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Abstract

  1. Top of page
  2. Abstract
  3. PATIENTS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. AUTHOR CONTRIBUTIONS
  7. ROLE OF THE STUDY SPONSORS
  8. REFERENCES

Objective

Varus thrust observed during gait has been shown to be associated with a 4-fold increase in the risk of medial knee osteoarthritis (OA) progression. Valgus thrust is believed to be less common than varus thrust; the prevalence of each is uncertain. Racial differences in risk factors may help explain variations in the natural history of knee OA. We undertook this study to determine the frequency of varus and valgus thrust in African Americans and Caucasians and to identify factors associated with thrust presence.

Methods

The Osteoarthritis Initiative cohort includes men and women who have knee OA or are at increased risk of developing it. Trained examiners assessed thrust presence by gait observation. Logistic regression with generalized estimating equations was used to identify factors associated with thrust presence, and odds ratios (ORs) with 95% confidence intervals (95% CIs) were calculated.

Results

Compared with Caucasians, African Americans had lower odds of varus thrust, controlling for age, sex, body mass index (BMI), injury, surgery, disease severity, strength, pain, and alignment in persons without knee OA (adjusted OR 0.50 [95% CI 0.36, 0.72]) and in those with knee OA (adjusted OR 0.46 [95% CI 0.34, 0.61]). Also independently associated with varus thrust were age, sex, BMI, disease severity, strength, and alignment. The odds of valgus thrust were greater for African Americans than for Caucasians in persons without knee OA (adjusted OR 1.69 [95% CI 1.02, 2.80]) and in those with knee OA (adjusted OR 1.98 [95% CI 1.35, 2.91]). Also independently associated with valgus thrust were disease severity and malalignment.

Conclusion

Compared with Caucasians, African Americans had lower odds of varus thrust and greater odds of valgus thrust. These findings may help explain the difference between these groups in the pattern of OA involvement at the knee.

Knee osteoarthritis (OA), a leading cause of functional limitation and disability in older persons, is believed to result from local mechanical factors acting within the context of a systemic susceptibility. Varus thrust is a local mechanical factor shown to be associated with a 4-fold increase in the odds of medial tibiofemoral OA progression over an 18-month followup period (1). Varus thrust is visualized during gait as the dynamic worsening or abrupt onset of varus (bow-leg) alignment as the limb accepts weight (stance phase), with a return to less varus and more neutral alignment during lift-off and the nonweightbearing (swing) phase of gait. A varus thrust indicates dynamic instability of the knee in the frontal plane and an acute increase in load transmitted to the medial compartment with each step of walking.

Valgus thrust is the worsening or abrupt onset of valgus (knock-knee) alignment visualized during the stance phase, with a return to less valgus (more neutral) alignment during lift-off and the swing phase of gait. In theory, a valgus thrust increases load transmitted to the lateral tibiofemoral compartment, potentially contributing to lateral OA disease onset and progression. While valgus thrust is thought to be less common than varus thrust, the frequency of varus thrust and valgus thrust in persons either with knee OA or at risk of developing it is not known for any racial group. In the Johnston County Osteoarthritis Project, African Americans were more likely than Caucasians to have lateral joint space narrowing (2), a finding that was not explained by differences in static varus–valgus alignment (3); a greater frequency of valgus thrust in African Americans could help to explain a predisposition to lateral compartment involvement.

The factors associated with varus or valgus thrust presence have not been identified. A thrust is a visible manifestation of excessive motion of one tibiofemoral surface in relation to the other under the dynamic conditions of gait. Thrust presence may represent capsuloligamentous damage and/or insufficient neuromuscular stabilization, conditions that may precede OA or develop or worsen as a result of disease. The capsular changes from static varus–valgus malalignment may predispose to thrust development. Knee injury may primarily or secondarily damage stabilizing joint tissues. In theory, greater axial load from obesity may intensify a thrust in knees with compromised frontal-plane stability. Quadriceps weakness or pain-induced quadriceps inhibition may impair mechanisms of neuromuscular stabilization.

