Ultimate fracture load of cadaver proximal humeri correlates more strongly with mean combined cortical thickness than with areal cortical index, DEXA density, or canal-to-calcar ratio

Abstract

Objectives: This investigation sought to advance the work published in our prior biomechanical study (Journal of Orthopaedic Research, 2016). We specifically sought to determine whether there are additional easy-to-measure parameters on plain radiographs of the proximal humerus that correlate more strongly with ultimate fracture load, and whether a parameter resembling the Dorr strength/quality characterisation of proximal femurs can be applied to humeri.

Materials and methods: A total of 33 adult humeri were used from a previous study where we quantified bone mineral density of the proximal humerus using radiographs and dual-energy x-ray absorptiometry (DEXA), and regional mean cortical thickness and cortical index using radiographs. The bones were fractured in a simulated backwards fall with the humeral head loaded at 2 mm/second via a frustum angled at 30° from the long axis of the bone. Correlations were assessed with ultimate fracture load and these new parameters: cortical index expressed in areas ("areal cortical index") of larger regions of the diaphysis; the canal-to-calcar ratio used analogous to its application in proximal femurs; and the recently described medial cortical ratio.

Results: The three new parameters showed the following correlations with ultimate fracture load: areal cortical index (r = 0.56, p < 0.001); canal-to-calcar ratio (r = 0.38, p = 0.03); and medial cortical ratio (r = 0.49, p < 0.005). These correlations were weaker when compared with those that we previously reported: mean cortical thickness of the proximal diaphysis versus ultimate fracture load (r = 0.71; p < 0.001); and mean density in the central humeral head versus ultimate fracture load (r = 0.70; p < 0.001).

Conclusion: Simple-to-measure radiographic parameters of the proximal humerus reported previously are more useful in predicting ultimate fracture load than are areal cortical index, canal-to-calcar ratio, and medial cortical ratio.Cite this article: J. G. Skedros, C. S. Mears, W. Z. Burkhead. Ultimate fracture load of cadaver proximal humeri correlates more strongly with mean combined cortical thickness than with areal cortical index, DEXA density, or canal-to-calcar ratio. Bone Joint Res 2017;6:1-7. DOI: 10.1302/2046-3758.61.BJR-2016-0145.R1.

Keywords: Canal-to-calcar ratio; Cortical index; Dual-energy x-ray absorptiometry; Fracture; Humerus; Osteoporosis.

Fig. 1

Anteroposterior view of a left cadaver humerus showing the four measurement locations (dashed white lines) of the metaphysis/diaphysis, where D1 is the surgical neck. From top to bottom, the dark squares indicate head (H)1, H2, H3 and D1 locations where mmAl measurements were made.

Fig. 2

In terms of the areas A-F that are shown diagrammatically in this radiograph, areal cortical index for the D2-D3 region is: [((A + B + C) – B) / (A + B + C)]. Areal cortical index for the D3-D4 region is: [((D + E + F) – E) / (D + E + F)]. The canal-to-calcar ratio is calculated as X/Y; as shown, these values were obtained from the D2 and D4 locations. The medial cortical ratio and an example of a linear cortical index measurement are shown in Figure 3. (D2, 2 cm below surgical neck; D3, 5 cm below surgical neck; D4, 7 cm below surgical neck.)

Fig. 3

Medial cortical ratio (MCR) is calculated as the medial cortex thickness (MCT) divided by the outer cortical diameter at D3 (shown here in D2 for clarity). The measurements used to calculate linear cortical index are shown at D3 in this drawing. Linear cortical index is defined as the difference between the outer (OD) and inner diameters (ID) of the bone, divided by the OD (OD - ID)/OD (lower cortical index values represent weaker bone). (D2, 2 cm below surgical neck; D3, 5 cm below surgical neck.)^,.