Elsevier

Joint Bone Spine

Volume 78, Issue 4, July 2011, Pages 358-363
Joint Bone Spine

Review
Detection of basic calcium phosphate crystals in osteoarthritis

https://doi.org/10.1016/j.jbspin.2010.10.008Get rights and content

Abstract

Osteoarthritis (OA) is the most common human joint disorder. Its complex pathogenesis remains poorly understood but appears multifactorial. To date, no specific pharmacological agent has been identified to alter the disease course of OA. Calcification of articular cartilage is a hallmark of OA and evidence suggests it contributes directly to joint degeneration. Calcium crystals are frequently found in OA synovial fluid but their exact role in the disease process is unclear. Basic calcium phosphate (BCP) crystals are the predominant crystal type found in OA and recent data indicate a pathogenic role for these crystals in OA. However, information on the exact frequency and distribution of BCP crystals vary considerably, mainly due to the lack of simple and reliable methods of detection. The purpose of this review is to describe the current available methods for detecting BCP crystals and to highlight their obvious advantages and limitations. Recent developments in the field are also discussed with particular reference to their potential clinical applicability. Improved methods of detection for BCP crystals could potentially aid the diagnosis of OA and the development of novel therapies.

Section snippets

BCP crystals in synovial fluid

Synovial fluid acts as both a lubricant and a source of biochemical nutrients to the relatively avascular articular surfaces. It is a viscous ultrafiltrate of plasma with a high glycoprotein and hyaluronic acid content, and its complex biological matrix presents significant analytical challenges [12]. BCP crystals are mainly composed of hydroxyapatite (HA), with smaller amounts of its precursors (octacalcium phosphate and tricalcium phosphate) [2]. Individual crystals are typically less than 1

Imaging methods

Imaging methods used in the detection of crystals in human joint tissue are primarily based on conventional microscopic analysis of synovial fluid samples, electron microscopy, or atomic force microscopy.

Spectroscopic methods

Spectroscopy relies on the identification of substances via the spectra emitted or absorbed by them and comparing these with specific emission/absorbance spectra of known compounds. Analysis of the absorbance, reflectance and transmittance properties of synovial fluid has been widely studied in OA research using a number of different techniques [12].

Calcium and phosphate analysis:

BCP crystals contain calcium, phosphate, hydroxyl and carbonate ions. Therefore, it seems reasonable to apply methods, which can determine the concentration of these ions in a given sample. Colorimetric/spectrophotometric methods involve the addition of various reagents (e.g. eriochrome blue, phosphonazo III) which then form a coloured complex with the specific element to be detected [12]. However, there are a number of problems with this approach when aiming to detect BCP crystals in synovial

Tetracycline staining of BCP crystals

The ability of tetracycline antibiotics to bind HA material and their fluorescence has enabled their use as labels of mineralisation in bones and teeth [41]. This property has recently been successfully adapted to the detection of HA/BCP crystals in synovial fluid [19]. The authors used oxytetracycline staining and ultraviolet light microscopy to identify and quantify increasing concentrations of synthetic BCP crystals in spiked porcine synovial fluid samples, as well as native BCP crystal

Conclusion

The clinical implications of improved detection methods for calcium-phosphate crystals in OA are wide-ranging. Calcification of articular cartilage is now well recognised as an indissociable feature of OA [6]. There is strong in vitro evidence, which demonstrates that such calcium deposition is inhibited by phosphocitrate [14], and is supported by complementary work using an animal model of OA [47]. To truly test the hypothesis that calcium-phosphate crystals play a causative role in OA, animal

Conflict of interest statement

None of the authors has any conflict of interest to declare.

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