To address these gaps in knowledge relating to varus and valgus thrust, we undertook a study ancillary to the Osteoarthritis Initiative (OAI) in which thrust was assessed by trained examiners. Here we analyze thrust data from African Americans and Caucasians, the 2 largest racial groups in the OAI sample. The objectives of our study were 1) to determine the frequency of varus thrust and valgus thrust in African Americans and Caucasians for persons at high risk of but without radiographic OA in either knee and for persons with radiographic OA in one or both knees and 2) to identify factors associated with the presence of varus and valgus thrust in persons without radiographic OA and those with radiographic OA in one or both knees.

PATIENTS AND METHODS

  1. Top of page
  2. Abstract
  3. PATIENTS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. AUTHOR CONTRIBUTIONS
  7. ROLE OF THE STUDY SPONSORS
  8. REFERENCES

Sample.

The OAI is a prospective, observational cohort study of incident and progressive knee OA in men and women ages 45–79 years, all with symptomatic radiographic knee OA or at increased risk of developing it, who were enrolled at 1 of 4 sites: Baltimore, MD; Columbus, OH; Pittsburgh, PA; and Pawtucket, RI (see http://www.oai.ucsf.edu/datarelease/About.asp). All racial/ethnic groups were eligible to enroll, and the recruitment goal was to have 23% of the cohort from racial/ethnic minority groups. To be eligible for the progression subcohort of the OAI, persons were required to have symptomatic radiographic knee OA, defined as the presence of both of the following in at least 1 native (i.e., nonreplaced) knee at baseline: pain, aching, or stiffness in or around the knee on most days for at least 1 month during the past 12 months; and a definite tibiofemoral osteophyte (osteophyte grade ≥1, using the Osteoarthritis Research Society International [OARSI] atlas [4]).

Persons were eligible for the incidence subcohort of the OAI if they did not have symptomatic radiographic OA in either knee at baseline, but had characteristics that placed them at increased risk of developing symptomatic radiographic knee OA during the study. Age-specific criteria for determining increased risk were identified from within the following set of established risk factors: symptoms in a native knee in the past 12 months; overweight, defined using age- and sex-specific cut points for weight; knee injury causing difficulty walking for at least 1 week; history of any knee surgery; family history of a total knee replacement for OA in a biologic parent or sibling; Heberden's nodes; repetitive knee bending at work or outside work; and age 70–79 years (see http://www.oai.ucsf.edu/datarelease/About.asp and Appendix B at that site for greater detail regarding the rationale and approach used to derive the criteria).

Exclusion criteria, applied to the entire OAI cohort, were as follows: rheumatoid arthritis or inflammatory arthritis; severe joint space narrowing in both knees on the baseline knee radiograph, or unilateral total knee replacement and severe joint space narrowing in the other knee; bilateral total knee replacement or plans to have bilateral knee replacement in the next 3 years; inability to undergo a 3.0T magnetic resonance imaging examination of the knee because of contraindications (including pacemaker, artificial valve, aneurysm clip or shunt, stent, implanted device, ocular metallic fragment) or inability to fit in the scanner or in the knee coil (including men weighing >285 pounds and women weighing >250 pounds); positive pregnancy test result; inability to provide a blood sample for any reason; use of ambulatory aides other than a single straight cane for >50% of the time in ambulation; comorbid conditions that might interfere with the ability to participate in a 4-year study; and current participation in a double-blind randomized trial.

In our study, all OAI participants were grouped according to the presence of radiographic knee OA as defined by the presence of a tibiofemoral osteophyte in at least 1 knee. Knee radiographs were acquired using a “fixed-flexion” knee radiography protocol (5), including bilateral, standing, posteroanterior knee films with knees flexed to 20–30° and feet internally rotated 10° using a plexiglass positioning frame (SynaFlexer; Synarc). Right and left knees were imaged together on 14 × 17–inch film using a focus-to-film distance of 72 inches. The rationale for this protocol and further detail may be found at http://www.oai.ucsf.edu/datarelease/OperationsManuals.asp.

Assessment of varus and valgus thrust.

