Abstract
Symptomatic knee osteoarthritis (OA) can be viewed as the end result of a molecular cascade which ensues after certain triggers occur and ultimately results in irreversible damage to the articular cartilage. The clinical phenotype that knee OA can produce is variable and often difficult to accurately predict. This is further complicated by the often poor relationship between radiographic OA and knee pain. As a consequence, it can be difficult to compare studies that use different definitions of OA. However, the literature suggests that while there are multiple causes of knee OA, two have attracted particular attention over recent years; occupation related knee OA and OA subsequent to previous knee injury. The evidence of a relationship, and the strength of this association, is discussed in this chapter.
Introduction
Osteoarthritis (OA) is a degenerative joint disease involving the cartilage and many of its surrounding tissues. In addition to damage and loss of articular cartilage, there is remodelling of subarticular bone, osteophyte formation, ligamentous laxity, weakening of periarticular muscles, and, in some cases, synovial inflammation. These changes may occur as a result of an imbalance in the equilibrium between the breakdown and repair of joint tissue. Primary symptoms of OA include joint pain, stiffness and limitation of movement. Disease progression is usually slow but can ultimately lead to joint failure with pain and disability.
OA may develop in any joint, but most commonly affects the knee, hip, hand, spine and foot. The incidence of knee OA increases with age, and women have higher rates than men, especially after the age of 50 years. A levelling off or decline occurs at all joint sites around the age of 80 years. Osteoarthritis (OA) of the knee is an extremely common condition globally, with over approximately 6 million sufferers in the UK alone. The prevalence of knee OA in adults aged over 80 years old in the UK approaches 1 in 2 . As the UK population life-expectancy gradually increases, so does the burden of symptomatic and functionally limiting osteoarthritis, which is thought to affect at least 15% of the OA patient population The implications of knee OA are not to be underestimated on either an individual or population scale, particularly given an aging working population.
In the absence of an extrinsic cause, such as prior knee injury, we refer to osteoarthritis as primary. The proportion of individuals within a specific OA population that have primary OA varies greatly, but increases with age. There are also differences by gender; in the Queensland Aboriginal communities it was found that 88% of women had primary OA, whereas 82% of men had secondary OA . We know that the risk of developing OA is determined by both systemic and local factors. Several systemic factors have been identified; these may act by increasing the susceptibility of joints to injury, by direct damage to joint tissues, or by impairing the process of repair in damaged joint tissue; occupational factors may hence be very relevant. Local factors are most commonly biomechanical in nature and adversely affect the forces applied to the joint; some sporting injuries, or injuries sustained at work, fall into this category.
The relationship between age and the risk of OA is likely multifactorial, as a consequence of numerous individual factors; these include oxidative damage, thinning of cartilage, muscle weakening, and a reduction in proprioception. Furthermore, basic cellular mechanisms that maintain tissue homeostasis decline with aging, leading to an inadequate response to stress or joint injury and resultant joint tissue destruction and loss. This may be relevant when considering the ability of the body to repair joint injury. The prevalence of OA and patterns of joint involvement vary among different racial and ethnic groups. Prolonged squatting and kneeling is associated with increased risk of moderate to severe radiographic knee OA .
While risk factors for osteoarthritis are well described in the literature, the link between occupation and osteoarthritis is less well documented, although a direct and causal link has been postulated. There also appears to be a significant link between previous knee injury and subsequent development of knee osteoarthritis .
We know that the mechanical alignment of the knee influences the distribution of load across the articular surfaces. In a normally aligned knee, 60–70% of weight-bearing load is transmitted through the medial compartment. Any shift in either a valgus or varus direction affects load distribution. Abnormal increases in compartmental loading are thought to increase stress on the articular cartilage, and other joint structures, subsequently leading to degenerative change. A recent systematic review confirmed that knee malalignment is an independent risk factor for the progression of knee OA . This may be relevant as knee alignment may influence risk of OA following injury or as a consequence of occupational factors.
Osteoarthritis of the knee and occupation
There has been research conducted into the potential link between occupation and the future risk of developing osteoarthritis. Notably, some of these studies postulating a link between occupation and arthritis contained inherent design features which make the findings less reliably clinically interpretable. For example, some of the studies used radiographic OA as their only diagnostic inclusion criterion related to OA . Given the discrepancy between radiographic and clinically symptomatic OA, this criterion alone could significantly influence interpretation of studies suggesting a link between occupation and OA.
However, despite some studies having potential problems in their design or diagnostic criteria, the overall consensus is that there is at least moderately strong evidence to support the hypothesis that certain occupational activities increase the risk of knee OA. Specifically, there is a significant body of evidence demonstrating that individuals who have subjected their knees to significant and recurrent squatting, bending, kneeling and loading are far more likely to develop OA . A recent systematic review considered the relationship between occupation and knee OA . They reported moderate evidence that combined heavy lifting and kneeling is a risk factor for knee OA, with odds ratios (OR) varying from 1.8 to 7.9, and limited evidence for heavy lifting (OR 1.4–7.3), kneeling (OR 1.5–6.9), stair climbing (OR 1.6–5.1), and occupational groups (OR 1.4–4.7) as risk factors. When examined by sex, moderate level evidence was found in men; however, the evidence in women was limited, and for this reason the authors highlighted the need for further research in this area. It has been estimated that about 5% of knee OA might result from occupations using repetitive knee use .
