Study data presented at the 2014 annual meeting of the American Orthopaedic Society for Sports Medicine indicate that anterior cruciate ligament (ACL) graft-bone tunnel healing is sensitive to changes in graft forces from knee motion. The paper, “The Effect of Dynamic Changes in ACL Graft Force on Soft Tissue ACL Graft-Tunnel Incorporation,” won the 2014 Aircast Award for Basic Science.

“We have very little understanding of how the knee’s mechanical environment and postoperative graft forces affect ACL graft healing after reconstruction,” according to lead author Richard Ma, MD, of the Missouri Orthopaedic Institute in Columbia, Mo. “Using a small-animal ACL reconstruction model, we found that soft-tissue ACL grafts that experienced higher graft forces have inferior early healing. Maintaining ACL graft isometry allowed one to avoid high graft forces and start early knee motion without affecting postoperative healing,” he said.

Study design
Dr. Ma and his colleagues performed unilateral ACL resection and soft-tissue autograft reconstruction on 100 male Sprague-Dawley rats. The rats were divided into the following groups:

• ACL reconstruction followed by knee immobilization for the duration of the study
• ACL reconstruction with a high-tension ACL graft that experienced an elevated graft force with daily knee motion
• ACL reconstruction with an isometric ACL graft that experienced minimal graft force with daily knee motion

Dr. Ma and his team were able to confirm the ACL graft isometry at the time of the ACL surgery. Intraoperative ACL graft force measurements showed a 1.6-fold increase in ACL graft force between the high-tension and isometric ACL grafts with intraoperative knee range of motion.

External fixators were then placed on each knee to eliminate external ACL graft loading during animal cage activity after surgery. Three days following surgery, the rats allocated to the motion groups began daily knee range of motion using a custom computerized knee flexion device. Biomechanical, microcomputed tomography, and histologic analyses were performed to assess the quality of ACL graft-tunnel healing at 3 and 6 weeks after surgery. “High ACL graft forces that occurred with early daily knee motion appeared to be deleterious to early ACL graft-tunnel incorporation,” said Dr. Ma.

Findings, conclusions
At 3 weeks after surgery, the load to failure for knees in the high-tension ACL graft group was significantly lower than in the isometric graft group (P = 0.01) and the immobilized group (P < 0.01). The isometric graft and immobilized groups also had greater mineralization around the ACL graft compared to the high-tension ACL graft group at both 3- and 6-week time points. Histologically, greater cellularity and collagen fiber gapping, particularly within the tibial tunnel, were seen in the ACL grafts that underwent motion. In addition, the researchers noted a higher prevalence of osteoclasts along the graft tunnel interface in high-tension ACL grafts.

Although the current study is a basic science study, it does raise clinical questions, particularly on current ACL reconstruction techniques. “We know that current anatomic ACL reconstructions—including anatomic single-bundle or even double-bundle techniques—result in some loss of ACL graft isometry. This likely translates to elevated ACL graft forces or strain once the operative knee begins to move,” said Dr. Ma.

“If this occurs, our study suggests it may impair the early bonding process between the ACL graft and bone socket,” he continued. “Our findings on the importance of ACL graft isometry when starting early knee motion, therefore, may have clinical implications for postoperative rehabilitation with modern anatomic ACL reconstructions.”

Dr. Ma’s coauthors are Michael Schaer, MD; Tina Chen, MS; Marco Sisto, BS; Clifford Voigt, MD; Joseph Nguyen, MPH; Lilly Ying, BS; Xiang-Hua Deng, MD; and Scott A. Rodeo, MD.

Disclosure information: Dr. Rodeo—Smith & Nephew, CAYENNE; Drs. Ma, Schaer, Voigt, Deng, Mr. Sisto, Mr. Nguyen, and Ms. Ying—no conflicts; Ms. Chen—no information available.

Maureen Leahy is assistant managing editor of AAOS Now. She can be reached at leahy@aaos.org

Bottom Line

• This basic science study demonstrated that the ACL graft-bone tunnel incorporation process is sensitive to changing graft forces from knee motion.
• ACL soft-tissue grafts that experienced higher ACL graft force and strain had inferior early healing properties.
• Maintaining ACL graft isometry during surgery may enable early postoperative knee range of motion without deleterious healing consequences.
• Additional studies are needed to evaluate the impact of post-ACL reconstruction mechanics on graft healing.