Knee injury types: how to tell them apart and which ones need surgery

This guide is for anyone who has hurt a knee and wants to know what the symptoms suggest, and for people with a diagnosis who want to know whether surgery is usual for it. It covers seven injury types, how each presents and is confirmed, which heal without surgery, and the signs that mean you should go to an emergency department.

The information comes from the American Academy of Orthopaedic Surgeons, the NHS, and peer-reviewed studies linked in the text.

How do I know what kind of knee injury I have?

Three details narrow it down: how the injury happened, what you felt at the time, and how fast the knee swelled. The AAOS notes that pain and swelling are the most common signs of any knee injury, that the knee may catch or lock, and that some injuries, especially ACL tears, cause instability, the feeling that the knee is giving way.

A knee that swells within hours of a twisting injury points to a ligament tear or a fracture. Swelling that builds over two or three days is more typical of a meniscus tear. Pain confined to the inner or outer edge after a sideways blow points to a collateral ligament. A kneecap that visibly shifted, even briefly, is a dislocation until proven otherwise.

What does an ACL tear feel like?

The anterior cruciate ligament runs through the centre of the knee. The AAOS says most ACL tears happen when a person is pivoting, cutting or jumping with the leg straightened, usually without contact. People often hear a pop and feel the knee give out, and the knee swells within 24 hours. A meniscus tear on the outer side of the knee commonly happens at the same time.

The surgeon examines the injured knee against the uninjured one. X-rays do not show ligaments but can reveal a bone fragment pulled off with the ligament; MRI gives the best view of the ligament and cartilage. Whether you need surgery depends on the severity of the tear, your age, how unstable the knee is, and what you want to do with it. A Swedish randomized trial of 121 young, active adults compared early reconstruction with rehabilitation plus optional later reconstruction. At five years the groups had the same knee scores, and 51% of the rehabilitation-first group had chosen surgery along the way. Reconstruction remains the usual choice for people returning to pivoting sport or whose knee keeps giving way. See Pathway’s ACL reconstruction page and ACL surgery recovery guide.

How is a PCL injury different?

The posterior cruciate ligament takes a bigger force to injure than the ACL. The AAOS gives two classic mechanisms: a bent knee hitting a dashboard, and a football player falling on a bent knee. Pain and swelling build steadily, the knee feels stiff, and it may sag backwards when bent. Imaging can look normal if the injury happened months earlier.

If only the PCL is injured, the AAOS says it may heal quite well without surgery: rest, ice, a brace, crutches and physiotherapy. Surgery is recommended for combined injuries and is almost always necessary after a knee dislocation with several torn ligaments.

Do MCL and LCL injuries need surgery?

The medial collateral ligament (MCL) runs along the inside of the knee and the lateral collateral ligament (LCL) along the outside. Both are injured by a force that pushes the knee sideways, most often in contact sport. The AAOS grades sprains from 1 (stretched but still stabilizing the joint) through 2 (stretched until loose) to 3 (torn in half or pulled off the bone). You feel pain on the affected side, swelling and, in higher grades, instability.

Most collateral injuries are diagnosed by examination; MRI confirms the grade and looks for other damage. A low-grade MCL injury with no other damage is managed in a hinged brace that lets the knee move while it heals. Stable grade 1 and 2 LCL injuries are also treated without surgery. Surgery is considered when a ligament has pulled off the bone or the LCL is torn along with other structures.

Will a meniscus tear heal on its own?

Each knee has two menisci, C-shaped cushions of cartilage between the thigh and shin bones. The AAOS describes two kinds of tear. Acute tears happen in sport, through twisting, cutting or being tackled. Degenerative tears happen in an ageing meniscus, where an awkward twist getting up from a chair may be enough. Many people feel a pop and keep walking; over two to three days the knee stiffens and swells, and it may catch, lock or give way.

Whether a tear can heal depends on where it is. The outer zone has a rich blood supply, so tears there may heal or be stitched. The inner zone has little blood flow, so tears there do not heal, and one that keeps causing symptoms is usually trimmed. The surgeon checks for joint-line tenderness and performs the McMurray test, bending and rotating the knee to put tension on the tear. X-rays rule out arthritis; MRI is the preferred test for the tear itself.

