Mirels' score calculator

Where the score comes from

Mirels published the score in Clinical Orthopaedics and Related Research in 1989. It was derived from 78 metastatic long bone lesions that had been irradiated without prophylactic fixation — so the derivation cohort is patients who were not operated on, followed to see which bones broke.

Of those 78 lesions, 51 did not fracture within six months and 27 did. The mean score was 7 in the non-fracture group and 10 in the fracture group. From this Mirels proposed that lesions scoring 7 or lower could be safely irradiated, while lesions scoring 8 or higher required prophylactic internal fixation before irradiation.

How well it actually performs

The score endures because it is quick, memorable and better than nothing. Its documented weaknesses are worth knowing in detail, because they determine when you should distrust it.

Observer agreement is poor

Eight independent observers — four orthopaedic surgeons and four radiologists — scored blinded radiographs from 47 patients, twice, twelve weeks apart.

The older, simpler system it was designed to replace was more reproducible on every measure. The components most responsible are the radiological ones — lesion size and appearance — which is where subjective judgement on a plain film does most of the work.

It performs badly in the upper limb

Across three dedicated humeral validation studies, sensitivity at the standard ≥9 threshold was approximately 14% — an area under the curve of 0.51 to 0.55, which is indistinguishable from chance. A larger cohort supported a revised threshold of ≥8, with an AUC of 0.82, and identified cortical breach as the dominant independent predictor of fracture, with an odds ratio of 21.0.

A separate series of 45 patients with upper limb metastases found an overall fracture rate of 76%, that a Mirels score of ≥9 did not accurately predict which lesions fractured, and — the detail most worth carrying — that fractures occurred at a median of 19 days from diagnosis, with 80% within three months. Humeral lesions also receive lower Mirels scores and less frequent staging CT than femoral lesions despite a higher fracture rate.

The practical lesson: in the humerus, look at the cortex and the timeline rather than the total.

Specificity is poor in the femur too

Mirels remains the most widely referenced method for the femur, but it is acknowledged to suffer from poor specificity — it over-calls. Finite-element work modelling lytic defects at 32 defined locations in the proximal femur has proposed a modified location component that predicts strength reduction better than the original three-tier site score, reflecting that where in the proximal femur a lesion sits matters more than the original score allows.

Why it needs extra caution in myeloma

Two reasons, and both come up in practice.

First, it was not derived in myeloma. The 78 lesions were solid-tumour metastases. Myeloma bone disease is diffusely lytic with suppressed osteoblast function, so the mechanical problem is not the same as a discrete metastasis in otherwise normal bone. Every myeloma lesion also scores maximum points on the lesion-type component by definition, which compresses the score's ability to discriminate.

Second, myeloma is markedly more responsive to systemic treatment and radiotherapy than most cancers that metastasise to bone, so a lesion's mechanical significance can change over weeks. There is a published case of a patient with a high-scoring impending hip fracture in whom chemotherapy made prophylactic nailing unnecessary. That is a single case report, not a rule — but it is a legitimate argument for making these decisions with the haematologist rather than from the score alone.

What to say when asked

A defensible position, and one you can support from the literature:

• Mirels is a screening prompt that reliably identifies bones worth thinking hard about. It is not a threshold to operate on.
• Functional pain — pain on loading — carries more weight than the total. It is the component with the least measurement error and the most direct mechanical meaning.
• Cortical breach and lesion size on CT should be assessed directly rather than inferred from a plain-film score, particularly in the humerus.
• The decision is multidisciplinary, and in myeloma it belongs with the haematology team because systemic treatment may alter the problem.
• If you are going to operate, the evidence favours doing it before the bone breaks — better survival at 90 and 360 days, fewer thromboembolic and infective complications, and more patients regaining walking function. But note that the best cost-adjusted study found prophylactic surgery cost-neutral, not cheaper; the gain shows up as independence, not savings.