Key points

Introduction

The standard definition of proprioception is “the reception of stimuli produced in (an) organism.” More colloquial use in orthopedic surgery has resulted in a general definition of proprioception as the ability to sense position of a joint in space. It provides crucial and fundamental somatosensory input for everyday functioning and is called on for the simple task of standing to more complex activities such as walking, running, and navigating unstable ground. Proprioceptive mechanoreceptors provide feedback for position in space and are, therefore, a critical asset for interacting with the surrounding three-dimensional world. Previous research has shown that proprioception is decreased with osteoarthritis (OA) of the knee, leading to an increased risk of falling. As the prevalence of OA increases with an aging population, so does the need for total knee arthroplasty (TKA), and knowing the effect of TKA on proprioception is essential. The purpose of this article is to review the relevant literature regarding proprioception and its relationship to balance, aging, and the effect of TKA.

Introduction

The standard definition of proprioception is “the reception of stimuli produced in (an) organism.” More colloquial use in orthopedic surgery has resulted in a general definition of proprioception as the ability to sense position of a joint in space. It provides crucial and fundamental somatosensory input for everyday functioning and is called on for the simple task of standing to more complex activities such as walking, running, and navigating unstable ground. Proprioceptive mechanoreceptors provide feedback for position in space and are, therefore, a critical asset for interacting with the surrounding three-dimensional world. Previous research has shown that proprioception is decreased with osteoarthritis (OA) of the knee, leading to an increased risk of falling. As the prevalence of OA increases with an aging population, so does the need for total knee arthroplasty (TKA), and knowing the effect of TKA on proprioception is essential. The purpose of this article is to review the relevant literature regarding proprioception and its relationship to balance, aging, and the effect of TKA.

Basic science of proprioception

Proprioception is made possible by length-sensing muscle spindle units in skeletal muscle, stretch receptors in the fibrous joint capsule, and Golgi organs in tendons and ligaments. The inputs from these various receptors are processed in the brain and integrated with visual and vestibular information to generate a sense of position and movement through space. A major component of proprioception is joint position sense, which can be examined by having patients determine the position of a limb or joint without use of their eyes or vestibular system. A commonly used method of quantifying proprioception is called joint position matching, during which a patient’s limb is held at a specific angle for a moment, then returned to the neutral position before asking the patient to recreate the angle. It is normal to have a slight and gradual decrease in proprioception as part of the aging process; however, sudden or gross decreases in proprioceptive abilities should be considered pathologic and investigated.

Mechanoreceptors

OA of the knee is accompanied by restricted mobility because of pain, altered joint mechanics, and muscle atrophy. These changes cause deficits in balance, which have been demonstrated to be important requirements for the independent and safe performance of the activities of daily life and in the prevention of falls. During TKA many intra-articular structures of the knee joint known to contribute to proprioception are resected. The menisci, cruciates, and other soft tissues about the knee have important functions in load transmission, gliding movements, nutrition of the articular cartilage, and stability of the knee joint. The population of nerve fibers and receptors within a meniscus provides the central nervous system with information related to mechanical stimulation, including motion and excessive loading, and may be related to protective reflexes. Mechanoreceptors are neural elements that mediate tactile and position sense. They convert stimuli into neural impulses, which are then deciphered and interpreted by the central nervous system. Although considerable controversy still exists about the types and exact locations of mechanoreceptors in human ligaments, it has been shown that they receive sensory input and mediate a response that regulates muscle tone and, therefore, coordination.

Effect of the posterior cruciate ligament

A suggestion of the proprioceptive role of the posterior cruciate ligament (PCL) is supported by the presence of varied mechanoreceptors found in aging and normal PCLs. One study demonstrated a large array of mechanoreceptors in the PCL, even in arthritic knees. In addition, because comparable clinical results have been demonstrated with either PCL retention or sacrifice, it may be desirable to consider retention to preserve its proprioceptive role to the knee, although it is unclear if mechanoreceptors persist after a PCL-retaining TKA. One analysis of the effect of the PCL on the biomechanics of TKA demonstrated no evidence of posterior edge loading on the medial aspect of the polyethylene insert, but edge loading was evident on the lateral side of the insert. This forward pattern of wear on the medial side of the polyethylene insert helps confirm that the PCL is involved in proper coronal balance of the knee and suggests that maintenance of the mechanoreceptors in a PCL-retaining knee may benefit the physiology and mechanics of the knee.

