Obesity is one of the most common comorbidities in patients seen in the knee clinic.  With obesity levels in the UK rising every year, over 50% of the population is set to be clinically obese by 2050. At such levels, it is projected that Obesity could cost the NHS nearly £50 billion per year.

The mechanical implications of obesity for the knee and other joints in the body are, perhaps, obvious.  However, overloading is not the only way that obesity can affect the knee.  The full range of issues associated with obesity can be grouped as follows:

• Mechanical factors
• Systemic factors
• Surgical factors
• Anaesthetic factors

Each of these areas is discussed briefly, below.

Mechanical Factors

The most recognised and well-studied association between obesity and knee pathology is that of osteoarthritis.  Obese patients place increased stresses on their articular cartilage, which drives earlier onset and more rapid degenerative changes.  Furthermore, there is a higher rate of varus alignment in the obese population, which places additional stress on the medial compartment of the knee joint. 

Apart from osteoarthritis, a recent study has shown that obese patients are more likely to suffer multi-ligamentous knee injuries due to low energy mechanisms.  Such injuries can be devastating for the function of the knee, particularly when considering the difficulties associated with surgery and obesity, which are discussed below.

Systemic factors

Overloading alone does not account for all of the increased morbidity associated with osteoarthritis in the obese population.  Over recent years, it has become increasingly evident that adipose tissue has a role as an endocrine organ, producing a variety of hormones and biochemical factors that affect the function of a wide variety of systems in the body.  Many of these chemicals – including chemokines, cytokines and adipokines – are pro-inflammatory and therefore exacerbate symptoms such as pain and swelling associated with arthritis.  They are also thought to disrupt the delicate balance of anabolic and catabolic processes within joints, which can lead to increased degeneration of cartilage and other soft tissues.  The role of adipose tissue helps explain why osteoarthritis of non load-bearing joints – e.g. in the hands and fingers – is also more common in obese patients.

Surgical factors

There are a number of ways in which obesity affects surgical procedures and outcomes.  It should come as no surprise that procedure time is increased for obese patients, as is the overall length of stay in hospital.  Obese patients are also likely to experience worse overall outcomes and satisfaction levels following their operation.  For example, in total knee replacement surgery for normal weight population, around 90% of patients will be satisfied with the result.  In the obese population, only 70% have a good outcome.  At least part of the reason for this is increased rate of complications, such as delayed wound healing, more post-operative pain and increased rates of deep vein thrombosis.

Anaesthetic factors

In general, obesity is a risk factor for a number of systemic diseases, such as cardio-respiratory (eg. hypertension, heart failure), metabolic (eg. diabetes) and gastrointestinal conditions (eg. gastro-oesophageal reflux disease), all of which can have anaesthetic implications.

Specific difficulties that the anaesthetist may encounter include:

• airway problems associated with anatomical restrictions and sleep apnoea, which can also cause perioperative drowsiness, complicating assessment and treatment.
• Ventilation problems due to difficulty intubating the patient, reduced lung capacities and low chest wall compliance.
• Inaccurate blood pressure readings using normally available cuff sizes
• Unpredictable pharmacokinetics leading to unknown rates of drug elimination, etc.
• Loss of landmarks for regional anaesthesia
• More difficult venous access

The Good News

For the reasons outlined above, obese patients can experience dramatic improvements in their symptoms and response to treatment if they are able to lose weight.  The effects of mechanical, systemic and other factors are additive, such that even a reduction of a few percent (5% – 10% is often targeted) can deliver significant benefits.  The systemic effect of adipose tissue means that greater benefit is derived if weight loss can be achieved through reduction of body fat percentage, rather than loss of muscle.  Studies have shown that good advice alone is not sufficient for successful weight loss – patients who are actively supported through the process tend to do much better.  In cases of extreme obesity, patients may wish to consider other weight—loss options, such as bariatric surgery, before electing for orthopaedic surgery.