Proprioception of the Knee
Anterior cruciate ligament (ACL) injury results in functional instability because of the increased translation of the tibia on the femur, as well as the rotatory instability of the knee. The stability of the knee is also dependent on one’s awareness of the joint’s position in space. Proprioception is the ability to perceive a joint’s position even without a visual reference. Nerve endings provide sensory feedback to the central nervous system that allows us to reference joint position. These nerve endings, called proprioceptors, are located in the joint capsule, cruciate ligaments, and menisci.
Without this feedback loop, the success of ligament reconstruction is diminished. In the 1980s with the advent of ACL reconstruction, rehabilitation efforts were focused on gaining range of motion and strength. At that time in sports medicine history, we were unaware that proprioception should be addressed during rehabilitation. Common complaints after knee ligament reconstructions were that the patient felt fatigue and the inability to cut and jump at his or her preinjury level. Most likely, these deficiencies were due to the use of inappropriate functional rehabilitation techniques. Kennedy et al. demonstrated that proprioceptors were present in the knee, and this discovery allowed us to change our rehabilitation strategy to include balance and coordination exercises in the regimen of postoperative care.
Many authors have studied the diminished proprioceptive function that occurs after ACL injury, and they have all concluded that a deficit is in fact present in the knee, probably because of the lack of feedback information provided by the ACL. Delayed hamstring firing could be a result of this lack of feedback. Capsular tension also may provide feedback to the central nervous system through proprioceptors that are present. When the capsule is injured during a knee injury, the normal capsular tension that is disrupted with an ACL injury may be lost.
The challenge of rehabilitation specialists is to stimulate other receptors in the surrounding area to make up for the loss of the feedback loop when the proprioceptor is injured. Rehabilitation specialists begin to stimulate the proprioceptive system as early as possible after ACL reconstruction.
Joint position sense involves several systems. The auditory, visual, tactile, and proprioceptive systems all contribute to joint position sense. To overload the damaged proprioceptive system, one or more of the normal systems can be removed. An example of this strategy is removing auditory input when tossing a ball to a patient ( Fig. 39-1 ). As the patient accepts the pass, he or she no longer hears the auditory input of the ball leaving the thrower’s hand or the sound of the ball hitting the hand as it is caught, which results in a more concerted effort to overload the proprioceptive system in hopes of recovering a degree of lost sensory feedback. Another example is having the patient close his or her eyes while balancing on the involved lower extremity. This removal of visual stimulation will force the patient to rely on his or her proprioception system to maintain stability on a single leg.
Proprioceptive input may begin early after a knee injury or surgery. Patients with a reconstructed ACL still maintain a degree of proprioceptive loss after the reconstruction. Even though capsular and ligament tension are restored, the feedback loop remains deficient. It is the job of the clinical rehabilitation specialist to safely overload the injured neuromuscular system to help the patient regain function through improved proprioceptive control. It is not known how proprioceptive loss is regained, but improved dynamic function by increased muscle control may be a factor. Proprioceptive nerve endings do not return to the graft material used to reconstruct the knee. Although the ACL and capsule may not have been injured, a proprioceptive deficit still exists in the postoperative knee because of the hemarthrosis and swelling present in the knee. Kennedy et al. found that in 10 subjects with normal knees, an experimental knee effusion produced with injection of 60 mL of fluid resulted in profound inhibition of reflex-evoked quadriceps contraction. Therefore reduction of swelling after surgery is a primary goal of rehabilitation.
The initial proprioceptive exercise performed after ACL reconstruction of the knee is instituted on postoperative day 2. This exercise involves simple weight shifting from the normal side to the involved side and back again. The patient is asked to assume a standing position while hand support is provided by leaning on a table or plinth. The knees are maintained in about 30 degrees of flexion. The patient then shifts the weight from side to side so that the involved knee accepts the full body weight alone, supported by the upper extremity. This activity is performed for 5 minutes. It is important to maintain the knee in a slightly bent position so that the quadriceps must fire to support the body weight ( Fig. 39-2 ).
Flexed knee ambulation is an abnormal postoperative gait assumed by many patients. This position may be assumed because of the lack of eccentric quadriceps control during the stance phase of the involved extremity as it progresses into the swing phase of the normal knee. The specific lack of control of the quadriceps occurs between the beginning of midstance through the beginning of the push-off phase of the gait cycle ( Fig. 39-3 ). It is during this phase that the quadriceps must eccentrically control the knee flexion moment that is created during this phase of the gait cycle.
It is critical that this eccentric control be established early in the rehabilitation process. Establishment of this control is helpful in maintaining a normal gait pattern, which will reduce altered neuromuscular modulation of the muscles that allow us to propel. If this issue is not addressed early in rehabilitation, patients will continue to assume this flexed-knee posture because this position provides a stable base of support during the stance phase as a result of the lack of need for muscle activity at 30 degrees of knee flexion. Another problem with the flexed-knee posture is increased patellofemoral compression. By maintaining this posture during gait, constant pressure is placed on the patellofemoral joint that could cause undue stress to the articular cartilage and the soft tissue structures that support it.
Another gait control activity is cone ambulation, which can be instituted 2 or 3 days after ACL reconstruction ( Fig. 39-4 ). The patient is required to use crutches for this activity. (Of course, patients who have weight-bearing restrictions as a result of associated knee injuries should not perform this activity.) Cone ambulation begins by placing four to six Styrofoam coffee cups on the ground about 18 inches apart. The subject is asked to step over the cone (the cup) with the involved extremity and place the foot between the next two cones. It is important that the rehabilitation specialist instruct the patient to maintain a slightly flexed position at the knee during this activity ( Fig. 39-5 ). The patient balances on the involved extremity alone and with crutches for 5 seconds and then proceeds to the next cone. We recommend four cycles of cone ambulation; up and back completes one cycle. Additional training can be performed by turning 90 degrees and performing lateral cone ambulation. As the knee flexion and quadriceps control improve, cones may be used to increase the challenge of the gait cycle.
Another important cue to maintenance of a normal gait pattern is to instruct patients to flex the knee as they begin lift-off of the weight-bearing extremity (the preswing phase of gait; see Fig. 39-3 ).
Criteria for discarding crutches are twofold. First, the patient must be able to ambulate with a normal gait with crutches. Second, the patient must have no sensation of the knee giving way. Too often patients are instructed to discard their crutches before they achieve a normal gait pattern in hopes that removal of the crutches will improve their strength. We have found the opposite to be true. Progression should be from two crutches to one crutch to no crutch. Proper gait training is the first phase of forcing the proprioceptive system to become engaged.
Continued proprioceptive input may be enhanced with the use of specialized equipment. The Monitored Rehab Systems (Haarlem, The Netherlands) Functional Squat provides stimulation of joint position sense along with strength training. This system allows the clinician to preset the range of motion to be used during the activity. The patient then controls the cursor position on a computer screen by performing a unilateral leg press with the involved lower extremity. As he or she extends the knee, the cursor moves horizontally on the screen. Six computer games may be performed to stimulate proprioceptive input. Accuracy is measured by calculation of error, and a score is given at the end of each exercise session. A typical session includes 60-second bouts of exercise and six sets ( Fig. 39-6 ).
Continued proprioceptive training evolves as lower extremity strength is increased. Functional strengthening continues with lunging activities using weighted balls as shown in Figure 39-7 . Added proprioceptive stimulation can be performed with use of a foam cushion, as shown.
Cone reaching is another method of providing proprioceptive training. This exercise requires the patient to reach across the midline and control balance and coordination while performing a single-leg squatting maneuver ( Fig. 39-8 ).