Posterolateral Knee
The goal of this study was to determine if untreated grade III injuries of posterolateral knee structures (PLS) contribute to increased force on an ACL reconstructed graft which could cause failure of the graft. This was accomplished through measuring the force in an ACL graft during limits-of-motion testing for an ACL reconstructed knee with otherwise intact structures and after selected cutting of specific posterolateral knee structures. Eight fresh frozen cadaveric knees were utilized. An ACL reconstruction using a central third patellar tendon graft was performed. The femoral bone block was secured with a 9 mm interference screw. The tibial bone plug was fixed to a tensioning jig (on the tibia) through a ligament augmentation device (LAD), which had a buckle transducer applied to it to measure force in the graft. The knee was then mounted in a joint testing machine, and an i nstrumented spatial linkage (ISL) machine was applied to measure 6 degrees-of-freedom. The graft force was standardized to the force present when a 67-N (15 lb) distal traction load was applied to the tibial portion of the graft with the knee in full extension (as would be done clinically). This graft force was recalibrated after each testing cycle. Limits-of-motion testing was then performed for the intact knee state and after sequential individual sectioning of the fibular collateral ligament (FCL), popliteofibular ligament (PFL), and popliteus tendon origin on the femur. Loads applied were 67-N of anterior or posterior load, 12-Nm of varus or valgus moment, and 5-Nm of internal or external rotation moment. Testing was performed for 3 cycles at each flexion angle (0º, 30º, and 90º) and averaged. The ACL graft force was significantly higher, 170% and 340% respectively, after cutting the FCL during varus loading at both 0º (p < .01) and 30º (p <.01) of knee flexion, than it was for the same loading of the intact PLS joint. In addition, coupled loading of varus and internal rotation at both 0º (p < .04) and 30º (p < .001) increased graft force. These findings remained significant with sequential cutting of the PFL and popliteus tendon. There was no significant increase in force on the ACL graft for any cutting cycle or knee flexion angle for an anterior applied, valgus applied, or an external rotation torque force. Analysis of the ISL data revealed that testing cycles were reproduced consistently. A significant increase in motion was seen, for all PLS cutting sequences, in internal rotation for internal tibial torque at 0º and 30º (p < .04); anterior translation for both an anteriorly applied force at 0º (p < .01) and for an internal rotation moment at both 0º and 30º (p < .05); and for varus opening at both 0º and 30º for both a varus moment (p < .0001) and a coupled varus and internal rotation moment (p < .03). In conclusion, we found that limits-of- motion testing identified a significant increase in force on an ACL graft during varus and coupled varus-internal rotation moments in knees with grade III PLS deficiency. We believe this study verifies the clinical observation that untreated grade III PLS injury contributes to ACL graft failure and suggests that the varus-thrust gait pattern seen in some of these patients places an ACL graft at significant risk for failure. It is recommended that grade III PLS injuries with evidence of varus instability be repaired/ reconstructed at the time of ACL reconstruction to decrease the chance of postreconstruction ACL graft failure.
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