Within the ancillary study, gait was observed for varus and valgus thrust presence by 2 trained examiners at each site in accordance with a detailed protocol that specified the following: participant clothing and footwear (footies with sole grips); use of a designated hallway marked by orange cones 10 meters apart; standardized script; observation of gait while the participant walked at a comfortable speed toward, away, and again toward the examiner; standardized sequence of observation steps performed by the examiner; and standardized approach to recording results. Varus thrust presence was defined as the dynamic worsening or abrupt onset of varus alignment as the limb accepted weight, with a return to less varus alignment during lift-off and the swing phase of gait. Valgus thrust was defined as the worsening or abrupt onset of valgus as the limb accepted weight, with a return to less valgus alignment during lift-off and the swing phase. Central training of examiners included a didactic session, observation of gait of volunteers with known varus and valgus thrust status, and certification testing. The examiners were blinded to the questions posed in this study. We previously reported intrarater reliability (κ = 0.81) (1) for examiners (whom we trained) assessing individuals with knee OA.

Assessment of covariates.

Race was ascertained by self-report. Persons who reported yes to “white or Caucasian” or “black or African American” were included in this study; other possible responses were “other nonwhite” or “Asian.” Knee injury, surgery, extensor strength, pain, and alignment were each assessed separately for each knee. Knee injury was defined by the report of yes to “ever injured so badly, that it was difficult to walk for at least one week.” Bilateral isometric knee extensor strength was measured at a knee angle of 60° from full extension using the Good Strength isometric strength chair (Metitur) (6, 7). Details of this protocol may be found at http://www.oai.ucsf.edu/datarelease/OperationsManuals.asp.

Pain was assessed using the Western Ontario and McMaster Universities Osteoarthritis Index (8) pain scale, adapted by the OAI to score pain separately for each knee. Standing varus and valgus malalignment were measured by physical examination in accordance with a carefully detailed protocol specifying the following: participant's foot position, knee position in double-leg stance, and weight distribution; landmarks for long-arm goniometer placement; and standardized reading and measurement procedures (http://www.oai.ucsf.edu/datarelease/OperationsManuals.asp). Using this approach, Cibere et al (9) found evidence of outstanding interobserver reliability. All examiners were trained and underwent a quality assurance check monthly. To assess OA disease severity within each tibiofemoral compartment, joint space was graded in the medial and lateral compartments separately using an adaptation of the OARSI atlas approach (4) in which 0 = none (OARSI grade 0), 1 = narrowed (OARSI grade 1 or 2), and 2 = severely narrowed (OARSI grade 3).

Statistical analysis.

Our analyses used OAI public data (V0.2.1 and V1.2.1) merged with ancillary study gait observation data. We categorized participants as having OA (i.e., in one or both knees) or not having knee OA in order to create 2 samples for analyses for the following reasons. First, from a public health perspective, we sought to ultimately generalize results to persons belonging to 1 of these 2 groups, not to their individual knees. Second, it is likely that the knees without radiographic OA from a person with no knee OA and the knee without radiographic OA from a person with OA in the contralateral knee have a different natural course and trajectory and do not have an identical risk factor profile for incident disease. A knee without OA that is contralateral to a knee with OA is at substantially higher risk of developing OA than the knees of a person with no knee OA; some have argued that the knees in the former group have OA in preradiographic stages. It is not clear that it is appropriate to combine these 2 groups of knees in analysis.

Person-level analysis.

Among persons without radiographic knee OA and separately among persons with radiographic knee OA, we computed the frequency of varus and valgus thrust presence in African American and Caucasian participants. Differences in the percentages of African Americans and Caucasians having varus and valgus thrust are reported with the associated 95% confidence intervals (95% CIs); a 95% CI that excludes zero denotes a statistically significant difference between the 2 racial groups. Descriptive statistics were used to characterize persons without and those with radiographic knee OA by thrust status (no thrust, varus thrust in one or both knees, valgus thrust in one or both knees).

Knee-based analyses.

Knee-based analyses included data from both knees of a participant. Multiple logistic regression with generalized estimating equations, to account for potentially correlated observations for knees from the same individual, was used to evaluate the relationship of race and other factors (i.e., age [continuous], sex, body mass index [BMI] [continuous], knee injury [yes/no], knee surgery [yes/no], radiographic knee OA severity [joint space grade = 0, versus >0], extensor strength [continuous], and static alignment [continuous]) to varus thrust presence (dependent variable, varus thrust versus no varus thrust) and, in separate models, to valgus thrust presence (dependent variable, valgus thrust versus no valgus thrust).

