The Partially Torn ACL

Not all ACL injuries represent complete ligament ruptures. In some cases only a portion of the ligament's fibers are torn, or the ligament has merely been permanently stretched out to some degree. When less than 50% of the ACL's fibers are torn and the remaining intact ones have not been severely stretched, the ACL has a reasonable chance of gradually reconstituting itself back to near-normal status. More severe partial tears, on the other hand, usually go on to behave like complete disruptions once the patient returns to physical activity. For that reason, severe partial tears should usually be treated as would a complete rupture. Most minor partial tears are best treated non-operatively, at least initially. The clinical behavior (symptoms) of the knee and the serial physical examination findings of a skilled knee surgeon will provide guidance as to how to proceed as time passes.

If a diagnostic knee arthroscopy is performed to assess the status of the ACL, great care should be taken because the visual appearance of a partially torn ACL can be very misleading. Sometimes an injured but still strong and stable ACL may look ominously lax and/or appear to have irregular, damaged fibers. Conversely, a severely compromised ACL that allows positive "pivot shift" joint instability to occur, will on occasion, be afflicted only with internal fiber disruption and generalized plastic deformation (permanent stretching), thus providing the appearance of merely being slack as opposed to being torn outright. This may cause a surgeon to underestimate the degree of ligament damage present.

Rather than basing a diagnostic opinion and/or recommendation for reconstructive surgery solely on the appearance of a partially torn ACL at arthroscopy, it is advisable for the surgeon to mechanically assess the ligament's functional integrity by:

a.	performing a manual ligament stress examination while the patient is under anesthesia, carefully comparing the injured knee to its normal mate, and;
b.	while viewing the ACL with the arthroscope, palpating and probing it with a blunt instrument as it is tightened (stretched) by external knee stress - if it becomes taut and rigid rather than remaining lax and soft, it has retained some significant portion of its mechanical integrity.

Thermal (heat-induced) ligament shrinkage/tightening procedures for loose or stretched out cruciate ligaments have not yet been proven effective over the long term and have occasionally been reported to cause ligament necrosis (tissue death) followed by complete dissolution or rupture. They should, therefore, be approached with caution.

Related Ligament Injuries and Complex Instabilities

FIGURE 10 -Diagrammatic illustration of how one or more collateral ligaments can be torn in combination with a cruciate ligament injury.

In particularly severe knee sprains, there is usually more damage than just a ruptured anterior cruciate ligament. In some cases additional ligaments such as the medial collateral (inner-side) ligament, posterior cruciate ligament, lateral collateral (outer-side) ligament, or portions of the joint's capsular (surrounding envelope) ligament are traumatically compromised as well (see FIGURE 10). The decision whether or not to perform surgical work on these additional damaged structures at the time of ACL reconstruction requires a good deal of insight and experience on the part of the surgeon, as this decision is often a "judgment call". Surgery to correct collateral ligament and capsular defects or laxities is known as "extra-articular" (external to the joint cavity) repair and/or augmentation, and is done in addition to the "intra-articular" anterior cruciate ligament reconstruction within the joint cavity. While formerly performed with great frequency (and often to the exclusion of intra-articular ACL reconstruction), supplemental extra-articular surgery today is not commonly performed. To some extent it has become a "lost surgical art." Few orthopedic textbooks describe methods of rebuilding a chronically torn medial collateral ligament, and very few surgeons have much experience in performing this type of surgical work. Various supplemental, extra-articular reconstructions involving "reefing" (capsule over-folding and tightening) and "tenodesis" (converting a nearby tendon into an auxiliary ligament) procedures may be needed when attempting to treat a knee that demonstrates a more severe (complex or multi-directional) instability as compared to a simple, isolated anterior cruciate ligament laxity. The older (more chronic) the ACL tear is, the more likely "complex" instability will be encountered. The experienced reconstructive knee surgeon will know when such supplemental procedures are likely to contribute to a successful outcome and which particular method to perform.