Background: Meniscal calcification is considered to play a relevant role in the pathogenesis of osteoarthritis of the knee. Little is known about the biology of acetabular labral disease and its importance in hip pathology. Here, we analyze for the first time the calcification of the acetabular labrum of the hip (ALH) and its relation to hip cartilage degeneration. Methods: In this cross-sectional post-mortem study of an unselected sample of the general population, 170 ALH specimens and 170 femoral heads from 85 donors (38 female, 47 male; mean age 62.1 years) were analyzed by high-resolution digital contact radiography (DCR) and histological degeneration grade. The medial menisci (MM) from the same 85 donors served as an intra-individual reference for cartilage calcification (CC). Scanning electron microscopy (SEM), energy dispersive analysis (ED) and Raman spectroscopy were performed for characterization of ALH CC. Results: The prevalence of CC in the ALH was 100% and that in the articular cartilage of the hip (ACH) was 96.5%. Quantitative analysis revealed that the amount of ALH CC was higher than that in the ACH (factor 3.0, p <0.001) and in the MM (factor 1.3, p < 0.001). There was significant correlation between the amount of CC in the fibrocartilage of the left and right ALH (r =0.70, p < 0.001). Independent of age, the amount of ALH CC correlated with histological degeneration of the ALH (Krenn score) (r =0.55; p < 0.001) and the ACH (Osteoarthritis Research Society International (OARSI), r =0.69; p < 0.001). Calcification of the ALH was characterized as calcium pyrophosphate dihydrate deposition. Conclusion: The finding that ALH fibrocartilage is a strongly calcifying tissue is unexpected and novel. The fact that ALH calcification correlates with cartilage degeneration independent of age is suggestive of an important role of ALH calcification in osteoarthritis of the hip and renders it a potential target for the prevention and treatment of hip joint degeneration. Keywords: Acetabular labrum, Hip, Chondrocalcinosis, Cartilage calcification, CPPD, Osteoarthritis * Correspondence: email@example.com; firstname.lastname@example.org Equal contributors Department of Orthopaedics, University Medical Center Hamburg-Eppendorf, Martinistraße 52, 20246 Hamburg, Germany Full list of author information is available at the end of the article © The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Hawellek et al. Arthritis Research & Therapy (2018) 20:104 Page 2 of 10 Background articular cartilage degeneration in the hip. Since we have Osteoarthritis (OA) is a major health problem and rep- recently described that there appears to be a systemic resents the most common joint disease in western popu- drive for CC [25, 26], here we used both medial menisci lations . OA affects the whole joint and therefore (MM) as a reference for the individual propensity to involves hyaline cartilage, subchondral bone, the joint develop CC. capsule, periarticular ligaments and fibrocartilage . Until now the understanding of the molecular events Methods that initiate and maintain OA pathogenesis remain in- A total of 170 ALH, femoral heads (FH) and MM were completely understood. One factor of interest is cartilage obtained from both hip and knee joints in an unselected calcification (CC). It is detectable in 100% of the hyaline sample of 85 individuals (hereafter referred to as “donors”) cartilage of end-stage hip and knee OA [3, 4]. Calcium who underwent autopsy at the Department for Legal crystals have the potential to induce a pro-inflammatory Medicine , University Medical Center Hamburg- intra-articular milieu [5–8] and also to alter the bio- Eppendorf. Only donors with bilaterally intact hip and mechanical properties of the cartilage [9, 10], both of knee joints without any signs of hip and/or knee disease which may finally result in OA . other than OA were included in this study. None of the Compared to articular hyaline cartilage, the fibrocarti- donors had evidence of previous hip and/or knee surgery. lage of the meniscus of the knee seems to be particularly Donors with history of tumors, infections or rheumatic prone to calcification [12, 13] and meniscal calcification diseases were excluded from the study