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Letters to the Editor |
Minneapolis, Minnesota
In the article entitled "The Effect of Reconstruction of the Medial Patellofemoral Ligament on Patellar Tracking" (Sandmeier et al., May/June 2000, pp 345–349), the clinically important problem of altered patellar tracking following patellar dislocation was studied using a cadaveric model. The authors chose specifically "... to evaluate the role of the medial patellofemoral ligament in patellar stability and to determine whether the ligament can be functionally reconstructed." Whenever a cadaveric model is used to investigate a clinical problem, the models’ underlying assumptions must be carefully considered. In a well-written research paper, these assumptions and associated limitations should be identified and their implications should be clearly summarized.
In the cadaveric model used in this study, the quadriceps tendon was loaded "... in line with the femoral shaft so that when the crosshead was raised, the knee was extended by the quadriceps tendon, duplicating a knee extension type of exercise." Tendon excursion, not force, was actually controlled. The forces required to extend the knee undoubtedly varied across knee flexion angles, but peak forces averaged 71 N. While these forces may approach those experienced during a simple unresisted knee extension exercise, forces in the quadriceps tendon routinely exceed 2000 to 3000 N during activities of daily living.1 Thus, the quadriceps loads applied in this study were well below physiologic levels for all but the most minimally demanding activities. This is important because there is a strong relationship between patellar tracking and patellofemoral contact force, which is related to quadriceps force.
The authors state that "No attempt was made to simulate the weight of the leg, as we believed that any weight added would be arbitrary ... the weight of the leg would have an equal effect in each testing condition and therefore should not have affected the results." This is an interesting, though unproven, assertion easily studied by varying weights at the distal end of the leg. The authors concede that "...decreased [quadriceps tendon] force would tend to exaggerate the contribution of the soft tissue restraints." Another important issue in the study regards the magnitude of the applied lateral forces acting to subluxate the patella. Lateral forces applied to the patella were 2.72 kg (26.7 N), nearly 40% of the applied quadriceps load, effectively altering the vector of quadriceps action (the Q angle) dramatically. No rationale is given for this choice of load magnitude. The magnitude of these loads relative to the quadriceps force would likewise tend to exaggerate the contribution of soft tissue restraints to patellar tracking.
The end result is that this method stacks the deck heavily in favor of the hypothesis that medial restraints are important in controlling patellar tracking. Perhaps controlling patellar tracking in the presence of a low quadriceps force at very specific knee flexion angles is the place where surgical reconstruction of the patellofemoral ligament may be of greatest benefit. But we believe that to extrapolate these cadaveric findings to the clinical arena, in the absence of more complete data clarifying the link between the reconstruction and the magnitude of the quadriceps load, is problematic. The findings reported are relevant only to activities with minimal quadriceps loading. A more thoughtful discussion of these issues would have contributed greatly to the value of this study.
REFERENCES
Portland, Oregon
Thank you for the opportunity to respond to the comments of Drs. Anderson and Guanche. I hope that I have addressed their concerns.
My colleagues and I agree that the quadriceps forces used in our study are orders of magnitude lower than the forces expected in activities of daily living or sports. We disagree with the assumption that our findings are relevant only to activities involving minimal quadriceps loading. While forces on the quadriceps tendon are undoubtedly higher than those used in our model during many activities, it is less clear that they are higher when a patellar dislocation occurs. A traumatic dislocation may be associated with quadriceps inhibition and a large decrease in the force on the quadriceps tendon.
The situation is similar to that of the lateral ankle ligaments. Ankle sprains happen during sports despite the evidence that the ligaments are not important in joint stability when the ankle joint is bearing weight. We believe that the primary role of the medial patellofemoral ligament is to serve as a guide for the patella when the forces compressing the patella in the trochlea are low, rather than acting as a restraint to large forces when the quadriceps are contracting.
If the forces on the extensor mechanism are higher at the time of a patellar dislocation, the most important factor is the direction of the net force vector on the patella. If the forces on the quadriceps tendon are increased, a larger lateral force is required to produce a patellar dislocation. Certainly in cases where a fracture of the lateral trochlea occurs in conjunction with patellar dislocation, the compressive forces holding the patella in the trochlea must have been substantial. The medial patellofemoral ligament is not a large structure and would not be strong enough to resist such forces.
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