Introduction

Over 200,000 anterior cruciate ligament (ACL) injuries occur in the USA annually [1]. It is estimated that more than half of these injuries undergo surgical reconstruction [2]. Following ACL reconstruction (ACLR), under the direction of the orthopedic surgeon, the rehabilitation specialist is charged with the responsibility of returning the patient to their pre-injury level of function. Post-operative rehabilitation programs have changed dramatically over the past couple of decades. Strict protocols based on time elapsed from surgery have been replaced by criteria based guidelines (Table 1). These guidelines follow a progression where selective criteria are met prior to advancement in the program. This paper will discuss the progression of rehabilitation following ACL reconstruction.

Table 1 Anterior cruciate ligament (BTB) rehabilitation guideline
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A functional progression was defined by Kegerreis [3] as an ordered sequence of activities enabling the acquisition or reacquisition of skills required for the safe and effective performance of athletic endeavors. In other words, the patient needs to master a simple activity before advancing to a more demanding activity. Programs are individualized, where some patients will be ready to advance sooner than others. Biological factors such as graft revascularization and maturation as well as fixation techniques are also considered to ensure a safe progression through the ACLR rehabilitation program.

Range of Motion

Following ACLR, achieving full knee extension range of motion (ROM) should be achieved as soon as possible. Extension loss results in abnormal joint arthrokinematics at both the tibiofemoral and patellofemoral joints. This in turn leads to abnormal articular cartilage contact pressures and quadriceps inhibition [4, 5•, 6].

Achieving full extension should ideally be achieved preoperatively. McHugh, et al. [7] found that patients with knee extension loss were 5× more likely to have extension loss issues after surgery.

Treatment strategies employed to achieve full extension include low load prolonged stretching (Fig. 1) and calf stretching. Patellofemoral joint mobilizations in a superior direction are utilized to encourage extension ROM [8] Sleeping in a post-operative brace locked at 0° extension is utilized to encourage extension and discourage the formation of a flexion contracture during the night hours. Full extension is one of several important criteria to meet to safely progress the patient off their crutches after surgery.

Fig. 1
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Passive low load prolonged stretching utilizing a rolled towel under the ankle

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ROM exercises to facilitate flexion begin immediately after ACLR. ROM flexion goals of 120° should be met 4 weeks following surgery and full symmetrical flexion achieved by 12 weeks. Treatment strategies begin with active-assisted ROM exercises off the side of a plinth or bed (Fig. 2). Treatment strategies employed to further progress gains in flexion ROM include wall slides, active-assisted ROM sitting or on a step, and one half moon movement on a stationary bicycle. A short crank ergometer (Fig. 3) is utilized allowing patients to cycle earlier in the rehabilitative process and thus facilitate gains in flexion ROM [9]. Fleming and colleagues [10] demonstrated relatively low ACL peak strain values in vivo during stationary cycling.

Fig. 2
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Active-assisted knee flexion/extension

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Fig. 3
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Short crank bicycle

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Inferior-guided patellofemoral joint mobilizations are utilized to encourage gains in knee flexion [8]. When 120° of knee flexion is demonstrated, quadriceps stretching off the side of a plinth (Fig. 4) and eventually in a prone position is introduced to the patient’s program. Soft tissue massage can be of particular benefit throughout the progression to restore full symmetrical flexion ROM.

Fig. 4
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Quadricep stretching off the side of a plinth

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Post-operative Weight Bearing

Weight bearing progression following ACLR is dictated by graft selection and surgeon preference. Advanced fixation techniques such as cancellous screw bone-to-bone fixation allow for immediate post-operative weight bearing. Following ACLR with an autologous bone-patellar tendon-bone (BTB) graft weight bearing is at first partial (50%) utilizing crutches and subsequently progressed to weight bearing as tolerated (WBAT) on successive days. This progression allows the knee joint to acclimate to increased loads. Ambulating in water, e.g., underwater treadmill (Fig. 5) can be utilized to gradually apply increased load through the knee joint and assist in the development of a normal gait pattern. Walking in chest-deep water results in a 60 to 75% reduction in weight bearing, while walking in waist-deep water results in a 40 to 50% reduction in weight bearing [11, 12].

Fig. 5
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Underwater treadmill (Hudson Aquatic Systems LLC, Angola, IN)

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A post-operative brace is initially locked at 0° for ambulation to protect the harvest site. The brace is opened when quadriceps control is demonstrated by the ability of the patient to straight leg raise (SLR) without a quadriceps lag or complaints of pain. Crutches are then discontinued upon meeting the criteria of demonstrating a normal non-antalgic gait.

A significant decrease in patellofemoral pain has been reported when an immediate progressive weight bearing guideline is utilized [13•]. ACLR’s utilizing hamstring or allografts are progressed at a slower rate secondary to the decreased strength of soft tissue fixation and biological considerations, respectively. Weight bearing may be delayed following ACLR’s with concomitant articular cartilage or meniscus repair procedures.