population. The is highly prevalent in knee OA [14, 15]. In addition, study was approved by the local ethics committee (refer- meniscal cells calcify more readily in OA than in healthy ence number PV4570) and is in compliance with the knees and calcification may alter the biomechanical Helsinki Declaration. The mean age was 62.1 years (range properties of the meniscus, which may further contribute 20–93 years); 38 of the donors were female and 47 male. to OA development . Accordingly, meniscal calcifica- Biometric characteristics of the donors are listed in tion is considered to play a relevant role in the pathogen- Table 1. Some of the data on hip articular cartilage calcifi- esis of knee OA [14–18]. cation in this larger cohort have previously been analyzed In contrast to the meniscus of the knee, little is known and published in another context . First the FH, ALH about calcification of the fibrocartilage of the hip, the ac- and MM were resected in toto. Any attached soft tissue etabular labrum (ALH) and its relation to cartilage de- was removed from the ALH, FH and MM. For FH ana- generation. Although the role of the acetabular labrum lysis, standardized 4 mm bone and cartilage slabs were cut in hip joint pathology has recently gained much atten- in the central coronal and axial planes, resulting in three tion [19, 20], knowledge about cellular mechanisms that standardized (central, anterior and posterior) slabs per govern labral function is scarce. It has recently been de- sample as published previously . ALH and MM were scribed that ALH cells appear to have a similar meta- kept in toto. bolic profile to meniscal cells , but to our knowledge there is only one study in which ALH calcification was an- Digital contact radiography (DCR) alyzed in a larger cohort (106 hip joints in 66 individuals) The ALH, the bone-cartilage slabs of the FH and the by computed tomography (CT), finding a prevalence of MM were washed with physiological solution to remove 18% . Of note, CC starts in the nanomicrometer to residual bone debris. Standardized radiographs were micrometer range and is therefore hard or almost im- taken (25 kV, 3.8 mAs, film focus distance 8 cm) using a possible to detect in the initial stages by standard radio- Table 1 Biometric characteristics of the study population (n =85) graphic methods with low-resolution x-ray, CT or magnetic resonance imaging (MRI). Therefore, little is Characteristic Value known about the actual prevalence of early initial Age in years 62.1 ± 19.3 crystallization in human joints in general, and in the Male 60.1 ± 18.6 ALH in particular . The most sensitive method Female 64.6 ± 20.0 available for the detection of such micro-calcifications is Height in cm high-resolution digital contact radiography (DCR) [4, 24]. Male 176.9 ± 7.1 A disadvantage of this method is that DCR can only be Female 164.7 ± 7.9 applied to tissue samples ex vivo. The goal of the present study was to describe the Body weight in kg prevalence of DCR-detectable ALH calcification in an Male 83.2 ± 18.3 unselected sample of the general population and to Female 72.3 ± 21.0 analyze the relationship between the amount of ALH Body mass index in kg/m 26.5 ± 6.0 calcification and the degree of fibrocartilage and Hawellek et al. Arthritis Research & Therapy (2018) 20:104 Page 3 of 10 high-resolution digital radiography device (Faxitron X- Ray, Illinois, USA). Quantitative computerized analysis of the areas of CC of each complete ALH, the three bone-cartilage slabs of the FH and the complete MM was performed with standard software (ImageJ 1.46, National Institutes of Health, Bethesda, USA) as pub- lished previously [3, 4, 28]. The percentage of calcifica- tion of the ALH and MM was determined by dividing the measured area of calcification by the total fibrocarti- lage area of the particular anatomical structure. The per- centage of CC in each of the three slabs of the FH was determined by dividing the measured area of calcifica- tion by the total cartilage area per slab. The mean amount of calcification measured from the three slabs of the FH was regarded to be representative of the entire articular cartilage of the hip (ACH). Classification of acetabular labrum calcification Based on previously