Many tears need no surgery. For degenerative tears, a 2017 BMJ clinical practice guideline made a strong recommendation against arthroscopic surgery for degenerative knee disease, including degenerative meniscal tears, because trials showed no important long-term difference in pain or function compared with exercise therapy. The guideline excluded truly locked knees, young patients with sports injuries, and major trauma. For those groups, Pathway’s surgeons perform meniscal repair when the tear sits in the vascular zone and partial meniscectomy when it does not. The AAOS puts recovery at about three to six weeks after a partial meniscectomy and three to six months after a repair.

What happens after a kneecap dislocation?

The kneecap (patella) sits in a groove at the front of the thigh bone, held on the inner side by the medial patellofemoral ligament (MPFL). The AAOS says dislocations usually follow a non-contact pivot, twist or awkward fall, and that a shallow groove, knock-knee alignment or rotational differences in the leg make them more likely. A 21-year population study found 23.2 first-time dislocations per 100,000 people a year, highest in 14- to 18-year-olds at 147.7 per 100,000.

The kneecap sometimes slides back on its own; otherwise a doctor returns it to the groove. X-rays check for bone fragments, and MRI or CT looks at the cartilage behind the kneecap and the alignment of the leg. For a first dislocation, the AAOS says doctors will most likely recommend bracing and exercises, with a return to normal activities within one to three months. When the kneecap keeps dislocating, Pathway’s surgeons perform MPFL reconstruction, rebuilding the ligament with a tendon graft. A meta-analysis of 14 studies found that 84.1% of patients returned to sport after isolated MPFL reconstruction, with a 1.2% rate of recurrent instability.

What is a cartilage injury?

Articular cartilage is the smooth white surface on the ends of the bones. An impact or twist can chip or shear it off, and the AAOS notes it is often damaged alongside meniscus or ligament tears. Cartilage contains no calcium, so it is invisible on X-ray; MRI shows the defect and any stress in the bone beneath it. Damaged cartilage causes pain with loading and can lead to arthritis over time.

Cartilage does not heal itself well. Restoration procedures, including microfracture, transfer of a cartilage plug from a non-weight-bearing part of the joint, and implantation of cultured cartilage cells, aim to fill the defect. The AAOS says most candidates are young adults with a single lesion.

How do I know if my knee is broken?

The kneecap is the bone most often broken around the knee, usually by a fall directly onto it or a dashboard impact. The AAOS lists pain and swelling at the front of the knee, bruising, difficulty straightening the knee or holding it straight, and inability to stand or walk. The ends of the thigh and shin bones can also break, usually with higher-energy trauma.

X-rays from several angles confirm the fracture, and CT is used when there are multiple fragments. Not every injured knee needs an X-ray: a decision rule validated in Ottawa identified 100% of clinically important fractures in 1,096 adults and could cut X-ray use by about 28%. When the bone pieces have not moved, a cast or splint holds the knee straight while it heals. Displaced pieces usually need surgery with screws, wires or plates, and an open fracture needs urgent surgery to prevent infection.

Which knee injuries need surgery?

• Usually surgical: displaced or open fractures; knee dislocations with several torn ligaments; a locked knee; recurrent kneecap dislocation; ACL tears in pivoting athletes or unstable knees; repairable meniscus tears in the vascular zone.
• Usually non-surgical first: MCL sprains; stable LCL sprains; isolated PCL injuries; first-time kneecap dislocations; degenerative meniscus tears; undisplaced kneecap fractures; ACL tears in lower-demand patients willing to rehabilitate first.
• Case by case: cartilage injuries, combined injuries, and any injury in someone whose work or sport demands a fully stable knee.

Red flags: when to go to an emergency department

Go to the nearest emergency department or call 911 if:

• The knee or leg looks deformed, or the kneecap is visibly out of place.
• You cannot put any weight on the leg or move the knee at all.
• Bone is visible through a wound, or there is a deep cut over the joint.
• The foot is cold, pale, numb or tingling, which can signal damage to the vessels or nerves behind the knee.
• The knee is hot, red and very swollen and you have a fever, which can signal a joint infection.

See a doctor within a day or two if you heard a pop and the knee gave out, the knee swelled within hours, it locks, catches or keeps giving way, or you are limping. The NHS gives the same urgent list and advises a routine appointment for pain that has not improved within a few weeks. Until you are seen, the AAOS advice is rest, ice for about 20 minutes at a time, a light compression wrap, and elevation.

Quick answers

How do I know if my knee injury is serious?

