When to order an investigation for a suspected ACL injury?

[physiobob]

Not sure about criteria suffice to say only if you suspect a total rupture would you both. As there was no large effusion then this is unlikely. As you suggest it may be lax on that side but why did she fall? Unstable ankles? Slippery ground.

Any impact on the knee directly may have caused some articular bruising and or meniscal irritation. Be careful not to cause to much loading with any exercise in the initial phases as this might aggravate the underlying tissue that is trying to heal. Weakness post injury is not actually real 'weakness' and so strengthening need not always be the desired course of action. i.e. was it weak before?

Just some ideas but hope it helps.

[roycar]

just a note, i myself had a complete acl tear, this was confirmed through arthroscopy and repaired, without having a large haemarthrosis/effusion. i never had more than a minor effusion.... until I had the surgery! That was a different story! an mri showed just "acl disruption". haemarthrosis is present in the vast majority of complete tears but not in 100% of same.

If a client of mine were reporting repeated instances of instability and objectively there was laxity on lachmans/anterior drawer I would be looking to see if an MRI was possible. especially if the client was very active etc.