An ACL reconstruction does not repair the torn ligament. It replaces it with a tendon graft that the body then turns into a new ligament over the following year. The operation is done through keyhole portals, takes about an hour, and is usually a day procedure. What differs between surgeons and between patients is the graft, what is done to the meniscus, and whether anything is added on the outside of the knee.
The graft
Four grafts are in common use, and each has a long track record.
Hamstring tendons (your own semitendinosus, sometimes with gracilis), folded to make a four-strand cord. The commonest graft in Australia. Trade-offs: a weaker hamstring for the first year, an ache at the back of the thigh and a numb patch below the scar, and, in the Scandinavian registries, a higher revision rate than a patellar tendon graft, with a hazard ratio of about 2.3 in the Norwegian registry. The size of the tendon matters: a graft of 8 mm or less in diameter has carried about seven times the failure risk in pooled studies, and some people simply have small hamstrings.
Patellar tendon (bone-patellar tendon-bone, the central third of the tendon below the kneecap with a block of bone at each end). The most stable graft on examination, with bone-to-bone healing and the lowest revision rate in the registries. Trade-offs: more pain at the front of the knee, pain with kneeling in a proportion of patients that is the main reason people decline it, and a small loss of extension and of quadriceps strength in the Cochrane review.
Quadriceps tendon (a strip of the tendon above the kneecap). The newest of the three in common use. In meta-analyses it matches the patellar tendon for stability and failure with about a quarter of the donor-site pain, and it has failed less often than hamstring in young patients in recent pooled data. The thigh muscle is slower to switch on afterwards, and a Swedish registry has reported more re-operations for stiffness and pain, so the picture is still forming.
Donor tendon (allograft), a tendon from a tissue donor. No donor site, so no hamstring weakness, no kneeling pain, a shorter operation and an easier first few weeks. The trade-off is in how the tissue was prepared and in who receives it. Allografts that have been irradiated or chemically sterilised fail more often, and most of the data showing allograft failing more are dominated by those grafts. In the largest registry analysis, non-processed donor tendons and those with minimal irradiation had no higher revision rate than hamstring autograft. In a systematic review of non-irradiated allograft against autograft, there was no difference in failure, laxity or knee scores, with the authors noting that this applied to patients in their late twenties and early thirties. In young, high-demand athletes the picture is different: a ten-year randomised trial in military patients with a median age of 26 found 26.5% failure with non-irradiated allograft against 8.3% with hamstring, and a large prospective cohort found about five times the re-tear odds with allograft, falling with each year of age.
In this practice a non-irradiated, non-chemically-processed donor tendon is the mainstay for most patients, including recreational athletes returning to pivoting sport, at any age. It avoids the small-hamstring problem altogether, it spares the front of the knee, and it is the graft of choice in revision and multi-ligament surgery. Professional and high-level competitive athletes typically have their own patellar tendon, because the high-demand cohorts are where allograft has failed more, and a lateral tenodesis is added where the knee warrants it. The distinction is the demand on the knee, not a birthday, and the numbers on both sides are part of the consent discussion. The full evidence, both sides of it, is on Which graft is best?.
How the graft is placed
Through two or three small portals, the inside of the knee is inspected and the menisci and cartilage are treated first. The remnants of the torn ligament are cleared, keeping the tissue at its attachments where that helps the graft heal. A tunnel is drilled in the femur and another in the tibia, each placed in the footprint where the original ligament attached, because tunnel position is what decides whether the graft controls rotation and lasts. The graft is pulled through, tensioned with the knee near full extension, and fixed at both ends, with a suspensory button or a screw depending on the graft. A single bundle is standard: the Cochrane review found no clinical difference between single- and double-bundle reconstruction.
The meniscus
Half of knees coming to reconstruction have a meniscal tear, and what is done about it matters more for the knee’s long-term health than the ligament does. A tear in the vascular outer zone is repaired, with sutures passed through the meniscus, and a repair heals better alongside a reconstruction than on its own: in a population study, re-operation after repair was 9.7% with a concurrent reconstruction against 16.7% without. A tear that cannot be repaired is trimmed to a stable edge, removing as little as possible, because meniscectomy is the strongest single predictor of arthritis after an ACL injury: 50% of patients with a meniscectomy had arthritis in one pooled analysis, against 16% of those without.
A meniscal repair changes the early rehabilitation: a hinged brace and protected weight bearing for four to six weeks, and no loaded deep bending for three to four months. The combined rehabilitation guide covers it.
The lateral extra-articular tenodesis
The ACL controls rotation as well as forward slide, and in some knees a graft inside the joint does not control rotation fully. A lateral extra-articular tenodesis is an additional step on the outside of the knee: a strip of the iliotibial band is routed beneath the lateral collateral ligament and fixed to the femur, acting as a rotational check-rein. In the STABILITY trial, 618 patients aged 14 to 25 having a hamstring reconstruction were randomised to have it or not. Over two years, graft rupture was 11% without it and 4% with it, and the broader measure of clinical failure fell from 40% to 25%. The cost was more pain in the first three to six months and a slightly larger scar. A Swedish registry found no difference in unselected patients at two years, which is why it is added selectively: the young athlete, the knee with a high-grade pivot shift, the revision, and the hyperlax knee.
Anaesthetic and the day
A general anaesthetic, with local anaesthetic placed in and around the knee at the end. Where a nerve block is added, it is an adductor canal block, which gives the same pain relief as the older femoral nerve block with better quadriceps strength in the first days, and a quadriceps that will not switch on is the one thing the first fortnight cannot afford. Most isolated reconstructions go home the same day; a reconstruction with a meniscal repair or a lateral procedure sometimes stays a night.
The risks, honestly
- Infection. About five in a thousand overall and about three in a thousand deep, in the largest registry, and higher with hamstring grafts than patellar tendon grafts (0.61% against 0.07% deep infection). An infection in a reconstructed knee is treated urgently with washout and antibiotics, and the graft can usually be saved.
- Blood clots. A symptomatic clot in about four in a thousand. Routine blood thinners are not used after an isolated reconstruction unless you have a particular risk factor; walking from day one is the prevention.
- Stiffness. A knee that loses its bend or, more importantly, its full extension. About 4% need a further procedure to free it, and the strongest risk factors are a knee that was stiff before surgery, a meniscal repair, and delayed rehabilitation afterwards, which is why the pre-operative program and the first fortnight matter so much.
- Graft failure. About 4% to 5% need a revision within five years across the registries. The figure is far higher in young athletes returning to pivoting sport, where graft rupture reached 18% at five years in an Australian cohort under 20, and it is the subject of chapter 5.
- Numbness. A patch of numb skin on the front of the shin below the scar after a hamstring graft is the rule rather than the exception; it shrinks over a year and rarely matters.
- Donor-site problems. Kneeling pain after a patellar tendon graft; hamstring weakness and a back-of-thigh ache after a hamstring graft; slower quadriceps recovery after a quadriceps tendon graft.
- In a growing child, disturbance of the growth plate, in about 2% to 5% of cases with modern techniques and almost always minor. See ACL injury in children and teenagers.
Your own risk profile, your age, your sport, your weight, diabetes and smoking, is part of the consent discussion. The surgeon’s job is to reduce every risk that can be reduced and to be honest about the ones that cannot.