Results from each model are reported as adjusted odds ratios (ORs) with associated 95% CIs; in these models, a 95% CI that excludes 1 represents a statistically significant association with thrust presence. Static alignment was handled as a continuous measure. For the varus thrust models, alignment in a varus direction was represented by a positive value and alignment in a valgus direction by a negative value; a significant OR >1 indicates that alignment increase in a varus direction is associated with greater odds of varus thrust. In the valgus thrust models, for ease of interpretation, alignment in a valgus direction was represented by a positive value and alignment in a varus direction by a negative value; a significant OR >1 indicates that alignment increase in a valgus direction is associated with greater odds of valgus thrust. Analyses were performed using SAS software version 9.2 (SAS Institute).

RESULTS

  1. Top of page
  2. Abstract
  3. PATIENTS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. AUTHOR CONTRIBUTIONS
  7. ROLE OF THE STUDY SPONSORS
  8. REFERENCES

Of 3,859 African American or Caucasian participants who underwent gait observation for thrust presence, 175 were excluded due to incomplete covariate data, and 92 were excluded for having varus thrust in one knee and valgus thrust in the other knee or for having evidence of both varus and valgus thrust within the same knee (i.e., motion in one direction followed by motion in the other direction within the same stance phase, or motion in one direction during one gait cycle and then in the other direction in the next gait cycle). The remaining 3,592 persons (594 African Americans and 2,998 Caucasians) comprised the final analysis sample. The 3,592 participants were classified into 2 groups: persons without radiographic OA in either knee (n = 1,566) and persons with radiographic OA in one or both knees (n = 2,026). Among persons without radiographic OA, the mean ± SD age was 61.0 ± 8.9 years, the mean ± SD BMI was 27.6 ± 4.6 kg/m2, and 58.7% were women. Among persons with radiographic OA, the mean ± SD age was 63.4 ± 9.1 years, the mean ± SD BMI was 29.4 ± 4.8 kg/m2, and 57.5% were women. Among persons without radiographic OA, 33.5% of African Americans and 40.3% of Caucasians had a thrust in at least one knee. Among persons with radiographic OA, 36.6% of African Americans and 47.8% of Caucasians had a thrust in at least one knee.

Frequency of varus and valgus thrust.

Varus thrust (in one or both knees) was present in 502 of 1,566 persons (32.1%) without radiographic knee OA and in 743 of 2,026 persons (36.7%) with radiographic knee OA. Valgus thrust (in one or both knees) was present in 113 of 1,566 persons (7.2%) without radiographic knee OA and in 185 of 2,026 persons (9.1%) with radiographic knee OA. The frequency of varus and valgus thrust among African Americans and Caucasians is shown in Figures 1 and 2. Varus thrust was more frequent than valgus thrust in both African Americans and Caucasians; this relationship held within subgroups of persons without and those with radiographic knee OA. However, varus thrust was significantly more common in Caucasians than in African Americans, among those without as well as among those with radiographic knee OA. Valgus thrust appeared to be more common in African Americans than in Caucasians, although this difference achieved significance only in persons with radiographic knee OA (Figures 1 and 2).

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Figure 1. Frequency of varus and valgus thrust in African Americans (AA) and Caucasians (C) without radiographic osteoarthritis (OA) in either knee. Among all persons without radiographic knee OA, 502 of 1,566 (32.1%) had varus thrust and 113 of 1,566 (7.2%) had valgus thrust. After stratification by race, varus thrust was significantly more frequent in Caucasians than in African Americans, with a difference of 10.3 (95% confidence interval [95% CI] 4.3, 16.3). The frequency of valgus thrust in African Americans exceeded that in Caucasians (difference = −3.5 [95% CI −7.6, 0.6]), although this was not statistically significant.