Quadriceps and patellar tendon

The quadriceps extensor mechanism of the knee is a major structure that contributes to knee stability. Proprioceptive information is transmitted to the nervous system via muscle spindles in the quadriceps. Researchers have reported that elderly individuals had less quadriceps muscle strength and a higher error in reproduction of joint position sense compared with young or middle-aged individuals; no correlation between quadriceps strength and knee proprioception was found. However, a study of the effects of patellar strap use showed slight improvements in proprioception, suggesting that the knee extensors indeed play a role in proprioception.

Balance, aging, and total knee arthroplasty

Balance requires the central integration of proprioceptive, somatosensory, visual, and vestibular inputs linked to appropriate musculoskeletal output for postural responses. Just as proprioception declines with age and/or OA, vestibular and visual inputs may also decline, resulting in an overall reduction in sensory input required for balance. A study by Teasdale and colleagues showed that disturbance of just 1 of these inputs was not of substantial significance. Because of an increased reliance on other inputs, however, disturbance of more than 1 is likely in the aging population. Therefore, after a TKA in this population, the negative effects on balance were far greater. The risk of falling before TKA is correlated with risk after TKA and, therefore, preoperative assessment of balance is an important consideration in postoperative rehabilitation.

As the human body ages, muscle mass declines to an extent of up to 20% to 40% by the seventh decade. This correlates with a loss of corrective postural musculature strength needed to maintain adequate balance. Postural musculature responds to cortical input via anticipatory postural adjustments (APA) and compensatory postural adjustments (CPA), which in combination aim to maintain center of mass or center of pressure over base support. Kanekar and Aruin demonstrated delays in APA with resultant increase in CPA in the elderly compared with younger controls, suggesting decreased control of posture and increased risk of falls. The risk of falling should not be confused with the fear of falling, which is often increased with multiple sensory deficits. Fear of falling affects up to 30% of community-dwelling geriatric adults. This can be as debilitating as an injury sustained in a fall and can lead to decline in physical and social function, depression, loss of independence, and decreased quality of life. Swinkels and colleagues demonstrated an overall improvement in balance confidence and reduction in depression symptomology after TKA in people who had not fallen preoperatively, but these results were not demonstrated in those who had fallen preoperatively. Furthermore, multiple studies have shown improvement in proprioception after TKA, whereas contradicting studies exist that argue decreased proprioception and balance after TKA could result in increased risk of falling. This topic is discussed in later sections. Although studies have shown balance strategies of TKA patients approach those of normal controls, they continue to remain asymmetric.

Introduction of an artificial joint disrupts the native proprioceptive system of the knee; however, this must be weighed against the inability of an anatomically deformed, painful, and weak knee to execute the necessary motor and functional demands required to maintain balance. In the native knee, intracapsular mechanoreceptors exist within structures such as the cruciate ligaments and menisci to provide proprioceptive information. In addition, performance of the knee has been shown to be related to the integrity of the anterior cruciate ligament (ACL). Often, the cruciates are resected in TKA, and this can contribute to alterations in proprioception after TKA. The proprioceptive input of extracapsular structures such as the collateral ligaments and surrounding musculature are not fully known, but integrity of these structures can be altered by deformities associated with severe OA. The exact relevance of the knee as a primary proprioceptive input is in doubt because of the suggestion that better balance performance in static and dynamic situations is typically seen with the use of anticipatory strategies using signals from the ankle rather than the hip/core. Studies using electromyography (EMG) have demonstrated hip/core proprioceptive signals to be the primary source of autonomic proprioceptive input, with the knee being supplemental for recruiting the gastrocnemius-soleus complex. The ambiguities in lower extremity proprioceptive systems are not entirely understood, but nonetheless remain an important link in dynamic balance control.

A recent study by Gauchard and colleagues used dynamic platform posturography in an attempt to independently analyze proprioception, vestibular, and proprioceptive inputs in patients who had recently undergone TKA and in age-matched controls. Their study demonstrated that patients with a recent TKA approached the performance of age-matched controls once they reached sufficient rehabilitation 34 and 41 days postoperatively; however, those in the early perioperative period between 17 and 20 days showed substantial deficits. Hip/core dominance was found in the early perioperative group, suggesting that early rehabilitation was insufficient for both motor strength and adaptive proprioceptive strategies. Of key interest is the result of altering proprioceptive input with the primary determinant of balance being vision, vestibular, or integration of vestibular with altered visual inputs. There was no significant difference between controls and rehabilitated patients, suggesting that any or all loss of articular proprioception may be compensated by an increase in muscular or alternate joint (hip, ankle) proprioception after appropriate rehabilitation. One limitation, however, was that the degree of OA was not evaluated in the age-matched controls. Overall, proprioceptive abilities are a result of many peripheral and central structures. Aging certainly plays a role in the ability of these structures to function properly and, as a result, balance can be affected.

