Is it an ACL injury or a meniscus tear?

Not automatically. The decision depends on your activity demands, knee stability during daily life and sport, and whether other structures (meniscus root tears, significant collateral injury) are involved. Structured rehabilitation first is a reasonable path for many patients, and reconstruction can still follow if the knee keeps giving way. Competitive pivoting athletes and people with persistent instability despite good rehab are stronger surgical candidates. I plan around your goals and how the knee behaves, not around the MRI image alone.

Often no, particularly for degenerative tears in middle-aged adults. Two major randomised trials (METEOR and ESCAPE) found exercise-based physiotherapy comparable to arthroscopic partial meniscectomy for degenerative and non-obstructive meniscal tears, at one year in METEOR and up to five years in ESCAPE. A third (FIDELITY) found the operation no better than sham surgery. The exceptions that still lean surgical are a truly locked knee (bucket-handle displacement), large traumatic tears in younger athletes, and some root tears where early repair protects the joint.

Fast swelling within six hours usually reflects bleeding into the joint, which comes from a vascular structure. The ACL is the most common cause, followed by intra-articular fracture and acute patellar dislocation. A classic meniscus tear typically produces slower joint effusion over twelve to forty-eight hours as synovial fluid accumulates. The timing is one of the most helpful pieces of history you can give me in the first visit.

Not reliably, and I would not try. Self-Lachman is difficult because you cannot relax the hamstrings on the injured leg while also applying the force. Do not try to prove the knee is stable by cutting, twisting, landing, or repeatedly testing it, because a knee that gives way can cause a fall or further damage. Notice whether it gives way during ordinary activity, and arrange an assessment, especially after a new injury or rapid swelling. Functional testing belongs later in rehab, once the knee has been assessed.

Yes, and more common than people expect. Degenerative meniscal tears appear on MRI in a large proportion of asymptomatic adults over 40, and the frequency rises with age. An MRI tear alone does not decide management. What matters is the clinical picture: joint-line tenderness, mechanical symptoms, response to an exercise trial, and whether the knee is functionally limiting you. Imaging guides, it does not lead.

Meniscus tear rehabilitation for non-surgical management often sees meaningful gains over 8 to 12 weeks with progressive strengthening exercises and load management. Post-meniscectomy rehab is similar. ACL timelines are longer because the tissue itself is more involved. Non-surgical ACL rehab commonly runs 4 to 9 months depending on activity goals, and post-operative ACL reconstruction rehab typically targets 9 to 12 months before return to cutting and pivoting sport, with the Aspetar 2023 guideline recommending objective return-to-sport criteria rather than time alone.