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thumbnail image

Figure 2. Frequency of varus and valgus thrust in African Americans and Caucasians with radiographic OA. Among all persons with radiographic knee OA, 743 of 2,026 (36.7%) had varus thrust and 185 of 2,026 (9.1%) had valgus thrust. After stratification by race, varus thrust was significantly more frequent in Caucasians than in African Americans (difference = 15.7 [95% CI 10.7, 20.7]), and valgus thrust was significantly more frequent in African Americans than in Caucasians (difference = −4.5 [95% CI −8.2, −0.8]). See Figure 1 for definitions.

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Factors associated with presence of varus and valgus thrust.

Table 1 shows the characteristics of persons without and those with radiographic knee OA by the presence of no thrust, varus thrust, and valgus thrust. Overall, persons with radiographic knee OA tended to be slightly older and have a greater BMI than those without radiographic knee OA. In both groups, a smaller percentage of persons with varus thrust (compared with persons with valgus thrust or no thrust) were women. In persons without radiographic knee OA, the percentage of women was highest among persons with a valgus thrust. The percentage of African Americans was highest among persons with a valgus thrust (and lowest among those with a varus thrust) in persons without and those with radiographic knee OA.

Table 1. Characteristics of the persons without and those with radiographic knee OA, by thrust status*
 Persons without radiographic OA in either knee (n = 1,566)Persons with radiographic OA in one or both knees (n = 2,026)
Persons with no thrust (n = 951)Persons with varus thrust only (n = 502)Persons with valgus thrust only (n = 113)Persons with no thrust (n = 1,098)Persons with varus thrust only (n = 743)Persons with valgus thrust only (n = 185)
  • *

    Persons with thrust had a varus thrust only in one or both knees or a valgus thrust only in one or both knees. Characteristics are in the persons described in the column heading. OA = osteoarthritis; BMI = body mass index.

Age, mean ± SD years60.2 ± 8.862.7 ± 8.960.6 ± 9.162.6 ± 8.964.3 ± 9.264.7 ± 9.2
Women, %61.052.467.363.448.957.3
African American, %16.511.021.220.711.424.9
BMI, mean ± SD kg/m227.4 ± 4.627.9 ± 4.727.1 ± 4.729.6 ± 5.129.3 ± 4.529.0 ± 4.8

Table 2 shows characteristics of knees from persons without and with radiographic knee OA, with no thrust, with varus thrust, and with valgus thrust. Knee injury and surgery tended to be more common in knees with radiographic OA. Alignment was most varus among knees with a varus thrust, especially those with radiographic OA, and most valgus among knees with a valgus thrust. Pain was worse in the knees of persons with radiographic knee OA.

Table 2. Characteristics of knees of the persons without and those with radiographic knee OA, by thrust status*
 Knees of persons without radiographic OA in either knee (both knees from 1,566 persons)Knees of persons with radiographic OA in one or both knees (both knees from 2,026 persons)
Knees with no thrust (n = 2,218)Knees with varus thrust only (n = 754)Knees with valgus thrust only (n = 160)Knees with no thrust (n = 2,697)Knees with varus thrust only (n = 1,105)Knees with valgus thrust only (n = 250)
  • *

    Radiographic knee osteoarthritis (OA) status was defined by definite osteophyte presence and did not rely on the presence/absence of joint space narrowing. WOMAC = Western Ontario and McMaster Universities Osteoarthritis Index.

  • Positive value indicates varus alignment and negative value indicates valgus alignment.

  • Joint space grading in the Osteoarthritis Initiative relied upon an adaptation of the Osteoarthritis Research Society International (OARSI) atlas approach (4), in which 0 = none (OARSI grade 0), 1 = narrowed (OARSI grade 1 or 2), and 2 = severely narrowed (OARSI grade 3).

