The anterior cruciate ligament, popularly known as the ACL, contains a wealth and variety of 'mechanoreceptors', nerve endings which relay information to the central nervous system about physical movement. These mechanoreceptors help to control and coordinate the position and movement of the knee during activity and thus protect the knee from injury.
The ACL spans the knee joint like a collagenous bridge, holding together and stabilising the knee. Unfortunately, the ACL is particularly prone to injury during sporting activity because of the high forces which can be placed on it by its associated long, lever-like bones (the femur and tibia). If you injure one of your ACLs seriously, you will be unable to continue your usual sports activity. The trouble is that since the ACL connects the tibia and femur, ACL damage alters knee kinematics - the movements between the tibia and femur as the knee flexes and extends. ACL injury also changes muscle-activation patterns around the knee, as the muscles attempt to adapt to the loss in stability; naturally, ACL impairments have a negative impact on running ability.
The physical disruption of normal knee activity associated with an ACL injury is bad enough, but what is sometimes forgotten is that ACL maladies consist of more than just damaged ligament cords; the mechanoreceptors within the ACL are usually hurt, too. This can lead to a significant loss in knee proprioception and can make it much more difficult for the nervous system to accurately determine the knee's position in space, its rate of motion, and its direction of movement. Altered knee kinematics after ACL injury can thus be the result of both weakened or ruptured collagen cords within the ACL and also poor sensory feedback from the mechanoreceptors in the ACL to the nervous system.
To minimise the negative, injury-related changes in knee kinematics and to reduce the risk of additional injury, athletes often brace their ACL-damaged knees with neoprene sleeves, elastic bandages, or functional braces, but not much is known about how these devices influence knee proprioception. Do they have any impact on a knee's ability to 'communicate' with its masters in the brain and spinal cord?
The logical argument
This may seem like a surprising question to you. After all, why should the mere attachment of a bandage or sleeve to a knee influence its proprioceptive capacity? The logical argument supporting this connection is that braces in effect make a knee stiffer than usual (less apt to move quickly and freely); this increase in stiffness should delay deformations in mechanoreceptors (mechanoreceptors 'fire up' and signal the nervous system when they are physically deformed by surrounding pressures or forces) and thus slow down the process of signalling the spinal cord that a change in knee position has been achieved. In turn, this could lead to delays in muscle contractions around the knee in response to knee motion, an effect which could be bad for an athlete attempting to recover from ACL injury (with less-quick muscular stabilisation of the knee joint, sudden, excessive motions might accidentally place too much force on an ACL which is attempting to heal).
What the research suggests
Somewhat surprisingly, however, the scientific evidence actually suggests that proprioception is improved with bandaging and the use of neoprene knee sleeves in a variety of different individuals. One study carried out with subjects with normal knees found a 25% improvement in knee-joint-position sense when elastic bandages were worn on the knees ('The Effect of Elastic Bandages on Human Knee Proprioception in the Uninjured Population,' American Journal of Sports Medicine, Vol. 23, pp. 251-255, 1995). Another piece of research, again with normal individuals, uncovered an 11% improvement in knee-joint-position sense when knee sleeves were worn ('Knee Bracing: Effects on Proprioception,' Arch Phys Med Rehabil, Vol. 77, pp. 287-289, 1996). Yet another study involving people suffering from osteoarthritis detected a 40% upswing in knee-position sense when the knees were cloaked with elastic bandages. This level of improvement was also apparent in individuals who had undergone knee-replacement surgery ('Joint Proprioception in Normal, Osteoarthritic, and Replaced Knees,' J Bone Joint Surg Br., Vol. 73, pp. 53-56, 1991).
Do athletes with actual ACL damage also benefit from wearing a knee sleeve or bandage? In one study, individuals with an ACL tear had significantly worse 'joint position sense' (the ability to detect the position of the knee in space), compared with control subjects with normal knees ('Knee Joint Proprioception in Normal Volunteers and Patients with Anterior Cruciate Ligament Tears, Taking Special Account of the Effect of a Knee Bandage,' Arch Orthop Trauma Surg, Vol. 115, pp. 162-166, 1996). When these ACL-injured subjects wore an elastic knee bandage, there was a significant improvement in position sense.
