Taking a Load Off: An Update on How Biomechanics Research is Shaping the Future of Conservative Treatment for Knee OA

By Monica R. Maly, PT, PhD

In a recent study in The Lancet, Cui et al estimated there are ~654 million people age 40 and older around the world who have knee osteoarthritis and an additional ~87 million individuals age 20 and older who have already been diagnosed. With such staggering numbers, understanding conservative treatments for this disease is a critical public health mandate.

Arthritis creates more pain, depression, immobility, disability, and unemployment than any other chronic condition in Canada (see “Arthritis in the United States,” page 29). The most common form is osteoarthritis (OA), which damages all tissues inside of and around moveable joints. This disorder is initiated by microscopic and joint-level injury. In response, maladaptive attempts to repair joint tissues spur on altered composition, structure and function of cartilage, bone and other joint tissues, which ultimately challenge and impair joint function.

OA affects the knee most often, likely due to its roles in weight-bearing and mobility. A greater prevalence and severity of knee OA occurs in older than younger adults and in females than males.⁵ More than any other joint, knee OA also preferentially affects people who are obese (body mass index [BMI] over 30 kg/m²). A dose-response relationship exists between BMI and knee OA incidence, such that those with a BMI >35 kg/m², have a 4.7-fold elevated risk for knee OA. This dose-response relationship also suggests that reducing BMI could be a powerful tool in lowering the risk for knee OA.

In Canada, OA prevalence will reach 10 million by 2041 and healthcare costs will rise to $8.1 billion by 2031. This dramatic increase in burden reflects an aging and increasingly obese Canadian population. It is important to reflect that these estimates were calculated before the pandemic. While we are uncertain of the real impact of COVID-19 on OA, there is good reason to expect these older estimates will be exceeded. First, sheltering-in-place for COVID-19 has resulted in alterations in lifestyle that worsen OA risk and disease. Less exercise, more energy intake and more screen time has increased BMI among adults. Second, care opportunities for people with knee OA are more challenging to access in the pandemic.

This review provides an update on (1) guidelines for managing knee OA without drugs or surgery, (2) research trends regarding the biomechanics underlying knee OA, and (3) biomechanical strategies to reduce pain and improve wellbeing with this disease.

Guidelines for Conservative Knee OA Management

In 2019, an updated set of clinical guidelines for the non-surgical management of knee OA was published by the Osteoarthritis Research Society International (OARSI). It reflects a comprehensive literature review and, very importantly, feedback from patients with OA. For the first time, these guidelines considered comorbidities common among people with knee OA:  gastrointestinal problems, cardiovascular diseases, frailty, and widespread pain and/or depression. Whether you have knee OA alone, or knee OA combined with these conditions, the OARSI guidelines strongly recommend (1) education and (2) structured, land-based exercise (strengthening/cardiovascular/balance/neuromuscular) or mind-body exercise (Tai Chi/yoga).

These updated OARSI recommendations diverge somewhat from previous versions in 2 important ways. For the first time, mind-body exercise approaches are recommended to reflect the importance of overall wellbeing. However, some biomechanical treatments (eg, cane, knee brace) were downgraded to “conditionally recommended” because these treatments did not align with patient preferences and did not have high quality evidence of efficacy. (It is important to note that canes, braces, and shoe orthoses can also be costly and many are not reimbursed by insurance.)

These OARSI recommendations are corroborated by the knee OA treatment guidelines published by the American College of Rheumatology and Arthritis Foundation (ACR/AF). Based on the literature review and patient input, the ACR/AF strongly recommends exercise, weight-management, self-management, Tai Chi, canes, and knee bracing. The ACR/AF also conditionally recommends promising interventions such as cognitive behavioural therapy and yoga, which currently lack high-quality clinical trials but may, in time, be shown helpful for people living with knee OA.

In the following, this review will examine current trends in biomechanical research, and discuss how this work could evolve conservative treatments for knee OA.

Biomechanical Pathology of Knee OA

Research over the last 10 years has provided compelling evidence that abnormal knee loading patterns worsen knee OA and advanced our knowledge on the root causes of abnormal loading.

A Traditional View:  Knee Adduction Moment

Traditional biomechanics research of knee OA pathology was driven by the notion that large loads initiate and/or worsen knee OA by exceeding the capacity of joint tissues. Much work has focused on the knee adduction moment (KAM). The KAM is the torque that rotates the tibia inward on the femur, producing the typical knee deformity caused by OA: a varus (or bow-legged) knee. In a static situation, the KAM can be calculated as the product of the (A) magnitude of the 3-dimensional ground reaction force vector projected upward from the point of contact under the foot and (B) the perpendicular distance between this force vector and the centre of the knee (lever arm). Traditionally KAM is thought to reflect a ratio of medial to total joint loading, where a greater KAM reflects an abnormally high proportion of loading on the medial knee compartment. Unfortunately, the more varus the knee becomes, the longer the KAM lever arm, and the greater the KAM.

