Background: Because the tension that exists in an anterior cruciate
ligament graft when the knee is unloaded (the initial tension) affects the
surgical outcome and because high initial tension has a number of adverse
consequences, the primary purpose of this study was to determine
quantitatively how much less initial tension was required for a high-stiffness
construct than for a low-stiffness construct. A secondary purpose was to
determine how the stiffness of the graft construct affects the anterior
load-displacement behavior of the knee from 0° to 90° of flexion.
Methods: Anterior-posterior load-displacement was measured in each
of ten intact cadaveric knee specimens, the anterior cruciate ligament was
excised, and the anterior cruciate ligament was reconstructed with a
double-loop bovine tendon graft. Graft constructs of different stiffness were
created with use of six springs, ranging in stiffness from 25 to 275 N/mm to
simulate the fixation stiffness. After adjusting the initial tension of the
graft so that the anterior-posterior laxity of the reconstructed knee matched
that of the intact knee, the 0-N posterior limit and the 225-N anterior limit
were measured at 0°, 30°, 60°, and 90° of flexion.
Results: The highest stiffness fixation (275 N/mm) required an
average of 73 N of initial tension, which was more than three times less than
the average of 242 N of initial tension required by the lowest stiffness
fixation (25 N/mm). The 225-N anterior limit was overconstrained an average of
1.0 mm with the highest stiffness fixation (275 N/mm), which was 3.6 mm less
than the overconstraint with the lowest stiffness fixation (25 N/mm).
Likewise, the posterior limit was overconstrained an average of 2.6 mm with
the highest stiffness fixation (275 N/mm), which was 3.8 mm less than the
overconstraint with the lowest stiffness fixation (25 N/mm).
Conclusions: The initial tension for a high-stiffness graft
construct is more than three times less than that for a low-stiffness
construct. The initial tension for a high-stiffness graft construct better
restores both the 225-N anterior limit and the 0-N posterior limit to normal
than the initial tension for a low-stiffness graft construct over the range of
flexion from 0° to 90°.
Clinical Relevance: Because a high-stiffness graft construct
requires substantially less initial tension than a low-stiffness graft
construct, the tension pattern in a high-stiffness graft construct better
matches the pattern in the intact anterior cruciate ligament. This tension
pattern may avoid adverse consequences to both the knee joint function and the
graft, which have been linked to high initial graft tension when the initial
tension is maintained postoperatively. When the initial tension is not
maintained postoperatively, a high-stiffness construct may be advantageous in
avoiding a recurrence of knee instability.