.

For more information:
Howell S. Current thinking and concerns on tensioning the grafts. Presented at ORTHOPEDICS TODAY NY 2005 - A comprehensive CME course. Nov. 19-20, 2005. New York.