• hip osteoarthritis
• knee osteoarthritis
• shoes

Osteoarthritis (OA) is among the most common types of joint disease and a frequent cause of pain and physical disability. It is the fourth most frequent predictor of health problems worldwide in women, and the eighth in men¹; in countries such as the United States, United Kingdom, or Canada its total economic costs have been estimated as 2% of the gross national income, second only to cardiovascular diseases.² OA is now considered as a dynamic process in which what appears to be the consequences of the older idea of mechanical wear and tear of the joint coincides with remodelling of joint structures, increased cartilage turnover, incomplete repair and new bone formation, more marked in the joint margins.^3,4 Joints at the lower extremity, mainly knees and hips, are frequently affected in primary OA, resulting in various degrees of walking pain and difficulty. Although the causes of primary OA remain undefined, it is well established that mechanical factors play an important part in its development and progression. Obesity is an important risk factor for symptomatic knee OA,⁵ and both men and women with a body mass index of 30 to 35 have a fourfold increase of knee OA compared with normal weight controls.⁶ Prospective data on women suggest that the risk for known OA increases by about 15% per each kg/m² of body mass index above 27, and that reduction of a similar weight resulted in a similar decrease of incident knee OA,⁷ proving that this is a modifiable factor. At the knee, being overweight is also a risk factor for the development of OA at the patellofemoral compartment.^8,9 Besides, longitudinal studies suggest that obese people with knee OA have a higher risk of experiencing disease progression than thinner people.¹⁰ The association between hip OA and overweight is not as strong as that of knee OA; in general the studies focusing on radiological hip OA have been inconsistent, while those reporting on symptomatic hip OA have in general found association.¹¹ Weight can act on the bearing joints through the obvious mechanism of increasing the forces across the bearing joint, but also the bearing force carried by the knee and hip joints during walking is two to three times the body weight,¹² and in the case of overweight the multiplied extra weight is a substantial added burden.

It seems that overcoming the mechanical tolerance of weight bearing joints—probably by damaging some of their less tolerant structures—is an important risk factor for OA. It is not surprising that the risk of knee OA is doubled by having a job involving at least medium physical demands and knee bending compared with a sedentary job.¹³ Increased risk for OA of the knee in persons with activities that entail repeated knee bending or squatting have also been found by others.^14,15 There is a link between knee injury and OA,^16,17 also found at the hip joint.¹⁶ Other data support the importance that mechanical factors seem to have for the incidence and progression of OA, such as knee alignment,¹⁸ or having suffered an open meniscectomy.¹⁹ The relevance of mechanical factors in the development of knee OA justify the search of a possible relation between this condition and a long history of high heel shoes wearing, as carried out in the paper by Dawson et al printed in this issue of the journal,²⁰ although such a relation was not found. Of interest, persons with lower bone mineral density osteoporosis have less OA in knees and hips than those with higher bone mass^21,22; this may be attributable to differences in mechanical stress through the bearing joints, and it has been suggested that more stiff subchondral bone with less deforming capacity under loading may lead to easier cartilage damage.²³ OA affects preferentially to persons in the later stages of life and is a common cause of pain and infirmity for them. The reasons probably include factors other than accumulated work on the bearing joints such as the increased vertical impact of the feet by walking in this age group,²⁴ declines in balance and lower extremity strength of older people with chronic knee pain,²⁵ decreased muscular strength at the leg muscles,²⁶ or a combination of all.

The goals in the management of OA are limited to symptom control to make the disease tolerable; it has to be kept in mind that for the patient the problem is not OA itself, but its symptoms.²⁷ Surgical salvage procedures in some joints—such as the hip and knee—can be used when symptom control fails. The attempts to pharmacologically modify the disease progress have not produced substantial results so far. Analgesics and non-steroidal anti-inflammatory drugs (NSAIDs) are widely used for this condition; because of the high prevalence of OA—and thus the size of the market—pharmaceutical companies have maintained efforts to develop new more effective or better tolerated drugs, and this has resulted in an important number of controlled studies and continuous attention to this essentially symptomatic therapeutic approach. The action of analgesics and NSAIDs on joint pain is quite independent of the mechanism of pain; one of the reasons for the success of these drugs in OA results from their efficacy relieving joint pain with independence of its cause. The origin of pain in the OA joint is poorly understood; a joint such as the knee may be affected by OA in different areas, and pain probably arises through different mechanisms—possibly related to the different ways in which a joint can be mechanically changed. In weight bearing joints, and particularly in the knee, symptomatic improvement also results from measures that decrease the load on the affected joint, such as weight reduction,⁷ or the simple use of canes, crutches, or walkers. Also patients generally note that pain increases with the use of the affected joint and may report that pain worsens if they carry weight, or when they gain it. Occasionally patients may report that comfortable shoes may reduce their symptoms. Shoes may help through different mechanisms, such as reducing the vertical impact of the feet, which is increased in older people,²⁴ acting as shock absorbers—as some sports shoes do—, helping to improve the balance, which is reduced in older people with knee pain,²⁵ or modifying the ways in which joints change. Wedged insoles have been found to reduce the load of the medial knee compartment²⁸—the most frequent location of OA in this joint—, and randomised controlled studies have resulted both in failure²⁹ and confirmation³⁰ of the symptomatic effect of this intervention. Shoes have received very little formal attention as therapeutic measures for symptomatic knee or hip OA. If one considers the relevance that mechanical factors have on the development and progression of OA and in the presence of symptoms in the affected joints, it seems worthwhile to evaluate whether shoes—by modifying mechanical factors—may result in lessening of the symptoms and perhaps disease progression. Indeed, ingeniousness is needed to develop properly designed trials to produce evidence of the benefits of shoes if there is any. Also better understanding of the mechanisms of pain in OA of knee and hip appear necessary, as well as a possibility of grouping the patients according to the overcharged or overstressed structure or structures from which the pain may arise, in order to design the shoes accordingly. But in the end, if shoes prove useful in reducing the symptoms of some types of lower limb—or perhaps lower spinal—OA, and the use of drugs for this aim can be reduced, the benefit will doubtless be for all of us.