A patient presents to the Emergency Department after sustaining a twisting knee injury while skiing. She felt a pop and was unable to bear weight afterward secondary to pain and a feeling of instability. Shortly after the injury, she noted increased swelling and pain. On examination, she has a moderate effusion and a positive Lachman test. An x-ray was obtained and is shown above (Image 1. Case courtesy of Mikael Häggström, M.D. – Author info – Reusing images, CC0, via Wikimedia Commons).
This patient has an anterior cruciate ligament (ACL) rupture. The x-ray is negative for bony abnormality.
Pearl: ACL rupture, a grade 3 sprain of the ligament , is a fairly frequent injury in sports. The classic mechanism is a twisting or hyperflexion motion during a fall with the weight backward. Patients often experience a “pop” followed by instability and rapid onset of effusion and increasing pain. There can be additional concomitant knee structures injured such as the medial collateral ligament or meniscus .
Pearl: A Segond fracture, a bony avulsion from the lateral tibial condyle, may be seen on x-ray (Image 2). When present, this is highly suggestive of ACL rupture.
Isolated ACL ruptures can be initially managed conservatively. Patients should be encouraged to bear weight as tolerated while protecting the knee. It is important for patients to deliberately engage the quadriceps and hamstrings muscles. An orthopedic referral should be placed for prompt follow-up (1-2 weeks). Patients should work aggressively on regaining full knee range of motion and focus on maintaining hamstrings and quadriceps strength. 
Pearl: Atrophy, loss of strength, and stiffness can set in quickly after a knee injury [6, 7]. Patients should aggressively combat this process to maintain strength and range of motion as much as possible immediately following an injury.
Pearl: The exercise bike is a great modality that encourages knee range of motion and quadriceps strengthening in addition to single-leg raises. Patients should avoid cutting and pivoting sports/activities.
No, she does not. The patient may be placed in a hinged knee brace if needed for comfort.
Pearl: Use of a knee immobilizer will propagate weakness, atrophy, and stiffness, all of which will be harmful to the patient regardless of whether or not the patient pursues surgical reconstruction. There is also an increased chance of deep venous thrombosis with the use of a knee immobilizer .
Anterior Cruciate Ligament (ACL) Injuries. American Academy of Orthopaedic Surgeons. https://orthoinfo.aaos.org/en/diseases–conditions/anterior-cruciate-ligament-acl-injuries/
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Posch, M., Schranz, A., Lener, M. et al. In recreational alpine skiing, the ACL is predominantly injured in all knee injuries needing hospitalization. Knee Surg Sports Traumatol Arthrosc (2020). PMID: 32803275.
Prins M. The Lachman test is the most sensitive and the pivot shift the most specific test for the diagnosis of ACL rupture. Australian Journal of Physiotherapy (2006). 52(1): 66. PMID: 16555409.
Biscarini A, Contemori S, Busti D, Botti FM, Pettorossi VE. Knee flexion with quadriceps cocontraction: A new therapeutic exercise for the early stage of ACL rehabilitation. J Biomech. 2016 Dec 8;49(16):3855-3860. PMID: 28573973.
Kilroe SP, Fulford J, Jackman SR, Van Loon LJC, Wall BT. Temporal Muscle-Specific Disuse Atrophy during One Week of Leg Immobilization. Medicine & Science in Sports & Exercise (2019). PMID: 31688656.