Anterior cruciate ligament injuries are common among athletes. Although the true natural history remains unclear, anterior cruciate ligament injuries are functionally disabling; they predispose the knee to subsequent injuries and the early onset of osteoarthritis. This article, the first in a 2-part series, was initiated with the use of the PubMed database and a comprehensive search of articles that appeared between January 1994 to the present, using the keywords anterior cruciate ligament. A total of 3810 citations were identified and reviewed to determine the current state of knowledge about the treatment of these injuries. Articles pertaining to the biomechanical behavior of the anterior cruciate ligament, the prevalence of anterior cruciate ligament injury, the natural history of the anterior cruciate ligament–deficient knee, injuries associated with anterior cruciate ligament disruption, risk factors for anterior cruciate ligament injury, indications for treatment of anterior cruciate ligament injuries, and nonoperative and operative treatments were obtained, reviewed, and served as the basis for part I. Part II, to be presented in another issue of this journal, includes technical aspects of anterior cruciate ligament surgery, bone tunnel widening, graft healing, rehabilitation after reconstruction, and the effect of sex, age, and activity level on the outcome of surgery. Our approach was to build on prior reviews and to provide an overview of the literature for each of the before-mentioned areas of study by summarizing the highest level of scientific evidence available. For the areas that required a descriptive approach to research, we focused on the prospective studies that were available; for the areas that required an experimental approach, we focused on the prospective, randomized controlled trials and, when necessary, the highest level of evidence available. We were surprised to learn that considerable advances have been made during the past decade regarding the treatment of this devastating injury.

Before studying the biomechanical effects of rehabili tation exercises on the reconstructed knee, it is impor tant to understand their effects on the normal anterior cruciate ligament. The objective of this investigation was to measure the strain behavior of this ligament dur ing rehabilitation activities in vivo. Participants were pa tient volunteers with normal anterior cruciate ligaments instrumented with the Hall effect transducer. At 10° and 20° of flexion, ligament strain values for active exten sion of the knee with a weight of 45 N applied to a sub ject's lower leg were significantly greater than active motion without the weight. Isometric quadriceps muscle contraction at 15° and 30° also produced a significant increase in ligament strain, while at 60° and 90° of knee flexion there was no change in ligament strain relative to relaxed muscle condition. Simultaneous quadriceps and hamstrings muscles contraction at 15° produced a significant increase in ligament strain compared with the relaxed state but did not strain the ligament at 30°, 60°, and 90° of flexion. Isometric contraction of hamstrings muscles did not produce change in ligament strain at any flexion angle. Exercises that produce low or un strained ligament values, and would not endanger a properly implanted graft, are either dominated by the hamstrings muscle (isometric hamstring), involve quad riceps muscle activity with the knee flexed at 60° or greater (isometric quadriceps, simultaneous quadri ceps and hamstrings contraction), or involve active knee motion between 35° and 90° of flexion.

The purpose of this investigation was to evaluate replacement of a torn anterior cruciate ligament with either a bone-patellar tendon-bone autograft or a two-strand semitendinosus-gracilis autograft to compare the results of clinical testing, patient satisfaction, activity level, functional status, and muscle strength. Fifty-six patients with a torn anterior cruciate ligament were enrolled in a prospective, randomized, controlled study. Twenty-eight underwent reconstruction with a bone-patellar tendon-bone autograft, and twenty-eight were treated with a two-strand semitendinosus-gracilis autograft. Patients were followed for an average of thirty-nine months (range, thirty-six to fifty-seven months). At the time of final follow-up, twenty-two patients in each group were evaluated in terms of clinical test findings, patient satisfaction, activity level, functional status, and isokinetic muscle strength. The objective outcome of replacement of the torn anterior cruciate ligament with a bone-patellar tendon-bone graft was superior to that obtained with a two-strand semitendinosus-gracilis graft. At the three-year follow-up interval, the patients in whom a hamstring graft had been used had an average of 4.4 mm of increased anterior knee laxity compared with the laxity of the contralateral, normal knee, whereas the patients in whom a bone-patellar tendon-bone graft had been used had an average of 1.1 mm of increased knee laxity. Fourteen percent (three) of the twenty-two patients with a hamstring graft had a mild pivot shift, and 27% (six) had a moderate pivot shift. Only 14% (three) of the twenty-two patients with a bone-patellar tendon-bone graft had a mild pivot shift, and none had a moderate pivot shift. At the same follow-up interval, the patients in whom a hamstring graft had been used had significantly lower peak knee-flexion strength than those who had a bone-patellar tendon-bone graft (p = 0.039). In contrast, the two treatments produced similar outcomes in terms of patient satisfaction, activity level, and knee function (ability to perform a one-legged hop, bear weight, squat, climb stairs, run in place, and duckwalk). After three years of follow-up, the objective results of anterior cruciate ligament replacement with a bone-patellar tendon-bone autograft were superior to those of replacement with a two-strand semitendinosus-gracilis graft with regard to knee laxity, pivot-shift grade, and strength of the knee flexor muscles. However, the two groups had comparable results in terms of patient satisfaction, activity level, and knee function.