Knee Ligament Injuries (Beyond the ACL)

Patient information — Dr Christopher Spelman, Orthopaedic Surgeon

Overview

As well as the anterior cruciate ligament (ACL), the knee is stabilised by several other ligaments, and each behaves differently when injured. Understanding which ligament is torn – its grade, and whether more than one is involved – is central to choosing the right treatment, because some of these injuries heal well without surgery while others need specialist reconstruction.

Posterior cruciate ligament (PCL)

The PCL is the strong ligament behind the ACL that stops the shin bone sliding backwards. It is usually injured by a direct blow to the front of the shin (a dashboard injury or a fall onto a bent knee) and is graded I to III by severity. Most isolated PCL injuries – even complete (grade III) ones – are managed successfully without surgery, using a specific dynamic PCL brace that supports the shin forwards and a structured, quadriceps-focused physiotherapy programme, with good outcomes maintained over several years. Reconstruction is reserved for higher-grade injuries with ongoing instability, or when the PCL is torn as part of a multiligament injury.

Medial collateral ligament (MCL)

The MCL, on the inner side of the knee, is the most commonly injured knee ligament, typically from a blow to the outer knee or a twist. It has a good blood supply and most injuries – including many complete tears – heal without surgery in a hinged brace with rehabilitation. Surgery is considered for certain severe tears, tears at the lower (tibial) attachment that can fold out of place and fail to heal, or when the MCL is injured together with a cruciate ligament, where the sequence of treatment matters.

Lateral collateral ligament (LCL) and posterolateral corner

The structures on the outer side of the knee (the LCL and the posterolateral corner) are less commonly injured but more likely to need surgical repair or reconstruction, particularly when torn alongside the ACL or PCL. A missed posterolateral corner injury is a well-recognised cause of failure of cruciate ligament reconstruction, so careful assessment of the outer side is important. Where surgery is needed, acute injuries may be repaired within the first two to three weeks, while later or more severe injuries are usually reconstructed.

Multiligament knee injuries and knee dislocation

When two or more major ligaments are torn – often following a knee dislocation – this is a serious, potentially limb-threatening injury. These are described using the Schenck classification according to which ligaments are involved. The priority is urgent assessment of the blood vessels and nerves behind the knee: the popliteal artery is injured in a significant proportion of true dislocations (around 18% overall, and up to about a third in certain patterns), and the common peroneal nerve (which lifts the foot) is injured in roughly 10–40%. Vascular status is checked clinically and, where there is any doubt, with imaging, because a missed arterial injury can lead to loss of the limb. Once the limb is safe, these injuries are generally treated surgically, with staged or combined reconstruction of the torn ligaments, followed by a prolonged rehabilitation.

What the evidence shows

Evidence supports non-operative management as the first-line treatment for most isolated PCL and MCL injuries, with good medium-term outcomes and low rates of later surgery. In contrast, posterolateral corner, multiligament injuries and knee dislocations generally require surgical treatment and specialist care, and their outcomes depend heavily on recognising the full pattern of injury (including the neurovascular structures) early. The key throughout is an accurate diagnosis of exactly which structures are injured.

Complications

Depending on the injury and treatment, risks include residual instability or looseness, stiffness (a particular problem after multiligament surgery), and the need for – or failure of – reconstruction. In knee dislocations and multiligament injuries there is the additional and serious risk of injury to the artery or nerve behind the knee; a peroneal nerve injury can cause a foot drop that may be temporary or permanent. This is why these injuries are treated as urgent and managed in a specialist setting.

Summary

Not all knee ligament injuries need surgery. Most isolated PCL and MCL injuries heal or stabilise with bracing and rehabilitation, while posterolateral corner, multiligament injuries and knee dislocations more often require surgery and urgent specialist assessment – including of the blood vessels and nerves. Dr Spelman will determine exactly which ligaments are injured and tailor treatment accordingly.