The distinction between acute and chronic anterior cruciate ligament (ACL) tears is important for several reasons, most notably management and medicolegal issues 1-3.
Anterior cruciate ligament tears are frequently classified as follows 1:
acute: (within 6 weeks of injury)
subacute: (between 6 weeks and 3 months from injury)
intermediate: (between 3 months and 1 year from injury)
chronic: (more than 1 year from injury)
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Diagnosis
In most clinical settings the distinction is usually based on patient history and clinical presentation. Still, it can also be done based on MR imaging findings and ligament changes observed by arthroscopy 1. MR imaging and arthroscopy play a crucial role in the setting of medicolegal matters as the patient's history may be distorted due to secondary gains 1,2
Clinical presentation
Symptoms of the ligament tear in the acute phase include pain and swelling of the joint. As time passes, pain relief occurs due to the disappearance of the hematoma.3 Insufficiency of the injured ACL, if not treated, leads to joint instability and degeneration.3,4
Radiographic features
MRI
These findings help differentiate between chronic and acute ACL tears 1:
edematous soft tissue in the intercondylar notch is the characteristic feature of acute ACL tear, characterized by a homogeneous low signal on proton density and an inhomogeneous mildly high signal on T2-weighted images 3
ACL morphology: an edematous and swollen ACL morphology (increased signal intensity on sagittal fat-saturated proton density images with a mass-like appearance) favors an acute tear, whereas chronic ACL tears usually have a hypointense and fragmented (secondary to fibrous scar) appearance or are absent in chronic tears 1; the chronically torn ACL fragments frequently scar to the lateral aspect of the posterior cruciate ligament 2,3
joint effusion: the presence is a non-specific marker of an acute intra-articular injury 1
posterior cruciate ligament angle: the PCL angle (measured as the angle between lines drawn through the central portion of the tibial and femoral insertions of the PCL on the sagittal MRI images) decreases with time after an ACL injury; an angle less than 105 is considered "PCL buckling", which consists a clue to chronic ACL tear 1,5
bone bruises: are found in the acute phase of the injury and gradually diminish from 3 weeks to several months after the injury 1
injury of the medial and lateral collateral ligaments: these findings, in association with an ACL tear, are significantly correlated with the acute phase of an ACL rupture 2
Treatment and prognosis
Similar to acute tears, management of chronically torn ACLs is decided based on the severity of the injury, patient demographics, and concomitant pathologies. The treatment options include conservative methods (immobilization), and reconstruction surgery (double-bundle arthroscopic surgery with patellar tendon or hamstring grafts) 6.
Practical points
some acute ACL tears may lack a characteristic edematous mass-like appearance, which could lead to the false diagnosis of a chronic tear 3
in some instances of chronic ACL tear, small poorly-defined areas of T2 hyperintensity in the intercondylar notch may lead to misdiagnosis as an acute tear. These areas are likely secondary to recent tears of the bridging fibrous scars 3
as fibrous scars have signal characteristics similar to the intact ligaments, a chronically torn ACL may be misinterpreted as normal. Focal angulations in the course of this low signal band are a clue to the correct diagnosis; however, they do not always exist 3
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