L. Stefan LOHMANDER,MD, PhD
Department of Orthopedics Clinical Sciences Lund
Lund University, SWEDEN
Research Unit for Musculoskeletal Function and Physiotherapy
Department of Orthopedics and Traumatology
University of Southern Denmark
Knee replacement for osteoarthritis: facts, hopes, and fears
Sweden and Denmark
Osteoarthritis (OA) affects hundreds of millions of people worldwide and is responsible for a huge burden of pain, functional limitations, and loss of quality-adjusted life expectancy. OA of the knee accounts for more than 90% of total knee replacements (TKR). By 2030, the incidence of TKR in the US is expected to increase by more than 6-fold. Pain and difficulty walking limiting participation in daily activities are the primary reasons for undergoing TKR. However, among patients with disabling OA, only a minority are willing to consider TKR. Reduced willingness to undergo TKR is associated with increasing age, being black (in the US), overestimation of the pain and disability needed to warrant TKR, and rejection of the medicalization of OA. The perception of the benefits of TKR is less positive among women and those of lower socioeconomic status. TKR is one of the most cost-effective medical interventions. Data from joint registries show that the 10-year reoperation rate for TKR is less than 5%. However, between 10% and 30% of patients undergoing TKR report little or no improvement following surgery, or are not satisfied. Patients’ expectations of joint replacement are relief of pain and improved mobility. Following TKR, the expectations regarding pain relief, walking ability, and the ability to perform daily activities are fulfilled to a greater extent than the expectations of being able to perform more physically demanding activities. To ensure optimal patient satisfaction, health professionals should provide sufficient information so that patients have realistic expectations of the results of TKR.
Osteoarthritis (OA) affects hundreds of millions of people worldwide and accounts for a huge burden of pain, functional limitations, loss of productivity, disability, and loss of quality-adjusted life expectancy.1,2 Any estimate of the burden of OA is strongly dependent on the criteria used, and the population and age group studied. The most recent estimates of the World Health Organization (WHO) Global Burden of Disease Study 2010 reported that OA of the knee and hip is now ranked as the 11th leading cause of years lived with disability (YLD [prevalence of a condition multiplied by the disability weight associated with that condition]), up from the 15th position in 1990. There was an estimated 64%increase in the global burden of OA-related YLD between the years 1990 and 2010. OA of the knee is responsible for four-fifths of the OA-related YLDs of the lower extremities. In the US, it is estimated that the direct and indirect costs of OA exceed US $100 billion per year.3-5 Excess mortality is observed in patients with OA and this is related to increasing functional impairment in those patients.6 OA most commonly affects the knees, hips, and hands, but may affect other joints as well, such as the shoulders, elbows, ankles, feet, and spine. The prevalence rises steeply between the ages of 50 and 60 for both women and men, so that the majority of patients with symptomatic OA are aged between 60 and 80 years. However, OA also occurs in younger and middle-aged patients. In fact, effective treatment of OA represents a particularly difficult challenge in patients aged between 30 and 60 years.
OA of the knee represents a major share of the OA disease burden, and OA is by far the most common reason for total knee replacement (TKR), usually accounting for more than 90% of knee replacement procedures. The incidence of TKR for OA is rising steeply, and should continue to rise dramatically, with a more than 6-fold increase expected by 2030 in the US.7 The following discussion will focus on knee replacements for OA, but will also, where appropriate, use evidence gathered from the study of hip replacement for OA.
Risk factors for knee OA
OA of the knee shares the same general risk factors as OA of the other joints in that each patient and each joint carries its own mix of genetic and environmental factors responsible for the development of OA. For the knee, individual genetic variability is thought to account for about 40% of the risk, with environmental factors accounting for the remaining 60%. Genetic variability may, for example, be expressed as individual differences in the quality of the cartilage matrix, reactivity of inflammation pathways, growth and repair pathways, and joint shape. Environmental risks are mostly biomechanical, affecting the dynamic loading of the joints and increasing stress on joint tissues. This in turn will result in the activation of inflammation and tissue degradation. Leading examples of such environmental “joint stressors” are overweight and obesity, joint injury, and occupational overload. Genetically determined variations in joint shape may also influence the risk of OA by increasing joint stress.
Epidemiology of knee OA
As already stated, the disease burden of OA is enormous. It is the most common form of progressive joint disease, and affects 50 million adults in the US and Europe,1,8 with OA of the knee representing a major share of this burden. In the US, knee OA accounts for more than 10 million quality-adjusted years of life lost, and almost 1 million hospital admissions per year.2,9,10 Although knee OA is usually perceived as a disease of the elderly, the mean age at diagnosis is actually 56 years.11 The prevalence of knee OA is rising due to the aging of the population and increasing rates of joint injury and, most importantly, obesity.