Strengthening

Re-establishing quadriceps control is an early goal of post-operative ACLR rehabilitation. Controlling post-operative effusion assists in discouraging quadriceps inhibition. Spencer et al. [14] identified that mechanoreceptors in the joint capsule respond to changes in tension and in turn inhibit motor nerves supplying the quadriceps muscles. Therapeutic interventions utilized include the use a commercial cold with compression device (Fig. 6), quadriceps setting with a towel under the knee, and weight bearing with the appropriate amount of load. Should a patient have difficulty eliciting a quadriceps contraction, a biofeedback unit or an electrical muscle stimulator can be used in conjunction with the quadriceps setting exercise to better facilitate a quadriceps contraction. Numerous studies [15,16,17] have demonstrated an earlier return of quadriceps strength after ACLR with the use of electrical stimulation. As quadriceps activation is demonstrated, a key observation in order to progress the patient is seeing the patient perform a straight leg raise without the assistance of the post-operative brace without any complaints of pain or quadriceps lag. When this criteria is met, the post-operative brace can be opened to allow normal knee range of motion during ambulation. Crutches are continued at this point until a non-antalgic gait is demonstrated. Closed kinetic chain exercises including leg press and squats inside a pain free arc of motion are introduced as these activities have been shown to minimize stress to the ACL [18,19,20,21, 22•, 23]. Limited evidence now demonstrates that open kinetic chain (OKC) exercises inside a 90°–0° arc of motion may not compromise graft laxity.[24,25,26]. Mikkelsen et al., randomized 44 ACLR (BTB) patients to either a closed chain rehabilitation only program and a closed chain program that added open chain exercises 6 weeks post-operatively. At 6-month follow up, KT-1000 knee ligament arthrometer values showed no significant difference in knee laxity between the groups. A significant increase in quadriceps strength in the open chain group was also identified. With the demonstration of 0°–130° ROM, OKC exercise progression begins with multiangle quadriceps isometrics inside a 90°–40° arc of motion (Fig. 7). Isometric exercises are progressed to isotonic exercises using progressive resistance (PRE). At 3 months, post-operatively isotonic exercise is allowed throughout a full arc of motion and progressed to isokinetic exercises utilizing moderate-fast speeds. Throughout this progression, the rehabilitation specialist should closely monitor the patellofemoral joint for crepitus and complaints of pain.

Fig. 6
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Gameready cold/compression device

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Fig. 7
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Isometric Knee Extension at 60° knee flexion

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Neuromuscular Training

After ACLR, surgery afferent information is altered, which results in a disruption in the pathway between the patient’s center of gravity and base of support [27]. Improving neuromuscular reaction time to imposed loads enhances dynamic stabilization around the knee and thus protects the static reconstructed tissue from overstress or re-injury [28]. As soon as the patient achieves 50% weight bearing, neuromuscular/balance training is initiated on a dynamic balance system (Fig. 8) or proprioception device (foam cushion, rocker board, etc.). Balance activities are then increased progressively to include unilateral weight bearing, use of multiplanar support surfaces, and perturbation training [29]. Activities should attempt to eliminate or alter sensory information from the visual, vestibular, and somatosensory systems so as to challenge the other systems.

Fig. 8
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Dynamic balance system (Biodex Corporation, Shirley, NY)

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Continued Progression

As range of motion and strength is demonstrated, the patient is instructed in a progressive step-up program, first mastering 6″ steps advancing to normal stair height 8″ steps. As further strength is demonstrated, a forward step down program is introduced (Fig. 9).

Fig. 9
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Forward step down exercise off an 8″ step

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After 3 months post-op, if ROM is within normal limits and sufficient strength is demonstrated via a pain-free 8″ step down without deviation, a running program is started. Backward running is preceded by forward running, as retrograde running has been shown to generate lower patellofemoral joint compression forces than forward running [30]. An Alter G treadmill (Alter G, Inc., Fremont, CA) (Fig. 10) is utilized to incrementally add load during a forward running progression. A running program is progressed with an emphasis on speed over shorter distances vs. slower distance running.

Fig. 10
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Unweighted treadmill (AlterG Inc., Fremont, CA)

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Plyometric training is then incorporated only if full ROM, an adequate strength base and flexibility, is demonstrated. Plyometric training should follow a progression with its components of speed, intensity, load, volume, and frequency being monitored and progressed accordingly. Activities should begin with simple drills and advance to more complex exercises (e.g., double leg jumping vs. box drills). Agility and deceleratory training are important interventions to include in the later phases of rehabilitation in preparation for return to sport.

Return to Sport

When to return to sport following ACLR is a controversial issue. Pinczewski et al. [31] reported that one in four patients undergoing an ACLR will suffer a second tear within 10 years of their first. Paterno et al. [32•] reported an incidence rate of a second ACL injury within 2 years after returning to sports was nearly 6× greater then healthy controls.

More and more surgeons and rehabilitation specialists are utilizing numerous forms of assessment in determining an athlete’s readiness to return to play. Subjective rating scales, knee laxity testing, isokinetic testing, functional hop testing, balance testing, and qualitative movement assessment (Fig. 11) are utilized to provide evidence in the decision making process. Acceptable scores on these assessments are required to safely return the athlete to sport. Following discharge from a formal rehabilitation program, volume of athletic exposures needs to be modified.

Fig. 11
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Quality movement assessment of a single leg squat exercise

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Several studies have demonstrated deficits in muscular strength, kinesthetic sense, balance, and force attenuation 6 months to 2 years following reconstruction [32•, 33,34,35]. Return to sport 6 months following ACLR, therefore is no longer the expected norm.

Summary

Rehabilitation following ACL reconstruction has shifted from a paradigm based on protocols to a progression based program with gradient increases in difficulty. It is the rehabilitation specialist’s responsibility to consider the forces placed on the healing ACL graft and patellofemoral contact pressures generated during specific exercises and activities. Early in the rehabilitative process, the focus needs to be on gaining full knee extension, decreasing edema, and developing quadriceps strength. Therapeutic exercises should progress in difficulty often being performed in a variety of positions and settings. Neuromuscular training should be implemented into the rehabilitation program as early as deemed appropriate and progressed accordingly throughout the rehabilitative process. ACLR rehabilitation progression should be based on objective criteria and not just time frames. In order to achieve a successful outcome the rehabilitation specialist must continually assess the patient and select exercises that challenge the patient properly. When the patient is ready to return to sports, objective criteria must be met in order to reduce the risk of further injury. This transition is now considered to take place at greater than 6 months. Exposure/volume to athletic activity needs to be controlled in the post-rehabilitation period.