Anterior Cruciate Ligament (ACL) Tear: Injury and Rehabilitation

The anterior cruciate ligament (ACL) is one of four major structures stabilizing the knee joint, mainly responsible for limiting forward movement of the shin bone relative to the thigh bone and for rotational stability of the knee. Its rupture is one of the most serious injuries in sports involving pivoting, sudden stops, and landing from jumps — soccer, basketball, volleyball, alpine skiing, and combat sports.

The injury most often occurs without contact with an opponent — a typical mechanism is a sudden change of running direction, landing from a jump with the knee collapsing inward (valgus) while the torso rotates, or a sudden stop with the foot planted. At the moment of the tear, patients often hear or feel a characteristic "pop" in the knee, followed by rapidly building swelling and a feeling of instability — the knee "gives way" beneath them.

For decades, the standard approach in active, young patients with an acute ACL tear was early surgical reconstruction, usually within a few weeks of the injury, once the acute swelling had subsided. This approach rested partly on a legitimate concern that knee instability would lead to secondary damage to the menisci and articular cartilage, and partly on the belief that without surgery a patient could not regain full function. The Swedish KANON trial, one of the few fully randomized trials in this area, substantially challenged that assumption.

Whether a patient undergoes surgical reconstruction or chooses conservative treatment, rehabilitation follows similar, sequential phases — with the difference that after surgery there's an additional constraint related to healing of the ligament graft.

This sequence has clinical significance: skipping or rushing any phase — for example, introducing pivoting drills before the quadriceps has regained symmetrical strength relative to the healthy leg — is one of the more commonly cited mistakes that lengthens rehabilitation or leads to reinjury.

Rather than a single test, the modern approach to assessing readiness relies on a battery of tests covering several independent domains: quadriceps and hamstring strength measured objectively (ideally with a dynamometer, not "by eye"), symmetry between the operated and healthy limb on single-leg hop tests, the quality of the movement pattern during sport-specific tasks, and — increasingly included — psychological readiness, meaning confidence and a lack of fear of reinjury during movement.

Freedom from fear of movement isn't a side issue: some research indicates that persistent fear of reinjury correlates with worse functional outcomes and a higher risk of reinjury, independent of purely physical parameters. That's why good rehabilitation programs increasingly incorporate work on movement confidence, not just strength training.

Healing of the ligament (whether one's own after conservative treatment, or a graft after reconstruction) and rebuilding muscle mass and strength after immobilization require adequate nutritional support. Adequate protein intake is key here, since a period of immobilization and reduced activity promotes muscle mass loss (so-called disuse atrophy) even when body weight appears to be maintained.

An ACL tear is no longer an injury with one obvious treatment path. The KANON trial, one of the few genuinely well-designed randomized trials in orthopedic surgery, showed that in some patients, rehabilitation with the option of delayed surgery gives medium-term outcomes comparable to early reconstruction — so the decision should be individual, based on athletic level, degree of instability, and patient preferences, rather than an automatic referral for surgery.

Regardless of the treatment path chosen, what most affects the long-term outcome is the quality and patience of the rehabilitation itself — specifically, avoiding the temptation to rush a return to sport before objective functional criteria are met, even when the knee "feels fine."