Detecting passive knee motion
Taking a slightly different approach, researchers from the University of Vermont College of Medicine, the University Hospital in Linkoping, Sweden, and the Ullevall University Hospital in Norway studied subjects with chronic tears of an anterior cruciate ligament ('The Effect of Bracing on Proprioception of Knees with Anterior Cruciate Ligament Injury,' Journal of Orthopaedic and Sports Physical Therapy, Vol. 32, pp. 11-15, 2002). In this study, the average time between the occurrence of the tear and the time of testing was 5.5 years, with a range of one to 22 years. Instead of looking at joint position sense, the researchers examined another indicator of knee proprioceptive prowess 'the threshold to detection of passive knee motion' (ie, how much passive movement could be induced in the knee before it 'realised' that it was actually in motion).
As it turned out, the knees with torn ACLs demonstrated significantly worse thresholds to detection of passive motion, compared with control knees (the individuals' normal knees were used as controls; if the ACL tear was in the right knee, for example, the left knee was used for comparison). However, the wearing of a neoprene sleeve on the ACL-deficient knee did not improve the threshold to detection of passive knee motion. This may not have been a real 'knock' against the use of neoprene sleeves, however. For one thing, the change in the threshold to detection of passive motion was quite small (just 0.28 degree), so the sleeves 'didn't have much to work with,' ie, improvements by definition would have to be very small. For another, the variable measured was the change in the threshold to detect passive motion, and the sleeve might function much more impressively during active motion initiated by muscles surrounding the knee.
At any rate, it's clear that ACL injury damages the collagen cords of the ligament and also harms proprioception of the knee; the proprioceptive loss can be long-lasting, although the magnitude of the long-term loss can be minor. Evidence suggests that the biggest loss in proprioception occurs during the two months immediately after ACL injury ('Proprioception After an Acute Knee Ligament Injury: A Longitudinal Study on 16 Consecutive Patients,' Journal of Orthopaedic Research, Vol. 15, pp. 637-644, 1997). After two months, the proprioceptive problems are persistent but small; research suggests, for example, that the negative change in the threshold to detect passive motion can last for five years or more but is usually less than one degree ('Proprioception in the Cruciate Deficient Knee,' J Bone Joint Surg Br., Vol. 74, pp. 247-250, 1992). It's not clear that this small loss has a major impact on knee function during athletic activity.
What about after surgery?
Athletes with ACL-deficient knees often undergo ACL reconstructions in hopes of returning their damaged knees to relatively normal function. How does the reconstruction process affect proprioception, and are braces of any value post-surgery?
One piece of research found that individuals who underwent ACL reconstruction with either a patellar-tendon autograft (in which a segment of their own patellar tendon was utilised to replace the ACL) or else an allograft (in which connective tissue from a cadaver was used to span the knee-joint space) had impaired thresholds to detection of motion during the two years after surgery ('Proprioception Following Anterior Cruciate Ligament Reconstruction,' Journal of Sports Rehabilitation, Vol. 1, pp. 188-196, 1992). In these subjects, wearing a neoprene sleeve over the knee produced a significant improvement in the threshold to detection of motion. If more than two years have passed following reconstruction, however, it is unlikely that use of a sleeve or knee bandage will improve proprioception; in fact, the reconstructed knee should be very close to having normal proprioception after that amount of time has gone by ('Proprioception after Anterior Cruciate Ligament Reconstruction with and without Bracing,' Knee Surg Sports Traumatol Arthrosc, Vol. 7, pp. 303-309, 1999).
Other effects of braces
As you can see, braces can improve proprioception of the knee in certain situations. Of course, braces can also have an effect on movements of the femur and tibia and thus on the amount of force placed on an injured or reconstructed ACL. To understand the size of this effect, scientists recently utilised a device which applied anterior-directed loads to the calves of subjects whose thighs were immobilised. When these subjects' knees were braced and their leg muscles were relaxed, 'anterior translation' (forward movement) of the tibia was reduced by up to 39% after force was applied to the calves; when the leg muscles were contracted, bracing produced as much as an 85% reduction in forward displacement of the tibia ('Anterior Cruciate Ligament Functional Brace Use in Sports,' American Journal of Sports Medicine, Vol. 24, pp. 539-546, 1996). Since excessive forward movement of the tibia can place undue stress on the ACL, it seems clear that the use of braces can be protective for individuals with ACL tears.
Individuals with ACL damage also have losses in proprioception of the knee, especially during the two months after injury. Using a brace during that time period should help with proprioception and may also control movement of the tibia and thus the forces placed on the ACL of the affected knee. Over longer periods of time, it is unclear that braces actually help with proprioception in ACL-deficient or ACL-repaired knees. It is also clear that the use of a brace changes the way in which muscles around the knee react to changes in knee position. Long-term, then, it is better to strengthen the ACL-recovered knee in a functional, sports-specific way and to rely on the improved strength to stabilise the knee, rather than using a brace as a permanent 'crutch'.