Dorsal Subluxation of the First Metacarpal During Thumb Flexion is an Indicator of Carpometacarpal Osteoarthritis Progression

Abstract

Background: Measurable changes in patients with progression of thumb carpometacarpal (CMC) osteoarthritis (OA) include joint space narrowing, osteophyte formation, subluxation, and adjacent-tissue changes. Subluxation, an indication of mechanical instability, is postulated as an early biomechanical indicator of progressing CMC OA. Various radiographic views and hand postures have been proposed to best assess CMC subluxation, but 3D measurements derived from CT images serve as the optimal metric. However, we do not know which thumb pose yields subluxation that most indicates OA progression.

Questions/purposes: Using osteophyte volume as a quantitative measure of OA progression, we asked: (1) Does dorsal subluxation vary by thumb pose, time, and disease severity in patients with thumb CMC OA? (2) In which thumb pose(s) does dorsal subluxation most differentiate patients with stable CMC OA from those with progressing CMC OA? (3) In those poses, what values of dorsal subluxation indicate a high likelihood of CMC OA progression?

Methods: Between 2011 and 2014, 743 patients were seen at our institutions for trapeziometacarpal pain. We considered individuals who were between the ages of 45 and 75 years, had tenderness to palpation or a positive grind test result, and had modified Eaton Stage 0 or 1 radiographic thumb CMC OA as potentially eligible for enrollment. Based on these criteria, 109 patients were eligible. Of the eligible patients, 19 were excluded because of a lack of interest in study participation, and another four were lost before the minimum study follow-up or had incomplete datasets, leaving 86 (43 female patients with a mean age of 53 ± 6 years and 43 male patients with a mean age of 60 ± 7 years) patients for analysis. Twenty-five asymptomatic participants (controls) aged 45 to 75 years were also prospectively recruited to participate in this study. Inclusion criteria for controls included an absence of thumb pain and no evidence of CMC OA during clinical examination. Of the 25 recruited controls, three were lost to follow-up, leaving 22 for analysis (13 female patients with a mean age of 55 ± 7 years and nine male patients with a mean age of 58 ± 9 years). Over the 6-year study period, CT images were acquired of patients and controls in 11 thumb poses: neutral, adduction, abduction, flexion, extension, grasp, jar, pinch, grasp loaded, jar loaded, and pinch loaded. CT images were acquired at enrollment (Year 0) and Years 1.5, 3, 4.5, and 6 for patients and at Years 0 and 6 for controls. From the CT images, bone models of the first metacarpal (MC1) and trapezium were segmented, and coordinate systems were calculated from their CMC articular surfaces. The volar-dorsal location of the MC1 relative to the trapezium was computed and normalized for bone size. Patients were categorized into stable OA and progressing OA subgroups based on trapezial osteophyte volume. MC1 volar-dorsal location was analyzed by thumb pose, time, and disease severity using linear mixed-effects models. Data are reported as the mean and 95% confidence interval. Differences in volar-dorsal location at enrollment and rate of migration during the study were analyzed for each thumb pose by group (control, stable OA, and progressing OA). A receiver operating characteristic curve analysis of MC1 location was used to identify thumb poses that differentiated patients whose OA was stable from those whose OA was progressing. The Youden J statistic was used to determine optimized cutoff values of subluxation from those poses to be tested as indicators of OA progression. Sensitivity, specificity, negative predictive values, and positive predictive values were calculated to assess the performance of pose-specific cutoff values of MC1 locations as indicators of progressing OA.

Results: In flexion, the MC1 locations were volar to the joint center in patients with stable OA (mean -6.2% [95% CI -8.8% to -3.6%]) and controls (mean -6.1% [95% CI -8.9% to -3.2%]), while patients with progressing OA exhibited dorsal subluxation (mean 5.0% [95% CI 1.3% to 8.6%]; p < 0.001). The pose associated with the most rapid MC1 dorsal subluxation in the progressing OA group was thumb flexion (mean 3.2% [95% CI 2.5% to 3.9%] increase per year). In contrast, the MC1 migrated dorsally much slower in the stable OA group (p < 0.001), at only a mean of 0.1% (95% CI -0.4% to 0.6%) per year. A cutoff value of 1.5% for the volar MC1 position during flexion at enrollment (C-statistic: 0.70) was a moderate indicator of OA progression, with a high positive predictive value (0.80) but low negative predictive value (0.54). Positive and negative predictive values of subluxation rate in flexion (2.1% per year) were high (0.81 and 0.81, respectively). The metric that most indicated a high likelihood of OA progression (sensitivity 0.96, negative predictive value 0.89) was a dual cutoff that combined the subluxation rate in flexion (2.1% per year) with that of loaded pinch (1.2% per year).

Conclusion: In the thumb flexion pose, only the progressing OA group exhibited MC1 dorsal subluxation. The MC1 location cutoff value for progression in flexion was 1.5% volar to the trapezium , which suggests that dorsal subluxation of any amount in this pose indicates a high likelihood of thumb CMC OA progression. However, volar MC1 location in flexion alone was not sufficient to rule out progression. The availability of longitudinal data improved our ability to identify patients whose disease will likely remain stable. In patients whose MC1 location during flexion changed < 2.1% per year and whose MC1 location during pinch loading changed < 1.2% per year, the confidence that their disease would remain stable throughout the 6-year study period was very high. These cutoff rates were a lower limit, and any patients whose dorsal subluxation advanced faster than 2% to 1% per year in their respective hand poses, were highly likely to experience progressive disease.

