Appendix B – Obesity and the Knee

Introduction

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.

Osteoarthritis of the Knee

Introduction

Total knee replacement (TKR) has now become a very common and successful procedure for managing the severely arthritic knee. Pain relief is generally excellent and the level of function that a patient can expect is improving with advances in both the surgical technique and in rehabilitation.

This means that knee replacement is being offered earlier in the process of deterioration of knee function associated with osteoarthritis and the aim of this chapter is to describe the place and indications for TKR in the management of knee OA.

It is important to remember however that knee replacement is the end stage of a long ladder of treatment options available for the management of early, mild and moderate knee osteoarthritis.

Symptoms of Knee OA

The main symptoms are pain, swelling, and stiffening in the knee. Symptoms tend to develop slowly over several years and are generally activity-related. Symptoms tend to be noticed after keeping the knee still for prolonged periods, only to be relieved by exercise but being made worse after too much activity. Often there is a history of repeated flare-ups with quiescent periods in between. Eventually, symptoms tend to affect sleep patterns.

Signs of Knee OA

The main signs are swelling, broadening of the knee joint and deformity. Diagnosis is generally made more on history than on the signs. A more in-depth account of history and examination of the knee is considered in the document ‘History and Examination of the Knee’, which is available for download at ww.timspalding.com.

Radiographs can be used to quantify the extent of the wear but are generally used to rule out other problems.

Non-Surgical Treatment Options

The mainstay of treatment for knee osteoarthritis is to improve the symptoms as it is generally not possible to alter the underlying disease process. As mentioned above, surgical treatment of knee osteoarthritis – in the form of Knee Replacement – is the end of a long ladder of treatment options. The following are all important in the management of knee OA, and are briefly described in turn, below.

Conservative Treatment
a) Activity and lifestyle modification
b) Physiotherapy.
c) Unloading type knee braces.
d) Walking aids

Medical Treatment
e) Analgesic and anti-inflammatory medications.
f) Nutritional Supplements, such as Glucosamine or Chondroitin

Injections into the Knee
g) Viscosupplementation injections.
h) Steroid Injections

a) Activity and Lifestyle Modification

Certain low-impact activities such as swimming, cycling and walking are excellent for building muscle, which then helps to support the knee and other joints of the body, preventing the unstable loading that can cause pain and further degenerative changes. Physical activities can also help reduce body weight, which further reduces load on the knee. Nutritional aspects are also very important in this context, and a balanced diet has numerous benefits for joint and general health.

b) Physiotherapy

Improvement in muscle control, balance, flexibility, strength, and gait can all help to unravel any detrimental effects of weakness or imbalance on loading of articular cartilage of the joint. The physiotherapist aims to ensure that forces are distributed evenly across the joint by assessing the patient holistically. The spine, hips, knees, ankles and feet can all impact on the forces transmitted through the knees, and should all be assessed.

c) Unloading Type Knee Braces

These braces are worn around the leg. They are called ‘unloading’ type because they unload a damaged area of the joint surface and redistribute the forces to another area. For this reason, they are particularly helpful for patients suffering from unicompartmental symptoms, such as those who have varus deformities of the knee.

d) Walking aids

Walking aids, such as sticks and canes, can be helpful for those whose symptoms limit their functional abilities. Uneven ground, for example, may present a particular problem to somebody with poor muscular control of the joint. In this situation, a cane to add stability when walking on the uneven surface could be considered.

e) Analgesic and anti-inflammatory medications

Painkillers taken orally – such as Paracetamol – or applied topically – such as Capsaicin, can be very effective at controlling the pain associated with osteoarthritis. Some patients express concern that taking painkillers is simply masking the underlying cause of their problems. Careful explanation of the benefits of symptom control and the fact that analgesia forms a valid part of the treatment ladder may therefore be necessary. Anti-inflammatory medications such as ibuprofen can also be taken orally or applied topically.

f) Nutritional Supplements

Sometimes referred to as ‘neutraceuticals’, these products are not classified as medications but have the potential to provide physical benefits. Two of the most commonly taken supplements are Glucosamine and Chondroitin, and they are often taken together. Both naturally occur within the body and are key components in the production of extracellular matrix (ECM). It is therefore thought that supplementation with these products can help delay degenerative connective tissue changes. There is some disagreement within the research community regarding the degree of benefit that can be derived, but the relief of moderate to severe joint pain has been shown at high doses (1500 mg/day for Glucosamine, 800-1200 mg/day for Chondroitin). Given their low side effect profiles and relative inexpense, they have a deserved place alongside or instead of other therapeutic options.