Computational modeling and instrumented joint replacements (that directly measure contact forces between the tibia and femur) have challenged whether the KAM truly represents the medial contact forces that occur between the tibia and femur. These studies identify that the KAM is not a surrogate for measuring actual forces within the knee; nonetheless this quantity still proves useful. Over the last 10 years, studies conducted with different cohorts, in different locations and over different follow-up time periods consistently show that the KAM during one walking stride predicts worsening knee tissue breakdown over time. These data provide a compelling rationale that treatments that reduce exposure to the KAM make sense for knee OA. Yet, there remains much more to learn about OA pathology beyond the KAM.

Multiple Biomechanical Variables Influence Knee OA Pathology

Knee OA is also linked with altered patterns in kinematics (eg, reduced knee flexion excursion, aka knee stiffening) and kinetics (eg, elevated knee flexion moments,²⁴ flattened vertical ground reaction force curves). Muscles crossing the knee joint play a pivotal role in modulating knee loads implicated in OA. Computational modeling shows muscle contraction alters the magnitudes and distributions of joint contact forces across the knee joint surface.  This work on the primary role of muscle contraction in knee joint loading emphasizes why exercise works in reducing pain and improving wellbeing for people living with knee OA.

Additional theories have emerged.

Cumulative Load and Improving Joint Tissue Health

It is theorized that all joint tissues will have a “sweet spot” (perhaps better articulated as the “sweet range”) of loading that promotes healthy structure and function; whereas chronic underloading (eg, due to inactivity) or overloading (eg, due to obesity) may both degrade joint tissues. To explore this theory, biomechanists have begun exploring cumulative loading – ie, the total exposure to loading across daily activity, rather than during a single step, as a feature in the pathway leading to OA. A recent study of 964 participants with knee OA in the Multicenter Osteoarthritis Study explored cumulative load as a mechanism for knee OA worsening. Compared to those with a low BMI (18-27 kg/m²), those with a BMI >31 kg/m² who walked a moderate (6,000-7,900) or high (>7,900) number of steps daily experienced a greater risk of medial knee OA worsening over 2 years, suggesting overloading could be detrimental. Importantly, underloading also increased risk. Among those with a low BMI, walking fewer than 6,000 steps daily was also associated with worsening OA. While this work points to the potential negative implications of overloading a joint affected by OA, there is also a positive viewpoint.

Despite repetitive exposure to very large peak loads during running, many studies show no elevated risk of knee OA among runners. This point is difficult to reconcile if our only concern is peak magnitudes. However, we now recognize that running also exposes the knee to low cumulative loads due to the relatively short duration of contact between the foot and the ground. This fresh perspective provides us with clues about strategies to promote knee joint tissue health by identifying the ideal range of cumulative loading. Within a healthy knee joint, more physical activity is associated with better quality cartilage. In fact, healthy young women who were more physically active with a low BMI showed tibial cartilage more resilient to change after running. This finding may suggest that we can actually improve the quality of knee cartilage with the right amount of loading.

Interactions of Biomechanics and Obesity

Since obesity is a risk factor for knee OA incidence, it is not surprising that biomechanists are now exploring how obesity and knee biomechanics may interact to worsen knee OA. Among adults with painful knee OA, KAM predicted knee cartilage loss over 2.5 years among the obese adults, but had no impact in those with a healthy BMI.  Medial tibial cartilage volume was reduced by 3.4 (6.0)% for normal/overweight and 14.1 (11.6)% for obese over this 2.5-year period. In fact, people with greater BMI have less cartilage, lower quality cartilage and softer cartilage under a mechanical load, compared to people with a normal range BMI.  These data suggest that combined interventions that tackle abnormal biomechanics concurrently with reducing inflammation (eg, through prudent diet) could multiply the effectiveness of either treatment alone.

This review will now turn focus to conservative intervention strategies with emphasis on how recent trends in biomechanics research could shape the future of conservative treatment.

Predicting the State-of-the-Art in Conservative Mechanical Interventions

The current trends in biomechanics research inspire fresh approaches that, in time, could offer new opportunities for conservative knee OA treatment.

