Biomechanical aspects of knee osteoarthritis and AposTherapy
Dr. Nimrod Rozen, Head of Orthopedic Surgery Department, HaEmek Medical Center, Afula, Israel
Over the past few decades there has been growing evidence on the importance of biomechanical factors in the pathogenesis of knee osteoarthritis (OA). Knee OA is characterized by decreased neuromuscular control, instability of the knee joint and weakness of the knee musculature, all of which lead to abnormal bracing of the knee in order to maintain stability. This bracing result in an increase in the knee adduction moment and knee flexion moment that together bring the knee joint toward a greater varus alignment than it was originally. As a result, all weight of the body is localized on the posterior medial aspect of the knee. The increase in knee joint loading causes further deterioration of the joint cartilage and worsens the symptoms of OA. These changes place the patient in a vicious cycle in which further deterioration of clinical symptoms, such as pain and function, is inevitable.
Many resources and efforts have been invested toward developing a solution that will stop and even reverse this cycle. Most of them focus on reducing loads from the medial aspect of the affected joint and returning the musculoskeletal properties of the knee back to healthy patterns.
Orthotics, braces and walking aids are some examples of biomechanical devices under examination today. Physical exercises and neuromuscular training, such as training with unstable surfaces, are also being examined. All these interventions, however, have had limited success from a clinical point of view and have yet to show evidence of stopping/changing the biomechanical deterioration of the knee.
Over the last several years, the Orthopedic Departments of HaEmek Medical Center, Sourasky Medical Center and Assaf Harofeh Medical Center as well as The Biorobotics and Biomechanics Laboratory of the Technion Israel Institute of Technology have examined a new biomechanical therapy for the management of knee OA called AposTherapy. Through a series of studies it has been shown that patients with knee OA who were treated with AposTherapy reported a significant reduction in pain and an improvement in function, quality of life and gait patterns. In addition, for the first time, a reversal of the biomechanical abnormalities was shown following AposTherapy.
AposTherapy is a non-invasive, biomechanical therapy that is intended to rehabilitate the pathological gait patterns of patients with knee OA. AposTherapy is carried out in the patient's own environment and during daily activities. The therapy is based on a unique technology that enables manipulation of the center of pressure under the foot which thereby modifies the moments acting on the joints of the lower extremity, pelvis and spine. Throughout therapy the patients are carefully monitored for changes in gait, pain and function.
This review summarizes the biomechanical aspects of knee OA and describes the mechanism of AposTherapy and its clinical application.
Over the past few years there has been growing evidence on the importance of biomechanics in the development and progression of knee osteoarthritis (OA). Biomechanical factors such as abnormal posture and mal-alignment of the joints lead to changes in the kinetics and kinematics that cause greater loads on the affected joint. Other factors, such as muscle weakness and neuromuscular deficits, lead to pathological compensatory mechanisms such as co-contraction and bracing that further harm the joint. Pain, which is a primary symptom in knee OA, arises in response to increased loads on the affected joint. High levels of pain lead to exercise avoidance and, when severe, even an avoidance of daily life activity such as walking and climbing stairs.
Knee OA is not just a joint disease but a disease of the soft tissue surrounding the joint as well. OA can also affect adjacent joints such as the ankle and hip. Adjacent muscles may also be weakened by knee OA, especially the Gluteus Maximus, Gluteus Medius and external rotators. Inflammation at the insertion point of tendons surrounding the knee joint (i.e. Pes Anserinus) or the hip (i.e. Trochanteric Bursitis) is also common findings in knee OA. There is also cartilage degeneration in the patellofemoral joint as well as changes in the soft tissue flexibility (such as in the posterior capsule). All these additional changes also contribute to the pain felt by a patient with knee OA.
Biomechanical changes at the knee joint
Knee OA affects the kinetics and kinematics of the knee joint. Loads are also transferred from the knee joint to the hip joint causing biomechanical changes there as well . These biomechanical changes are the primary source for pain and functional limitation and therefore preventing their progression is critical in the management of knee OA.
The medial compartment of a normal knee joint bears approximately 70% of body weight whereas the lateral compartment bears the remaining weight. This is a result of the trajectory of the ground reaction force (GRF) vector at the knee joint. The GRF trajectory passes medially and posterior to the knee joint. The moment created by this force at the knee is made up of an adduction and flexion moment. Patients with knee OA can be characterized by a significant increase in knee adduction moment  (Figure 1). The magnitude of the knee adduction moment directly correlates with joint space narrowing, medial joint capsule loosening and levels of pain and functional limitation . The phenomenon of joint capsule loosening is also known as "pseudo-laxity" . In order to overcome the sensation of joint instability, the muscles surrounding the medial aspect of the knee adopt a bracing mechanism by which they contract as a whole to stabilize the medial aspect of the knee joint. This bracing also increases the loads at the medial compartment and accelerated the degenerative changes at the knee joint.
