3-D model provides critical parameters to prevent physeal injury
Among young athletes, the number of anterior cruciate ligament (ACL) ruptures is rising. Because these patients are often skeletally immature, ACL reconstruction risks damaging the growth plate and causing long-term growth disturbances. With the use of cutting-edge technology—three-dimensional, magnetic resonance imaging (3-D MRI) simulations of the procedure—James Kercher, MD, was able to identify critical parameters for safer, more precise ACL reconstruction.
He presented the results of his findings during the AAOS 75th Annual Meeting in San Francisco.
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Simulator uncovers new findings
James Kercher, MD
Another important finding related to the graft tunnel. “In the past” said Dr. Kercher, “it was taught that drilling a vertical tunnel was an important factor in reducing the amount of injury to the growth plate, but we found it to be of minimal importance in that respect.” Although a vertical tunnel provides more room for fixation of the graft, the study showed that “the diameter of the graft tunnel contributed more to the zone of injury than the vertical line of drilling.”
MAARS: The ACL simulator
The study used a proprietary interactive software program—Module for Adolescent ACL Reconstruction Surgery (MAARS)—to create 3-D models of the knee (Fig. 1). The software used sagittal and coronal T1-weighted images from the standard MRI scan used to evaluate the ACL injury. At least a 1.5T magnet and an adequate view of both the proximal tibial and distal femoral physes are required.
“From these images, the physis was isolated using a technique called thresholding (Fig. 2). The scan was then segmented on its slice axis to provide an accurate 3-D model, which was used to obtain precise volumetric measurements of the physis,” explained Dr. Kercher. “Based on the anatomic path of the native ACL, we performed simulations of transphyseal tunnels, using an 8-mm tunnel as our standard. MAARS calculated the volume of physis removed and the optimum angle for the trajectory.”
The MRI knee studies of 31 patients (21 male, 10 female), ranging in age from 10 years to 15 years, were used. All other identifiers were removed from the studies. MAARS adjusted for the graft’s diameter, its trajectory across the growth plate, and the angle for drilling the tibial tunnel so an interference screw could safely be placed outside the physis.
“We could manipulate all of these variables,” said Dr. Kercher. “After the diameter of the tunnel and distance from the growth plate were input and the true intra-articular ACL trajectory was reconstructed, the module ran a program that would seek out a trajectory to satisfy these inputs.
“For example, if you wanted to drill an 8-mm tunnel and use a 20-mm interference screw for graft fixation on the tibial side,” he continued, “the program would ‘search’ the tibia until it found a path. Then, the graft tunnel was simulated and the program calculated the percentage of the physis removed.” The team determined that, on average, a 68-degree sagittal trajectory was needed for drilling the tibial tunnel to allow placement of a 20-mm screw.
A ‘safe’ or ‘optimal tunnel’ must be of sufficient length to allow interference screw fixation that does not violate the physis, and the surgical path must be as anatomically close to the true ACL as possible. “Because the program generated a large range of responses and because the growth plate configurations were very irregular, we did not find a reproducible trajectory that would allow interference screws to be safely placed without careful planning,” said Dr. Kercher. “This is important because the biggest risk for premature physeal closure during reconstructive procedures is placing hardware, such as screws, across the growth plate.”
Critical parameters identified
Extensive simulations of multiple variables also identified the diameter of the graft tunnel as a critical parameter affecting the volume of physeal injury. Varying the graft diameter from 6 mm to 11 mm increased the volume of injury from 2.3 percent to 7.8 percent—an average of 1.1 percent for every incremental increase of 1 mm. “We found that as the transphyseal tunnel diameter began to exceed 10 mm, the volume percent of the growth plate removed approached 7 percent,” said Dr. Kercher.
Making the tunnel more vertical only decreased the amount of injury by 1 percent. As the tunnel drill angle increased from 45 degrees to 70 degrees, the amount of physis removed decreased from 4.1 percent to 3.1 percent—an average of 0.2 percent for each 5 degree increase in drill angle.
Benefits of surgical simulations
Using a tool such as MAARS enables the surgeon to evaluate various scenarios based on several parameters before performing surgery. This allows the surgeon to tailor the reconstuction to minimize injury to the growth plates on an individual basis.
“A simulator can provide critical information, such as the exact tunnel placement, angle for drilling, and maximum tunnel diameter,” said Dr. Kercher.
“The MAARS module may be helpful in that preoperative planning,” he concluded. He would like to see this simulator used in prospective, randomized controlled studies in the future. Using this technology to analyze growth plate injury in transphyseal ACL reconstructions would help determine the validity of the ‘threshold of injury.’
Co-authors for “ACL reconstruction in the skeletally immature: An anatomic study utilizing 3-D MRI reconstruction” are JohnW. Xerogeanes, MD; Allen R. Tannenbaum, BA, PhD; Ramsey A. Al-Hakim, BS; James C. Black, BA; John Zhao, BS; and Johnny Greene, BS. Disclosure information on the authors can be found online at www.aaos.org/disclosure
Annie Hayashi is the senior science writer for AAOS Now. She can be reached at firstname.lastname@example.org