Figure 1. A stepwise approach to the management of osteoarthritic
Management options for knee OA
A wide range of treatments are used for knee OA, including nonpharmacological, pharmacological, and surgical approaches.12-14 OA management strategies are determined by disease severity, patient preferences, local resources, and established treatment modalities.
A stepwise approach to the management of osteoarthritis is widely recommended, starting with patient empowerment and self-management, and, if these simple approaches are unsuccessful, the addition of specific medical or surgical interventions (Figure 1). With the exception of knee replacement, the effect size for most therapeutic interventions in knee OA is only small to moderate.
Basic management of osteoarthritis includes education, exercise, and weight loss (for overweight people), which can be complemented as needed by simple analgesics, and, for hand and knee joints, topical nonsteroidal anti-inflammatory drugs (NSAIDs). Additional studies are needed to tailor the different programs to different patients. Common sense suggests that a combination of treatments benefits most patients.
Oral NSAIDs and paracetamol (acetaminophen) are often used by patients with OA. However, gastrointestinal, renal, and cardiovascular side effects are of particular concern with NSAIDs, since many patients with OA are elderly and have comorbidities, and thus are at increased risk of some or all of these side effects. These concerns add complexity to the task of pre- scribing pain relievers for patients with OA. The utility of opioids for OA is limited by their side effects. Corticosteroid injections are frequently used, but their benefit is short-lived (1 to 4 weeks). In addition to these treatments, patients with knee OA use a wide range of alternative therapies.
The surgical management of knee OA commonly includes arthroscopic surgery of the knee upon suspicion of a meniscus tear or cartilage derangement amenable to surgical treat treatment. However, high-quality trials comparing arthroscopic surgical debridement (including meniscus resection) with sham surgery, medical care as usual, or exercise programs found no difference between the groups compared,15-18 thereby showing that arthroscopic surgery is of no added benefit for these patients. Over the age of 35, meniscus lesions are often part of the early stages of the osteoarthritic process, and the diagnosis of a meniscus tear by magnetic resonance imaging (MRI) of the osteoarthritic knee does not correlate with the presence of symptoms.19
Valgus osteotomy of the tibia can provide long-lasting pain relief and functional improvement in physically active patients of less than 60 years of age in whom OA is mainly limited to the medial knee compartment. If and when needed, the osteotomy can later be converted to a knee replacement. In a large national Swedish series of about 3000 patients who had undergone tibial osteotomy for knee OA at a mean age of 52 years, the 10-year “conversion rate” to TKR was 30%.20 This suggests that osteotomy should remain a viable option for patients in this group, thereby delaying or obviating the need for knee replacement by more than 10 years in most cases.
Figure 2. (A) Annual number of knee replacements in Sweden for osteoarthritis (OA), rheumatoid arthritis (RA), and posttraumatic osteoarthritis
between the years 1975 and 2010. (B) Annual incidence of knee replacements (all replacements) for men and women.
Practice variations in the use of knee replacement and future projections for TKR use
Joint replacement for OA of the knee may be the only intervention with a large effect size, but it is only appropriate for patients with advanced disease or substantial symptoms that do not respond to other treatments. This group is but a minority of all those with knee OA, but still represents a very substantial number of patients. Rates of TKR are increasing worldwide, and in 2010, more than 600 000 procedures were carried out in the US alone, with rates tripling over the last decade in those aged between 45 and 64.9,21 At over $20 000 per procedure, the annual cost of TKR in the US now exceeds $13 billion.5,22,23
National knee replacement registry data from the Scandinavian countries, UK, and Australia also confirm the continuous and rapid increase in the incidence of knee replacement for OA. For example, results from the Swedish Knee Arthroplasty Register show that the annual rate of primary TKR for OA in Sweden doubled between 2000 and 2010 (Figure 2), while the annual rate of TKR in those younger than 55 increased 5-fold.24,25 The dramatic increase seen in younger patients may in part reflect a change in surgical practice from the use of osteotomy and unicompartmental knee replacement to TKR in younger patients.25
The mean age for TKR has changed over time, and in Sweden it now shows a decreasing trend, being about 69 years for both women and men (Figure 3, page 184).24 Reflecting the current routine widespread use of TKR as a procedure to treat severe knee OA, 1 in 14 elderly women in Sweden has now had a TKR.24 Similarly, nearly 1 in 20 Americans over 50 years of age have had knee replacement surgery, which represents more than 4 million people.9,21
Figure 3. (A) Temporal shifts in the mean age at primary total knee replacement surgery for women and men in Sweden between 1975
and 2010. (B) Comparison of the 2000 and 2010 prevalence rates of knee replacement according to age for women and men in Sweden.