Figure 1. Knee adduction moment
The knee adduction moment is a result of the magnitude of the ground reaction force (GRF) times the distance (i.e. moment arm) from the center of rotation (GRF*LA). The graph of the knee adduction moment during a gait cycle in a patient with knee OA is characterized by an increase in the peak and impulse (the area under the curve) of the moment (Presented by Mali M., 2007).
Previous studies have examined the correlation between muscle performance and knee OA [5-9]. A weak correlation was found between muscle weakness, especially in the quadriceps, and radiographic changes of knee OA. This weak correlation was found in patients who did not report any knee pain or functional limitation . In fact, it has been shown that even in a supposedly healthy population, evidence of radiographic changes can be found in the presence of muscle weakness even before the appearance of knee OA symptoms. Furthermore, it has been shown that the muscles surrounding the knee joint function abnormally. For example, muscles are active for longer periods of time and at higher magnitudes but produce less efficient and defective patterns such as co-contractions [11-12]. Lawek et al., reported that the muscles at the medial compartment of knee (vastus medialis, medial gastrocnemius and medial hamstrings) present with a pathological protective patterns. These pathologic muscle patterns increase the compressive forces and moments acting on the affected compartment .
Alongside the changes in muscle activation changes are observed in the sensory system as well. In and surrounding the knee joint are thousands of sensory receptors that are responsible for detecting changes in joint positioning (proprioception) and transmitting the afferent information to the central nerve systems. Efferent information is transferred from the central nerve system to the peripheral system to create a motor respond. This reflexive system prevents falls and injuries. In the event of damage to the proprioceptive system, a person is at a high risk to injury. In knee OA, almost 50% of patients report a sense of instability in their symptomatic knee, such as a feeling of "giving way", an inability to trust the knee while carrying out daily activities and a feeling that the leg is "not cooperating" . This sensation of knee joint instability highly correlates with dysfunction and immobility .
Treating the biomechanical factors of knee OA
Due to the well-established evidence on the effect of biomechanical factors on knee OA, there is a growing experimental and developmental effort in finding an adequate biomechanical intervention for knee OA. The overall goal is to find a solution that will stop the deterioration of the disease and hopefully even reverse its pathogenesis. Current research focuses on finding biomechanical methods by which to reduce loads from the affected joint and rehabilitate muscle activity. Clinicians have attempted to reduce knee joint loads through biomechanical devices such as orthotics and knee braces [15-18]. These interventions aim at reducing the knee adduction moment. The principle for muscle rehabilitation is based on training and improving neuromuscular control . To date, biomechanical therapies have had limited clinical success and have yet to show evidence of reversing the abnormal biomechanical patterns of knee OA. Furthermore, none of the therapies integrates both load reduction and muscle rehabilitation.
Current biomechanical therapies
Knee braces, orthotics, walking aids and similar intervention are the primary biomechanical therapies being investigated today. Exercises on unstable surfaces are also being examined as therapies for improving neuromuscular control. These therapies have all had limited clinical success and not shown evidence of reversing the abnormal biomechanical patterns of knee OA.
Some researchers hypothesize that using orthotics and knee braces cause a decrease in knee adduction moment. Researches believe, however, that these cause only static changes at the knee joint and as such do not create new motor learning or improvement in neuromuscular system. In addition, the decrease in knee adduction moment is not accompanied by a relief in symptoms such a reduction in pain or improvement in function.
Neuromuscular exercises on unstable surfaces are limited to date and mostly include exercises on a stationary wobble board. Such exercises do not allow for control of the biomechanical alignment in the knee and as such may cause pain during exercise. Exercising while in pain affects proper motor learning, may even cause implementation of abnormal compensatory motor patterns. There may also be poor compliance. Another limitation of stationary exercises is that they cannot be carried out during activity, as is required for motor learning. Neuromuscular training exercises should instead be carried out under proper biomechanical alignment in which the patient reports reduced or no pain. Exercise should be carried while performing daily activities. An added plus would be exercises in closed kinematic movements that include weight bearing activities and high numbers of repetitions .