published soft tissue classifications [29–31], the distribution of ALH calcification was cate- gorized as three different patterns (singular, spotted or streaky) on DCR images (Fig. 1). Histology Histological degeneration of the fibrocartilage of the superior-anterior part of each ALH and of the hyaline Fig. 2 a Representative digital contrast radiography (DCR) images cartilage of the main load-bearing zone of each FH (presented in original size and × 4 magnification (red boxes)) of the (central zone, directly adjacent to the central slab acetabular labrum (L = left, R = right) from one donor showing distinct plane) was assessed. All specimens were fixed in 4% cartilage calcification and the corresponding histological images in paraformaldehyde (PFA) for 24 h, dehydrated in 80% al- which cartilage calcifications (black) were confirmed histochemically by cohol, embedded in paraffin and 4-μmsectionswere von Kossa staining. b DCR images (presented in original size and × 4 magnification (red boxes)) with increasing cartilage calcifications (from prepared. Sections of the ALH were stained with left to the right) of the acetabular labrum from different donors and hematoxylin and eosin (Fig. 2b) and samples of the FH the corresponding rising histological degeneration grade, which was were stained with 1% Safranin-O to evaluate the histo- evaluated by the Krenn score (0–3) on hematoxylin-eosin (HE) staining logical degeneration grade of the tissue sample according to the Krenn-score for fibrocartilage (grade 0–3; Table 2)  and the Osteoarthritis Research Society International (OARSI) osteoarthritis cartilage histopathology assess- ment system for hyaline cartilage (grade 0–6) . Calcifi- cations identified by DCR were confirmed to represent calcium-phosphate crystal deposition by von Kossa staining (Fig. 2a). Characterization of acetabular labrum calcification To characterize the physicochemical nature of ALH CC detected by DCR and confirmed by von Kossa staining, 10 randomly selected labral specimens were processed for further analysis. Fig. 1 Exemplary samples of digital contact radiography images of the acetabular labrum from six different donors with distinct cartilage calcification. Cartilage calcification was detected as radiopaque spots Scanning electron microscopy within the fibrocartilage of the acetabular labrum. The cartilage For morphology studies semiquantitative electron probe calcification was classified into three typical calcification patterns microanalysis and scanning electron microscopy (SEM) (singular, spotted and streaky) of secondary electrons (SE) were performed on a Hawellek et al. Arthritis Research & Therapy (2018) 20:104 Page 4 of 10 Table 2 Krenn score – histopathological evaluation of the collected Raman spectra were compared to reference − 1 degeneration grade of the fibrocartilage (0–3) spectra for hydroxyapatite (960 cm ) and calcium − 1 Grade Features pyrophosphate dihydrate (1050 cm ). 0 Normal histological morphology � Isomorphic chondrocytes � Homogeneously eosinophil-stained matrix Statistical analysis � Regular cellularity The biometric characteristics of donors are reported as 1 Low-grade degeneration mean values ± standard deviations. For descriptive ana- � Low reduction of cellularity (small areas) � Inhomogeneous stained matrix lysis, mean cartilage calcification values for the hyaline � Small fissures in the matrix cartilage of the femoral head were used. Data were loga- 2 Moderate degeneration rithmically transformed if appropriate. For categorical � Moderate reduction of cellularity (large areas) data Fisher’s and McNemar’s tests were used. A linear � Variable size and shape of chondrocytes mixed model was used to analyze the difference between � Moderate fissures in the matrix the mean amount of cartilage calcification in the ALH, 3 High-grade degeneration FH and MM considering side and joint as fixed effects. � Strong reduction of cellularity � Large areas of complete loss of chondrocytes Subject was used as a random effect with a compound � Reticular/basophilic stained matrix symmetry covariance structure. In addition, the mixed (mucoid degeneration) model assumptions were checked using residual plots. � Large fissures in the matrix (pseudocysts) To