Seek care promptly if you heard a pop and the knee gave way, it swelled within hours, you cannot straighten it or put weight on it, it looks deformed, or it is hot and red with a fever. Fast swelling after a twist often means a torn ligament or a fracture inside the joint.

Which knee injuries heal without surgery?

Most MCL sprains, most isolated PCL injuries, many first-time kneecap dislocations, and many meniscus tears, especially degenerative tears in people over 40, are treated with bracing, physiotherapy and time. Cracked kneecaps whose pieces have not moved apart can also heal in a cast or splint.

Which knee injuries usually need surgery?

Fractures where the bone pieces have moved apart, a knee that is locked and cannot straighten, dislocations with several torn ligaments, kneecaps that keep dislocating, and ACL tears in people who want to return to pivoting sport or whose knee keeps giving way.

Do I need an MRI for a knee injury?

Often, but not always. X-rays come first when a fracture is possible. MRI is the preferred test for ligament, meniscus and cartilage injuries because X-rays do not show soft tissue. Your surgeon orders it when the result would change the treatment plan.

Can an ACL tear heal on its own?

A complete ACL tear does not reliably heal, but it does not always need reconstruction. In a randomized trial of 121 young active adults, those who started with rehabilitation and had surgery only if the knee stayed unstable did as well at five years as those who had early surgery, and about half never needed the operation. Pivoting athletes and people with persistent giving way usually choose reconstruction.

Questions to ask your surgeon

• Which structures are injured, and is anything else damaged alongside the main injury?
• Is this injury one that usually heals without surgery, and what would a trial of rehabilitation look like?
• If I choose surgery, what are the options, and what does each one mean for my recovery time?
• What is my risk of re-injury or arthritis with each option?

Pathway sports medicine surgeons Dr. Jihad Abouali, who performs more than 200 ACL reconstructions a year, and Dr. Jas Chahal, Associate Professor at the University of Toronto, treat the injuries described here. Pathway’s guide to ACL repair vs. reconstruction explains when each applies.

References

1. American Academy of Orthopaedic Surgeons (OrthoInfo). Common Knee Injuries. Accessed October 21, 2026.
2. American Academy of Orthopaedic Surgeons (OrthoInfo). Anterior Cruciate Ligament (ACL) Injuries. Accessed October 21, 2026.
3. American Academy of Orthopaedic Surgeons (OrthoInfo). Posterior Cruciate Ligament Injuries. Accessed October 21, 2026.
4. American Academy of Orthopaedic Surgeons (OrthoInfo). Collateral Ligament Injuries. Accessed October 21, 2026.
5. American Academy of Orthopaedic Surgeons (OrthoInfo). Meniscus Tears. Accessed October 21, 2026.
6. American Academy of Orthopaedic Surgeons (OrthoInfo). Unstable Kneecap. Accessed October 21, 2026.
7. American Academy of Orthopaedic Surgeons (OrthoInfo). Articular Cartilage Restoration. Accessed October 21, 2026.
8. American Academy of Orthopaedic Surgeons (OrthoInfo). Patellar (Kneecap) Fractures. Accessed October 21, 2026.
9. NHS. Knee pain. Last reviewed December 21, 2023. Accessed October 21, 2026.
10. Frobell RB, Roos HP, Roos EM, Roemer FW, Ranstam J, Lohmander LS. Treatment for acute anterior cruciate ligament tear: five year outcome of randomised trial. BMJ. 2013;346:f232.
11. Siemieniuk RAC, Harris IA, Agoritsas T, et al. Arthroscopic surgery for degenerative knee arthritis and meniscal tears: a clinical practice guideline. BMJ. 2017;357:j1982.
12. Sanders TL, Pareek A, Hewett TE, Stuart MJ, Dahm DL, Krych AJ. Incidence of first-time lateral patellar dislocation: a 21-year population-based study. Sports Health. 2018;10(2):146–151.
13. Schneider DK, Grawe B, Magnussen RA, et al. Outcomes after isolated medial patellofemoral ligament reconstruction for the treatment of recurrent lateral patellar dislocations: a systematic review and meta-analysis. Am J Sports Med. 2016;44(11):2993–3005.
14. Stiell IG, Greenberg GH, Wells GA, et al. Prospective validation of a decision rule for the use of radiography in acute knee injuries. JAMA. 1996;275(8):611–615.

Health information notice: This article provides general information about common knee injuries and cannot replace individual medical advice. A surgeon diagnoses your injury by examining your knee and reviewing your imaging, and sets your treatment plan with you.