Proprioception, osteoarthritis, and total knee arthroplasty

Normal bony, capsular, ligamentous, and muscular structures about the knee are vital for native knee proprioception, and alterations in these structures bring variations in proprioceptive abilities. OA of the knee leads to physiologic changes about the knee resulting in a cycle of pain, disuse, body weight imbalance, muscle atrophy, and ultimately loss of proprioception. Numerous studies have shown a definitive link between OA and proprioceptive loss. Loss of joint space, an essential component of OA, has been associated with decreased joint position sense. Barrett and colleagues described decreased joint position sense in 45 patients with knee OA compared with nonarthritic controls. Other studies have described loss of joint position sense (measured by various techniques) in patients with osteoarthritic knees. Not only does loss of proprioceptive mechanisms about the knee lead to OA, the inverse also can be true: OA can cause loss of proprioception.

The human body is remarkable in its ability to compensate for physiologic stresses. Loss of major joint proprioception may result in a compensatory attempt to maintain balance. In their review, Pua and colleagues described the importance of skeletal muscle about the knee. As OA leads to pain and, therefore, decreased conscious use of the knee joint, the body may increase swaying on that knee as an attempt by the central nervous system to “ensure an adequate level of sensory input.” This swaying recruits skeletal muscles about the knee and the result is a greater excursion distance in the center of pressure of the knee. Pua and colleagues noted that increased sway was associated with better physical function in patients with OA of the knee. This is further supported by other studies showing that OA leading to knee joint instability may induce recruitment of additional dynamic stabilizers about the knee. Other contrasting instability-reducing strategies also have been postulated. Reduction in the excursion of the center of pressure resulting in a “postural stiffening gait” has been described as a different strategy of the central nervous system to combat knee instability caused by OA.

Proprioception and total knee arthroplasty

Knee arthroplasty continues to be the gold standard for definitive treatment of knee arthritis. Pain relief, improved knee range of motion, and increased mobility are generally the results of this procedure; however, maintenance of fine balance and proprioception has been questioned. This continues to be studied across multiple specialties, including orthopedics, gerontology, and physical therapy. Multiple sources have shown that TKA significantly restores the ability to maintain upright balance. Single-leg standing balance was improved after TKA when compared with patients with high tibial osteotomy. This suggests that restoration of joint space and not simply mechanical realignment of the knee plays an important role in proprioception and balance. Another study evaluated 11 patients who had TKA and found that standing balance was improved as measured by a 59% decrement in postural sway. When Ishii and colleagues looked at the effect of unilateral and staged bilateral TKAs, both groups had a benefit from the operation with regard to balance. In the unilateral group, the center of balance remained on the operative extremity, which may offload the contralateral extremity. In patients with staged bilateral TKAs, improved balance was found after the second TKA. Joint position sense improvement after TKA is further supported by a study in which 21 TKA knees were compared with normal, control knees, and osteoarthritic knees. The group undergoing TKA showed improved postoperative joint position sense. Another study revealed an improvement in recruiting the quadriceps muscles after TKA, which may contribute to improved proprioception. However, decreased EMG amplitudes in the vastus lateralis and biceps femoris have been found in patients with TKA compared with control patients with OA. This suggests that certain dynamic knee stabilizers may be over-recruited, whereas others are under-recruited after TKA. This may explain the postural stiffening gait sometimes seen early after TKA.

Other studies have shown no difference in proprioception between knees with TKA and knees with OA. Barrack and colleagues and Skinner and colleagues compared TKA knees with age-matched controls and with a group of younger control patients. They noted no improvement of proprioception in any group as measured by the ability to detect passive motion or to reproduce knee flexion angles. They stated that loss of proprioception after TKA may be implicated in some TKA failures because of increased stress placed on the implanted components. In a similar study, 28 TKA knees were compared with controls and similar physical tests were performed; no difference was found between the 2 groups overall, but a significant decrease in proprioceptive ability of the TKA knees was identified at 60° of flexion.