Knee injury, %25.125.622.530.834.628.0
Knee surgery, %8.28.64.415.422.419.6
Alignment, mean ± SD−0.5 ± 3.4−0.1 ± 3.6−1.8 ± 3.4−0.2 ± 3.71.0 ± 4.2−2.1 ± 4.4
Medial joint space grade, %      
 082.277.286.364.745.471.6
 117.522.313.830.040.424.8
 20.30.50.05.314.23.6
Lateral joint space grade, %      
 092.889.993.181.881.564.0
 17.110.16.315.716.523.2
 20.10.00.62.52.112.8
Isometric extensor strength, mean ± SD N339.7 ± 127.6332.0 ± 127.8317.8 ± 119.0342.4 ± 133.5352.9 ± 134.7322.3 ± 123.1
WOMAC pain score (0–20), mean ± SD1.4 ± 2.51.6 ± 2.61.7 ± 2.62.4 ± 3.42.7 ± 3.23.0 ± 3.6

Knee-based analyses to identify factors associated with varus and valgus thrust were performed separately in the 2 subgroups of persons (without and with radiographic knee OA) and are shown in Tables 3 and 4. Adjusting for all other factors shown in Table 3, in persons without radiographic knee OA, being African American, a woman, and having greater knee extensor strength were each associated with a reduced odds of varus thrust presence during gait. Greater age, BMI, and varus malalignment were each associated with the presence of varus thrust. In persons with radiographic knee OA, being African American and a woman again were associated with a reduced odds of varus thrust presence, and greater medial joint space narrowing and varus alignment were each associated with the presence of a varus thrust.

Table 3. Factors associated with varus thrust presence (dependent variable)*
 All knees of persons without radiographic OA in either knee, OR (95% CI) for varus thrustAll knees of persons with radiographic OA in one or both knees, OR (95% CI) for varus thrust
  • *

    Shown are the results of knee-based analyses adjusted for all other factors in the table. OA = osteoarthritis; OR = odds ratio; BMI = body mass index; WOMAC = Western Ontario and McMaster Universities Osteoarthritis Index.

  • Significant result (i.e., 95% confidence interval [95% CI] excludes 1).

African American (vs. Caucasian)0.50 (0.36, 0.72)0.46 (0.34, 0.61)
Age (per year)1.02 (1.01, 1.03)1.01 (1.00, 1.02)
Female (vs. male)0.56 (0.44, 0.72)0.66 (0.53, 0.82)
BMI (per unit)1.05 (1.02, 1.07)1.01 (0.99, 1.03)
Knee injury (yes vs. no)1.02 (0.82, 1.27)0.94 (0.78, 1.13)
Knee surgery (yes vs. no)0.97 (0.70, 1.35)1.12 (0.90, 1.40)
Medial joint space narrowing (yes vs. no)1.18 (0.95, 1.47)1.73 (1.53, 1.96)
Knee extensor strength (per 20 units)0.96 (0.94, 0.99)0.99 (0.97, 1.00)
WOMAC pain score (per unit)1.03 (0.99, 1.07)1.02 (0.99, 1.04)
Varus malalignment (per degree)1.04 (1.01, 1.07)1.06 (1.04, 1.09)
Table 4. Factors associated with valgus thrust presence (dependent variable)*
 All knees of persons without radiographic OA in either knee, OR (95% CI) for valgus thrustAll knees of persons with radiographic OA in one or both knees, OR (95% CI) for valgus thrust
  • *

    Shown are the results of knee-based analyses adjusted for all other factors in the table. OA = osteoarthritis; OR = odds ratio; BMI = body mass index; WOMAC = Western Ontario and McMaster Universities Osteoarthritis Index.

  • Significant result (i.e., 95% confidence interval [95% CI] excludes 1).

African American (vs. Caucasian)1.69 (1.02, 2.80)1.98 (1.35, 2.91)
Age (per year)1.00 (0.97, 1.02)1.01 (0.99, 1.03)
Female (vs. male)1.08 (0.63, 1.85)0.67 (0.45, 1.00)
BMI (per unit)0.97 (0.93, 1.02)0.96 (0.93, 1.00)
Knee injury (yes vs. no)1.00 (0.67, 1.51)0.72 (0.51, 1.04)
Knee surgery (yes vs. no)0.54 (0.24, 1.19)1.22 (0.82, 1.83)
Lateral joint space narrowing (yes vs. no)0.89 (0.46, 1.69)1.85 (1.45, 2.36)
Knee extensor strength (per 20 units)1.00 (0.96, 1.04)1.00 (0.97, 1.03)
WOMAC pain score (per unit)1.02 (0.96, 1.08)1.02 (0.98, 1.07)
Valgus malalignment (per degree)1.14 (1.07, 1.21)1.15 (1.09, 1.20)

Table 4 summarizes the adjusted analyses for valgus thrust. Among persons without radiographic knee OA, being African American and having greater knee valgus malalignment were each associated with the presence of a valgus thrust. For persons with radiographic knee OA, being African American and having greater valgus alignment and greater lateral joint space narrowing were each associated with the presence of a valgus thrust. Secondary analyses with the dependent variables of varus thrust versus no varus thrust and of valgus thrust versus no valgus thrust yielded similar results.

