According to what is previously described (fourth traffic light), we allow the athlete to RTS only if certain criteria are completely satisfied. These criteria represent our RTS criteria, and we follow a potential strategy in order to accomplish each of them.

1. Recovery of muscular strength is certainly a milestone in rehabilitation, both in the literature and in our experience. Quadriceps femoris weakness is very common after ACLR and persists at long follow-up [35, 36]. We also know that strength weakness alters knee joint biomechanics and may lead to early osteoarthritis [37, 38]. It is indeed mandatory to reach the symmetry between the two limbs (100% both for extensor and flexor strength) evaluated with the isokinetic test (Fig. 51.2). In case of strength deficit, the test must be repeated until the complete recovery.

2. Metabolic recovery also plays a crucial role and has to be considered because fatigue leads to a potential risk of reinjury by altering the neuromuscular control [39]. We suggest checking aerobic and anaerobic lactate thresholds through specific tests (Fig. 51.3). Customized threshold training is subsequently proposed to guarantee a proper metabolic reconditioning. Before RTS the player has to reach the right values of aerobic and anaerobic threshold depending on the type of sport.

3. The on-field rehabilitation (OFR) is the most critical and important part of the recovery process. Sport-specific movements and drills are progressively reintroduced, and aerobic/anaerobic reconditioning is completed. Della Villa et al. demonstrated that a program of OFR allows earlier RTS without jeopardizing functional outcome at 5-year follow-up [40]. We also know that OFR may safely lead to complete functional recovery and return to sport [41]. The strategy we propose is to perform sport-specific supervised exercises both indoor (synthetic field) and outdoor (natural field) (Fig. 51.4). The protocol is progressive in terms of loading, complexity of the proposed exercises, and velocity of the agility drills. Regarding the duration of the OFR, it mainly depends on the clinical issue.

4. Movement patterns restoration also needs to be pursued. We know that specific movement patterns are frequently associated with a certain type of injury. For example, a dynamic knee valgus may predict primary ACL injury, and altered neuromuscular control may predict second ACL injury after ACLR [42, 43]. These dangerous patterns have to be avoided in order to reduce the reinjury rate. Patients presenting with some kind of movement impairments need to be pro-habilitated to a more correct movement strategy. This is the main reason why we suggest performing a sport-specific movement analysis test (MAT) and correct the neuromuscular impairments (Fig. 51.5).

Apart from these well-established criteria, we know that prevention and psychological aspects are other “key points” in the modern rehabilitation landscape. The prevention concept should be early introduced in the recovery process: from the first specific intervention in the pool to the more specific neuromuscular programs to be performed on the field. The programs may be really effective in primary prevention, with a reduction up to 30% of injuries, in case of maximal compliance to the program [44]. Plus, educating the patient to a neuromuscular prevention program (to be performed at least three times a week) can be very effective in reducing the risk of reinjury. Psychological factors have been already studied; it seems that both fear (fear of reinjury and kinesiophobia) and innate personality traits play a role in the return to sport decision [45].

There have been numerous studies that focus more specifically on clinical measures to assess functional performance following ACL reconstruction. A recent systematic literature review found that concentric or isometric strength and the single-hop leg test for distance were most commonly used [46]. Myer et al. provided a more recent analysis on an athlete’s single-limb performance using the single-limb symmetry index [47]. In their Level 3 case control study, single-limb vertical jump height and maximum vertical ground reaction force were measured on a portable force plate, but deficits were independent of time after reconstruction [47]. These measures can be difficult to extrapolate from the clinic to the playing field given the complexity of knee kinematics during athletic competition and considerations in regard to patient effort during clinical examination. Lentz et al. recently compared physical impairment and functional and psychosocial measures 6 months and 1 year following ACL reconstruction [48]. They found that elevated pain-related fear of movement and reinjury, quadriceps weakness, and reduced IKDC scores at 6 months post-op placing patients at risk for failure to return to sports at 1 year [48].

Reports in the early 1900s showed that the ACL also plays a role as a restraint to rotation of the knee [49]. Slocum and Larson first described a clinical examination that assessed rotatory knee stability [49]. Further work by Jakob and eventually Lemaire et al. on the basis of previous studies coined the term “pivot shift” to describe the anterolateral rotation laxity seen with ACL insufficiency [49]. This physical exam maneuver is characterized by abnormal anterior rotatory subluxation of the lateral tibial plateau when the medially rotated limb is under load in a few degrees of flexion. While still under load, spontaneous reduction occurs as the knee is flexed to 30° or 40°.

Despite a lack of standardization in the literature, the pivot shift is the most specific test to establish the diagnosis of ACL insufficiency before and after surgery [50–52]. On most occasions, pivot shift test grading in the clinic setting is subjective but used as an objective outcome tool to test dynamic laxity of the ACL. Standardizing the pivot shift test to improve inter-tester reliability has been the focus of recent work at our institution [53]. Several different approaches have been developed to assist in improving the pivot shift test: (1) measurement of knee laxity, (2) quantification of knee dynamics by acceleration, and (3) mechanization and instrumentation of the test [54]. A meta-analysis in 2012 found the KT-1000 arthrometer performed with maximum manual force has the highest sensitivity, specificity, accuracy, and positive predictive value for diagnosis of ACL rupture [26]. We utilize the KT-1000 arthrometer (MEDmetric Corporation) at our institution.

Improvements in measurement technology have allowed for quantification of dynamic knee motion with an electromagnetic motion tracking system. This permits characterization of the pivot shift by tibial anterior translation and/or tibial acceleration [54, 55]. The system (FASTRAK, Polhemus, Colchester, VT) at our institution uses an electromagnetic field with three receivers to measure the 6° of freedom of the knee at a high sampling rate (Fig. 51.6). To enhance repeatability, previous studies have shown that the pivot shift can be mechanized [56–58]. In the clinical setting, an examiner requires proprioceptive feedback to control the force and moment arm for each individual knee which introduces variability with regard to the examiner’s unique testing maneuver. Standardized technique at our institution has been designed on the basis of the Galway and MacIntosh procedure [59]. A recent review described the most common torques used to simulate the pivot shift were 10-Nm valgus and 5-Nm internal rotation at 30 degrees of knee flexion [49]. However, great variability in technique remains, and no methodology can currently be defined as the gold standard. As such, further work is necessary before defining return to play criteria on the basis of the pivot shift maneuver.