- See:
-
Valgus Stress Test:
-
Lateral Collateral Ligament:
- Torn Collateral Tested in Flexion:
- when tested in flexion which relaxes posterior capsule, same ligamentous laxity will result
in a much greater degree of instability;
- at 30 deg flexion, the cruciates are in their most relaxed state, and pathologic laxity palpated is capsular laxity;
-
varus instability in flexion:
- role of
LCL increases w/ joint flexion, as posterolateral structures become lax;
- w/ joint flexion, resistance by
ACL decreases, but large forces are found in
PCL at 90 degrees of flexion;
- LCL is primary restraint to varus stress at 5 deg & 25 deg flexion;
- lateral capsular structure provide secondary support;
- iliotibial band &
popliteus muscles have dynamic stabilizing role;
- Torn Collateral Tested in Extension:
- intact cruciate ligaments and posterior capsule are taut & little abduction or adduction instability is detectable;
- instability w/ varus or
valgus stress testing suggests cruciate ligament disruption in addition to collateral ligament disruption;
-
varus laxity in hyperextension;
- instability to varus angulation indicates damage to arcuate complex &
PCL;
- cruciate ligaments (primarily ACL) resist approx 25% of moment at full extension;
- One Plane Lateral Instability:
- one plane lateral instability is apparent on varus stress testing when knee opens on the lateral side;
- indicates disruption of lateral capsular ligament,
LCL,
biceps tendon, iliotibial band,
arcuate &
popliteus complex,
PCL, and possibly the
ACL (this is major instability approaching severity of
disloation);
- Postero-Lateral Instability:
- expect to find increased varus rotation and external rotation at 30 deg of knee flexion;
- similarly, the same knee flexed to 90 deg will demonstrate decreased varus and external rotaion moments;
- in contrast, when there is increased varus rotation and external rotation moments at both 30 and 90 deg of flexion,
then there are tears of the PCL and posterolateral corner;
Limits of movement in the human knee. Effect of sectioning the posterior cruciate ligament and posterolateral structures.
