On fragmenting, densely mineralised acellular protrusions into articular cartilage and their possible role in osteoarthritis

Abstract

High density mineralised protrusions (HDMP) from the tidemark mineralising front into hyaline articular cartilage (HAC) were first described in Thoroughbred racehorse fetlock joints and later in Icelandic horse hock joints. We now report them in human material. Whole femoral heads removed at operation for joint replacement or from dissection room cadavers were imaged using magnetic resonance imaging (MRI) dual echo steady state at 0.23 mm resolution, then 26-lm resolution high contrast X-ray microtomography, sectioned and embedded in polymethylmethacrylate, blocks cut and polished and re-imaged with 6-lm resolution X-ray microtomography. Tissue mineralisation density was imaged using backscattered electron SEM (BSE SEM) at 20 kV with uncoated samples. HAC histology was studied by BSE SEM after staining block faces with ammonium triiodide solution. HDMP arise via the extrusion of an unknown mineralisable matrix into clefts in HAC, a process of acellular dystrophic calcification. Their formation may be an extension of a crack self-healing mechanism found in bone and articular calcified cartilage. Mineral concentration exceeds that of articular calcified cartilage and is not uniform. It is probable that they have not been reported previously because they are removed by decalcification with standard protocols. Mineral phase morphology frequently shows the agglomeration of many fine particles into larger concretions. HDMP are surrounded by HAC, are brittle, and show fault lines within them. Dense fragments found within damaged HAC could make a significant contribution to joint destruction. At least larger HDMP can be detected with the best MRI imaging ex vivo.

Pyrophosphate: a key inhibitor of mineralisation

“…Chondrocytes, the resident cell type in cartilage, release ATP constitutively [70], display high levels of NPP1 activity and can produce large amounts of extracellular pyrophosphate [71,72]. In the degenerative joint disease osteoarthritis (OA), aberrant articular cartilage calcification may occur and damage the surrounding tissue [73]. NPP1 levels are reported to be lower in cartilage from patients with severe OA [74] and Enpp1 polymorphisms have been associated with hand OA [75].…”

A role for interleukins in ochronosis in a chondrocyte in vitro model of alkaptonuria

“…There have been advances in understanding the end-stage manifestations of AKU and its use as a model for documenting new advances in understanding Bcommon^os-teoarthritis (OA), and other arthropathies are shedding new light on disease processes [8,9,15]. There also appears to be a potential therapy for AKU.…”

Evaluation of laser ablation microtomy for correlative microscopy of hard tissues

“…Femoral head with prominent HDMPs from AKU case presenting with severe OA pain, necessitating the arthroplasty: case presented in Boyde et al . (). (A) Background = BSE SEM of block surface after staining with ammonium triiodide.…”

“…In every instance we had employed BSE SEM of topography free PMMA block surfaces. Most blocks were re‐imaged using BSE SEM after iodine staining – wet, using a triiodide solution, or dry, by iodine vapour to reveal cells and uncalcified tissue like cartilage, tendon, ligament, osteoid and muscle (Boyde, ; Boyde et al ., ,b; Ley et al ., ). Many of the samples had also been studied by confocal fluorescence and reflection imaging (Boyde et al ., ) and several by in‐house high‐contrast x‐ray microtomography (XMT: Boyde et al ., ) or standard commercial microtomography (μCT: Boyde et al ., ).…”

“…It was later discovered that this high‐density mineral infill (HDMI) material also extends into hyaline articular cartilage (HAC) as high‐density mineralised protrusions (HDMPs) which fragment to make a hard and sharp abrasive material within HAC – also lost to sight and mind after decalcification – and constitute one, as yet, hardly recognised method of joint destruction in osteoarthritis (OA). HDMPs were first clearly demonstrated in equine joints (Boyde, ; Ley et al ., ; Laverty et al ., ) and later found in human hip (Boyde et al ., ) and knee joints (unpublished).…”