OTA 1997 Posters - Knee Injury
Strain Rate Sensitivity of Traumatic Knee Ligament Injuries
Animesh Agarwal, MD, Robert C. Schenck, Jr., MD, Ian S. Kovach, PhD, MD, Russell Brummett, BA, Kyriacos A. Athanasiou, PhD, PE
UTHSCSA, San Antonio, Texas, USA
Purpose: The purpose of this study was to evaluate two clinically applicable strain rates and evaluate cruciate ligament failure mechanisms and morphology of torn knee restraints. We selected straight hyperextension as the mechanism failure as it would absolutely tear both cruciates, and injure the ligaments in one injury pattern that is seen clinically as in both sporting and high energy trauma situations.
Methods: We created an experimental model to evaluate the effects of strain rate on the injury mechanism of the anterior and posterior cruciate ligaments. Utilizing knee hyperextension to rupture the anterior and posterior cruciates, two strain rates (100%, 5400%) were selected to reproduce two clinical injury patterns: low (sporting) and high energy (pedestrian-motor vehicle accident). A Materials Testing System (MTS) was used to produce the slow rate pattern utilizing a frame allowing one axis of rotation, with the knee pushed into hyperextension with the MTS plunger. The fast rate pattern was created by a drop tower using gravity (22.6 kg from 1.22 meters, ~ 14 mph at impact). Ten pairs of fresh frozen cadaveric knee (average age 61, range 38-81 years) were tested to 45 degrees hyperextension, a total of 5 cm vertical distance. Injury morphology was defined grossly and histologically.
Results: Of the specimens tested at high strain rate (impact loading) the femur of one knee fractured from its potting cast. Of the remaining nine high rate loaded specimens, all suffered an identical avulsion/stripping injury from the femoral origin of the PCL. No fractures of the medial femoral subchondral bone or bony fragment occurred. The ACL tore in a more variable pattern at a high rate with a predominance of midsubstance tears. Injury patterns included midsubstance tears (four of nine), mixed injuries (three of nine) and avulsion from the tibia in two of nine. Out of the midsubstance tears, three were in-continuity. High energy injuries produced ruptures of the capsule off the femoral origin.
Low rate injuries produced a predominance of midsubstance PCL tears (eight of ten). Two low rate injuries produced pure femoral avulsions in a pattern similar to that seen in the high rate injuries. ACL injury patterns were equally distributed between midsubstance tears in five of ten specimens, and avulsions in five of ten (two tibial, three femoral). In those ACL avulsion injuries from the tibia, the morphology was that of a large bony fragment off the tibial eminence. In contrast, low energy hyperextension injuries left the capsular structures intact.
Cadaveric Knee Injury Results
| Posterior Cruciate Ligament | Anterior Cruciate Ligament | |||||||
| Low Rate | Impact | Low Rate | Impact | |||||
| Specimen #(Age) | Side | Type | Side | Type | Side | Type | Side | Type |
| 29495 (38) | L | Mid | R | A-F | L | Mid | R | Mix |
| 31548 (83) | R | Mid | L | A-F | R | Mid | L | Mix |
| 29489 (38) | L | A-F | R | A-F | L | Mid | R | A-T |
| 31528 (79) | R | Mid | L | A-F | R | A-F | L | A-T |
| 34420 (66) | R | Mid | L | A-F | R | A-T | L | Mid |
| 34207 (63) | R | A-F | L | A-F | R | A-F | L | Mix |
| 34430 (75) | R | Mid | L | A-F | R | A-T | L | MidC |
| 33596 (60) | L | Mid | R | A-F | L | Mid | R | MidC |
| 31048 (67) | R | Mid | L | A-F | R | Mid | L | MidC |
| 34435 (81) | L | Mid | R | * | L | A-F | R | * |
Discussion and Conclusion: Although velocity and energy are used interchangeably to describe injuries of the musculoskeletal system, energy is the most universal in injury description. Low energy injuries of the knee are frequently seen in sporting activities and clinically produce midsubstance tears of both cruciate ligaments. High energy trauma to the knee such as seen with a pedestrian-motor vehicle accident produces high strain rates and clinically produces avulsion injuries of ligaments.
Classic biomechanical teaching of ligament failures, which is based upon two strain rates (0.67% and 67%), indicates that avulsions occur in response to a "slow" rate of loading and midsubstance tears occur under "fast" rates. The goal of this study was to apply a clinically appropriate strain rate (sporting versus high energy trauma) in a clinically applicable injury model (knee hyperextension), such that both energy and hyperextension could be related to clinical experience.
In our model, a clinically slow rate produced consistently midsubstance PCL, tears, and a clinically high energy fast rate produced consistently PCL avulsion injuries with stripping off the femoral origin. The ACL appeared to tear predominantly in a midsubstance pattern at both high and low rates of injury. From this cadaveric study, it appears that the PCL is strain rate sensitive. The ACL tears clearly did not avulse more frequently at higher strain rates, as was seen with injury to the PCL.
In summary, this study demonstrates that clinically relevant fast rates of hyperextension loading of the knee result in PCL avulsions. In contrast, slow loading rates such as those seen in sporting events, result in PCL mid-substance tears. The ACL appears to be insensitive to the rate of loading in this hyperextension model of knee injury. Its failure may be due to notch impingement.