The severity of knee OA usually is assessed using different measures of specific and often limited aspects of joint structure, function, and pain. However, the relationships between these measures are unclear. Our goals were to: (1) examine the relationships between knee structure (Flex-Ext ROM, radiographic TF angle, and medial JS), function (KOOS values, peak adduction angle, and moment), and pain; and (2) identify the variables that best predict knee pain. The rationale was that such information would be helpful to better understand the pathomechanics of primary knee OA.
This was a preliminary study with several limitations. First, we used a small sample size (n = 15), recruited through advertisements in the New York City area; and therefore, future studies should be conducted on larger samples from the general population. However, we consider these initial results important because the data suggest a link between structure, function, and pain in the patient with primary knee OA. Second, our study was limited to evaluating the macromechanical characteristics and knee pain in patients with primary medial knee OA. Other painful knee conditions such as meniscus and cruciate ligament injuries were beyond the scope of our study. Also, the degenerative changes in knee OA, including subarticular bone attrition, bone marrow lesions, synovitis, and effusion, have been associated with knee pain [34
]; however, evaluating these relationships was not the focus of our study. Third, our findings are limited to assessment at one instant in time, whereas the relationships between variables might vary during the natural course of the disease.
There are inconsistent reports regarding relationships between variables of knee structure, function, and pain. For example, medial JS has been correlated with changes in cartilage volume on MRI [9
], bone mineral density [46
], body mass [52
], and severity of knee OA symptoms [15
], and it has been used routinely in clinical practice to assess the status and progression of medial knee OA [8
]. However, substantial discordance also has been observed between the severity of patient’s symptoms, and radiographic evidence of degenerative changes in knee OA (including JS narrowing). [7
] Likewise, peak knee adduction moment during gait has been correlated with medial knee OA pain [36
], varus alignment [17
], mechanical axis [52
], load distribution [5
], compartmental bone mineral content [37
], and radiographic evidence of disease progression [4
]. However, conflicting reports exist related to knee adduction moment. Heiden et al. reported that larger adduction moment was correlated with lower self-perceived knee pain and other OA symptoms [29
], whereas Maly et al. observed no correlation between knee adduction moment and knee OA pain [47
]. Also, a lack of correlation between knee adduction moment and limb varus/valgus alignment has been reported [57
]. McNicholas et al. observed in patients who had a total meniscectomy, that knee adduction moment did not correlate with either limb alignment or radiographic severity of OA [48
The means and standard deviations for all variables of interest in this study (Table ) were comparable to those in several studies [3
], as were the mean knee adduction angle and moment curves during gait (Fig. 3) [53
]. All subjective measurements were correlated (|r| ≥ 0.54) with one another (Table ), as were most of the objective measurements (|r| ≥ 0.56) except for the peak knee adduction moment which did not correlate with any variable. The lack of correlation of the peak adduction moment to VAS knee pain and other variables was puzzling as it was contrary to the expected outcomes based on several previous reports [29
]. However, other researchers also have observed a lack of correlation of knee adduction moment with knee pain, and other variables pertinent in knee OA. This may be attributable to differences in research methods.
Numerous variables have been studied regarding their usefulness for predicting the progression of knee OA (Table )[13
]; however, amid conflicting reports, few predictive variables have strong supporting evidence [51
]. We found that the radiographic medial knee JS and the peak knee adduction angle were the best predictors of knee pain in our study; and that these variables accounted for approximately three-quarters of the model variance (r2
= 0.73). Our analysis included a combination of a static measurement (medial JS) indicative of knee structure, and a dynamic measurement (peak knee adduction angle) indicative of knee function. The peak knee adduction angle during gait corresponds to the ‘varus thrust’, sudden lateral bowing of the knee during the diagonal weight shift in patients with medial knee OA [12
]. The peak knee adduction angle is not used routinely in clinical practice, although it has been recommended as an important clinical index for knee OA [45
]; our data support this recommendation.
Structural and functional predictors of pain in medial knee OA
The treatment goal in varus knee OA, by means of surgical (eg, high tibial osteotomy) [16
] and nonsurgical methods (eg, wedging foot orthoses, unloading knee braces) [10
], is to reduce deviation of the mechanical axis of the lower extremity from the knee joint center. However, the ‘mechanical axis’ is a static 2-D radiographic measurement that varies with foot position [32
] and weightbearing status (eg, single versus double-limb support) [70
], and it might not always correlate with complex 3-D loading or dynamic function of the knee [2
]. Based on our preliminary results, we believe that a combination of static measurement (eg, medial JS) and dynamic (or functional) measurement might be more predictive of the patient’s perceived knee pain than any one type of variable, and that such a mixed model might be useful for monitoring progression of knee OA and for assessing treatment outcomes.
We determined the relationships among several measurements of knee structure, function, and pain. Among the examined variables, radiographic medial knee JS (a structural variable) and peak knee adduction angle (a functional variable) were most predictive of patient-perceived knee pain. Therefore, the value of the peak knee adduction angle as a tool for assessing knee OA severity and treatment efficacy warrants further investigation.