Inferior strength reported with single-strand Achilles constructs for ACL tibial fixation

Cyclic displacement showed 4 mm of creep in Achilles model vs. <1 mm for other constructs.

Research indicates that single-stranded Achilles tendon constructs have poor biomechanical properties for ACL tibial fixation compared to two- and four-stranded constructs, and investigators warn that a secondary fixation may be needed when an Achilles allograft is used.

To determine the biomechanical properties of different soft tissue ACL allografts for tibial fixation and whether tibial fixation devices are graft-specific, LCDR Matthew T. Provencher, MD, MC, USN, and colleagues randomized 64 porcine tibiae to receive three different graft types using three different fixation devices.

The graft constructs consisted of a single-stranded Achilles tendon, a two-stranded posterior tibialis and a four-stranded semitendinous graft. The fixation devices included an interference screw (Delta, Arthrex), an osteoconductive interference screw (Calaxo, Smith & Nephew) and the Intrafix device (Mitek).

“The Intrafix device was designed to accommodate four graft limbs in separate channels and it was the hypothesis that all devices might not perform as well with either two or one limb,” Provencher told Orthopedics Today.

Not equal

The investigators discovered that the single-stranded Achilles tendon construct showed significantly greater mean cyclic displacement, lower overall stiffness and a lower load-to-failure rate compared to the other constructs. While they found no significant biomechanical differences for each graft type among the devices, they found that the single-stranded Achilles construct demonstrated the worst results.

“Not all soft tissue graft types demonstrated equal tibial fixation,” Provencher said during his presentation at the American Orthopaedic Society for Sports Medicine 2008 Annual Meeting. “The one-stranded Achilles demonstrated the poorest stiffness and strength, and two- and four-stranded [constructs] provided excellent fixation properties. The Intrafix demonstrated the best performance with a four-stranded construct.

“[Tibial] fixation for Achilles allograft should probably be investigated further, and backup fixation may be warranted when using this as a soft tissue graft on the tibial side,” he said.

Study procedure

The investigators performed bone mineral density scans on the specimens prior to surgery and prepped the grafts to mirror a clinical scenario. They drilled the tunnels to the graft diameter in 0.5 mm increments.

In fixation, all of the screws were made one size larger than the tunnel and the Intrafix device was the largest available, the 8- to 10-mm size, Provencher said.

After preconditioning, the constructs underwent cyclic loading to test for creep, displacement and cyclic stiffness. The investigators also tested pullout stiffness and load to failure, he added.

Limitations

They discovered significantly greater overall stiffness in the two- and four-stranded constructs as well as a greater load to failure for the multi-stranded constructs compared to the Achilles tendon model. The overall cyclic displacement showed about 4 mm of creep in the Achilles model compared to less than 1 mm for the other constructs.

“[For each graft type], there were no significant differences among all devices,” Provencher said. “However, the Achilles performed poorly among all of them. However, the Intrafix was slightly better for four-stranded versus a two-stranded and one-stranded construct.”

They also found that both of the multiple-stranded constructs performed well with both the interference and Intrafix device.

“The four-strand was the best biomechanical construct [with Intrafix] and was comparable to prior studies of only one graft type in terms of pullout and stiffness,” Provencher said.

He cited the time-zero study design as a study limitation. “We attempted to reproduce a clinical scenario which is not always possible in the lab and we did not test backout fixation,” Provencher said.

For more information:
LCDR Matthew T. Provencher, MD, MC, USN, can be reached at the U.S. Naval Medical Center, San Diego, Dept. of Orthopaedic Surgery, 34800 Bob Wilson Drive, San Diego, CA 92134-112; 619-532-8427; e-mail: matthew.provencher@med.navy.mil. He has no direct financial interest in any product or company mentioned in this article.
The views expressed in this article are those of the study authors and do not reflect the official policy or position of the Department of the Navy, Department of Defense, or the United States government.

Reference:
Fogel H, Park D, Verma N, et al. Biomechanical analysis of soft tissue ACL tibial fixation: Comparison of one-, two-, and four-stranded allograft constructs. Presented at the American Orthopaedic Society for Sports Medicine 2008 Annual Meeting. July 10-13, 2008. Orlando, Fla.