The primary role of the anterior cruciate ligament is to resist anterior subluxation of the tibia (Chan et al, 1994). The anterior drawer test, Lachman test, and stress arthrometry testing are often used clinically in order to assess anterior laxity of the knee resulting from anterior cruciate ligament tears. These tests offer sensitivities and specificities of 78-95% and 100%, 75-89% and 95-100%, and 92% and 95%, respectively for diagnosing complete tears of the anterior cruciate ligament (Lee et al, 1988). However, they are somewhat subjective and dependent on the experience of the examiner. Also, muscular guarding by the patient may limit clinical accuracy in the acute setting (Iversen, 1989).

Magnetic resonance (MR) imaging, an additional diagnostic technique, allows direct visualization of the anterior cruciate ligament and it’s surrounding soft tissues non-invasively and has improved accuracy of preoperative evaluation of the injured knee. Conventional MR imaging criteria focus on the morphology of the ligament itself, without considering joint alignment. The conventional MR imaging methods have lower diagnostic sensitivities and specificities than the methods considering joint alignment that have been proposed by Chan et al. (1994). Routine MR examination is performed with the knee in 15 degrees of external rotation (Lee et al, 1988). The extended position of the knee is ideal for measuring anterior laxity, as the anterior cruciate ligament is normally taught in extension (Kennedy et al., 1974). Previous biomechanical studies have shown that anterior tibial subluxation is greater in extension than flexion in patients with anterior laxity of the knee (Butler et al., 1980). 

Chan et al, (1994), conducted a study using MR imaging to determine the efficacy of MR images of anterior subluxation of the tibia for diagnosing complete tears of the anterior cruciate ligament. In their study, the presence of a torn anterior cruciate ligament was established on the basis of measurement of the position of the lateral tibial plateau relative to the lateral femoral condyle on a sagittal MR image. The records of 120 consecutive patients who underwent MR imaging of the knee and subsequently underwent surgery were analyzed (Chan et al., 1994). From this study it was concluded that an anterior subluxation greater than 5-mm (grade 2 or greater) is a simple objective measurement that helps in the diagnosis of complete tears of the anterior cruciate ligament. It was also determined that the 5-mm measurement also provided improved discrimination of complete from partial tears of the anterior cruciate ligament (Chan et al., 1994).

Disruption of the anterior cruciate ligament (ACL) results in abnormal arthrokinematics of the tibiofemoral joint. This leads to instability of the joint. Repeated episodes of instability lead to stretching of the soft tissue, injury to the menisci, and deterioration of the joint surfaces (Irrgang, 1993). If left untreated, ACL injury can lead to further complications such as progressive instability, recurrent pain, and degenerative changes of the joint.

ACL rehabilitation practices in past decades have included leg immobilization for 12 weeks post-operation and the use of crutches for 16 weeks (Mangine & Kremchek, 1997). These protocols were developed due to the idea that intra-articular recovery was a long-term process (Noyes, 1976). It is also well understood that as immobilization time increases intra-articular ligament strength decreases (Noyes, 1977). Due to the increase in scientific knowledge of reconstruction, graft selection, and the effects of joint motion the use of immobilization was decreased during the 1980's (Mangine & Kremchek, 1997). Rehabilitation began to incorporate immediate post-operative range of motion. Although the decrease use of immobilization shortened rehabilitation time, protocols continued to call for lengthy rehabilitation. It was in the mid-1980's that the "non-compliant" patient was discovered to be accelerating through the rehabilitation faster then the compliant patients (Shelbourne & Nitz, 1990). The non-compliant patients were those that went against instructions and progressed however they felt they could through the rehabilitation (Shelbourne & Nitz, 1990). From these non-compliant patients accelerated protocols were developed that decreased the rehabilitation time for the reconstructed ACL.

In the past 20 years, numerous scientific and clinical studies have focussed on the anterior cruciate ligament deficient knee and improved treatment. As our understanding of the short and long-term effects caused by the torn anterior cruciate ligament has improved, and the morbidity of surgical reconstruction has decreased, the indications of reconstruction have increased (Fu & Schulte, 1996). Several different techniques have been developed to improve the outcome of surgery. The patellar tendon autograft has been established as the "gold standard" of graft choice, but several other choices are available, and have produced similar results in early follow-up (Fu & Schulte, 1996). Studies have shown that the patellar tendon autograft is able to withstand forces developed in early rehabilitation activities (Mangine & Kremchek, 1997). It has been documented that the patellar tendon graft is 166% the strength of the anterior cruciate ligament (Mangine & Kremchek, 1997). This advance, along with others, has improved the success rate of ACL reconstruction to what is believed to be the 90^th percentile (Mangine & Kremchek, 1997).

One of the major advances in anterior cruciate ligament reconstruction has been the acceptance of early range of motion and controlled endurance and strength training during the postoperative period (Fu and Schulte, 1996). The goal of rehabilitation following ACL repair is to maximize dynamic stability of the joint to allow return to activity without recurrent episodes of instability (Irrgang, 1993). This requires the development of strength and endurance of muscles surrounding the knee joint as well as learning when and how to recruit those muscles with the appropriate timing and force to maintain stability of the joint (Irrgang, 1993).

Due to individual variation, it would not be prudent to develop specific protocols that detail each step of the rehabilitation program following ACL injury or reconstruction. It is possible, however, to establish guidelines for protocols that are based upon scientific research. Rehabilitation following ACL reconstruction focuses on treatment of impairments and functional limitations (Irrgang & Harner 1997). The protocol should also consider the effects of exercise and activity have on tibial translation as well as on strain and load of the ACL (Irrgang, 1993).

Many factors must be considered when designing a protocol for a given athlete, as they will all have a role in the success or failure of the protocol. These factors include conservative versus accelerated approach, surgical considerations such as graft type and source, gender differences, and perhaps most importantly the motivation of the athlete to return to competition.

The purpose of this literature review is to present an objective view of the available research in an attempt to aid the practitioner in developing a comprehensive anterior cruciate ligamet rehabilitation protocol.