DISCUSSION

  1. Top of page
  2. Abstract
  3. PATIENTS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. AUTHOR CONTRIBUTIONS
  7. ROLE OF THE STUDY SPONSORS
  8. REFERENCES

In the OAI cohort, varus thrust was more frequent than valgus thrust, both in African Americans and in Caucasians; this relationship held within subgroups of persons without and those with radiographic knee OA. Varus thrust was significantly more frequent among Caucasians than among African Americans, for both disease subgroups. Valgus thrust was significantly more frequent among African Americans than among Caucasians in persons with knee OA; a similar finding in those without knee OA was not statistically significant. The odds of varus thrust were significantly lower for African Americans compared with Caucasians in both disease subgroups, after controlling for potential confounders. Other factors associated with varus thrust presence in persons without knee OA were age, BMI, varus malalignment, sex (reduced odds in women), and knee extensor strength (reduced odds with greater strength) and, in persons with knee OA, more severe medial tibiofemoral OA disease, varus malalignment, and sex (reduced odds in women). The odds of valgus thrust were significantly greater for African Americans compared with Caucasians, in persons without and those with knee OA. Also associated with valgus thrust presence in persons without knee OA were valgus malalignment and, in persons with knee OA, more severe lateral disease and valgus malalignment.

To our knowledge, this is the first report on the frequency of varus and valgus thrust and on the differences in the frequency of thrust presence between African Americans and Caucasians. The OAI afforded an opportunity to use gait observation on a large scale and to estimate thrust frequency in these 2 racial groups, and specifically within persons with radiographic knee OA or at high risk of developing it.

Previous studies have explored the effect of opening wedge high tibial osteotomy (10) and wedged insoles (11) on varus thrust in OA knees, or have investigated thrust in the setting of anterior cruciate ligament deficiency (12–14). Our results confirmed the clinical belief that varus thrust is more common than valgus thrust. The frequency of thrust presence, ranging from 32% to 37% for varus thrust and from 7% to 9% for valgus thrust, underscores the importance of studies to investigate its role in the natural history of knee OA.

A visible thrust represents a severe derangement in knee mechanics, implying an acute increase in load—medial load with a varus thrust and lateral load with a valgus thrust—with each step of walking. This derangement during the most common weightbearing locomotor activity may help to drive the advancement of knee OA. In the Mechanical Factors in Arthritis of the Knee (MAK) study of a cohort of persons with knee OA, the presence of a varus thrust was associated with a 4-fold increase in the odds of radiographic medial tibiofemoral OA progression in the subsequent 18 months (age-, sex-, BMI-, and pain-adjusted OR 3.96 [95% CI 2.11, 7.43]) (1). In a substudy of MAK participants who underwent quantitative gait analysis, the peak external knee adduction moment was substantially greater in knees with than in those without a varus thrust, supporting the concept that knees with a varus thrust sustain greater medial load (1). The small number of knees with a valgus thrust precluded analysis of its relationship with lateral progression in the MAK study.

Previous studies showed that compared with Caucasians, African Americans had a higher prevalence of symptomatic and radiographic knee OA, and this difference was more pronounced in women than in men (15–17). Relatively little is known about any difference in local mechanical risk factors between racial groups. In our study, African Americans had a lower risk (based on the estimated OR) of having a varus thrust and a greater risk of having a valgus thrust compared with Caucasians, after accounting for static varus–valgus alignment and other potential confounders. As described in reports involving the Johnston County Osteoarthritis Project, African Americans were more likely than Caucasians to have lateral tibiofemoral joint space narrowing (2), a difference that could not be explained by greater static valgus alignment (3). Their findings and our results introduce the possibility, which should be examined in future studies, that a higher prevalence of valgus thrust in African Americans may be contributing to a greater risk of developing lateral knee OA.