Direct comparisons of the rates of TKR for OA between different countries are hindered by the limited availability of high quality specific data for OA procedures, as well as by differences in population structure. The 2007-2009 estimated rates of primary TKR for all diagnoses per 100 000 people varied between 9 for Romania and 213 for the USA.26 Even between countries with seemingly similar socioeconomic conditions, national health care systems, and population structures, the TKR rates per 100 000 varied greatly, ranging from 188 for Germany, 178 for Finland, 112 for Sweden, to 98 for France.26 The reasons for these wide variations in usage are not clear, but are likely to be due to differences on both the “supply side” (health care services) and the “demand side” (patients).
Time-related trends in TKR use
The projections for future TKR rates have regularly been outdone by reality. In the US, the demand for primary TKR was, in 2005, expected to grow by more than 600%, to reach 3.48 million procedures by 2030.27 The growth of total knee revision surgeries was projected to be proportional, with the demand for revision procedures expected to double between 2005 and 2015. If these predictions hold true, both health care infrastructures and the training of orthopedic surgeons will need to expand accordingly. The dramatic increase in TKR rates inevitably leads to questions about financial feasibility in the future: who will pay? So what are the possible reasons for the steeply increasing demand for TKR seen over the last decade? The reasons for this increase are most certainly multifactorial. Population growth, the increasing number of elderly people, and the increase in average body mass index (BMI) are, no doubt, important factors.
The increase in the age-standardized prevalence of knee OA (for which there is limited evidence) may be driven by the now global epidemic of overweight and obesity (for which there is strong evidence), and by an increase in joint injury rates. These two “environmental” risk factors may together be responsible for up to 50% of knee OA cases in some societies.21 The importance of overweight and obesity in the risk of severe knee OA leading to TKR was investigated in a prospective population-based cohort study (Figure 4); a mean BMI of 30 was associated with an 8-fold increase in TKR risk when compared with a “normal” BMI of about 22.28 Even an increase in BMI from 21-22 to 24-25 (ie, within a BMI range that is considered normal) was associated with a 3-fold risk increase in the rate of TKR for OA. In fact, in this large population-based study, very few TKR procedures were carried out on patients in the lowest BMI quartile (Figure 4). In the US, the major impact of obesity on the risk of knee OA and TKR can also be illustrated by estimating the number of TKRs averted by “reversing” the prevalence of obesity to the levels of 10 years ago. This would correspond to a mean BMI reduction of 0.6 or a reduction in body weight of less than 2 kg for a person of average height. This means that more than 100 000 TKRs would be averted over the remaining life span of this population.2
Population growth, aging, and an increase in BMI all seem to result in an increase in the demand for TKR, even if these factors do not fully explain the recent steep increase in demand.21 However, increased availability on the “supply side” is a further important and possible reason for the increase in the incidence of TKR surgery, as total joint replacements have changed from the early days of being “boutique” procedures to now being a generally available “commodity” in many settings.29,30
Figure 4. Kaplan-Meier survival analysis of population fraction
without total knee replacement (TKR) or osteotomy for knee osteoarthritis.
TKR or osteotomy for knee osteoarthritis. Individual survival plots show body
mass index (BMI) quartiles (population, n=11026 [men] + 16934 [women]), with
a statistically significant difference between each of the four survival curves.
Each BMI quartile consisted of 2756 men and 4233 women on average. The
median BMI values of the population quartiles were 22.5/21.1 (men/women)
(green), 25.0/23.6 (gray), 27.0/26.0 (blue), and 30.1/30.1 (red), respectively, for
BMI quartiles 1 to 4.
Modified from reference 28 with permission: Lohmander et al. Ann Rheum
Dis. 2009;68:490-496. © 2008, BMJ Publishing Group Ltd & European League
What determines patient demand for TKR?
Pain and difficulty walking limiting mobility and participation in normal daily activities are the primary reasons for undergoing TKR for OA. However, several studies have indicated that among patients with a marked need for joint replacement, ie, those with disabling OA, only a minority (around one-third) were willing to consider joint replacement as a treatment option.31-33 Patients’ willingness to undergo total joint replacement is, therefore, a critical predictor in determining the demand for this procedure.34,35
Reduced willingness to consider joint replacement was shown to be associated with increasing age, being black rather than white (in the US), overestimation of the levels of pain and disability needed to warrant joint replacement, and rejection of the medicalization of OA—ie, considering OA as a natural and inevitable part of aging.34,35 The perception of the benefits of joint replacement for OA was shown to be less positive among women than men, and among those of lower socioeconomic status.36 Although blacks suffer from more severe OA than whites in the US, they are less likely to consider surgery as a solution to their problem, and this is associated with less positive beliefs about the benefits of surgical procedures.37 A person’s knowledge and beliefs regarding TKR appears to be largely shaped by his or her interactions with family and friends. The differences summarized here may at least in part explain the inequities observed in joint disease care.38,39
Given the low proportion of patients with OA in whom TKR is indicated who are actually willing to undergo the procedure, changes in willingness to undergo TKR seem to be an important contributing factor in driving the recent increase in the incidence of TKR, and also in driving a future increase in the demand for TKR. Increased expectations regarding the maintenance of an active lifestyle into older age may influence willingness, as may direct-to-consumer advertising, which is now common in many countries. Studies investigating temporal changes and changes in willingness to undergo TKR in different settings would be of considerable interest.