Clinical relevance: Our findings suggest that in patients with early signs of CMC OA, nonoperative interventions aimed to reduce further dorsal subluxation or operative treatments that spare the trapezium and limit subluxation may be effective. It remains to be determined whether our subluxation metrics can be rigorously computed from more widely available technologies, such as plain radiography or ultrasound.

Fig. 1

This figure shows a visual summary of the data that were used as input into the linear mixed-effects models. First metacarpal locations (mean values) are plotted with respect to the trapezial articular surface (y-axis), at each thumb pose (x-axis), for each follow-up timepoint (darkening of plot line with time of follow-up), for controls (black lines) and patients who were classified into stable OA (green and blue) and progressing OA groups (yellow and red). (A) Thumb poses are plotted left to right from the highest to lowest mean dorsal MC1 location of the control values (black solid line). The stable OA group (blue hues) displayed trends that are qualitatively similar to those of the control cohort over the 6-year study period. Patients with progressing OA (orange hues) exhibited increasing dorsal subluxation over time and, uniquely, were dorsally subluxed in thumb flexion. Examples of trapezium (gray) and first metacarpal (sand) bone models for the (B) progressing OA and (C) stable OA subgroups in flexion at Year 0 and 6 are shown. The coordinate system–aligned region (shaded gray band in plot) depicts the first metacarpal locations within ± 5% of the trapezium’s origin.

Fig. 2

Hand and thumb poses acquired and analyzed in this study are shown in this figure as volume reconstructions from CT scans. Polycarbonate fixtures that were used to standardize hand and thumb pose are rendered in gray. Top, left to right: (A) neutral, (B) adduction, (C) abduction, (D) flexion, and (E) extension. Bottom, left to right: (F) grasp, (G) jar, and (H) pinch. Grasp, jar-top twist, and pinch images were acquired when each study participant was relaxed and again at 80% of the individual’s maximum effort. A load cell was embedded in the jigs, and its output was displayed on a laptop screen to provide a visual target during the loaded scans. A color image accompanies the online version of this article.

Fig. 3

First metacarpal location was calculated with respect to a coordinate system defined by the articular surface of the trapezium. (A) First metacarpal and trapezium bone surface models (off-white) and delineated articular facets (blue) are shown. (B) Exploded joint view of the first metacarpal and trapezium articular facets (blue), illustrating the articular curvature-based coordinate systems oriented in the volar-dorsal (red), proximal-distal (green), and radial-ulnar (blue) anatomic directions. Dorsal subluxations are reported as positive values. (C) A typical first metacarpal from each of the 11 CT scan thumb poses was superimposed onto a mathematically fixed trapezium: palmar view (left) and sagittal view (right). (D) First metacarpal volar-dorsal locations are shown by thumb pose. The volar-dorsal location of the first metacarpal was defined with respect to the radial-ulnar axis of the trapezium coordinate system. These values from a control participant highlight first metacarpal locations in extension (30% dorsal) and flexion (15% volar).

Fig. 4

Results of the linear mixed-effects model (mean and 95% CIs) are plotted in this figure by pose and disease severity. (A) Differences in first metacarpal location (%) (volar: -, dorsal: +) between stable OA (blue) and progressing OA (red) (p < 0.001) were the greatest and had the highest C-statistics (0.70) in the jar and flexion thumb poses (highlighted) at enrollment. All poses, except extension, were associated with a more dorsal first metacarpal location in progressing OA than in stable OA. (B) The rate of dorsal subluxation (% per year) in progressing OA was the greatest in flexion and pinch loading (highlighted), and these thumb poses had the greatest differences from those of controls and those with stable OA. The upper-case L on the x-axis labels denotes loaded thumb poses. A color image accompanies the online version of this article.

Fig. 5

This figure shows cutoff values for MC1 location and subluxation rate that indicate the likelihood of CMC OA progression. (A) The first metacarpal location on the trapezium (%) is shown in the thumb flexion pose. Solid lines indicate the linear mixed-effects model means (95% CI, shaded) of the first metacarpal location at enrollment (Year 0) and at Year 6. The dashed black line is the computed subluxation cutoff value (1.5% volar) for subgroup categorization. The red arrow indicates progressing OA values, and the blue arrow indicates stable OA values. (B) The first metacarpal location (%) for the jar pose by group and computed cutoff (dashed line) is shown. (C) The mean (95% CI) of the rate of dorsal subluxation (% per year) in the thumb flexion pose was computed using a linear mixed-effects model. The dashed line represents the cutoff value of 2.1% subluxation per year. (D) The mean (95% CI) of the rate of dorsal subluxation in the pinch-loaded pose was computed using a linear mixed-effects model. The dashed line displays the subluxation rate cutoff value of 1.2% subluxation per year.