Good normal knee first.

The Lachman is more of a finesse maneuver and can be done gently. I would rest their knee on top of my bent knee so it’s resting on my knee at about 30° of flexion. I would put my left hand on their joint line to feel how many millimeters their tibia comes forward. And the critical thing is probably more than how far the knee translates forward is the endpoint. An intact ACL will come to a rapid stop snap like hitting a wall or coming to the end of a leash with a firm snap end point. And this can be done in a very easy gentle painless manor. Occasionally I’ll have to go back-and-forth from the good knee to the injured knee to get a real good feel on the side to side difference.

If I feel the knee slide forward against my thumb and index finger without a firm endpoint the ACL is torn. It’s not too uncommon to have the patient and family members watch and they can see across the room how different the exam is from side to side and generally the patient and family can appreciate the difference.

If the knee only moves a millimeter or two with a very firm symmetric endpoint then the ACL is not torn.

The mistake I often see is when they try to try to do it as a real forceful maneuver and they don’t appreciate how far the knee is sliding forward nor the snap of the endpoint.

If someone is shadowing me I’ll try to show them how gentle a maneuver it is and I’ll put their hands on top of the knee so they can feel the snap of the endpoint which is the key.

Granted it is often difficult and impossible to elicit a pivot shift due to pain and guarding. I will try to make them feel as comfortable as possible with gentle passive movement for a few cycles and kind of sneak up on them with it. But you probably only get one good attempt then they won’t let you try again. If someone has had a long standing chronic ACL deficiency with significant instability you can usually get a good pivot shift in the office.

If someone was just two days from injury I may try once for a pivot shift and I’d be more surprised if I was able to elicit it. It would be probably futile to try in an acute setting.

Pivot shift under anesthesia though is key to assessing severity of instability (along with other tests)

If I get a good pivot shift while the patient is under anesthesia, I will immediately harvest the graft as they need stabilization. If I can’t get a good pivot shift under anesthesia I will assess the ACL first arthroscopically and perhaps not do the reconstruction if the pivot shift is stable under anesthesia