report the association between continuous variables Pearson’s(r) or Spearman’s(r ) rank correlation coeffi- cient was calculated. To test the correlation between JEOL 8900RL (JEOL, Ltd., Akishima, Japan) to obtain cartilage calcification, histological degeneration and age, information about the surface of the ALH cartilage the mean value of the left and right femoral head and and minerals. The accelerating voltage was set to the mean value of the left and right labrum were calcu- 10 kV and a beam current of 1.5 nA was used for the lated for each individual. To avoid spurious correlation, quantitative analysis. SEM images were performed at a test of partial correlation was performed adjusting for 10 kV accelerating voltage and a variable beam the respective excluded parameters (cartilage calcifica- current between 0.1 and 0.25 nA. tion, histological OA grade and age). All statistical ana- To determine the chemical elements in the samples lyses were performed with statistical software R , and to analyze the qualitative and semiquantitative version 3.1.1. P values less than 0.05 were considered chemical compositions raw x-ray counts were acquired statistically significant. by a SiriusSD® (SGX Sensortech Ltd.) energy dispersive (ED) silicon drift detector. For calibration a natural apatite crystal (Ca (OH,Cl,F)(PO ) ) was used for cal- Results 5 4 3 cium and phosphate and albite (NaAlSi O ) was used Prevalence of cartilage calcification 3 8 for sodium. Prior to the measurement the samples were The prevalence of CC of the ALH was 100% (85/85) coated with a thin layer of gold (20 nm). (95% CI 0.96, 1.00), of the ACH it was 96.5% (82/85) (95% CI 0.90, 0.99) and of the MM it was 98.8% (84/85) (95% CI 0.94, 1.00) (Table 3). The prevalence of bilateral Raman spectroscopy CC of the ALH was 100% (85/85) (95% CI 0.96, 1.00), of Raman spectroscopy was performed on unstained la- the ACH it was 80.0% (68/85) (95% CI 0.70, 0.88) and of brum specimens. Raman spectra were obtained using a the MM it was 92.9% (79/85) (95% CI 0.85, 0.97) Horiba Jobin Yvon HR 800 UV Raman spectrometer (Table 3). Von Kossa stained histological sections con- with an attached Olympus BX41 microscope. The samples firmed that DCR-detectable CC actually represents were excited using a 488 nm laser line of a Coherent calcium-phosphate crystal depositions at the histological Sapphire solid-state laser, with 50 mW at the laser exit. level (Fig. 2a). CC was detected in 100% of the left and The use of a holographic grating with 600 lines/mm and a right ALH (85/85), in 88.2% of the left and right ACH CCD-detector with 1024 × 256 pixels yielded a spectral (75/85), in 94.1% of the left MM (80/85) and in 97.6% of dispersion of better than 2.2/cm per pixel. Raman spectra the right MM (83/85). There was no significant prepon- were collected in three spectral windows in the range 200/ derance of CC according to left or right side in the ALH cm to 4000/cm with an acquisition time of 2 × 2 s for each (p = 1.0), ACH (p = 1.0) and MM (p = 0.38) (Table 3). spectral window. The Raman spectra were frequency cor- There was no significant difference in the prevalence of rected using silicon (Si band at 520.4/cm), which was CC by sex in the ALH (p = 1.0), FH (p = 0.09) and MM measured directly after the sample measurement. The (p = 1.0). Hawellek et al. Arthritis Research & Therapy (2018) 20:104 Page 5 of 10 Table 3 Prevalence of DCR-detectable cartilage calcification (n = 85) Acetabular labrum Femoral head Medial meniscus Number Percentage Number Percentage Number Percentage Total calcified cartilage (CC) 85/85 100 82/85 96.5 84/85 98.8 Bilateral CC 85/85 100 68/85 80.0 79/85 92.9 Unilateral CC 0/85 0 14/85 16.5 5/85 5.9 Left CC 85/85 100 75/85 88.2 80/85 94.1 Right CC 85/85 100 75/85 88.2 83/85 97.7 Quantitative analysis of ALH calcification reveals a higher significantly higher than that in the ACH by factor 3.0 degree of calcification than in articular and meniscal (p < 0.001) and higher than that in the MM by factor 1.3 cartilage (p < 0.001) per tissue volume unit (Fig. 3d). There was significant correlation between the amount of CC in the fibrocartilage of the left and right ALH (r =0.70, Calcification of the ALH correlates with histological hip