Published 4/1/2016
Jennie McKee

Award-Winning Research Focuses on ACL Reconstruction

Investigator honored for studies of graft placement under in vivo loading conditions
When Louis E. DeFrate, ScD, joined the department of orthopaedic surgery at Duke University Medical Center a decade ago, he and colleague William E. Garrett Jr, MD, PhD, discussed the relative lack of data on how anterior cruciate ligament (ACL) graft placement affects joint function under in vivo loading conditions. Soon after, with the assistance of radiologist Charles E. Spritzer, MD, Dr. DeFrate developed groundbreaking new strategies for assessing ACL graft placement and its effects on joint motion and cartilage thickness.

"The findings of my studies have directly influenced clinical practice at Duke University and at other academic centers nationally," wrote Dr. DeFrate, the Frank H. Bassett Endowed Chair and associate professor of orthopaedic surgery, biomedical engineering, and mechanical engineering and materials science at Duke University. Dr. DeFrate received the 2016 Kappa Delta Young Investigator Award at the AAOS Annual Meeting for the extensive research summarized in his paper, "The effects of ACL graft placement on in vivo knee function and cartilage thickness distributions."

Studying ACL reconstruction outcomes
According to Dr. DeFrate, a great deal of research has found good short-term outcomes after ACL reconstruction, but data on long-term outcomes paint a less-clear picture. Long-term studies have noted that some patients sustain a high incidence of joint degeneration after undergoing the procedure. In addition, osteoarthritis remains a significant long-term concern. Researchers have suggested that altered joint kinematics, among other mechanisms, likely contribute to joint degeneration after a patient sustains an ACL injury.

"One important factor that may affect the ability of a reconstruction to restore normal function is anatomic placement of the ACL graft," asserted Dr. DeFrate. "In particular, investigators have reported that a common problem is nonanatomic placement of the graft on the femur, with the most frequent error being anterior placement of the femoral tunnel."

Despite concerns regarding the important role of graft placement in subsequent joint function, few researchers have evaluated the effects of graft placement on joint motion under in vivo loading conditions. Dr. DeFrate designed a series of studies to obtain data on this important area.

Analyzing two surgical techniques
Dr. DeFrate first focused on two different reconstruction techniques and their ability to achieve anatomic femoral placement. The two reconstruction techniques considered were a single-incision transtibial technique, in which the femoral tunnel is placed through the tibial tunnel, and a two-incision transtibial tunnel independent technique, in which the graft is placed independently of the tibial tunnel.

To evaluate these techniques, Drs. Spritzer and DeFrate used MRI and three-dimensional (3D) modeling to quantify the placement of the graft relative to the native ACL footprint. Specifically, the researchers used an innovative 3D modeling technique to visualize the native ACL anatomy of the contralateral knee, thus removing the need to use bony landmarks to determine graft placement. In a study of 16 patients, half of whom had undergone a unilateral transtibial ACL reconstruction and half of whom had undergone a two-incision tibial tunnel independent ACL reconstruction, the investigators measured the ability of the two reconstruction techniques to place the femoral tunnel anatomically relative to the ACL footprint. Findings indicated that the transtibial technique resulted in graft placement farther from the center of the native ACL attachment site. It was also linked to greater variability compared to the tibial tunnel independent technique, in which the femoral tunnel was placed anatomically on the femur.

"The transtibial technique resulted in a more anterior and superior placement of the tunnel compared to the tibial tunnel independent technique," wrote Dr. DeFrate, noting that the findings suggest that "the position and orientation of the tibial tunnel may constrain placement of the femoral tunnel when transtibial techniques are used.

"Great care should be taken when performing transtibial techniques to ensure that anatomic graft placement is being achieved," he cautioned.

Restoring knee motion
Next, the researchers measured how graft placement affects the ability of ACL reconstruction to restore native joint kinematics as well as ACL length and orientation in patients under in vivo weight-bearing loading conditions.

The researchers recruited the same patients who had participated in the earlier study and several additional patients, for a total of 22 patients who had undergone unilateral ACL reconstruction. Of those, 12 patients received transtibial reconstructions resulting in anteroproximal placement of the graft on the femur, while 10 patients underwent tibial tunnel independent reconstructions, resulting in anatomic graft placement. Subjects underwent multiplanar, high-resolution MRI of the injured knee as well as the intact knee to create 3D models. Participants performed a single leg lunge, during which the investigators used biplanar fluoroscopy to record each subject's knee motion. Using the fluoroscopic images and 3D models, the researchers evaluated the length and orientation of the ACL and graft as well as the motion of the knee.

These results, noted Dr. DeFrate, suggest that grafts placed anatomically more closely reproduce the orientation and length of the native ACL during in vivo knee function. As a result, they are more effective in restoring normal knee function. Grafts placed anteroproximally, however, result in a more vertical orientation that does not sufficiently restrain motions in the transverse plane.

Maintaining normal cartilage thickness
Finally, the investigators developed a novel method for identifying whether anatomic graft placement plays an important role in maintaining normal cartilage thickness after ACL reconstruction. They studied all 10 subjects from the previously studied anatomic placement group and all 12 subjects from the previously studied anteroproximal graft placement group. After evaluating all patients with sagittal MRI, the investigators segmented the articular surface of the cartilage on the images to create 3D models and measured cartilage thickness distributions across the tibiofemoral joint.

Patients who had undergone anteroproximal graft placement had significantly decreased cartilage thickness along the lateral aspect of the medial condyle in the reconstructed knee, compared to the intact knee. Patients who underwent anatomic graft placement, however, had no statistically significant differences in cartilage thickness between the reconstructed and intact knees.

Dr. DeFrate concluded that abnormal motion associated with anteroproximal graft placement on the femur may affect normal cartilage loading and lead to disrupted cartilage homeostasis and decreased cartilage thickness along the lateral aspect of the medial femoral condyle. Because patients who had undergone anatomic graft placement did not have evidence of significant cartilage thinning, Dr. DeFrate suggested that restoring normal knee motion might help maintain cartilage.

"All of these studies quantified the effects of femoral graft placement on graft function, knee kinematics, and cartilage thickness in the same population of patients," wrote Dr. DeFrate. "Our results demonstrate that achieving anatomic graft placement is critical to restoring native ACL function and normal knee kinematics—and, ultimately—may help slow cartilage degeneration following ACL reconstruction."

Dr. DeFrate's disclosure information can be accessed at www.aaos.org/disclosure

Jennie McKee is managing editor of AAOS Now. She can be reached at mckee@aaos.org