Gait Retraining

While too new to be currently recommended by treatment guidelines in knee OA, gait retraining is a motor learning intervention that aims to modify kinematics during gait to reduce the risk for injury. Gait retraining has potential for reducing KAM by shortening the lever arm for KAM. Common strategies include trunk lean (ie, shifting the body’s center of mass closer to the knee), “medial thrust” (ie, forcing the knee medially during gait), and altering the foot progression angle (ie, toe-in or toe-out to apply the ground reaction force closer to the knee). To-date, data suggest that trunk lean and medial thrust gait patterns are most effective at reducing the KAM. However, many questions remain regarding implementation of gait retraining in the real-world. First, smaller sample sizes, fair to moderate study quality, variation in retraining and methodological techniques, and a relatively small number of publications to-date show that this field is still growing, without clear answers about whether these strategies will feasibly improve pain and other clinical outcomes in people with knee OA. Second, not enough data exist to evaluate the impact of altering gait on joints other than the knee. Third, methods of training, for example provision of biofeedback, vary greatly between studies with no clear recommendations on the ideal approach. Future work will provide direction to patients and clinicians regarding the potential of gait retraining for making meaningful improvements in people with knee OA.

Knee Bracing

Unloader knee braces for knee OA are designed to apply external forces around the knee to redistribute internal joint contact forces away from areas of joint degeneration, most often the medial compartment. Medial compartment unloader braces effectively reduce the KAM as well as direct measurements of medial contact force. Importantly, research evidence confirms that these braces can effectively reduce pain and improve wellbeing among people with unicompartmental knee OA.  Accordingly, the OARSI and ACR/AF guidelines conditionally or strongly recommend knee braces. OARSI did note, however, that patients may not adhere to brace use in the long-term, perhaps due to minor complications such as skin irritation, slipping, and poor fit. Perhaps a further consideration is that many patients with knee OA will have disease in more than one compartment. To this end, computational modelling is advancing a new idea: a knee brace that unloads the medial, lateral, and patellofemoral all at once.  This approach could prove critical to expanding the number of people with knee OA who benefit from this technology.

Weight Management

Traditionally, weight loss was thought to be treat knee OA simply by reducing the overall loading magnitude on the knee. For every unit of body mass lost via diet and exercise, a 4-unit reduction in compressive knee forces was observed.  However, recent work has emphasized that managing obesity in knee OA likely improves the clinical picture through multiple pathways. Surgical or conservative weight loss actually improve the quality of cartilage. In the multi-centred Osteoarthritis Initiative cohort, participants who lost >5% of their body mass had slower cartilage degeneration at both 4-year and 8-year follow-up, compared to those with a stable weight. It is impressive to note these improvements were experienced even in the presence of knee OA. We now understand weight loss improves inflammatory and immune responses, which if left unchecked, degrade tissues. Even more compelling, diet alone can improve physical function – and when diet is combined with exercise, improve physical function and pain.

However, weight loss through diet restriction is associated with a loss of lean (including muscle) mass – a loss that may disrupt the ability of muscle to modulate knee loads. Fortunately, more sophistication in dietary intervention for knee OA is on the horizon. First, the composition of food intake is important. In obese adults with risk or established knee OA, low fiber intake was associated with more severe knee pain. Following a Western diet (high in saturated fat and sugar) was associated with great risk for knee OA; while following a prudent diet (high in fish, vegetable/fruit, legumes and whole grains) lowered the risk of OA – associations that persisted even after accounting for BMI. Second, behaviours around food intake are also important. Nutrition risk screening identifies characteristics, including behavioral, that are associated with malnutrition (ie, deficiency or excess of essential nutrients). Nutrition risk screening examines risk factors such as poor appetite, physical challenges while eating such as swallowing, lack of socialization, and poor participation in meal preparation. Among Canadians with OA, those with a greater nutrition risk had poorer mobility and general health. Thinking about what you eat, and how you eat, could be important new facets in conservative OA management.

Exercise

The benefits of exercise for knee OA are established, explaining why properly prescribed exercise using multiple approaches (strength, aerobic fitness, neuromuscular, balance, and mind-body) are strongly recommended for knee OA. Very encouraging is recent data showing that strengthening can improve clinical outcomes likely without further damage to cartilage or synovial tissue (though more corroborating data are needed). OA researchers are now facing new challenges. Recent OA literature now aims to find the ideal exercise approach through direct comparison. For example, both weight-bearing and non-weight-bearing knee extensor strengthening is recommended for people with medial knee OA and comorbid obesity, though those completing weight-bearing exercise actually experienced fewer adverse events and greater improvements in health-related quality of life.

Improving adherence to treatment in chronic disease may have a greater impact on patient outcomes than any other innovation. Adherence is challenging in OA, particularly among those with pain – arguably those with the greatest need. Unfortunately, pain relief disappears after ceasing rehabilitation and only 33% of seniors with chronic conditions maintain home exercise. Technology may be a powerful antidote to poor adherence, with promising data emerging. For example, a web-based exercise program improved adherence to home exercise among patients with musculoskeletal conditions. This type of technology support of exercise is an exciting area, particularly in the context of the pandemic, to provide new avenues to keep people with knee OA moving.

Monica R. Maly, PT, PhD, is an associate professor in the Department of Kinesiology and Health Sciences at the University of Waterloo in Ontario, Canada.