p < 0.001, 95% CI 0.57 0.79) (Fig. 3a) and between the degeneration independent of age ALH and the MM (r =0.66, p < 0.001, 95% CI 0.52 0.76) When adjusted for age there was significant correlation (Fig. 3b) and the ALH and the ACH (r =0.48, p <0.001, between the amount of CC in the ALH and the histo- 95% CI 0.30 0.63) (Fig. 3c). logical degeneration grade of the ALH according to the A significant difference was found between the Krenn score (r = 0.55, p < 0.001) (Figs. 2b and 4a). The amount of CC in the three distinct cartilage tissues, with distribution of the degeneration grade with detailed data the amount of CC in the fibrocartilage in the ALH being on the donors and according to the mean amount of CC Fig. 3 a–c Logarithmic scatter plots show significant correlation for the mean amount of calcified cartilage (CC) in percentage of total cartilage area between the left and right acetabular labrum (r = 0.70, 95% CI 0.57 0.79), p <0.001) (a), the medial meniscus and the acetabular labrum (r =0.66, 95% CI 0.52 0.76), p <0.001) (b) and the femoral head and the acetabular labrum (r = 0.48, 95% CI 0.30 0.63), p < 0.001) (c). Data points are jittered to avoid over plotting. Logarithmic scatter plots are shown with the blue orthogonal regression line and with the corresponding correlation coefficient (r). d Logarithmic effect plot of the mean amount of CC in percentage of total cartilage area for the femoral head, the acetabular labrum and the medial meniscus. The amount of CC in the fibrocartilage of the acetabular labrum was significant larger compared to the amount of CC in the medial meniscus (factor of 1.3, p < 0.001) and the amount of CC in the acetabular labrum was significant larger compared to the amount of CC in the hyaline cartilage of the femoral head (factor of 3.0, p < 0.001) Hawellek et al. Arthritis Research & Therapy (2018) 20:104 Page 6 of 10 ab cd Fig. 4 Logarithmic scatter plots show significant correlation between the mean amount of calcification (percentage of total cartilage area) of the acetabular labrum of the hip (ALH) and the histological degeneration grade (Krenn) of the ALH (r = 0.55, p < 0.001) after adjustment for age (a), the mean amount of ALH calcification (percentage of total cartilage area) and the histological osteoarthritis (OA) grade for the hyaline cartilage (Osteoarthritis Research Society International (OARSI)) of the femoral head (r = 0.35, p < 0.001) after adjustment for age (b) and the histological degeneration grade (Krenn) of the ALH and age (r =0.35, p < 0.001) after adjusting for cartilage calcification (d). c There was no correlation between the mean amount of ALH calcification (percentage of total cartilage area) and age (p = 0.72) after adjusting for the histological degeneration grade. Data points are jittered to avoid over plotting. Logarithmic scatter plots are shown with the blue orthogonal regression line and with the corresponding correlation coefficient (r) in the ALH is shown in Table 4. Moreover, there was Classification of acetabular labrum calcification significant correlation between the amount of CC in the In 56.5% of all analyzed samples calcification could be ALH and the histological OA grade of the ACH classified as a singular calcification pattern. In 28.8% and (OARSI) after adjustment for age (r = 0.35, p < 0.001) 14.7% of the samples, respectively, a spotted or a streaky (Fig. 4b). There was significant correlation between the calcification pattern was detected (Fig. 1). In samples with amount of CC in the ALH and age (r = 0.32, p = 0.003), a singular calcification pattern the mean amount of calcifi- which was no longer significant after adjustment for the cation was 0.38% (SD ±0.68) and the mean histological de- histological ALH degeneration grade (r = 0.04, p = 0.72) generation grade was 1.1 (SD ±0.8). In samples with a (Fig. 4c). There was significant correlation between the spotted or a streaky calcification pattern, respectively, the histological degeneration grade of the ALH and age mean amount of calcification was 0.91% (SD ±1.54) and (r = 0.50, p < 0.001), which persisted after adjustment 10.01% (SD ±12.2) and the mean histological degeneration for CC (r =0.35, p < 0.001) (Fig. 4d). grade was 1.5 (SD ±0.9) and 2.0 (SD ±1.0) (Table 5). Table 4 Distribution of the degeneration grade of the acetabular labrum (n = 170) Grade Number Percentage Male Percentage Female Percentage Mean amount of calcified cartilage 0 26/170 15.3 17/94 18.1 9/76 11.8 0.25 (SD ±0.31) 1 79/170 46.5 37/94 39.4 42/76 55.3 0.37 (SD ±0.77) 2 44/170 25.9 28/94 29.8 16/76 21.1 1.69 (SD ±4.40) 3 19/170 11.2 10/94 10.6 9/76 11.8 11.70 (SD ±12.25) Hawellek et al. Arthritis Research & Therapy (2018) 20:104 Page 7 of 10 Table 5 Calcification pattern of the acetabular labrum (n = 170) These data are entirely novel and shed new and unex- with the corresponding mean histological degeneration grade pected light on the potential clinical relevance of ALH (Krenn score) and the mean amount of cartilage calcification calcification. Given that in the existing literature the Pattern Krenn Score Mean calcified cartilage prevalence of ALH calcification is estimated to be Number Percentage Mean SD Percentage SD smaller than 20% , the prevalence of 100% reported here was highly unexpected. This first-sight discrepancy Singular 96/170 56.5% 1.1 ±0.8 0.38 ±0.68 is likely to be explained by the unequalled sensitivity of Spotted 49/170 28.8 1.5 ±0.9 0.91 ±1.54 CC detection by DCR as compared to native CT . Streaky 25/170 14.7 2.0 ±1.0 10.01 ±12.2 Taking this into account, it was still striking to observe that both the prevalence and the amount of CC per unit Characterization of acetabular labrum calcification of tissue volume were significantly higher in the ALH Well-developed crystals of prismatic and rhomboid habit, than in the ACH or the MM within the same individuals the typical form of calcium pyrophosphate dihydrate (Fig. 3d). Most interestingly in this context, we observed (CPPD) crystals, were detected by SEM imaging (Fig. 5a). CC in histologically healthy labral tissue. Thus, labral Using energy dispersive qualitative element analysis (ED) calcification cannot just be a result or byproduct of de- oxygen, phosphorous, calcium and small amounts of so- generative tissue changes, but rather seems to precede dium were detected. The mean calcium/phosphate (Ca/P) the degeneration. In light of the correlation between the molar ratio was approximately 1.0 in all samples, indicat- amount of CC in the ALH and histological degeneration ing the presence of CPPD crystals. By Raman spectros- of both the labrum and the articular cartilage independent copy only spectra could be detected, which can be of age (Fig. 4), these data open the possibility (although assigned to triclinic calcium pyrophosphate dihydrate (t- speculative at the present time) that the calcification CPPD, Ca P O ·2H O) (Fig. 5b). In conclusion, when crystals may be involved as a causative factor. We have 2 2 7 2 assessed by SEM, ED and Raman spectroscopy, only previously reported that articular cartilage calcification CPPD crystals were detected in the ALH samples. We did can be looked at as the result of a systemically driven not find evidence of the appearance of basic calcium process [25, 26], which was reconfirmed by the present phosphate crystals in the analyzed samples. data, exemplified by correlation between the amount of CC in the ALH and that in the MM and the contralateral ALH in this unselected cohort of donors (Fig. 3a, b). Discussion The results of this study support the idea that if the Here we demonstrated that fibrocartilage calcification of degree of CC in the ALH even in young individuals ex- the ALH is highly prevalent in an unselected sample of ceeds some yet to be defined threshold, this may trigger the general population, including in histologically labral and subsequently total hip joint pathologic healthy tissue. The amount of ALH calcification corre- change. Further research will be needed to support this lates significantly with fibrocartilage and articular cartil- concept, in particular in regard to the underlying cellular age degeneration of the hip independent of age. and molecular mechanisms and in regard to the Fig. 5 a Exemplary sample of scanning electron microscopy (SEM) imaging of the acetabular labrum of the hip (ALH) fibrocartilage showing well- developed rhomboid and prismatic crystals in various sizes and spatial arrangements, indicating the presence of