Bilateral clubfeet are highly correlated: a cautionary tale for researchers.

BACKGROUND: Congenital talipes equinovarus, or clubfoot, is a common pediatric orthopaedic condition of unknown origin. In many clubfoot clinical trials, interventions are assigned to a patient, but response to treatment is assessed separately in each foot. Trials commonly report x patients with y feet where y is greater than x (eg, 35 patients with 56 feet). However, common statistical tests assume that each data point is independent. Although data from unilateral cases of clubfoot are independent, it is unknown if each foot of patients with bilateral clubfeet are correlated. QUESTIONS/PURPOSES: The purpose of this study was to assess the correlation in the feet of patients with bilateral clubfeet by (1) evaluating the degree of severity between lower limbs of each patient with bilateral clubfeet at baseline; (2) determining if right and left feet of each patient responded to intervention in the same way; (3) determining the proportion of bilateral relapse; and (4) determining the proportion of right and left feet which required the same intervention to correct bilateral relapse.
METHODS: We performed a chart review of the records of 33 patients with bilateral clubfeet (66 feet). Baseline severity was assessed using the Pirani score. The number of Ponseti serial casts to correct the deformity, the proportion of patients who underwent bilateral Achilles tenotomy, the proportion of bilateral relapse, and the treatment to correct bilateral relapse were examined.
RESULTS: The degree of severity between right (Pirani score mean, 5.2; SD, 0.8) and left (Pirani score mean, 5.2; SD, 0.5) feet for each patient at baseline was highly correlated (r=0.76, p<0.001). Response to intervention between lower limbs was highly correlated for the number of Ponseti casts required for initial correction (right mean, 5.2, SD, 1.1; left mean, 5.2, SD, 1.3) (r=0.89, p<0.001) and the proportion of patients who underwent bilateral Achilles tenotomy (right, 17/18; left, 16/18) (r=0.94, p<0.001). In the nine patients who experienced relapse, eight experienced bilateral involvement. In all cases of bilateral relapse, the right and left foot of each patient required the same intervention to correct the relapse.
CONCLUSIONS: In patients with bilateral clubfeet, baseline severity, response to initial Ponseti treatment, Achilles tenotomy, and relapse outcomes were highly correlated in the right and left feet of each patient. Pooling clinical results of patients who present with bilateral clubfeet is statistically inappropriate, since results in two limbs of the same patient do not represent independent observations. These results support analogous work in other specialties suggesting that patients with bilateral presentations should not be analyzed as independent data points. LEVEL OF EVIDENCE: Level IV, therapeutic study. See the Instructions for Authors for a complete description of levels of evidence.