g) Viscosupplementation injections

Viscosupplementation injections in the knee, such as Synvisc One, have shown promise in affecting symptoms. We have found that 50% of patients with mild to moderate arthritis show an improvement in their symptoms after Synvisc injections. This lasts for up to nine months. The active ingredient in the injections is hyaluronic acid or hyaluronan. It is thought to work both mechanically, by acting as a viscous lubricant of the articular surface – and therefore decreasing the coefficient of friction – and biochemically via its anti-inflammatory properties. Synvisc One is a single injection and Ostenil involves a course of 3 injections with a week’s gap.

h) Steroid Injections

Corticosteroids have anti-inflammatory properties and can therefore be helpful in controlling acute flare-ups of osteoarthritis. They are often combined with a local anesthetic to reduce the discomfort of the injection itself. The local anesthetic will start to work within a few minutes, but will not generally last longer than around half an hour. The anti-inflammatory effect of the steroid generally starts within a few hours to a few days of the injection – depending on the specific corticosteroid used – and lasts between four to eight weeks.

Surgical Options

The variety of surgical options includes:

a) Arthroscopy
b) Osteotomy – high tibial or distal femoral
c) Partial resurfacing
d) Isolated patello femoral arthroplasty.
e) Uni-compartmental knee replacement.
f) Total knee replacement.

a) Arthroscopy

Arthroscopy at its most simple involves looking inside the knee by inserting a camera through a small incision. There are a variety of interventions that can subsequently be carried out during the same procedure, based on the findings. These are considered in more detail in Appendix A. In the context of the degenerative changes seen in osteoarthritis, arthroscopic techniques can be used in the treatment of degenerative tears, tight adhesions, unstable flaps of articular cartilage and removal of inflamed synovium.

b) High tibial or distal femoral osteotomy

Osteotomy is performed to re-align the leg where there is mal alignment and arthritis developing in the overloaded medial or lateral compartment. Two types are generally undertaken – high tibial, which is the most common and is used to correct a varus (“bow leg”) deformity, and distal femoral, which is used to correct a valgus (“knock-kneed”) deformity. Not all legs need to be corrected as these alignments can be physiological. However if there has been an injury to the knee and part of the meniscus has been removed then the joint is prone to rapid deterioration.

The main indication is for early wear and tear affecting the varus or valgus knee in the under fifty year old. It is best done before the joint surface is completely worn down to bare bone but this is often the phase where patient has less symptoms and does not relish the prospect of two months on crutches and possibly up to four to six months getting back to outdoor activities.

However, in the right patient the concept of “buying time” and slowing down the wear process is appropriate and in general, as a rule of thumb, for isolated medial or lateral degeneration osteotomy would be advised for the under 50 year old to preserve their own joint as long as possible, and arthroplasty would be indicated for the over 60 year old with sufficient symptoms.

For patients between aged 50 and 60 a decision is made based on level of symptoms, intended lifestyle, occupation, hobbies and patient plus surgeon preferences.

Managing the athletically active arthritic knee remains an interesting challenge and so far we have over 150 osteotomy patients in our database.

c) Partial resurfacing

If only a very small area of articular cartilage is damaged, it may be possible to avoid removal of all or half of the joint surface with metal and plastic. Two new partial resurfacing options are available. The smallest – called Hemicap – is a metal implant that fills in an area of articular cartilage damage on the femur. The manufacturer likens their product to a filling for a tooth. The healthy surrounding cartilage remains in place. A slightly larger component – a Unicap – is also available. This involves resurfacing both the femur and the tibia. Both of these can be useful options for younger patients who wish to avoid – or at least delay – the need for a partial or total knee replacement. It can also be used in those who have suffered focal chondral damage as a result of trauma.

d) Isolated patello-femoral arthroplasty

Arthritis that is localised to the anterior compartment of the knee – the patella-femoral joint – can occur. It is particularly common in women, as a result of minor mis-alignment and maltracking that is more common in females. Whilst total knee replacement will predictably remove any pain associated with this condition, it is an undesirable choice for many younger, active, patients. For this reason, isolated patella-femoral arthroplasty is proving to be a very good alternative for management. The procedure involves resurfacing the patella with a plastic button and the articulating femur surface with a metal conforming implant. The procedure is now very popular, with good results reported.

e) Unicompartmental knee replacement

Uni-compartmental knee replacement involves resurfacing usually the medial part of the joint (femur and tibial surfaces). It is possible to resurface the lateral part but this is a much less common procedure.