calcium pyrophosphate dihydrate − 1 (CPPD) crystals. b Corresponding sample of measured Raman spectra (green line) with a peak count at 1050 cm confirming the presence of CPPD crystals in comparison to the reference spectra of t-CPPD (red line) Hawellek et al. Arthritis Research & Therapy (2018) 20:104 Page 8 of 10 epidemiology of the degree of CC in early symptomatic standardized planes, but only a small part of the articu- labral tissue in young adults without the presence of lating surface of the joint in absolute terms, which, the- advanced OA. oretically, opens up the possibility of sampling error. To further analyze the ALH calcification we developed None of the mentioned limitations is likely to have had a classification referring to previously published classifi- any profound impact on the major new findings and cations of other types of soft tissue calcification [29–31]. conclusions that we draw from the present study. By using three calcification patterns (single, spotted and streaky), we observed that streaky calcification is the pat- Conclusions tern in ALH calcification with the highest mean amount Calcification of the acetabular labrum of the hip is unex- of CC. Samples with a singular calcification pattern had pectedly highly prevalent and occurs even in healthy labral the lowest mean amount of CC in ALH calcification. tissue, but the amount of labral calcification significantly Samples with a spotted calcification pattern had a mean correlates with overall hip joint degeneration independent amount of CC that was between the other two patterns. of age. Calcification of the acetabular labrum can be classi- Interestingly ALH samples with a streaky calcification fied into three typical patterns (singular, spotted and pattern had more histological evidence of degeneration streaky) and is mainly induced by calcium pyrophosphate and samples with a singular calcification pattern had less dihydrate deposition. We propose that acetabular labral degeneration on average. Moreover, samples with a spot- calcification deserves further detailed study as a poten- ted calcification pattern displayed moderate degener- tially causative factor in labral pathological change and ation. To further characterize ALH calcification we early osteoarthritis of the hip. performed scanning electron microscopy, energy disper- Abbreviations sive analysis and Raman spectroscopy. By these analyses ACH: Articular cartilage of the hip; ALH: Acetabular labrum of the hip; we found evidence for the deposition of calcium pyro- BCP: Basic calcium phosphate; CC: Cartilage calcification; CT: Computed phosphate dihydrate (CPPD) crystals only, but not of tomography; CPPD: Calcium pyrophosphate dihydrate; DCR: High-resolution digital contact radiography; ED: Energy dispersive analysis; FH: Femoral head; basic calcium phosphate (BCP). Several studies have MM: Medial meniscus; OA: Osteoarthritis; OARSI: Osteoarthritis Research characterized the calcification of hyaline articular cartil- Society International; SEM: Scanning electron microscopy age in the hip  and knee [4, 36–39] joints. In these studies the detection of BCP and CPPD crystals was re- Acknowledgements We like to thank Prof. Michael Amling, IOBM University Medical Center Hamburg, ported. To our knowledge there are only few studies in Germany, for helpful discussions and support in this project. Moreover we would which calcification in the meniscus has been character- like to thank Elke Leicht for expert technical assistance. ized [39, 40] and there are no studies available that have Funding characterized calcification in the ALH. Using Fourier This project was supported by Deutsche Arthrose-Hilfe e.V. Grant-Nr.: P336- transform infrared (FTIR) spectroscopy, Dessombz et al. A117-Rüther-EP5-hawe2-schulter-pr-III-10 k-2016-16. detected CPPD crystals in one human meniscus (fibro- cartilage) while CPPD crystals and carbonated apatite Availability of data and materials All data generated or analyzed in this study are available on request from were detectable at the same time in the other meniscus the first or corresponding authors. that was analyzed . Kiraly et al. analyzed the type of crystals in 10 menisci by histological examination . Authors’ contributions TH