Mirels’ score for upper limb metastatic lesions: do we need a different cutoff for recommending prophylactic fixation?

Abstract

The aim of this study was to investigate the reproducibility, reliability and accuracy of Mirels’s score in upper limb bony metastatic disease and validate its use in predicting pathological fractures. Methods 45 patients with upper limb bony metastases met the inclusion criteria (62% male 28/45). Mean age was 69 years (SD 9.5) and commonest primaries were lung (29% 13/45), followed by prostate and hematological (each 20% 9/45). The most commonly affected bone was the humerus (76% 35/45), followed by the ulna (6.5% 3/45). Mirels’s score was calculated in 32 patients; with plain radiographs at index presentation scored using Mirels’s system by 6 raters. The radiological aspects (lesion size and appearance) were scored twice by each rater (2-weeks apart). Intra- and interobserver reliability were calculated using Fleiss’ kappa test. Bland-Altman plots compared the variances of both individual components and total Mirels’s score. Results The overall fracture rate of upper limb metastatic lesions was 76% (35/46) with a mean follow-up of 3.6 years (range 11 months-6.8 years). Where time from diagnosis to fracture was known (n=20), fractures occurred at a median 19 days (IQR 60-10) and 80% (16/20) occurred within 3-months of diagnosis. Mirels’s score of =9 did not accurately predict lesions that fractured (fracture rate 11% 5/46 for Mirels’s =9 versus 65% 30/46 for Mirels’s =8, p<0.001). Sensitivity was 14% and specificity was 73%. When Mirels’s cut-off was lowered to =7, patients were more likely to fracture than not (48% 22/46 versus 28% 13/46, p=0.045), sensitivity rose to 63% but specificity fell to 55%. Kappa values for interobserver variability were k=0.358 (fair, 95% confidence interval CI 0.288-0.429) for lesion size, k=0.107 (poor, 95% CI 0.02-0.193) for radiological appearance and k=0.274 (fair, 95% CI 0.229-0.318) for total Mirels’s score. Values for intraobserver variability were k=0.716 (good, 95% CI 0.432-0.999) for lesion size, k=0.427 (moderate, 95% CI 0.195-0.768) for radiological appearance and 0.580 (moderate, 95% CI 0.395-0.765) for total Mirels’s score. Conclusions This study demonstrates moderate to substantial agreement between and within raters using Mirels’s score on upper limb radiographs. However, Mirels’s score had a poor sensitivity and specificity in predicting upper extremity fractures. Until a more valid scoring system has been developed, based on our study, we recommend a Mirels’s threshold of =7/12 for considering prophylactic fixation of impending upper limb pathological fractures. This contrasts with the current =9/12 cut-off, which is recommended for lower limb pathological fractures.