Surgery is performed through a small incision, about 10cm long and as such hospital stay is about 3 – 4 days.

To undertake this surgery the arthritis has to be limited to the medial compartment and there should be no significant patella-femoral symptoms. Also, the ACL needs to be intact for the mechanism to work.

The long term results are as good as total knee replacement but it can still take two to three months to fully get over. It is usually much more comfortable in the early phase and this, combined with the short hospital stay, makes it a very appealing option for the appropriate patient.

f) Total knee replacement.

The indications for knee replacement are symptoms of:

• Pain
• Loss of function.
• Abnormal movement.

These symptoms are, of course, all relative and the level of symptoms which will justify the risks for undergoing knee replacement have evolved over recent years.

Each decision is based on balancing the following factors.

1. Pain.

This is the main factor. Pain should be poorly controlled by analgesics or anti-inflammatory medication. There should be night pain and difficulty in getting to sleep because of knee pain.

2. Loss of Function.

Loss of function without pain is not an indication for knee replacement. Loss of function is usually associated with pain and the level of restriction that is considered to justify knee replacement is a walking tolerance of less than a mile, difficulty climbing stairs and a feeling that ‘life is on hold’ due the knee symptoms.

3. Abnormal Movement.

Severely varus or valgus knees or knees with significant instability may mean that knee replacement is employed earlier rather than waiting for further deformity.

4. Age.

The life span of the knee replacement implant is unfortunately limited and therefore the intention of surgery is for the implant life span to extend beyond the life span of the individual without revision. In individuals aged over sixty one can expect a 95% ten year survival of the knee replacement extending to 90% at fifteen years and 85% at twenty years.

In individuals under age sixty one can only expect that 70% of knee replacements will still be satisfactory after ten years.

To summarise, over age 60 we would expect one knee replacement to see the patient out whereas under age 60 a patient should expect revision. The results of revision surgery are far less predictable.

5. Obesity.

Obesity is not a contraindication to knee replacement. There is no clear effect on survival rates and it has been shown on various occasions that the relative benefit of knee replacement in the obese patient is greater. Obesity is a very important topic in orthopaedics, and its relevance to the knee is considered in more detail in Appendix B.

6. Expectations.

This is perhaps the second most important factor after the level of pain. It is hard to define expectations but the following phrases summarise the issues:

• Total knee replacement gives you the knee function of a 70 year old not a 20 year old.
• With a knee replacement you can walk five miles and perhaps play light doubles tennis, provided you can say ‘yours’ and ‘nice shot’.
• It is a metal and plastic knee that does not feel like a normal knee.
• Nine out of ten go extremely well but one in ten can have severe problems after surgery making you potentially worse off.
• Mild pain is normal after knee replacement and 10 – 15% of patients can have persistent moderate pain at one to two years.

Post Operative Recovery

Since the introduction of the technique of Mini-Incision knee replacement in November 2003 and, more recently, an enhanced care program, the length of recovery in hospital has been substantially shortened such that we aim for most patients to go home on the 4th day after surgery. This is nearly half the previous length of stay. In the technique the patella is slid to the side rather than everted which traditionally required a longer cut in the quadriceps tendon. The usual incision length is now 12 – 14 cm compared with 18cm before.

Much of the progress toward earlier recovery and discharge home has been made in the better control of the discomfort after surgery and the earlier mobilisation. On discharge patients should expect to be able to cope with stairs and should be using crutches or sticks. It is usual to progress onto one stick easily by the six week stage and to discard support after the six to eight week outpatient review.

Patients are advised that it generally takes up to three months to get over a knee replacement and this usually equates with return of good function and a feeling that the swelling has gone down with the re-emergence of a normal contour to the knee.

Warmth in the knee however usually persists for up to six months and is not a cause for concern. Reassurance is sometimes required.

Patients will often say that their knee felt better even at a year following surgery but there is generally little change after that stage.

Range of Movement

The average range of movement achieved after knee replacement, according to our recent analysis, is 106° . A lot of patients therefore are achieving greater than this range and this has a substantial bearing on knee function. It is possible to achieve full range of movement (heel to buttock) but this is usually dependant on the amount of bend in the knee that was present prior to surgery. Achieving this range, in turn, is very much dependant on the efforts of the patient adhering to the rehabilitation programme.

In functional terms, a knee that only bends to 90° means that the patient has to push up on the arms of the chair in order to stand. Bend beyond the 100° allows the patient to tuck their heels behind themselves, under the chair, and stand up easily. We are generally unhappy therefore if the patient has only achieved 90° !