and JH contributed to the conception and design of the study, They reported that 80% of the calcified meniscal tissue acquisition of data, analysis and interpretation of data and drafting and contained CPPD crystals and 20% BCP crystals. They revision of the manuscript. SH was responsible for statistical analysis and noticed that both crystal types can be found in the interpretation of the data. MK contributed to DCR and histological analysis and interpretation of the data. JB contributed to the conception and design meniscal tissue but the large amount of crystals within of the study and interpretation. BCS and AK contributed to analysis and the fibrocartilage of the knee appeared to be CPPD crys- interpretation of calcification analysis and revision of the manuscript. KP tals. Currently, it remains speculative whether BCP crys- contributed to the acquisition of data. WR contributed to the conception and design of the study. AN contributed to the conception and design of tals can be found in ALH calcification. In our ALH the study, analysis and interpretation of data and drafting and revision of the samples that underwent detailed physico-chemical crys- manuscript. All authors read and approved the final manuscript. tal characterization (n = 10), we found only CPPD and no BCP crystals. We conclude that calcification of the Ethics approval and consent to participate This cross-sectional study was approved by the local Ethics Committee of ALH seems to appear mainly by CPPD deposition, but the Medical Association Hamburg, Germany (Ärztekammer Hamburg, reference this need to be confirmed in future studies. number: PV4570) and was carried out according to existing rules and regulations Limitations of the present study include limited avail- of the University Medical Center Hamburg-Eppendorf. Informed consent to the removal and use of the joints for scientific purposes was obtained from the able information on the medical history of the donors. family members. Moreover there was no information about clinical symptoms of hip or knee pain and function. The stan- Competing interests dardized slab specimens of the FH reflect representative The authors declare that they have no competing interests. Hawellek et al. Arthritis Research & Therapy (2018) 20:104 Page 9 of 10 Publisher’sNote 17. MacMullan PA, McCarthy GM. The meniscus, calcification and osteoarthritis: Springer Nature remains neutral with regard to jurisdictional claims in a pathologic team. Arthritis Res Ther. 2010;12(3):116. published maps and institutional affiliations. 18. Stone AV, Vanderman KS, Willey JS, Long DL, Register TC, Shively CA, Stehle JR Jr, Loeser RF, Ferguson CM. Osteoarthritic changes in vervet monkey knees Author details correlate with meniscus degradation and increased matrix metalloproteinase Department of Orthopaedics, University Medical Center and cytokine secretion. Osteoarthr Cartil. 2015;23(10):1780–9. Hamburg-Eppendorf, Martinistraße 52, 20246 Hamburg, Germany. 19. Bsat S, Frei H, Beaulé PE. The acetabular labrum: a review of its function. Department of Medical Biometry and Epidemiology, University Medical Bone Joint J. 2016;98-B(6):730–5. Center Hamburg-Eppendorf, Hamburg, Germany. Department of Osteology 20. Rankin AT, Bleakley CM, Cullen M. Hip joint pathology as a leading cause of and Biomechanics, University Medical Center Hamburg-Eppendorf, Hamburg, groin pain in the sporting population: a 6-year review of 894 cases. Am J Germany. Department of Orthopaedic Surgery, Otto-von-Guerricke-University Sports Med. 2015;43(7):1698–703. Magdeburg, Magdeburg, Germany. Centrum of Geoscience, 21. Dhollander AA, Lambrecht S, Verdonk PC, Audenaert EA, Almqvist KF, Pattyn Georg-August-University Göttingen, Göttingen, Germany. Department of C, Verdonk R, Elewaut D, Verbruggen G. First insights into human acetabular Legal Medicine, University Medical Center Hamburg-Eppendorf, Hamburg, labrum cell metabolism. Osteoarthr Cartil. 2012;20(7):670–7. Germany. 22. Cooke WR, Gill HS, Murray DW, Ostlere SJ. Discrete mineralisation of the acetabular labrum: a novel marker of femoroacetabular impingement? 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Published: May 30, 2018
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