ACL reconstruction: what happens, risks and recovery

The anterior cruciate ligament sits in the middle of the knee and stops the shin sliding forward under the thigh bone. Once it has torn completely it does not heal back together on its own. Some people cope well without it; others find the knee buckles on turning, stepping off a kerb or changing direction in sport.

Surgery is therefore aimed at instability, not at the scan finding. People whose knee feels solid in everyday life and who do not play pivoting sports are often advised to try structured physiotherapy first. Surgery tends to be discussed when the knee keeps giving way despite good rehabilitation, when other structures such as the meniscus are also torn, or when the person needs to return to a sport that involves twisting and cutting.

Strengthening exercises and activity changes are the main alternative. Choosing them first does not close the door on reconstruction later.

Many surgeons ask for a period of "prehab" before the operation: getting the swelling down, regaining full straightening of the knee and rebuilding thigh muscle strength. A knee that is calm and moves well going in tends to be easier to rehabilitate afterwards.

At pre-assessment the team checks your general health, medicines and allergies and explains fasting rules for the anaesthetic. Follow the written instructions you are given. Bring comfortable loose clothing that fits over a bulky dressing, and arrange for crutches to be fitted before you leave.

• Tell the team about blood thinners, diabetes medicines and anything that affects healing, including smoking or vaping.

• Plan transport home and someone to help for the first few days.

• Set up a bed or chair arrangement at home that keeps the leg elevated and avoids stairs where possible.

The operation is normally done under general anaesthetic, sometimes with a nerve block or spinal to ease pain afterwards; your anaesthetist will explain which suits you. According to one NHS trust leaflet it takes around an hour and a half.

Using a camera and fine instruments through small cuts, the surgeon clears what remains of the torn ligament. A tendon graft, typically taken from the hamstring through a small additional incision near the shin, is threaded through tunnels drilled in the shin bone and thigh bone and held with buttons and screws. Other graft choices exist, and some surgeons use the kneecap tendon; the choice affects which exercises are delayed afterwards.

You then wake in recovery, are helped to stand by a physiotherapist and, once pain is manageable and you can use crutches safely, you can usually go home the same day.

The leaflets consulted for this page list infection, stiffness, re-rupture of the graft, nerve injury and numbness around the graft harvest site as recognised complications. None of the pages retrieved gave percentages, so none are quoted here; ask your surgeon for figures that reflect their own practice and your personal risk.

More common problems are bruising, swelling and a long period of muscle weakness. Stiffness is often addressed by working hard at early range of movement, which is one reason the first weeks matter. Less common but serious problems include a blood clot in the leg, wound infection and a graft that stretches or tears again.

A Cambridge-based rehabilitation leaflet sets out a sequence: in week one the aim is control of the thigh muscle, less swelling and full straightening, with some bending; bending targets rise through weeks two to four. From about six weeks to twelve, strengthening continues while running, pivoting, jumping and cutting are avoided until cleared.

After twelve weeks the same leaflet moves from time-based to criteria-based progression, and straight-line jogging can start around three months if control on a single-leg dip is good. Hamstring-graft patients are told to avoid resisted open-chain hamstring work for twelve weeks.

• Crutches up to about three weeks in the Leeds leaflet, then walking without them.

• Office work roughly two to three weeks; heavy physical work later, as advised by your team.

• Driving and swimming (not breaststroke) from around three to four weeks once you can make an emergency stop; stationary bike from about six weeks.

• Return to sport only after strength and hop tests, not just after a calendar date.

A warm, increasingly swollen knee that is more painful without any change in activity, a fever or chills, or redness spreading from a wound should be reported promptly to the hospital or your GP, or to NHS 111 out of hours. A hot, swollen, tender calf, or sudden breathlessness or chest pain, needs emergency help: call 999.

Some soreness after exercises is expected. The Cambridge leaflet advises that if exercise raises knee temperature, swelling or pain, you reduce what you are doing, use ice and elevation and talk to your physiotherapist.

• Which graft do you recommend for me and why?

• Is my meniscus damaged, and will it be repaired at the same time?

• What weight-bearing and brace instructions apply to me?

• Who runs my physiotherapy, and how soon does it start?

• What are the criteria for returning to my sport, and who signs them off?

The operation and the graft options are broadly the same in both settings. Differences are usually practical: how long you wait, whether you can choose the surgeon, how much physiotherapy is bundled into the package and how many follow-up visits are included. On the NHS, physiotherapy capacity varies by area, so ask early how and where you will be seen. If you are paying, ask exactly what is covered if rehabilitation takes longer than planned.