We found that other factors were also related to thrust presence. Older age was modestly associated with a greater likelihood of varus thrust presence. Given that older adults have on average weaker knee muscles (18–21) functioning in an environment of increased varus/valgus laxity (22), a varus thrust may reflect an age-related diminution in muscular mechanisms of knee stabilization during ambulation. Extensor strength was related only to varus thrust and only in those without radiographic OA; the low prevalence of valgus thrust may have limited our ability to detect an association with strength. Nevertheless, these findings suggest that strength may not capture relevant muscle activity that helps to prevent a thrust from occurring. Static malalignment was related to varus and valgus thrust presence, possibly reflecting the asymmetry in tibiofemoral load distribution in malaligned knees that—via its contribution to stretching of the capsule and ligaments, meniscal degeneration, disease progression with loss of bone and cartilage height—may exacerbate dynamic instability evident as a thrust in the same direction as the static malalignment. Women without radiographic knee OA as well as those with knee OA had a reduced likelihood of varus thrust. Women are more likely than men to have a greater Q-angle (23), potentially protecting them to some extent from developing a varus thrust. Further study is needed to examine the relationship between the Q-angle and varus and valgus thrust.

The finding that knees with more moderate-to- severe knee OA were more likely to have a varus thrust is not surprising; as knee OA progresses, loss of cartilage and bone height and stretching of soft tissue restraints may exacerbate dynamic instability. In turn, the abrupt and large displacements between opposing articular surfaces from a thrust may increase shear and compressive stresses, resulting in disease progression. Like other local mechanical risk factors, varus thrust may develop at any point in the course of OA disease at the knee and appears to increase the risk of progression subsequent to its development (1). Of note, we found that 32% of persons at high risk of but without radiographic knee OA had a varus thrust, indicating that it is not solely a consequence of OA disease. The development of methods to quantify the elements of a thrust, such as its force and loading rate, will facilitate further investigation to better understand the mechanism of a thrust effect.

There are limitations to our study that must be acknowledged. Persons without knee OA in the OAI were all at high risk of developing it, and so our frequency estimates cannot be applied to the general population without knee OA. However, persons at high risk of knee OA constitute a group of high importance from a public health perspective and worthy of targeted investigation. Also, these analyses were cross-sectional and were therefore most useful for suggesting hypotheses for further investigation in longitudinal studies.

In conclusion, in the OAI cohort, compared with Caucasians, African Americans had lower odds of varus thrust and greater odds of valgus thrust, after controlling for potential confounders. These findings may help to explain the reported differences between these groups in the pattern of OA involvement of the tibiofemoral joint.

AUTHOR CONTRIBUTIONS

  1. Top of page
  2. Abstract
  3. PATIENTS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. AUTHOR CONTRIBUTIONS
  7. ROLE OF THE STUDY SPONSORS
  8. REFERENCES

All authors were involved in drafting the article or revising it critically for important intellectual content, and all authors approved the final version to be published. Dr. Sharma had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.

Study conception and design. Chang, Hochberg, Dunlop, Nevitt, Hayes, Eaton, Bathon, Jackson, Kwoh, Sharma.

Acquisition of data. Hochberg, Nevitt, Eaton, Bathon, Jackson, Sharma.

Analysis and interpretation of data. Chang, Hochberg, Song, Dunlop, Chmiel, Nevitt, Eaton, Sharma.

ROLE OF THE STUDY SPONSORS

  1. Top of page
  2. Abstract
  3. PATIENTS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. AUTHOR CONTRIBUTIONS
  7. ROLE OF THE STUDY SPONSORS
  8. REFERENCES

None of the funding partners played a role in the design or conduct of the study, collection, management, analysis, or interpretation of the data, or preparation, review, or approval of the manuscript.

REFERENCES

  1. Top of page
  2. Abstract
  3. PATIENTS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. AUTHOR CONTRIBUTIONS
  7. ROLE OF THE STUDY SPONSORS
  8. REFERENCES
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