Copyright © 2009 by The Journal of Bone and Joint Surgery, Inc.
Commentary & Perspective
Commentary & Perspective by
David R. Diduch, MS, MD*,
University of Virginia, Charlottesville, Virginia
Posted October 2009
What happens in the athlete's leg to cause a pivot shift? How does a seemingly
innocuous landing from a rebound suddenly result in a season-ending anterior
cruciate ligament injury? What was different about that particular event and
how the athlete landed? Are there ways to educate and train athletes to avoid
these provocative positions? These are some of the key questions that Dr. Barry
Boden and colleagues address with this latest study amidst an outstanding volume
of investigative work on the mechanics of anterior cruciate ligament injury.
The current study, "Tibiofemoral Alignment: Contributing Factors to Noncontact
Anterior Cruciate Ligament Injury," is the third in a series of studies examining
the position of the hip, knee, and ankle as it relates to the risk of experiencing
a pivot-shift type of subluxation event. The work of Dr. Boden and colleagues
began with examining video, collected over a period of twelve years, of anterior
cruciate ligament tears during competition and making comparisons with normal
controls during similar events. In their first publication in 20071,
they observed that hip flexion during a landing position was common and that
female athletes were five times more likely to experience a valgus knee collapse
event than male athletes were. They also observed that these "noncontact" events
were often preceded by some alteration of movement, such as being pushed from
behind just prior to anterior cruciate ligament injury.
The next study2, published in early 2009, really identified the "at-risk
landing position" that occurred when the hip was flexed but the foot remained
relatively flat, a position in which the calf muscles could not as effectively
absorb the landing forces2. In that study, they determined the average
hip, knee, and ankle angles at impact for "safe"
and "provocative" one-limb landing positions. This background led to the current
study as the authors sought to determine what about this position might lead
to an anterior rotatory subluxation or a pivot-shift event. To enhance statistical
power, they added a third "exaggerated" position for the knee, which further
increased hip flexion and knee extension.
The current study methods included subjecting twenty-five healthy, uninjured
volunteers to magnetic resonance imaging scans with the leg fixed in either the
safe, provocative, or exaggerated landing position by means of a fiberglass splint.
A partial weight-bearing load was applied during the scan to observe translation
of the tibia. Through analysis of the magnetic resonance images, the authors
examined tibiofemoral alignment with specific attention to the posterior tibial
slope relative to the femur (i.e., they sought to determine how vertical the
weight-bearing surface of the tibia was as gravitational forces went through
the knee upon landing). They also examined anterior tibial translation by measuring
the point of tibiofemoral joint contact relative to the femoral sulcus, and they
noted where contact occurred on the lateral femoral condyle relative to its shape.
They hypothesized that the leg alignment in a provocative position would predispose
the knee to anterior subluxation via gravitational forces.
Their results offer objective and intuitive validation of the events that
occur during the moment of anterior subluxation and anterior cruciate ligament
tear. As the hip flexes and the knee extends, the tibial articular surface becomes
more vertically oriented relative to the ground, resulting in effectively more
tibial slope. The more vertically oriented tibial slope results in a more anteriorly
directed subluxation force, as previously demonstrated under gravity loads3.
As load is applied upon landing or cutting, the femur slides down the hill and
the tibia is thrust forward, rupturing the anterior cruciate ligament. This principle
is well understood by our veterinary counterparts, who commonly employ "leveling" osteotomies to decrease the posterior tibial slope to treat anterior cruciate ligament insufficiency
in canines4.
In addition, Dr. Boden's group helps us understand what happens specific to
the lateral side of the knee as the pivot shift occurs. In the more provocative
positions, the point of contact moves anteriorly, closer to the sulcus on the
femur and where bone bruises typically occur. As contact moves toward this flatter
part of the joint surface and away from the rounder portion of the posterior
lateral femoral condyle, sliding is more likely to occur rather than rolling,
especially against the convex or flat lateral tibial plateau. The medial side
enjoys greater contact area and tends to roll rather than slide. Thus, they present
the events of a pivot shift quite logically. The ankle is relatively neutral,
such that a large load is transmitted through the knee, just as the hip is flexed
and the knee extended, making the tibial slope oriented more vertically. Gravitational
forces acting through the vertical tibial slope translate the tibia anteriorly
as the lateral tibial plateau slides along the flat femoral surface toward the
sulcus, until the anterior cruciate ligament can take it no longer.
The methodology employed was sound and the study appropriately powered, demonstrated
by achieving statistical significance for the results for all parameters measured.
Observer bias was limited by blinding and by repeating the measurements five
months later, yet intrarater reliability was high. While an entire range of joint
positions is possible during athletic-related injuries, the authors effectively
simplified the conditions on the basis of prior extensive video analysis, using
averages to arrive at three distinctly different positions for testing. This
was very effective and speaks to their years of experience in this area of research.
While frontal plane alignment and rotational differences may well be important
factors in anterior cruciate ligament tears, these were beyond the scope of the
study due to time constraints for holding the limb in one position, and they
remain open to investigation in future studies.
Potential limitations of the study include the fact that the subjects were
not analyzed for variations in lower-limb alignment or for altered positions
for hip abduction or tibial rotation. These remain open for future research.
Also, there is the potential for errors in lower-limb and trunk positioning,
given that the subjects were set in place only with a goniometer. However, as
noted in the statistical significance for each parameter measured, the trends
were very consistent among the three positions.
Dr. Boden and colleagues are to be congratulated for helping us understand
the events contributing to and occurring during a pivot shift. The next step
is to translate this understanding into preventive training strategies.
*The author did not receive any outside funding or grants in support of his research for or preparation of this work. Neither he nor a member of his immediate family received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity.
References
1. Krosshaug T, Nakamae A, Boden BP, Engebretsen L, Smith G, Slauterbeck JR, Hewett TE, Bahr R. Mechanisms of anterior cruciate ligament injury in basketball: video analysis of 39 cases. Am J Sports Med. 2007;35:359-67.
2. Boden BP, Torg JS, Knowles SB, Hewett TE. Video analysis of anterior cruciate ligament injury: abnormalities in hip and ankle kinematics. Am J Sports Med. 2009;37:252-9.
3. Dejour H, Bonnin M. Tibial translation after anterior cruciate ligament rupture. Two radiological tests compared. J Bone Joint Surg Br. 1994;76:745-9.
4. Shahar R, Milgram J. Biomechanics of tibial plateau leveling of the canine cruciate-deficient stifle joint: a theoretical model. Vet Surg. 2006;35:144-9.
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