Indications for TKR as perceived by health professionals and patients
Several attempts have been made to develop consensus criteria for TKR indications. As in all such efforts, the composition of the group and the method used to form the consensus determine the result. Well-known examples of such criteria are those developed in New Zealand and Canada.40,41 Components of these scores include pain, functional impairment, problems in care giving, and radiographic severity. However, attempts to determine cut-off points leading to an “appropriate” indication for TKR by correlating preoperative patient reported symptoms with recommendations for placement on a TKR waiting list have not been successful.42 This suggests that these more easily measured components explain only a minor proportion of the decisions to place patients on waiting lists for surgery.
Most TKR criteria have been dominated by the views of health professionals, often orthopedic surgeons. The views of patients on the appropriateness of TKR have been explored to some extent, and suggest that they do not always agree with the views of health professionals. The concept of the “capacity to benefit” from TKR suggests that the benefits should outweigh any likely risks or unintended consequences by a considerable margin for TKR to be indicated.43,44
Effectiveness of knee replacement: what determines outcome and patient satisfaction after TKR for OA?
It has been said that total joint replacement “doesn’t bring years to life, but brings life to years.” This is true in the sense that patient-reported outcomes of joint replacement are on average “good to excellent,” and it is particularly pertinent in comparison with most nonsurgical treatments for severe OA. The Scandinavian national joint registries have for many years reported on the continuous improvement of outcomes over time as a result of improved surgical, anesthesiological, and care procedures and materials.45 In Sweden, the 10-year revision (reoperation) rate is now estimated to be less than 5% for patients operated with TKR for OA at the beginning of this century.24 This means that for more than 95 out of 100 patients undergoing TKR for OA, the patient will be survived by the implant. Reoperation rates from other countries have generally been somewhat higher, perhaps because of the choice of implants, surgical techniques, and other factors.26 However, it is well-known that the risk of reoperation is considerably higher for younger patients operated with TKR, than for older patients.24 Higher revision rates for younger patients operated with TKR are expected in light of the changing demographics of TKR surgery, with rapidly increasing numbers of younger patients undergoing joint replacement.46 The Swedish national registry reports no difference in revision rates between men and women operated with TKR for OA in the 2000s.24
When considering these excellent results, it should be noted that they represent reoperation rates, not patient-reported outcomes. In fact, between 10% and 30% of patients undergoing joint replacement report little or no improvement following surgery, or are not satisfied with the outcome.47-50
Factors associated with a less than optimal patient-reported outcome of TKR at 6 months include worse preoperative pain and function, increased anxiety/depression, and living in poorer areas.51 Multiple joint involvement negatively influences the outcome after TKR,52 while severe obesity results in slower recovery after surgery, but with no difference at 3 years.53 It should be noted that the determinants of pain or function following TKR may not be the same in all studies.51,54 The most important expectations of patients undergoing joint replacement surgery are pain relief and improved mobility. It appears that at 5 years following TKR, patients’ expectations of pain relief, walking ability, and ability to perform the activities of daily living are fulfilled to a greater extent than their expectations of being able to perform more physically demanding activities.50 For example, 41% of patients expected to be able to perform activities such as golfing and dancing following TKR, while only 14% were in fact able to perform these activities 5 years after undergoing TKR. To ensure optimal patient satisfaction, it is important that health care professionals provide adequate information before surgery so that patients have realistic expectations of the results of TKR.
The expectations, results, and patient satisfaction with TKR surgery for OA summarized above reflect the current “case mix” of patients with regard to disease severity, expectations of outcome, age, and active life expectancy. TKR stands out as one of the most effective and cost-effective medical interventions. However, whether this outstanding record will be upheld or not given the changing demographics of patients undergoing this type of procedure, with those aged between 45 and 64 years being the most rapidly increasing group, remains to be seen. Innovations and new implant designs have at times been introduced before evidence of their efficacy and safety could be established. Improved regulatory and clinical processes for the introduction of new devices are badly needed.
Another pressing question is that of health economics. The projected dramatic increase in TKR use, if it becomes a reality, will demand increased orthopedic and hospital resources, among others, and this will result in sharply increased costs for patients and society. _
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Keywords: indication; osteoarthritis; total joint replacement; willingness