The "50% Rule"

The "50% rule" in arthroscopic and orthopaedic surgery states that tendon and ligament injuries with structural involvement of less than 50% are best treated by observation, while those with involvement of greater than 50% are best treated by surgical repair or reconstruction.

The rule appears to have come out of studies on the management of hand flexor tendon injuries and transferred over to arthroscopy in the shoulder and knee. It should be remembered that the rule is derived from expert opinion and has never been subjected to a randomized controlled clinical trial nor validated for accuracy, reliability, or reproducibility. The role of imaging in all this is as murky if not more so.

References

Pedowitz RA, Higashigawa K, Nguyen V. The "50% rule" in arthroscopic and orthopaedic surgery. Arthroscopy. 2011 Nov;27(11):1584-7.