Anterior cruciate ligament injury

For Young Learners

Definition

The anterior cruciate ligament, or ACL, is a short tough band deep inside the knee. It runs at an angle from the thigh bone to the shin bone. Its job is to stop the shin sliding forward, and it helps stop the knee twisting too far.

Causes and Risk Factors

Most ACL tears happen with nobody touching the player. A footballer plants a foot and turns. A netballer lands from a jump slightly off balance. The body twists and the knee does not follow.

Girls and women tear the ACL more often than boys and men playing the same sport. The reasons are still being studied, and they are not simply about strength.

How It Presents

There is often a loud pop. The knee swells within a few hours, which happens because the tear bleeds inside the joint. Afterwards the knee can feel like it wants to give way when the person turns on it.

Warm up programmes that train landing and turning lower the risk. They work best when a whole squad does them regularly through the season rather than only at the start.

For Patients

Definition

The anterior cruciate ligament (ACL) is a strong cord inside the knee joint. It is the main restraint stopping the shin bone sliding forward on the thigh bone, and it helps control rotation when you turn.

How It Presents

A tear usually happens in a twisting movement rather than a collision. People describe a pop, swelling within a few hours, and later a knee that feels unreliable when they change direction.

Walking in a straight line is often comfortable quite early. That is one reason some tears are missed at first.

Investigations and Imaging

A clinician examines the knee first. An x-ray looks for a fracture rather than for the ligament itself. An MRI scan confirms the tear and, more usefully, shows what else was injured at the same time.

Management Overview

Not every tear needs an operation. The decision rests on how unstable the knee feels, what you need it to do, and whether anything else inside the joint was injured too.

Someone whose knee gives way during pivoting sport is in a different position from someone whose knee is steady during daily activity. Surgery replaces the torn ligament with a graft rather than stitching it back together.

Rehabilitation Outline

Recovery is measured in months rather than weeks, whichever route is taken. Rehabilitation matters as much as any operation, and it is the part you control. Returning to sport is judged on what the knee can do rather than on the calendar.

For Medical Students

Definition

The ACL is an intracapsular, extrasynovial ligament running from the posteromedial aspect of the lateral femoral condyle to the anterior intercondylar area of the tibia. It carries two functional bundles, anteromedial and posterolateral, named for their tibial insertions.

It is the primary restraint to anterior tibial translation and a secondary restraint to internal rotation.

Causes and Risk Factors

The classical mechanism is non contact: deceleration or landing with the knee near extension, valgus and internal rotation of the tibia. Female sex, previous injury, neuromuscular control and anatomical factors including tibial slope are recognised contributors.

How It Presents

An audible pop, immediate loss of confidence in the knee, and a haemarthrosis developing over the first few hours. Later the complaint is rotatory instability on pivoting rather than pain.

Examination

Lachman is performed at about twenty to thirty degrees of flexion and is the most sensitive clinical test. The anterior drawer at ninety degrees is less sensitive, particularly in the acute knee where hamstring guarding masks translation. The pivot shift reproduces the rotatory instability patients describe.

MRCS Level

Examination

Examine the whole knee, because the associated injuries change the management. Assess the collaterals, the posterolateral corner and the extensor mechanism. Guarding limits the acute examination, and assessment under anaesthetic is sometimes required before a plan can be made.

Investigations and Imaging

On plain radiographs look for a Segond fracture, an avulsion of the lateral tibial rim, which implies a cruciate rupture until proven otherwise. In a skeletally immature patient look for a tibial spine avulsion rather than a midsubstance tear. A deep lateral femoral notch sign is a further indirect clue.

MRI confirms the tear and grades the associated injury: lateral compartment bone bruising, meniscal tears including ramp lesions of the posteromedial meniscocapsular junction, chondral damage, and collateral or corner injury.

Management Overview

The immediate priorities are to exclude a knee dislocation with vascular injury, to restore range of movement, and to settle the effusion. Definitive management follows once the knee is quiet and the whole injury pattern is understood.

FRCS Level

Management Overview

Reconstruct on instability and demand rather than on the scan. The clear indication is a patient whose knee gives way during the activities they need and who has failed or declined a structured rehabilitation programme.

Graft choice trades re rupture risk against donor site morbidity and is age dependent. Hamstring autograft, bone patellar tendon bone autograft and quadriceps tendon autograft each have a defensible case. Allograft in a young pivoting athlete carries a higher failure rate.

Complications and Prognosis

Arthrofibrosis is associated with operating on a stiff, swollen knee with poor quadriceps control. Graft failure, donor site pain, and persistent rotatory laxity are the recognised early concerns.

Failure modes worth knowing for a viva: non anatomic tunnel placement, missed posterolateral corner injury, unaddressed ramp lesion or meniscal deficiency, increased posterior tibial slope, and return to sport before criteria are met.

Controversies and Evidence

The case for adding a lateral extra articular procedure in selected high risk patients is now reasonably strong, and its role in routine primary reconstruction is not settled.

Fellowship Level

Management Overview

Revision starts with why the first reconstruction failed. Separate a traumatic re injury from a knee that was never stable. Obtain long leg alignment views, a true lateral to assess posterior tibial slope, and computed tomography for tunnel position and diameter.

Tunnel management decides whether this is one operation or two. Acceptably placed tunnels of modest diameter allow a single stage revision. Confluent, widened or badly malpositioned tunnels need bone grafting and a staged approach, with the interval set by graft incorporation rather than a fixed number of months.

Complications and Prognosis

Treat the knee rather than the ligament. Meniscal deficiency, uncorrected coronal malalignment and increased posterior tibial slope each load a revision graft, and leaving them is a common route to a second failure.

Revision outcomes sit below primary outcomes for return to sport and for patient reported scores. Counsel the patient before the operation rather than after it.

Where slope is the factor being addressed, the operation that reduces it is an anterior closing wedge osteotomy. Whether the additional operation is justified is weighed in the consultant block rather than here.

Consultant Perspective

Controversies and Evidence

Four decisions sit in front of a consultant here, and the literature settles none of them.

The first is whether to repair a proximal avulsion rather than reconstruct it. Repair offers a native ligament with its innervation and no graft harvest. It risks a second operation in a young pivoting athlete, where reconstruction has a known medium term result. Tissue quality and age move the weight. The early series are not long enough to move it for anybody.

The second is when an increased posterior tibial slope justifies correcting it. Correction reduces the load the graft carries. An anterior closing wedge osteotomy adds its own recovery and its own complications to an operation the patient already needs. Previous failure moves the weight further than slope magnitude does, which is the part most often reversed.

The third is what clears a patient to return to sport. Criterion based batteries test what looks relevant, and their effect on re rupture in adolescents is not established. Time based clearance alone is inadequate, which is the only part everyone agrees on. What moves the weight is the sport, and whether anyone will see the patient again.

The fourth is how far registry data should reach into an individual decision. It is good enough to shape graft choice across a population and stays blunt for the person in clinic. The question a department can actually answer is narrower: whether its own outcomes match the registry, and whether anyone is checking.

All four turn on the same five things, and a trial population carries none of them. The patient's age and physiology, which set healing and tissue quality. Their medical comorbidities, because smoking, diabetes and body mass change operative risk and graft healing without changing the mechanics. The demands they put on the knee, which is occupation and sport rather than age. Their psychological state, because fear of reinjury governs what the reconstructed knee is actually used for. And their own ideas, concerns and expectations, which decide whether the operation that is statistically right is the one this patient should have.

Take all five explicitly rather than by impression, and counsel the patient with them in view. A graft choice that is right for a cohort can be wrong for the person in front of you on any one of them.