AposTherapy is a biomechanical therapy that was developed in Israel and has the ability to combine the two biomechanical treatments for knee OA: reducing loads from the affected joint [21-22] and training of neuromuscular system [23-24]. The Apos system is calibrated individually to each patient in order to obtain a healthier biomechanical alignment in which less pain is reported and gait patterns are more symmetrical. The patient is then instructed to exercise with the system while performing his or her daily activities at home or at work. This exercise regiment has enables the patient to adopt and implement a proper gait patterns without pain. In addition, AposTherapy has been shown to strengthen the muscles of the knee and improve the timing of muscle activation . In addition, patients report an increased sense of joint stability.
The Apos system is comprised of convex adjustable biomechanical elements placed under the hind-foot and fore-foot regions of each foot. The elements are attached via a platform in the form of a shoe. This platform enables the clinician to calibrate the biomechanical elements to a customized location for each patient. The ideal location for each patient centers the pressure in the foot to a position that properly aligns the lower limb (Figure 2).
Figure 2. Apos system
Haim et al. showed that changing the location of the center of pressure (COP) through positioning of the biomechanical elements causes changes in the moments acting on the lower extremity joints. For example, shifting the biomechanical elements to the lateral aspect of the foot causes a lateral shift of the COP and a decrease in knee adduction moment . In addition to proper alignment of the lower limb and a reduction in knee adduction moment, the convex nature of the elements the elements promotes controlled perturbation. Since the system can be used during walking throughout the day, the alignment and controlled perturbation are applied repetitively and dynamically throughout all phases of the step-cycle.
Bar ziv et al.  examined the effect of AposTherapy on the clinical symptoms of patients with knee OA. This was a prospective, controlled, double blind study. The study examined the changes in self-evaluation questionnaires (WOMAC, SF-36, Knee society score) and in functional tests (Aggregated locomotor function) in two comparative groups over two months. The first group received AposTherapy and the second group received a sham control device (i.e. the shoe platform without the biomechanical elements and calibration). The patients in the first group reported a significant improvement in their levels of pain and function as well as improvement in their quality of life. No significant changes were found in the patients from the control group. Recently, preliminary results of a two year follow-up study on these patients were presented. Patients from the research group maintained the positive improvement seen after just two months of therapy and patients from the control group again showed no significant improvement. Similar findings were presented by Elbaz et al. in a study that examined the effect of three months of AposTherapy on the gait patterns and clinical measurements of patients with knee OA. In addition to the improvement in their levels of pain and function, the patients reported a significant improvement in gait velocity, step length and single limb support (a phase in the gait cycle where the body weight is entirely supported by one limb while the contralateral limb swings forward).
In order to fully understand the effect of AposTherapy on the biomechanics of knee OA and on the symptoms of patients with knee OA a collaborative multicenter study (Technion Institution, HaEmek Medical Center and Sourasky Medical Center) was conducted. The first stage of the study included a full analysis of the kinetics, kinematics and muscle activity at different positions of the biomechanical elements in a healthy population [21-22, 25]. Two papers present the effect of the Apos system on the knee adduction moment  and knee flexion moment  while walking. These studies show that by changing the location of the center of pressure by shifting the biomechanical elements, the moments acting on the lower extremity, pelvis and spine can be controlled. Similarly, it was shown that the muscle activation patterns also change when the biomechanical elements are shifted .
The second stage of the study examined the changes in kinetic, kinematic and muscle activation patterns of patients with knee OA treated with AposTherapy. Results supported previous findings and showed that shifting the biomechanical elements can change the COP. A change in the location of the COP caused a reduction in the knee adduction moment of these patients by shifting of the GRF vector trajectory and reducing the lever arm from the center of the knee joint to the GRF vector. A medial shift of the biomechanical element led to a 16% decrease in the magnitude of the knee adduction moment. Furthermore, the kinetics, kinematics muscle activity and clinical symptoms of these patients were examined after nine months of AposTherapy. As in the studies by Bar-Ziv et al.  and Elbaz et al.  a significant decrease in pain and improvement in function was observed after nine months of therapy. Furthermore, AposTherapy led to a significant reduction in the peak value and the impulse value (area under the curve) of the knee adduction moment. These results illustrate that, for the first time, a reversal of the pathological biomechanics of knee OA is achievable.
To the best of our knowledge, this is the first time that a therapy has lead to a significant improvement in the gait patterns of patients with knee OA in combination with significant improvements in clinical symptoms. In other words, this is the first time a therapy was shown to stop the vicious cycle and reveres it.
In conclusion, AposTherapy can improve the pain, function and quality of life of patients with knee OA. Furthermore, AposTherapy has been shown to improve the gait patterns and biomechanical characteristics of knee OA that correlate with disease severity.
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