Partial Rupture of the Anterior Cruciate Ligament

D P Johnson and O Basso

There is confusion regarding the diagnosis and clinical outcome following partial rupture of the anterior cruciate ligament. The diagnosis is often made when there is a moderate degree of instability or when the arthroscopic appearances of the cruciate do not demonstrate complete rupture. There is conflicting opinion as to whether this injury results in chronic instability, increasing laxity and degenerative change as has now been demonstrated in complete anterior cruciate ligament. No previous attempt has been made to clarify the diagnosis and assess the clinical outcome of this injury.

This study analysed prospectively 26 cases followed for a mean duration of 2 years in which the diagnosis was made following clinical examination, plain radiography, MRI and arthroscopic assessment by a single surgeon. Clinical criteria for the diagnosis included a negative pivot shift, a positive anterior draw or Lachmans test and a partial rupture or laxity of the anterior cruciate ligament at arthroscopy. Patients were assessed at final follow up by clinical examination by the same surgeon, KT 1000 arthrometry, plain radiography and isokinetic dynamometry. The radiographs were analysed independently by two blinded radiographers for the extent of the ligament rupture. The results were submitted to statistical analysis.

The results demonstrated that the experienced knee MRI radiographers were unable to differentiate a partial rupture from a complete rupture (p<0.01). Two of the 26 patients had persistent instability and undergone successful anterior cruciate reconstruction. In both of these patients it was noted at the initial arthroscopy that less than 50% of the ligament remained (p<0.01). In the other 24 patients in which more than 50% of the ligament was intact at the initial arthroscopy no further episodes of instability occurred and none required further surgical intervention. After two year follow up none were limited by their knee in sporting activities, there was no increased laxity in the knees on clinical examination or on KT 1000 testing. Isokinetic analysis demonstrated a mean 13% reduction in quadriceps strength in the affected knee.

This study demonstrates that following partial rupture of the anterior cruciate ligament in which the pivot shift is negative. MRI is unable to differentiate the partially torn ACL from the completely torn ligament. If greater than 50% of the ligament is intact there is no increased laxity or functional restriction over a 2 year period and these patients should undergo an intensive period of rehabilitation rather than surgical reconstruction.

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