By acknowledging and recognising the concept of occupation-related knee OA, one can target the specific risk factors and ideally prevent development of OA of the knees . The concept of avoiding risk-related occupational activity is likely to require a very specific and tailored approach in different occupational settings. Further high quality research examining heavy lifting and/or kneeling, especially in the female population, could provide valuable additional information which may substantiate the above findings.
Differences in rates of occupation related OA in males and females
It remains somewhat unclear as to whether there is a true difference in the rate of occupation-related knee OA between males and females. This uncertainty arises due to a multitude of factors. For example, vague definitions of occupational related knee activity and lack of inclusion of “domestic duties” may all potentially contribute to any perceived discrepancy between rates of occupation related knee OA in males and females ( Table 2 ).
| Occupational activity | Number of eligible studies | Positive relationship to knee OA Number (%) (OR) | Number of high quality studies | Level of evidence |
|---|---|---|---|---|
| Heavy lifting | 17 (18) | 11/18 (61) (1.4–7.3) | 10 | Limited |
| Kneeling or knee bending | 16 (15) | 11/15 (73) (1.5–6.9) | 9 | Limited |
| Combined heavy lifting and kneeling | 6 | 5/6 (83) (1.8–7.9) | 4 | Moderate |
| Climbing stairs | 8 | 7/8 (88) (1.61–1.51) | 7 | Limited |
| Occupational activity | Number of eligible studies | Positive relationship to knee OA Number (%) (OR) | Number of high quality studies | Level of evidence |
|---|---|---|---|---|
| Occupation involving heavy lifting and/or kneeling | (12) | 9/12 (75) (1.4–4.7) | 3 | Limited |
| Occupation related physical loading – males | 21 (22) | 20/22 (91) (1.4–7.9) | 13 | Moderate |
| Occupation related physical loading – females | 15 | 9/15 (60) (1.4–7.3) | 10 | Limited |
However, there have been at least 2 meta-analyses concluding that there was an association of knee OA in males with certain occupational activity exposure but this could not be demonstrated in females In partial agreement with these findings, a more recent meta-analysis concluded that the evidence for a clear association between occupation and knee OA in males was “moderate” whereas in females there was deemed to be only “limited” evidence for a clear association .
Overall data from 3 meta-analyses appears to highlight some doubt over the existence of any link between female occupational activity and subsequent knee OA. The relative paucity of high-quality studies involving female participants may well be a factor in this Those studies which have included female participants have not been rated as high quality (results in 3 of these studies were statistically non-significant with multivariate analysis) .
As an interesting aside, some recent research has suggested that females may have a higher cumulative joint loading than their male counterparts in occupational activities . How this might potentially translate into risk of knee OA remains unclear.
Sporting activity, knee injury and osteoarthritis
Sporting activity is known to be directly implicated with the development of subsequent knee OA. For example, physical activity undertaken by professional sportsmen/sportswomen is a clear demonstrable risk factor for knee OA. This was the conclusion of a large international systematic review group . A recent systematic review reported that specific sports with a significantly higher prevalence of knee osteoarthritis were soccer (OR = 3.5), elite-level long-distance running (OR = 3.3), competitive weight lifting (OR = 6.9), and wrestling (OR = 3.8). Elite-sport (soccer or orienteering) and non-elite-sport (soccer or American football) participants without a history of knee injury had a greater prevalence of knee osteoarthritis than non-exposed participants who did not report sports participation. This association will reflect a combination of joint loading; use and minor injury.
Obesity, occupation and knee osteoarthritis
In addition to being an individual risk factor for knee OA, being obese also appears to add further risk when associated with occupations involving recurrent kneeling, bending and squatting . The incidence rates appear to be higher in subjects who also reported occupational lifting, and these latter factors appeared to interact multiplicatively with the risk conferred by obesity. This interaction is important when counselling patients regarding ways to reduce risk of knee OA ( Table 1 ).
Possible mechanisms of knee injury in risk related occupational activities
Various theories have been postulated regarding the likely mechanistic cause of OA in the context of repeated occupational exposure to kneeling, bending or squatting. One theory is based around direct cumulative damage through loading of the joint and the subsequent detrimental effect on the integrity of articular cartilage. Repeated loading has been demonstrated as being damaging to articular cartilage in animal models. It is known that repeated loading of the joint stimulates proteoglycan production but the overall consequences of the molecular cascade which follows loading of the knee joint are not yet fully established .
Another possible theory relates to inadvertent progressive damage to either knee ligaments or to the menisci through the aforementioned repeated occupation related activities. Notably, both meniscal and ligamentous injury are already known to be strongly associated with a higher risk of subsequent knee OA .
Osteoarthritis of the knee and injury
Although a direct link between prior knee injury and subsequent OA has been postulated, the validity of this link has been the subject of debate. One may speculate that significant injury (implying intra-articular fractures or meniscal injury) to a joint would render that area more susceptible to further degenerative change and symptoms in the future. It is thought that the molecular inflammatory pathways, which ensue after injury, may at least partly explain how indirect injury or a history of trauma could result in knee OA in the future.
A meta-analysis conducted in 2014 included 13 relevant cohort studies that considered injury and knee OA . The overall conclusion from this meta-analysis was one of a clear and direct association between previous knee injury and the subsequent development of osteoarthritis (pooled odds ratio 2.83, 95% confidence interval 1.91–4.19 l2 = 89.1%) (see Fig. 1 ).
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