A meta-analysis of the diagnostic accuracy of Hounsfield units on computed topography relative to dual-energy X-ray absorptiometry for the diagnosis of osteoporosis in the spine surgery population

Abstract

BACKGROUND

The preoperative identification of osteoporosis in the spine surgery population is of crucial importance. Limitations associated with dual-energy x-ray absorptiometry, such as access and reliability, have prompted the search for alternative methods to diagnose osteoporosis. The Hounsfield Unit(HU), a readily available measure on computed tomography, has garnered considerable attention in recent years as a potential diagnostic tool for reduced bone mineral density. However, the optimal threshold settings for diagnosing osteoporosis have yet to be determined.

METHODS

We selected studies that included comparison of the HU(index test) with dual-energy x-ray absorptiometry evaluation(reference test). Data quality was assessed using the standardised QUADAS-2 criteria. Studies were characterised into 3 categories, based on the threshold of the index test used with the goal of obtaining a high sensitivity, high specificity or balanced sensitivity-specificity test.

RESULTS

9 studies were eligible for meta-analysis. In the high specificity group, the pooled sensitivity was 0.652 (95% CI 0.526 – 0.760), specificity 0.795 (95% CI 0.711 – 0.859) and diagnostic odds ratio was 6.652 (95% CI 4.367 – 10.133). In the high sensitivity group, the overall pooled sensitivity was 0.912 (95% CI 0.718 – 0.977), specificity was 0.67 (0.57 – 0.75) and diagnostic odds ratio was 19.424 (5.446 – 69.275). In the balanced sensitivity-specificity group, the overall pooled sensitivity was 0.625 (95% CI 0.504 – 0.732), specificity was 0.914 (0.823 – 0.960) and diagnostic odds ratio was 14.880 (7.521 – 29.440). Considerable heterogeneity existed throughout the analysis.

CONCLUSION

In conclusion, the HU is a clinically useful tool to aide in the diagnosis of osteoporosis. However, the heterogeneity seen in this study warrants caution in the interpretation of results. We have demonstrated the impact of differing HU threshold values on the diagnostic ability of this test. We would propose a threshold of 135 HU to diagnose OP. Future work would investigate the optimal HU cut-off to differentiate normal from low bone mineral density.

Introduction

Osteoporosis (OP) is a systemic skeletal disease characterised by low bone mass and a progressive microarchitectural deterioration of bone tissue, leading to enhanced bone fragility and a consequent increased risk of fracture [1]. Osteoporotic fractures result in a considerable socioeconomic and healthcare burden, due to the associated loss of independence and increased mortality rate [[2], [3], [4], [5], [6]]. Furthermore, they are associated with a significant complication profile. Notably, the risk of pseudarthrosis and subsequent mechanical failure has been reported to be as high as 35 % in the setting of osteoporosis, and this is of particular concern in an elderly patient cohort [[3], [4], [5]]. Moreover, the decreased pull-out strength, cut-out and insertional torque, associated with instrumenting osteoporotic patients results in an increased risk of perioperative vertebral fracture and postoperative instrumentation failure [5]. Therefore, the preoperative diagnosis of OP is crucial to ensure timely preoperative optimisation and aide in surgical planning [5,6].

The gold standard method to diagnose OP, as defined by the World Health Organisation, is a T-score of - 2.5 (ie, more than 2.5 standard deviations below the average of a 25 year old adult) obtained by dual-energy X-ray absorption densitometry (DXA) [7]. However, access to DXA varies considerably internationally [8] and potential sources of error exist through improper patient positioning and scan interpretation [9]. These limitations have prompted the search for other techniques to diagnose OP. The Hounsfield Unit (HU), described by Schreiber et al [10] has recently emerged as a readily available alternative measure of bone mineral density on computed tomography (CT). In recent years, there have been a number of clinical studies exploring the diagnostic utility of HU. A considerable portion of the spine surgery population undergo CT imaging as routine preoperative planning to accurately assess surgical anatomy for pedicle screw placement, providing an ideal opportunity for OP screening [11]. Moreover, in the broader healthcare context, patients commonly undergo CT imaging for a wide spectrum of clinical indications, therefore allowing for opportunistic screening.

As with all diagnostic tests, sensitivity and specificity are inextricably linked and are dependent on the set threshold of the diagnostic test. At present, there is a lack of consensus on the threshold for the HU definition of OP, with varying thresholds used throughout the literature. The purpose of this meta-analysis is to synthesise the literature to-date to ascertain the overall sensitivity, specificity, and diagnostic power of the HU for OP.